Comparative functional morphology indicates niche partitioning among sympatric marine reptiles

Mesozoic marine ecosystems were dominated by diverse lineages of aquatic tetrapods. For over 50 Ma in the Jurassic until the Early Cretaceous, plesiosaurians, ichthyosaurians and thalattosuchian crocodylomorphs coexisted at the top levels of trophic food webs. We created a functional dataset of continuous craniomandibular and dental characters known from neontological studies to be functionally significant in modern aquatic tetrapods. We analysed this dataset with multivariate ordination and inferential statistics to assess functional similarities and differences in the marine reptile faunas of two well-sampled Jurassic ecosystems deposited in the same seaway: the Oxford Clay Formation (OCF, Callovian-early Oxfordian, Middle-Late Jurassic) and the Kimmeridge Clay Formation (KCF, Kimmeridgian-Tithonian, Late Jurassic) of the UK. Lower jaw-based macroevolutionary trends are similar to those of tooth-based diversity studies. Closely related species cluster together, with minimal overlaps in the morphospace. Marine reptile lineages were characterized by the distinctive combinations of features, but we reveal multiple instances of morphofunctional convergence among different groups. We quantitatively corroborate previous observations that the ecosystems in the OCF and KCF were markedly distinct in faunal composition and structure. Morphofunctional differentiation may have enabled specialization and was an important factor facilitating the coexistence of diverse marine reptile assemblages in deep time.

Late Cretaceous coprolite from the Opole area (southern Poland) as evidence for a variable diet in shell-crushing shark Ptychodus (Elasmobranchii: Ptychodontidae)

Background

Coprolites, i.e., fossilized faeces, are an important source of knowledge on the diet and food processing mechanisms in the fossil record. Direct and indirect evidences for the dietary preferences of extinct sharks are rare in the fossil record. The first coprolite attributable to Ptychodus containing prey remains from the European Cretaceous is documented here.

Methods

A coprolite from the Late Cretaceous of Opole (southern Poland) was scanned using micro-computed tomography to show the arrangement of the inclusions. In addition, the cross-section was examined under the SEM/EDS to analyse the microstructure and chemical composition of the inclusions.

Results

Brachiopod shell fragments and foraminiferan shells are recognized and identified among the variously shaped inclusions detected through the performed analysis.

Conclusions

The extinct shell-crushing shark Ptychodus has been identified as the likely producer of the examined coprolite. The presence of brachiopod shell fragments indicates that at least some species of this durophagous predatory shark may have preyed on small benthic elements on the sea bottom.

The Cretaceous Neornithine record and new Vegaviidae specimens from the López de Bertodano Formation (Upper Maastrichthian) of Vega Island, Antarctic Peninsula

A worldwide revision of the Cretaceous record of Neornithes (crown birds) revealed that unambiguous neornithine taxa are extremely scarce, with only a few showing diagnostic features to be confidently assigned to that group. Here we report two new neornithine specimens from Vega Island (López de Bertodano Formation). The first is a synsacrum (MN 7832-V) that shows a complex pattern of transversal diverticula intercepting the canalis synsacri, as in extant neornithines. Micro-CT scanning revealed a camerate pattern of trabeculae typical of neornithines. It further shows the oldest occurrence of lumbosacral canals in Neornithes, which are related to a balance sensing system acting in the control of walking and perching. The second specimen (MN 7833-V) is a distal portion of a tarsometatarsus sharing with Vegavis iaai a straight apical border of the crista plantaris lateralis. Osteohistologically the tarsometatarsus shows a thick and highly vascularized cortex that lacks any growth marks, resembling Polarornis gregorii. The cortex is osteosclerotic as in other extinct and extant diving neornithines. These new specimens increase the occurrences of the Cretaceous avian material recovered from the Upper Cretaceous strata of the James Ross Sub-Basin, suggesting that a Vegaviidae-dominated avian assemblage was present in the Antarctic Peninsula during the upper Maastrichtian.

High phenotypic plasticity at the dawn of the eosauropterygian radiation

The initial radiation of Eosauropterygia during the Triassic biotic recovery represents a key event in the dominance of reptiles secondarily adapted to marine environments. Recent studies on Mesozoic marine reptile disparity highlighted that eosauropterygians had their greatest morphological diversity during the Middle Triassic, with the co-occurrence of Pachypleurosauroidea, Nothosauroidea and Pistosauroidea, mostly along the margins of the Tethys Ocean. However, these previous studies quantitatively analysed the disparity of Eosauropterygia as a whole without focussing on Triassic taxa, thus limiting our understanding of their diversification and morphospace occupation during the Middle Triassic. Our multivariate morphometric analyses highlight a clearly distinct colonization of the ecomorphospace by the three clades, with no evidence of whole-body convergent evolution with the exception of the peculiar pistosauroid Wangosaurus brevirostris, which appears phenotypically much more similar to nothosauroids. This global pattern is mostly driven by craniodental differences and inferred feeding specializations. We also reveal noticeable regional differences among nothosauroids and pachypleurosauroids of which the latter likely experienced a remarkable diversification in the eastern Tethys during the Pelsonian. Our results demonstrate that the high phenotypic plasticity characterizing the evolution of the pelagic plesiosaurians was already present in their Triassic ancestors, casting eosauropterygians as particularly adaptable animals.

Ecological signal in the size and shape of marine amniote teeth

Amniotes have been a major component of marine trophic chains from the beginning of the Triassic to present day, with hundreds of species. However, inferences of their (palaeo)ecology have mostly been qualitative, making it difficult to track how dietary niches have changed through time and across clades. Here, we tackle this issue by applying a novel geometric morphometric protocol to three-dimensional models of tooth crowns across a wide range of raptorial marine amniotes. Our results highlight the phenomenon of dental simplification and widespread convergence in marine amniotes, limiting the range of tooth crown morphologies. Importantly, we quantitatively demonstrate that tooth crown shape and size are strongly associated with diet, whereas crown surface complexity is not. The maximal range of tooth shapes in both mammals and reptiles is seen in medium-sized taxa; large crowns are simple and restricted to a fraction of the morphospace. We recognize four principal raptorial guilds within toothed marine amniotes (durophages, generalists, flesh cutters and flesh piercers). Moreover, even though all these feeding guilds have been convergently colonized over the last 200 Myr, a series of dental morphologies are unique to the Mesozoic period, probably reflecting a distinct ecosystem structure.

The cranial anatomy and relationships of Cardiocorax mukulu (Plesiosauria: Elasmosauridae) from Bentiaba, Angola

We report a new specimen of the plesiosaur Cardiocorax mukulu that includes the most complete plesiosaur skull from sub-Saharan Africa. The well-preserved three-dimensional nature of the skull offers rare insight into the cranial anatomy of elasmosaurid plesiosaurians. The new specimen of Cardiocorax mukulu was recovered from Bentiaba, Namibe Province in Angola, approximately three meters above the holotype. The new specimen also includes an atlas-axis complex, seventeen postaxial cervical vertebrae, partial ribs, a femur, and limb elements. It is identified as Cardiocorax mukulu based on an apomorphy shared with the holotype where the cervical neural spine is approximately as long anteroposteriorly as the centrum and exhibits a sinusoidal anterior margin. The new specimen is nearly identical to the holotype and previously referred material in all other aspects. Cardiocorax mukulu is returned in an early-branching or intermediate position in Elasmosauridae in four out of the six of our phylogenetic analyses. Cardiocorax mukulu lacks the elongated cervical vertebrae that is characteristic of the extremely long-necked elasmosaurines, and the broad skull with and a high number of maxillary teeth (28-40) which is characteristic of Aristonectinae. Currently, the most parsimonious explanation concerning elasmosaurid evolutionary relationships, is that Cardiocorax mukulu represents an older lineage of elasmosaurids in the Maastrichtian.

Synchrotron microtomography of a Nothosaurus marchicus skull informs on nothosaurian physiology and neurosensory adaptations in early Sauropterygia

Nothosaurs form a subclade of the secondarily marine Sauropterygia that was well represented in late Early to early Late Triassic marine ecosystems. Here we present and discuss the internal skull anatomy of the small piscivorous nothosaur Nothosaurus marchicus from coastal to shallow marine Lower Muschelkalk deposits (Anisian) of Winterswijk, The Netherlands, which represents the oldest sauropterygian endocast visualized to date. The cranial endocast is only partially encapsulated by ossified braincase elements. Cranial flattening and lateral constriction by hypertrophied temporal musculature grant the brain a straight, tubular geometry that lacks particularly well-developed cerebral lobes but does potentially involve distinguishable optic lobes, suggesting vision may have represented an important sense during life. Despite large orbit size, the circuitous muscular pathway linking the basisphenoidal and orbital regions indicates poor oculomotor performance. This suggests a rather fixed ocular orientation, although eye placement and neck manoeuvrability could have enabled binocular if not stereoscopic vision. The proportionally large dorsal projection of the braincase endocast towards the well-developed pineal foramen advocates substantial dependence on the corresponding pineal system in vivo. Structures corroborating keen olfactory or acoustic senses were not identified. The likely atrophied vomeronasal organ argues against the presence of a forked tongue in Nothosaurus, and the relative positioning of external and internal nares contrasts respiratory configurations proposed for pistosauroid sauropterygians. The antorbital domain furthermore accommodates a putative rostral sensory plexus and pronounced lateral nasal glands that were likely exapted as salt glands. Previously proposed nothosaurian 'foramina eustachii' arose from architectural constraints on braincase development rather than representing functional foramina. Several modifications to brain shape and accessory organs were achieved through heterochronic development of the cranium, particularly the braincase. In summary, the cranium of Nothosaurus marchicus reflects important physiological and neurosensory adaptations that enabled the group's explosive invasion of shallow marine habitats in the late Early Triassic.

Exceptionally prolonged tooth formation in elasmosaurid plesiosaurians

Elasmosaurid plesiosaurians were globally prolific marine reptiles that dominated the Mesozoic seas for over 70 million years. Their iconic body-plan incorporated an exceedingly long neck and small skull equipped with prominent intermeshing ‘fangs’. How this bizarre dental apparatus was employed in feeding is uncertain, but fossilized gut contents indicate a diverse diet of small pelagic vertebrates, cephalopods and epifaunal benthos. Here we report the first plesiosaurian tooth formation rates as a mechanism for servicing the functional dentition. Multiple dentine thin sections were taken through isolated elasmosaurid teeth from the Upper Cretaceous of Sweden. These specimens revealed an average of 950 daily incremental lines of von Ebner, and infer a remarkably protracted tooth formation cycle of about 2–3 years–other polyphyodont amniotes normally take ~1–2 years to form their teeth. Such delayed odontogenesis might reflect differences in crown length and function within an originally uneven tooth array. Indeed, slower replacement periodicity has been found to distinguish larger caniniform teeth in macrophagous pliosaurid plesiosaurians. However, the archetypal sauropterygian dental replacement system likely also imposed constraints via segregation of the developing tooth germs within discrete bony crypts; these partly resorbed to allow maturation of the replacement teeth within the primary alveoli after displacement of the functional crowns. Prolonged dental formation has otherwise been linked to tooth robustness and adaption for vigorous food processing. Conversely, elasmosaurids possessed narrow crowns with an elongate profile that denotes structural fragility. Their apparent predilection for easily subdued prey could thus have minimized this potential for damage, and was perhaps coupled with selective feeding strategies that ecologically optimized elasmosaurids towards more delicate middle trophic level aquatic predation.

Functional anatomy of the head of the large aquatic predator Rhomaleosaurus zetlandicus (Plesiosauria, Reptilia) from the Toarcian (Lower Jurassic) of Yorkshire, England

The skull and mandible of the type specimen of the large pliosauroid plesiosaur Rhomaleosaurus zetlandicus from the Toarcian of England are elongate, and adapted for powerful predatory activity in water. The mandible contains all elements found in primitive reptilian mandibles. The broadly caniniform dentition suggests that Rhomaleosaurus fed on a wide range of active prey, and forcibly dismembered larger prey by shaking and twisting them. The cranial musculature is reconstructed for the first time in plesiosaurs. It was adapted for feeding in water. The adductor musculature included a large anterior pterygoideus attached to the suborbital fenestra, a large posterior pterygoideus, and a group of large dorsal muscles including the adductor mandibulae externus. The anterior pterygoideus exerted maximum torque when the jaws were wide open, snapping them shut quickly, and the dorsal muscle mass exerted maximum torque when the jaws were closed on prey to subdue and dismember it. The role of the posterior pterygoideus is uncertain or intermediate. The musculature combines elements of the "kinetic inertial’ system ascribed to aquatic tetrapods by Olson (1961), with his ‘static, pressure’ system ascribed to terrestrial tetrapods. Olson suggested that the large pterygoideus musculature typical of the ‘kinetic inertial’ system functioned to confer kinetic energy on the mandible. However, its function may instead have been to compensate for the inertia and drag of the mandible. The depressor musculature comprised the depressor mandibulae and the longitudinal pharyngeal muscles, and opened the jaw quickly against drag. The cervical musculature cannot be reconstructed in detail. There was a strong nuchal ligament. The forces within the head are analysed by using box and girder beams as analogues. Gross form, shape of constituent bones, and sutural morphology confirm adaptations to resist great bending moments arising from the action of the muscles when biting on prey. When the jaws were closed, the pterygoid flange supported the mandible against the inward component of the adductor muscle force. Rhomaleosaurus was a visual predator. The eyes were large. The stapes is present. Underwater olfaction was likely. There is no evidence for an eardrum, but it is not known whether this is the plesiomorphic reptilian state or secondarily derived from a tympanate ancestor. The ears were not acoustically isolated from the braincase, so underwater directional hearing was poor, and sonar was not possible. The structure of the head of Rhomaleosaurus is a functional compromise between the needs to maximize structural strength and to maximize swimming and feeding efficiency. Especially important were the ability to sustain large muscle and reaction forces to provide an adequate bite force at the end of a long snout, and the wide gape allowing the swallowing of large pieces of prey. Even larger items were dismembered into smaller pieces by shake and twist feeding. The major unresolved problems are the effects of scaling factors, and the torsional loadings induced when biting asymmetrically, or twisting large prey to pieces.

Stomach stones for feeding or buoyancy? The occurrence and function of gastroliths in marine tetrapods

Gastroliths or ‘stomach stones’ occur frequently in some, but not all, groups of fossil and living m arine tetrapods. C om parative analysis of gastrolith distribution suggests a role in buoyancy control rather than food processing. Once accidental ingestion by bottom-feeding animals is excluded, gastroliths occur in most tetrapods which ‘fly’ underw ater with hydrofoil limbs, including plesiosaurs, penguins, and otariid pinnipeds, but not the m arine chelonians. They do not usually occur in cetaceans, ichthyosaurs, mosasaurs, and odobenid and phocid pinnipeds, which swim with a caudal fin or the equivalent. Occurrence in amphibious forms is variable; crocodilians often have gastroliths, but nothosaurs and placodonts do not. T he correlation of gastroliths and underw ater flight is corroborated by a com parative analysis which takes phylogenetic factors into account. There is no correlation with diet. Consideration of function and occurrence in terrestrial forms suggests that the use of gastroliths in digestion would not be useful, and might even be harmful, to a carnivorous m arine tetrapod. Gastroliths are more efficient than skeletal bone (as in pachyostosis) in terms of sinking force per unit of added mass or volume. As well as driftwood and ice, m arine tetrapods should be considered as a potential source of erratic stones in freshwater and marine sediments. Gastroliths may have evolved by the accidental ingestion of stones, the retention into adulthood of stones used by juveniles to process insect or plant food, or as a com pensatory replacem ent for dense bones habitually filling the stomach. Their presence or absence should be more carefully recorded and further studies should be carried out on their function.