Two skulls representing new forms of carnivorous cynodonts were discovered recently in the Upper Triassic (Carnian) Ischigualasto Formation of northwestern Argentina. These specimens are the first complete carnivorous cynodonts described from the Late Triassic of Argentina, and provide critical evidence for the evolution of cranial form between well known Middle Triassic cynodonts, such as Probainognathus, and the first mammals in the latest Triassic.

The first skull (PVSJ 422) is proportionately long, with a posterior skull width only 35 per cent of basal skull length (9.5 cm.). Other distinctive features of the skull include the form of the postcanine crowns, which have two cusps posterior to the principal cusp and fine serations on their leading edges, and which overlap each other in an imbricate pattern. Postcanines 5-7 are subequal in size, overlap the dentary laterally, and are substantially larger than postcanines 2-4. The dental formula in the upper jaw is I4-C1-PC7, with the small first postcanine separated fore and aft by small diastemas. As in other advanced eucynodonts, no pineal foramen is present. Some postcranial elements, as yet unstudied, were also found with this specimen.

The second skull (PVSJ 411) has broader proportions, the width between quadrates almost half that of basal skull length (7.5 cm.). These proportions, in combination with deep, curved zygomatic arches, resemble other Middle Triassic cynodonts such as Probelesodon. Distinctive features of this skull include bicusped postcanines in the upper jaw similar to those of Probelesodon, and a mandibular symphysis that extends back to the level of the fourth postcanine. The dental formula in the upper jaw is I4-C1-PC6. As in the first skull, a pineal foramen is absent.

Several advanced cranial features, such as a fused mandibular symphysis, occur in both skulls and strongly suggest that these new forms belong within the Eucynodontia (Hopson, 1991). Other advanced features, such as the absence of a pineal foramen, further suggest that these new forms are closer to mammals than are traversodontid cynodonts. In contrast, other characters appear to be curiosly primitive, such as a short hard palate in the first skull. These new cynodonts thus may introduce previously unseen features and character combinations to the study of cynodont cranial evolution.

A well preserved archaeocyath fauna occurs in the Forteau Formation in Southern Labrador, Canada. Following on from the confirmation that archaeocyatha are aspiculate calcified sponges, detailed ecological work is now being undertaken on this fauna, a representative of the first reefal ecosystem in the fossil record.

One species, Metaldetes profundus, shows considerable variability in growth form, from open bowl-shaped forms in lagoonal settings, to a branching morphology in biohermal habitats. This species also appears to colonise a variety of substrate types and for this purpose often uses exothecal structures and holdfasts. M. profundus is also the dominant species numerically, comprising 80% of the fauna, except at the top of the bioherm horizon, where Archaeocyathus atlanticus becomes dominant. In contrast to previous studies' findings, bowl-shaped Metaldetes at the top of the bioherm horizon occupy the basal region of the bioherms, not the upper areas and provide a surface on which the bioherms can develop.

The unique form of Retilamina amourensis endows it with the ability to stabilise substrates so facilitating the colonization of a substrate. It is more common at the base of bioherms, providing a surface on which other archaeocyaths can settle. The calcimicrobes Renalcis and Epiphyton also appear to have played a major role in the initiation of the bioherms, occurring as 3–5cm thick layers on erosion surfaces, within which small, juvenile, upright M. profundus sticks, <1cm in diameter, are found, surrounded and supported by the dense calcimicrobe aggregate.

Digital 3-D analysis is being used to document the behaviour of the archaeocyaths, together with vertical and lateral zonation within the bioherms, especially with regard to the timing of calcimicrobe encrustation. Volumetric abundances of the archaeocyaths are also being calculated.

There are small faunal composition changes over the area: Archaeosycon billingsi, a solitary stick-like form is more abundant in the west than in the east and Archaeocyathus atlanticus, another stick-like form occurs in dense clusters in lagoonal environments, elsewhere being solitary.

General.—Ebridians, in many ways, are “neglected step-children” in the family of siliceous microplankton. Reduced to only three living species and never particularly abundant througout their history, ebridians do show up in marine, diatom-bearing sediments from practically every region of the globe.

The title of this chapter makes a promise that cannot be kept: that somehow there could be summarized in its few pages all the paleoenvironments of dinosaur-bearing strata through time. Dinosaurs were –as far as can be determined – ubiquitious in the terrestrial realm. It would be impossible to summarize the Recent terrestrial settings of birds (avian dinosaurs), let alone 135 million years of terrestrial settings. For this reason, this paper will be restricted to generalizations about ancient environments, highlighting particularly interesting or productive dinosaur-bearing localities in North America.

Biological parameters, in addition to physical parameters, are important in determining past ecology, taphonomy and the effects of human intervention. Research conducted on a Recent community of gastropods and two late Pleistocene fossil assemblages from Puerto Penasco, Mexico, reveal a complex pattern of interrelationships among gastropod shell users. First, shell representation is biased in the intertidal of Puerto Penasco, Mexico, because of a complex mosaic of secondary shell occupants. Hermit crabs (five species) represent almost half (47%) of the intertidal gastropod shell resource available throughout the year. Living snails are represented by 17 out of the 32 gastropod taxa. Additionally, hermit amphipods (three species) occupy ten gastropod taxa. Hermit crabs and hermit amphipods retain the shells in anomalous habitats (that differ from the living snail). Second, physical factors act as a temporal component which affects shell use and availability during the seasons at Puerto Penasco. Late winter storms mix-up the intertidal distribution of living gastropods and hermit crabs. Subtidal to low intertidal shells appear in the high intertidal; living snails are buried under a thick bed of sand. Most importantly, empty shells become available, and the hermit amphipod population peaks. Thus, physical factors contribute to the demise of living snails (i.e, burial by sand) and the mixing of shells. However, the organisms (hermit crabs and amphipods) maintain this motif by retaining the shells in the anomalous habitats.

Third, all hermit crab species (Paguristes anahuacus, Pagurus lepidus, Paguristes roseus), except for one (the high intertidal, Clibanarius digueti), have epi-and endobionts associated with the gastropod shell. More than 20 species of invertebrates bore into or encrust the hermitted shells at Penasco. Of these, the encrusting bryozoans Hippothoa, Hippopodinella adpressa, ?Floridia antiqua, Lichenopora, Antropora tincta and the boring spionid polychaetes (Polydora commensalis, Polydora, Boccardia) and spirorbid polychaetes (Spirorbis; Serpula) are important bionts to use in recognizing hermit crab shell use in the fossil record of the northern Gulf of California. The encrusting bryozoans (H. adpressa and A. tincta) are present on Pleistocene gastropods at the unusual Pelican Point terrace deposit (large gastropod shells preserved among large bryozoan encrusted cobbles) indicating hermit crab inhabitation. These bryozoans appear to protect the gastropods from taphonomic alteration.

Finally, reworked fossil shells occur within the hermit crab guild and the beach drift assemblage. Hermit crabs retain fossil shells of the moon snail, Polinices, (n=two occurrences) and Turritella (n=3 occurrences). These species are common in the coquina beach rock which makes up the intertidal substrate of Puerto Penasco. However, reworking of fossil coquina is quite substantial in the beach drift assemblage. Three sampling periods (=150 samples) indicate the following: three species of fossil bivalves (Chione, Trachvcardium and Glycimeris) and five species of fossil gastropods (Oliva, Polinices, Muricanthus, Nassarius, and Turritella) dominated the beach drift assemblage (over 16, 600 fossil whole shells/fragments). Fossil Chione represented the most shells (958 valves;>15,557 fragments). Recent bivalves were represented by 1115 shells/fragments (representing 12 species) and Recent gastropods contained mostly fragments (1069 pieces; 30 species). Additionally, the fossil gastropods were large, unlike the species that occur today, which have been picked over by humans. Thus, a large part of active beach deposition at Puerto Penasco contains late Pleistocene shells, taphonomically altered by secondary occupants and beachcombers.

The marine vicariant event resulting from the Pliocene emergence of the Central American Isthmus presents a unique opportunity for calibrating rates of molecular evolution. The synchronous fragmentation of the ranges of previously widespread taxa into Western Atlantic and Eastern Pacific components (geminates) enables one to make comparisons of rates among higher taxa on the same time scale and to evaluate the regularity of rates of molecular evolution among all species sampled. Other advantages of this approach are that the time scale (approximately 3 Ma) is one of particular interest for evolutionary biologists concerned with speciation and one that minimizes the ambiguities associated with augmentation of divergence values to account for multiple hits at a site. The divergence values derived for geminate pairs are independent, allowing statistical evaluation of variance in rates.

The current popularity of the relative rates test as the final arbiter of questions regarding rates and rate variation is primarily a matter of convenience and not a reflection of methodological superiority. A review of the commonly used techniques for calibrating rates of molecular evolution shows that each approach has limitations. Temporally based calibrations of rates are necessary complements to time-independent comparisons.

Interpretation of transisthmian molecular comparisons in the literature have in many cases been unduly influenced and confused by molecular clock assumptions and the restriction of studies to single higher-level taxa. Analysis of the apparently contradictory published data as well as new results from sequence comparisons of fishes, urchins and snails suggests a synthesis: taxon specific rates of molecular evolution, with reduced variance within taxonomic groups and great variance among all groups sampled.

Brachiopods contain several classes of intracrystalline molecules, secreted during the growth of the organism, and subsequently incorporated in the shell during biomineralization. The function of these molecules is not clear, although in the Mollusca, molecules in similar sites are thought to be involved with biologically controlled mineralization. The order of amino acids, and hence the quantity of each amino acid in proteins is determined by the genome of the organism, and study of such molecules should reveal information regarding the genomes of the fossil organisms.

Previous studies of fossil molecules from brachiopods have concentrated on the web of intracrystalline molecules which surrounds the crystallites of the secondary fibres of the shell. Work with Recent articulate brachiopods has shown that this web decays in less than a year, and hence any remnants of this web in the fossil record are likely to be highly degraded and unrepresentative of the original composition of the protein.

In contrast to this, intracrystalline molecules will be protected, encased within the calcium carbonate of the shell and analogous in many ways to fluid inclusions found in inorganic minerals. In such sites, contaminant and degradative factors such as microorganisms and large concentrations of water will be excluded and the effects of the enclosing sediments negated. Any breakdown of the molecules will be in situ, and the detectable products analysed.

In this study, intracrystalline molecules have been extracted from the shells of Recent and fossil brachiopods from the Plio-Pleistocene South Wanganui Basin, New Zealand. The molecules were analysed for free and peptide bound amino acids from both the soluble and insoluble fractions. The state of preservation of the molecules and their taxonomic specifity were analysed.

It was found that, even in these protected sites, individual amino acids were up to 90% in the free state i.e. that the proteins had mostly been hydrolyzed by the action of time and any water remaining in the shell structure after 0.12 Ma. Amino acids which are particularly sensitive to degradative reactions, such as serine and threonine were totally lost from the soluble fraction. However, the use of multivariate statistical analyses on data from the soluble remains of the protein indicates that decay does not render the molecules useless for taxonomy; groupings can be made on the basis of analyses alone.

Progressively older samples tend to have lower yields of amino acid in the soluble fraction, and this tends to correspond with an increase in the relative proportion of intracrystalline insoluble compounds. Although there is no direct linear relationship between the age of a sample and the proportion of insoluble compounds, older samples tend to have more insolubles. It is clear that other factors must play a part in the formation of these insoluble compounds. Preliminary studies of their amino acid composition indicates the presence of sensitive amino acids such as serine, indicating the stabilising effects of these diagenetic compounds.

The degradation of amino acids follows complex decay pathways, with some decaying to non-amino compounds and others to other amino acids, the most common being alanine from the dehydration of serine. The increasing concentrations of these amino acids can be a distorting factor in molecular taxonomy from fossil molecules.

ACKNOWLEDGEMENTS. This work was completed during the tenure of a UK NERC studentship to DW (GT4/89/GS/42) and funding from the Royal Society to GBC.

I measured the settling velocities and observed the settling behavior of sixteen molluscan species from Bahia la Choya. Bivalve shells stabilize in a concave-up fall position, and exhibit slower settling velocities than gastropod shells of similar weight and volume. Within individual species settling rates increase as the ratio of shell weight to maximum cross-sectional area increases. Five distinct fall-patterns occur: straight-fall, gliding, rotation, rocking, and oscillation.

At the end of the Ordovician many marine benthonic and planktonic faunas underwent a global extinction that has been attributed to climatic changes and glacio-eustatic fluctuations in sea-level (Berry and Boucot, 1973; Sheehan, 1973, 1975; Brenchley, 1984). Raup and Sepkoski (1982) found that the Late Ordovician-Early Silurian extinction event was one of the five major Phanerozoic mass extinctions.