Science Literature

A previous blog on Devonian tetrapods remarked on their aquatic lifestyles and noted their suite of mosaic characters that make discussion of evolutionary trajectories highly speculative. Few fossils from the lower Carboniferous were known, but the diversified forms from the middle and upper Carboniferous were clearly components of terrestrial faunas. So, we find a group of aquatic amphibians in the Upper Devonian and a diversified group of terrestrial amphibians in the middle-upper Carboniferous. The puzzle is the lack of any terrestrial fossil material in the lower Carboniferous, leaving evolutionary palaeontologists little or no data to work with. The absence of evidence has been so noticeable that this part of the record has been labelled Romer's Gap (after the distinguished American palaeontologist from the last Century). In 2006, Ward et al. proposed an explanation for the lack of terrestrial fossils that invoked low concentrations of atmospheric oxygen. This, they surmised, inhibited the evolutionary development of ecosystems on land. Since that paper, more discoveries have been made in Scotland in rocks representing Romer's Gap, and "a wealth of new tetrapod and arthropod fossils" have been recovered. The inference can be made that the Romer's Gap ecosystems were not impoverished but, for various reasons, only recently have palaeontologists discovered the evidence needed to warrant this conclusion.

"Rather than beginning immediately following "Romer's Gap", we can now test the hypothesis that diversification and terrestrialization of tetrapods had been taking place during the 15 or more million years that it represents. Our discoveries and other recent new records from elsewhere certainly suggest that many tetrapod lineages have their origins much earlier than previously appreciated, and their earliest appearances may well be extended back in time as the result of further research."

Five toes found near the Scottish fishing village Burnmouth belong to the new caches of fossils. (Credit: J. Clack, Source here)

It is undoubtedly exciting to have this new data to work with. Inevitably, the question to be answered is how the new fossil data affects our understanding of Devonian and Carboniferous tetrapods. The research paper provides an initial insight and the last sentence quoted above gives the gist of the findings. What follows attempts to highlight evidences documented in the research paper. Readers should be aware that the Tournasian stage is the lowest in the Carboniferous, and above this is the Visean stage. Romer's Gap extends from the base of the Tournasian to the middle part of the Visean.

The first locality to be reported is Burnmouth. Isolated tetrapod remains have been found in several horizons.

"The most significant of these is a small (10 mm across the metapodial series) pentadactylous autopod (identity as manus or pes cannot yet be determined). [. . .] Its morphology strongly suggests that its owner was a terrestrial tetrapod. [. . .] The proportions of the metapodials and preserved phalanges, being elongate and gracile, are most similar to those of the Visean forms Silvanerpeton, Eldeceeon, and Balanerpeton, as well as the Late Carboniferous anthracosaur Gephyrostegus, all of which are usually considered to have been terrestrial."

A few metres higher in the sequence, fossil material has been found that is very similar to Crassigyrinus.

"Crassigyrinus is a large tetrapod previously known only from the late Visean and early Namurian of Scotland. The newly discovered jaw ramus is almost exactly the same size as the known specimens, has almost identical external ornamentation, and differs from the known specimens in only minor details of the internal structure. [. . .] This Burnmouth horizon confirms the presence, by this early date, of large vertebrates whose affinities are with later Carboniferous rather than Devonian forms."

The same Formation outcropping at Burnmouth appears again at Willie's Hole to the south-west. Three distinct horizons have revealed tetrapods and many other fossils. In all, there are "100 samples of large and small semiarticulated tetrapod skeletons and isolated bones". In bed 1, reference is made to a small individual that can be provisionally reconstructed. "Its proportions most closely resemble those of the Visean Silvanerpeton or the Late Carboniferous Gephyrostegus.” Bed 2 has one of the largest specimens, but it cannot be assigned to an existing genus. There are indications that this animal was a mosaic of different characters allowing some similarities with other species to be recognised: "but only further study will elucidate their relationships". Another terapod from this bed had characters "reminiscent of that of a temnospondyl"(an upper Carboniferous group).

"[. . .] only further study could confirm or refute such an assignment. If corroborated, it would represent the earliest member of the group by about 15 million years."

Other sites are mentioned more briefly, but the fossil material does not change the position already noted. The authors are confident that their discoveries reveal Romer's Gap to be a collection failure. Consequently, it is not necessary to postulate low atmospheric oxygen levels to explain the absence of fossils.

"Our new records, combined with those from trackways, suggest that tetrapods appear to have recovered relatively rapidly from the EDME [End-Devonian Mass Extinction] by the mid-Tournaisian. Fish groups had evolved, or reevolved, into new large forms (e.g., rhizodonts, lungfishes). By the mid-Visean, not only had tetrapods appeared that are usually considered terrestrial and the base of the crown group been established, but highly specialized secondarily aquatic forms had also evolved."

The important point to note is that the new finds do not reveal an evolutionary trajectory linking the Devonian forms with the Carboniferous forms. Romer's Gap does not have transitional forms but documents the earlier appearance of five-toed Carboniferous forms. As is so frequently found, new fossil finds do not document evolutionary transitions but extend the range of the more "modern" life forms. This is the pattern recognised by advocates of Punctuated Equilibria, not the pattern predicted by Darwinists. For the researchers, the real work is before them.

"These finds will allow us to put forward refined hypotheses, testable by further finds and analyses. The wealth of material from several different sites and environments will provide the opportunity to investigate the causes and consequences of the EDME. Our initial results suggest that reestablishment of at least some components of the tetrapod fauna was achieved within 10 million years. We have established that pentadactyly arose about 20 million years earlier than previously documented. Studies may now examine the interlinkage of environmental and atmospheric changes to faunal turnover, the timing of ecosystem recovery, the sequence of acquisition of terrestrial characters by tetrapods, resolution of the problems of relationships among early tetrapods (and thus the recalibration of the phylogenetic tree), and the time of appearance of crown group tetrapods, based on the presence, rather than the absence, of fossil data."

The last sentence is worthy of note. However, it is in tension with Figure 6 in their paper, which has a family tree of tetrapods. All the fossil evidence shows discontinuity, but evolutionary linkages are marked (all located within Romer's Gap) that are devoid of supporting data. We are still a long way from a science that majors "on the presence, rather than the absence, of fossil data".

Earliest Carboniferous tetrapod and arthropod faunas from Scotland populate Romer's Gap
Timothy R. Smithson, Stanley P. Wood, John E. A. Marshall, and Jennifer A. Clack.
Proceedings of the National Academy of Sciences, published 5 March 2012 | doi:www.pnas.org/cgi/doi/10.1073/pnas.1117332109

Devonian tetrapods (limbed vertebrates), known from an increasingly large number of localities, have been shown to be mainly aquatic with many primitive features. In contrast, the post-Devonian record is marked by an Early Mississippian temporal gap ranging from the earliest Carboniferous (Tournaisian and early Visean) to the mid-Visean. By the mid-Visean, tetrapods had become effectively terrestrial as attested by the presence of stem amniotes, developed an essentially modern aspect, and given rise to the crown group. Up to now, only two localities have yielded tetrapod specimens from the Tournaisian stage: one in Scotland with a single articulated skeleton and one in Nova Scotia with isolated bones, many of uncertain identity. We announce a series of discoveries of Tournaisian-age localities in Scotland that have yielded a wealth of new tetrapod and arthropod fossils. These include both terrestrial and aquatic forms and new taxa. We conclude that the gap in the fossil record has been an artifact of collection failure.

See also:

Fossil discoveries fill crucial gap in land animal evolution, by Tamera Jones (Planet Earth Online, 7 March 2012)

Fossil pushes back land-animal debut, by Devin Powell (ScienceNews: Monday, March 5th, 2012)

Tetrapod family tree looks like a bush, by David Tyler (ARN Literature blog, 29 April 2009)