Dinosaur vs. hominid speciation

[Phillip Bigelow <n8010095@henson.cc.wwu.edu>]

>From: jshields@iol.ie (James Shields)
 :> One thing we have a habit of forgetting is how few species we actually
 :> know about from fossil evidence.

Stan Friesen <swf@elsegundoca.attgis.com> wrote:  
>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).


The first reason (the recent-ness of the hominid fossil record) has always
bothered me.  It is not necessarily the recent age of the
deposits, but rather the *geographical extent* of the rock exposures that
would make our knowlege of hominid evolution relatively well known.  This
doesn't appear to be the case in Africa, where 100% of the pre-Homo erectus
fossils were found in the African Rift valley.  That leaves roughly 80% of
the African continent that hasn't produced hominid fossils. 
  There are a couple likely reasons for this.  In many places,
Plio-Pleistocene age rocks never had been deposited in the first place,
or were deposited, but were subsequently eroded away.  Another reason is that
Plio-Pleistocene age rocks are present, but are buried in the subsurface.
   The correlary to all of this is that, almost certainly, hominids were
present in the parts of Africa where their remains will never be found. 
Therefore, we can never be absolutely sure how many different genera and
species of hominids existed in Africa during the Plio-Pleistocene.
   The same logic applies to dinosaurs.  How many families, genera or
species of dinosaurs do we know of that existed in mountainous habitats?
None.  It's a preservation problem, folks.  The Mountain Gorilla probably
will never make it into the fossil record, simply because it doesn't live in
a depositional basin setting.  Were there mountain-dwelling hominids?  We
likely will never know. 
   All of the above leads me to think that, both in hominids and in
dinosaurs, we are vastly underestimating the great diversity of these
animals.