A fish-quadruped missing link

Rich as the fossil record is, it is terribly incomplete, for the obvious reason that the chance of preservation over fragmentation and destruction of body parts is extremely small. That is especially the case for the high-energy and oxidising land and freshwater environments. Each fossil species can easily be assumed to be a one-off, appearing, thriving for a short while and then disappearing: ripe for the assumption of divine creation, as Linnaeus assumed. Very rarely indeed, specimens emerge that fill in the many gaps needed by evolutionary theory, the most celebrated being Archaeopterix that bridged the gap between dinosaurs and birds. That transition has been enriched by a whole series of older fossils from Chinese lagerstätten that show the transition in sublime detail.

The comparative anatomy of fish and land vertebrates suggests a common ancestry, and the Devonian to Early Carboniferous terrestrial record has yielded tantalising fish with lobed fins (e.g. Eusthenopteron and Panderichthys) and almost fish-like animals with four rudimentary limbs (e.g. Acanthostega and Ichthyostega). Yet a gap remained to be filled in the apparent transition from aquatic to land-dwelling vertebrates. US palaeobiologists engaged in seeking candidates from the Late Devonian of Arctic Canada have found one that reduces any uncertainty tremendously (Daeschler, E.B et al. 2006. A Devonian tetrapod-like fish and the evolution of the tetrapod body plan. Nature, v. 440, p. 757-763. Shubin, N.H. et al. 2006. The pectoral fin of Tiktaalik roseae and the origin of the tetrapod limb. Nature, v. 440, p. 764-771). The fossil, prepared with lengthy and painstaking care, shows such amazing anatomical detail as to demonstrate clearly that the fin and shoulder girdle are indeed intermediate between fish and tetrapods, whereas previous candidates supporting a transition are either definitely fish or tetrapods. Tiktaalik slots nicely into the time gap too, about 2 Ma younger than the most tetrapod-like fish Panderichthys and slightly older than fish-like quadrupeds. The outcome of a deliberate search for an animal to fit the gap, Tiktaalik above all demonstrates the predictive capacity of palaeontology, which counters a common epithet flung by those bent on divine intervention and/or intelligent design. Based on this outstanding success, fossil hunters will be encouraged to sift on a stratigraphically finer scale for yet more steps in vertebrate evolution, including our own.

See also: Ahlberg, P.E. & Clack, J.A. 2006. A firm step from water to land. Nature, v. 440, p. 747-749.


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