"Were the very first tetrapods like Ichthyostega or like Acanthostega or like neither?" study author Ahlberg asked.
Ventastega provides a valuable third example. Its features are similar to those of Tiktaalik and Acanthostega but do not resemble Ichthyostega's at all.
"This suggests that Ichthyostega is a specialized offshoot of early tetrapod evolution, whereas Ventastega and Acanthostega are closer to the main line of transition from water to land," Alhberg said.
Awkward Creature
Ventastega was likely not entirely comfortable in water or on land. But the animal already bore some of the adaptations that would allow its descendants to one day abandon the sea completely.
Its shoulder girdle, where the forelimbs attached to the body, had acquired the characteristic tetrapod shape, and the creature's pelvis was attached to the backbone, suggesting Ventastega's paired hind fins had already turned into limbs.
In addition, the shape of its head and the pattern of its teeth are intermediate between those of Tiktaalik and Acanthostega, hinting at a gradual change in feeding behavior.
Ventastega is one of about a dozen known fossil species representing the transition from fish to tetrapods, comparable to the number known for the evolution of land mammals into whales, scientists point out. (Related: "Whales Evolved From Tiny Deerlike Mammals, Study Says" [December 19, 2007].)
So researchers have a rough idea of the major evolutionary changes that took place but still have their work cut out for them when it comes to filling in the gaps.
Daeschler, of the Academy of Natural Sciences, compared studying the fish-to-tetrapod transition to building a house.
"We've got the frame built. We know what the rooms are shaped like," he said. "But we haven't put in the electricity, installed the lamps, or put Sheetrock on the walls."
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