29.4 Reptiles
3 min read•june 14, 2024
Amniotes revolutionized land colonization with their specialized egg, enabling embryos to develop outside water. This adaptation, along with waterproof skin and efficient respiratory systems, allowed reptiles to thrive in diverse terrestrial habitats.
Reptiles evolved diverse skull structures and unique characteristics like ectothermy and scaly skin. Their ability to conserve water and regulate body temperature through behavior helped them adapt to various environments, from deserts to forests.
Amniote Evolution and Reptile Characteristics
Key features of amniotes
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- enables embryonic development on land by containing extraembryonic membranes
- protects embryo in fluid-filled cavity
- facilitates gas exchange between embryo and environment
- stores metabolic waste and aids in respiration
- provides nutrition for developing embryo (chicken egg)
- Waterproof skin with scales or prevents desiccation in terrestrial environments (desert reptiles)
- Efficient respiratory and circulatory systems facilitate oxygen uptake and delivery to tissues
- Evolutionary significance: enabled reptiles to become fully terrestrial and colonize diverse land habitats (deserts, forests, grasslands)
Skull structures in reptile groups
- have no (openings) in the skull (turtles, Eunotosaurus)
- have a single temporal fenestra behind the eye socket
- Includes extinct "mammal-like reptiles" and modern mammals (Dimetrodon, Cynognathus)
- have two on each side of the skull
- Includes most modern reptiles and birds (lizards, snakes, crocodilians, )
Defining characteristics of reptiles
- (cold-blooded) rely on external heat sources for body temperature regulation (basking in the sun)
- Scaly, dry skin prevents water loss and provides protection
- Three-chambered heart (except crocodilians) allows for partial separation of oxygenated and deoxygenated blood
- efficiently excrete uric acid, conserving water (white, chalky urine)
- Diverse tooth types () adapted for various feeding strategies (carnivorous, herbivorous)
- Amniotic eggs with leathery or hard, calcified shells provide protection and allow for embryonic development on land (buried in sand or soil)
- serves as a common opening for digestive, urinary, and reproductive tracts
Thermoregulation and Reptile Diversity
- is crucial for maintaining optimal body temperature
- Behavioral adaptations include basking in the sun or seeking shade
- Physiological adaptations such as changing skin color or altering blood flow
- includes diverse groups of animals
- (lizards and snakes) are the largest and most diverse group of modern reptiles
- Crocodilians, turtles, and tuataras are other major reptile groups
Reptile Evolution and Diversity
Evolutionary milestones of reptiles
- Carboniferous Period (359-299 million years ago)
- Appearance of early amniotes, such as and
- Permian Period (299-252 million years ago)
- Diversification of and early diapsids
- Appearance of the first true reptiles, like
- Triassic Period (252-201 million years ago)
- Emergence of major reptilian lineages
- (crocodilians, , and dinosaurs)
- (lizards, snakes, and tuataras)
- Turtles
- Emergence of major reptilian lineages
- Jurassic and Cretaceous Periods (201-66 million years ago)
- Dominance of dinosaurs (Tyrannosaurus rex, Stegosaurus) and marine reptiles (, )
- Diversification of lizards and snakes
- Cretaceous-Paleogene Extinction (66 million years ago)
- Extinction of non-avian dinosaurs and many other reptilian lineages
- Cenozoic Era (66 million years ago to present)
- Adaptive radiation of modern reptiles into new species and ecological niches (Komodo dragons, chameleons, sea turtles)
Key Terms to Review (46)
Allantois: The allantois is an extra-embryonic membrane involved in the exchange of gases and handling of liquid waste in reptilian embryos. It plays a crucial role in the development and survival of the embryo within the egg.
Allantois: The allantois is a membranous sac that is involved in the storage of waste and gas exchange in the developing embryos of reptiles and some other vertebrates. This structure plays a vital role in the development of the embryo by providing a means for waste management and facilitating respiration, allowing for the efficient exchange of gases like oxygen and carbon dioxide between the embryo and the surrounding environment.
Amnion: The amnion is a membranous sac that surrounds and protects the embryo in reptiles, birds, and mammals. It creates an aquatic environment necessary for embryo development on land.
Amnion: The amnion is a membranous sac that surrounds and protects the developing embryo in amniotic animals, such as reptiles. This structure plays a crucial role in providing a stable environment by containing amniotic fluid, which cushions the embryo and helps prevent desiccation. The evolution of the amnion was significant for reptiles, as it allowed them to reproduce on land without the need for a water environment, providing greater reproductive success in diverse habitats.
Amniotic egg: An amniotic egg is a type of egg that contains a protective amniotic membrane, allowing for the development of embryos in a terrestrial environment. This adaptation is crucial for reptiles, as it provides a moist environment and prevents desiccation, enabling them to reproduce away from water bodies. The evolution of the amniotic egg marks a significant step in vertebrate evolution, allowing reptiles to colonize a variety of habitats.
Anapsids: Anapsids are a group of reptiles characterized by their lack of temporal fenestrae, which are openings in the skull behind the eyes. This feature distinguishes them from other reptiles like diapsids, which have two such openings. Anapsids include early forms of reptiles and are significant in the evolutionary history of vertebrates, particularly in understanding the lineage leading to modern reptiles.
Archosaurs: Archosaurs are a clade of diapsid reptiles that include modern birds and crocodilians, as well as their extinct relatives such as dinosaurs and pterosaurs. They are characterized by certain features such as a distinct skull structure, an upright posture, and adaptations for bipedalism in some groups. Archosaurs are significant in the evolutionary history of reptiles and play an essential role in understanding the diversification of vertebrates.
Brumation: Brumation is a period of dormancy in reptiles, akin to hibernation in mammals. It is characterized by decreased metabolic activity and reduced physiological functions during colder months.
Carapace: A carapace is a hard, protective outer shell found in certain animal species, such as arthropods and reptiles. It provides structural support and defense against predators and environmental hazards.
Chorion: The chorion is an outer fetal membrane in amniotic eggs, including those of reptiles. It plays a crucial role in gas exchange between the embryo and the external environment.
Chorion: The chorion is the outermost membrane that surrounds an embryo in reptiles, birds, and some mammals. It plays a crucial role in protecting the developing embryo and facilitating gas exchange between the embryo and its environment. This adaptation allows for efficient development outside of water, enabling reptiles to reproduce on land more effectively.
Cloaca: A cloaca is a common cavity into which the intestinal, urinary, and reproductive tracts open in birds, reptiles, amphibians, and some fish. It functions as an exit for waste products and reproductive cells.
Cloaca: The cloaca is a single opening in certain animals that serves multiple functions, including excretion of waste and reproductive purposes. It acts as a common chamber for the digestive, urinary, and reproductive tracts, allowing for efficient elimination of bodily waste as well as the passage of eggs or sperm. This anatomical feature is a significant evolutionary trait in various groups of animals, connecting their systems in a unique way.
Crocodilia: Crocodilia is an order of large, predatory, semi-aquatic reptiles that includes crocodiles, alligators, caimans, and gharials. They are characterized by a long flattened snout, powerful jaws, and a robust body covered in thick scales.
Diapsids: Diapsids are a group of reptiles characterized by having two temporal fenestrae on each side of their skulls. This unique skull structure allows for greater muscle attachment and jaw strength, which is essential for their diverse feeding strategies. Diapsids include modern reptiles such as lizards, snakes, and crocodilians, as well as birds, which evolved from theropod dinosaurs, showcasing the evolutionary significance of this group within the broader context of reptiles.
Dinosaurs: Dinosaurs are a diverse group of reptiles that dominated terrestrial ecosystems during the Mesozoic Era. They are characterized by unique anatomical features such as upright limbs and diverse adaptations for different modes of life.
Ectothermic: Ectothermic organisms rely on external environmental conditions to regulate their body temperature. These animals often exhibit behaviors such as basking in the sun or seeking shade to maintain optimal body functioning.
Ectothermic: Ectothermic refers to organisms that rely primarily on external environmental heat sources to regulate their body temperature. This adaptation allows ectotherms to thrive in various habitats, but they often face limitations in colder environments where they become sluggish or inactive. Understanding ectothermy is crucial when examining the physiological and behavioral traits of different species, especially those that have evolved in diverse climates.
Ectotherms: Ectotherms are organisms that regulate their body temperature primarily through external sources rather than internal metabolic processes. They rely on environmental heat sources to maintain physiological functions.
Endotherms: Endotherms are organisms that maintain a constant body temperature through internal metabolic processes. They can generate heat to stay warm or cool down by expelling excess heat.
Heterodont dentition: Heterodont dentition refers to the presence of different types of teeth within the same species, which can serve various functions such as biting, tearing, and grinding food. This type of dental arrangement is crucial for animals that have diverse diets, allowing them to effectively process different kinds of food. In reptiles, heterodont dentition is often seen in species that have adapted to specific feeding strategies, showcasing their evolutionary adaptations.
Homeotherms: Homeotherms are organisms that maintain a stable internal body temperature regardless of external conditions. This trait is crucial for sustaining consistent metabolic processes.
Hylonomus: Hylonomus is recognized as one of the earliest known reptiles, dating back to the Carboniferous period, around 300 million years ago. This small, lizard-like creature is significant as it represents a crucial evolutionary step from amphibians to reptiles, showcasing adaptations that allowed vertebrates to thrive in terrestrial environments. Its discoveries have provided valuable insights into the early evolution of reptiles and the transition to fully land-based lifestyles.
Ichthyosaurs: Ichthyosaurs were marine reptiles that thrived during the Mesozoic era, resembling modern dolphins in body shape. They were well-adapted for life in the ocean, showcasing features such as streamlined bodies and long, slender limbs transformed into flippers, allowing them to swim efficiently. Their unique adaptations make ichthyosaurs a significant group within the broader category of reptiles.
Jacobson's organ: Jacobson's organ, also known as the vomeronasal organ, is a chemoreceptor organ involved in the detection of pheromones and other chemical signals. It plays a critical role in various behaviors such as mating, territory marking, and prey detection.
Lepidosaurs: Lepidosaurs are a clade of reptiles that include modern lizards, snakes, and the tuatara, characterized by their unique scale structure and skin shedding processes. They represent a significant group within reptiles, showcasing diverse adaptations that have allowed them to thrive in various environments, from deserts to rainforests.
Metanephric kidneys: Metanephric kidneys are a type of kidney that develops in higher vertebrates, including reptiles, from the metanephros stage of kidney development. They are characterized by their advanced structure and function, allowing for efficient regulation of water and electrolyte balance in organisms that often inhabit terrestrial environments.
Paleothyris: Paleothyris is an extinct genus of early reptiles that lived during the Permian period, characterized by its unique skull structure and distinct features that provide insight into the evolution of reptiles. This genus is significant as it showcases some of the transitional characteristics between more primitive amniotes and the more derived reptiles that followed, highlighting the evolutionary pathways that led to modern reptiles.
Plastron: The plastron is the ventral (bottom) part of a turtle or tortoise shell. It provides protection to the organism's underside and houses several key anatomical features.
Plesiosaurs: Plesiosaurs were a group of marine reptiles that lived during the Mesozoic era, specifically from the late Triassic to the late Cretaceous period. Known for their distinctive long necks, small heads, and large bodies, plesiosaurs were powerful swimmers that thrived in ancient oceans, showcasing diverse adaptations in their morphology and lifestyle.
Protorosaurus: Protorosaurus is an extinct genus of early reptile that lived during the Late Permian to the Early Triassic period, recognized as a significant link in the evolutionary history of reptiles. This creature provides crucial insights into the transition from amphibian-like ancestors to fully terrestrial reptiles, showcasing adaptations that paved the way for later groups of reptiles.
Pterosaurs: Pterosaurs were flying reptiles that lived during the Mesozoic Era, from the late Triassic to the end of the Cretaceous period. They are known for their wings formed by a membrane stretching from an elongated fourth finger to their body and legs.
Reptilia: Reptilia refers to a class of cold-blooded vertebrates that primarily include turtles, lizards, snakes, crocodilians, and birds. Members of this class are characterized by their scaly skin, the presence of lungs for breathing, and usually laying eggs with hard shells. Reptilia is significant for its evolutionary adaptations that allow these animals to thrive in a variety of terrestrial habitats.
Scutes: Scutes are hard, bony plates or scales found on the skin of various reptiles, particularly turtles and crocodilians. These structures serve as protective armor, helping to safeguard these animals from predators and environmental hazards. Scutes can also play a role in thermoregulation and are important for the overall health and durability of the reptile's integumentary system.
Sphenodontia: Sphenodontia is an order of reptiles that includes only one living genus, the tuatara. These reptiles are distinct from lizards and have unique anatomical features.
Squamata: Squamata is the largest order of reptiles, comprising lizards, snakes, and amphisbaenians. They are characterized by their scaled skin and ability to move their upper jaws independently of their heads.
Squamates: Squamates are a diverse group of reptiles that include lizards, snakes, and amphisbaenians. They are characterized by their scaly skin and a unique jaw structure that allows for a wide range of feeding behaviors. This group represents the largest order of reptiles, demonstrating a remarkable adaptability to different habitats and ecological niches.
Synapsids: Synapsids are a group of amniotes that includes mammals and their extinct relatives. They are characterized by a single temporal fenestra, an opening in the skull behind each eye.
Synapsids: Synapsids are a group of vertebrates characterized by having a single temporal fenestra in their skulls, allowing for a more powerful jaw structure and greater muscle attachment. This evolutionary trait distinguishes them from other groups, such as reptiles, and plays a crucial role in the development of mammals, showcasing the transition from early amniotes to modern mammals.
Temporal fenestrae: Temporal fenestrae are openings in the skull behind the eyes of some vertebrates. They serve to reduce the weight of the skull and provide attachment sites for muscles.
Temporal fenestrae: Temporal fenestrae are openings in the skull located behind the eyes, characteristic of certain reptiles, especially archosaurs and their descendants. These openings are significant because they reduce the weight of the skull while allowing for stronger jaw muscles to attach, enhancing feeding efficiency. This adaptation is crucial for the evolutionary success of these reptiles, as it contributes to their predatory capabilities and overall ecological diversity.
Testudines: Testudines, also known as turtles and tortoises, are an order of reptiles characterized by a bony or cartilaginous shell developed from their ribs. They are one of the oldest reptile groups, having existed for over 200 million years.
Thermoregulation: Thermoregulation is the process by which animals maintain their internal body temperature within a tolerable range despite external fluctuations. It is a critical component of homeostasis that ensures optimal functioning of physiological processes.
Thermoregulation: Thermoregulation is the process by which animals maintain their body temperature within a certain range, despite changes in environmental temperatures. This ability is crucial for ensuring optimal metabolic functions and overall homeostasis. Different animal groups have evolved various strategies for thermoregulation, influencing their physiological adaptations, behaviors, and ecological niches.
Yolk sac: The yolk sac is a membranous structure attached to the embryo that provides early nourishment in the form of yolk. It is crucial for the development of certain vertebrates, including reptiles, during their embryonic stages.
Yolk Sac: The yolk sac is an embryonic structure that provides nourishment to the developing embryo in many vertebrates, including reptiles. It plays a crucial role in early development by supplying essential nutrients and energy until the developing organism can establish its own means of sustenance. In reptiles, the yolk sac is typically well-developed and rich in yolk, supporting the embryo as it grows within the protective eggshell.