Wrist bones became heavily remodeled somewhere along the way between early dinosaurs and modern birds. Researchers combining new analyses of modern bird embryos and fossils reveal how wrists were modernized, if you will, over millions of years. These previously undetected processes of loss, fusion, and even the “re-evolution” of a lost bone are described in a PLOS Biology study this week. 

Early dinosaurs had as many as nine wrist bones but birds these days have only four, arranged roughly in a two-by-two square grid. Wrists also went from being straight to bent and hyperflexible, which allows birds to fold their wings against their bodies when they’re not using them to fly. How the number became halved and how the shapes changed are hotly debated topics. Even the names of the various wrist bones are different, depending on if you ask a paleontologist or a developmental biologist. 

But understanding an organism’s developmental processes often helps explain the evolutionary relationships between ancestral and derived forms. So, in a clever combination of paleontology and embryology, a University of Chile team led by Alexander Vargas reexamined about a dozen fossils across the dinosaur-bird transition from several museum collections, and they also collected developmental data from seven different species of modern birds. By staining the embryonic cartilaginous skeleton and studying specific proteins, the team was able to look at how wings develop in a growing embryo. Their interdisciplinary dataset ranges from Archaeopteryx and Bambiraptor fossils to fertilized chicken, pigeon, and finch eggs.

Work from the 1970s revealed that birds and some dinosaurs have a very similar, half-moon shaped bone called the semilunate, which resulted from the merging of two bones present in earlier, non-flying dinosaurs. These new findings confirm that the bird semilunate (labeled SL) was formed by the fusion of two dinosaur bones: distal carpal 1 and 2 (green and yellow). Another two bones -- the radiale (purple) and the intermedium (orange) -- fused to form the bird wrist bone called the scapholunare (labeled SC). 

The distal carpel 3 (blue) in birds used to be called element x (light blue) by embryologists, who thought it took the place of a bone called the ulnare (brown), which was lost in bird development. However, the ulnare briefly shows up in embryonic birds before it’s completely gone in adults. As it turns out, distal carpel 3 is not new bone, and it doesn’t replace the ulnare. Not only do the ulnare and element x coexist in the embryos of birds, but also the embryonic position of element x corresponds with distal carpal 3.

And lastly (or fourthly), the team found that the pisiform (red), which was lost in dinosaurs, was actually re-acquired in the early evolution of birds. They think this “re-evolution” was likely an adaptation for flight, with the pisiform transmitting force on the downstroke while restricting flexibility on the upstroke. And it’s a rare example of an evolutionary reversal. 

[Via PLOS Biology synopsis, PLOS release]

Images: 2014 PLOS Biology