摘要:

The phenomenon of iterative evolution of homeomorphs in identical temporal sequences is exhibited by Aalenian and Bajocian ammonite faunas in the south German depositional basin. Within each evolutionary cycle, inflated, evolute, and ornamented shells with complex suture lines are successively replaced with discoidal, involute, smooth shells with simpler sutures. Three distinct and identical cycles of morphological change occur, and involve ammonites belonging to four different familial/subfamilial groups: the Leioceratinae, Graphoceratinae, Hammatoceratidae, and Sonniniidae. Geometric and statistical analyses of morphological change within each of these higher taxa reveal remarkable iteration in both the pattern and proportion of change with time. The iterative morphological cycles were not driven by orthogenesis or anagenetic transformation within lineages, as has been previously suggested. A clear correlation exists between the successive ammonite faunas and repeated cyclical changes in the physical marine environment of the depositional basin. DAmmonoidea, Aalenian, Bajocian, south Germany, iterative evolution.

ISSN:0024-1164    

DOI:10.1111/j.1502-3931.1984.tb00661.x

出版商:Blackwell Publishing Ltd    

年代:1984

数据来源: WILEY

摘要:

Selected brachiopod specimens were placed on a well‐sorted medium grained sand (0.5 mm) in a recirculating flume and subjected to a constant unidirectional current of 26–27 cm/sec. The specimens were placed in each of six feasible life‐orientations. In three orientations, the valves were reclining with either the lateral, anterior or posterior profile upcurrent. In three orientations, the commissural plane was perpendicular to the substrate with either the pedicle or brachial valve upcurrent, or a lateral margin upcurrent. Destabilization indices were calculated for each specimen in each orientation based on the elapsed time until the specimen reoriented. Nonstrophic biconvex brachiopods are most stable with the valves reclining on the substrate. Among orientations with the valves erect, the pedicle valve upcurrent and the brachial valve upcurrent are the most and least stable orientations, respectively. Shell stability increased among nonstrophic specimens that were more equi‐biconvex. Biconvex strophic brachiopods are also more stable if the valves were equi‐biconvex, cither compressed (biplanate) or inflated, provided that the specimen was propped up on its hingeline with a lateral margin upcurrent. Catacline interareas afforded greater stability than procline, apsacline or orthocline interareas when the valves were perpendicular to the substrate. Accentuated plication reduced shell stability. Alate biconvex forms were often more stable if the valves were vertical to the substrate rather than reclining. Alate biconvex geometries are more stable than nonstrophic and strophic biconvex forms if the commissural plane is perpendicular to the substrate and parallel to the current. Broad catacline interareas increased stability in all orientations. Plano‐convex and dorsi‐biconvex alate forms are usually less stable than equi‐biconvex or ventri‐biconvex alate geometries. Concavo‐convex geometries are stable in all orientations except with the valves nearly vertical to the substrate and parallel to the current. Spines greatly retard sediment‐scour and maintain concavo‐convex specimens in orientations with the valves elevated above the substrates. DBrachiopoda, shell shape,

ISSN:0024-1164    

DOI:10.1111/j.1502-3931.1984.tb00662.x

出版商:Blackwell Publishing Ltd    

年代:1984

数据来源: WILEY

摘要:

Although patterns of dispersion are a fundamental attribute of modern populations, few attempts have been made to evaluate such spatial patterns in the rock record, evidently because of taphonomic uncertainties concerningin situpreservations of fossils. Unlike body fossils, however, ichnofossils almost inevitably remainin situ, and in some instances are indicative of contemporaneous populations of burrowers. Studies of dispersion among such ichnofossils, by means of the coefficient of dispersion and distance to nearest neighbour techniques, may reveal not only the spacing but also the specific trophic adaptations ‐ and even the general identity ‐ of the tracemakers. In this study, a population of the vertical shaftSkolithos linearisfrom the Lower Cambrian Bradorc Formation of Labrador exhibits an aggregated or clumped dispersal as a whole; yet individuals within the population exhibit random dispersion at low population densities and uniform dispersion at high population densities. These collective patterns, by analogy with modern tracemakers, suggest suspension feeders, either lophophorate phoronids or tentacular crowned polychaetes, as the ancient tracemaker. In contrast, aS. linearispopulation from the Middle Silurian Thorold Formation of Ontario exhibits random dispersion at all densities observed, suggesting that the ancient tracemaker was a polychaete that utilized either mucous nets or low, narrow tentacular crowns in suspension feeding. Populations of the vertical U‐shaped burrowDiplocraterionsp., from the Thorold Formation, exhibit uniform to random dispersion, suggesting that it represents a dwelling burrow of a suspension‐feeding polychaete. The uniform pattern resulted from a competition between individuals for living space. □Trace fossils, spatial distribution, Skolithos, Diplocraterion,trophic analysis. P

ISSN:0024-1164    

DOI:10.1111/j.1502-3931.1984.tb00663.x

出版商:Blackwell Publishing Ltd    

年代:1984

数据来源: WILEY

ISSN:0024-1164    

DOI:10.1111/j.1502-3931.1984.tb00664.x

出版商:Blackwell Publishing Ltd    

年代:1984

数据来源: WILEY

摘要:

The Late Ordovician paleogeographic changes in the Yangtze Platform indicate a widespread regression and consequent subaerial exposure probably caused by the ‘locking up’ of substantial amounts of marine waters during the Late Ordovician glaciation. The subsequent rapid paleogeographic changes on the Yangtze Platform during the earliest Silurian suggest a rapid melting of the ice. The relatively cool waters in the partially closed Yangtze basin during the latest Ordovician were replaced by open and warm waters during the earliest Silurian. Sedimentation rates and composition of faunal assemblages underwent marked changes during latest Ordovician and Early Silurian.Glaciation, Ordovician, Silurian, Paleogeography, sedimentary rate, faunal cha

ISSN:0024-1164    

DOI:10.1111/j.1502-3931.1984.tb00665.x

出版商:Blackwell Publishing Ltd    

年代:1984

数据来源: WILEY

ISSN:0024-1164    

DOI:10.1111/j.1502-3931.1984.tb00666.x

出版商:Blackwell Publishing Ltd    

年代:1984

数据来源: WILEY

摘要:

On the basis of precise trilobite examples, the difficulties and interest in cladistic analysis are emphasized. The problem of distinguishing plesiomorphic and apomorphic characters is exemplified inMorgatiawith reference to auxiliary impressions of the glabella and vincular furrows. InCrozonaspis, cladograms by themselves do not clarify the distinction of primitive versus derived features; the stratigraphical distribution of species, although not conclusive, is helpful in clarifying relationships. Although stratigraphical data are essential for reconstructing species lineages, they may be misleading when dealing with taxa above the species level. For all phylogenetical reconstructions, the completeness of the palaeontological and biogeographical record must be taken into consideration. DTrilobita, Phacopida, cladistics, Ordovician.

ISSN:0024-1164    

DOI:10.1111/j.1502-3931.1984.tb00667.x

出版商:Blackwell Publishing Ltd    

年代:1984

数据来源: WILEY

摘要:

A specimen of the dalmanitacean trilobiteRhenopssp. cf.R. anserinuswith ventral appendages is described and discussed. It is shown to have 1 pair of antennae, 4 pairs of cephalic legs, 11 pairs of thoracic legs, and 9(?) pairs of pygidial legs. The pygidial legs differ clearly from those of the thorax in their construction and are also smaller and more closely spaced. Of the outer appendage branch a few sets of lamellar spines are visible. DTrilobita, appendages, segmentation.Vom dalmanitiden TrilobitenRhenopssp. cf.R. anserinuswird ein Exemplar mit ventralen Extremitaten beschrieben und diskutiert. Es hat 1 Antennenpaar, 4 Kopfbeinpaare, 11 Thorakalbeinpaare, und 9(?) Pygidium‐Beinpaare. Die Pygidium‐Beinpaare unterscheiden sich in ihrer Konstruktion klar von den Thorakalbeinpaaren. Sie sind kiirzer, und der Abstand zueinander ist schmaler. Lamellendornen (sogen‐annte ‘Kiemenanhange’) sind an einigen wenigen Aussenasten

ISSN:0024-1164    

DOI:10.1111/j.1502-3931.1984.tb00668.x

出版商:Blackwell Publishing Ltd    

年代:1984

数据来源: WILEY

摘要:

The Crustoidea are important in discussions of graptolite origins, because they represent a morphoecological type intermediate between the Rhabdopleurida and the Dendroidea. TEM and SEM studies of their rhabdosomes provide new data on the structural variation displayed by the graptolite periderm. Since the crustoid fusellar fabric does not differ markedly from the fuseller fabric of other graptolites and is dissimilar to that of pterobranchs, it is supposed that the fundamental graptolite fibril pattern was attained at the crustoid stage of graptolite evolution. The crustoid cortex is made of paracortex (the multiple deposition of sheets separated by an intersheet material in the form of condensed meshwork of fibrous material) and of pseudocortex (the accumulation of sheets and intersheet material devoid of fibrous character). The presence of sporadically genuine cortex is noted. Presumably the mechanisms of cortical fibrilogenesis were attained at the crustoid stage, but only in an incipient and incomplete form. Modes of periderm corticization arc discussed. Sheets, common elements in secondary deposits of pterobranchs and graptolites, arc compared, but some problems still remain unclear. ?Crustoidea, Pterobranchia, phytogeny, ultrastructure, cortex, morphogenesis, homology.

ISSN:0024-1164    

DOI:10.1111/j.1502-3931.1984.tb00669.x

出版商:Blackwell Publishing Ltd    

年代:1984

数据来源: WILEY

摘要:

The three plates characteristic ofAptychopsishave been described as a jaw apparatus in this orthocone cephalopod as well as an operculum for it. Due to the limited material, interpretations have in various cases been made from photographic illustrations and those results are discussed in this article. The area of the aptychopsid plates is compared with the apertural area of the cephalopod and the results show a great similarity. Arguments for and against soft parts outside the aptychopsid plates are discussed as well as the position of the plates. Material from a Silurian shale in southern Sweden is described. ? Aptychopsis,aptychopsid plates. Cephalopoda, operculum, jaw apparatus, Silurian.

ISSN:0024-1164    

DOI:10.1111/j.1502-3931.1984.tb00670.x

出版商:Blackwell Publishing Ltd    

年代:1984

数据来源: WILEY