Belemnites had a strong bullet-shaped guard on the end of their shells, which most likely acted as armour to protect against being eaten by the giant marine reptiles that lived at the time. The elegant coiled ammonites, also found in Yorkshire, but more famously in Dorset, are frequently found in male and female forms. A single bed of rock may contain identical small ammonites, which probably represent juveniles. Then, in larger specimens, there are two types of outer whorls. Some are very large and plain, while others are smaller and ornate, with strange protuberances at the edge of the shell.
Most palaeontologists consider the larger shells to have been female and the smaller ones male.
He has very recently discovered what appear to be eggs in the outer chamber of a larger conch, adding credence to the idea of a large dominant female, attended by peacock-like but diminutive males. While this sounds like an interesting, if not rather attractive, form of social biology, Steve Etches, along with all other palaeontologists, is quick to point out that the ammonites were wiped out along with the dinosaurs 65 million years ago. One might imagine that the complexity of sexual activity and behaviour would have increased over geological time and that sexual traits would be less obvious in more ancient fossils. Under the microscope, some rocks from this period reveal the shells of small creatures called ostracods, related to the daphnia used to feed goldfish today. According to David Siveter, of Leicester University, these tiny crustaceans probably sported the most ostentatious sexual apparatus of all time.