Khankhuuluu

MEANING: Dragon prince

PERIOD: Late Cretaceous

CONTINENT: Asia

Khankhuuluu is a basal tyrannosauroid whose discovery shed new light on the evolution of the tyrannosaur family, as well as their dispersal between Asia and North America. Smaller and more gracile than its more derived relatives, Khankhuuluu was about 4 m in length, and weighed approximately 750 kg. It had a slender snout and was likely a swift runner.

Abstract from paper: Eutyrannosaurians were large predatory dinosaurs that dominated Asian and North American terrestrial faunas in latest Cretaceous times. These apex predators arose from smaller-bodied tyrannosauroids during the ‘middle’ Cretaceous that are poorly known owing to the paucity of fossil material1–3 . Here we report on a new tyrannosauroid, Khankhuuluu mongoliensis gen. et sp. nov., from lower Upper Cretaceous deposits of Mongolia that provides a new perspective on eutyrannosaurian origins and evolution. Phylogenetic analyses recover Khankhuuluu immediately outside Eutyrannosauria and recover the massive, deep-snouted Tyrannosaurini and the smaller, gracile, shallowsnouted Alioramini as highly derived eutyrannosaurian sister clades. Khankhuuluu and the late-diverging Alioramini independently share features related to a shallow skull and gracile build with juvenile eutyrannosaurians, reinforcing the key role heterochrony had in eutyrannosaurian evolution. Although eutyrannosaurians were mainly infuenced by peramorphosis or accelerated growth4–10, Alioramini is revealed as a derived lineage that retained immature features through paedomorphosis and is not a more basal lineage as widely accepted11–19. Our results reveal that Asian tyrannosauroids (similar to Khankhuuluu) dispersed to North America, giving rise to Eutyrannosauria in the mid-Late Cretaceous. Eutyrannosauria diversifed and remained exclusively in North America until a single dispersal to Asia in the latest Cretaceous that established Alioramini and Tyrannosaurini. Stark morphological diferences between Alioramini and Tyrannosaurini probably evolved due to divergent heterochronic trends—paedomorphosis versus peramorphosis, respectively—allowing them to coexist in Asia and occupy diferent ecological niches.

Khankhuuluu is from the Late Cretaceous. The Cretaceous is the third and final geological period of the Mesozoic Era, with the Late Cretaceous making up roughly the second half of it, lasting from about 100 to 66 million years ago. It was a time of significant evolutionary change, with dinosaurs reaching their greatest diversity before the mass extinction at the end of the Cretaceous.

The Cretaceous was a period with a relatively warm climate, though the Late Cretaceous experienced a global cooling trend, caused by falling levels of carbon dioxide in the atmosphere. The continents were nearing their present positions, but high sea levels flooded low-lying regions, turning Europe into an archipelago, and forming the Western Interior Seaway in North America. These seas were home to a variety of marine reptiles, including mosasaurs and plesiosaurs, while pterosaurs and birds shared the skies.

On land, dinosaurs continued to thrive and diversify during the Late Cretaceous, producing many of the most well-known goups, including tyrannosaurs, hadrosaurs, and pachycephalosaurs. Established Cretaceous dinosaur clades like the ceratopsians, ankylosaurs, and dromaeosaurs continued to flourish. Sauropod species consisted almost exclusively of titanosaurs, which seemed to be confined to the Southern Hemisphere for much of the Late Cretaceous. Flowering plants and grasses diversified and spread, becoming the dominant flora similar to what we see today.

The Cretaceous (along with the Mesozoic) ended with the Cretaceous-Paleogene (K-Pg) extinction event, a large mass extinction in which many groups, including non-avian dinosaurs, pterosaurs, and large marine reptiles, died out. This event, likely triggered by an asteroid impact, is marked by the abrupt K-Pg boundary, a distinct geologic layer separating the Mesozoic and Cenozoic Eras. In its aftermath, mammals and avian dinosaurs rapidly diversified, becoming the dominant land animals of the Cenozoic Era.

Khankhuuluu is a tyrannosaur. Tyrannosauroidea is a group of theropod dinosaurs known for often being the largest predators in their ecosystems, at the top of the food chain. They evolved throughout the Cretaceous, starting as relatively small predators before giving rise to some of the largest carnivores to ever walk the Earth. Early tyrannosaurs from the Jurassic and Early Cretaceous were generally small-bodied and built for speed, with long arms and three-fingered hands. Some of these early forms are known to have been feathered. Early tyrannosaurs lived alongside larger predatory dinosaurs like carcharodontosaurs, which dominated apex predator niches in many regions.

As the Cretaceous progressed, tyrannosaurs gradually increased in size, with later forms evolving more robust skulls, stronger bite forces, and reduced arms. By the Late Cretaceous, the most derived members of the group had become massive apex predators with thick, deep skulls specialized for crushing bone. These tyrannosaurids had only two functional fingers on each hand, a trait that distinguished them from earlier relatives. By this time, tyrannosaurs had largely replaced other large theropods in North America and Asia, cementing their role as dominant predators by the end of the Mesozoic.