8.3 Reptile evolution

Reptile evolution marks a pivotal moment in vertebrate history. These early land-dwellers emerged during the Carboniferous, developing key adaptations like the amniotic egg. This allowed them to thrive in terrestrial environments, setting the stage for their dominance in the Mesozoic.

Reptiles diversified into several major groups, including anapsids, synapsids, and diapsids. Each lineage developed unique skull structures and adaptations, leading to the evolution of turtles, mammal-like reptiles, and the ancestors of modern reptiles and birds. This diversity shaped vertebrate evolution for millions of years.

Origins of reptiles

• Reptiles emerged during the Carboniferous period, approximately 320-310 million years ago, as the first fully terrestrial vertebrates
• The evolution of the amniotic egg was a key adaptation that allowed reptiles to become independent of water for reproduction, unlike their amphibian ancestors
• Reptiles descended from early tetrapods, with the oldest known reptile being Hylonomus lyelli, a small, lizard-like creature from the Late Carboniferous of Nova Scotia

Amniotic egg development

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• The amniotic egg consists of several membranes that provide protection and nourishment to the developing embryo, including the amnion, chorion, and allantois
• The shell of the amniotic egg is made of calcium carbonate and provides a barrier against desiccation and mechanical damage, allowing eggs to be laid on land
• The yolk sac provides nutrition for the embryo, while the albumen acts as a shock absorber and provides additional moisture and protein

Transition from amphibians

• Reptiles evolved from early tetrapods, likely temnospondyls, during the Late Carboniferous period
• Key adaptations that differentiated reptiles from amphibians include the amniotic egg, more efficient locomotion, and better osmoregulation
• The evolution of scales and a waterproof skin helped reptiles conserve moisture and thrive in drier environments compared to their amphibian ancestors

Carboniferous period emergence

• The Carboniferous period (359-299 million years ago) saw the rise of the first reptiles, such as Hylonomus and Paleothyris
• This period was characterized by a warm, humid climate with extensive coal swamps, providing an ideal environment for the diversification of early reptiles
• By the end of the Carboniferous, reptiles had begun to diverge into several distinct lineages, setting the stage for their dominance in the Permian and Mesozoic eras

Anapsid reptiles

• Anapsids are characterized by a skull with no temporal fenestrae, or openings behind the eye sockets
• This primitive skull structure is thought to be the ancestral condition for all reptiles
• Anapsids were among the earliest reptiles to appear, with fossils dating back to the Late Carboniferous period

Primitive skull structure

• The anapsid skull is solid and heavily built, lacking any temporal openings
• This skull structure provides excellent protection for the brain and jaw muscles but limits the size and attachment points for these muscles
• Examples of anapsid reptiles include the extinct pareiasaurs and the living turtles

Turtles as living anapsids

• Turtles are the only surviving lineage of anapsid reptiles, having evolved during the Triassic period
• The turtle shell is a unique adaptation that provides protection from predators and supports the body during locomotion
• Despite their anapsid skull, molecular evidence suggests that turtles may be more closely related to diapsid reptiles, with the anapsid condition being a secondary adaptation

Diversity in Permian period

• During the Permian period (299-252 million years ago), anapsid reptiles diversified into various forms, including the herbivorous pareiasaurs and the aquatic mesosaurs
• Pareiasaurs were large, heavily built reptiles with leaf-shaped teeth adapted for herbivory, while mesosaurs were small, semi-aquatic reptiles with long, slender jaws
• The end-Permian mass extinction severely impacted anapsid diversity, with only the turtles surviving into the Mesozoic era

Synapsid reptiles

• Synapsids are characterized by a single temporal fenestra, or opening, behind the eye socket
• This skull structure allows for larger jaw muscles and more efficient biting and chewing, which may have contributed to the success of synapsids as the dominant terrestrial vertebrates of the Permian period
• Synapsids are often referred to as "mammal-like reptiles" due to their many mammalian characteristics, such as differentiated teeth and upright posture

Mammal-like characteristics

• Synapsids evolved several mammal-like traits, including a more upright posture, differentiated teeth (incisors, canines, and molars), and a secondary palate
• Some later synapsids, such as the cynodonts, developed hair, endothermy, and a more mammal-like jaw joint between the dentary and squamosal bones
• These adaptations allowed synapsids to occupy a wide range of ecological niches and laid the groundwork for the evolution of mammals

Pelycosaurs of Permian

• Pelycosaurs were the earliest and most primitive group of synapsids, appearing in the Late Carboniferous and diversifying during the Early Permian
• They include famous taxa such as Dimetrodon and Edaphosaurus, which are known for their large, sail-like structures on their backs
• Pelycosaurs ranged in size from small, insectivorous forms to large, apex predators like Dimetrodon, which could reach lengths of up to 4 meters

Therapsids of Triassic

• Therapsids were a more advanced group of synapsids that appeared in the Middle Permian and became the dominant terrestrial vertebrates of the Late Permian and Early Triassic
• They include several diverse subgroups, such as the herbivorous dicynodonts and the carnivorous cynodonts, which were the direct ancestors of mammals
• Therapsids survived the end-Permian mass extinction and radiated into various forms during the Triassic period, before declining in the face of competition from archosaurs

Diapsid reptiles

• Diapsids are characterized by two temporal fenestrae, or openings, behind the eye socket
• This skull structure allows for even larger jaw muscles and more efficient biting and feeding compared to synapsids and anapsids
• Diapsids include the majority of modern reptiles, such as lizards, snakes, crocodilians, and birds, as well as extinct groups like dinosaurs and pterosaurs

Two temporal fenestrae

• The presence of two temporal openings in the diapsid skull is a key adaptation that allows for the attachment of large jaw muscles
• The lower temporal fenestra is located below the postorbital and squamosal bones, while the upper temporal fenestra is located above these bones
• The arrangement of these fenestrae varies among different diapsid lineages, with some groups losing one or both openings secondarily

Lepidosaurs vs archosaurs

• Diapsids are divided into two major lineages: Lepidosauria, which includes modern lizards, snakes, and tuataras, and Archosauria, which includes crocodilians, dinosaurs, and pterosaurs
• Lepidosaurs are characterized by a more primitive diapsid skull, with a lower temporal bar separating the two fenestrae
• Archosaurs have a more derived skull structure, with the lower temporal bar reduced or absent, allowing for an even larger gape and stronger bite force

Permian to Triassic diversity

• Diapsids first appeared in the Late Permian period and radiated into various forms during the Triassic
• Early diapsids included small, lizard-like forms such as Youngina and Petrolacosaurus, as well as larger, aquatic reptiles like Claudiosaurus
• During the Triassic period, diapsids diversified into several major lineages, including lepidosaurs, archosaurs, and marine reptiles like ichthyosaurs and plesiosaurs

Lepidosaurs

• Lepidosauria is a clade of diapsid reptiles that includes lizards, snakes, and tuataras
• Lepidosaurs are characterized by a primitive diapsid skull with a lower temporal bar, as well as several other unique features such as a transverse cloacal slit and a forked tongue
• Lepidosaurs first appeared in the Late Permian and radiated into various forms during the Mesozoic era

Lizards and snakes

• Lizards and snakes (squamates) are the most diverse group of lepidosaurs, with over 10,000 living species
• Lizards are characterized by their four limbs, external ear openings, and movable eyelids, while snakes have lost their limbs and external ears secondarily
• Squamates have evolved a wide range of adaptations for different lifestyles, such as adhesive toe pads in geckos, venom in some snakes and lizards, and infrared sensing in pit vipers

Sphenodontians and tuataras

• Sphenodontians are a group of lepidosaurs that were diverse during the Mesozoic era but are now represented by a single living species, the tuatara (Sphenodon punctatus) of New Zealand
• Sphenodontians are characterized by their acrodont dentition (teeth fused to the jaw bones), a complete lower temporal bar, and a third eye on the top of the head (the parietal eye)
• Tuataras are unique among lepidosaurs in having a slower metabolism, longer lifespan, and a more primitive skull structure compared to lizards and snakes

Adaptations for land

• Lepidosaurs have evolved several adaptations for life on land, including a waterproof skin with scales, a more efficient respiratory system, and a three-chambered heart
• Many lepidosaurs are also adapted for climbing, with long, slender bodies, clawed toes, and a prehensile tail in some species
• Some lepidosaurs, such as the Komodo dragon and the extinct mosasaurs, have secondarily adapted to an aquatic lifestyle, with modifications such as paddle-like limbs and a streamlined body shape

Archosaurs

• Archosauria is a clade of diapsid reptiles that includes crocodilians, dinosaurs, and pterosaurs
• Archosaurs are characterized by a more derived diapsid skull, with the lower temporal bar reduced or absent, as well as several other unique features such as a fourth trochanter on the femur and a more upright posture
• Archosaurs first appeared in the Early Triassic and became the dominant terrestrial vertebrates of the Mesozoic era

Ruling reptiles

• The name "Archosauria" means "ruling reptiles," reflecting their dominance during the Mesozoic era
• Archosaurs include some of the largest and most iconic extinct animals, such as Tyrannosaurus rex, Brachiosaurus, and Quetzalcoatlus
• Archosaurs also include the only living descendants of the dinosaurs, the birds, which are now the most diverse group of terrestrial vertebrates

Pseudosuchians vs avemetatarsalians

• Archosaurs are divided into two major lineages: Pseudosuchia, which includes crocodilians and their extinct relatives, and Avemetatarsalia, which includes dinosaurs, pterosaurs, and birds
• Pseudosuchians are characterized by a more heavily built skull and a sprawling posture, while avemetatarsalians have a more upright posture and a lighter, more agile build
• The two lineages diverged in the Early Triassic and evolved independently throughout the Mesozoic era

Triassic period dominance

• During the Triassic period, archosaurs radiated into various forms and became the dominant terrestrial vertebrates
• Early archosaurs included small, agile forms like Euparkeria and Ornithosuchus, as well as larger, more heavily built forms like Desmatosuchus and Saurosuchus
• By the end of the Triassic, archosaurs had diversified into several major lineages, including crocodylomorphs, pterosaurs, and dinosaurs, setting the stage for their dominance in the Jurassic and Cretaceous periods

Marine reptiles

• During the Mesozoic era, several groups of reptiles independently adapted to an aquatic lifestyle, giving rise to a diverse array of marine reptiles
• These groups include the ichthyosaurs, plesiosaurs, mosasaurs, and marine crocodylomorphs, among others
• Marine reptiles played important roles as predators and prey in Mesozoic marine ecosystems and evolved a wide range of adaptations for life in the water

Ichthyosaurs and plesiosaurs

• Ichthyosaurs were a group of fish-shaped marine reptiles that appeared in the Early Triassic and persisted until the Late Cretaceous
• They were characterized by a streamlined body, a long snout with conical teeth, and paddle-like limbs, and some species could reach lengths of up to 20 meters
• Plesiosaurs were another group of marine reptiles that appeared in the Late Triassic and diversified during the Jurassic and Cretaceous periods
• They were characterized by a long neck, a small head, and paddle-like limbs, and ranged in size from small, agile forms to large, apex predators like Elasmosaurus and Kronosaurus

Adaptations for aquatic life

• Marine reptiles evolved several adaptations for life in the water, including a streamlined body shape, paddle-like limbs, and a powerful tail for swimming
• Many marine reptiles also had a layer of blubber or other insulation to maintain body heat in cold water, as well as salt glands to excrete excess salt
• Some groups, such as ichthyosaurs and mosasaurs, were viviparous (giving birth to live young) and had a dorsal fin and a tail fluke for more efficient swimming

Mesozoic era success

• Marine reptiles were highly successful during the Mesozoic era, occupying a wide range of ecological niches and reaching large sizes
• They played important roles as predators, with some species like Tylosaurus and Kronosaurus being among the largest predators in Mesozoic marine ecosystems
• Marine reptiles also served as prey for other marine predators, such as sharks and other marine reptiles, and their remains are often found in the stomach contents of these animals
• Most marine reptile lineages went extinct at the end of the Cretaceous period, along with the non-avian dinosaurs, but some groups, such as sea turtles and marine iguanas, have persisted to the present day

Flying reptiles

• During the Mesozoic era, several groups of reptiles independently evolved the ability to fly, giving rise to a diverse array of flying reptiles
• The most successful and well-known group of flying reptiles were the pterosaurs, which appeared in the Late Triassic and persisted until the end of the Cretaceous
• Other groups of flying reptiles include the gliding lizards (e.g., Draco) and the extinct kuehneosaurids, although these groups were less diverse and successful than the pterosaurs

Pterosaurs of Mesozoic

• Pterosaurs were a group of flying reptiles that appeared in the Late Triassic and radiated into various forms during the Jurassic and Cretaceous periods
• They were characterized by a lightweight, hollow skeleton, a large head with a long snout, and a wing membrane stretched between the body and an elongated fourth finger
• Pterosaurs ranged in size from small, sparrow-sized forms like Nemicolopterus to the largest known flying animals of all time, such as Quetzalcoatlus and Hatzegopteryx, with wingspans of up to 10-12 meters

Adaptations for flight

• Pterosaurs evolved several adaptations for flight, including a lightweight, hollow skeleton with air sacs, a keeled breastbone for the attachment of flight muscles, and a large brain with enlarged optic lobes for processing visual information
• The wing membrane of pterosaurs was made of layers of skin, muscle, and fibrous tissue and was supported by the elongated fourth finger and a series of small, rod-like bones called actinofibrils
• Some pterosaurs, such as the anurognathids and the rhamphorhynchids, had a long, stiffened tail that acted as a rudder and stabilizer during flight

Diversity in size

• Pterosaurs exhibited a wide range of sizes, from small, insectivorous forms to large, apex predators
• The smallest known pterosaur is Nemicolopterus crypticus from the Early Cretaceous of China, with a wingspan of only 25 cm and an estimated weight of 10 grams
• The largest known pterosaurs, such as Quetzalcoatlus northropi and Hatzegopteryx thambema, had wingspans of up to 10-12 meters and estimated weights of 200-250 kg
• Pterosaurs also showed a wide range of dental and cranial adaptations, reflecting their diverse diets and feeding strategies, which included insectivory, piscivory, and carnivory

Dinosaurs

• Dinosaurs were a diverse group of archosaurs that appeared in the Late Triassic and dominated terrestrial ecosystems throughout the Mesozoic era
• They were characterized by an upright posture, a sacrum composed of three or more fused vertebrae, and a perforate acetabulum (hip socket)
• Dinosaurs ranged in size from small, agile forms like Compsognathus to the largest known terrestrial animals, such as Argentinosaurus and Patagotitan

Saurischians vs ornithischians

• Dinosaurs are divided into two major lineages based on