What do fossils tell us about sex and reproduction?
Fossils can help scientists identify many things about extinct organisms. Body fossils can provide evidence of what a species may have looked like, whether an individual suffered from disease, and how an organism developed.
However, identifying the behavior is much more difficult. Traces of fossils, such as preserved footprints and poo, can help identify how an animal may have moved or what it may have eaten, but other behaviors are more difficult to study.
Graduate student Sarah Losso, the study’s lead author, explains, “Once in a while you’ll get fossil specimens that died and were preserved during the act of copulation, while some insects were preserved during the act of copulation. ‘coupling. However, other than that, it is difficult to infer mating behaviors in extinct animals.
It can also be difficult to distinguish the sex of fossilized animals, complicating any study of reproduction. For trilobites, differences in size and morphology, such as the shape of the caudal plate (called the pygidium), have been suggested as examples of sexual dimorphism.
However, these features could also be representative of different species, particularly if the different morphs do not consistently appear together at different sites.
In 2016, the discovery of fossilized trilobite eggs suggested the presence of a brood pouch, where eggs and hatchlings can develop inside the parent’s body for a period of time.
Although usually a characteristic of female arthropods, brood care is also a characteristic of male spider crabs, so this characteristic cannot be used to identify sex with complete certainty. The new discovery of clasp-like appendages could help put the sex of trilobites beyond doubt for the first time.
“This study is important because the question of trilobite sexual dimorphism has not been clearly resolved,” says Greg. “This is the first instance where trilobite sexual dimorphism has not been recognized in exoskeleton or body size, but rather in appendage morphology.”