A team of scientists led by Stony Brook paleontologist David Krause, Ph.D., discovered a 66- to 70-million-year-old mammalian skull in a block of sandstone in Madagascar that could reshape the current understanding of mammalian evolution.
“The discovery was somewhat of a surprise as we had no idea that the skull of the mammal was in this block,” Krause, a SUNY distinguished service professor in the Department of Anatomical Sciences at SBU, said. “There are only two other skulls from the entire Southern Hemisphere in the late Cretaceous Period, which this mammal is from.”
When the sandstone block, collected by one of Krause’s former graduate students in 2010, underwent a CT scan in the Department of Radiology at Stony Brook University, Krause “saw the skull of a fossil mammal staring back at us.”
The fossilized mammal is named Vintana, which means luck, for the circumstances under which the fossil was found.
The mammal is thought to be a groundhog-like herbivore due to the anatomical characteristics that Krause and his team were able to define.
The process of removing and characterizing the fossil began with a technician who spent about six months removing one sand grain at a time from the skull.
The skull then underwent additional CT scanning under a micro CT scanner, which allowed for finer resolution images of greater detail.
In a process that took “thousands upon thousands of hours,” technicians identified each individual voxel, or 3D pixel, and the anatomical structure it belonged to until all of the voxels were characterized at a very fine level, Krause said.
“We were able to not only see things that were hard to see on the outside, but we could see the sutures between individual elements and document them,” Krause said. “Most importantly we could see into the inside of the cranium, which means that we could look inside the nasal cavity, the brain case and the inner ear, and all of those are important features for figuring out how the animal lived and how it was related to other Mesozoic mammals.”
Through fossil analysis, Krause and his team found that Vintana had large olfactory bulbs and likely had a keen sense of smell, as the bulbs took up 14 percent of the mammal’s brain volume, while they only take up 0.01 percent of human brain volume.
They also found that Vintana had large eye sockets, suggesting that the mammal had large eyes, allowing it to see in low-light conditions.
Vintana had high-crowned teeth similar to the teeth of modern-day rodents, suggesting that the fossil mammal was an herbivore.
Krause and his team also found that Vintana had semicircular canals with large radii that were orthogonal to each other.
Krause compared this characteristic to extant or living mammals and found that very active mammals have similar semicircular canals.
“We’re talking about a kind of sensory machine here, with big eyes, good hearing, good sense of smell and probably quite active as well,” Krause said.
Additionally, Vintana is the second largest mammal known from the age of dinosaurs anywhere in the world, Krause said.
“It’s a surprise to find something this big in the age of dinosaurs,” Krause said. “The thought is that mammals were small at that time because they were living in the shadow of dinosaurs and would be too small for dinosaurs to eat, but this guy [Vintana] would have been a great hors d’oeuvre for a dinosaur.”
Through phylogenetic analysis of the fossil mammal, Krause found that Vintana belongs to the group Gondwanatheria, with “theria” meaning beast and “Gondwana” referring to the southern supercontinent that existed in the Cretaceous Period.
This phylogenetic classification is important because it strongly suggests that, “mammals that live on Madagascar in the late cretaceous period went extinct and did not give rise to those that live on Madagascar today,” Krause said. “Those animals that live on Madagascar today evolved from ancestors that got there after Madagascar was completely isolated in the Indian Ocean and Vintana and fellow Gondwanatherians were completely extinct.”
Prior to the discovery of Vintana, knowledge of mammals during the Mesozoic period was based largely on isolated teeth and bone fragments.
Additionally, phylogenetic analysis showed a relationship between Vintana and an earlier group thought not to be mammalian. These findings therefore “bring back the origin of mammals to 30 million years earlier than we knew before,” Krause said.
Krause and his team have also discovered several other well-preserved fossils in the same site that Vintana was found, including the fossils on display in the Stony Brook Administration Building and in the lobby of Stony Brook Medicine.
“It’s been going for 21 years now and in many ways I feel we’ve just knocked off the tip of the iceberg,” Krause said. “We hope to continue to do more and more fieldwork and find more and more fossils. The exciting thing about science is that you can answer some question and you end up raising a lot more.”