Evaluating the Significance of Paleophylogeographic Species Distribution Models in Reconstructing Quaternary Range-Shifts of Nearctic Chelonians

Abstract

The climatic cycles of the Quaternary, during which global mean annual temperatures have regularly changed by 5–10°C, provide a special opportunity for studying the rate, magnitude, and effects of geographic responses to changing climates. During the Quaternary, high- and mid-latitude species were extirpated from regions that were covered by ice or otherwise became unsuitable, persisting in refugial retreats where the environment was compatible with their tolerances. In this study we combine modern geographic range data, phylogeny, Pleistocene paleoclimatic models, and isotopic records of changes in global mean annual temperature, to produce a temporally continuous model of geographic changes in potential habitat for 59 species of North American turtles over the past 320 Ka (three full glacial-interglacial cycles). These paleophylogeographic models indicate the areas where past climates were compatible with the modern ranges of the species and serve as hypotheses for how their geographic ranges would have changed in response to Quaternary climate cycles. We test these hypotheses against physiological, genetic, taxonomic and fossil evidence, and we then use them to measure the effects of Quaternary climate cycles on species distributions. Patterns of range expansion, contraction, and fragmentation in the models are strongly congruent with (i) phylogeographic differentiation; (ii) morphological variation; (iii) physiological tolerances; and (iv) intraspecific genetic variability. Modern species with significant interspecific differentiation have geographic ranges that strongly fluctuated and repeatedly fragmented throughout the Quaternary. Modern species with low genetic diversity have geographic distributions that were highly variable and at times exceedingly small in the past. Our results reveal the potential for paleophylogeographic models to (i) reconstruct past geographic range modifications, (ii) identify geographic processes that result in genetic bottlenecks; and (iii) predict threats due to anthropogenic climate change in the future.

Latitudinal diversity gradients in Mesozoic non-marine turtles

“…In the Cretaceous, the peak in both raw and subsampled generic richness was further north at 30–45° N (rather than 15–30° N as seen today [6]), and the raw data suggest that it may have shifted even further north to 45–60° N during the Maastrichtian although this peak is less pronounced in the subsampled data (figure 1 c ). These results are consistent with other studies that have demonstrated that LBGs are not constant through time, but are dynamic patterns that are strongly affected by changes in global climatic regimes [4,15]. They cannot be regarded as fixed global patterns for all taxa, contrary to widely held assumptions, nor can we expect future LBGs to necessarily retain a tropical to subtropical peak [4], within the climatic constraints imposed by the biological limitations of ectothermy and obligate ovipary [30].…”

“…This range expansion might have been driven by short-term continental scale temperature increases from the Early to Late Maastrichtian that enabled some turtles to increase their ranges [31,32]. The persistence of peak richness at mid-latitudes in the Northern Hemisphere throughout the Mesozoic, despite turtles having a wider latitudinal range than at present [18], and the evidence for a propensity for range expansion in response to climate change in both the Cretaceous [31] and more recent times [15], suggests that persistent abiotic mechanisms control these latitudinal patterns.…”

“…Rather the more equable climates of the Mesozoic allowed expansion of the latitudinal range of turtles [18] and probably led to novel combinations of the abiotic factors that influence freshwater ecosystems [41]. Comparison of the latitudinal richness patterns of Mesozoic and modern turtles strongly suggests that today's pattern is one that became established early in testudinate history, potentially as early as the Jurassic, and the limited geographical shifts we observe may reflect extended niche conservatism through time [15,47–51]. Moreover, climate appears to have had a profound influence on turtle distributions and may account for the asymmetrical global distribution of the turtle LBG around the equator, in the Mesozoic at least.…”

Slow and steady: the evolution of cranial disparity in fossil and recent turtles

“…This trend once again conspicuously mirrors global temperature estimates, which show stark cooling following the Early Eocene climatic optimum [66,67]. In contrast to the rise in disparity throughout the Mesozoic (where climate change might have played a subliminal role), we find a climatically controlled decrease in Tertiary disparity to be highly plausible (see the electronic supplementary material, S2, Text S3 and tables S20 and S22), as global cooling has otherwise been shown to have significant impact on the diversity of turtles, at least as demonstrated in North America where Palaeogene turtle faunas are well sampled and well understood [87][88][89]. As the principal mechanism, we suggest habitat loss resulting from the global drying that accompanied global cooling [90].…”

Turtles and Tortoises of the World During the Rise and Global Spread of Humanity: First Checklist and Review of Extinct Pleistocene and Holocene Chelonians.

“…15) and interglacial warming periods and changes leading to landscape aridification and sea level changes have caused fluctuating distributions of many species and created refugia where speciation has occurred. Rödder et al (2013) modeled range extensions and contractions of 59 extant North American freshwater and semi-terrestrial turtle species through three Quaternary glacial and interglacial cycles from 320,000 ybp through the present. They found that although fossils of these species usually occurred within predicted thermal niches, based on extant populations of the same species, they sometimes occurred outside these niches, suggesting that the species' thermal niches might be much more flexible and less directly climate-dependent than previously assumed.…”