An Entire Branch of the Tree of Life Was Lost for 15 Million Years. Scientists Just Found It.

3 min readHere’s what you’ll learn when you read this story:Looking back into time, paleontologists discovered a new extinct order of marsupials—Keeunamorphia—including three new extinct species and two new extinct genera.Keeuna woodburni, Phantasmodon travouilloni, and Phantasmodon minuferox are thought to have been mouselike insectivores that scurried around Australia’s ancient rainforests.Keeunamorphia may be one of the oldest and longest-lived lineages of marsupials. Kangaroos. Koalas. Wallabies. Wombats. Australia is crawling with fantastically weird marsupials, which arrived ‘down under’ around 55 million years ago as the supercontinent of Gondwana was breaking apart. But while their geographic history is relatively well-understood, the prehistoric origins of these creatures are still somewhat unclear. Which lineages did they evolve from, and how did they diversify into so many wildly different species?Paleontologist Tim Churchill and his team of researchers from the University of New South Wales (UNSW) have identified three extinct species and two genera that belonged to an order of marsupials that had previously gone unknown. Keeunamorphia—which encompasses genus Keeuna and genus Phantasmodon—has now been added to the existing five orders under marsupial supergroup Australidelphia. It includes small insectivores that lived around 18 million years ago, darting between the trees and vines of humid rainforests in what is now northern Queensland, before the region (and most of Australia) dried out. Fragmented fossils surfaced in shallow cave pools, where entire skeletons are rare. Churchill thinks Keeunamorphia might be the most ancient lineage from which modern-day Australian marsupials are descended.“Keeunamorphia may represent some of Australia’s earliest marsupials, with possible origins in Gondwana and a record spanning more than 35 million years before its apparent extinction in the Middle Miocene,” he said in a study recently published in Journal of Paleontology.With only teeth and jaw fragments to work with, the researchers were able to figure out where the two species they discovered fit into the lineage of marsupials. The teeth appeared unrelated to those of known marsupials—while the molars of these species are tribosphenic (made to slice and grind), they have an unusual cusp on the primary crest, or ectoloph. This feature is also seen in two other extinct species: Ankotarinja tirarensis, and Djarthia murgonensis.Australidelphians only have two known synamorphies (features exclusive to the supergroup)—characteristics of their tarsal (heel) and ankle bones—and three other possible synamorphies. Though these bones from the new species have been lost to time and erosion, the synamorphies that distinguish them as marsupials are thought to have begun with their mouselike relative Djarthia murgonensis. Churchill found thirteen aspects of their teeth that matched other those of species within Keeunamorphia, but are not seen in any other marsupial, extant or extinct (though the group Marsupialiformes is the most closely related).Some Keeunamorphia species have more similarities than others. Phantasmodon travouilloni’s upper molars, for instance, resemble those of Keeuna woodburni, despite being in another genus. Phantasmodon and Keeuna also share a strange positioning of crests and ridges on their teeth—a derived trait which is present in related species, but not in their last common ancestor (Djarthia murgonensis lacks this feature). But even with these parallels, the new species still differ significantly from Keeuna woodburni. Keeunamorphians have other dental features that align more closely with dasyuromorphians, which were carnivors that hunted both vertebrate sand invertebrates.Churchill thinks that the links between Keeunamorphians and other marsupials are indicators of a marsupial lineage that diverged early on, continuing to survive and evolve for tens of millions of years before it faded out. If he’s correct, this could make it both the oldest and longest-lived lineage of marsupials, though he sees Keeunamorphians as having been more primitive than other marsupials they coexisted with in what was once a lush Australia. There is still a 20-million-year period that has vanished from the marsupial fossil record, which may mean there are still other lineages that have not been discovered yet, whether or not they shared a common ancestor.“By the Miocene, most small carnivorous niches were occupied by dasyuromorphians, peramelemorphians, or diminutive thylacoleonids,” the researchers said. “This may explain why keeunamorphians are relatively rare in the Miocene fossil record […] which otherwise exhibits an extremely rich fossil diversity for most metatherian lineages.”Elizabeth Rayne is a creature who writes. Her work has appeared in Popular Mechanics, Ars Technica, SYFY WIRE, Space.com, Live Science, Den of Geek, Forbidden Futures and Collective Tales. She lurks right outside New York City with her parrot, Lestat. When not writing, she can be found drawing, playing the piano or shapeshifting.