Minjinia and the origin of endoskeletons

[Halichoeres]

People often make the mistake of thinking our distant relatives are the way we used to be. For example, people often assume that because sharks have cartilage skeletons, we must have once had cartilaginous ancestors. A careful look at vertebrate phylogeny would predict that sharks came from ancestors with not only dermal bone (like what placoderm armor, your skull roof, and many fish scales are made of), but endochondral bone (what your limb bones, vertebrae, and ventral braincase are made of). Here's a Devonian fish on the gnathostome stem, just slightly more derived than most 'placoderms,' with a braincase composed of endochondral bone, showing that this trait was present in the common ancestor of sharks and tunas.

A CT scan of the skull roof and braincase of Minjinia turgenensis:


Paper is paywalled by the profiteers at Nature Ecology & Evolution, but don't let that stop you. https://www.nature.com/articles/s41559-020-01290-2

[ceratopsian]

A salutary reminder about making assumptions.

[stargatedalek]

Was I a weirdo for assuming it was a given that sharks originally had bony skeletons and replaced them with cartilage? I'm now not fully clear if that's correct either though?

[Halichoeres]

Was I a weirdo for assuming it was a given that sharks originally had bony skeletons and replaced them with cartilage? I'm now not fully clear if that's correct either though?

Not at all, a bony skeleton as the primitive condition is what comparative anatomists have been predicting based on gnathostome phylogeny for a long time. But you still see a cartilaginous skeleton regarded as the primitive condition by, for example, developmental geneticists and others who are less familiar with the fossil record of fishes. I suppose I should be very precise here and note that sharks proper have always had cartilaginous skeletons, and that's probably true for crown chondrichthyans as well. Endochondral bone was probably lost near the root of 'acanthodians.'

[Brontozaurus]

Was I a weirdo for assuming it was a given that sharks originally had bony skeletons and replaced them with cartilage? I'm now not fully clear if that's correct either though?

Not at all, a bony skeleton as the primitive condition is what comparative anatomists have been predicting based on gnathostome phylogeny for a long time. But you still see a cartilaginous skeleton regarded as the primitive condition by, for example, developmental geneticists and others who are less familiar with the fossil record of fishes. I suppose I should be very precise here and note that sharks proper have always had cartilaginous skeletons, and that's probably true for crown chondrichthyans as well. Endochondral bone was probably lost near the root of 'acanthodians.'

I'm curious, what benefit would they have gotten from trading a bony skeleton for a cartilaginous one?

[Halichoeres]

Was I a weirdo for assuming it was a given that sharks originally had bony skeletons and replaced them with cartilage? I'm now not fully clear if that's correct either though?

Not at all, a bony skeleton as the primitive condition is what comparative anatomists have been predicting based on gnathostome phylogeny for a long time. But you still see a cartilaginous skeleton regarded as the primitive condition by, for example, developmental geneticists and others who are less familiar with the fossil record of fishes. I suppose I should be very precise here and note that sharks proper have always had cartilaginous skeletons, and that's probably true for crown chondrichthyans as well. Endochondral bone was probably lost near the root of 'acanthodians.'

I'm curious, what benefit would they have gotten from trading a bony skeleton for a cartilaginous one?

Great question, I'm not really sure. The ancestors of chondrichthyans were probably quite small, and it's possible that under a scenario of nutrient limitation (calcium or phosphorus or both) there might have been a growth/reproduction advantage favoring mutations that broke perichondral bone growth, and at small sizes a cartilage endoskeleton is mostly as good as a bone one. Sometimes pupfishes or other small fishes reduce their ossification under those sorts of conditions. You might already know this, but larger chondrichthyans often have non-bone mineral deposits on their skeletons that offer it some rigidity.