1. An extinct group of early amniote reptiles from the Carboniferous period, characterized by a distinctive cup-shaped socket in the vertebrae; considered ancestral to later reptile groups.
Usage: paleontology; plural form; singular is cotylosaur; also called stem-amniotes

Examples

• Cotylosaurs were among the first vertebrates to develop the amniotic egg, allowing them to reproduce away from water.
• Paleontologists study cotylosaurs to understand the early evolution of reptiles.
• The cotylosaurs flourished during the Carboniferous period before giving rise to more specialized reptile lineages.
• Unlike modern reptiles, cotylosaurs had relatively simple skeletal structures.
• Fossil evidence of cotylosaurs has been found in North America and Europe.