| 11. |
- Ahlberg, Per, 1963-
(författare)
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Follow the footprints and mind the gaps : a new look at the origin of tetrapods
- 2019
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Ingår i: Earth and environmental science transactions of the Royal Society of Edinburgh. - : CAMBRIDGE UNIV PRESS. - 1755-6910 .- 1755-6929. ; 109:1-2, s. 115-137
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Tidskriftsartikel (refereegranskat)abstract
- The hypothesis that tetrapods evolved from elpistostegids during the Frasnian, in a predominantly aquatic context, has been challenged by the discovery of Middle Devonian tetrapod trackways predating the earliest body fossils of both elpistostegids and tetrapods. Here I present a new hypothesis based on an overview of the trace fossil and body fossil evidence. The trace fossils demonstrate that tetrapods were capable of performing subaerial lateral sequence walks before the end of the Middle Devonian. The derived morphological characters of elpistostegids and Devonian tetrapods are related to substrate locomotion, weight support and aerial vision, and thus to terrestrial competence, but the retention of lateral-line canals, gills and fin rays shows that they remained closely tied to the water. Elpistostegids and tetrapods both evolved no later than the beginning of the Middle Devonian. The earliest tetrapod records come from inland river basins, sabkha plains and ephemeral coastal lakes that preserve few, if any, body fossils; contemporary elpistostegids occur in deltas and the lower reaches of permanent rivers where body fossils are preserved. During the Frasnian, elpistostegids disappear and these riverine-deltaic environments are colonised by tetrapods. This replacement has, in the past, been misinterpreted as the origin of tetrapods.
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| 12. |
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| 13. |
- Ahlberg, Per, et al.
(författare)
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Furongian (Cambrian) agnostoids of Scandinavia and their implications for intercontinental correlation
- 2012
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Ingår i: Geological Magazine. - 0016-7568. ; 149:6, s. 1001-1012
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Tidskriftsartikel (refereegranskat)abstract
- Out of the 14 agnostoid species/subspecies so far recorded from the Furongian of Scandinavia, seven are excellent biostratigraphical indices and important for correlation between Baltica and other palaeocontinents. Glyptagnostus reticulatus, Aspidagnostus lunulosus and Agnostus (Homagnostus) obesus first appear at the base of the G. reticulatus Zone, allowing a precise correlation of the base of the Paibian Stage into Scandinavia. Tomagnostella orientalis and Psettdagnostus cyclopyge appear near the base of the Parabolina brevispina Zone, suggesting a correlation with the uppermost Paibian through the lowermost Jiangshanian stages. Lotagnostus americanus and Pseudagnostus rugosus have an intercontinental distribution and their first appearance in Scandinavia allows for a correlation with one of the most favourable levels for defining the base of Cambrian provisional Stage 10. In the Furongian of Scandinavia, agnostoids are conspicuously assembled in three different intervals: the lower Glyptagnostus reticulatus Zone, the lower Psettdagnostus cyclopyge Zone and the Lotagnostus americanus through lower Trilobagnostus holmi zones. The agnostoid-barren and largely unfossiliferous succession separating the lower and middle agnostoid-bearing intervals can be explained by means of subsequent dissolution of the calcareous fauna and/or a hostile environment. The middle agnostoid-bearing interval is succeeded by an anomalous succession dominated by the orthide brachiopod Orusia lenticularis, reflecting a regressive event coupled with increasing levels of oxygen at the sediment/water interface. This shallowing evidently resulted in unfavourable conditions for agnostoids.
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| 14. |
- Ahlberg, Per, et al.
(författare)
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Geological fieldwork in the Kirwanveggen area
- 1992
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Ingår i: Swedish Antarctic research programme : a cruise report. 1991/92, Dronning Maud Land, Livingston Island, South Georgia. - 9197076988 ; , s. 13-21
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Bokkapitel (refereegranskat)
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| 15. |
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| 16. |
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| 17. |
- AHLBERG, PER, et al.
(författare)
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Integrated Cambrian biostratigraphy and carbon isotope chemostratigraphy of the Grönhögen-2015 drill core, Öland, Sweden
- 2019
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Ingår i: Geological Magazine. - 0016-7568. ; 156:06, s. 935-949
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Tidskriftsartikel (refereegranskat)abstract
- The Grönhögen-2015 core drilling on southern Öland, Sweden, penetrated 50.15 m of Cambrian Series 3, Furongian and Lower–Middle Ordovician strata. The Cambrian succession includes the Äleklinta Member (upper Stage 5) of the Borgholm Formation and the Alum Shale Formation (Guzhangian–Tremadocian). Agnostoids and trilobites allowed subdivision of the succession into eight biozones, in ascending order: the uppermost Cambrian Series 3 (Guzhangian) Agnostus pisiformis Zone and the Furongian Olenus gibbosus, O. truncatus, Parabolina spinulosa, Sphaerophthalmus? flagellifer, Ctenopyge tumida, C. linnarssoni and Parabolina lobata zones. Conspicuous lithologic unconformities and the biostratigraphy show that the succession is incomplete and that there are several substantial gaps of variable magnitudes. Carbon isotope analyses (δ13Corg) through the Alum Shale Formation revealed two globally significant excursions: the Steptoean Positive Carbon Isotope Excursion (SPICE) in the lower–middle Paibian Stage, and the negative Top of Cambrian Excursion (TOCE), previously referred to as the HERB Event, in Stage 10. The δ13Corg chemostratigraphy is tied directly to the biostratigraphy and used for an improved integration of these excursions with the standard agnostoid and trilobite zonation of Scandinavia. Their relations to that of coeval successions in Baltoscandia and elsewhere are discussed. The maximum amplitudes of the SPICE and TOCE in the Grönhögen succession are comparable to those recorded in drill cores retrieved from Scania, southern Sweden. The results of this study will be useful for assessing biostratigraphic relations between shale successions and carbonate facies on a global scale.
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| 18. |
- Ahlberg, Per, et al.
(författare)
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Jan Bergström 1938-2012
- 2013
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Ingår i: Kungliga Fysiografiska Sällskapet i Lund Årsbok 2011-2012. - 1402-1277. ; 2011-2012, s. 113-115
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Bokkapitel (refereegranskat)
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| 19. |
- Ahlberg, Per, et al.
(författare)
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Lotagnostus? : mystacinus, a rare agnostid from the Upper Cambrian of Sweden
- 2000
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Ingår i: Acta Palaeontologica Polonica. - 0567-7920. ; 45:1, s. 91-96
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Tidskriftsartikel (refereegranskat)abstract
- Agnostids afford the best means of correlating Cambrian strata, and they are widely used for intercontinental correlations. They are most common in open-marine deposits and reached a maximum diversity during the Middle and early Late Cambrian. About twenty species of agnostids are known from the Upper Cambrian of Scandinavia. Lotagnostus? mystacinus Tjernvik, 1953 is a rare agnostid from the Upper Cambrian Agnostus pisiformis Zone of south-central Sweden. It is redescribed following examination of the holotype and an additional pygidium. The outline and morphology of especially the pygidial posteroaxis, which is distinctly trilobate by a pair of deep notular furrows, suggest that the species is best classified as a species of Lotagnostus. If correctly assigned to Lotagnostus, it would be the oldest known species of that genus.
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| 20. |
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