Pregnant plesiosaur with giant foetus hints at caring parents

By ED YONG - NOT EXACTLY ROCKET SCIENCE
Added: Thu, 11 Aug 2011 22:50:52 UTC

In 1987, Charles Bonner discovered the fossilised bones of a large sea reptile on his family ranch. It was a flipper-limbed plesiosaur, probably Polycotylus, and one of many such fossils recovered from Logan County in Kansas. But this specimen was special – there was a smaller one inside it. This plesiosaur was pregnant.

The Bonners donated the find to the Natural History Museum of LA County, where it languished for years. No one had the resources to prepare and study it until Luis Chiappe decided to include the specimen in the new exhibit halls he was preparing. He brought in Robin O’Keefe from Marshall University to analyse the historic find.

The mass of small bones inside the adult is haphazardly arranged, but O’Keefe and Chiappe think that it was clearly a foetus. The skeleton shows signs of incomplete growth, as it includes the distinctive forearm bone of Polycotylus. It shows no signs of having been chewed or exposed to stomach acid, so it wasn’t a youngster that the adult had cannibalised.

O’Keefe and Chiappe’s discovery suggests that this reptile gave birth to live young. This isn’t that surprising. Most modern reptiles lay eggs but many groups have abandoned this strategy in favour of live births. Throughout reptile history, live births have evolved has happened no fewer than 80 times, and that’s just among living species. Several extinct groups also gave birth in the same way, including the fearsome mosasaurs and the dolphin-like ichthyosaurs. We know this because people have found fossils of pregnant females, and a particularly extraordinary one of an ichthyosaur killed in the act of giving birth.

But until now, despite hundreds of fossils and over 200 years of collection, no one had found a pregnant plesiosaur. Even their close relatives, the nothosaurs, have yielded a pregnant specimen even though we have far fewer nothosaur fossils. The British palaeontologist Harry Seeley claimed to have found a plesiosaur embryo in 1895, but when others examined the specimen, it became clear that he had actually found fossilised shrimp burrows! The Kansas plesiosaur finally fills in that longstanding gap.