DUBLIN: A pregnant plesiosaur fossil has confirmed that these predatory marine reptiles gave birth to single, large offspring – contrary to other marine reptiles.
Reptilian reproduction typically involves a large number of small babies, but the fossil evidence says that plesiosaurs did things differently, according to Robin O’Keefe of Marshall University in West Virginia, co-author of a new paper published in Science today.
“Plesiosaurs were apparently more like mammals and social lizards, giving birth to one or a few large young. We speculate that this may indicate that plesiosaurs displayed maternal care and social behaviour,” he said.
No evidence for live birth had been found in plesiosaurs despite an excellent fossil record and a collection history spanning almost 200 years. Plesiosaurs had sometimes been portrayed crawling out of water to lay eggs in the manner of sea turtles, but experts had long suspected that their anatomy was not compatible to movement on land.
A plesiosaur fossil found on a Kansas ranch in 1987 has now confirmed their suspicions. The fossilised bones of the mother plesiosaur, which measured over 4 m long, enclosed a 1.5 m unborn young.
Other marine species such as whales and dolphins that give birth to large young care for them extensively. O’Keefe and his colleague, museum palaeontologist Luis Chiappe, suggest that plesiosaurs may have displayed similar behaviours on the basis of their ecological and reproductive similarity.
“This paper will change the way we think about plesiosaur reproductive biology, and will inform how we speculate how they lived socially. The paper gives us some of the first hard evidence we’ve had to start to answer some of the questions,” said O’Keefe.
The pregnant plesiosaur fossil dates from the Late Cretaceous and is a large, relatively short-necked species belonging to the genus Polycotylus. Other marine reptiles of the time had a more typical pattern of reptilian reproduction involving a larger number of small babies, noted O’Keefe. Plesiosaurs were highly adapted for swimming and are the only known vertebrate organisms to possess two pairs of fins used for propulsion.
The paper suggests that plesiosaurs did not lay eggs and spent their entire life cycle in the water. Giving birth to a single, large individual is a reproductive strategy often associated with gregarious behaviour and parental care, so the speculation is that perhaps plesiosaurs made for excellent parents too.
“Many marine reptiles gave birth to live young, but this study makes plesiosaurs the odd ones out when it comes to reproductive behaviour,” said Adam Smith, a plesiosaur expert and curator at Thinktank, Birmingham Science Museum in the UK. “The hypothesis that plesiosaurs may have behaved more like whales and engaged in parental care is fascinating and would be quite unusual for reptiles.”
A baby, not a meal
Michael Benton of the University of Bristol in the UK described it as “an excellent chance discovery that proves, at last, that plesiosaurs, like ichthyosaurs, bore live young”. Ichthyosaurs were dolphin-like marine reptiles.
Benton noted that the authors carefully argue that the small specimen in the stomach is a foetus and was not consumed by the larger animal. “I believe they are right,” he said. “The important biological consequence, and what was unexpected, is that plesiosaurs, unlike ichthyosaurs, bore limited numbers of young, and so presumably invested much effort in their nurture.” This is the so-called K-selected strategy seen also in mammals.
The fossil plesiosaur described had a relatively short neck with a large head and long snout which was lined with teeth. “Polycotylus was one of the last types of plesiosaurs to evolve. It was a highly derived swimming reptile with a torpedo-shaped body outline and four wing-like flippers,” said Smith. “It would have been a very fast and agile swimmer.”
The fossil was donated by the Bonner family to the Natural History Museum of Los Angeles. O’Keefe noted that the fossil was large and complex which made for a big and expensive job in terms of detailed preparation. The impetus came from Chiappe, he said, who made the decision to include the fossil on display in the brand-new exhibit halls at the Natural History Museum in LA County. The new hall opened in July this year.
Plesiosaurs are an intriguing group of extinct reptiles. All were carnivorous and many were top predators in their ecosystem. Some looked similar to the fabled Loch Ness Monster of Scotland. They are unrelated to the dinosaurs but died out at the same time. Plesiosaurs left no living descendants.
Original paper in Science
Footage showing the Plesiosaur exhibit at the Natural History Museum
Robin O’Keefe homepage
Luis Chiappe homepage