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Celebration of Scholarship and Creative Activity 2021

Cranial Fusion in Ursus Americanus and their Ontogenetic Utility

Katrina E. Hornung

Junior, Geology

Abstract

Ursus americanus, also known as the American black bear, has roamed North America ever since the middle of the Pleistocene Epoch 1.6 million years ago. This makes them an ideal candidate for studying changes in growth over time through comparing fossilized specimens to those of the modern day. When studying the skull sutures of ten modern specimens collected in Wisconsin and analyzed using PAUP V. 4.0, a cladistics software, five growth stages have been able to be determined through adulthood, and with it basal length has been shown to be complementary to being a proxy for growth with maximum length being achieved by stage 2 at 26cm. When using this information, a fossilized Pleistocene skull from Texas was compared to the modern skulls and was determined to be 10% larger than those that shared the same growth stage in this study, reaching 29.3cm in basal length while being between growth stages 2 and 3. To further build upon this study, sexual dimorphisms’ impact on U. americanus skulls may be necessary to understand changes made in adulthood to a skull’s sutures or other characteristics.

Project Background 

I have always liked fossils, so I was interested in pursuing that when this project came up. I find it interesting that I can study prehistoric examples of a species that still exists. The pandemic resulted in a delay in my research, but I was fortunately able to move forward and persevere. I find it fascinating that I can learn so much about a species like the black bear by studying something as small as a suture in its skull.

What Do You Think? 

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5 Comments

  1. Meghan Krueger

    Good job Katrina! Your poster looks awesome.

    Reply
  2. Eric Hiatt

    Excellent poster and summary! Your work shows that such analyses could be useful on other fossil groups.

    Reply
  3. Timothy Paulsen

    Nice presentation Katrina. Interesting.

    Reply
  4. Ben Hallett

    Thank you Katrina, great job! Is there any likely explanation why these bears might be smaller today than they were in the Pleistocene?

    Reply
    • Katrina E Hornung

      Thank you Ben!
      While we don’t know for sure, one of the possibilities behind this change in size may be due to a changing climate, with temperatures having generally increased since the Pleistocene. It’s been noted that there’s a pattern in which animals in colder regions tend to be larger than those found in warmer regions, which is known as Bergmann’s rule. Another possibility that was thought of was how there used to be other animals such as the short-faced bear in North America during the Pleistocene, so being larger might have been more advantageous when competing for necessary resources.

      Reply

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