February 26, 2009
With small heads, thick limbs, spiked tails, and backs decked with plates, stegosaurs were among the most bizarre creatures ever to have evolved. A new discovery, however, shows that some were even stranger than the weird genera already known. Yesterday a new paper in Proceedings of the Royal Society B by Octavio Mateus, Susannah Maidment, and Nicolai A. Christiansen announced that Miragaia, a long-necked stegosaur, lived about 150 million years ago in what is now Portugal.
Most stegosaurs had relatively short necks for their body size. Miragaia was different. It had 17 neck vertebrae, eight more than earlier ornithischian dinosaurs from which the stegosaurs evolved. This is especially interesting because some of its close relatives, like Stegosaurus, had an increased number of neck vertebrae even though their necks were shorter than that of Miragaia. This suggests that the evolution of the long neck in Miragaia was allowed by evolutionary changes that were already underway among stegosaurs for some time.
To compare Miragaia with the long-necked sauropod dinosaurs like Apatosaurus, you need to go down to the bones. There is not just one way a long neck can evolve. One way, seen in many sauropods, occurred through the lengthening individual vertebrae. The long neck of Miragaia, by contrast, evolved through the addition of vertebrae to the neck.
Some of these were “borrowed” from vertebrae in line behind the neck region, meaning that they once formed part of the upper back but evolved to function like neck vertebrae. This, too, is seen in some sauropod dinosaurs, but how did it happen? The authors propose that a well-known kind of regulatory gene important to organizing the body plan of an animal, called a hox gene, may have triggered the variation that eventually allowed back vertebrae to become neck vertebrae. Unfortunately we cannot test this directly because we do not have preserved Miragaia DNA, but it is an intriguing hypothesis.
Another perplexing question is what selective pressures led long-necked stegosaurs to evolve. The researchers entertained two possibilities: that it was the result of sexual selection or that it allowed stegosaurs with slightly longer necks to browse from a wider array of foliage. Further study will be required to determine whether either of these ideas is correct (or if there is some other cause paleontologists have not thought of yet). Regardless of what the answer turns out to be I am looking forward to the debate and discussion this amazing new fossil stirs up.
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