DEEP TIME CONSERVATION PALEOBIOLOGY: VERTEBRATE FOSSIL TESTS OF ECOLOGICAL NICHE CONSERVATION AND RELATIONSHIP TO ANTHROPOGENIC CLIMATE CHANGE

Niche conservation theory is a significant component of predictions of biotic response to future climate change, but historical tests of niche conservation are often limited by the quality of the fossil record, especially for deep-time divergences across large geographic areas. This is especially problematic for extant squamate reptiles, where a taxonomically ambiguous fossil record consisting primarily of isolated mandibular and vertebral elements has not been sufficient to test numerous molecular divergence timing and predictive habitat distribution models.

New discoveries of fossils of the extant boid snake Charina from the early and middle Miocene of Nebraska provide deep-time data on the historical relationship between distribution and environment in extant populations. Extant Charina is a uniquely cold-tolerant boid, primarily inhabiting grasslands and forests California, the Pacific Northwest, Montana, Wyoming, and Utah. The taxon possesses a unique, diagnostic caudal vertebral morphology consisting of hypertrophied, bifid neural spines, tall pterapophyses, and fusion of distal vertebrae to form a caudal club that is employed for defense. Fossil precaudal vertebrae from the Paleogene and early Neogene have previously been assigned to both Boidae and Charinaon the basis of general form similarity, but multiple clubs and caudal vertebrae from the Great Plains early Miocene represent the oldest apomorphy-based record for the clade.

Atmospheric pCO₂ and temperature estimates derived from local and global proxies coeval with the Great Plains Charina record allow comparison with modern climate parameters. Combining niche model results with distributions predicted from paleoclimates indicates a greater temperature range tolerance than demonstrated by either dataset, and that modern distributions are relicts of much wider ranges. Limits on modern distributions appear correlated with precipitation and may additionally depend on competitive exclusion by allopatric sister-taxon Lichanura. Fossil data combined with metabolic studies in Charina suggest that the current northern distribution is achieved through physiological adaptations for rapid body temperature elevation, a process that will facilitate range expansion, as opposed to shifts, during future warming.