Earliest tetrapods may have never left the water

According to most anthropologists, tetrapods were the first animals to ditch the sea for land. The group evolved from fishes, and once on land, they gave rise to amphibians, reptiles, birds and mammals.

New research suggests the transition from water to land was slow, and that the earliest tetrapods may have never abandoned the sea. In a new paper, published this week in the journal Nature, scientists describe a well-preserved tetrapod fossil unearthed in Russia, the earliest of its kind.

Most of what researchers know about tetrapods was gleaned from the Ichthyostega and Acanthostega genera, which spread across dry land at the end of the Devonian period, which ended some 360 million years ago. Ichthyostega and Acanthostega fossils are regularly found complete. But scientists have found fragments of older tetrapod species and tetrapod footprints dating back 390 million years.

The newly named Parmastega aelidae is the earliest tetrapod to show up whole in the fossil record. The fossil was found in limestone created by an ancient tropical lagoon. The animal's lateral line canals, sensory organs for detecting vibrations in the water, provided clues as to what life was like for the early tetrapod.

The canals appear well-developed along the jaw, snout and sides of the head, but they dissipate just before the creature's eyes. The pattern suggests the tetrapod spent a lot of time at the surface of the water, with its eyes just above -- like a crocodile.

Scientists suspect there were millipedes or sea scorpions to snatch from the water's edge. Crocodiles lurk with their eyes just above waterline to spot antelope and other large mammals coming for a drink.

While the creature's jaw structure is similar to later tetrapods -- adapted to scooping up prey on land -- its body pattern, specifically the design of its shoulder girdle, suggests its skeleton features a lot of cartilage, a sign the species likely never left the water.

Scientists used sophisticated statistical analysis to compare the newly discovered tetrapods to hundreds of other specimens. The results suggest Parmastega aelidae belonge to a sister group of tetrapods.

Researchers admit they can't know exactly how the new species of tetrapod behaved, but the evidence suggests the first tetrapods weren't exactly renegades -- more like careful prototypers. The first groups may have test piloted a variety of later tetrapod features, but never left the shallows.

"Far from presenting a progressive cavalcade of ever more land-adapted animals, the origin of tetrapods is looking more and more like a tangled bush of ecological experimentation," scientists concluded.

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