Researchers have identified a 90 million year old fossil that helps solve a long standing mystery about a strange group of prehistoric animals. The discovery was led by University of Minnesota Twin Cities scientist Peter Makovicky along with Argentine paleontologist Sebastian Apesteguía.
Their findings, published in the peer reviewed journal Nature, describe a nearly complete skeleton of Alnashetri cerropoliciensis. This dinosaur belonged to a peculiar group of bird like theropods called alvarezsaurs. These animals are known for their tiny teeth and unusually short arms that end in a single enlarged thumb claw.
For decades, scientists struggled to understand this group because most well preserved fossils had been discovered in Asia. Fossils from South America were often incomplete, leaving major gaps in the evolutionary story.
Patagonia Discovery Provides a Crucial Specimen
The almost complete Alnashetri fossil was uncovered in 2014 in northern Patagonia, Argentina, at a fossil rich site famous for exceptionally preserved Cretaceous animals. The species had originally been named several years earlier based on fragmentary remains, but the new skeleton provided a far clearer view of the animal’s unusual body structure.
Preparing the specimen was a slow and careful process. Over the past decade, researchers meticulously cleaned and assembled the delicate bones to prevent damage to the small and fragile skeleton.
“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 of the study and a professor in the University of Minnesota Department of Earth and Environmental Sciences. “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 fossil is providing scientists with valuable insight into how this lineage of dinosaurs evolved, became smaller, and spread across ancient continents.
Insights Into the Evolution of Tiny Dinosaurs
The skeleton reveals that Alnashetri differed from its later relatives in several ways. It had longer arms and larger teeth, showing that some alvarezsaurs had already evolved very small body sizes before developing the specialized features that later species used for what scientists believe was an “ant-eating” diet.
Microscopic examination of the bones also showed that the animal was fully grown and at least four years old. These dinosaurs rank among the smallest known non avian dinosaurs, and they remained small throughout their lives. Even the largest members of the group only reached about the size of an average human, which is tiny compared with most dinosaurs. Alnashetri itself weighed less than 2 lbs, making it one of the smallest dinosaurs discovered in South America.
By studying additional alvarezsaur fossils preserved in museum collections across North America and Europe, the team also found evidence that these animals appeared much earlier than scientists previously believed. Their widespread distribution likely occurred when the continents were still connected as the supercontinent Pangaea. The later breakup of Earth’s landmasses explains how the animals became scattered across the globe rather than migrating across oceans.
Fossil Site Continues To Reveal Ancient Life
The well preserved skeleton came from the La Buitrera fossil area, a location that has produced many scientifically important discoveries. Previous finds from the site include early snakes and small saber toothed mammals.
“After more than 20 years of work, the La Buitrera fossil area has given us a unique insight into small dinosaurs and other vertebrates like no other site in South America,” said Apesteguía, a researcher at Universidad Maimónides in Buenos Aires, Argentina.
Scientists are still actively studying fossils from the same region, and more discoveries may soon add to the story of these unusual dinosaurs.
“We have already found the next chapter of the alvarezsaurid story there, and it is in the lab being prepared right now,” added Makovicky.
International Research Team and Support
The research involved an international collaboration of scientists. In addition to Makovicky and Apesteguía, the team included Jonathan S. Mitchell from Coe College in Iowa; Jorge G. Meso and Ignacio Cerda from Instituto de Investigación, Universidad Nacional de Río Negro and Museo Provincial; and Federico A. Gianechini from Instituto Multidisciplinario de Investigaciones Biológicas de San Luis.
Funding for the research was provided 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.