Hand Morphology and Biomechanics in the Gray Fossil Site Mastodon with Implications for Giant Body Size in Terrestrial Animals

Activity: Talk typesOral presentation


Introduction: An early and species-rich proboscidean lineage was Mammut (mastodons), forest browsing elephant relatives that appeared in North America during the late Miocene and became widespread through the Pleistocene, with the American mastodon (Mammut americanum) being the most abundant and well-known in North America. Mammut were typically 5-8 tons and 2.3-2.8 meters tall at the shoulder. However, a discovery of a giant mastodon at the Tennessee Gray Fossil Site (GFS) highlights subtle differences in the hand that make elephant forefeet characteristics worth exploring into more detail. With the GFS representing a forested refugium in a time of changing environments 4.9-4.5 mya, the site is critical to understanding late Hemphillian-age faunal diversity in the east. An incredibly large mastodon living in steep forests would have challenges navigating a dense habitat. Pertinent key features of the GFS mammutid are its unusually large size of 13-16 tonnes, its trifurcated terminal phalanges, and unique thumb (pollex) apparently angled laterally from the rest of the manus. With the GFS acting as a refugium amongst changing environments, and the GFS proboscidean representing an early mastodon, discovering what makes it different is crucial to understanding early mastodon species and how they compare to other proboscideans.

Methods: To understand comparative manus mechanics of the GFS mastodon, we describe its osteology and apply finite element analysis (FEA). We start by reconstructing the hand musculoskeletal arrangement. We estimate forces on the manus based on the animal’s body mass, duty factors during locomotion, and muscle forces to act on the manual elements in various motions from standing, walking, and running gaits. For FEA we apply these forces and bone material properties virtually to the specimen and simulate stress and strain of locomotion.

Results: The GFS mastodon has a splayed manus. Phalanges attached to the first metacarpal (MC1) are oriented at a lateral 45o, allowing the manus to have an overall wider, flatter structure for stability than seen in typical proboscideans lacking the lateral orientation shift. Compared to other proboscidean taxa where digit I is more elevated in the fat pad, the GFS mastodon’s digit I is a more active digit, connecting more firmly with the ground, with very large, flat MC1 articular surfaces for proximal and anterior extended mobility during compression. The second metacarpal also lacks a connection with MC1, further demonstrating the lateral extension MC1 exhibits and the absence of an articular limit for a narrow range of movement. FEA maximum stresses of 8 MPa in a walking gait indicates that the manus had high safety factors at low speeds, suggesting the broad manus and divergent first digit enhanced potential capability to traverse high relief terrain.

Conclusions: The mastodon’s thumb was potentially a means to support its incredibly large self on steep terrain since it lived within the Appalachian Mountains roughly 5 million years ago.
Period15 Feb 2024
Event title
Oklahoma State University Center for Health Sciences Research Week 2024
Event typeConference
LocationTulsa, United States, OklahomaShow on map


  • mastodon
  • stability
  • adaptation