Tuesday, February 28, 2017

Vanolimicola, Rail or Jacana?

Animals that live in or near water usually have an edge when it comes to preserving as fossils. After all, the very habitats they live in are depositional environments. As a result, one might think that we would have an excellent fossil record of the charadriiforms. In addition to living in environments favorable to fossil preservation, charadriiforms are tremendously diverse. True to their common name of "shorebirds", many charadriiforms do forage by walking around on shores (e..g: most plovers), but there are also those that wade into the water (e.g.: avocets), swim on the water surface (e.g.: phalaropes), dive underwater (e.g.: auks), hunt from the air (e.g.: skuas), and even a few that feed on dry land, sometimes far from water (e.g.: buttonquails).

Yet, the early fossil record of charadriiforms is surprisingly sparse. Some bird fossils from near the end of the Cretaceous have been considered charadriiforms, but these specimens are so fragmentary that it is difficult to be certain of their classification. Even if they were charadriiforms, they would have little to tell us about the ancestral morphology of the group. One clade of charadriiforms that has a decent early record, however, are the jacanas.

Comb-crested jacana, photographed by "Djambalawa", licensed.

Extant jacanas live in freshwater lakes, where they use their astonishingly long toes to walk on floating vegetation. (For this reason, they are also known as lily trotters.) In most jacanas, the females are larger than the males, and the latter are in large part responsible for rearing their young. Unlike other living charadriiform groups, jacanas are known from identifiable fossils going back to the Eocene. A recently-described fossil appears to continue this trend... maybe.

The holotype of Vanolimicola, from Mayr (in press).

Vanolimicola longihallucis comes from Messel in Germany, known for being a treasure trove of well-preserved Eocene fossils. The holotype of Vanolimicola is far from the cream of the crop by Messel standards, but it's complete enough to show that it's a small, long-legged bird. (It's rather striking how frequently the description refers to it as being "fragmentary". Had it been discovered almost anywhere else, it likely would have been considered a decent find.) Its sandpiper-like beak and the proportions of its pedal phalanges suggest charadriiform affinities. However, it also has a very long hallux, which would be unusual for most charadriiforms... but is typical in jacanas! Though the feet of Vanolimicola aren't quite as disproportionately large as in modern jacanas, might it represent an early stem-jacana that lacked such specializations?

Perhaps, but the case is far from watertight. Rails are another group of birds that often live and feed on the margins of water bodies. As such, they are superficially similar to shorebirds in many ways, despite being more closely related to cranes. As is well known (at least among paleornithologists), a large diversity of rail-like birds was present in the Eocene, some (such as Songzia from China) being anatomically very similar to Vanolimicola. Proportions of the forelimb bones are a reliable way to distinguish between the skeletons of rails and shorebirds, but unfortunately, the wings of Vanolimicola are poorly-preserved.

Even given these ambiguities, it would have been nice had the description included a phylogenetic analysis to directly test these different possibilities. As of now, the affinities of Vanolimicola remain tantalizing but uncertain. Nonetheless, the fact that it is one of the few semi-aquatic birds known from the Messel makes it a somewhat notable find.

Reference: Mayr, G. In press. A small, "wader-like" bird from the Early Eocene of Messel (Germany). Annales de Paléontologie in press. doi: 10.1016/j.annpal.2017.01.001

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