A new soft-bodied fauna: The Pioche Formation of Nevada

ABSTRACT-A new Burgess Shale-type soft-bodied fauna crossing the Lower-Middle Cambrian boundary in the Comet Shale Member of the Pioche Formation in Lincoln County, Nevada, contains common remains of soft-bodied ecdysozoan taxa. These fossils provide important new information about the nature and variety of Cambrian soft-bodied organisms. Arthropod taxa include one species of Canadaspis Novozhilov in Orlov, 1960, one species of ?Perspicaris Briggs, 1977, three species of Tuzoia Walcott, 1912, and at least two species of Anomalocaris Whiteaves, 1892. A priapulid referable to Ottoia Walcott, 1911a, was also recovered. A comprehensive review of Tuzoia is given. Some specimens from Early Cambrian sections are replaced by hematite, resulting in iron staining similar to that in such other Early Cambrian soft-bodied faunas as the Kinzers Formation in Pennsylvania. Some taxa in the Comet Shale, and in other Early and Middle Cambrian soft-bodied faunas, have prodigious geographic ranges that spanned much of Laurentia and even other Cambrian cratons. Moreover, these taxa ranged across the Early-Middle Cambrian boundary relatively unscathed. This is in contrast to many trilobite taxa that had narrow geographic ranges in the Early Cambrian and high levels of extinction at the Early-Middle Cambrian boundary.

INTRODUCTION

THE PALEOBIOLOGICAL significance of Cambrian soft-bodied faunas is considerable. They have inspired extensive research and analyses (e.g., Whittington, 1974, 1985; Briggs, 1977, 1979; Collins et al., 1983; Conway Morris, 1985, 1989a, 1992, 1993, 2000; Briggs and Fortey, 1989; Gould, 1989, 1991; Briggs et al., 1992, 1994; Budd, 1995, 1999; Chen et al., 1995; Collins, 1996; Fortey et al., 1996; Edgecombe, 1998; Fortey and Thomas, 1998, and other references given below). Here, many of the ecdysozoans (metazoans that molt, including arthropods and priapulids) from a new Cambrian fossil conservation deposit sensu Shields (1998) in the Pioche Formation of Nevada are illustrated and described. Many of the important Cambrian soft-bodied faunas are also from Laurentia, and the Pioche Formation joins the other Laurentian soft-bodied faunas in being situated in the concentric belt around North America and Greenland. As with many of these other faunas, the Pioche Formation was also deposited in a position on the seaward margin of the carbonate platform (Garcia-Bellido Capdevila and Conway Morris, 1999). Modes of preservation in Burgess Shale and similar faunas has been analyzed by such authors as Whittington (1985), Butterfield (1990), Briggs and Nedin (1997), Orr et al. (1998), and Babcock (2001). Babcock et al. (2001) recently considered the nature of Burgess Shale-type faunas and how, where, and why they came to be preserved.

The new soft-bodied fauna is in the Comet Shale Member (formerly the C-Shale Member) of the Pioche Formation in eastern Nevada. The lithostratigraphy and biostratigraphy of these members have been reviewed by Palmer (1998) and Sundberg and McCollum (2000). The Comet Shale Member contains a fissile clay shale, with a few thin siltstone lenses near its base and a few bioclastic limestones near its top. It varies in thickness from 17 m-34 m and, depending on location, contains faunas of the upper Olenellus or Eokochaspis nodosa zones (Sundberg and McCollum, 2000). Based on traditional Laurentian chronostratigraphy, the member is latest Early Cambrian and earliest Middle Cambrian in age, and thus is at least a few million years older than the classic Burgess Shale, though it is somewhat younger than the Chengjiang.

The specimens of the soft-bodied Pioche fauna often co-occur with mineralized olenelloid or ptychoparioid trilobites but consist of thin, biomineralized body cuticle akin to the preservational style of specimens from the Middle Cambrian Spence and Marjum Formations of Utah (see Robison and Wiley, 1995). The olenelloid trilobites are extremely abundant in the Early Cambrian part of the member (see Palmer, 1998), while the ptychoparioid trilobites are fairly common in the Middle Cambrian part of the member (see Sundberg and McCollum, 2000). The elements of the soft-bodied fauna are, by contrast, much rarer. Some specimens from the Early Cambrian part of the Comet Shale Member typically have their tissues replaced by a reddish-orange mineral. None of the specimens from the Middle Cambrian part of the Comet Shale has this style of preservation. In certain cases, particularly for specimens of Tuzoia Walcott, 1912, this mineral has relief to it and seems to trace out aspects of anatomy. X-ray diffraction studies of the red to orange soft-bodied specimens reveal the presence of quartz, calcite, illite, and iron-bearing hematite. By contrast, specimens lacking the reddish-orange color lack calcite and, significantly, hematite.

The preservation of the soft-bodied fauna in the Early Cambrian part of the Comet Shale Member of the Pioche Formation is similar to that of three other important late Early Cambrian soft-bodied faunas: the Kinzers Formation of Pennsylvania; the Latham Shale of the Marble Mountains of southern California; and the Eager Formation of Cranbrook, British Columbia, Canada. All have exoskeletons and soft tissues replicated by iron bearing minerals (e.g., Briggs and Mount, 1982). All of these faunas are probably roughly coeval. The similarity in taphonomic style and the associated iron staining suggest a potentially interesting set of oceanographic conditions that may have prevailed during the late Early Cambrian.