A longirostrine tyrannosauroid from the Early Cretaceous of China

Abstract

The fossil record of tyrannosauroid theropods is marked by a substantial temporal and morphological gap between small-bodied, Barremian taxa, and extremely large-bodied taxa from the latest Cretaceous. Here we describe a new tyrannosauroid, Xiongguanlong baimoensis n. gen. et sp., from the Aptian-Albian Xinminpu Group of western China that represents a phylogenetic, morphological, and temporal link between these disjunct portions of tyrannosauroid evolutionary history. Xiongguanlong is recovered in our phylogenetic analysis as the sister taxon to Tyrannosauridae plus Appalachiosaurus, and marks the appearance of several tyrannosaurid hallmark features, including a sharp parietal sagittal crest, a boxy basicranium, a quadratojugal with a flaring dorsal process and a flexed caudal edge, premaxillary teeth bearing a median lingual ridge, and an expanded axial neural spine surmounted by distinct processes at its corners. Xiongguanlong is characterized by a narrow and elongate muzzle resembling that of Alioramus. The slender, unornamented nasals of Xiongguanlong are inconsistent with recent hypotheses of correlated progression in tyrannosauroid feeding mechanics, and suggest more complex patterns of character evolution in the integration of feeding adaptations in tyrannosaurids. Body mass estimates for the fullgrown holotype specimen of Xiongguanlong fall between those of Late Cretaceous tyrannosaurids and Barremian tyrannosauroids, suggesting that the trend of increasing body size observed in North American Late Cretaceous Tyrannosauridae may extend through the Cretaceous history of Tyrannosauroidea though further phylogenetic work is required to corroborate this.

A long-snouted, multihorned tyrannosaurid from the Late Cretaceous of Mongolia

“…The elongate and gracile skull of Alioramus is similar to that of Xiongguanlong, a recently described basal tyrannosauroid from the Aptian-Albian (Early Cretaceous) of China (27). Xiongguanlong is more primitive, smaller, and more than 35 million years older than Alioramus, and is a non-tyrannosaurid that occupies a basal position on the tyrannosauroid stem.…”

“…The nearly complete and remarkably preserved skull and substantially complete postcranium of A. altai clearly demonstrate that Alioramus was a small, gracile, heavily ornamented, longirostrine carnivore. The skull is lightly built, and the upper jaw is more than five times longer than it is deep above the antorbital fenestra, an extraordinary ratio otherwise only seen in spinosaurids (26), the small basal tyrannosauroid Xiongguanlong (27), and a few coelurosaurs (28) among theropods. Four sets of cranial horns-one on each lacrimal and postorbital and two on each jugal-are present, as well as a series of discrete rugosities on the nasal.…”

New tyrannosaur from the mid-Cretaceous of Uzbekistan clarifies evolution of giant body sizes and advanced senses in tyrant dinosaurs

“…Along with the phylogenetic proximity of the two taxa, this suggests that the holotypic individual of T. euotica was roughly the same body size as Xiongguanlong, whose mass has been estimated at 170-270 kg (14,15). Although most sutures on the holotype of T. euotica are fused, part of the broken parabasisphenoid was not fused to the remainder of the braincase, perhaps suggesting that the individual was not yet osteologically mature and adults of the species may have been somewhat larger.…”

“…The supraoccipital bears a pronounced midline ridge with a deep fossa on each side, as in basal tyrannosauroids (14) and close outgroups (17,18) but unlike the flatter supraoccipital of derived tyrannosaurids that expands dorsally into tab-like processes (19,20). Ventrally, the supraoccipital terminates in a large diamondshaped process that does not extend to the dorsal rim of the foramen magnum.…”

A monodactyl nonavian dinosaur and the complex evolution of the alvarezsauroid hand

“…This documents a decoupling, representing mosaic evolution on a small scale, between two different types of specialization in the parvicursorine hand: functional refinement of digit II and reduction of digits III and IV. Mosaic evolution on various scales has also been suggested in other dinosaur groups, such as the Tyrannosauroidea (29,30) and the Sauropoda (31). The classic exemplar of mosaic evolution is the avialan dinosaur Archaeopteryx, which is bird-like in some details of the skeleton and in having asymmetric flight feathers but more typically reptilian in retaining such features as a long bony tail and unfused metapodial elements (32).…”