Fossil Hill Member

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. 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. Other fossils include bony fish, hybodont sharks, and invertebrates with ceratitid ammonoids being especially abundant.

History

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. 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. 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. 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.

Geology

Stratigraphy

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. 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.

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.

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. 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.

Paleobiota

Ichthyosaurs

Sauropterygians

A partial femur attributed to a nothosaurid (possibly ''Corosaurus'') and an interclavicle putatively attributed to a thalattosaur have been uncovered from outcroppings of the Fossil Hill Member in the Favret Formation.

Pseudosuchians

Actinopterygians

Various bones including opercular remains, jaw fragments, and scales have been found that are not attributable to any specific taxon.

Chondrichthyans

Molluscs

;Ammonites

;Bivalves
*'' Daonella''
**''D. americana''
**''D. dubia''
**''D. moussoni''
**''D. sturi''
;Nautiloids
*'' Aulametacoceras humboldtensis''N. J. Silberling and K. M. Nichols. 1982. Middle Triassic Molluscan Fossils of Biostratigraphic Significance from the Humboldt Range, Northwestern Nevada. Geological Survey Professional Paper 1207
*'' Germanonautilus furlongi''
*'' Grypoceras whitneyi''
*'' Paranautilus smithi''

See also

* List of fossiliferous stratigraphic units in Nevada

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 of the United States
Fossiliferous stratigraphic units of North America
Anisian Stage
Limestone formations of the United States
Geologic members of the United States