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.
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.
► The first record of a Santonian inoceramid,
Inoceramus amakusensis in upright life position. ► This species inhabited storm- and wave-influenced, well-oxygenated sandy bottoms during mid-ontogeny. ► Its life position is comparable to that of endobyssate mytilids or semi-infaunal sand stickers. ►
I. amakusensis, possibly by changing its life habit, could have adapted to various marine environments.
Abstract
The heteromorph ammonite
P
ravitoceras sigmoidale
from the
U
pper
C
retaceous
S
eidan
F
ormation (
I
zumi
G
roup) in south‐west
J
apan 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
D
idymoceras 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.
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 incised-valley-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.
The Upper Cretaceous Himenoura Group is exposed in Oshima, Amakusa Islands, Kyushu, southwest Japan. It is composed of inner shelf, shoreface, bay, and various tidal deposits, which are characterized ...by wave-and tide-dominated facies. In addition, abundant bivalve fossils are divided into five fossil assemblages and these correspond to each depositional facies : Inoceramus - Sphenoceramus - Nanonavis sachalinensis (inner shelf), Glycymeris amakusensis - Loxo japonica (shoreface), Yaadia japonica (shoreface), "Ostrea" sp. - Septifer ushibukensis (bay) and Corbula ushibukensis - Crassostrea sp. (tidal flat and tidal channel) assemblages. Characteristic species of these assemblages show autochthonous or indigenous mode of occurrences, and their habitats are reconstructed from the modes of occurrence, shell preservations and depositional facies. These assemblages can be considered as indicators of the Upper Cretaceous shallow-marine environments.
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.
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