Every profession has its in-jokes, and palaeontology is no exception. One opportunity to depart from its solemnities is the convention of naming a newly described species in Latin. In this short article there are a number of forbidding names, so it may be as well to explain that Whatcheeria refers to the town near its find spot, What Cheer, in Iowa; Casineria is dog Latin for ‘Cheese Bay’, where it was found, and Eldeceeon was named after the Livingston Development Corporation (LDC), which contributed to the purchase of the specimen. The fossil Pederpes finneyae was named after its finder Peder Aspen and its preparer Sarah Finney.

Although few will have heard of it, Pederpes, according to Kutschera and Niklas, proves the evolutionary transition from aquatic to terrestrial. It dates to the first subdivision of the Carboniferous period, the Tournasian, and thus occurs at the right time: after the aquatic tetrapods Acanthostega and Ichthyostega and before terrestrial tetrapods such as Casineria. Whereas the swimming tetrapods had paddle-like feet that pointed back or to the side, the feet of Pederpes, it was suggested (Clack 2002), pointed forward, consistent with locomotion on land.

This picture has since turned out to be rather speculative. The environment in which the tetrapod was fossilised was marginal marine, while its closest relative, Whatcheeria, seems to have inhabited coastal lakes and swamps. Although slightly more aquatic in character than Pederpes (Clack & Finney 2005), Whatcheeria was a later species, showing that amongst the whatcheeriids there was no trend of increasing terrestriality. Furthermore, there is also evidence that whatcheeriids were in existence at the same time as Acanthostega and Ichthyostega, i.e. in the late Devonian. Thus if Pederpes was contemporary with the earliest aquatic tetrapods it cannot be cited as an evolutionary intermediate.

A character analysis of early tetrapods performed for the initial report seemed to confirm that whatcheeriids were closest to the Devonian examples. However, the analysis was subsequently conceded to be ‘not particularly robust’ (Clack & Finney 2005), and a more comprehensive analysis by other workers the following year (Ruta et al 2003) suggested that they were more advanced.

In their more detailed report Clack and Finney looked into why their conclusions were so different. One exercise involved aligning their dataset as closely as possible with that of Ruta et al. The result was surprising. After Acanthostega, the next most primitive tetrapods were a pair consisting of Ichthyostega and the fish Eusthenopteron. Then came Crassigyrinus, a mysterious creature that had previously been interpreted by Clack as ‘secondarily aquatic’, i.e. it had evolved from a fish, then re-acquired some of its fish-like features as it returned to the water (or deeper water) and its limbs shrunk. Then came Pederpes, followed by 17 progressively more advanced tetrapods.

The results made little sense. Although the exercise assumed that all such animals were related, these first four were obviously very different from each other. As Ruta et al. had commented previously, ‘there is no well-supported, let alone stable, hypothesis of early tetrapod phylogeny.’ The early tetrapods were too diverse to be shoe-horned into the same evolutionary tree. Clack and Finney described their attempt to get to the bottom of the discrepancies as ‘inconclusive’, suspecting ‘some unconscious bias in the way characters are selected and formulated’.

The Tournasian is almost devoid of tetrapod fossils, mainly due to a global extinction event at the end of the preceding period. Were it not for the consequent crash in numbers of individuals amongst the surviving species, we would probably have seen close to the same diversity in that epoch as we see in the succeeding Visean, and the problem of how to link the early tetrapods into a single phylogeny would be even greater. The whatcheeriids are far from alone. Other groups that suddenly appear in the Visean include aïstopods, adelogyrinids, anthracosaurs, microsaurs, colosteids and temnospondyls. They are all highly ‘derived’ (remote from the presumed ancestral condition) when they appear. ‘Some were strictly aquatic, others probably as truly terrestrial as any amphibian could be. Some had greatly elongated bodies with hundreds of vertebrae, others as few as 15. Some presumably lived inconspicuously in the leaf litter, others were highly adapted toward burrowing habits, while still others were agile forms, suited for living in the forest or open ground.’ (Carroll & Gaskill 1978). The snake-like aïstopods had no limbs, nor even limb girdles, but are interpreted as tetrapods because of certain features of the skull, vertebrae and ribs.

If it is true that aïstopods evolved from animals with limbs, then ‘the existence of such a specialized animal so early in the fossil record of tetrapods shows that a great deal of evolutionary diversification and adaptation must have taken place in the time since tetrapods first evolved’ (Clack 2002). But when was that? The time when aïstopods had limbs could hardly be later than mid Devonian, and that would have been after the tetrapod line split into the branches that became Acanthostega, Ichthyostega, whatcheeriids, Crassigyrinus, aïstopods, adelogyrinids, anthracosaurs, microsaurs, colosteids and temnospondyls. On this basis one ought to be looking for the origin of tetrapods in the Ordovician or earlier.

Terrestrial forms amongst these groups include Eldeceeon (an anthracosaur), Westlothiana (an anthracosaur or microsaur) and Balanerpeton (a temnospondyl), all from the East Kirkton Limestone quarry in Bathgate, between Glasgow and Edinburgh. Temnospondyls were the most abundant and diverse of the early tetrapods and over time became progressively more, not less, aquatic. Casineria, also from Scotland, was another terrestrial tetrapod from the Visean, a mouse-sized creature notable for having the earliest known manus (front foot) with five fingers, as against the six to eight of the Devonian tetrapods. The fingers and curved claws were designed for gripping. Again, the existence of such an ‘advanced’ tetrapod so early in the record is evidence against the presumption that terrestrial vertebrates – amniotes in particular – descended from aquatic ones. As Marcel van Tuinen and Elizabeth Hadly put it, rather cryptically, ‘At face value, the fossil record suggests that amniotes diversified rapidly from uncertain immediate ancestors.’ Perhaps that is another in-joke.

There is nothing inconceivable about the idea that aquatic tetrapods might have evolved a terrestrial lifestyle. The question is whether, specifically during the Tournasian, they did. As things stand, no Lower Carboniferous tetrapod can be linked phylogenetically to either Acanthostega or Ichthyostega, so the transition is undocumented. Pederpes is not an ‘intermediate grade between primary aquatic Upper Devonian amphibians and early tetrapods’. And that, no doubt, is why it continues to be a fossil known only to specialists.

See also:
The first tetrapods