When dinosaurs ruled the land, other groups of prehistoric reptiles dominated the waters. Their bones have also fossilised and they reveal much about how these ‘sea dragons’ lived. They tell us about the shape of their bodies, the things they ate and even how they determined their sex. And according to Aurélien Bernard from the University of Lyon, they can tell us whether these reptiles could control their body temperature.
The majority of reptiles are ‘cold-blooded’. Unlike mammals and birds, they can’t generate and retain their own heat, and their body temperature depends on their surroundings. But Bernard thinks that at three groups of marine reptiles – the dolphin-shaped ichthyosaurs, the crocodile-shaped mosasaurs, and the the paddle-flippered plesiosaurs – bucked this trend. Whether in tropical or cold waters, they could maintain a constant body temperature that reached as high as 35-39 degrees Celsius.
For humans and most other mammals, sex is a question is chromosomes. Two X chromsomes makes us female while an X and a Y makes us male. Birds use a similar but reversed system, where males are ZZ and females are ZW. But for reptiles, including crocodiles, turtles and many lizards, sex is determined not by genes, but by temperature.
In crocodiles, males hatch from eggs incubated at cooler temperatures while warmer conditions produce females. In turtles, it’s the other way around, and lizards use a variety of criteria including some very complicated combinations of genes, temperature and even size of egg.
But what did extinct reptiles do? It’s not exactly easy to tell for genes and temperature don’t fossilise. However, fossils can tell us about how such animals reproduced. For example, we know that three groups of marine reptiles – the serpentine mosasaurs, the dolphin-shaped ichthyosaurs, and the long-necked, paddle-flippered sauropterygians – gave birth to live young through a series of stunning fossils of pregnant females, and even one case of a birth in progress (see below). Around 20% of living reptiles (excluding birds) do the same and the ability probably evolved at least 100 times in lizards and snakes alone.
Chris Organ from Harvard University thinks that, in all three lineages of marine reptiles, the evolution of genetic sex determination preceded the evolution of live births. By studying the reproductive styles and sex-determining methods of 94 species of amniotes (mammals, birds and reptiles), he showed that the two traits often co-evolve. Live birth almost always depends on first having a gene-based method of assigning sex, while egg-layers can use either chromosomes or temperature.
Organ suggests that these important changes were instrumental for the success of these prehistoric swimmers, allowing them spread throughout the open oceans, where temperatures can’t be relied on to determine sex. And without the need to return to land to lay their eggs, they were free to live permanently at sea and evolve more extreme physical adaptations to an aquatic life, including paddle-like limbs, back fins, streamlined bodies and fluked tails.