Peter J. Harries

Phanerozoic trends in the global diversity of marine invertebrates.

It has previously been thought that there was a steep Cretaceous and Cenozoic radiation of marine invertebrates. This pattern can be replicated with a new data set of fossil occurrences representing 3.5 million specimens, but only when older analytical protocols are used. Moreover, analyses that employ sampling standardization and more robust counting methods show a modest rise in diversity with no clear trend after the mid-Cretaceous. Globally, locally, and at both high and low latitudes, diversity was less than twice as high in the Neogene as in the mid-Paleozoic. The ratio of global to local richness has changed little, and a latitudinal diversity gradient was present in the early Paleozoic.

The early Toarcian (Early Jurassic) and the Cenomanian–Turonian (Late Cretaceous) mass extinctions: similarities and contrasts

Abstract The early Toarcian (eTo) and Cenomanian–Turonian (C–T) mass extinctions are comparable from a wide range of scales and perspectives. From a broad standpoint, their similarities include: virtually identical extinction intensity at the familial and generic levels, widespread basinal facies deposited during sea-level highstands, an overall greenhouse climate, and anoxia as an important causal mechanism. The high-resolution, macroinvertebrate data analyzed here, consisting of stratigraphic ranges, diversity and abundance, point to smaller-scale similarities. The two events resulted in significant ecological disruption and, in both cases, the biotic responses were very similar. Taxa inhabiting the upper water column were unaffected by anoxia and included ammonites and, in the eTo, belemnites. In addition, epifaunal taxa adapted to low-oxygen conditions, such as the buchiids, posidoniids and inoceramids, flourished in the post-extinction environment during the survival interval. As conditions ameliorated, the biota became more diverse and gradually began to resemble pre-extinction biotas. Furthermore, the δ13C curves predict the end of the survival interval and suggest that the period characterized by carbon isotope excursions represent disrupted environmental conditions. This points to the potential application of δ13C as a tool for determining the repopulation modes and timing for other mass extinctions.

Models for biotic survival following mass extinction

Abstract Mass extinction intervals are characterized by three dynamic processes: extinction, survival, and recovery. It has been assumed that the taxa surviving a mass extinction are composed predominantly of eurytopic groups and opportunistic/disaster species. However, high-resolution stratigraphic and palaeontological analyses of several mass extinction intervals show that the repopulation of the global ecosystem takes place among ecologically and genetically diverse and complex taxa and occurs far too rapidly to be solely attributed to rapid radiation from a few ecological generalists. We suggest a number of potential survival mechanisms or strategies (sensu Fryxell 1983) which have evolved in diverse taxa and which could have allowed them to survive mass extinction intervals. These mechanisms consist of: rapid evolution, preadaptation, neoteny/progenesis, protected and/or unperturbed habitat, refugia species, disaster species, opportunism, broad adaptive ranges, persistent trophic resources, widespread and rapid dispersion, dormancy, bacterial-chemosymbioses, skeletonization requirements, reproductive mechanisms, larval characteristics and chance. Because of the wide variety of potential survival mechanisms, the range of survivors may be far higher than previously hypothesized. This would account, in part, for the diversity and evolutionary state of Lazarus taxa and for the rapid re-establishment of some complex ecosystems following many mass extinction intervals, without calling on “explosive” radiation from generalist/opportunist stocks following a mass extinction interval.

A synoptical classification of the Bivalvia (Mollusca)

Preface This classification summarizes the suprageneric taxonomy of the Bivalvia for the upcoming revision of the Bivalvia volumes of the Treatise on Invertebrate Paleontology, Part N.

The importance of crisis progenitors in recovery from mass extinction

Abstract Progenitor taxa are defined as species or lineages which arise, commonly through punctuated or macroevolutionary processes, during the main phases of a mass extinction interval, and which then survive to seed the evolution of dominant groups during ensuing radiation and ecosystem recovery. Their success in surviving the severe environmental perturbations commonly associated with mass extinctions and their immediate aftermath lies in the fact that they are initially adapted in their evolution to these dynamically changing environments. This differentiates them from other surviving clades of ecological generalists, opportunists, disaster taxa, taxa with specialized survival mechanisms, etc., all of which may have a long pre-extinction evolutionary history. Progenitor taxa characterize those ecosystems which are most severely affected by mass extinction processes (perturbations and feedback loops), e.g. those of tropical to warm temperate climate zones. Progenitor taxa are rarer in those ecosystems with relatively minor response to environmental perturbations of mass extinction intervals (deep sea and more poleward areas), where many established pre-extinction lineages survive the extinction event(s) with little change. In several published records of ‘explosive radiation’ among new lineages following mass extinctions, high-resolution stratigraphic sampling has shown that many of these ‘new’ recovery taxa actually had their origins as small, relatively rare progenitor taxa during the preceding mass extinction intervals. Examples from Cretaceous mass extinction intervals are presented (Cenomanian-Turonian, Cretaceous-Tertiary).

Effect of diagenesis on the Sr, O, and C isotope composition of late Cretaceous mollusks from the Western Interior Seaway of North America

Evaluating the effects of diagenesis on the isotopic compositions of Sr, O, and C in marine carbonates is critical to their use as proxies in reconstructing information on the salinity, temperature and dissolved inorganic carbon of ancient oceans. We have analyzed a series of samples of mollusk shells from the Baculites compressus zone (late Campanian) of the Pierre Shale of South Dakota. Samples included outer shell material and septa of cephalopods collected inside and outside concretions. Preservation was evaluated using light microscopy, scanning electron microscopy (SEM), trace element analysis and X-ray diffraction. All of the material consists of aragonite based on X-ray diffraction. An SEM preservation index (PI) was established based on comparison of the microstructure of the fossil material with that of modern Nautilus. Excellent preservation (PI = 5) was characterized by well-defined nacreous plates with discrete, angular boundaries. In contrast, samples showing fused nacreous plates with indistinct boundaries were rated poor (PI = 1). 87Sr/86Sr ratios vary with preservation and average 0.707648 ± .000021 (n = 10) for excellent preservation (PI ≈ 5), 0.707615 ± .000028 (n = 5) for good preservation (PI ≈ 3), 0.707404 ± .000074 (n=7) for fair preservation (PI ≈ 2), and 0.707261 ± .000053 (n=8) for poor preservation (PI ≈ 1). These data suggest that as the quality of the preservation declines, the mean 87Sr/86Sr ratio decreases and the standard error of the mean increases. Oxygen and carbon isotope analyses of the same specimens also show decreases with preservation, and δ18O, δ13C and 87Sr/86Sr are well correlated, suggesting that these tracers are all altered as the PI decreases. The Sr/Ca ratio increases as preservation decreases, indicating that Sr is added to the shell material during diagenesis. In contrast, Mg/Ca shows no trend with preservation. If the increasing Sr concentration (and decreasing 87Sr/86Sr) of the shell material with decreasing preservation represents the addition of Sr to the shell during diagenesis, we calculate that the added Sr had 87Sr/86Sr ranging from 0.707582 to 0.707032. Potential sources of the added Sr include older marine carbonates and weathering of volcanic ash layers present in the shale. The mechanisms of alteration likely include epitaxial growth of strontianite on the original shell aragonite and isotopic exchange of C and O between alteration fluids and shell carbonate. We conclude that SEM preservation criteria are effective in screening shell material that records original isotopic values and that variations in Sr, O and C isotope composition in well-preserved material can be used to assess paleoenvironmental parameters, such as salinity and temperature. Our results also indicate that assessing preservation is a critical prerequisite to the determination of numerical ages of shell material using strontium isotope stratigraphy.

Paleoecology of giant Inoceramidae ( Platyceramus ) on a Santonian (Cretaceous) seafloor in Colorado

Abstract Giant Middle Coniacian to Lower Campanian Platyceramus Seitz is among the largest Cretaceous bivalves, commonly reaching an axial length of over 1 m, and occasionally over 2–3 m in size. The genus is characterized by its large size, very low convexity, normal inflation limited mostly to the umbonal area, and flattened flanks. It is especially common in moderately deep calcareous shale facies, as well as in chalks and limestones of the Niobrara Formation and equivalents. Preferred facies contain abundant pyrite, elevated total organic carbon (TOC), and very low biotic diversity. The genus maintains its giant size in these facies, and becomes more abundant. It clearly prefers dysoxic facies. As such, it probably is chemosymbiotic; photosymbiosis is almost ruled out because of inferred water depths of 200–350 m. It is also found more sparsely, and of smaller size, in oxygenated facies, including shoreface sandstone. The study area contains over 81 giant-sized Platyceramus platinus (Logan, 1898) on a single bedding plane; there are very few small ones. This allows spacing, orientation, and size analysis to be performed on an adult population.

Methane seeps as ammonite habitats in the U.S. Western Interior Seaway revealed by isotopic analyses of well-preserved shell material

Methane seep deposits are common in the Upper Cretaceous Pierre Shale of the U.S. Western Interior. They contain a rich fauna including ammonites, bivalves, gastropods, sponges, corals, echinoids, crinoids, and fish. In an effort to understand the role of ammonites in these ecosystems, we examined a seep from the upper Campanian Didymoceras cheyennense Zone in Custer County, South Dakota, that contains molluscs with well-preserved shell material permitting isotopic analyses. Values of δ13C of the micritic limestone at the seep range from −46.94‰ to −11.49‰, confirming the influence of anaerobic oxidation of methane on the isotopic composition of the dissolved inorganic carbon reservoir. The ammonites also consistently display light values of δ13C ranging from −13.71‰ to 0.68‰. These values are generally lighter than those in nonseep specimens from age-equivalent rocks elsewhere in the basin (–1.75‰ to 3.42‰). In a single specimen of Baculites corrugatus from the seep, light δ13C values occur throughout ontogeny. These data suggest that ammonites incorporated isotopically light methane-derived carbon in their shells and lived near the vent fluids and methane-oxidizing bacteria. Both juvenile and adult specimens are present, implying that these ammonites spent their entire lives at the seep and formed an integral part of an interwoven community. The values of 87Sr/86Sr in the limestone and well-preserved fossils at the seep (0.707690–0.707728) are higher than that of the open ocean at this time (0.707659). These elevated values suggest that the seep fluids were imprinted with a radiogenic Sr signature, perhaps derived from equilibration with granitic deposits at depth during the initial uplift of the Black Hills.

LATE CRETACEOUS GASTROPOD DRILLING INTENSITIES: DATA FROM THE MAASTRICHTIAN FOX HILLS FORMATION, WESTERN INTERIOR SEAWAY, USA

Abstract Drillholes interpreted as the products of gastropod predation have become the most widely applied proxy for predator-prey interactions in the fossil record. In a broader sense, they have also become accepted as reflecting the evolution of many aspects of predator-prey interactions; e.g., intensity, behavior, and efficiency. Because of the relative paucity of published studies, the generality of long-term trends revealed by recent compilations of drilling intensity data has not been widely tested. The present study examines predatory gastropod drilling intensity in bivalves from the late early to early late Maastrichtian Fox Hills Formation within the Western Interior Seaway (WIS). These data allow a new comparison to be made to drilling intensity values reported previously from coeval United States Coastal Plain collections. 6,910 valves and 4,343 articulated specimens representing 53 bivalve species were examined for evidence of drilling predation. Although the approach employed was designed to maximize the number of drillholes recorded, only 236 unequivocal drillholes, 20 possible drillholes, 11 partial drillholes, and a single specimen with multiple drillholes were found, resulting in a maximum drilling intensity of 3.3%. These drillholes are thought to have been largely produced by naticids, which are abundant in these deposits. When compared to other published studies of Maastrichtian gastropod drilling, these data document that drilling intensities in the latest Cretaceous varied considerably both spatially and temporally, and that the WIS record provides a lower baseline for Cretaceous drilling than that derived from coeval Coastal Plain localities.

REEF RECOVERY FOLLOWING THE FRASNIAN–FAMENNIAN (LATE DEVONIAN) MASS EXTINCTION: EVIDENCE FROM THE DUGWAY RANGE, WEST-CENTRAL UTAH

ABSTRACT The temporally extensive late Middle through Late Devonian biotic crisis involved at least three distinct peaks of elevated extinction intensity during an interval spanning ∼25 myr and resulted in the preferential elimination of certain shallow-marine, warm-water taxa, especially members of reef communities. By the end of the second peak, delimited by the Frasnian–Famennian (F–F) boundary, the stromatoporoids, members of the dominant constructor guild in mid-Paleozoic reefal ecosystems, had ceased building reefs in most parts of the world. The northern Dugway Range in west-central Utah, United States, however, represents one of the few locations globally where stromatoporoids continued reef building into the Famennian. Two measured sections there, which are constrained biostratigraphically using conodonts, indicate that the biohermal sequences occur within the middle Palmatolepis crepida biozone and are early Famennian in age. The post-F–F extinction Dugway reefal faunas are depauperate and dominated by labechiid and stylostromid stromatoporoids, as is characteristic of other early Famennian reefs. In this region, evidence for reefal development is episodic, with stromatoporoid-bearing units interbedded with peloidal and coated-grain carbonate units lacking evidence of reef construction. The stromatoporoid survivors, although fairly minor constituents of Frasnian reef communities, belong to long-ranging clades and may represent so-called extinction-resistant taxa that flourished, albeit locally in Laurentia, following the F–F mass extinction.