13-11 Paleogeographic Reconstruction of Eastern California: Implications for Distribution and Isolation of Relict Toad Species (Anaxyrus Exsul and Anaxyrus Nelsoni)

Session: Earth Life Sciences across the Cordillera

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Abstract:

Paleogeographic reconstructions utilizing geologic data are important elements in determining the timing of species isolation and divergence. The variation and genetic relations among various species of pupfish (Cyprinodon) is a well-known geological and biological puzzle of eastern California and western Nevada. Less well-known, but equally important and in the same region, are two sister species of toads Anaxyrus exsul and Anaxyrus nelsoni. A. exsul is commonly known as the black toad of Deep Springs Valley, which is an essentially aquatic, threatened toad that exists in small, spring-fed ponds only in Deep Springs Valley, California. A. nelsoni, closest genetic relation to A. exsul, is critically endangered and found 115 km to the south at Darwin Falls in the Argus Range along the eastern edge of Panamint Valley, California. Speculation based on genetic data suggest that these two, isolated, “relict” species diverged from a common ancestor as long ago as early Pleistocene.

We date the development of critical drainage divides that now separate the two species using ⁴⁰Ar/³⁹Ar, tephrochronology on rhyolite tephra and whole-rock XRF on volcanic rocks. The divides presently separate Deep Springs-Eureka/Saline Valleys, Saline/Death Valleys, Death/Panamint Valleys. Conglomerate including the 3.13 Ma tuff of Mesquite Flat confirms a fluvial connection to the southeast from Deep Springs Valley into Eureka/Saline Valley. A 4.517 Ma andesite cone within a canyon crossing the Last Chance Range is an avenue for west-to-east flow into Death Valley until the canyon was filled by a 3.921 Ma basalt flow that flowed west-to-east from the Saline Range toward Death Valley. To the south, the Nova Formation documents a fluvial connection between Death Valley to the Argus Range based on 5.13 Ma to 3.8 Ma basalt flows that erupted in the Argus Range prior to the development of Panamint Valley. The combination of these geologic data supports a fluvial connection between the present-day isolated habitats of A. exsul and A. nelsoni prior to 3.9 Ma or during the Pliocene, which is consistent with the biological data.

Hydrological isolation of the basins containing the two toads was the result of post-3.9 Ma movement along transtensional faults that form the mountain escarpments separating the basins. Coincidence of fault-controlled mountain uplift and toad speciation supports that intraplate tectonics forced the evolution.

Geological Society of America Abstracts with Programs. Vol. 58, No. 3, 2026

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