Divergent morphological responses to millennia of climate change in two species of bats from Hall’s Cave, Texas, USA

Abstract

How species will respond to ongoing and future climate change is one of the most important questions facing biodiversity scientists today. The fossil record provides unparalleled insight into past ecological and evolutionary responses to climate change, but the resource remains virtually untapped for many organisms. We use geometric morphometrics and a 25,000 year fossil record to quantify changes in body size and mandible shape through time and across climate regimes for two bat species present in Quaternary paleontological deposits of central Texas: Myotis velifer, a bat distributed throughout the Southwestern US and Mexico that is still found in central Texas today, and Eptesicus fuscus, a bat widely distributed throughout North America that has been extirpated in central Texas. Because of ecogeographic rules like Bergmann’s rule, which posits that endotherms are larger in colder environments, we hypothesized that both species were larger during cooler time intervals. Additionally, we hypothesized that both species would show variation in dental morphology across the studied sequence as a response to climate change. While we found a decrease in centroid size–a proxy for ­­body size–through time for both species, we could not establish a clear relationship between centroid size and temperature alone. However, we did find that specimens from drier environments were significantly larger than those from wetter ones. Furthermore, we found significant dental shape variation between environments reflecting different temperature levels for both species. Yet only M. velifer exhibited significant variation between environments of varying precipitation levels. This result was surprising because present-day populations of E. fuscus are highly variable across both temperature and precipitation gradients. We determined that the morphological change experienced by M. velifer through time, and between warmer and cooler temperatures, was associated with the coronoid process, condylar process, and the mandibular symphysis. These parts play a pivotal role in bite force, so changes in these features might relate to changes in diet. We show that long-term datasets derived from fossil material provide invaluable insight not only into the validity of ecogeographic rules, but also into the adaptive capacities of extant taxa when faced with environmental changes. Our results highlight diverging responses to a variety of climate factors that are relevant to consider in biodiversity research given ongoing global change.

Morphometric analyses of the vertebrae of Ambystoma (Tschudi, 1838) and the implications for identification of fossil salamanders

“…Morphometric approaches are often utilized in fossil identifications when morphological differences are small or otherwise not easily described qualitatively and have been applied across many groups of organisms (e.g., Marramà & Kriwet, 2017; Moroz et al, 2021; Scarsbrook et al, 2021). Although morphometrics can help researchers easily visualize morphological differences, the value of using morphometrics for fossil identifications can be variable, as we have shown here.…”

“…Fossil identifications are the foundation upon which paleoecological analyses are based and often play an important role in evolutionary analyses. As examples, fossil identifications can have substantial impacts on evolutionary and ecological inferences because they can directly affect investigations relating morphological evolution to past environmental changes (e.g., Moroz et al, 2021), investigations on the mechanisms and timing of lineage diversification through time (e.g., Scarpetta, 2020), and investigations on ecosystem changes over time (e.g., Smith et al, 2016). Therefore, the methods by which we identify fossils require detailed study and attention to recognize limits in our abilities to identify fossil material (Bell et al, 2010).…”

“…One approach that has not been used to study osteological variation in the vertebrae of Ambystoma is geometric morphometrics. Geometric morphometrics employs landmarks to quantify shape differences, and it has been used by biologists to differentiate species (e.g., Scarsbrook et al, 2021), characterize sexual dimorphism (e.g., Bonnan et al, 2008), relate form to function (e.g., Gray et al, 2019), and study morphological variation through time (e.g., Moroz et al, 2021). Geometric morphometrics allows us to consider aspects of vertebrae that are not captured through linear measurements alone and quantifies many of the qualitative descriptions of features that previous researchers made.…”

Looking Back for the Future: The Ecology of Terrestrial Communities Through the Lens of Conservation Paleobiology

“…Morphological analysis of fossils can demonstrate shifts in features associated with ecologically relevant traits. In a study of bat fossils from Quaternary paleontological sites across central Texas, Moroz et al (2021) found that changes in mandible shape of Myotis velifer were linked to precipitation changes and likely associated with dietary shifts. These data suggest that the range-restricted M. velifer may quickly adapt to climate change, which is useful knowledge for conservation practitioners who often prioritize range-restricted species over widespread species.…”