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>> >Amphibians are tetrapods but not amniotes. Amniotes are those tetrapods
>> >that lay shelled eggs (or, for you more strict cladists, all descendants
>> >of the last common ancestor of, say, me and a hoatzin).
>> I'm still in the process of switching from Linneanism to cladistics. As
>> I recall, amniotes are characterized by an egg with a fluid filled
>> (amniotic) sack. Would this not, then, include amphibians? I am assuming
>> that the choice of "amniota" in cladistics has a similar connotation, even
>> though it's based on descent rather than physical characteristics.
>No, really, Amniota is tetrapods minus amphibians, or all descendants of
>the common ancestor of mammals and birds (or lizards, or turtles, etc.).
Clarification time here: Clades are based ON COMMON DESCENT, although an
individual may be recognized as a member of the clade because it shares
Amniota is now typically defined as the most recent common ancestor of
Mammalia and Reptilia (the clade uniting turtles, lepidosaurs, and
archosaurs) and all of that ancestor's descendants.
Sauropsida is the clade of all taxa closer to Reptilia than to Mammalia.
Synapsida is the clade of all taxa closer to Mammalia than to Reptilia.
Thus, Amniota is also defined as the node uniting Sauropsida and Synapsida.
Thomas R. Holtz, Jr.
Dept. of Geology
University of Maryland
College Park, MD 20742