Devonian – The Earliest Body Fossils of Tetrapods

Australia: The Land Where Time Began

Devonian – The Earliest Body Fossils of Tetrapods

The earliest known genera of tetrapods, Obruchevichthys and Elginerpeton, found so far are of Frasnian age, and are body fossil material, mostly of lower jaws, and in 1 case, some limb and girdle fossils. Relatively recently they has been recognised from fossil materials in museum collections where they have been since the last century, but the identity of the fossils was not known until newer tetrapod fossils from the Devonian that are better preserved were recovered allowing the earlier found fossils could be identified.

The original description of Obruchevichthys was as a sarcopterygian fish dating from the Frasnian of Latvia and western Russia (Ahlberg, 1991b). The only known fossils of this animal are lower jaw fragments from an animal that had a skull that was about 400 mm long. This animal is suggested by the slender, elongate jaws to have had a rather flattened skull, though the shape of the head can only be guessed at. The remains occur together with many other fishes that were typical of the Late Devonian, which included placoderms, lungfishes, acanthodians, as well as some fragmentary lobefins. There are some features of the lower jaw that are now considered to be characteristic of tetrapods, though there is no known limb or girdle material of this animal. Clack¹ suggests the position of this animal on the cladogram is likely to remain unchanged, close to animals that have limbs, even if it is eventually found not to have had limbs. Together with Elginerpeton it is recognised as a tetrapod and they constitute the earliest known body fossil evidence of the group.

A picture that is more complete has been built up of Elginerpeton as it is known from a larger number of fossils than Obruchevichthys (Ahlberg, 1991b, 1995). Many of these specimens had been labelled as “unknown sarcopterygian” and left in museum draws in the UK until 1991 and had not been described and in some cases not even catalogued. The material was recovered from Scat Craig, a tiny locality where the rocks containing them are exposed in a stream cutting near Elgin in Scotland. More material was collected from this locality in 1992, though not much of it proved to be of Elginerpeton, which like Obruchevichthys has been dated to the late Frasnian.

Material of upper and lower jaw fragments, that includes the premaxilla, and some elements of the limb and girdle, have been recovered, therefore it is the earliest known tetrapod that had limbs. Among these recovered elements were a tibia, a femur, pectoral and pelvic girdle fragments, and a humerus that is less certainly established (Ahlberg, 1998). The skull length is suggested by the recovered material to have been about 400 mm, with a flat head and a snout that is somewhat pointed. The massive pectoral and pelvic girdle elements are in accord with the size of the skull. In particular, the pelvic girdle is of comparable morphology to that of Ichthyostega that is better known. The shape of the femur, that is short and flattened, is suggestive of it being held in such a way that it is unlikely to have supported weight, being used instead as a paddle. If it is assumed that the humerus came from the same animal, and as Clack¹ says it is not even certain it is a humerus, and seems to have been somewhere between Panderichthys and a juvenile Ichthyostega according to its morphology.

The remains of Elginerpeton were recovered from coarse sandstone, and its place of origin cannot be established as the bones are disarticulated and water worn, apparently being transported from another locality before being fossilised. Other remains that have been recovered from this locality include some fishes that are typical of the Devonian such as placoderms, a lungfish, a porolepiforms, as well as so agnathan (jawless) fishes. Elginerpeton is suggested by the limbs to have been an aquatic animal, though not much of its habitat or lifestyle can be obtained from the sediments in which it was found.

Other vertebrate material from the Late Devonian has also been recovered from other quarries not far from Scat Craig in the Elgin area, such as Rosebrae Quarry. This quarry contained lungfish, Rhynchodipterus in particular, which is known from an individual that is almost complete. In this quarry the bone material has been preserved as holes or natural moulds in the rock, the original bone having been dissolved completely. This kind of preservation has proven to be amenable to MRI techniques if the specimen is immersed in water, because of the nature of the rock. There is also a jaw from a large tetrapodomorph that has been recovered from this locality, which suggests the possibility of more tetrapodomorph fossils being found there, though the circumstances of the quarry mean it would be expensive and not cost-effective in terms of the likelihood of finding them, if they are actually there.

The Devonian rocks in this quarry, composed of coarse yellow-brown sandstone are overlain unconformably by rocks dating to the Triassic, which are coincidently formed of yellow-brown sandstone, which makes it very difficult to see where the division between the 2 rock layers is by observing the sedimentary sequence. It is still considered to be a mystery why these 2 rock series of such different ages look so similar, though Clack¹ suggests that the explanation of the similarity of the 2 rock series may be the fact that both sediments were deposited in deserts. As a result of the earlier confusion crocodile-like reptiles and mammallike reptiles were previously believed to have come from the Devonian rocks but were eventually shown to have come from the Triassic.

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