摘要:

Graptoblasts are redefined as widenings of the sealed terminal portions of graptolite stolothecae resulting in ovoid bodies composed of an outer fusellar layer, named here as blastotheca, and an inner secondary layer of electron dense material, named the blastocrypt. In their proximal portion a number of graptoblasts display a scar with fuselli stripped and the crassal material of the blastotheca exposed and displaying a rough and dull surface. The boundary of this area, termed the talus, is edged by a distinct escarpment, probably the margin of the preserved part of the blastotheca. The outer wall of the talus is usually abruptly terminated. Only exceptionally is the outer wall conical and gradually widening. Thus the transverse partition between the parental stolotheca and the graptoblast was produced by the blastocrypt without fusellar covering. It seems, therefore, that graptoblasts were most probably lacking any internal portion of their blastothecae which could possibly be compared with the internal parts of a dendroid auto‐theca. It is concluded that graptoblasts housed a zooid that never functioned as an autozooid, but suffered arrested development and encystation. The biological role of graptoblasts as dormant bodies is discusse

ISSN:0024-1164    

DOI:10.1111/j.1502-3931.1986.tb00718.x

出版商:Blackwell Publishing Ltd    

年代:1986

数据来源: WILEY

ISSN:0024-1164    

DOI:10.1111/j.1502-3931.1986.tb00719.x

出版商:Blackwell Publishing Ltd    

年代:1986

数据来源: WILEY

摘要:

Late Paleozoic faunas found in sediments of dysaerobic origin represent a unique community type that remained essentially unchanged from the Middle Devonian to the Early Permian. This dysaerobic community had the following unique characteristics: (1) dominance by vagile molluscs, with suspension‐feeding brachiopods, bryozoans, corals and echinoderms usually subordinate; (2) a trophic structure dominated by deposit feeders and carnivorous or scavenging cephalopods; (3) small body size as the result of high juvenile mortality and possibly stunting; and (4) preservation of most individuals as pyritic stein‐kerns. All these characteristics are directly linked to the low oxygen levels of the dysaerobic environment. The relative stability of the deeper water, dysaerobic environment is consistent with Sanders' Stability‐Time Hypothesis. The dysaerobic environment apparently offered a refuge for more archaic biotas that evolved in nearshore environments of the early Pale

ISSN:0024-1164    

DOI:10.1111/j.1502-3931.1986.tb00720.x

出版商:Blackwell Publishing Ltd    

年代:1986

数据来源: WILEY

ISSN:0024-1164    

DOI:10.1111/j.1502-3931.1986.tb00721.x

出版商:Blackwell Publishing Ltd    

年代:1986

数据来源: WILEY

摘要:

Evaluation of taphonomic sample bias in tempestites is an important prerequisite for paleoecologic analysis in storm‐dominated shelf settings. Data from tempestites in the Upper Cambrian Bison Creek Formation of southern Alberta indicate that size, sorting greatly influences trilobite relative abundance patterns. Shifts in relative abundances of taxa both within and between beds are matched by changes in trilobite size. That is, collections fall into two intergrading types of assemblage which are dominated by taxa which reach large and small adult sizes, respectively. These shifts in dominance are interpreted as taphonomic variability within a single trilobite biofacies. Biofacies differentiation is expressed as abrupt changes in relative abundance patterns that are independent of trilobite size. Qualitative general models for size‐sorting in tempestites are developed. Size‐sorting in graded tempestites produces vertical shifts in relative abundances of taxa that mimic those expected in short‐term autogenic ecologic succession. Size‐sorting between beds in a single section could lead to an overestimation of the number of biofacies present. Size‐sorting between proximal and distal portions of a depth gradient might generate abundance patterns that mimic those expected for ‘depth‐related communities’, even in the complete absence of any underlying ecologic differentiation. The models represent taphonomic null hypotheses which must be rejected prior to making paleoecologic inferences in storm‐dominated shelf environments. 0 Taphonomy, tempestites, Trilobita, Upper Cam

ISSN:0024-1164    

DOI:10.1111/j.1502-3931.1986.tb00722.x

出版商:Blackwell Publishing Ltd    

年代:1986

数据来源: WILEY

摘要:

A cluster of thirteen elements of Panderodus unicosfatus (Branson&Mehl) from the Llandoverian of Po‐dolia, Ukraine, is shown to represent an almost complete apparatus; only one element is lacking. The restored original arrangement of the elements is consistent with the model of the conodont apparatus with seven pairs of elements ordered in two parallel rows, with cusps opposing in every pair. The apparatus of Panderodus is closely similar to that of the Cambrian ‘protoconcdonts’ and their Recent chaetognath successors. Nevertheless, general trends in the evolution of elements of the Panderodontidae towards increased elongation, smoothness of the cusp curvature, and depth of the basal cavity indicate that the grasping apparatus of Panderodus developed independently of, and convergently to, that of the Chaetog

ISSN:0024-1164    

DOI:10.1111/j.1502-3931.1986.tb00723.x

出版商:Blackwell Publishing Ltd    

年代:1986

数据来源: WILEY

ISSN:0024-1164    

DOI:10.1111/j.1502-3931.1986.tb00724.x

出版商:Blackwell Publishing Ltd    

年代:1986

数据来源: WILEY

摘要:

Experimental work has been conducted on the formation of plant compression fossils using a simple compression apparatus in which actual plant material was compressed in wet sediment. Pressure was applied through a single perforated piston which only allowed the passage of water upwards out of the compacting sediment. Several lines of investigation were followed to establish the principal factors that dictate the form of a plant compression fossil. It is demonstrated that these include the grain size and the compressibility of the sediment, the degree of sediment fill of hollow structures within the plant organ prior to the compression, and finally the rigidity or degree of decay of the plant tissues before the deformation. The results of this work indicate that the horizontal dimensions of the plant organ change during compression. This is directly related to the compressibility of the sediment the plant was buried in; stems buried in clay matrices increased in horizontal dimensions, whereas stems buried in sand matrices underwent reductions in horizontal dimensions. This is associated with the difference in the response of these sediments to the hydrostatic stress that exists within the system during the initial stages of compaction. The results of this experimental study are used to interpret the structure and form of some plant compression fossils.

ISSN:0024-1164    

DOI:10.1111/j.1502-3931.1986.tb00725.x

出版商:Blackwell Publishing Ltd    

年代:1986

数据来源: WILEY

ISSN:0024-1164    

DOI:10.1111/j.1502-3931.1986.tb00726.x

出版商:Blackwell Publishing Ltd    

年代:1986

数据来源: WILEY

摘要:

A set of 18 measurements of the dermal armour of Bothriolepis canadensis Whiteaves (Placodermi, Anti‐archa) is analysed with respect to allometric growth patterns. The strongest allometric patterns were found for the orbital fenestra and premedian plate of the head‐shield. and the anterior median dorsal plate of the trunk‐shield. These are all areas of the greatest importance in antiarch phylogeny and imply a role for ontogenetic effects such as paedomorphosis in the evolution of antiarchs. It is suggested that this is partly a result of the severe constraints on growth in a closed box such as the armour of placoderms, and may be generally true of such arrange

ISSN:0024-1164    

DOI:10.1111/j.1502-3931.1986.tb00727.x

出版商:Blackwell Publishing Ltd    

年代:1986

数据来源: WILEY