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ScienceWeek
PALEONTOLOGY: ON THE FISH-TETRAPOD TRANSITION
The following points are made by Jennifer A. Clack (Science 2004 304:57):
1) How did animals get from fins to fingers, from an animal that swims in water to one that walks on land? This key evolutionary innovation was apparently first made during the Devonian period, approximately 370 to 360 million years ago, and it can be succinctly described as the "fish-tetrapod transition". In recent years, the fossil evidence for this event has increased almost exponentially in quality and quantity, stimulating fresh ideas and changing perspectives (1).
2) Shubin et al (2) revealed how even fragmentary finds can be used to draw inferences about the nature and sequence of changes that must have taken place during the evolution of terrestrial locomotion by tetrapods. Shubin et al (2) describe a humerus bone from the Devonian period that they discovered in Pennsylvania. The morphology of this limb bone proclaims it to be not only from a tetrapod, but from one predicted to be of a completely new and unusual form. From this humerus, the researchers infer some of the anatomical and functional changes that took place in the tetrapod lineage during the Devonian period, and these changes in turn suggest some possible routes by which terrestriality was attained.
3) The lobe-finned relatives of early tetrapods had complex internal skeletons and a suite of muscles in their paired fins, features eventually exploited to produce weight-bearing limbs. Among the immediate relatives of tetrapods, the most proximal three elements of their limbs can be readily identified as related to the humerus, radius, and ulna of the forelimb, and the femur, tibia, and fibula of the hindlimb. The humeri of Devonian tetrapod-like fish such as Eusthenopteron and Panderichthys bear particular resemblance to those of early tetrapods, and several key features are common to them all. However, fins and limbs are conspicuously different with respect to orientation, range of movement, and function. The new humerus found by Shubin et al (2) provides some clues as to the timing and sequence in which these differences arose, although it poses yet more intriguing questions.
4) The new humerus, like those of other Devonian tetrapods and their close relative Panderichthys, is flattened dorsoventrally, and the shoulder joint appears to have had a greatly restricted range of movement. However, in Panderichthys, as in other fish, the fin and shoulder joint face posteriorly, whereas in tetrapods they are reoriented to face laterally. As a result of this reorientation, the attitude of the limb to the body is essentially horizontal rather than vertical; the operational space in which the limb acts is level with the shoulder joint rather than posterior to it; and the direction in which its muscles pull is approximately at right angles to the body rather than at an acute angle to it. Shubin et al (2) suggest that features such as the orientation of a ridge on the ventral side of this new humerus show that some of these changes had already taken place. The net result was probably a limb that was effective in the dorsoventral plane and could prop up the relatively large plated head of an early tetrapod, but whose front-to-rear movement, necessary for walking, was very limited.
References (abridged):
1. J. A. Clack, Gaining Ground: The Origin and Early Evolution of Tetrapods (Indiana Univ. Press, Bloomington, IN, 2002)
2. N. H. Shubin, E. B. Daeschler, M. I. Coates, Science 304, 90 (2004)
3. E. Jarvik, Fossils Strata 40, 1 (1996)
4. J. A. Clack, H. Blom, P. E. Ahlberg, J. Vertebr. Paleontol. 23, 41A (2003)
5. J. E. Jeffery, Biol. J. Linn. Soc. 74, 217 (2001)
Science http://www.sciencemag.org
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Related Material:
ON EARLY LAND VERTEBRATES
The following points are made by Robert Carroll (Nature 2002 418:35):
1) The transition between fish and land vertebrates was a turning point in the history of life. Early stages in the evolution from aquatic lobe-finned fish to tetrapods -- animals with limbs capable of locomotion on land -- are seen in many fossils from the Upper Devonian(1), just before 363 million years ago. In contrast, few remains are known from the next 30 million years, when the ancestors of the major tetrapod lineages differentiated from one another, an interval that falls in the early part of the Carboniferous period. So striking is the hiatus that Coates and Clack(2) coined the term "Romer's Gap" for it, in reference to the long search of Alfred Sherwood Romer (1894-1973) for fossils dating to that time. It is fitting that Clack (3) should be the first to recognize a well-preserved specimen of a new amphibian species from Romer's Gap. The fossil, Pederpes finneyae, comes from 350-million-year-old deposits at Dumbarton, Scotland.
2) Although the full length of the tail of Pederpes is not known, the animal was probably nearly a meter in length. It was a short-limbed, large-skulled predator, resembling an especially ungainly crocodile. But it almost certainly reproduced in the water, somewhat like modern aquatic salamanders. Grooves in the skull for lateral-line canals ù a characteristic of fish ù suggest that it lived partly in the water. The foot structure, however, indicates it could walk on land. Pederpes is advanced over its Devonian antecedents in having only five toes on the foot, yet has a relict of a tiny finger on the forelimb reminiscent of the supernumerary digits of the best-known amphibians -- Ichthyostega and Acanthostega -- from the Upper Devonian.
3) Discovery of a nearly complete skeleton in the middle of Romer's Gap should help in establishing the pattern of evolutionary change among early tetrapods. It might also provide context for understanding the interrelationships of all later land vertebrates. Clack demonstrates that Pederpes and other members of the Whatcheeriidae -- the family to which it is assigned -- occupied an intermediate grade between the primarily aquatic Upper Devonian amphibians and later tetrapods. In particular, the feet show major advances towards effective locomotion on land. On the other hand, whatcheeriids have no skeletal features that indicate specific affinities to either of the major groups of "conservative" amphibians, the temnospondyls or anthracosaurs, that dominated the later Carboniferous.(4,5)
References (abridged):
1. Clack, J. A. in Amphibian Biology (eds Heatwole, H. & Carroll, R. L.) 979-1029 (Surrey Beatty, Chipping Norton, Australia, 2000)
2. Coates, M. I. & Clack, J. A. in Studies on Early Vertebrates (eds Arsenault, M., Leliþvre, H. & Janvier, P.) 373-388 (Bulletin du Museum National d'Histoire Naturelle, Paris, 1995)
3. Clack, J. A. Nature 418, 72-76 (2002)
4. Carroll, R. L. in Amphibian Biology (eds Heatwole, H. & Carroll, R. L.) 1198-1269 (Surrey Beatty, Chipping Norton, Australia, 2000)
5. Anderson, J. S. Systematic Biol. 50, 170-193 (2001)
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Related Material:
AN EARLY TETRAPOD FROM "ROMER'S GAP".
The following points are made by J.A. Clack (Nature 2002 418:72):
1) The fossil record of early tetrapods has been increased recently by new finds from the Devonian period and mid-late Early Carboniferous period. Despite this, understanding of tetrapod evolution has been hampered by a 20-million-year gap ("Romer's Gap") that covers the crucial early period when many key features of terrestrial tetrapods were acquired.
2) The author describes the only articulated skeleton of a tetrapod, Pederpes, yet found from the Tournaisian epoch (354-344 million years ago (Myr)). The new taxon includes a pes with five robust digits, but a very small, possibly supernumerary digit preserved on the manus suggests the presence of polydactyly. Polydactylous early tetrapods may have survived beyond the end of the Devonian and pentadactyly cannot be assumed for the pes. However, the pes has characteristics that distinguish it from the paddle-like feet of the Devonian forms and resembles the feet of later, more terrestrially adapted Carboniferous forms.
3) Pederpes is the earliest-known tetrapod to show the beginnings of terrestrial locomotion and was at least functionally pentadactyl. With its later American sister-genus, Whatcheeria, it represents the next most primitive tetrapod clade after those of the Late Devonian, bridging the temporal, morphological and phylogenetic gaps that have hitherto separated Late Devonian and mid-Carboniferous tetrapod faunas.
Nature http://www.nature.com/nature
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