Tenontosaurus tilletti Ostrom, 1970

Thomas, DA, 2015, The cranial anatomy of Tenontosaurus tilletti Ostrom, 1970 (Dinosauria, Ornithopoda), Palaeontologia Electronica 11 (1), pp. 1-98 : 5

Tenontosaurus tilletti Ostrom, 1970

Holotype. AMNH 3040 : a partial skeleton, lacking pectoral girdle, cervicals, and skull.

Paratypes. YPM-PU 16338 : a partial skeleton, lacking pectoral girdle, cervicals, and skull; YPM 5456: a partial skeleton with skull.

Referred Specimens. MCZ 4087, 4166, 4205, 4385, 4388, 4390, 7556-7558; OMNH 2526, 2531, 4164, 8137, 10132, 10144, 16562, 16563, 32838, 32842, 32845, 32847, 34191, 34782, 58340, 62990, 63525; BB 1; YPM-PU 16514; YPM 4882, 4904, 5099, 5117, 5146, 5195, 5299, 5399, 5410, 5411, 5413, 5416, 5417, 5422, 5424, 5426-5428, 5457-5471, 5473-5481, 5483, 5523, 5533-5535; AMNH 3010-3014, 3017, 3020, 3022, 3023, 3031, 3034, 3043-3045, 3050, 3061-3063, 5854.

Horizon and Distribution. Little Sheep Mudstone and Himes Member of the Lower Cretaceous (Upper Aptian–Lower Albian) Cloverly Formation (Units V, VI, and VII of Ostrom, 1970) of the Bighorn Basin area of northern Wyoming and southcentral Montana; Lower Cretaceous Antlers Formation (Aptian–Albian) of southeast Oklahoma .

Revised Cranial Diagnosis (emended from Ostrom, 1970; for revised postcranial diagnosis, see Forster, 1990). Skull with very large external nares, long slitlike antorbital fenestra and supplementary inferior temporal fenestra beneath the usual lateral fenestra. Orbit roughly triangular, with dorsoventrally straight anterior and sharply rounded posterior margins (emended by this study), and larger than either lateral fenestra. Premaxilla, which nearly encircles the nares, flares inferiorly into broad, U-shaped edentulous beak, opposed by shallow, horseshoe-shaped predentary with pseudo-tooth projections along upper margin. Mandibles bear a long, curved retroarticular process. Quadrate long, very narrow transversely; posterior margin convex rather than concave. Paroccipital processes hook-shaped, and downturned at extremity. Dentary teeth with very prominent vertical keel, maxillary teeth without keels but with numerous nonparallel, subequal minor ridges. The following characteristics of Tenontosaurus tilletti were recovered in the systematic analysis described below. Characters are followed by “(character number:state present),” found in Appendices 2 and 3. The first set of character states are presumed plesiomorphies of T. tilletti : lack of contact between the jugal and quadrate (20:0), lack of quadrate foramen (25:0), large quadratojugal (26:1), contact between nasal and lacrimal (28:0), articulation of the jugal with the posterior maxilla (33:0), lack of quadrate notch (37:0), lack of contact between the parietal and prootic (49:1), dorsal edge of the ilium above the ischial peduncle not thickened and beveled (87:0), straight ischial shaft (92:0), and ischial shaft flattened in cross section (93:0). Tenontosaurus tilletti shares the following apomorphic characters with other members of Iguanodontia : frontal contributes to less than half of dorsal orbital rim (8:1), parallel dorsoventral borders of the dentary (54:1), single surangular foramen (69:2), dorsal neural spines rectangular with height more than twice width (72:1), manus digit III bearing three phalanges (84:1), and a pubic shaft shorter than the ischium, with no pubic symphysis (91:1). The following characters are apomorphic to the genus Tenontosaurus : nasal overlapped by nasal process of the premaxilla (13:1), postorbital process comprises posteriormost tip of jugal (22:1), triangular outline of parietal in dorsal aspect (45:1), denticulate occlusal margin of predentary (62:1), laterally reflected boss on anterior margin of the scapula (75:1), laterally compressed and bar-like prepubic process of the pubis (90:1), and an obturator process present near the midshaft of the ischium (94:1). Finally, the following suite of characters is autapomorphic to T. tilletti : lack of premaxillary teeth (3:1), length of suture between frontals and parietal equal to about two-thirds the combined transverse width of the frontals (7:1), axial length of the nasals one and one-half times axial length of the frontals (10:1), posteriorly curved and distally flared scapular blade (74:2), distal phalanges of manus digit II claw-like and digit III a nub (83:2), and straight distal shaft of the femur (96:1). Two autapomorphies for T. tilletti (internal contacts of the squamosal with the jugal and of the basioccipital with the laterosphenoids) were not coded in this analysis as their status is unable to be investigated in the other taxa analyzed at this time.