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Pictured: Illustration demonstrates Brontotheres’ large body size.

Across land, air, and sea, this summer 91社区 News is highlighting some of the recent studies published by the聽College of Osteopathic Medicine鈥檚 (NYITCOM)聽Department of Anatomy. The 鈥淏eyond the Bones鈥� series is a snapshot of just a few of the projects helping to shape scientific understanding about evolution.聽

In this latest installment, a researcher explores the fossil record an ancient relative of the rhinos to help explain why natural selection might favor larger animals more often than smaller animals.

After dinosaurs became extinct, the bodies of many mammals became larger. However, the causes for this change have been widely debated. Now, a May 11 study published in聽, co-authored by Associate Professor聽Matthew Mihlbachler, Ph.D., sheds new light on this topic.聽

Mihlbachler and researchers from the University of Alcal谩 (Madrid, Spain) examined the fossil record of brontotheres, large herbivorous mammals related to horses and rhinos that lived from 53 to 34 million years ago in North America and Asia. Brontotheres experienced a significant increase in body size throughout their lineage and, due to their large mass, are often referred to as 鈥渢hunder beasts.鈥� While the smallest brontotheres are believed to have been the size of modern coyotes, the largest ones are estimated to have been 14 feet long and eight feet tall and weighed thousands of pounds.

To analyze the animals鈥� size over time, Mihlbachler and the team estimated body mass from fossilized tooth measurements and applied statistical models that traced body size evolution throughout the brontothere family tree. While many scientists believe that species tend to get larger as they evolve (a theory called Cope鈥檚 Rule), the team found that brontotheres did not follow this expected pattern. In fact, their lineage evolved in both directions to include smaller and larger species. However, the smaller species were more extinction-prone, likely due to competition and predation. At a certain point, this left only the larger-bodied species to carry on the lineage. 

While large brontotheres outlived their smaller counterparts, their massive bodies may have prevented them from adapting to the increasingly cooler conditions of the earth, ultimately leading to their demise. The animals鈥� extinction coincided with a marked global cooling event, that resulted in the formation of polar ice caps and loss of forest habitats in temperate latitudes. 

鈥淭he final extinction of brontotheres underscores the vulnerability of large species to environmental change and habitat loss, which is a phenomenon we still see today with the dwindling populations of elephants, rhinos, and other large mammals that, without tremendous conservation efforts, would be extinct,鈥� says Mihlbachler. 

鈥淚n the short term, larger species may be better at competing with other coexisting species, explaining why larger species tend to dominate over time. However, evolution of large body size may be a kind of evolutionary trap because larger species may be more vulnerable to abrupt climate change, such as the global cooling event at the end of the Eocene [era] about 34 million years ago that led to the extinction of the brontotheres.鈥�

Learn more聽about innovative research from the Department of Anatomy鈥檚 esteemed faculty.