Charles E. Mitchell

Black shales of the Coroico Formation are part of a thick succession of Lower and Middle Ordovician strata that were deposited in the Cordillera Oriental foreland basin along the margin of West Gondwana. The basin was inhabited primarily by a cool-water, Atlantic-type graptolite fauna. Newly discovered material from rocks that crop out near the town of Consata in NW Bolivia include unexpected warm-water or Pacific-type elements such asParisograptus caduceusandPseudotrigonograptuswithin an assemblage dominated by abundant pendentDidymograptusspecimens,Cryptograptus schaeferi, and diplograptaceans such asOelandograptus oelandicusandHustedograptus bulmanin. sp.Parisograptus caduceushas generally been considered to be restricted to oceanic depths below those of the epipalagic realm that occupied continental shelves, whereas the remainder of the assemblage is more characteristic of relatively shallow water, epicratonic sites. We interpret this mixed assemblage to be the Atlantic Province, West Gondwanan equivalent of the off-shore isograptid biofacies that is much more widely known from low latitude sites around the globe. These results suggest that in this region of West Gondwana, the properties of local water masses (productivity and physical features such as temperature, salinity, or oxygenation) strongly influenced graptolite species distribution and led to biofacies differentiation among coeval assemblages. It also suggests that some isograptids inhabited the epipelagic biotope in mid to high latitude regions. The new speciesHustedograptus bulmaniis described herein.

Xu Chen , Jiayu Rong, Junxuan Fan, Renbin Zhan , Charles E. Mitchell, David A.T. Harper, Michael J. Melchin, Ping'an Peng, Stan C. Finney, and Xiaofeng Wang. Episodes 2006;29:183-96. https://doi.org/10.18814/epiiugs/2006/v29i3/004

We have studied the pattern of graptolite species turnover during the latest Ordovician mass extinction based on four continuous Ashgillian to earliest Llandovery sections together with data from more than 30 other published sections. The studied sections represent relatively shallow-water and deeper-water belts in the Yangtze Platform region. Using temporally scaled range data, species diversities and extinction and origination probabilities have been calculated using several analytical methods, including a capture-mark-recapture method. We test the statistical significance of these results and the apparent taxonomic selectivity of extinction and origination via Monte Carlo simulations and contingency analysis. Graptolite species diversity within the Yangtze Platform rose steadily during the late Ashgill, until in the mid-late Paraorthograptus pacificus Chron, when rising extinction risk overtook origination. Diversity dropped to very low levels during the early Hirnantian when extinction probab...

Late Ordovician rocks of the Qilang and Yingan formations from the Kalpin area in the Tarim region of western Xinjiang, China (Tarim palaeoplate) contain a moderately diverse graptolite fauna. The fauna from the Qilang Formation contains Corynoides calicularis Nicholson, 1867; Dicranograptus clingani resicis Williams and Bruton, 1983; Lasiograptus costatus Lapworth, 1873; Pseudoclimacograptus scharenbergi (Lapworth, 1876); and Glossograptus sp. among other species. This assemblage most likely corresponds to the upper Climacograptus (Climacograptus) bicornis Zone? to lower C. (Diplacanthograptus) lanceolatus Zone of Australia. The Qilang Formation also yields the new taxon, Amplexograptus maxwelli spinousus new subspecies. The overlying Yingan Formation yields a more diverse assemblage that includes Climacograptus (Diplacanthograptus) spiniferus Ruedemann,1912; C. (D.) lanceolatus VandenBerg, 1990; Orthograptus quadrimucronatus (Hall, 1865); Amplexograptus praetypicalis Riva,1987; Dicellograptus p...