摘要:

Size‐frequency analysis of over 5,000 Ordovician trilobites from the Teretiusculus Shales of the Builth inlier, central Wales, has revealed size distributions with counter intuitive shapes. Not only do most species show normal or slightly skewed distributions, despite the preponderance of moults, but there is no evidence of instar peaks. Such features can, however, be explained by reference to steady‐state population structures of Recent marine arthropods, in which small individuals often form only a minor proportion of the post‐larval population structure. Trilobite steady‐state population structures would have differed in detail from species to species, but certain distribution shapes may have been characteristic of particular environments. These findings necessitate a reappraisal of previous work on trilobite size‐frequency distributions, survivorship and recognition of instars. The Builth data also show the first clear evidence of phyletic size increase and parallel size changes in trilobites. ▭ Trilobites, size‐frequency distributions, steady‐state populations, instars, phylet

ISSN:0024-1164    

DOI:10.1111/j.1502-3931.1988.tb01759.x

出版商:Blackwell Publishing Ltd    

年代:1988

数据来源: WILEY

摘要:

New material of early growth stages of the Silurian (Llandovery) trilobite Acernaspis is described. Pre‐adult ontogenetic stages of this genus closely resemble adults of the post‐Llandovery genus ***Ananaspis. A heterochronic descent of Ananaspis from Acernaspis is proposed. ***Ananaspis is interpreted as pae‐domorphic, having arisen largely through neoteny. Neotenic changes already appear in the lineage in the last Acernaspis species, and Ananaspis then underwent continuous ncotcnic change throughout its known Silurian history. ▭ Heterochrony, neoteny, Silurian, Trilobita, Phacopidae, Acernaspis. An

ISSN:0024-1164    

DOI:10.1111/j.1502-3931.1988.tb01760.x

出版商:Blackwell Publishing Ltd    

年代:1988

数据来源: WILEY

摘要:

Emsian (upper Lower Devonian) limestones exposed on Kasaan, Round, and Wadleigh Islands in southeastern Alaska are part of an allochthonous suitc of island are deposits preserved within the accrctionary Alexander Terrane. Ten benthic marine communities, including several new brachiopod associations, are defined on the basis of field data correlated with a cluster analysis. Biologic and sedimentologic evidence is integrated with community group assignments in order to substantiate the paleoecology and evolutionary history of each community. These faunas were adapted to a spectrum of quiet and rough water habitats, including restricted, shallow subtidal areas nearshore, offshore biostromal banks, and open and restricted portions of a lagoon or shelf. Comparisons with coeval assemblages from the Cordillera and elsewhere reveal that these communities are unique to southeastern Alaska. Their origin within an early Paleozoic island are appears to have led to biogeographic isolation from other parts of the Cordillera and the development of a distinct biota at a site offshorc from the ancient continental margin. ▭Emsian, paleocommunities, community groups, cluster analysis, Alexander Terran

ISSN:0024-1164    

DOI:10.1111/j.1502-3931.1988.tb01761.x

出版商:Blackwell Publishing Ltd    

年代:1988

数据来源: WILEY

摘要:

The presence of organisms whose bodies have low preservation potential may be deduced by searching for the traces produced by them. The addition of predatory gastropods and soft‐bodied epizoans to Quaternary marine faunas dominated by bivalves was facilitated by an examination of borings in bivalve shells. Borings attributed to predatory gastropods (ichnogenusOichnus) were observed in shells ofAstartespp.,Hiatella arcticaandMacoma calcarea. Astarte, HiatellaandMacomawere preyed upon in preference to other members of a diverse suspension‐feeding bivalve community. Borings attributed to epizoans (ichnogenusCautostrepsis) were observed in bivalve shells(Astartespp.Hiatella arctica), calcareous algae and limestone clasts. Biotic interactions revealed by trace fossils are employed, for the first time, to reconstruct the trophic structure of arctic Quaternary marine benthic faunas. ▭ Arctic molluscs, palaeoecology, Oichnus, Caulostr

ISSN:0024-1164    

DOI:10.1111/j.1502-3931.1988.tb01762.x

出版商:Blackwell Publishing Ltd    

年代:1988

数据来源: WILEY

摘要:

In recent ecological studies, opportunistic species have been defined as those species that can respond rapidly to an unexploited niche. Such species are characterized by a lack of equilibrium population size, a density independent mortality, an ability to increase abundance rapidly, a relatively poor competitive ability, high dispersal ability, and devoting a high proportion of resources to reproduction. The occurrence ofSkolithosin the Cardium Formation (Turonian) provides ichnofossil evidence for opportunistic behavior in the geological record. The Cardium Formation in the Pembina oilfield area of west‐central Alberta consists predominantly of mudstones. which coarsen upward into fine‐grained sandstone beds; the sequence is interpreted as a storm‐dominated shelf deposit. The mudstones contain a deposit‐feeding iehnofossil assemblage indicative of deposition in low energy conditions:Thalassinoides, Chondrites, Zoophycos, Terebellina, Rhizocorallium, Planolites, TeichichnusandAsterosoma.These mudstones are interrupted by frequent sharp‐based thin sandstone beds. which commonly contain abundantSkolithosand which are the result of rapid storm deposition. Such storm activity would be capable, through scouring and rapid deposition, of completely destroying the normal benthic fauna Modern studies of opportunistic recolonization have shown that the most common opportunistic colonists in open marine settings are tube‐dwelling or shallow‐burrowing suspension‐feeding polychactes. Opportunistic forms are frequently smaller and found in higher densities than the equilibrium counterparts. TheSkolithosfrom the storm sandstones of the Cardium Formation are thus inferred to represent the activities of shallow‐burrowing opportunistic colonists following storm depopulation. ▭Opportunistic, ichnology, S

ISSN:0024-1164    

DOI:10.1111/j.1502-3931.1988.tb01763.x

出版商:Blackwell Publishing Ltd    

年代:1988

数据来源: WILEY

摘要:

Straight‐sided, tubular fossils occur together with a diverse assemblage of deep‐water trace fossils in linegrained Franciscan turbidites at Point Saint George. Pacific Coast of California. Based on comparison with living abyssal organisms and on skeletal microstructure, these tubes are interpreted as recrystallized siliceous tests of the large abyssohenthic foraminiferid,Bathysiphon.Tubes found preserved in‐place show that life orientation of the Franciscan species was vertical with the broader, mature end protruding several millimetres above the muddy seafloor. Positioned in this way the organisms probably were suspension feeders or carnivores.Bathysiphonand trace fossils taken together reflect a diverse deep‐water benthic community that was disrupted (or locally eliminated) repeatedly by turbidity currents reaching the outer margins of a submarine fan. ▭ Bathysiphon,deep‐water benthos, outer fan/basin floor, trace fossils, Franciscan Complex, flysch,

ISSN:0024-1164    

DOI:10.1111/j.1502-3931.1988.tb01764.x

出版商:Blackwell Publishing Ltd    

年代:1988

数据来源: WILEY

ISSN:0024-1164    

DOI:10.1111/j.1502-3931.1988.tb01765.x

出版商:Blackwell Publishing Ltd    

年代:1988

数据来源: WILEY

摘要:

In the animal kingdom evolutionary size changes involved increasing, decreasing and stationary patterns. Planktic and benthic Foraminifera chiefly increased their size during evolution. This increase, however, did not always occur gradually, but could be interrupted by periods when the animals maintained or even decreased in size. The rate of the size increase is different for the various species examined, some benthic forms grew only 10% during the Oligocene‐Pleistocene interval, while for others this figure was up to 96%. Some benthic species increased in size in certain areas, but not in others. It is not improbable that some phylogenetic trends of planktic Foraminifera representing, according to stratigraphers, the evolution of one species into another, represent in reality, from the biological point of view, specimens of the same species which changed their size and in addition some minor morphological traits which are encompassed by the normal span of intraspecific variability. A comprehensive understanding and explanation of the size change of Foraminifera needs much additional research. ▭Foraminifera, size cha

ISSN:0024-1164    

DOI:10.1111/j.1502-3931.1988.tb01766.x

出版商:Blackwell Publishing Ltd    

年代:1988

数据来源: WILEY

摘要:

Studies of cuttlebone ofSepia officinalisL. from the English Channel show a direct and linear correlation between ambient water temperature and septal spacing. The application of this observation to fossil shallow‐water nautiloids and ammonoids is complicated by the equatorial habitat of many of these shells, their declining rate of chamber production through ontogeny and a paucity of large and complete specimens. But annual temperature variations are the most likely cause of variations in septal spacinga priori, temperature apparently being the prime determinant of feeding and growth rates of individuals at a particular ontogenetic stage. ▭Nautiloids, annual growth, Ordovician, Sepia,English Chan

ISSN:0024-1164    

DOI:10.1111/j.1502-3931.1988.tb01767.x

出版商:Blackwell Publishing Ltd    

年代:1988

数据来源: WILEY

摘要:

Only ten gastropods have been recovered and a single hyolith found in the radiographic study of the Hunsrück fauna. Four gastropods are interpreted as retaining soft parts: the radula may be identified in one and perhaps the liver and digestive organs in the others. These gastropods may have moved into the area on floating vegetation. The hyolith probably lived on the bottom. Soft parts are present within its shell, but they are not readily interpreted. A tentacular mass anterior to a possible mantle cavity is present, with the bulk of the shell being filled by enigmatic tissue. ▭Gastropoda, Hyolitha, soft parts, radula, pyritizati

ISSN:0024-1164    

DOI:10.1111/j.1502-3931.1988.tb01768.x

出版商:Blackwell Publishing Ltd    

年代:1988

数据来源: WILEY