摘要:

The word ‘stromatolite’ should only be applied to organosedimentary structures predominantly accreted by sediment trapping, binding and/orin situprecipitation as a result of the growth and metabolic activities of benthic, principally prokaryotic, micro-organisms. Structures of uncertain origin that resemble stromatolites should be called ‘stromatoloids’. This cautious approach would eliminate the currently common assumption that structures with mesoscopic morphological similarities to microbially accreted sedimentary structures must be biogenic, a misconception that hampers investigations into the antiquity of life.

ISSN:0311-5518    

DOI:10.1080/03115518108566999

出版商:Taylor & Francis Group    

年代:1981

数据来源: Taylor

摘要:

The Dunderberg Shale and Halfpint Members of the Nopah Formation in the southern Nevada portion of the Great Basin consist of interbedded shale, limestone, and dolostone. In four measured sections these two members range in thickness from 23 to 82 m and contain abundant sedimentary structures and macro- and microfossils. Trilobites and inarticulate brachiopods indicate the rocks are of medial Late Cambrian (Dresbachian and Franconian) age and correlate with other sections described from the Great Basin. Conodonts were found in limestones and dolostones that contain trilobites of theDunderbergiaand overlyingElviniaBiozones; approximately 3,740 conodonts were recovered and were assigned to one protoconodont and 13 paraconodont single-element form-species. New form-species ofFurnishinaandProoneotoduswere recognized but not named. Characteristics and distribution of the lithofacies, fossils and organo-sedimentary structures indicate that subtidal sediments formed towards the northwest on a broad, shallow, westward sloping shelf, and intertidal channel and algal bank deposits formed towards the southeast in a craton margin setting.

ISSN:0311-5518    

DOI:10.1080/03115518108567000

出版商:Taylor & Francis Group    

年代:1981

数据来源: Taylor

摘要:

The lower part of the Bungonia Limestone in the Bungonia-Marulan South area, N.S.W., has previously been dated as Ludlovian. New fossil localities from the upper part of the formation yield faunas includingScabriscutellumsp. cf.S. scabrum, Schizophoria, Eospirifer eastoniandSpathognathodussp. cf.S. remscheidensiswhich indicate a Lochkovian age. The Bungonia Limestone is intruded by parts of the Marulan Batholith (398 ± 7m.y.) thus giving a minimum age for the Silurian-Devonian boundary.

ISSN:0311-5518    

DOI:10.1080/03115518108567001

出版商:Taylor & Francis Group    

年代:1981

数据来源: Taylor

摘要:

Heterochrony, that is phyletic change in the onset or timing of morphological development, is considered to have been the principal factor affecting the evolution of a number of xystridurine trilobite species during the Middle Cambrian.Galahetesshows morphological and size characteristics which indicate that precocious maturation, that is progenesis, was the process responsible for its paedomorphic evolution during the Templetonian Stage. The morphological characteristics of adultGalahetesparallel those found in juvenile stages ofXystridura templetonensis, whose morphology is essentially that of the conservative, structurally ancestral xystridurine stem form which persisted through the Ordian and Templetonian Stages. BothX. alteraandX. dunstaniare also considered to have evolved from this stem form by paedomorphosis during the Templetonian Stage. In these species, however, the process is thought to have been neoteny, whereby retardation of somatic development resulted in retention of characters which occurred in juveniles of their progenitors. Retardation in onset of maturity, which often accompanies neoteny, allowed the attainment of a larger size.

ISSN:0311-5518    

DOI:10.1080/03115518108567002

出版商:Taylor & Francis Group    

年代:1981

数据来源: Taylor

摘要:

Five faunal provinces based on vertebrates are proposed for Early Devonian time, as follows: a cephalaspid province (Euramerica), an amphiaspid province (Siberia), a tannuaspid province (Tuva); a galeaspid-yunnanolepid province (South China), and a wuttagoonaspid-phyllolepid province (East Gondwana). It is suggested that certain major groups of early vertebrates differentiated in isolation on these continental areas as freshwater environments were invaded, and the interrelationships of some agnathans and placoderms endemic to these areas are analyzed using the cladistic vicariance biogeographic method of Platnick & Nelson (1978). Two possible episodes of biotic dispersal are identified; between Laurentia and Baltica at the end of the Silurian, and between Gondwana and Euramerica during the Late Devonian. Amongst placoderm fishes a Gondwana origin is suggested forPhyllolepis, antiarchs indicate affinity between Gondwana and South China, and euarthrodires may have evolved in Euramerica. Phylogenetic analysis of other test groups is required to resolve conflicting evidence of area interconnections provided by agnathans and placoderms, and to establish the existence of general patterns in the Devonian distribution of endemic vertebrate taxa.

ISSN:0311-5518    

DOI:10.1080/03115518108567003

出版商:Taylor & Francis Group    

年代:1981

数据来源: Taylor