Dodo - Taxonomy and Evolution

Taxonomy and Evolution

The Dodo has been variously declared a small ostrich, a rail, an albatross, and a vulture. In 1842, Johannes Theodor Reinhardt proposed that Dodos were ground pigeons, based on studies of a Dodo skull he had discovered in the royal Danish collection at Copenhagen. This view was met with ridicule, but was later supported by Hugh Edwin Strickland and Alexander Gordon Melville in their 1848 monograph The Dodo and Its Kindred, which attempted to separate myth from reality. After dissecting the preserved head and foot of the specimen at the Oxford University Museum and comparing it with the few remains then available of the extinct Rodrigues Solitaire (Pezophaps solitaria) they found that the two were closely related. Strickland stated that although not identical, these birds shared many distinguishing features of the leg bones, otherwise known only in pigeons.

The Dodo was anatomically similar to pigeons in many features. Strickland pointed to the very short keratinous portion of the beak, with its long, slender, naked basal part. Other pigeons also have bare skin around their eyes, almost reaching their beak, as in Dodos. The forehead was high in relation to the beak, and the nostril was located low on the middle of the beak and surrounded by skin, a combination of features shared only with pigeons. The legs of the Dodo were generally more similar to those of terrestrial pigeons than of other birds, both in their scales and in their skeletal features. Depictions of the large crop hinted at a relationship with pigeons, in which this feature is more developed than other birds. Pigeons generally have very small clutches, and the Dodo is said to have laid a single egg. Like pigeons, the Dodo lacked the vomer and septum of the nostrils, and it shared details in the mandible, the zygomatic bone, the palate and the hallux. The Dodo differed from other pigeons mainly in the small size of the wings and the large size of the beak in proportion to the rest of the cranium.

For many years the Dodo and the Rodrigues Solitaire were placed in a family of their own, the Raphidae (formerly Dididae), because their exact relationships with other pigeons were unresolved. Each was placed in its own monotypic family (Raphidae and Pezophapidae, respectively), as it was thought that they had evolved their similar features independently. Osteological and molecular data has since led to the dissolution of the family Raphidae, and the Dodo and Solitaire are now placed in their own subfamily, Raphinae, in the family Columbidae.

Comparison of mitochondrial cytochrome b and 12S rRNA sequences isolated from a Dodo tarsal and a Rodrigues Solitaire femur confirmed their close relationship and their placement within the Columbidae. The genetic evidence was interpreted as showing the Southeast Asian Nicobar Pigeon to be their closest living relative, followed by the Crowned Pigeons of New Guinea and the superficially Dodo-like Tooth-billed Pigeon from Samoa. The generic name of the latter is Didunculus ("little Dodo"), and it was called "Dodlet" by Richard Owen. The following cladogram, from Shapiro and colleagues (2002), shows the Dodo's closest relationships within the Columbidae, a clade consisting of generally ground-dwelling island endemics.

A similar cladogram was published in 2007, inverting the placement of Goura and Dicunculus and including the Pheasant Pigeon and the Thick-billed Ground Pigeon at the base of the clade. The 2002 study indicated that the ancestors of the Solitaire and the Dodo diverged around the Paleogene-Neogene boundary. The Mascarene Islands (Mauritius, Réunion, and Rodrigues), are of volcanic origin and are less than 10 million years old. Therefore, the ancestors of both birds probably remained capable of flight for a considerable time after the separation of their lineage. The lack of mammalian herbivores competing for resources on these islands allowed the Solitaire and the Dodo to attain very large sizes. The DNA obtained from the Oxford specimen is degraded, and no usable DNA has been extracted from subfossil remains, so these findings still need to be independently verified.

Throughout the 19th century, several species were classified as congeneric with the Dodo, including the Rodrigues Solitaire and the Réunion Solitaire, as Didus solitarius and Raphus solitarius, respectively (Didus and Raphus being names for the Dodo genus used by different authors of the time). An atypical 17th-century description of a Dodo and bones found on Rodrigues, now known to have belonged to the Rodrigues Solitaire, led Abraham Dee Bartlett to name a new species, Didus nazarenus, in 1852. Based on Solitaire remains, it is now a synonym of that species. Crude drawings of the Red Rail of Mauritius were also misinterpreted as Dodo species, Didus broeckii and Didus herberti. Another large, flightless pigeon, the Viti Levu Giant Pigeon, was described in 2001 from subfossil material from Fiji. It was only slightly smaller than the Dodo and the Solitaire, and it too is thought to have been related to the Crowned Pigeons.