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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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