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, "Dryptosaurus?" 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,Frederickson, J. A., Lipka, T. R., & Cifelli, R. L. (2016). A new species of the lungfish Ceratodus (Dipnoi) from the Early Cretaceous of the eastern USA. Journal of Vertebrate Paleontology, e1136316. 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 ImagesEgertonodusE. basanus MarylandTeeth, spineA hybodont shark.PlanohybodusP. ensis MarylandTeethA hybodont shark, formerly placed in Hybodus.

Ray-finned fish Ray-finned fish reported from the Arundel Formation Genus Species Location Stratigraphic position Material Notes Imagescf. Lepidotes MarylandTeeth, jawA semionotid.cf. Vidalamiinae MarylandTeethAn amiid.

Lobe-finned fish Lobe-finned fish reported from the Arundel Formation Genus Species Location Stratigraphic position Material Notes ImagesCeratodusC. kranzi MarylandTooth plateA ceratodontid lungfish.

Reptiles

Dinosaurs

Dinosaurs reported from the Arundel Formation Genus Species Location Stratigraphic position Material Notes ImagesAcrocanthosaurusA. 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.Allosaurus"3.25 Maryland, United States; 1. Arundel Clay," in Weishampel, et al. (2004). Page 556."A." medius Maryland"Tooth.""Table 4.1," in Weishampel, et al. (2004). Page 78.An indeterminate theropod tooth.Astrodon"3.25 Maryland, United States; 1. Arundel Clay" and "3.34 Washington D. C., United States; 1. Arundel Clay," in Weishampel, et al. (2004). Page 556.A. johnstoni Maryland Washington D.C."3.34 Washington D. C., United States; 1. Arundel Clay," in Weishampel, et al. (2004). Page 556."Tooth.""Table 13.1," in Weishampel, et al. (2004). Page 270."Capitalsaurus"Kranz, D. 1998. Mostly Dinosaurs: A Review of the Vertebrates of the Potomac Group (Aptian Arundel Formation), USA, in Lucas, Kirkland and Estep, eds., 1998: 235-238."C." potens Washington D.C."Vertebra."A neotheropod possibly synonymous with Acrocanthosaurus.Coelurus"C." gracilis"Manual ungual and teeth."A dromaeosaurid synonymous with Deinonychus.Creosaurus"C." potensReclassified as "Capitalsaurus" potenscf. DeinonychusIndeterminate MarylandA dromaeosauridDryosaurus"D." grandis Maryland"Limb elements.""Table 6.1," in Weishampel, et al. (2004). Page 139.An indeterminate member of Ornithomimosauria.MagulodonKranz, P. (1996). Notes on the sedimentary iron ores of Maryland and their dinosaurian faunas. Maryland Geological Survey Special Publications 3:87–115.M. muirkirkensis Maryland"Tooth"Likely an ornithischian, this genus is a nomen nudum that has not been formally published.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.Ornithomimus"O." affinisJunior synonym of "Dryosaurus" grandisPleurocoelusP. altus Maryland"Tibia [and] fibula.""Table 13.1," in Weishampel, et al. (2004). Page 266.A sauropod synonymous with Astrodon.P. nanus MarylandPriconodonP. crassus Maryland"Teeth, tibia.""Table 17.1," in Weishampel, et al. (2004). Page 368.A large nodosaurid.cf. RichardoestesiaIndeterminate Maryland"Teeth"A small theropod.cf. TenontosaurusListed as "?Tenontosaurus sp." in "3.25 Maryland, United States; 1. Arundel Clay," in Weishampel, et al. (2004). Page 556.Indeterminate Maryland

Pterosaurs Unassigned pteradactyloid tracks.Lockley, M.; Harris, J.D.; and Mitchell, L. 2008. "A global overview of pterosaur ichnology: tracksite distribution in space and time." Zitteliana. B28. p. 187-198. .

Pterosaurs of the Arundel Formation Genus Species Location Stratigraphic position Abundance Notes ImagesPteraichnus

Turtles Turtles reported from the Arundel Formation Genus Species Location Stratigraphic position Material Notes ImagesArundelemysA. dardeni Maryland"A single, incomplete skull lacking the lower jaws and cheek region"A baenid.GlyptopsG. caelatus Maryland"Carapace fragments probably pertaining to a single individual"A pleurosternid.NaomichelysN. sp. Maryland"Shell fragments"A helochelydrid.

Crocodylomorphs Crocodylomorphs reported from the Arundel Formation Genus Species Location Stratigraphic position Material Notes Imagescf. Bernissartiidae indet.Indeterminate MarylandToothA likely bernissartiid.cf. Goniopholididae indet.Indeterminate MarylandScutes, teethA likely goniopholidid, the most common crocodylomorph from the formation.cf. Pholidosauridae indet.Indeterminate MarylandTeethA likely pholidosaurid.

Mammals Mammals reported from the Arundel Formation Genus Species Location Stratigraphic position Material Notes ImagesArgillomysA. marylandensis MarylandMolar toothA multituberculate.ArundelconodonA. hottoni MarylandDentary with teethA 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.Brenner, Gilbert J., 1963, The spores and pollen of the Potomac Group of Maryland: Maryland Geological Survey Bulletin, no. 27, 215 p.

Arundel Formation

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