Newly discovered fossil is a clue to early mammalian evolution

A newly discovered 66-70 million-year-old groundhog-like creature, massive in size compared to other mammals of its era, provides new and important insights into early mammalian evolution. Stony Brook University paleontologist David Krause, PhD, led the research team that unexpectedly discovered a nearly complete cranium of the mammal, which lived alongside Late Cretaceous dinosaurs in Madagascar. The findings, which shake up current views on the mammalian evolutionary tree, will be published in the journal Nature on November 5.

Cast of the skull of the mammal Vintana sertichi, left, made by Joseph
 Groenke, and a life reconstruction Gary Staab with Staab Studios. The 
skull of the previously unknown mammal was found in Madagascar
 [Credit: Stony Brook University/Joseph Groenke]

"We know next to nothing about early mammalian evolution on the southern continents," said Dr. Krause, a SUNY Distinguished Service Professor in the Department of Anatomical Sciences at Stony Brook. "This discovery, from a time and an area of the world that are very poorly sampled, underscores how very little we know. No paleontologist could have come close to predicting the odd mix of anatomical features that this cranium exhibits."

The new fossil mammal is named Vintana sertichi. Vintana belongs to a group of early mammals known as gondwanatherians, which previously were known only from isolated teeth and a few scrappy jaw fragments. The well-preserved skull allows the first clear insight into the life habits and relationships of gondwanatherians.

"The discovery of Vintana will likely stir up the pot," added Krause. "Including it in our analyses reshapes some major branches of the 'family tree' of early mammals, grouping gondwanatherians with other taxa that have been very difficult to place in the past."

The skull is huge, measuring almost five inches long, twice the size of the previously largest known mammalian skull from the entire Age of Dinosaurs of the southern supercontinent of Gondwana. At a time when the vast majority of mammals were shrew- or mouse-sized, living in the shadows of dinosaurs, Vintana was a super heavyweight, estimated to have had a body mass of about 20 pounds, twice or even three times the size of an adult groundhog.

Cast of the skull of the mammal Vintana sertichi. The skull of the 
previously unknown mammal was found in Madagascar 
[Credit: Stony Brook University/Joseph Groenke]

Adding to the intrigue is the fact that the cranium of Vintana has a peculiar shape, being very deep, with huge eye-sockets, and long, scimitar-shaped flanges for the attachment of massive chewing muscles.

The initial discovery was made in 2010 and, like many in science, came about by chance.

Vintana means luck and refers to the circumstances that its discoverer, Joseph Sertich, a former graduate student of Dr. Krause's at Stony Brook University, had in finding the fossil in 2010. Sertich, now a curator at the Denver Museum of Nature & Science, collected a 150 block of sandstone filled with fish fossils.

When the block was CT scanned in Stony Brook's Department of Radiology the images revealed that something exceedingly rare lay inside -- a nearly complete cranium of a previously unknown ancient mammal. The specimen represents only the third occurrence of mammalian skulls from the Cretaceous of the entire Southern Hemisphere, the other two being from Argentina.

"When we realized what was staring back at us on the computer screen, we were stunned," said Joe Groenke, Krause's technician and the first to view the CT images. Groenke spent the next six months extracting the skull from the surrounding rock matrix, one sand grain at a time.

A finite element analysis result, showing how stress is distributed through 
the skull under an incisor bite at a wide gape angle 
[Credit: UMass Amherst]

Dr. Krause and his team conducted a comprehensive analysis of the skull, much of it using micro-computed tomography and scanning electron microscopy to reveal minute aspects of its anatomy, including areas like the braincase, nasal cavity, and inner ear that are poorly known in almost all early mammals. They compared the skull to those of hundreds of other fossil and extant mammals.

Various features of its teeth, eye sockets, nasal cavity, braincase, and inner ears revealed that Vintana was likely a large-eyed herbivore that was agile, with keen senses of hearing and smell. These and other features were also used to analyze its relationships to other early mammals. This phylogenetic analysis demonstrated that Vintana and other gondwanatherians were close relatives of multituberculates, the most successful mammalian contemporaries of dinosaurs on the northern continents.

Gondwanatherians and multituberculates also grouped with another enigmatic taxon, the Haramiyida. The analysis by Krause's team is the first to find strong evidence for clustering these three groups together, primarily because the cranial anatomy of gondwanatherians was previously completely unknown.

Dr. Krause emphasizes that a major question remains for scientists: How did such a peculiar creature evolve?

Artist's reconstruction of an ancient mammal Vintana sertichi. The skull of the
 previously unknown mammal was found in Madagascar 
[Credit: Nature/Luci Betti-Nash]

With its long-term isolation from the rest of the world, Madagascar had been an island for over 20 million years prior to the time in which the strata containing Vintana were deposited. Dr. Krause and his team theorize that the very primitive features of the cranium are holdovers from when the ancient lineage that ultimately produced Vintana was marooned on the island. It was this isolation, first from Africa, then Antarctica/Australia, and finally the Indian subcontinent, that allowed for the evolution of unique and bizarre features amidst Vintana's primitive foundation of characteristics.

The research by Dr. Krause and colleagues on Vintana sertichi is supported by the National Science Foundation and the National Geographic Society.

Dr. Zhe-Xi Luo, a leading expert on early mammalian evolution from the University of Chicago, reviewed the manuscript for Nature.

He hailed the Vintana as "the discovery of the decade for understanding the deep history of mammals; it offers the best case of how plate tectonics and biogeography have impacted animal evolution -- a lineage of mammals isolated on a part of the ancient Gondwana had evolved some extraordinary features beyond our previous imagination. This new study of Vintana is a giant leap forward toward resolving the long-standing mystery of gondwanatherian mammals, which has puzzled paleontologists for decades."

Luo went on to say, "Vintana is also a galvanizing discovery for the future decades. With features so remarkably different from those of other mammals previously known to science, this fossil tells us how little we knew about the early evolution of mammals -- it will stimulate paleontologists to conduct more field exploration in order to advance the frontier of deep time history and evolution."

Dr. Guillermo Rougier from the University of Louisville, another expert who also reviewed the Nature paper, concurred, calling the study "a remarkable achievement" and predicted that the paper "will shake the field upon publication; the specimen is exceptional."

"This is the first discovery of a cranial fossil from a very enigmatic extinct group of mammals called Gondwanatheria in the Southern Hemisphere," says Dr. Yusheng (Chris) Liu, Program Director in the National Science Foundation (NSF)'s Division of Earth Sciences, which funded the research. "This important find will help us better understand the early evolution of gondwanatherians and their relatives."

Source: Stony Brook University [November 05, 2014]