Re: Amniotes

["T. Michael Keesey" <mightyodinn@yahoo.com>]

--- Aegyptiacus@aol.com wrote:
> Dear listmembers
> I've seen in a cladogram that amniotes where at the base of sauropsids and 
> synapsids. What evidence is there that it (the amniotic egg) didn't appear 
> before within the reptilomorph lineage (eg seymouriamorphs or anthracosaurs 
> or embolomeri)?

Sauropsids and synapsids *are* amniotes.

Clade _Amniota_ is not defined by the presence of the amniotic egg, which would
be near-impossible to determine in certain forms. Instead, it's defined as a
crown clade (a node-based clade anchored on extant specifiers). You could
define it thusly: "The last common ancestor of _Homo sapiens_ and _Lacerta
agilis_, plus all descendants." (Or select other specifiers.)

Doubtless the first amniotic egg did appear sometime before the first amniote
(sensu stricto).

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