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15.10: Evolutionary History of Reptiles - Biology

15.10: Evolutionary History of Reptiles - Biology


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Reptiles originated approximately 300 million years ago during the Carboniferous period. One of the oldest known amniotes is Casineria, which had both amphibian and reptilian characteristics. One of the earliest undisputed reptiles was Hylonomus. Soon after the first amniotes appeared, they diverged into three groups—synapsids, anapsids, and diapsids—during the Permian period.

The Permian period also saw a second major divergence of diapsid reptiles into archosaurs (predecessors of crocodilians and dinosaurs) and lepidosaurs (predecessors of snakes and lizards). These groups remained inconspicuous until the Triassic period, when the archosaurs became the dominant terrestrial group due to the extinction of large-bodied anapsids and synapsids during the Permian-Triassic extinction. About 250 million years ago, archosaurs radiated into the dinosaurs and the pterosaurs.

Although they are sometimes mistakenly called dinosaurs, the pterosaurs were distinct from true dinosaurs (Figure 1). Pterosaurs had a number of adaptations that allowed for flight, including hollow bones (birds also exhibit hollow bones, a case of convergent evolution). Their wings were formed by membranes of skin that attached to the long, fourth finger of each arm and extended along the body to the legs.

The dinosaurs were a diverse group of terrestrial reptiles with more than 1,000 species identified to date. Paleontologists continue to discover new species of dinosaurs. Some dinosaurs were quadrupeds (Figure 2); others were bipeds. Some were carnivorous, whereas others were herbivorous. Dinosaurs laid eggs, and a number of nests containing fossilized eggs have been found. It is not known whether dinosaurs were endotherms or ectotherms. However, given that modern birds are endothermic, the dinosaurs that served as ancestors to birds likely were endothermic as well. Some fossil evidence exists for dinosaurian parental care, and comparative biology supports this hypothesis since the archosaur birds and crocodilians display parental care.

Dinosaurs dominated the Mesozoic Era, which was known as the “age of reptiles.” The dominance of dinosaurs lasted until the end of the Cretaceous, the last period of the Mesozoic Era. The Cretaceous-Tertiary extinction resulted in the loss of most of the large-bodied animals of the Mesozoic Era. Birds are the only living descendants of one of the major clades of dinosaurs.

Visit this site to see a video discussing the hypothesis that an asteroid caused the Cretaceous-Triassic (KT) extinction.

Evolution and paleontology

The first land vertebrates, the Tetrapoda, appeared about 397 million years ago, near the middle of the Devonian Period. Despite having limbs rather than fins, early tetrapods were not completely terrestrial because their eggs and larvae depended upon a moist aquatic habitat. The first tetrapods apparently soon diverged one lineage became the amphibians (which retained the requirement for moisture-associated reproduction), whereas a second lineage yielded the Amniota during the Early Pennsylvanian Epoch (318 million to 312 million years ago). Fossils of these early amniotes are lacking. However, they must have appeared at this time because, for the Middle Pennsylvanian Epoch (312 million to 307 million years ago), fossils of synapsids (mammal-like reptiles) and early reptiles occur together in the same fossil beds. These earliest known synapsids and reptiles had already developed some traits that would persist in their descendants, modern mammals and reptiles. One example of a feature both groups held in common was the presence of extra-embryonic membranes (essentially, the amniotic sac) in early development, an adaptation that permitted the shift to a fully terrestrial egg.


15.10: Evolutionary History of Reptiles - Biology

Reptiles originated approximately 300 million years ago during the Carboniferous period. One of the oldest known amniotes is Casineria, which had both amphibian and reptilian characteristics. One of the earliest undisputed reptiles was Hylonomus. Soon after the first amniotes appeared, they diverged into three groups—synapsids, anapsids, and diapsids—during the Permian period.

Figure 1. Pterosaurs, which existed from the late Triassic to the Cretaceous period (210 to 65.5 million years ago), possessed wings but are not believed to have been capable of powered flight. Instead, they may have been able to soar after launching from cliffs. (credit: Mark Witton, Darren Naish)

The Permian period also saw a second major divergence of diapsid reptiles into archosaurs (predecessors of crocodilians and dinosaurs) and lepidosaurs (predecessors of snakes and lizards). These groups remained inconspicuous until the Triassic period, when the archosaurs became the dominant terrestrial group due to the extinction of large-bodied anapsids and synapsids during the Permian-Triassic extinction. About 250 million years ago, archosaurs radiated into the dinosaurs and the pterosaurs.

Although they are sometimes mistakenly called dinosaurs, the pterosaurs were distinct from true dinosaurs (Figure 1). Pterosaurs had a number of adaptations that allowed for flight, including hollow bones (birds also exhibit hollow bones, a case of convergent evolution). Their wings were formed by membranes of skin that attached to the long, fourth finger of each arm and extended along the body to the legs.

Figure 2. Edmontonia was an armored dinosaur that lived in the late Cretaceous period, 145.5 to 65.6 million years ago. (credit: Mariana Ruiz Villareal)

The dinosaurs were a diverse group of terrestrial reptiles with more than 1,000 species identified to date. Paleontologists continue to discover new species of dinosaurs. Some dinosaurs were quadrupeds (Figure 2) others were bipeds. Some were carnivorous, whereas others were herbivorous. Dinosaurs laid eggs, and a number of nests containing fossilized eggs have been found. It is not known whether dinosaurs were endotherms or ectotherms. However, given that modern birds are endothermic, the dinosaurs that served as ancestors to birds likely were endothermic as well. Some fossil evidence exists for dinosaurian parental care, and comparative biology supports this hypothesis since the archosaur birds and crocodilians display parental care.

Dinosaurs dominated the Mesozoic Era, which was known as the “age of reptiles.” The dominance of dinosaurs lasted until the end of the Cretaceous, the last period of the Mesozoic Era. The Cretaceous-Tertiary extinction resulted in the loss of most of the large-bodied animals of the Mesozoic Era. Birds are the only living descendants of one of the major clades of dinosaurs.


Evolutionary history of spiny-tailed lizards (Agamidae: Uromastyx) from the Saharo-Arabian region

Karin Tamar, Institute of Evolutionary Biology (CSIC-Universitat Pompeu Fabra), Barcelona, Spain.

Institute of Evolutionary Biology (CSIC-Universitat Pompeu Fabra), Barcelona, Spain

Allwetterzoo Münster, Münster, Germany

Department of Herpetology & Ichthyology, Natural History Museum of Geneva (MHNG), Geneva, Switzerland

CNRS-UMR 5175, Centre d’Écologie Fonctionnelle et Évolutive (CEFE), Montpellier, France

EPHE, CNRS, UM, SupAgro, IRD, INRA, UMR 5175 Centre d’Écologie Fonctionnelle et Évolutive (CEFE), PSL Research University, Montpellier, France

Institute of Evolutionary Biology (CSIC-Universitat Pompeu Fabra), Barcelona, Spain

Institute of Evolutionary Biology (CSIC-Universitat Pompeu Fabra), Barcelona, Spain

Karin Tamar, Institute of Evolutionary Biology (CSIC-Universitat Pompeu Fabra), Barcelona, Spain.

Institute of Evolutionary Biology (CSIC-Universitat Pompeu Fabra), Barcelona, Spain

Allwetterzoo Münster, Münster, Germany

Department of Herpetology & Ichthyology, Natural History Museum of Geneva (MHNG), Geneva, Switzerland

CNRS-UMR 5175, Centre d’Écologie Fonctionnelle et Évolutive (CEFE), Montpellier, France

EPHE, CNRS, UM, SupAgro, IRD, INRA, UMR 5175 Centre d’Écologie Fonctionnelle et Évolutive (CEFE), PSL Research University, Montpellier, France

Institute of Evolutionary Biology (CSIC-Universitat Pompeu Fabra), Barcelona, Spain

Abstract

The subfamily Uromastycinae within the Agamidae is comprised of 18 species: three within the genus Saara and 15 within Uromastyx. Uromastyx is distributed in the desert areas of North Africa and across the Arabian Peninsula towards Iran. The systematics of this genus has been previously revised, although incomplete taxonomic sampling or weakly supported topologies resulted in inconclusive relationships. Biogeographic assessments of Uromastycinae mostly agree on the direction of dispersal from Asia to Africa, although the timeframe of the cladogenesis events has never been fully explored. In this study, we analysed 129 Uromastyx specimens from across the entire distribution range of the genus. We included all but one of the recognized taxa of the genus and sequenced them for three mitochondrial and three nuclear markers. This enabled us to obtain a comprehensive multilocus time-calibrated phylogeny of the genus, using the concatenated data and species trees. We also applied coalescent-based species delimitation methods, phylogenetic network analyses and model-testing approaches to biogeographic inferences. Our results revealed Uromastyx as a monophyletic genus comprised of five groups and 14 independently evolving lineages, corresponding to the 14 currently recognized species sampled. The onset of Uromastyx diversification is estimated to have occurred in south-west Asia during the Middle Miocene with a later radiation in North Africa. During its Saharo-Arabian colonization, Uromastyx underwent multiple vicariance and dispersal events, hypothesized to be derived from tectonic movements and habitat fragmentation due to the active continental separation of Arabia from Africa and the expansion and contraction of arid areas in the region.

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Conclusions

Based on our analyses, the ancestors of crown and total-group snakes were nocturnal stealth hunters that foraged widely for soft-bodied prey in warm, mild, well-watered, and well-vegetated ecosystems (Figure 9). Prey size was relatively small compared to prey regularly consumed by snakes exhibiting the macrostomatan condition, but large relative to the size of prey targeted ancestrally by non-snake lizards. It was unlikely that they employed constriction to subdue prey. The earliest snakes were likely active primarily on the ground surface (even if beneath cover), although they may have also exhibited semi-fossorial habits. Ancestral snakes are unequivocally inferred to have originated on land, rather than in aquatic settings. The biogeographic origin of snakes is less clear than their early ecology and behavior however, our results suggest that the ancestor of crown snakes most likely originated on the Mesozoic supercontinent of Gondwana, and indicate the possibility that the ancestor of total-group snakes arose instead on Laurasia. A conclusive resolution of the biogeographic origin of total-group snakes will require both reevaluation of the controversial fossil snake Coniophis precedens, and the discovery of new fossils of stem-group snakes.

Reconstruction of the ancestral crown-group snake, based on this study. Artwork by Julius Csotonyi.

The snake total-group, or at least the Coniophis-node, is inferred to have arisen in the middle Early Cretaceous, with the crown originating about 20 million years later, during the Albian stage. A series of rapid divergences in their early evolutionary history suggests that snakes may have been participants in the hypothesized Early Cretaceous Terrestrial Revolution. Our results further suggest that henophidian diversity, which includes the bulk of extant snake species, radiated entirely after the K-Pg mass extinction.

These results paint the clearest picture yet of the early evolution of snakes, shedding light on their ecological, behavioral, biogeographic, and macroevolutionary origins. Both the ancestors of total-group and crown-group snakes were apparently similar in ecology and behavior to many basal macrostomatans surviving today. This conclusion, dependent on the inclusion of fossil stem snakes in our analysis, would be unexpected if only extant snakes were considered, given the sister-position of highly derived scolecophidians to all other extant crown snakes. Thus, the importance of fossil intermediates for illuminating macroevolutionary processes cannot be understated. Furthermore, our results demonstrate that the inclusion of phenotypic and fossil data can affect the inference of phylogenetic topologies, even when such data are vastly outnumbered by genetic sequence data. Fossils afford unprecedented glimpses into the grand tapestry of evolutionary history, and can inform inferences well beyond those that can be drawn from the fortuitous survivors comprising Earth’s modern biota. Transitional fossils are therefore invaluable for both phylogenetic analyses and for the accurate reconstruction of ancestral states.


How can we study mammalian evolution through morphology?

To understand mammalian evolution we need to be able to accurately identify what defines a mammal—but this is somewhat difficult, especially in evolutionary history as observed in the fossil record. Most of the specialisations mammals have are shared by other groups, and so are not on their own sufficient to identify a mammal. Mammals belong to the aminote clade—tetrapod vertebrates that protect their developing embryos—either in an egg or in the mother—in a membrane called the amnion. Other amniotes include the birds and reptiles, and one needs to be able to distinguish mammals from their amniote relatives. While almost all mammals are warm-blooded (the naked mole rat is a possible exception) so are birds, so this can’t be used as a defining feature. It is likely that the common ancestor of mammals and birds was cold blooded, so the presence of endothermy in these two groups is another example of convergent evolution. Most mammals have live births however, some reptile species such as Zootoca vivipara and Pseudemoia entrecasteauxii also give birth to live young, while the extant monotreme species (the platypus and two echidna species) lay eggs but are still mammals. All mammals produce milk and most have fur, but these features are not useful since they are not usually preserved in fossils. However, a useful defining feature to identify mammals and distinguish them from other amniotes like reptiles and birds is a specialised middle ear and jaw joint—and this is often easier to find in the fossil record.


Details

Laurie Vitt

Dr. Vitt is a reptile ecologist who received his Ph.D. from Arizona Sate University in 1976. He was a Professor at UCLA for 8 years and Professor and Curator at the Sam Noble Museum at the University of Oklahoma for 21 years. He currently maintains Emeritus status. He has had extensive field experience in American deserts and New World tropics, especially Brazil. He has published more than 250 research articles and 8 books. Awards include appointment as a George Lynn Cross Research Professor at the University of Oklahoma, membership in the Brazilian Academy of Scientists, Distinguished Alumnus (Western Washington University), Distinguished Herpetologist (Herpetologist League), and two book awards.

Affiliations and Expertise

Sam Noble Museum and Biology Department University of Oklahoma Norman, Oklahoma

Janalee Caldwell

Dr. Caldwell is an amphibian biologist who received her Ph.D. from the University of Kansas in 1974. She was a Professor of Biology and Curator at the Sam Noble Museum at the University of Oklahoma for 21 years, where she received recognition for outstanding research. She is now Professor Emeritus and Curator Emeritus. Dr. Caldwell conducted field research in tropical forests in Brazil and other South American countries that resulted in publication of numerous scientific articles. She served as President of the Society for the Study of Amphibians and Reptiles and as editor of several scientific journals. She participated is various projects with the goal of encouraging young people, especially girls, to choose careers in science.

Affiliations and Expertise

Sam Noble Museum and Biology Department University of Oklahoma Norman, Oklahoma


Origin of Reptiles

Reptiles evolved from amphibians of Carboniferous period, which depended on water bodies for laying eggs and development of larval stages and hence could not exploit arid habitats far away from water bodies. They invented a large yolk-laden shelled egg that could be laid on land and in which an amniotic sac contained fluid in which embryo could develop to an advanced stage, capable of fending for itself when hatched. The following anatomical changes transformed the ancestral amphibians into land adapted reptiles:

  • Body developed a covering of epidermal scales to prevent loss of body moisture, and skin glands were lost.
  • Skull became monocondylic for better movement and flexibility. Atlas and axis vertebrae together permitted skull movement in all directions.
  • Limb bones and girdles became stronger but limbs were attached on the sides of body, and belly touched the ground during creeping mode of locomotion.
  • Sacral region involved two strong and fused vertebrae to support the body weight on hind legs.
  • Pentadactyle limbs developed claws that helped in climbing on rocks and trees.
  • Lung respiration became more efficient.
  • As a water conservation strategy, metanephros kidneys excreted uric acid which did not require water for excretion.
  • Reptiles continued to be ectothermal since ventricle was not completely partitioned by a septum and blood mixed in heart.
  • Internal fertilization evolved as a large cleioid shelled egg was laid on land.
  • Embryonic membranes amnion, allantois and yolk sac evolved to enable embryonic development in arid conditions.

ANCESTORS OF REPTILES

THE COTYLOSAURS

They were the most primitive stem reptiles that evolved from the labyrithodont amphibians (Embolomeri) in Carboniferous period.

Seymoria was a lizard-like animal, with pentadactyle limbs and a short tail. It had homodont labyrinthine teeth on the jaw bones as well as on vomer and palatine bones. Presence of lateral line indicates its amphibious habits. Skull was monocondylic for better movement of head. Seymoria indicates gradual transition from labyrinthodont amphibians to reptiles. Another 5 foot long cotylosaur fossil, Limnoscelis was found in Mexico that had large premaxillary teeth and long tail.

THE PARAPSIDS

They possessed superior temporal vacuity in the skull and were adapted for aquatic mode of life.

Plesiosaurus was marine long-necked, fish-eating animal with 15 metre long fusiform body, short tail and paddle-like limbs modified for swimming. The skull was euryapsid type with a superior temporal vacuity. The fossils are from lower Jurassic (about 180 million years) and they are believed to have become extinct in end-Cretaceous mass extinction.

Ichthyosaurus had fish-like body with fore limbs modified into paddle-like fins and hind limbs disappeared. There was a fleshy dorsal fin too. Caudal fin was large and bilobed. Jaws projected into an elongated snout and teeth were homodont, an adaptation for fish-catching. Skull was parapsid type with additional postfrontal and supratemporal bones behind the eye orbit. Vertebral column became secondarily simplified with amphicoelous vertebrae.

THE SYNAPSIDS

Synapsids split off from the primitive reptilian stock very early in evolution, perhaps in the middle carboniferous period. Synapsids had started developing mammalian characteristics that enabled them to be fleet-footed and active predators. Their legs commenced to move under the body. Heterodont dentition and false palate started developing in pelycosaurs and had been completely formed in therapsids. Two types of synapsids occurred from carboniferous to Permian, namely, the primitive Pelycosaurs and advanced therapsids.

Pelycosaurs are represented by Dimetrodon whose fossils were discovered from North America and Russia from the late Carboniferous to Permian periods. They were primitive reptile-like animals in which limbs had moved under the body but not completely and each limb had 5 digits with claws. Neural spines on the back were excessively long stretching highly vascularized skin between them that formed a fin-like or sail-like structure. They had heterodont dentition with incisors, canines and molars clearly defined but the false palate had not been completely formed.

Therapsids were more advanced and active synapsids which were perhaps endothermic animals with high rate of metabolism. Heterodont dentition with false palate allowed these animals to chew and grind food for quick digestion in the gut so that high metabolic demand of the body could be fulfilled. Jaw muscles were attached to zygomatic arch to make chewing effective. Carnivore therapsids were called Cynodonts (ex. Cynognathus) and herbivores were Dicynodonts.

THE THECODONTS

They evolved from the sauropsid Archosauria, a group of insignificant lizard-like reptiles that survived the Triassic mass extinction. They evolved into bipedal and highly agile predators.

Euperkeria and Ornithosuchus fossils were unearthed from South Africa and Europe. They were about 2 ft long bipedal lizard-like animals with small head but very long tail for balancing while they chased flying insects by rapid running. Endothermy must have evolved in thecodonts to meet the extraordinary energy demands of their predatory life style.

THE SAURISCHIANS

They were dinosaurs with lizard-like pelvic girdle in which ischium and pubis bones radiated away from each other. They were both bipedal and quadrupedal and carnivores as well as herbivores.

THE ORNITHISCHIANS

They were dinosaurs with bird-like pelvic girdle in which ischium and pubis bones were directed towards posterior as found in modern birds. These were also highly diversified carnivores as well as herbivores and both bipedal and quadruped.

THE PTEROSAURIA

They were flying or gliding dinosaurs of Mesozoic that varied in size from sparrow-sized to some species, like Pteranodon, having a wing span of 8 meters. They had pneumatic bones. Last digit of the fore limb was extraordinarily long and served to attach the membranous patagium between fore limb, hind limb and the body. Hind limbs were used for clinging on to the rocks and cliffs and 3 digits of fore limbs also had curved claws, an adaptation for clinging. Their jaws were modified into beak that possessed homodont dentition but Pteranodon did not have teeth.


Did adaptive radiations shape reptile evolution?

Animals sampled in the analysis. Colors indicates rates of evolution: warm colors high rates and cool colors low rates Credit: Tiago R. Simões

Some of the most fundamental questions in evolution remain unanswered, such as when and how extremely diverse groups of animals—for example reptiles—first evolved. For seventy-five years, adaptive radiations—the relatively fast evolution of many species from a single common ancestor—have been considered as the major cause of biological diversity, including the origins of major body plans (structural and developmental characteristics that identify a group of animals) and new lineages. However, past research examining these rapid rates of evolution was largely constrained by the methods used and the amount of data available.

In a paper out today in Nature Communications, a research team lead by Harvard University examined the largest available data set of living and extinct major reptile groups (such as marine reptiles, turtles, lizards, and the ancestors of dinosaurs and crocodiles) to tackle the longstanding question of how adaptive radiations have shaped reptile evolution. Using DNA information from modern species and hundreds of anatomical features from both modern and fossil species for statistical analysis, the study detected that periods of fast anatomical change during the origin of reptile groups often predate when those groups diversified into hundreds or thousands of species. This contradicts long-held ideas of adaptive radiation in evolution biology.

"Our findings suggest that the origin of the major reptile groups, both living and extinct, was marked by very fast rates of anatomical change, but that high rates of evolution do not necessarily align with taxonomic diversification" said first author Dr. Tiago Simões, Postdoctoral Fellow in in the lab of Stephanie Pierce, AssociateProfessor in the Department of Organismic and Evolutionary Biology at Harvard University.

Simões and Pierce revealed that rates of evolution and morphological variety in reptiles prior to the Permian-Triassic Mass Extinction—the biggest mass extinction of all time—were equally high, or even higher, than after the event. As reptile species diversity was much lower during the Permian compared to Triassic, these results indicate that fast rates of evolution do not need to coincide with rapid taxonomic diversification as predicted by the classical theory of adaptive radiation. The two can be decoupled.

The team, which also included Ph.D. student Oksana Vernygora and Professor Michael Caldwell at the University of Alberta, further discovered that accelerated rates of evolution correspond to the origin of unique reptile body plans, but that very similar functional adaptations in reptiles can arise through varying rates of evolution.

"Surprisingly," Pierce said, "reptiles that evolved similar protective armour like turtles or serpentine bodies like snakes, show radically different rates of evolution, indicating the origin and evolution of unique body plans is heterogeneous through evolution."

"Our results also show that the origin of snakes is characterized by the fastest rates of anatomical change in the history of reptile evolution," said Simões. "But, that this does not coincide with increases in taxonomic diversity [as predicted by adaptive radiations] or high rates of molecular evolution."

The mismatch between morphological and molecular evolution supports the idea that protein coding DNA sequences do not seem to be correlated with broad-scale changes in anatomy. Although much more research is needed to understand how body plans evolve, the team hypothesizes that non-protein coding regions of the genome may be responsible for rapid morphological change, as these parts are more free to mutate and take on new functional roles.

"It is clear to us that to advance our understanding of the major patterns in evolution we need further studies capable of measuring phenotypic and molecular evolutionary rates, times of origin, and phenotypic diversity across large timescales" said Simões.

Simões and colleagues continue to develop new methods and are expanding their data set back in time to look at the origins of amniotes, the group that includes both reptilesand mammals. Of particular interest is pinpointing when in geological time these two groups of animals diverged and how extinction, diversification, and adaptation have shaped their evolutionary history over the last 300+ million years.

"I'm excited to continue my research to unravel the early evolutionary dynamics of the two most successful groups of animals on the planet," Simões said. "I'm also focusing on improving available protocols to analyze morphological data and construct more robust evolutionary trees, including the timing of origin of major vertebrate lineages."


Evolution of eggshell structure in relation to nesting ecology in non-avian reptiles

Liliana D'Alba, Department of Biology, EON-Unit, Universiteit Gent, Gent, Belgium.

Contribution: Conceptualization, Data curation, Formal analysis, Methodology, Resources, Writing - original draft

Department of Biology, EON-Unit, Universiteit Gent, Ghent, Belgium

Contribution: Data curation, Formal analysis

Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, California, USA

Contribution: Data curation, Funding acquisition

Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, California, USA

Contribution: Data curation, Supervision

Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, California, USA

Contribution: Data curation

Department of Biology, EON-Unit, Universiteit Gent, Ghent, Belgium

Contribution: Methodology, Writing - review & editing

Department of Biology, EON-Unit, Universiteit Gent, Ghent, Belgium

Contribution: Funding acquisition, Writing - original draft, Writing - review & editing

Department of Biology, EON-Unit, Universiteit Gent, Ghent, Belgium

Liliana D'Alba, Department of Biology, EON-Unit, Universiteit Gent, Gent, Belgium.

Contribution: Conceptualization, Data curation, Formal analysis, Methodology, Resources, Writing - original draft

Department of Biology, EON-Unit, Universiteit Gent, Ghent, Belgium

Contribution: Data curation, Formal analysis

Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, California, USA

Contribution: Data curation, Funding acquisition

Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, California, USA

Contribution: Data curation, Supervision

Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, California, USA

Contribution: Data curation

Department of Biology, EON-Unit, Universiteit Gent, Ghent, Belgium

Contribution: Methodology, Writing - review & editing

Department of Biology, EON-Unit, Universiteit Gent, Ghent, Belgium

Contribution: Funding acquisition, Writing - original draft, Writing - review & editing

Funding information: Fonds Wetenschappelijk Onderzoek, Grant/Award Number: G0A7921N Human Frontier Science Program, Grant/Award Number: RGP0047 U.S. DOE Office of Science User Facility, Grant/Award Number: DE-AC02-05CH11231

Abstract

Amniotic eggs are multifunctional structures that enabled early tetrapods to colonize the land millions of years ago, and are now the reproductive mode of over 70% of all terrestrial amniotes. Eggshell morphology is at the core of animal survival, mediating the interactions between embryos and their environment, and has evolved into a massive diversity of forms and functions in modern reptiles. These functions are critical to embryonic survival and may serve as models for new antimicrobial and/or breathable membranes. However, we still lack critical data on the basic structural and functional properties of eggs, particularly of reptiles. Here, we first characterized egg shape, shell thickness, porosity, and mineralization of eggs from 91 reptile species using optical images, scanning electron microscopy, and micro computed tomography, and collected data on nesting ecology from the literature. We then used comparative analyses to test hypotheses on the selective pressures driving their evolution. We hypothesized that eggshell morphology has evolved to protect shells from physical damage and desiccation, and, in support, found a positive relationship between thickness and precipitation, and a negative relationship between porosity and temperature. Although mineralization varied extensively, it was not correlated with nesting ecology variables. Ancestral state reconstructions show thinning and increased porosity over evolutionary time in squamates, but the opposite in turtles and crocodilians. Egg shape, size, porosity and calcification were correlated, suggesting potential structural or developmental tradeoffs. This study provides new data and insights into the morphology and evolution of reptile eggs, and raises numerous questions for additional research.