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A TIME-TRAVELLING dolphin, finding itself transported back 150 million years,
would be in for a shock. It would discover that the Jurassic oceans were teeming
with dolphin lookalikes. Admittedly, these creatures might be four times the
size of our temporally challenged cetacean, and have teeth that would put a
crocodile to shame. But so much else would be familiar: the streamlined body,
the shape of the tail—if not its orientation—the dorsal fin, even
the fact that the monsters gave birth in the water. Our visitor could be
forgiven for embracing the ichthyosaur as a long-lost relative.

That would be a mistake. Ichthyosaurs were reptiles, their closest living
descendants being snakes and lizards. But their striking resemblance to a modern
mammal is no coincidence. Dolphins and ichthyosaurs may not share a common
ancestor, but both are perfectly adapted to the same lifestyle. If you consider
evolution as a form of problem solving, then it is hardly surprising that some
solutions crop up more than once. “There are only so many ways you can do the
same job,” says Angela Milner of the Natural History Museum in London.

Finding that the dolphin has a prehistoric doppelgänger is pretty weird
but, at the level of individual body parts, convergence of form is ubiquitous in
nature. This phenomenon is at once the scourge of taxonomists trying to tease
out the relationships between species and a godsend to evolutionary biologists
who want to understand the lifestyles of long-extinct animals.

Taking the plunge

More than 245 million years ago the ichthyosaur’s terrestrial ancestor made
the plunge back into a watery world. With a body designed for walking on land,
getting around underwater would have been a major challenge. By shortening and
flattening the long bones of its limbs and extending and covering the existing
fingers and toes, evolution carved out a set of paddles front and rear. Fossils
of the earliest ichthyosaurs have five distinct rows of bones in each. Later,
the number varied from three to seven, with up to 100 individual bones in each
paddle. But ichthyosaurs needed more than oars and a streamlined shape if they
were to take their place as top predators.

The solution was obvious: ditch the reptilian tail and replace it with a
model with a good track record for speed. Why go back to the drawing board when
sharks had solved that problem millions of years earlier? With a little
tinkering, the ichthyosaur’s existing tail could be bent downward near the end
to support the lower fluke of its new tail fin. Constructing a scaffold for the
mirror image lobe on top would have been more tricky, but a blade of
cartilaginous material did the job. With another lump of cartilage on the back
to use as a stabilising dorsal fin—also similar to a shark’s—the
transformation was complete.

Life finds a way, as the saying goes. But there are limits to adaptability.
Ichthyosaurs never overcame the need to breathe air. Most of their fossil
remains are found in near-shore sediments suggesting that, like dolphins, they
stuck to shallow waters. Nevertheless, ichthyosaurs thrived for 140 million
years. No one knows why they became extinct 85 million years ago. But around 30
million years later the dolphin’s terrestrial ancestor made the plunge back into
a watery world…Who says history never repeats itself?