Mesozoic mammals were fascinating little beasts. They burrowed, climbed, glided, and swam through the Age of Dinosaurs, not as underdogs waiting for their moment to be free of the great reptiles, but as varied, successful creatures. And they keep getting stranger. Mammals previously known only from their dentition are starting to come into view thanks to the discovery of skulls and skeletons. The latest to debut is Vintana sertichi – what looks like a Mesozoic muskrat with some evolutionary tales to tell.
Vintana was a lucky find. The mammal’s skull was hidden inside a 150 pound chunk of sandstone collected from the 72-66 million year old rock of Madagascar by then-graduate-student, now Denver Museum of Nature and Science paleontologist, Joseph Sertich. A CT scan of the block is what gave Vintana away and allowed paleontologists a rare look at a lineage of prehistoric beasts previously known from teeth and bits of jaw.
In the big picture of mammalian evolution, Stony Brook University paleontologist David Krause and colleagues write in their description, Vintana was a gondwanathere. That name comes from the lost, southern supercontinent Gondwana that included South America, Antarctica, Africa, and other landmasses where these mammals have been found. And by comparing the skull – the only specimen of Vintana yet known – with other recent fossil finds, Krause and coauthors concluded that the gondwanatheres had a close relationship to a prolific group of superficially squirrel-like mammals called multituberculates that lived among the northern continents. This, Oklahoma State University paleontologist Anne Weil points out, adds increasing support for a grouping of mammals that got a mention in a They Might Be Giants song but has long been controversial among researchers – the Allotheria.
Not that Vintana can be taken as a typical gondwanathere. This mammal was weird.
First of all, Vintana was pretty large for its time. From front to back, the skull of Vintana measures a little under five inches. That’s not especially big in absolute terms – you could hold the mammal’s skull in the palm of your hand – but it’s surprisingly large for a Mesozoic mammal. Among known fossil mammals Vintana is second in size only to Repenomamus, the badger-sized beast that snacked on baby dinosaurs.
Vintana had a different diet, though. From the shape of the teeth and the microscopic wear patterns upon them, Krause and coauthors suspect that Vintana was an herbivore that chewed large or especially tough plant food like roots or seeds. The mammal’s flaring cheeks are in line that that notion. These wings of bone would have allowed for bigger and more powerful chewing muscles, allowing Vintana to crack open, and chew into, foods inaccessible to other mammals.
Krause and colleagues were even able to draw out some clues about the mammal’s senses. Vintana had relatively large eyes for its size – useful for seeing in low light or with better acuity in better-lit conditions – and an inner ear suited to keeping those eyes stable during quick movements of the head. The ears of this marmot-sized mammal were also attuned to a relatively narrow range of high-frequency calls, and, as indicated by the large impressions for the olfactory bulbs on the inside of the skull, Vintana probably had a sharp sense of smell. Good eyesight, the ability to move fast, and a nuanced sense of smell seem like they’d all be advantages with snaggle-toothed dinosaurs around.
But what makes Vintana so remarkable isn’t what we know about it. It’s what we don’t know just yet. While Vintana shows some specialized traits not seen among Mesozoic mammals before, Krause and coauthors point out, aspects of the mammal’s ear and braincase more closely resemble those of protomammals that lived over 130 million years earlier. That makes Vintana of “mosaic” of archaic and derived traits that point to an unusual evolutionary history. Isolation on islands may explain why.
When Vintana lived on Madagascar, between 72 and 66 million years ago, the island had recently gone through a couple of major breakups. The first was about 112 to 115 million years ago. This is when the combined chunk of Madagascar and India split from Cretaceous Africa. Then, around 88 million years ago, Madagascar and India broke off their connection. Following this pattern, Krause and coauthors suggest that Vintana is the descendant of gondwanatheres that became increasingly isolated as these landmasses split and shifted. Rather than being the rule for gondwanatheres, Vintana might be an island oddity – the specialized descendant of a relatively archaic ancestral lineage that managed to survive in an isolated pocket. Testing this idea will take more time and more fossils, but, for now, Vintana is beginning to whisper an untold evolutionary tale.
Krause, D., Hoffman, S., Wible, J., Kirk, E., Schultz, J., von Koenigswald, W., Groenke, J., Rossie, J., O’Connor, P., Seiffert, E., Dumont, E., Holloway, W., Rogers, R., Rahantarisoa, L., Kemp, A., Andriamialison,H. 2014. First cranial remains of a gondwanatherian mammal reveal remarkable mosaicism. Nature. doi: 10.1038/nature13922
Weil, A. 2014. A beast of the southern wild. Nature. doi: 10.1038/nature13940