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segunda-feira, 25 de novembro de 2019

A Terra está à beira de sua sexta extinção em massa e a culpa é nossa

A taxa na qual as espécies de vertebrados estão morrendo excede em muito a norma

Jan Zalasiewizc

Caballos e colegas analisaram essas dificuldades e apresentaram provavelmente a estimativa mais robusta até agora de quão grave é a crise moderna.
Eles foram deliberadamente conservadores - estão bem conscientes dos perigos do lobo chorando em um tópico de tanta importância e onde as paixões são tão altas. Para começar, eles se limitam ao grupo de organismos mais estudado, os vertebrados. Então, eles fazem uma estimativa alta das extinções de fundo para comparar, para tornar as figuras modernas o mais dramáticas possível. E então, eles consideram apenas os animais conhecidos como extintos (o cenário "altamente conservador") ou acrescentam as extinções na natureza que provavelmente aconteceram, mas ainda não foram verificadas.

Mesmo com essa cautela, os números ainda são chocantes. Em vez das nove extinções entre vertebrados que seriam esperadas em circunstâncias geológicas normais desde 1900, sua estimativa conservadora acrescenta outras 468 extinções, espalhadas entre mamíferos, aves, répteis, anfíbios e peixes. Exemplos de espécies perdidas incluem o golfinho Yangtze e o sapo dourado da Costa Rica . Dependendo do grupo, as taxas de extinção são 10 vezes a mais de 100 vezes mais altas que o normal. Uma sexta extinção em massa, portanto, está começando. Eles estimam que ele iria rivalizar com a última grande catástrofe do passado, quando os dinossauros e muito mais desapareceram 65 milhões de anos atrás, em menos de três vidas humanas.
Mais uma vez, esta é uma estimativa conservadora. Ele simplesmente considera os mecanismos de morte em operação atualmente, de perda de habitat, predação, poluição e assim por diante. A projeção de Caballos não tenta levar em consideração, por exemplo, os efeitos do aquecimento global ou da acidificação do oceano. Uma vez que eles entrem em ação, eles varrerão muitas espécies de suas zonas de habitabilidade e aumentarão ainda mais a taxa de extinção.

Em termos de escala, estamos vivendo agora um daqueles episódios breves e raros da história da Terra, quando o quadro biológico da vida é desmantelado. É em todos os sentidos uma tragédia - mas, por si só, pode ser visto como apenas mais um episódio de destruição biológica na história do nosso planeta. A Terra já esteve aqui antes - e estará aqui novamente, antes que sua vida seja completamente extinta daqui a um bilhão de anos ou mais.

Essa perturbação específica da biosfera, no entanto, tem algumas características muito especiais. De fato, não houve nada remotamente parecido na história do nosso planeta. Por coincidência, um dos autores do estudo de Caballos, Anthony Barnosky, da Universidade da Califórnia em Berkeley, esteve envolvido em outro estudo publicado na mesma semana, um estudo que procurava identificar exatamente o que é tão diferente - e tão estranho. , em termos planetários, para não enfatizar muito bem o que está acontecendo com a biosfera no momento.

Este segundo estudo, liderado por Mark Williams, paleontólogo da Universidade de Leicester, identificou algumas novidades extraordinárias no coração dos eventos atuais. Primeiro, extinções passadas foram motivadas pelo que agora estão se tornando cavaleiros muito familiares de um apocalipse planetário: explosões vulcânicas maciças para sufocar a atmosfera e envenenar os mares; o caos causado por um grande impacto de asteroide; e os efeitos devastadores das rápidas mudanças climáticas. Nada disso realmente ocorreu na atual crise biológica - nem mesmo as mudanças climáticas, que ainda estão apenas em seus estágios iniciais.

Em vez disso, as extinções estão sendo conduzidas pelos efeitos de apenas uma única espécie, o Homo sapiens . Essa extinção em massa nunca havia ocorrido antes (com a possível exceção, 2,5 bilhões de anos atrás, quando um tipo de micróbio evoluiu a fotossíntese para liberar oxigênio, um gás que seria altamente tóxico para os outros micróbios que viviam na época, e isso teria foram empurrados para as margens da vida na Terra - onde ainda permanecem). Ainda mais extraordinariamente, essa espécie única vive em terra, mas também conseguiu se tornar o principal predador nos oceanos, causando o colapso de populações de baleias e peixes.

Ao todo, nossas espécies isoladas agora comandam algo entre 25% e 40% da produtividade primária na Terra. É uma produtividade que, em grandes áreas de terra, é “hiper fertilizada” pela extração de milhões de toneladas de nitrogênio do ar, no processo Haber-Bosch , e pela escavação de quantidades comparáveis de fosfato do solo.

Essas culturas superalimentadas são alimentadas, de maneira altamente eficiente, aos animais da fazenda, que comemos por sua vez. A escala desta operação é uma grande razão para a escala da extinção em massa de outros organismos.

O cientista Vaclav Smil, da Universidade de Manitoba, calculou que, simplesmente medidos em massa, os humanos agora compõem um terço dos vertebrados terrestres e os animais que mantemos para comer - vacas, porcos, ovelhas e outros - compõem a maioria dos outros dois terços. Todos os animais selvagens - elefantes, girafas, tigres e assim por diante - são agora menos de 5% em massa. É uma medida de como eles foram levados às margens pelos humanos.

Os seres humanos mudam as coisas de outras maneiras - agora direcionam a evolução dos animais que são úteis para eles, através da criação e da engenharia genética: novamente, é uma novidade planetária. A energia que nossa espécie obtém da fotossíntese não é suficiente e, portanto, extraímos energia fotossintética do solo, como hidrocarbonetos, em grandes quantidades, e a usamos para alimentar nossas máquinas.

Essas máquinas - carros, aviões, computadores e muito mais - foram, juntamente com seu software humano, denominadas tecnosfera pelo geólogo Peter Haff, da Universidade Duke. Ele o vê como um sistema emergente com sua própria dinâmica interna (e que os humanos atualmente dirigem, mas realmente não controlam) - na verdade, uma ramificação da biosfera. Seja o que for, está evoluindo na velocidade da luz em comparação com a evolução biológica.

As mudanças na biologia da Terra incluem, portanto, um evento de extinção em massa de rápido desenvolvimento, conforme traçado por Gerardo Caballos e seus colegas.

Mas isso pode ser visto como parte de uma transformação muito mais profunda. Novos padrões fundamentais estão surgindo que podem ser comparados, digamos, com a mudança, meio bilhão de anos atrás, quando uma biosfera composta apenas por micróbios deu lugar a um dominado por animais multicelulares.

Poderia esse novo padrão planetário se desenvolver - talvez bem o suficiente para ajudar a impedir uma extinção em massa? Atualmente, a tecnosfera é mais um parasita do que um parceiro da biosfera - é terrível na reciclagem, por exemplo.

Mas alguns aspectos podem ajudar a aliviar os piores efeitos do aquecimento global. Por exemplo, os seres humanos causaram a maior migração de espécies da história. Algumas dessas espécies invasoras podem ser bem adaptadas a novas temperaturas mais altas. E um melhor uso de energia e materiais pode reduzir a pressão sobre os ecossistemas naturais restantes.

Evitar uma extinção em massa ainda é possível - mas não temos muito tempo.
O autor é professor de paleobiologia na Universidade de Leicester

NÃO É A PRIMEIRA VEZ

Extinções em massa anteriores

A história geológica inclui muitos períodos em que as espécies morreram em grande número. Em cada um dos seguintes, mais da metade das espécies da Terra desapareceu:

1 Ordoviciano Final, 443 milhões de anos atrás.

Isso coincide com glaciação muito rápida; o nível do mar caiu mais de 100 metros, devastando ecossistemas marinhos rasos; menos de um milhão de anos depois, houve uma segunda onda de extinções quando o gelo derreteu, o nível do mar subiu rapidamente e os oceanos se esgotaram.

2 Devoniano tardio, c 360 milhões de anos atrás.

Um evento prolongado e confuso, novamente atingindo a vida em mares rasos com muita força e uma extinção que provavelmente se deveu às mudanças climáticas.

3 Extinção em massa Permiano-Triássica, c 250 milhões de anos atrás.

O maior de todos, 'O Grande Morrer', de mais de 95% das espécies, está fortemente ligado a enormes erupções vulcânicas na Sibéria que causaram, entre outros efeitos, um breve episódio selvagem do aquecimento global.

4 Extinção em massa Triássico-Jurássico, cerca de 200 milhões de anos atrás.

Isso tem sido associado a outra enorme explosão de vulcanismo.

5 Extinção em massa do período cretáceo-paleógeno há 65 milhões de anos.

Isso matou os dinossauros e muito mais; um impacto de asteróide no México provavelmente causou o dano, mas o ecossistema do mundo pode ter sido enfraquecido por explosões vulcânicas no que é hoje a Índia.

segunda-feira, 15 de outubro de 2018

The Smallest-Known Neonate Individual of Tylosaurus (Mosasauridae, Tylosaurinae) Sheds New Light on the Tylosaurine Rostrum and Heterochrony

Takuya Konishi, Paulina Jiménez-Huidobro & Michael W. Caldwell

Received 11 Aug 2017, Accepted 08 Jun 2018, Published online: 11 Oct 2018

Abstract

We here report on the smallest-known, neonate-sized Tylosaurus specimen, FHSM VP-14845, recovered from the lower Santonian portion of the Niobrara Chalk exposed in Kansas, U.S.A. Lacking any associated adult-sized material, FHSM VP-14845 comprises fragmentary and associated cranial bones, here considered to represent a single neonatal individual with an estimated skull length of 30 cm. Despite its small size, a suite of cranial characters diagnoses FHSM VP-14845 as a species of Tylosaurus, including the elongate basisphenoid morphology. At the same time, FHSM VP-14845 unexpectedly lacks a conical predental rostrum on the premaxilla, generally regarded as diagnostic of this genus. Further, the first and the second premaxillary teeth are closely spaced, with the second set positioned posterolateral to the first, contributing to the overall shortness of the dentigerous premaxilla. Because a conical predental rostrum is already present in ontogenetically young specimens of T. nepaeolicus and T. proriger with respective skull lengths of approximately 40 and 60 cm, formation of such a rostrum must have taken place very early in postnatal ontogeny. Our recognition of a neonate-sized Tylosaurus specimen without an elongate predental rostrum of the premaxilla suggests hypermorphosis as a likely heterochronic process behind the evolution of this iconic tylosaurine feature.
Citation for this article: Konishi, T., P. Jiménez-Huidobro, and M. W. Caldwell. 2018. The smallest-known neonate individual of Tylosaurus (Mosasauridae, Tylosaurinae) sheds new light on the tylosaurine rostrum and heterochrony. Journal of Vertebrate Paleontology. DOI: 10.1080/02724634.2018.1510835.

INTRODUCTION

The fossil record of Tylosaurus (Mosasauridae: Tylosaurinae) begins unequivocally in the late Coniacian (Everhart, 2001 Everhart, M. J. 2001. Revisions to the biostratigraphy of the Mosasauridae (Squamata) in the Smoky Hill Chalk Member of the Niobrara Chalk (Late Cretaceous) of Kansas. Transactions of the Kansas Academy of Science 104:59–78.[Crossref], [Google Scholar], 2005a Everhart, M. J. 2005a. Tylosaurus kansasensis, a new species of tylosaurine (Squamata, Mosasauridae) from the Niobrara Chalk of western Kansas, U.S.A. Netherlands Journal of Geosciences 84:231–240.[Crossref], [Web of Science ®], [Google Scholar]). Because the first occurrence of a taxon means that the species evolved before that point in time, it is not surprising that possible Tylosaurus are recognized from early Coniacian rocks (Everhart, 2005b Everhart, M. J. 2005b. Earliest record of the genus Tylosaurus (Squamata; Mosasauridae) from the Fort Hays Limestone (Lower Coniacian) of western Kansas. Transactions of the Kansas Academy of Science 108:149–155.[Crossref], [Google Scholar]) to even older units of rock and time (i.e., Turonian) (Polcyn et al., 2008 Polcyn, M. J., G. L. Bell Jr., K. Shimada, and M. J. Everhart. 2008. The oldest North American mosasaurs (Squamata: Mosasauridae) from the Turonian (Upper Cretaceous) of Kansas and Texas with comments on the radiations of major mosasaur clades; pp. 137–155 in M. J. Everhart (ed.), Proceedings of the Second Mosasaur Meeting, Hays, Kansas. Fort Hays Studies Special Issue 3. May 3–6, 2007. Fort Hays State University, Hays, Kansas. [Google Scholar]; Flores, 2013 Flores, A. L. 2013. Occurrence of a tylosaurine mosasaur (Mosasauridae; Russellosaurina) from the Turonian of Chihuahua State, Mexicao. Boletín de la Sociedad Geológica Mexicana 65:99–107.[Crossref], [Web of Science ®], [Google Scholar]). Thereafter, the known temporal range for the genus extends at least to the early Maastrichtian, making Tylosaurus one of the longest-existing mosasaur genera known (Caldwell and Diedrich, 2005 Caldwell, M. W., and C. G. Diedrich. 2005. Remains of Clidastes Cope, 1868, an unexpected mosasaur in the upper Campanian of NW Germany. Netherlands Journal of Geosciences 84:213–220.[Crossref], [Web of Science ®], [Google Scholar]; Bullard, 2006 Bullard, T. S. 2006. Anatomy and systematics of North American tylosaurine mosasaurs. M.Sc. thesis, University of Alberta, Edmonton, Alberta, Canada, 208 pp. [Google Scholar]; Bullard and Caldwell, 2010 Bullard, T. S., and M. W. Caldwell. 2010. Redescription and rediagnosis of the tylosaurine mosasaur Hainosaurus pembinensis Nicholls, 1988, as Tylosaurus pembinensis (Nicholls, 1988). Journal of Vertebrate Paleontology 30:416–426.[Taylor & Francis Online], [Web of Science ®], [Google Scholar]; Konishi and Caldwell, 2011 Konishi, T., and M. W. Caldwell. 2011. Two new plioplatecarpine (Squamata, Mosasauridae) genera from the Upper Cretaceous of North America, and a global phylogenetic analysis of plioplatecarpines. Journal of Vertebrate Paleontology 31:754–783.[Taylor & Francis Online], [Web of Science ®], [Google Scholar]; Jiménez-Huidobro and Caldwell, 2016 Jiménez-Huidobro, P., and M. W. Caldwell. 2016. Reassessment and reassignment of the early Maastrichtian mosasaur Hainosaurus bernardi Dollo, 1885, to Tylosaurus Marsh, 1872. Journal of Vertebrate Paleontology 36:3, DOI: 10.1080/02724634.2016.1096275[Taylor & Francis Online], [Web of Science ®], [Google Scholar]; Konishi et al., 2016 Konishi, T., M. W. Caldwell, T. Nishimura, K. Sakurai, and K. Tanoue. 2016. A new halisaurine mosasaur (Squamata: Halisaurinae) from Japan: the first record in the western Pacific realm and the first documented insights into binocular vision in mosasaurs. Journal of Systematic Palaeontology 14:809–839.[Taylor & Francis Online], [Web of Science ®], [Google Scholar]). Throughout their known stratigraphic and paleobiogeographic range, Tylosaurus attained the largest body size among sympatric members of the family and, based on their gastric contents, were apex predators that fed upon various marine vertebrates including other mosasaurs and plesiosaurs (Martin and Bjork, 1987 Martin, J. E., and P. R. Bjork. 1987. Gastric residues associated with a mosasaur from the Late Cretaceous (Campanian) Pierre Shale in South Dakota. Dakoterra 3:68–72. [Google Scholar]; Everhart, 2004 Everhart, M. J. 2004. Plesiosaurs as the food for mosasaurs; new data on the stomach contents of a Tylosaurus proriger (Squamata; Mosasauridae) from the Niobrara Formation of western Kansas. The Mosasaur 7:41–46. [Google Scholar]). Numerous medium- to large-sized specimens presumably pertaining to subadult and adult Tylosaurus are known, including KU 5033, whose skull and total body length is estimated to have been approximately 1.8 m and nearly 13 m, respectively (e.g., Everhart, 2005c Everhart, M. J. 2005c. Oceans of Kansas. Indiana University Press, Bloomington, Indiana, 322 pp. [Google Scholar]; pers. observ.). Conversely, small (<50 and="" are="" class="ref-lnk" cm="" documented="" genus="" in="" length="" literature="" not="" of="" rare="" recently="" skull="" span="" specimens="" the="" total="" until="" verhart="" well="">2005a Everhart, M. J. 2005a. Tylosaurus kansasensis, a new species of tylosaurine (Squamata, Mosasauridae) from the Niobrara Chalk of western Kansas, U.S.A. Netherlands Journal of Geosciences 84:231–240.[Crossref], [Web of Science ®], [Google Scholar]

; Konishi and Caldwell, 2007a Konishi, T., and M. W. Caldwell. 2007a. Ecological and evolutionary implications of ontogenetic changes in the marginal dentition of Tylosaurus proriger (Squamata: Mosasauridae). Journal of Vertebrate Paleontology 27(3, Supplement):101A.[Web of Science ®], [Google Scholar]; Jiménez-Huidobro et al., 2016 Jiménez-Huidobro, P., T. R. Simoes, and M. W. Caldwell. 2016. Re-characterization of Tylosaurus nepaeolicus (Cope, 1874) and Tylosaurus kansasensis Everhart, 2005: ontogeny or sympatry? Cretaceous Research 65:68–81.[Crossref], [Web of Science ®], [Google Scholar]; but see Sheldon, 1993 Sheldon, M. A. 1993.

Ontogenetic study of selected mosasaurs of North America. M.S. thesis, University of Texas, Austin, Texas, 184 pp. [Google Scholar]). Konishi et al. (2010 Konishi, T., M. W. Caldwell, and G. L. Bell Jr. 2010. Redescription of the holotype of Platecarpus tympaniticus Cope, 1869 (Mosasauridae: Plioplatecarpinae), and its implications for the alpha taxonomy of the genus. Journal of Vertebrate Paleontology 30:1410–1421.[Taylor & Francis Online], [Web of Science ®], [Google Scholar]) also suggested that the holotype of Platecarpus anguliferus (Cope, 1874 Cope, E. D. 1874. The Vertebrata of the Cretaceous period found west of the Mississippi River. Bulletin of the United States Geological and Geographical Survey of the Territories 1:3–48. [Google Scholar]) belongs to Tylosaurus sp., hinting at the possibility that some small Tylosaurus material may have been misidentified as various contemporary plioplatecarpines, a group of small- to medium-sized russellosaurines sensu Polcyn and Bell (2005 Polcyn, M. J., and G. L. Bell Jr. 2005. Russellosaurus coheni n. gen., n. sp., a 92 million-year-old mosasaur from Texas (U.S.A.), and the definition of the parafamily Russellosaurina. Netherlands Journal of Geosciences 84:321–333.[Crossref], [Web of Science ®], [Google Scholar]). A particularly diminutive mosasaur specimen, FHSM VP-14845, the subject of the current study, is one such example.

Specimen FHSM VP-14845, an exceptionally small-sized mosasaur specimen consisting of fragmentary jaw and cranial elements, was collected in 1991 from the Smoky Hill Chalk Member in western Kansas. With the transverse width of the alveolar margin barely reaching 1 cm, the specimen likely represents a neonatal individual. The material had been informally assigned to Platecarpus, although it has never been formally described.

In this contribution, we first describe FHSM VP-14845 as representing the smallest-known specimen of Tylosaurus, its generic assignment augmented through comparison with other, larger North American specimens that are unequivocally assignable to the genus. We then discuss its bearings on Tylosaurus ontogeny. Finally, we present evidence for the developmental stage of this individual to be neonatal and discuss the heterochronic nature of the iconic and conical premaxillary rostrum characteristic of the genus Tylosaurus and the subfamily Tylosaurinae.

Institutional Abbreviations

AMNH, American Museum of Natural History, New York, New York, U.S.A.; ANSP, Academy of Natural Sciences Philadelphia, Philadelphia, Pennsylvania, U.S.A.; CMC VP, Cincinnati Museum Center, Cincinnati, Ohio, U.S.A.; FHSM VP, Fort Hays Sternberg Museum, Hays, Kansas, U.S.A.; IPB, Goldfuss-Museum im Institut für Paläontologie, Bonn, Germany; KU, University of Kansas Natural History Museum, Lawrence, Kansas, U.S.A.; MCZ, Museum of Comparative Zoology, Harvard University, Cambridge, Massachusetts, U.S.A.; RMM, Red Mountain Museum (currently McWane Science Center), Birmingham, Alabama, U.S.A.; TMM, Texas Memorial Museum, University of Texas, Austin, Texas, U.S.A.; UCMP, University of California Museum of Paleontology, Berkeley, California, U.S.A.; YPM, Yale Peabody Museum of Natural History, New Haven, Connecticut, U.S.A.

SYSTEMATIC PALEONTOLOGY

REPTILIA Linnaeus, 1758 Linnaeus, C. 1758. Systema Naturae, Secundum Classes, Ordines, Genera, Species cum Characteribus, Differentiis, Synonymis, Locis. Tomus I. Editio Decima, Reformata. Laurentii Salvii, Stockholm, 824 pp. [Google Scholar]

SQUAMATA Oppel, 1811 Oppel, M. 1811. Die Ordnungen, Familien, und Gattungen der Reptilien als Prodrom Einer Naturgeschichte Derselben. Joseph Lindauer, Munich, 86 pp.[Crossref], [Google Scholar]

MOSASAURIDAE Gervais, 1852 Gervais, P. 1848-1852. Zoologie et Paléontologie Françaises (Animaux Vertébrés), first edition. Libraire de la Société de Géographie, Paris, 271 pp. [Google Scholar]

TYLOSAURINAE Williston, 1897 Williston, S. W. 1897. Range and distribution of the mosasaurs. Kansas University Quarterly 6:177–189. [Google Scholar]

TYLOSAURUS Marsh, 1872b Jiménez-Huidobro, P., and M. W. Caldwell. 2016. Reassessment and reassignment of the early Maastrichtian mosasaur Hainosaurus bernardi Dollo, 1885, to Tylosaurus Marsh, 1872. Journal of Vertebrate Paleontology 36:3, DOI: 10.1080/02724634.2016.1096275[Taylor & Francis Online], [Web of Science ®], [Google Scholar]

Liodon (in part) Cope, 1869 Cope, E. D. 1869. Remarks on Holops brevispinus, Ornithotarsus immanis and Macrosaurus proriger. Proceedings of the Academy of Natural Sciences of Philadelphia 21:123. [Google Scholar]–1870:200.

Rhinosaurus Marsh, 1872a Marsh, O. C. 1872a. On the structure of the skull and limbs of mosasauroid reptiles, with descriptions of new genera and species. American Journal of Science 3rd series 3:448–464.[Crossref], [Google Scholar]:17.

Rhamphosaurus Cope, 1872 Cope, E. D. 1872. Remarks on discoveries recently made by Prof. O. C. Marsh. Proceedings of the Academy of Natural Sciences of Philadelphia 24:140–141. [Google Scholar]:141.

Tylosaurus Marsh, 1872b Marsh, O. C. 1872b. Note on Rhinosaurus. American Journal of Science 3rd series 4:147. [Google Scholar]:47.

Type species

Macrosaurus proriger Cope (1869 Cope, E. D. 1869. Remarks on Holops brevispinus, Ornithotarsus immanis and Macrosaurus proriger. Proceedings of the Academy of Natural Sciences of Philadelphia 21:123. [Google Scholar]).

Holotype

MCZ 4374.

Generic Diagnosis

See Russell (1967 Russell, D. A. 1967. Systematics and morphology of American mosasaurs. Bulletin of the Peabody Museum of Natural History 23:1–241. [Google Scholar]:171–173) and Jiménez-Huidobro and Caldwell (2016 Jiménez-Huidobro, P., and M. W. Caldwell. 2016. Reassessment and reassignment of the early Maastrichtian mosasaur Hainosaurus bernardi Dollo, 1885, to Tylosaurus Marsh, 1872. Journal of Vertebrate Paleontology 36:3, DOI: 10.1080/02724634.2016.1096275[Taylor & Francis Online], [Web of Science ®], [Google Scholar]).
TYLOSAURUS sp.
(Figs. 2–5, 7−9, 12)

Referred Specimens

FHSM VP-14845: dentigerous portion of premaxilla with broken base of internarial bar, other tooth-bearing jaw fragments, right and left splenial fragments, right and left partial coronoids, incomplete quadrates on both sides, right and left partial pterygoids, and partial braincase (basisphenoid). FHSM VP-14840: anterior portion of premaxilla, broken at the second tooth row. FHSM VP-14843: premaxilla, with broken posterior portion of internarial bar.

Locality and Horizon

FHSM VP-14845: approximately 2.4 km (1.5 miles) south of Castle Rock, southeastern Gove County (coordinates: SW1/4, Sec. 14, T14S, R26W [approximately 38°49′51.65″N, 100°11′14.25″W]) in western Kansas, from above Hattin’s (1982 Hattin, D. E. 1982. Stratigraphy and depositional environment of Smoky Hill Chalk Member, Niobrara Chalk (Upper Cretaceous) of the type area, western Kansas. Kansas Geological Survey Bulletin 225:1–108. [Google Scholar]) Marker Unit 7 (M. J. Everhart, pers. comm., Oct. 15, 2013) in the Smoky Hill Chalk Member of the Niobrara Chalk (Fig. 1; Everhart, 2001 Everhart, M. J. 2001. Revisions to the biostratigraphy of the Mosasauridae (Squamata) in the Smoky Hill Chalk Member of the Niobrara Chalk (Late Cretaceous) of Kansas. Transactions of the Kansas Academy of Science 104:59–78.[Crossref], [Google Scholar]). Lower to middle Santonian, Upper Cretaceous (Everhart, 2001 Everhart, M. J. 2001. Revisions to the biostratigraphy of the Mosasauridae (Squamata) in the Smoky Hill Chalk Member of the Niobrara Chalk (Late Cretaceous) of Kansas. Transactions of the Kansas Academy of Science 104:59–78.[Crossref], [Google Scholar]). FHSM VP-14840: Gove County, western Kansas, Smoky Hill Chalk Member, Niobrara Chalk. Upper Coniacian–lower Santonian (Everhart, 2001 Everhart, M. J. 2001. Revisions to the biostratigraphy of the Mosasauridae (Squamata) in the Smoky Hill Chalk Member of the Niobrara Chalk (Late Cretaceous) of Kansas.

Transactions of the Kansas Academy of Science 104:59–78.[Crossref], [Google Scholar]). FHSM VP-14843: Gove County, western Kansas, Smoky Hill Chalk Member, Niobrara Chalk. Upper Coniacian–lower Santonian (Everhart, 2001 Everhart, M. J. 2001. Revisions to the biostratigraphy of the Mosasauridae (Squamata) in the Smoky Hill Chalk Member of the Niobrara Chalk (Late Cretaceous) of Kansas. Transactions of the Kansas Academy of Science 104:59–78.[Crossref], [Google Scholar]). More precise provenances for the latter two specimens are unknown.

FIGURE 1. Locality and horizon of FHSM VP-14845. A, map of continental U.S.A. with the Santonian (ca. 85 Ma) coastlines of the Western Interior Seaway superimposed, showing the approximate specimen locality (white star) in Gove County, western Kansas; see below for references. B, map of Kansas showing the Niobrara Chalk exposed (in gray), accompanied by a stratigraphic section of the chalk. Within the Niobrara Chalk, FHSM VP-14845 was derived from the lower Smoky Hill Chalk Member just above Marker Unit 7 (MU 7) of Hattin (1982 Hattin, D. E. 1982. Stratigraphy and depositional environment of Smoky Hill Chalk Member, Niobrara Chalk (Upper Cretaceous) of the type area, western Kansas. Kansas Geological Survey Bulletin 225:1–108. [Google Scholar]) and is early–middle Santonian (ca. 85 Ma) in age (Ogg et al., 2004 Ogg, J. G., F. P. Agterberg, and F. M. Gradstein. 2004. The Cretaceous Period; pp. 344–383 in F. M. Gradstein, J. G. Ogg, and A. Smith (eds.), A Geologic Time Scale. Cambridge University Press, Cambridge, U.K. [Google Scholar]). Base map in A from U.S. Geological Survey National Map Viewer (http://nmviewogc.cr.usgs.gov/viewer.htm); the seaway coastlines after Deep Time Maps and Smith et al. (1994 Smith, A. G., D. G. Smith, and B. M. Funnell. 1994. Atlas of Mesozoic and Cenozoic Coastlines. Cambridge University Press, Cambridge, U.K., 99 pp. [Google Scholar]). B modified from Kansas Geological Survey Map M-118, Surficial Geology of Kansas, and the thickness of the chalk after Hodson and Wahl (1960 Hodson, W. G., and K. D. Wahl. 1960. Geology and ground-water resources of Gove County, Kansas. Kansas Geological Survey Bulletin 145:1–126. [Google Scholar]). Abbreviations: FHLM, Fort Hays Limestone Member; KS, Kansas; MU, Marker Unit; SHCM, Smoky Hill Chalk Member; WIS, Western Interior Seaway.

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DESCRIPTION AND COMPARISON OF FHSM VP-14845

Premaxilla

The rostrum is mostly incomplete except for its distal end, which indicates that the predental rostrum was only about 3 mm long, barely equivalent to the basal diameter of the premaxillary tooth crowns (Fig. 2). Inferred from the remaining bone surfaces, and also from the posterolateral positioning of the second premaxillary tooth relative to the first one (see below), the reconstructed outline of the dentigerous portion of the premaxilla in dorsal aspect describes a gently pointed arc that, compared with adult Tylosaurus specimens, is proportionally much shorter. As shown in Table 1, FHSM VP-14845 is the only Tylosaurus specimen we examined that showed the distance across the widest part of the premaxilla exceeding the snout length anterior to that part of the bone (length:width ratio = 0.86; Fig. 2, Table 1). The outline also differs sufficiently from that of adult forms of Plesioplatecarpus and Platecarpus known from the Niobrara Chalk in exhibiting a pointed anterior end instead of a transversely straight one (e.g., Russell, 1967 Russell, D. A. 1967. Systematics and morphology of American mosasaurs. Bulletin of the Peabody Museum of Natural History 23:1–241. [Google Scholar]:fig. 83). In adult forms of Ectenosaurus clidastoides and the mosasaurine Clidastes, the two sides of the snout converge at a more acute angle than in FHSM VP-14845 (compare Russell, 1967 Russell, D. A. 1967.

Systematics and morphology of American mosasaurs. Bulletin of the Peabody Museum of Natural History 23:1–241. [Google Scholar]:figs. 72 and 86; ANSP 10193 [C. propython holotype, pers. observ.]). Polcyn et al. (2008 Polcyn, M. J., G. L. Bell Jr., K. Shimada, and M. J. Everhart. 2008. The oldest North American mosasaurs (Squamata: Mosasauridae) from the Turonian (Upper Cretaceous) of Kansas and Texas with comments on the radiations of major mosasaur clades; pp. 137–155 in M. J. Everhart (ed.), Proceedings of the Second Mosasaur Meeting, Hays, Kansas. Fort Hays Studies Special Issue 3. May 3–6, 2007. Fort Hays State University, Hays, Kansas. [Google Scholar]) reported on a tylosaurine premaxilla TMM 40092-27 from the Turonian portion (Arcadia Park Shale) of the Eagle Ford Shale exposed in northeastern Texas. The seemingly complete dentigerous portion of this geologically older material is essentially equidimensional (length:width ratio = 0.98; Fig. 2C), and is characterized by readily converging sides forming an abbreviated predental rostrum, the length of which is comparable to the basal diameter of the premaxillary teeth (see also Polcyn et al., 2008 Polcyn, M. J., G. L. Bell Jr., K. Shimada, and M. J. Everhart. 2008. The oldest North American mosasaurs (Squamata: Mosasauridae) from the Turonian (Upper Cretaceous) of Kansas and Texas with comments on the radiations of major mosasaur clades; pp. 137–155 in M. J. Everhart (ed.), Proceedings of the Second Mosasaur Meeting, Hays, Kansas. Fort Hays Studies Special Issue 3. May 3–6, 2007. Fort Hays State University, Hays, Kansas. [Google Scholar]:fig. 5C). Consequently, the overall dorsal outline of the dentigerous portion of FHSM VP-14845 bears the closest similarity to that of the Turonian-aged TMM 40092-27, particularly concerning its overall shortness and abbreviated predental rostrum.

FIGURE 2. Dentigerous portion of tylosaurine premaxillae. A–B, FHSM VP-14845, Tylosaurus sp. in A, dorsal and B, ventral views. C, TMM 40092-27, Tylosaurinae, in ventral view. Broken lines in A and B indicate reconstructed outlines of the element. C based on Polcyn et al. (2008 Polcyn, M. J., G. L. Bell Jr., K. Shimada, and M. J. Everhart. 2008. The oldest North American mosasaurs (Squamata: Mosasauridae) from the Turonian (Upper Cretaceous) of Kansas and Texas with comments on the radiations of major mosasaur clades; pp. 137–155 in M. J. Everhart (ed.), Proceedings of the Second Mosasaur Meeting, Hays, Kansas. Fort Hays Studies Special Issue 3. May 3–6, 2007. Fort Hays State University, Hays, Kansas. [Google Scholar]:fig. 5C). Abbreviations: inb, internarial bar; rp, resorption pit; rs, predental rostrum; tb1, first tooth base; tb2, second tooth base; vp, vomerine process (of premaxilla). Scale bars equal 1 cm (A and B) and 5 cm (C).

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TABLE 1. Length: width ratio of dentigerous portion of premaxilla among small Tylosaurus specimens from North America. Note that TMM 40092-27, whose corresponding ratio is 0.98 (not shown here), is not only larger but is also stratigraphically much older (Turonian, Eagle Ford Fm., Texas; Polcyn et al., 2008 Polcyn, M. J., G. L. Bell Jr., K. Shimada, and M. J. Everhart. 2008. The oldest North American mosasaurs (Squamata: Mosasauridae) from the Turonian (Upper Cretaceous) of Kansas and Texas with comments on the radiations of major mosasaur clades; pp. 137–155 in M. J. Everhart (ed.), Proceedings of the Second Mosasaur Meeting, Hays, Kansas. Fort Hays Studies Special Issue 3. May 3–6, 2007. Fort Hays State University, Hays, Kansas. [Google Scholar]) than any of the specimens listed herein.

CSV Display Table

Somewhat unexpectedly, both pairs of premaxillary teeth project anteriorly and laterally at the base (Fig. 2B), implying a procumbent nature atypical of tylosaurines (e.g., Bell, 1997 Bell, G. L., Jr. 1997. A phylogenetic revision of North American and Adriatic Mosasauroidea; pp. 293–332 in J. M. Callaway and E. L. Nicholls (eds.), Ancient Marine Reptiles. Academic Press, San Diego, California.[Crossref], [Google Scholar]:fig. 5C).

Also unusual are closely spaced first and second premaxillary teeth, where the second pair is also located posterolateral to the first pair (Fig. 2B). In larger individuals, the anterior and posterior pairs are well separated anteroposteriorly, where the posterior pair occurs more or less posterior to the anterior set (see below). Nevertheless, the premaxillary tooth crowns are smooth and bicarinate, exhibiting a ‘D’-shaped cross-section (Fig. 2B), the latter being consistent with the intercarinal angle reported by Konishi and Caldwell (2007a Konishi, T., and M. W. Caldwell. 2007a. Ecological and evolutionary implications of ontogenetic changes in the marginal dentition of Tylosaurus proriger (Squamata: Mosasauridae). Journal of Vertebrate Paleontology 27(3, Supplement):101A.[Web of Science ®], [Google Scholar]) in Tylosaurus of about 120° on the labial side. The same angle would be close to 180° in a given subadult and adult plioplatecarpine tooth, including Ectenosaurus (e.g., Konishi and Caldwell, 2011 Konishi, T., and M. W. Caldwell. 2011. Two new plioplatecarpine (Squamata, Mosasauridae) genera from the Upper Cretaceous of North America, and a global phylogenetic analysis of plioplatecarpines. Journal of Vertebrate Paleontology 31:754–783.[Taylor & Francis Online], [Web of Science ®], [Google Scholar]), and marginal teeth of adult Clidastes spp. are longitudinally elongate ovals in cross-section, with predominantly fore-and-aft carinal orientation (e.g., CMC VP 7554 [C. liodontus] and YPM 40350 [Clidastes sp.]; T.K., pers. observ.).

Quadrate

Whereas the lateral side of the preserved quadrates show signs of cortical bone loss postmortem, the more intact medial surface clearly exhibits a very large stapedial pit, whose vertical length is comparable to that of the suprastapedial process and its width is approximately 50% of the anteroposterior width of the shaft (Fig. 3A). Departing from a typical abbreviated morphology of the suprastapedial process in Tylosaurus, the immature tylosaur exhibits a suprastapedial process that is slender but substantially long relative to the rest of the quadrate, its length appearing to be almost half the estimated quadrate height based on observation of FHSM VP-15632 (T. kansasensis sensu Everhart, 2005a Everhart, M. J. 2005a. Tylosaurus kansasensis, a new species of tylosaurine (Squamata, Mosasauridae) from the Niobrara Chalk of western Kansas, U.S.A. Netherlands Journal of Geosciences 84:231–240.[Crossref], [Web of Science ®], [Google Scholar]; T. nepaeolicus sensu Jiménez-Huidobro et al., 2016 Jiménez-Huidobro, P., T. R. Simoes, and M. W. Caldwell. 2016. Re-characterization of Tylosaurus nepaeolicus (Cope, 1874) and Tylosaurus kansasensis Everhart, 2005: ontogeny or sympatry? Cretaceous Research 65:68–81.[Crossref], [Web of Science ®], [Google Scholar]) and RMM 5610 (T. proriger), in which the ventral limit of the stapedial pit corresponds to the midheight of the quadrate (e.g., Everhart, 2005a Everhart, M. J. 2005a. Tylosaurus kansasensis, a new species of tylosaurine (Squamata, Mosasauridae) from the Niobrara Chalk of western Kansas, U.S.A. Netherlands Journal of Geosciences 84:231–240.[Crossref], [Web of Science ®], [Google Scholar]:fig. 3d). In FHSM VP-14845, the suprastapedial process curves down gently and projects posteriorly as well as ventrally from the long axis of the shaft and also that of the stapedial pit, forming a large meatus (i.e., stapedial notch). In larger specimens attributed to various Tylosaurus species, the suprastapedial process projects more ventrally and the corresponding meatus is proportionally narrow (e.g., Russell, 1967 Russell, D. A. 1967. Systematics and morphology of American mosasaurs. Bulletin of the Peabody Museum of Natural History 23:1–241. [Google Scholar]:fig. 94A [T. proriger]; Bell, 1997 Bell, G. L., Jr. 1997. A phylogenetic revision of North American and Adriatic Mosasauroidea; pp. 293–332 in J. M. Callaway and E. L. Nicholls (eds.), Ancient Marine Reptiles. Academic Press, San Diego, California.[Crossref], [Google Scholar]:fig. 7B [T. nepaeolicus]; and Everhart, 2005a Everhart, M. J. 2005a. Tylosaurus kansasensis, a new species of tylosaurine (Squamata, Mosasauridae) from the Niobrara Chalk of western Kansas, U.S.A. Netherlands Journal of Geosciences 84:231–240.[Crossref], [Web of Science ®], [Google Scholar]:fig. 3 [T. ‘kansasensis’]). As a shared Tylosaurus feature, the anterior border of the cephalic condyle is shallowly excavated (Fig. 3A, arrow). Unfortunately, at least the ventral half of the element was lost postmortem.

FIGURE 3. FHSM VP-14845, Tylosaurus sp., incomplete quadrates. Right quadrate in A, lateral, B, medial aspects and left quadrate in C, lateral, and D, medial aspects. Arrow indicates posterior notching along anterior border of cephalic condyle. Note the large stapedial pit whose length equals that of the suprastapedial process and is nearly as wide as the quadratic shaft. Quadrate shafts are distally incomplete on both sides. Abbreviations: ccd, cephalic condyle; spt, stapedial pit; ssp, suprastapedial process. Scale bar equals 1 cm.

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Pterygoids

The main dentigerous portion is preserved on both sides, and a distal portion of the left quadrate ramus is also preserved (Fig. 4). As is typical in tylosaurine pterygoids, the pterygoid tooth row is straight and wider anterior to the ectopterygoidal process (e.g., KU 28705, Tylosaurus sp.). In adult plioplatecarpines, such as Plesioplatecarpus planifrons known from the lower Smoky Hill Chalk Member in Kansas (e.g., Konishi and Caldwell, 2007b Konishi, T., and M. W. Caldwell. 2007b. New specimens of Platecarpus planifrons (Cope, 1874) (Squamata: Mosasauridae) and a revised taxonomy of the genus. Journal of Vertebrate Paleontology 27:59–72.[Taylor & Francis Online], [Web of Science ®], [Google Scholar]:fig. 3), the pterygoid tooth row follows a gentle sinusoid curvature, closely following the lateral edge of the pterygoid anterior to the level of the ectopterygoidal process (the tooth row runs more or less along a straight midline of the pterygoid in Tylosaurus) (Konishi and Caldwell, 2011 Konishi, T., and M. W. Caldwell. 2011. Two new plioplatecarpine (Squamata, Mosasauridae) genera from the Upper Cretaceous of North America, and a global phylogenetic analysis of plioplatecarpines. Journal of Vertebrate Paleontology 31:754–783.[Taylor & Francis Online], [Web of Science ®], [Google Scholar]). In Clidastes, numerous, closely packed pterygoid teeth occur along a pronounced vertical ridge, which Tylosaurus lacks (e.g., Bell, 1997 Bell, G. L., Jr. 1997. A phylogenetic revision of North American and Adriatic Mosasauroidea; pp. 293–332 in J. M. Callaway and E. L. Nicholls (eds.), Ancient Marine Reptiles. Academic Press, San Diego, California.[Crossref], [Google Scholar]:character 42[1]; KU 1022). As expected, resorption pits occur posterolaterally to the functional tooth positions.

FIGURE 4. FHSM VP-14845, Tylosaurus sp., incomplete pterygoids in ventral aspect. Dentigerous portion of A, right and B, left pterygoids. Quadratic ramus of C, left pterygoid. Functional tooth crowns have been dissolved. Note the anteroposteriorly linear tooth alignment and also these teeth originating from a flat bony base, not along an elevated ridge. Abbreviations: ecpp, ectopterygoidal process; qr, quadrate ramus. Scale bar equals 1 cm.

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Basisphenoid

Overall, the basisphenoid is more elongate than those in plioplatecarpines (cf. Russell, 1967 Russell, D. A. 1967. Systematics and morphology of American mosasaurs. Bulletin of the Peabody Museum of Natural History 23:1–241. [Google Scholar]:33) or in Clidastes (CMC VP 7554 and KU 1022; pers. observ.). Anteriorly, the sella turcica terminates as a pair of smooth, vertical surfaces for articulation with the trabecular cartilage (e.g., Oelrich, 1956 Oelrich, T. M. 1956. The anatomy of the head of Ctenosaura pectinata (Iguanidae). Miscellaneous Publications, Museum of Zoology, University of Michigan 94:1–122. [Google Scholar]:fig. 14; Fig. 5A). In adult plioplatecarpines, these surfaces are posteriorly inclined and less well defined (Russell, 1967 Russell, D. A. 1967. Systematics and morphology of American mosasaurs. Bulletin of the Peabody Museum of Natural History 23:1–241. [Google Scholar]:fig. 10). In RMM 5610, a juvenile specimen of Tylosaurus proriger with a skull length of about 50 cm, a pair of elongate ovoid foramina for the internal carotid branches pierce the sella turcica just in front of the dorsum sellae (Fig. 6A). An identical condition is discernible on FHSM VP-14845 (Fig. 5A). In the adults of the plioplatecarpine Platecarpus, these foramina occur more anteriorly, around the midlength of the sella turcica (Russell, 1967 Russell, D. A. 1967. Systematics and morphology of American mosasaurs. Bulletin of the Peabody Museum of Natural History 23:1–241. [Google Scholar]:fig. 10). Unlike Platecarpus, no distinct foramina for the basilar artery are present on the floor of the sella turcica in FHSM VP-14845 or RMM 5610.

FIGURE 5. FHSM VP-14845, Tylosaurus sp., basisphenoid in A, dorsal and B, ventral views. Abbreviations: ala, alar process; ba, exit for basilar artery branch; bpp, basipterygoid process; ds, dorsum sellae; ftrc, facet for trabecular cartilage; icb, exit for internal carotid branch; plp, posterolateral process; sel, sella turcica. Scale bar equals 1 cm.

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FIGURE 6. RMM 5610, Tylosaurus proriger, base of braincase (basioccipital and basisphenoid) in A, dorsal and B, ventral views. Note that basisphenoid morphology is virtually identical to that of FHSM VP-14845. Abbreviations: ala, alar process; ba, exit for basilar artery branch; bpp, basipterygoid process; ds, dorsum sellae; ftrc, facet for trabecular cartilage; icb, exit for internal carotid branch; plp, posterolateral process (of basisphenoid); ppr, parasphenoid process; sel, sella turcica. Scale bar equals 1 cm.

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As is the case with Tylosaurus generally, the alar process is markedly longer than the sella turcica (Figs. 5A, 6A), whereas the opposite condition characterizes Platecarpus (Russell, 1967 Russell, D. A. 1967. Systematics and morphology of American mosasaurs. Bulletin of the Peabody Museum of Natural History 23:1–241. [Google Scholar]:fig. 10). In another plioplatecarpine, Selmasaurus johnsoni, and in a mosasaurine, Clidastes, these two features are about equal in length (FHSM VP-13910 [S. johnsoni holotype], ANSP 10193 [C. propython holotype], and CMC VP 7554 [C. liodontus]; pers. observ.). Near the posterior edge of the dorsum sellae on the dorsal surface is a single, irregularly shaped median foramen that may have served as an exit for the basilar artery. This region is also pierced by a circular foramen in RMM 5610. The base of the posterolateral process is preserved on the left side of FHSM VP-14845, indicating that its main axis trended more posteriorly than laterally, a typical Tylosaurus character (cf. Fig. 5B). On the ventral surface, the base of the left basipterygoid process is preserved, showing that the process projected ventrally at its base (Fig. 5B), consistent among Tylosaurus specimens, including RMM 5610 (Fig. 6B) (see also Jiménez-Huidobro and Caldwell, 2016 Jiménez-Huidobro, P., and M. W. Caldwell. 2016. Reassessment and reassignment of the early Maastrichtian mosasaur Hainosaurus bernardi Dollo, 1885, to Tylosaurus Marsh, 1872. Journal of Vertebrate Paleontology 36:3, DOI: 10.1080/02724634.2016.1096275[Taylor & Francis Online], [Web of Science ®], [Google Scholar]:fig. 7A). The same process projects anterolaterally in contemporary plioplatecarpines (e.g., FHSM VP-401, Ectenosaurus clidastoides; FHSM VP-13910, Selmasaurus johnsoni; AMNH 1820, Platecarpus tympaniticus; also see Rieppel and Zaher, 2000 Rieppel, O., and H. Zaher. 2000. The braincases of mosasaurs and Varanus, and the relationships of snakes. Zoological Journal of the Linnean Society 129:489–514.[Crossref], [Web of Science ®], [Google Scholar]:fig. 2C) and more anteriorly than laterally, but barely ventrally, in mosasaurines (e.g., Russell, 1967 Russell, D. A. 1967. Systematics and morphology of American mosasaurs. Bulletin of the Peabody Museum of Natural History 23:1–241. [Google Scholar]:fig. 12; Clidastes sp., cf. C. liodontus [KU 1022, pers. observ.]; Plotosaurus bennisoni [holotype, UCMP 32778, pers. observ.]; Prognathodon kianda [Schulp et al., 2008 Schulp, A. S., M. J. Polcyn, O. Mateus, L. L. Jacobs, and M. L. Morais. 2008. A new species of Prognathodon (Squamata, Mosasauridae) from the Maastrichtian of Angola, and the affinities of the mosasaur genus Liodon; pp. 1–12 in M. J. Everhart (ed.), Proceedings of the Second Mosasaur Meeting, Hays, Kansas. Fort Hays Studies Special Issue 3. May 3–6, 2007. Fort Hays State University, Hays, Kansas. [Google Scholar]:fig. 6H, I]). These vertically oriented bases of the paired basipterygoid processes occur close to the midsagittal plane, forming a distinct median cleft between them (Figs. 5B, 6B).

Splenials

Both splenials are preserved near their articular cotyle, and the better-preserved right element is described here. On the medial aspect, the base of the medial wing is broken postmortem and has been pushed up against the lateral wing (Fig. 7A). On the articular cotyle, the surface medial to the vertical keel is wider and more strongly excavated than the surface lateral to it, exposing the medial surface of the keel (Fig. 7A, C). In lateral aspect, although the cortical layer has been eroded postmortem, it is nonetheless discernible that the lateral border of the splenial cotyle is vertically straight (Fig. 7B). Such a cotylar morphology would have allowed the splenial to pivot medially with a greater angle than laterally with respect to the angular, consistent with the hypothesis that the mandible bent outward at the intramandibular joint in mosasaurs (e.g., Lee et al., 1999 Lee, M. S. Y., G. L. Bell Jr., and M. W. Caldwell. 1999. The origin of snake feeding. Nature 400:655–659.[Crossref], [Web of Science ®], [Google Scholar]:fig. 1). As mentioned already, much of the outer surface of the lateral wing is eroded postmortem.

FIGURE 7. FHSM VP-14845, Tylosaurus sp., right splenial in A, medial, B, lateral, and C, posterior views. Scale bar equals 1 cm.

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Tooth-Bearing Elements

There are more than a dozen fragments of tooth-bearing elements that are neither fragments of the premaxilla nor the pterygoid (Fig. 8). None of these jaw fragments preserve functional tooth crowns, and the only crown that is preserved intact occurs in a single replacement pit (Fig. 8B). Two long sections of jaw rami, here identified as those pertaining to the right dentary and the left maxilla, can be assembled out of some of these jaw fragments. In stark contrast to the premaxillary dentition, adjacent functional tooth crowns in jaw rami are separated from each other by a gap at least 50% greater than a single basal crown diameter (Konishi and Caldwell, 2007a Konishi, T., and M. W. Caldwell. 2007a. Ecological and evolutionary implications of ontogenetic changes in the marginal dentition of Tylosaurus proriger (Squamata: Mosasauridae). Journal of Vertebrate Paleontology 27(3, Supplement):101A.[Web of Science ®], [Google Scholar]; Fig. 8A, B, double-headed arrows). On both the maxilla and the dentary, the medial dental margin is approximately at the same level as the lateral margin and forms a complete ovoid alveolus. In contrast, alveoli in contemporary plioplatecarpines are circular (e.g., Konishi and Caldwell, 2007b Konishi, T., and M. W. Caldwell. 2007a. Ecological and evolutionary implications of ontogenetic changes in the marginal dentition of Tylosaurus proriger (Squamata: Mosasauridae). Journal of Vertebrate Paleontology 27(3, Supplement):101A.[Web of Science ®], [Google Scholar]:fig. 3).

FIGURE 8. FHSM VP-14845, Tylosaurus sp., tooth-bearing elements (TBEs) compared with those of RMM 5610, Tylosaurus proriger. A, partial right dentary of FHSM VP-14845, anterior to the right; B, partial left maxilla of the same in occlusal view, anterior to the left; C, premaxilla and left maxilla of RMM 5610 in lateral view showing eighth and ninth maxillary teeth; D, close-up view of the same teeth, showing the interdental distance that is approximately 35% greater than the basal crown length of adjacent teeth. Abbreviations: c, basal crown length; d, interdental distance; rp, resorption pit; t, tooth. Arrows indicate interdental gap. Scale bars equal 2 cm.

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Coronoids

On both right and left coronoids (Fig. 9), the coronoid processes are tall and acutely angled in profile. On the left coronoid, the posterior border is apparently complete and is vertically straight, lacking a distinct posteroventral process that would merge with the coronoid buttress of the surangular. In RMM 5610 (Tylosaurus proriger), such a process is well developed and projects posteriorly beyond the posterior edge of the coronoid process (Fig. 10). The lack of this process in FHSM VP-14845 is not preservational, however; both the lateral and medial borders forming the base of the element meet posteriorly to form the posteroventral corner of the coronoid, much as in RMM 5610 (Figs. 9D [arrow], 10B [arrow]). The medial wing is largely incomplete on both sides, whereas the short lateral wing is complete on the left element. In dorsoventral aspect, the coronoid exhibits a gentle medial curvature, even though both coronoids are incomplete anteriorly.

FIGURE 9. FHSM VP-14845, Tylosaurus sp., coronoids. Right coronoid in A, lateral, B, medial, and C, dorsal views. Left coronoid in D, lateral, E, medial, F, dorsal, and G, ventral views. Arrow indicates posteroventral corner. Scale bar equals 1 cm.

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FIGURE 10. RMM 5610, Tylosaurus proriger, right coronoid in A, lateral and B, ventral views. Arrow indicates posteroventral corner. Scale bar equals 2 cm.

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DISCUSSION

Biostratigraphy of Tylosaurus in Western Kansas

Collected from above Marker Unit (MU) 7 (Hattin, 1982 Hattin, D. E. 1982. Stratigraphy and depositional environment of Smoky Hill Chalk Member, Niobrara Chalk (Upper Cretaceous) of the type area, western Kansas. Kansas Geological Survey Bulletin 225:1–108. [Google Scholar]; M. J. Everhart, pers. comm.) of the Smoky Hill Chalk Member exposed in western Kansas, U.S.A., FHSM VP-14845 not only represents the smallest Tylosaurus specimen known to date, but it also bridges a significant biostratigraphic gap that existed between MU 5 and MU 9 for the genus (Everhart, 2001 Everhart, M. J. 2001. Revisions to the biostratigraphy of the Mosasauridae (Squamata) in the Smoky Hill Chalk Member of the Niobrara Chalk (Late Cretaceous) of Kansas. Transactions of the Kansas Academy of Science 104:59–78.[Crossref], [Google Scholar]). Among the two nominal and the one then unnamed Tylosaurus species known from upper Coniacian–lower Campanian strata of the member, Everhart (2001 Everhart, M. J. 2001. Revisions to the biostratigraphy of the Mosasauridae (Squamata) in the Smoky Hill Chalk Member of the Niobrara Chalk (Late Cretaceous) of Kansas. Transactions of the Kansas Academy of Science 104:59–78.[Crossref], [Google Scholar]) indicated for the respective taxa the following stratigraphic ranges: Tylosaurus ‘kansasensis’ sensu Everhart, 2005a Everhart, M. J. 2005a. Tylosaurus kansasensis, a new species of tylosaurine (Squamata, Mosasauridae) from the Niobrara Chalk of western Kansas, U.S.A. Netherlands Journal of Geosciences 84:231–240.[Crossref], [Web of Science ®], [Google Scholar] (MUs 1–5; upper Coniacian–lowermost Santonian), T. nepaeolicus (MUs 1–5; upper Coniacian–lowermost Santonian), and T. proriger (MUs 9–23; middle Santonian–lower Campanian) (e.g., Fig. 1). Everhart (2005a Everhart, M. J. 2005a. Tylosaurus kansasensis, a new species of tylosaurine (Squamata, Mosasauridae) from the Niobrara Chalk of western Kansas, U.S.A. Netherlands Journal of Geosciences 84:231–240.[Crossref], [Web of Science ®], [Google Scholar]:233) later confined the age of T. kansasensis to the late Coniacian. Recently, Jiménez-Huidobro et al. (2016 Jiménez-Huidobro, P., T. R. Simoes, and M. W. Caldwell. 2016. Re-characterization of Tylosaurus nepaeolicus (Cope, 1874) and Tylosaurus kansasensis Everhart, 2005: ontogeny or sympatry? Cretaceous Research 65:68–81.[Crossref], [Web of Science ®], [Google Scholar]) synonymized Tylosaurus kansasensis Everhart, 2005, with T. nepaeolicus (Cope, 1874 Cope, E. D. 1874. The Vertebrata of the Cretaceous period found west of the Mississippi River. Bulletin of the United States Geological and Geographical Survey of the Territories 1:3–48. [Google Scholar]) based on the sympatry of the two species, and on their significant morphological overlap on supposed key diagnostic characters of T. kansasensis. Jiménez-Huidobro et al. (2016 Jiménez-Huidobro, P., T. R. Simoes, and M. W. Caldwell. 2016. Re-characterization of Tylosaurus nepaeolicus (Cope, 1874) and Tylosaurus kansasensis Everhart, 2005: ontogeny or sympatry? Cretaceous Research 65:68–81.[Crossref], [Web of Science ®], [Google Scholar]) also suggested that specimens assigned to T. kansasensis were likely juveniles of T. nepaeolicus (Cope, 1874 Cope, E. D. 1874. The Vertebrata of the Cretaceous period found west of the Mississippi River. Bulletin of the United States Geological and Geographical Survey of the Territories 1:3–48. [Google Scholar]).
The fact that FHSM VP-14845 is from the MU 7 (lower– middle Santonian) renders it possible that it could represent any one of these Tylosaurus species known from Kansas and increases the possibility that stratigraphically older ‘Lower Chalk’ T. nepaeolicus (sensu Jiménez-Huidobro et al., 2016 Jiménez-Huidobro, P., T. R. Simoes, and M. W. Caldwell. 2016. Re-characterization of Tylosaurus nepaeolicus (Cope, 1874) and Tylosaurus kansasensis Everhart, 2005: ontogeny or sympatry? Cretaceous Research 65:68–81.[Crossref], [Web of Science ®], [Google Scholar]) and younger ‘Upper Chalk’ T. proriger briefly coexisted in the Western Interior Seaway (cf. Everhart, 2001 Everhart, M. J. 2001. Revisions to the biostratigraphy of the Mosasauridae (Squamata) in the Smoky Hill Chalk Member of the Niobrara Chalk (Late Cretaceous) of Kansas. Transactions of the Kansas Academy of Science 104:59–78.[Crossref], [Google Scholar]). In fact, Everhart (2001 Everhart, M. J. 2001. Revisions to the biostratigraphy of the Mosasauridae (Squamata) in the Smoky Hill Chalk Member of the Niobrara Chalk (Late Cretaceous) of Kansas. Transactions of the Kansas Academy of Science 104:59–78.[Crossref], [Google Scholar]) hypothesized that the taxon range zone for T. proriger may have extended to include MU 5 (upper Coniacian), based on the occurrences of two large tylosaurine specimens (FHSM VP-13742 and 13908) from between MU 4 and MU 5. Although taxonomic assignment of FHSM VP-14845 to one or another species of Tylosaurus remains equivocal at the moment, its fine-scale morphological similarities to the basisphenoid of RMM 5610, a T. proriger juvenile, supports the possibility that FHSM VP-14845 could be assigned to T. proriger. It is noteworthy that the coronoid process in at least one adult and two juvenile specimens of Tylosaurus nepaeolicus sensu Jiménez-Huidobro et al. (2016 Jiménez-Huidobro, P., T. R. Simoes, and M. W. Caldwell. 2016. Re-characterization of Tylosaurus nepaeolicus (Cope, 1874) and Tylosaurus kansasensis Everhart, 2005: ontogeny or sympatry? Cretaceous Research 65:68–81.[Crossref], [Web of Science ®], [Google Scholar])—AMNH 124, FHSM VP-2295 [T. kansasensis holotype], and VP-2495, respectively—overhangs the posterior border of the element (Fig. 11, arrow), whereas it does not extend beyond the posterior border of the element in FHSM VP-14845, a condition it shares with both small/juvenile (RMM 5610; Fig. 10) and large/adult (FHSM VP-3) T. proriger specimens.

FIGURE 11. FHSM VP-2495, Tylosaurus nepaeolicus sensu Jiménez-Huidobro et al., 2016 Jiménez-Huidobro, P., T. R. Simoes, and M. W. Caldwell. 2016. Re-characterization of Tylosaurus nepaeolicus (Cope, 1874) and Tylosaurus kansasensis Everhart, 2005: ontogeny or sympatry? Cretaceous Research 65:68–81.[Crossref], [Web of Science ®], [Google Scholar], left postdentary complex showing overhanging posterior coronoid process (arrow). Scale bar equals 5 cm.

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Cranial Ontogeny in Tylosaurus

Premaxilla and Predental Rostrum

Everhart (2005 Everhart, M. J. 2005a. Tylosaurus kansasensis, a new species of tylosaurine (Squamata, Mosasauridae) from the Niobrara Chalk of western Kansas, U.S.A. Netherlands Journal of Geosciences 84:231–240.[Crossref], [Web of Science ®], [Google Scholar]) reported that the relative length of the predental rostrum on the premaxilla in Tylosaurus kansasensis ranged from 2.5% to 3.0% of mandibular length, smaller than the same ratio in T. nepaeolicus (4.2%) and in T. proriger (4.8%). Considering T. nepaeolicus to be a senior synonym of T. kansasensis, where specimens assigned to the latter are generally smaller in size than those assigned to the former, Jiménez-Huidobro et al. (2016 Jiménez-Huidobro, P., T. R. Simoes, and M. W. Caldwell. 2016. Re-characterization of Tylosaurus nepaeolicus (Cope, 1874) and Tylosaurus kansasensis Everhart, 2005: ontogeny or sympatry? Cretaceous Research 65:68–81.[Crossref], [Web of Science ®], [Google Scholar]) hypothesized that the predental rostrum grew longer ontogenetically in Tylosaurus, although they did not characterize rostral lengthening in much detail. In FHSM VP-14845, the edentulous rostrum beyond the first premaxillary tooth pair is present but much smaller (≪10 mm in length) than is observed in other specimens of Tylosaurus of Santonian–Campanian age (Figs. 2, 12; Thurmond, 1969 Thurmond, J. T. 1969. Notes on mosasaurs from Texas. Texas Journal of Science 21:69–80.[Web of Science ®], [Google Scholar]; Sheldon, 1993 Sheldon, M. A. 1993. Ontogenetic study of selected mosasaurs of North America. M.S. thesis, University of Texas, Austin, Texas, 184 pp. [Google Scholar]). In addition, the outline of the projection describes a gentle parabolic arc in dorsoventral aspect, which is in stark contrast to the more triangular and more elongate morphology that is typical of a Tylosaurus rostrum. Notwithstanding, the presence of a small but distinct predental projection in FHSM VP-14845 precludes it from being Platecarpus, a ‘short-snouted’ plioplatecarpine, as it was originally considered to be.

FIGURE 12. Rostral and alveolar elongation in early Tylosaurus ontogeny. All the FHSM specimens are referable to Tylosaurus sp., whereas RMM 5610 pertains to T. proriger. Note progressively more anteroposterior alignment of two pairs of premaxillary teeth in association with alveolar elongation, which shows slowing down (here between FHSM VP-14840 and RMM 5610) while the rostrum maintains positive allometric growth. Abbreviation: t2, second premaxillary tooth. Scale bar equals 2 cm.

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Fortunately, there are two Tylosaurus specimens in the Fort Hays Sternberg Museum collections that, like FHSM VP-14845, exhibit a very small predental rostrum that is well below 10 mm in length (Fig. 12). Also found in Gove County, FHSM VP-14843 and 14840 not only show a progressively longer predental rostrum compared with FHSM VP-14845, but they also show a corresponding increase in the longitudinal dimension of the first premaxillary tooth alveolus (Fig. 12). Accompanying this seemingly ontogenetic alveolar elongation, it is also apparent that the second tooth pair migrates progressively with respect to the first tooth pair, from a more posterolateral position in FHSM VP-14845 to a more posterior position in FHSM VP-14840 (t2 in Fig. 12). This in turn results in an increase in the length:width ratio of the dentigerous premaxilla, where the entire dentigerous portion including the predental portion becomes elongate. As Tylosaurus ontogeny progresses beyond the size class represented by FHSM VP-14840, the alveolar elongation seems to slow down while the elongation of the predental rostrum continues, so that the length of the rostrum and that of the first premaxillary alveolus become nearly equal (RMM 5610; Fig. 12). In FFHM 1997-10, a 1.2-m-long skull referable to T. proriger (Everhart, 2001 Everhart, M. J. 2001. Revisions to the biostratigraphy of the Mosasauridae (Squamata) in the Smoky Hill Chalk Member of the Niobrara Chalk (Late Cretaceous) of Kansas. Transactions of the Kansas Academy of Science 104:59–78.[Crossref], [Google Scholar]), the predental rostrum is even longer than the first premaxillary alveolus, suggesting that in Tylosaurus the rostrum continued lengthening at a greater rate than the rest of the premaxilla, and likely the rest of the skull, beyond the ontogenetic stage represented by RMM 5610 (e.g., FFHM 1997-10).
This growth pattern of the rostrum suggests a relatively late offset (cessation) of rostral development through Tylosaurus life history relative to its hypothetical, russellosaurine ancestor, in which the rostrum was either short or absent. In terms of a heterochronic pattern of evolution, we therefore recognize that rostral development in the Tylosaurus premaxilla exhibited hypermorphosis.

Dentition

In Tylosaurus proriger, Konishi and Caldwell (2007a Konishi, T., and M. W. Caldwell. 2007a. Ecological and evolutionary implications of ontogenetic changes in the marginal dentition of Tylosaurus proriger (Squamata: Mosasauridae). Journal of Vertebrate Paleontology 27(3, Supplement):101A.[Web of Science ®], [Google Scholar]) suggested that there was positive allometry in the basal crown diameter of marginal teeth relative to the jawbones, where slender juvenile tooth crowns with substantial interdental gaps become stout and conical in adults, closing such gaps. At the level of the crown base, the juvenile maxilla (RMM 5610) exhibits an interdental gap that is nearly 1.5 times greater than the anteroposterior basal length of an adjacent tooth crown (Fig. 8C, D). The gaps exhibited on the maxilla and dentary of FHSM VP-14845 are even more substantial, becoming 1.7–2.0 times as long as the anteroposterior basal length of adjacent crowns (Fig. 8A, B, double-headed arrows), lending further support to Konishi and Caldwell’s (2007a) hypothesis. In contrast, the interdental gap between the first and the second teeth on the premaxilla of FHSM VP-14845 is distinctly smaller than the basal crown diameter of the adjacent teeth (Fig. 2B), here considered associated with its very early ontogenetic stage preceding alveolar elongation (Fig. 12). Morphologically, tooth crowns in both the premaxilla and other jawbones of FHSM VP-14845 are slender, as in RMM 5610.

Quadrates

The preserved portions of the quadrates augment the ontogenetic argument of Jiménez-Huidobro et al. (2016 Jiménez-Huidobro, P., T. R. Simoes, and M. W. Caldwell. 2016. Re-characterization of Tylosaurus nepaeolicus (Cope, 1874) and Tylosaurus kansasensis Everhart, 2005: ontogeny or sympatry? Cretaceous Research 65:68–81.[Crossref], [Web of Science ®], [Google Scholar]) for this element in Tylosaurus: namely, the smaller or younger the animal, the proportionately more slender the suprastapedial process and the greater the size of the stapedial notch (Jiménez-Huidobro et al., 2016 Jiménez-Huidobro, P., T. R. Simoes, and M. W. Caldwell. 2016. Re-characterization of Tylosaurus nepaeolicus (Cope, 1874) and Tylosaurus kansasensis Everhart, 2005: ontogeny or sympatry? Cretaceous Research 65:68–81.[Crossref], [Web of Science ®], [Google Scholar]:78). Specimen FHSM VP-14845 exhibits a substantial gap between the suprastapedial process and the shaft, the former being elongate and deflected medially (Fig. 3). The stapedial pit is enormous, as long as the suprastapedial process itself, which is noteworthy given its small relative size in a typical adult Tylosaurus quadrate (Russell, 1967 Russell, D. A. 1967. Systematics and morphology of American mosasaurs. Bulletin of the Peabody Museum of Natural History 23:1–241. [Google Scholar]; Bell, 1997 Bell, G. L., Jr. 1997. A phylogenetic revision of North American and Adriatic Mosasauroidea; pp. 293–332 in J. M. Callaway and E. L. Nicholls (eds.), Ancient Marine Reptiles. Academic Press, San Diego, California.[Crossref], [Google Scholar]:fig. 7B). Also of note is that, at least in medial aspect, the quadrate shaft of FHSM VP-14845 is only slightly wider than that of the stapedial pit. It thus seems that the entire quadrate developed with positive allometry relative to the suprastapedial pit in both vertical and horizontal dimensions. In sum, a tylosaurine quadrate early in ontogeny is characterized as possessing a long, slender suprastapedial process, a wide stapedial notch, and a very large stapedial pit.

Ontogenetic Status of FHSM VP-14845: Prenatal or a Neonate?

Based on the dentary tooth crown diameter, and under the assumption that tooth crowns grow isometrically to overall body size, Field et al. (2015 Field, D. J., A. LeBlanc, A. Gau, and A. D. Behlke. 2015. Pelagic neonatal fossils support viviparity and precocial life history of Cretaceous mosasaurs. Palaeontology 58:401–407.[Crossref], [Web of Science ®], [Google Scholar]) estimated the size of the smallest Clidastes sp. specimen they reported (YPM 058126) to be 0.66 m, or about 22% the length of a 3-m-long adult. As Konishi and Caldwell (2007a Konishi, T., and M. W. Caldwell. 2007a. Ecological and evolutionary implications of ontogenetic changes in the marginal dentition of Tylosaurus proriger (Squamata: Mosasauridae). Journal of Vertebrate Paleontology 27(3, Supplement):101A.[Web of Science ®], [Google Scholar]) reported in Tylosaurus proriger, however, tooth crowns in Tylosaurus exhibit positive allometry relative to the respective tooth-bearing elements, except for those on the premaxilla at a very early ontogenetic stage (i.e., FHSM VP-14845; this study). Jaws of a juvenile possessing slender tooth crowns and large interdental gaps (see above) are later filled by enlarged, and not by additional, adult tooth crowns. Comparison between the smaller and larger specimens Field et al. (2015 Field, D. J., A. LeBlanc, A. Gau, and A. D. Behlke. 2015. Pelagic neonatal fossils support viviparity and precocial life history of Cretaceous mosasaurs. Palaeontology 58:401–407.[Crossref], [Web of Science ®], [Google Scholar]) identified as Clidastes reveals that the interdental gaps are indeed proportionately larger in smaller specimens (e.g., YPM 058126, the gap larger than the basal crown diameter) than in larger specimens (YPM 1314, the gap smaller than basal crown diameter). Hence, it is possible that Field et al. (2015 Field, D. J., A. LeBlanc, A. Gau, and A. D. Behlke. 2015. Pelagic neonatal fossils support viviparity and precocial life history of Cretaceous mosasaurs. Palaeontology 58:401–407.[Crossref], [Web of Science ®], [Google Scholar]) underestimated the total body length of YPM 058126, which renders the neonate status of this specimen that Field et al. (2015 Field, D. J., A. LeBlanc, A. Gau, and A. D. Behlke. 2015. Pelagic neonatal fossils support viviparity and precocial life history of Cretaceous mosasaurs. Palaeontology 58:401–407.[Crossref], [Web of Science ®], [Google Scholar]) suggested less unequivocal. Also of note, YPM 1253, the third smallest specimen of Clidastes identified by Field et al. (2015 Field, D. J., A. LeBlanc, A. Gau, and A. D. Behlke. 2015. Pelagic neonatal fossils support viviparity and precocial life history of Cretaceous mosasaurs. Palaeontology 58:401–407.[Crossref], [Web of Science ®], [Google Scholar]), indeed pertains to a Platecarpus-like plioplatecarpine: a short premaxillomaxillary suture (about two and a half alveoli long), a round basal tooth crown cross-section, and clear presence of hemapophyses (i.e., a hemal arch–spine complex not fused to the caudal vertebra) are all characteristic of this short-snouted plioplatecarpine common in the Smoky Hill Chalk Member (T.K., pers. observ., 2005).
By estimating both the skull length (SL) and the total body length (TBL), we further evaluated the possible developmental stage of FHSM VP-14845 at the time of its death. To do this, we used data from two large Tylosaurus skeletons collected from the Kansas Chalk: AMNH FR-221, an 8.83-m-long, nearly complete and articulated Tylosaurus proriger skeleton (Osborn, 1899a Osborn, H. F. 1899a. A complete mosasaur skeleton, osseous and cartilaginous. Memoirs of the American Museum of Natural History 1:167–188. [Google Scholar], 1899b Osborn, H. F. 1899b. A complete mosasaur skeleton, osseous and cartilaginous. Science 10:919–925.[Crossref], [PubMed], [Google Scholar]), and FHSM VP-3, another articulated, partially reconstructed skeleton of T. proriger that is slightly smaller than AMNH FR-221 (Russell, 1967 Russell, D. A. 1967. Systematics and morphology of American mosasaurs. Bulletin of the Peabody Museum of Natural History 23:1–241. [Google Scholar]). By comparing six-alveolus lengths between AMNH FR-221 and FHSM VP-14845, the skull length (SL) of the latter was estimated to be about 30 cm. Comparison between SLs of FHSM VP-14845 and AMNH FR-221 was made subsequently, yielding the estimated total body length (TBL) of 2.23 m for FHSM VP-14845 (Appendix 1). This estimated TBL is 25.3% that of AMNH FR-221 and 17.2% that of KU 5033 (the ‘Bunker tylosaur’), the largest-known T. proriger specimen collected from Kansas at an estimated TBL of 13 m (Everhart, 2002 Everhart, M. J. 2002. New data on cranial measurements and body length of the mosasaur, Tylosaurus nepaeolicus (Squamata; Mosasauridae), from the Niobrara Formation of western Kansas. Transactions of the Kansas Academy of Science 105:33–43.[Crossref], [Google Scholar], 2005c Everhart, M. J. 2005c. Oceans of Kansas. Indiana University Press, Bloomington, Indiana, 322 pp. [Google Scholar]; pers. observ.), and 24.8–27.9% of an estimated maximum TBL for T. nepaeolicus at 8–9 m (Everhart, 2002 Everhart, M. J. 2002. New data on cranial measurements and body length of the mosasaur, Tylosaurus nepaeolicus (Squamata; Mosasauridae), from the Niobrara Formation of western Kansas. Transactions of the Kansas Academy of Science 105:33–43.[Crossref], [Google Scholar]). Based on Caldwell and Lee (2001 Caldwell, M. W., and M. S. Y. Lee. 2001. Live birth in Cretaceous marine lizards (mosasauroids). Proceedings of the Royal Society B, Biological Sciences 268:2397–2401.[Crossref], [PubMed], [Web of Science ®], [Google Scholar]:fig. 2, and references therein), the estimated TBL for FHSM VP-14845 falls well within the neonate TBL range expected for extant varanoid lizards, at approximately 10–40% maternal TBL.
Still, given the exceptionally large adult size of Tylosaurus compared with extant Varanus, it may be possible that FHSM VP-14845 was a prenatal individual close to parturition, a possibility that Field et al. (2015 Field, D. J., A. LeBlanc, A. Gau, and A. D. Behlke. 2015. Pelagic neonatal fossils support viviparity and precocial life history of Cretaceous mosasaurs. Palaeontology 58:401–407.[Crossref], [Web of Science ®], [Google Scholar]) did not consider for any of the small Clidastes specimens they analyzed. Although direct evidence supporting or countering such a possibility is lacking in FHSM VP-14845, we present here an argument favoring the likelihood that FHSM VP-14845 was a neonate. First, FHSM VP-14845, consisting of associated fragmentary bones pertaining to a single individual, was discovered without any associated adult or juvenile bones (M. J. Everhart, pers. comm., 2018), indicating that it was preserved by itself ex utero (e.g., O’Keefe and Chiappe, 2011; Field et al., 2015 Field, D. J., A. LeBlanc, A. Gau, and A. D. Behlke. 2015. Pelagic neonatal fossils support viviparity and precocial life history of Cretaceous mosasaurs. Palaeontology 58:401–407.[Crossref], [Web of Science ®], [Google Scholar]). Second, even if FHSM VP-14845 was an offspring from an exceptionally large Tylosaurus proriger individual such as KU 5033, the TBL ratio of 17.2% between the two specimens still exceeds the equivalent ratio of 15% estimated in Carsosaurus marchesetti, a basal mosasauroid (Caldwell and Lee, 2001 Caldwell, M. W., and M. S. Y. Lee. 2001. Live birth in Cretaceous marine lizards (mosasauroids). Proceedings of the Royal Society B, Biological Sciences 268:2397–2401.[Crossref], [PubMed], [Web of Science ®], [Google Scholar]). Among extant cetaceans, another clade of secondarily aquatic tetrapods that are universally viviparous, the TBL ratio between the neonate and the mother is negatively correlated with the maternal TBL across a variety of whale species, both in mysticetes (R² = 0.417) and in odontocetes (R² = 0.315) and when both clades are combined (R² = 0.620; Fig. 13). In extant viviparous/ovoviviparous chondrichthyan taxa without embryonic cannibalism, some of the smaller species (e.g., Squalus acanthias) exhibit the longest gestation period of up to 24 months, indicating that smaller taxa give birth to a small number (2–14 in S. acanthias) of large offspring relative to maternal size (Pough et al., 2013 Pough, F. H., C. M. Janis, and J. B. Heiser. 2013. Vertebrate Life, ninth edition. Pearson Education, Inc., Glenview, Illinois, 634 pp. [Google Scholar]:fig. 5-11). Indeed, the whale shark (Rhincodon typus), the largest extant shark species growing up to at least 12 m in TBL, is known with the litter size of 300, the largest recorded among extant sharks (Stevens, 2007 Stevens, J. D. 2007. Whale shark (Rhincodon typus) biology and ecology: a review of the primary literature. Fisheries Research 84:4–9.[Crossref], [Web of Science ®], [Google Scholar], and references therein). Perinatal embryos of a 10.6-m gravid whale shark ranged from 58 to 64 cm in TBL, which amounts to 5.5–6.0% of the maternal TBL (Joung et al., 1996 Joung, S. J., C. T. Chen, E. Clark, S. Uchida, and W. Y. P. Huang. 1996. The whale shark, Rhincodon typus, is a livebearer: 300 embryos found in one ‘megamamma’ supreme. Environmental Biology of Fishes 46:219–223.[Crossref], [Web of Science ®], [Google Scholar]). The same ratio in Squalus acanthias becomes fivefold, where we recorded 26.5–31.7% based on 10 perinatal embryos from three females (T.K., pers. observ.). Although circumstantial, these lines of evidence lend more support to FHSM VP-14845 having been a precocial neonate ex utero.

FIGURE 13. Extant cetacean neonate size distribution against maternal body length for A, mysticetes, B, odontocetes, and C, mysticetes and odontocetes combined. Note the general decline in neonate size relative to maternal body size. Data compiled from Boyd et al. (1999 Boyd, I. L., C. Lockyer, and H. D. Marsh. 1999. Reproduction in Marine Mammals; pp. 218–286 in J. E. Reynolds III and S. A. Rommel (eds.), Biology of Marine Mammals. Smithsonian Institution Press, Washington, D.C. [Google Scholar]) and Jefferson et al. (2008 Jefferson, T. A., M. A. Webber, and R. L. Pitman (eds.). 2008. Marine Mammals of the World. Academic Press, Amsterdam, The Netherlands, 573 pp. [Google Scholar]).

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Whether FHSM VP-14845 is T. nepaeolicus or T. proriger, the arguments given above indicate that Tylosaurus was born with a short, round predental rostrum on the premaxilla. During the subsequent stages of early ontogeny, it is predicted that such a rostrum underwent a rapid positive allometric growth to develop into a long, conical structure characteristic of the subfamily. In T. proriger, such a robust conical rostrum was already present, although still growing, even as the individual’s skull reached 60 cm in length (RMM 5610; Fig. 12; Appendix 1), and in T. nepaeolicus sensu Jiménez-Huidobro et al. (2016 Jiménez-Huidobro, P., T. R. Simoes, and M. W. Caldwell. 2016. Re-characterization of Tylosaurus nepaeolicus (Cope, 1874) and Tylosaurus kansasensis Everhart, 2005: ontogeny or sympatry? Cretaceous Research 65:68–81.[Crossref], [Web of Science ®], [Google Scholar]), a conical rostrum is evident on a specimen with a skull length as short as 40 cm (IPB R322; Jiménez-Huidobro et al., 2016 Jiménez-Huidobro, P., T. R. Simoes, and M. W. Caldwell. 2016. Re-characterization of Tylosaurus nepaeolicus (Cope, 1874) and Tylosaurus kansasensis Everhart, 2005: ontogeny or sympatry? Cretaceous Research 65:68–81.[Crossref], [Web of Science ®], [Google Scholar]:fig. 3B).
In 2016, Jiménez-Huidobro et al. (p. 78) suggested that the premaxillary rostrum “seems to be shorter” in small specimens of T. nepaeolicus that Everhart (2005) regarded as T. kansasensis. Nevertheless, the exact onset of the conical premaxillary rostrum development in Tylosaurus ontogeny remained unclear and its universal presence has been assumed for T. nepaeolicus and T. proriger. Recognizing here that FHSM VP-14845 can be assigned to Tylosaurus despite the lack of the conical rostrum allows formulation of the following hypotheses: (1) at least certain Tylosaurus species were born without a conical premaxillary rostrum; (2) alveolar, as well as predental, elongation continued and contributed to development of a prow-like dentigerous premaxilla in Tylosaurus; and (3) the onset of rostrum morphogenesis began exceptionally early in Tylosaurus postnatal ontogeny. Finally, from an evolutionary perspective, we further conclude that hypermorphosis is a major heterochronic driver behind the evolution of a conical tylosaurine rostrum, given the lack of such a feature in plioplatecarpines, a generally well-supported sister clade of tylosaurines (e.g., Konishi and Caldwell, 2011 Konishi, T., and M. W. Caldwell. 2011. Two new plioplatecarpine (Squamata, Mosasauridae) genera from the Upper Cretaceous of North America, and a global phylogenetic analysis of plioplatecarpines. Journal of Vertebrate Paleontology 31:754–783.[Taylor & Francis Online], [Web of Science ®], [Google Scholar]; Simões et al., 2017 Simões, T. R., O. Vernygora, I. Paparella, P. Jimenez-Huidobro, and M. W. Caldwell. 2017. Mosasauroid phylogeny under multiple phylogenetic methods provides new insights on the evolution of aquatic adaptation in the group. PLoS ONE 12:e0176773. doi: 10.1371/journal.pone.0176773.[Crossref], [PubMed], [Web of Science ®], [Google Scholar]). At the same time, we reject the possibility of sexual selection as a driver of tylosaurine rostrum evolution, given its presence exceptionally early in their postnatal ontogeny. It is a possibility that the bony rostrum was selected for a sex-independent function in tylosaurines, such as for ramming, which killer whales today employ when hunting cetaceans of various sizes (Ford et al., 1998 Ford, J. K. B., G. M. Ellis, L. G. Barrett-Lennard, A. B. Morton, R. S. Palm, and K. C. Balcomb III. 1998. Dietary specialization in two sympatric populations of killer whales (Orcinus orca) in coastal British Columbia and adjacent waters. Canadian Journal of Zoology 76:1456–1471.[Crossref], [Web of Science ®], [Google Scholar], 2005 Ford, J. K. B., D. R. Matkin, K. C. Balcomb, D. Briggs, and A. B. Morton. 2005. Killer whale attacks on minke whales: prey capture and antipredator tactics. Marine Mammal Science 21:603–618.[Crossref], [Web of Science ®], [Google Scholar]; Visser et al., 2010 Visser, I. N., J. Azeschmar, J. Halliday, A. Abraham, P. Ball, R. Bradley, S. Daly, T. Hatwell, T. Johnson, W. Johnson, L. Kay, T. Maessen, V. McKay, T. Peters, N. Turner, B. Umuroa, and D. S. Pace. 2010. First record of predation on false killer whales (Pseudorca crassidens) by killer whales (Orcinus orca). Aquatic Mammals 36:195–204.[Crossref], [Web of Science ®], [Google Scholar]).
Handling editor: Patrick Druckenmiller.

ACKNOWLEDGMENTS

We sincerely thank M. J. Everhart for his kind hospitality in accommodating our collections visits at the FHSM and elsewhere in Kansas, and for providing us with additional information on and photographs of FHSM VP-14845. We also thank incisive comments provided by our reviewers, A. Schulp and M. Polcyn. S. Garvey assisted T.K. with measurements of Squalus acanthius specimens. This research was in part funded by NSERC (Natural Sciences and Engineering Research Council of Canada) Discovery Grant no. 238458, NSERC Accelerator Grant no. 412275, and a Faculty of Science Chairs Research Allowance to M.W.C.

APPENDIX 1.

Skull and body length estimates for FHSM VP-14845. To estimate the skull length (SL) and the total body length (TBL) of FHSM VP-14845, we used selected measurements of FHSM VP-3 and AMNH FR-221 from Russell (1967 Russell, D. A. 1967. Systematics and morphology of American mosasaurs. Bulletin of the Peabody Museum of Natural History 23:1–241. [Google Scholar]:appendix A) and Osborn (1899a Osborn, H. F. 1899a. A complete mosasaur skeleton, osseous and cartilaginous. Memoirs of the American Museum of Natural History 1:167–188. [Google Scholar]), respectively, as follows.

Skull length (SL) FHSM VP-3 = 1058 mm; length between first and sixth dentary teeth (D1–D6) measured across tooth bases = 225 mm
(SL) FHSM VP-14845 = (D1–D6) FHSM VP-14845 × 1058 mm/225 mm — A

We calculated (D1–D6) FHSM VP-14845 using the more complete left maxilla because tooth size does not vary greatly between upper and lower jaws (but see Russell’s [1967 Russell, D. A. 1967. Systematics and morphology of American mosasaurs. Bulletin of the Peabody Museum of Natural History 23:1–241. [Google Scholar]] very likely measuring error for FHSM VP-4 (= VP-3), where the distance between the first and sixth teeth for a maxilla is more than twice as long as that for a dentary. These values are similar in other specimens). Based on this we obtained:

(D1–D6) FHSM VP-14845 = 64 mm — B

Inserting B into A, we derive:

(SL) FHSM VP-14845 = 300.9 mm — C

3. Total body length (TBL) AMNH FR-221 = 8830 mm, whereas (SL) AMNH FR-221 = 1190 mm. From C, then, (TBL) FHSM VP-14845 can be obtained as follows:

(TBL) FHSM VP-14845 = (TBL) AMNH FR-221 × C/(SL) AMNH FR-221 = 8830 mm × 300.9 mm/1190 mm = 2232.7 mm

Thus, approximately, (TBL) FHSM VP-14845 = 2.23 m

Similarly, using the M1–M6 length as the best proxy for D1–D6, we attained the following SL estimate for juvenile T. proriger (RRM 5610):

4. (SL) RMM 5610 = (M1–M6) RMM 56 × 1058 mm/225 mm

= 130 mm × 1058 mm/225 mm

= 611 mm

≈ 60 cm

According to the above calculations, the skull of RMM 5610 is approximately twice as long as that of FHSM VP-14845.

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segunda-feira, 25 de novembro de 2019

A Terra está à beira de sua sexta extinção em massa e a culpa é nossa

Seres humanos criam a sexta grande extinção de espécies animais, dizem cientistas

NÃO É A PRIMEIRA VEZ

Extinções em massa anteriores

1 Ordoviciano Final, 443 milhões de anos atrás.

2 Devoniano tardio, c 360 milhões de anos atrás.

3 Extinção em massa Permiano-Triássica, c 250 milhões de anos atrás.

4 Extinção em massa Triássico-Jurássico, cerca de 200 milhões de anos atrás.

5 Extinção em massa do período cretáceo-paleógeno há 65 milhões de anos.

segunda-feira, 15 de outubro de 2018

The Smallest-Known Neonate Individual of Tylosaurus (Mosasauridae, Tylosaurinae) Sheds New Light on the Tylosaurine Rostrum and Heterochrony

Abstract

INTRODUCTION

Institutional Abbreviations

SYSTEMATIC PALEONTOLOGY

Type species

Holotype

Generic Diagnosis

Referred Specimens

Locality and Horizon

DESCRIPTION AND COMPARISON OF FHSM VP-14845

Premaxilla

Quadrate

Pterygoids

Basisphenoid

Splenials

Tooth-Bearing Elements

Coronoids

DISCUSSION

Biostratigraphy of Tylosaurus in Western Kansas

Cranial Ontogeny in Tylosaurus

Premaxilla and Predental Rostrum

Dentition

Quadrates

Ontogenetic Status of FHSM VP-14845: Prenatal or a Neonate?

ACKNOWLEDGMENTS

APPENDIX 1.

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