Fossil Oysters Research Articles

Non-destructive Analytical Study of Raman Spectra Variations and Mechanisms of Calcite and Aragonite in Modern and Fossilized Oysters.

Oyster fossils are some of the most common bivalve mollusk fossils found all over the world, they are different from other fossils because the oyster is still alive in the present day, and the body structure of modern oyster is almost the same as that of ancient one. Therefore, we designed a control experiment comparing the Raman spectra of minerals from both modern oysters and fossil oysters to explore the mechanism of oyster's fossilization process, which is considered to be helpful for investigating biological evolution or paleoenvironment. The oyster fossil sample was found in Nagi-Cho, Okayama Prefecture, Japan. We focused on the variations of band position and full width half-maximum of ν1 Raman band (symmetric stretching mode) of calcite (CaCO3) from modern and fossil oysters and the mineral conversion between calcite and aragonite (CaCO3) around the adductor muscle inside the oyster. Compared to modern oysters, the ν1 band at around 1086 cm-1 of calcite from oyster fossils shifted to a high wavenumber region, and the possible reason for this phenomenon is considered an elemental substitution between Ca2+ and Mg2+. As for aragonite around adductor muscle in fossil oysters, it has been found by Raman spectra that most of the aragonite has been converted into calcite because calcite has a relatively more stable structure.

Fossil oysters from the southernmost coast of Brazil: Bioerosion and palaeoenvironmental implications

Fossil oysters are commonly found in Rio Grande do Sul Coastal Plain (RSCP), where the main species are Ostrea cf. puelchana d’Orbigny, 1841 and Crassostrea praia Ihering, 1907. These fossils are transported towards the beach by storm events throughout the year, being available in the swash zone. The objective of this article is to reconstruct the Holocene palaeoenvironment by analysing these fossils. A total of 1898 valves, collected along Cassino and Hermenegildo beach, both in southernmost Brazil, were described. It was observed a prevalence of C. praia at Cassino beach (39.20%) and O. cf. puelchana at Hermenegildo beach (89.02%). Hermenegildo beach sample were statistically greater for C. praia height ( p-value: 0.1736) and length ( p-value: 0.4604) and for O. cf. puelchana height ( p-value: 0.0602) and length ( p-value: 0.07394). The predominance for northern and southern regions for each species indicates a palaeoenvironment setting that favoured these organisms, probably related to temperatures and salinity variation. The most common ichnogenera found in the oysters were Caulostrepsis and Entobia present in both sites, observed in over 40% of internal and external sides of the samples’ shell. According to radiocarbon modelled age, the studied samples coexisted during the Mid-Holocene (8.290–8.130 cal. yr. BP), with the subsequent sea-level rise and fall the shells became bioavailable. The lack of samples with closed valves and high percentage of bioerosion in the inner side of the valve, indicates that the palaeoenvironment in both Cassino and Hermenegildo beaches gradually changed, not as result of an extreme event.

Evolution of the paleo-Daesan Bay (Nakdong River, South Korea) as a result of Holocene sea level change

AbstractTo explore sea level transgression in low-lying inland areas and its possible influence on prehistoric cultures, we investigated the physical and geochemical features of 20-m-long sedimentary cores from the previously seawater-filled Daesan Basin located in the middle reach of the present Nakdong River in Korea as proxies for seawater transgression deep inland areas due to Holocene sea level rise. Based on the relationships among grain size, total sulfur content (TS%), and carbon/sulfur (C/S) ratio, the first transgressive event was detected at ca. 8500 cal yr BP, caused by seawater influx along the present Nakdong River. Higher TS% (0.8–1%) and interbedded fossil oysters at 8000–6000 cal yr BP indicate marine environments, supporting a paleo-Daesan Bay with water depth of ~10–8 m. The common peaks in TS%, in both inland paleo-Daesan Bay and a present coastal area (Suncheon Bay) in southern Korea (e.g., at 3200 and 4700 cal yr BP), may indicate intervals of higher salinity, which suggests simultaneous responses to changes in sea level or hydroclimate. The duration of marine environment (paleo-Daesan Bay) in the remote inland from ca. 8000–3200 cal yr BP provides an analog for inland paleo-bay studies in East Asia.

Reconstruction of the evolutionary biogeography reveal the origins and diversification of oysters (Bivalvia: Ostreidae)

Oysters (Bivalvia: Ostreidae Rafinesque, 1815) live in the intertidal and shallow subtidal areas worldwide. Despite their long evolutionary histories, abundant fossil records, global distribution, and ecological significance, a systematic time-dependent biogeographical analysis of this family is still lacking. Using combined mitochondrial (COI and 16S rRNA) and nuclear (18S rRNA, 28S rRNA, H3 and ITS2) gene makers for 80% (70/88) of the recognized extant Ostreidae, we reconstructed the global phylogenetic and biogeographical relationships throughout the evolutionary history of oysters. The result provided a holistic view of the origin, migration and dispersal patterns of Ostreidae. The phylogenetic results and fossil evidence indicated that Ostreidae originated from the circum-Arctic region in the Early Jurassic. The widening of the Atlantic Ocean and changes in the Tethys Ocean further facilitated their subsequent diversification during the Cretaceous and the Palaeogene periods. In particular, Crassostrea and Saccostrea exhibited relatively low dispersal abilities and their major diversifications were consistent with the tectonic events. Environmental adaptations and reproductive patterns, therefore, should play key roles in the formation of oyster distribution patterners, rather than the dispersal ability of their planktonic larvae. The diversity dynamics inferred by standard phylogenetic are consistent with the fossil record, however, further systematic classification, especially for fossil genus Ostrea, would enhance our understanding on extant and fossil oysters. The present study of the historical biogeography of oysters provides new insights into the evolution and speciation of oysters. Our findings also provide a foundation for the assessment of evolutionary patterns and ecological processes in intertidal and inshore life.

Reef-building Pacific oysters record seasonal variations in water mass-properties of tidal basins from the Central Wadden Sea (North Sea)

High-resolution proxy records are needed to evaluate models simulating past, present and future climatic conditions. Before such records can be reliably employed, calibration studies have to be conducted to assess the confidence intervals of the proxies. Here, we use shells of the fast growing Pacific oyster Magallana gigas from the Central Wadden Sea, North Sea, a temperate barrier island-backbarrier tidal flat-salt marsh system with large seasonal changes of water mass-properties, for the calibration of geochemical proxies. M. gigas represents a non-native invasive species that rapidly develops oyster reefs. Calcite shells of two specimens from the intertidal and subtidal zones were sampled in high resolution yielding sub-monthly data sets. The time period represented in the shell, based on δ18O age modeling, was estimated at 8–10 years and the growth of the shells was restricted from (late) spring to (early) autumn of each year. Mg/Ca, Mn/Ca and Sr/Ca ratios of the intertidal and subtidal specimens show similar seasonal patterns. Mg/Ca and Sr/Ca ratios are investigated as high-resolution sea surface temperature (SST) proxies. Important ontogenetic effects (i.e., increasing time-averaging with increasing age) as well as inter-specimen variability are discussed as limiting factors for the proxy development. Intertidal Mg/Ca ratios show only a significant correlation to the high-resolution SST record of the Central Wadden Sea when the early ontogenetic stage is considered. Sr/Ca ratios were comparable in terms of absolute values and amplitudes to those of M. gigas in the Northern Wadden Sea, but amplitudes were decreasing with increasing age. These findings seriously hamper the application of Mg/Ca and Sr/Ca for reliable palaeotemperature reconstructions regardless of ontogenetic stage. The Mn/Ca ratios were investigated as proxy for Mn cycling in tidal basins, where it is interrelated with seasonal changes in primary production. In addition to the generally observed seasonal variability of the Mn/Ca oyster records, the subtidal Mn/Ca is significantly elevated compared to intertidal Mn/Ca. The subtidal Mn/Ca offset likely reflects differences in Mn cycling in tidal settings and could, therefore, serve in the palaeorecord as indicator to differentiate inter- and subtidal habitats in the same embedding sedimentary facies. This habitat effect has to be considered as an important factor besides environmental change when interpreting the high-resolution proxy record of fossil oysters.

The complex case of Chesapeake Bay restoration

Amid encouraging signs, researchers and activists are struggling to make progress in the face of agricultural inputs, climate change, and relentless development . On a warm October day, six researchers ventured out to a reef in Baines Creek, a tributary of Virginia’s Elizabeth River. Back in 2014, the Army Corps of Engineers had dumped tons of fossil oyster shells into the water in the hopes of attracting more of one of the key inhabitants of the Chesapeake Bay—oysters. Now the researchers, wearing masks and socially distancing as they wade through the water at low tide, are measuring the progress. Numerous tributaries feed into the Chesapeake Bay, such as the Severn River near Annapolis, MD. They enhance the beauty of the Bay’s watershed—they also shuttle pollutants that imperil the Bay’s ecosystems. Image credit: Shutterstock/Ladinn. What they are finding is just astounding, says Romuald Lipcius, professor of fisheries science at the Virginia Institute of Marine Science (VIMS) at Gloucester Point, and also the School of Marine Science at William & Mary University. The team counts hundreds of oysters growing on each square meter of reef. Some oysters had reached the ripe old age of five, once considered unachievable because of diseases that had been introduced into the Bay; oysters’ fecundity increases exponentially with size and age. “All these oysters peeking above the water is a beautiful sight,” says William & Mary professor of geology Rowan Lockwood, who studies fossil oysters. “It is a glimpse of what the Bay was like 6,000 years ago.” And maybe, what it could be like again. But that ideal future is still a long shot. A couple of weeks after Lipcius plumbed the depths of Baines Creek, Matt Pluta piloted his small boat through a dense fog on the Choptank River, which flows into the Bay from …

A conservation palaeobiological perspective on Chesapeake Bay oysters

The eastern oyster plays a vital role in estuarine habitats, acting as an ecosystem engineer and improving water quality. Populations of Chesapeake Bay oysters have declined precipitously in recent decades. The fossil record, which preserves 500 000 years of once-thriving reefs, provides a unique opportunity to study pristine reefs to establish a possible baseline for mitigation. For this study, over 900 fossil oysters were examined from three Pleistocene localities in the Chesapeake region. Data on oyster shell lengths, lifespans and population density were assessed. Comparisons to modern Crassostrea virginica, sampled from monitoring surveys of similar environments, reveal that fossil oysters were significantly larger, longer-lived and more abundant than modern oysters from polyhaline salinity zones. This pattern results from the preferential harvesting of larger, reproductively more active females from the modern population. These fossil data, combined with modern estimates of age-based fecundity and mortality, make it possible to estimate ecosystem services in these long-dead reefs, including filtering capacity, which was an order of magnitude greater in the past than today. Conservation palaeobiology can provide us with a picture of not just what the Chesapeake Bay looked like, but how it functioned, before humans. This article is part of a discussion meeting issue 'The past is a foreign country: how much can the fossil record actually inform conservation?'

The Tyrannosaur Chronicles: The Biology of the Tyrant Dinosaurs by DavidHone. Bloomsbury Sigma, London, 2016. No. of pages: 304. Price: UK£12‐99. ISBN 978‐1‐4729‐1126‐1 (paperback).

Dinosaurs. Again. I am not really complaining. Honest. One of the favourite groups of ancient organisms deserves its popularity, and David Hone has written an exposé of the most loved of these, the tyrannosaurs. Hone writes in a comfortable ‘bedside’ manner, just right for a book that is aimed for an informed yet not necessarily specialist audience. Illustrations, including colour plates, are good. Some numbering of figures is eccentric – fig. 5a is on p. 46, yet 5b is on p. 57. Errors in the text are few but include wrongly italicised Ankylosaurs (p. 40) and fragements (p. 53). Preliminary chapters include, unusually, an informative ‘Note from the illustrator’, Scott Hartman, and a brief introduction to tyrannosaur osteology. Names of bones that are also figured are mainly in bold, but nasals are not in bold but illustrated (p. 17), whereas the atlas vertebra is bold yet not labelled on the relevant diagram (p. 18). All chapters are well written and informative, but I am unimpressed that Hone explains the importance of radiometric dating and downplays the relevance of biostratigraphic correlation (pp. 73–74). Some aspects are also imprecise in their explanation; ‘Small fossils are harder for us to find than large ones …’ (p. 76) contradicts a comment on the same page, ‘… small species tend to be more numerous than large ones’. Hone's book is divided into four sections encompassing 19 chapters. Part 1 is introductory, covering obvious and relevant aspects such as history of research, diversity, relationships and biogeography. Hone introduces the history of dinosaur research with a suitably broad brush and even major figures, like Barnum Brown, who excavated the first Tyrannosaurus rex (Dingus and Norrell, 2010), receives scant mention. Yet this is understandable; Hone's focus is on what we know, not what we knew. He is in a rush to get to the hard facts. Part 2, ‘Morphology’, is divided, logically, into six chapters such as ‘Skull’ and ‘Limbs’ and forms an informative whole. Hone's explanations can be sparkling, even Wodehousian, none more so than when discussing the function of bone trabeculae: ‘… would provide strength – a little like the spokes strengthen a bike wheel’ (p. 105). The problems posed by track sites (whodunit?) are understood (p. 119), not a universal attainment of dinosaur workers. Yet Hone can be repetitious, such as when he uses feathered – feathers – feathery four times in one sentence (p. 127). Part 3, ‘Ecology’, is less satisfactory, essentially because the data is scantier and more difficult to interpret. A bed of fossil oysters, with borings, encrusters and other obvious organism-organism interactions, may yield more solid, reproducible palaeoecological data than all the tyrannosaur fossils known. Such is the nature of the fossil record; marine invertebrates are a richer source of ecological information than terrestrial vertebrates, although I salute workers on tyrannosaurs for gleaning the not inconsiderable dataset discussed by Hone. But Part 4, ‘Moving forwards’, is largely speculative and provides a weak finish to Chronicles. If these speculations had been integrated into earlier chapters and the book ended with a short, sharp finale, the ending would undoubtedly been stronger. Overall, this is a book that will please armchair dinosaur buffs, but which will also be widely read amongst the experts. Similarly informed volumes on other dinosaur groups would be welcome additions to the literature.

Burial estimates constrained by clumped isotope thermometry: example of the Lower Cretaceous Qishn Formation (Haushi-Huqf High, Oman)

Abstract Accurate determination of the thermal history of sedimentary basins is critical to constrain the timing of diagenetic processes. Here, clumped isotope palaeothermometry is used to estimate minimum burial depth of the Lower Cretaceous Qishn Formation in east central Oman. Fossil oysters were collected in a soft argillaceous unit, thin-sectioned, and studied under petrographical and cathodoluminescence microscopy, revealing a variable state of shell preservations with early silica replacement of carbonate and late-stage calcite cementation. Clumped isotopes values varied from 0.602 to 0.666‰, with the best-preserved oyster shell yielding a temperature of 37 ± 4°C and a calculated oxygen isotope ratio in seawater (δ 18 O seawater ) compatible with Cretaceous seawater having experienced moderate evaporation (1.0–1.5‰ VSMOW). The new minimum estimates for the burial depth of the Qishn Formation is 1.0–1.2 km, based on the temperature difference between the well-preserved oyster and the partially neomorphosed oyster recording the highest burial temperature (63 ± 4°C). This is in excess of what was predicted by previous studies (<400 m), but compatible with conodonts alteration index temperatures (<80°C). This study highlights the potential of clumped isotopes as a quantitative tool to estimate temperature and burial depth in fine-grained carbonate succession where fluid inclusions are absent, and offers a new tool to constrain the thermal histories of sedimentary basins.

Rapid relative sea-level fall along north-eastern Australia between 1200 and 800 cal. yr BP: An appraisal of the oyster evidence

A fast-paced post-glacial sea-level rise and subsequent mid-Holocene sea-level highstand are well documented at several far field locations away from the presence of former ice sheets but sea-level development during the late Holocene remains ambiguous. In this study, we present new data from modern and fossil oysters attached to shoreline rocks along the north-eastern Australian coastline that reveal new constraints on the nature and timing of relative sea-level change over the past 2500yr. Surveyed elevations of various contemporary oyster zones contextualize modern oyster growth forms in relation to sea-level datum and build the reference for our fossil oyster data. Based on survey data and field observations we developed a robust set of criteria for measuring fossil oysters to determine their relative sea-level position and constrain the uncertainties associated with these reconstructions. Thick (>10cm) fossil oyster visors above the equivalent modern growth suggest higher relative sea-levels in the past (i.e. >1200cal.yrBP). Radiocarbon analyses of the modern oyster visors suggest continuous lateral accumulation over the past ~800yr which implies relatively stable sea-level over this period. The modern and fossil dataset defines a distinct and rapid relative ~1m sea-level fall between 1200 and 800cal.yrBP. Whether the sea-level fall was stepped or followed a broader smooth/monotonic pattern is unclear. The timing coincides with the initiation of some inshore fringing coral reefs in the Great Barrier Reef region and other major geomorphological changes along the coastal zone. A combination of various factors may have been the driving mechanism behind this relative sea-level fall with rates between 1.0 to 5.2mmyr−1.

Holocene Antarctic melting and lithospheric uplift history of the southern Okinawa trough inferred from mid- to late-Holocene sea level in Iriomote Island, Ryukyu, Japan

Relative sea level deduced from fossil oysters and mangroves are reported for Iriomote Island, Ryukyus, southwestern Japan. Radiocarbon-dated fossil oysters, as well as previously analyzed mangrove muds, are compared to geophysical and glacio-hydro-isostatic adjustment (GIA) models that describe Earth deformation due to changing surface loading. The Holocene-high-stand (HHS) inferred from oyster fossils (Saccostrea echinata and Saccostrea malaboensis) is 2.7 m at ca. 3500 years ago, after which sea level gradually fell to present level. The HHS magnitude attributed to GIA for the last ca. 4000 is between 1 and 1.5 m above present day sea level, and the residual indicates the long-term lithospheric uplift rate of the island. The timing of peak HHS also indicates that late Holocene melting mainly from Antarctica ceased by approximately 4000 years ago.

Ichnology of fossil oysters (Bivalvia, Ostreidae) from the southern Brazilian coast

Ichnological features over fossil oysters from southern Brazilian coast are described herein. In the deposits Ostrea puelchana d’Orbigny, 1841 is the dominant oyster, followed by O. equestris Say, 1834 and Crassostrea virginica Gmelin, 1791. Although these taxa are found along the entire coast, they exhibit some geographical variations regarding the presence of borings. The most conspicuous ichnogenera are Caulostrepsis and Entobia, with Gastrochaenolites being common in fossils from the central sector of the coast, and scarce in the southern one. Maeandropolydora and incrustations (made by fouling organisms, e.g. cirripeds and bryozoans) are scarce. Most of the borings are found on the external side of the valves which suggest a dominant bioerosion activity during the life span of the organisms.Key words: Quaternary, Ostreidae, bioerosion, Entobia, Caulostrepsis, Gastrochaenolites.

Birth of the modern Chesapeake Bay estuary between 7.4 and 8.2 ka and implications for global sea-level rise

Two major pulses of sea-level rise are thought to have taken place since the last glacial maximum — meltwater pulses (mwp) 1A (12 cal ka) and 1B (9.5 cal ka). Between mwp 1B and about 6 cal ka, many of the complex coastal ecosystems which ring the world’s oceans began to form. Here we report data for rhenium, carbon isotopes, total organic carbon, and fossil oysters from Chesapeake Bay which span the transition from fresh to brackish water conditions in the bay in the mid-Holocene. These data constrain sea-level change and resulting environmental change in the bay. They indicate that the transition was rapid, and that it was produced by (1) a third pulse of rapid eustatic sea-level rise, or (2) a geometry of the prehistoric Chesapeake Bay basin which predisposed it to a nonlinear response to a steadily rising sea level. Similar nonlinear changes in vulnerable coastal environments are likely to take place in the future due to polar warming, regardless of the timing or rate of sea-level rise.

Of coiled oysters and big brains: how to rescue the terminology of heterochrony, now gone astray.

During the past decade, the terminology of heterochrony, heretofore consistent and workable, has become internally illogical and incoherent as the unfortunate result of an extension of terms, properly devised to describe shifts in developmental timing of shapes and features, to the rates and timings that cause these shifts. All the resulting, and extensive, confusion in the literature arises as a pure consequence of this error in logic and nomenclature, and not at all from disagreement about the important empirical questions described by this central concept and phenomenon in the integration of evolution and development. In particular, the claim that the same feature in human evolution (the paedomorphic shape of the human cranium) expresses either neoteny or the apparently opposite phenomenon of hypermorphosis only records the terminological error, and not any factual disagreement-for this neotenic feature has probably arisen by a prolongation of juvenile growth patterns inappropriately designated as "hypermorphosis of rate." I show that a prominent and unchallenged case of neoteny in fossil oysters arises by exactly the same evolutionary mode. When we restore the terminology of heterochrony by the "paedomorphic" intellectual event of dropping these inadaptive terminal accretions (the illogical extension of shape categories to describe rates), then the concept of heterochrony will again make proper distinctions by designating a clearly meaningful category of evolutionary changes originating by shifts in timing for features already present in ancestors. "It's not all het- erochrony"-and this particular statement of "less is more" represents heterochrony's strength as an interesting subset with definite meaning, rather than an illogical hodge-podge apparently applicable to all phenomena, and therefore explaining nothing.

Geology and Zoology — A Symbiosis Darwin's Beagle Voyage and Galapagos Experience

Darwin's South American fossil collections yielded many insights. in one section he recorded the gradual vertical change in shell characteristics of fossil oysters. The giant fossil vertebrates brought the realization that several were precursors of living species. Contrary to accepted Catastrophism Theory, he inferred that all taxa found fossilized did not become extinct together since the living agouti, among others, had fossil precursors. He envisioned the way an existing molluscan fauna and its sedimentary matrix could be transformed into fossiliferous rocks reminiscent of the Tertiary, and also speculated about the kinds of fossils that the Galapagos biota would yield in some future rock column. Volcanic islands (Galapagos Archipelago and others) were linked in Darwin's thought to the appearance of "peculiar" taxa inhabiting these "mere spots" in the sea. Were these migrants from the nearest continents? If so, why were they so distinctive while carrying the imprint of their place of origin? If not, how did these four-legged and other terrestrial creatures get to the islands? Were the archipelagos ever united to the continents? Gradually Darwin was led to a continent-archipelago-continent concept. Continents were elevated archipelagos. (He had the Andean volcanoes in mind.) Archipelagos were submerged remnants of continents. He incorporated into this concept the rise and extinction of species: elevation of archipelagos were the "best nurseries" for appearance of new taxa, while subsidence of continents generally led to extinction of extant forms.

Occurrence of Ostrea eilatensis: a New Oyster Species from the Cenomanian of Southern Israel and Egypt

THE Cenomanian sections in Central and Southern Israel are rich in ostreid fauna. The majority of fossil oysters represented are attributed to Exogyra; however, I have lately collected many specimens of morphologically variable species, unquestionably referred to Ostrea, from various localities in the Negev, Southern Israel.

Australian Tertiary Paleotemperatures

Tertiary fossil oysters and clams from sites in southern Victoria, Australia, of about lat. 38° S. have been used to determine paleotemperatures from their $$O^{18}/O^{16}$$ ratio. High temperatures were found in the Upper Oligocene-Lower Miocene with a fairly sharp drop into the Middle Miocene.

The Problem in Oyster Taxonomy

HEDGPETH (1954) has resurrected an old question concerning oyster taxonomy and evolution has not been alive since days of d'Orbigny. was settled by Leymerie (1840, 1874) a hundred years ago, although he was not fully vindicated until some fifteen years ago. Nevertheless, lest biologists not acquainted with oyster taxonomy be led astray, I must take up pen and reply. The points upon argument rests seem to be two. 1. Oysters from four different zones of Jurassic (Liassic) show gradations from ordinary oyster shape to extreme ramshorn upward curl of beak of Gryphaea, and these populations may be arranged in a statistical series with intermediate stages being most numerous. 2. There exists a ramshorn specimen of Japanese oyster, Crassostrea gigas, of a photograph is shown, and Packard (1923) found an Eocene specimen of of Exogyra type (lower valve laterally curved). From this, Hedgpeth suggests that factors which induced Gryphaea lines, are still latent in estuarine stock of (p. 24) and says the names we choose to apply to our modern stocks of oysters may be more a matter of taste (and predisposition) than a reflection of genetic circumstances. He then suggests that Gryphaea be retained for both Mesozoic fossils and living group, as originally used by Lamarck. This suggestion imaginatively propounds confusion, and whole argument is untenable. One small fact has been skillfully arrayed against a vast mass of contrary evidence, most of was not mentioned. In early days of taxonomy it was common to confuse unrelated species. Lamarck did not separate fossil Gryphaea from living Crassostrea and indeed it was only on basis of fairly recent knowledge that latter genus was clearly separated from Ostrea (cf. Gunter, 1950). A similar instance was Walbaum's inclusion of paddlefish, Polyodon, with sharks. There are hundreds of others. Hedgpeth would have us return to this early stage of taxonomic knowledge of oysters. Within certain limits, defined by fact that shells consist of two hinged valves, oysters are among most plastic organisms known. They vary in all sorts of ways within those limits, a fact attested by many workers. The writer once found one shaped like Cinderella's glass slipper and sent it off to Washington. Nelson (1938) called attention to a specimen was mistaken for Spondylus by a worker familiar with latter group. He also said that certain shells of Virginia oyster in his collection came well within range of five other species, belong to two genera. Ranson (1942) said distinction of a genus based upon sculpture of shell is artificial. The writer (1950, p. 438) said, It is difficult to separate genera and exceedingly difficult to separate species of oysters on basis of shell characteristics alone. Incidentally, some paleontologists have stepped into this pitfall. I once saw, in a well-known museum, a group of fossil oysters had not been fully separated from their matrix and were obviously a block from one reef. They showed only variations exhibited by many single species. Yet they had been carefully divided into seven species and were so labeled. Thus, in no group of animals is there more taxonomic necessity for using full statistical series, adequate to show wide variations, than in oysters. The writer (Gunter, 1950) previously sug-

The feeding mechanism of the oyster. I. On the pallium and the branchial chambers of Ostrea virginica, O. Edulis and O. Angulata, with comparisons with other species of the genus

AbstractSeparation of the mantle from the visceral mass on the right side in deeply cupped species of oyster provides a shorter channel for egress of water from the right demibranchs. Correlated with this change is reduction in size of the right epibranchial chamber beneath the adductor together with backward displacement of the adductor itself. This water passage, designated the promyal chamber, develops during the first week after attachment of the oyster larva. The greatest displacement of the adductor and the largest promyal chamber occur in O. frons. The mantle border shows three reduplications of which the innermost, the pallial curtain, controls the amount and the place of entrance and egress of water. The importance of pallial curtains and promyal chamber to survival in turbid waters is discussed; oysters with the promyal chamber have invaded the river mouths, the flat oysters without this chamber have remained in the clear waters of high salinity near the sea. The bearing of these factors on the disappearance of fossil oysters is considered. It is proposed to relate to the genus Ostrea all flat, larviparous oysters which lack the promyal chamber and to raise the subgenus Gryphaea to generic rank to include all deeply cupped oviparous oysters with the promyal chamber. Anatomical and histological features of the pallium and branchial chambers are considered in detail.

Remarkable Fossil Oysters from Syria

Remarkable Fossil Oysters from Syria