In addition to being vertebrates, do we have in common with fish? The types of joints we (and most vertebrates) share are probably most likely born from the same common ancestor. But that’s not a feature we share with all vertebrates.
Humans, other terrestrial animals, and jaw-filled fish have synovial joints. The lubricating cavities within these joints allow bones and cartilage to slide against each other without friction, resulting in greater mobility and stability and promote movement.
The origin of these joints was uncertain. Currently, University of Chicago biologist Neelima Sharma and her colleagues are looking at fish that form this type of joint. The synovial joint is known to be present in the jaw, but it is not a jawless fish. This left the question of whether they are merely features of the common bone skeleton, or whether they can also be seen in fish with cartilage skeletons such as sharks and skates (no terrestrial animals with cartilage skeletons).
As Sharma and her team discovered, jaw-free cartilage fish such as skate embryos they studied develop these joints, while jawless fish such as the wolf and Hagfish do not have them.
So, what does this mean? If a chin-fitting fish has a synovial joint that is common to all jaw-fitting vertebrates, including us, it must have evolved in our shared ancestors.
Something like a fish in our past
The common ancestors of vertebrates with synovial joints remain mysterious, but the oldest specimens with evidence of these joints are Bothriolepis canadensis, A fish that lived around 387-360 million years ago between the mid-Devonian period.
When investigating using a CT scan Bothriolepis Fossil, Sharma observed a joint cavity between the shoulder and the thoracic fin. I don’t know if the cavity is filled with synovial fluid, but in any case, she thinks it appears to function like a synovial joint. In contrast, early jawless fish fossils do not have any signs of synovial joints.
