SYNAPSIDA 

Some of the animals featured in my YT video on Synapsids, many more are included there. This piece is now also available HERE for prints and more

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Not Dinosaurs mug

A mug featuring iconic and charismatic prehistoric animals that too often get mislabeled as "dinosaurs". Marine reptiles, pterosaurs, and the relatives of mammals and crocodiles are all here!

252mya.com/notdinosmug

Filikomys reconstruction by Misaki Ouchida. Multituberculates are now thought to have been intelligent mammals, so this discussion is worth having.

Modern placental mammals have something marsupials and monotremes lack: a corpus callosum. This allows for connectivity between the brain hemispheres and thus higher intelligence (though marsupials might be more clever than we give them credit for, and birds lack a corpus callosum and are next in line for civilization). Its thus worth to ask: did extinct non-placental groups have a corpus callosum?

On one hand, the fact that only placentals out of all amniotes have one would imply that this is a feature exclusive to them. On the other hand, marsupials and monotremes both produce undeveloped, fetal young, so they could have secondarily lost the corpus callosum as part of their simplified development. Extinct groups like multituberculates not only were as intelligent as placental mammals but had parts of their braincase not seen in other mammal groups, so it's not unreasonable to assume either the corpus callosum was a mammal synapomorphy and lost on multiple groups or it developed independently. Or maybe multies and co didn't have one and simply got smart via other ways like birds.