Arundel Formation

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.^[3] This rock unit had been economically important as a source of iron ore, but is now more notable for its dinosaur fossils.^[4] It is named for Anne Arundel County, Maryland.^[5]

Arundel Formation
Stratigraphic range: late Aptian or early Albian PreꞒ Ꞓ O S D C P T J K Pg N ↓
Outdoor excavation on an exposed portion of the Arundel clays at Dinosaur Park at Laurel, Maryland, USA
Type	Geological Formation
Unit of	Potomac Group
Underlies	Patapsco Formation (Unconformity)
Overlies	Patuxent Formation
Thickness	up to 125 feet (40 m)[1]
Lithology
Primary	Claystone, mudstone
Other	Siderite nodules
Location
Region	Maryland, Washington D. C.
Country	United States
Type section
Named for	Anne Arundel County, Maryland
Named by	W. B. Clark, 1897[1]

It consists of clay lenses within depressions in the upper part of the Patuxent Formation that may represent oxbow swamp facies.^[4] 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.^[3]

Vertebrate paleofauna

Dinosaurs present include the large theropod Acrocanthosaurus,^[6][7][8] the giant sauropod Astrodon, the possible ornithischian Magulodon,^[6] the poorly known theropods "Allosaurus" medius, "Creosaurus" potens, and "Coelurus" gracilis, the ornithomimosaurian "Dryosaurus" grandis,^[9] as well as another indeterminate ornithomimosaurian (though it most likely is Nedcolbertia),^[10] the nodosaurid Priconodon,^[11] a possible basal ceratopsian,^[12] and potentially the ornithopod Tenontosaurus.^[4] Other vertebrates are not as well known from the formation, but include a freshwater shark, a lungfish,^[13] at least three genera of turtles, and several crocodilians.^[4]

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.^[14] 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.^[3]

Cartilaginous fish

Cartilaginous fish reported from the Arundel Formation
Genus	Species	Location	Stratigraphic position	Material	Notes	Images
Egertonodus	E. basanus	Maryland[3]		Teeth, spine	A hybodont shark.
Planohybodus	P. ensis	Maryland[3]		Teeth	A hybodont shark, formerly placed in Hybodus.

Ray-finned fish

Ray-finned fish reported from the Arundel Formation
Genus	Species	Location	Stratigraphic position	Material	Notes	Images
cf. Lepidotes		Maryland[3]		Teeth, jaw	A semionotid.
cf. Vidalamiinae		Maryland[3]		Teeth	An amiid.

Lobe-finned fish

Lobe-finned fish reported from the Arundel Formation
Genus	Species	Location	Stratigraphic position	Material	Notes	Images
Ceratodus	C. kranzi	Maryland[3]		Tooth plate	A ceratodontid lungfish.[15]

Reptiles

Dinosaurs

Color key Taxon Reclassified taxon Taxon falsely reported as present Dubious taxon or junior synonym Ichnotaxon Ootaxon Morphotaxon		Notes Uncertain or tentative taxa are in small text; crossed out taxa are discredited.

Dinosaurs reported from the Arundel Formation
Genus	Species	Location	Stratigraphic position	Material	Notes	Images
Acrocanthosaurus[6][8]	A. cf. atokensis[8]	Maryland[2]		"Teeth",[6] "incomplete skeleton"[8]	A large carcharodontosaurid theropod. Presence long suspected but uncertain, but confirmed in 2024 following the discovery of more complete remains.[8]	Acrocanthosaurus
Allosaurus[16]	"A." medius[16]	Maryland[16]		"Tooth."[17]	An indeterminate theropod tooth.	Astrodon Deinonychus Tenontosaurus
Astrodon[18]	A. johnstoni[18]	Maryland[16] Washington D.C.[19]		"Tooth."[20]
"Capitalsaurus"[21]	"C." potens	Washington D.C.[16]		"Vertebra."[17]	A neotheropod possibly synonymous with Acrocanthosaurus.[22]
Coelurus[16]	"C." gracilis			"Manual ungual and teeth."[17]	A dromaeosaurid synonymous with Deinonychus.[22]
Creosaurus[16]	"C." potens[16]				Reclassified as "Capitalsaurus" potens
cf. Deinonychus[16]	Indeterminate[16]	Maryland[16]			A dromaeosaurid
Dryosaurus	"D." grandis	Maryland[16]		"Limb elements."[23]	An indeterminate member of Ornithomimosauria.
Magulodon[24]	M. muirkirkensis[24]	Maryland[24]		"Tooth"[24]	Likely an ornithischian, this genus is a nomen nudum that has not been formally published.
Neoceratopsia indet.[12]	Indeterminate[12]	Maryland[16]		"Teeth"[12]	An indeterminate member of Neoceratopsia. Initially believed to have belonged to an indeterminate member of Dryosauridae or the genus Tenontosaurus.
Ornithomimus	"O." affinis				Junior synonym of "Dryosaurus" grandis
Pleurocoelus[16]	P. altus[16]	Maryland[16]		"Tibia [and] fibula."[25]	A sauropod synonymous with Astrodon.
	P. nanus[16]	Maryland[16]
Priconodon[16]	P. crassus[16]	Maryland[16]		"Teeth, tibia."[26]	A large nodosaurid.
cf. Richardoestesia[3]	Indeterminate[3]	Maryland[3]		"Teeth"	A small theropod.
cf. Tenontosaurus[27]	Indeterminate[27]	Maryland[16]

Pterosaurs

Unassigned pteradactyloid tracks.^[28]

Color key Taxon Reclassified taxon Taxon falsely reported as present Dubious taxon or junior synonym Ichnotaxon Ootaxon Morphotaxon		Notes Uncertain or tentative taxa are in small text; crossed out taxa are discredited.

Pterosaurs of the Arundel Formation
Genus	Species	Location	Stratigraphic position	Abundance	Notes	Images
Pteraichnus[28]				[28]

Turtles

Turtles reported from the Arundel Formation
Genus	Species	Location	Stratigraphic position	Material	Notes	Images
Arundelemys	A. dardeni	Maryland[3]		"A single, incomplete skull lacking the lower jaws and cheek region"	A baenid.
Glyptops	G. caelatus	Maryland[3]		"Carapace fragments probably pertaining to a single individual"	A pleurosternid.
Naomichelys	N. sp.	Maryland[3]		"Shell fragments"	A helochelydrid.

Crocodylomorphs

Crocodylomorphs reported from the Arundel Formation
Genus	Species	Location	Stratigraphic position	Material	Notes	Images
cf. Bernissartiidae indet.	Indeterminate	Maryland[3]		Tooth	A likely bernissartiid.
cf. Goniopholididae indet.	Indeterminate	Maryland[3]		Scutes, teeth	A likely goniopholidid, the most common crocodylomorph from the formation.
cf. Pholidosauridae indet.	Indeterminate	Maryland[3]		Teeth	A likely pholidosaurid.

Mammals

Mammals reported from the Arundel Formation
Genus	Species	Location	Stratigraphic position	Material	Notes	Images
Argillomys	A. marylandensis	Maryland[3]		Molar tooth	A multituberculate.
Arundelconodon	A. hottoni	Maryland[3]		Dentary with teeth	A triconodontid.

Other fossils

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]

See also

• Earth sciences portal
• Paleontology portal
• Dinosaurs portal

• List of dinosaur-bearing rock formations