Mammal body-size responds to climate change in ancient Wyoming

To unravel the link between past climates and animal faunas requires an exceptional fossil record. Chew, an associate professor at Western University of Health Sciences, California, used fossils from the Bighorn Basin of Wyoming, a nearly complete record of around 5 million years of mammalian evolution, to study responses of mammal communities through time. "The Bighorn Basin fossil record, particularly from this part of the basin, is one of the best early Cenozoic terrestrial records in the world." remarks Chew. "My colleagues have been assembling the fossil samples on which this work is based for more than 30 years. Their efforts have produced a superb, highly resolved, thoroughly studied record that is unparalleled. This record allows us to examine more sophisticated questions about faunal response to climate and environmental change than was previously possible."

Information on past climate in the Bighorn basin comes from the structure of carbon atoms, known as isotopes, preserved in the rock. This technique revealed three global warming events. The first occurred 55 million years ago, and has been previously linked to decreasing body-size. However, data from the next two events, occurring two million years later, is required to test if this forms part of a larger evolutionary pattern. "No other terrestrial record exists with the density of fossils necessary to test faunal response to the later hyperthermals [climatic warming]. The central Bighorn Basin record essentially documents a set of repeated, natural experiments in climate warming." Chew explains.

Chew examined the size of over 7500 fossil teeth from over one hundred types of mammals, and compared them before, during and after the climatic warming events. On average, the Bighorn Basin teeth were 10-20% smaller during the warm periods. Some lineages of mammals became smaller themselves, in a process known as dwarfing. However, mostly size change was driven by an increase in the abundance of small species relative to large ones in the basin during warm periods.

These findings add to the increasing evidence for the strong link between climatic change and animal populations. "The ability to compare faunal response between events is critical for establishing mechanisms of change and predicting the consequences of future warming."

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Citation: Chew, A. E. "Mammal faunal response to the Paleogene hyperthermals ETM2 and H2." Climate of the Past Discussions 11 (2015): 1371-1405.

AMY CHEW
Western University of Health Sciences
Pomona, CA, USA
achew@westernu.edu
909-282-0338

John Bloch
University of Florida
jbloch@flmnh.ufl.edu

Ross Secord
University of Nebraska at Lincoln
rsecord2@unl.edu

CHEW, Amy, Western University of Health Sciences, Pomona, CA, USA

Decreasing body size has been described as the 'third universal response' to warming. Examples include wide-spread size decreases in the early Eocene mammals of the Willwood Formation, Bighorn Basin, WY, at the Paleocene-Eocene Thermal Maximum (PETM), a well-known episode of geologically rapid, intense global warming. Nearly half of all Willwood FM mammal lineages were smaller during the PETM than before and/or after the event, compared with only 1-2 larger taxa. These changes occurred through the immigration of small species and the temporary dwarfing of lineages, probably via metabolic effects.

Two additional, smaller hyperthermals, ETM2 and H2, are also recorded in the Willwood FM ~2 million years after the PETM, providing the opportunity to test the universality of body size response to rapid warming. The geochemical signatures of ETM2 and H2 have been identified in the northern part of the Bighorn Basin. Previous analysis extrapolated their stratigraphic position to the southern part of the basin where dense mammal samples span each event. More than 32000 mammal fossils from >100 lineages are known from a 220-meter thick stretch of stratigraphic section along the Fifteenmile Creek that brackets and includes the ETM2 and H2 levels. From these specimens, the length and width of >7500 complete lower first molars were previously measured and natural log-transformed occlusal surface areas were calculated as a proxy for body size.

Mean molar area for the entire Willwood fauna is 10-20% smaller during ETM2 and H2 than before and after the events. The same pattern is exhibited in randomized, standardized subsamples of the data and the differences are statistically significant. This change is superficially similar to the decline in body size seen at the PETM, but detailed examination of individual lineages reveals different mechanisms. At ETM2 and H2, there are no documented immigrants and new species appearing through morphological innovation are both larger and smaller than their close relatives. Dwarfing is apparent in only a few lineages (Cantius, Microsyops). Instead, the overall size decreases at ETM2 and H2 are driven by shifts in (standardized, proportional) relative abundance, which favor the smaller species in eight of the 10 most common families. Many of these abundance shifts begin at Biohorizon B, a faunal event immediately preceding ETM2, and reverse after H2. Biohorizon B is a profound episode of faunal change apparently related to the onset of warming at the Early Eocene Climatic Optimum that sets the context in which ETM2 and H2 occurred.