Re: Filling Romer's Gap

["Steve Brusatte" <dinoland@lycos.com>]

On Wed, 3 Jul 2002 14:23:20   
 Williams, Tim wrote:
>
>In this week's Nature:
>
>Carroll, R. (2002).  Palaeontology: Early land vertebrates.  Nature 418:
>35-36.
> 
>Clack, J.A. (2002).  An early tetrapod from 'Romer's Gap'.  Nature 418:
>72-76. 
> 
>Clack describes a wonderful new skeleton of an early tetrapod named
>_Pederpes_, the first to show adaptations for walking on land.  The fossil
>was actually discovered back in 1971 but misidentified as a fish!

May I be the first in this group to shout out for joy? :-)  I have a particular 
fondness for these early tetrapods, perhaps because I wish I could find some 
nice Late Pennsylvanian ones in the largely unexplored (for tetrapods) rock 
units in central Illinois.  

I take it this specimen is the "new Tournaisian tetrapod" that Clack described 
in her book _Gaining Ground_.  If so, this specimen comes from near Dumbarton, 
Scotland, and includes a nearly articulated skeleton lacking only a few parts 
of the limbs, skull, lower jaws, and tail.  A photo of it is printed in her 
book, but some of the smaller elements are difficult to see.  She mentions that 
its humerus resembles the Pennsylvanian genus _Baphetes_, which at least one 
cladistic analysis has shown as very close to _Eucritta_.  However, its skull 
is very much like that of the Mississippian (Visean) _Whatcheeria_ from, of all 
places, Iowa!

Only two manual digits are preserved, but there are indeed five pedal digits.  
There is only brief mention of the small manual digit in _Gaining Ground_, but 
the specimen was still in preparation when it was written.  Interestingly, 
Clack compared this small digit to the atrophied digit that characterizes the 
leading edge of the hindlimb of _Ichthyostega_.  The hindlimbs are very robust, 
and do not show the flattening of the tibia, fibula, and certain phalanges that 
characterize not only _Whatcheeria_, but also many other earlier tetrapods that 
were much better adapted for swimming (by using their somewhat flattened limbs 
as paddles).

I'm quite pleased that this new specimen has been named.  It is most definitely 
among the five or six most important early tetrapods known.  As Clack mentioned 
in the excerpt that Tim posted, it bridges a very important gap.  Basically, 
there are three major tetrapod "divisions."  There are a few genera known from 
the Frasnian (_Livoniana_ and _Obruchevichthys_ among them), but many of these 
went extinct at the Frasnian-Famennian extinction.  A better-known, but more 
conservative, division characterizes the Famennian, with such genera as 
_Acanthostega_, _Ventastega_, _Hynerpeton_ (and _Densignathus_, which is only 
known from one lower jaw found at the _Hynerpeton_ site in Pennsylvania), and 
_Metaxygnathus_.  Once again, some of these forms seem to go extinct.

After this extinction we are faced with "Romer's Gap," but _Whatcheeria_ and 
now _Pederpes_ are filling in the considerable temporal gap between the 
Famennian forms and later Carboniferous genera such as _Crassigyrinus_ and 
_Greererpeton_.

In brief: a very important specimen that not only fills a temporal gap, but 
also shows some important functional adaptations.  It's very nice to see that 
it has been named and published.

Steve

---
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Steve Brusatte-DINO LAND PALEONTOLOGY
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