摘要:

The genusArchaeoellipsoidesHorodyski&Donaldson comprises large (up to 135 μ long) ellipsoidal and rod‐shaped microfossils commonly found in silicified peritidal carbonates of Mesoproterozoic age. Based on morphometric and sedimentary comparisons with the akinetes of modern bloom‐formingAnabaenaspecies,Archaeoellipsoidesis interpreted as the fossilized remains of akinetes produced by planktic heterocystous cyanobacteria. These fossils set a minimum date for the evolution of derived cyanobacteria capable of marked cell differentiation, and they corroborate geochemical evidence indicating that atmospheric oxygen levels were well above 1% of present day levels 1,500 million years ago.Akinetes, atmospheric oxygen, cyanobacteria, heterocyst, microbial fossils, nitrogen fixation, peritidal, Protero

ISSN:0024-1164    

DOI:10.1111/j.1502-3931.1995.tb01817.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY

摘要:

A tiny arthropod with a thin, possibly poorly mineralized, bivalved carapace and a pair of annulated, uniramous, probable frontal appendages is described from lower Ordovician marine mudstones in boreholes from central England. It represents only the fifth Ordovician example of a conservation deposit with soft integument preserved. Its systematic position is unresolved, but it may belong to the Ostracoda; if so, it is a rare example of an ostracod with fossilized appendages.Arthropoda, Ostracoda, appendages, Tremadoc Series, Ordovician, England.

ISSN:0024-1164    

DOI:10.1111/j.1502-3931.1995.tb01818.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.tb01819.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY

摘要:

Two common Upper Ordovician crinoids,XenocrinusandDendrocrinus, had distinctive columns that showed marked contrasts of both form and function.Xenocrinus baeri(Meek) had a tetragonal facet geometry but functioned in the normal manner for a symplectially articulated column. Column flexure was by bending, the four directions of curvature being limited by the tetragonal arrangement of crenulae. In contrast, columns ofDendrocrinus caseiMeek, with a characteristic pentastellate symmetry, were able to twist, a functional adaptation not previously reported from crinoids; this was controlled by the unusual geometry of the crenularium. This suggests that some or all crinoid columns may be subject to twisting stresses, perhaps associated with changes in the current direction, that crenulae may resist passively.Functional morphology, column, crinoids, Xenocrinus, Dendrocrinus

ISSN:0024-1164    

DOI:10.1111/j.1502-3931.1995.tb01820.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.tb01821.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY

摘要:

The base of theUndulograptus austrodentatusBiozone appears to be a synchronous event that is widely recognizable within graptolitic facies around the world. It occurs within an interval in which graptolite species ranges are now well known and in which there is a rapid turnover in the composition of graptolite faunas. This turnover reflects the rapid evolutionary radiation of the Diplograptacea simultaneously with the appearance of several distinctive pseudisograptid and glossograptid species. These events provide the basis for the recognition of two thin but widely applicable subzones; a lowerArienigraptus zhejiangensisSubzone and an upperU. sinicusSubzone. The occurrence of the lower boundary of theU. austrodentatusBiozone within a succession of first appearances also permits accurate and reliable identification of the boundary as well as assessment of stratigraphic completeness across the boundary interval in correlated sections. Diverse graptolite faunas of late Yapeenian and early Darriwilian age occur in association with theHistiodella altifronsBiozone of the North American midcontinent conodont zonation and theParoistodus originalisandMicrozarkodina parvabiozones of the North Atlantic conodont zonation. They also occur in association with the shelly‐fossil zonations developed for several different continents. These features of the base of theU. austrodentatusBiozone make it a suitable level for use as the boundary level for a global stage. Its stratigraphic position within the Ordovician System relative to other likely global stages as well as its coincidence with one of the major events in graptolite evolutionary history suggest that this level also may be a suitable level for the base of a global Middle Ordovician Series.Ordovician System, Ordovician stages, graptolite zonation, chronostratigraphy, international correlation.Charles E. Mitchell and Jörg Maletz, Department of Geology, State University of New York at Buffalo, Buffalo, New York 14260‐1550, USA; 13th July, 1994; revised 22nd May,

ISSN:0024-1164    

DOI:10.1111/j.1502-3931.1995.tb01822.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.tb01823.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY

摘要:

Morphometric analysis of growth inAulacopleura koninckireveals several important features: (1) morphological variability is approximately constant throughout growth, increasing slightly in later ontogeny; (2) shape is more tightly constrained than the numbers of postcephalic segments, which can be quite variable; and (3) a major ontogenetic transition occurs at glabellar lengths of about 1.7 mm. This transition divides the ontogeny into two distinct growth phases, is smooth rather than abrupt, and is expressed as changes in growth trajectories, especially in the pygidium. The transition is not strictly correlated with the number of thoracic or pygidial segments. These results suggest a re‐evaluation of the concepts of meraspid and holaspid growth stages inA. konincki, using growth trajectories rather than thoracic segment number to define the stages. Developmental flexibility in holaspid segment numbers in this phylogenetically advanced trilobite suggests that environmental factors, rather than phylogenetic constraint, may explain variations in segment production patterns within some advanced Trilobita.Morphometrics, trilobites, ontogeny, variability, segmentation, Silurian, Czech Republi

ISSN:0024-1164    

DOI:10.1111/j.1502-3931.1995.tb01824.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.tb01825.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY

摘要:

Three fragmentary maxillae and an isolated upper molar from the Upper Eocene locality of Wai Lek (Krabi Basin, South Thailand) belonging to the small anthracotheriid Siamotherium krabiense display strongly abnormal morphologies that are particularly unusual among the dental anomalies recorded in both extant and fossil mammals. Variation and atavism can be ruled out, and the other hypothetical origins of these anomalies, particularly the occurrence of odontomes and inbreeding, are discussed.Dental anomalies, Anthracotheriidae, Asia, atavism, variation, inbreeding.Stéphane Ducrocq and Jean‐Jacques Jaeger, Laboratoire de Paléontologie, Institut des Sciences de l'Evolution, Case Courrier 064, Université Montpellier II, Place Eugéne Bataillon, F‐34095 Montpellier Cédex 5, France (SD's current address: Staatliches Museum für Naturkunde, Rosenstein 1, D‐70191 Stuttgart, Germany); Yaowalak Chaimanee and Varavudh Suteethorn, Department of Mineral Resources, Geological Survey, Rama VI Road, Bangkok 10400, Thailand; 14th November, 1994; revised 27th

ISSN:0024-1164    

DOI:10.1111/j.1502-3931.1995.tb01826.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY