摘要:

A large sample of the bizarre bellerophontiform molluscPterothecafrom the Upper Llandovery Wether Law Linn Formation in the Scottish Pentland Hills shows marked polymorphic asymmetry. A survival strategy that presents to a predator a mosaic of different search images is suggested to account for the marked variability of thePterothecamorphology. Multivariate comparisons withPterotheca mullochensisfrom the Llandovery of Girvan,P. simplexfrom the Caradoc of Girvan,P.cf.simplexfrom the Caradoc of the Northern Belt of the Southern Uplands andP.aff.mullochensisfrom the Llandovery in the Pentland Hills have helped clarify the morphological differences between these groups. A new species,Pterotheca trimerelloides, is established for the specimens from the Pterotheca beds in the Wether Law Linn Formation. □Mollusca, Silurian, Ordovician, Pterotheca,taxonomy, palaeoecolog

ISSN:0024-1164    

DOI:10.1111/j.1502-3931.1995.tb01600.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY

摘要:

One of the long‐standing problems in North American graptolite biostratigraphy is the distinct differences in assemblages of post‐Climacograptus bicornisage between the classical graptolite sequences in the New York ‐ Quebec and Marathon, west Texas, regions. These have been attributed either to faunal provincialism or to the presence of a major hiatus between the Woods Hollow and Maravillas formations in Texas. New collections from the key Marathon Picnic Grounds section contain diagnostic Late Ordovician graptolites that confirm the existence of a major stratigraphic gap below the Maravillas Formation. The lower Maravillas Formation (Zone 13) has a Late Ordovician, low‐diversity Pacific Province graptolite fauna that includes the biostratigraphically diagnostic speciesClimacograptus nevadensis, C. tubuliferus, Orthograptus fastigatusandDicellograptus ornatus.Zone 13 graptolite assemblages from the Marathon region correlate with theC. tubuliferustoD. ornatuszones in the Trail Creek, Idaho, succession, the Ea4‐Bo2 interval in Victoria, theO. fastigatusZone in the Canadian Arctic Islands, theO. quadrimucronatustoD. ornatuszones of the Canadian Cordillera, and theD. complanatustoD. ancepszones in Scotland. The hiatus between the Woods Hollow and Maravillas formations spans an interval corresponding to at least the Eal Ea3 interval in Australia, theC. americanusto upperA. manitoulinensiszones in the New York ‐ Quebec succession, and theD. clinganiandP. lineariszones in Scotland. These results agree with the magnitude of the hiatus previously indicated by conodont biostratigraphy. Late Ordovician graptolite distribution patterns in North America can be explained by an extension of Cooper, Fortey&Lindholm's (1991;Lethaia 24) Lower Ordovician graptolite biofacies model into the Upper Ordovician, which incorporates both lateral water‐mass specificity and depth stratification. Using this model, we recognize in Laurentia two separate biofacies among tropical‐zone Late Ordovician Pacific Province graptolite faunas, a cosmopolitan Oceanic biofacies, and a cratonic Laurentian biofacies. The lower Maravillas Formation graptolite fauna is clearly part of the Oceanic biofacies, whereas the coeval Appalachian faunas represent the Laurentian biofacies. □Graptolites, Ordovician, biostratigraphy, Texas, biofac

ISSN:0024-1164    

DOI:10.1111/j.1502-3931.1995.tb01601.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY

摘要:

The various past uses of the assemblage‐zone and biozone concepts are reviewed and a modern definition of biozone is given. Silurian biozones are related to the recentideas on standard reference (graptolite) zones (SRZs) and primary biozones (PBs). Use of evolving lineages, interregna, numerical notation systems, and chronozones are discussed. Precision in correlation of the Silurian biozones is described in terms of the Ordovician‐Silurian boundary and the Wenlock‐Ludlow (Silurian) boundary. A number of recommendations are made with respect to all zonal concepts, essentially giving prime importance to the use of biozones. □Biozones, standard reference zones, primary biozones, chronozones, grap

ISSN:0024-1164    

DOI:10.1111/j.1502-3931.1995.tb01602.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY

ISSN:0024-1164    

DOI:10.1111/j.1502-3931.1995.tb01603.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY

摘要:

The in‐life and post‐mortem orientations of the Lower Devonian brachiopodMeristella atokafrom the Haragan Formation (Lower Devonian; south‐central Oklahoma) are inferred from the distribution of epizoic bryozoans and the orientations of base plates of epizoic corals. Three bryozoans,Cyphotrypa corrugata, Fistuliporella maynardiandLeioclema pulchellum, and one coral,Favosites conicus, are considered. Most zoaria that contact the commissure terminate at the commissurc, and a few zoaria terminate at growth lines. This suggests that the bryozoans were primarily life associates ofMeristella atoka.Collectively, these three bryozoan species most extensively encrusted marginal sectors of the brachial valve and are very rare on the posteromedian sector (the umbo) of the pedicle valve. This distributional pattern indicates that the preferred living orientationof Meristella atoka wasumbo‐down (posteromedian sector of the pedicle valve resting on or buried in the substratum) with the commissure steeply inclined to the sediment water interface. Most coralla ofFavosites conicusthat contacted the commissure encrusted over the commissure. This indicates thatFavosites conicuseither preferentially encrustedMeristella atokapost‐mortem or colonized living brachiopods but subsequently caused them to die. Furthermore,Favosites conicusmost extensively encrusted anterior sectors of the brachial valve, especially the fold. The lateral and anterior orientations of the commissure with respect to the base plates (holothecae) ofFavosites conicusindicates that the brachiopods were oriented approximately horizontal with respect to the base plates. This suggests that the preferred post‐mortem orientation ofMeristella atokawas resting nearly horizontally on the substratum. These data and interpretations confirm previously inferred in‐life orientations ofMeristella atokaand are consistent with post‐mortem orientations hypothesized for elongate‐oval athyrid brachiopods. □Brachiopoda, Athyridacea, Meristella atoka, palaeoecology, autecology, epizoans, bryozoans, corals, sedimentology, Lower Devonian, Nort

ISSN:0024-1164    

DOI:10.1111/j.1502-3931.1995.tb01604.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY

ISSN:0024-1164    

DOI:10.1111/j.1502-3931.1995.tb01605.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY

摘要:

A remarkable diversification of several independent ammonoid lineages with high evolutionary rates occurred in the Late DevonianWocklumeriaStufe. Many speciation events led to paedomorphic ammonoids that display a striking range of conch shapes, sculpture, and ornamentation. In the goniatite family Prionoceratidae, the transition from normalMimimitocerasspecies to paedomorphicBalviaspecies provides an example of rapid size decrease combined with an early character developmental offset arising from progenesis. Adults of earlyBalviaspecies largely have the preadult ancestral morphology ofMimimitoceras, but later evolving species acquire distinct conch and ornamentation types. Progenetic ammonoid species also appeared within the clymeniid family Kosmoclymeniidae and probably in the Glatzielliidae. In the clymeniid family Parawocklumeriidae, evolution is characterized by the extension of tri‐segmented and triangularly coiled whorls found only in juveniles of earlier species, to the adults of later species. This is interpreted as resulting from neoteny. The distribution of paedomorphic ammonoids in the Late DevonianWocklumeriaStufe is closely correlated to relative sealevel changes. The regressive trend in the lower two‐thirds of theWocklumeriaStufe is interpreted as the cause of a diversification of the pelagic habitat during unstable conditions, and as an extrinsic factor inducing heterochronic change. Some ammonoids reacted by rapid maturation and faster reproductive rates, giving the opportunity to exploit a wider range of niches. The apparent consequence was the formation of several allopatric species. □Ammonoidea, Late Devonian, evolution, heterochrony, sealevel ch

ISSN:0024-1164    

DOI:10.1111/j.1502-3931.1995.tb01606.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY

ISSN:0024-1164    

DOI:10.1111/j.1502-3931.1995.tb01607.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY

摘要:

The widespread assumption that sutural complexity in ammonites is mainly proportional to water depth is revisited. Fractal analysis has been used for the precise morphometric evaluation of sutural complexity in 131 Upper Jurassic ammonites. Suture lines belonging to twelve families have been analyzed, account being taken of shell structure (coiling, shape of whorl section), sculpture and paleoenvironments. Fractal dimensions obtained in epicontinental and epioceanic ammonites show the unlikelihood of precise relationships between suture complexity and depth, and/or the absence of major differences in habitat depth if bathymetry played any significant role in the configuration of intricate septa. Suture complexity appears to be better related to shell structural types. □Fractal analysis, ammonite sutures, Upper Jurassi

ISSN:0024-1164    

DOI:10.1111/j.1502-3931.1995.tb01608.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY

ISSN:0024-1164    

DOI:10.1111/j.1502-3931.1995.tb01609.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY