Carbon isotopes are used as a standard stratigraphic tool. Through geological time d 13 C reflect... more Carbon isotopes are used as a standard stratigraphic tool. Through geological time d 13 C reflects past changes within different carbon reservoirs (e.g. global biomass, atmospheric and marine
Morphometric analysis of graptolites from the persculptus and acuminatus biozones of central Wale... more Morphometric analysis of graptolites from the persculptus and acuminatus biozones of central Wales identifies four successive morphospecies of normalograptids. These graptolites can be used for biostratigraphical subdivision of these strata as follows: (a) an early persculptus Biozone interval containing broad forms with geniculate thecae that have the morphology of Persculptograptus persculptus with an early insertion point for the full median septum (theca 1 1); (b) an supradjacent level of early persculptus Biozone age, with narrower, parallel-sided forms that have been reffered to as Normalograptus? cf. parvulus and a slightly later insertion point for the full median septum (theca 1 2); (c) a third interval, encompassing the later part of the persculptus Biozone to the early acuminatus Biozone, with ?Normalograptus parvulus, which has the full median septum delayed to the level of theca 3-7; and (d) a younger interval, in the mid-acuminatus Biozone with Persculptograptus persculptus? specimens which do not display the median septum on its reverse side. These taxa can be used for refined biostratigraphy and correlation in the late Ordovician and early Silurian of central Wales. The progressive delay in the insertion of the median septum in these taxa may have wider application for the correlation of the interval immediately after the Hirnantian glacial maximum.
Graptolites are common fossils in Early Palaeozoic strata, but little is known of their soft-part... more Graptolites are common fossils in Early Palaeozoic strata, but little is known of their soft-part anatomy. However, we report a long-overlooked specimen of Dicranograptus aff. ramosus from Late Ordovician strata of southern Scotland that preserves a strongly polymorphic, recalcitrant, organic-walled network hitherto unseen in graptoloid graptolites. This network displays three morphologies: proximally, a strap-like pattern, likely of flattened tubes; these transform distally into isolated, hourglass-shaped structures; then, yet more distally, revert to a (simpler) strap-like pattern. The network most likely represents a stolon-like system, hitherto unknown in graptoloids, that connected individual zooids. Its alternative interpretation, as colonial xenobionts that infested a graptoloid colony and mimicked its architecture, is considered less likely on taphonomic and palaeobiological grounds. Such polymorphism is not known in non-graptolite pterobranchs, which are less diverse and mo...
Normalograptus kufraensissp. nov. occurs as monospecific assemblages in the Tanezzuft Formation a... more Normalograptus kufraensissp. nov. occurs as monospecific assemblages in the Tanezzuft Formation at the western margin of the Kufra Basin (Jabal Eghei), southern Libya. These graptolites have parallel-sided rhabdosomes with long, straight virgellae, climacograptid thecae and a full straight median septum.N. kufraensisis intermediate between Ordovician graptolites from theN. angustus(Perner) lineage and the younger sister speciesN. ajjeri(Legrand) andN. arrikiniLegrand.N. kufraensisdiffers from these taxa as follows: it is broader thanN. angustus; it has greater thecal spacing thanN. ajjeriorN. arrikini. A table comparing measurements ofN. kufraensiswith 44 otherNormalograptustaxa differentiates it from other members of this morphologically conservative group. Even thoughN. angustusandN. ajjeriare very long-ranging graptolites, a stratophenetic approach suggests that the specimens from Jabal Eghei may be of late Hirnantian or younger age. The faunal composition and preservation sugges...
The Hirnantian glaciation of West Gondwana produced a glacially sculpted topography, which is dra... more The Hirnantian glaciation of West Gondwana produced a glacially sculpted topography, which is draped by organic-rich latest Ordovician and early Silurian 'hot shales'. Although these are the most important Early Palaeozoic source rock in North Africa, organic enrichment is distributed unevenly. For example, in Al Kufrah Basin, Libya, 'hot shales' are elusive, but outcrop analysis at the western basin demonstrates why this is the case. The topmost Mamuniyat Formation, of Hirnantian age, comprises glaciogenic sandstones, passing upward into mixed facies of the Tanezzuft Formation, which has a latest Ordovician-early Silurian age. The basal Tanezzuft Formation contains a shelly carbonate (cool-water deposits accumulated under oxygenating conditions) and bioturbated sandstone succession. Above, hummocky cross-bedded and graded sandstone intervals are intercalated with shale and siltstone (storm influx onto a muddy shelf). These are interrupted by several lonestone-bearing intervals (ice-rafted debris), a striated pavement (of subglacial origin), and manganese oxide crusts and concretions. The concretions and bioturbation imply oxygenation of the sea floor during transgression. These putative glacial deposits were deposited following the main phase of the Hirnantian glaciation, at the same stratigraphic level as 'hot shales' elsewhere in northern Gondwana. Lingering ice caps may have produced well-oxygenated marine waters precluding 'hot shale' deposition.
Earth and Environmental Science Transactions of the Royal Society of Edinburgh, 2008
ABSTRACTThough little is known of the graptoloid reproductive mechanism, graptolites with putativ... more ABSTRACTThough little is known of the graptoloid reproductive mechanism, graptolites with putatively sac-like appendages, supposedly ovarian vesicles, have been known from the Moffat Shales Group, Southern Uplands, Scotland, for over 150 years. Locally, these co-occur with isolated, two-dimensional, discoidal or ovato-triangular fossils. In the 1870s, Nicholson interpreted these isolated fossils as being graptoloid ‘egg-sacs’ detached from their parent and existing as free-swimming bodies. He assigned them to his genusDawsonia, though the name was pre-occupied by a trilobite, and named four species:D. campanulata,D. acuminata,D. rotundaandD. tenuistriata. A reassessment of Nicholson’s type material from the Silurian of Moffatdale, Scotland, and from the Ordovician Lévis Formation of Quebec, Canada, shows thatDawsoniaNicholson comprises the inarticulate brachiopodsAcrosaccus?rotundus,Paterula?tenuistriataandDiscotretacf.levisensis, the tail-piece of the crustaceanCaryocaris acuminata...
... These elements are fairly immobile during metamorphism, perhaps migrating only over distances... more ... These elements are fairly immobile during metamorphism, perhaps migrating only over distances of several hundred micrometres to tens of millimetres (Kingsbury et al. 1993; Franz et al. ... British Geological Survey 2003. Cardigan and Dinas Island, Solid and Drift Geology. ...
The Early Palaeozoic Icehouse (Late Ordovician-Early Silurian, c. 455-425 Ma) was a remarkable ev... more The Early Palaeozoic Icehouse (Late Ordovician-Early Silurian, c. 455-425 Ma) was a remarkable event in the Earth's climatic history, marked by extensive glaciations occurring at a time of elevated atmospheric CO2. The oceanography of the Early Palaeozoic Icehouse was markedly different from that of modern oceans, with frequent episodes of oceanic anoxia and high concentrations of CO2 which may have acidified the oceans and restricted carbonate burial. Thus, the marine organic carbon reservoir may have more strongly influenced long-term changes in atmospheric CO2 than at present. We suggest that deposition of black shales represented a major sink for atmospheric carbon. Sequence stratigraphy reveals that widespread black shale deposition occurred in transgressions, whereas regressions are characterized by deposition of bioturbated facies, allowing changes in lithofacies and deep-water redox conditions to be related to the Early Palaeozoic carbon cycle. Assuming increased tempera...
Despite the Burgess Shale's (British Columbia, Canada) paleobiologicalimportance, there is li... more Despite the Burgess Shale's (British Columbia, Canada) paleobiologicalimportance, there is little consensus regarding its taphonomy.Its organic fossils are preserved as compressions associatedwith phyllosilicate films ("clay templates"). Debate focuseson whether these templates were fundamental in exceptional preservationor if they formed in metamorphism, meaning that it is importantto establish the timing of their formation relative to decay.An early diagenetic origin has been
A new component of the Early Palaeozoic arthropod fauna is described from a monospecific accumula... more A new component of the Early Palaeozoic arthropod fauna is described from a monospecific accumulate of carapaces in a Late Ordovician (Katian) hemipelagic mudstone from the Cardigan district of southwest Wales (UK). Its non-biomineralized carapace is preserved as a carbonaceous residue, as is more labile anatomy (soft-parts) including the inner lamella and sub-ovate structures near its antero-dorsal margin, which we interpret to be putative eyes. The depositional context and associated fauna indicate that the arthropods inhabited an area of deep water and high primary productivity above a pronounced submarine topography. The preserved density of carapaces suggests the arthropods may have congregated into shoals or been transported post-mortem into depressions which acted as detritus traps. The accumulate provides a rare example of soft-part preservation in hemipelagic mudstones and highlights the role of organic material as a locus for authigenic mineralization during metamorphism.
The Early Palaeozoic Icehouse (Late Ordovician-Early Silurian, c. 455-425 Ma) was a remarkable ev... more The Early Palaeozoic Icehouse (Late Ordovician-Early Silurian, c. 455-425 Ma) was a remarkable event in the Earth's climatic history, marked by extensive glaciations occurring at a time of elevated atmospheric CO 2 . The oceanography of the Early Palaeozoic Icehouse was markedly different from that of modern oceans, with frequent episodes of oceanic anoxia and high concentrations of CO 2 which may have acidified the oceans and restricted carbonate burial. Thus, the marine organic carbon reservoir may have more strongly influenced long-term changes in atmospheric CO 2 than at present. We suggest that deposition of black shales represented a major sink for atmospheric carbon. Sequence stratigraphy reveals that widespread black shale deposition occurred in transgressions, whereas regressions are characterized by deposition of bioturbated facies, allowing changes in lithofacies and deep-water redox conditions to be related to the Early Palaeozoic carbon cycle. Assuming increased temperature is a function of increased atmospheric CO 2 , and that glacioeustatic sea-level can serve as a proxy for temperature due to changing ice volume, we infer that the deposition of transgressive black shales may have acted as a negative feedback mechanism, drawing down CO 2 and preventing the onset of runaway greenhouse conditions.
Carbon isotopes are used as a standard stratigraphic tool. Through geological time d 13 C reflect... more Carbon isotopes are used as a standard stratigraphic tool. Through geological time d 13 C reflects past changes within different carbon reservoirs (e.g. global biomass, atmospheric and marine
Morphometric analysis of graptolites from the persculptus and acuminatus biozones of central Wale... more Morphometric analysis of graptolites from the persculptus and acuminatus biozones of central Wales identifies four successive morphospecies of normalograptids. These graptolites can be used for biostratigraphical subdivision of these strata as follows: (a) an early persculptus Biozone interval containing broad forms with geniculate thecae that have the morphology of Persculptograptus persculptus with an early insertion point for the full median septum (theca 1 1); (b) an supradjacent level of early persculptus Biozone age, with narrower, parallel-sided forms that have been reffered to as Normalograptus? cf. parvulus and a slightly later insertion point for the full median septum (theca 1 2); (c) a third interval, encompassing the later part of the persculptus Biozone to the early acuminatus Biozone, with ?Normalograptus parvulus, which has the full median septum delayed to the level of theca 3-7; and (d) a younger interval, in the mid-acuminatus Biozone with Persculptograptus persculptus? specimens which do not display the median septum on its reverse side. These taxa can be used for refined biostratigraphy and correlation in the late Ordovician and early Silurian of central Wales. The progressive delay in the insertion of the median septum in these taxa may have wider application for the correlation of the interval immediately after the Hirnantian glacial maximum.
Graptolites are common fossils in Early Palaeozoic strata, but little is known of their soft-part... more Graptolites are common fossils in Early Palaeozoic strata, but little is known of their soft-part anatomy. However, we report a long-overlooked specimen of Dicranograptus aff. ramosus from Late Ordovician strata of southern Scotland that preserves a strongly polymorphic, recalcitrant, organic-walled network hitherto unseen in graptoloid graptolites. This network displays three morphologies: proximally, a strap-like pattern, likely of flattened tubes; these transform distally into isolated, hourglass-shaped structures; then, yet more distally, revert to a (simpler) strap-like pattern. The network most likely represents a stolon-like system, hitherto unknown in graptoloids, that connected individual zooids. Its alternative interpretation, as colonial xenobionts that infested a graptoloid colony and mimicked its architecture, is considered less likely on taphonomic and palaeobiological grounds. Such polymorphism is not known in non-graptolite pterobranchs, which are less diverse and mo...
Normalograptus kufraensissp. nov. occurs as monospecific assemblages in the Tanezzuft Formation a... more Normalograptus kufraensissp. nov. occurs as monospecific assemblages in the Tanezzuft Formation at the western margin of the Kufra Basin (Jabal Eghei), southern Libya. These graptolites have parallel-sided rhabdosomes with long, straight virgellae, climacograptid thecae and a full straight median septum.N. kufraensisis intermediate between Ordovician graptolites from theN. angustus(Perner) lineage and the younger sister speciesN. ajjeri(Legrand) andN. arrikiniLegrand.N. kufraensisdiffers from these taxa as follows: it is broader thanN. angustus; it has greater thecal spacing thanN. ajjeriorN. arrikini. A table comparing measurements ofN. kufraensiswith 44 otherNormalograptustaxa differentiates it from other members of this morphologically conservative group. Even thoughN. angustusandN. ajjeriare very long-ranging graptolites, a stratophenetic approach suggests that the specimens from Jabal Eghei may be of late Hirnantian or younger age. The faunal composition and preservation sugges...
The Hirnantian glaciation of West Gondwana produced a glacially sculpted topography, which is dra... more The Hirnantian glaciation of West Gondwana produced a glacially sculpted topography, which is draped by organic-rich latest Ordovician and early Silurian 'hot shales'. Although these are the most important Early Palaeozoic source rock in North Africa, organic enrichment is distributed unevenly. For example, in Al Kufrah Basin, Libya, 'hot shales' are elusive, but outcrop analysis at the western basin demonstrates why this is the case. The topmost Mamuniyat Formation, of Hirnantian age, comprises glaciogenic sandstones, passing upward into mixed facies of the Tanezzuft Formation, which has a latest Ordovician-early Silurian age. The basal Tanezzuft Formation contains a shelly carbonate (cool-water deposits accumulated under oxygenating conditions) and bioturbated sandstone succession. Above, hummocky cross-bedded and graded sandstone intervals are intercalated with shale and siltstone (storm influx onto a muddy shelf). These are interrupted by several lonestone-bearing intervals (ice-rafted debris), a striated pavement (of subglacial origin), and manganese oxide crusts and concretions. The concretions and bioturbation imply oxygenation of the sea floor during transgression. These putative glacial deposits were deposited following the main phase of the Hirnantian glaciation, at the same stratigraphic level as 'hot shales' elsewhere in northern Gondwana. Lingering ice caps may have produced well-oxygenated marine waters precluding 'hot shale' deposition.
Earth and Environmental Science Transactions of the Royal Society of Edinburgh, 2008
ABSTRACTThough little is known of the graptoloid reproductive mechanism, graptolites with putativ... more ABSTRACTThough little is known of the graptoloid reproductive mechanism, graptolites with putatively sac-like appendages, supposedly ovarian vesicles, have been known from the Moffat Shales Group, Southern Uplands, Scotland, for over 150 years. Locally, these co-occur with isolated, two-dimensional, discoidal or ovato-triangular fossils. In the 1870s, Nicholson interpreted these isolated fossils as being graptoloid ‘egg-sacs’ detached from their parent and existing as free-swimming bodies. He assigned them to his genusDawsonia, though the name was pre-occupied by a trilobite, and named four species:D. campanulata,D. acuminata,D. rotundaandD. tenuistriata. A reassessment of Nicholson’s type material from the Silurian of Moffatdale, Scotland, and from the Ordovician Lévis Formation of Quebec, Canada, shows thatDawsoniaNicholson comprises the inarticulate brachiopodsAcrosaccus?rotundus,Paterula?tenuistriataandDiscotretacf.levisensis, the tail-piece of the crustaceanCaryocaris acuminata...
... These elements are fairly immobile during metamorphism, perhaps migrating only over distances... more ... These elements are fairly immobile during metamorphism, perhaps migrating only over distances of several hundred micrometres to tens of millimetres (Kingsbury et al. 1993; Franz et al. ... British Geological Survey 2003. Cardigan and Dinas Island, Solid and Drift Geology. ...
The Early Palaeozoic Icehouse (Late Ordovician-Early Silurian, c. 455-425 Ma) was a remarkable ev... more The Early Palaeozoic Icehouse (Late Ordovician-Early Silurian, c. 455-425 Ma) was a remarkable event in the Earth's climatic history, marked by extensive glaciations occurring at a time of elevated atmospheric CO2. The oceanography of the Early Palaeozoic Icehouse was markedly different from that of modern oceans, with frequent episodes of oceanic anoxia and high concentrations of CO2 which may have acidified the oceans and restricted carbonate burial. Thus, the marine organic carbon reservoir may have more strongly influenced long-term changes in atmospheric CO2 than at present. We suggest that deposition of black shales represented a major sink for atmospheric carbon. Sequence stratigraphy reveals that widespread black shale deposition occurred in transgressions, whereas regressions are characterized by deposition of bioturbated facies, allowing changes in lithofacies and deep-water redox conditions to be related to the Early Palaeozoic carbon cycle. Assuming increased tempera...
Despite the Burgess Shale's (British Columbia, Canada) paleobiologicalimportance, there is li... more Despite the Burgess Shale's (British Columbia, Canada) paleobiologicalimportance, there is little consensus regarding its taphonomy.Its organic fossils are preserved as compressions associatedwith phyllosilicate films ("clay templates"). Debate focuseson whether these templates were fundamental in exceptional preservationor if they formed in metamorphism, meaning that it is importantto establish the timing of their formation relative to decay.An early diagenetic origin has been
A new component of the Early Palaeozoic arthropod fauna is described from a monospecific accumula... more A new component of the Early Palaeozoic arthropod fauna is described from a monospecific accumulate of carapaces in a Late Ordovician (Katian) hemipelagic mudstone from the Cardigan district of southwest Wales (UK). Its non-biomineralized carapace is preserved as a carbonaceous residue, as is more labile anatomy (soft-parts) including the inner lamella and sub-ovate structures near its antero-dorsal margin, which we interpret to be putative eyes. The depositional context and associated fauna indicate that the arthropods inhabited an area of deep water and high primary productivity above a pronounced submarine topography. The preserved density of carapaces suggests the arthropods may have congregated into shoals or been transported post-mortem into depressions which acted as detritus traps. The accumulate provides a rare example of soft-part preservation in hemipelagic mudstones and highlights the role of organic material as a locus for authigenic mineralization during metamorphism.
The Early Palaeozoic Icehouse (Late Ordovician-Early Silurian, c. 455-425 Ma) was a remarkable ev... more The Early Palaeozoic Icehouse (Late Ordovician-Early Silurian, c. 455-425 Ma) was a remarkable event in the Earth's climatic history, marked by extensive glaciations occurring at a time of elevated atmospheric CO 2 . The oceanography of the Early Palaeozoic Icehouse was markedly different from that of modern oceans, with frequent episodes of oceanic anoxia and high concentrations of CO 2 which may have acidified the oceans and restricted carbonate burial. Thus, the marine organic carbon reservoir may have more strongly influenced long-term changes in atmospheric CO 2 than at present. We suggest that deposition of black shales represented a major sink for atmospheric carbon. Sequence stratigraphy reveals that widespread black shale deposition occurred in transgressions, whereas regressions are characterized by deposition of bioturbated facies, allowing changes in lithofacies and deep-water redox conditions to be related to the Early Palaeozoic carbon cycle. Assuming increased temperature is a function of increased atmospheric CO 2 , and that glacioeustatic sea-level can serve as a proxy for temperature due to changing ice volume, we infer that the deposition of transgressive black shales may have acted as a negative feedback mechanism, drawing down CO 2 and preventing the onset of runaway greenhouse conditions.
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