For decades, researchers have puzzled over where mysterious, extinct mammal relatives called haramiyids fit in the mammalian family tree. Their placement is critical for figuring out the initial appearances of major animal groups we have today. Now, a reanalysis of 210 million-year-old fossils suggests that the diversification of mammals occured in the Jurassic period around 175 million years ago – more than 30 million years after mammal precursors like haramiyids diversified in the Triassic. The findings were published in Proceedings of the National Academy of Sciences this week.
Haramiyids are mostly known through isolated teeth found throughout Europe. That is, until 1995, when the nearly complete lower jawbone with intact molars of Haramiyavia clemmenseni – one of the earliest known mammal forerunners – was uncovered in the Tait Bjerg Beds of the Fleming Fjord Formation in East Greenland. Yet it remained unclear if haramiyids belong on the so-called crown mammal branch (from which all modern mammals descended) or if they occupy a separate, more ancestral position at the base of the family tree.
Using high-resolution CT scans and 3D reconstructions, University of Chicago’s Neil Shubin and colleagues reanalyzed the Late Triassic slabs that contained Haramiyavia. "As the earliest known haramiyid, Haramiyavia is the key piece of evidence for inferences about the timeline of early mammalian evolution," study author Zhe-Xi Luo from the University of Chicago says in a statement. "With CT and other new technologies, we can extract anatomical insights that were not possible to obtain in the past, allowing us to more accurately interpret mammalian evolution." You can watch a cool video showing the original fossil and the subsequent 3D reconstruction, as well as an animation of their teeth, here.
Based on these new details, the team confirm that haramiyids are in a separate lineage from mammals. Think of Haramiyavia as a very close cousin of early mammals, Shubin explains to IFLScience, not a direct ancestor but a proto-mammal. Primitive structures of the jaw place them at the very base of the mammalian family tree.
Specifically, the presence of two anatomical structures – the dentary condyle in the jaw hinge and the postdentary trough, which helps attach the lower jawbone to the middle ear – suggests that haramiyids are distinct from an extinct group of rodent-like mammals called multituberculates. These two were previously grouped in the same lineage, which would have meant that mammalian diversification occurred in the Late Triassic.
Additionally, based on scanning electron microscopy of wear patterns, their teeth moved differently than that of multituberculates. With incisors for cutting and complex cheek teeth for grinding plant matter, haramiyids were adapted for an omnivorous or herbivorous diet – not an insect- or worm-based one. This suggests that diverse feeding and chewing adaptations were a major factor in the evolution of proto-mammals.
Image in the text: An illustration of Haramiyavia (top) with a 3D reconstruction of the 210 million-year-old fossil jaw superimposed (bottom). April Neander