Aurornis xui: a new piece in the bird evolution puzzle

feather_ArchaeopteryxAs more and more stunning feathered reptile fossils are unearthed in China the tale of the origin of birds and the evolution of flight becomes more and more complicated. Did powered flight, as in the birds, evolve once only or does it represent one of the most astonishing examples of convergence yet reported? A new analysis of a Jurassic bird-like dinosaur named Aurornis xui may shed some new light on the story.

Aurornis_xuiArchaeopteryx, known from Jurassic rocks around 150 million years old is the best known candidate for earliest primitive bird, at the base of the lineage leading to today’s ‘true’ birds (Euornithines). This iconic status has been shaken up with the discovery of feathered paravian (bird-like) dinosaurs that appear to be primitive birds very much like Archaeopteryx but may be around 10 million years older. Godefroit et al. recently presented Aurornis xui and suggest that it and its relative Anchiornis precede Archaeopteryx in the Avialae – the group that leads to and includes the true birds. This destroys Archaeopteryx‘s position as the earliest bird and, in their new grouping of avialids, infers that powered flight using feathered forearms only evolved once. So much for convergence? Perhaps, but then again perhaps not. Read on for a few highlights from the complex debate…

A rich diversity of feathered reptiles is now known to exist, particularly from Late Jurassic and Early Cretaceous fossils of China. These reptiles belong almost exclusively to the Paraves, a group that includes Deinonychosauria (dromeosaurs such as the famous Velociraptor and troodontids) and also Avialae. Feathered forewings are common to all paravians (even the hefty Velociraptor has quill knobs for long feather attachment) and many deinonychosaurs also show feathered hindwings. Primitive paravians (e.g. Eosinpteryx), dromeosaurs and troodontids could not fly; perhaps their feathers evolved through sexual selection, as adaptations for display. Microraptor_fossilIt seems clear that, in addition to a bipedal gait, the transformation of scales into pennaceous feathers was indeed a pre-requisite for flight, but feathers also occur in a huge diversity of non-flying lineages.

One interesting evolutionary pattern among the pre-avialids is the appearance, in the dromeosaurs, of species with well developed hind-wings. Microraptor gui is a key example here, being described as capable of bi-plane style of gliding, with a U-shaped glide trajectory like certain gliding mammals. Gliding feathered reptiles evolved by convergence earlier in reptile history; Longisquama insignis is a Late Triassic archosaur whose feathers are different in structure than those of Microraptor; not surprising given that it is very distantly related. Longisquama‘s discovery sparked  debate about possible convergent evolution of feathers and even of birds.

While Godefroit puts all the powered fliers in one avialid group certain members of his proposed lineage are still disputed. For example, other analyses placed Aurornis and Anchiornis in the adjacent group Troodontidae,  and another supposed avialid named Xiaotingia, some believe could be a primitive dromeosaur. The clumsy flier Rahonavis is generally accepted to be an early bird, and yet some argue that it belongs with the dromeosaurs, again indicating possible convergent evolution of forearm-powered flight and ‘birds’. Archaeopteryx_lithographicaWork by Xu et al. in 2011 even places Archaeopteryx outside the Avialae, supporting a possible double origin of powered flight.  The intense diversification of feathered reptiles evident from the Late Jurassic to Early Cretaceous makes the hunt for solid answers a tough challenge.

Given the clear ecological advantages of powered flight, perhaps it is hardly surprising that evolution found a way to put a number of reptiles, the insects and even bats into the skies. Whatever the relationships among the bird-like dinosaurs turn out to be, at the largest scale of the tree of life the evolution of flight remains an inspiring case of convergence at its most fundamental.

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