About a year and a half ago, as my first post on Dinosaur Tracking, I wrote about the discovery of a tiny, termite-eating dinosaur called Albertonykus. It belonged to one of the strangest groups of dinosaurs recognized to date. Called the Alvarezsaurids, these dinosaurs were covered in feathers, had stout forelimbs tipped in an enormous claw, and appear to have been the dinosaur equivalent of anteaters. But how do these creatures relate to other dinosaurs, and how did they evolve?
Described last week in the journal Science, the dinosaur Haplocheirus sollers has provided scientists with an idea of what the progenitors of the later Alvarezsaurids looked like. Discovered in 159- to 161-million-year-old rock in the northwestern corner of China, the strange new dinosaur was represented by an almost entirely complete skeleton, an extraordinary find since Haplocheirus is over 60 million years older than any previously discovered alvarezsaurid. Given such a wide time gap, though, it is not surprising that Haplocheirus is very different from its later relatives, and its appearance may help scientists understand when the ancestors of birds appeared.
As is presently understood, the alvarezaurids were maniraptoran dinosaurs, a diverse group that contains a variety of forms from birds to the herbivorous therizinosaurs. Many of the most well-known, and bizarre, members of this group have been found in Cretaceous strata, but if all the disparate types were truly related then they should have had ancestors stretching back into the Late Jurassic. For the alvarezsaurids, at least, Haplocheirus confirms that this is true. It lacks some of the specializations seen in its later relatives, such as the stubby arms adapted for digging, and instead looks like a more generalized maniraptoran dinosaur. Furthermore, it suggests that the alvarezsaurids diverged from their maniraptoran cousins relatively early, meaning that the "bird" traits seen in later alvarezsaurids were evolved independently.
Like many other dinosaurs recently discovered in China, Haplocheirus has answered some questions and raised entirely new ones. This new dinosaur confirms that early maniraptoran dinosaurs were already present in the Jurassic, but the huge gap between it and its Cretaceous relatives means that there were many other as-yet-undiscovered forms. Scientists are constantly working to fill in these gaps, and it will be exciting to find out what sort of creatures connect the early bird-like dinosaurs with their specialized descendants. Choiniere, J., Xu, X., Clark, J., Forster, C., Guo, Y., & Han, F. (2010). A Basal Alvarezsauroid Theropod from the Early Late Jurassic of Xinjiang, China Science, 327 (5965), 571-574 DOI: 10.1126/science.1182143