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Re: mammalian reptiles
>What specific details determine that a fossil is from a reptilian mammal?
I'm not sure what you mean by a "reptilian mammal". The crucial diagnostic
feature for true mammals is the presence of the dentary-squamosal jaw
articulation and the absence of the temporal/articular one except as middle
ear ossicles (one reptile, Diarthrognathus, had both). Beyond that a
synapsid skull condition defines the line of reptiles leading to mammals,
but that includes pelycosaurs like Dimetrodon as well as therapsids and
other "mammal-like reptiles".
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