Thursday, April 26, 2012

[Paleontology • 2009] Raptorex kriegsteini • Tyrannosaurid skeletal design first evolved at small body size

Fig. 1: Skull, endocast, and premaxillary teeth of the Early Cretaceous tyrannosauroid R. kriegsteini. 

Nearly all of the large-bodied predators (>2.5 tons) on northern continents during the Late Cretaceous were tyrannosaurid dinosaurs. We show that their most conspicuous functional specializations—a proportionately large skull, incisiform premaxillary teeth, expanded jaw-closing musculature, diminutive forelimbs, and hindlimbs with cursorial proportions—were present in a new, small-bodied, basal tyrannosauroid from Lower Cretaceous rocks in northeastern China. These specializations, which were later scaled up in Late Cretaceous tyrannosaurids with body masses approaching 100 times greater, drove the most dominant radiation of macropredators of the Mesozoic.

Fig. 2: Postcranial features of the Early Cretaceous tyrannosauroid R. kriegsteini.

Fig. 4: Temporal, geographic, and phylogenetic patterns among tyrannosauroids. 
(A) Temporally calibrated phylogeny of tyrannosauroids based on phylogenetic analysis, showing an early diversity of basal tyrannosauroids (shaded) from localities across Laurasia and plotted on an Early Cretaceous paleogeographic map. Proceratosaurus and Xiongguanlong were excluded from this plot because of limited available morphologic data and age uncertainty, respectively. 
(B) Phylogram scaled to the amount of character change (delayed transformation) 
Circled nodes 1 to 3 outline major transformations in tyrannosauroid evolution. Taxonomic definitions of Tyrannosauroidea, Tyrannosauridae, and other taxa follow. 
Abbreviations for biogeographic area and localities: A, Asia; E, Europe; NA, North America; 
1, Stokesosaurus; 2, Proceratosaurus and Eotyrannus; 3, Aviatyrannis; 4, Guanlong; 5, Dilong; 6, Xiongguanlong; 7, Raptorex.

Sereno, P.; Tan, L.; Brusatte, S. L.; Kriegstein, H. J.; Zhao, X. & Cloward, K. (2009). "Tyrannosaurid skeletal design first evolved at small body size". Science. 326 (5951): 418–422. doi:10.1126/science.1177428