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The evolutionary origin of centriole/kinetosomes, centrosomes, and other microtubule organizing centers (MTOCs), whether by direct filiation or symbiogenesis, has been controversial for >50 years. Centrioles, like mitochondria and chloroplasts, duplicate independently of the nucleus and constitute a heritable system independent of chromosomal DNA. Nucleic acids endogenous to the MTOC would support evolutionary origin by symbiogenesis. To date, most reports of centrosome-associated nucleic acids have used generalized reagents such as RNases and nucleic acid dyes. Here, from a library of RNAs extracted from isolated surf clam (Spisula solidissima) centrosomes, we describe a group of centrosome-associated transcripts representing a structurally unique intron-poor collection of nuclear genes skewed toward nucleic acid metabolism. Thus, we resolve the debate over the existence of centrosome-associated RNA (cnRNA). A subset of cnRNAs contain functional domains that are highly conserved across distant taxa, such as nucleotide polymerase motifs. In situ localization of cnRNA65, a molecule with an RNA polymerase domain, showed it is present in the intact oocyte nucleus (germinal vesicle). Its expression, therefore, precedes the appearance of [gamma]-tubulin-containing centrosomes. At this stage, the in situ signal resembles the nucleolinus, a poorly understood organelle proposed to play a role in spindle formation. After oocyte activation and germinal vesicle breakdown, cnRNA65 persists as a cytoplasmic patch within which [gamma]-tubulin-stained centrosomes can be seen. These observations provoke the question of whether cnRNAs and the nudeolinus serve as cytological progenitors of the centrosome and may support a symbiogenetic model for its evolution. centrosome | evolution | nucleolinus

The Mysidiellidae are morphologically isolated among Triassic bivalves but share important characters with Late Paleozoic Ambonychioidea. Apart from a great similarity in the general shape of the shell, the most primitive mysidiellid genus Promysidiella resembles ambonychioids in the presence of a duplivincular-opisthodetic ligament system. Within the Mysidiellidae, this ligament type evolved into the transitional ligament system that characterizes Late Triassic Mysidiella. The phyletic polarity indicates that this evolution probably took place by paedomorpbosis. New examinations of the shell microstructure of Mysidiella demonstrate the presence of simple prismatic and possibly foliated structures in the calcitic outer shell layer, which further supports an ambonychioid affinity. Therefore, the Mysidiellidae are removed from the Mytiloidea and assigned to the Ambonychioidea. The poorly known genus Protopis, which was originally included in the Mysidiellidae, probably had a parivincular ligament system and was hence a member of the Heteroconchia. Joannina, which was previously considered a junior synonym of Protopis, is re-established. The hinge margin of Joannina carries a well developed nymph but lacks teeth. These characters as well as its modioliform shape, anterior shell lobe, and pronounced diagonal carina link Joannina with the Late Triassic genus Healeya (Modiomorphoidea). Both taxa are herein placed in the new family Healeyidae, which differs from the morphologically similar Kalenteridae in the absence of elaborated hinge teeth. Protopis, as well as the recently described genera Leidapoconcha, Waijiaoella, and Qingyaniola, are tentatively assigned to the Healeyidae.

A high-resolution record of Eocene paleotemperature variation is preserved within the high southern latitude, marine shelf succession of the La Meseta Formation on Seymour Island, off the NE side of the Antarctic Peninsula. [sup.87] Sr[sup.86] Sr ratios of bivalve shell carbonate indicate that the La Meseta Formation spans virtually the entire Eocene, and suggest the presence of an early middle Eocene unconformity. Paleoclimatic and paleoceanographic inferences are based on the stable oxygen and carbon isotope values of two genera of bivalves collected with a high degree of stratigraphic resolution within the formation, and with multiple replicate samples from each horizon. [[delta]sup.18] O data indicate roughly 10[degrees] C of cooling from the early Eocene climatic optimum (~15[degrees] C) through the end of the Eocene (minimum ~5 [degrees] C), much of which took place in two comparatively short intervals at ca. 52 and ca. 41 Ma. Many features of the isotope curves generated from this Eocene shelf section are apparent in [[delta]sup.18] O and [[delta]sup.13] C data from the Southern and global oceans, including warm intervals that likely correspond to the early and middle Eocene climatic optima (EECO and MECO). A rapid middle Eocene shift to much more positive values is the most significant in the section and reflects a drop to universally cooler temperatures in the late middle and late Eocene that might also be associated with a short-lived glacial advance. However, even using a somewhat depleted value for [[delta]sup.18] O of seawater in the Antarctic peninsular region, average Seymour Island shelf-water paleotemperatures did not reach freezing before the end of the Eocene. [[delta]sup.13] C data similarly reflect the documented middle Eocene surface ocean enrichment followed by more negative values, but depletion is much more pronounced on Seymour Island and persists for the remainder of the Eocene, suggesting a combination of upwelling, metabolic effects, and/or atypical carbon cycling on the shelf in this region. Isotope data capture information about changes in the paleoenvironment that also had consequences for the biota, as published paleontological records document marked change in the nature of terrestrial and marine biota at this time. The fact that middle Eocene cooling and biotic turnover in the Peninsular region correspond well in time to the proposed initial opening of Drake Passage suggests that the formation of gateways, in addition to changes in [pCO.sub.2], had significant consequences for the Earth's climate system during the Paleogene. Keywords: stable isotope, Eocene, climate, bivalve, Antarctica, strontium.

Critical reinvestigation of the bivalve Buchia cf. blanfordiana Jeletzky, 1965 from the Upper Jurassic of western British Columbia indicates that it is morphologically distinct from B. blanfordiana Stoliczka, 1866 of the Indo-Pacific region. We analyzed collections from relatively coeval sections containing multiple buchiid species from the Northern and Southern Hemispheres and used ten morphological characters to describe Buchia shell shape and size. We tested for taxonomic differences on left and right valves using a traditional morphometric approach with both linear and angular measurements and Fourier (outline) analyses. Phenetic discrimination revealed considerable overlap in the morphospace, but high classification rates between the two groups when compared with a step-wise discriminant analysis. This study supports the retention of the Indo-Pacific genera Australobuchia Zakharov, 1981 and Malayomaorica Jeletzky, 1963 and confirms that Buchia (=Australobuchia) blanfordiana is restricted to the Southern Hemisphere. Consequently, we propose the new name Buchia columbiana n. sp. for material assigned to Buchia cf. blanfordiana Jeletzky, 1965. Our results have implications for bipolarity and migration of the genus Buchia, supporting previous suggestions that buchiids formerly identified as Buchia from the Southern Hemisphere should be grouped as a separate genus.

The specimens of Gastrochaena cuneiformis Spengler, 1783, with Spengler-written labels at the Zoologisk Museum, Copenhagen, did not come from Spengler's type locality in the Nicobar Islands, and may instead be syntypes of Chemnitz's (1788) West Indies 'Pholas hians'. The identity of Gastrochaena cuneiformis as a senior synonym of Gastrochaena gigantea (Deshayes, 1830) is established on the basis of Spengler's original descriptions and illustrations, and by examination of specimens from the type locality. A neotype for G. cuneiformis is designated and illustrated, and its genus is revised to exclude Rocellaria Blainville, 1829, and Lamychaena Freneix in Freneix and Roman, 1979. Gastrochaena Spengler, 1783 is the most plesiomorphic of these three genera, as shown by its simple boring, short siphons, and diffuse, poorly differentiated anterior pedal muscles. Rocellaria evolved from a close common ancestor with Gastrochaena, and is characterized by a ventral shift and fusion of the posteroventral pallial sinus with the posteroventral pallial band, low, irregular posterior commarginal lamellae, and well defined anterior pedal retractor muscles generally supported by myophores. Lamychaena evolved from Rocellaria during the Oligocene, extending its ctenidia far posterior into the siphonal part of the boring, and, in some species, uniting its anterior pedal retractor and protractor muscles as they approach the byssus apparatus.

A bivalve fauna from the Permian Triassic interval of the Dongpan section, Guangxi, South China is described herein. This bivalve fauna includes 16 species belonging to 10 genera. New taxa are Euchondria fusuiensis, Palaeoneilo qinzhouensis, and Claraia liuqiaoensis. Bed 12 of the interval (Talung Formation) contains the typical late Late Permian ammonoids: Huananoceras sp., Laibinoceras cf. L. compressum Yang, and Qiangjiangoceras sp. Bed 13 (Luolou Formation) contains typical Early Triassic bivalves Claraia dieneri Nakazawa, C. cf. C. wangi (Patte), and C. griesbachi (Bittner), which coexist with the earliest Triassic ammonoid Ophiceras sp. The bivalve and ammonoid distributions at the Dongpan section indicate that bed 12 should be assigned to the Late Permian, and bed 13 should be assigned to the Early Triassic. From the early Changhsingian to the Induan, the byssal notches of Claraia species tend to become progressively narrower, and change from being ventrally extended to being horizontally extended. The species level diversity of Claraia also increases through this interval.

Shells of freshwater bivalves are commonly used for radiocarbon dating late Pleistocene archaeological and vertebrate fossil sites, thus providing important constraints on late Pleistocene human dispersal and megafauna extinction hypotheses. The reliability of bivalve shells for dating rests partly on the ease with which subsequent diagenetic alteration can be recognized; typically, wherein original shell aragonite is replaced by calcite in meteoric environments. Here we document late Pleistocene freshwater bivalve shells wherein meteoric diagenesis involved syntaxial overgrowth of aragonite cement on original aragonite shell biocrystals. Aragonite cement was identified in situ using Raman microspectroscopy and formed rather than calcite as a result of unusually high Mg:Ca ratios in local groundwaters. Thus, altered shells contain diagenetic [sup.14.C], rendering their dates unreliable, but they may slip past common vetting techniques because (1) epitaxial cements are not readily apparent petrographically because they are in optical continuity with adjacent biocrystals; (2) X-ray diffraction indicates that no calcite is present; (3) alteration is not apparent in cathodoluminescence studies; and (4) stable isotopes of C and O are difficult to interpret in shells that originate in terrestrial-meteoric environments. Hence, although freshwater with a high Mg:Ca ratio is not common, groundwater chemistry should be considered before accepting bivalve-based radiocarbon dates uncritically. More broadly, meteoric diagenesis in carbonate rocks is generally characterized by the dissolution of aragonite or its conversion to calcite. Our data show that such is not invariably the case, even in fully terrestrial, freshwater systems. Keywords: radiocarbon dating, meteoric diagenesis, bivalve shell, cement, Mg:Ca ratio, Pleistocene.

The ellipse, Venustaconcha ellipsiformis (Bivalvia: Unionidae), was first recorded in Minnesota from the Straight River in 1987, but we knew little of its distribution in the state, brooding behavior, glochidial host fish relationships or status. To examine these questions, we followed standard procedures to conduct an extensive, qualitative statewide mussel survey, described mantle flapping behavior from field and laboratory observations and identified suitable glochidia hosts in the laboratory and from naturally infested fishes. We found extant populations in five Mississippi River tributaries in southeastern Minnesota: the Cannon, Cedar, Root, Upper Iowa and Zumbro rivers. This appears to be the extent of its historic range in Minnesota as no valid records were found elsewhere. Among these, the largest population occurred in the headwaters of the Root River drainage, which was also the only drainage where we observed recent recruitment. Brooding V. ellipsiformis quickly flap a small mantle extension, often in response to passing shadows or jarring of the substrate, or their mantle extensions may be slowly undulated. We identified 11 fish species as suitable hosts for V. ellipsiformis glochidia in the laboratory: brook stickleback (Culaea inconstans), mottled sculpin (Cottus bairdii), slimy sculpin (C. cognatus), logperch (Percina caprodes), mud darter (Etheostoma asprigene), rainbow darter (E. caeruleum), Iowa darter (E. exile), fantail darter (E. flabbelare), Johnny darter (E. nigrum), banded darter (E. zonale) and blackside darter (P. maculata). Rainbow darter, fantail darter and blackside darter were also found naturally infested with V. ellipsiformis glochidia. Venustaconcha ellipsiformis should remain classified as a 'Threatened' species in Minnesota and management should include conserving populations within drainages due to its restricted range and likely low dispersal ability.

Myophorellid bivalves (Trigonioida, Myophorellacea) with conspicuously tuberculate shells are abundant in Lower Cretaceous rocks of the Neuquen Basin represented by the Gondwanian genus Steinmanella Crickmay, 1930. Publications concerning Neuquen trigonioids deal only with taxonomy, and until now the sedimentary environments of the Steinmanella bearing facies have not been interpreted. The present study analyzes morphology, taxonomy, stratigraphy, and paleobiology of Steinmanella species collected from five sections of the Lower Member of the Agrio Formation, including the type locality. Two species are differentiated: Steinmanella pehuenmapuensis (Leanza, 1998) from the lower part of the member (Pseudofavrella angulatiformis Zone) and Steinmanella transitoria (Steinmann, 1881) from the middle and upper part of the member (Holcoptychites neuquensis, Hoplitocrioceras gentilii, and Weavericeras vacaensis Zones). Specimens of both species have been recorded in intermediate to high-energy shoreface and low-energy offshore deposits. The inferred paleoecologic model is that Steinmanella lived semi-infaunally and burrowed on sandy substrates in the shoreface and on muddy substrates in the offshore. Colonization of low-energy subenvironments in the offshore probably occurred during increased seafloor oxygenation and reduced net sedimentation or brief omission periods.

We used 24 fossil and Recent species to construct character states of both composite and exemplar taxa for phylogenetic analyses of Neogene genera and subgenera of the Corbulidae from tropical America. All characters were conchologic and two matrices, which differed in the manner that commarginal-rib characters were coded, were analyzed using branch and bound searches and maximum parsimony. Character polarity was determined using Corbula sensu strictu as an outgroup. These analyses produced a limited number of robust and well-resolved cladograms that require only one ghost lineage. Such stable results indicate a high level of congruency among characters, and demonstrate that conchologic data sets can yield highly resolved cladograms. Tropical American corbulids are not monophyletic, and include two major clades. Crown groups within these clades are endemic, and all genera endemic to tropical America first appear in the Miocene. In fact, generic diversity and body size peak in the upper Miocene of the Caribbean/western Atlantic. Range restrictions and extinctions of large-bodied genera from both corbulid clades contribute to a post-Miocene decline in body size in this region. The eastern Pacific does not experience a similar decline in diversity and body size. Diversity and morphologic trends in Caribbean corbulids coincide with regional environmental changes, in particular decreases in seasonality and productivity. Except for the extinction of Bothrocorbula, however, corbulid extinctions apparently predate faunal turnover reported for other molluscs. Associated with these extinctions, we found evidence of geographic range restriction, but not range shifts, in corbulid genera, indicating that the geologic development of environmental refugia contributed more to survival than eurytopy. Large-bodied genera of the southern Caribbean Gatunian Province (Tenuicorbula, Panamicorbula, and Hexacorbula) became restricted to the eastern Pacific. Range restriction to this high productivity refugium (i.e., paciphilic genera), rather than origination of new taxa, produced several corbulid genera now endemic to the region. Large-bodied genera from the Caloosahatchian Province of the southeastern U.S. (Both-rocorbula and Bicorbula), however, underwent global extinction. These subtropical and warm temperate taxa are presumably more eurytopic than their tropical counterparts, but do not exhibit range shifts in response to Neogene environmental change.