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The Daily Cardinal Est. 1892
Friday, June 09, 2023

No bones about it: Comparative skeletal anatomy in gnathostomes

There’s lots of common ground between your skeleton and a bird skeleton.

Unless you are a sentient worm reading the papers (in which case, please contact us for an interview), you have some bones. A whole skeleton’s worth, hopefully. At first glance, there’s a substantial difference between human skeletons and other skeletons — birds and bats have wings; stingrays have a terrifying bone fan. However, despite gross morphological differences, the basis of all vertebrate skeletons is actually the same. 

A vertebrate is an animal with a vertebral column, or backbone. The vertebral column evolved to replace the notochord found in chordates. 

Vertebrates arose during the Cambrian explosion, a period about 541 million years ago during which animals underwent a massive radiation of evolution. They are a clade, meaning they all evolved from one common ancestor. In this case, that common ancestor was likely in the genus Myllokunmingia, a weird, lumpy slug-lookin’ thingy whose fossils have been found in China. 

99% of living vertebrates, including humans and other recognizable groups such as birds and reptiles, are contained within a subclass of the clade called the gnathostomes — from Greek gnath, meaning jaw, and stoma, meaning mouth. The gnathostomes all have jaws, as well as certain other distinguishing traits. This relationship means there’s even more similarity between your skeleton and the skeleton of other gnathostomes. Yay!

Gnathostome skeletons, with all their variations, share a basis that stems back to the same common ancestor. As a result, bones that are homologous (having the same value or function) to each other are fairly recognizable across species. 

Take a look at your hand. Wiggle your fingers, flex and extend your wrist. Now, move upward — bend your elbow, and extend it. Move your arms up and down. The bones that allow you to make all these fun movements are essentially the same bones that make up the wings of birds! 

Just like you, birds have a humerus that articulates with the shoulder blade. They have a radius and an ulna, and tarsal bones leading down to digits. They even have little digits, fingerbones that are directly connected to their primary feathers towards the end of the wing. In birds, some digits have been lost, but the base structure of the arm relies on the same bones. Remember that the next time you’re out for wings.

Birds have another unique skeletal structure that corresponds to the human sternum. Flying requires some pretty big pecs, and big pecs means … you guessed it, a bigger skeletal anchor for the pectoral musculature! Birds therefore have a keel that runs axially along their breastbone for the muscles to attach to. If you’re ever holding a bird for some reason, you can feel the keel right underneath the skin. 

As further proof of the relationship between form and function, flightless birds like penguins lack a keeled skeleton. Sucks for them. 

Let’s move to the hips. How many directions can you (safely) move your leg? How far can you abduct it? Compare this to the movement of your arms from the shoulder. Now imagine a cow. It can be any color you want. Could a cow move it’s front limbs like you can move your arms? Probably not, unless it was a weird super cow. But you and the cow should have a pretty similar range of motion from the hips. 

Given that a cow’s leg and arm bones are pretty similar to yours — why is this? Think about the cow again — or a different cow, if you like. Cows, unlike primates (including humans), do not need to do a lot of swinging or movement from the shoulder in general — they just need to hold themselves up while walking or running around doing cow things. Even if the cows wanted to, they couldn’t start swinging around on trees, as their skeletons aren’t designed for that. Both the shoulder and hip girdles of bovine skeletons provide stability to the joints-just like your hip girdle provides stability to your legs. However, this stability comes at the expense of range of motion. 

Getting to know your skeleton, as well as the skeletons of other creatures, can be lots of fun, and a great way to learn about evolution, comparative anatomy and physiology. If you’re ever feeling bored, start looking up skeletons, and comparing them to your own (or any other human skeleton). You may be surprised by the similarities you find. 

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