Fossil Hill Member

Fossil Hill Member
Stratigraphic range: Anisian PreꞒ Ꞓ O S D C P T J K Pg N
Type	Geological member
Unit of	Prida Formation, Favret Formation
Lithology
Primary	Calcareous Shale
Other	Limestone
Location
Region	Nevada
Country	United States
Type section
Named by	Kathryn Nichols and Norman Silberling

The Fossil Hill Member is a Middle Triassic-aged rock unit. The Fossil Hill unit occurs as a member of both the Prida and Favret formations. It outcrops in multiple locations across Northwestern Nevada including the western Humboldt Range, Tobin Range, Augusta Mountains, and China Mountain. Calcareous shale, mudstone, and black limestones are the most common lithologies present within the unit.^[1] The member was named for Fossil Hill, Nevada, a locality within the Humboldt Mountains where large quantities of Anisian-aged marine fossils were discovered in the early 20th century. Fossils are common throughout the Fossil Hill, and the unit is well known for preserving the remains of some of the earliest marine reptiles, including several genera of ichthyosaurs and a pistosaur.^[2] Other fossils include bony fish, hybodont sharks, and invertebrates with ceratitid ammonoids being especially abundant.^[3]

The Fossil Hill Member was first recognized for its paleontological resources at the end of the 19th century. John Campbell Merriam and Eustace Furlong from the University of California Berkeley led the Saurian Expedition of 1905 to Fossil Hill in the Humboldt Mountains with the intention to search for Ichthyosaur fossils. The expedition was joined by the venerable heiress Annie Alexander who financed the expedition and provided most of the written documentation through journal entries and photographs.^[4] The expedition excavated the remains of 25 Triassic Ichthyosaurs, many of which were collected and shipped back to Berkeley. These skeletons, along with those collected from subsequent expeditions by UC Berkeley in the early 20th century, were the most complete Ichthyosaur specimens from North America at the time, and Merriam's study of these fossils provided some of the earliest knowledge on early ichthyosaur evolution.

Sketches representing the skull and fossil skeleton of UCMP 9950, one of the specimens of C. petrinus having been discovered during expeditions led by the University of California at the beginning of the 20th century

In the latter half of the 20th century, the section of Fossil Hill Member outcropping in the Augusta Mountains was identified for its ammonoid record to be used in biostratigraphic definition and correlation of Triassic rocks.^[5] Field observations of vertebrate fossils in the Augusta Mountains drew the attention of paleontologist Martin Sander who has led numerous teams to collect marine reptile fossils including specimens representing 7 new species. Paleontological fieldwork and research in the Augusta Mountains is ongoing.

The Favret Formation and the Prida Formation, constitute two of the recognized geological formations within the Star Peak Group, of Northwestern Nevada. The Fossil Hill member links these two formations.

Tectonic fragmentation of the region has broken apart most stratigraphic units of the Star Peak Group. Because of this outcrop discontinuity, the Fossil Hill Member is part of different formations in different parts of the outcrop area.^[2] In the Prida Formation, this member outcrops west of the Humboldt Range, and in the Favret Formation, it outcrops in the Augusta Mountains and southern extent of the Tobin Range. Although differing in precise age, the strata assigned to this member form a laterally uniform unit of offshore calcareous rocks. The various occurrences of the Fossil Hill member are also united by a distinct pelagic Anisian paleofauna preserved in the rocks.^[1]

The Fossil Hill varies in thickness throughout its outcrops. In the China Mountain and Tobin Range sections, erosion and dolomitization from overlying units have rendered the Fossil Hill usually thin or locally absent. The unit within the Prida Formation varies from 60 to 120 meters thick while the section in the Augusta Mountains exceeds 250 meters.^[1]

Silty shale is the prevailing lithology throughout the Fossil Hill Member outcropping in the Augusta Mountains with alternating facies of fissile shaley limestone and massive black limestones common throughout the entire section. Specific bioclastic beds dominated by Daonella fossils occur at multiple intervals within the shaley limestones. Compressed ammonoid fossils are also common.^[1] Three dimensionally preserved ammonoids occur less commonly but are still present in multiple areas. These three dimensional ammonoids are less prone to weathering and are useful for biostratigraphy.^[5]

Genus	Species	Material	Notes	Images
Cymbospondylus	C. petrinus[6]	Multiple specimens of excellent quality from both the Favret and Prida Formations including UCMP 9950, a near-complete specimen from Fossil Hill.	A large cymbospondylid ichthyosaur varying in size from 8–12 metres (26–39 ft).
	C. nichollsi[7]	FMNH PR2251, from the Favret Formation, consists of the back half of a skull, the first 28 vertebrae, several ribs, and part of the shoulder girdle	Mid-sized species of Cymbospondylus, estimated to be about 7.6 metres (25 ft) long
	C. duelfiri[8]	LACM DI 158109 consists of a nearly complete skull with articulated and disarticulated post cranial material. Diminutive strings of articulated fetal vertebrae in the trunk.	The smallest species of the genus at 4.3 metres (14 ft) long, holotype is among the earliest examples of viviparity
	C. youngorum[2]	LACM DI 157871, consists of a large complete skull, some cervical vertebrae, the right humerus as well as fragments of the shoulder girdle.	A giant, 17 metres (56 ft) long ichthyosaur, the largest animals known up to this point in time
Omphalosaurus	O. nevadanus[9]	Multiple specimens from both the Fossil Hill locality and the Favret formation.	A medium-sized—5 metres (16 ft)—basal ichthyosaur with dental batteries of button shaped teeth and a presumed durophage
Phalarodon	P. fraasi[10]	Multiple specimens from the Fossil Hill and Augusta Mountain localities	A small, 1 metre (3.3 ft) long mixosaur
	P. callowayi[10]		Holotype found in the same beds as Augustasaurus
Thalattoarchon	T. saurophagis[11]	FMNH PR 3032, consists of a major part of the skull and the axial skeleton, including parts of the pelvic girdle and rear flippers	A large ichthyosaur with large, laterally compressed teeth, interpreted as a macropredator

Genus	Species	Material	Notes	Images
Augustasaurus	A. hagdorni[12]	Single specimen. FMNH PR 1974 is a partial articulated skeleton containing the posterior neck, trunk, shoulder girdle, and both forelimbs and a disassociated skull,	A pistosauroid from the Favret Formation

Genus	Species	Material	Notes	Images
Benggwigwishingasuchus	B. eremicarminis[13]	A single, mostly complete skeleton (LACM-DI 158616)	A poposauroid pseudosuchian from the Favret Formation, specimen was probably a terrestrial animal washed out to sea[13]

• List of fossiliferous stratigraphic units in Nevada