Khankhuuluu

The Khankhuuluu fossil material was discovered in 1972–1973 in outcrops of the Bayanshiree Formation (Baishin-Tsav locality) in southeastern Mongolia. The collected specimens are now accessioned at the Mongolian Academy of Sciences. The first specimen MPC-D 100/50, comprises a partial skeleton including cranial bones (the , incomplete left , parts of both , a , and parts of both and ), several vertebrae (three and the last 17 ), a furcula, the left , and the left III. The second specimen, MPC-D 100/51, also includes cranial bones (the right , , and quadrate), a partial left ( and partial ), part of the right hindlimb (proximal part of the and four toe bones), and part of the left hindlimb (incomplete ). These remains were first described by Mongolian paleontologist Altangerel Perle in 1977. He assigned them to the Chinese species Alectrosaurus olseni, known from the Iren Dabasu Formation, claiming that the bones provided evidence that these two formations were synchronous. In 2012, Tsuihiji and colleagues described an isolated left bone, MPC-D 102/4, from the Bayanshiree Formation (Tsaagan Teg region). In a separate paper published that year, Averianov and Hans-Dieter Sues described several isolated cranial and postcranial tyrannosauroid remains (later named as a new taxon, Timurlengia) from the Bissekty Formation of Uzbekistan. Citing tyrannosaur researcher Thomas Carr, they noted that the Bayanshiree "Alectrosaurus" material could not be definitively referred to this genus. == Description ==

Khankhuuluu is a medium-sized tyrannosauroid. None of the known specimens preserve a complete skull. However, the cranial bones of the two more complete specimens, MPC-D 100/50 and MPC-D 100/51, suggest a total skull length between long. MPC-D 100/51 is slightly smaller than the holotype and has a femur long. Both specimens likely belong to mature individuals. Sexual maturity is suggested by the development of ornamentation on the nasal and lacrimal. Skeletal maturity is implied by the close of the of the dorsal vertebrae, as well as the interdigitating frontal sutural surface of the postorbital. Like most eutyrannosaurians ('true tyrannosaurs', the clade including giant Asian and North American taxa such as the tyrannosaurids), the fused midline bones are rugose and have many small bosses. The has a rugose cornual process (bony protuberance), another typical eutyrannosaurian feature. However, unlike tyrannosaurids, in which this horn-like process has a dorsal component, it is strictly laterally directed in Khankhuuluu. In contrast to adult tyrannosaurids, which have a prominent cornual process on the lateral surface of the , this bone is smooth in Khankhuuluu. This is more similar to juvenile tyrannosaurids and the more basal tyrannosauroid Xiongguanlong. As evidenced by the morphology of the postorbital, the posterior margin of the orbit is round, lacking the suborbital flange seen in both juvenile and mature tyrannosaurids. While the orbit is also round in juvenile tyrannosaurids, it becomes more angular through growth with the increased development of the suborbital flange. There are two rows of (small holes) on the anterior (front) end of the maxilla, which merge into a single row posteriorly (toward the back). Postcranial skeleton The have a that is deeper than it is long and tall that exceed the height of the centra. While this is typical for eutyrannosaurians, it is different from the earlier Xiongguanlong which lacks the first character, and Timurlengia which lacks both. The of the ilium tapers toward the back when viewed from the side, while in tyrannosaurids it is long and rectangular. This is more similar to the earlier-diverging genera Juratyrant and Stokesosaurus. The ischium has a prominent tubercle on the posterior surface and it lacks expansion on the distal end. Both of these features are more similar to eutyrannosaurians than earlier taxa. Among the bones described by Perle in 1977 but not located by Voris and colleagues in 2025 were a manual (hand claw), likely one from the first digit, and the proximal end of a . The ungual is long. These bones presumably indicate large claws and forelimb proportions compared to later tyrannosaurids like Albertosaurus. Like Xiongguanlong and later taxa, the of the femur is as tall as the . While the tibia is incomplete, it was likely notably longer than the femur. The leg bones of alioramins, immature tyrannosaurids, and the earlier genera Moros and Dilong are proportionally similar in this sense. However, eutyrannosaurians excluding alioramins have a tibia that is shorter than, or comparable in length to, the femur. == Classification ==

Prior to the formal description and naming of Khankhuuluu, some studies commented on the relationships of the Bayanshiree fossil material. When included in a phylogenetic analysis, the Bissekty tyrannosauroid Timurlengia was recovered in a position similar to that later found for Khankhuuluu in its 2025 description, as a late-diverging tyrannosauroid outside of the Tyrannosauridae. == Paleoecology ==

Age and palaeoenvironment (Khankhuuluu shown in red, left) Khankhuuluu is known from the 'Baishin-Tsav' and 'Tsaagan Teg' localities of the Bayanshiree Formation. There may be two distinct levels of the formation based on faunal differences: a lower part lasting from the Cenomanian to early Turonian ages and an upper part lasting the late Turonian to Santonian ages. Fluvial, lacustrine and caliche-based sedimentation indicates that the Bayanshiree environment would have had a semi-arid climate, with large lakes. Largescale cross-stratification in many of the sandstone layers at the Baynshire and Burkhant localities may indicate the presence of large meandering rivers, which may have drained the eastern part of the Gobi Desert. Contemporary biota A vast faunal diversity is known in the formation, comprising fossils of dinosaurs and various other animals. Named theropods besides Khankhuuluu include several therizinosaurs (Duonychus, Erlikosaurus, Enigmosaurus, and Segnosaurus), the large dromaeosaurid Achillobator, and the ornithomimosaur Garudimimus. Herbivorous dinosaurs are represented by the ankylosaurs Talarurus and Tsagantegia, the small marginocephalians Amtocephale (a pachycephalosaur) and Graciliceratops (a ceratopsian), the hadrosauroid Gobihadros, and the sauropod Erketu. Other fauna include small mammals, azhdarchid pterosaurs, semiaquatic reptiles like crocodylomorphs and nanhsiungchelyid turtles, and various fish. Numerous fossilized fruits have been recovered from the Bor Guvé and Khara Khutul localities. == References ==