Fishy Ancestors of Humans Surprisingly Diverse

June 25, 2008

The fishlike ancestors of humans and other land animals were a surprisingly diverse bunch, according to a new fossil reconstruction of the transitory species Ventastega curonica.

The aquatic creature, which lived during the late Devonian period about 365 million years ago, represented an evolutionary midpoint between Tiktaalik, one of the earliest fish to clamber onto land, and primitive four-legged land animals, or tetrapods.

"Ventastega gives clues to what the very earliest tetrapods looked like," said study leader Per Ahlberg of Uppsala University in Sweden.

Ventastega was first described from a few bone fragments unearthed in Latvia in 1994, but it took additional years of excavation and the discovery of remains from many more individuals before scientists had a good idea of what the creature looked like.

The latest portrait to emerge, from an especially well-preserved find, reveals an animal with a part-fish, part-tetrapod skull and a full-fledged tetrapod body. It would have spent the majority of its time on water and been clumsy on land.

While Ventastega in many ways fulfills scientists' expectations of what an early water-land transition animal should look like, it also shows substantial morphological differences from other similar species living around the same time.

These differences could help explain the seemingly swift rate at which later land animals diverged into separate species, scientists say.

"It's kind of remarkable that when we start to see true limbed animals, they've already diversified and are filling various special niches within this aquatic ecosystem," said Ted Daeschler, a paleontologist at the Academy of Natural Sciences in Philadelphia not affiliated with the new study.

The work is detailed in the June 26 issue of the journal Nature.

Third Example

Prior to Ventastega's discovery, scientists had characterized the earliest tetrapods mostly on fossils of two primitive examples: Acanthostega and Ichthyostega.

Both land creatures lived during the late Devonian. But they looked nothing like each other.

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