U scientists unearth the oldest Tyrannosaur to Date

Oct. 23, 2013 – A remarkable new species of tyrannosaur has been unearthed in Grand Staircase-Escalante National Monument (GSENM), southern Utah.  The huge carnivore inhabited Laramidia, a landmass formed on the western coast of a shallow sea that flooded the central region of North America, isolating western and eastern portions of the continent for millions of years during the Late Cretaceous Period, between 95-70 million years ago. The newly discovered dinosaur, belonging to the same evolutionary branch as the famous Tyrannosaurus rex, was announced today in the open-access scientific journal PLoS ONE andunveiled on exhibit in the Past Worlds Gallery at the Natural History Museum of Utah at the Rio Tinto Center in Salt Lake City, Utah.

Among tyrannosaurs, a group of small to large-bodied, bipedal carnivorous dinosaurs including T. rex that lived during the Jurassic and Cretaceous periods, the newly discovered species, Lythronax argestes, possesses several unique features, a short narrow snout with a wide back of the skull with forward-oriented eyes.  Lythronax translates as “king of gore,” and the second part of the name, argestes, refers to its geographic location in the American Southwest. Previously, paleontologists thought this type of wide-skulled tyrannosaurid only appeared 70 million years ago, whereas Lythronax shows it had evolved at least 10 million years earlier.

The study, funded in large part by the Bureau of Land Management and the National Science Foundation, was led by Mark Loewen, research associate at the Natural History Museum of Utah, and adjunct assistant professor in the department of geology and geophysics at the University of Utah.  Additional collaborative authors include Randall Irmis, Natural History Museum of Utah and department of geology and geophysics, University of Utah; Joseph Sertich, Denver Museum of Nature & Science; Philip Currie, University of Alberta; and Scott Sampson, Denver Museum of Nature & Science. The skeleton was discovered by BLM employee Scott Richardson, and excavated by a joint NHMU-GSENM team.

Lythronax lived on Laramidia, along the western shores of the great seaway that separated North America; this landmass hosted a vast array of unique dinosaur species and served as the crucible of evolution for iconic dinosaur groups such as the horned and duck billed dinosaurs. This study also indicates that tyrannosaurid dinosaurs (the group of tyrannosaurs that includes T. rex) likely evolved in isolation on this island continent. Lythronax stands out from its contemporaries in having a much wider skull at the eyes and a narrow short snout, similar to its relative T. rex, which lived 10 to 12 million years later. Mark Loewen, the study’s lead author, noted, “The width of the back of the skull of Lythronax allowed it to see with an overlapping field of view—giving it the binocular vision— very useful for a predator and a condition we associate with T. rex.”  Previously, paleontologists thought this type of wide-skulled tyrannosaurid only appeared approximately 70 million years ago, whereas Lythronax shows it had evolved at least 10 million years earlier.

Paleontologists have recently determined that the dinosaurs of southern Laramidia (Utah, New Mexico, Texas and Mexico), although belonging to the same major groups, differ at the species level from those on northern Laramidia (Montana, Wyoming, the Dakotas and Canada).  Lythronax and its tyrannosaurid relatives on southern Laramidia are more closely related to each other than the long snouted forms from northern Laramidia. Joseph Sertich, a co-author of the study, stated that, “Lythronax may demonstrate that tyrannosaurs followed a pattern similar to what we see in other dinosaurs from this age, with different species living in the north and south at the same time.”

These patterns of dinosaur distribution across Laramidia lead the researchers to ask what might have caused the divisions between the north and south, given that an enterprising dinosaur could have walked from Alaska to Mexico if given enough time. Randall Irmis, a study co-author, explained that by analyzing the evolutionary relationships, geologic age and geographic distribution of tyrannosaurid dinosaurs, the team determined that “Lythronax and other tyrannosaurids diversified between 95 to 80 million years ago, during a time when North America’s interior sea was at its widest extent. The incursion of the seaway onto large parts of low-lying Laramidia would have separated small areas of land from each other, allowing different species of dinosaurs to evolve in isolation on different parts of the landmass.” As the seaway gradually retreated after 80 million years ago, these differences in dinosaur species may have been reinforced by climate variations, differences in food sources (different prey and plants), and other factors.  This hypothesis explains why the iconic Late Cretaceous dinosaurs of western North America are so different from those of the same age on other continents.

A Treasure Trove of Dinosaurs on the Lost Continent of Laramidia

Lythronax was discovered in Grand Staircase-Escalante National Monument (GSENM), which encompasses 1.9 million acres of high desert terrain in south-central Utah.  This vast and rugged region, part of the National Landscape Conservation System administered by the Bureau of Land Management (BLM), was the last major area in the lower 48 states to be formally mapped by cartographers. Today GSENM is the largest national monument in the United States. Co-author Scott Sampson proclaimed that, “Grand Staircase-Escalante National Monument is the last great, largely unexplored dinosaur boneyard in the lower 48 states.”

During the past 14 years, crews from the Natural History Museum of Utah, GSENM, the Denver Museum of Nature & Science, and several other partner institutions (for example, the Raymond Alf Museum of Paleontology and Utah Geological Survey) have unearthed a new assemblage of more than a dozen species dinosaurs in GSENM.  In addition to Lythronax, the collection includes a variety of other plant-eating dinosaurs—among them duck-billed hadrosaurs, armored ankylosaurs, dome-headed pachycephalosaurs, and two other horned dinosaurs—Utahceratops and Kosmoceratops—together with carnivorous dinosaurs great and small, from “raptor-like” predators such as Talos, to another large tyrannosaur named Teratophoneus. Amongst the other fossil discoveries are fossil plants, insect traces, snails, clams, fishes, amphibians, lizards, turtles, crocodiles and mammals. Together, this diverse bounty of fossils is offering one of the most comprehensive glimpses into a Mesozoic ecosystem. Remarkably, virtually all of the identifiable dinosaur remains found in GSENM belong to new species.

Philip Currie, another co-author, stated that, “Lythronax is a wonderful example of just how much more we have to learn about the world of dinosaurs. Many more exciting fossils await discovery in Grand Staircase-Escalante National Monument.”

Study Summary

Dinosaurs were the dominant members of terrestrial ecosystems during the Mesozoic. However, the pattern of evolution and dispersal of these groups and other terrestrial vertebrates during the end of the Mesozoic remains poorly understood. We report the discovery of the earliest member of the group of large-bodied tyrannosaurs that led to Tyrannosaurus rex and illustrate that this clade diversified during the Late Cretaceous on the island continent of western North America and later dispersed to Asia. Our new phylogenetic analysis of tyrannosaurid dinosaurs reveals a previously unrecognized common pattern of diversification among Late Cretaceous vertebrates (that is, turtles, crocodylians, ceratopsians), and we hypothesize that this pattern was directly linked to sea level-related isolation during the last 15 to 20 million years of the Mesozoic. Utilizing new data from a new 80 million year old Utah tyrannosaurid (Lythronax), new material of another Utah taxon (Teratophoneus), a novel and comprehensive phylogenetic analysis of theropod dinosaurs, and a novel quantitative biogeographic analysis, our results indicate that tyrannosaurid dinosaurs and other non-marine tetrapods originated in isolation on western North America (Laramidia) following separation from eastern North America (Applalachia) by incursion of the Cretaceous Interior Seaway, diversified in isolated basins during maximum extent of the seaway, and dispersed across western North America and into Asia only after a major drop in sea level near the end of the Cretaceous.

Fact Sheet: Major Points of the Paper

(1) A remarkable new species of tyrannosaur, Lythronax argestes, has been unearthed in Grand Staircase-Escalante National Monument, southern Utah.

(2) Lythronax is distinguished by a number of unique features, including a relatively narrow short snout with a wide back of the skull that would have resulted in binocular vision.

(3) At 80 million years old, Lythronax is the geologically-oldest tyrannosaurid dinosaur ever discovered. Tyrannosauridae is the group of tyrannosaurs most closely related to T. rex; the really large-bodied forms with small arms.  Earlier tyrannosaurs were much smaller and more lightly built.

(4) The fact that the geologically-oldest tyrannosaurid (Lythronax) is most closely related to the geologically-youngest tyrannosaurids (T. rex and Tarbosaurus at 70 to 66 million years old) indicates that most of the group must have diversified prior to 80 million years ago, so there are a lot of undiscovered tyrannosaurids out there waiting to be found.

(5) This diversification of tyrannosaurids prior to 80 million years ago appears to correlate with a time of high sea levels, which may have isolated tyrannosaurids in different pockets of Laramidia, allowing them to differentiate and diverge into separate lineages, or separate branches of the family tree.

(6) This seems to be a common pattern for many Laramidian Late Cretaceous vertebrate animals, so sea levels may have played an important role in explaining why we are finding so many different species in each Laramidian basin in rocks from 80 to 74 million years old.

(7) Our analysis indicates that tyrannosaurids originated in northern Laramidia (western North America), with multiple species moving into southern Laramidia over time, and some species immigrating to Asia towards the end of the Cretaceous between 75 to 70 million years ago.

Study Design

• Our comparison of the bones with all other known tyrannosaurid dinosaurs indicate that the new tyrannosaurid possessed unique features of the upper jaw (maxilla), snout, and side of the skull.

• On the basis of these features, the scientific team named it a new genus and species of tyrannosaurid dinosaur, Lythronax argestes, translating to “gore king from the southwest”.

• Lythronax is particularly notable for its slender snout but very wide back of the skull, features shared only with Tyrannosaurus rex and its close cousin Tarbosaurus bataar.

• To reconstruct the evolutionary relationships (family tree) of Lythronax, we analyzed 501 skeletal features for 54 different species of carnivorous dinosaurs, and discovered that Lythronax is most closely related to Tyrannosaurus rex and Tarbosaurus bataar.

• We then used this family tree and information about the geologic age and geographic location of different tyrannosaurid dinosaurs to reconstruct the biogeography of the group – that is, the evolution of their distribution across different continental landmasses.

New Dinosaur Name: Lythronax argestes

• The first part of the name, Lythronax, (LYE-thro-nax) can be translated as the “king of gore” or “gore king” in reference to its large teeth and carnivorous lifestyle. The second part of the name argestes (ar-GES-tees) comes from the poet Homer’s southwest wind, in reference to the southwestern geographic location of the specimen.

Size

• Lythronax was approximately 24 feet (8 meters) long and weighed around 2.5 tons.

Relationships

• Lythronax belongs to a group of carnivorous dinosaurs called “tyrannosaurids,” the same group as the famous Tyrannosaurus rex.

Anatomy

• Lythronax was a two-legged carnivore, the largest carnivore in its ecosystem.

• Like other tyrannosaurid dinosaurs, Lythronax had a large head full of sharp teeth.

Age and Geography

• Lythronax lived during the Campanian stage of the Late Cretaceous period, which spanned from approximately 84 million to 70 million years ago. This animal lived about 80 million years ago.

• Lythronax lived in a swampy, subtropical coastal setting on the “island continent” of western North America, known as “Laramidia.”

• Lythronax represents the earliest member of the tyrannosaurid group from Laramidia.

• During the Late Cretaceous, the North American continent was split in two by the Western Interior Seaway.  Western North America formed an island continent called Laramidia, stretching from Mexico in the south to Alaska in the north.

• Lythronax is a member of a group or relatively short-snouted tyrannosaurs that were restricted to the southern part of Laramidia at the same time long-snouted tyrannosaurs were living in the northern parts of Laramidia. This finding presents strong evidence of dinosaur provincialism on Laramidia—that is, the formation of northern and southern dinosaur assemblages during a part of the Late Cretaceous.

Discovery

• Lythronax was found in a geologic unit known as the Wahweap Formation, abundantly exposed in GSENM, southern Utah.

• The Lythronax discovery site is in the southern part of BLM-administered Grand Staircase-Escalante National Monument (GSENM) in Kane County, northeast of Big Water, Utah.

• Lythronax was first discovered by BLM employee Scott Richardson in 2009. Scott also found the first specimen of Kosmoceratops, a bizarre horned dinosaur from GSENM.

• Lythronax specimens are permanently housed in the collections and on public display at the Natural History Museum of Utah in Salt Lake City, Utah.

• These discoveries are the result of a continuing collaboration between the Natural History Museum of Utah, the Denver Museum of Nature & Science, and the Bureau of Land Management.

Excavation

• The skeleton of Lythronax was excavated in March and May of 2009 by Natural History Museum of Utah crews in in collaboration with paleontologists from GSENM.

• The bones of Lythronax that were discovered include parts of the skull (maxilla, nasal, jugal, quadrate, frontal, laterosphenoid, palatine, dentary, splenial, surangular, prearticular), hips (pubis), and leg (tibia, fibula, metatarsals) and tail (chevron).

• Other fossils found along with Lythronax at the site include parts of turtle shell, crab claws, leaf impressions, and a few molluscs.  The bizarre horned dinosaur Diabloceratops eatoni was recovered from nearby rocks of the same age.

Preparation

• It required approximately 10 months to fully prepare all of the bones of Lythronax.

• Nearly all preparation was done by NHMU volunteers Marilyn Harris, Jerry Golden and Randy Johnson.

Other

• The fossil record of tyrannosaurid dinosaurs from the southern part of Laramidia has been very poorly known. The discovery of this new dinosaur in Utah helps to fill a major gap in our knowledge.

• Lythronax is part of a previously unknown assemblage of dinosaurs discovered in GSENM over the past 12 years.

• The skull of Lythronax is on permanent display at the Natural History Museum of Utah.

• The Bureau of Land Management manages more land—253 million acres—than any other federal agency, and manages paleontological resources using scientific principles and expertise.

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