Alexander Vargas of the University of Chile has solved the mystery of how dinosaur wrists evolved into bird wings. To do so, he and his team united the fields of embryology and paleontology. By approaching this problem in a new way, he could explain some aspects of bird evolution, such as how the number of wrist bones changed and how the wrists became bent and flexible (1).

A diagram of the bird wing, including the bird wrist and its constituent bones.

Vargas combined information from these disparate fields in his approach to learning more about the dinosaur-to-bird evolution process. Vargas and his team examined both fossils from different points in the evolution from dinosaurs to birds and the embryological development data from seven modern bird species.

The results of these studies clarified the origins of the four wrist bones of birds from the nine found in early dinosaurs.

First, the findings confirm work from the 1970s that suggested that two dinosaur bones, known as distal carpal 1 and distal carpal 2, combined, or “ossificatied,” to form the semi-lunate bone in birds. This bone gets its name from its half-moon shape. Additionally, the study found that two dinosaur bones, the radiale and the intermedium, merged to create a single bird bone called the scapholunare. The previous name of this bone, the radiale+intermedium, mistakenly suggests that the bone is formed by a fusion in the embryonic development, rather than by an evolutionary process. For this reason, the authors of the study suggested “scapholunare” as a more accurate name (2).

The third bone in bird wrists is known to embryologists as “element x.” Originally, embryologists thought that it replaced the dinosaur bone known as the ulnare. However, developmental data from the Vargas study found that the ulnare and element x coexist in bird embryos. Further, they found that element x corresponds to the dinosaur bone known as distal carpal 3 (2).

The researchers discovered that the fourth bone in bird wrists, the pisiform, had a rare evolutionary history. They found that it was lost in dinosaurs, but re-evolved in early birds. The purpose of the re-evolution of the pisiform was probably to facilitate flight. The pisiform enables a forceful downbeat of wings, and restricts the flexibility of the wings on the upbeat (2).

The combination of paleontological and developmental research successfully shed light on the origins of the four bird wrist bones, as well as how the pisiform created the flexibility that enabled flight in birds.

Sources:

1. Botelho JF, Ossa-Fuentes L, Soto-Acuña S, Smith-Paredes D, Nuñez-León D, et al. (2014). New Developmental Evidence Clarifies the Evolution of Wrist Bones in the Dinosaur–Bird Transition. PLoS Biol 12(9): e1001957. doi: 10.1371/journal.pbio.1001957

2. (2014, September 30). How dinosaur arms turned into bird wings. ScienceDaily. Retrieved October 6, 2014 from www.sciencedaily.com/releases/2014/09/140930144157.htm