TheHornerstown Formation is a latestCretaceous to earlyPaleocene-aged geologicformation inNew Jersey. It preserves a variety of fossil remains, including those of dinosaurs, and contains direct evidence of themass mortality that occurred at theCretaceous-Paleogene boundary.^[2][3][4]

Outcrops of the Hornerstown Formation are known from sites such asEdelman Fossil Park.^[5]

The age of the Hornerstown deposits have been controversial. While most fossils are of animal taxa known from the earliestCenozoic era, several fossils of otherwise exclusivelyCretaceous age have been found. These include remains of the sharkSqualicorax, several types of non-avian dinosaurs, the teleost fishEnchodus, several species ofammonite, and marine lizards referred to the genusMosasaurus. Some of these remains show signs of severe abrasion and erosion, however, implying that they may be re-worked from older deposits. Most of these fossils are restricted to the lowest point in the formation, one rich in fossils and known as the Main Fossiliferous Layer, or MFL. Other explanations for the out-of-place fossils in the MFL is that they represent a time-averaged assemblage that built up and remained unburied during a time of low sediment deposition, or that they were stirred up from deeper in the sediment and deposited together during atsunami.^[6] Biochemical analyses done on mosasaur bones from the Hornerstown Formation and the underlying, purely CretaceousNew Egypt Formation have found differing chemical signatures in the content ofrare earth elements depending on whether the bones derive from the New Egypt or the Hornerstown Formation. This provides evidence against the idea that the presence of these remains in the Hornerstown is just the result of reworking, and supports the Hornerstown Formation including Cretaceous strata.^[7]

To account for these Cretaceous fossils, the Hornerstown Formation is generally treated as including the last portion of theMaastrichtian shortly before the Cretaceous-Paleogene boundary, and is thus divided into three sections: the section below the MFL (entirely Maastrichtian), the MFL itself (at the K-Pg boundary), and the section above the MFL (entirely Danian). However, other studies continue to treat the Hornerstown Formation as a Paleocene formation that saw significant reworking of Cretaceous fossils into itself.^[8] In addition, recent studies have found evidence of highiridium concentration andshocked quartz within the MFL, suggesting that the MFL represents athanatocoenosis formed from an ecosystem collapse during theCretaceous-Paleogene extinction event. This makes the Hornerstown Formation one of the few geological formations to contain direct evidence of the immediate impact of the extinction event. The Hornerstown Formation is important paleontologically, as it shows the impact of the K-Pg extinction on the coastal waters of easternAppalachia, contains many of the last known records of taxa that went extinct at the K-Pg boundary, while also providing evidence of survivorship for the taxa that managed to survive the extinction event.^[3]

Based on Case (1996) and Boleset al (2024):^[3][9]

Chimaeras of the Hornerstown Formation
Genus	Species	Location	Stratigraphic position	Abundance	Material	Notes	Images
Edaphodon	E. agassizi		Danian		Tooth plates	Acallorhinchid chimaera.
	E. mantelli		Danian
	E. mirificus		Maastrichtian
	E. stenobyrus		Maastrichtian
Ischyodus	I. bifurcatus		Maastrichtian, potentially Danian		Tooth plates, jaw elements	A callorhinchid chimaera.
	I. thurmanni		Maastrichtian
	I. williamsae		Danian

Sharks of the Hornerstown Formation
Genus	Species	Location	Stratigraphic position	Abundance	Material	Notes	Images
Araloselachus	A. cuspidatus		Maastrichtian		Teeth	Asand shark.[10]
Carcharias	C. teretidens		Danian			A relative of thesand tiger shark.
Cretalamna	C. appendiculata		Maastrichtian & Danian			Amegatooth shark.
Heptranchias	H. howellii		Maastrichtian			A relative of thesharpnose sevengill shark.
Hexanchus	H. microdon		Danian			Asixgill shark.
	H. sp.		Maastrichtian & Danian
Otodus	O. obliquus		Danian			A megatooth shark.
Odontaspis	O. sp.		Danian			A sand shark.
Palaeocarcharodon	P. sp.		Danian			A megatooth shark.
Palaeogaleus	P. vincenti		Maastrichtian & Danian			Ahoundshark.
Palaeohypotodus	P. rutoti		Danian			A sand shark.
Pseudocorax	P. affinis		Maastrichtian			Apseudocoracid shark.
Scapanorhynchus	S. texanus		Maastrichtian			Agoblin shark.[11]
Scyliorhinus	S. gilberti		Danian			Acatshark.
Sphenodus	S. lundgreni		Maastrichtian			Aorthacodontid shark.
Squalicorax	S. pristodontus		Maastrichtian			Acrow shark.
Squalus	S. minor		Maastrichtian & Danian			Aspurdog.
	S. sp
Squatina	S. sp.		Maastrichtian			Anangelshark.
Weltonia	W. ancistrodon		Danian			Acow shark.
Xampylodon	X. brotzeni		Maastrichtian			A cow shark.

Rays of the Hornerstown Formation
Genus	Species	Location	Stratigraphic position	Abundance	Material	Notes	Images
Aetomylaeus	A. striatus		Maastrichtian		Teeth	Aneagle ray.[10]
Dasyatis	D. crosswickense		Danian			Awhiptail stingray. Type locality for species.
Hypolophites	H. hutchinsi		Danian			A whiptail stingray. Type locality for species.
Hypolophodon	H. sylvestris		Danian			A whiptail stingray.
Ischyrhiza	I. mira		Maastrichtian			Asclerorhynchidsawskate.[10]
Rhinoptera	R. sp.		Maastrichtian			Acownose ray.
Rhombodus	R. laevis		Maastrichtian			Arhombodontid ray.
Viperecucullus	V. kuehnei		Danian			A whiptail stingray, type locality for the species and genus.

Actinopterygii of the Hornerstown Formation
Genus	Species	Location	Stratigraphic position	Abundance	Material	Notes	Images
Acipenser	A. cf.albertensis		Maastrichtian			Asturgeon.[3]
Anomoeodus	A. phaseolus		Maastrichtian, potentially Danian		8 teeth	Apycnodont. One tooth is known from the Danian section of the formation, which would be the latest record of this genus and prove its occurrence in theCenozoic; however, it may have been reworked from lower layers.[3]
Atractosteus	A. sp.		Maastrichtian & Danian		11 teeth	Agar. First record of gars from eastern North America during the Paleocene.[3]
cf.Bananogmius	cf.B. sp.		Maastrichtian			Aplethodidtselfatiiform.[3][10]
Dercetidae indet.			Maastrichtian & Danian		27 flank scales	Adercetidaulopiform. The first evidence in eastern North America of the Dercetidae surviving the K-Pg extinction event.[3]
Enchodus	E. ferox		Maastrichtian			An enchodontid aulopiform. TheE. gladiolus remains are the first record of the species in eastern North America.[3]
	E. gladiolus				8 teeth
Iridopristis	I. parrisi	Sewell, New Jersey	Danian		3 partial articulated specimens, including a near-complete skull.[8]	A stem-lineage member ofHolocentridae. The earliest known definitive holocentrid. Type locality for genus and species.[8]
Paralbula	P. marylandica		Maastrichtian & Danian		23 teeth	Aphyllodontidelopomorph. First evidence ofP. marylandica existing during the Cretaceous, indicating that it survived the extinction event.[3]
Phyllodus	P. paulkatoi		Maastrichtian		1 tooth plate	A phyllodontid elopomorph. First known occurrence in eastern North America.[3]
Saurocephalus	S. lanciformis		Danian		1 tooth	Asaurodontid ichthyodectiform. First known occurrence in eastern North America and first known occurrence of ichthyodectiforms as a whole in the Paleocene and Cenozoic.[3]

A number of fossil birds are known from the greensands of the formation. The Hornerstown serves as the type locality for all these genera and species:

The tyrannosauroidDryptosaurus is sometimes referred to this formation, as its remains were found in theNew Egypt Formation, which is sometimes considered a part of the Hornerstown.^[15]

Crocodylomorphs of the Hornerstown Formation
Genus	Species	Location	Stratigraphic position	Abundance	Material	Notes	Images
Borealosuchus	B. threeensis		Maastrichtian		Lower jaw, postcranial remains	Aeusuchian. Type locality of species.[3][16]
Bottosaurus	B. harlani		Maastrichtian & Danian		Remains including lower jaw of a juvenile individual[17]	An earlycaiman.[3][17]
	B. tuberculatus[10]		Maastrichtian
Hyposaurus	H. rogersii		Maastrichtian & Danian			Adyrosaurid.[3][18]
cf.Procaimanoidea	P. sp.		Maastrichtian			A caiman.[3]
Thoracosaurus	T. neocesariensis		Maastrichtian & Danian			Agavialoid.[3]

Testudines of the Hornerstown Formation
Genus	Species	Location	Stratigraphic position	Abundance	Notes	Images
Adocus	A. agilis[19]		Maastrichtian		Anadocid.[3]
	A. beatus
	A. syntheticus[19]
Agomphus	A. pectoralis		Maastrichtian		Akinosternoid related toDermatemys.[3][21]
Bothremys	B. sp.		Maastrichtian		Abothremydidside-necked turtle.[3]
Euclastes	E. wielandi		Maastrichtian & Danian		A sea turtle.[3]
Lytoloma	L. jeanesii		Maastrichtian		A sea turtle.[19]
Osteopygis	O. emarginatus		Maastrichtian		Amacrobaenid.[19]
Peritresius	P. ornatus		Maastrichtian		A sea turtle.
Taphrosphys	T. sulcatus		Maastrichtian		A bothremydid side-necked turtle.[3]
	T. strenuus

Mosasaurs of the Hornerstown Formation
Genus	Species	Location	Stratigraphic position	Abundance	Material	Notes	Images
Halisaurus	H. platyspondylus		Maastrichtian			Ahalisaurine. Type locality of the genus and species.
Mosasaurus	M. hoffmanni (=Nectoportheus validus)[19]		Maastrichtian		Pterygoid, teeth, vertebrae[3][7]	Amosasaurine. This formation contains some of the last known remains, comprising individuals killed during the mass mortality event that formed the MFL.[3] A very large specimen is known.[7]
Plioplatecarpus	P. sp.		Maastrichtian			Aplioplatecarpine.

• List of dinosaur-bearing rock formations

• Weishampel, David B.; Dodson, Peter; and Osmólska, Halszka (eds.): The Dinosauria, 2nd, Berkeley: University of California Press. 861 pp. ISBN 0-520-24209-2.

• Danian Stage
• Maastrichtian Stage of North America
• Paleocene Series of North America
• Cretaceous geology of New Jersey
• Paleogene geology of New Jersey
• Cretaceous–Paleogene boundary
• Shallow marine deposits

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