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Alphacrinus mansfieldi new genus and species from the Middle Tremadoc Series (Early Ibexian), near the base of the Ordovician, is the oldest known disparid crinoid. A new family, Alphacrinidae, receives this monospecific genus. Alphacrinus's character mosaic includes primitive traits unknown among other disparids, auguring for disparid origin from a more complexly plated, less standardized antecedent, and echoing the evolutionary progression documented for camerates and cladids. Disparids are diagnosed as those crinoids expressing an arm-like branch from the C ray. Morphologic progression indicates this distinctive trait evolved by modification of CD interray plates, not as an outgrowth from the C ray.

The Romaine Formation of the Mingan Islands, Québec, contains a limited trilobite fauna of five genera (Bolbocephalus, Peltabellia, Petigurus, Strigigenalis, Strotactinus) with Lower Ordovician Ibexian (Floian) affinity, succeeded by two genera (Acidiphorus, Pseudomera) of early Middle Ordovician Whiterockian (Dapingian) affinity. Trilobites of later Whiterockian (Darriwillian) affinity are more abundant in the overlying Mingan Formation, with 29 genera (Amphilichas, Bathyurus, Calyptaulax, Ceraurinella, Cybeloides, Dolichoharpes, Encrinuroides, Eobronteus, Eorobergia, Failleana, Glaphurina, Glaphurus, Hibbertia, Hyboaspis, Illaenus, Isotelus, Kawina, Nieskowskia, Phorocephala, Pandaspinapyga, Pliomerops, Remopleurides, Sphaerexochus [including S. valcourensis n. sp.], Sphaerocoryphe, Stenopareia, Thaleops, Thulincola, Uromystrum, and Vogdesia). Lectotypes are selected for Mingan species of Illaenus, Peltabellia, Petigurus, Sphaerexochus, Stenopareia, and Thaleops. A number of genera form biofacies which can be related to lithofacies. Taken together with Chazy Group and Northwest Territories trilobite faunas, Mingan trilobites provide critical information on late Whiterockian trilobite distributions in Laurentia in far more detail than seen in the stratotype area of Nevada.

Storm-generated event beds are an important source of paleoecological information, especially in Paleozoic strata. Storm deposition and subsequent physical and biological modification can potentially alter the diversity structure of death assemblages significantly. To examine the effects of storm deposition on fossil assemblage composition, storm beds are compared with co-occurring beds representing background sedimentation in 67 samples from six Ordovician mixed carbonate-clastic units deposited above the maximum storm wave base. In the great majority of pairwise comparisons, evenness and sampled richness are higher in storm beds than in background beds. This effect is not explained by differences in lithification, skeletal fragmentation, or in the proportions of aragonitic or multielement skeletons. The elevated diversity of storm beds can result from homogenization of fine-scale faunal patchiness preserved in background beds or may be due to taphonomic feedback. The relative importance of these two end-member scenarios can be evaluated with detrended correspondence analysis. In shallow, carbonate-dominated environments, the former appears to predominate, while the latter is more important in a deeper setting dominated by fine-grained clastics. The disparity between background beds and storm beds suggests that, at least in the Lower Paleozoic, background beds may record a higher-resolution paleoecological signal while storm beds record a more complete census of alpha diversity. Because post–Middle Ordovician increases in the depth and intensity of bioturbation may have diminished the temporal resolution and increase the faunal completeness of background beds, this disparity is not necessarily expected in younger strata.

Relative abundance data are of primary importance in paleoecology, but it is not always obvious how they should be interpreted. Because relative abundance is expressed as a proportion of the total sample, change in the abundance of one group necessarily changes the relative abundance of all groups in the sample. There are two possible interpretations for a trend in the relative abundance of a taxon: an “active” scenario in which the trend reflects change in the population density of the group itself, or a “passive” scenario in which the change is driven by population changes in other taxa. To discriminate between these scenarios it is necessary to collect absolute abundance data (abundance expressed as a function of sample area or volume). We examine both absolute and relative abundance trends through a major paleoecological transition: the shift from trilobite-dominated to brachiopod-dominated paleocommunities in shallow marine carbonates spanning the Lower/Middle Ordovician boundary in western Utah and eastern Nevada. We sampled 61 carbonate mudstone and wackestone beds from the upper Ibex Series (Lower Ordovician) and lower Whiterock Series (Middle Ordovician) at three sections that span the boundary. All samples come from the shallow subtidal Bathyurid trilobite biofacies. Samples were broken into small pieces, and all skeletal fragments >2 mm were identified to the finest possible taxonomic level. Consistent with previous work on this interval, the relative abundance of trilobites declines sharply across the boundary, while the relative abundance of brachiopods increases. Absolute abundance data indicate that the decline in trilobite abundance is genuine and not an artifact of normalization. The trend is not easily explained by sampling bias, facies distribution, taphonomic regime, or sedimentation style. The dramatic shift in abundance contrasts with relatively minor changes in relative genus richness across the boundary. This is partly ascribable to differences in the relative abundance structure of trilobite faunas. Though comparable numbers of trilobite and brachiopod genera occur above and below the boundary, the trilobite fauna from the upper Ibex Series has lower evenness then the lower Whiterock Series fauna. Hence sampled trilobite richness is high in the lower Whiterock despite the small number of specimens. This highlights the importance of collecting abundance data. Although these data suggest that in at least some cases richness and abundance patterns are not closely coupled, more robust richness data are necessary to confirm this conclusion.

Seven genera and eight species of lingulate brachiopods were recovered from the House Limestone and lower Fillmore Formation, Ibex area, Utah, USA. These strata are assigned to the upper Skullrockian Stage and lower Stairsian Stage of the Ibexian Series (Iapetognathus Conodont Zone to Low Diversity Interval) and are correlated with the Tremadocian Series of the Acado–Baltic Faunal Province. The fauna includes two new linguloid species, Spinilingula prisca and Wahwahlingula sevierensis, one new siphonotretoid species, Schizambon obtusus, and two new acrotretoid species, Eurytreta fillmorensis and Ottenbyella ibexiana. The last species is the first record of the genus in North America and suggests a correlation of the basal Fillmore Formation with the Ceratopyge Limestone in Sweden. A Siphonobolus? covered by long hollow spines may be one of the oldest siphonotretides with such ornament. This fauna and those described previously from older Utah strata document the biodiversification of the Cambrian–Ordovician lingulate brachiopods and demonstrate their potential for regional and intercontinental correlation.

New evidence of fossil macroborings in the Lower Ordovician (Ibexian) of western Utah demonstrates that the macroboring behavioral strategy was firmly established in the earliest stages of the great Ordovician diversification of the marine biosphere. In Utah, borings were excavated in hardgrounds that had developed on sponge-algal mounds and flat-pebble conglomerates in the Fillmore Formation (Ibexian). The most complete specimens possess a neck up to 1 cm in length that opens into a teardrop-shaped chamber with a maximum diameter of 1 cm. The chamber terminates at a depth of 3-4 cm below the hardground surface. These borings belong to the ichnogenus Gastrochaenolites. The organisms responsible for creating the borings are unknown. Sedimentologically, the effect of boring on hardgrounds was to break them into pebble- and cobble-sized clasts. The endolithic lifestyle represented by the borings may have evolved in response to ecologic pressures such as predation or competition for food resources. The macroborings from the Fillmore Formation represent an innovative strategy that may have resulted in the later development of new body plans and the early establishment of endolithic macroinvertebrates.

Bearriverops n. gen. is a distinctive clade of small, vaulted trilobites from the Lower Ordovician (Ibexian Series; Stairsian Stage) of Utah and Idaho. The genus includes at least seven new species known from silicified material of which five are well enough known to name: B. alsacharovi, B. borderinnensis, B. deltaensis, B. ibexensis, and B. loganensis (the type species). All are known from the lower Fillmore Formation in western Utah; B. alsacharovi and B. loganensis are also known from the Garden City Formation of southeastern Idaho. Both units record deposition in shallow subtidal environments above storm wave base. Bearriverops is characterized particularly by a suite of pygidial apomorphies apparently related to spiral enrollment. Its close relatives include a large group of mostly undescribed Skullrockian and Stairsian species with more conventional spinose pygidia. Together, the taxa are referred to Dimeropygidae, which is considered a senior synonym of Toernquistiidae. Cladistic parsimony analysis of Bearriverops indicates that B. alsacharovi and B. borderinnensis are sister taxa, and that B. loganensis, B. deltaensis, and the plesiomorphic B. ibexensis are successive sister taxa to this clade.

Eriaster ibexensis n. gen. and sp., from the Ibexian (Lower Ordovician) of Utah, is the oldest-known body-fossil taxon of the class Asteroidea. Although important features are not preserved, the external form of E. ibexensis is suggestive of certain living asteroids and unlike approximately coeval somasteroids. The similar ages of Eriaster and the oldest-known somasteroid challenge the candidacy of the latter as basal to asteroids. Trace fossils assigned to Asteriacites have been recovered from strata as ancient as Lower Cambrian. Asteriacites from younger strata are considered to represent resting traces of asterozoans. Mode of formation of early representatives is problematic; however, their simple existence raises the possibility of extended pre-Ibexian asterozoan evolution, which could explain the morphological disparity found among the earliest-known body-fossil asterozoans.

Lower Ordovician sections in the type Ibexian area of western Utah contain a considerably more diverse trilobite fauna than has previously been reported. Reinvestigation of these faunas, based on new field sampling, allows a reassessment of the dimeropygid genera Ischyrotoma Raymond, 1925, and Dimeropygiella Ross, 1951. These taxa have been considered synonyms, but parsimony analysis indicates each is a well supported clade, and they are best recognized as sister genera. The number of species known from Ibex has been doubled, from four to eight, and morphological information is now available for most parts of the exoskeleton. New species include Ischyrotoma juabensis (Juab Formation), I. wahwahensis (Wah Wah Formation), Dimeropygiella fillmorensis (Fillmore Formation), and D. mccormicki (Fillmore Formation). The previously named species Dimeropygiella caudanodosa, D. blanda, and D. ovata are fully revised on the basis of abundant new material. Pseudohystricurus is a paraphyletic group, with species distributed as a basal grade of the Ischyrotoma/Dimeropygiella group.

Collections from upper Ibexian (Tulean Stage) rocks of western United States, from the Canning Basin in western Australia, and from the Argentine Precordillera contain a seximembrate apparatus of multidenticulate conodonts whose elements have been included by authors in species of Prioniodus Pander, 1986, and Reutterodus Serpagli, 1974. The individual elements as well as the complete apparatus are not consistent with assignment of the species to either of these genera or to any other extant genus. A new generic name, Stiptognathus Ethington, Lehnert, and Repetski, is proposed with Reutterodus borealis Repetski, 1982, as type species. The apparatus consists of Pa, Pb, Sa-c, and M elements; the genus represents either the Prioniodontidae or the Periodontidae.