Discovery of a Transitional in Romer's Gap
The oldest known tetrapods (vertebrate animals with four limbs, such as amphibians, reptiles, birds, and mammals) are Icthyostega, Tulerpeton and Acanthostega dated from the late Devonian about 365My ago. They appear to be primarily aquatic. At the beginning of the Upper Carboniferous, at 335My ago, several groups appear in the record including temnospondyls and anthracosaurs (which have amphibian features), six groups of lepospondyls which superficially resemble snakes, salamanders and lizards, and a precursor to the amniotes (which is the lineage from which modern reptiles, dinosaurs, birds and mammals ultimately derived). Between 335My ago and 365My ago, no tetrapod fossils have so far been found, so the derivation of the multiple lineages in the Upper Carboniferous from the early primitive tetrapods in the late Devonian is something of a mystery. It is known as Romer's Gap, and it is mysterious because the fossils on the older side of the gap (late Devonian) bear little direct resemblance to those on the more recent side (Lower to Upper Carboniferous transition). For a very detailed discussion of the evolution of tetrapods, see Jennifer Clack's superb book, Gaining Ground (1)
Now, in 2002, Jennifer Clack describes a tetrapod fossil from the middle of Romer's Gap (2) . It comes from Dumbarton, Scotland, from 350My old deposits and so is plumb in the middle of Romer's Gap. She has named it Pederpes finneyae.
It shows some key transitional features between the earliest
known tetrapods (which are themselves transitional between lobe finned fish
and tetrapods generally) and the later Carboniferous lineages. It also shows
a mixture of primitive features (ie features found only in fish or
earlier tetrapods) and derived features. Primitive features (there
are several additional ones descrobed in the paper) include the following:
1) The normal tetrapod arrangement is pentadactyly (ie they have five digits on fore and hind-limb). Of course, in some species, adults have fewer (the horse has one for example), but all tetrapods (reptiles, birds, amphibians, mammals, dinosaurs etc) have five digits in the embryonic limb, and the condition of less than five digits in adult amniotes is a secondary adaptation. Pentadactly is a conserved arrangement. However, early tetrapods such as Icthyostega, Acanthostega and Tulerpeton have more than five (six, seven or eight), a condition known as polydactyly. And what is the situation for Pederpes? The fossil preserves five digits on the hind limb, and although we cannot be sure that there were not more, it seems that there were five functional ones. The front limb is poorly preserved and only two digits have survived. However one of these is tiny and the inference is that the tiny digit represents a sixth supernumerary digit allied to a normal pendactylous arrangement, as in earlier polydactylous species.
stapes, which in more recent terrestrial animals is a bone which forms part
of the arrangement for conducting sound in the middle ear is here a relatively
massive structure as it is in stem tetrapods such as Acanthostega, and is not
associated with a tympanum or ear drum as it is in more derived tetrapods.
3) There is evidence for lateral lines
(sense organs which fish use to detect vibrations in the water) in tubes
through bone which are not found
in modern adult tetrapods but are routinely found in fish
Derived and transitional features include the fact that the shapes and asymmetries in the hind-foot show a more specific adaptation to terrestrial locomotion than in the earlier tetrapods. The form of the phalanges in the earliest tetrapods was more symmetrical resulting in a more paddle-like and less well adapted limb for terrestrial locomotion and one that was held laterally from the body. The asymmetries in the bones of the hind foot of Pederpes suggest that the foot was rotated to face forward as is the case with tetrapods today, an arrangement more suited to terrestrial locomotion than the paddle like feet of the earliest tetrapods.
|Reconstruction of pedes of various taxa. Pederpes, Greererpeton, Silvanerpeton and Proterogyrinus show asymmetrical metatarsals (see arrows) compared with those of the Devonian forms Acanthostega, Ichthyostega and Tulerpeton. In Acanthostega and Ichthyostega, the metatarsals are not clearly differentiated, and in Tulerpeton, if correctly interpreted, they are cylindrical but include some interarticulations. Scale bars, 10 mm. After reference (2)|
Note that this fossil is mentioned in Jennifer Clack's book, Gaining Ground, but the book was published before the paper and before the specimen was named. In the book it is called simply the 'Tournaisian specimen'.
For further information on tetrapod evolution, see this review paper (3)
So we have a transitional between the very earliest and later tetrapods. The scale of the evolution of tetrapods is getting finer.
There is an excellent summary of the Pederpes finneyae discovery and a reference
to Jennifer Clack's book, Gaining Ground, at Bob Patterson's fantastic website
The base page for terrestrial vertebrates on the Tree of Life website can be found here (5)
1. Clack, Jennifer, Gaining Ground, The Origin and Evolution of Tetrapods, Indiana University Press, 2002, ISBN 0-253-34054-3
(Jennifer Clack is Reader in Vertebrate Palaeontology and Senior Assistant Curator, University Museum of Zoology, Cambridge, UK Return to text
2. Clack, An early tetrapod from Romer's Gap, Nature 418, 72 - 76 Return to text
3. Laurin et al, Early Tetrapod Evolution, Tree 15, 118 - 123
http://www.ese.u-psud.fr/epc/conservation/Publi/abstracta/AE_TREE2000.pdf Return to text
5. The base page of terrestrial vertebrates on the Tree of Life website - go here