Re: Sauropod Ancestors

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

>From: jshields@iol.ie (James Shields)
 > >I have seen similar results in cladistic analysis of hominids,
 > >claiming that none of the known fossil species is ancestral to
 > >Homo sapiens!  There cannot be *that* many undiscovered hominids!
 > 
 > One thing we have a habit of forgetting is how few species we actually
 > know about from fossil evidence.

However, with regard to hominids, we are in am uch bvetter than
average position, for several reasons:
        1. hominids are very recent (Plio-Pleistocene), and thus
           there are more fossils and subfossils to be found.
        2. due to special interest, more searching has been done
           for hominids than for most other groups (except perhaps
           dinosaurs).

In fact in the australopithicine/habiline group of fossils, we have
so many specimens with so many different combinations of characters
that it is difficult to sort the specimens into species.

I think it is very likely that the genus Homo originated from within
the group of species we classify as Australopithecus, even if the
actual immediate predecessor is unknown.

And within the genus Homo, I would be *very* surprised to discover
more than one or two additional species, and even then only at the
habiline grade.  Starting with the the erectus grade, I am fairly
confident that we have all of the distinct morphotypes in our
current sample of fossils. (Note, there is still some room for
uncertainty about the number of species these specimens represent;
most workers see only two species, but some, including me, see four).

 > I book I read (can't remember which one)
 > suggested approximately 1 in 10,000.

While this may be true "in the large", it is not necessarily true fo
any specific group.  In the case of dinosaurs, an estimate of the
total number of actual species was made a few years ago based on
sampling theory.  The result was that we know between 5% and 20% of
the total number of dinosaur species that ever actually existed.
[Not a very prrecise estimate, I know, but there were lots of
unknowns in the equation].

Now, when you get to coelenterates, or flatworms, and such like,
we probably know rather *fewer* than 1 in 10,000.

The result is that the average may well be about 1 in 10,000 potential
fossil species are known.
 > 
 > Presumably, more recent species are better known than ancient ones, but
 > there could still be a lot of unknown hominids...

Well, I could see doubling the number of australopithecines in general,
and, if you only recognize three species of Homo now, I could see
*maybe* doubling that number. (But since I recognize 6 or 7 species
of Homo, I do not see how that number could be doubled).

[I split what many call Homo sapiens into three species:
Homo sapiens, Homo neanderthalensis, and Homo heidelbergensis;
the last also containing some specimens often considered advanced
Homo erectus. I also do not see how all of the habiline grade
specimens can be accomodated in one species, so Homo habilis
needs to be split into at *least* two species].


With regard to dinosaurs, I do not see how many major groups
containing large species can be left to be discovered.  The
last major group to be discovered was the segnosaurs, and even
there it has turned out we have known of one species of that
group for many decades.

I would find the discovery of another sauropodomorph subgroup
to be *very* surperising.  New species and genera, and even an
occasional new family, are one thing, but a new higher category
is quite another.

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

The peace of God be with you.