Amniotes are important. You are an amniote (I promise). Birds are amniotes. Turtles are amniotes. Crocodiles are amniotes. Dinosaurs were amniotes. Dogs and cats are amniotes. Lizards are amniotes.
But amphibians (like the spring peepers that have been so noisy of late) are not amniotes. Fishes aren’t amniotes either.
So what is an amniote? Amniotes are vertebrates that are able to reproduce away from water. Amniotes don’t need a pond in which to lay their eggs. Chickens use a dry nest. We as mammals just keep the egg inside while it develops.
You see, a developing embryo needs to be bathed in water. It needs to stay moist, which allows it to grow before it develops the structures necessary to keep it from dehydrating. It also allows early off for the diffusion of nutrients and waste in and out of the embryo. And the water provides a protective buffer around the embryo to keep it safe from shocks. All vertebrates need this.
Fish have it easy. They just lay eggs in the water they swim in, preferably in a safe place so they don’t get eaten. Amphibians, despite being able to get out of the water and walk around on land, must also lay their eggs in water, which is why you typically see them in wet places where ponds are.
But everything changed with the evolution of the amnion. The amnion is a protective sac around the embryo. It encases the embryo in its own pond. This amnion is further surrounded by a shell, which provides protection and keeps the amnion from leaking. If it does leak the embryo is at risk of desiccation or infection.
The development of a shelled egg with an amnion to protect the embryo marks the origin of reptiles.
But here’s the challenge, and the point of this post: The amnion is a soft structure, and the original eggs probably were leathery and not mineralized like chicken eggs are. Because they are soft structures, they are not likely to be preserved in the rock record. Furthermore, they’re not likely to be preserved in the animal that lays them, even if they are preserved.
Then how can we recognize an amniote when all we have are bones?
First, let’s make sure we all know what an amphibian is. Amphibians are terrestrial vertebrates (that is, they have limbs with hands and feet for walking around on land) that must return to water to reproduce. They can still have adaptations for swimming, such as flattened tails with fin rays, but full-blown hands and feet are a defining feature of amphibians. Spotting hands and feet is pretty easy in the fossil record, but the need to have access to water for reproduction is not. You can read more about the transition from fish to amphibian here.
We have the advent of limbs for walking to help us distinguish amphibians from fish, but what do we have to distinguish amniotes from amphibians?
My first impulse is to say, “Sit down and weep.”
But then I take a breath. There are differences in the bone anatomy that we can use. Similarities shared among all amniotes that are distinct from amphibians. Unfortunately, these are all pretty technical. But lets have a look anyway.
According to Benton (2015, Vertebrate Paleontology, 4th ed), there are four important characteristics that arise that distinguish all amniotes from amphibians.
• Frontal contacts orbit
• Rounded occipital condyles
• 3 ossification in scapulocoracoid
What’s that in English, please?
Frontal contacts orbit:
The frontal bones are equivalent to our forehead bones. In amniotes, these are in contact with the orbit or the eye socket. In amphibians they are separated by another bone.
Rounded occipital condyles:
Occipital condyles are what connect the skull to the spinal column.
Three ossifications in the scapulocoracoid:
The scapulocoracoid is part of the pectoral girdle, or the shoulder part of the forelimb. It connects the scapula to the sternum. The scapula is also where the front limb attaches, and has a ‘blade’ (equivalent to our shoulder blade) that goes toward the back.
In more primitive vertebrates, the scapulocoracoid is formed when different patches of bone (ossifications) grow together. In amniotes, the scapulocoracoid is derived from three ossifications.
The astragalus is one of my favorite bones ever. This is the bone that connects the shin bone (tibia) to the foot. In mammals, the astragalus often looks like a foam packing peanut.
This single bone in amniotes represents the fusion of three bones in amphibians: the the tibiale, which contacts the tibia, the intermedium, and the fourth centrale. Therefore, if an organism has an astragalus, it is an amniote.
I’ve presented here four skeletal ways to distinguish amniotes from amphibians. Two characteristics are in the skull, one is in the forelimb, and one is in the hindlimb. Hopefully, if you find a fossil, you can find evidence of at least one of these things to demonstrate if what you have is an amniote or not.
Now, go forth and find me an amniote!