Evidence for biotic interaction is not obtainable easily in the fossil record. Epizoans cemented to the skeletons of host organisms provide an opportunity to study possible host–epizoan interactions and the ecological structure of hard substratum communities. One such possibility is sponges and their associated encrusters, but few such occurrences have been documented from the fossil record. Early Ordovician limestones of the San Juan Formation in the Argentine Precordillera bear an important sponge fauna consisting mainly of demosponges. This Early Llanvirn assemblage has the highest diversity, and most sponges were subjected to epizoan encrustations. A total of nineteen species included in eleven genera of sponges were examined to record epizoans encrustations. Type and relative abundance of epizoans were compiled for each sponge species. Five kinds of epizoans were recorded: bryozoans, stemmed echinoderms, brachiopods, other sponges, and scars and borings of undetermined organisms. Crinoids and blastozoans are widespread common epizoans on sponges. Small brachiopods are attached to sponges showing no location preference; rarely, juvenile sponges appear on conspecific adults. Sinuous scars and worm-like organisms are found in almost all the sponges. Deep borings, like those found in Devonian or younger sponges, are absent. The analysis of epizoan distribution on each sponge species shows a fairly homogeneous distribution of echinoderms on sponges but host specificity of bryozoans. Here, surface texture seems to have been a significant controlling factor. Almost all the trepostome bryozoa adhere to Rhopalocoelia tenuis Carrera. This taxon is a cylindrical sponge whose smooth surface texture contrasts sharply with the rough and open textured spiculation observed among other species in the fauna. This feature seems to have been a key factor for the mutual relationship. Rhopalocoelia tenuis shows a reaction to epizoans, modifying the shape of growth and changing the direction of the spicular structure in the vicinity of the epibiont, providing evidence that encrustation occurred while the sponge was alive.

Diversity, composition and encrusting strategies of the Argentine epizoan community reflect the early evolutionary stage of this biotic relationships in the Early Paleozoic. This contrasts with the more varied and diversified relationships among organisms that were able to encrust, bore, and chemically perforate hard parts after the Devonian. The Early to Middle Ordovician was a time of rapid evolutionary diversification among hard substrate communities. The Argentinean epizoan community appears to be transitional between the first hard substrate encrusters (Late Cambrian), which were eocrinoid dominated and were not accompanied by macroborers or bryozoans, and the later Paleozoic communities, in which bryozoans were the most prolific utilizers of attachment and macroborers were common.

In Glacier Bay, Alaska, Silurian limestones record the development and demise of a stromatolite reef complex in the Alexander terrane. These microbial deposits are of regional and paleontological significance because they contain paleogeographically distinctive biotas and yield important insights into Phanerozoic stromatolites that inhabited normal-marine subtidal environments. Willoughby limestones exposed on Drake Island reveal that stromatolite growth at the platform margin influenced platform dynamics with the protection of peritidal and lagoonal habitats behind a reef-fringed rim, which experienced early lithification by the precipitation of synsedimentary marine cements. Relatively low-energy subtidal conditions in a restricted, shallow-marine lagoon are implied by the peloidal and mollusk-rich wackestones, packstones, and grainstones. At the platform margin, stromatolite boundstones and cementstones capped a reef-like mound by forming a thick microbial-cement crust on a core of outer lagoonal sediments. Well laminated, fenestral mudstones and peloidal wackestones-grainstones have an abrupt lateral contact with the microbial boundstones, indicating their formation in peritidal flats adjacent to the stromatolite reefs. Foreslope breccias consist of clasts derived from shallow subtidal environments on the shelf and at its rim. Cyclic repetition of these deposits indicate that sea level fluctuations and tectonic instability in the Alexander terrane contributed to periodic shedding of shallow-marine detritus to the upper slope, followed by the rebuilding of stromatolite reefs at the platform margin. Culminating stages in the Klakas orogeny induced large-scale, catastrophic collapse of the carbonate platform. Silurian stromatolites characterized by similar habitats and microbial-sponge biotas in southwestern Alaska and the Ural Mountains, Russia, together with paleomagnetic data, detrital zircon evidence, and the timing of late-stage Caledonide events, firmly establish genetic and temporal links of the Alexander terrane with other areas along the Uralian Seaway.

The Bromacker section of the Lower Permian, Tambach Formation, in central Germany, yields an important fossil-vertebrate assemblage that was deposited in an upland setting near the center of a small, internally-drained paleo-graben. The fossil-vertebrate assemblage shares many taxa in common with others that are well-documented from North America, but is atypical in the: (1) unusually large abundance of the terrestrial herbivore Diadectes; (2) complete absence of aquatic and semi-aquatic vertebrates; and (3) rarity of medium-to-large carnivorous synapsids. The graben setting and the low-diversity, terrestrial, fossil-vertebrate assemblage together comprise a unique upland paleoecosystem, heretofore undocumented in the Early Permian. The composition of and relative abundances within the assemblage at Bromacker suggest that experiments with “high-fiber” vertebrate herbivores as the dominant or significant basal component of vertebrate food webs had begun by the Early Permian, but only in settings with few or no aquatic and semi-aquatic vertebrates.

The combined stratigraphic section at Bromacker consists of portions of two conformable stratigraphic intervals—the Lower and the Upper beds. Depositional events in both were dominated by seasonal-to-subseasonal cycles of flooding in an ephemeral, alluvial-to-lacustrine setting that was hot year-round with annual precipitation similar to that of a wet-and-dry tropical or wetter climate. Excellently preserved, articulated and disarticulated fossil vertebrates indicate subaerial exposure times of short duration and limited reworking. In the case of articulated specimens, death and burial were probably coeval events, most likely caused by floods.

A −1.5‰ to −2‰ carbon isotope excursion immediately above the clay layer that defines the Cretaceous-Tertiary (K/T) boundary has been reported in marine sediments world wide. This paper reports a similar −1.5‰ to −2.8‰ carbon isotope excursion recorded by C3 land plants from three temporally-controlled, stratigraphically-constrained terrestrial sections in the Western Interior of North America (Garfield County, Montana, and Slope County, North Dakota). Carbon isotope measurements of bulk sedimentary organic carbon were well-correlated with those of isolated plant cuticle, suggesting that the terrestrial organic carbon signature in these sediments parallels that of plant cuticle. Carbon isotope signatures were also independent of rock type and depositional environment, showing that the carbon isotope signature of plants, although altered, is not biased taphonomically. Because the signature in terrestrial facies records the isotope composition of paleoatmospheric CO₂, this record—combined with that from marine sections—offers additional insight into changes in carbon cycling underlying the K/T negative carbon isotope excursion. For example, radiometric age determinations from the Hell Creek Road locality in Montana bracket the atmospheric carbon isotopic recovery between 65.00 ± 0.05 Ma and 65.16 ± 0.04 Ma. This reflects a more rapid recovery for the terrestrial biosphere than for that of the marine realm, perhaps due to lower extinction rates in land plants than in marine primary producers.

The La Paz area in the southwestern Gulf of California, Mexico, provides an ideal site for studying Recent warm-temperate to subtropical carbonate environments. Carbonate factories include small pocket bays, a rhodolith dominated carbonate shelf, and a mixed-carbonate siliciclastic high-energy beach. Underwater mapping and constituent analysis have revealed free-living coralline red algae in the form of rhodoliths to be the main carbonate producers, contributing 33% of the biogenic constituents to the sediment. Other significant contributions come from corals (20%), molluscs (18%), echinoderms (5%), and benthic foraminifera (4%). The benthic foraminiferal community includes mixtures of tropical and temperate species. This compositional pattern stands in marked contrast to (1) better-studied coral and green algae dominated tropical carbonate systems and (2) foraminifer, bryozoan, and mollusc dominated cool-water carbonates. Cluster analyses of biogenic constituents and benthic foraminifera revealed distinctive coral, coralline red algal, and molluscan microfacies. However, subdivisions characterized by benthic foraminifera more closely reflect the bottom facies observed during underwater mapping as opposed to patterns defined by cluster analysis of biogenic components. Diagnostic features for interpreting similar environments found in the fossil record include the (1) co-occurrence of coralline red algae and corals together with the absence of calcareous green algae, (2) presence of 5–10 genera of larger benthic foraminifera, (3) laterally and vertically not extensive character of environments, and (4) changes of microfacies over short distances.

Eggshell taphonomy often is acknowledged to be worthy of investigation, but few studies have examined factors influencing eggshell preservation. In this study eggshell weathering, fragmentation, dispersal, orientation, and predation were characterized in field and laboratory studies. Modern gull eggshell fragment dispersal and orientation patterns were compared with analogous patterns at a non-avian dinosaur egg clutch site. Among other findings: (1) hatched and depredated gull eggshells protected by an exclosure, but exposed to two years of colony weathering, fragmented relatively slowly, suggesting that colony resident activity is responsible for the relatively rapid disappearance of eggshell on the colony surface; (2) depredated eggshells could be distinguished from hatched eggshells due, in part, to differences in fracture patterns; (3) both gull and non-avian dinosaur eggshell fragments were most abundant close to nest centers, whereas this was not true for bones and mollusc shells in gull territories; (4) both gull and non-avian dinosaur eggshell fragments within nest areas were oriented concave up more commonly than concave down, in contrast to transported eggshell fragments; (5) chicken eggs experimentally placed in a gull colony were more likely to be taken by egg predators in areas of short or sparse vegetation than in areas of tall vegetation; and (6) the extent of fracturing was greater in fresh than in hollow chicken eggshells following compression beneath simulated sediment loads. Eggshell weathering, fragmentation, orientation, and dispersion patterns should be characterized during the excavation of fossilized eggs and nest sites.

Unusual structures, observed on taphonomically-reworked ammonite molds collected in the Oolithe Ferrugineuse de Bayeux Formation (Bajocian, Normandy), are described. They appear as depressions in the mold, located below “holes” in the shell, and they are filled with a sediment plug texturally different from the mold. These structures are called “pseudoborings” because of their similarity in both external shape and size (from 2 mm to over 10 mm in diameter) to bivalve borings. Detailed analyses of polished surfaces and thin sections cut through the mold show that the depth of penetration of pseudoborings and the irregularity of the plug-mold boundary exclude a bioerosional origin. The origin of pseudoborings is hypothesized to be due to erosion of unlithified sediments and exhumation of sediment-filled ammonite shells, previously damaged by attacks by cephalopod predators, resulting in a partial emptying of the conch.