Lucky Find Uncovers a Marvelous Fossil Mammal

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.

Vintana, with a menacing Majungasaurus in the background. Art by Luci Betti-Nash.
Vintana, with a menacing Majungasaurus in the background. Art by Luci Betti-Nash.

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.

References:

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

4 thoughts on “Lucky Find Uncovers a Marvelous Fossil Mammal

  1. Interesting article. I am a veterinarian, and this animal reminds me of a large Prairie Dog. Many of the animals I treat are what are considered exotics, and quite a few of them have some pretty unusual traits. It’s interesting to watch the lines of development as animals pass through their evolutionary development. At times, I am not sure that they, along with some of us, have traveled all that far.

    That “it’s surprisingly large for a Mesozoic mammal”, may be because we aren’t all that familiar with Mesozoic animals. Although there are great fossil finds, those fossils are many millions of years old, so in actuality, we are guessing on many things.

  2. Mesozoic mammals also have a nasty tendency to leave nothing but their teeth behind. There are precious few known from skeletal material. The big jugal flanges on this guy remind me of those in sloths both large and small.

  3. “surprisingly large” is a reasonable assessment, because whatever the age, in almost all conditions we expect the fossil record to be biased towards the biggest and most robust jaws and teeth: they’re more likely to survive predation, scavenging, transport, diagenesis, erosion (and reworking, where it all happens over again from ‘transport’), and easier to notice on the ground, in a quarry or in the lab. Ecologically we expect small animals to have been more abundant in every paleofauna, but the preservational bias should show us more and better specimens (on average, per species) of the bigger members of any group.
    If you look at a phylogeny of Mesozoic mammals and stem-mammals like the ones at http://en.wikipedia.org/wiki/Evolution_of_mammals, the majority of groups named at the tips consist (as far as we yet know) entirely of mouse-sized (or smaller) species: animals bigger than a kilo or so like Repenomamus, Steropodon and Vintana stand out a long way from their close relatives, and it is ‘obvious’ (based on parsimony) that the common ancestors along the stem linking all those groups were very likely tiny as well. There’ve been recent papers analysing evolution of body size in the stem lineage of birds, it’s probably time for someone to do this (again) for mammals.

  4. Dear Brian,

    This mammal reminds me of the marmots which live in the Alps. Marmots have very keen senses, and use a peculiar system of communication, consisting in a series of squeals and chirps. They warn each other with this “language” in times of danger. I wonder if Vintana, too, might have had the ability to communicate vocally with his own kind?

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