A team co-led by University of Minnesota researcher Peter Makovicky and Argentinean colleague Sebastian Apesteguía has identified a 90-million-year-old fossil that provides the "missing link" for a mysterious group of prehistoric animals.
The study, published in Nature, details the discovery of a complete skeleton of Alnashetri cerropoliciensis. Alnashetri belongs to a group of bird-like dinosaurs, known as alvarezsaurs, that are famous for their tiny teeth and stubby arms ending in a single large thumb claw. For decades, they have remained a mystery because most of the well-preserved fossils were found in Asia, while records from South America were fragmented and difficult to interpret.
"Going from fragmentary skeletons that are hard to interpret, to having a near complete and articulated animal is like finding a paleontological Rosetta Stone," said Peter Makovicky, lead author and a professor in the College of Science and Engineering. "We now have a reference point that allows us to accurately identify more scrappy finds and map out evolutionary transitions in anatomy and body size."
The discovery of this nearly complete skeleton opens up a new understanding of how its lineage evolved, shrank and spread across the ancient world.
- Unlike its later relatives, Alnashetri had long arms and larger teeth. This proves that some alvarezsaurs evolved to be tiny long before they developed these specialized features thought to be adaptations for an "ant-eating" diet.
- Bone analysis confirmed this specimen was a four-year-old adult weighing less than 2 lbs, making it one of the smallest dinosaurs ever found in South America.
- By identifying previously found alvarezsaurs fossils in museum collections from North America and Europe, the team showed that these dinosaurs originated on the supercontinent Pangaea and were separated by shifting landmasses rather than ocean crossings.
Their work is far from over, as the scientists continue to discover and study fossils from the same area where they discovered Alnashetri. "We have already found the next chapter of the alvarezsaurid story there, and it is in the lab being prepared right now," added Makovicky.
The research was supported by the National Scientific and Technical Research Council (CONICET), The Field Museum, National Geographic, University of Minnesota, United States National Science Foundation and the Fulbright U.S. Scholar program.
