Richard A. Robison

A global carbon isotope excursion (SPICE) during the Late Cambrian: relation to trilobite extinctions, organic-matter burial and sea level

Abstract The Steptoean positive carbon isotope excursion (SPICE) marks a global oceanographic event that confirms intercontinental correlations between different biogeographic realms based on agnostids and other blue-water trilobites. The SPICE excursion is documented from sections in Laurentia, Kazakhstan, China, and Australia where it begins with the mass extinction at the base of the Pterocephaliid Biomere (Steptoean Stage) in Laurentia and at coeval extinction horizons in Gondwana and periGondwana terranes. The peak of SPICE (+5‰) coincided with a time of maximum regression in Laurentia. SPICE is similar in this regard to excursions that coincide with glacio-eustatic falls, such as in the Late Ordovician. A plausible scenario involves the transformation of ocean circulation between two states, which led to enhanced coastal upwelling and benthic extinctions. The lack of evidence for glaciation indicates that the coeval sea level fall (Sauk II–Sauk III event) resulted from tectonic or hydrologic changes that remain poorly understood at this time.

Global Standard Stratotype-section and Point (GSSP) of the Furongian Series and Paibian Stage (Cambrian)

The Global Standard Stratotype-section and Point (GSSP) of the Furongian Series (uppermost series of the Cambrian System) and the Paibian Stage (lowermost stage of the Furongian Series), has been recently defined and ratified by the International Union of Geological Sciences (IUGS). The boundary stratotype is 369 metres above the base of the Huaqiao Formation in the Paibi section, northwestern Hunan Province, China. This point coincides with the first appearance of the cosmopolitan agnostoid trilobite Glyptagnostus reticulatus, and occurs near the base of a large positive carbon isotopic excursion (SPICE excursion).

Middle Cambrian Stratigraphy of the House, Wah Wah, and Adjacent Ranges in Western Utah

Geologic mapping and paleontologic evaluation of Middle Cambrian strata in western Utah have extended the usage of several formations first defined in the House Range by Walcott and have shown that new units are needed outside of the House Range for upper Middle Cambrian strata of different facies. The following usage is proposed for the House Range: Lower Cambrian — Prospect Mountain Quartzite and the lower member of the Pioche Formation; Middle Cambrian — Tatow Member of the Pioche Formation, Millard Member and upper members of the Howell Limestone, Chisholm Formation, Dome Limestone, Whirlwind Formation, Swasey Limestone, Wheeler Shale, Marjum Formation, and the lower part of the Weeks Limestone. New formations defined in the Wah Wah Mountains include the Eye of Needle Limestone (Wheeler Shale equivalent), Pierson Cove Formation (lower Marjum Formation equivalent), and the Wah Wah Summit Formation (Weeks Limestone near equivalent). The Trippe Limestone is extended from the Deep Creek Range into west-central Utah, and a new member, Fish Springs Member, is defined as a thin but widely mappable unit. The House Range contains the most continuously fossiliferous trilobite sequence, but most ranges in western Utah contain the following faunas: Albertella , Glossopleura , Ehmaniella , Ptychagnostus gibbus , Bathyuriscus fimbriatus , and Eldoradia .

Biogeographical significance of Cambrian trilobites from the Carolina slate belt

Abundant middle Middle Cambrian trilobites of the Ptychagnostus atavus Interval-zone are described from the upper Asbill Pond formation (informal name) near Batesburg, South Carolina. The fauna, containing at least nine species, has strong affinities with cool-water faunas of both Armorica and Baltica, but little affinity with warm-water, shelf faunas of Laurentia. This is compatible with previous conclusions that the Carolina slate belt is an exotic terrane that was accreted to North America during the early or middle Paleozoic. Further evaluation, however, introduces an alternative possibility that the fauna could have lived in deep, cool-water environments along the periphery of Laurentia, and subsequent displacement need not necessarily have been great. Combined faunal and stratigraphic components of the Carolina slate belt do not precisely match those of any known region, but may be closest to those of Bohemia. Although a southpolar location in proximity to Armorica, Baltica, and Gondwana seems likely, a definitive placement in a reconstruction of the Cambrian world is not yet possible. Included in the trilobite fauna are the agnostoids Hypagnostus mammillatus , H. parvifrons , Peronopsis fallax , Ptychagnostus sp.,and Tomagnostus fissus . The polymeroids include Agraulos sp., Paradoxides cf. polonicus , Paradoxides sp., and Skreiaspis ? sp.

Systematics, Paleobiology, and Taphonomy of Some Exceptionally Preserved Trilobites from Cambrian Lagerstätten of Utah

ABSTRACT Mid-Cambrian biotas of the Spence, Wheeler, Marjum, and Weeks formations in Utah are exceptionally diverse. Continued collecting has produced additional trilobites here assigned to one new genus, Meniscopsia, and 25 new species. The new species, all known from complete or nearly complete dorsal exoskeletons, are Amecephalus jamisoni, Athabaskia gladei, Bolaspidella jarrardi, Bolaspidella reesae, Burlingia halgedahlae, Coosella kieri, Ehmaniella whitei, Glossopleura campbelli, Glossopleura yatesi, Ithycephalus stricklandi, Kootenia randolphi, Kootenia youngorum, Meniscopsia beebei, Menomonia sahratiani, Modocia comforti, Modocia kohli, Modocia whiteleyi, Norwoodia boninoi, Olenoides skabelundi, Olenoides vali, Polypleuraspis cooperi, Ptychoparella lloydi, Ptychoparella mettae, Zacanthoides liddelli, and Zacanthoides marshalli. Descriptions of two other species, Triadaspis bigeneris and Zacanthoides typicalis, are expanded to include new information from complete dorsal exoskeletons. The ventral cephalic presence of a functional rostral-hypostomal suture in the corynexochoid Zacanthoides marshalli further demonstrates that a fused rostral-hypostomal plate is not a unique defining character of the order Corynexochida, adding to evidence that the Corynexochida may be a polyphyletic taxon. The ventral cephalic presence of a median suture, associated with a rostellum in Norwoodia boninoi, order Ptychopariida, is further evidence for a polyphyletic origin of the median suture, previously cited as a unique apomorphic character defining the order Asaphida, which needs revision. The presence of a mineralized alimentary tract and digestive glands in some specimens of Meniscopsia beebei and Coosella kieri suggests that the gut was fluid filled at the time of burial and that the species were predaceous. Collapsed glabellas are interpreted as further evidence of fluid-filled gut tracts in some of the new species. Many of the articulated trilobites preserved in mid-Cambrian Lagerstätten of Utah were probably buried under rapidly deposited sediment clouds on a storm-prone marine shelf. Occasionally, trilobite clusters were buried. Encrustation of fully articulated individuals with calcitic cone-in-cone deposits in the Wheeler and Marjum formations is a likely consequence of concretionary-type growth within a biofilm shortly after death.

Geological implications of Middle Cambrian boulders from the Haymond Formation (Pennsylvanian) in the Marathon basin, west Texas

Fossiliferous Middle Cambrian boulders from a previously unknown boulder unit in the Haymond Formation of Pennsylvanian age in the southeastern part of the Marathon basin, Texas, contain trilobites, brachiopods, and mollusks indicative of the seaward margin of a late Middle Cambrian carbonate platform. The source area was to the southeast of present outcrops. Paleogeographic relocation of the source, about 100 to 200 km southeast of the present Marathon basin, provides the first clear evidence for the position of the seaward margin of the earliest Paleozoic carbonate platform along the southern sector of the United States. Metamorphic rocks of the “interior zone” of the late Paleozoic Ouachita orogen now lie well inboard of the seaward edge of the carbonate platform and are most likely allochthonous on a scale of tens of kilometres.

New records of Burgess Shale-type taxa from the middle Cambrian of Utah

Abstract. Cambrian strata of the Laurentian craton contain numerous examples of Burgess Shale—type faunas. Although displaying a more or less concentric distribution around the cratonal margin, most faunal occurrences are in present-day western North America, extending from the Northwest Territories to California. Nevertheless, the soft-bodied and lightly skeletalized fossils in most of these Lagerstätten are highly sporadic. Here, we extend knowledge of such Middle Cambrian occurrences in Utah with reports of four taxa. An arthropod from the Marjum Formation, Dytikosicula desmatae gen. et sp. nov., is a putative megacheiran. It is most similar to Dicranocaris guntherorum, best known from the younger Wheeler Formation, but differs primarily in the arrangement of pleurae and overall size. Along with a specimen of ?Yohoia sp, a new species of Yohoia, Y. utahana sp. nov., is described. It differs from the type and only known species, Y. tenuis, principally in its larger size and shorter exopods; it is the first description of this genus from outside the Burgess Shale. A new species of a stem-group lophotrochozoan from the Spence Shale, Wiwaxia herka sp. nov., possesses a palisade of dorso-lateral spines that are more robust and numerous than the type species of Wiwaxia, W. corrugata. Another notable taxon is Eldonia ludwigi from the Marjum Formation, which is interpreted as a primitive ambulacrarian (assigned to the cambroernids) and a new specimen of the ?cnidarian Cambrorhytium from the Wheeler Shale is illustrated.

Character Displacement in Cambrian Agnostid Trilobites: ABSTRACT

The phenomenon known as character displacement develops when the ranges of 2 or more closely related species overlap geographically. Differences between the species are accentuated in the zone of sympatry, but remain less pronounced in the parts of their ranges outside that zone. Characters displaced may be morphologic, ecologic, behavioral, or physiologic. Biologists have shown that size is one of the most common morphologic characters displaced. A moderate difference in size--on the order of 1.3--appears to be sufficient to cause obligated feeding on different kinds of food, and thus permits sympatric species to avoid competitive elimination. Although character displacement in modern faunas has been well documented, to my knowledge no examples have been described from the fossil record. Analysis of several extensive collections of Cambrian fossils suggests that size displacement was common among agnostid trilobites. Where 2 or more agnostid trilobites are found together, the interspecific ratio of maximum size usually is close to 1.3. Also, maximum size generally is more uniform where given species are separated than where they are found together. The similarity of these patterns to those displayed by certain modern faunas indicates that sympatric agnostids were structurally specialized to feed on resources of different sizes. This conclusion helps to explain how superficially similar species of Cambrian agnostids may have coexisted without competitive elimination. The examples further indicate that character displacement was operating early in the history of metazoans. End_of_Article - Last_Page 962------------

Revision of Cambrian Stratigraphy, Silver Island Mountains, Utah: NOTES

On the basis of new fossil collections, there appears to be few, if any, exposed rocks of Middle Cambrian age in the Silver Island Mountains of Utah, and rocks of Dresbachian age are at least 2,400 feet thick. New correlations and nomenclature are presented.