The Cambrian represents the first appearance of abundant hard-bodied fossils in the fossil record as well as the first appearance of most major phyla. This geologically sudden appearance of hard fossils is termed the "Cambrian Explosion." Fossil lagerstatten (fossil deposits of exceptional preservation and/or abundance) such as the Burgess Shale (Briggs et al., 1994) and the Chengjiang deposits (Hou et al., 2007) have greatly increased our knowledge of soft-bodied Cambrian animals that are normally not preserved in the fossil record, and that outnumbered their hard-bodied contemporaries.
Cambrian fossils occur in rocks in the northern and southeastern portions of the state. During this time Idaho was underwater, being located along the western edges of the continent and forming part of a broad oceanic shelf. For a detailed paleogeographical reconstruction of North America at this time, see this Ron Blakey reconstruction.
Cambrian fossils occur in rocks in the northern and southeastern portions of the state. During this time Idaho was underwater, being located along the western edges of the continent and forming part of a broad oceanic shelf. For a detailed paleogeographical reconstruction of North America at this time, see this Ron Blakey reconstruction.
Cambrian marine scene by talented paleoartist Douglas Henderson (used here with permission). Anomalocaris, Leanchoilia, and various other soft-bodied animals as well as typical hard-bodied fossils are portrayed. Fossils such as these are known from the Spence Shale in Utah and would have lived in the Cambrian seas of Idaho as well.
Fossil bearing formations in the northern part of the state include the Rennie Shale and Lakeview Limestone, near Pend Orielle Lake. They have been noted a containing abundant Cambrian fossils such as hyolithids, brachiopods, and trilobites such as Agnostus, Pagetia, and Oryctocephalus (Reeser, 1939a).
The Cambrian Elkhead Limestone occurs near Pocatello, and molted fossil trilobite parts are very common in some outcrops of this limestone near Mink Creek.
The Langston Dolomite, Ute Limestone, Bloomington Formation, Nounan Limestone, and Saint Charles Limestone have been noted to have at least modest fossil content consisting of trilobites, brachiopods, and invertebrate burrows (Mansfield, 1927).
The most noted and probably most productive fossil bearing Cambrian formation in Idaho is the Spence Shale, a member of the Langston Formation. The Spence Shale was first described by Walcott (1908) and the age is middle Cambrian, near the boundary of Series 2 and Series 3 (see timescale). The fauna of the Spence was later described in more detail by Reeser (1939b). The Spence Shale crops out in the Malad and Bear River Ranges, with the type location, Spence Gulch (see photo above) occuring near Liberty, Idaho. The Spence Shale also outcrops near Brigham City, Utah. Rocks in the Spence Shale at Spence Gulch are very fossiliferous and consist of soft fissile shale, with the lower portion being very dark and organic, and the upper shale layers being more tan and olive-green. A good reference on the geology and taphonomy of the Spence Shale is Liddell (1997).
The Spence Shale has produced a great diversity of fossils in Utah and Idaho. In Utah a diverse soft-bodied fauna of Burgess Shale-type organisms is known. Unfortunately, most soft-bodied fossils known from the Utah outcrops are so far unreported from the Spence in Idaho. The Spence Shale in Idaho has a very diverse fauna, with the trilobites Agnostus, Amecephalus, Zacanthoides, and possibly Elrathina being among the most common (see photos below). Other fossils include the eocrinoid Gogia, brachiopods, hyolithids, and tubes of the worm Selkirkia (see below).
The Cambrian Elkhead Limestone occurs near Pocatello, and molted fossil trilobite parts are very common in some outcrops of this limestone near Mink Creek.
The Langston Dolomite, Ute Limestone, Bloomington Formation, Nounan Limestone, and Saint Charles Limestone have been noted to have at least modest fossil content consisting of trilobites, brachiopods, and invertebrate burrows (Mansfield, 1927).
The most noted and probably most productive fossil bearing Cambrian formation in Idaho is the Spence Shale, a member of the Langston Formation. The Spence Shale was first described by Walcott (1908) and the age is middle Cambrian, near the boundary of Series 2 and Series 3 (see timescale). The fauna of the Spence was later described in more detail by Reeser (1939b). The Spence Shale crops out in the Malad and Bear River Ranges, with the type location, Spence Gulch (see photo above) occuring near Liberty, Idaho. The Spence Shale also outcrops near Brigham City, Utah. Rocks in the Spence Shale at Spence Gulch are very fossiliferous and consist of soft fissile shale, with the lower portion being very dark and organic, and the upper shale layers being more tan and olive-green. A good reference on the geology and taphonomy of the Spence Shale is Liddell (1997).
The Spence Shale has produced a great diversity of fossils in Utah and Idaho. In Utah a diverse soft-bodied fauna of Burgess Shale-type organisms is known. Unfortunately, most soft-bodied fossils known from the Utah outcrops are so far unreported from the Spence in Idaho. The Spence Shale in Idaho has a very diverse fauna, with the trilobites Agnostus, Amecephalus, Zacanthoides, and possibly Elrathina being among the most common (see photos below). Other fossils include the eocrinoid Gogia, brachiopods, hyolithids, and tubes of the worm Selkirkia (see below).
A specimen of Naraoia from near Pocatello. From Robison (1984).
One type of soft-bodied animal known from the Burgess Shale, as well as the Chengjiang fauna, has been reported from the Gibson Jack Formation south of Pocatello. This is the trilobite-like arthropod
Naraoia. A few specimens have been found near Pocatello and were
reported in Robison (1984).
Naraoia. A few specimens have been found near Pocatello and were
reported in Robison (1984).