Animantarx

Taxobox|
name = "Animantarx"
fossil_range = Early Cretaceous
regnum = Animalia
phylum = Chordata
classis = Sauropsida
superordo = Dinosauria
ordo = Ornithischia
subordo = Thyreophora
superfamilia = Ankylosauria
familia = Nodosauridae
genus = "Animantarx"
species = "A. ramaljonesi"
binomial = "Animantarx ramaljonesi"
binomial_authority = Carpenter "et al.", 1999

"Animantarx" (pronEng|ˌænɨˈmæntɑrks AN-i-MAN-tahrks; "living citadel") is a genus of nodosaurid ankylosaurian dinosaur from the Upper Cretaceous of western North America. Like other nodosaurs, it would have been a slow-moving quadrupedal herbivore covered in heavy armor scutes, but without a tail club. The skull measures approximately 10 inches (25cm) in length, suggesting the animal as a whole was no more than 10 feet (3 meters) long.

The generic name is composed of the Latin words "animatus" ("living" or "animated") and "arx" ("fortress" or "citadel"), referring to its armored nature. In particular, the name is a reference to a comment made by paleontologist R. S. Lull about ankylosaurs, that as "an animated citadel, these animals must have been practically unassailable..." [Lull, R.S. 1914. Rulers of the Mesozoic. "Yale Review" 3: 352-363.] The type species is the only one known so far, and is called "A. ramaljonesi" after its discoverer, Ramal Jones. His wife, Carol Jones, also discovered the contemporaneous dinosaur "Eolambia" nearby.

Discovery and species

Only one specimen of "Animantarx" has so far been recovered. The remains include the lower jaw and back half of the skull, along with neck and back vertebrae, and various limb elements. "Animantarx" is characterized by a unique combination of features, including a highly domed skull back, small horns on the postorbital and quadratojugal bones of the skull, and a mandible which is only armoured on half of its length.

These fossil remains were discovered in the Mussentuchit Member of the Cedar Mountain Formation in the eastern portion of the U.S. state of Utah. This section of the formation is believed to represent the late Albian through early Cenomanian stages of the Late Cretaceous Period, or about 106 to 97 million years ago. At least 80 other vertebrate species are known from the Mussentuchit, including fish, frogs, lizards, snakes, crocodilians, dinosaurs, birds, and mammals, although not all are complete enough to name. Many dinosaur groups are represented by fossils from this member, including carnivorous theropods as well as several different herbivorous types, including the iguanodont "Eolambia". The presence of aquatic animals like fish and frogs, as well as the mudstone in which their fossils are found, suggests that this was a floodplain environment.Kirkland, J.I., Britt, B., Burge, D.L., Carpenter, K., Cifelli, R., DeCourten, F., Eaton, J., Hasiotis, S., and Lawton, T. 1997. Lower to Middle Cretaceous dinosaur faunas of the Central Colorado Plateau: a key to understanding 35 million years of tectonics, sedimentology, evolution, and biogeography. "Brigham Young University Geology Studies" 42:69-103.]

Earlier layers within the Cedar Mountain Formation contain different nodosaur species. The oldest layer, known as the Yellow Cat Member, contains "Gastonia", while the intermediate Poison Strip and Ruby Ranch Members contain remains which may belong to "Sauropelta". The Mussentuchit, which is the youngest member of the Cedar Mountain, contains only "Animantarx". While there is still a lot of exploration left to be done, this division of nodosaur species corresponds with that of other dinosaur groups and provides support for the hypothesis of three separate faunas in the Cedar Mountain Formation. The Mussentuchit fauna includes many taxa which may be of Asian origin and suggests a dispersal event may have occurred from Asia into North America around this time.Kirkland, J.I., Britt, B., Burge, D.L., Carpenter, K., Cifelli, R., DeCourten, F., Eaton, J., Hasiotis, S., and Lawton, T. 1997. Lower to Middle Cretaceous dinosaur faunas of the Central Colorado Plateau: a key to understanding 35 million years of tectonics, sedimentology, evolution, and biogeography. "Brigham Young University Geology Studies" 42:69-103.] [Carpenter, K., Kirkland, J.I., Burge, D.L., & Bird, J. 1999. Ankylosaurs (Dinosauria: Ornithischia) of the Cedar Mountain Formation, Utah, and their stratigraphic distribution. In: Gillette, D. (Ed.) "Vertebrate Paleontology in Utah". "Utah Geological Survey Miscellaneous Publication 99-1". Pp. 243-251.]

Fossils in this region are often slightly radioactive, and remains of "Animantarx" were actually discovered following a radiological survey of the area performed by Ramal Jones, which located a higher level of radioactivity at a certain location. Subsequent excavation at this site turned up the fossil skeleton of "Animantarx"; no bones had been exposed on the surface. [Jones, R.D. & Burge, D.L. 1995. Radiological surveying as a method for mapping dinosaur bone sites. "Journal of Vertebrate Paleontology" 15: 38A.]

Classification

"Animantarx" is universally thought of as a nodosaurid ankylosaur, although its precise relationships within that family are uncertain. The most recent cladistic analysis of ankylosaur phylogeny does not include "Animantarx", although the authors recognize the genus as Nodosauridae "incertae sedis" because of its rounded supraorbital protrusions and a "knoblike" acromion on the scapula. [Vickaryous, M.K., Maryanska, T., & Weishampel, D.B. 2004. Ankylosauria. In: Weishampel, D.B., Dodson, P., & Osmólska, H. (Eds.). "The Dinosauria" (2nd edition). Berkeley: University of California Press. Pp. 363-392.] Two separate studies have found "Animantarx" to be the sister taxon of "Edmontonia" within Nodosauridae. [Carpenter, K. 2001. Phylogenetic analysis of the Ankylosauria. In: Carpenter, K. (Ed.). "The Armored Dinosaurs". Bloomington: Indiana University Press. Pp. 454–483.] [Hill, R.V., Witmer, L.M., Norell, M.A. 2003. A New specimen of "Pinacosaurus grangeri" (Dinosauria: Ornithischia) from the Late Cretaceous of Mongolia: ontogeny and phylogeny of ankylosaurs. "American Museum Novitates" 3395: 1-29.]

References