Identity plates

IN MANY living species, the sexes look different from one another. Presumably that was true in the past, too, but knowing for certain is hard because pigments and soft tissues that might be sexually dimorphic are rarely preserved in fossils. Among dinosaurs in particular, even the durable fighting parts more usually sported by males than females have proved a disappointment to those seeking dimorphism. Famously horned dinosaurs such as Triceratops show no convincing evidence of such differences. Only the hadrosaurs, with their distinctive cranial crests (now known to have been horns of an acoustic rather than a combative variety), seem to differ between males and females.

If Evan Saitta of Bristol University, in England, is right, however, another well-known dinosaur was indeed sexually dimorphic. Mr Saitta has been investigating Stegosaurus, a Jurassic beast that sported rows of bony plates on its back. He worked on one particular species, Stegosaurus mjosi, of which five specimens have recently been unearthed in Montana to join the half dozen previously known. These finds show that Stegosaurus plates come in two shapes: broad, oval ones and tall, narrow ones about a third smaller than the oval ones. Mr Saitta’s study, just published in PLOS ONE, nails down the question of whether the different sorts of plate were sported by different sexes.