Reviewing the relationship between the molars of small mammals and climate change during the Paleocene-Eocene transition (~55.5 million years ago)

Evaluating the responses of three closely related small mammal lineages to climate change across the Paleocene–Eocene thermal maximum

Summarized by Matthew Eisenson, a geology major at the University of South Florida (USF) at the Tampa campus. Currently, he is in his third year. He plans on attending graduate school to earn his PhD in volcanic hazards. From that, he plans on working on different volcanoes and examining their hazards so that he can help the people in those areas during times of natural disasters caused by volcanic eruptions. He may potentially become a university professor after working in the field for a few years. When he is not studying for school, he likes to play tabletop roleplaying games with people or play games with friends.

What was the hypothesis being tested? The main hypothesis that was being tested in this study was whether abiotic- climate change (driven (i.e., nonliving variables like temperature, precipitation, salinity, and humidity) can be traced in the dental molars of three mammalian species: cf. Colpocherus sp , Macrocranion junnei, and Talpavoides dartoni. These are all small mammals that were rodent-like in appearance. 

What data was used? The data used in this study were fossils of the three most common species of stem erinaceids (the group containing hedgehogs) that were alive during the Paleocene-Eocene Thermal Maximum (PETM), listed above. As the name suggests. The Paleocene-Eocene Thermal Maximum, the climate warmed drastically on Earth. From these, this study looked at the teeth (molars) of these animals. They got the specimens used for this experiment from the Florida Museum of Natural History (FMNH), Gainesville, FL., and National Museum of Natural History (USNM), Washington, D.C. These teeth were taken from a range of time surrounding the PETM: early, mid, late, and post. The three teeth belonging to each species can be seen in Figure 1. 

Methods: This study used several methods to study changes in the molars of the listed species through time. One method used was done by measuring the size change of the molars. This was done by calculating the log transformed crown area (length × width). The other strategy used was measuring the shape change of the molars. This was done by looking at three-dimensional model analysis of the shape change with dental topographic metrics and 3D geometric morphometrics. The shape change was also measured by univariate parameters (or based on one attribute) created from the linear and angular measurements (Figure 1).

Results: This experiment had results that neither supported the hypothesis that abiotic change as a direct driver of altered dental morphology or the null hypothesis that biotic change as a direct driver of altered dental morphology. This came as a surprise, as previous studies have shown that molar teeth can change due to a changing climate. Much of the data was limited by sample size, making most seen changes fall within an error range (i.e., that the size changes are not significant enough to indicate true change). Even though there were changing climates during this period, there were no significant changes in these animals’ molars. 

The figure above shows the way the researchers broke down the measurements for the molars. Shown are three views of the tooth with the measurements marked on them (A= top down view; B= side view; C= angled view). First, they measured the crown area by doing the natural log of the length times the width. Next was the relative talonid by dividing the width by the length. Next, they measured the relative metaconid length by dividing the metaconid length by the length. They also measured the relative metaconid-entoconid intercusp by dividing the metaconid-entoconid intercusp by the length. They measured the relative hypoconid−hypoconulid intercusp distance by dividing the hypoconid−hypoconulid intercusp distance by the length. Finally, they measured the relative trigonid height by diving the trigonid height by the length.
Figure 1. This image shows how the researchers did their linear and angular measurements to get their univariate parameters. This was done by measuring several parts of each molar and putting it through a code to get a univariate parameter that could be used. Each tooth was measured 3 separate times. The acronyms are CA, crown area; HHID, hypoconid−hypoconulid intercusp distance; L, length; MEID, metaconid−entoconid intercusp distance; ML, metaconid length; R-, relative; TH, trigonid height; TW, talonid width; W, width. A= top down view; B= side view; C= angled view

Why is this study important:  This study is important to look at because it showed contradicting data from what has been found before. As stated, these species seemed to have been minimally affected (at least in their molar shape and size) by climate change, while other studies on other species have shown far larger effects from climate change. More thought, analysis, and retesting is needed in this area for a more correct answer to be brought forth.

Broader Implications beyond this study?  The broader implications for this study all relate to climate change. As this study looks at relating certain characteristics across time and climate change, there is an implication about using it to better understand our geologic past, by using data gathered here to correlate molars with climate change. Another implication is looking at current climate change and how mammals will/are responding to it. Looking at how mammals may respond to the current global climate change can give us a lot of information. We can also see how past events predict what are seeing with current climate change and use that information for conservation purposes. 

Citation: Vitek, N. S., Morse, P. E., Boyer, D. M., Strait, S. G., & Bloch, J. I. (2021). Evaluating the responses of three closely related small mammal lineages to climate change across the Paleocene–Eocene thermal maximum. Paleobiology, 47(3), 464-486.