The pile of crumbling dinosaur bones languished in a drawer at the Museo del Desierto in Saltillo, Mexico, for two decades.
“When you see that thing sitting in a museum drawer, it looks pretty underwhelming,” said Nick Longrich, a paleontologist from the University of Bath in England. “It looks like a pile of rubble.”
But the dire condition of the bones, discovered in 2000 in northern Mexico, concealed a secret: They belonged to a close relative of Tyrannosaurs rex. Unlike its heavily built cousin, this animal was long-legged and lightly built, with big eyes that may have helped it hunt in low light and a heavy snout for dispatching helpless prey.
It is only the second tyrannosaur species ever found in Mexico. The species has been named Labocania aguillonae after Martha Carolina Aguillón, the local paleontologist who discovered it. Longrich and Héctor Rivera-Sylva of the Museo del Desierto described the species in the journal MDPI Fossil Studies, calling it a key piece of evidence of a southern tribe of tyrannosaurs that were distinct from northern specimens.
About 70 million to 80 million years ago, North America’s west was home to multiple tyrannosaur species. Most of them — animals like Albertosaurus, Daspletosaurus and the eventual emperor of the continent, Tyrannosaurus rex — are known from specimens found in the Great Plains or Canada.
Over the past two decades, however, a handful of tyrannosaur species have been discovered in Utah and New Mexico, Longrich said. The fossil record of Mexican tyrannosaurs has been scrappy. The first known species — the enigmatic Labocania anomala — is known only from jaw bones discovered in Baja California. The Coahuila specimen in the new study, however, includes parts of the skull, vertebral column, hips and limbs, making it “particularly significant for the region,” Rivera said.
The team’s analysis makes the case that the new species and other southern tyrannosaurs belong to a group known as Teratophonii. This lineage was more closely related to Tyrannosaurus rex than northern animals like Albertosaurus. “We don’t have a complete skeleton for any of these animals, but when you find places they do overlap, they look like each other,” Longrich said. “They have distinct skulls, distinct anatomy, distinct limbs. They’re doing something a bit different than the other tyrannosaurs.”
Labocania’s presence in Mexico suggests — at least before the arrival of T. rex — a greater diversity of tyrannosaurs in the late Cretaceous period in North America than researchers had assumed. Tyrannosaurs seem to have had small geographic ranges, with as many as five to 10 distinct species in different parts of the continent, Longrich said. Such diversity is notable because mammalian predators like lions, wolves, leopards and cheetahs once had large and diverse ranges. “We’re finding that these tyrannosaurs don’t really conform to that,” Longrich said.
Some paleontologists, however, aren’t sure whether the Labocania fossils are complete enough to make reliable comparisons with other tyrannosaurs. Thomas Holtz, a paleontologist at the University of Maryland who did not participate in the study, said that while the hypothesis was worth considering, “the very fragmentary nature of both Labocania species means that support of this taxonomic conclusion is weak.” — ASHER ELBEIN/NYT
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