Fossilized Jaw Bone Reveals Early Evolution of Mammals

A new high-tech analysis of the fossilized jaw bone of Haramiyavia clemmenseni, one of our earliest ancestors, is shedding new light on the mammalian family tree. Scientists from the University of Chicago, Harvard University and Brown University believe their findings could change the timeline for when mammals first emerged and diversified.

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University of Chicago paleontologist Neil Shubin is one of the lead authors of the study and he joins us to share more information about the findings.

Haramiyavia clemmenseni (April Neander / University of Chicago)Haramiyavia clemmenseni (April Neander / University of Chicago)

Using high-resolution computer tomography (CT) scans and 3-D reconstruction, scientists examined the jaw of the 210 million-year-old, mouse-sized Haramiyavia—one of the earliest known proto-mammals that was discovered in Greenland in 1995 by a team that included Shubin—and identified complex teeth and chewing motions adapted for a plant-based, herbivorous diet, which indicates that diverse feeding adaptations evolved early among proto-mammal ancestry.

However, when comparing the primitive structures of the jaw to a primitive middle ear, scientists suggested that Haramiyavia and its relatives were not mammals, and instead occupied more of an ancestral position on the mammalian evolutionary tree.

“It’s one of the most important early mammal jaws ever discovered,” Shubin said. “It’s the first of its kind and it was the earliest one known, and we wanted to settle the controversy. The controversy was, ‘is this a real mammal, or is this a cousin of mammals?’ It doesn’t sound like much, but it’s very important in evolutionary circles to know that.

“When we scanned it we discovered two important facts: One, it has the teeth of a mammal, but it has the jaw bone and the ear of a reptile. So it clearly is a hybrid between reptile and mammal—it’s not a true mammal, it’s the closest cousin of mammals. And what that tells us then is that mammals, when they first diversified—when they had their explosion of evolution—happened much later than the Triassic, it happened in the Jurassic.”

A photo from Shubin’s expedition to Greenland, where Haramiyavia was unearthed.A photo from Shubin’s expedition to Greenland, where Haramiyavia was unearthed.

Published in the Proceedings of the National Academy of Sciences on Nov 16, 2015, the study confirms previous suggestions that mammal diversification occurred during the Jurassic Period around 175 million years ago, which is more than 30 million years after Haramiyavia and other mammal ancestors adapted during the Triassic Period.

Below, a 3-D reconstruction of the jaw of Haramiyavia clarifies the early evolution of mammals:

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