Marine bivalves of the family Myalinidae are an important benthic constituent of the Permian Reef Complex of West Texas and New Mexico. We describe and summarize the myalinids from Lower and Middle Permian reef and near-reef settings and infer living habits as either epifaunal or semi-infaunal byssally attached suspension feeders. The six myalinid species described are exceptionally preserved as silica pseudomporphs. Included in the fauna are two new taxa: Myalina lamellosa, a species with distinctive commarginal lamellae, and Myalina plicata, the only known myalinid with prominent radial plicae. The family Myalinidae is placed in the Ambonychioidea (Order Pterioida) and an emended diagnosis incorporates ligament characters and details of shell ultrastructure.
ABSTRACT: We describe a diverse molluscan fauna of silicified fossils from
two localities of the Taylor Mountains D-3 quadrangle of southwestern
Alaska. The molluscan fauna consists of at least 8 species of
bivalves, including 1 new species. Cassianella cordillerana
McRoberts n.sp., and at least 11 species of gastropods, including 2
new species, Neritaria nuetzeli Blodgett n.sp. and
Andangularia wilsoni Blodgett n.sp. Bivalve and gastropod
affinities suggest an early Norian age with taxonomic similarities to
several southern Alaskan tectonostratigraphic terranes (for example,
Alexander and Chulitna), as well as to the South American Cordillera
of Peru. The mollusks are associated with numerous brachiopods that
also support a Norian age and similar biogeographic affinities.
ABSTRACT: Marine bivalves document the long-term increase in generic richness through the early Mesozoic. Following the end-Permian crisis, the Early Triassic was marked by a gradual recovery in generic richness (57 Induan and 66 Olenekian genera). Diversity slowly increased in the Middle Triassic (98 Anisian and 121 Ladinian genera) and peaked in the Late Triassic (171 Carnian, 165 Norian, and 143 Rhaetian genera). These data support earlier hypotheses that the recovery following the end-Permian extinction was very gradual and was not completed (in terms of both richness and ecologic complexity) until the Ladinian. Although a Carnian-Norian extinction is not evident in the data and may be a regional event limited to the Tethyan Realm, the end-Triassic extinction is profound--less than 30 genera (<35%) survived into the Jurassic. Diversity metrics are not equally distributed among bivalve living habits. The generally epifaunal Pteriomorphia and Isofilibranchia exhibit higher extinction rates compared to the ordinarily infaunal Heteroconchia (especially the Veneroida and Trigonoida). This pattern of selective extinction led to a gradual increase in generic richness of infaunal suspension feeders through most of the Triassic. Contrary to previous hypotheses, this increase in infaunalization may not have been related to the evolutionary expansion of major predatory groups (e.g., shell-crushing cephalopods, crustaceans, sharks, fish, and reptiles), which had typically low abundances and limited distribution during the Triassic. Drilling predators, although present during the Triassic, are not considered to be prominent causes of mortality among bivalves. Instead, the infaunalization of bivalves during the Triassic may have been due to several interconnected abiotic and biotic causes associated with the recovery after the end-Permian mass extinction.
ABSTRACT: A distinctive myalinid bivalve Elversella rugosa new genus
and species from the Middle Permian of west Texas is described.
Elversella rugosa is characterized as being inequivalved, with
a larger left valve covered rhythmically by coarse rugae and a smooth
right valve that is somewhat smaller and less convex.
ABSTRACT: Black Bear Ridge on Williston Lake is one of several important
North American reference sections for intercalibrating the ranges of
Upper Triassic ammonoids, conodonts, bivalves, ichthyoliths, and
brachiopods. The section, largely comprising the Ludington and
Pardonet formations, consists of four sequences (BB-I through IV)
spanning the uppermost Carnian through Hettangian of the Lower
Jurassic. These sequences record deposition on the distal slope and
adjacent abyssal plain, west (seaward) of the Pangean continental
shelf. Fossil fauna and sequence stratigraphy serve to identify
significant biological and sedimentary events in the history of the
Late Triassic. Each of the four sequences consist of a comparably
coarse- grained lowstand succession which grades up through a thick
transgressive succession marked by peak abundances of the conodont
(Norigondolella) and ichthyolith (Birgeria). Highstand systems tracts
within the study interval are thin, consisting of condensed intervals
with few fossiliferous limestone beds. The Carnian-Norian boundary
interval lies within sequence BB-1 and the transitional beds between
the Ludington and Pardonet Formations, wherein a lineage of several
Metapolygnthaus conodont species provide two potential datums for
boundary definition: the base of the Communisti and Primitius zones.
Within the same interval, several ammonoids and bivalves occur, and
there are several changes in the ichthyolith fauna.
ABSTRACT: Black Bear Ridge on Williston Lake is one of several important
North American reference sections for intercalibrating the ranges of
Upper Triassic ammonoids, conodonts, bivalves, ichthyoliths, and
brachiopods. The section (Ludington and Pardonet formations) spans
the uppermost Carnian through Hettangian of the Lower Jurassic. The
co-occurrence of several fossil groups facilitates an integrated
biozonation of the interval. Fossil fauna and biofacies serve to
identify significant biological and sedimentary events in the history
of the Late Triassic. Conodont and ichthyolith faunal change is
gradual through the Carnian-Norian boundary, whereas conodont and
ammonoid changes are dramatic across the Lower-Middle Norian
boundary. Major transgressions are recognized in both the Lower and
Middle Norian, and are marked by a conodont (Norigondolella) and
ichthyolith (Birgeria) biofacies.
ABSTRACT: Halobia daonellaformis new species is described
from the lowermost Carnian of northeast British Columbia. Halobia
daonellaformis is regarded as a primitive Halobia characterized
by external ornamentation similar to Daonella lommeli,
but with a poorly developed anterior auricle. Morphologic characters
of H. daonellaformis suggest that Halobia may not be a
natural taxon, but a polyphyletic group with one or more ancestors
from Daonella and Aparimella and/or other
posidoniid(s). The sudden appearance of Halobia throughout the
marine Triassic suggests a rapid dispersal mechanism following a
Ladinian origin. Larval shell morphology indicates a planktotrophic
developmental strategy for H. daonellaformis, and by extension
to other halobiids, may explain the widespread distribution of many
halobiid species.
ABSTRACT: Classification of scleractinian corals based upon gross
morphological features has been found unsatisfactory in the light of
additional information from skeletal microarchitecture and
microstructure. It is necessary to investigate microstructural
details and limits in morphologic variation within and between
different coral clades before a revised classification is
constructed.
Variations in morphologic characters and microstructural details from
a population of Dimorpharaea (Family Microsolenidae) from
Upper Bathonian (Jumara Dome) strata in Kachchh are described. The
data used include the diameter (D) and height (H) of the corallum,
number of corallites in the colony (NC), number of septa in the
mother corallite at the center of the colony (NS), minimum distance
between centers of central corallite and corallite of the inner ring
(C1), minimum distance between corallite centers of the outer ring
(C2), septal density (DS) and trabecular density (DT). The principal
components analysis reveals that most of the variation is explained
by "size" related characters (D and H) while corallite density (NC
and C1) and septal structures (DS and DT) contribute to the second
and third principal component axes, respectively. The
microarchitecture and distribution of characters observed in the
Kachchh Dimorpharaea require a re-evaluation of
familial-specific concepts and suggest that the population belongs to
a single species, Dimorpharaea stellans, rather than four
morphospecies (D. stellans, D. distincta, D. continua and
D. orbica) as has been assumed by previous authors.
ABSTRACT: The Norian-Rhaetian bivalve fauna from the Upper part of
Antimonio Formation is taxonomically and ecologically more diverse
than previously reported. Of the ten bivalve taxa described herein,
eight have never been previously described from Sonora and at least
four represent previously unknown forms. Two new species are formally
recognized: (i) ?Lopha cordillerana n. sp., a large ostreid
from a Norian bioherm facies, and (ii) Xiaoschuiculana tozeri
n. sp., a nuculanid from fine-grained sand and siltstones of probable
Rhaetian age. Other taxa, while ecologically important, are too few
in number or too poorly preserved to be adequately assigned to
species. Although the Antimonio bivalve fauna shows strong taxonomic
affinities with South American faunas, similarity to other North
American localities, such as from the Luning-Gabbs sequence of Nevada
or co-eval rocks in the Cordilleran terranes of California and
Oregon, cannot be assessed due to inadequacy in species-level
taxonomy.
ABSTRACT: A sequence spanning the Triassic-Jurassic boundary is described from near the village of Lorüns in the Vorarlberg region of western Austria. At Lorüns, the latest Triassic is characterized by bedded carbonates of the Kössen Formation supporting a stenotopic fauna indicative of a shallow sub-tidal environment of normal marine salinity. The Triassic-Jurassic boundary may be represented as a sequence boundary developed on top of a 1.1 m thick boundary interval of red mudstones of the lower Schattwald Shale which is interpreted to have been deposited in a marginal marine environment and quite possibly a mud flat. Above the boundary interval, the upper Schattwald Shale is characterized by thin-bedded marl and dark limestone beds with a Hettangian macrofauna dominated by epifaunal filter-feeding bivalves including ostreids and mytilids, which suggest a shallow, subtidal, salinity-controlled environment typical of an interplatform lagoon. Carbonate production rejuvenated in the later Early Hettangian with development of the Lorüns Oolite, a shallow subtidal oolitic and oncolitic unit bearing echinoderms indicative of normal marine conditions.
Lithologic and geochemical evidence for the spread of anoxic
waters during marine transgression is absent from Lorüns where
Th/U values, determined by gamma-ray spectrometry, are above 5 in the
boundary interval. A negative excursion in
d13C in the boundary interval can be
interpreted in two different ways: (i) a short-term decrease in
primary productivity or (ii) precipitation of caliche during paleosol
development. The first alternative is favored due to the positive
excursion in d18O within the same interval,
even though such a change may have been short-lived. Although
short-term changes in paleotemperature may have occurred within the
boundary interval, comparison of d18O values from the Kössen
Formation and Lorüns Oolite indicate no significant long-term
changes in paleotemperature.
ABSTRACT. Novaculapermia boydi gen. et sp. nov. is a
remarkable Lower Permian vertically elongate bivalve that
superficially resembles 'razor' clams of the superfamily Solenoidea.
Our 'razor' clam possessed a duplivincular ligament and early
ontogeny of the Myalinidae. The flattened, equiconvex form suggests
that Novaculapermia was a shallow vertical burrower in soft
sediments possibly anchored by byssal attachment; evidently it was
not a reef dweller, but lived in near-reef environments.
ABSTRACT: Past attempts to substantiate the species-area effect by correlating changes in sea-level and marine diversity have met with limited success. Partial rank correlation and concordance analyses are used as two complementary numerical methods to examine the association between sea-level changes and diversity as predicted by the species-area effect. When applied to Early Jurassic bivalve species from northwestern Europe, the numerical analyses failed to discriminate an association (r = -0.298 for the partial rank correlation and p = 0.45 for the concordance probability). Additional analysis using subsets of the data or recoding periods of anoxic water as periods of reduced habitable area (in addition to marine regression) also failed to show a significant association. The absence of significant correlation is likely to be due to numerous biotic and abiotic factors that cannot directly be measured from sediments or fossil assemblages. A multitude of interrelated cause and effect relationships renders the species-area effect a poor predictor of the influence of sea-level changes on marine diversity.
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