Tuesday, August 9, 2011

Xiaotingia zhengi or is Archaeopteryx still a bird?


Fig 1.- A reconstruction of Xiaotingia zhengi.

Xiaotingia zhengi is a new small carnivorous theropod dinosaur discovered in the Late Jurassic Tiaojishan Formation of the western Liaoning province of China. It is known from a single articulated skeleton missing the tail and part of the left leg, and preserved in a slab that also contains some feather impressions.

Fig 2.- Anchiornis huxleyi, a taxon previously described as a troodont, is closely related to Xiaotingia and Archaeopteryx.

The intriguing part of the story published last month in the journal Nature is less the description of the new fossil itself than the resulting cladistic analysis that followed. Xiaotingia was found to be closely related to the famous dinosaur-bird transitional fossil Archaeopteryx lithographica from Bavaria, Germany, and to another taxon described in 2009, Anchiornis huxleyi. Anchiornis had its share of fame last year when it became the first dinosaur to reveal its true colors from the examination of fossilized pigments found in the feather impressions. But Anchiornis was at that time classified as a flying dinosaur belonging to the troodontid family.

Fig 3.- Archaeopteryx lithographica, the "ancestral" bird, was actually a deinonychosaurian dinosaur related to Velociraptor. Note, the position of the wings is too high in this reconstruction.

The new cladistic tree proposed by Xing Xu and colleagues places Archaeopteryx, Anchiornis and the novel taxon Xiaotingia within the same family, the Archaeopterygidae and nests them at the base of the Deinonychosaurians, the group that contains such celebrities as Velociraptor, Deinonychus, Troodon and Utahraptor, and away from the ‘sister” branch (the Avialae) that lead to the modern birds. Archaeopterygidae is then a third and the most basal family that constitutes the Deinonychosaurians, along with the more derived Troodontids (Troodon and friends) and Dromaeosaurs (Velociraptor and friends). The real story behind the discovery of Xiaotingia is therefore that it knocks Archaeopteryx out of the genealogical tree that lead to the modern birds, a position that it held for a century and half! The true ancestors of birds, were more closely related to such oddities as the strange Scansoriopteryids and perhaps other dinosaurs such as the beaked Oviraptosaurs. This leads to the interesting possibility that an herbivorous rather than a carnivorous diet was an ancestral trait for birds.

Fig 4.- Phylogenetic tree of the ancestor of birds and their closest relatives (After Xu et al., 2011). Archaeopteryx does no longer belongs to Aves (Birds in the traditional sense).

As a matter of fact, the position of Archaeopteryx as the ancestor of all birds was precarious for some times now. The close relationship of Archaeopteryx with the Deinonychosaurians has been suspected for quite a while, with such discoveries as the four-winged flying sensation called Microraptor and other small “dromies”. The new cladistic analysis is just confirming a trend that started some two decades ago. Overall the new analysis by Xu and colleagues is making perfect sense: a number of traits that Archaeopteryx possesses are more akin to Deinonychosaurians than to Avialians. Archaeopteryx for instance has a slender shallow snout like the Deinonychosaurians and quite unlike basal avialians such as Epidexipteryx and Sapeornis and Oviraptosaurs who tend to have short and blunt snouts. Many details of the skeletal anatomy, such as the dimension and shape of the different openings in the skull (antorbital fenestra, premaxillary fenestra, …) shape and proportions of different bones (such as the lacrimal in the skull, the pubis, metatarsals, etc…) all point to Deinonychosaurian affinities for the Archaeopterygids.

Now to the vexed question as whether Archaeopteryx could still be considered a bird or not, the most we can say is that it all depends on how you define “bird”. In the traditional sense (as “birds”=Aves), Archaeopteryx is not a bird anymore. But using a broader definition (for instance, “birds”= Paraves (Avialae+ Deinonychosauria)), Archaeopteryx would still be considered a bird, but so should be all Archaeopteryx closest relatives, Velociraptor, Microraptor, Deinonychus and co. Anyway, the fact that a small feathered dinosaur related to Velociraptor could have been mistaken for the  “Urvogel”, the ancestral bird for well more than a century, tellingly shows how intimate the connection between the two groups is. Birds really are the last surviving group of dinosaurs, so get over it and move on, folks!


References:
Xing Xu, Hailu You, Kai Du and Fenglu Han. 2011. An Archaeopteryx-like theropod from China and the origin of Avialae. Nature 475: 465–470.


Original artworks on Paleoexhibit are copyrighted to Nobu Tamura. Do not use without permission.

2 comments:

  1. And then the new Samrukia paper comes out and has Archie as a basal paravian.

    By the way, Archaeopteryx is an avian in the broad sense by definition (and so would Velociraptor, Microraptor, et al. in the Xiaotingia cladogram). Archaeopteryx isn't an avian in the crown group sense though (but neither would Hesperornis and Ichthyornis).

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  2. Fantastic post Nobumichi! Keep up the great work on here, always great to read. There is always going to be this issue with Archaeopteryx and the title of 'the first bird', its what the media do...

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