| Arundel Formation | |
| Type: | Geological Formation |
| Period: | Aptian |
| Prilithology: | Claystone, mudstone |
| Otherlithology: | Siderite nodules |
| Namedfor: | Anne Arundel County, Maryland |
| Namedby: | W. B. Clark, 1897[1] |
| Region: | Maryland, Washington D. C. |
| Country: | United States |
| Unitof: | Potomac Group |
| Underlies: | Patapsco Formation (Unconformity) |
| Overlies: | Patuxent Formation |
| Thickness: | up to 125feet |
The Arundel Formation, also known as the Arundel Clay, is a clay-rich sedimentary rock formation, within the Potomac Group, found in Maryland[2] of the United States of America. It dates to the Early Cretaceous, and is of late Aptian or (more likely) early Albian age. This rock unit had been economically important as a source of iron ore, but is now more notable for its dinosaur fossils.[3] It is named for Anne Arundel County, Maryland.[4]
It consists of clay lenses within depressions in the upper part of the Patuxent Formation that may represent oxbow swamp facies. The Arundel Formation contains a high number of terrestrial fauna, indicating that it was deposited in a freshwater fluvial environment, likely representing slow-moving river channels and oxbows. The Arundel Formation is the only major source for Early Cretaceous terrestrial vertebrates in eastern North America, and provides the best record of the dinosaurs that inhabited the region at the time.
Dinosaurs present include the large theropod Acrocanthosaurus,[5] [6] the giant sauropod Astrodon, the possible ornithischian Magulodon, the poorly known theropods "Allosaurus" medius, "Creosaurus" potens, and "Coelurus" gracilis, the ornithomimosaurian "Dryosaurus" grandis,[7] as well as another indeterminate ornithomimosaurian (though it most likely is Nedcolbertia),[8] the nodosaurid Priconodon,[9] a possible basal ceratopsian,[10] and potentially the ornithopod Tenontosaurus. Other vertebrates are not as well known from the formation, but include a freshwater shark, a lungfish,[11] at least three genera of turtles, and several crocodilians.
The dinosaurian fauna of the Arundel Formation is very similar to that found in the concurrent, more comprehensive geological formations from further west (i.e. the Antlers, Cloverly, and Cedar Mountain Formations). This supports the idea of a largely homogenous dinosaur fauna stretching across North America during the Early Cretaceous, until the formation of the Western Interior Seaway divided the continent and led to major faunal changes on both halves.[12] In contrast, among other vertebrate taxa, there are major differences between the Arundel and these western formations; in the Arundel, crocodylomorph and shark remains are far more common than those of bony fishes, whereas the opposite is true for the western formations. This may owe to differing environmental conditions on the Atlantic coast compared to the North American interior.
| Cartilaginous fish reported from the Arundel Formation | |||||||
|---|---|---|---|---|---|---|---|
| Genus | Species | Location | Stratigraphic position | Material | Notes | Images | |
| Egertonodus | E. basanus |
| Teeth, spine | A hybodont shark. | |||
| Planohybodus | P. ensis |
| Teeth | A hybodont shark, formerly placed in Hybodus. | |||
| Ray-finned fish reported from the Arundel Formation | |||||||
|---|---|---|---|---|---|---|---|
| Genus | Species | Location | Stratigraphic position | Material | Notes | Images | |
| cf. Lepidotes |
| Teeth, jaw | A semionotid. | ||||
| Teeth | An amiid. | |||||
| Dinosaurs reported from the Arundel Formation | ||||||
|---|---|---|---|---|---|---|
| Genus | Species | Location | Stratigraphic position | Material | Notes | Images |
| Acrocanthosaurus[15] | A. cf. atokensis |
| "Teeth", "incomplete skeleton" | A large carcharodontosaurid theropod. Presence long suspected but uncertain, but confirmed in 2024 following the discovery of more complete remains. | ||
| "A." medius |
| "Tooth."[17] | An indeterminate theropod tooth. | |||
| Astrodon[18] | A. johnstoni |
| "Tooth."[20] | |||
| "Capitalsaurus"[21] | "C." potens |
| "Vertebra." | A neotheropod possibly synonymous with Acrocanthosaurus.[22] | ||
| "C." gracilis | "Manual ungual and teeth." | A dromaeosaurid synonymous with Deinonychus. | ||||
| "C." potens | Reclassified as "Capitalsaurus" potens | |||||
| cf. Deinonychus | Indeterminate |
| A dromaeosaurid | |||
| "D." grandis |
| "Limb elements."[23] | An indeterminate member of Ornithomimosauria. | |||
| Magulodon[24] | M. muirkirkensis |
| "Tooth" | Likely an ornithischian, this genus is a nomen nudum that has not been formally published. | ||
| Neoceratopsia indet. | Indeterminate |
| "Teeth" | An indeterminate member of Neoceratopsia. Initially believed to have belonged to an indeterminate member of Dryosauridae or the genus Tenontosaurus. | ||
| "O." affinis | Junior synonym of "Dryosaurus" grandis | |||||
| Pleurocoelus | P. altus |
| "Tibia [and] fibula."[25] | A sauropod synonymous with Astrodon. | ||
| P. nanus |
| |||||
| Priconodon | P. crassus |
| "Teeth, tibia."[26] | A large nodosaurid. | ||
| cf. Richardoestesia | Indeterminate |
| "Teeth" | A small theropod. | ||
| cf. Tenontosaurus[27] | Indeterminate |
| ||||
Unassigned pteradactyloid tracks.[28]
| Turtles reported from the Arundel Formation | |||||||
|---|---|---|---|---|---|---|---|
| Genus | Species | Location | Stratigraphic position | Material | Notes | Images | |
| Arundelemys | A. dardeni |
| "A single, incomplete skull lacking the lower jaws and cheek region" | A baenid. | |||
| Glyptops | G. caelatus |
| "Carapace fragments probably pertaining to a single individual" | A pleurosternid. | |||
| Naomichelys | N. sp. |
| "Shell fragments" | A helochelydrid. | |||
| Crocodylomorphs reported from the Arundel Formation | |||||||
|---|---|---|---|---|---|---|---|
| Genus | Species | Location | Stratigraphic position | Material | Notes | Images | |
| cf. Bernissartiidae indet. | Indeterminate |
| Tooth | A likely bernissartiid. | |||
| cf. Goniopholididae indet. | Indeterminate |
| Scutes, teeth | A likely goniopholidid, the most common crocodylomorph from the formation. | |||
| cf. Pholidosauridae indet. | Indeterminate |
| Teeth | A likely pholidosaurid. | |||
| Mammals reported from the Arundel Formation | |||||||
|---|---|---|---|---|---|---|---|
| Genus | Species | Location | Stratigraphic position | Material | Notes | Images | |
| Argillomys | A. marylandensis |
| Molar tooth | A multituberculate. | |||
| Arundelconodon | A. hottoni |
| Dentary with teeth | A triconodontid. | |||
William Bullock Clark (1897) described lignitized trunks of trees often found in upright positions with their roots still intact.[1]
G. J. Brenner (1963) described spores and pollen within the formation.[29]