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SEX: Naughty female stegosauruses offered it on a PLATE

These beasts really DID have two backs

Differences in stegosauruses' armour plating may have had distinctly sexual origins, the University of Bristol's Evan Thomas Saitta suggests.

Sophie, the teenage Stegosaurus starlet on display at the Natural History Museum, has 19 plates along her back. These are quite thin, suggesting no particular utility in defence.

Dinoboffins think they could have been used to regulate her temperature, or possibly for mating displays - the latter claim being supported by Saitta's new paper.

Titled "Evidence for Sexual Dimorphism in the Plated Dinosaur Stegosaurus mjosi (Ornithischia, Stegosauria) from the Morrison Formation (Upper Jurassic) of Western USA", the study focused on every schoolboy's favourite dino, the stegosaurus, and specifically a collection of five individuals who were buried in a single quarry in Montana.

The site is believed to demonstrate co-existence (and possibly suggests sociability) between two different forms of Steggy that are only differentiable through the dimorphism in their plates.

"Without evidence for niche partitioning, it is unlikely that the two morphs represent different species," writes Saitta, who notes that the differences only occur in fully-grown individuals.

Based on what the dinoboffin considered the "display-oriented morphology" of the plates, it seems as if female mate choice was likely the driving evolutionary mechanism rather than male-male competition.

Saitta notes that dinosaur ornamentation could well have served similar functions to the ornamentation of modern species, and suggests comparisons with the ornamentation involved in sexual selection of species — such as the horns of bovids (everything from buffalo to sheep) — may be "appropriate in predicting the function of some dinosaur ornamentation".

The paper may cause a reappraisal of previous dino bones, with questions raised about whether creatures previously considered juvenile were actually something quite different. ®

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