25 September, 2009

The evolution of troglobitism and more on typhlochactid phylogeny

Recently, there have been several papers published on the troglomorphic scorpions of North America (see recent blog postings). Prendini, Francke & Vignoli have now published an article that focus on the evolution of troglobitic and troglomorphic scorpions and discuss if troglobitism can be considered an evolutionary dead-end. The paper also take a look at typhlochactid trichobothriotaxy and phylogeny.

A summary of the paper can be found below, but it is interesting to note that the authors conclude that troglobitism is not an evolutionary dead-end: that an ancestral troglobitic condition can evolve into a troglomorphic, endogean (above surface) condition. The authors belive that this is the case for the troglomorphic, litter dwelling species of Typhlochactas. In plain text: Specialized, troglobitic cave dwelling scorpions can go back to the surface and during evolution evolve into new species adapted to a more variable and demanding environment. So troglobitism is not necessarly an evolutionary dead-end.

The scorpion family Typhlochactidae Mitchell, 1971 is endemic to eastern Mexico and exclusively troglomorphic. Six of the nine species in the family are hypogean (troglobitic), morphologically specialized for life in the cave environment, whereas three are endogean (humicolous) and comparably less specialized. The family therefore provides a model for testing the hypotheses that ecological specialists (stenotopes) evolve from generalist ancestors (eurytopes) and that specialization (in this case to the cavernicolous habitat) is an irreversible, evolutionary dead-end that ultimately leads to extinction. Due to their cryptic ecology, inaccessible habitat, and apparently low population density, Typhlochactidae are very poorly known. The monophyly of these troglomorphic scorpions has never been rigorously tested, nor has their phylogeny been investigated in a quantitative analysis. We test and confirm their monophyly with a cladistic analysis of 195 morphological characters (142 phylogenetically informative), the first for a group of scorpions in which primary homology of pedipalp trichobothria was determined strictly according to topographical identity (the ‘‘placeholder approach’’). The phylogeny of Typhlochactidae challenges the conventional wisdom that ecological specialization (stenotopy) is unidirectional and irreversible, falsifying Cope's Law of the unspecialized and Dollo's Law of evolutionary irreversibility. Troglobitism is not an evolutionary dead-end: endogean scorpions evolved from hypogean ancestors on more than one occasion.

Prendini L, Francke OF, Vignoli V. Troglomorphism, trichobothriotaxy and typhlochactid phylogeny (Scorpiones, Chactoidea): more evidence that troglobitism is not an evolutionary dead-end. Cladistics. 2010;26(2):117-142. [Subscription required for full text]

Thanks to Oscar Francke for sending me this paper!

Family Typhlochactidae

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