New scans of old jaws uncover clues to mammal evolution

"When you look at the entirety of the Haramiyavia jaw and its primitive features, it's clear that this group sat at the very base of the mammalian family tree," says Neil Shubin. (Credit: University of Chicago Medicine and Biological Sciences)

New technology has offered a fresh glimpse at some old evidence: a set of fossilized jaws found in Greenland nearly 20 years ago. The findings take a big step toward clarifying mammal evolution.

On expedition to Greenland in the summer of 1995—their fifth season of digging in the tundra—researchers found a pair of one-inch-long jaws embedded in limestone.

“When I first picked up the cell-phone sized slab of rock containing the two jaws, I could tell that it was something important—multi-rooted teeth with complex cusps were obvious, both advanced features,” says Stephen Gatesy, professor of ecology and evolutionary biology at Brown University.

“But after finding the counter-slab—the matching rock containing more bone—nearby, we spent days looking for more with very little success. It was more than a bit anticlimactic because you can’t do much with a tiny specimen like that in the field; you wrap it up and wait to get it back to the lab.

“Once more details were revealed by manual preparation with fine needles under a microscope, we realized that we’d found the first haramiyid fossil that was more than just isolated teeth. Having teeth together in jaws let us interpret their chewing motion and infer diet.”

[Teeth alter date of grassy diet for pre-humans]

Haramiyids, it turns out, were early precursors to mammals that lived 210 million years ago. The team’s interpretation, published in Nature in 1997, was that the fossil represented an example of proto-mammal diversification occurring well before the origin of “crown mammals.” Over the next 20 years, the idea proved to be a subject of considerable debate.

jaw of Haramiyavia clemmenseni
A detailed analysis of the jaw of Haramiyavia clemmenseni gives scientists a clearer idea of the early mammalian evolutionary timeline. (Credit: University of Chicago Medicine and Biological Sciences)

Now the team has published a new analysis in the Proceedings of the National Academy of Sciences, using technologies such as high-resolution computer tomography and 3D computer reconstruction, which weren’t available two decades ago. Neil Shubin’s lab at the University of Chicago led the work.

The analysis revealed complex teeth and chewing motions adapted for an herbivorous diet. This indicates that diverse feeding adaptations were evolving in the Triassic era among these proto-mammals. On the other hand, primitive structures of its jaw provided evidence that the species and its relatives were not “crown mammals,” which emerged later in the Jurassic era after the end-Triassic extinction event.

“This fossil is a unique representative from an incredibly important era in the evolution of mammals; the ecosystem of the whole world changed as the Triassic transitioned into the Jurassic.

[Free app lets amateurs find an I.D. fossils]

“When you look at the entirety of the Haramiyavia jaw and its primitive features, it’s clear that this group sat at the very base of the mammalian family tree, much in the same way that Tiktaalik rosea sat at the base of the tetrapod tree,” Shubin says.

“Micro-CT scanning allowed extremely detailed 3D models to be created of each tooth and bone,” Gatesy says. “We then combined evidence from all the material into a composite jaw. I don’t think this would have been possible without digital tools. Our anatomical conclusions largely corroborate the findings of our earlier paper but also increase our confidence in key characters.”

In other words, new technology allowed the team to make the most of the small, hard-earned find made after years of digging near the top of the world.

“It’s always satisfying when better methods come along to offer a fresh glimpse at old evidence,” Gatesy says. “I also found the revival of our old collaboration both nostalgic and bittersweet. Sadly, we lost two of our five coauthors within the last three years and they are sorely missed.”

In addition to Gatesy, Shubin and the late Farish Jenkins, the paper’s other authors are lead author Zhe-Xi Luo, professor of organismal biology and anatomy at the University of Chicago, and the late William Amaral.

Source: Brown University / University of Chicago