Dinos: Another look for "you" and "him"

Stegosaurs provide evidence of sex dimorphism in dinosaurs

Stegosaurus skeleton - striking are the back plates. © thinkstock
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The back plates have betrayed it: Even with dinosaurs, there were external differences between males and females. The first evidence of such a sexual dimorphism is now provided by the backplates of Stegosaurus fossils discovered in Utah. Some of these animals have large, broad plates, while others carry smaller, narrower ones. Since all animals are the same age and belong to the same species, gender differences are the most valid explanation.

Males and females look different - this is almost normal in today's animals. However, proving this sexual dimorphism in extinct species is difficult. Because often only a few fossil specimens of a species are known, sometimes even only parts of a skeleton. And if two finds differ in one or a few characteristics, then this could also be because they are simply two different species or a juvenile and an adult.

Five new Stegosaurus skeletons

"For this reason, there is still no clear evidence for a sexual dimorphism in the dinosaurs, " said Evan Saitta from the University of Bristol. But now the paleontologist has come to the aid of excavation. For in the US state of Montana he and his colleagues came across five new specimens of the species Stegosaurus mjosi. Typical of these dinosaurs are a series of upright plates on the back and two pairs of long spines on the tail tip.

Further studies showed that all five individuals died about 150 million years ago at the same time, they lie close together in the same layer of rock, as the researchers reported. The special feature: some of these stegosaurs have broad, rounded back plates, while the others are smaller and rather elongated

Probably the Stegosaurus males were those with the larger back plates © Evan Saitta

At the age it is not

In order to find out what lies behind these differences, Saitta was almost detective: he analyzed both the physiological age and other skeletal characteristics of the fossil so all "suspects" speak explanatory to test for these differences. As he explains, the absence of intermediate forms and sizes argues that it is a non-gendered intraspecific variation. "Then one would expect such intermediate forms, " ​​says Saitta. display

Because both variants also occur in clearly adult animals, it can not be age-related differences. "Plates of both forms can be found in both sexually mature, young adults and in older adults, " says the researchers. "One form can not simply be the immature variant of the other."

No different types

And the wide and narrow backplates were not part of two different types of stegosaurs, as Saitta explains. "The position of the finds indicates that the individuals died together, " says the researcher. This suggests that they also coexisted with each other during their lifetime and probably formed a social group.

If they belonged to two different species and still occur in the same place at the same time, then there should be further differences in the fossils. Because such a coexistence of different species only works if both realize a slightly different niche for example by eating a different food. "In such a case, we would expect features in the fossils that reflect this incorporation, " says Saitta. "But instead, the only difference was between the fossils in the backplates."

Presumably, the stegosaurs were not the only dinosaurs with sexual dimorphism - here a Stegosaurus model from the Dino Park M nchehagen German Ole / CC-by-sa 3.0

Clear case of sexual dimorphism

In the opinion of Saitta, this leaves only one conclusion: The stegosaurs with the various backplates are male and female and thus the first clearly documented case of sexual dimorphism in dinosaurs. "If all alternative hypotheses are excluded, then sexual dimorphism is the most likely explanation, " says the researcher.

While it's not yet clear who wore which plates, Saitta dares to say, "Since males typically invest more in their ornamentation, the wider, larger plates are likely to come from males." The striking spine comb might impress them and for attracting females. The females, on the other hand, wore narrower plates which, while less bulky, could have been a good defense against predatory dinosaurs. (PLOS ONE, 2015; doi: 10.1371 / journal.pone.0123503)

(PLOS, 23.04.2015 - NPO)