A 'living fossil' from Central Asia: The morphology of Pseudochactas ovchinnikovi Gromov, 1998 (Scorpiones: Pseudochactidae), with comments on its phylogenetic position

Pseudochactas ovchinnikovi Gromov, 1998, arguably the most remarkable scorpion discovered during the last century, inhabits an isolated, mountainous region of southeastern Uzbekistan and southwestern Tajikistan, Central Asia. This scorpion displays several morphological characters unique among Recent (extant) scorpions, including a unique trichobothrial pattern and a mixture of other characters, some potentially synapomorphic with Buthidae C.L. Koch, 1837, others with the nonbuthid scorpion families, particularly Chaerilidae Pocock, 1893. Consequently, a monotypic family, Pseudochactidae Gromov, 1998 was created to accommodate it. Although there is widespread agreement that Pseudochactas Gromov, 1998 is basal within Recent scorpions, its precise phylogenetic position remains a matter of debate. Three competing hypotheses have been proposed to account for its position: (1) sister group of all Recent scorpions; (2) sister group of Buthidae; (3) sister group of Chaerilidae. Despite the importance of Pseudochactas in determining the basal relationships among Recent scorpions, several important character systems, including the hemispermatophore and the ovariuterus, have not yet been studied in the genus. There are also several misconceptions regarding some of the character systems (e.g., trichobothria and carinae) that have been studied. In this contribution, we provide a detailed, fully illustrated reexamination of the morphology of Pseudochactas, including the first descriptions of its hemispermatophore, ovariuterus, and pectinal peg sensillae. We discuss the implications of these and other characters for the phylogenetic position of this 'living fossil' and conclude that Hypothesis 2, sister group of Buthidae, is the most plausible of the alternatives, all of which await further testing in a rigorous phylogenetic analysis.