2. Amphibian to reptile

PederpesEvery profession has its in-jokes, and palaeontology is no exception. The convention of naming a newly described species in Latin offers a deliciously cryptic occasion to depart from its solemnities. Since this short article is peppered with forbidding names, 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. Unfortunately there are no homely names for long extinct animals.

From a biblical standpoint, the transition from primarily aquatic to fully terrestrial animals is of particular interest, because the cataclysm that destroyed the Earth blotted out only terrestrial animals. Fish and other marine creatures could have evolved from larva-like forms, and amphibians could have evolved from fish (‘could have’ because nature is inherently wonderful, and the tadpole-to-frog cycle shows what the genetic code is capable of). Terrestrial animals, however, could not have survived the bombardment. Were it not for the construction of a protective ark that saved breeding pairs of every species, land environments could not have been re-stocked. Terrestrial species also underwent huge evolutionary change as they recolonised the earth, but their evolution did not begin in the sea.

Thus the story of how ‘animals conquered the land’ divides into two parts, one a transition from fish to marine amphibian, the other a transition from amphibian to terrestrial amniote. While the first has been well publicised, the second remains befogged. Who has heard of Pederpes, the animal that according to Kutschera and Niklas, demonstrates the second transition? Dating to the earliest subdivision of the Carboniferous period, the Tournasian, the fossil occurs at just the right time: after the first aquatic tetrapods Acanthostega and Ichthyostega and before terrestrial tetrapods such as Casineria. Whereas the swimming tetrapods had polydactyl paddle-like feet that pointed back or to the side, the feet of Pederpes had five digits and pointed forward, consistent with locomotion on land. Why has more not been made of this creature?

Mainly because the scenario of progressive adaptation to life out of water has turned out to be rather problematic. The closest relative of Pederpes, Whatcheeria, is slightly more aquatic in form and seems to have inhabited lakes and swamps, but it is a later species, showing that amongst the whatcheeriids there was no trend of increasing terrestriality. Furthermore, there is evidence that whatcheeriids were contemporary with Acanthostega and Ichthyostega, in the late Devonian. If Pederpes were also to date back to the time of these amphibians, the case for interpreting it as an evolutionary intermediate would founder.

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

CrassigyrinusClack 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 to evolve in their reconstructed ancestral tree were a pair consisting of the tetrapod Ichthyostega and the fish Eusthenopteron. Then came Crassigyrinus, a mysterious creature that has always 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 no 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’.

Tetrapod fossils from the Tournasian used to be extremely scarce, possibly because of a global extinction event at the end of the preceding period. Pederpes, until recently, was the only one. Now, however, the Scottish Borders are beginning to yield a few more fossils (Smithson et al. 2012). The environments show that the animals were terrestrial, for plants, myriapods and scorpions (animals that had long preceded them) were also fossilised there.

Were it not for the crash in numbers of individuals following the extinction event, it is likely that we would see close to the same diversity as we see in the succeeding Visean, and the problem of how to link the early tetrapods into a phylogeny that included the Tournasian and Visean animals would be even greater. The whatcheeriids are far from alone. Other groups that suddenly appeared – or re-appeared – in the Visean include aïstopods, adelogyrinids, anthracosaurs, microsaurs, colosteids and temnospondyls, all of them highly ‘derived’ (remote from the presumed ancestral condition). ‘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 suggest a tetrapod past 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 – not the late Devonian as per current orthodoxy.

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. Some would later evolve into frogs. Another terrestrial tetrapod from the Visean was Casineria, also from Scotland: a mouse-sized creature notable for having the earliest known pentadactyl foot, as against the six to eight digits of the Devonian tetrapods. The digits 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, ‘At face value, the fossil record suggests that amniotes diversified rapidly from uncertain immediate ancestors.’ Perhaps that rather cryptic confession is another in-joke! Robert Carroll is more explicit: ‘The numerous lineages that appear at the end of the Lower Carboniferous are diverse and distinct from one another and do not present features that permit the confident assignment of relationships either to the Devonian taxa [notably Acanthostega and Ichthyostega] or among the several lineages themselves.’

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 actually 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