Re: Sauropod Ancestors

[Stan Friesen <swf@ElSegundoCA.ATTGIS.COM>]

jshields@iol.ie wrote:
 > I wrote:
 > >With regard to dinosaurs, I do not see how many major groups
 > >containing large species can be left to be discovered.
 > 
 > I'm getting WAY out of my depth here, but here goes...
 > 
 > As I understand it, the conditions for fossilisation are pretty rare.
 > Furthermore, there are environments in which it simply does not take place.
 > Isn't it possible that there are dinosaurs and groups of dinosaurs which
 > lived their lives in such places and simply did not wander into places where
 > they were likely to be preserved.

This is why I qualified it as "major" groups with "large" species.
A major group - one with many species - is going to be found in
a wide variety of habitats, and thus at least *some* species of
that group will be living in places where fossilization can occur.

Also, while fossilization is rare, large species fossilize relatively
more often than small ones, since the bones last longer after death.

The largest new group iscovered in recent years was the pachycephalo-
saurs, and they are barely what I would call a "major group", being
classified in just two families.
 > 
 > Couple this with the number of species we know must exist but don't have any
 > examples of. For example, we know saurischians and orinithischians must have
 > a common ancestor, but (unless I am mistaken) we have no idea of what that 
 > was.

Maybe, maybe not.  Cladistics has an unfortunate tendency to treat
ancestral groups as sister groups.  I suspect that the common ancestor
may have had derived features later lost in all descendents.  This
happens very frequently, yet under cladistic analysis such a situation
is resolved by reating the ancestor as a side branch.

A possible example of this is Eoraptor, which is treated cladistically
as an early member of the theropod clade.  However, the few derived
features uniting it with the theropods are mostly carnivore
specializations which would be lost entirely in any herbivores
derived from such an ancestor.  Thus the conclusion that it is
on the theropod clade is, at best, equivocal. [It cannot, itself,
be the last common ancestor of dinosaurs, since it is contemproaneous
with a definate theropod (Herrerasaurus) and an ornithischian.
 > 
 > I suspect that many of the specimens which share a name are actually
 > seperate related species. The only way we can be (reasonably) sure that two
 > fossils are from the same species is if they were found together. I'm not
 > suggesting that every new fossil find should be given a new name, rather
 > questioning whether we can give many fossils species names (or in some cases
 > even genus names) at all.

This may be less frequent than you think.  Many dinosaurs have pronounced
secondary sexual characters, which are a reliable way of distinguishing
species.  Thus in the euceratopsians, the pachycephalosaurs, the
lambeosaurine hadrosaurs, ceratosaurs, tyrannosaurids, and allosaurs,
at the very least, distinct species are likely to be easily discerned.
[In fact this may be more reliable than being found together, since
species *do* mix it up in nature].

But then, this is a seperate issue from the discovery of new major
groups.

 > 
 > What I'm saying is that some species which are known from only a few teeth
 > or assorted bones cannot accurately be named other than calling them "A
 > tyrannosaur".
 > 
 > By the way, how is the number of known species estimated?

By counting the species not listed as "nomen dubium" (aka doubtful),
and then adding in published accounts of distinct, but unnamed, forms.


swf@elsegundoca.attgis.com              sarima@netcom.com

The peace of God be with you.