As old as the hills - How did the species diversity in Southwestern USA originate?

Robert Bryson Jr and co-workers have recently published an extensive and important paper investigating the historical diversification of montane scorpions in the Vaejovis vorhiesi group (Vaejovidae) in Southwestern USA and comparing this with landscape- and paleohistory of the highlands of this area. I refer to the detailed abstract below for more details.

Abstract:
Background: The age of lineages has become a fundamental  datum in studies exploring the interaction  between geological transformation and biotic diversification. However, phylogeographical  studies are often biased  towards lineages that are younger than the geological features of the landscapes  they inhabit. A  temporally deeper historical biogeography framework may be required to address episodes of biotic diversification  associated with  geologically older landscape changes. Signatures of such associations may be retained in the genomes of ecologically specialized  (stenotopic) taxa with limited vagility. In the study presented here, genetic data from montane  scorpions in the Vaejovis vorhiesi group, restricted to humid rocky habitats in mountains  across southwestern  North America, were used to explore the  relationship between scorpion diversification and regional geological history.

Results: Strong phylogeographical  signal was evident within the vorhiesi group, with 27  geographically cohesive lineages inferred from a mitochondrial phylogeny. A time-calibrated  multilocus species tree revealed a pattern of Miocene and Pliocene (the Neogene period) lineage  diversification. An estimated 21 out of 26 cladogenetic events probably occurred prior to the onset  of the Pleistocene, 2.6 million years ago. The best-fit density-dependent model suggested  diversification rate in the vorhiesi group gradually decreased through time.

Conclusions: Scorpions of the vorhiesi group have had a long history in the highlands of  southwestern North America. Diversification among these stenotopic scorpions appears to have  occurred almost entirely within the Neogene period, and is temporally consistent with the dynamic  geological history of the Basin and Range, and Colorado Plateau physiographical provinces. The  persistence of separate lineages at small spatial scales suggests that a combination of ecological  stenotopy and limited vagility may make these scorpions particularly valuable indicators of  geomorphological evolution.

Reference:
Bryson Jr RW, Riddle BR, Graham MR, Smith BT, Prendini L. As Old as the Hills: Montane Scorpions in Southwestern North America Reveal Ancient Associations between Biotic Diversification and Landscape History. PLoS One. 2013;8(1):e52822. [Free full text]