Dromomeron gregorii Nesbitt 2009
Creators: name: Lessner, Emily J.
name: Parker, William G.
name: Marsh, Adam D.
name: Nesbitt, Sterling J.
name: Irmis, Randall B.
name: Mueller, Bill D.
Publisher: name: Zenodo
Year: 2018
Subjects: Biodiversity
Taxonomy
Animalia
Chordata
Lagerpetonidae
Dromomeron
Dromomeron gregorii

description: DROMOMERON GREGORII Nesbitt et al., 2009c
Referred Specimen— TTU-P 11186, distal end of left femur (Fig. 10L–N).
Locality— MOTT 3869 (Boren Quarry).
Description and Rationale for Assignment— TTU -P11186 preserves the robust distal end of a femur and was briefly assigned to Lagerpetidae by Nesbitt et al. (2009c: pg. 508) (Fig. 10L–N). The anterior surface of TTU-P 11186 preserves a mediolaterally-oriented ridge
that arcs proximally onto the lateral side of the femur, though this ridge is less defined in TTU-P 11186 than in TTU-P 10866, described above. The ridge separates unfinished bone (dorsally) from finished bone (ventrally) (Fig. 10N), and Nesbitt et al. (2009c) identify this ridge as the distal origin of the M. femorotibialis externus. The medial surface is flat in the specimen, and the antero- medial corner is about 90 ° (Fig. 10M). On the distal end of the femur, the presence of the concave posterolateral surface of the crista tibiofibularis, the scar on the anterior surface for the origin of the M. femorotibialis externus and the 90° anteromedial corner are autapomorphic for Dromomeron (Nesbitt et al. 2009c, Nesbitt 2011; 327:1) (modification to the placement within Lagerpetidae by Nesbitt et al. (2009c)).. In distal view, the anterior edge of TTU-P 11186 is flat (Fig. 10M). The crista tibiofibularis is larger than the medial, tibial condyle in TTU-P 11186, is broad mediolaterally, and is separated from the lateral, fibular condyle on the distal surface by a distinct groove. The presence of a large crista tibiofibularis is autapomorphic for Lagerpetidae (Nesbitt 2011; 326:1), and the inflated morphology of the crista tibiofibularis is more similar to that of Dromomeron (Irmis et al. 2007 a). The medial condyles of the specimen are square in distal view and the lateral condyle is rounded (Fig. 10M). The distal articular surface of the lateral condyle of TTU- P11186 is covered with small grooves and ridges that are less defined than that of TTU-P 10866. TTU-P 11186 lacks autapomorphies of the distal end of the femur of D. romeri, including the presence of a sharp, flaring ridge on the anteromedial edge and a lateral tuberosity on the anterolateral edge (Nesbitt et al. 2009c; 130:1). We assign this specimen to D. gregorii on the basis of the scar on the anterior surface, the 90° anteromedial corner, and the absence of the anteromedial flare present in D. romeri.
DINOSAURIFORMES Novas, 1992
Referred Specimen— TTU-P 11412B, distal end of left tibia (Fig. 10O, P).
Locality— MOTT 3898 (Headquarters South).
Description and Rationale for Assignment— TTU- P11412B preserves the distal end of a left tibia and the portion of a midshaft (Fig. 10O, P). The specimen is rounded posteriorly and has two laterally extending processes in distal view (Fig. 10P). The posterolateral process (=descending process of Langer 2004) tapers to a point laterally in distal view, extends proximally, but stops just distal to the midshaft. The anterolateral process (=astragalar process of Langer (2004)) is rounded anteriorly in distal view. The posterolateral process of
the distal end is slightly concave and extends farther distally than the anterolateral process (Fig. 10O); this is a dinosauriform apomorphy (Nesbitt 2011; 334:1). On the lateral surface of TTU-P 11412B, the processes are separated by a proximodistally-oriented groove that does not extend onto the midshaft, a character state present in some dinosauriforms and all dinosaurs (Nesbitt 2011; 338:1). The anterior and posterior margins of the distal end of the tibia are straight. The medial face of the distal end of the tibia is rounded, smooth, and featureless. Based on the morphology of the posterolateral process and the presence of a proximodistally-oriented groove on the lateral surface, we assign TTU-P 11412B to Dinosauriformes.
DINOSAURIA Owen, 1842
sensu Padian & May, 1993
Referred Specimens— Tibiae: TTU-P 11289, proximal end of left tibia (Fig. 10Q, R); TTU-P 11412D, proximal end of right tibia (Fig. 10S, T); TTU-P 11405B, distal end of right tibia (Fig. 10U, V).
Locality— MOTT 3898 (Headquarters South).
Description and Rationale for Assignment— TTU- P11289and TTU-P 11412D preserve the proximal ends of tibiae and TTU-P 11412D preserves a portion of the midshaft as well (Fig 10Q–T). The midshaft of TTU-P 11412D is subelliptical in cross-section and tapers anteriorly. The specimens are nearly identical and therefore described together, noting any differences. The proximal end is slightly convex and preserves a cnemial crest anteriorly and two posterior condyles that taper posteriorly and are equal in size (Fig. 10R, T). A prominent cnemial crest is an apomorphic trait of Dinosauriformes (Novas 1992; 4:1). The posterolateral condyle is worn in TTU- P11412D (Fig. 10R). Like other early dinosaurs (Scutellosaurus Colbert, 1981, Saturnalia, Tawa Nesbitt et al., 2009b, Dilophosaurus Welles, 1954, Coelophysis) the posterolateral condyle is offset anteriorly relative to the posteromedial condyle (Langer and Benton 2006: fig. 13, Nesbitt 2011; 331:1). The posterolateral condyle is rounded laterally in TTU-P 11289 and separated from the cnemial crest by a groove on the lateral surface that extends from the distal end onto the midshaft in both specimens (Fig. 10 Q-T). The medial surface is flat and featureless, lacking the fibular crest of many theropods. The cnemial crest tapers anterolaterally and forms a sharp ridge on the anterior surface of the proximal end of the tibia that extends distally onto the midshaft (Fig. 10Q, S). An anterolaterally-tapering cnemial crest is a character shared by all dinosaurs (Nesbitt 2011; 328:2). Therefore, based on the presence of this character and absence of theropod apomorphies discussed below, we assign TTU-P 11289 and TTU-P 11412D to Dinosauria.
TTU-P 11405B preserves the distal end of the right tibia and a portion of the midshaft. The midshaft is almost circular in cross section with the exception of a flat anterolateral side (Fig. 10U, V). TTU-P 11405B is subrectangular in distal view (Fig. 10V), a synapomorphy of Dinosauromorpha (Sereno 1991: pg. 37). The posteromedial edge of the midshaft is rounded, whereas the anteromedial surface has a sharp, proximodistally-oriented ridge that terminates just proximal to the distal end (Fig. 10S). This is a character state present in ornithischians, sauropodomorphs, and neotheropods (Nesbitt 2011; 336:1). The medial border is rounded and the anteromedial corner of the proximal surface is downturned. The lateral portion of TTU-P 11405B preserves two surfaces for articulation that are level at their lateral border (Fig. 10V). The proximal surface of the anterior articular surface (=astragalar process) is slightly depressed distally in comparison to the proximal surface of the posterior articular surface (descending process). Based on the ridge on the anteromedial surface, we assign TTU-P 11405B to Dinosauria.
SAURISCHIA Seeley, 1887 sensu Gauthier, 1986
Referred Specimens— Tibia: TTU-P 11412C, distal end of right tibia (Fig. 10W, X). Femora: TTU-P 11139, distal end of left femur (Fig. 10Y, Z); TTU-P 11409C, distal end of left femur (Fig. 10 AA, AB); TTU-P 11274, distal end of right femur (Fig. 10 AC, AD).
Localities— Tibia: MOTT 3898 (Headquarters South). Femora: MOTT 3899 (Headquarters NW); MOTT 3898 (Headquarters South).
Description and Rationale for Assignment— Tibia — TTU-P 11412C preserves the distal end of a right tibia as well as a portion of the midshaft (Fig. 10W, X). Upon initial inspection, TTU-P 11412C is very similar to TTU-P 11412B (assigned to Dinosauriformes above, Fig. 10O, P), though slight but important differences are discussed below. The tibia is nearly square in distal view. The posterolateral surface (=descending process of Langer (2004)) of the distal end extends further distally than the anterolateral surface (=astragalar process of Langer (2004)), and the two surfaces level out on the medial surface of the distal end (Fig. 10W). The posterior margin of the distal end of the tibia is slightly concave (Fig. 10X), an apomorphy of saurischians (Nesbitt 2011; 335:1). The anterolateral and anteromedial margins are rounded, whereas the posteromedial margin of the distal end is formed by a proximodistally-oriented ridge, which is a dinosaur character state (Nesbitt 2011; 336:1). The medial side of the distal end of the tibia is smooth and featureless. The lateral side of the distal end has a proximodistally-oriented groove between the descending and astragalar processes (Fig. 10X), a plesiomorphy of dinosaurs (Nesbitt 2011; 338:1). Based on this combination of characters found in dinosaurs and saurischians, we assign TTU-P 11412C to Saurischia.
Femora— TTU-P 11139, TTU-P 11274, and TTU- P11409C preserve the distal ends of femora, and TTU- P11139 and TTU-P 11409C also preserve a portion of the midshaft (Fig. 10Y –AD). These specimens are otherwise identical and therefore described together. In distal view, the femur is concave posteriorly and convex anteriorly (Fig. 10Z, AB, AD). The femur preserves a medial, tibial condyle on the posteromedial surface of the distal end that tapers posteriorly (worn in TTU-P 11139), a rounded lateral, fibular condyle on the anterolateral surface of the distal end, and a small, square crista tibiofibularis on the posterolateral surface of the distal end (Fig. 10Z, AB, AD). The crista tibiofibularis and lateral condyle are separated on the distal surface by a groove that deepens to a fossa in the center of the distal end. The presence of the groove is a character state of dinosauromorphs, Effigia, Poposaurus, Fasolasuchus, Postosuchus, and crocodylomorphs (Nesbitt 2011; 322:1). Among dinosauromorphs this character state is shared by Dromomeron (which exhibits a squared anteromedial corner and an inflated crista tibiofibularis) and saurischians (Nesbitt 2011). The distal surface of the femur is otherwise flat. In distal view, the angle between the lateral condyle and the crista tibiofibularis on the lateral surface of the distal end is obtuse (Fig. 10Z, AB, AD) as in avian-line archosaurs, whereas this angle is about 90° in Effigia, Batrachotomus, Fasolasuchus, Postosuchus, and crocodylomorphs (Nesbitt 2011; 319:1). As in avian-line archosaurs the obtuse angle between the lateral condyle and crista tibiofibularis in this femur is accompanied by a semicircular, rounded lateral condyle (Nesbitt 2011; 319:1). On the posterior surface, there is a wide intercondylar groove created by the medial condyle medially and the crista tibiofibularis laterally that extends from the distal end and shallows proximally onto the midshaft (Fig. 10 AA–AD). The medial condyle extends proximally, ending in a roughened surface angling onto the posteromedial side of the midshaft. The crista tibiofibularis extends proximally as well, though less proximally than the medial condyle, and ends proximally in a distinct step onto the midshaft (Fig. 10Y, AA, AC). The anterior surface of the distal end of the femur is smooth. TTU-P 11139, TTU-P 11409C, and TTU- P11274 are very similar to Tawa hallae (GR 244, Nesbitt et al. 2009b) though they do not preserve any apomorphies specific to Tawa. We assign these specimens to Saurischia based on the groove and angle between the lateral condyle and crista tibiofibularis.
THEROPODA Marsh, 1881 sensu Gauthier, 1986
Referred Specimens— Tibiae: TTU-P 11405A, proximal end of left tibia (Fig. 10 AE, AF); TTU-P 11397A, proximal end of left tibia (Fig. 10 AG, AH).
Locality— MOTT 3898 (Headquarters South).
Description and Rationale for Assignment— TTU- P11405A and TTU-P 11397A preserve the proximal ends of left tibiae as well as portions of the midshaft (Fig. 10 AE–AH). TTU-P 397A is smaller and preserves more of the midshaft than TTU-P 11405A. The midshaft of TTU-P 11397 is nearly oval in cross section. The proximal ends of both tibiae preserve the cnemial crest anteriorly and two posterior condyles (Fig. 10 AF, AH). A prominent cnemial crest is a derived trait of Dinosauriformes (Novas 1992; 4:1). The proximal surface of TTU-P 11405A has a slight concavity posterior to the cnemial crest and anterior to the medial posterior condyle, and in TTU- P11397A the proximal surface is slightly concave as well. Among theropods, Coelophysis has a shallow concavity on the proximal surface of the tibia, whereas the surface is deeply concave in some other theropods (e.g., Allosaurus Marsh, 1877) (Nesbitt 2011; 329:1). The cnemial crest in both specimens tapers and curves anterolaterally in proximal view (Fig. 10 AF, AH), a character state shared by all dinosaurs (Nesbitt 2011; 328:2). In TTU-P 11397A, the lateral and medial posterior condyles are rounded, whereas in TTU-P 11405A the medial condyle tapers posteriorly and the lateral condyle is rounded. In both specimens, the lateral and medial condyles each form half of the posterior border in proximal view and are separated by a groove that originates on the proximal end and continues onto the posterior surface. In proximal view, the lateral condyle is offset anteriorly relative to the medial condyle (Fig. 10 AF, AH), which is plesiomorphic for theropods (Nesbitt 2011; 331:1). The lateral margin of the lateral condyle in TTU-P 11405A is flat as in the- ropods (Nesbitt 2011; 332:1), and this surface is nearly flat, but worn, in TTU-P 11397A. The medial surface of the proximal end is smooth in TTU-P 11405A and expands slightly laterally on the cnemial crest just distal to the proximal surface of TTU-P 11397A. On the anterolateral surface of both TTU-P 11405A and TTU-P 11397A, there is a wide groove paralleled posteriorly by a sharp, dorsoventrally-oriented ridge originating at the proximal end and ending on the midshaft (Fig. 10 AE, AG). This ridge (=fibular crest) is observed in Saturnalia, Heterodontosaurus Crompton and Charig, 1962, Silesaurus, Sacisaurus Ferigolo and Langer, 2007, and neotheropods (Nesbitt 2011; 333:1). The flat lateral margin of the lateral tibial condyle and the presence of a fibular crest lead us to assign TTU-P 11405A and TTU-P 11397 to Theropoda.
description: Published as part of Lessner, Emily J., Parker, William G., Marsh, Adam D., Nesbitt, Sterling J., Irmis, Randall B. & Mueller, Bill D., 2018, New insights into Late Triassic dinosauromorph-bearing assemblages from Texas using apomorphy-based identifications, pp. 1-41 in PaleoBios 35 on pages 29-31, DOI: 10.5070/P9351039960