Think less sea monsters, more doting parents: the long-necked plesiosaurs that roamed the seas during the dinosaur era gave birth to live young. They probably cared for their offspring and may even have lived in large social groups, like modern-day whales.
Plesiosaurs were reptiles, which as a group tend to lay eggs rather than giving birth. Other prehistoric marine reptiles were known to be exceptions to that rule, but until now fossil evidence that plesiosaurs did the same has been frustratingly elusive. 鈥淧eople have looked and looked,鈥� says of Marshall University in Huntington, West Virginia.
Last year O鈥橩eefe was called in to help prepare a fossil plesiosaur for display in the Natural History Museum of Los Angeles County. Originally excavated by a Kansas family on their ranch in 1987, it had sat in the museum basement ever since, until , director of the museum鈥檚 dinosaur institute, decided to take a closer look.
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Baby on board
Cleaning up the fossil revealed that it was an adult female with a fetus inside it. O鈥橩eefe identified both as the same species, Polycotylus latippinus. This species had a very distinctive arm bone, making identification easy: 鈥淲e got lucky there,鈥� he says. There is also no evidence that the adult ate the juvenile, such as damage from stomach acid.
The evidence for pregnancy is 鈥渁bsolutely convincing鈥�, says plesiosaur expert , natural science curator at , a science museum in Birmingham, UK. 鈥淚t鈥檚 been a long time coming,鈥� he adds.
O鈥橩eefe says it鈥檚 not surprising that plesiosaurs gave birth: reptilian eggs are hard-shelled and must be laid on land, and plesiosaurs were too big to clamber out onto the shore.
Caring moms
What did come as a surprise was the finding of a single, large fetus. Based on the fossil and the developmental stage of the fetus, the expectant mother 笔.听濒补迟颈辫辫颈苍耻蝉 was 4.7 metres long, and the baby would have reached at least 1.6 metres if it had been born at full term. 鈥淎ll other Mesozoic marine reptiles had several small babies,鈥� O鈥橩eefe says. From this fossil, however, plesiosaurs appear to have had one big baby.
Other animals that give birth to single, large babies tend to provide them with a lot of care. They also commonly live in large social groups based around extended families. So O鈥橩eefe says plesiosaurs may have lived in pods, like modern whales.
That鈥檚 still speculation at this point, cautions Smith: there is no direct evidence of parental care or social living. However, he says there is some evidence that young plesiosaurs lived in shallow coastal waters, before heading out into deeper waters as adults. 鈥淵oung individuals may have lived together in pods,鈥� he says. 鈥淏ut none of this is empirically verified.鈥�
Social animals tend to be smarter than solitary ones, because of the cognitive demands of group living, so if plesiosaurs did live in groups they ought to show signs of intelligence. 鈥淭he problem is, they鈥檙e really stupid,鈥� O鈥橩eefe says: their skulls were quite small, without much room for brains.
Journal reference: , vol聽333, p聽870; DOI: 10.1126/science.1205689
