Gobekko cretacicus(Reptilia: Squamata) and its bearing on the interpretation of gekkotan affinities

Fossil-informed biogeographic analysis suggests Eurasian regionalization in crown Squamata during the early Jurassic

Background

Squamata (lizards, snakes, and amphisbaenians) is a Triassic lineage with an extensive and complex biogeographic history, yet no large-scale study has reconstructed the ancestral range of early squamate lineages. The fossil record indicates a broadly Pangaean distribution by the end- Cretaceous, though many lineages (e.g., Paramacellodidae, Mosasauria, Polyglyphanodontia) subsequently went extinct. Thus, the origin and occupancy of extant radiations is unclear and may have been localized within Pangaea to specific plates, with potential regionalization to distinct Laurasian and Gondwanan landmasses during the Mesozoic in some groups.

Methods

We used recent tectonic models to code extant and fossil squamate distributions occurring on nine discrete plates for 9,755 species, with Jurassic and Cretaceous fossil constraints from three extinct lineages. We modeled ancestral ranges for crown Squamata from an extant-only molecular phylogeny using a suite of biogeographic models accommodating different evolutionary processes and fossil-based node constraints from known Jurassic and Cretaceous localities. We hypothesized that the best-fit models would not support a full Pangaean distribution (i.e., including all areas) for the origin of crown Squamata, but would instead show regionalization to specific areas within the fragmenting supercontinent, likely in the Northern Hemisphere where most early squamate fossils have been found.

Results

Incorporating fossil data reconstructs a localized origin within Pangaea, with early regionalization of extant lineages to Eurasia and Laurasia, while Gondwanan regionalization did not occur until the middle Cretaceous for Alethinophidia, Scolecophidia, and some crown Gekkotan lineages. While the Mesozoic history of extant squamate biogeography can be summarized as a Eurasian origin with dispersal out of Laurasia into Gondwana, their Cenozoic history is complex with multiple events (including secondary and tertiary recolonizations) in several directions. As noted by previous authors, squamates have likely utilized over-land range expansion, land-bridge colonization, and trans-oceanic dispersal. Tropical Gondwana and Eurasia hold more ancient lineages than the Holarctic (Rhineuridae being a major exception), and some asymmetries in colonization (e.g., to North America from Eurasia during the Cenozoic through Beringia) deserve additional study. Future studies that incorporate fossil branches, rather than as node constraints, into the reconstruction can be used to explore this history further.

A Morrison stem gekkotan reveals gecko evolution and Jurassic biogeography

Geckos are a speciose and globally distributed clade of Squamata (lizards, including snakes and amphisbaenians) that are characterized by a host of modifications for nocturnal, scansorial and insectivorous ecologies. They are among the oldest divergences in the lizard crown, so understanding the origin of geckoes (Gekkota) is essential to understanding the origin of Squamata, the most species-rich extant tetrapod clade. However, the poor fossil record of gekkotans has obscured the sequence and timing of the assembly of their distinctive morphology. Here, we describe the first North American stem gekkotan based on a three-dimensionally preserved skull from the Morrison Formation of western North America. Despite its Late Jurassic age, the new species already possesses several key characteristics of the gekkotan skull along with retained ancestral features. We show that this new stem gekkotan, and several previously named species of uncertain phylogenetic relationships, comprise a widespread clade of early crown lizards, substantiating faunal homogeneity in Laurasia during the Late Jurassic that extended across disparate ecological, body-size and physiological classes.

The affinities of the Late Triassic Cryptovaranoides and the age of crown squamates

Most living reptile diversity is concentrated in Squamata (lizards, including snakes), which have poorly known origins in space and time. Recently, †Cryptovaranoides microlanius from the Late Triassic of the United Kingdom was described as the oldest crown squamate. If true, this result would push back the origin of all major lizard clades by 30-65 Myr and suggest that divergence times for reptile clades estimated using genomic and morphological data are grossly inaccurate. Here, we use computed tomography scans and expanded phylogenetic datasets to re-evaluate the phylogenetic affinities of †Cryptovaranoides and other putative early squamates. We robustly reject the crown squamate affinities of †Cryptovaranoides, and instead resolve †Cryptovaranoides as a potential member of the bird and crocodylian total clade, Archosauromorpha. Bayesian total evidence dating supports a Jurassic origin of crown squamates, not Triassic as recently suggested. We highlight how features traditionally linked to lepidosaurs are in fact widespread across Triassic reptiles. Our study reaffirms the importance of critically choosing and constructing morphological datasets and appropriate taxon sampling to test the phylogenetic affinities of problematic fossils and calibrate the Tree of Life.

A new gecko from the earliest Eocene of Dormaal, Belgium: a thermophilic element of the 'greenhouse world'

We here describe a new gekkotan lizard from the earliest Eocene (MP 7) of the Dormaal locality in Belgium, from the time of the warmest global climates of the past 66 million years (Myr). This new taxon, with an age of 56 Myr, together with indeterminate gekkotan material reported from Silveirinha (Portugal, MP 7) represent the oldest Cenozoic gekkotans known from Europe. Today gekkotan lizards are distributed worldwide in mainly warm temperate to tropical areas and the new gecko from Dormaal represents a thermophilic faunal element. Given the Palaeocene-Eocene thermal maximum at that time, the distribution of this group in such northern latitudes (above 50° North - the latitude of southern England) is not surprising. Although this new gekkotan is represented only by a frontal (further, dentaries and a mandibular fragment are described here as Gekkota indet. 1 and 2-at least two gekkotan species occurred in Dormaal), it provides a new record for squamate diversity from the earliest Eocene 'greenhouse world'. Together with the Baltic amber gekkotan Yantarogekko balticus, they document the northern distribution of gekkotans in Europe during the Eocene. The increase in temperature during the early Eocene led to a rise in sea level, and many areas of Eurasia were submerged. Thus, the importance of this period is magnified by understanding future global climate change.

Surface sculpturing in the skull of gecko lizards (Squamata: Gekkota)

It has previously been stated that geckos are characterized by smooth cranial bones bearing no sculpturing; however, there are many exceptions. Here we systematically characterize variation in sculpturing in cranial bones across all seven gekkotan families and examine patterns of evolutionary transitions in these traits on a multigene molecular gekkotan phylogeny to elucidate trends in phenotypic diversification in bone sculpturing. Over 195 species were reviewed using specimens where smooth, grooved, pitted and rugose sculpturing patterns were found. Of the 26 cranial bones, only seven (premaxilla, maxilla, nasal, prefrontal, frontal, parietal and postorbitofrontal) were found to bear sculpturing across more than three species. Sculpturing was found to extend beyond these seven bones onto either the dentary, surangular and/or quadrate within five species. Phylogenetic analysis showed that sculpturing evolved recently and repeatedly in several distinct lineages. The remaining 19 skull bones were smooth, except in the five species above, supporting the suggestion that smooth skull bones were ancestral in gekkotans. There is no apparent relationship between body size and the presence of bone sculpturing. The functional significance, if any, of sculpturing requires further investigation.

Embryonic skull development in the gecko, Tarentola annularis (Squamata: Gekkota: Phyllodactylidae)

Tarentola annularis is a climbing gecko with a wide distribution in Africa north of the equator. In the present paper, we describe the development of the osteocranium of this lizard, from the first appearance of the cranial elements up to the point of hatching. This is based on a combination of histology and cleared and stained specimens. This is the first comprehensive account of gekkotan pre-hatching skull development based on a comprehensive series of embryos, rather than a few selected stages. Given that Gekkota is now widely regarded as representing the sister group to other squamates, this account helps to fill a significant gap in the literature. Moreover, as many authors have considered features of the gekkotan skull and skeleton to be indicative of paedomorphosis, it is important to know whether this hypothesis is supported by delays in the onset of cranial ossification. In fact, we found the sequence of cranial bone ossification to be broadly comparable to that of other squamates studied to date, with no significant lags in development.

You Can't Run, but You Can Hide: The Skeleton of the Sand-Swimmer Lizard Calyptommatus leiolepis (Squamata: Gymnophthalmidae)

Squamates exhibit a vast diversity of body plans, which directly determines habitat use and preference. Here the skeleton of the sand-swimmer burrower gymnophthalmid, Calyptommatus leiolepis, is analyzed to investigate how its peculiar fossorial locomotion affects its osteology. Calyptommatus leiolepis is a limb-reduced, short-intermediate tailed lizard. Although there are other studies on its general anatomy, we performed a detailed description of its skeleton. Using high-resolution computer tomography, each bone element within the skeleton was digitally segmented and a detailed description rendered. Anatomical features related to burrowing include the head having a shovel-like snout with a well-developed horizontal soft tissue ridge, nasal cartilages that exclude sand from the nostrils, reduced eyes covered by a brille, lack of forelimbs, extreme reduction of hind limbs, and imbricated scales among others. The genus Calyptommatus has unique features such as a triradiate jugal (with digit-like posterior projections), a reduced pectoral girdle and forelimbs, parasternal processes that interconnect the ribs, and a single digit in the hind limbs. When comparing this species with other gymnophthalmid lizards including, fossorial species, it is clear that Calyptommatus exhibits the highest number of structural modifications within the family. Despite its specialized morphology, it still retains characters that link this genus to other members of Gymnophthalmidae when included in a phylogeny based solely on phenotypic data. Anat Rec, 303:1305-1326, 2020. © 2019 American Association for Anatomy.

Off the scale: a new species of fish-scale gecko (Squamata: Gekkonidae: Geckolepis) with exceptionally large scales

The gecko genus Geckolepis, endemic to Madagascar and the Comoro archipelago, is taxonomically challenging. One reason is its members ability to autotomize a large portion of their scales when grasped or touched, most likely to escape predation. Based on an integrative taxonomic approach including external morphology, morphometrics, genetics, pholidosis, and osteology, we here describe the first new species from this genus in 75 years: Geckolepis megalepissp. nov. from the limestone karst of Ankarana in northern Madagascar. The new species has the largest known body scales of any gecko (both relatively and absolutely), which come off with exceptional ease. We provide a detailed description of the skeleton of the genus Geckolepis based on micro-Computed Tomography (micro-CT) analysis of the new species, the holotype of G. maculata, the recently resurrected G. humbloti, and a specimen belonging to an operational taxonomic unit (OTU) recently suggested to represent G. maculata. Geckolepis is characterized by highly mineralized, imbricated scales, paired frontals, and unfused subolfactory processes of the frontals, among other features. We identify diagnostic characters in the osteology of these geckos that help define our new species and show that the OTU assigned to G. maculata is probably not conspecific with it, leaving the taxonomic identity of this species unclear. We discuss possible reasons for the extremely enlarged scales of G. megalepis in the context of an anti-predator defence mechanism, and the future of Geckolepis taxonomy.

Mid-Cretaceous amber fossils illuminate the past diversity of tropical lizards

Modern tropical forests harbor an enormous diversity of squamates, but fossilization in such environments is uncommon and little is known about tropical lizard assemblages of the Mesozoic. We report the oldest lizard assemblage preserved in amber, providing insight into the poorly preserved but potentially diverse mid-Cretaceous paleotropics. Twelve specimens from the Albian-Cenomanian boundary of Myanmar (99 Ma) preserve fine details of soft tissue and osteology, and high-resolution x-ray computed tomography permits detailed comparisons to extant and extinct lizards. The extraordinary preservation allows several specimens to be confidently assigned to groups including stem Gekkota and stem Chamaleonidae. Other taxa are assignable to crown clades on the basis of similar traits. The detailed preservation of osteological and soft tissue characters in these specimens may facilitate their precise phylogenetic placement, making them useful calibration points for molecular divergence time estimates and potential keys for resolving conflicts in higher-order squamate relationships.

Peramorphic traits in the tokay gecko skull

Traditionally, geckos have been conceived to exhibit paedomorphic features relative to other lizards (e.g., large eyes, less extensively ossified skulls, and amphicoelous and notochordal vertebrae). In contrast, peramorphosis has not been considered an important process in shaping their morphology. Here, we studied different sized specimens of Gekko gecko to document ontogenetic changes in cranial anatomy, especially near maturity. Comparison of this species with available descriptions of other geckos resulted in the identification of 14 cranial characteristics that are expressed more strongly with size increase. These characteristics become move evident in later stages of post-hatching development, especially near maturation, and are, therefore, attributed to peramorphosis (hyperossification). ACCTRAN and DELTRAN character optimizations were applied to these characters using a tree of 11 genera derived from a gekkotan molecular phylogeny. This analysis revealed that G. gecko expresses the majority of these putative peramorphic features near maturity, and that some of these features are also expressed in species closely related to G. gecko. The characters studied have the potential to be applied in future phylogenetic and taxonomic studies of this group of lizards.

The oldest known snakes from the Middle Jurassic-Lower Cretaceous provide insights on snake evolution

The previous oldest known fossil snakes date from ~100 million year old sediments (Upper Cretaceous) and are both morphologically and phylogenetically diverse, indicating that snakes underwent a much earlier origin and adaptive radiation. We report here on snake fossils that extend the record backwards in time by an additional ~70 million years (Middle Jurassic-Lower Cretaceous). These ancient snakes share features with fossil and modern snakes (for example, recurved teeth with labial and lingual carinae, long toothed suborbital ramus of maxillae) and with lizards (for example, pronounced subdental shelf/gutter). The paleobiogeography of these early snakes is diverse and complex, suggesting that snakes had undergone habitat differentiation and geographic radiation by the mid-Jurassic. Phylogenetic analysis of squamates recovers these early snakes in a basal polytomy with other fossil and modern snakes, where Najash rionegrina is sister to this clade. Ingroup analysis finds them in a basal position to all other snakes including Najash.

On the fossil record of the Gekkota

Gekkota is often interpreted as sister to all remaining squamates, exclusive of dibamids, or as sister to Autarchoglossa. It is the only diverse lineage of primarily nocturnal lizards and includes some of the smallest amniotes. The skeleton of geckos has often been interpreted as paedomorphic and/or "primitive" but these lizards also display a wide range of structural specializations of the postcranium, including modifications associated with both scansorial locomotion and limb reduction. Although the concept of "Gekkota" has been variously applied by different authors, we here apply a rigorous apomorphy based definition, recent advances in gekkotan morphology and phylogenetics, and diverse comparative material to provide a comprehensive assessment of 28 known pre-Quaternary geckos, updating the last such review, published three decades ago. Fossils evaluated include both sedimentary fossils and amber-embedded specimens. Known Cretaceous geckos are exclusively Asian and exhibit character combinations not seen in any living forms. Cenozoic gekkotans derive from sites around the world, although Europe is especially well represented. Paleogene geckos are largely known from disarticulated remains and show similarities to Sphaerodactylidae and Diplodactylidae, although resemblances may be plesiomorphic in some cases. Many Neogene gekkotans are referable to living families or even genera, but their geographic occurrences are often extralimital to those of modern groups, as is consistent with paleoclimatic conditions. The phylogenetic placement of fossil gekkotans has important repercusions for timetree calibration, but at present only a small number of fossils can be confidently assigned to even family level groupings, limiting their utility in this regard.

Key innovations and island colonization as engines of evolutionary diversification: a comparative test with the Australasian diplodactyloid geckos

The acquisition of key innovations and the invasion of new areas constitute two major processes that facilitate ecological opportunity and subsequent evolutionary diversification. Using a major lizard radiation as a model, the Australasian diplodactyloid geckos, we explored the effects of two key innovations (adhesive toepads and a snake-like phenotype) and the invasion of new environments (island colonization) in promoting the evolution of phenotypic and species diversity. We found no evidence that toepads had significantly increased evolutionary diversification, which challenges the common assumption that the evolution of toepads has been responsible for the extensive radiation of geckos. In contrast, a snakelike phenotype was associated with increased rates of body size evolution and, to a lesser extent, species diversification. However, the clearest impact on evolutionary diversification has been the colonization of New Zealand and New Caledonia, which were associated with increased rates of both body size evolution and species diversification. This highlights that colonizing new environments can drive adaptive diversification in conjunction or independently of the evolution of a key innovation. Studies wishing to confirm the putative link between a key innovation and subsequent evolutionary diversification must therefore show that it has been the acquisition of an innovation specifically, not the colonization of new areas more generally, that has prompted diversification.