juvenile tyrannosaurid, Osborn, 1906

Carr, T. D. & Williamson, T. E., 2000, A review of Tyrannosauridae (Dinosauria, Coelurosauria) from New Mexico, New Mexico Museum of Natural History and Science Bulletin 17, pp. 113-146 : 133-134

Late Maastrichtian

Several tyrannosaurids have been documented in Late Maastrichtian sediments of western North America, including Aublysodon cf. A. mirandus ( Molnar and Carpenter, 1989), Albertosaurus lancensis ( Molnar, 1980) (= A. megagracilis Paul, 1988 ), and Tyrannosaurus rex ( Osborn, 1905) . All but T. rex are problematic assignments.

Aublysodon mirandus was the name restricted to a nondenticulate premaxillary tooth by Marsh (1892) included in the type series of Deinodon horridus ( Leidy, 1856) . Although Osborn (1905) abandoned Aublysodon , Carpenter (1982) rediagnosed Aublysodon mirandus based on new teeth collected in Lancian sediments of Wyoming. Since then, one partial skull ( LACM 28471 ) from late Maastrichtian beds of Montana has been referred to Aublysodon cf. A. mirandus by Molnar and Carpenter (1989) who expanded the diagnosis of the taxon to include skeletal material for the first time. Later, Lehman and Carpenter (1990) referred a partial skeleton (OMNH 10131; Figs. 5 , 6A-K , 10A-I , 11M-U , 12F- I , 13A- J ) from late Campanian strata of New Mexico to Aublysodon cf. A. mirnndus , and expanded the skeletal diagnosis. As demonstrated above, the latter specimen is referable to Daspletosaurus . The referral of the skeletal material to Aublysodon in both cases was based on associated nondenticulate premaxillary teeth.

More recently, Aublysodon teeth have been documented by Currie et al. (1990) from the Dinosaur Park Formation. They also note identical teeth from the Iren Dabasu Formation in the People's Republic of China that might be derived from Alectrosaurus ( Currie et al., 1990). Eaton et al. (1999 a) reported ”? Aublysodon sp. ” from a Santonian locality in southwestern Utah, although the nature of the material is not indicated. Eaton et al. (1999b) report ” cf. Aublysodon sp. from the middle to late Turonian Smoky Hollow Member of the Straight Cliffs Formation of Utah ( Parrish, 1999). These occurrences give Aublysodon a maximum temporal range of 26 million years in North America, an unprecedented duration among dinosaurs.

In our view, the small Late Maastrichtian theropod ( LACM 28471 ), referred to Aublysodon by Molnar and Carpenter (1989) differs in no significant way from other tyrannosaurids. The authors list several primitive features that are actually typical of juvenile tyrannosaurids ( Carr, 1999), including long low muzzle, frontals longer than wide, smooth nasals, and a wedge-like frontoparietal suture ( Molnar and Carpenter, 1989). With regard to the diagnostic skeletal characters of Aublysodon , the acute angle of the rostroventral and alveolar borders of the dentary is typical of the shallow bone in juvenile tyrannosaurids. Also, the lingually placed Carina of the first dentary tooth, different extents of the mesial and distal denticles along the tooth crown, and relatively finer mesial than distal denticles are also typical of tyrannosaurids in general ( Carr, 1996; Table 2). We are unsure of the significance of the "step" in the alveolar border of the dentary at the third alveolus, but this region of the margin is typically convex in tyrannosaurids, and the step may simply reflect the change from a concave to a convex margin.

Finally, there is nothing in the published photographs and figures (e.g., Molnar, 1978) to prevent identification of LACM 28471 as a juvenile tyrannosaurid .

The absence of denticles on the associated premaxillary tooth of LACM 24871 , generally small size of " Aublysodon " teeth, et. ( Currie et al., 1990), and widespread geographic and temporal occurrence ( Currie et al., 1990; Eaton et al. 1999a,b; Parrish, 1999), strongly suggests that a lack of denticles reflects relative development, as in other coelurosaurs such as velociraptorine dromaeosaurids. Pending personal examination of the fossil, we consider the specimen to represent a juvenile indeterminate tyrannosaurid.

Figures

FIGURE 5. Daspletosaurus sp. (OMNH 10131) from the upper Fruitland or Kirtland (exclusive of Naashoibito Member) formations. Left postorbital in A, lateral, B, caudal, C, medial, and D, dorsal views; frontoparietal region in E, dorsal, F, ventral, G, right lateral, and H, rostral views; partial left dentary in I, lateral, and I, medial views; left femur in K, proximal, L, rostral, M, medial, N, caudal, and O, lateral views; right femur in P, proximal, Q, rostral, R, medial, S, lateral, and T, caudal views; distal half of right tibia in U, rostral, V, medial, W, lateral, and X, caudal views.

FIGURE 6. Daspletosaurus sp. (ONINH 10131) from the upper Fruitland or Kirtland (exclusive of Naashoibito Member) formations. Right metatarsal H in A, rostral, D, distal, E, lateral, G, caudal, and I, medial views; right metatarsal HI in B, rostral, C, distal, F, lateral, and I JI, medial views; K, various ribs in rostral and caudal views. L-Q NMIVINH P250891, indeterminate tyrannosaurid femur from the De-na-zin Member of the Kirtland Formation in L, rostral, M, medial, N, proximal, O, caudal, P, distal, and Q, lateral views.

FIGURE 10. Tyrannosaurid skull bones. A-I, Daspletosaurus sp. (OMNH 10131), with left postorbital in A, lateral, B, dorsal, and C, caudal views; frontoparietal region in D, dorsal, E, ventral, F, right lateral, and G, left lateral views; and left dentary in H, medial and I, lateral views. I-M, Tyrannosaurus rex (NMMNH P-3698), with vomerine process of rightpalatine in], medial, and L, lateral views; and basal portion in K, medial and M, lateral views. Scale bars equal 10 cm.

FIGURE 11. Comparison of tyrannosaurid femora. A-F, P-22976, right femur in A, lateral, B, medial, C, rostral, D, caudal, E, proximal, and F, distal views. G-L, P—25083, right femur in G, lateral, H, medial, I, rostral, I, caudal, K, proximal, and L, distal views. M-U, OMNH 10131, right femur in M, lateral, O, medial, Q, roslral, S, caudal, and V, proximal views; left femur in N, lateral, P, medial, R, rostral, T, caudal, and U, proximal views. Scale bar equals 10 cm.

FIGURE 12. Comparison of tyrannosaurid tibiae. A-E NMMNH P-25085, right tibia in A, lateral, B, medial, C, rostral, D, caudal, and E, proximal views. F-I OMNH 10131, distal end of right tibia in F, rostral, G, caudal, H, lateral, and I medial views. Scale bar equals 10 cm.

FIGURE 13. Comparison of tyrannosaurid metatarsals. A-J, OMNH 10131, right metatarsal II in A, rostral, C, caudal, E, medial, G, lateral, and J, distal views; metatarsal III in B, rostral, D, caudal, F, medial, H, lateral, and I distal views. K-O, NMMNH P—25049, right metatarsus in K, rostral, L, lateral, M caudal N medial and O, distal views. Scale bars in I and J equal 5 cm, all others equal 10 cm.