Palaeozoic stromatoporoids are calcified sponges common between Middle Ordovician and Late Devonian times in reefs and related facies.Taxonomic work is well known,but controversial because of conflict between classifi...Palaeozoic stromatoporoids are calcified sponges common between Middle Ordovician and Late Devonian times in reefs and related facies.Taxonomic work is well known,but controversial because of conflict between classification schemes based on the calcareous skeleton versus spicules(which are almost completely lacking in Palaeozoic stromatoporoids);however,lower-level taxonomy(at genus-level) of the calcareous skeleton is considered reliable to be applied in palaeobiological study.Knowledge of stromatoporoid ecology is poorly developed,such that comprehensive information is available for only a few case studies,in some Silurian and Devonian examples.Thus an overall understanding of stromatoporoid responses to environmental conditions has not yet been achieved,although stromatoporoids were likely able to deal with fine grained sediment where they mostly occur.Many stromatoporoid genera have only certain growth forms,so future focus on the use of low-level taxonomy in ecological studies,by comprehensive sampling in high-resolution studies,may establish the relationships between stromatoporoids and their environments.Intergrown organisms and growth banding in stromatoporoids are aspects that have great potential in such work.Mineralogy of stromatoporoids remains poorly understood.Regardless of their apparent state of preservation(ranging from apparently well-preserved to complete loss of calcareous skeleton features) all stromatoporoids are in fact substantially recrystallized.They underwent a peculiar diagenesis,whereby the calcareous skeleton and gallery cements of all stromatoporoids are overprinted by irregular elongated calcite crystals arranged normal to the growth laminations,most clearly visible in cross-polarized light.Stromatoporoids cooccur with mollusc shells that are always either fully recrystallized or dissolved(present as internal and external moulds);this difference means that while molluscs are likely under-represented in the fossil record,stromatoproids are not,providing confidence for palaeoecological work on their assemblages.Stromatoporoids lack characters which would readily classify them as being originally aragonite or low-Mg calcite;they may have been high-Mg calcite but the evidence is circumstantial.Their peculiar diagenetic fabric also has implications for the debate about the relationship between stromatoporoids and the concept of aragonite/calcite seas,which requires more work.展开更多
Stromatoporoid sponges were very abundant during the middle Palaeozoic Era and are thought to disappear at the end of the Devonian Period in the Hangenberg Crisis. However, there are records of organisms with stromato...Stromatoporoid sponges were very abundant during the middle Palaeozoic Era and are thought to disappear at the end of the Devonian Period in the Hangenberg Crisis. However, there are records of organisms with stromatoporoid-type structure in Carboniferous strata, the subject of this study. The Viséan fossil Labechia carbonaria Smith 1932 has been discussed previously in literature and its affinity has not been confirmed. In this study, the type material of L. carbonaria collected from the middle part of the Frizington Limestone Formation(previously called Seventh Limestone), Holkerian Substage, stored in the Natural History Museum(London, UK) and British Geological Survey(Keyworth, UK) was re-examined. The Holkerian Substage, in which L. carbonaria was found, lies between ca 335–339 Ma, and the Frizington Limestone Formation ranges from topmost Arundian to upper Holkerian, so middle Frizington Limestone Formation is likely approximately 337 Ma. L. carbonaria comprises thick long pillars connected by thin curved cyst plates consistent with the structure of the stromatoporoid genus Labechia. However, a common opinion is that L. carbonaria fossils may be mistaken for fragments of rugose corals,but there are problems with assigning it to the Rugosa. In vertical section(VS) L. carbonaria could be mistaken for a transverse section(TS) of a Carboniferous rugose coral. However, in TS L. carbonaria shows the rounded cross sections of stromatoporoid pillars. If it was a coral, septal sheets of the VS of a coral should be seen. For a rugose affinity to still apply, a coral structure would have to be composed of free trabeculae, but these are not known after the middle of the Devonian Period;there are no corals of Early Carboniferous age with the structure of L.carbonaria. Another interpretation, that L. carbonaria is a chaetetid, is discounted because it lacks calicles and is very different in structure from chaetetids. We conclude that L. carbonaria is a stromatoporoid. Because the beginning of the Carboniferous Period was ca 359 Ma, stromatoporoids thus occur approximately 22 million years after their purported disappearance at the end-Devonian Hangenberg Crisis. L. carbonaria, together with other rare occurrences in Carboniferous strata of stromatoporoid-form sponges Newellia mira(Newell) in USA, and uncertain taxa Komia Korde and Palaeoaplysina Krottow that have been attributed to stromatoporoids by some authors,supports some published views that end-Devonian stromatoporoid extinction may not have been as final as is traditionally interpreted. Thus Mesozoic stromatoporoids may represent resurgence of sponge lineages that survived the late Palaeozoic, perhaps in uncalcified form. Palaeogeographically, during the Early Carboniferous, the UK was positioned in low latitudes and in a central location of global distribution of reefal buildups during the late Palaeozoic continental assembly towards Pangaea. Thus it is curious that L. carbonaria is found in only one place;future search may determine its true palaeographic distribution, with potential reconsideration of the extinction of stromatoporoids at the end of the Devonian Period.展开更多
文摘Palaeozoic stromatoporoids are calcified sponges common between Middle Ordovician and Late Devonian times in reefs and related facies.Taxonomic work is well known,but controversial because of conflict between classification schemes based on the calcareous skeleton versus spicules(which are almost completely lacking in Palaeozoic stromatoporoids);however,lower-level taxonomy(at genus-level) of the calcareous skeleton is considered reliable to be applied in palaeobiological study.Knowledge of stromatoporoid ecology is poorly developed,such that comprehensive information is available for only a few case studies,in some Silurian and Devonian examples.Thus an overall understanding of stromatoporoid responses to environmental conditions has not yet been achieved,although stromatoporoids were likely able to deal with fine grained sediment where they mostly occur.Many stromatoporoid genera have only certain growth forms,so future focus on the use of low-level taxonomy in ecological studies,by comprehensive sampling in high-resolution studies,may establish the relationships between stromatoporoids and their environments.Intergrown organisms and growth banding in stromatoporoids are aspects that have great potential in such work.Mineralogy of stromatoporoids remains poorly understood.Regardless of their apparent state of preservation(ranging from apparently well-preserved to complete loss of calcareous skeleton features) all stromatoporoids are in fact substantially recrystallized.They underwent a peculiar diagenesis,whereby the calcareous skeleton and gallery cements of all stromatoporoids are overprinted by irregular elongated calcite crystals arranged normal to the growth laminations,most clearly visible in cross-polarized light.Stromatoporoids cooccur with mollusc shells that are always either fully recrystallized or dissolved(present as internal and external moulds);this difference means that while molluscs are likely under-represented in the fossil record,stromatoproids are not,providing confidence for palaeoecological work on their assemblages.Stromatoporoids lack characters which would readily classify them as being originally aragonite or low-Mg calcite;they may have been high-Mg calcite but the evidence is circumstantial.Their peculiar diagenetic fabric also has implications for the debate about the relationship between stromatoporoids and the concept of aragonite/calcite seas,which requires more work.
文摘Stromatoporoid sponges were very abundant during the middle Palaeozoic Era and are thought to disappear at the end of the Devonian Period in the Hangenberg Crisis. However, there are records of organisms with stromatoporoid-type structure in Carboniferous strata, the subject of this study. The Viséan fossil Labechia carbonaria Smith 1932 has been discussed previously in literature and its affinity has not been confirmed. In this study, the type material of L. carbonaria collected from the middle part of the Frizington Limestone Formation(previously called Seventh Limestone), Holkerian Substage, stored in the Natural History Museum(London, UK) and British Geological Survey(Keyworth, UK) was re-examined. The Holkerian Substage, in which L. carbonaria was found, lies between ca 335–339 Ma, and the Frizington Limestone Formation ranges from topmost Arundian to upper Holkerian, so middle Frizington Limestone Formation is likely approximately 337 Ma. L. carbonaria comprises thick long pillars connected by thin curved cyst plates consistent with the structure of the stromatoporoid genus Labechia. However, a common opinion is that L. carbonaria fossils may be mistaken for fragments of rugose corals,but there are problems with assigning it to the Rugosa. In vertical section(VS) L. carbonaria could be mistaken for a transverse section(TS) of a Carboniferous rugose coral. However, in TS L. carbonaria shows the rounded cross sections of stromatoporoid pillars. If it was a coral, septal sheets of the VS of a coral should be seen. For a rugose affinity to still apply, a coral structure would have to be composed of free trabeculae, but these are not known after the middle of the Devonian Period;there are no corals of Early Carboniferous age with the structure of L.carbonaria. Another interpretation, that L. carbonaria is a chaetetid, is discounted because it lacks calicles and is very different in structure from chaetetids. We conclude that L. carbonaria is a stromatoporoid. Because the beginning of the Carboniferous Period was ca 359 Ma, stromatoporoids thus occur approximately 22 million years after their purported disappearance at the end-Devonian Hangenberg Crisis. L. carbonaria, together with other rare occurrences in Carboniferous strata of stromatoporoid-form sponges Newellia mira(Newell) in USA, and uncertain taxa Komia Korde and Palaeoaplysina Krottow that have been attributed to stromatoporoids by some authors,supports some published views that end-Devonian stromatoporoid extinction may not have been as final as is traditionally interpreted. Thus Mesozoic stromatoporoids may represent resurgence of sponge lineages that survived the late Palaeozoic, perhaps in uncalcified form. Palaeogeographically, during the Early Carboniferous, the UK was positioned in low latitudes and in a central location of global distribution of reefal buildups during the late Palaeozoic continental assembly towards Pangaea. Thus it is curious that L. carbonaria is found in only one place;future search may determine its true palaeographic distribution, with potential reconsideration of the extinction of stromatoporoids at the end of the Devonian Period.
