The extinct cave-rails were not rails

Ancient DNA analyses also uncover an interesting biogeographic connection.

For my MSc thesis at Antwerp University, I studied the phylogenetic relationships between several snail species (genus Plutonia) on the Azores (the island group that belongs to Portugal). Using a few genes, I hoped to determine the evolutionary position of Plutonia atlantica, a carnivorous species within a group of herbivores. One of my supervisors, however, was not very supportive of this approach. António Frias Martins told me that he does “not trust the molecules” and preferred to focus on morphological characters, such as detailed differences in genitalia. We agreed to disagree and I finished my molecular analyses. More than 10 years later, I still have to disagree with António. I would even argue that I don’t trust the morphology. Convergent evolution – unrelated species evolving similar features – complicates phylogenetic analyses and often leads to wrong conclusions. Molecular data can usually shine some light on the morphological smokescreen of convergent evolution and pinpoint the actual phylogenetic relationships. A recent study in the journal Biology Letters nicely illustrates the power of genetic data.

Ancient DNA

The Greater Antilles used to house three peculiar species of flightless cave-rails. Bones of the Antillean Cave-rail (Nesotrochis debooyi) were found in archeological sites on the Virgin Islands and in caves on Puerto Rico. The other two species were known from fossil remains on Cuba (the Cuban cave-rail, N. picipicensis) and Hispaniola (the Haitian cave-rail, N. steganinos). Originally, scientists classified the three species as rails in the family Rallidae. However, more recent work indicated that these birds also share some features with flufftails (family Sarothruridae). Given the morphological and ecological convergence between rails and flufftails, it is extremely difficult to determine the evolutionary history of Nesotrochis using only morphological data.

That is why Jessica Oswald and her colleagues turned to ancient DNA. They managed to obtain a nearly complete mitochondrial DNA sequence from the Haitian cave-rail. Comparing this sequence with the DNA of several other species revealed that the cave-rails are not closely related to rails, but cluster with the flufftails and the extinct adzebills (family Aptornithidae) from New Zealand. The exact relationships between these three groups – cave-rails, flufftails, and adzebills – could not be resolved, but we can already conclude that the cave-rails were not rails. The scientists that described these species were misled by convergent evolution.

Phylogenetic analyses of mitochondrial DNA indicates that the cave-rails (Nesotrochis) are closely related to flufftails (Sarothruridae) and adzebills (Aptornithidae). From: Oswald et al. (2021).

Long-distance Dispersal?

Apart from pinpointing the phylogenetic position of the cave-rails, this study also uncovered an interesting biogeographical pattern. As explained above, the cave-rails were restricted to the Greater Antilles, a group of islands in the Caribbean Sea. Their closest relatives, however, occurred on different continents: the flufftails can be found in Africa and New Guinea, while the adzebills lived on New Zealand. How did the cave-rails end up in the Caribbean region? The authors offer two main hypotheses:

Nesotrochis could be a relictual taxon that survived in the Caribbean after extinction on the adjacent mainland, or an example of long-distance dispersal from the Old World to the Caribbean.

Based on the high incidence of convergent evolution on this branch of the evolutionary tree, I would argue that morphological data will not be very helpful to solve this biogeographical mystery. My money is on more detailed analyses of ancient and modern DNA. But I would be happy to be proven wrong. Who knows what morphological insights remain to be discovered?

Geographical distribution of cave-rails (green), flufftails (blue) and adzebills (red). From: Oswald et al. (2021).


Oswald, J. A., Terrill, R. S., Stucky, B. J., LeFebvre, M. J., Steadman, D. W., Guralnick, R. P., & Allen, J. M. (2021). Ancient DNA from the extinct Haitian cave-rail (Nesotrochis steganinos) suggests a biogeographic connection between the Caribbean and Old World. Biology Letters17(3), 20200760.

Featured image: Bones of the Antillean Cave-rail (Nesotrochis debooyi) © Wetmore | Wikimedia Commons

One thought on “The extinct cave-rails were not rails

  1. When I read this revelation as it came out, I was just as fascinated as you appear to have been. Not that *Nesotrochis* had really been on my mind before (it was just part of the big mental heap of extinct rails for me) but adzebills certainly struck a chord with me since I first found out about them and that *Nesotrochis* turned out to be other relatives to them, made these ‘rails’ much more interesting to me.

    I love it when such discoveries are made, also because they show how previously unknown lineages can be hiding in plain sight and that makes me wonder what other creatures are still awaiting to be unmasked for what they really are (sylviornithids being another great example) and also just how many comparably small, isolated clades have disappeared relatively recently without having even left a trace (so far?). Especially with birds, where the fossil record is so patchy for most groups.

    As for the biogeographical pictures in what I’d call ‘sarothruroids’ for now: a relictual distribution pattern with only a few species surviving here and there across the globe seems by far the likeliest scenario to me but so far, this is impossible to say with even near-certainty, as you pointed out.

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