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Quadruped Sailbacks - Synapsids

On Tue, 25 Apr 1995, James Shields wrote:

> >Okay - Dimetrodon, Edaphosaurus, and its relatives are not dinosaurs, nor
> >reptiles (under cladistic taxonomy), but are primitive representatives of
> >the synapsids (mammals and their ancestors).
> Weren't the Synapsids reptiles (hence the name "mammel-like reptiles")?

They are reptiles in the traditional sense - that is they are amniotes but 
not birds or mammals.  Some taxonomists refer to the (putative) clade 
containing chelonians (turtles) and diapsids (lizards and their relatives, 
crocodiles, dinosaurs, birds, pterosaurs, and probably icthyosaurs and 
plesiosaurs) as reptiles.  In this sense synapsids are not reptiles.

> This leads me to the question, where do we draw the line between reptiles
> and mammels? What features of modern mammels were known to be present in
> synapsids?

Early synapsids (like Dimetrodon) share little with the mammals except one or
two features of the skull which many mammals have lost anyway.  But later ones
developed mammalian features like an upright stance, whiskers (so presumably 
hair, so presumably homeothermy), lips (so presumably milk), buttocks and 
finally the mammalian ear.  Many of the latest "mammal-like reptiles" 
probably looked, walked and smelt just like mammals.  

There are two ways of drawing the line.  Some prefer to define mammals as 
descendants of the latest common ancestor of all recent mammals.  This is a
very logical method, but it makes it very difficult to tell whether a 
particular fossil is mammalian or not.  The alternative is to pick some key 
feature to separate mammals from (other) synapsids, and the feature chosen 
is the jaw articulation.  Mammals can move their lower jaw in almost any
direction, and chew on one side at a time, moving the jaw inwards, then 
downwards, then back where it started from (go on, try it).

> I hope this isn't getting too far away from dinosaurs to be of interest to
> this group.

I think anything to do with tetrapod palaeontology is on-topic.

                                                                Bill Adlam