1. Vertebrates: All three groups are vertebrates, meaning they have a backbone (spinal column) made of bone. This provides support and allows for movement.
2. Amniotes: They are all amniotes, meaning they produce eggs that are surrounded by a protective membrane called the amnion. This allows them to reproduce on land, as the amnion keeps the embryo moist and prevents it from drying out.
3. Endothermic or Ectothermic: While birds and mammals are endothermic (warm-blooded), reptiles are ectothermic (cold-blooded). However, all three groups are capable of regulating their body temperature to some extent.
Let me know if you'd like to explore any of these characteristics in more detail!