Discovery of Lythronax argestes pushes back story of the dinosaur group that led to Tyrannosaurus rex by 10m years
A newly-discovered dinosaur, which has been christened “King of gore” by the scientists who have studied it, is the oldest known member of the dinosaur group that gave rise to the mighty Tyrannosaurus rex.
The 80m-year-old fossil of Lythronax argestes was found in the Grand Staircase-Escalante National Monument in the desert of southern Utah. The two-legged carnivorous beast was eight metres long and weighed around 2.5 tonnes. It had a head full of sharp teeth and lived during the Late Cretaceous period, between 95m and 65m years ago.
Lythronax, which derives from the Greek words lythron (gore) and anax (king), had a short, narrow snout, forward-pointing eyes and a wide back to its skull – similar in shape to its later relative T rex. The second part of the animal’s name, argestes, is derived from the Greek poet Homer’s south-west wind, a reference to the geographic location of the fossil.
Palaeontologists had thought that this type of tyrannosaurid dinosaur only evolved around 70m years ago, but the Lythronax discovery pushes the earliest appearance of these creatures back at least 10m years.
“The width of the back of the skull of Lythronax allowed it to see with an overlapping field of view – giving it binocular vision – very useful for a predator and a condition we associate with T rex,” said Dr Mark Loewen, a research associate at the Natural History Museum of Utah and a lead author on the study, which is published in the journal PLOS ONE.
Dr Corwin Sullivan at the Institute of Vertebrate Palaeontology and Palaeoanthropology in Beijing, who was not involved in the study, said: “It’s always great to meet a new Cretaceous tyrant, and Lythronax might just be a particularly noteworthy one. Despite being the oldest known tyrannosaurid, it’s by no means a primitive member of the group, which tells us two interesting things: that tyrannosaurids started their evolutionary radiation sooner than we thought, and that a fair bit of their early record is still missing.”
The animal lived in a place scientists call Laramidia, an island of swampy, subtropical land that stretched from Mexico to Alaska. It formed on the western side of a sea that flooded the central region of North America between 95m and 70m years ago.
That sea isolated the western and eastern portions of the North American continent for millions of years, and scientists think that separation allowed different species to evolve in different places, each in relative isolation. Lythronax is the earliest member of the tyrannosaurid group found from Laramidia.
Co-author Prof Randall Irmis, also at the Natural History Museum of Utah, said that Lythronax and other tyrannosaurids diversified between 95m and 80m years ago, at 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,” he said.
Isolation of populations of organisms often leads to their evolving into new forms – such as the finches discovered by Charles Darwin that evolved into numerous different species on the various islands of the Galapagos. The separation of Laramidia into numerous different regions might have similarly assisted the diversification of the tyrannosaurids. While Lythronax and its closest relatives are found in Utah and surrounding regions, new lines arose in the north including animals such as the slender-snouted Daspletosaurus and eventually Tyrannosaurus, which is known primarily from Montana in the US and Alberta in Canada, and its nearest relatives, which spread to Asia via Alaska.
Dr Thomas Holtz, an expert in tyrannosaur evolution at the University of Maryland, said Lythronax represented a previously unknown phase of tyrannosaurid evolution, which would lead palaeontologists to rethink their ideas about the history of these giant dinosaurs. “Prior to its discovery it seemed that we could trace the origin of the truly giant, massive-toothed, broad-snouted forms such as T rex and Tarbosaurus bataar through medium-sized, smaller toothed, less-rounded snouted tyrants, such as Daspletosaurus of 75m years ago. Lythronax shows that Daspletosaurus is not on the main line to Tyrannosaurus, and represents its own branch of the family tree.
“Instead, Lythronax shows that the massive-toothed round-snouted forms go quite far back in the tyrant lineage. This means that the extremely powerful puncture-and-pull feeding apparatus of T rex was already well developed by 80m years ago, rather than arriving late on the scene.”
Sullivan said he was intrigued, but not yet fully persuaded, by the suggestion that the high sea levels around Laramidia stimulated the diversification of tyrannosaurids. “That kind of geographic splitting can certainly create opportunities for speciation, so it’s a plausible mechanism, but I’d like to see a more extensive and fine-grained review of the evidence than Mark and his coauthors could cram into their paper – one that gets into the nitty-gritty of where the basins were, when the marine barriers between them would have appeared and disappeared, and what lived in them.”
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