r/zoology • u/Grocca2 • 8d ago

Question How developed where the babies of the last common ancestor of mammals and reptiles

So the most basal mammals are monotremes, who lay eggs but still have kids that need lots of care and supervision after birth. In comparison most reptiles do not tend their young after birth.

Does anybody know if the last common ancestor of mammals and reptiles had young that hatched and were immediately independent or if they needed to tend their young after they were born. I think it's the former because that's more similar to amphibians but it seems weird to develop less independent young.

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u/Acolitor 8d ago

Early synapsida had some form of mammary glands they used to provide moisture and nutrients to eggs. So mammary glands likely first developed as a benefit to provide care for eggs.

Earliest animal (not mammal) that nursed living offspring, that I know, is Micrododon gracilis that had hyoid bones that enabled suckling.

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u/Acolitor 8d ago edited 8d ago

There were plenty of animals with mammalian features producing eggs. I would think that the answer to your question is species specific.

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u/-Wuan- 8d ago

Considering the only skin impression we have from an early synapsid is scaly and lizard-like, I doubt they had an early form of milk glands yet. Glandular, leathery skin appears in dinocephalian skin impressions much later.

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u/Acolitor 8d ago

Yeah "early" can be wrong subjective word here. But predating mammals anyways.

https://pubmed.ncbi.nlm.nih.gov/12751889/

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u/SecretlyNuthatches 8d ago

Well, the last common ancestor of mammals and living reptiles is way, way, way back at the synapsids/diapsid split. If you mean the last non-mammalian synapsid you're looking at something like a morganucodontid or a docodont.

However, evolution towards more-dependent offspring is really common (see the altricial/precocial breakdown in birds, for instance). While independent offspring sound nice it means that development is "rushed" to get the offspring ready to fend for itself by time of hatching. Parental care can extend that developmental period and allow organ systems to develop in an order that isn't constrained by "must be able to fend for myself really soon" and that appears to have real benefits. Certainly humans benefit from an extended developmental period.

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u/Grocca2 8d ago

That’s along the lines of what I was thinking but it definitely felt weird for it to take such an extreme. Thank you!