Marta S. Fernández

THE ELASMOSAURID PLESIOSAUR ARISTONECTES CABRERA FROM THE LATEST CRETACEOUS OF SOUTH AMERICA AND ANTARCTICA

Abstract Aristonectes parvidens Cabrera, 1941, and Morturneria seymourensis (Chatterjee and Small, 1989), two plesiosaurs from the Maastrichtian of South America and Antarctica whose phylogenetical position is controversial, are reviewed and found to be congeneric and conspecific. Most differences between the two type specimens are interpreted as related to ontogenetic growth: Morturneria is based on an immature, whereas Aristonectes is based on an adult, probably an old individual. Aristonectes exhibits an unique set of characters among Plesiosauria: a low and wide ogival skull, a paired vomero-nasal fenestra, a mandible high anteriorly with a very short and high symphysis, a homodont dentition composed of numerous, strongly outwardly directed and poorly ornamented teeth (dental formula: 10–13 premaxillary, at least 51 maxillary and probably 60–65 dentary teeth, depending upon individual ontogeny). Moreover, Aristonectes shares several synapomorphies with the elasmosaurid clade, mainly strongly binocular-shaped and platycoelous cervical centra with lateral ridges. In contrast to cryptoclidids, it retains some plesiomorphic characters (e.g., horizontal jugal and poorly ventrally excavated cheek, glenoid fossa at about the same level as the alveolar row). The dental morphology and peculiar occlusal pattern, forming an interlocking trap, suggest that Aristonectes strained a diet of small, soft organisms from the water.

The Cranial Osteology and Feeding Ecology of the Metriorhynchid Crocodylomorph Genera Dakosaurus and Plesiosuchus from the Late Jurassic of Europe

Background Dakosaurus and Plesiosuchus are characteristic genera of aquatic, large-bodied, macrophagous metriorhynchid crocodylomorphs. Recent studies show that these genera were apex predators in marine ecosystems during the latter part of the Late Jurassic, with robust skulls and strong bite forces optimized for feeding on large prey. Methodology/Principal Findings Here we present comprehensive osteological descriptions and systematic revisions of the type species of both genera, and in doing so we resurrect the genus Plesiosuchus for the species Dakosaurus manselii. Both species are diagnosed with numerous autapomorphies. Dakosaurus maximus has premaxillary ‘lateral plates’; strongly ornamented maxillae; macroziphodont dentition; tightly fitting tooth-to-tooth occlusion; and extensive macrowear on the mesial and distal margins. Plesiosuchus manselii is distinct in having: non-amblygnathous rostrum; long mandibular symphysis; microziphodont teeth; tooth-crown apices that lack spalled surfaces or breaks; and no evidence for occlusal wear facets. Our phylogenetic analysis finds Dakosaurus maximus to be the sister taxon of the South American Dakosaurus andiniensis, and Plesiosuchus manselii in a polytomy at the base of Geosaurini (the subclade of macrophagous metriorhynchids that includes Dakosaurus, Geosaurus and Torvoneustes). Conclusions/Significance The sympatry of Dakosaurus and Plesiosuchus is curiously similar to North Atlantic killer whales, which have one larger ‘type’ that lacks tooth-crown breakage being sympatric with a smaller ‘type’ that has extensive crown breakage. Assuming this morphofunctional complex is indicative of diet, then Plesiosuchus would be a specialist feeding on other marine reptiles while Dakosaurus would be a generalist and possible suction-feeder. This hypothesis is supported by Plesiosuchus manselii having a very large optimum gape (gape at which multiple teeth come into contact with a prey-item), while Dakosaurus maximus possesses craniomandibular characteristics observed in extant suction-feeding odontocetes: shortened tooth-row, amblygnathous rostrum and a very short mandibular symphysis. We hypothesise that trophic specialisation enabled these two large-bodied species to coexist in the same ecosystem.

Salt glands in a Tithonian metriorhynchid crocodyliform and their physiological significance

Our knowledge of Mesozoic tetrapods is based mainly on osteological evidence. The discussion of the evolution of any homeostatic system is highly speculative because direct non-osteological evidence is uncommon. Here we report an extraordinarily well-preserved cast of a pair of lobulated protuberances in the skull of the marine metriorhynchid crocodiliform Geosaurus from the Tithonian (Jurassic) of Patagonia (Argentina). These protuberances are interpreted as representing salt glands. Based on their topology, these glands are identified as the nasals. Optimization of this character on a phylogenetic tree permits us to infer the ancestral condition for archosaurs. The relationship between salt gland and diet is also analysed. The presence of hypertrophied salt glands in the skull of Geosaurus suggests that as early as 140 million years ago, some Mesozoic marine reptiles had evolved an extra-renal osmoregulatory system. This achievement was an important clue in the successful colonization of marine environments. Salt glands preclude the risk of lethal dehydration and allow marine reptiles to include an important amount of invertebrates in their diet.

Salt glands in the Jurassic metriorhynchid Geosaurus: implications for the evolution of osmoregulation in Mesozoic marine crocodyliforms

The presence of salt-excreting glands in extinct marine sauropsids has been long suspected based on skull morphology. Previously, we described for the first time the natural casts of salt-excreting glands in the head of the Jurassic metriorhynchid crocodyliform Geosaurus araucanensis from the Tithonian of the Vaca Muerta Formation in the Neuquén Basin (Argentina). In the present study, salt-excreting glands are identified in three new individuals (adult, a sub-adult and a juvenile) referable to the same species. New material provides significant information on the salt glands form and function and permit integration of evolutionary scenarios proposed on a physiological basis in extant taxa with evidence from the fossil record. G. araucanensis represents an advanced stage of the basic physiological model to marine adaptations in reptiles. G. araucanensis salt glands were hypertrophied. On this basis, it can be hypothesized that these glands had a high excretory capability. This stage implies that G. araucanensis (like extant pelagic reptiles, e.g. cheloniids) could have maintained constant plasma osmolality even when seawater or osmoconforming prey were ingested. A gradual model of marine adaptation in crocodyliforms based on physiology (freshwater to coastal/estuarine to estuarine /marine to pelagic life) is congruent with the phylogeny of crocodyliforms based on skeletal morphology. The fossil record suggests that the stage of marine pelagic adaptation was achieved by the Early Middle Jurassic. Salt gland size in the juvenile suggests that juveniles were, like adults, pelagic.

First diagnostic marine reptile remains from the Aalenian (Middle Jurassic): a new ichthyosaur from southwestern Germany.

Background The Middle Jurassic was a critical time in the evolutionary history of ichthyosaurs. During this time interval, the diverse, well-studied faunas of the Lower Jurassic were entirely replaced by ophthalmosaurids, a new group that arose sometime prior to the Aalenian-Bajocian boundary and by the latest middle Jurassic comprised the only surviving group of ichthyosaurs. Thus, the Middle Jurassic Aalenian-Bathonian interval (176–165 million years ago) comprises the time frame during which ophthalmosaurids not only originated but also achieved taxonomic dominance. However, diagnostic ichthyosaur remains have been described previously from only a single locality from this interval, from the Bajocian of Argentina. Methodology/Principal Findings In this paper, we describe a new species of ichthyosaur based on a partial articulated specimen from the Middle Jurassic of southwestern Germany. This specimen was recovered from the Opalinuston Formation (early Aalenian) and is referable to Stenopterygius aaleniensis sp. nov. reflecting features of the skull and forefin. The genus Stenopterygius is diverse and abundant in the Lower Jurassic of Europe, but its presence has not previously been confirmed in younger (Middle Jurassic) rocks from the northern hemisphere. Conclusions/Significance This specimen represents the only diagnostic ichthyosaur remains reported from the Aalenian. It bears numerous similarities in size and in morphology to the Lower Jurassic species of the genus Stenopterygius and provides additional evidence that the major ecological changes hypothesized to have occurred at the end of the Toarcian took place sometime after this point and most likely did not occur suddenly. There is currently no evidence for the presence of ophthalmosaurids in the northern hemisphere during the Aalenian-Bathonian interval.

Marine reptiles from the Late Cretaceous of northern Patagonia

Abstract During the Campanian–Maastrichtian, Patagonia was flooded by the Atlantic and reduced to an archipelago. Several localities of northern Patagonia have yielded marine reptiles. Analysis of several assemblages suggests that the diversity and abundance of pelagic marine reptiles in northern Patagonia was higher by the end of the Cretaceous than previously thought. Several plesiosaurids, including Aristonectes parvidens and the polycotylid Sulcusuchus , and the first remains of mosasaurinae have been found. The Cretaceous marine reptile record from South America is scanty. Nevertheless, materials described here suggest that Tethyan and Weddelian forms converged in northern Patagonia, as seen with invertebrates.

A metriorhynchid crocodyliform braincase from northern Chile

ABSTRACT A three-dimensionally preserved metriorhynchid braincase from the Oxfordian of northern Chile is described. The specimen is referred to the metriorhynchid Metriorhynchus cf. M. westermanni. The excellent preservation provides clear sutures and a detailed description, and X-ray computed tomographic (CT) scanning provides internal anatomical details. The general pattern of the orbitotemporal region is consistent with that of the basal thalattosuchian Pelagosaurus typus as described recently. The specimen from northern Chile shares with other metriorhynchids (e.g., Cricosaurus araucanensis, Metriorhynchus westermanni, M. casamiquelai, and Dakosaurus andiniensis) a dorsally exposed laterosphenoid, a laterosphenoid-prootic suture forming a blunt crest separating the supratemporal fenestra into two fossae for muscular attachment, and the quadrate incompletely sutured to the braincase. Thus, these features characterize not only basal but derived Thalattosuchia, as suggested by previous authors. The main difference in the orbitotemporal region is that in the specimen described herein, and in the other metriorhynchids examined, the trigeminal fossa is developed mainly caudal to the trigeminal foramen, whereas in P. typus the fossa is developed rostral and caudal to the trigeminal foramen. CT scanning indicates the presence of enlarged dorsal dural venous sinuses overlying the brain, as it has been described recently in Steneosaurus pictaviensis, and a well-developed sinus within the quadrate. The large foramen ventrolateral to the occipital condyle, which characterizes metriorhynchids, is confirmed as the entry for the internal carotid artery.

ICHTHYOSAURIAN EYES: PALEOBIOLOGICAL INFORMATION CONTENT IN THE SCLEROTIC RING OF CAYPULLISAURUS (ICHTHYOSAURIA, OPHTHALMOSAURIA)

Abstract In the eyes of many extant reptiles there is an osseous ring embedded within the sclera and around the cornea. This sclerotic ring was well developed and is frequently preserved in ichthyosaurs. Significant information can be drawn from it, such as eyeball diameter and relative size of the cornea. Estimation of these parameters in extinct forms is significant as they are closely related to vision capability. This paper presents insights into the growth pattern and vision capabilities of some Jurassic ichthyosaurs as inferred from the analysis of the sclerotic rings and orbit sizes. The suggested growth pattern of ichthyosaur eyes is similar to that of human eyes. Thus, the cornea did not grow as much as the rest of the eye. The relative size of the sclerotic ring and orbits can be used as a complementary criterion for estimating relative age in some ichthyosaurs. In juveniles the aperture of the sclerotic ring represents approximately 20% of the orbital area. This percentage decreases in adults.

Tithonian Marine Reptiles of the Eastern Pacific

Publisher Summary This chapter presents an up-to-date analysis of the South American Tithonian reptile fauna in a biogeographical context and interprets the pattern of the marine reptile distribution during the Late Jurassic. The temporal sequence of the Tithonian marine reptiles record begins in the early Tithonian levels of Cerro Lotena. All the Tithonian thalattosuchians from the eastern Pacific are Metriorhynchidae. It is found that in spite of their relative abundance, the diversity of ichthyosaurs in the Tithonian of South America is low. The Tithonian marine reptiles of the Neuquen Basin are distributed in sedimentary facies assigned to basinal environments. In European Tithonian basins, turtles, teleosaurids, and metriorhynchid crocodiles are characteristic of nearshore and lagoonal environments. During the Late Jurassic, probably before, and during the Middle Jurassic, exchange events between reptiles from the Andean region and the West Tethys were favored by the opening of the Hispanic Corridor. Callovian crocodiles in Chile and southwestern Mexico, Callovian plesiosaurs in the Neuquen Basin, and Oxfordian plesiosaurs and crocodiles in Cuba support this hypothesis. The links to the Tethyan fauna could also be favored during the Late Jurassic by the opening of a new seaway between South America, Africa, and Antarctica. The discovery of ichthyosaur remains and fishes from the Upper Jurassic outcrops in the northeast of the Antarctic Peninsula is the first evidence of marine vertebrates in this seaway.

REVISION OF PLATYPTERYGIUS HAUTHALI von HUENE, 1927 (ICHTHYOSAURIA: OPHTHALMOSAURIDAE) FROM THE EARLY CRETACEOUS OF PATAGONIA, ARGENTINA

Abstract Most of the Cretaceous ichthyosaurian materials are referable to the single genus Platypterygius, which includes seven species. Although Cretaceous ichthyosaurs were probably cosmopolitan in their distribution, South American records are scarce. In recent revisions, the name Platypterygius hauthali has been retained for Early Cretaceous material from South America, but no specific diagnosis has been provided. The species P. hauthali was proposed on the basis of fragmentary material from the Neocomian of Santa Cruz Province (Patagonia, Argentina). However, analysis of the forefin features of the holotype shows that the pattern of carpal ossifications is different from that of other species of the genus and, furthermore, that this pattern is sufficient for specific diagnosis. Based on these data and additional findings of ichthyosaurs in Neuquén, Argentina, it can be concluded that there are three valid ichthyosaur taxa present in the Early Cretaceous of South America: Caypullisaurus bonapartei (Berriasian of Neuquén, Argentina), Platypterygius hauthali (Early Barremian of Santa Cruz, Argentina) and P. sachicarum (Barrenian-Aptian of Colombia).