ScienceDaily (Mar. 18, 2010) — Blind scorpions that live in the stygian depths of caves are throwing light on a long-held assumption that specialized adaptations are irreversible evolutionary dead-ends. According to a new phylogenetic analysis of the family Typhlochactidae, scorpions currently living closer to the surface (under stones and in leaf litter) evolved independently on more than one occasion from ancestors adapted to life further below the surface (in caves).
Typhochactas mitchelli is among the smallest known scorpions and part of the Typhlochactidae family of cave scorpions, endemic to Mexico. Like all scorpions, it fluoresces in long-wave ultraviolet light as this image of its ventral side highlights. (Credit: V. Vignoli)
"Our research shows that the evolution of troglobites, or animals adapted for life in caves, is reversible," says Lorenzo Prendini, Associate Curator in the Division of Invertebrate Zoology at the American Museum of Natural History. "Three more generalized scorpion species living closer to the surface evolved from specialized ancestors living in caves deep below the surface."
Scorpions are predatory, venomous, nocturnal arachnids that are related to spiders, mites, and other arthropods. About 2,000 species are distributed throughout the world, but only 23 species found in ten different families are adapted to a permanent life in caves. These are the specialized troglobites.
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Volume 26 Issue 2, Pages 117 - 142
Published Online: 10 Sep 2009
© 2010 The Willi Hennig Society
Troglomorphism, trichobothriotaxy and typhlochactid phylogeny (Scorpiones, Chactoidea): more evidence that troglobitism is not an evolutionary dead-end
Lorenzo Prendini a,*, Oscar F. Francke b and Valerio Vignoli c
a Division of Invertebrate Zoology, American Museum of Natural History, Central Park West at 79th Street, New York, NY 10024-5192, USA ; b Departmento de Zoologia, Instituto de Biología, Universidad Nacional Autónoma de México, Apto Postal 70-153, Coyoacán, 04510 México ; c Department of Evolutionary Biology, University of Siena, Via Aldo Moro 2-53100, Siena, Italy
*Corresponding author:
E-mail address: lorenzo@amnh.org
ABSTRACT
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.
© The Willi Hennig Society 2009.
Accepted 8 May 2009
DIGITAL OBJECT IDENTIFIER (DOI)10.1111/j.1096-0031.2009.00277.x
© The Willi Hennig Society 2009.
Accepted 8 May 2009
DIGITAL OBJECT IDENTIFIER (DOI)10.1111/j.1096-0031.2009.00277.x
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