With technologies like CT scans and 3-D printing, a team of scientists reported that it had solved a mystery about the family tree of mammals that started with a single tooth a century and a half ago. The tooth, found in Germany in 1847, was tiny and distinctive in shape — not quite reptile, not quite mammal. More fossils of that kind were found around Europe, but always just single teeth. Scientists named this group of animals haramiyids — Arabic for “trickster.”
The teeth were embedded in rocks as old as 210 million years, an era that ancestors of the first mammals were evolving. “These were some of the most enigmatic fossils for years,” said Neil H Shubin, a professor of organismal biology and anatomy at the University of Chicago. In the late 1980s, Neil, then a graduate student, was part of a four-scientist team led by Farish Jenkins, a Harvard paleontologist, that searched for fossils in East Greenland. “You’re looking for tiny teeth in this vast Arctic landscape,” Neil said. The researchers found one particularly intriguing specimen, which they named Haramiyavia. Avia is Latin for grandmother — this was the grandmother of the trickster.
After a couple of years of meticulously clearing away much of the limestone surrounding the fossil, they reported on part of the Haramiyavia jawbone, revealing that the animal was indeed a proto-mammal. What was not clear was whether Haramiyavia was a direct part of the family tree of mammals — that would push the emergence of mammals back to more than 200 million years ago — or an evolutionary branch that split off before the emergence of the common ancestors of mammals, the view of paleontologists who believe that the first mammals evolved 170 million to 160 million years ago.
About two years ago, Neil decided to re-examine the slab of limestone from Greenland that enshrouded the Haramiyavia fossil. Clearing away more limestone would jeopardise the fragile fossil. Instead, Neil and his colleagues placed it in CT scanners to peer through the rock at a mostly complete jawbone and many of the teeth. “This kind of work used to be unimaginable,” said Zhe-Xi Luo, another University of Chicago paleontologist who joined Neil on the new analysis. Their conclusion: Haramiyavia, and thus all haramiyids, were not mammals, but belonged to a more ancestral side branch.
The crucial evidence they cite, reported in the Proceedings of the National Academy of Sciences, is a trough in the lower jaw of Haramiyavia. In mammals, the trough is absent, because two bones connected to the trough migrated to the middle ear to form part of the three-bone hearing mechanism. “This thing had a very primitive ear,” Neil said. “That is the piece that is sort of the smoking gun.” From the scans of the jaw and the teeth, the researchers created three-dimensional enlargements of the fossils, studying them like puzzle pieces to see how they fit together. Haramiyavia, a few inches long and rodent-like in appearance, ate plants by grinding leaves between broad teeth.
Timothy Rowe, a professor of geology at the University of Texas at Austin who was not involved in the new research, praised the work. “They really stepped out and squeezed every last bit of information that they could from these fossils,” he said.
Timothy said there was no longer evidence that the earliest divergence of mammals occurred during the Triassic Period more than 200 million years ago. “The oldest date that’s based on real evidence is 30 or 40 million years younger than that,” he said.
Not everyone agrees. “It’s a very great work, but I don’t think I’m totally convinced that is the case,” said Jin Meng, the curator of fossil mammals at the American Museum of Natural History in New York. Jin is a member of a team that in the last couple of years has described more recent species of haramiyids that lived in China about 160 million years ago. The well-preserved Chinese fossils, nearly complete, possessed the characteristics of true mammals. The mammalian characteristics include the absence of a trough in the jawbone, Jin said in an interview. “If we accept the conclusion of this study, many of those mammalian structures must have evolved independently,” he said. “I still think the other hypotheses remain alive.”
Published 23 November 2015, 15:51 IST