Evolution of the freshwater ecosystem: The fossil record

The fossil priapulid worms Ottoia prolifica, Selkirkia willoughbyi n. sp., Selkirkia spencei, and Selkirkia sp. are illustrated from the Middle Cambrian of Utah. New records of O. pro ca from the Spence Shale and Marjum Formation represent notable geographic and stratigraphic extensions of its previously unique occurrence in the Stephen Formation of British Columbia. O. prolifica has a range through much of the Middle Cambrian (?15 Ma), during which time it shows minimal morphological change. New records of S. spencei augment previous finds in the Spence Shale. S. willoughbyi n. sp. occurs in the Marjum Formation and Wheeler Formation. It differs from the type species S. columbia in details of tube size and degree of tapering, although the poorly known soft parts appear to be broadly similar. These occurrences extend significantly the stratigraphie range of Selkirkia, and are augmented by the discovery of Selkirkia sp. in the Wheeler Formation. A unique specimen of the possible annelid worm Palaeoscolex, P. cf. P. ratcliffei, is described from the Middle Cambrian of Spain, thereby extending the geographic range from previously known occurrences in England, Utah, and South Australia. Papillate ornamentation of various species of Palaeoscolex is compared, and the new class Palaeoscolecida is erected. These descriptions of soft-bodied organisms provide further information on the diversity of Cambrian life. The possibility that present information on soft-bodied and lightly skeletized biotas is biased toward deeper water, possibly conservative, forms is discussed. Brief descriptions of possible soft-bodied worms and trace fossils (burrows and questionable coprolites) from Utah are also given. Manuscript received July 23, 1985. 2 The University of Kansas Paleontological Contributions—Paper 117 OF THE MANY BIASES that distort our perception of the fossil record, probably none is as significant as the almost total loss of the soft-bodied component of a fauna. As yet there is no clear indication of the relative change in this component through geologic time, and so it may be unwise to infer that a given percentage of softbodied animals is absent from a fossil fauna. Indeed, even were percentages of change known, it remains to be demonstrated whether taphonomic factors have themselves shown systematic changes through the Phanerozoic that might either favor or inhibit the chances of soft-part preservation. Trace fossils can at best hint at former levels of diversity, and rarely provide reliable estimates of a missing soft-bodied component. Accordingly, any information of actual soft-part preservation that helps to supplement our knowledge of this missing fraction of a fauna is of considerable significance. For the Middle Cambrian, the exemplar of a soft-bodied fauna is from the Burgess Shale. It has been suggested that the unusual character of this fauna lies in its favored history of preservation rather than the original biotic assemblage (Conway Morris, 1981; Conway Morris and Robison, 1982). If this is correct then such softbodied faunas may provide a guide to original diversity and paleoecologic insights more reliable than those provided by normal shelly assemblages, for which a substantial fraction of the fauna failed to fossilize. Recent discoveries of exceptionally preserved Middle Cambrian fossils in the immediate vicinity of the Burgess Shale (Collins, Briggs, and Conway Morris, 1983) and in similar open-shelf lithofacies of Idaho and Utah (Willoughby and Robison, 1979; Rigby, 1980, 1983; Gunther and Gunther, 1981; Robison and Richards, 1981; Conway Morris and Robison, 1982; Briggs and Robison, 1984; Robison, 1984b, 1985) support the notion that softbodied organisms comprised a significant proportion of these faunas. As information on softbodied and lightly skeletized taxa from Middle Cambrian localities in Utah and Idaho has accumulated in the last few years, it has become apparent that the great majority of soft-bodied genera so described are known also from the Burgess Shale or nearby sites in the Stephen Formation. Thus, of the approximately 40 genera of arthropods, sponges, priapulids, annelids, medusoids, incertae sedis, and algae known from these localities, almost 75 percent occur in the Burgess Shale [excluding the poorly known Dioxycaris argenta (see Briggs, 1976; Briggs and Robison, 1984) and assuming various questionable assignments to Perspicaris, Emeraldella, Leanchoilia, Sidneyia (Briggs and Robison, 1984), and Stephenoscolex (Conway Morris, 1979a) are correct]. Although new genera will no doubt continue to be described, the overall generic diversity of the soft-bodied component from moderately deepwater biofacies in the western Cordillera appears to be tolerably well documented. Brief descriptions follow for the priapulids Ottoia and Selkirkia from Utah and the possible annelid Palaeoscolex from Spain. This information extends known geographic and stratigraphic ranges. Mention is made also of other softbodied material and trace fossils. Although data are scanty, it appears that some lightly skeletized genera such as Anomalocaris (Briggs and Mount, 1982; Briggs and Robison, 1984) and the aberrant trilobite Naraoia (Robison, 1984b) have lengthy stratigraphic ranges with examples known from the upper Lower Cambrian (Bonnia-Olenellus Zone) to upper Middle Cambrian (Ptychagnostus punctuosus Zone). Robison (1984b) noted that the stratigraphic range of Naraoia exceeds that of most other Cambrian trilobite genera. A parallel exists, however, in the trilobite Ogygopsis, which has a similar stratigraphic range (reviewed in Conway Morris, 1985). Recently, there has been speculation on whether certain habitats promote generic and specific longevity (e.g., Jablonski and Valentine, 1981). Although some data are contradictory, in the context of Lower Paleozoic marine communities Fortey's (1980) documentation of many long ranging Lower Ordovician genera being confined to "deep-water sites with low oxygen concentration, and probably beneath the thermocline" could be directly relevant to explaining the longevity of such Cambrian forms as Ogygopsis, Anomalocaris, perhaps Naraoia, and by implication a number of other genera. In this context the occurrence of Ogygopsis is of particular interest. Several workers (Nelson, 1963; Palmer and Halley, 1979) have commented on the likelihood that the sporadic appearance of this trilobite in the rock record reflects the onset of a particular set of environmental conditions, specifically paleooceanographic factors involving access to the open ocean, or bathymetric changes, or both. One such possibility is occasional and Conway Morris & Robison—Middle Cambrian Fossils from Utah & Spain 3 nonperiodic incursions of deeper and presumably cooler waters into proximal regions of the open shelf. It is suggested that at least some of the soft-bodied taxa known from the Cambrian of the western Cordillera represent conservative forms inhabiting relatively deep water. It may be no coincidence that if their occurrences are linked to more poorly aerated water masses, then the proximity of anoxic conditions may be a major factor in promoting soft-part preservation. Our knowledge of Cambrian biotas with a relatively extensive soft-part component may be biased toward those deeper water faunas that, given certain oceanographic conditions, were able to migrate shoreward. Equally diverse assemblages may have flourished in shallower and more aerated water, but there the chances of extensive soft-part preservation were correspondingly reduced, making it less likely that paleontologists will be afforded the insights provided by the Burgess Shale and analogous deeper water deposits. Nevertheless, even if further evidence suggests the existence of relatively conservative deepwater Cambrian faunas, there is little reason to suppose they were biotically homogeneous. Thus, although Ogygopsis occurs in association with soft-part preservation, including Anomalocaris, in part of the Stephen Formation, elsewhere Anomalocaris and Naraoia occur inde-

Middle Cambrian priapulids and other soft-bodied fossils from Utah and Spain

The carbonate shelf, shelf edge, and outer shelf basin were the sites of deposition of a facies complex that shifted in response to transgressive-regressive (T-R) cycles. In the Silurian-Lochkovian, three T-R cycles are identified, and seven are identified from mid–Lower Devonian (Pragian) to high Upper Devonian (middle Famennian). The last of these produced a thick, upward-deepening sequence truncated above by a Famennian-lower Kinderhookian hiatus, due to foreland effects of the Antler orogeny. The middle and upper members of the Pilot Shale compose an eighth cycle, which spans the Devonian-Mississippian boundary on the carbonate shelf. Most of the short-lived T-R cycles resulted from rapid deepening and progradation of the carbonate-shelf edge. Exceptions occurred during the late Early Devonian, when the carbonate-shelf edge was a ramp, and during the Taghanic and Frasnian onlaps, when transgressions are known to have been strong. The dolomite front tended to coincide approximately with the carbonate-shelf edge, but it was situated cratonward during extensive onlaps and basinward during the pre–Middle Devonian regression.

Time-rock model for Siluro-Devonian continental shelf, western United States

The Pre-Cambrian history of the western United States is fragmentary, and the radio-metric dates so far measured give no obvious clues to ancient orogenic belts or to systematic continental accretion. From Cambrian until Devonian time the western part of the area of the present Cordillera was geosynclinal, the eastern part the site of intermittent shelf seas. The geosyncline was compound, the western part eugeosynclinal, the eastern miogeosynclinal. In Early Mississippian time the Antler orogenic belt rose near the axis of the geosyncline through Nevada and Idaho in an alpine mountain chain at least 500 miles long. The former geosyncline was split into two, the western segment remaining eugeosynclinal, the eastern miogeosynclinal. Notable orogenies took place in the western area in Pennsylvanian, Permian, Triassic, and Jurassic time, across Colorado in Pennsylvanian to Early Permian time, and across southern Arizona in Triassic time. At the end of the Jurassic, after the Nevadan orogeny, the western geosyncline was displaced still farther to the west, the eastern one was drained, and a new one formed on the site of the former craton, in the area of the present eastern Cordillera. Over a million cubic miles of largely clastic sediment accumulated in this new geosyncline. In mid-Cretaceous time orogenic pulses began on the western side of the Cretaceous geosyncline and gradually extended eastward until by the end of the Eocene the entire belt was involved. On the west side of the trough great thrusts were formed from Arizona and Nevada to Montana and beyond into Canada; toward the east, uplift of anticlinal areas and depression of synclinal ones occurred, shortening the surficial crust only slightly but creating structural relief as great as eight or ten miles. The western trough has frequently been much disturbed and broken up by orogenic episodes since the beginning of the Cretaceous. Some structures formed by the ‘Mid-Pleistocene’ Pasadenan orogeny are still highly active. The Colorado Plateau became established as a structural unit by mid-Eocene time but its uplift was delayed until Pliocene and Quaternary time. It may owe its elevation to an iso-static response to the transfer of crustal material by currents in the mantle. The Basin Range structural province is dominated by normal faulting and extends far beyond the region of internal drainage. The distension of the crustal segment indicated by the normal faults may also be attributed to differential drag by currents in the mantle. Transcurrent (strike-slip) faults are important only in California, Nevada, and Idaho. The great San Andreas fault in California (whose movement may have begun in Cretaceous time and is still continuing) seems to demand drag of crustal segments by differential movements in the mantle as the immediate cause. Volcanic activity has occurred in the western part of the area during every period of the Phanerozoic. The Cambrian and Ordovician extrusive rocks are chiefly andesitic or basaltic, but siliceous lavas became abundant during the Silurian and have since been conspicuous. The greatest volcanic field (Permian-Lower Triassic) was submarine; eruptions extended from southern California to Alaska. The extrusion of the flood-basalts of the north-west began in Eocene time, when the greatest outpouring took place. The Columbia River basalts are Miocene and early Pliocene; basaltic flows continued in the Snake River region until Pleistocene or even Recent time. Most of the Tertiary volcanic rocks of the Great Basin are siliceous; the total bulk of the rhyolitic welded tuffs and lavas of this area probably equals that of the Columbia River flood-basalts. Plutonic activity has been dominated by the great batholiths of the Peninsular Range, the Sierra Nevada, and Idaho, with their satellitic intrusions. Most of the radiometric dates are Cretaceous, but many small intrusions are Triassic, Jurassic, and Eocene. The youngest large granitic intrusions are of Miocene age. In Cretaceous time plutons probably a thousand times larger than those of all the rest of the Phanerozoic were emplaced. The records of sedimentation, tectonics, and surface volcanism are uniformitarian, but plutonic activity has been catastrophic—completely lacking during the Palaeozoic and since trivial except during the mid-Cretaceous and Miocene. Plutonic activity is evidently not a necessary accompaniment of normal orogenic processes. The great plutons have arisen on the site of a persistently eugeosynclinal crustal segment. Their volume demands supply either from huge volumes of differentiating mantle or by remelting of large volumes of sial; either mechanism requires differential movement of crust and mantle on a vast scale. The abrupt boundary of the continent against the ocean basin and the complete independence of the large strike-slip faults of the two domains also suggest relative movement of continental crust and mantle. Either the continent is drifting westward over the ocean floor, or the ocean floor (with the sialic segments that formerly lay off-shore) is moving eastward under the continent. The local mechanics would be the same in either case, though the wider mechanisms would be quite different. It is probable that the continent as a whole is moving away from a widening Atlantic.

The tectonic evolution of the western United States SEVENTEENTH WILLIAM SMITH LECTURE

Latest Precambrian to latest Devonian time; Development of a continental margin

_____________________ Introduction. _--___-______--___Acknowledgments--.-_---___-_-. Structural setting._______________ Economic significance. _-__._. Cambrian system.________________ Prospect Mountain quartzite.. Pioche shale_______--_-_-_.__. Eldorado dolomite___________ Geddes limestone.___________ Secret Canyon shale._________ Lower shale member. .... Clarks Spring member.._ Hamburg dolomite.___-_.____ Dunderberg shale.___________ Windfall formation.__________ Catlin member._________

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T.B. Nolan

Papers

The stratigraphic section in the vicinity of Eureka, Nevada