The Fossil Hill Member is a

Middle Triassic In the geologic timescale, the Middle Triassic is the second of three epochs of the Triassic period or the middle of three series in which the Triassic system is divided in chronostratigraphy. The Middle Triassic spans the time between Ma and ...

-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 Nevada ( ; ) is a U.S. state, state in the Western United States, Western region of the United States. It is bordered by Oregon to the northwest, Idaho to the northeast, California to the west, Arizona to the southeast, and Utah to the east. N ...

including the western

Humboldt Range The Humboldt Range is a largely north-south running range of mountains in northwest Nevada, USA, that extend from the town of Imlay in the north to the junction with the West Humboldt Range in the south. It is bordered continuously by Interst ...

Tobin Range The Tobin Range is a mountain range in eastern Pershing County, Nevada. The linear range is typical of the basin and range structure with an approximate length and width of by with a NNE orientation.Fish Creek Mts (1982) and Winnemucca (1986) 30x ...

Augusta Mountains The Augusta Mountains are a small mountain range in Pershing, Churchill and Lander counties of Nevada.''Nevada Atlas and Gazetteer,'' DeLorme, 8th ed., 2012, p. 37. To the northeast the range merges with the Fish Creek Range. To the north acros ...

, and China Mountain. Calcareous shale, mudstone, and black limestones are the most common

lithologies The lithology of a rock unit is a description of its physical characteristics visible at outcrop, in hand or core samples, or with low magnification microscopy. Physical characteristics include colour, texture, grain size, and composition. Lit ...

present within the unit. The member was named for Fossil Hill,

, a locality within the Humboldt Mountains where large quantities of

Anisian In the geologic timescale, the Anisian is the lower stage or earliest age of the Middle Triassic series or epoch and lasted from million years ago until million years ago. The Anisian Age succeeds the Olenekian Age (part of the Lower Triassic Ep ...

-aged marine

fossils A fossil (from Classical Latin , ) is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserved in ...

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 Genus ( plural genera ) is a taxonomic rank used in the biological classification of living and fossil organisms as well as viruses. In the hierarchy of biological classification, genus comes above species and below family. In binomial nomenclat ...

of ichthyosaurs and a

pistosaur Pistosauroidea is a group of marine reptiles within the superorder Sauropterygia that first appeared in the latter part of the Early Triassic and were the ancestors of plesiosaurs. Pistosauroids are rare in Triassic marine assemblages, and are re ...

. Other fossils include bony fish, hybodont sharks, and

invertebrates Invertebrates are a paraphyletic group of animals that neither possess nor develop a vertebral column (commonly known as a ''backbone'' or ''spine''), derived from the notochord. This is a grouping including all animals apart from the chordate ...

with ceratitid ammonoids being especially abundant.

Geology

Stratigraphy

The

Favret Formation The Favret Formation is a Triassic geologic formation. The Favret outcrops in the Augusta, New Pass and Fish Creek ranges of north central Nevada and consists of limestone, shale and siltstone. Fossil Hill Member The Favret Formation alon ...

and the

Prida Formation The Prida Formation is a geologic formation in Nevada. It preserves fossils dating back to the Triassic period. See also * List of fossiliferous stratigraphic units in Nevada * Paleontology in Nevada Paleontology in Nevada refers to paleontol ...

, constitute two of the recognized

geological formations A geological formation, or simply formation, is a body of rock having a consistent set of physical characteristics ( lithology) that distinguishes it from adjacent bodies of rock, and which occupies a particular position in the layers of rock exp ...

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. In the Prida Formation, this member outcrops west of the

, 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. The Fossil Hill varies in thickness throughout its outcrops. In the China Mountain and

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 60-120 meters thick while the section in the Augusta Mountains exceeds 200 metres.

Paleobiota

Ichthyosaurs

Sauropterygians

Pseudosuchians

References

{{Chronostratigraphy of Nevada Triassic geology of Nevada Geologic formations of the United States Natural history of Nevada Stratigraphy of Nevada Triassic System of North America Shale formations Fossiliferous stratigraphic units of North America Anisian Stage