The Journal of Geology

Bush, AndrewM.; Bambach, RichardK.

doi: 10.1086/424576pmid: N/A

AbstractWe estimate the effects of three biases on the observed alpha diversity of paleocommunities from the Middle Paleozoic and Late Cenozoic. The first bias results from the preferential dissolution of aragonite relative to calcite; this bias can lower the relative abundance and preserved diversity of aragonitic taxa, potentially lowering the rarefied diversity of an entire fossil assemblage. We model the effects of this bias by analytically reinserting aragonitic specimens and taxa into Paleozoic assemblages that have been described in the literature. The aragonitic specimens are inserted using a wide range of reasonable assumptions about the original local paleocommunity composition. Although the dissolution bias is probably not as severe as has been argued by some, our analytical modeling indicates that the average Paleozoic assemblage may have lost up to 29% of its total diversity. The second bias results from the higher diversity of the tropics relative to temperate latitudes, but the Late Cenozoic collections we analyzed from the literature represent temperate assemblages whereas the Paleozoic collections were tropical in origin (the northward drift of North America and Europe through time caused this difference). On the basis of latitudinal diversity gradients in the Late Cenozoic, the diversity of the temperate Late Cenozoic samples should be at least doubled for an accurate comparison to the tropical Paleozoic samples. The third bias is environmental: our Late Cenozoic samples tend to come from more onshore, stressed habitats than the Paleozoic samples. In our study, this factor should reduce the apparent diversity of Late Cenozoic paleocommunities by about 9%. After correcting for these biases, standardized alpha diversity appears to increase by a factor of 3.03.7 from the Middle Paleozoic to the Late Cenozoic. Previous studies that did not correct for these biases suggested that alpha diversity increased by a factor of 2.5 times; the earlier studies produced approximately correct results because (by chance) the effects of the biases largely cancel out. In the consensus article on marine diversity history, an observed increase in alpha diversity was taken as powerful support for an increase in global diversity from the Paleozoic to the Cenozoic. Although we do not test all conflating factors, this study provides new rigor to this longstanding view on alpha diversity change in the Phanerozoic.

Rees, P.McAllister; Noto, ChristopherR.; Parrish, J.Michael; Parrish, JudithT.

doi: 10.1086/424577pmid: N/A

AbstractThe Jurassic and Cretaceous are considered to have been warmer than today on the basis of various climate data and model studies. Here, we use the available global record of climatesensitive sediments, plants, and dinosaurs to infer broadscale geographic patterns for the Late Jurassic. These provide a context for our more detailed accounts of the Morrison and Tendaguru Formations in North America and East Africa. At the global scale, evaporites predominated in low latitudes and coals in mid to high latitudes. We ascribe these variations to a transition from drier to wetter conditions between the equator and poles. Plant diversity was lowest in equatorial regions, increasing to a maximum in midlatitudes and then decreasing toward the poles. Most dinosaur remains are known from lowlatitude to marginally midlatitude regions where plant fossils are generally sparse and evaporites common. Conversely, few dinosaur remains are known from mid to high latitudes, which have higher floral diversities and abundant coals. Hence, there is an obvious geographic mismatch between known dinosaur distributions and their primary food source. This may be due to taphonomic bias, indicating that most dinosaur discoveries provide only a small window on the diversity and lifestyles of this group. On the basis of our global and localscale studies, we suggest that dinosaur preservation was favored in environments toward the drier end of the climate spectrum, where savannas rather than forests predominated. A holistic approach, incorporating climate and vegetation as well as geography, is required to better understand patterns of dinosaur ecology and evolution.

Raymond, Anne; Metz, Cheryl

doi: 10.1086/424580pmid: N/A

AbstractComparison of the duration, onset conditions, and biotic response to the four Phanerozoic glaciations suggests that there are two Phanerozoic glacial modes. Late Ordovician glaciation typifies shortduration, high atmospheric CO2 events, characterized by cosmopolitan faunal distributions and two episodes of catastrophic extinction. PennsylvanianPermian and Cenozoic glaciation typify longduration, low atmospheric CO2 events, characterized by abundant biogeographic differentiation and stable or rising biotic diversity. Late Devonian glaciation appears most similar to Late Ordovician glaciation: it had a short duration; Late Devonian biotas were cosmopolitan; and Late Devonian glaciation was associated with increased extinction at the DevonianCarboniferous boundary. On the basis of biotic response, we would predict high atmospheric CO2 levels at the time of Devonian glaciation. However, the Berner GeoCarb curve suggests relatively low atmospheric CO2 levels in the Late Devonian.

Johnson, MarkesE.; Jiayu, Rong; Wenbo, Su

doi: 10.1086/424575pmid: N/A

AbstractThe regional geology of southern Inner Mongolia records relationships between an upper Silurian coastline and a small island on what was formerly the continental shelf of the SinoKorean Plate (North China Block). In both cases, rocky shorelines are defined by unconformities between strata of Ludlow age in the Xibiehe Formation and underlying igneous rocks of Ordovician age. The paleoisland occurs near Bater Obo about 50 km south of the ChineseMongolian border. The diorite core of this geological inlier is oriented on an axis N 55 E. A simple dichotomy of exposed windward versus sheltered leeward environments accounts for contrasting carbonate facies on the flanks of the paleoisland. Wider corroboration of a prevailing Silurian wind field is supported by evidence from other unconformities about 30 km to the southwest, where a linear paleoshore extends for 15 km on an average bearing of N 60 E. Siliciclastic sandstone derived from a granodiorite source in the Gushan area now sits in close proximity to basaltic bedrock in the Baoerhantu area. Local transport of sand is the result of alongshore currents driven by winds that were consistent in direction with the facies pattern around the paleoisland. Globalscale interpretations of Silurian paleogeography typically place the SinoKorean Plate in low latitudes north of the paleoequator, where it would receive trade winds out of the northeast. In terms of present geography, however, the Silurian winds of Inner Mongolia are aligned with a source out of the northwest. None of the published scenarios of SinoKorean paleogeography accommodates the physical data from the Bater Obo, Baoerhantu, and Gushan areas of Inner Mongolia. Based on these relationships, the SinoKorean Plate must be rotated 90 clockwise from its present orientation in order to approximate its original Silurian orientation under a prevailing atmospheric system associated with northeasterly trade winds.

Spicer, RobertA.; Herman, AlexeiB.; Kennedy, ElizabethM.

doi: 10.1086/424579pmid: N/A

AbstractThe extent to which the leaves of woody dicots encode in their physiognomy the climatic conditions that exist during dormancy was tested by sampling 20 sites along an approximately westeast transect across European Russia, the Crimean Peninsula, Western Siberia, and central Eastern Siberia. This transect encompassed the most extreme mean annual temperature range recorded in the modern world where vegetation exists. Climate Leaf Analysis Multivariate Program (CLAMP) revealed little change in calibration of the warm month mean temperature compared with the PHYSG3AR data set derived from less extreme sites primarily in North America and Japan, but significant change with respect to the cold month mean temperature (CMMT) calibration. Although CLAMP underestimated the CMMT by up to 9C in the coldest sites, the addition of the transect sites improved CLAMPs performance at low temperatures. This suggests that winter cold is encoded in foliar physiognomy even though the leaves are functional only during the late spring and summer months. This increase in performance was, however, at the cost of decreasing precision. Precipitation predictive capabilities were only slightly affected, but calibration of key climatic variables such as enthalpy, used in determining palaeoaltitude, remained more or less unchanged after the inclusion of the cold transect samples.

GierlowskiKordesch, ElizabethH.; Park, LisaE.

doi: 10.1086/424578pmid: N/A

AbstractLake faunal diversity has been the focus of many modern studies; however, there have been few studies comparing the lacustrine diversity in the modern and fossil record. Whether or not this comparison is possible is dependent on preservation. By comparing the exceptional preservation of the Konservat Lagersttten of the Eocene Green River Formation with its various modern analogs in East Africa, unique factors controlling diversity within a lake system can be determined. Such factors would include lake surface area, lake longevity, productivity, and lake type. In order to effectively compare lake diversity in the modern and fossil record, we first estimated the potential preservational bias of the fauna from modern Lake Tanganyika and determined that, at maximum, approximately 43.8% of species, 59.3% of genera, and 65.8% of families would be identifiable. With this taphonomic filter in place, we compared fish species diversity versus lake area; no strong relationship was found. Faunal species diversity versus lake longevity was shown to correlate in longlived lakes both in a modern and geologic context. When including modern lakes into this comparison, the correlation is not as strong, possibly because of the limited life span of these lakes. Lake Tanganyika was comparable to lakes in the geologic record, after the taphonomic filter was applied. Finally, to test the relationship between species and lake type and productivity, the underfilled, balancedfilled, and overfilled lake basin model of Carroll, Bohacs, and coworkers was used as a basis for comparison between the fauna found in lake deposits within the Green River Formation and modern lakes from East Africa.