[Paleontology • 2016]
Makhaira rossica • Peculiar Macrophagous Adaptations in A New Cretaceous Pliosaurid
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Makhaira rossica
Fischer, Arkhangelsky, Stenshin, Uspensky, Zverkov & Benson, 2015
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Abstract
During the Middle and Late Jurassic, pliosaurid plesiosaurs evolved
gigantic body size and a series of craniodental adaptations that have
been linked to the occupation of an apex predator niche. Cretaceous
pliosaurids (i.e. Brachaucheninae) depart from this morphology, being
slightly smaller and lacking the macrophagous adaptations seen in
earlier forms. However, the fossil record of Early Cretaceous
pliosaurids is poor, concealing the evolution and ecological diversity
of the group. Here, we report a new pliosaurid from the Late Hauterivian
(Early Cretaceous) of Russia. Phylogenetic analyses using reduced
consensus methods recover it as the basalmost brachauchenine. This
pliosaurid is smaller than other derived pliosaurids, has tooth alveoli
clustered in pairs and possesses trihedral teeth with complex serrated
carinae. Maximum-likelihood ancestral state reconstruction suggests
early brachauchenines retained trihedral teeth from their ancestors, but
modified this feature in a unique way, convergent with macrophagous
archosaurs or sphenacodontoids. Our findings indicate that Early
Cretaceous marine reptile teeth with serrated carinae cannot be
unequivocally assigned to metriorhynchoid crocodylomorphs. Furthermore,
they extend the known diversity of dental adaptations seen in Sauropterygia, the longest lived clade of marine tetrapods.
Systematic description
Plesiosauria Blainville, 1835
Pliosauridae Seeley, 1874
Thalassophonea Benson & Druckenmiller, 2014
Makhaira rossica gen. et sp. nov.
LSIDs: urn:lsid:zoobank.org:pub:2C95C409-72C0-45FE-BF58-608657D5382F (Publication);
urn:lsid:zoob- ank.org:act:F19A595F-D739-4361-9088-84B7B947DC93 (Makhaira);
urn:lsid:zoobank.org:act:258CFACB-27D3-44CF-8B04-A12FDDECA55C (Makhaira rossica)
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Figure 2. Rostrum of YKM 68249/1-10.
(a–c) Right premaxilla, in (a) lateral, (b) medial and (c) ventral
views. Numbers indicate the position of each alveolus. The ventral
premaxilla–maxilla suture is located at the 6th alveolus. Note the
procumbent 1st alveolus. (d–e) Anterior part of the symphysis, in (d)
anterior, (e) ventral and (f) posterior views.
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Figure 3. Dentition and mandible of YKM 68249/1-10.
(a) Mandible in dorsal view. (b) Replacement tooth in the 2nd alveolus,
showing the trihedral cross section. (c) Base of the 3rd alveolus
crown, showing the marked mesiolabial carina (the crown fragment has
been glued slightly off its original position). (d) 1st or 2nd
post-symphysis replacement tooth, showing distal ridges and serrated
carinae. (e,f) Successive zooms of the mesiolabial carinae of a broken
off crown lying on the ventral surface of the symphysis (figure 2). (h)
Schematic diagram of the carination, drawn from (f). Note the serrated
crenulations.
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Holotype, Horizon and Locality: YKM 68249/1-10, a slightly
immature fragmentary skeleton consisting of a partial right premaxilla,
the anterior part of the mandible, several teeth, three dorsal vertebrae
in anatomical connection, a partial left ischium and a partial right
ilium. It is preserved in three dimensions in a series of pyritic
limestone nodules found along the banks of the Volga River, 600 m to the
north of Slantsevy Rudnik, Ulyanovsk Oblast, Russian Federation (figure
1). The precise level within the section is unknown, but the section
only contains Upper Hauterivian (Lower Cretaceous) strata of the
Speetoniceras versicolor Zone in this locality.
Etymology: From Latinized Ancient Greek ‘μάχαɩρα’ (mákhaira): a blade with a curved outline and Latin ‘rossica’: Russian.
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Figure 4. Postcranial remains of YKM 68249/1-10.
(a–f) Dorsal centrum, in (a) anterior, (b) right lateral, (c) posterior,
(d) left lateral, (e) ventral and (f) dorsal views. (g–k) Right ilium
in (g) dorsal, (h) medial, (i) lateral, (j) posterior and (k) ventral
views. (l–q) Left ischium in (l) anterolateral (glenoid), (m) dorsal,
(n) posterolateral, (o) medial, (p) anteromedial and (q) ventral views.
(r) Reconstruction of Makhaira rossica based on Late Jurassic
pliosaurids and mid-Cretaceos brachauchenines; the orange coloured parts
indicate fossils preserved in YKM 68249/1-10.
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Discussion and Conclusion
Ecology of Early Cretaceous pliosaurids
Makhaira rossica shares morphological features with both Late
Jurassic and Mid-Cretaceous pliosaurids, detailing the tempo of
morphological evolution in the early history of Brachaucheninae.
Osteological features often associated with macrophagy, and widely
present in Middle–Late Jurassic pliosaurids such as the spatulate
rostrum and the expanded caniniform teeth were seemingly lost early in
the evolution of brachauchenines. However, the incompletely resolved
phylogenetic position of Makhaira rossica within Brachaucheninae,
and the presence of these features in some other Cretaceous pliosaurid
specimens whose phylogenetic affinities were not resolved by our
analysis raises a number of questions regarding the evolution and
biodiversity of early members of that clade. Specifically, it seems that
Early Cretaceous pliosaurids exhibit multiple ecomorphologies that are
in need of further study. Because of the poor record of Early Cretaceous
pliosaurids, it is still unclear whether trihedral, strongly carinated
teeth constitute the ancestral condition of derived thalassophoneans or
were acquired convergently in Makhaira rossica, Pliosaurus and currently enigmatic taxa such as ‘Pliosaurus’ rossicus.
Parsimony-based methods are ambiguous while likelihood methods suggest
that trihedral teeth are a synapomorphy of Pliosaurus + Brachaucheninae,
that was subsequently lost within Brachaucheninae. In this scenario, Makhaira rossica thus retained the ancestral state of that trait, but modified it via a unique serration pattern.
Makhaira rossica departs from both Late Jurassic and Cretaceous
thalassophoneans by its smaller size: the largest dorsal centrum is
72 mm wide. Nevertheless, fusion of neurocentral suture suggests
osteological maturity for this specimen. For comparison, the last
cervical centrum of the late Barremian ‘Brachauchenius’ sp. is 117 mm wide, the largest dorsal centrum of Brachauchenius lucasi is 90 mm wide and the width of those of Kronosaurus queenslandicus and ‘Kronosaurus’ boyacensis exceed 150 mm and 170 mm, respectively. Makhaira rossica markedly
differs from Cretaceous thalassophoneans by having relatively large
teeth and dental adaptations reminiscent of macrophagous predators such
as theropod dinosaurs or thalattosuchians crocodyliforms. Unexpectedly,
because of their densely serrated and wave-like pattern, the carinae of
YKM 68249/1-10 appear larger and more complex than in other macrophagous
marine tetrapods such as Mosasaurus hoffmani (V. Fischer 2015, personal
observation on ULg PA.25119), Dakosaurus maximus ([54]; V. Fischer 2015, personal observation on ULg PA.6600) or Geosaurus, the latter being regarded as having ‘hypercarnivorous’ adaptations. Makhaira rossica is
also unique among plesiosaurs in having trihedral but moderately widely
spaced teeth. Contrary to carination and serration, previous authors
have not generally assigned a specific functional interpretation to the
presence of wide interalveolar spacing. However, we note that the
carinated teeth of macrophagous marine reptiles are usually closely
spaced.
Makhaira rossica thus indicates that pliosaurids explored
previously unrecognized niches during the Early Cretaceous, with the
presence of a smaller bodied taxon possessing clear yet distinctive
macrophagous adaptations. By being the first sauropterygian to develop
complex serration of its carinae, Makhaira rossica further exemplifies
the profound diet-driven morphofunctional convergences that evolved
among Mesozoic marine reptiles.
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Figure 5. Phylogenetic position of Makhaira rossica and
ancestral state reconstructions of character 139, related to crown
shape. (a) Strict consensus of the maximum-parsimony analysis of the
full dataset. (b) Strict consensus of the maximum-parsimony analysis of
the reduced dataset. (c) Results of maximum-parsimony method for
ancestral state reconstruction (using MESQUITE). (d) Results of
likelihood method for ancestral state reconstruction (using CLADDIS).
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Implications for metriorhynchid extinction
An isolated crown from the Aptian of Sicily (MSNC 4475) has been
recently regarded as evidence for the late survival of geosaurine
metriorhynchid crocodyliforms, several million years after their
supposed extinction [61]. However, although they do not yet co-occur
within a single pliosaurid taxon, all the features of MSNC 4475
described in [61] can now be shown to have been present among Cretaceous
pliosaurids (‘The conical shape of the tooth crown, noticeable lingual
curvature, presence of mesial and distal carinae, and microscopic
denticles along the carinae’ [61], p. 610). We also note that MSNC 4475
appears weakly trihedral in apical view ([61]; figure 2f). Moreover,
fine, smooth and widely spaced apicobasal ridges restricted to one
surface of the tooth and the triangular or approximately triangular
cross section of the crown are other features shared between MSNC 4475
and Makhaira rossica. Differences between these two specimens are also
present: the apicobasal ridges are not located on the curved side in the
large tooth of Makhaira rossica (but such ridges are present in one
small replacement tooth (figure 3) and thus possibly variable with
dental development in Makhaira rossica), and the weak development of a
trihedral cross section in the Sicilian tooth. It is not currently
possible to make a definitive statement on the affinities of MSNC 4475,
which clearly is an important specimen and potentially illustrates the
profound convergence of Makhaira rossica with macrophagous archosaurs.
However, future discoveries are likely to clarify whether MSNC 4475 is a
late-surviving, low-latitude metriorhynchid or a brachauchenine
pliosaurid.
Valentin Fischer, Maxim S. Arkhangelsky, Ilya M. Stenshin, Gleb N.
Uspensky, Nikolay G. Zverkov and Roger B. J. Benson. 2015. Peculiar
Macrophagous Adaptations in A New Cretaceous Pliosaurid.
Royal Society Open Science. DOI:
10.1098/rsos.150552
Os paleontologistas encontraram os ossos na vila de Krzyżanowice, perto das montanhas Świętokrzyskie. Nos
dias do pliosaurus, essa parte da Polônia era um arquipélago de ilhas
tropicais, com lagoas quentes e reservatórios marinhos, onde os animais
marinhos viviam. 
As criaturas podem atingir mais de 10 m de comprimento e pesar várias dezenas de toneladas. 
Os ossos pertencem tanto ao pilossauro quanto às criaturas que ele comeu. 
