The Origins Of Vertebrates Part 4: Domination
So, we have skeletons, we have a central nervous system, we have backbones, are we there yet? Well, first of all, we are, because we’re already vertebrates; in terms of in these blogs however… the skeletons are made of cartilage at the moment, but they’re still in the club. So, sharks and rays – the cartilaginous fish – are one of the most touted examples of living fossils, due to the relatively small number of changes that have occurred in their physiology since they first emerged about 450 millions years ago. Due to the fact that their skeletons are made of cartilage, we have relatively few fossils of them, so aren’t we lucky then, that the group that split off from our ancestry at this point, were so well suited to their environment, that evolution only had a little work to do to polish them up?
Having said that they lack true bone, their teeth and sometimes their vertebrae, are calcified. However, another important distinction is that their skeleton is essentially dead by the time it’s functional, whereas in our skeleton, there’s still a whole host of active, biological processes happening within our bones until we’ve kicked the bucket.
Now, I remember being surprised when I was younger to learn that sharks were a type of fish, and they are closely related to the other groups of fish, which I will get to in a moment, but the definition of fish depends on where you start counting. Should lampreys be counted as fish? Placoderms? Hagfish? Evolution is a fluid process; there’s no distinctive point we can look at and say that the first fish happened x million years ago; the lines are fuzzy, and despite what Ray Comfort might tell you, evolution doesn’t tell us that a cat should, given enough time, give birth to a dog.
Nor will there probably ever be a crocoduck.
Because that’s ridiculous.
What you would probably never mistake for anything other than a fish as a child however, is a ray-finned fish. These guys encompass a brilliantly diverse set of fish; seahorses, eels, clownfish to name a few, and as well as the adaptations obvious from the variety of shapes and sizes, with the ray-finned fish came more flexible jaws, less armor, and most importantly: the swim bladder, allowing fish to be able to stop moving in the water by balancing out pressure on them from inside, and it was this adaptation, that allowed lungs to develop. The lobe-finned fish, featuring everyone’s favorite living fossil, the coelacanth; had more muscular fins that pointed downwards, superficially resembling limbs. Some, like the ancestors of the lungfish, evolved a lung-like gas bladder where their swim bladder once was, and in fact, catfish ancestors used this system, later adapting the organ back into a swim bladder, as it is found in extant catfish.
Next comes arguably the penultimate drastic evolutionary mark in vertebrate history, the ultimate being the evolution of flight, which has happened in three separate events that I plan to cover in the next and last post in this series: the transition from aquatic to terrestrial life.
The creatures that are probably most closely survived by today’s amphibians (frogs, newts and the like) are some of the most incredible fossils we can see, particularly Tiktaalik roseae, a creature with a flattened skull, incredibly strong fins that are so leg-like it begs the question of how creationists are still a thing, and probably the best example of a so called ‘missing link’ since Archaeopteryx. This creature, and its relatives, bridged the gap between lobe finned fish and amphibians, developing lungs and limbs that became better suited to life on the land the more they hung around, leading to the huge radiation of tetrapods (creatures with four legs) that we see today.
Next week, we’ll explore the tetrapods, and the variation that led to birds, humans and the largest animal on earth.
[image credits: redorbit, treehugger, fossilsforkids]