Genus: Dromomeron Irmis, Nesbitt, Padian, Smith, Turner, Woody, and Downs, 2007

Classification: Archosauria - Dinosauromorpha - Lagerpetonidae

Etymology: Greek, dromas, 'running', and Greek, meros, 'femur'.

Synonyms:

Type species: D. romeri Irmis, Nesbitt, Padian, Smith, Turner, Woody, and Downs, 2007

Other Species: D. gregorii Nesbitt, Irmis,Parker, Smith, Turner, and Rowe, 2009

Diagnosis: Differs from all other dinosauromorphs in possessing the following synapmorphies: 1) a concave posterolateral surface of the crista tibiofibularis of the distal end of the femur; 2) distinct scar on the anterior surface of the distal end of the femur; and 3) posterolateral condyle of the proximal portion of the tibia is ventrally deflected or “hooked.” (Nesbitt et al., 2009)

Species: D. romeri Irmis, Nesbitt, Padian, Smith, Turner, Woody, and Downs, 2007

Etymology: In honor of Alfred Sherwood Romer, who first described dinosaur precursors from Argentina, including Lagerpeton.

Synonyms:

Holotype: GR 218: Complete left femur.

Paratypes:
GR 219: Right femur.
GR 220: Left tibia.
GR 221: Partial left femur.
GR 222: Left tibia.
GR 223: A complete astragalocalcaneum.
GR 234: A nearly complete femur.
NESBITT, IRMIS, PARKER, SMITH, TURNER & ROWE, 2009
GR 238: Partial articualted skeleton.
GR 239: Isolated right tibia (enemial crest crushed).
AMNH FR 2721: Distal portion of a femur.
AMNH FR 30648: Distal portion of a right tibia.
AMNH FR 30649: Distal portion of right tibia.

Referred Specimens:

NMMNH P-35379: Astragalocalcaneum.

Time: Early Norian, Late Triassic.

Horizon: Petrified Forest Formation, Chinle Group. (Petrified Forest Member, Chinle Formation).

Location: Ghost Ranch, Rio Arriba County, New Mexico, USA.

Total length:

Mass:

Diagnosis: Differs from all other dinosauromorphs except Lagerpeton chanarensis in possessing a hook-shaped femoral head, a lateral emargination ventral to the femoral head, an enlarged posteromedial tuber on the proximal portion of the femur, an enlarged crista tibiofibularis on the distal end of the femur, a posteromedial crest on the distal end of the tibia, and an astragalus with a posterior ascending process; differs from Lagerpeton in possessing a much larger crista tibiofibularis; and differs from all other basal dinosauromorphs in the absence of a fourth trochanter, the presence of a sharp ridge on the anteromedial edge of the distal end of the femur, and a large crest on the anteromedial edge of the astragalus (Irmis et al., 2007).
Differs from Dromomeron gregorii and all other basal dinosauromorphs in possessing the following autapomorphies: 1) absence of a distinct ridge for the attachment of the M. caudifemoralis longus (=4th trochanter); 2) presence of a sharp ridge on the anteromedial edge of the distal end of the femur; 3) presence of a lateral tuberosity on the anterolateral edge of the distal end of the femur; and 4) a large crest on the anteromedial edge of the astragalus and associated anteromedial concavity on the distal tibia (Nesbitt et al., 2009).

Comments: By the beginning of the Middle Triassic, the clade Archosauria had diversified into two lineages, Pseudosuchia and Ornithodira. Pseudosuchian archosaurs include the phytosaurs, aetosaurs, “rauisuchians,” and crocodylomorphs, whereas the Ornithodira includes the pterosaurs, a variety of basal dinosauromorphs and dinosauriforms, and dinosaurs (including birds). Throughout the Triassic Period, the pseudosuchians dominated ornithodirans in terms of size and diversity. By the end of the Triassic, all pseudosuchian clades were extinct except for Crocodylomorpha, and ornithodirans became taxonomically diverse and globally distributed. The early history of Pseudosuchia is well documented throughout Gondwana and Laurasia, but the early history of Ornithodira is only well-known from the Middle Triassic of Argentina. This material includes the incomplete skeletons of the earliest known basal dinosauromorphs Lagerpeton Romer, 1971, Marasuchus Sereno and Arcucci, 1994, and Pseudolagosuchus Arcucci, 1987.

Irmis et al. (2007) reported the first occurrence of a Late Triassic non-dinosauriform dinosauromorph. This taxon, Dromomeron romeri, from the Chinle Formation of New Mexico clearly indicates that non-dinosauriform dinosauromorphs were contemporaneous with dinosaurs and pseudosuchians in North America during the Late Triassic and had a broader geographic distribution than previously believed. Nessbit et al, 2009 described the hindlimb of Dromomeron romeri, name and describe a second species of Dromomeron, and demonstrated that non-dinosauriform dinosauromorphs had an extensive evolutionary history in the Late Triassic of North America. Furthermore, a growth series of femora from D.gregorii provides a better understanding of the ontogeny of character states in nondinosauriform dinosauromorphs.
When Dromomeron was first described (Irmis et al., 2007), no unambiguously associated specimens were found. Because Lagerpeton is only known from the hindlimb, pelvis, and dorsal and caudal vertebrae, only those elements could be identified from isolated remains and assigned to Dromomeron. Each element (femur, tibia, astragalocalcaneum) can be independently assigned to a Lagerpeton-like taxon using unambiguous synapomorphies. Taxonomic differences between D. romeri and D. gregorii are noted throughout the description. If only Dromomeron is mentioned, the feature is present in both D. romeri and D. gregorii. In the summer of 2007, a partially articulated specimen of Dromomeron romeri (GR 238) was discovered at the Hayden Quarry (holotype locality). Although the specimen does not preserve the ankle, it confirms that the tibia and femur, as hypothesized by Irmis et al. (2007), belong to the same taxon. The partial fibula and phalanges from GR 238 are described by Nesbitt et al. (2009); other portions of the skeleton will be described elsewhere after preparation is complete.

Illustrations:

The holotype left femur of Dromomeron romeri (GR 218) in A, anterolateral view; B, posteromedial view; C, proximal view; D, distal view. Scale bar 1 cm. Arrow indicates anterior direction. (from Irmis, R. B., Nesbitt, S. J., Padian, K., Smith, N. D., Turner, A. H., Woody, D., and Downs, A., 2007)

Species: D. gregorii Nesbitt, Irmis,Parker, Smith, Turner, and Rowe, 2009

Etymology: In honor of the late Joseph T. Gregory, who studied and described many of the fossil vertebrates from the Otis Chalk quarries.

Synonyms:

Holotype: TMM 31100-1306: Complete right femur.

Paratypes:
TMM 31100-464, -1308, -1234, -764: Right femora.
TMM 31100-278: Right tibia.
TMM 31100-1324: Left tibia.

Referred Specimens:

TMM 31100-82: Left femur.
TMM 31100-101: Distal portion of right femur.
TMM 31100-199: Left tibia.
TMM 31100-1314: Left tibia.
TMM 31100-1321: Proximal portion of right tibia.
TMM 31100-1321: Left tibia.
TMM 31185-99: Left tibia.
TMM 31185-100: Distal portion of a left tibia.

UCMP 25815: Distal portion of left femur.

Time: Carnian, Late Triassic.

Horizon: Colorado City Formation, Upper Dockum Group. Lower Bluewater Creek Formation (Lower Petrified Forest Formation).

Location: Otis Chalk Quarry 3 (TMM loality 31100), near Otis Chalk, Howard County, Texas, USA. Placerias quarry (NMMNH locality 858, UCMP A269, MNA 207C), Big Hollow Wash near Romero Springs, a small valley 6 1/2 miles (approximately 10.4 km) S 65"W of St. Johns, Apache County, Arizona, USA.

Total length:

Mass:

Diagnosis: Differs from Dromomeron romeri in possessing a distinct ridge for the attachment of the M. caudifemoralis longus (=4th trochanter), the presence of an anterior trochanter and trochanteric shelf, robust proximal and distal ends of the femora, the intercondylar groove of the distal femur is reduced to a slit in larger specimens (possible autapomorphy), and the lack of an anteromedial concavity on the distal end of the tibia (Nesbitt et al., 2009).

Comments: Dromomeron gregorii , D. romeri, Lagerpeton form the sister group to all other dinosauromorphs and demonstrate that this clade, Lagerpetidae, persisted well into the Norian. Lagerpetidae is supported by several synapomorphies: femoral head hookshaped in medial and lateral views; ventral emargination on the anterolateral side of the femoral head; an enlarged posteromedial tuber of the proximal end of the femur; femoral crista tibiofibularis larger than the medial condyle; anteromedial corner of the distal end of the femur forms 90o or acute (>90o) angle; and a posterior ascending process of the astragalus. An ontogenetic series of the femur of Dromomeron indicates that some character states previously used in phylogenetic analyses of early dinosaurs may be ontogenetically variable.

The discovery of Dromomeron romeri demonstrated that basal dinosauromorphs survived well into the Late Triassic and coexisted with early dinosaurs (Irmis et al., 2007). It also demonstrated that basal dinosauromorphs had a wider geographic range than previously thought. However, with D. romeri only known from one locality, it was unclear how widespread temporally and geographically this taxon was. The discovery of D. gregorii and other remains assignable to Dromomeron provide important new data for addressing these questions. Furthermore, they record the existence of a clade of Lagerpeton-like animals, here named Lagerpetidae, and the discovery of D. gregorii reinforces the conclusions of Irmis et al. (2007) regarding the tempo of the rise of dinosaurs in North America.

Illustrations: The holotype right femur of Dromomeron gregorii (TMM-31100-1306) in A, anterolateral view; B, posteromedial view; C, proximal view; D, distal view. Scale bar 1 cm. Arrow indicates anterior direction. (from Nesbitt, S. J., Irmis, R. B., Parker, W. G., Smith, N. D., Turner, A. H., and Rowe, T., 2009)

Ontogenetic sequence of the femur of Dromomeron gregorii. All lefts elements in proximal (top), anterolateral (middle), and distal (bottom) views. TMM-31100-764 A, TMM-31100-1234 B, TMM-31100-1308 C, TMM-31100-464 D, and TMM-31100-1306 E. Scale bar 1 cm. Six Dromomeron gregorii right femora of different sizes allow a discussion of the differentiation of character states in a growth series of basalmost ornithodirans. The preservation of each femur varies considerably from pristine to severely crushed. Nonetheless, careful comparisons of several character states illuminate ontogenetic trends in D. gregorii. ( (from Nesbitt, S. J., Irmis, R. B., Parker, W. G., Smith, N. D., Turner, A. H., and Rowe, T., 2009)

References:

Irmis, R. B., Nesbitt, S. J., Padian, K., Smith, N. D., Turner, A. H., Woody, D., and Downs, A., 2007, A Late Triassic dinosaurmorph assemblage from New Mexico and the rise of dinosaurs: Science, v. 317, p. 358-361.

Nesbitt, S. J., Irmis, R. B., Parker, W. G., Smith, N. D., Turner, A. H., and Rowe, T., 2009, Hindlimb osteology and districution of basal Dinosauromorphs from the Late Triassic of North America: Journal of Vertebrate Paleontology, v. 29, n. 2, p. 498-516.

© Tetrapoda Database Roman Ulansky roman.ulansky@gmail.com or adios85@mail.ru