# Hornerstown Formation

The Hornerstown Formation is a latest Cretaceous to early Paleocene-aged geologic formation in New Jersey. It preserves a variety of fossil remains, including those of dinosaurs, and contains direct evidence of the mass mortality that occurred at the Cretaceous-Paleogene boundary.
Outcrops of the Hornerstown Formation are known from sites such as Edelman Fossil Park.

## Age & significance

The age of the Hornerstown deposits have been controversial. While most fossils are of animal taxa known from the earliest Cenozoic era, several fossils of otherwise exclusively Cretaceous age have been found. These include remains of the shark Squalicorax, several types of non-avian dinosaurs, the teleost fish Enchodus, several species of ammonite, and marine lizards referred to the genus Mosasaurus. 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 a tsunami. Biochemical analyses done on mosasaur bones from the Hornerstown Formation and the underlying, purely Cretaceous New Egypt Formation have found differing chemical signatures in the content of rare 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.
To account for these Cretaceous fossils, the Hornerstown Formation is generally treated as including the last portion of the Maastrichtian 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. In addition, recent studies have found evidence of high iridium concentration and shocked quartz within the MFL, suggesting that the MFL represents a thanatocoenosis formed from an ecosystem collapse during the Cretaceous-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 eastern Appalachia, 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.

## Vertebrate paleofauna
### Cartilaginous fishes

Based on Case (1996) and Boles et al (2024):

#### Chimaeras

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

#### Sharks

Sharks of the Hornerstown Formation :
| Genus | Species | Stratigraphic position | Material | Notes |
| Araloselachus | A. cuspidatus | Maastrichtian | Teeth | A sand shark. |
| Carcharias | C. teretidens | Danian | Teeth | A relative of the sand tiger shark. |
| Cretalamna | C. appendiculata | Maastrichtian & Danian | Teeth | A megatooth shark. |
| Heptranchias | H. howellii | Maastrichtian | Teeth | A relative of the sharpnose sevengill shark. |
| Hexanchus | H. microdon | Danian | Teeth | A sixgill shark. |
| Hexanchus | H. sp. | Maastrichtian & Danian | Teeth | A sixgill shark. |
| Otodus | O. obliquus | Danian | Teeth | A megatooth shark. |
| Odontaspis | O. sp. | Danian | Teeth | A sand shark. |
| Palaeocarcharodon | P. sp. | Danian | Teeth | A megatooth shark. |
| Palaeogaleus | P. vincenti | Maastrichtian & Danian | Teeth | A houndshark. |
| Palaeohypotodus | P. rutoti | Danian | Teeth | A sand shark. |
| Pseudocorax | P. affinis | Maastrichtian | Teeth | A pseudocoracid shark. |
| Scapanorhynchus | S. texanus | Maastrichtian | Teeth | A goblin shark. |
| Scyliorhinus | S. gilberti | Danian | Teeth | A catshark. |
| Sphenodus | S. lundgreni | Maastrichtian | Teeth | A orthacodontid shark. |
| Squalicorax | S. pristodontus | Maastrichtian | Teeth | A crow shark. |
| Squalus | S. minor | Maastrichtian & Danian | Teeth | A spurdog. |
| Squalus | S. sp | Maastrichtian & Danian | Teeth | A spurdog. |
| Squatina | S. sp. | Maastrichtian | Teeth | An angelshark. |
| Weltonia | W. ancistrodon | Danian | Teeth | A cow shark. |
| Xampylodon | X. brotzeni | Maastrichtian | Teeth | A cow shark. |

#### Rays

Rays of the Hornerstown Formation :
| Genus | Species | Stratigraphic position | Material | Notes |
| Aetomylaeus | A. striatus | Maastrichtian | Teeth | An eagle ray. |
| Dasyatis | D. crosswickense | Danian | Teeth | A whiptail stingray. Type locality for species. |
| Hypolophites | H. hutchinsi | Danian | Teeth | A whiptail stingray. Type locality for species. |
| Hypolophodon | H. sylvestris | Danian | Teeth | A whiptail stingray. |
| Ischyrhiza | I. mira | Maastrichtian | Teeth | A sclerorhynchid sawskate. |
| Rhinoptera | R. sp. | Maastrichtian | Teeth | A cownose ray. |
| Rhombodus | R. laevis | Maastrichtian | Teeth | A rhombodontid ray. |
| Viperecucullus | V. kuehnei | Danian | Teeth | A whiptail stingray, type locality for the species and genus. |

### Ray-finned fishes

Actinopterygii of the Hornerstown Formation :
| Genus | Species | Location | Stratigraphic position | Material | Notes |
| Acipenser | A. cf. albertensis | | Maastrichtian | | A sturgeon. |
| Anomoeodus | A. phaseolus | | Maastrichtian, potentially Danian | 8 teeth | A pycnodont. 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 the Cenozoic; however, it may have been reworked from lower layers. |
| Atractosteus | A. sp. | | Maastrichtian & Danian | 11 teeth | A gar. First record of gars from eastern North America during the Paleocene. |
| cf. Bananogmius | cf. B. sp. | | Maastrichtian | | A plethodid tselfatiiform. |
| Dercetidae indet. | | | Maastrichtian & Danian | 27 flank scales | A dercetid aulopiform. The first evidence in eastern North America of the Dercetidae surviving the K-Pg extinction event. |
| Enchodus | E. ferox | | Maastrichtian | | An enchodontid aulopiform. The E. gladiolus remains are the first record of the species in eastern North America. |
| Enchodus | E. gladiolus | | Maastrichtian | 8 teeth | An enchodontid aulopiform. The E. gladiolus remains are the first record of the species in eastern North America. |
| Iridopristis | I. parrisi | Sewell, New Jersey | Danian | 3 partial articulated specimens, including a near-complete skull. | A stem-lineage member of Holocentridae. The earliest known definitive holocentrid. Type locality for genus and species. |
| Paralbula | P. marylandica | | Maastrichtian & Danian | 23 teeth | A phyllodontid elopomorph. First evidence of P. marylandica existing during the Cretaceous, indicating that it survived the extinction event. |
| Phyllodus | P. paulkatoi | | Maastrichtian | 1 tooth plate | A phyllodontid elopomorph. First known occurrence in eastern North America. |
| Saurocephalus | S. lanciformis | | Danian | 1 tooth | A saurodontid ichthyodectiform. First known occurrence in eastern North America and first known occurrence of ichthyodectiforms as a whole in the Paleocene and Cenozoic. |

### Reptiles
#### Birds

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:
Birds of the Hornerstown Formation :
| Genus | Species | Stratigraphic position | Notes |
| Anatalavis | A. rex | ?earliest Paleocene | A waterfowl potentially related to the magpie-goose. |
| Graculavus | G. velox | ?earliest Paleocene | A potential wader. |
| Laornis | L. edwardsianus | ?earliest Paleocene | A laornithid wading bird. |
| Novacaesareala | N. hungerfordi | ?earliest Paleocene | A potential tropicbird. |
| Palaeotringa | P. littoralis | ?earliest Paleocene | A potential wader. |
| Palaeotringa | P. vagans | ?earliest Paleocene | A potential wader. |
| Telmatornis | T. priscus | ?earliest Paleocene | A potential wader. |
| Tytthostonyx | T. glauconiticus | ?earliest Paleocene | A potential seabird. |

#### Non-avian dinosaurs

Dinosaurs of the Hornerstown Formation :
| Genus | Species | Location | Stratigraphic position | Abundance | Notes |
| Hadrosauridae indet. | | | Maastrichtian | | A hadrosaurid ornithischian. |
The tyrannosauroid Dryptosaurus is sometimes referred to this formation, as its remains were found in the New Egypt Formation, which is sometimes considered a part of the Hornerstown.

#### Crocodylomorphs

Crocodylomorphs of the Hornerstown Formation :
| Genus | Species | Stratigraphic position | Material | Notes |
| Borealosuchus | B. threeensis | Maastrichtian | Lower jaw, postcranial remains | A eusuchian. Type locality of species. |
| Bottosaurus | B. harlani | Maastrichtian & Danian | Remains including lower jaw of a juvenile individual | An early caiman. |
| Bottosaurus | B. tuberculatus | Maastrichtian | Remains including lower jaw of a juvenile individual | An early caiman. |
| Hyposaurus | H. rogersii | Maastrichtian & Danian | | A dyrosaurid. |
| cf. Procaimanoidea | P. sp. | Maastrichtian | | A caiman. |
| Thoracosaurus | T. neocesariensis | Maastrichtian & Danian | | A gavialoid. |

#### Plesiosaurs

Plesiosaurs of the Hornerstown Formation :
| Genus | Species | Location | Stratigraphic position | Abundance | Material | Notes | Images |
| "Plesiosaurus" | "P." brevifemur | | Maastrichtian | | | An indeterminate plesiosaur known from a well-documented specimen, now lost. | |

#### Turtles

Testudines of the Hornerstown Formation :
| Genus | Species | Stratigraphic position | Notes |
| Adocus | A. agilis | Maastrichtian | An adocid. |
| Adocus | A. beatus | Maastrichtian | An adocid. |
| Adocus | A. syntheticus | Maastrichtian | An adocid. |
| Agomphus | A. pectoralis | Maastrichtian | A kinosternoid related to Dermatemys. |
| Bothremys | B. sp. | Maastrichtian | A bothremydid side-necked turtle. |
| Euclastes | E. wielandi | Maastrichtian & Danian | A sea turtle. |
| Lytoloma | L. jeanesii | Maastrichtian | A sea turtle. |
| Osteopygis | O. emarginatus | Maastrichtian | A macrobaenid. |
| Peritresius | P. ornatus | Maastrichtian | A sea turtle. |
| Taphrosphys | T. sulcatus | Maastrichtian | A bothremydid side-necked turtle. |
| Taphrosphys | T. strenuus | Maastrichtian | A bothremydid side-necked turtle. |

#### Mosasaurs

Mosasaurs of the Hornerstown Formation :
| Genus | Species | Stratigraphic position | Material | Notes |
| Halisaurus | H. platyspondylus | Maastrichtian | | A halisaurine. Type locality of the genus and species. |
| Mosasaurus | M. hoffmanni (=Nectoportheus validus) | Maastrichtian | Pterygoid, teeth, vertebrae | A mosasaurine. This formation contains some of the last known remains, comprising individuals killed during the mass mortality event that formed the MFL. A very large specimen is known. |
| Plioplatecarpus | P. sp. | Maastrichtian | | A plioplatecarpine. |