Fishes With Legs
Until the 1980s, the fossil record of early tetrapods was essentially limited to Ichthyostega, a Late Devonian tetrapod from eastern Greenland. (Another Greenland form and an Australian form were known only from fragmentary remains.) But the early tetrapod record has expanded dramatically since 1987. Moreover, the fossil record of their fish ancestors has also been greatly enlarged in recent years. These enhanced records, together with findings from other scientific disciplines has engendered a new understanding of how tetrapods evolved. The first tetrapods are now seen as fishes with legs.
The new understanding also decouples a long-standing approach for thinking about this important chapter in vertebrate evolution. Instead of thinking of a transition between aquatic fishes and terrestrial tetrapods, the new understanding considers both the transition between fish and tetrapod and the transition between aquatic and terrestrial.
Perhaps the most important findings to alter our understanding of tetrapods evolution followed the discovery and analysis of Acanthostega in the late 1980s and early 1990s. A skull roof of this tetrapod was first discovered by Gunar Säve-Söderbergh and Erik Jarvik in 1933, but the significance of this animal was not realized until after additional material was recovered by Jenny Clack in 1987. Together with Michael Coates, Clack realized that this animal was clearly a tetrapod, but that it was a poor excuse for a land animal. Its legs were ill suited to support its weight and the wrists were absent. Yet, it sported well developed digits (fingers and toes). Surprisingly, the forelimbs possessed eight digits rather than the anticipated five digits, while the hindlimbs possessed seven.
Additional features from the spine, ribs, pelvis and tail corroborated the notion that Acanthostega would not be able to support itself on land. Since it was essentially contemporaneous with the apparently more terrestrial Ichthyostega, Acanthostega, like modern-day dolphins and whales, may have secondarily lost the skeletal features needed for life on land. This possibility, however, is undermined by the presence of internal fish-like gills. (It also breathed with its lungs.) Acanthostega, with its four limbs, pelvic girdle and assorted other features, was clearly a tetrapod, but it probably never left the water.
The unexpected characteristics of Acanthostega have led to reinterpretations of Ichthyostega by Per Ahlberg, Jennifer Clack and Michael Coates. They found an animal that was less adept to life on land than the one depicted by Erik Jarvik, but they also found one that was probably less aquatic than Acanthostega. Ichthyostega forelimbs were more robust than those of Acanthostega and were probably able to lift the front half of the body. However, its hindlimbs probably functioned more like paddles than legs. Like Acanthostega, it had seven digits on its hindlimbs, but it lacked evidence of internal fish-like gills.
The work on Acanthostega and subsequent work on Ichthyostega renewed interest in other Late Devonian tetrapods. It prompted a re-examination of already known tetrapods (e.g., Tulerpeton) and the discovery of new species (e.g., Hynerpeton). Moreover, the enhanced understanding of early tetrapod anatomy facilitated the re-evaluation of assorted museum specimens previously identified as lobe-fins (e.g., Elginerpeton and Ventastega).
The remains of these other Late Devonian tetrapods are considerably less complete than those of Acanthostega and Ichthyostega. Of these, Tulerpeton and Ventastega are the two best preserved. The other typically consist of a single or only a few isolated elements, although some of these elements are particularly informative (e.g., Hynerpeton and the Red Hill humerus).
In addition to the new findings on Late Devonian tetrapods, paleontologists have also learned more about their closest fishy relatives, Panderichthys and Elpistostege. These Late Givetian to Early Frasnian (early Late Devonian) fishes —also known as Elpistostegelians— share a variety of features with the early tetrapods. In fact, the fossil skull of one these species, Elpistostege, was originally classified as a tetrapod. But they differ from early tetrapods in several crucial ways.
Most recently (2006), the discovery of another tetrapod-like fish, Tiktaalik, has substantially enriched our understanding of the transition of lobe-fins to tetrapods. This species exhibit a number of characters intermediate between those of Panderichthyes and the Famennian tetrapods.
The span between these tetrapod-like fishes and the Devonian tetrapods such as Acanthostega and Ichthyostega is about 10-15 million years. But there's a gap of about 15 million years in the fossil record following the close of the Famennian, after which time the record shows a very diverse group of aquatic and terrestrial tetrapods.
(million years ago)
|Livoniana||Late Givetian/Early Frasnian||Latvia & Estonia|
|Tiktaalik||Early to Middle Frasnian||Nunavut, Canada|
|Elginerpeton||Late Frasnian (368)||Scotland|
|Obruchevichthys||Late Frasnian (368)||Latvia|
|Metaxygnathus||Late Frasnian to Late Famennian||New South Wales, Australia|
|Jakubsonia||Early Famennian||Oryol Region, Russia|
|Strud jaw||Late Famennian (365)||Belgium|
|Acanthostega||Late Famennian (363)||East Greenland|
|Ichthyostega||Late Famennian (363)||East Greenland|
|Tulerpeton||Late Famennian (363)||Tula Region, Russia|
|Hynerpeton||Late Famennian (361)||Pennsylvania, USA|
|Densignathus||Late Famennian (361)||Pennsylvania, USA|
|Red Hill humerus||Late Famennian (361)||Pennsylvania, USA|
|Sinostega||Late Famennian||Ningxia Hui, China|
|Tetrapod Trackways||Frasnian and Famennian||Australia, Ireland & Scotland|
Most early tetrapod remains have been recovered from Euramerica, a paleocontinent consisting of cratonic North America, Baltic Europe, northern Britain and European Russia. The only exceptions come from Australia (Metaxygnathus and suspected tetrapod trackways) which was then located on the far end of Gondwana, and China (Sinostega), which was located just north of the eastern part of Gondwana. Whether this biogeographic concentration is the result of sampling bias or other reasons is unknown. But all of the early tetrapod localities were tropical to subtropical. In addition, with the exceptions of Tulerpeton and possibly Jakubsonia, which were recovered from estuarine deposits, all of the Late Devonian tetrapods and the Elpistostegelians inhabited freshwater habitats.
These freshwaters and their surrounding landscapes had undergone a dramatic transformation. The harsh, infertile, unstable and unpromising freshwater and terrestrial environments of the Middle Silurian gave way to ones that were profoundly modified by the emergence and evolution of vascular plants. By the Late Devonian opportunity knocked and tetrapods answered.
- Discover Magazine Online: Coming Onto the Land.
- G.R. Morton's Transitional Forms: Fish to Amphibian:
- Clack, J.A. 2002. Gaining Ground: The Origin and Early Evolution of Tetrapods. Bloomington: Indiana Univ. Press.
- Janvier, P. 1996. Early Vertebrates. Oxford: Claredon Press.
- Long, J.A. 1995. The Rise of Fishes: 500 Million Years of Evolution. Baltimore and London: John Hopkins Univ. Press.
- Maisey, J.G. 1996. Discovering Fossil Fishes. New York: Henry Holt & Co.
- Zimmer, Carl. 1998. At the Waters' Edge. Touchstone Press.
- Discover Magazine. June 1995: "Coming Onto the Land".
- National Geographic. May 1999. "From Fins to Feet"
- New Scientist. 19 August 2000. "One Small Step for Fish, One Giant Leap for Us."
- 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.
- 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. Academic Press . San Diego, London.
- Daeschler, E.B. and N. Shubin. 1995. "Tetrapod origins". Paleobiology 21(4): 404-409.
- Long, J.A. and M.S. Gordon. 2004. "The greatest step in vertebrate history: a paleobiological review of the fish-tetrapod transition." Physiological and Biochemical Zoology 77(5): 700-719.