Eoneophron

Eoneophron Temporal range:Late Cretaceous,late
Fossil material known fromEoneophron
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Chordata
Clade:	Dinosauria
Clade:	Saurischia
Clade:	Theropoda
Family:	†Caenagnathidae
Genus:	†Eoneophron
Species:	†E. infernalis
Binomial name
†Eoneophron infernalis Atkins-Weltmanet al.,2024

Eoneophron(meaning "dawnNeophron") is anextinctgenus ofcaenagnathidtheropod dinosaur from theLate CretaceousHell Creek Formationof South Dakota, US. The genus contains asingle species,E. infernalis,known from a partial hindlimb.

Discovery and naming

Osteological thin section of one of the holotype metatarsals ofEoneophron,with observed LAGs outlined in light blue and red

TheEoneophronfossil material was discovered in sediments of theHell Creek FormationinMeade County,South Dakota,United States. The specimen, which consists of a partial right hindlimb, waspreparedand initially listed for commercial sale before being purchased in 2020 for $5,000 by Kyle Atkins-Weltman. He then donated the specimen to theCarnegie Museum of Natural Historyfor research. Preliminary observation of the material had suggested that the bones belonged to a juvenile specimen ofAnzu,a related dinosaur from the same formation. However, after analyzing thehistologyof the bones, Atkins-Weltman and colleagues determined that the individual would have been skeletally mature when it died. Based on the observedlines of arrested growth(LAGs), they further estimated that the animal was in the sixth year of life when it died.

In 2024, Atkins-Weltmanet al.describedEoneophron infernalisas a new genus and species of caenagnathid oviraptorosaur based on these fossil remains. CM 96523, theholotypespecimen, consists of bones from the right leg, including thefemur,thetibiawith fusedastragalocalcaneum,andmetatarsalsIII and IV.

Thegeneric name,Eoneophron,combines theAncient Greekword "èos", meaning "dawn", with "Neophron",the generic name of theEgyptian vulture,which has been referred to as "pharaoh's chicken". This name was chosen in part to honor the lead author's petnile monitor,named Pharaoh. Thespecific name,"infernalis",is derived from theLatinword for Hell, referencing the discovery of the fossil material in the Hell Creek Formation. The fullbinomial namealso references the press nickname— "chicken from hell" —given to the relatedAnzu wyliei.

Description

After accounting for taphonomic distortion, Atkins-Weltmanet al.determined that the femur ofEoneophronwould have had a circumference of 10.58 centimetres (4.17 in). Using this, they estimated thatEoneophronhad a body mass of approximately 78 kilograms (172 lb). This is larger than several other caenagnathids, such asAnomalipes,Apatoraptor,Chirostenotes,andElmisaurus,all of which weighed under 70 kilograms (150 lb). The contemporaryAnzuwas even larger thanEoneophron,weighing between 202–342 kilograms (445–754 lb).

Left femur of the holotype

Classification

In theirphylogenetic analysis,Atkins-Weltmanet al.(2024) recoveredEoneophronas thesister taxonto a clade comprisingCitipesandElmisaurus.The followingcladogramrepresents the phylogenetic results of a majority rule consensus tree built on 504 most parsimonious trees.

Oviraptorosauria	Incisivosaurus Caudipteryxspp. Avimimusspp. Oviraptoridae Caenagnathidae Microvenator Gigantoraptor Caenagnathasia Nomingia Epichirostenotes Anomalipes Chirostenotes Hagryphus Eoneophron Citipes Elmisaurus Apatoraptor Caenagnathus Anzu

Paleoenvironment

Life restorationofEoneophron

Eoneophronis known from the Late Cretaceous Hell Creek Formation of South Dakota, United States, which dates to the latestMaastrichtian.Many other dinosaurs, including the fellow caenagnathidAnzu,are also known from the formation.