June 8, 2022

Mammal enamel: Toothy analysis IDs vegetation of ancient Nebraska

Pocket Science: Exploring the 'What,' 'So what' and 'Now what' of Husker research

Hooved mammal fossils
University of Nebraska State Museum

University of Nebraska State Museum
A collection of fossils, including teeth, from hooved mammal species that resided in Nebraska between 18 million and 12 million years ago.

Welcome to Pocket Science: a glimpse at recent research from Husker scientists and engineers. For those who want to quickly learn the “What,” “So what” and “Now what” of Husker research.

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Between 18 million and 12 million years ago, the Great Plains supported an unprecedented variety of hooved mammal species that browsed on leafy vegetation — up to three times more than in any modern ecosystem, including the densest rainforests. That diversity, ecologists figured, could only have emerged in a landscape similarly blanketed with woody, foliage-rich vegetation that employed a form of photosynthesis, C3, favored by roughly 85% of plant species.

But some recent studies have hinted that the Great Plains may actually have featured more open, savanna-like swaths topped by grasses that would later come to dominate the region. Those grasses evolved a water-saving form of photosynthesis known as C4.

So what?

Nebraska’s Willow Nguy and Ross Secord sought to clarify the confusion by turning to teeth — specifically, the amount of a particular carbon atom, or isotope, found in the enamel of fossilized choppers. The C3 and C4 plants produce differing amounts of the carbon-13 isotope, which accumulates and is preserved in the enamel of herbivores consuming those plants. That enamel then becomes a mineralized record of the vegetation covering a landscape millions of years ago.

After analyzing the enamel of 183 teeth from a representative sample of extinct mammal species, Nguy and Secord concluded that the Nebraska of 18 million to 12 million years ago resembled a fairly dry savanna or woodland peppered with denser forests. Carbon-13 concentrations and the sheer variety of hooved mammal species suggests that even those open expanses featured more flowering, leafy plants than the grassland-rich Nebraska of today, the researchers said. Still, a year-round growing season may have allowed some species to subsist on sparser vegetation.

Now what?

Refining the model that informed the team’s estimates could help reconcile a disparity between the amount of C4 grasses on ancient landscapes and the surprisingly limited consumption of that vegetation, Nguy said.