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| 2. |
- Budd, Graham E., et al.
(författare)
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The lobes and lobopods of Opabinia regalis from the middle Cambrian Burgess Shale
- 2012
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Ingår i: Lethaia. - : Scandinavian University Press / Universitetsforlaget AS. - 0024-1164 .- 1502-3931. ; 45:1, s. 83-95
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Tidskriftsartikel (refereegranskat)abstract
- Despite many papers devoted to it, the morphology of the Burgess Shale animal Opabinia regalis continues to excite controversy. In particular, the trunk region remains incompletely understood, leading to several recent attempts to interpret the fossil in radically different ways. New material of Opabinia from the Royal Ontario Museum and the Smithsonian collection, together with the recent description of comparative material of the Burgess Shale anomalocaridid Hurdia, help clarify details of its morphology, in particular with regards to the lateral lobes and setal blades. A recent reconstruction of the trunk lobes is rejected, and further evidence for the presence of trunk limbs is presented. Despite disagreements over its morphology, the phylogenetic placement of Opabinia is now relatively uncontroversial, although various derived aspects of its morphology complicate placing it precisely.
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| 3. |
- Ebbestad, Jan Ove R., et al.
(författare)
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Predation on bellerophontiform molluscs in the Palaeozoic
- 2009
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Ingår i: Lethaia. - : Scandinavian University Press / Universitetsforlaget AS. - 0024-1164 .- 1502-3931. ; 42:4, s. 469-485
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Tidskriftsartikel (refereegranskat)abstract
- Shell repair assumed to result from failed predation is documented in 66 specimens of Ordovician-Carboniferous bellerophontiform tergomyan and gastropod molluscs to examine the relationship between the distribution and appearance of injuries, shell morphology and the internal anatomy of the molluscs, as well as the attack strategies of the presumed predators. Furthermore, the distribution of repaired injuries from failed attacks along the apertural margin as a reflection of the nature of the margin and emarginations is investigated. Bellerophontiform molluscs are ideal for this study because of their distinctive isostrophic morphology and the possibility to directly compare broad and narrow conchs with either deep or shallow medial emarginations. The results show that taxa with a deep medial emargination in the form of a slit have significantly more medial injuries than lateral ones. Near-equal frequencies of lateral and medial injuries in specimens with a shallow emargination (slit or sinus) suggest random distribution. Shell form (narrow or broad) does not exert overall control on the distribution of injuries except, perhaps, in some broad explanate shells with an insignificant medial emargination. While this suggests that it is the type of medial emargination that governs distribution of injuries in these forms, it is not clear if this is a result of passive selection due to structural geometry or preferential targeting by predators (i.e. site-specific mode of attack). Predation strategies on bellerophontiform molluscs thus seem to be dependent on the morphological features of the shells rather than their interpretation as tergomyan or gastropod.
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| 4. |
- Frisk, Åsa M., et al.
(författare)
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Palaeoenvironmental aspects of Late Ordovician Sericoidea shell concentrations in an impact crater, Tvären, Sweden
- 2011
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Ingår i: Lethaia. - : Scandinavian University Press / Universitetsforlaget AS. - 0024-1164 .- 1502-3931. ; 44:4, s. 383-396
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Tidskriftsartikel (refereegranskat)abstract
- Numerous studies have reported the presence of the small, thin-shelled cosmopolitan rhynchonelliformean Sericoidea as being environmentally controlled and, together with its close relatives, characteristic of deep-water, distal, clastic Ordovician and Silurian settings. Assemblages of Sericoidea have been analysed from post-impact strata in a newly formed Late Ordovician impact crater. In the crater succession, colonization of benthic faunas can be monitored through the post-impact limestone, demonstrating a number of environmental preferences. Consequently, the crater, as a result of its restricted area, provides an experimental arena for faunal distributions to be correlated with specific environments. The continuous infilling of the crater following its formation reveals a transition from argillaceous mudstones to carbonates deposited in deeper-water environments to shallower regimes. Rhynchonelliformean brachiopods inhabited the crater depression very late after the impact and are entirely represented by the genus Sericoidea, occurring abundantly in the upper third of the existing crater infill. The deep-water regime that existed in the depression during the initial interval of crater formation had been substantially reduced. Clearly Sericoidea-bearing associations associated with shaly substrates did not merely favour and occupy deep-water environments as previously suggested. The unfavourable conditions triggered by the impact and the inhospitable aftermath allowed Sericoidea to exploit a less-crowded ecospace. This reorganization, following the catastrophe, from a deep-water related ecological niche to considerable shallower settings suggests that Sericoidea was a pioneer colonist displaying an opportunist r-strategy. The shell beds analysed are related to shallower water and this may, moreover, help unravel the dilemma of the general absence of Sericoidea in the deeper-water Foliomena fauna.
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| 5. |
- Luksevics, Ervins, et al.
(författare)
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Frasnian vertebrate taphonomy and sedimentology of macrofossil concentrations from the Langsede Cliff, Latvia
- 2012
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Ingår i: Lethaia. - : Scandinavian University Press / Universitetsforlaget AS. - 0024-1164 .- 1502-3931. ; 45:3, s. 356-370
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Tidskriftsartikel (refereegranskat)abstract
- The siliciclastic sequence of the Upper Devonian of Kurzeme, Western Latvia, is renowned for abundant vertebrate fossils, including the stem tetrapods Obruchevichthys gracilis and Ventastega curonica. During the first detailed taphonomic study of the vertebrate assemblage from the Ogre Formation cropping out at the Langsede Cliff, Imula River, abundant vertebrate remains have been examined and identified as belonging to one psammosteid, two acanthodian and three sarcopterygian genera; the placoderm Bothriolepis maxima dominates the assemblage. Besides fully disarticulated placoderm and psammosteid plates, separate sarcopterygian scales and teeth, and acanthodian spines, partly articulated specimens including complete distal segments of Bothriolepis pectoral fins, Bothriolepis head shields and sarcopterygian lower jaws have been found. The size distribution of the placoderm bones demonstrates that the individuals within the assemblage are of approximately uniform age. Distinct zones have been traced within the horizontal distribution of the bones. These linear zones are almost perpendicular to the dominant dip azimuth of the cross-beds and ripple-laminae and most probably correspond to the depressions between subaqueous dunes. Concavity ratio varies significantly within the excavation area. The degree of fragmentation of the bones and disarticulation of the skeletons suggest that the carcasses were reworked and slightly transported before burial. Sedimentological data suggest deposition in a shallow marine environment under the influence of rapid currents. The fossiliferous bed consists of a basal bone conglomerate covered by a cross-stratified sandstone with mud drapes, which is in turn overlain by ripple laminated sandstone, indicating the bones were buried by the gradual infilling of a tidal channel. All the MiddleUpper Devonian vertebrate bone-beds from Latvia are associated with sandy to clayey deposits and have been formed in a sea-coastal zone during rapid sedimentation episodes, but differ in fossil abundance and degree of preservation.
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| 6. |
- Löwemark, Ludvig, 1969-
(författare)
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Ethological analysis of the trace fossil Zoophycos: Hints from the Arctic Ocean
- 2012
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Ingår i: Lethaia. - : Scandinavian University Press / Universitetsforlaget AS. - 0024-1164 .- 1502-3931. ; 45:2, s. 290-298
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Tidskriftsartikel (refereegranskat)abstract
- The distribution of the trace fossil Zoophycos in Quaternary marine sediments from the Arctic Ocean was studied in twelve piston and gravity cores retrieved during the Swedish icebreaker expeditions YMER80, Arctic Ocean-96 and LOMROG I & II. The sampled cores span an area from the Makarov Basin to the Fram Strait. Zoophycos was only found in two cores taken at more than 2 km water depth on the slope of the Lomonosov Ridge, but was absent in cores obtained at shallower depth, confirming earlier observations of the trace maker’s bathymetric preferences. The two cores containing Zoophycos are characterized by quiet sedimentation and slightly enhanced food flux compared with the general Arctic. The occurrence of Zoophycos in these cores in a setting that is characterized by extreme seasonal variations in food flux due to the total ice coverage during winters and high primary productivity during the long summer days, is interpreted to be a cache-behaviour response to pulsed flux of food to the benthic realm. □Arctic Ocean, ethology, Quaternary, spreiten, trace fossils, Zoophycos.
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| 7. |
- Moczydlowska-Vidal, Malgorzata, 1951-, et al.
(författare)
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Micro- and nano-scale ultrastructure of cell walls in Cryogenian microfossils : revealing their biological affinity
- 2010
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Ingår i: Lethaia. - : Scandinavian University Press / Universitetsforlaget AS. - 0024-1164 .- 1502-3931. ; 43:2, s. 129-136
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Tidskriftsartikel (refereegranskat)abstract
- Recently established protocols and methods in advanced microscopy and spectrometry applied to studies of ancient unicellular organic-walled microfossils of uncertain biological affinities (acritarchs) provide new evidence of the fine ultrastructure of cell walls and their biochemistry that support the interpretation of some such microfossils as photosynthesizing microalgae. The micro-scale and nanoscale ultrastructure of the cell walls of late Cryogenian sphaeromorphic acritarchs from the Chichkan Formation (Kazakhstan) revealed by the advanced techniques and studied originally by Kempe et al. (2005) is here further analyzed and compared to that of modern microalgal analogues. On the basis of such comparison, we interpret the preserved cell wall ultrastructure to reflect original layering and lamination within sublayers of the fossil wall, rather than being a result of taphonomic and diagenetic alteration. The outer thick layer represents the primary wall and the inner layer the secondary wall of the cell, whereas the laminated amorphous sub-layers, 10-20 nm in thickness and revealed by transmission electron and atomic force microscopy, are recognized as trilaminar sheath structure (TLS). Because two-layered cell walls, trilaminar sheaths, and the position of the TLS within the fossil cell wall are characteristic of the mature developmental state in cyst morphogenesis in modern microalgae, we infer that the Chichkan sphaeromorphs are likely resting cells (aplanospores) of chlorophyceaen green microalgae from the Order Volvocales.
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| 8. |
- Paterson, John R., et al.
(författare)
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Oikozetetes from the early Cambrian of South Australia : implications for halkieriid affinities and functional morphology
- 2009
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Ingår i: Lethaia. - : Scandinavian University Press / Universitetsforlaget AS. - 0024-1164 .- 1502-3931. ; 42:2, s. 199-203
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Tidskriftsartikel (refereegranskat)abstract
- Paterson, J.R., Brock, G.A. & Skovsted, C.B. 2009: Oikozetetes from the early Cambrian of South Australia: implications for halkieriid affinities and functional morphology. Lethaia, Vol. 42, pp. 199-203 Shells of Oikozetetes and isolated halkieriid sclerites from a section of the lower Cambrian Mernmerna Formation in the Flinders Ranges, South Australia, are tentatively considered as being derived from the same scleritome. Details of shell morphology and the possible combination of biomineralized shell and sclerites suggest that Oikozetetes, if interpreted correctly, is closely related to Halkieria. A new interpretation of Oikozetetes shell morphology, in addition to the first report of paired muscle scars on the interior surface, sheds new light on the possible functional morphology of halkieriid shells and the means of attaching the shell to the body. The occurrence of Oikozetetes in South Australia extends its biostratigraphic range to the lower Cambrian and biogeographic range to East Gondwana.
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| 9. |
- Popov, L. E., et al.
(författare)
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Earliest ontogeny of Early Palaeozoic Craniiformea : compelling evidence for lecithotrophy
- 2012
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Ingår i: Lethaia. - : Scandinavian University Press / Universitetsforlaget AS. - 0024-1164 .- 1502-3931. ; 45:4, s. 566-573
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Tidskriftsartikel (refereegranskat)abstract
- The early ontogeny of Palaeozoic Craniiformea (Brachiopoda) remains controversial, with conflicting reports of evidence indicating lecithotrophic versus planktotrophic larval stages. Further compelling evidence for lecithotrophy in Palaeozoic craniiforms is described here. Newly obtained, well-preserved Silurian specimens of craniiforms, including Craniops (Craniopsida), and Lepidocrania? and Orthisocrania (Craniida) from Gotland and the St. Petersburg region, form the basis for this study. The new material demonstrates that the characters of shell structure and shell formation provide evidence of early differentiation of an adult dorsal mantle, and the presence of a distinctive primary layer with a characteristic lath-like pattern indicates that these craniiforms underwent a lecithotrophic larval stage, more or less identical to that of living.
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| 10. |
- Popov, Leonid E., et al.
(författare)
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Earliest ontogeny of Early Palaeozoic Craniiformea : implications for brachiopod phylogeny
- 2010
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Ingår i: Lethaia. - : Scandinavian University Press / Universitetsforlaget AS. - 0024-1164 .- 1502-3931. ; 43:3, s. 323-333
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Tidskriftsartikel (refereegranskat)abstract
- Well preserved specimens of the Early Palaeozoic craniiform brachiopods Orthisocrania and Craniops retain clear evidence of a lecithotrophic larval stage, indicating the loss of planktotrophy early in their phylogeny. The size of the earliest mineralized dorsal shell was <100 mu m across, and the well preserved shell structure in these fossil craniiforms allows their earliest ontogeny to be compared directly with that of living Novocrania, in which the first mineralized dorsal shell (metamorphic shell) is secreted only after settlement of the lecithotrophic larvae. Immediately outside this earliest shell (early post-metamorphic or brephic shell) and in the rest of the dorsal valve the primary layer in both fossil and living craniiforms has characteristic radially arranged laths, which are invariably lacking in the earliest dorsal shell. The ventral valve of the fossil specimens commonly preserves traces of an early attachment scar (cicatrix), which is equal in size to the dorsal metamorphic shell, and the brephic post-metamorphic ventral valve also has a primary shell with radially arranged laths. However, a primary shell with radial laths is completely lacking in the ventral valve of living Novocrania, indicating that heterochrony may have been involved in the origin of the encrusting mode of life in living craniids; the entire ventral valve of Recent craniids (with the possible exception of Neoancistrocrania) may correspond to the earliest attachment scar of some fossil taxa such as Orthisocrania. It is also probable that the unique absence of an inner mantle lobe as well as the absence of lobate cells in Novocrania could be the result of heterochronic changes. The dorsal valve of both fossil and living craniiforms has a marked outer growth ring, around 500 mu m across, marking the transition to the adult, and a significant change in regime of shell secretion. The earliest craniiform attachment is considered to be homologous to the unique attachment structures described recently in polytoechioids (e.g. Antigonambonites) and other members of the strophomenate clade. However, unlike the craniiforms, polytoechioids and strophomenates all have planktotrophic larvae, and planktotrophy is most probably a plesiomorphic character for all Brachiopoda.
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