Toads to tapirs, turtles to turtle-doves, the diversity of land vertebrates, or tetrapods, seems enormous. Human beings are tetrapods, as are most of the animals with which we are familiar: we are descendants of lobe-finned fishes that evolved legs and subsequently came ashore, more than 360 m.y. ago.

This wide range of form conceals a deceptively simple tale of evolution. All modern tetrapods can be placed at the end-points of one or other of two lineages that diverged from each other long ago, not long after tetrapods first invaded the land -- or even, as a few have claimed, before that.

One lineage includes the amphibians. The amphibians we see today are the somewhat specialized remnants of a once large and diverse group of creatures. Modern amphibians are perhaps most closely related to a large, extinct group called temnospondyls, which included large, land-faring creatures as well as aquatic animals reminiscent more of alligators than newts. The other lineage includes the amniotes. The roots of the amniotes lie among another extinct group, the anthracosaurs.

Jennifer Clack of the University of Cambridge paints a picture of the tetrapod past in the 2 July 1998 issue of Nature. She describes fossils of a strange, salamander-like animal found in rocks at East Kirkton, in southern Scotland, that were laid down close to a tropical lake, around 335 Ma. There was active volcanoes in the area then, and there might even have been hydrothermal activity in the lake itself.

Yet animals and plants thronged on the lakeshore. The earliest-known anthracosaurs and temnospondyls, as well as the earliest-known amniote (Westlothiana lizziae, or "Lizzie") come from East Kirkton, as well as a good crop of giant scorpions, millipede-like animals and other creepy-crawlies. The picture must have been quite horrific‹these primeval monsters patrolling the boiling sulphurous scenery. Clack has given a fitting name to her subject: Eucritta melanolimnetes, the Creature from the Black Lagoon.

Eucritta is strange because it combines in the same animal features of the skull and skeleton that we would separately associate with temnospondyls, or with anthracosaurs. It seems to belong to a rather enigmatic, shadowy group of animals called the baphetids, until recently only known from skulls whose eye-sockets had a curious shape like an old-fashioned keyhole. It seems as if baphetids were tetrapods of the third kind: not temnospondyls, not anthracosaurs, but something different again.

Although it is simple to tell the difference between amniotes and amphibians nowadays, the members of these two lineages have had more than 340 m.y. of separate evolutionary history to call on. But when Eucritta was alive, the split between amniotes and amphibians was still quite new. Their evolutionary courses were yet to be written, and anthracosaurs, temnospondyls and baphetics looked very similar, adopting a rather generalized, salamandrine form.

While it was the destiny of amphibians and anthracosaurs to thrive and diversify, baphetids became extinct, perhaps before their full potential was realized. One can only guess at the forms that evolution would have wrought from their frames, had they, too, survived to the present day.

© Macmillan Magazines Ltd 1998 - NATURE NEWS SERVICE

Note: This item from the Nature News Service is mounted on this Web page by special permission of Nature.

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