Taro Kumagae - Academia.edu (original) (raw)
Papers by Taro Kumagae
Paleontological Research, 2017
To clarify the process of producing the striations in the trace fossil Asteriacites, we observed ... more To clarify the process of producing the striations in the trace fossil Asteriacites, we observed the behavior of living ophiuroids and asteroids in aquariums. When ophiuroids stopped crawling, they buried themselves shallowly in the substratum, removing substratum under their arms and discs by using their tube-feet. The basal-arm tube-feet produce well spaced, fine, parallel striations that are perpendicular to the arm axis. The oral tube-feet produce fine, radial striations in the central depression. When the ophiuroids resumed crawling, they raised their disc and four arms above the substratum and dragged one arm backward. The one backward arm erased the striations, and parallel fine striations remained in four arm depressions. Similarly, asteroids also produced wide and shallow striations perpendicular to the arm axis by tube-feet movement. When the asteroids started to move again, they bulldozed the substratum under the one preceding arm, where the striations were erased. Since the asteroids crawled raising the other four arms by their tube feet which produced rough and deep striations, the wide and shallow striations remained only in a half of each arm depression. The striations of Asteriacites lumbricalis and A. quinquefolius were similar in shape to the striations produced by movement of tube feet of living ophiuroids and asteroids, respectively.
Paleontological Research, 2016
Five Spathian (late Olenekian) ammonoids are newly reported from the Zhitkov Formation in the Art... more Five Spathian (late Olenekian) ammonoids are newly reported from the Zhitkov Formation in the Artyom section, South Primorye, Russian Far East. They include the early Spathian taxa Bittnerites pacificus and Tirolites longilobatus from the middle part of the formation, and Ussuriphyllites amurensis, Leiophyllites sp. and Keyserlingites sp. from the upper part. The Ussuriphyllites amurensis Zone was previously correlated with the lower Anisian, but we herein correlate it with the upper Spathian. As a consequence of this age assignment, the weakly bioturbated offshore mudstone in the middle and upper parts of the formation is correlated with the Spathian. In contrast, the Smithian (early Olenekian) offshore laminated mudstone in the lower part of the formation lacks any sign of bioturbation. These mudstones suggest that the oxygen-deficient sea floor was transformed into an aerobic environment inhabitable to benthic organisms at the Smithian-Spathian boundary.
Kyoto University (京都大学)0048新制・課程博士博士(理学)甲第16642号理博第3754号新制||理||1543(附属図書館)29317京都大学大学院理学研究科地球惑星科学... more Kyoto University (京都大学)0048新制・課程博士博士(理学)甲第16642号理博第3754号新制||理||1543(附属図書館)29317京都大学大学院理学研究科地球惑星科学専攻(主査)准教授 前田 晴良, 教授 平田 岳史, 教授 大野 照文学位規則第4条第1項該
The stratigraphy and paleontology of a Lower Triassic section exposed within a quarry along the n... more The stratigraphy and paleontology of a Lower Triassic section exposed within a quarry along the northeastern coast of Abrek Bay, South Primorye has been the subject of an intense investigation. This 165 meter thick section is divided lithostratigraphically into the Lazurnaya Bay and Zhitkov formations in ascending order. The Lazurnaya Bay Formation, which unconformably overlies the Permian Abrek Formation, consists mainly of sandstone exhibiting an upward-fining sequence, whereas the overlying Zhitkov Formation consists of dark gray, laminated mudstone intercalated with thin, turbiditic fine sandstone beds. The Lazurnaya Bay Formation consists of the following three facies types in ascending order: a distal deltaic facies deposited during a transgressive event; a shelf environment above storm wave base; and a transitional facies from a shelf to a slope environment. A lower slope environment represents the lower part of the Zhitkov Formation, while the main part consists of a proximal basin-floor facies. This thick monotonous succession of barely bioturbated, laminated mudstone suggests deposition in a stable anoxic basin-floor setting. Aside from the lower part of the Lazurnaya Bay Formation, the remaining Lower Triassic strata are very fossilferous. Ammonoids, nautiloids, gastropods, bivalves, brachiopods, conodonts and shark fossils are abundant throughout the sequence, while crinoids and scaphopods are present only in the upper part. Nine ammonoid zones (taxon-rang zones) and beds, Lytophiceras sp. Zone, Gyronites subdharmus Zone, Ambitoides fuliginatus Zone, Clypeoceras spitiense "bed", Paranorites varians Zone, Clypeoceras timorense Zone, Radioprionites abrekensis "bed", Balhaeceras balhaense "bed", and Arctoceras subhydaspis "bed", as well as three conodont zones, Neogondolella carinata Zone, Neospathodus dieneri-N. pakistanensis Zone, and Neospathodus ex gr. waageni-N. novaeholladiae Zone are recognized in ascending order. Based on these ammonoids and conodonts, the sequence ranges in age from Early Induan (Griesbachian) to middle Early Olenekian (middle Smithian). Even though the ammonoid faunas contain a few species that are common to other realms, the majority of ammonoids are essentially endemic. In contrast, the bivalve fauna appears to be more related to the Maizuru fauna of Southwest Japan. While the grypoceratid nautiloids exhibit diverse shell forms and siphuncle positions, evidence suggests that they diversified primarily in the western Panthalassa area during the Early Triassic. The existence of Holocrinus in beds of early Smithian age, which represents the oldest known occurrence in the world, suggests that crinoids experienced an earlier recovery in South Primorye following the Permian-Triassic (P/Tr) mass extinction. The transition from the Lazurnaya Bay Formation to the Zhitkov Formation occurred during the early Late Induan (early Dienerian). Sedimentation appears to be continuous from the Late Induan to the Early Olenekian, and faunal successions during this interval are relatively complete and exceptionally well preserved. This phenomenon likely resulted from deposition under much deeper and quieter environmental conditions than other areas in the western part of the "Ussuri Basin". Although this fining-upward sequence is for the most part typical throughout South Primorye, the datum planes of fossil zones obviously extend across lithostratigraphic boundaries. In the Abrek Bay area, the Induan/Olenekian boundary exists within the Zhitkov Formation, whereas in other western areas of the region, it occurs in the Lazurnaya Bay Formation. These facts support an eastward-deepening setting of the Triassic System in the basin. Ammonoids are rare in the laminated mudstone of the Zhitkov Formation, but they are common in the very fine sandstone beds that are intercalated within the mudstone. These occurrences suggest that these ammonoids were not indigenous to the slope-basin environment, but rather were of allochthonous origin. Most of the ammonoids likely lived in shallower facies, and after death their empty shells were transported from their biotope to the anoxic basin-floor by low-density turbidite gravity flows. Sandstones exhibit a strong monazite age peak at 500 Ma and a subordinate peak at 270 Ma. These age patterns indicate that Triassic sediments in the Abrek Bay area were derived from the Khanka Block and/or the adjacent Jiamusi Block, with no contribution from the nearby Sino-Korea Craton. Similar age patterns have been recognized in Permian and Lower to Middle Triassic sandstones in the Maizuru Belt of southwest Japan, which suggests that both areas belonged to either the Khanka Block or the Jiamusi Block during the Early Triassic. This assumption is also supported by the faunal similarity of bivalves between South Primorye and the Maizuru belt. Eight new genera
Abstract. The stratigraphic distribution and modes of occurrence of Early Triassic Bellerophontoi... more Abstract. The stratigraphic distribution and modes of occurrence of Early Triassic Bellerophontoidea (Gastropoda) are studied at seven sections in South Primorye, Russian Far East, where depositional environments ranging from nonmarine, shoreface, to distal basin plain settings are recorded. Warthia zakharovi and Dicellonema abrekensis are abundant in Induan (Griesbachian and Dienerian) fine- to medium-grained, hummocky cross-stratified (HCS) sandstone beds occasionally intercalated with wavy-mudstone layers, whereas they are absent in coarser-grained cross-stratified successions. This observation suggests that bellerophontoids inhabited a lower shoreface environment above the storm wave base and possibly an inner shelf environment as well during this particular stage. Olenekian (Smithian and Spathian) bellerophontoids have not been found in the storm-induced sandstone beds, but W. zakharovi occurs in the lower Smithian sandstone beds of distal turbidites intercalated in the laminat...
Paleontological Research, 2015
Palaeontology, 2013
ABSTRACT The heteromorph ammonite Pravitoceras sigmoidale from the Upper Cretaceous Seidan Format... more ABSTRACT The heteromorph ammonite Pravitoceras sigmoidale from the Upper Cretaceous Seidan Formation (Izumi Group) in south-west Japan is frequently encrusted by sessile anomiid bivalves. Fossils of P. sigmoidale with anomiids are often concentrated at the top of or just above turbidite sandstones. Projecting retroversal hooks and apertures of P. sigmoidale are usually intact, and some individuals are associated with jaw apparatuses near apertures. Anomiids are found on both sides and ventral peripheries of P. sigmoidale conchs, attached predominantly to body chambers. These modes of occurrence suggest that the encrustation by anomiids occurred not on post-mortem floating or sunken carcasses but on live conchs and that these organisms were rapidly buried by turbidity current deposits shortly after death. Attachment to both flanks and ventral peripheries of the retroversal hooks may indicate that at least adult individuals of P. sigmoidale did not lie on the sea floor and did not drag their body chambers. It is suggested that fully mature individuals of this ammonite species lived for a long period of time after having formed the retroversal hook because a few generations of anomiids have colonized a single body chamber. Such colonization by anomiids is also observed on Didymoceras awajiense, which is considered to be the closely related ancestral species of P. sigmoidale. This anomiid–heteromorph ammonite commensal relationship might continue to persist in descendants during the course of evolution of these heteromorph ammonites.
Paleontological Research, 2010
Cretaceous Research, 2008
The depositional environments and bivalve assemblages are determined for the Upper Cretaceous Hin... more The depositional environments and bivalve assemblages are determined for the Upper Cretaceous Hinoshima Formation of the Himenoura Group, Kamishima, Amakusa Islands, Kyushu, Japan. The Hinoshima Formation is characterized by a thick transgressive succession that varies from incisedvalley-fill deposits to submarine slope deposits with high aggradation rates of depositional systems. The incised valley is filled with fluvial, bayhead delta, brackish-water estuary, and marine embayment deposits, and is overlain by thick slope deposits. Shallow marine bivalves are grouped into five fossil assemblages according to species composition: Glycymeris amakusensis (foreset beds of a bayhead delta), Nippononectes tamurai (foreset beds of a bayhead delta), Ezonuculana mactraeformis-Nucula formosa (central bay), Glycymeris amakusensis-Apiotrigonia minor (slope), and Inoceramus higoensis-Parvamussium yubarensis (slope). These bivalve assemblages all represent autochthonous and parautochthonous conditions except for a Glycymeris amakusensis-Apiotrigonia minor assemblage found in debris flow and slump deposits. The life habitats of these bivalves and the compositions of the assemblages are discussed in terms of the ecological history of fossil bivalves of the mid-to Late Cretaceous.
Paleontological Research, 2010
Based on materials from the Krasnoyarka Formation in the Naiba area in south Sakhalin, Russia, ta... more Based on materials from the Krasnoyarka Formation in the Naiba area in south Sakhalin, Russia, taphonomic histories of a large Campanian ammonoid, Canadoceras kossmati Matsumoto, 1954, were closely investigated. Large Canadoceras shells exceeding 30 cm in diameter are usually embedded horizontally and solitarily in muddy sandstone. A thin, lenticular calcareous concretion envelopes the shell (= envelope concretion). Their body chambers are mostly lost. The inner whorls comprising the center of the umbilicus completely disappear without exception, and only two or three outer whorls are preserved. The body and air chambers are somewhat compressed by compaction and are filled with sediments. Phycosiphon burrows are common not only in open body chambers but also in inner air chambers, which were originally closed. These observations suggest that the thin-shelled inner whorls and organic-rich siphuncular tubes degraded before final burial of the shell, and sediment infilling to the inside of the chambers followed. The early loss of inner whorls and siphuncular tubes gave rise to "draft-through currents." The continuous supply of oxygen and nutrients by the draft-through currents supported the Phycosiphon producers in the inner air chambers. Compared with other calcareous concretions containing intact fossils, values of minus-cement porosity (MCP) remain relatively low (63-74%) and vary by areas even in the same envelope concretion. This indicates that the envelope concretions were cemented under a progressive increase of compaction during the later diagenetic stage. The formation of the envelope concretion appears to be a long-term phenomenon. Various events at different stages have been overprinted in a single large ammonoid fossil.
Cretaceous Research, 2011
The taphonomic features and paleoecology of this species were investigated focused on vertically ... more The taphonomic features and paleoecology of this species were investigated focused on vertically embedded individuals of articulated Inoceramus amakusensis Nagao et Matsumoto. In the Hinoshima Formation, Himenoura Group of Kyushu, Japan, this Santonian (Late Cretaceous) inoceramid bivalve characteristically occurs in incised-valley fill siliciclastic marine deposits. Modes of I. amakusensis occurrence and preservation, from in situ (= occurrence in life position) to allochthonous shell fragments, are strongly affected by its paleoecology and depositional environments. Several I. amakusensis (up to 25 cm in shell height) were recovered from bioturbated sandstones associated with storm-influenced deposits. Their commissural planes are almost perpendicular to the bedding plane, with the anterior face oriented downward and the posteroventral portion extending upward. Furthermore, I. amakusensis is morphologically comparable to endobyssate mytilid bivalves today. These results suggest that this Cretaceous species was an orthothetic sand sticker at least during mid-ontogeny that preferentially inhabited a well-oxygenated, nearshore seafloor. I. amakusensis was distributed in various depositional environments and has been regarded as a recliner in offshore muddy substrate. However, the present discovery suggests that it was also well adapted, with an upright life position, to high-energy shallow clastic environments characterized by high sediment supply.
Paleontological Research, 2017
To clarify the process of producing the striations in the trace fossil Asteriacites, we observed ... more To clarify the process of producing the striations in the trace fossil Asteriacites, we observed the behavior of living ophiuroids and asteroids in aquariums. When ophiuroids stopped crawling, they buried themselves shallowly in the substratum, removing substratum under their arms and discs by using their tube-feet. The basal-arm tube-feet produce well spaced, fine, parallel striations that are perpendicular to the arm axis. The oral tube-feet produce fine, radial striations in the central depression. When the ophiuroids resumed crawling, they raised their disc and four arms above the substratum and dragged one arm backward. The one backward arm erased the striations, and parallel fine striations remained in four arm depressions. Similarly, asteroids also produced wide and shallow striations perpendicular to the arm axis by tube-feet movement. When the asteroids started to move again, they bulldozed the substratum under the one preceding arm, where the striations were erased. Since the asteroids crawled raising the other four arms by their tube feet which produced rough and deep striations, the wide and shallow striations remained only in a half of each arm depression. The striations of Asteriacites lumbricalis and A. quinquefolius were similar in shape to the striations produced by movement of tube feet of living ophiuroids and asteroids, respectively.
Paleontological Research, 2016
Five Spathian (late Olenekian) ammonoids are newly reported from the Zhitkov Formation in the Art... more Five Spathian (late Olenekian) ammonoids are newly reported from the Zhitkov Formation in the Artyom section, South Primorye, Russian Far East. They include the early Spathian taxa Bittnerites pacificus and Tirolites longilobatus from the middle part of the formation, and Ussuriphyllites amurensis, Leiophyllites sp. and Keyserlingites sp. from the upper part. The Ussuriphyllites amurensis Zone was previously correlated with the lower Anisian, but we herein correlate it with the upper Spathian. As a consequence of this age assignment, the weakly bioturbated offshore mudstone in the middle and upper parts of the formation is correlated with the Spathian. In contrast, the Smithian (early Olenekian) offshore laminated mudstone in the lower part of the formation lacks any sign of bioturbation. These mudstones suggest that the oxygen-deficient sea floor was transformed into an aerobic environment inhabitable to benthic organisms at the Smithian-Spathian boundary.
Kyoto University (京都大学)0048新制・課程博士博士(理学)甲第16642号理博第3754号新制||理||1543(附属図書館)29317京都大学大学院理学研究科地球惑星科学... more Kyoto University (京都大学)0048新制・課程博士博士(理学)甲第16642号理博第3754号新制||理||1543(附属図書館)29317京都大学大学院理学研究科地球惑星科学専攻(主査)准教授 前田 晴良, 教授 平田 岳史, 教授 大野 照文学位規則第4条第1項該
The stratigraphy and paleontology of a Lower Triassic section exposed within a quarry along the n... more The stratigraphy and paleontology of a Lower Triassic section exposed within a quarry along the northeastern coast of Abrek Bay, South Primorye has been the subject of an intense investigation. This 165 meter thick section is divided lithostratigraphically into the Lazurnaya Bay and Zhitkov formations in ascending order. The Lazurnaya Bay Formation, which unconformably overlies the Permian Abrek Formation, consists mainly of sandstone exhibiting an upward-fining sequence, whereas the overlying Zhitkov Formation consists of dark gray, laminated mudstone intercalated with thin, turbiditic fine sandstone beds. The Lazurnaya Bay Formation consists of the following three facies types in ascending order: a distal deltaic facies deposited during a transgressive event; a shelf environment above storm wave base; and a transitional facies from a shelf to a slope environment. A lower slope environment represents the lower part of the Zhitkov Formation, while the main part consists of a proximal basin-floor facies. This thick monotonous succession of barely bioturbated, laminated mudstone suggests deposition in a stable anoxic basin-floor setting. Aside from the lower part of the Lazurnaya Bay Formation, the remaining Lower Triassic strata are very fossilferous. Ammonoids, nautiloids, gastropods, bivalves, brachiopods, conodonts and shark fossils are abundant throughout the sequence, while crinoids and scaphopods are present only in the upper part. Nine ammonoid zones (taxon-rang zones) and beds, Lytophiceras sp. Zone, Gyronites subdharmus Zone, Ambitoides fuliginatus Zone, Clypeoceras spitiense "bed", Paranorites varians Zone, Clypeoceras timorense Zone, Radioprionites abrekensis "bed", Balhaeceras balhaense "bed", and Arctoceras subhydaspis "bed", as well as three conodont zones, Neogondolella carinata Zone, Neospathodus dieneri-N. pakistanensis Zone, and Neospathodus ex gr. waageni-N. novaeholladiae Zone are recognized in ascending order. Based on these ammonoids and conodonts, the sequence ranges in age from Early Induan (Griesbachian) to middle Early Olenekian (middle Smithian). Even though the ammonoid faunas contain a few species that are common to other realms, the majority of ammonoids are essentially endemic. In contrast, the bivalve fauna appears to be more related to the Maizuru fauna of Southwest Japan. While the grypoceratid nautiloids exhibit diverse shell forms and siphuncle positions, evidence suggests that they diversified primarily in the western Panthalassa area during the Early Triassic. The existence of Holocrinus in beds of early Smithian age, which represents the oldest known occurrence in the world, suggests that crinoids experienced an earlier recovery in South Primorye following the Permian-Triassic (P/Tr) mass extinction. The transition from the Lazurnaya Bay Formation to the Zhitkov Formation occurred during the early Late Induan (early Dienerian). Sedimentation appears to be continuous from the Late Induan to the Early Olenekian, and faunal successions during this interval are relatively complete and exceptionally well preserved. This phenomenon likely resulted from deposition under much deeper and quieter environmental conditions than other areas in the western part of the "Ussuri Basin". Although this fining-upward sequence is for the most part typical throughout South Primorye, the datum planes of fossil zones obviously extend across lithostratigraphic boundaries. In the Abrek Bay area, the Induan/Olenekian boundary exists within the Zhitkov Formation, whereas in other western areas of the region, it occurs in the Lazurnaya Bay Formation. These facts support an eastward-deepening setting of the Triassic System in the basin. Ammonoids are rare in the laminated mudstone of the Zhitkov Formation, but they are common in the very fine sandstone beds that are intercalated within the mudstone. These occurrences suggest that these ammonoids were not indigenous to the slope-basin environment, but rather were of allochthonous origin. Most of the ammonoids likely lived in shallower facies, and after death their empty shells were transported from their biotope to the anoxic basin-floor by low-density turbidite gravity flows. Sandstones exhibit a strong monazite age peak at 500 Ma and a subordinate peak at 270 Ma. These age patterns indicate that Triassic sediments in the Abrek Bay area were derived from the Khanka Block and/or the adjacent Jiamusi Block, with no contribution from the nearby Sino-Korea Craton. Similar age patterns have been recognized in Permian and Lower to Middle Triassic sandstones in the Maizuru Belt of southwest Japan, which suggests that both areas belonged to either the Khanka Block or the Jiamusi Block during the Early Triassic. This assumption is also supported by the faunal similarity of bivalves between South Primorye and the Maizuru belt. Eight new genera
Abstract. The stratigraphic distribution and modes of occurrence of Early Triassic Bellerophontoi... more Abstract. The stratigraphic distribution and modes of occurrence of Early Triassic Bellerophontoidea (Gastropoda) are studied at seven sections in South Primorye, Russian Far East, where depositional environments ranging from nonmarine, shoreface, to distal basin plain settings are recorded. Warthia zakharovi and Dicellonema abrekensis are abundant in Induan (Griesbachian and Dienerian) fine- to medium-grained, hummocky cross-stratified (HCS) sandstone beds occasionally intercalated with wavy-mudstone layers, whereas they are absent in coarser-grained cross-stratified successions. This observation suggests that bellerophontoids inhabited a lower shoreface environment above the storm wave base and possibly an inner shelf environment as well during this particular stage. Olenekian (Smithian and Spathian) bellerophontoids have not been found in the storm-induced sandstone beds, but W. zakharovi occurs in the lower Smithian sandstone beds of distal turbidites intercalated in the laminat...
Paleontological Research, 2015
Palaeontology, 2013
ABSTRACT The heteromorph ammonite Pravitoceras sigmoidale from the Upper Cretaceous Seidan Format... more ABSTRACT The heteromorph ammonite Pravitoceras sigmoidale from the Upper Cretaceous Seidan Formation (Izumi Group) in south-west Japan is frequently encrusted by sessile anomiid bivalves. Fossils of P. sigmoidale with anomiids are often concentrated at the top of or just above turbidite sandstones. Projecting retroversal hooks and apertures of P. sigmoidale are usually intact, and some individuals are associated with jaw apparatuses near apertures. Anomiids are found on both sides and ventral peripheries of P. sigmoidale conchs, attached predominantly to body chambers. These modes of occurrence suggest that the encrustation by anomiids occurred not on post-mortem floating or sunken carcasses but on live conchs and that these organisms were rapidly buried by turbidity current deposits shortly after death. Attachment to both flanks and ventral peripheries of the retroversal hooks may indicate that at least adult individuals of P. sigmoidale did not lie on the sea floor and did not drag their body chambers. It is suggested that fully mature individuals of this ammonite species lived for a long period of time after having formed the retroversal hook because a few generations of anomiids have colonized a single body chamber. Such colonization by anomiids is also observed on Didymoceras awajiense, which is considered to be the closely related ancestral species of P. sigmoidale. This anomiid–heteromorph ammonite commensal relationship might continue to persist in descendants during the course of evolution of these heteromorph ammonites.
Paleontological Research, 2010
Cretaceous Research, 2008
The depositional environments and bivalve assemblages are determined for the Upper Cretaceous Hin... more The depositional environments and bivalve assemblages are determined for the Upper Cretaceous Hinoshima Formation of the Himenoura Group, Kamishima, Amakusa Islands, Kyushu, Japan. The Hinoshima Formation is characterized by a thick transgressive succession that varies from incisedvalley-fill deposits to submarine slope deposits with high aggradation rates of depositional systems. The incised valley is filled with fluvial, bayhead delta, brackish-water estuary, and marine embayment deposits, and is overlain by thick slope deposits. Shallow marine bivalves are grouped into five fossil assemblages according to species composition: Glycymeris amakusensis (foreset beds of a bayhead delta), Nippononectes tamurai (foreset beds of a bayhead delta), Ezonuculana mactraeformis-Nucula formosa (central bay), Glycymeris amakusensis-Apiotrigonia minor (slope), and Inoceramus higoensis-Parvamussium yubarensis (slope). These bivalve assemblages all represent autochthonous and parautochthonous conditions except for a Glycymeris amakusensis-Apiotrigonia minor assemblage found in debris flow and slump deposits. The life habitats of these bivalves and the compositions of the assemblages are discussed in terms of the ecological history of fossil bivalves of the mid-to Late Cretaceous.
Paleontological Research, 2010
Based on materials from the Krasnoyarka Formation in the Naiba area in south Sakhalin, Russia, ta... more Based on materials from the Krasnoyarka Formation in the Naiba area in south Sakhalin, Russia, taphonomic histories of a large Campanian ammonoid, Canadoceras kossmati Matsumoto, 1954, were closely investigated. Large Canadoceras shells exceeding 30 cm in diameter are usually embedded horizontally and solitarily in muddy sandstone. A thin, lenticular calcareous concretion envelopes the shell (= envelope concretion). Their body chambers are mostly lost. The inner whorls comprising the center of the umbilicus completely disappear without exception, and only two or three outer whorls are preserved. The body and air chambers are somewhat compressed by compaction and are filled with sediments. Phycosiphon burrows are common not only in open body chambers but also in inner air chambers, which were originally closed. These observations suggest that the thin-shelled inner whorls and organic-rich siphuncular tubes degraded before final burial of the shell, and sediment infilling to the inside of the chambers followed. The early loss of inner whorls and siphuncular tubes gave rise to "draft-through currents." The continuous supply of oxygen and nutrients by the draft-through currents supported the Phycosiphon producers in the inner air chambers. Compared with other calcareous concretions containing intact fossils, values of minus-cement porosity (MCP) remain relatively low (63-74%) and vary by areas even in the same envelope concretion. This indicates that the envelope concretions were cemented under a progressive increase of compaction during the later diagenetic stage. The formation of the envelope concretion appears to be a long-term phenomenon. Various events at different stages have been overprinted in a single large ammonoid fossil.
Cretaceous Research, 2011
The taphonomic features and paleoecology of this species were investigated focused on vertically ... more The taphonomic features and paleoecology of this species were investigated focused on vertically embedded individuals of articulated Inoceramus amakusensis Nagao et Matsumoto. In the Hinoshima Formation, Himenoura Group of Kyushu, Japan, this Santonian (Late Cretaceous) inoceramid bivalve characteristically occurs in incised-valley fill siliciclastic marine deposits. Modes of I. amakusensis occurrence and preservation, from in situ (= occurrence in life position) to allochthonous shell fragments, are strongly affected by its paleoecology and depositional environments. Several I. amakusensis (up to 25 cm in shell height) were recovered from bioturbated sandstones associated with storm-influenced deposits. Their commissural planes are almost perpendicular to the bedding plane, with the anterior face oriented downward and the posteroventral portion extending upward. Furthermore, I. amakusensis is morphologically comparable to endobyssate mytilid bivalves today. These results suggest that this Cretaceous species was an orthothetic sand sticker at least during mid-ontogeny that preferentially inhabited a well-oxygenated, nearshore seafloor. I. amakusensis was distributed in various depositional environments and has been regarded as a recliner in offshore muddy substrate. However, the present discovery suggests that it was also well adapted, with an upright life position, to high-energy shallow clastic environments characterized by high sediment supply.