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segunda-feira, 16 de setembro de 2019

Registros vivos do tempo

Novas espécies de invertebrados marinhos descobertas no litoral paulista ajudam a entender a evolução dos oceanos
MARCELLO GUIMARÃES SIMÕES / UNESP
Cada vez que precisa coletar os pequenos animais marinhos invertebrados usados em sua pesquisa, o paleontólogo Marcello Guimarães Simões sofre com as viagens ao mar. Assim que o Progresso, um pesqueiro de camarões adaptado para pesquisas, zarpa do cais de Ubatuba, no litoral norte paulista, começam os enjôos, que só cessam na volta a terra firme. Mas nem o mal-estar o faz desistir do seu objetivo: capturar pequenos invertebrados conhecidos como braquiópodes – semelhantes a moluscos encontrados nas praias, com os quais têm em comum apenas as conchas – que estão ajudando a compreender as alterações que o ambiente marinho sofreu nos últimos milhares de anos e, no futuro, devem permitir propor formas de recuperação para essas áreas.
 
Vencendo o mal-estar e muitas vezes o mar revolto, Simões e Adilson Fransozo, ambos do Instituto de Biociências (IB) da Universidade Estadual Paulista (Unesp), em Botucatu, conseguiram coletar entre 2001 e este ano cerca de 5 mil exemplares de uma das espécies menos estudadas de braquiópodes vivos, a Bouchardia rosea – tarefa da qual participou a equipe de Michal Kowalewski, do Instituto Politécnico da Virgínia, nos Estados Unidos.

Por serem morfologicamente muito semelhantes a representantes da mesma linhagem que viveram há 60 milhões de anos, as Bouchardia são consideradas fósseis vivos. Com duas conchas quase simétricas e articuladas, semelhantes àquelas que viram brinquedo nas mãos das crianças nas praias, esses invertebrados são uma espécie de janela aberta simultaneamente para o passado e para o futuro, pois estão ajudando a reconstituir, com detalhes, o ambiente marinho em eras remotas e a desenvolver a paleoecologia marinha aplicada, ciência voltada para avaliar a deterioração ambiental e impedir seu progresso.

Desde o início dos trabalhos com os braquiópodes coletados no litoral paulista, a equipe da Unesp, coordenada por Simões, derrubou alguns dogmas. Os resultados mais recentes, publicados em julho na revista Palaios, uma das mais importantes da área, e apresentados em outubro no encontro anual da Sociedade Geológica dos Estados Unidos, em Denver, mostraram que a atual fauna de braquiópodes articulados da plataforma continental brasileira é mais diversificada do que se imaginava. Além de Bouchardia rosea, invertebrado tipicamente brasileiro e o único braquiópode articulado até então conhecido, os pesquisadores encontraram representantes de Platidia anomioides, Terebratulina sp e Argyrotheca cuneata, espécies que apresentam afinidades com as da Antártica, do Caribe, do Mediterrâneo e da porção sul-africana dos oceanos Atlântico e Índico.

Essa diversidade lança algumas questões sobre a distribuição dos braquiópodes pelo planeta. Duas perguntas que os pesquisadores pretendem responder, tão logo quanto possível, são por que a Bouchardia rosea só existe no Brasil e como as outras espécies chegaram até aqui. Já sabem que parentes remotos de Bouchardia rosea – como a B. conspicua, a B. antarctica e a B. transplatina – possuem uma longa história geológica: algumas habitaram os oceanos há cerca de 60 milhões de anos, após a extinção dos dinossauros. Atualmente, fósseis de espécies de Bouchardia e outras formas aparentadas, como Bouchardiella , podem ser encontrados em rochas da Antártica, da Austrália, da Argentina e do Uruguai.

Com cerca de 15 milímetros de comprimento e revestido por conchas róseas de carbonato de cálcio, às vezes com minúsculas listras brancas, a B. rosea é a única espécie viva conhecida da família Bouchardiidae que ainda habita os mares, espalhando-se pelo litoral dos estados de São Paulo, Paraná, Rio de Janeiro e Espírito Santo. Até o momento, os pesquisadores não sabem exatamente quando a espécie surgiu.

“Por motivos ainda pouco entendidos, enquanto as populações de Bouchardia rosea apresentam uma distribuição mais ao norte, ao longo da margem oriental da América do Sul, outras espécies fósseis do mesmo gênero têm uma distribuição mais ampla nos continentes do Hemisfério Sul e são encontradas na Austrália, na Antártica e, no continente sul-americano, na Argentina e no Uruguai”, ressalta Simões. “Outros estudos sugerem que a distribuição das espécies decorra das modificações nas correntes oceânicas ocorridas nos últimos milhões de anos, durante a evolução do Atlântico.”
Outra novidade é a profundidade em que podem ser encontradas as populações de Bouchardia, às vezes com milhares de indivíduos em apenas 1 metro quadrado. Falava-se que viviam em águas rasas, com cerca de 20 metros de profundidade, com raras ocorrências em águas com profundidade superior a 100 metros, mas as pesquisas do grupo da Unesp e as amostras coletadas pelo Programa de Avaliação do Potencial Sustentável de Recursos Vivos na Zona Econômica Exclusiva (Revizee) demonstraram que esses organismos podem viver a até 550 metros. A descoberta amplia o alcance de futuros estudos, ao mostrar que uma espécie antes encontrada apenas em águas rasas pode ocupar hábitats mais profundos e pouco estudados.

Pode haver também algumas correções de rota nas pesquisas. Com base nos escassos dados sobre a biologia de Bouchardia rosea, os especialistas interpretavam o modo de vida das espécies fósseis do gênero como sendo similares à de Bouchardia rosea . Dessa forma, viram muitas ocorrências fósseis como geradas em condições deposicionais de águas rasas, já que, até então, a distribuição da espécie vivente estava, em grande parte, restrita às águas rasas. “Como agora sabemos que a espécie ocupa fundos a profundidades em torno dos 500 metros, seria interessante reexaminar as ocorrências fósseis à luz dos novos dados e comparar com os advindos de outras ciências, como a estratigrafia, que estuda a origem e a composição das camadas de rochas”, comenta o pesquisador da Unesp.

Os estudos de Simões sugerem também que as correntes oceânicas de ressurgência (de águas muito frias) podem ser fundamentais para o desenvolvimento de populações de braquiópodes. 

Mais uma vez, as evidências caminham na contramão do que se sabia: imaginava-se que essas correntes, ricas em nutrientes, trariam prejuízos para esses invertebrados, já que poderiam entupir o aparelho filtrador, chamado lofóforo, que drena a água e seleciona a matéria nutritiva. Por fim, verificou-se ainda a preferência dos braquiópodes por determinados tipos de substratos – as Bouchardia crescem em áreas do fundo oceânico em que a concentração de carbonato de cálcio nos sedimentos varia de 40% a 70%.

As constatações sobre as correntes e o substrato em que crescem mais facilmente são pistas importantes para o mapeamento global dessas populações, indicando de maneira mais clara onde podem se manifestar e as áreas que podem ajudar a recuperar. “As conchas oferecem registros confiáveis sobre a história recente e as modificações ambientais e biológicas vividas por aquela região da plataforma brasileira”, reafirma Kowalewski. “A partir de dados do passado, conseguimos informações sobre os estragos causados pela ação humana na natureza, redirecionando a atuação de ecologistas, geólogos, paleontólogos, biólogos e de agências ambientais.”

Kowalewski cita como exemplo um trabalho da Universidade do Arizona, também nos Estados Unidos, que, em meados da década de 90, em colaboração com a equipe de Virgínia, jogou luzes sobre o potencial da chamada paleoecologia marinha aplicada, uma área de pesquisa em que fósseis vivos como os braquiópodes e os registros de épocas passadas têm servido como referência para projetos de recuperação de áreas degradadas. Com esse tipo de informação, por exemplo, já foi possível desenvolver um plano de recuperação da biodiversidade aquática do delta do Rio Colorado, próximo à fronteira dos Estados Unidos com o México, área que sofreu intensa transformação ambiental com a construção de hidrelétricas.

As barragens erguidas desde 1930 reduziram bastante a quantidade de água que chegava à foz do Rio Colorado. Em consequência, houve um aumento da salinidade, o que colocou em risco de extinção espécies de invertebrados como a Mulinia coloradoensis, um molusco que servia de alimento para pequenos peixes, pássaros e para a população local. Para complicar, havia poucos registros históricos dos parâmetros ambientais do delta do rio, no período anterior às barragens.

Foi nesse ponto que a paleoecologia entrou em ação. Os pesquisadores coletaram amostras de conchas dos invertebrados ameaçados, que formam densas acumulações (depósitos superficiais) ao longo do delta. Depois, em laboratório, determinaram a idade dessas conchas, que variavam de recentes a até 7,3 mil anos. Esse dado é essencial porque, como os animais secretam o carbonato de cálcio e formam as conchas em equilíbrio químico com a água do mar, permite aos pesquisadores acessar as informações geoquímicas de conchas formadas em épocas distintas.

Essa análise revela, por exemplo, como as condições ambientais – temperatura e salinidade, entre outras – variaram de centenas a milhares de anos atrás. Ficou claro, assim, qual deveria ser a quantidade de água a ser descarregada no delta, a fim de manter as condições adequadas para a sobrevivência da Mulinia.

A equipe da Unesp procura seguir caminho similar. Embora embrionárias, as pesquisas são tão ambiciosas quanto os trabalhos feitos pelos norte-americanos. “A paleoecologia marinha aplicada é uma área de enorme relevância social que só recentemente passou a receber a atenção merecida”, afirma Simões. “O estudo do Rio Colorado é um marco.” Seu laboratório guarda amostras de acúmulos de conchas de Bouchardia rosea mortas e moluscos bivalves encontradas em acumulações com milhares de exemplares. Foram coletadas em 46 estações marítimas, com até 45 metros de profundidade, localizadas a cerca de 40 quilômetros da costa de Ubatuba, Caraguatatuba e São Sebastião, aonde chegavam somente após quatro horas de viagem de barco.

De volta ao laboratório, os pesquisadores analisaram as conchas do ponto de vista de uma área da ciência que estuda os padrões de preservação dos restos orgânicos, como os graus de articulação das conchas e de fragmentação, alterações na cor ou as taxas de incrustação ocorridas durante a vida ou adquiridas após a morte desses invertebrados – a chamada tafonomia. Conseguiram assim descobrir como se formaram esses acúmulos e a procedência das conchas depositadas no fundo do mar. Depois, as conchas seguiram para a equipe da Universidade de Virgínia, que conferiu as características tafonômicas básicas.

De lá, foram enviadas para o Woods Hole Oceanographic Institution’s National Ocean Sciences Accelerator Mass Spectrometry Facility (Nosams) e datadas com a técnica de carbono 14, mais indicado para datações de materiais com até 70 mil anos. Outra remessa foi de Virgínia para a Universidade George Washington, onde os pesquisadores cuidaram das datações com aminoácidos, que funcionam como relógios biológicos e indicam o tempo transcorrido desde a morte do organismo. “Embora possua limitações, a datação por aminoácidos é um método mais barato e mais rápido que o do carbono 14”, comenta Simões. Conclusão: as idades das conchas de Bouchardia encontradas na costa norte de São Paulo variaram de zero (o organismo tinha acabado de morrer) a 20 mil anos, com predomínio absoluto das que têm até 500 anos.

“Surpreendentemente, é possível estabelecer séries completas de idades a cada 50 anos, até a faixa em torno dos 500 anos, e determinar como os padrões ambientais variavam”, diz o pesquisador.
Na próxima etapa do trabalho, as equipes da Unesp e de Virgínia pretendem investir no estudo geoquímico das conchas já datadas, dentro das séries de idades até 500 anos ou mais – e os segredos que guardam deverão ser revelados pela análise de isótopos estáveis. Os especialistas vão comparar as proporções entre os elementos químicos – como carbono 13 e 12 e oxigênio 18 e 16 – das águas oceânicas com os registrados pelas conchas de Bouchardia de diferentes épocas (atuais, com 500, 3 mil e 20 mil anos, por exemplo).

O objetivo é construir uma linha histórica de parâmetros ambientais, como salinidade e temperatura. Simões sabe que nesse caminho não haverá apenas tempestades e enjôos. “Se estivéssemos falando de uma nova espécie de dinossauro, o tema seria rapidamente reconhecido”, afirma. “No caso de conchas tão pequenas, o processo é mais lento.”

O Projeto
Tafonomia de Braquiópodes e Bivalves em Ambientes Siliciclásticos da Costa Norte do Estado de São Paulo: Variação Ambiental nas Assinaturas Tafonômicas, Estilo Bioestratinômico e Mistura Temporal entre Tafocenoses
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sexta-feira, 29 de junho de 2018

Exceptionally preserved crustaceans from western Canada reveal a cryptic Cambrian radiation

Thomas H. P. Harvey, Maria I. Vélez, and Nicholas J. Butterfield
PNAS January 31, 2012. 109 (5) 1589-1594; https://doi.org/10.1073/pnas.1115244109
  1. Edited by Steven M. Stanley, University of Hawaii, Honolulu, HI, and approved December 16, 2011 (received for review September 16, 2011)

Abstract

The early history of crustaceans is obscured by strong biases in fossil preservation, but a previously overlooked taphonomic mode yields important complementary insights. Here we describe diverse crustacean appendages of Middle and Late Cambrian age from shallow-marine mudstones of the Deadwood Formation in western Canada. The fossils occur as flattened and fragmentary carbonaceous cuticles but provide a suite of phylogenetic and ecological data by virtue of their detailed preservation. In addition to an unprecedented range of complex, largely articulated filtering limbs, we identify at least four distinct types of mandible. Together, these fossils provide the earliest evidence for crown-group branchiopods and total-group copepods and ostracods, extending the respective ranges of these clades back from the Devonian, Pennsylvanian, and Ordovician. Detailed similarities with living forms demonstrate the early origins and subsequent conservation of various complex food-handling adaptations, including a directional mandibular asymmetry that has persisted through half a billion years of evolution. At the same time, the Deadwood fossils indicate profound secular changes in crustacean ecology in terms of body size and environmental distribution. The earliest radiation of crustaceans is largely cryptic in the fossil record, but “small carbonaceous fossils” reveal organisms of surprisingly modern aspect operating in an unfamiliar biosphere.
Crustaceans are the dominant arthropods in the modern marine realm and are renowned for their diversity, disparity, complexity, and ecologic range (1, 2). Their fossil record, however, is heavily skewed toward biomineralizing post-Cambrian forms (3), obscuring the higher-level relationships of crustaceans and their terrestrial mandibulate relatives, the myriapods and hexapods (4). Nonmineralizing (pan)crustaceans have been documented in the Cambrian fossil record but, until recently, have been represented almost exclusively by “Orsten-type” taxa of minute body size (< 2 mm) and limited appendage differentiation (5, 6). In contrast, the larger-bodied crustacean-like forms preserved in Burgess Shale-type and other macroscopic assemblages are either assignable to much deeper phylogenetic positions (1, 6, 7), or have yet to reveal key diagnostic characters among the inner leg branches and mouthparts (8, 9). Notably, the only macroscopic Cambrian fossil to exhibit convincing mandibles (“jaws”) is a Late Cambrian euthycarcinoid, a probable stem-group mandibulate (10).

Despite this limited record, the identification of disarticulated but unambiguously crustacean body parts among small carbonaceous fossils (SCFs) (11) in the Early Cambrian Mount Cap Formation of NW Canada (12, 13) points to a cryptic but significant diversity of Cambrian crustaceans. Here we describe extensive SCF assemblages of exceptionally preserved filtering appendages and mouthparts (mandibles) from the Middle and Upper Cambrian Deadwood Formation of western Canada (∼488 to 510 Ma; Cambrian Series 3—Furongian) (14). By bridging a major taphonomic gap in body size and preservational resolution, the Deadwood fossils provide crucial phylogenetic and ecologic datapoints for charting a major Cambrian radiation of crustaceans.

Geological Context

The Deadwood Formation (broadly defined, to include the Earlie and Finnegan formations) encompasses a broad expanse of shallow-marine, Middle to Late Cambrian sandstones and mudstones extending through eastern parts of the Western Canada Sedimentary Basin, the Williston Basin, and into the Black Hills of South Dakota, its type locality (15, 16). In Canada, the formation occurs primarily in the subsurface, with all of the specimens in this study recovered from petroleum exploration drillcores in southwest Saskatchewan and southeast Alberta. Unoxidized mudstones from Ceepee Riley Lake 3-4-39-13W3 and Ceepee Reward 4-28-38-24W3 (Middle/Late Cambrian, Saskatchewan) (16) and Rio Bravo Ronald 1-6-38-15W4 (Late Cambrian, Alberta) (15) were gently dissolved in hydrofluoric acid and the isolated SCFs individually collected from the rinsed residues (see Materials and Methods and SI Text for details of sample distributions and age). Among the several thousand recovered specimens are significant subpopulations of cuticle fragments that bear distinctively arthropodan spines and setae, including an exceptionally rich diversity of crustacean body parts.

Fossil Description and Identification

The Deadwood crustaceans are distinguished from other arthropodan remains by diagnostic cuticular ornamentations. They come from nine samples representing three separate assemblages, one from each drillcore (Table S1). Mandibles are the most widely distributed elements and fall into four distinct categories: branchiopod-type, copepod-type, ostracod-type, and an unidentified morphology. Other crustacean remains include comparatively delicate arrays of spines and setae, which are generally less abundant and informative, although one sample horizon has yielded a rich assemblage of extensively articulated branchiopod-type limbs.

Branchiopod-Type Mandibles.

The first of two types of mandible from the Riley Lake assemblage is distinguished by an extensive, D-shaped grinding (molar) surface (n = 17) (Fig. 1 A–H). The specimens fall into at least three distinct “morphotypes” that appear to be independent of both size and preservational orientation/resolution. In the first morphotype (n = 6) (Fig. 1 A–D), scaly lineations extend across the width of the molar surface, forming deep ridges at the straight/concave margin and a protruding fringe (sometimes also strong teeth) along the opposite edge (Fig. 1B). The second morphotype (n = 2) (Fig. 1 E and F) is distinguished by its opposite polarity (which is evident once images have been corrected for the “way-up” of slide-mounted specimens) and by lineations that do not extend across the width of the molar surface, but become confluent with an unornamented region bounded by marginal nodes (Fig. S1). The third molar morphotype (n = 3) (Fig. 1 G and H) features a region with disconnected, poorly aligned scales and no discrete bounding margin. In all three morphotypes the mandibular profile, as far as it is preserved, appears to be similar: one or more long setae and a single stout spine are inserted in line with the more acute end of the molar surface, beyond which the mandibular margin curves away forming a pronounced “shoulder” (Fig. 1 A, C–E, G, and H).
Fig. 1.
Fig. 1.
Fossil crustacean mandibles from the Middle and Late Cambrian Deadwood Formation. (A–H) Branchiopod-type mandibles from the Riley Lake assemblage. Morphotypes one (A–D) and two (E and F) are interpreted as the right and left mandibles from a single taxon, and morphotype three (G and H) as a distinct form. See Fig. S1 for detailed images of A, E, and F. (I–O) Copepod-type mandibles from the Riley Lake assemblage; detail I′ shows the platform and dorsal seta. (P) An ostracod-type mandible from the Rio Bravo Ronald assemblage; detail P′ magnifies the gnathal edge. Images have been reversed from slide-orientation in C, E, F, and H to show true polarity, and in J, K, N, and O for purposes of comparison. Grains of diagenetic pyrite show as opaque objects. See Table S2 for specimen numbers. (Scale bar, 50 μm for A–P; 30 μm for I′ and P′.)
Mandibles with extensive, scaly molar surfaces are known from among hexapods and myriapods as well as branchiopods, malacostracans, and remipedes (17). However, in both overall shape and detailed ornamentation the fossil molars are conspicuously similar to those of branchiopod crustaceans (Fig. 2 A and B). The pronounced posterior “shoulder” is characteristic of the post-molar profile in branchiopod mandibles (21, 22), and confirms that a distinct incisor process was absent during life. This condition is shared with branchiopods crown-wards of Rehbachiella (23), a Cambrian stem-group form (see character 14 in ref. 24). Moreover, the first and second fossil morphotypes show striking similarities to the right- and left-handed mandibles, respectively, of various extant anostracan branchiopods (Fig. 2 A and B), which suggests that they come from a single taxon displaying a complex pattern of mandibular asymmetry adapted for enhanced food-grinding efficiency (18, 21, 25). A comparable pattern of continuous scale rows on the right molar vs. a smooth region adjacent to dorsal marginal nodes on the left is a recognized synapomorphy (see character 15 in ref. 24) of extant anostracans and Lepidocaris, a stem-anostracan from the Devonian Rhynie Chert (24, 25). The third fossil morphotype is sufficiently distinct to represent a separate—although still branchiopodan—taxon (18).
Fig. 2.
Fig. 2.
Mandibles from modern crustaceans. (A and B) Gnathal edges (molars) from the right and left mandibles, respectively, of Chirocephalus diaphanus (Branchiopoda: Anostraca); reprinted with permission from ref. 18 (copyright 1991, Koninklijke Brill NV). Labels indicate anterior (A), posterior (P), dorsal (D), and ventral (V); a and b indicate matched opposing regions. (C) Gnathal edge of Calanus propinquus (Copepoda; cranial side of female right mandible; image reversed); arrow indicates dorsal seta. Image courtesy of Jan Michels. (D) Coxa with articulated palp of Macropyxis alanlordi (Ostracoda: Podocopa: Macrocyprididae). Image courtesy of Simone Nunes Brandão (19). (E) Mandibular (coxal) gnathal edge of Danielopolina exuma (Ostracoda: Myodocopa: Halocypridina); redrawn from ref. 20. (Scale bars, 100 μm for A, B, and D; 50 μm for C; E is not drawn to scale.)
Overall, the Deadwood molars range up to at least 230 μm long, predicting a maximum body length of at least 10–15 mm based on scaling relationships in extant anostracans (see figure S3 in ref. 13). The presence in the first and second morphotypes of a moderately sized posterior tooth and an asymmetric “tooth-groove” system points to an ecology of mixed benthic scraping and suspension feeding, as opposed to more exclusive predation or suspension feeding (21).

Copepod-Type Mandibles.

A contrasting type of mandible from the Riley Lake assemblage occurs as cuticle fragments bearing arc-shaped arrays of up to six robust teeth (n = 32) (Fig. 1 I–O). The tooth row terminates in a protruding, bristly wedge-shaped platform (n = 12) (Fig. 1 I, J, L, and M), confirming that the fossils represent entire gnathal edges rather than fragmentary incisors. Below the platform is inserted a papposerrate seta that is conspicuously longer and more robust than adjacent setae, and projects in line with the tooth row (n = 5) (Fig. 1 I′ and M). Variation in tooth outline (from broadly conical to narrow and strongly bicuspidate) and in the degree of secondary ornamentation (on the apical ridge and the basal slopes) depends in part on the angle of fossil compression, which varies from side-on to oblique or near-“vertical,” but also exhibits a trend toward more robust and highly ornamented teeth in larger specimens. This observation aside, large and small specimens exhibit similar numbers of teeth and similar relative proportions of the toothed edge and bristly platform, and are reasonably interpreted as ontogenetic variants of a single species.
Broadly comparable mandibles are widespread among crustaceans, but this particular combination of fine-scale elaborations (teeth, platform, and protruding seta), their numbers, positions, and proportions, and their overall ontogenetic consistency, are shared only with copepods—among which close matches for the fossils are numerous (26–29) (Fig. 2C). In particular, the prominent projecting seta (Fig. 1I′) is comparable in form and position to the potentially homologous “dorsal seta” (sometimes a pair of setae) found in every major order of nonparasitic copepods [i.e., Calanoida, Cyclopoida, Platycopioida, Misophrioida, Harpacticoida and Mormonilloida (27, 29)] (Fig. 2C).
In contrast, comparisons with mandibles in other crustacean groups appear to be superficial. Certain cirripedes possess a tooth row that ends in a protrusive bristly region, although the teeth are fewer and more robust and there is no projecting seta (30). The series of cusped teeth found in some branchiopods are either restricted to very early ontogenetic stages [e.g., in anostracans (31)], or are much broader and closely packed, and unaccompanied by terminal platforms or setae [in notostracans and laevicaudatans (22, 32)]. Among the fossil Orsten-type crustaceans, the mandibles of Skara and Bredocaris are broadly similar in profile but do not exhibit bifurcated tooth cusps, platforms, or dorsal setae (33, 34), whereas Rehbachiella is distinguished by the disproportionate expansion through ontogeny of a flattened grinding region (23).
The specific similarities to copepod mandibles allow predictions of body size and diet in the Deadwood species. Scaling relationships between gnathal edge and body length in various living copepods (35, 36) predict a prosome length of around 4.5–7 mm (and a body length ∼1–2 mm more) for the largest intact fossil (gnathal edge length ∼270 μm) (Fig. 1J). The fragmentary remains of larger mandibles (Fig. 1 N and O) point to still larger individuals, possibly in the centimetric range. A correlation between diet and mandibular morphology is well-established for living planktic calanoid copepods [Itoh's “Edge Index” (37)]. Comparisons with the similar adaptations seen in the fossil taxon, notwithstanding its comparatively large body size and unknown planktic or benthic habit, predict a largely herbivorous diet for the larger specimens with comparatively robust teeth, and a more omnivorous diet for the smaller specimens with elongate cusps, a possible ontogenetic distinction.

Ostracod-Type Mandible.

The third type of Deadwood mandible is represented by a single specimen from the Rio Bravo Ronald borehole that uniquely preserves the entire proximal mandibular body (coxa) along with its intact gnathal edge (Fig. 1P). The coxa exhibits an elongate overall shape that narrows to an acute apex, a large proximal opening (the insertion point in life for soft tissues), and a palp foramen (for the attachment of more distal parts, which have not been preserved). The gnathal edge is particularly complex: it bears a raised toothed blade (or possibly two superimposed blades) adjacent to three long setae set back from the edge, an intermediate region with short setae alongside a series of toothed cusps and a stout hooked tooth, and a bristly protruding platform (Fig. 1P′). Other mandibular remains in this assemblage are limited to two isolated gnathal edges that likely represent a fourth distinct type of Deadwood mandible, and are not considered further (Fig. S2).
In both overall morphology and details of the gnathal edge, the more complete Rio Bravo Ronald mandible compares most closely to those of ostracod crustaceans (Fig. 2 D and E). Similarly shaped, markedly elongate coxae with palp foramina of equivalent size and position are characteristic of both major living subgroups, Myodocopa and Podocopa, presumably reflecting the distinctive orientation, musculature, and articulation of ostracod mandibles (38). The complexity and form of the gnathal edge appear to be shared in particular with halocyprid myodocopes, some of which express a similar suite of characters including a raised toothed blade with adjacent long setae, an intermediate region with a hook-shaped spine, and a protruding grinding surface (Fig. 2E) (20, 39). The size of the fossil is consistent with an overall body (carapace) length of around 2 mm (19).

Branchiopod-Type Limbs.

A contrasting assemblage of SCFs, from a single thin (∼5 mm) horizon in the Ceepee Reward borehole, lacks mandibles but contains delicate setal armatures in unrivalled abundance and degree of articulation (n = 150) (Fig. 3). Most conspicuously, crustacean-type “filter plates” formed from a series of coplanar plumose setae with intersetule distances of ∼1 μm, plus accessory setae, occur commonly as isolated structures (n > 45) (Fig. 3 A and B) and sometimes within extensive setal arrays up to 800 μm across that reveal their wider anatomical context (n ∼11) (Fig. 3 C and D and Fig. S3). Specimens that preserve a continuous underlying cuticle are demonstrably derived from a single appendage (Fig. 3D) and show that filter plates were borne on limbs with a series of up to five nodose lobes (Fig. 3D′), along with a diversity of contrasting armatures composed variously of pappose, coarse plumose and, most distinctively, bifurcating serrated (“saw-toothed”) setae (Fig. 3 C and D and Fig. S3). The absence of articulations between the nodose lobes identifies them as the endites of either an undivided limb stem or a poorly segmented limb branch.
Fig. 3.
Fig. 3.
Fossil branchiopod-type limbs from the Middle Cambrian Deadwood Formation (Reward assemblage). (A and B) Isolated filter plates plus accessory setae; detail (A′) shows the diagnostic setulation of plumose filtering setae. (C and D) More extensive setal arrays preserving filter plates and other armatures in situ on limbs. (C) An array representing one or more appendages; detail C′ shows a filter plate (from center-right of image; rotated). (D) Part of a single appendage that preserves a filter plate (to left) and three protruding endites (arrowed); detail D′ shows the middle endite. See Table S2 for specimen numbers. (Scale bar, 60 μm for A and B; 15 μm for A′; 100 μm for C and D; 25 μm for C′; and 35 μm for D′.)
Filter plates are widespread and multiply convergent structures among crustaceans, but the arcuate outlines of the Deadwood examples and their arrangement on extensive lobose appendages are shared only with the phyllopodous thoracic filters of branchiopods (24). In contrast, the mouthpart filters found in certain malacostracans and the filter-like structures in various ostracods are much larger in proportion to the overall appendage (40, 41), whereas the thoracic filters of euphausiacean malacostracans (“krill”) and leptostracans/phyllocarids are linear rather than arcuate, and are not associated with such diverse accessory armatures (42–44). Among branchiopods, the Deadwood filters share a strictly coplanar setal arrangement with crown-group forms, in contrast to the more 3D armatures of Rehbachiella (23); similar combinations of filter plates and protuberant endites are known, for example, in the notostracan/diplostracan-like Castracollis from the Devonian Rhynie Chert (24, 45). Reconstructing the Deadwood fossils as a branchiopod crustacean with a long series of filtering thoracic appendages, an overall body length of at least several millimeters is likely for the more articulated arrays, although a centimetric body size is suggested by isolated filters constructed from substantially larger setae. A mixed scraping/filtering ecology (rather than a wholly planktic mode of life) is suggested by the juxtaposition of filter plates and saw-toothed armatures (25).

Discussion

Cambrian arthropods have sometimes been “shoehorned” into modern clades, despite having character combinations that support deeper, more stem-ward phylogenetic positions (7, 46). Conversely, the Deadwood fossils risk being assigned to inappropriately derived positions because of their “modern” appearance but disarticulated condition. Therefore, we conservatively assign them to comparatively inclusive clades, identifying crown groups via a synapomorphy shared with a subset of the crown (46).
To summarize, the Middle/Late Cambrian branchiopod-type fossils can all be assigned to a subset of the branchiopod total-group that excludes Rehbachiella. Furthermore, the mandibles that express anostracan-type right-left differentiation—a directional asymmetry (47) conserved across half a billion years of evolution—can be assigned to the crown. The Deadwood fossils thus extend the known range of crown-group branchiopods, as well as those crown-wards of Rehbachiella, back some 80–100 Myr from the Lower Devonian Rhynie Chert (48). Furthermore, filter plates and scraping armatures that are strikingly similar to those preserved in the Deadwood assemblage occur in the Mount Cap Formation (13) (Fig. S4), extending the known range of total-group branchiopods back to the late Early Cambrian (∼510 Ma).
The Late Cambrian ostracod-type mandible can be assigned to the ostracod total-group and perhaps to the crown, based on the halocyprid-like construction of the gnathal edge. Ostracod-type carapaces are known from the Early Ordovician and may extend back to the Cambrian in the guise of particular bradoriids (49). However, the Deadwood mandible provides the only appendage-based evidence for ostracods before the Silurian Herefordshire Lagerstätte, a unit that it predates by some 70 Myr (50).
The Middle/Late Cambrian copepod-type mandibles are assigned to the copepod total-group (stem or crown) based on a combination of characters including isometric growth and a dorsal seta. The Deadwood fossils thus extend the record of copepods (broadly defined) back some 190–210 Myr from fragments extracted from a Pennsylvanian (∼303 Ma) bitumen clast (51); other pre-Holocene records are restricted to the Miocene and Cretaceous (27).
Taken together, our results provide unambiguous evidence for a substantial branching by the Late Cambrian of within-crown (pan)crustacean lineages—a largely cryptic component of the Cambrian “explosion”—and offer key calibrations for molecular clocks and time-scaled phylogenies (48).

Complementary Taphonomic Modes.

A Cambrian radiation of crustaceans is not evident in either the conventional “shelly” fossil record or, apparently, macroscopic Burgess Shale-type biotas (1). However, it is revealed to a limited extent by the small-bodied (< 2 mm) forms preserved in Orsten-type assemblages (5, 6). Among these forms, Rehbachiella has been interpreted as a stem-branchiopod (24) and others, notably Skara, Yicaris, Bredocaris, and (possibly) the metanauplius Wujicaris, as stem-group members of various higher-level “entomostracan” taxa (52–55); pentastomid-like Orsten fossils may also be crustaceans (48). Debates over the phylogenetic affinities of the Orsten taxa have emphasized the difficulty in interpreting larvae and miniaturized adults (56, 57), and it is conceivable that their apparently more plesiomorphic positions are an artifact of their developmental stage or smaller size (13).
Importantly, the Deadwood fossils, like those of the Mount Cap (13), reveal the microscopic anatomies of both micro- and macroscopic (millimetric to centimetric) individuals and therefore circumvent a major taphonomic bias. That said, even the smallest Deadwood and Mount Cap individuals exhibit previously unseen morphologies, perhaps because they lived in comparatively shallow-marine environments that are undersampled by both Burgess Shale-type and Orsten-type preservation. Mandibles, at least, are emerging as a widespread and reasonably abundant component of SCF assemblages, conceivably as indigestible remains sedimented via fecal pellets (26, 35, 36) or simply as biostratinomically recalcitrant seabed detritus (58). In any case, they offer clear potential for reconciling the Orsten forms with adults and larger-bodied relatives for a new, high-definition narrative of early mandibulate evolution.

Evolving Crustacean Form and Function.

The fresh taphonomic perspective of SCFs provides the only direct evidence for sophisticated particle-handling in larger-bodied Cambrian arthropods. This characteristically crustacean-type ecology at the interface of micro- and macroscopic nutrient cycling has otherwise been loosely inferred from overall body form (1) and the proxy record of phytoplankton diversification (59). The detailed adaptations described here represent the acme of Cambrian differentiation within appendages, an alternative (and potentially correlative) measure of evolving arthropod complexity to the larger-scale tagmosis that has been the focus of previous studies (e.g., ref. 2).
In part, the new fossils reinforce a picture of early origination and subsequent conservation in crustacean form and function (60). At the same time, however, the small carbonaceous record provides evidence for unanticipated ecologic turnover. In the modern oceans, branchiopods are represented by a just a few species of small, secondarily marine cladocerans; larger forms, comparable in size to those of the Deadwood (up to ∼15 mm or more) and Mount Cap (∼50 mm), are now entirely nonmarine (24). Furthermore, modern free-living copepods are almost all much smaller than the ∼5- to 10-mm (plus) Deadwood taxon (27). In the modern world, visual predators—especially teleost fish—drive down body size in planktic freshwater crustacean communities (61) and strongly constrain the complex behaviors and distribution patterns of krill (62, 63), a group that shares with the Cambrian branchiopods the attributes of centimetric body size, marine habitat, and (by convergence) thoracic filtering. Significantly, the Deadwood and Mount Cap fossils reveal a contrasting pattern of crustacean distribution in the comparatively “unescalated” Cambrian biosphere.

Materials and Methods

Washed mudstone samples of 5–20 g were immersed in 40% hydrofluoric acid for 2–5 d before being flushed with water over a 30- or 63-μm sieve. Individual microfossils were picked from residues suspended in water using a pipette, rinsed in distilled water, and transferred to glass coverslips for mounting on glass microscope slides (using epoxy resin). Specimens were studied using transmitted light microscopy, and final images assembled from digital photographs taken at different focal planes. Figured specimens are stored at the Geological Survey of Canada (GSC), 601 Booth Street, Ottawa, ON, Canada, numbered sequentially from GSC 135369 to GSC 135393 (Table S2).

Acknowledgments

We thank staff at the Geological Subsurface Laboratory, Regina, and Energy Resources Conservation Board, Calgary for help with core sampling; geoLOGIC for generous access to subsurface data; Pier Binda for discussion of Deadwood microfossils; and Jan Michels, Simone Nunes Brandão, and Graziella Mura for providing images. This work is supported by Sidney Sussex College, Cambridge, and Natural Environment Research Council Grant NE/H009914/1.

Footnotes

  • Author contributions: T.H.P.H., M.I.V., and N.J.B. performed research; T.H.P.H. analyzed data; and T.H.P.H. and N.J.B. wrote the paper.
  • The authors declare no conflict of interest.
  • This article is a PNAS Direct Submission.
  • This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.1073/pnas.1115244109/-/DCSupplemental.

References

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  21. ↵
  22. ↵
  23. ↵
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  25. ↵
  26. ↵
  27. ↵
  28. ↵
  29. ↵
  30. ↵
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  39. ↵
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  53. ↵
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  55. ↵
  56. ↵
  57. ↵
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  61. ↵
  62. ↵
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