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