摘要:

Raup, D. M.: Depth inferences from vertically imbedded cephalopods.Vertically imbedded cephalopods are not uncommon in the fossil record. Experiments with modernNautilusshow that after death the shell cannot maintain a vertical orientation on the sea floor unless the depth is less than about 10 m. At greater depths, hydrostatic pressure causes flooding of the phragmocone sufficient to make the shell fall over. The depth limit applies whether the shell floats after death or not. Calculations made on data from six ammonite species indicate that the same depth limit can be applied to fossil coiled cephalopods. Vertical preservation can occur in deeper water only if the shell is vertically oriented upon impact and if the sediment is such as to trap the shell in that position.

ISSN:0024-1164    

DOI:10.1111/j.1502-3931.1973.tb01195.x

出版商:Blackwell Publishing Ltd    

年代:1973

数据来源: WILEY

摘要:

Heinberg C.: The internal structure of the trace fossilsGyrochorteandCurvolithus.An extremely mica‐rich sandstone from the Lower Cretaceous of Milne Land, East Greenland has yielded new data on the internal construction of certain trace fossils. The sediment contains the following forms:Gyrochorte, Curvolithus, Siphonites, Planolites, and small endichnia. Mica orientation withinCurvolithusandGyrochortepermits a detailed analysis of the manner in which the animals burrowed in order to produce the characteristic back‐fill structures. Weiss' interpretation ofGyrochorteas the burrow of a worm which progressed obliquely through the sediment, is supported on the basis of the internal struct

ISSN:0024-1164    

DOI:10.1111/j.1502-3931.1973.tb01196.x

出版商:Blackwell Publishing Ltd    

年代:1973

数据来源: WILEY

摘要:

Halleck, Margaret S.: Crinoids, hardgrounds, and community succession: The Silurian Laurel‐Waldron contact in southern Indiana.The uppermost surface of the Silurian Laurel Limestone at its contact with the Waldron Shale in southeastern Indiana was a hardground lithified prior to the deposition of the Waldron. Evidence for this conclusion is the presence of attached palmate crinoid roots, auloporid corals, and craniid brachiopods on the Laurel surface; the irregularity of the contact with the Waldron; and a pyritic veneer at this contact. The hardground apparently had a submarine origin. In addition to the attached epifauna mentioned above, algal‐sediment ‘clods’ formed on this surface. Some of these accumulated around the crinoid stems, causing them to produce cirral extensions. The resulting community was a crinoid ‘meadow’ with algal growths forming sediment traps around and between the crinoids. Later stages of Waldron Shale deposition led to the development of a soft‐bot

ISSN:0024-1164    

DOI:10.1111/j.1502-3931.1973.tb01197.x

出版商:Blackwell Publishing Ltd    

年代:1973

数据来源: WILEY

摘要:

Amsden, T. W: The pseudodeltidium inTriplesiaandPlacotriplesia.North American species representingTriplesiarange from the Middle Ordovician into the Lower Silurian, and possess a distinctive pseudodeltidial apparatus. The pseudodeltidium begins as a solid ridge of shell material just in front of the pedicle foramen and extends forward, opening into a hoodlike structure which partly covers a delthyrial notch; the cardinal process shaft bears a well‐developed collar which fits into the delthyrial notch. In contrast, North American representatives ofPlacotriplesia, which are at present known only from early Late Silurian (Wenlockian) strata, have no delthyrial notch, and the pseudodeltidial ridge or fold is absent at all stages of growth; the cardinal process collar is vestigial or absent. Wright's observation that the type species ofPlacotriplesia, P. praecipta(Ulrich&Cooper) has a pseudodeltidial fold is incorrect, and his implication thatTriptesiaandPlacotriplesiaare connected by gradational forms is not supported by the evidenc

ISSN:0024-1164    

DOI:10.1111/j.1502-3931.1973.tb01198.x

出版商:Blackwell Publishing Ltd    

年代:1973

数据来源: WILEY

摘要:

Yochelson, E. L., Flower, R. H.&Webers, G. F.: The hearing of the new Late Cambrian monoplacophoran genusKnightoconusupon the origin of the Cephalopoda.Knightoconus, a new genus of the Hypseloconidae (Mollusca: Monoplacophora) from rocks of early Franconian age in Antarctica, is multiseptate. The multiple septa are a criticàl feature to be expected in a form ancestral to cephalopods. Fossil cephalopods, however, invariably have a siphuncle as well as septa; some gastropods, some hyolithids, and some monoplacophorans also have septa but lack a siphuncle. Therefore, only the siphuncle can be considered a unique and particularly significant feature of the cephalopod shell. Hypothetical reconstructions of molluscan anatomy support the notion that cephalopods may have been derived directly from a hypseloconid having a high, slightly curved, multiseptate, bilaterally symmetrical shell, by the subsequent development of a siphuncle

ISSN:0024-1164    

DOI:10.1111/j.1502-3931.1973.tb01199.x

出版商:Blackwell Publishing Ltd    

年代:1973

数据来源: WILEY

摘要:

[Anders Martinsson:] Editor's column: Cryptozoic and Phanerozoic.The use ofCryptozoicas the antithesis of the widely accepted termPhanerozoicis recommended.Precambrianhas been used to suppress the former term, but is less suited semantically and is derived from a different term system.

ISSN:0024-1164    

DOI:10.1111/j.1502-3931.1973.tb01200.x

出版商:Blackwell Publishing Ltd    

年代:1973

数据来源: WILEY