Joanne Kluessendorf

A Silurian soft-bodied biota

A new Silurian (Llandoverian) biota from Wisconsin with a significant soft-bodied and lightly sclerotized component is dominated by arthropods and worms. The fauna includes the earliest well-preserved xiphosure, a possible marine uniramian, three new arthropods of uncertain affinity, and possibly the first Paleozoic leech. This may be only the second locality to yield a conodont animal. Lack of a normal shelly fauna suggests an unusual environment. The discovery adds significantly to the few such exceptionally preserved faunas known from Lower Paleozoic rocks.

Pohlsepia Mazonensis, An Early ‘Octopus’ From The Carboniferous Of Illinois, USA

Pohlsepia mazonensis gen. et sp. nov. from the Mazon Creek Konservat Lagersta¨tte (Carboniferous) of Illinois is an exceptionally preserved soft-bodied fossil coleoid, with well-defined body and arms. Lacking an internal shell and possessing eight subequal and two modified arms, Pohlsepia can be compared with both the living cirrate octopods and the decabrachian sepiardarids, both of which lack a well-developed internal skeleton. Given its sac-like body, lack of a well-defined head and presence of fins, Pohlsepia can be safely compared with modern cirrate octopods. It is the oldest known completely soft-bodied coleoid and as such has great significance with respect to the phylogeny of the group, given that both the octobrachian and decabrachian clades have previously been thought to have evolved in the Jurassic. Key words: Coleoidea, Octobrachia, Konservat Lagersta¨tte, Mazon Creek, Carboniferous.

GRAPTOLITES FROM NEAR THE ORDOVICIAN-SILURIAN BOUNDARY IN ILLINOIS AND IOWA

Abstract The Wilhelmi Formation of Illinois and the Mosalem Formation of Iowa contain monospecific assemblages of the stratigraphically important species Normalograptus parvulus Lapworth. This species is confined elsewhere to the uppermost Ordovician Normalograptus persculptus Biozone and the lower part of the lowermost Silurian Parakidograptus acuminatus Biozone. The presence of this graptolite raises the possibility that the lowermost parts of the Wilhelmi and Mosalem formations are of late Ordovician age rather than of early Silurian age as previously thought.

STASIS AND EXTINCTION OF SILURIAN (LLANDOVERY-WENLOCK) TRILOBITE ASSOCIATIONS RELATED TO OCEANIC CYCLICITY

Silurian trilobites of the central United States belong to a series of temporally-successive associations which appeared abruptly, maintained taxonomic stasis for a time, and then disappeared abruptly. Their disappearance resulted from global perturbations of short-term duration and moderate magnitude, which caused substantial taxonomic replacement but no reorganization of major ecosystems. The most significant extinction and replacement in Silurian trilobite associations in the study area occurs near the Llan- dovery-Wenlock boundary. This turnover in trilobite associations appears to correspond to Jeppssons Ireviken Event in his model of oceanic and climatic cyclicity. Major sea-level changes earlier in the Llandovery did not have a similar impact on trilobite associations. the Silurian rocks of the central United States (Mikulic, 1979; in press), which show distinct patterns of abrupt appear- ance, followed by taxonomic stasis, and then abrupt extinction (Kluessendorf and Mikulic, 1997). Some of these associations are restricted to either a reef or nonreef setting; however, key taxa may range through a variety of environments, demonstrat- ing that the succession of associations is an evolutionary feature and not an environmental one. The best-documented of these associations are found in the upper Llandovery-lower Wenlock rocks of this region where non-trilobite biostratigraphic control and depositional history are well established. Within this time interval, evidence indicates that extinction events may be related to the Ireviken Event (Jeppsson, 1997), a stressful transitional period between a primo and secundo oceanic episode (Jeppsson,

Upright crinoids of the Thornton Reef, Wenlock (Silurian) of Illinois, USA

The rare preservation of columns perpendicular to bedding may provide convincing evidence that ancient crinoids adopted an upright attitude during life. However, taphonomic and sedimentological analyses are important in determining whether such occurrences are truly in situ or represent unusual allochthonous accumulations. The Crinoid Biosome of the Thornton Reef Complex, Silurian of Illinois, USA, includes pluricolumnals preserved perpendicular to bedding, in association with more common specimens parallel or angled to bedding. Upright pluricolumnals are relatively shorter (<150 mm) than the longest non-upright specimens; they sometimes occur as imbricate accumulations and lack attachment structures (although these are preserved separately). Brachiopod valves and colonial corals may also be preserved perpendicular to bedding. These features suggest that the Thornton beds were formed as viscous mass flow deposits and that the upright crinoid columns are allochthonous or, at best, parautochthonous.