Amniote Origins and Characteristics

The Transition from Amphibians: The evolutionary transition from amphibians to amniotes is characterized as being less dramatic than the preceding transition from fish to tetrapods.
Ancestry: Most systematists conclude that the clade Amniota is derived from the anthracosaur amphibians.
Gephyrostegus: This genus is noted because anthracosaurs like it were already very amniote-like in their general characters prior to the formal emergence of the Amniota group.

Geographic Origin: The earliest known amniote fossils are located in the Joggins Formation of Nova Scotia.
Geological Context: * The Joggins Formation preserves the remains of an ancient swamp forest. * The age of this formation is mid-Pennsylvanian, dating back approximately $300\,\text{myo}$ .
Fossil Preservation Mechanisms: * The formation contains many fossilized tree stumps that were preserved in situ (in their original place). * Some of these stumps became hollow over time. * Small animals entered these hollow stumps, died, and their remains were preserved within them.

Hylonomus: * Identified as the earliest known and one of the most primitive amniotes. * It possessed an anapsid skull condition.
Other Joggins Discoveries: * The tree stumps yielded other amniote specimens, including a primitive synapsid and another anapsid similar to Hylonomus. * Note: While these are the earliest known fossils, they are not considered the most structurally primitive amniotes.
Carbonodraco: An early amniote found in Ohio.

Parareptiles (Stem Reptiles): * This group evolved prior to the derivation of modern reptilian groups and is placed on the branch below the split between archosaurs and lepidosaurs. * Pareiasaurs: A group of terrestrial species within the parareptiles. * Example: Scutosaurus, a pareiasaur from the later Permian of eastern Europe. * Milleretids: * Example: Millerosaurus. * Procolophonida: * Example: Sclerosaurus.
Mesosaurs: * A primitive group of aquatic marine reptiles from the Permian age. * Physical Characteristics: Narrow, elongate snouts with long, interlocking teeth. * Classification: Historically considered to be part of the Parareptiles.

The Cleidoic Egg: A defining feature of amniotes, which contains four essential extraembryonic membranes: 1. Amnion 2. Chorion 3. Allantois 4. Yolk sac
Amniote Integument (Skin): Differing significantly from amphibian skin, amniote skin is characterized by: * A thicker, many-layered epidermis. * Cornification via the deposition of abundant keratin. * Integumentary thickenings known as scales, which are particularly dense and provide high levels of waterproofing. * A significant reduction in the number of skin glands.

Reptilian Keratins: 1. Alpha keratin: Characterized as pliable and supple; found throughout the skin and is the exclusive keratin type in the hinge regions between scales. 2. Beta keratin: Characterized as tough and hard; found abundantly in the scale areas of the reptile epidermis.
Epidermal Growth and Shedding: * Stratum Germinativum: This lower layer of the epidermis produces upper layers through mitosis. * Turtles and Crocodilians: Mitosis occurs continuously at low levels (excluding hibernation periods). Skin cells are produced from below and shed from above in small patches. * Lepidosauria (Lizards and Snakes): * Possess two distinct cellular layers in the epidermis: an outer generation and an inner generation. * Ecdysis: The outer generation is shed all at once. The inner generation is replaced from below following episodic periods of mitotic activity in the stratum germinativum.

The main groups of amniotes are distinguished primarily by the arrangement of bones in the temporal region of the skull, specifically involving the postorbital and squamosal bones.
Skull Conditions: * Anapsid: No temporal openings (e.g., the primitive condition seen in Hylonomus). * Emarginated Anapsid: A variation of the anapsid condition. * Synapsid: A single temporal opening (includes early and late synapsid forms). * Diapsid: Two temporal openings (includes early diapsids like Araeoscelis and late diapsids). * Euryapsid: A specific skull configuration often found in certain marine reptiles. * Modified Diapsid: Advanced configurations found in modern lizards and snakes.

Diapsida Overview: Most animals categorized as reptiles are diapsids. They underwent an early radiation of lizard-like forms such as Araeoscelis and then split into two major lineages.
Lineage 1: Lepidosauria * Included Taxa: Lizards, Snakes, Amphisbaenians, and Tuataras (Sphenodon). * Diagnostic Characters: 1. Transverse cloacal opening. 2. Periodic ecdysis (skin shed all at once). 3. Presence of autotomy planes in the tail (found at least in primitive members).
Lineage 2: Archosauria * Included Taxa: Crocodilians, Aves (birds), and Dinosauria. * Diagnostic Characters: 1. Longitudinally oriented cloacal opening. 2. Continuous ecdysis (skin shed continuously in small clumps). 3. Lack of autotomy planes in the tail.