At a time when the vast majority of mammals were the size of shrews and mice, Vintana sertichi was a super heavyweight. Researchers have unearthed the skull of a primitive mammal with remarkably keen senses who lived in Madagascar 66 to 72 million years ago. This giant groundhog-like critter belongs to a mysterious group of mammals called the gondwanatherians, who roamed the ancient supercontinent Gondwana at around the same time as dinosaurs. The work was published in Nature this week.
Gondwanatherians have only been known from isolated teeth and jaw fragments, so the details of their appearance and evolutionary relationships were hard to determine. Now, a team led by Stony Brook’s David Krause describe a fossilized cranium (no lower jaw) uncovered in an Upper Cretaceous formation in Madagascar. This is only the third mammalian skull recovered from the Cretaceous in the southern hemisphere.
Vintana is the Malagasy word for “luck,” and it refers to the fossil’s chance discovery. In 2010, Joseph Sertich of the Denver Museum of Nature & Science collected a sandstone block filled with fish fossils without knowing that the skull of a previously unknown mammal was inside. They also named the species, sertichi, after him.
This 125-millimeter-long skull is twice the size of the previous largest-known mammal skull from southern Gondwana during the age of dinosaurs. The team estimate a body mass of nine kilograms (that’s nearly 20 pounds), making it the biggest primitive mammal on that supercontinent during its time -- and up to three times the size of adult groundhogs today.
Micro-CT scans and scanning electron microscopy of the braincase, nasal cavity, and inner ear reveal that Vintana had good high-frequency hearing and a keen sense of smell. In fact, 14 percent of its brain was devoted to interpreting odors, Science reports. Deep, huge sockets indicate big eyes, and all of these features suggest it was fast and agile.
The team also modeled the mechanics of Vintana’s chewing by reconstructing the cranium from CT scans and studying the anatomy of living rodents. The image on the right shows how stress is distributed through the skull under an incisor bite at a wide gape angle. (Warm colors indicate low stress, cool colors indicate high stress.) Those long, saber-like flanges were for the attachment of massive chewing muscles, suggesting that Vintana enjoyed a mixed diet of roots, seeds, and nut-like fruits.
The new findings help clarify where gondwanatherians fit in the mammalian family tree, reshaping a few branches even. “We know next to nothing about early mammalian evolution on the southern continents,” Krause says in a news release. “The discovery of Vintana will likely stir up the pot.” Gondwanatherians, they found, were closely related to multituberculates, a long-lived, successful group of rodent-like mammalian contemporaries of dinosaurs on the northern continents.
Madagascar had been isolated for over 20 million years before Vintana’s time. The researchers think its primitive features are holdovers from when the island was cut off from Africa, then Antarctica and Australia, and finally the Indian subcontinent. Isolation for that long was plenty of time for the evolution of these unique mosaic of features.
Images: Luci Betti-Nash (top), Joseph Groenke via NSF (middle), UMass Amherst (bottom)
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