The Arundel Formation, also known as the Arundel Clay, is a clay

Clay

Clay is a general term including many combinations of one or more clay minerals with traces of metal oxides and organic matter. Geologic clay deposits are mostly composed of phyllosilicate minerals containing variable amounts of water trapped in the mineral structure.- Formation :Clay minerals...

-rich sedimentary rock formation, within the Potomac Group, found in Maryland

Maryland

Maryland is a U.S. state located in the Mid Atlantic region of the United States, bordering Virginia, West Virginia, and the District of Columbia to its south and west; Pennsylvania to its north; and Delaware to its east...

  of the United States of America. It is of Aptian

Aptian

The Aptian is an age in the geologic timescale or a stage in the stratigraphic column. It is a subdivision of the Early or Lower Cretaceous epoch or series and encompasses the time from 125.0 ± 1.0 Ma to 112.0 ± 1.0 Ma , approximately...

 age (Lower Cretaceous). This rock unit had been economically important as a source of iron ore, but is now more notable for its dinosaur

Dinosaur

Dinosaurs are a diverse group of animals of the clade and superorder Dinosauria. They were the dominant terrestrial vertebrates for over 160 million years, from the late Triassic period until the end of the Cretaceous , when the Cretaceous–Paleogene extinction event led to the extinction of...

 fossil

Fossil

Fossils are the preserved remains or traces of animals , plants, and other organisms from the remote past...

s. Although often considered a formation, it may be only oxbow

Oxbow

An oxbow is a U-shaped wooden or metal frame that fits under and around the neck of an ox or bullock, with its upper ends passing through the bar of the yoke and held in place with a metal key, called a bow pin. The wood most often used is hardwood steamed into shape, like elm or hickory and...

 swamp facies

Facies

In geology, facies are a body of rock with specified characteristics. Ideally, a facies is a distinctive rock unit that forms under certain conditions of sedimentation, reflecting a particular process or environment....

 within the Potomac Group.

Dinosaurs

Dinosaurs present include possibly the large theropod Acrocanthosaurus

Acrocanthosaurus

Acrocanthosaurus is a genus of theropod dinosaur that existed in what is now North America during the Aptian and early Albian stages of the Early Cretaceous. Like most dinosaur genera, Acrocanthosaurus contains only a single species, A. atokensis. Its fossil remains are found mainly in the U.S...

, the poorly known theropods "Allosaurus

Allosaurus

Allosaurus is a genus of large theropod dinosaur that lived 155 to 150 million years ago during the late Jurassic period . The name Allosaurus means "different lizard". It is derived from the Greek /allos and /sauros...

 (or "Dryptosaurus

Dryptosaurus

Dryptosaurus was a genus of primitive tyrannosaur that lived in Eastern North America during the middle Maastrichtian stage of the Late Cretaceous period. Although largely unknown now outside of academic circles, a famous painting of the genus by Charles R...

") medius", "Creosaurus (or "Dryptosaurus") potens" (also known as "Capitalsaurus

Capitalsaurus

"Capitalsaurus" is the informal name given to a genus of dinosaur from North America. Its fossils were discovered in Washington, DC. It was a theropod, and it lived during the Cretaceous...

"), and "Coelurus

Coelurus

Coelurus is a genus of coelurosaur dinosaur from the Late Jurassic period . The name means "hollow tail", referring to its hollow tail vertebrae...

 gracilis", the ornithomimosauria

Ornithomimosauria

The Ornithomimosauria, ornithomimosaurs or ostrich dinosaurs were theropod dinosaurs which bore a superficial resemblance to modern ostriches. They were fast, omnivorous or herbivorous dinosaurs from the Cretaceous Period of Laurasia...

n "Coelosaurus affinis", the sauropods Astrodon

Astrodon

Astrodon was a genus of large herbivorous sauropod dinosaur, related to Brachiosaurus, that lived in what is now the eastern United States during the Early Cretaceous period. Its fossils have been found in the Arundel Formation, which has been dated through palynomorphs to the Aptian-Albian...

and Pleurocoelus, the nodosaurid Priconodon

Priconodon

Priconodon is an extinct genus of dinosaur , known from its large teeth. Its remains have been found in the Aptian-Albian age Lower Cretaceous Arundel Formation of Muirkirk, Prince George's County, Maryland.-History:...

, a possible basal

Basal (phylogenetics)

In phylogenetics, a basal clade is the earliest clade to branch in a larger clade; it appears at the base of a cladogram.A basal group forms an outgroup to the rest of the clade, such as in the following example:...

 ceratopsia

Ceratopsia

Ceratopsia or Ceratopia is a group of herbivorous, beaked dinosaurs which thrived in what are now North America, Europe, and Asia, during the Cretaceous Period, although ancestral forms lived earlier, in the Jurassic. The earliest known ceratopsian, Yinlong downsi, lived between 161.2 and 155.7...

n, and potentially the ornithopod

Ornithopod

Ornithopods or members of the clade Ornithopoda are a group of ornithischian dinosaurs that started out as small, bipedal running grazers, and grew in size and numbers until they became one of the most successful groups of herbivores in the Cretaceous world, and dominated the North American...

 Tenontosaurus

Tenontosaurus

Tenontosaurus is a genus of medium- to large-sized ornithopod dinosaur. The genus is known from the late Aptian to Albian ages of the middle Cretaceous period sediments of western North America, dating between 115 to 108 million years ago...

. Other vertebrate

Vertebrate

Vertebrates are animals that are members of the subphylum Vertebrata . Vertebrates are the largest group of chordates, with currently about 58,000 species described. Vertebrates include the jawless fishes, bony fishes, sharks and rays, amphibians, reptiles, mammals, and birds...

s are not as well-known from the formation, but include a freshwater shark

Shark

Sharks are a type of fish with a full cartilaginous skeleton and a highly streamlined body. The earliest known sharks date from more than 420 million years ago....

, a lungfish

Lungfish

Lungfish are freshwater fish belonging to the Subclass Dipnoi. Lungfish are best known for retaining characteristics primitive within the Osteichthyes, including the ability to breathe air, and structures primitive within Sarcopterygii, including the presence of lobed fins with a well-developed...

, at least three genera of turtle

Turtle

Turtles are reptiles of the order Testudines , characterised by a special bony or cartilaginous shell developed from their ribs that acts as a shield...

s, and at least one crocodilia

Crocodilia

Crocodilia is an order of large reptiles that appeared about 84 million years ago in the late Cretaceous Period . They are the closest living relatives of birds, as the two groups are the only known survivors of the Archosauria...

n.

Dinosaur Dinosaur Dinosaurs are a diverse group of animals of the clade and superorder Dinosauria. They were the dominant terrestrial vertebrates for over 160 million years, from the late Triassic period until the end of the Cretaceous , when the Cretaceous–Paleogene extinction event led to the extinction of... s reported from the Arundel Formation
Genus	Species	Location	Stratigraphic position	Material	Notes	Images
Allosaurus Allosaurus Allosaurus is a genus of large theropod dinosaur that lived 155 to 150 million years ago during the late Jurassic period . The name Allosaurus means "different lizard". It is derived from the Greek /allos and /sauros...	A. medius	Maryland		"Tooth."
Astrodon Astrodon Astrodon was a genus of large herbivorous sauropod dinosaur, related to Brachiosaurus, that lived in what is now the eastern United States during the Early Cretaceous period. Its fossils have been found in the Arundel Formation, which has been dated through palynomorphs to the Aptian-Albian...	A. johnstoni	Maryland Washington D.C.		"Tooth."
Coelosaurus	C. affinis	Maryland		"Limb elements."
"Coelurus"	"C." gracilis			"Manual ungual and teeth."	Probably a dromaeosaurid.
Creosaurus	C. potens	Maryland		"Vertebra."
Deinonychus Deinonychus Deinonychus was a genus of carnivorous dromaeosaurid dinosaur. There is one described species, Deinonychus antirrhopus. This 3.4 meter long dinosaur lived during the early Cretaceous Period, about 115–108 million years ago . Fossils have been recovered from the U.S...	Indeterminate	Maryland
Pleurocoelus	P. altus	Maryland		"Tibia [and] fibula."
	P. nanus	Maryland
	Indeterminate	Washington D. C.
Priconodon Priconodon Priconodon is an extinct genus of dinosaur , known from its large teeth. Its remains have been found in the Aptian-Albian age Lower Cretaceous Arundel Formation of Muirkirk, Prince George's County, Maryland.-History:...	P. crassus	Maryland		"Teeth, tibia."
Tenontosaurus Tenontosaurus Tenontosaurus is a genus of medium- to large-sized ornithopod dinosaur. The genus is known from the late Aptian to Albian ages of the middle Cretaceous period sediments of western North America, dating between 115 to 108 million years ago...	Indeterminate	Maryland

Pterosaurs

Unassigned pteradactyloid tracks.

Pterosaur Pterosaur Pterosaurs were flying reptiles of the clade or order Pterosauria. They existed from the late Triassic to the end of the Cretaceous Period . Pterosaurs are the earliest vertebrates known to have evolved powered flight... s of the Arundel Formation
Genus	Species	Location	Stratigraphic position	Abundance	Notes	Images
Pteraichnus Pteraichnus Pteraichnus is an ichnogenus that has been attributed to pterosaurs. It has been found in, among other units, the Lower Jurassic Aztec Sandstone....

Other fossils

W. B. Clark (1897) described lignitized

Lignite

Lignite, often referred to as brown coal, or Rosebud coal by Northern Pacific Railroad,is a soft brown fuel with characteristics that put it somewhere between coal and peat...

trunks of trees often found in upright positions with their roots still intact.

G. J. Brenner (1963) described spores and pollen within the formation.