Mini tyrannosaur fills big evolutionary gap | Science News for Students

Mini tyrannosaur fills big evolutionary gap

This deer-sized T. rex relative shows how quickly tyrannosaurs got big
Mar 22, 2019 — 6:45 am EST
an illustration of Moros intrepidus

About the size of a deer, the newly discovered Moros intrepidus (illustrated) is one of the smallest tyrannosaurs known.

Jorge Gonzalez, © L. Zanno

Even the giant Tyrannosaurus rex had humble beginnings. A new fossil shows that an early ancestor was only about the size of a deer. Its discovery helps fill a 70-million-year gap in the evolution of giant tyrannosaurs such as T. rex.

Lindsay Zanno is a paleontologist at North Carolina State University in Raleigh. She and her colleagues dug for 10 years around Emery County in Utah. They were searching for clues to solve a longtime dino mystery: When and how did tyrannosaurs get their famous bulk?

Early tyrannosaurs were much smaller. Teeth from petite species have been found in rocks in North America dating to around 150 million years ago. At that time, in the Late Jurassic Period, large allosaurs topped the food chain. The next time tyrannosaurs showed up in the North American fossil record was 70 million years later, during the Cretaceous Period. By then, they had become the colossal top predators best known today.

Zanno and her team were looking for clues to what happened in between when they found a long, thin leg bone. It dated to around 96 million years ago. They determined the fossil came from a new species of tyrannosaur. It’s the oldest one known from the Cretaceous. They named the species Moros intrepidus, or “omen of doom.”

M. intrepidus is one of the smallest tyrannosaurs from the Cretaceous. Analyses of the fossil leg show it would have stood about 1.2 meters (4 feet) tall at the hip. It probably weighed about 78 kilograms (172 pounds). That’s about the size of a mule deer. The find was described February 21 in Communications Biology.

The bone’s long, thin shape suggests M. intrepidus was a swift runner. Later titanic tyrannosaurs were likely far less speedy.

“What Moros shows is that the ancestral stock of the big tyrannosaurs was small and fast,” says Thomas Carr. He studies tyrannosaurs at Carthage College in Kenosha, Wis. He wasn’t part of the new study. But the new fossil also suggests something big — literally — happened after Moros, Carr says. “The tyrannosaurs became giant sometime in that 16-million-year stretch” between Moros and T. rex, he notes.

The researchers used traits of the new fossil to see where M. intrepidus fit into the tyrannosaur family tree. They determined that M. intrepidus came from Siberia in Asia. It could have reached modern-day Alaska when sea levels were low, the authors say. Many other animals followed a similar route from Asia. That great migration included mammals, lizards and other dinosaurs.

The Cretaceous Period’s warming climate probably killed off the allosaurs, Zanno says. But not the tyrannosaurs. “They rapidly increase in size and go on really quickly to become the dominant predators,” she says.

M. intrepidus leaves plenty of questions about how tyrannosaurs evolved. “It’s great that [the new fossil] helps fill in part of the history,” says Thomas Holtz Jr. He’s a tyrannosaur expert at the University of Maryland in College Park. Scientists still need to find the rest of the skeleton for M. intrepidus. Other tyrannosaurs from the gap between M. intrepidus and its giant descendants could help pinpoint when the creatures exploded in size.

Concludes Holtz: “The story of tyrannosaurs is definitely not over.”

Power Words

(more about Power Words)

allosaurs     (also known as allosauroids) A group of two-legged, meat-eating dinosaurs named for one of its oldest species, Allosaurus .

biology     The study of living things. The scientists who study them are known as biologists.

climate     The weather conditions that typically exist in one area, in general, or over a long period.

colleague     Someone who works with another; a co-worker or team member.

Cretaceous     A geologic time period that included the end of the Age of Dinosaurs. It ran from roughly 145.5 million years ago until 65.5 million years ago.

descendant     A blood relative of a person who lived during a previous time.

dinosaur     A term that means terrible lizard. These ancient reptiles lived from about 250 million years ago to roughly 65 million years ago. All descended from egg-laying reptiles known as archosaurs. Their descendants eventually split into two lines. For many decades, they have been distinguished by their hips. The lizard-hipped line are believed to have led to the saurichians, such as two-footed theropods like T. rex and the lumbering four-footed Apatosaurus (once known as brontosaurus). A second line of so-called bird-hipped, or ornithischian dinosaurs, appears to have led to a widely differing group of animals that included the stegosaurs and duckbilled dinosaurs. But a new 2017 analysis now calls into question that characterization of relatedness based on hip shape.

evolution     (v. to evolve) A process by which species undergo changes over time, usually through genetic variation and natural selection. These changes usually result in a new type of organism better suited for its environment than the earlier type. The newer type is not necessarily more “advanced,” just better adapted to the particular conditions in which it developed. Or the term can refer to changes that occur as some natural progression within the non-living world (such as computer chips evolving to smaller devices which operate at an ever faster speed).

fossil     Any preserved remains or traces of ancient life. There are many different types of fossils: The bones and other body parts of dinosaurs are called “body fossils.” Things like footprints are called “trace fossils.” Even specimens of dinosaur poop are fossils. The process of forming fossils is called fossilization.

Jurassic     Lasting from about 200 million to 145.5 million years ago, it’s the middle period of the Mesozoic Era. This was a time when dinosaurs were the dominant form of life on land.

literally     A term that the phrase that it modifies is precisely true. For instance, to say: "It's so cold that I'm literally dying," means that this person actually expects to soon be dead, the result of getting too cold.

lizard     A type of reptile that typically walks on four legs, has a scaly body and a long tapering tail. Unlike most reptiles, lizards also typically have movable eyelids. Examples of lizards include the tuatara, chameleons, Komodo dragon, and Gila monster.

mammal     A warm-blooded animal distinguished by the possession of hair or fur, the secretion of milk by females for feeding their young, and (typically) the bearing of live young.

migration     (v. migrate) Movement from one region or habitat to another, especially regularly (and according to the seasons) or to cope with some driving force (such as climate or war). An individual that makes this move is known as a migrant.

paleontologist     A scientist who specializes in studying fossils, the remains of ancient organisms.

predator     (adjective: predatory) A creature that preys on other animals for most or all of its food.

sea level     The overall level of the ocean over the entire globe when all tides and other short-term changes are averaged out.

Siberia     A region in northern Asia, almost all of which falls within Russia. This land takes its name from the language of the Tatar people, where Siber means sleeping land. This region is vast. It has become famous for its long, harsh winters, where temperatures can fall to −68° Celsius (−90° Fahrenheit).

species     A group of similar organisms capable of producing offspring that can survive and reproduce.

trait     A characteristic feature of something. (in genetics) A quality or characteristic that can be inherited.

tyrannosaur     A line of meat-eating dinosaurs that began during the late Jurassic Period, about 150 million years ago. These species persisted into the late Cretaceous Period, about 65 million years ago. The best known member of these species: the late Cretaceous’ Tyrannosaurus rex, a 12-meter (40 foot) long top predator of its time.

Tyrannosaurus rex     A top-predator dinosaur that roamed Earth during the late Cretaceous period. Adults could be 12 meters (40 feet) long.


Journal: L. Zanno et al. Diminutive fleet-footed tyrannosauroid narrows the 70-million-year gap in the North American fossil record. Communications Biology. Published online February 21, 2019. doi:10.1038/s42003-019-0308-7.