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Filling Romer's Gap



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!

"The fossil record of early tetrapods has been increased recently by new
finds from the Devonian period and mid¨Clate Early Carboniferous period.
Despite this, understanding of tetrapod evolution has been hampered by a
20-million-year gap ('Romer's Gap') that covers the crucial, early period
when many key features of terrestrial tetrapods were acquired.  Here I
describe the only articulated skeleton of a tetrapod, _Pederpes_, yet found
from the Tournaisian epoch (354¨C344 million years ago (Myr)). The new taxon
includes a pes with five robust digits, but a very small, possibly
supernumerary digit preserved on the manus suggests the presence of
polydactyly.  Polydactylous early tetrapods may have survived beyond the end
of the Devonian and pentadactyly cannot be assumed for the pes.   However,
the pes has characteristics that distinguish it from the paddle-like feet of
the Devonian forms and resembles the feet of later, more terrestrially
adapted Carboniferous forms.  _Pederpes_ is the earliest-known tetrapod to
show the beginnings of terrestrial locomotion and was at least functionally
pentadactyl.  With its later American sister-genus, _Whatcheeria_, it
represents the next most primitive tetrapod clade after those of the Late
Devonian, bridging the temporal, morphological and phylogenetic gaps that
have hitherto separated Late Devonian and mid-Carboniferous tetrapod
faunas."


Tim