After two centuries of scientific endeavor, scientists have concluded that the age of the Earth is 4.54 billion years. This value, which is based on the relationships between lead isotopes in meteorites and in the Earth, has an uncertainty of less than 1 percent and is consistent with numerous radiometric age measurements on ancient rocks found on the Earth and Moon as well as on meteorites. In addition, the antiquity of the Earth is consistent with evidence indicating that the Milky Way Galaxy and the universe are of the order of 14-16 billion years (abbreviated Ga) in age.

Much effort has been expended in determining the geological utility of conodont elements; little has been spent on determining their usefulness to the conodont animal. Most conodonts had an apparatus made up of a number of morphologically distinct, complex elements. Such apparatuses were borne by members of the Ozarkodinida, Prioniodinida, and Prioniodontida, together accommodating ~75 percent of known conodont genera.

The ozarkodinid apparatus consisted of an anterior battery of S and M elements behind which lay opposed pairs of Pb and Pa elements. Debate over its function has centered on two main hypotheses. These have considered the apparatus either as a filter feeding system, or as teeth. Ontogenetic analysis reveals that ozarkodinid apparatus growth was incompatible with a filtering function. The rate of increase in size of the S and M elements (the postulated filtering system) was insufficient to have met the increasing food requirements of the growing animal.

Like the apparatus of ozarkodinids, that of prioniodinids was differentiated into an anterior S and M element series and posterior, paired Pa and Pb elements. Despite these similarities in element arrangement, the prioniodinid apparatus does not exhibit the same degree of morphological differentiation as that of ozarkodinids. The Pa and Pb elements were probably involved in swallowing rather than cutting and grinding food.

The prioniodontid apparatus is poorly known but it is clear that although the morphology of elements is comparable to ozarkodinids, their arrangement in the apparatus was markedly different. Until the architecture of the apparatus has been determined, it's mode of function can only be guessed at. Preliminary knowledge of the apparatus, however, suggests that it operated differently to those of ozarkodinids and prioniodinids.

Understanding of the function of conodont elements requires that their morphology is studied within the context of an integrated multielement apparatus. There is substantial variation in apparatus composition and differentiation, and element morphology between clades. This variation undoubtedly reflects, to a large extent, differences in apparatus adaptability and versatility, functional flexibility, and food specificity; all factors that in other groups have had a major influence on evolutionary patterns.

In the Proterozoic Clemente Formation at Cerro Rajon, south of Caborca, Sonora, we have collected sandstone beds that contain many small, sediment-filled structures. Each structure is a thin-walled, elongate sack, about 1.5 cm tall, oriented perpendicular to bedding. The sacklike structures are laterally flattened and roughly elliptical in transverse section, with the major axis of the ellipse measuring several mm and the minor axis 2-3 mm. The walls are not mineralized and consist of a thin layer of cryptocrystalline clay. One or both walls of each specimen is undulatory with a wavelength of 3-4 mm. The sacks are closed at the bottom, but appear to be open at the top, like an uninflated toy balloon. On the bottom surface of the sandstone bed, beneath each vertical sack structure, is a prolate ellipsoid-shaped sole mark. The long axis of each ellipsoid sole mark is parallel to the long axis of the associated sack structure, and the sole marks are oriented sub-parallel to one another on the surface of the bed. We tentatively interpret these structures to be the holdfasts of gregarious, attached, benthic organisms oriented sub-parallel to one another on the sea floor.

A preliminary study of carbon and oxygen isotope stratigraphy was carried out on the 2 km-thick Proterozoic through Lower Cambrian section at Cerro Rajon. Samples were collected every 50 m, and analyses were done in the laboratory of J.M. Hayes at Indiana University, whose assistance is gratefully acknowledged. The isotope record shows a primary signature for most of the section. Although the wide spacing of sample collection does not permit an unequivocal intercontinental correlation, a prominent negative δ13C excursion near the base of the Papalote Formation may correlate with similar excursions at the Vendian-Cambrian boundary on several continents. This would place the base of the Cambrian of the Caborca area several hundred meters lower than previously thought, but still several hundred meters above the possible body fossils of the Clemente Formation.

The late Quaternary was a time of rapid environmental fluctuations. The last glacial maximum was reached about 20 ka with continental glaciers covering most of Canada as well as the northeastern and upper midwestern United States (U.S.). Glacial ice physically displaced entire terrestrial biomes and the cooler climates altered distributions of species outside of the glacial limits. About 14 ka, the climate began to warm rapidly and glacial ice retreated northward, opening new landscapes for colonization by terrestrial biotas. Maximum warmth was reached between 9–5 ka with a time transgressive progression from west to east.

Radiocarbon chronologies allow for fine scale (100's to 1000's of years) resolution of mammal responses to these changes. Mammal communities did not respond as intact units but individual species shifted diachronically along environmental gradients. As a result, many late Pleistocene mammal communities contain associations of extant species that do not occur together today and appear to be ecologically incompatible. Pleistocene mammal communities also had a greater diversity of species than either Holocene or modern ones. This greater diversity was, in part, due to the existence of a diverse megafauna that became extinct at the end of the Pleistocene (10 ka). However, Pleistocene small mammal guilds with extant species, especially insectivores and microtine rodents, were also more diverse. Rapid changes in small mammal species distributions, diversity patterns, and clinal shifts around 10 ka strengthens environmental models for the end-Pleistocene extinction.

Modern mammal communities began to appear at the end of the Pleistocene and into the Holocene. In the eastern U.S., the species composition of these communities has been stable for the last 10 ka, although vegetational communities have shown change throughout the Holocene. In other parts of the U.S., middle Holocene warming caused some species to shift their geographic ranges. However, species composition of communities was not significantly altered. Understanding these changes not only provides a better perspective for viewing mammal communities of the past but it may also give insight into those of the future as climate will continue to vacillate, whether induced naturally or anthropogenically.

Holocene patch-reefs occur throughout the shallow marine platform, to the lee of the barrier reef in northern and southern Belize, Central America. Patch reefs on the northern shelf that occur within an areally extensive patch reef complex (Mexico Rocks) indicate that differences exist between reefs here and well-studied patch reefs on the southern shelf that have been used by workers as a general model for patch reef development throughout Belize. This model proposes that patch reefs on the Belizean shelf are dominated by typical Atlantic-Caribbean, biotically-zoned coral assemblages of Acropora palmata and A. cervicornis that kept up or caught up with Holocene sea level rise during the last 8000 years to form large “keep up” or in some instances “catch up” reefs.

In contrast to those in the south, the northern patch reefs are not biotically zoned, are dominated by Montastrea annularis rather than Acropora spp., and are much younger (400 years old) than those in the south. In addition, northern shelf patch reefs developed predominantly by lateral growth in a milieu of static sea level and are herein called “accretion” type reefs. These differences in biotic and sedimentologic parameters between reefs on the northern and southern shelves imply fundamentally different ecologic and sea level history controls on patch reef formation from north to south. A leading contributor to the variation among the reefs along the Belizean shelf may be species-specific growth rates of the coral species that initiate each patch reef, and response to sea level fluctuation versus stasis through time.

The Lower Ordovician through Upper Jurassic record of the phylum Bryozoa consists almost entirely of the class Stenolaemata, excepting a very few Gymnolaemata (ichnofossils of the order Ctenostomata and, in the uppermost Upper Jurassic, Pyriporopsis, which is the oldest known Cheilostomata). Along with the decline to eventual extinction of most Paleozoic stenolaemates beginning with the end-Permian crisis, the stenolaemate order Cyclostomata radiated in Late Triassic and Jurassic to include the diversity of growth forms and locally the abundance common for Paleozoic bryozoan faunas. By late Cretaceous, however, cheilostomes had diversified and had come to dominate in bryozoan faunas; they have continued to dominate to the present.

In the abundant and diverse bryozoan fauna of the northern Adriatic Sea, cheilostomes are four to five times as diverse and are orders of magnitude more abundant than are cyclostomes. Within this fauna, encrusting sheet-like cheilostomes consistently overgrow adjacent encrusting cyclostomes. Different ontogenetic patterns of skeleton and organs, as well as larger zooid sizes at colony margins, produce patterns of feeding currents that allow cheilostomes to deliver their filtered water to and to overtop competing cyclostomes at interacting margins. These Bauplan differences apparently constitute a key evolutionary innovation characterizing the Cheilostomata. Along with higher growth rates in most cheilostomes, this key evolutionary innovation should have given cheilostomes a competitive edge since the origin of multiserial colonies within the clade in Early Cretaceous. It is hypothesized that superior overgrowth capability of cheilostomes has been an important factor in their success and the relative decline of cyclostomes.

The “primitive,” “minor” or “lesser” classes of the Mollusca comprise four groups that are typically recognized at the class level in most classification schemes. These classes are: (1) the Aplacophora, (2) the Monoplacophora, (3) the Polyplacophora, and (4) the Scaphopoda. Members of these classes are bilaterally symmetrical and typically possess a ventral muscular foot and a dorsal epidermal tissue (the mantle) that secretes a calcareous covering. Between these two structures lie the viscera. The space that develops between the mantle and the foot of the mollusc is referred to as the mantle cavity. Into this cavity open the ducts from the excretory, alimentary, and reproductive systems; gills are also typically located in this space. The anterior end of these molluscs is marked by a muscular head in which most of the ganglia are concentrated. The mouth is situated ventrally and opens internally into the pharynx which contains, at its anterior end, the radula and its associated musculature and support structures. The intestinal tract is typically arranged in complex loops.

Early ontogeny is still the least well-known part of the molluscan life cycle, despite its unarguable significance in ecology, biogeography, and evolution. The past decade has seen an acceleration in biological research on molluscan embryonic and larval development, and paleontologists have recently begun to explore this field as well. Although the very earliest phases of ontogeny are inaccessible to the paleontologist, a remarkable amount of information can be derived from the preserved details of larval shell morphology. The fossil record thus affords rich opportunities to study directly the development of extinct species, both from a phylogenetic standpoint and for tracing the evolutionary and biogeographic effects of these early stages in life history. Access to long-extinct clades and an ability to document rates and patterns of evolution allow paleontologists to test hypotheses that can be addressed only indirectly using Recent data. Here I will emphasize these paleontological aspects of molluscan development (see also Jablonski and Lutz, 1983); for the more strictly biological aspects of reproduction and embryology, see Giese and Pearse (1977, 1979), Verdonk et al. (1983) and Tompa et al. (1984).

Radiolarians are marine zooplankton possessing a tough, central capsular membrane that divides the cytoplasm into intracapsular (containing the nucleus, organelles, and food reserves) and extracapsular (with food-gathering rhizopodia and digestive vacuoles) portions (Figure 1). They bear two kinds of pseudopodia, the axopodia and filopodia. The axopodia extend radially through the ectoplasm and capsular membrane to the interior of the endoplasm. The axopodia are inserted into a special structure, the axoplast (Figure 1). The development of the axoplast and its complex is of fundamental importance in radiolarian taxonomy. For a detailed description of radiolarian cytology, biology, and reproduction, see Anderson (1983).

Taphonomic studies of marine fossil assemblages have achieved new levels of sophistication in the past several years, but field-based investigations of time-averaged skeletal accumulations continue to suffer from an unavoidable problem: to date, a time machine has yet to be invented that would permit the researcher to observe directly the formation of the assemblage under study. Even when working with Recent subfossil accumulations on the sea floor, where it is theoretically possible to view the activity of (possibly myriad) taphonomic processes, practical considerations prevent the extensive, day-to-day monitoring that would be required to “bear witness”, in a kinetic sense, to the formation of skeletal assemblages. At best, we are sometimes provided with opportunities for comparative glimpses of skeletal accumulations before and after events of potential taphonomic significance, such as hurricanes (e.g., Miller et al., 1992).

Trilobites, like most arthropods, grew through a series of discrete increments (known as instars). The moulted sclerites accumulated in the sediment and were potentially preserved. This mode of growth provides us with a tangible record of morphological attributes that characterize a given stage in an individual's ontogeny, and a record of changes that occur between successive stages.

The concept of organic evolution implies descent of all living and extinct forms from one or more common ancestors in the distant past of earth history. During recent decades, traditional phylogenetic deductions from the anatomy of living and fossil organisms have been supplemented by a vast and growing new body of molecular evidence. Each living creature contains a genome consisting of linear sequences of DNA bases, millions or billions of them, which determine all the developmental, anatomical and physiological characteristics of that species. This DNA is transcribed into messenger RNA (mRNA) which uses the machinery of the cell to make proteins. The sequence of mRNA bases determines the sequence of amino acids of the resultant protein (Lowenstein, 1986).

The study of paleoecologic processes through geologic time requires careful analysis of the stratigraphic distribution of fossils. Collections from stratigraphic units of varying thickness provide data needed to describe the spatial and temporal patterns of fossil species, populations, and paleocommunities. Obviously, collections that have broad stratigraphic spacing will reveal only long-term trends. Narrowing the stratigraphic distance between collections may reveal some aspects of shorter term biological processes, but it is well established (Gould, 1980; Schindel, 1980; McCall and Tevesz, 1983) that the lower limit of paleontologic resolution is reached when biological processes operate more quickly than the rate of formation of final undisturbed bedding units.

Size-frequency distributions and size-survivorship curves were constructed for the following seven species of living gastropods from Bahia la Choya: Cerithideamazatlanica, Cerithideaalbonodosa, Cerithiumstercusmuscarum, Nassariusiodes, Nassariusbrunneostoma, Theodoxusluteofasciatus and Tegularugosa. Size-frequency distributions and size-survivorship curves of empty shells of Cerithideamazatlanica and Cerithiumstercusmuscarum were constructed to evaluate the effects of taphonomic processes on survivorship curves.

All size-survivorship curves are convex-up in shape: small size classes show low mortality and mortality increases with increasing size. Despite post-mortem processes, survivorship curves for empty shells mimic the shape of curves based on live individuals.