# 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 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. It is named for Anne Arundel County, Maryland.
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.

## Vertebrate paleofauna

Dinosaurs present include the large theropod Acrocanthosaurus, the giant sauropod Astrodon, the possible ornithischian Magulodon, the poorly known theropods "Allosaurus" medius, "Creosaurus" potens, and "Coelurus" gracilis, the ornithomimosaurian "Dryosaurus" grandis, as well as another indeterminate ornithomimosaurian (though it most likely is Nedcolbertia), the nodosaurid Priconodon, a possible basal ceratopsian, and potentially the ornithopod Tenontosaurus. Other vertebrates are not as well known from the formation, but include a freshwater shark, a lungfish, 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. 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

Cartilaginous fish reported from the Arundel Formation :
| Genus | Species | Location | Stratigraphic position | Material | Notes | Images |
| Egertonodus | E. basanus | Maryland | | Teeth, spine | A hybodont shark. | |
| Planohybodus | P. ensis | Maryland | | 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 | | Teeth, jaw | A semionotid. | |
| cf. Vidalamiinae | | Maryland | | 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 | | Tooth plate | A ceratodontid lungfish. | |

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

#### Pterosaurs

Unassigned pteradactyloid tracks.
| 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 | | | | | | |

#### Turtles

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

#### Crocodylomorphs

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

### Mammals

Mammals reported from the Arundel Formation :
| Genus | Species | Location | Stratigraphic position | Material | Notes | Images |
| Argillomys | A. marylandensis | Maryland | | Molar tooth | A multituberculate. | |
| Arundelconodon | A. hottoni | Maryland | | 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.
G. J. Brenner (1963) described spores and pollen within the formation.