A Partial Skeleton of the Tyrannosaurid Dinosaur Aublysodon from the Upper Cretaceous of New Mexico Author Thomas M. Lehman Author Kenneth Carpenter text Journal of Paleontology 1990 64 6 1026 1032 http://www.jstor.org/stable/1305741 journal article 10.1017/S0022336000019843 48399970-b4aa-4f89-9739-3644a0f1d596 1037493 Aublysodon cf. A. mirandus Leidy, 1868 Figures 1- 6 Frontal and parietal.— The posterior parts of both frontals, and the anterior part of the fused parietals, are preserved as a single, thoroughly co-ossified mass ( Figures 1.1, 2.1, 2.2 ). Ad­ ditional fragments of the sidewalls of the braincase are crushed into the endocranial cavity and cannot be described. The frontals, from the midline to the postorbital suture, are much narrower than in other tyrannosaurids ( Figure 3 ). Although the frontals are incomplete anteriorly, enough is preserved to show a deep vertical notch above the orbital border, a feature that Russell (1970) considered diagnostic of Albertosaurus . Although a similar notch is also present in Daspletosaurus , it occurs between the lacrimal and the frontal, and is not incised into the frontal. In the New Mexico specimen, the notch penetrates even more deeply into the frontal than in A. libratus , and is more prominent than in any other tyrannosaurid ( Figure 3 ). In a small, presumably juvenile, specimen referred to cf. Aublysodon sp. (TMP 80.16.485) the frontal notch is not as well developed. The notch may become more prominent with growth, as the buttress for articulation with the postorbital expands. The frontal notch is developed immediately anterior to where the frontal expands laterally, forming a buttress for articulation with the postorbital. This buttress is separated into a rounded anterior lobe and an elongate posterior lobe, both of which articulated with the postorbital. A similar condition is observed in a specimen referred to IAlbertosaurus sp . (TMP 81.10.1). The edge of the frontal bordering the supratemporal fenestra is inclined sagittally, as in Albertosaurus , and not transversely as in the broad-skulled Daspletosaurus or Tyrannosaurus . The frontal is much narrower transversely than in Daspletosaurus , and also narrower than in most specimens referred to Albertosaurus ( Figure 3 ). Its relative width is comparable to that observed in other specimens referred to Aublysodon (LACM 28471, TMP 80.16.485), although Nanotyrannus also has narrow frontals ( Figure 3 ). Likewise, the preserved anterior part of the fused parietals forms a constricted sagittal crest, somewhat narrower than that seen in Daspletosaurus or Tyrannosaurus , and more closely resembling that of Albertosaurus . The parietals extend forward, separating the frontals posteriorly, and resulting in a prominent V-shaped frontal-parietal suture. Postorbital. — Part ofthe left postorbital, lacking the distal end of the descending process (jugal ramus) and most of the squamosal process, is preserved ( Figures 1.2, 1.3, 2.3, 2.4 ). The horizontal ramus of the bone bears a blunt rugosity for contact with the frontal. This sutural surface is, like the corresponding surface on the frontal, divided into two parts; one, a rounded cup-shaped depression, lies anterior and dorsal to the other, which is an elongate recess. A shallow pit below the frontal suture probably received the antotic process of the laterosphenoid. The descending process of the postorbital forms an overlapping suture with the jugal, and bears a slightly thickened ridge along the orbital margin where a low flange may have extended into the orbit. Owing to incomplete preservation, however, the extent to which this process was developed is unknown. Judging from the dimensions of the broken edge, the projection was much less developed than that seen in many Tyrannosaurus specimens (see Osborn, 1912 ). Development ofthe orbital flange in Tyrannosaurus is, however, ontogenetically controlled, and shows considerable variation among adults ( Carpenter, in press b ). Too little of the squamosal process is preserved to reveal its form. Figure 1—Preserved skull fragments of New Mexico Aublysodon cf. A. mirandus ( OMNH 10131 ). 7, parts of the frontal-parietal mass; 2, 3, left postorbital; 4, left dentary. General skull outline adapted from Russell (1970). Scale bar 10 cm . The size and shape of the supraorbital rugosity on the postorbital vary among tyrannosaurids. In the New Mexico specimen, this rugose boss has a prominent crescent shape similar to that seen in Daspletosaurus , and is considerably more prominent than that seen in many specimens of Albertosaurus . Development of the postorbital rugosity is, however, subject to a great deal of individual variation, even between right and left sides of a single individual (e.g., A. libratus , FMNH PR308); hence its utility for taxonomic purposes is limited. Dentary. — The posterior part ofthe left dentary contains eight alveoli with teeth in various stages of eruption ( Figures 1.4, 2.5, 2.6 ). The last tooth is pushed posteriorly and is abnormally positioned. Well-developed interdental plates line the lingual side of the dentary and rest on a rounded supradentary plate (the lingual “bar” of Madsen, 1976 ). The supradentary plate is bounded below by a narrow and shallow Meckelian groove, and ends posteriorly where the Meckelian fossa expands behind the last alveolus. The external surface of the dentary exhibits two rows of foramina, one of which extends along the middle of the dentary, and the other just above the ventral edge of the bone. The preserved portion of the ventral margin of the dentary defines a smooth curve like that in Albertosaurus , and is not strongly deflected ventrally as in Daspletosaurus or Tyrannosaurus . This condition is, however, variable among specimens referred to Albertosaurus . The dentary is very similar in size and general form to the specimen (USNM 8346) referred to Gorgosaurus (= Albertosaurus ) by Gilmore (1916 , Pl. LXXIII, fig. 1; 1935 ). Dentition.— The teeth preserved in the dentary are more strongly inclined posteriorly than in either Albertosaurus or Daspletosaurus , perhaps owing in part to post-mortem deformation. Serration counts (10-12 serrations per 5 mm on anterior carinae, and 9-10 per 5 mm on posterior carinae) are similar to those ofother small tyrannosaurids. Like Albertosaurus , and to a lesser extent Daspletosaurus , the dentary teeth become more strongly curved posteriorly at their tips, and have anterior carinae that are deflected lingually along their bases. Two additional isolated teeth are also preserved with the skull fragments. One, probably a maxillary tooth, 75 mm in length, has serrations comparable to those of the dentary teeth but is not as strongly curved at its tip. The other, a premaxillary tooth, 52 mm in length, exhibits the incisiform “D”-shaped cross section typical of tyrannosaurids, with posteromedial and posterolateral carinae on either side of a faceted lingual surface ( Figure 4 ). The carinae lack denticulations and converge toward one another about midway down the length of the tooth. There is a well-developed bilobed median ridge between the carinae on the lingual surface of the tooth. A small wear facet is present on the tip ofthe crown. Although larger, this tooth is comparable to those referred by others to Aublysodon ( Carpenter, 1982 ; Paul, 1988 ; Molnar and Carpenter, 1989 ). The tooth is also similar to another (USNM 8355) figured by Gilmore (1916, Pl. LXXIII, fig. 4) from the Farmington Sandstone Member of the Kirtland Shale. Gilmore’s specimen may likewise be referable to Aublysodon (Molnar and Carpenter, 1989 ). Figure 2— Aublysodon cf. A. mirandus ( OMNH 10131 ). Frontal-parietal mass in 1, dorsal view, and 2, right lateral view; left postorbital in 3 , medial view , and 4, lateral view; left dentary in 5, medial view, and 6, lateral view. Abbreviations: cl, cleft in frontal; fr, frontal and suture for frontal; j, suture for jugal; lsp, suture for laterosphenoid; p, parietal; po, suture for postorbital; sq, broken squamosal process. Scale bar 10 cm. Ribs and gastralia. —Four fragments of ribs are preserved. These include a distal end, a proximal end lacking the tuberculum, and two shaft fragments. A single, nearly complete, right gastralium is also present. The gastralium measures 636 mm along its ventral curve. It is slender and flattened at its medial end, as in Albertosaurus , and is unlike the heavy thickened element in Tyrannosaurus . The gastralia of Albertosaurus are well known in the type specimens of A. libratus (GSC 2120) and A. arctunguis (ROM 807). The present specimen is comparable in form and length to those from the middle part of the series (numbers 5 -12). Pubis. — The preserved portion ofthe pubis includes the pubic foot and parts ofboth shafts ( Figure 5.7 ). The specimen is poorly preserved and obliquely crushed towards the right, so that the ventral surface is visible from the left side. The pubic foot measures 555 mm along the ventral surface of the symphysis, and has a maximum thickness of 165 mm just posterior to the pubic shaft. Neither the left nor right shaft is complete, the longest one ( left ) extending 285 mm above the foot. Although the longer shaft appears to be slightly bowed ventrally, this may reflect post-mortem deformation. The form and size of the pubis are comparable to that observed in Albertosaurus libratus and A. arctunguis, but the anterior end of the foot is proportionally larger. Femur.— Both femora are preserved, although neither is complete. The left femur is truncated just above the distal condyles, crushed anteroposteriorly, and weathered along its posterior surface ( Figure 5.1, 5.2 ). As preserved, it is 856 mm long and has a minimum circumference just below the fourth trochanter of 344 mm. The center of the fourth trochanter is 348 mm below the top ofthe femoral head, and the lesser trochanter is separated from the femoral head by a gap 46 mm deep. The restored length of the femur is estimated to be 1,080 mm. The right femur is considerably more damaged, particularly on its posterior surface, and the fourth trochanter cannot be located with confidence. It has also been anteroposteriorly crushed. The preserved portion is 670 mm long and has a minimum circumference of 370 mm . The gap separating the lesser trochanter from the femoral head is 45 mm deep. These femora compare well with an isolated specimen from the Fruitland Formation (UNM B-828) and with those of Albertosaurus , but are not as robust as in Daspletosaurus or Tyrannosaurus . As restored ( Figure 5 ), the circumference of the femur is equal to about 31 percent of its length, compared with 34-37 percent in Albertosaurus and 38-41 percent in Daspletosaurus ( Russell, 1970 ) . Tibia.— Only the distal half of the right tibia is preserved ( Figure 5.3 ). It is slightly crushed anteroposteriorly. The preserved portion is 478 mm long and 216 mm across at the distal end. The shaft is 110 mm in transverse width. The anterior surface has a well-developed suture for the ascending process of the astragalus and a flattened facet to receive the distal end of the fibula. The distal end of the tibia bears of deep medial emargination for reception of the astragalus, such that the astragalar suture is markedly oblique relative to the shaft, and the bone appears to lack a medial “malleolus” ( Figure 6 ). This appears to be a natural condition, not a result of crushing, and the entire distal surface of the tibia is grooved for reception of the astragalus. Although the astragalar facet is also oblique relative to the shaft of the tibia in Albertosaurus , it does not approach the condition observed in this specimen, which is unique among large theropods ( Figure 6 ). The deep medial emargination does not appear to represent a juvenile condition, based on comparison with juvenile Albertosaurus libratus specimens (e.g., AMNH 5458 and 5664) and the seemingly adult proportions of the present specimen. A similar condition is found, however, in primitive theropods such as Dilophosaurus ( Welles, 1984 ), Coelophysis ( Padian, 1986 ; Colbert, 1989 ), and Liliensternus ( Huene, 1934 ) where instead the lateral surface is deeply emarginated. As restored ( Figure 5 ), the length ofthe tibia would be about 82 percent the length of the femur, compared with 87 percent in Daspletosaurus , 88 percent in Tyrannosaurus , and 90-100 percent in Albertosaurus ( Russell, 1970 ) . Figure 4— Aublysodon cf. A. mirandus ( OMNH 10131 ). Premaxillary tooth in 1, lateral view, and 2, posterior view; 3, cross sections at indicated levels with anterior end to the right. Scale bar 10 mm . Figure 3—Comparison of tyrannosaurid frontal bones in dorsal view. 1, Aublysodon mirandus (LACM 28471). 2, cf. Aublysodon sp. (TMP 80.16.485). 3, Aublysodon cf. A. mirandus ( OMNH 10131 ). 4, Nanotyrannus lancensis (CMNH 5741). 5, Albertosaurus libratus (AMNH 5664). 6, Albertosaurus libratus (USNM 12814). 7, Daspletosaurus torosus (NMC 8506). Sutures for adjacent bones are shown: f, frontal; fn, notch in frontal (also indicated by arrow); 1, lacrimal; n, nasal; p, parietal; pf, prefrontal; po, postorbital; stf, border of supratemporal fenestra. Scale bar 10 cm. 1 and 5 are adapted from Molnar (1978), 2 from Currie (1987), 4 from Bakker et al. (1988), 6 and 7 from Russell (1970). Metatarsals. — The left fourth metatarsal is complete, 461 mm in length, and 70 mm wide at the distal articular surface ( Figure 5.4, 5.6 ). It compares well with a specimen described by Lehman (1981) from the upper Kirtland Shale, but is shorter and stockier. It is substantially shorter than the comparable element in Al- bertosaurus libratus (546 mm) and A. arctunguis (558 mm). The length of metatarsal IV is about 43 percent the length of the femur as restored ( Figure 5 ), compared with 50-60 percent in Albertosaurus , 49 percent in Daspletosaurus , and 46 percent in Tyrannosaurus . The distal third of metatarsal III is also preserved ( Figure 5.5, 5.6 ). The preserved portion is 252 mm long and 85 mm wide at the distal articular surface. Enough is preserved to indicate that metatarsal III is constricted proximally between metatarsals II and IV, as is typical of tyrannosaurids.