Xiaotingia zhengi

A lovely new dinosaur fossil from China is described in Nature today: it's named Xiaotingia zhengi, and it was a small chicken-sized, feathered, Archaeopteryx-like beast that lived about 155 million years ago. It shares some features with Archaeopteryx, and also with some other feathered dinosaurs.

(Click for larger image)

a, b, Photograph (a) and line drawing (b). Integumentary structures in b are coloured grey. cav, caudal vertebra; cv, cervical vertebra; dv, dorsal vertebra; fu, furcula; lc, left coracoid; lfe, left femur; lh, left humerus; li, left ilium; lis, left ischium; lm, left manus; lp, left pes; lpu, left pubis; lr, left radius; ls, left scapula; lu, left ulna; md, mandible; rfe, right femur; rfi, right fibula; rh, right humerus; ri, right ilium; rm, right manus; rr, right radius; rt, right tibiotarsus; ru, right ulna; sk, skull; ss, synsacrum.

Now here's why this particular fossil has some paleontologists in a dither. Systematics uses a set of objective, computer-based tools to objectively build phylogenetic trees: you plug a set of character parameters for a set of organisms into it, and it analyzes them and determines the most likely or most parsimonious tree to describe their relationships. Plugging in data from modern birds, Archaeopteryx, and dromeosaurs, for instance, generates trees in which Archaeopteryx clusters with the birds, and not the dromeosaurs. Archaeopteryx was not a direct ancestor of modern birds, but was thought to be related to the basal avians — so it was a kind of close cousin.

When Xiaotingia's data is tossed into the calculation, though, the results change. Xiaotingia doesn't cluster so tightly with birds; it's a more distant relative. However, Archaeopteryx shares enough significant features with Xiaotingia that they now cluster together, pulling Archaeopteryx out of the basal Aves and into a new classification. It says that Archaeopteryx is now a kind of second cousin, a little less closely related to the birds than previously thought.

(Click for larger image)

Archaeopteryx has historically been regarded as the most basal bird (avialan), but the discovery of the closely related Xiaotingia led Xu et al.1 to pull these archaeopterygids out of avialans (birds) and into deinonychosaurs along with dromaeosaurids and troodontids. This new grouping better accounts for the evolution of feeding strategies among bird-like dinosaurs. Previous research suggested that herbivory was common among this group, as reflected in the tall, boxy skulls of oviraptorosaurs and basal avialans such as Epidexipteryx. The triangular, sharp-toothed skull of Archaeopteryx was incongruous among basal avialans, but fits better among the carnivorous dromaeosaurids and troodontids.

I have to say that I think it's extremely cool that we have a new fossil from down around the roots of the bird family tree, and it does sharpen our knowledge of what was going on down there in the middle and late Jurassic. There was a whole assortment of delicate-boned, feathered, bipedal dinosaurs that were flourishing and diversifying in that window of time, and we've now got enough data that we can distinguish details in the family tree, which is absolutely fabulous.

However, a lot of the fuss over the specimen as somehow radically changing the importance of Archaeopteryx is a bit overblown. The relative status of Archaeopteryx and Xiaotingia is a bit of taxonomic detail — important details in working out the specific history of life — but it's the equivalent of deciding that a fossil belongs in one pigeonhole rather than the pigeonhole next to it. Its shift in status means that there's a bigger gap in the early history of the true birds than we thought, and it also means that there was a greater diversity of bird-like forms than we expected in the Jurassic. One other suggestion is that removing the carnivorous Archaeopteryx from the base of the bird family tree opens up the possibility that modern birds might have descended from the vegetarian side of the family — if the last common ancestor of birds was an herbivore, that has interesting implications for the paths evolution took.

But don't worry, Archaeopteryx still represents a beautiful example of a transitional form. This new fossil is just another transitional form discovered. Creationists cannot take any consolation from it: Archaeopteryx isn't suddenly gone, it's become a part of a richer picture of bird evolution.

Xu X, You H, Du K Han F (2011) An Archaeopteryx-like theropod from China and the origin of Avialae. Nature 475, 465-470.