Susan E. Evans

At the feet of the dinosaurs: the early history and radiation of lizards

Lizards, snakes and amphisbaenians together constitute the Squamata, the largest and most diverse group of living reptiles. Despite their current success, the early squamate fossil record is extremely patchy. The last major survey of squamate palaeontology and evolution was published 20 years ago. Since then, there have been changes in systematic theory and methodology, as well as a steady trickle of new fossil finds. This review examines our current understanding of the first 150 million years of squamate evolution in the light of the new data and changing ideas. Contrary to previous reports, no squamate fossils are currently documented before the Jurasis. Nonetheless, indirect evidence predicts that squamates had evolved by at least the middle Triassic, and diversified into existing major lineages before the end of this period. There is thus a major gap in the squamate record at a time when key morphological features were evolving. With the exception of fragmentary remains from Africa and India, Jurassic squamates are known only from localities in northern continents (Laurasia). The situation improves in the Early Cretaceous, but the southern (Gondwanan) record remains extremely poor. This constrains palaeobiogeographic discussion and makes it difficult to predict centres of origin for major squamate clades on the basis of fossil evidence alone. Preliminary mapping of morphological characters onto a consensus tree demonstrates stages in the sequence of acquisition for some characters of the skull and postcranial skeleton, but many crucial stages – most notably those relating to the acquisition of squamate skull kinesis – remain unclear.

Assessment of the role of sutures in a lizard skull: a computer modelling study

Sutures form an integral part of the functioning skull, but their role has long been debated among vertebrate morphologists and palaeontologists. Furthermore, the relationship between typical skull sutures, and those involved in cranial kinesis, is poorly understood. In a series of computational modelling studies, complex loading conditions obtained through multibody dynamics analysis were imposed on a finite element model of the skull of Uromastyx hardwickii, an akinetic herbivorous lizard. A finite element analysis (FEA) of a skull with no sutures revealed higher patterns of strain in regions where cranial sutures are located in the skull. From these findings, FEAs were performed on skulls with sutures (individual and groups of sutures) to investigate their role and function more thoroughly. Our results showed that individual sutures relieved strain locally, but only at the expense of elevated strain in other regions of the skull. These findings provide an insight into the behaviour of sutures and show how they are adapted to work together to distribute strain around the skull. Premature fusion of one suture could therefore lead to increased abnormal loading on other regions of the skull causing irregular bone growth and deformities. This detailed investigation also revealed that the frontal–parietal suture of the Uromastyx skull played a substantial role in relieving strain compared with the other sutures. This raises questions about the original role of mesokinesis in squamate evolution.

Integration of molecules and new fossils supports a Triassic origin for Lepidosauria (lizards, snakes, and tuatara)

BackgroundLepidosauria (lizards, snakes, tuatara) is a globally distributed and ecologically important group of over 9,000 reptile species. The earliest fossil records are currently restricted to the Late Triassic and often dated to 227 million years ago (Mya). As these early records include taxa that are relatively derived in their morphology (e.g. Brachyrhinodon), an earlier unknown history of Lepidosauria is implied. However, molecular age estimates for Lepidosauria have been problematic; dates for the most recent common ancestor of all lepidosaurs range between approximately 226 and 289 Mya whereas estimates for crown-group Squamata (lizards and snakes) vary more dramatically: 179 to 294 Mya. This uncertainty restricts inferences regarding the patterns of diversification and evolution of Lepidosauria as a whole.ResultsHere we report on a rhynchocephalian fossil from the Middle Triassic of Germany (Vellberg) that represents the oldest known record of a lepidosaur from anywhere in the world. Reliably dated to 238–240 Mya, this material is about 12 million years older than previously known lepidosaur records and is older than some but not all molecular clock estimates for the origin of lepidosaurs. Using RAG1 sequence data from 76 extant taxa and the new fossil specimens two of several calibrations, we estimate that the most recent common ancestor of Lepidosauria lived at least 242 Mya (238–249.5), and crown-group Squamata originated around 193 Mya (176–213).ConclusionA Early/Middle Triassic date for the origin of Lepidosauria disagrees with previous estimates deep within the Permian and suggests the group evolved as part of the faunal recovery after the end-Permain mass extinction as the climate became more humid. Our origin time for crown-group Squamata coincides with shifts towards warmer climates and dramatic changes in fauna and flora. Most major subclades within Squamata originated in the Cretaceous postdating major continental fragmentation. The Vellberg fossil locality is expected to become an important resource for providing a more balanced picture of the Triassic and for bridging gaps in the fossil record of several other major vertebrate groups.

A sphenodontine (Rhynchocephalia) from the Miocene of New Zealand and palaeobiogeography of the tuatara (Sphenodon)

Jaws and dentition closely resembling those of the extant tuatara (Sphenodon) are described from the Manuherikia Group (Early Miocene; 19–16 million years ago, Mya) of Central Otago, New Zealand. This material is significant in bridging a gap of nearly 70 million years in the rhynchocephalian fossil record between the Late Pleistocene of New Zealand and the Late Cretaceous of Argentina. It provides the first pre-Pleistocene record of Rhynchocephalia in New Zealand, a finding consistent with the view that the ancestors of Sphenodon have been on the landmass since it separated from the rest of Gondwana 82–60 Mya. However, if New Zealand was completely submerged near the Oligo-Miocene boundary (25–22 Mya), as recently suggested, an ancestral sphenodontine would need to have colonized the re-emergent landmass via ocean rafting from a currently unrecorded and now extinct Miocene population. Although an Early Miocene record does not preclude that possibility, it substantially reduces the temporal window of opportunity. Irrespective of pre-Miocene biogeographic history, this material also provides the first direct evidence that the ancestors of the tuatara, an animal often perceived as unsophisticated, survived in New Zealand despite substantial local climatic and environmental changes.

A giant frog with South American affinities from the Late Cretaceous of Madagascar

Madagascar has a diverse but mainly endemic frog fauna, the biogeographic history of which has generated intense debate, fueled by recent molecular phylogenetic analyses and the near absence of a fossil record. Here, we describe a recently discovered Late Cretaceous anuran that differs strikingly in size and morphology from extant Malagasy taxa and is unrelated either to them or to the predicted occupants of the Madagascar–Seychelles–India landmass when it separated from Africa 160 million years ago (Mya). Instead, the previously undescribed anuran is attributed to the Ceratophryinae, a clade previously considered endemic to South America. The discovery offers a rare glimpse of the anuran assemblage that occupied Madagascar before the Tertiary radiation of mantellids and microhylids that now dominate the anuran fauna. In addition, the presence of a ceratophryine provides support for a controversial paleobiogeographical model that posits physical and biotic links among Madagascar, the Indian subcontinent, and South America that persisted well into the Late Cretaceous. It also suggests that the initial radiation of hyloid anurans began earlier than proposed by some recent estimates.

Paramacellodid lizard skulls from the Jurassic Morrison Formation at Dinosaur National Monument, Utah

ABSTRACT New skull material of the Jurassic lizard Paramacellodus from the Morrison Formation of Dinosaur National Monument, Utah, permits a more comprehensive description of the anatomy of this lizard, previously known mainly from jaw and postcranial remains. The Morrison form cannot be satisfactorily distinguished from its British counterpart and the specimens are referred to Paramacellodus sp., cf. P. oweni. Cladistic analysis places Paramacellodus, and thus related genera attributed to the family Paramacellodidae Estes, 1983, as a sister taxon of Scincoidea (Cordyliformes + Scincidae), rather than of Cordyliformes alone.

Fossil lizards from the Jurassic Kota Formation of India

Abstract The Mesozoic lizard fauna of Gondwana is virtually unknown. We report here on a lizard assemblage from the Upper Member of the Kota Formation of peninsular India, usually considered to be of Early–Middle Jurassic age. The dominant form, Bharatagama rebbanensis, gen. et sp. nov., has a predominantly acrodont dentition. Comparison with living and extinct taxa suggests that this new genus is a primitive acrodont iguanian distinct from the Cretaceous priscagamids. It predates known records of iguanian lizards by some 80 Ma, and provides evidence that iguanians had begun to diversify before the break-up of Pangea. A second fully pleurodont taxon is known from the same deposit. It is tentatively attributed to the Squamata but is too fragmentary for further determination.

The earliest known Salamanders (Amphibia, Caudata):A record from the Middle Jurassic of England

Abstract Marmorerpeton gen. nov. represented by M. kermackisp. nov. and M. freemani sp. nov., is the earliest known genus of fossil salamander from the Upper Bathonian, Middle Jurassic of Kirtlington, Oxfordshire, England. Marmorerpeton is more primitive than any other known salamander in the absence of intravertebral spinal nerve foramina in the atlantal centrum, but in other features it resembles members of the family Scapherpetontidae, neotenous salamanders otherwise known from the Upper Cretaceous and Palaeocene. The Kirtlington herpetofauna is a unique freshwater assemblage of Middle Jurassic small amphibians and reptiles, several of which represent the earliest known occurrences of their respective groups.

Frogs and salamanders from the Upper Jurassic Morrison Formation (Quarry Nine, Como Bluff) of North America

ABSTRACT Previously undescribed frog and salamander bones from the Morrison Formation, at Quarry Nine, Como Bluff, Wyoming, are reported. One anuran ilium is diagnostically discoglossid and forms the basis of Enneabatrachus hechti, gen. et sp. nov. A second ilium is diagnostically pelobatid and is the earliest record for the family Pelobatidae, although it is indeterminate below family level. Salamander vertebrae appear to represent two morphological types but neither can be diagnosed critically. The previously described anuran taxa Eobatrachus agilis Marsh and Comobatrachus aenigmatis Hecht and Estes and the salamander Comonecturoides marshi Hecht and Estes are all based on non-determinate elements and must be considered nomina dubia at our present state of knowledge.

First definitive record of Mesozoic lizards from Madagascar

Abstract We report here the first unequivocal record of a pre-Late Pleistocene lizard from the island of Madagascar, based on a nearly complete lower jaw, elements of both the pectoral and pelvic girdles, several vertebrae and ribs, and numerous osteoderms of what is presumed to be a single individual. The specimen, recovered from the Upper Cretaceous (Maastrichtian) Anembalemba Member, Maevarano Formation, Mahajanga Basin, northwestern Madagascar, is identified as a scincoid scincomorph and, more specifically, a new genus and species of ?Cordylidae (Cordyliformes), based on a combination of gnathic, dental, and postcranial characters. The new taxon is the first identifiable lizard from the Late Cretaceous of the African continent (sensu lato). If the new taxon is correctly attributed to the Cordylidae, it constitutes a significant temporal and geographic range extension for the clade since cordylids have no definite representatives in the fossil record and extant forms are restricted to the sub-Saharan portion of mainland Africa. This new record also indicates that cordylids, after their occurrence in the Maastrichtian, became extinct on Madagascar, leaving only zonosaurine Gerrhosauridae as extant representatives of Cordyliformes on the island. Owing to limited knowledge concerning the time of divergence for cordylids and gerrhosaurids relative to the tectonic separation of Africa and Madagascar, and in light of the paucity of Mesozoic lizard fossils in general, and from Gondwana in particular, the discovery of the new taxon in the Late Cretaceous of Madagascar does little to otherwise constrain scenarios concerning the biogeographic history of early cordyliforms. Finally, we document here the observation that lizards appear to have been much less speciose than snakes in Late Cretaceous faunas of Gondwana, whereas the reverse is true in Laurasia. Lizards do not appear to become common in Gondwana until the Early Tertiary.