Mother and child

THE Mesozoic land was dominated by dinosaurs. At sea, though, the most abundant reptiles were the ichthyosaurs and plesiosaurs. Roughly speaking, these animals filled the ecological niches now occupied by toothed cetaceans such as dolphins and killer whales. Ichthyosaurs, indeed, looked somewhat like dolphins, though plesiosaurs—with their long necks and diamond-shaped paddles—were unlike anything now alive.

Ichthyosaurs also resembled cetaceans in another way: unlike most living reptiles, which lay eggs, they gave birth to live young. For years, palaeontologists have wondered if the same was true of plesiosaurs. Now they have found out that it was—but with an interesting twist.

Robin O'Keefe of Marshall University in West Virginia has analysed a plesiosaur fossil found in Kansas in 1987, which palaeontologists had suspected was a pregnant female about 4.7 metres (15 feet) long, but which had not been cleaned up and studied until last year. As he reports in Science, he found an array of tiny bones, apparently belonging to a small specimen of the same species, in the fossil's abdominal cavity (see above; the extra bones are highlighted). It is unlikely that these were the remains of a meal because the bones are not broken down in the way that would be expected if they were partly digested. Instead, Dr O'Keefe believes he has discovered evidence that plesiosaurs, too, gave birth to live young.

What is more, all the baby bones come from a single individual, estimated to have been 1.5 metres long. Ichthyosaurs and other contemporary viviparous species, by contrast, gave birth to multiple offspring. The fetus's level of development indicates that it was at most two-thirds mature. Had it survived to birth it would have been about 1.8 metres in length, and about one and a half times as heavy (relative to parental weight) as the offspring of other viviparous species of the time.

That it was so heavy, and also alone, is of great interest. Georges Cuvier, an early palaeontologist, made his reputation by predicting the anatomies of newly discovered fossil species from scant evidence, such as single bones. He did so by applying to fossils the principles of comparative anatomy, asking what light the body shapes of living animals could cast on the shapes of creatures from the past. Many modern palaeontologists try to do something similar, except that what they reconstruct is behaviour.

Dr O'Keefe has performed such an analysis on his find. He starts from a fundamental observation about reproduction: you can go for quantity or quality. Having one child at a time is the ultimate expression of quality. It implies huge parental investment in the offspring since, if you lose it, you lose everything. Often, too, it implies membership of a social group, within which favours can be traded to spread the load of parenthood. Most speculatively, it might even imply a degree of intelligence—for the most intelligent mammals and birds are generally those that live in groups.

All this is a lot to load on a single fossil, of course. But it would make sense. It would mean plesiosaurs not only occupied a similar ecological niche to whales, but behaved like them, too.