Recent findings have revealed a diverse assortment of Late Devonian tetrapods. Two genera (Elginerpeton and Obruchevichthys) have been recorded from the late Frasnian, while nine others (e.g., Acanthostega and Hynerpeton) have been found in Famennian rocks. In addition, the fossils of three plausibly transitional lobe-fins (Elpistostege, Livoniana and Panderichthys) have been recovered from the late Middle Devonian (Givetian) or earliest Late Devonian (early Frasnian), while a more demonstably transitional form (Tiktaalik) has been recovered from the early to middle Frasnian.
These findings have led to a widely accepted scenario in which tetrapods diverged from their lobe-fin ancestors sometime during the Frasnian, and became widely distributed in tropical and subtropical localities by the end of the Devonian. Significantly, the Devonian tetrapods appear to have been predominantly if not exclusively aquatic. They were, in a sense, fishes with legs.
The "Tournaisian Gap"
Although these findings have substantially altered and enriched our understanding of early tetrapod evolution, there are several nagging gaps in the fossil record. Perhaps the greatest of these is the "Tournaisian Gap" or "Romer's Gap", a 20 million year gap between the latest Devonian tetrapods and a very diverse assortment of primitively aquatic, secondarily aquatic and terrestrial tetrapods from the Middle and Upper Viséan (Lower Carboniferous). In fact, some of the Viséan tetrapods are early members of the two major clades that include all modern tetrapods: amphibians and reptilomorphs (including anthracosaurs and the egg-laying amniotes).
A major breakthrough with respect to the Tournaisian Gap occurred in 2002 with the publication of work by Jennifer Clack on Pederpes finneyae. This Tournaisian tetrapod exhibits features indicating terrestrial locomotion and pentadactyl limbs. Dated at 348-344 million years ago, Perdepes reduces the gap by about 5 million years. However, there still was an approximately 15 million year span during which the more-derived, pentadactyl, and fully terrestrial tetrapods of the late Tournasian and Viséan became established.
The "Frasnian Gap"
Although not quite as large as the "Tournaisian Gap", the span between the late Givetian (late Middle Devonian) to early Frasnian (early Late Devonian) and the Famennian is of particular interest because its the interval during which most authorities believe tetrapods differentiated from the lobe-fins.
Studies of the elpistostegalian Panderichthyes provided significant insight into the early stages of tetrapod evolution, and with the discovery of Tiktaalik, our understanding has improved dramatically. However, little is known of the earliest true tetrapods. We have fragmentary fossils from two late Frasnian tetrapods (Elginerpeton and Obruchevichthys), but these are widely regarded as members of a short-lived evolutionary side-branch. A better understanding of these —and hopefully other, yet to be discovered— Frasnian tetrapods may help resolve the bewildering array of primitive and derived characters found in the Famennian tetrapods.
The Geographic Gap
Nearly all of the early tetrapod record comes from Europe and eastern North America. In fact, much of what we know originated from only a few localities. For example, the two most complete Famennian tetrapods, Ichthyostega and Acanthostega, come from eastern Greenland, while most of the Viséan tetrapods come from Scotland.
Europe and eastern North America, once part of the paleocontinent Euramerica, will continue to yield important discoveries. For instance, Ted Daeschler, Neil Shubin, and Farish Jenkins —the discoverers of Tiktaalik— continue to investigate the extensive but poorly known Frasnian deposits of the Canadian Arctic. In addition, Jennifer Clack, Per Ahlberg, Henning Blom, and others continue to investigate materials from Greenland and the Baltic.
Nonetheless, major findings may someday come from outside of Euramerica. For example, Steve Scheckler and Brigitte Meyer-Berthaud recovered remarkably informative plant fossils (Archaeopteris) from a Frasnian locality in Morocco, which was once part of the northern margin of the paleocontinent Gondwana. This and other Moroccan sites may someday yield tetrapod remains.
One promising region for future discoveries is Australia (then part of eastern Gondwana. Australian investigators have already contributed a significant body of knowledge about marine and freshwater Devonian and Carboniferous vertebrates. They also have an early tetrapod record, which includes the fragmentary fossils of a Devonian tetrapod (Metaxygnathus), a fragmentary Viséan tetrapod, and what are probably Famennian-aged tetrapod trackways.
Another interesting region is China. It's been the source of a series of dramatic paleontological discoveries involving Cretaceous birds, dinosaurs and mammals, as well as two remarkable assemblages of Cambrian invertebrates. The recent discovery of a Famennian tetrapod jaw (Sinostega) from northwest China suggests future Devonian finds.
Familiar and New
Findings within the last two decades have dramatically enriched our knowledge of tetrapod evolution and the Devonian Transformation. Work by familiar names (e.g, Ted Daeschler, Jennifer Clack and Per Ahlberg) will certainly yield additional significant findings. New finds will also come from unfamiliar graduate students and other researchers. After all, where were these familiar names 20 years ago?
- Palaeos.com's (www.palaeos.com) web pages on aïstopods, anthracosaurs, baphetids, crassigyrinids, Greererpeton, and temnospondyls.
- Tree of Life's (tolweb.org) web pages on baphetids (loxommatids), Crassigyrinus and temnospondyls.
- Clack, J.A. 2002. Gaining Ground: The Origin and Early Evolution of Tetrapods. Bloomington: Indiana Univ. Press.
- National Geographic. May 1999. "From Fins to Feet."
- Scientific American. December 2005. "Getting a Leg Up on Land."
- Scientific Papers
- Clack, J.A. 1997. "Devonian tetrapod trackways and trackmakers: a review of the fossils and footprints." Paleogeography, Paleoclimatology, Paleoecology 130: 227-250.
- Clack, J.A. 1998. "A new Early Carbonferous tetrapod with a melange of crown-group characters." Nature 394: 66-69.
- Clack, J.A. 2002. "An early tetrapod from Romer's Gap." Nature 418: 72-76.
- Clack, J.A. and Finney, S.M. 2005. "Pederpes finneyae, an articulated tetrapod from the Tournaisian of Western Scotland." J. of Systematic Palaeontology 2: 311-346.
- Cloutier, R. and P.E. Ahlberg. 1996. "Morphology, characters, and interrelationships of basal sarcopterygians". pp 445-479. In: M.L.J Stiassny, L. R. Parenti, and G.D. Johnson. (eds.) Interrelationships of Fishes. San Diego, London: Academic Press.
- Daeschler, E.B. and N. Shubin. 1995. "Tetrapod Origins." Paleobiology 21(4): 404-409.
- Paton, R. L., T.R. Smithson and J.A. Clack. 1999. "An amniote-like skeleton from the Early Carboniferous of Scotland". Nature 398: 508-513.
- Smithson, T. R. 1985 "Scottish Carboniferous amphibian localities". Scottish J. of Geol. 21:123-142.