Fossil Hill Member | |
Type: | Geological member |
Prilithology: | Calcareous Shale |
Otherlithology: | Limestone |
Namedby: | Kathryn Nichols and Norman Silberling |
Region: | Nevada |
Country: | |
Unitof: | Prida Formation, Favret Formation |
Period: | Triassic |
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.[1] Other fossils include bony fish, hybodont sharks, and invertebrates with ceratitid ammonoids being especially abundant.[2]
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 60-120 meters thick while the section in the Augusta Mountains exceeds 200 metres.
Cymbospondylus | C. petrinus[3] | 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 NaNm (-2,147,483,648feet). | ||
C. nichollsi[4] | 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 7.6m (24.9feet) long | |||
C. duelfiri[5] | 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 4.3m (14.1feet). Holotype is among the earliest known viviparous animals. | |||
C. youngorum[6] | 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, 17m (56feet) long ichthyosaur, the largest known animal up to this point in time. | |||
Omphalosaurus | O. nevadanus[7] | Multiple specimens from both the Fossil Hill locality and the Favret formation. | A medium-sized—5m (16feet)—basal ichthyosaur with dental batteries of button shaped teeth and durophagous feeding strategy. | ||
O. nettarhynchus[8] | Articulated, but fragmented dentaries from the Spathian beds of the lower Prida Formation | Fewer, but larger teeth than other species of Omphalosaurus | |||
Phalarodon | P. fraasi[9] | Multiple specimens from the Fossil Hill and Augusta Mountain localities | A small, 1m (03feet) long mixosaur | ||
P. callowayi | Holotype found in the same beds as Augustasaurus | ||||
Thalattoarchon | T. saurophagis[10] | 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 and interpreted macropredatory habit. |
Augustasaurus | A. hagdorni[11] | 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 |
Benggwigwishingasuchus | B. eremicarminis[12] | 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 |