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Fossil helps document shift from sea to land

Fossils yield insight into early tetrapod evolution

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12:07pm, June 25, 2008
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New fossils of an ancient, four-limbed creature help fill in the blanks of the evolutionary transition between fish and the first land-adapted vertebrates.

Fossils of creatures that span the water-to-land transition of vertebrates are few and far between. One of those pioneers, Ventastega curonica, was first described in 1994 but previously has been known only from fragmentary remains unearthed from 365-million-year-old rocks at a site in western Latvia. Fossils found at the site during subsequent excavations now allow scientists to more fully reconstruct the creature, says Per Ahlberg, a paleontologist at Uppsala University in Sweden.

The new remains — including most of the creature’s skull, the braincase, half of the bones in its forelimb and a quarter of its pelvic girdle — suggest that Ventastega was an evolutionary intermediate between Tiktaalik, a four-limbed fish that lived about 382 million years ago (SN: 6/17/06, p. 379), and subsequent tetrapods such as Acanthostega, which were capable of walking on land.

The size and proportions of the new fossils hint that Ventastega probably measured between 1 and 1.3 meters in length. Most features of the creature’s skull match those of Tiktaalik, which lived millions of years earlier, but the overall shape of the skull and braincase “is characteristically ‘early tetrapod,’” Ahlberg says. Likewise, the creature’s lower jawbone was shaped like that of early tetrapods but was adorned with fangs like those found in its fishy predecessors, he notes. “Ventastega was a mosaic of features.”

Ventastega lived approximately during the same era as Acanthostega, but its features were more primitive, a sign that Ventastega may have been an evolutionary holdover, Ahlberg says. Nevertheless, the size and shape of Ventastega’s limb bones, particularly those of its forelimbs, suggest that the creature’s limbs ended in digits, not fins.

The fossil record suggests that the evolutionary transition between fish and early tetrapods was smooth. Over millions of years, these creatures’ eyes grew larger and their snouts became broader while the overall size of the skull decreased somewhat, Ahlberg and his colleagues report in the June 26 Nature.

The new fossils of Ventastega “are great,” says Neil Shubin, a paleontologist at the University of Chicago. Although the newly described remains include just a few bones, “they’re very informative,” he adds. The earliest tetrapods probably evolved between 5 million and 7 million years before Tiktaalik, he notes, and the new fossils will help researchers predict what those creatures would have looked like.

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