Wednesday, August 21, 2013

The Plating Game: Probable sexual dimorphism in stegosaurs

Famed (unfairly so) for their small intellectual potential and vast body size, as well as two rows of bony plates running down the back and a tail tipped with spikes, stegosaurs are among the most easily recognizable dinosaurs out there. They co-dominated the Jurassic Period alongside sauropods as the most abundant herbivorous dinosaurs in the world, stolidly grazing on fern prairies all across Laurasia, the northern landmass.

As famous and popular as they are, we know very little about stegosaur biology. Infant stegosaurs are known from a few extremely fragmentary remains, and eggs are practically, if not totally, unknown. However, a recent discovery (thanks to CT scanning) made by Evan Saitta, a senior at Princeton, has revealed that there is a surefire way to differentiate between male and female stegosaurs: by looking at their plates.

A male-female pair of stegosaurs in repose. Another male lingers in the background. Illustration by Connor Ross.
Many dinosaurs, especially ornithischians, are instantly recognizable by their outlandish anatomical features. It's true that no one pays as much attention to a relatively "bland" little hypsilophodontian than they to do the frills of ceratopsids, the crests of lambeosaurs, or, in this case, the plates of stegosaurs. These features are sometimes identified as "nametags" within species or thermoregulatory structures; however, it is now becoming more and more clear that these bizarre appendages were probably used in sexual display. Such structures are sexually selected, a process of evolution differing from natural selection*. Those individuals with larger, brighter, or generally more impressive ________ (frills, horns, antlers, plates, sails, etc.) will pass on those impressive genes to their offspring, making them irresistible to the opposite sex.

So, how does one tell the difference between male and female stegosaurs by solely examining fossils? It turns out that one gender sported broader, more ovoid plates than the other, which carried more taller, more slender plates. Which plates belong to which gender remains to be seen, but I wouldn't be surprised if the males had the wider, larger plates than the females. When it comes to sexual selection, this is generally the case; males sport larger and flashier features than females.

While other uses for stegosaur plates, including thermoregulation, have not been ruled out, and still may be very likely, but confirming at least one suspicion about the still-very-mysterious biology of stegosaurs is one step in the right direction to uncovering just how these animals lived.