''Dromomeron'' (meaning "running
femur
The femur (; : femurs or femora ), or thigh bone is the only long bone, bone in the thigh — the region of the lower limb between the hip and the knee. In many quadrupeds, four-legged animals the femur is the upper bone of the hindleg.
The Femo ...
") is a
genus
Genus (; : genera ) is a taxonomic rank above species and below family (taxonomy), family as used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In bino ...
of
lagerpetid avemetatarsalian which lived around 220 to 211.9 ± 0.7 million years ago.
The genus contains species known from
Late Triassic
The Late Triassic is the third and final epoch (geology), epoch of the Triassic geologic time scale, Period in the geologic time scale, spanning the time between annum, Ma and Ma (million years ago). It is preceded by the Middle Triassic Epoch a ...
-age rocks of the
Southwestern United States
The Southwestern United States, also known as the American Southwest or simply the Southwest, is a geographic and cultural list of regions of the United States, region of the United States that includes Arizona and New Mexico, along with adjacen ...
and northwestern
Argentina
Argentina, officially the Argentine Republic, is a country in the southern half of South America. It covers an area of , making it the List of South American countries by area, second-largest country in South America after Brazil, the fourt ...
. It is described as most closely related to the earlier ''
Lagerpeton'' of
Argentina
Argentina, officially the Argentine Republic, is a country in the southern half of South America. It covers an area of , making it the List of South American countries by area, second-largest country in South America after Brazil, the fourt ...
, but was found among remains of true dinosaurs like ''
Chindesaurus
''Chindesaurus'' ( ) is an extinct genus of basal saurischian dinosaur from the Late Triassic (213-210 million years ago) of the southwestern United States. It is known from a single species, ''C. bryansmalli'', based on a partial skeleton recov ...
'', indicating that the first dinosaurs did not immediately replace related groups.
[
Based on the study of the overlapping material of ''Dromomeron'' and '' Tawa hallae'', Christopher Bennett proposed that the two taxa were conspecific, forming a single growth series of ''Dromomeron''.] However, noting prominent differences between their femurs which cannot be attributed to variation with age, Rodrigo Muller rejected this proposal in 2017. He further noted that, while ''D. romeri'' is known from juveniles only, it shares many traits in common with ''D. gigas'', which is known from mature specimens.
Description
It is known from partial remains, largely from the hindlimbs, which indicate an animal with an overall length of .
The brain of ''D. gregorii'' is characterised by a floccular lobe of the cerebellum that projects within the space of the semicircular canals, which are large relative to other archosauromorphs. The anterior canal is circularly shaped.
Classification
The bones of ''Dromomeron'' are most similar to those of the older pterosauromorph ''Lagerpeton'',[ and the two animals have been classified together in a ]clade
In biology, a clade (), also known as a Monophyly, monophyletic group or natural group, is a group of organisms that is composed of a common ancestor and all of its descendants. Clades are the fundamental unit of cladistics, a modern approach t ...
Lagerpetonidae
Lagerpetidae (; originally Lagerpetonidae) is a family (biology), family of basal (phylogenetics), basal Avemetatarsalia, avemetatarsalians (early-diverging members of the reptile lineage leading to birds and other dinosaurs). Though traditionall ...
.[Baron, M.G., Norman, D.B., and Barrett, P.M. (2017). A new hypothesis of dinosaur relationships and early dinosaur evolution. ''Nature'', 543: 501–506. ]
Cladogram
A cladogram (from Greek language, Greek ''clados'' "branch" and ''gramma'' "character") is a diagram used in cladistics to show relations among organisms. A cladogram is not, however, an Phylogenetic tree, evolutionary tree because it does not s ...
simplified after Kammerer, Nesbitt & Shubin (2012):
Discovery and species
''Dromomeron romeri''
The species name ''romeri'' honors influential 20th-century vertebrate paleontologist Alfred Sherwood Romer.[ ''Dromomeron'' and ]type species
In International_Code_of_Zoological_Nomenclature, zoological nomenclature, a type species (''species typica'') is the species name with which the name of a genus or subgenus is considered to be permanently taxonomically associated, i.e., the spe ...
''D. romeri'' are based on GR 218, a complete left thigh bone from the Hayden Quarry at Ghost Ranch, New Mexico
New Mexico is a state in the Southwestern United States, Southwestern region of the United States. It is one of the Mountain States of the southern Rocky Mountains, sharing the Four Corners region with Utah, Colorado, and Arizona. It also ...
. The rocks there are in the lower portion of the Petrified Forest Member of the Chinle Formation, and are Norian
The Norian is a division of the Triassic geological period, Period. It has the rank of an age (geology), age (geochronology) or stage (stratigraphy), stage (chronostratigraphy). It lasted from ~227.3 to Mya (unit), million years ago. It was prec ...
in age. Additional hindlimb bones, some probably from the same individual, are also known,[ and a partial skeleton has been recovered from Hayden Quarry, but has not yet been fully prepared. A few other specimens have been recovered from nearby localities, including the Snyder Quarry.][ Other specimens from the Chinle Formation of ]Arizona
Arizona is a U.S. state, state in the Southwestern United States, Southwestern region of the United States, sharing the Four Corners region of the western United States with Colorado, New Mexico, and Utah. It also borders Nevada to the nort ...
and a roughly contemporaneous part of the Dockum Group of Texas
Texas ( , ; or ) is the most populous U.S. state, state in the South Central United States, South Central region of the United States. It borders Louisiana to the east, Arkansas to the northeast, Oklahoma to the north, New Mexico to the we ...
also have been assigned to this genus. These have been assigned to a second species ''D. gregorii'', named in 2009.[
]
''Dromomeron gregorii''
''D. gregorii'', named for Joseph T. Gregory, is based on TMM 31100–1306, a right femur (thigh bone) from the Otis Chalk Quarry, Colorado City Formation, (Dockum Group), near Otis Chalk, Texas. Several other limb bones from the quarry, and a partial femur (thigh bone) from the '' Placerias'' Quarry of eastern Arizona
Arizona is a U.S. state, state in the Southwestern United States, Southwestern region of the United States, sharing the Four Corners region of the western United States with Colorado, New Mexico, and Utah. It also borders Nevada to the nort ...
have been assigned to this species. The rocks that ''D. gregorii'' is known from are older than those ''romeri'' has been found in. As with the Hayden Quarry, the Otis Chalk Quarry has at least one specimen of a herrerasaurid.[
]
''Dromomeron gigas''
A third species, ''D. gigas'', was described by Martínez ''et al''. (2016) on the basis of fossils recovered from the Norian Quebrada del Barro Formation in northwestern Argentina.
Paleoecology
Also found at the Hayden Quarry are the remains of phytosaurs, aetosaur
Aetosaurs () are heavily armored reptiles belonging to the extinct order (biology), order Aetosauria (; from Ancient Greek, Greek, (aetos, "eagle") and (, "lizard")). They were medium- to large-sized Omnivore, omnivorous or Herbivore, herbivor ...
s, rauisuchia
"Rauisuchia" is a paraphyletic group of mostly large and carnivorous Triassic archosaurs. Rauisuchians are a category of archosaurs within a larger group called Pseudosuchia, which encompasses all archosaurs more closely related to crocodilians ...
ns, and several types of dinosaurs and dinosaur relatives, including a '' Silesaurus''-like animal, the herrerasaurid ''Chindesaurus
''Chindesaurus'' ( ) is an extinct genus of basal saurischian dinosaur from the Late Triassic (213-210 million years ago) of the southwestern United States. It is known from a single species, ''C. bryansmalli'', based on a partial skeleton recov ...
'', and the basal theropod ''Tawa''. Finding the remains of four types of dinosaurs and dinosaur relatives (including ''Dromomeron'' itself) is noteworthy because it shows that dinosaurs did not immediately replace their dinosauromorph predecessors; that some of these groups, like the lagerpetonids, persisted (for longer than previously known) and diversified; and that dinosaurian replacement may have occurred at different times in different areas.[
]
References
External links
Online supplementary material
for the Irmis ''et al.''. article (PDF).
for the Irmis ''et al.''. article.
{{Taxonbar, from=Q2566615
Lagerpetidae
Late Triassic archosaurs of North America
Late Triassic reptiles of South America
Fossil taxa described in 2007
Triassic Argentina