Willwood Formation

It consists of fine grained clastic rocks (mudstone and shale) interbedded with medium grained clastic rocks (sandstone) and sporadic conglomerates. The formation underlies portions of the Bighorn Basin of Big Horn, Hot Springs, Park and Washakie counties of Wyoming.^[3]

Radiometric dating of volcanic tuffs, combined with comparisons with other formations using magnetostratigraphy, using numerous samples from various levels of the formation suggest an age range of 55 – 52 million years ago, placing the Paleocene-Eocene boundary near the base of the formation.^[4]

Trace fossils have been found in the Willwood Formation.^[5] Fossil birds include Gastornis, Neocathartes and Paracathartes. A fossil alligatorid, namely Orthogenysuchus, was also found in this formation.

Bats reported from the Willwood Formation
Genus	Species	Presence	Material	Notes	Images
cf. Icaronycteris	cf. I. sp.	Locality SC-29, Park County, Wyoming.[8]	3 fragmentary specimens.[8]

Cimolestans reported from the Willwood Formation
Genus	Species	Presence	Material	Notes	Images
Azgonyx	A. gunnelli	Localities SC-67, SC-308 and SC-351.[9]		A tillodont.
	A. sp.	Locality SC-67.[9]		A larger tillodont than A. gunnelli.
Coryphodon	C. sp.	Localities FG-61, SC-67 and SC-139.[9]	[10]	A pantodont.
Didelphodus	D. absarokae	Lower Gray Bull to the upper part of the Lysite zones.[7]		A palaeoryctid.
Ectoganus	E. bighornensis	Localities HG-3 and SC-67.[9]		A taeniodont.
	E. copei		USGS 3838, with postcrania.[10]	A taeniodont.
	E. gliriformis		A fragmentary femur (PU 13173).[10]	A taeniodont.
Esthonyx	E. bisculatus		Jaws, teeth and a few postcranial remains.[10]	A tillodont.
	E. spatularius	Bighorn Basin,[11] and locality SC-67.[9]	Fragmentary left dentary.[9]	A tillodont.
Megalesthonyx	M. hopsoni	Buffalo Basin.[12]		A tillodont.
cf. Palaeoryctes	cf. P. sp.		"USGSD 1661, 2151, 3711".[7]	A palaeoryctid.
Palaeosinopa	P. incerta	Lower to upper Gray Bull zone,Bighorn Basin.[7][13]		A pantolestid also found in the Wind River and San Jose formations.[7]
	P. lutreola	Middle and upper Gray Bull zone, Bighorn Basin.[7][13]		A pantolestid.
	P. veterrima	Lower Gray Bull through Lysite zones, Bighorn Basin.[7][13]	More than 50 specimens.[7]	A pantolestid.
	P. sp.	Lower part of the formation.[6]	UM 69722 (a partial skull and mandible).[6]	A pantolestid.
cf. Pararyctes	cf. P. sp.	"Yale locality 363 at the 190-m level of the Willwood Formation in the Elk Creek Facies".[7]	UW 7028.[7]	A palaeoryctid.

Ferae reported from the Willwood Formation
Genus	Species	Presence	Material	Notes	Images
Acarictis	A. ryani	Locality SC-67.[9]	A broken right molar and a left molar.[9]	A hyaenodont.
Alocodontulum	A. atopum		YRM 30790 and USGS 7208.[10]	A palaeanodont.
Arfia	A. junnei	Localities SC-67, SC-69, SC-79, SC-121, SC-308 and SC-348.[9]		A hyaenodont.
	A. sp.	Southern Bighorn Basin.[10]	Fragmentary postcranial remains.[10]	A hyaenodont.
Cf. Asiabradypus	Cf. A. sp.	Locality SC-67.[9]	An edentulous right lower jaw (UM 87859m).[9]	A palaeanodont.
Didymictis	D. leptomylus	Localities MP-40, SC-67 and SC-79.[9]	Skull and teeth.[9]	A viverravid.
	D. protenus		Multiple postcranial specimens.[10]	A viverravid.
Dipsalidictis	D. platypus	Localities SC-67 and SC-141.[9]		An oxyaenid.
	D. transiens	Localities SC-79 and SC-67.[9]	An upper left canine and a trigonid of a molar.[9]	An oxyaenid.
Miacis	M. winkleri	Localities SC-67, SC-348 and SC-350.[9]	3 fragmentary dentaries.[9]	A miacid.
	cf. M. sp.		USGS 7161.[10]	A miacid.
Oxyaena	O. forcipata		Relatively complete skeletal remains.[10]	An oxyaenid.
	O. gulo		USGS 7186.[10]	An oxyaenid.
	O. intermedia		Fragmentary postcrania.[10]	An oxyaenid.
	O. platypus		[10]	Now moved to the genus Dipsalidictis.
Palaeanodon	P. ignavus		Partial skeletons including postcrania.[10]	A palaeanodont.
	P. nievelti	Localities SC-67 and SC-121.[9]		A palaeanodont.
Palaeonictis	P. occidentalis		"Hind foot skeleton and associated fragments".[10]	An oxyaenid.
	P. sp.	Locality SC-67.[9]	Trigonid of a right molar.[9]	An oxyaenid.
Prolimnocyon	P. atavus	Bighorn Basin.[16]	Partial skeleton.[16]	A hyaenodont.
	P. eerius	Localities SC-67 and possibly SC-139.[9]	Crushed skull.[9]	A hyaenodont.
Prototomus	P. deimos	Localities SC-67 and SC-349.[9]	4 fragmentary specimens.[9]	A hyaenodont.
	P. sp.		4 specimens.[10]	A hyaenodont.
Tritemnodon	T. sp.		2 specimens with postcrania.[10]	A hyaenodont.
?Uintacyon	?U. sp.		[10]	A miacid.
Viverravus	V. bowni	Locality SC-67,[9] and locality SC-29, Park County, Wyoming.[8]		A viverravid.
	V. politus	Locality SC-67.[9]	A left maxilla.[9]	A viverravid.
Vulpavus	cf. V. canavus		3 specimens.[10]	A miacid.

Marsupials reported from the Willwood Formation
Genus	Species	Presence	Material	Notes	Images
Mimoperadectes	M. labrus	Localities SC-67 and SC-69.[9]	Left dentary.[9]
Peradectes	P. cf. P. chesteri		2 isolated teeth.[8]

Ungulates reported from the Willwood Formation
Genus	Species	Presence	Material	Notes	Images
Anacodon	A. ursidens	[10]		An arctocyonid.
Bunophorus	cf. B. grangeri		USGS 16470 (a very incomplete skeleton).[10]	A dichobunid originally listed as cf. Wasatchia dorseyana.
Cardiolophus	C. radinskyi		UM 64913.[24]	An isectolophid.
Copecion	C. davisi	Localities BR-2, SC-51, SC-67, SC-69, SC-79, SC-139, SC-308, SC-345, SC-348, SC-349 and SC-351.[9]		A phenacodontid.
Chriacus	C. badgleyi	Localities SC-67, SC-79, SC-121 and SC-342.[9]		An arctocyonid.
	C. sp.		Several skeletal specimens.[10]	An arctocyonid.
Diacodexis	D. ilicis	Localities SC-67 and SC-308.[9]		A dichobunid.
	D. metsiacus		Several specimens, including a near-complete skeleton.[10]	A dichobunid.
Dissacus	D. praenuntius	Park County, Wyoming.[9][25]	Teeth, jaw elements & bone fragments.[9][25]	A mesonychid.
	D. serior	USGS locality D-1754, Bighorn Basin, Wyoming.[25]	Left dentary fragments (USGS 27612).[25]	A mesonychid.
	D. willwoodensis	Bighorn Basin, Wyoming.[25]	Mandibles.[25]	A mesonychid.
Ectocion	E. osbornianus	Locality SC-67 and SC-351.[9]	2 fragmentary dentaries.[9] and a single postcranial association.[10]	A phenacodontid.
	E. parvus	Localities HG-3, SC-67, SC-69, SC-121, SC-139, SC-182, SC-308 and SC-351.[9]		A phenacodontid.
Hapalodectes	H. anthracinus	University of Wyoming locality V-73086, Wyoming.[25]	Teeth (USGS 9628).[25]	A mesonychian
	H. leptognathus		A fragmentary specimen (USGS 5912).[10]	A mesonychian.
Heptodon	cf. H. calciculus	Wind River Basin.[10]	Remains of a subadult with postcrania.[10]
Homogalax	H. protapirinus	Bighorn Basin.[26]	A skull (YPM(PU) 16168),[26] and several other specimens.[27][24]	An isectolophid.
	cf. H. protapirinus		At least 2 specimens.[10]	An isectolophid.
Hyopsodus	H. loomisi	Localities SC-67, SC-69, SC-79, SC-121, SC-308 and SC-342.[9]		A hyopsodontid.
	H. powellianus		"AMNH 4147, right mandible fragment with M1 through M3".[6]	A hyopsodontid.
Hyracotherium	H. grangeri	Locality SC-67.[9]	2 isolated teeth.[9]	An equid.
	H. sandrae	Localities FG-61, SC-67, SC-69, SC-79, SC-121 and SC-139.[9]		An equid.
	H. sp.		Skeletal remains with postcrania.[10]	An equid.
Meniscotherium	M. priscum	Locality SC-67(?)[9]	Left dentary fragment.[9]	A phenacodontid.
Pachyaena	P. gigantea	YPM localities 131 & 149, Wyoming.[25]	Jaw elements & teeth.[25]	A mesonychid.
	P. gracilis	USGS locality D-1640.[25]	Maxillary fragments (USGS 7185).[25]	A mesonychid.
	P. ossifraga	Locality SC-348.[9]	Trigonid of a right molar.[9]	A mesonychid.
	P. sp.		Postcranial material.[10]	A mesonychid.
Phenacodus	P. cf. P. intermedius	Localities SC-67 and SC-141.[9]		A phenacodontid.
	P. primaevus		Near complete skeletons.[10]	A phenacodontid.
	P. vortmani		Near complete skeletons.[10]	A phenacodontid.
Princetonia	P. yalensis	Locality SC-121(?)[9]	Right dentary.[9]	An arctocyonid.
Thryptacodon	T. antiquus	[10]		An arctocyonid.
	T. barae	Localities MP-38 and SC-121.[9]		An arctocyonid.
	T. olseni	[10]		An arctocyonid.
	T. sp.		2 partial skeletons.[10]	An arctocyonid.
Wasatchia	cf. W. dorseyana		USGS 16470 (a very incomplete skeleton).[10]	Now a junior synonym of Bunophorus.
Xenicohippus		Lower part of the Upper Willwood Formation.[28]		A hyracotheriine.

Birds reported from the Willwood Formation
Genus	Species	Presence	Material	Notes	Images
Anachronornis	A. anhimops	Clark Quadrangle, Park County, Wyoming.[29]	USNM 496700.[29]	An anseriform.
Anseriformes Familia Incertae sedis		Clark Quadrangle, Park County, Wyoming.[29]	USNM 496701 (right quadrate missing the pterygoid condyle); USNM 496702 (fragmentary right femur consisting of only the proximal and distal ends).[29]	Generally similar to Anachronornis & Danielsavis.[29]
Aves indet.		Localities SC-67 and SC-121.[9]	A cervical vertebra and proximal part of a carpometacarpus.[9]	An indeterminate bird.
Calcardea	C. junnei		"UM 76882, anterior part of sternum, partial left coracoid. nearly complete right coracoid, partial left humerus. two nearly complete vertebrae. fragmentary left and right tarsometatarsi, and several phalanges, all probably representing a single individual bird."[8]	Originally thought to be a heron, now thought to be related to Vastanavis.[30]
Diatryma	D. gigantea		Several specimens.[31]	Now deemed a junior synonym of Gastornis.
Eogeranoides	E. campivagus		"Fragmentary tibiotarsus and tarsometatarsus remains".[32]	A dubious geranoidid, may be a junior synonym of Paragrus prentici.
Gastornis	G. gigantea		Several specimens.[31]	A gastornithiform.
Geranoides	G. jepseni	Elk Creek, Bighorn Basin.[32]		A geranoidid, now deemed a junior synonym of Palaeophasianus meleagroides.
Microolithus	M. wilsoni		Partial eggs.[33]	Fossil bird eggs.
Palaeophasianus	"P." incompletus		"A fragmentary distal end of a tarsometatarsus".[32]	A very large possible geranoidid, may actually belong to the genus Paragrus or be outside Geranoididae entirely.
	P. meleagroides	Elk Creek, Bighorn Basin.[32]		A geranoidid.
	?P. sp.		AMNH 5156.[32]	A geranoidid.
Paracathartes	P. howardae	[34]		A lithornithid.
Paragrus	P. prentici	[32]		A geranoidid.
Primoptynx	P. poliotauros		Specimen including all major postcranial bones.[35]	A large owl.
Sandcoleus	S. copiosus	Clark Quadrangale, Park County, Wyoming.[36]	"Two articulated partial skeletons and a number of other elements".[36]	A sandcoleid mousebird.

Crocodilians reported from the Willwood Formation
Genus	Species	Presence	Material	Notes	Images
Allognathosuchus	A. sp.	Localities SC-67, SC-139, SC-342 and SC-349.[9]		An alligatorid.
Crocodilia indet.		Localities SC-67, SC-69 and SC-121.[9]	Dermal scutes and vertebral centra.[9]	May represent either large Allognathosuchus or small Leidyosuchus.
Leidyosuchus	L. sp.	Localities SC-67, SC-79, SC-139, SC-342, SC-345 and SC-350.[9]
Orthogenysuchus	O. olseni	[37]		A caiman.
Pristichampsus	P. sp.	Locality SC-348.[9]	A single tooth (UM 83817m).[9]

Testudines reported from the Willwood Formation
Genus	Species	Presence	Material	Notes	Images
Chelonia indet.		Localities SC-67, SC-121 and SC-348.[9]	Several pieces of the carapace and plastron.[9]	Indeterminate medium to large turtles.
Echmatemys	E. sp.	Locality SC-67.[9]	Much of the plastron and several peripherals or pleurals from the carapace (UM 83627), and a peripheral (UM 66617m).[9]
Gomphochelys	G. nanus	Southeastern Bighorn Basin, Washakie County, Wyoming.[40]		A dermatemydid.
Cf. Plastomenus	Cf. P. sp.	Localities SC-67 and SC-350.[9]	Fragmentary pieces of the carapace and plastron.[9]	A trionychid.

Fish reported from the Willwood Formation
Genus	Species	Presence	Material	Notes	Images
Amia	A. sp.	Localities SC-67 and SC-342.[9]	Isolated cervical vertebrae, and a concretion of broken bones including 1 or 2 tooth-bearing bones likely belonging to Amia.	A bowfin.
Cuneatus	C. maximus		[41]	A large gar.
Lepisosteus	L. sp.		"Mostly diamond-shaped ganoid scales, but some specimens include distinct opisthocoelous vertebrae, basioccipitals and other cranial elements."[9]	A gar.

Invertebrate ichnotaxa reported from the Willwood Formation
Ichnogenus	Ichnospecies	Presence	Material	Notes	Images
Camborygma	C. litonomos	Polecat Bench.[42]	Prismatic cylindrical structures.[42]	Likely represent crayfish burrows.[43][42]
Cocoon traces		Polecat Bench.[44]
Cf. Cylindricum	Cf. C. isp.	Polecat Bench.[44]	"Predominantly vertical, straight to sinuous, unbranched, unlined, and smooth walled shafts with circular cross sections and gently rounded, unenlarged terminations".[42]	Burrows possibly created by beetles, hymenopterans, emerging cicada nymphs, spiders or molluscs.[42]
Edaphichnium	E. lubricatum	Polecat Bench.[44]	"Tubular burrows with ellipsoidal fecal pellets".[45]	Formed by substrate feeding by Earthworms or other invertebrates.
Naktodemasis	N. bowni	Bighorn Basin.[46]	"Burrows composed of nested ellipsoidal packets backfilled with thin, tightly spaced, menisci subparallel to the bounding packet."[46]	Burrows likely constructed by burrowing bugs or cicada nymphs.[42][46]
cf. Planolites	cf. P. isp.	Polecat Bench.[44]		Traces made by the feeding of worm-like animals.
cf. Steinchnus	S. isp.	Polecat Bench.[44]		Traces made by invertebrates, possibly beetles or mole crickets.

Gastropods reported from the Willwood Formation
Genus	Species	Presence	Material	Notes	Images
Cf. Albertanella	Cf. A. minuta		[8]	A pupillid.
Discus	D. ralstonensis		Many individuals (some juvenile specimens).[8]	An endodontid.
Elimia or Pleurocera	E. or P. sp.		Some 25 specimens, many are internal molds.[9]	A pleurocerid.
Grangerella	cf. G.(?) phenacodorum		2 specimens originally referred to Protoboysia complicata.[8]	A grangerellid.
	G. sinclairi		[8]	A grangerellid.
Cf. Gyraulus	Cf. G. sp.	Locality SC-69.[9]	A poorly-preserved planispiral internal mold.[9]	A pulmonatan.
Hendersonia	H. evanstonensis		Several specimens, none preserving the operculum.[8]	A helicinid.
Holospira	H. sp.	Localities SC-67, SC-139, SC-345 and SC-349.[9]	7 individuals.[9]	An urocoptid.
"Hydrobia"	"H." sp.	Locality SC-69.[9]	"A very small, dextral, high-spired shell with 9 whorls".[9]	A hydrobiid.
Cf. Microphysula	Cf. M. sp.		A single individual.[8]	A sagdid.
Oreohelix	O. megarche		A single specimen (UM(I) 65404).[8]	An oreohelicid.
Cf. Pupilla	Cf. P. sp.		UM(I) 654071.[8]	A pupillid.

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