Geology 115: Life of the Past - The Amniotes and the Amniotic Egg

The Philosophical Question: "Which came first, the chicken or the egg?"
- In the traditional sense, the answer is the egg.
- The amniotic egg evolved approximately $310$ million years ago.
- The chicken as a species evolved only a few million years ago
Lecture Focus: This lecture will cover:
- The development of the amniotic egg.
- Its purpose and the environment in which it evolved.
- The successful characteristics that define it.

Origin: Amniotes are a group that evolved from stem tetrapods known as anthracosaurs.
Anthracosaurs Characteristics:
- Relatively large and diverse group of early tetrapods.
- Predominantly adapted for an aquatic life.
- Possessed a long snout and long bodies, characteristic of ambush predators (similar to modern crocodiles).
- Jaws evolved for slamming shut on prey, similar to alligators and crocodiles.
- Limbs were not very sturdy; well-adapted for swimming but not very effective for terrestrial locomotion.
- Unlike modern amniotes, they were not well-adapted for land.
Amniote Divergence: The general consensus among paleontologists is that amniotes evolved somewhere at or near the anthracosaur group, likely within it.
- A key distinction: most anthracosaurs were relatively large, whereas the earliest amniotes were relatively small.
Seymouria - An Early Amniote Example:
- Considered a good representative of the earliest amniotes.
- Characterized by a light skeleton and a relatively slender body, similar to modern reptiles.
- Ears were still well-adapted for hearing in water, suggesting some capacity for swimming and an aquatic existence.
- However, the organization and placement of its bones were much more akin to amniotes than other basal tetrapod groups.

The Paradox: Amniotes are defined as "egg layers," yet the oldest fossilized eggs date from the Early Jurassic, approximately $190$ million years ago.
- This contradicts the evolution of the amniotic egg at $310$ million years ago.
Low Preservation Potential: Early amniotic eggs had a relatively low preservation potential, making fossil discovery difficult.
Inferring Early Egg Laying: Our understanding that eggs evolved $310$ million years ago comes from:
- The presence of early amniotes at that time who, by definition, must have laid eggs.
- The terrestrial morphology of these early amniote skeletons.
- The sedimentary environments in which these fossils are preserved, which indicate a terrestrial setting.
- These factors imply that early amniotes did not rely on an aquatic source for reproduction.
Single Origin of the Amniotic Egg: Further verification for the ancient origin of the amniotic egg comes from modern comparative anatomy:
- Animals like turtles, lizards, snakes, crocodiles, birds, and mammals (via internal fertilization) all share common structures within their eggs or reproductive systems.
- This indicates that the amniotic egg arose once in evolutionary history.
- Tracing back the relationships of these organisms leads to a common ancestor, the likely candidate for the first animal to lay an amniotic egg around $310$ million years ago.

The amniotic egg represents a significant advancement over the fish-like eggs of amphibians, incorporating several new structures to facilitate terrestrial development.
Key Structures and Their Functions:
- Embryo: The actively growing and developing young within the egg. The egg acts as a self-contained environment for metamorphosis from an embryonic stage to an animal closer to an adult, eliminating the need for an aquatic larval stage.
- Amnion (Amniotic Sac and Fluid): A fluid-filled sac surrounding the embryo.
  - Function: Cushions and protects the embryo from physical shock and forces (analogous to astronaut cushioning in a space shuttle)
- Yolk Sac
- Allantois: the waste that is being put off, separated from the embryo
- Chorion
- Shell

was a way for these animals to reproduce without returning to the water
- without the fluid in the egg, it would dry out
early amniotes were small
oxygen level was 35% (21% today)
evolved for life on the forest floor
- hunting arthropods and other amniotes
- hiding eggs/nests and could be protected by a shell

Hylonomus - early amniote
- remains found fossilized in a 312 MYA upright tree trunk