Water and air pose completely different challenges to animals on a number of levels.

Only animals that evolved to live solely on land ever developed claws. The earliest vertebrates -- fish and amphibians — never developed hard nails and remained dependent on watery environments ...

How Ancient Reptile Footprints Are Rewriting the History of When Animals Evolved to Live on Land WASHINGTON (AP) — Scientists in Australia have identified the oldest known fossil footprints of a ...

Only animals that evolved to live solely on land ever developed claws. The earliest vertebrates -- fish and amphibians – never developed hard nails and remained dependent on watery environments ...

Only animals that evolved to live solely on land ever developed claws. The earliest vertebrates — fish and amphibians – never developed hard nails and remained dependent on watery environments ...

Only animals that evolved to live solely on land ever developed claws. The earliest vertebrates -- fish and amphibians – never developed hard nails and remained dependent on watery environments ...

Only animals that evolved to live solely on land ever developed claws. The earliest vertebrates — fish and amphibians – never developed hard nails and remained dependent on watery environments ...

Only animals that evolved to live solely on land ever developed claws. The earliest vertebrates -- fish and amphibians – never developed hard nails and remained dependent on watery environments ...

WASHINGTON (AP) — Scientists in Australia have identified the oldest known fossil footprints of a reptile-like animal, dated to around 350 million years ago.. The discovery suggests that after the ...