Evidence for a Tyrannosaurus rex from southeastern Colorado

Abstract

Order SauriSchia Seeley 1888 Family TyrannoSauridae Osborn 1905 Genus Tyrannosaurus Osborn 1905 cf. T. rex Osborn 1905 Referred specimens—TSJC 2008.1.1; TSJC 2008.1.2; TSJC 2008.1.3. Description—Tooth: A tyrannosaurid tooth (TSJC 2008.1.1) most closely resembles the teeth of “Nanotyrannus lancensis” (or adolescent T. rex, Carr and Williamson 2004) based on crown shape and serration density (N. Larson, pers. comm. 2007). In straight-line measurement, the tooth is approximately 3.5 cm from tip to base. The serration density along the anterior carina is 12/cm, justifying the diagnosis as T. rex (Farlow et al. 1991). The tooth is slightly recurved, blunt, bears denticles along its anterior carina, and is broken at its base. Femur: A limb bone fragment (TSJC 2008.1.2) is also tentatively identified as cf. T. rex. Theropod bones commonly exhibit onion-skin layering in cross section (K. Carpenter, pers. comm. 2007), and this bone seems to lack this feature, although the bone has distinct, differentiable layers. Whereas the bone itself may not be exclusively attributed to a theropod based on its gross histological properties alone, there are quantitative reasons supporting this diagnosis. First, the bone fragment is circular in cross section, and intersecting perpendicular bisectors of lines secant to its intact, periosteal surface suggest a radius of 90 ± 0.1 mm. As long bone circumference is used to estimate a live animal’s weight (Anderson et al. 1985; Alexander 1989), a 90 mm radius corresponds to a biped weighing approximately 5.23 metric tons. In Maastrichtian time in western North America, this value is exclusive to T. rex as far as mass estimates based on long bone circumferences for bipeds are concerned (Alexander 1989; Horner et al. 2004). Second, as T. rex femora have circular cross sections at their midpoint (Farlow et al. 1995; Holtz 2004) and the published growth curve for T. rex is based on midshaft femoral measurements (Erickson et al. 2004), it is possible to quantitatively compare the published growth curve for T. rex with the growth pattern observed in this specimen. Measuring the midpoint of each growth line to an accuracy of ± 0.5 mm along a radial transect, T. rex annual growth markers are exceptionally good predictors for the growth lines observed in this bone (cχ2 = 1.44, df = 6, p-value > 0.95). As T. rex is the only animal known to exhibit this characteristic pattern of growth (Erickson et al. 2004; ChinsamyTuran 2005), this is believed to be a fairly powerful diagnostic test. Significance—Estimated to be about 71 m.y. old based on the age of the Baculites eliasi ammonite zone, these fossils may be among the oldest known T. rex fossils (D. Wolberg, pers. comm. 2007). Institutional abbreviation—TSJC, Trinidad State Junior College Louden-Henritze Archeology Museum.