tiktaalik skeleton

Tiktaalik roseae

Tiktaalik roseae is the most recent discovery in the study of early tetrapod evolution and arguably the most dramatic find in more than a decade. After several years of prospecting remote sections of Nunavut Territory in the Canadian Arctic, Ted Daeschler, Neil Shubin, and Farish Jenkins were able to report on three articulated and numerous isolated specimens of an elpistostegalian lobe-fin that exhibits a remarkable suite of characters transitional between the earliest tetrapods and their fishy precursors. Indeed, it has appropriately been dubbed a "fishapod".

Tiktaalik was a relatively large fish. The articulated specimens exceed a meter in length and extrapolation from isolated bones indicate that some individuals probably exceeded 2 meters. It vaguely resembles modern crocodilians with its flattened head and body, raised snout, closely-spaced eyes on the top of its head, limb-like fins, and bony scales. Its overall body form suggests that it spent —as do crocodiles— much of its time in shallow water.

As already mentioned, Tiktaalik exhibits a variety of transitional features. Its skull more closely resembles those of early tetrapods than does Panderichthyes, another elpistostegalian long regarded as a transitional form between tristocopterid lobe-fins and tetrapods. Proportionally its snout is longer while the post-orbial region is shorter than it is in Panderichthyes. The relationships of individual skull bones are also more comparable to tetrapods than to its fellow elpistostegalian. On the other hand, the lower jaw and revealed aspects of its palate more closely resembles Panderichthyes; much of the palate in the Tiktaalik specimens is presently obscured by lower jaw and branchial elements and the braincase is yet to be studied.

The branchial skeleton of Tiktaalik is well developed, indicating that it internal gills were important for respiration. But, its spiracular notch —an element scientists suspect augmented respiration in early tetrapods— is more developed than it is in Panderichthyes. More striking, however, is the loss of bony gill coverings in Tiktaalik. The loss of these elements effectively separated the pectoral girdle from the skull. In other words, Tiktaalik —like tetrapods— had a neck.

Except for the absence of a bony connection to the skull, the pectoral girdle of Tiktaalik is more similar to that of Panderichthyes than it is to either tristocopterid lobe-fins or early tetrapods. The pectoral appendage, with its ray-like lepidotrichia, evidently functioned as a fin, although the lepidotrichia are conspicuously shorter than those in Panderichthyes. The internal, endochondral, skeleton, however, suggest several proto-tetrapod characteristics. It's well-developed and contains some distal elements that are homologous with those in tetrapods, but it also contains others that are more comparable with elements in other lobe-fins. Strikingly, it had a series of distal joint planes that suggest a functional analog to the wrist. Tiktaalik could probably prop itself up of the bottom with its pectoral fins. They were almost, but not quite limbs.

The axial skeleton is unusual. The vertebrae of two of the three articulated specimens were unossified. Those in the third ossified specimen suggest some ossification of the centra, but this will require further study. The ribs, on the other hand are well-ossified and much larger than they are in other lobe-fins and are even more-developed than they are in the tetrapod Acanthostega. Moreover, they exhibit proximal expansion of the caudal margin that results in a slight overlap with the next rib. This imbrication is comparable to that found in another tetrapod, Ichthyostega, although the ribs of the two animals differ considerably in detail; it's been suggested that the overlap of ribs in both Ichthyostega and Tiktaalik served to provide extra trunk support in the absence of buoyant water.

The spine of Tiktaalik is also unusual in that it contains considerably more pre-sacral vertebrae (~ 45) than either Acanthostega (~ 30) or Ichthyostega. The pelvic girdle and fin are poorly known.

Tiktaalik was recovered from sediments of the Fram Formation. This formation is interpreted to be the deposits of meandering lowland streams. Palynomorph investigations indicate that this formation formed in the early and middle Frasnian (early Late Devonian). Associated fauna includes an antiarch placoderm (Asterolepis), porolepiform lobe-fins (including Laccognathus), as well as osetolepidid and tristichopterid lobe-fins.

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Web:
National Geographic Society's web page on Tiktaalik:
news.nationalgeographic.com/news/ 2006/04/0405_060405_fish.html
National Science Foundation's (US) web page on Tiktaalik:
www.nsf.gov/news/news_summ.jsp?cntn_id=106807
University of Chicago's web page on Tiktaalik:
tiktaalik.uchicago.edu/
Scientific Papers
Ahlberg, P.E. and J.A. Clack. 2006. "A firm step from water to land." Nature 440(6): 747-749.
Daeschler, E.B., N.H. Shubin, and F.A. Jenkins, Jr. 2006. "A Devonian tetrapod-like fish and the evolution of the tetrapod body plan." Nature 440(6): 757-763.
Shubin, N.H., E.B. Daeschler, and F.A. Jenkins, Jr. 2006. "The pectoral fin of Tiktaalik roseae and the origin of the tetrapod limb. Nature 440(6): 764-771.

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