Fossil Hill Member | |
---|---|
Stratigraphic range: | |
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 lead 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.
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 lead 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 60-120 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 [9] | 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 [10] | 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 [11] | Multiple specimens from the Fossil Hill and Augusta Mountain localities | A small, 1 metre (3.3 ft) long mixosaur | |
P. callowayi [11] | Holotype found in the same beds as Augustasaurus | |||
Thalattoarchon | T. saurophagis [12] | 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 [13] | 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 [14] | 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 [14] |
Shonisaurus is a genus of very large ichthyosaurs. At least 37 incomplete fossil specimens of the type species, Shonisaurus popularis, have been found in the Luning Formation of Nevada, USA. This formation dates to the late Carnian-early Norian age of the Late Triassic, around 227 million years ago. Other possible species of Shonisaurus have been discovered from the middle Norian deposits of Canada and Alaska.
Cymbospondylus is an extinct genus of large ichthyosaurs, of which it is among the oldest representatives, that lived from the Lower to Middle Triassic in what are now North America and Europe. The first known fossils of this taxon are a set of more or less complete vertebrae which were discovered in the 19th century in various mountain ranges of Nevada, in the United States, before being named and described by Joseph Leidy in 1868. It is in the beginning of the 20th century that more complete fossils were discovered through several expeditions launched by the University of California, and described in more detail by John Campbell Merriam in 1908, thus visualizing the overall anatomy of the animal. While many species have been assigned to the genus, only five are recognized as valid, the others being considered synonymous, doubtful or belonging to other genera. Cymbospondylus was formerly classified as a representative of the Shastasauridae, but more recent studies consider it to be more basal, view as the type genus of the Cymbospondylidae.
Mixosaurus is an extinct genus of Middle Triassic ichthyosaur. Its fossils have been found near the Italy–Switzerland border and in South China.
Utatsusaurus hataii is the earliest-known ichthyopterygian which lived in the Early Triassic period. It was nearly 2.5–3 metres (8.2–9.8 ft) long with a slender body. The first specimen was found in Utatsu-cho, Miyagi Prefecture, Japan. It is the only described species in the genus Utatsusaurus and the only member of the family Utatsusauridae. The name Utatsusaurus was given after the city. The fossils have been found from the Early Triassic Osawa Formation of Miyagi Prefecture, Japan and British Columbia, Canada.
Shastasauridae is an extinct family of Triassic ichthyosaurs. The family contains the largest known species of ichthyosaurs, which include some of and possibly the largest known marine reptiles.
Chaohusaurus is an extinct genus of basal ichthyosauriform, depending on definition possibly ichthyosaur, from the Early Triassic of Chaohu and Yuanan, China.
Himalayasaurus is an extinct genus of ichthyosaur from the Late Triassic Qulonggongba Formation of Tibet. The type species Himalayasaurus tibetensis was described in 1972 on the basis of fragmentary remains, including teeth, limb bones, and vertebrae. The entire body length of Himalayasaurus is estimated to have been over 15 metres (49 ft) in length. Himalayasaurus has since been considered a nomen dubium or "dubious name" because of the lack of features that set it apart from other ichthyosaurs, although the presence of distinct cutting edges on its teeth have more recently been proposed as a unique feature of the genus. Himalayasaurus belongs to the family Shastasauridae, which includes other large-bodied Triassic ichthyosaurs like Shonisaurus.
Omphalosaurus is an extinct genus of marine reptile from the Early Triassic to Middle Triassic, thought to be in the order of Ichthyosauria. Most of what is known about Omphalosaurus is based on multiple jaw fragments, ribs, and vertebrae. Specimens of Omphalosaurus have been described from the western United States, Poland, Austria and the island of Spitsbergen off the northern coast of Norway.
Augustasaurus is an extinct genus of sauropterygians that lived during the Anisian stage of the Middle Triassic in what is now North America. Only one species is known, A. hagdorni, described in 1997 from fossils discovered in the Favret Formation, Nevada, USA.
Phalarodon is an extinct genus of mixosaurid ichthyosaur known from the Middle Triassic. Its name is derived from the Greek φάλαρα (phálara) and odon ("tooth"). The genus has had a tumultuous history since its classification in 1910, with different workers describing species under different genera or declaring the genus to be a nomen dubium. Currently three species are recognized, but more have been identified in the past.
The Guanling Formation is a Middle Triassic geologic formation in southwestern China.
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.
Barracudasauroides is a genus of mixosaurid ichthyosaur which lived during the Middle Triassic. Fossils of this genus have been found in Guizhou Province, China. It is known from GMPKU-P-1033, a partial skeleton recovered from the Upper Member of the Guanling Formation of Yangjuan village, Xinmin area; this rock unit dates to the Pelsonian substage of the Anisian stage. It was named by Michael W. Maisch in 2010, and the type species is Barracudasauroides panxianensis.
Guanlingsaurus is an extinct genus of shastasaurid ichthyosaur from the Late Triassic of China. It grew up to 8.3 metres (27 ft) in length and has a wide, triangular skull with a short and toothless snout.
Thalattoarchon is a genus of large, Middle Triassic predatory ichthyosaur from North America, containing the single species T. saurophagis. The taxon was described in 2013 from a single specimen discovered in the Favret Formation in Nevada. The generic name, meaning "ruler of the seas", refers to its status as an apex predator, while the specific epithet, meaning "lizard eater", alludes to its carnivorous diet. The classification of this genus within the ichthyosaurs is much debated, being either classified within the clade Merriamosauria or in the more basal family Cymbospondylidae.
The Prida Formation is a geologic formation in northwestern Nevada. It preserves fossils dating back to the Triassic period.
This timeline of ichthyosaur research is a chronological listing of events in the history of paleontology focused on the ichthyosauromorphs, a group of secondarily aquatic marine reptiles whose later members superficially resembled dolphins, sharks, or swordfish. Scientists have documented ichthyosaur fossils at least as far back as the late 17th century. At that time, a scholar named Edward Lhuyd published a book on British fossils that misattributed some ichthyosaur vertebrae to actual fishes; their true nature was not recognized until the 19th century. In 1811, a boy named Joseph Anning discovered the first ichthyosaur fossils that would come to be scientifically recognized as such. His sister Mary would later find the rest of its skeleton and would go on to become a respected fossil collector and paleontologist in her own right.
The Besano Formation is a geological formation in the southern Alps of northwestern Italy and southern Switzerland. This formation, a thin but fossiliferous succession of dolomite and black shale, is famous for its preservation of Middle Triassic (Anisian–Ladinian) marine life including fish and aquatic reptiles. It is exposed in the Monte San Giorgio and Besano area. It is among the formations responsible for the area being designated as a UNESCO World Heritage Site. In Switzerland, it is also known as the Grenzbitumenzone. The Anisian-Ladinian boundary lies in the upper part of the Besano Formation.
Nadia Belinda Fröbisch is a German vertebrate paleontologist and developmental biologist who specializes in the evolution and development of amphibians. She is currently a professor at the Museum für Naturkunde Berlin in the Leibniz Institute for Research on Evolution and Biodiversity.