Saturday, January 22, 2011

How to determine the sex of a pterosaur?

Unless this is a difference between adult and young, you probably know how to identify a male in a species of pterosaur: it has a crest. This was the conclusion of the study of a comprehensive fossil found in China and associated with what appears to actually be an egg.
The study of extinct species is difficult, especially for closely related species that no longer exist today. That's why many questions about the biology and ethology of pterosaur, those flying reptiles that were not of the dinosaurs, remain unanswered. One of them concerned the difference between a female and a pterosaur pterosaur male. It could be assumed that the presence of a ridge, sometimes large, some members of a species, indicating that it was actually a male, not female who would be deprived.
This kind of sexual dimorphism is not uncommon in nature: see deer and antlers. However, the male is not always the biggest and we can not draw hasty conclusions from the study of a fossil without the risk of being wrong.
It seems that the discovery in China of a well-preserved fossil of a pterosaur species living there are 160 million years is likely to raise doubts. At least with regard Darwinopterus, who lived during the Jurassic period, it appears very likely that the distinctive sign for a male was indeed the presence of a ridge that had not females. The fossil remains of Mrs. T with a performance artist in the lower left devoid of a female and a male ridge. The egg is clearly visible on the image top right. The fossil remains of Mrs. T with a performance artist in the lower left devoid of a female and a male ridge. The egg is clearly visible on the image top right. © From left to right in descending Science / AAAS, Mark Witton / University of Portsmouth, Lu Junchang
Mrs. T and her egg
Mrs. T called the first Chinese explorers in reference to Mrs. Pterodactyl in English poorly controlled, the specimen found was indeed found with an egg. Indeed, the egg-shaped structure was found just below the pelvis and its width is exactly that of the latter element reinforcing the idea of a female about to lay eggs. In addition, hip width is greater for the specimen without peak in the case of the 40 fossil Darwinopterus (with crest) recovered to date since the discovery of the species in 2009.
In the article published in Science by David Unwin and his Chinese colleagues, the researchers suggest that the eggshell is composed of calcium carbonate. However, taphonomy (the discipline of paleontology that studies all the processes that occur after the death of an organism to its fossilization and the formation of fossil deposits) applied to the study of sediments in which the fossil is shows that the conservation of calcareous shell could occur. It is not in the presence of an egg like a bird but rather the type of egg flexible associated with reptiles.
Paleontologist Kevin Padian of the University of Berkeley, however, is not convinced. For him, the absence of peak at some of the fossils Darwinopterus is perhaps not due to sexual dimorphism, but simply a difference of age. He recalls that found in dinosaurs, but not only them, a change in skull shape with age. In addition, the egg is not found can not be a he said. Reptiles lay several eggs at once and in this case, the egg is so big it can only indicate that spawning in this pterosaur was done with a single egg.
It will probably be more discoveries to remove all doubts. For now, only four cases of egg clutches are known for pterosaurs, who had at least 130 species.

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