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The direction of mastication revealed by dental microwear was studied in relation to changes in the structure of the masticatory apparatus in the Issiodoromyinae (Rodentia, Mammalia). Our results offer a rare opportunity to assess the order of establishment of the morphological characters that are related to the acquisition of propalinal mastication in an extinct lineage. Elfomys and Pseudoltinomys show cuspidate tooth crowns, a high mandibular condyle, and a significant lateromedial orientation of the masticatory muscles (superficial masseter and internal pterygoid) in association with oblique chewing movements. In contrast, the Issiodoromys lineage is characterized by a flat molar occlusal surface, a low mandibular condyle, and posterior orientation of the masseter and pterygoid associated with slightly oblique to propalinal chewing. We show that striking convergences with some members of Ctenohystrica have occurred in the evolution of the Issiodoromyinae.
A total of 289 belemnite guards was analyzed from the Boreal Barremian (northeastern England and northwestern Germany) for their trace elements and isotopic composition. The oxygen-isotope signature shows similar paleotemperature variations for all seven sections investigated. Three different phases of climatic evolution can be recognized. An earliest Barremian cool phase (Praeoxyteuthis pugio belemnite Zone) is followed by a distinctive warming event in the late early Barremian (Aulacoteuthis spp. belemnite Zone). This Aulacoteuthis warm pulse has a relatively short-term peak (~500 kyr), which accompanies widespread anoxic bottom-water conditions and records the highest temperature throughout the Valanginian–Barremian in the Boreal Realm. The late Barremian (Oxyteuthis brunsvicensis, O. germanica, and O. depressa belemnite Zones) is characterized by relatively positive δ18O values reflecting a cooling phase. This paleotemperature interpretation of the oxygen data is supported by the Mg/Ca trends. The carbon isotope curve of the Barremian shows an increase from 1‰ in the early to 3‰ in the late Barremian. This trend may reflect the global signal of the carbon budget of Barremian oceans and be related to the widespread deposition of TOC-rich sediments in the southern part of the Boreal Realm and the progradation of the Urgonian carbonate platforms in the Tethys. There is evidence, however, that paleobiological factors may have played a much more important role in the incorporation of stable carbon isotopes than hitherto thought. In a monospecific belemnite population collected from one 30-cm-thick interval, the δ13C signal shows a variation of 1.5‰. Ontogenetic changes in the metabolic activity linked to temperature or food changes may explain changes in the fractionation rate, resulting in an ontogenetically controlled biofractionation.
Spectacular accumulations of abundant bryozoan nodules (bryoliths) occur in the early Pleistocene coarse-grained, mixed carbonate-siliciclastic deposits of the Cape Arkhangelos Calcarenite of Rhodes, Greece. These bryoliths are always associated with numerous bivalves (mostly ostreids and pectinids) and serpulid worms, together with other rarer skeletal organisms. The spheroidal bryoliths vary in diameter from 1 cm to 16 cm, and those with a more ellipsoidal shape range from 0.8 cm to 22 cm in length and from 0.7 cm to 16 cm in width; an elongated bryolith of exceptional size (40 cm) was even found. The bryoliths are composed almost exclusively of the anascan cheilostome Calpensia nobilis and are nucleated on gravels, pebbles, or bivalve shells. After settlement of bryozoan crusts, the bryoliths increased in size by repeated overturning of these mobile hard substrates by longshore currents. The bryoliths developed at depths of 20–40 m on a flat shoal between emerged areas where bottom currents episodically accelerated during storms. The currents also transported the bryoliths, together with other skeletal elements, into deeper settings (40–80 m) where they could continue their growth, upward or downward, or be colonized by other organisms.
Despite the considerable knowledge gained of the patterns and processes shaping the completeness and fidelity of fossil bivalve faunas, it is still hard to generalize these patterns to the species level and to different regions around the globe. Here I analyze the completeness and fidelity of Quaternary bivalve faunas of the temperate Pacific coast of South America, summarizing >120 years of paleontological studies in the region. The degree of completeness, measured as percentage of extant forms, is only moderate. Only 45% of the 93 extant species are preserved in the Quaternary fossil record. When completeness was measured as the discoverable fraction of species (according to a non-parametric richness estimation), however, values were >88%. Missing species were not biased by shell mineralogy nor concentrated in particular taxonomic groups. Completeness was highly selective for other species traits; small size, geographic restriction, and forms inhabiting deeper areas of the shelf had lower chances of being present in the fossil record, in agreement with previous studies. A generalized linear model revealed that the effect of these variables was multiplicative—i.e., the effects of geographic range and bathymetric distribution were significant only for small-sized forms. Therefore, the moderate degree of completeness may be the consequence of a limited fossilization potential (e.g., via low preservation potential, lack of deep-shelf outcrops, or preburial down-slope transport), rather than poor sampling quality. Results suggest that the sampling effort made in the region has reached a plateau and that further progress in the completeness of species' inventories may occur only at a very slow rate.
Modification of event beds by the burrowing nereidid polychaete Alitta virens (Sars) was examined using laboratory microcosms, to assess its importance as an ecosystem engineer in pristine sediments. In all microcosms, the nereidids modified their environment to permit long-term occupation, but different behavioral strategies and burrow morphologies were observed based on sediment characteristics and nutrient availability. Alitta virens utilized scavenging, surface deposit feeding, suspension feeding, microbial gardening, deposit feeding at depth, and cannibalism. Suspension feeding using mucus nets is used by many nereidids but has not been documented previously in A. virens; extended use of the technique may indicate low availability of biotic sediments for deposit feeding. Alitta virens typically produced burrows similar to Arenicolites and Skolithos, but morphologies resembling Polykladichnus, Planolites, Palaeophycus, and Thalassinoides were formed under differing sedimentary conditions and over different time scales. In the rock record, such ichnological diversity might be interpreted as indicating paleoecological diversity, rather than the response of one taxon to changing conditions. Alitta virens is an allogenic ecosystem engineer, its behavior changing the physical and geochemical characters of its environment. These changes, combined with the widespread occurrence and population longevity of A. virens, demonstrate that burrowing polychaetes are important ecosystem engineers in shallow marine environments, and are likely to have been so over geological time scales.
Burrow-wall micromorphologies produced by nine different intertidal invertebrates were characterized by scanning electron microscopy. Minimally disturbed burrows and host sediment associated with Heteromastus filiformis, Saccoglossus bromophenolosus, Corophium volutator, Clymenella sp., Phoronopsis viridis, Cirriformia luxuriosa, Arenicola marina, Nereis virens, and Upogebia pugettensis were collected from various locales along the western coast of Washington and California. Scanning electron micrographs show several unique sedimentary characteristics present in the burrow lining of each invertebrate. Three traits—grain orientation and grain-size distribution, the nature of the mucus lining, and the presence of wall sculpture—appear significant in describing the microscale burrow lining. In several burrow types, the burrow structure was distinguishable from the matrix by changes in the modal grain size and sorting of the burrow-wall sediment. Variations in the type of packing—cubic versus rhombohedral packing—between the burrow and sediment often accompanied grain-size variations. In addition to the alterations to grain distribution and orientation, most burrows surveyed showed mucus-stabilized walls. The style of the mucus secretion—globules, threads, or ribbons—and the resultant structure—smooth blankets or woven mats—used to stabilize the burrow wall can characterize burrow linings. Wall sculpture (microbioglyphs or microsurficial morphology) was observed rarely but when present was highly distinctive of Corophium volutator burrow linings. The observed variation in burrow-wall micromorphologies provides evidence of trace-maker behavior, burrow-preservation potential, and sediment diagenesis.