Thor A. Hansen

Effects of sampling standardization on estimates of Phanerozoic marine diversification.

Global diversity curves reflect more than just the number of taxa that have existed through time: they also mirror variation in the nature of the fossil record and the way the record is reported. These sampling effects are best quantified by assembling and analyzing large numbers of locality-specific biotic inventories. Here, we introduce a new database of this kind for the Phanerozoic fossil record of marine invertebrates. We apply four substantially distinct analytical methods that estimate taxonomic diversity by quantifying and correcting for variation through time in the number and nature of inventories. Variation introduced by the use of two dramatically different counting protocols also is explored. We present sampling-standardized diversity estimates for two long intervals that sum to 300 Myr (Middle Ordovician-Carboniferous; Late Jurassic-Paleogene). Our new curves differ considerably from traditional, synoptic curves. For example, some of them imply unexpectedly low late Cretaceous and early Tertiary diversity levels. However, such factors as the current emphasis in the database on North America and Europe still obscure our view of the global history of marine biodiversity. These limitations will be addressed as the database and methods are refined.

A Tsunami Deposit at the Cretaceous-Tertiary Boundary in Texas

At sites near the Brazos River, Texas, an iridium anomaly and the paleontologic Cretaceous-Tertiary boundary directly overlie a sandstone bed in which coarse-grained sandstone with large clasts of mudstone and reworked carbonate nodules grades upward to wave ripple-laminated, very fine grained sandstone. This bed is the only sandstone bed in a sequence of uppermost Cretaceous to lowermost Paleocene mudstone that records about 1 million years of quiet water deposition in midshelf to outer shelf depths. Conditions for depositing such a sandstone layer at these depths are most consistent with the occurrence of a tsunami about 50 to 100 meters high. The most likely source for such a tsunami at the Cretaceous-Tertiary boundary is a bolidewater impact.

Predator—Prey Interactions in the Fossil Record

qPredator-prey interactions are among the most significant of all organism-organism interactions....It will only be by compiling and evaluating data on predator-prey relations as they are recorded in the fossil record that we can hope to tease apart their role in the tangled web of evolutionary interaction over time. This volume, compiled by a group of expert specialists on the evidence of predator-prey interactions in the fossil record, is a pioneering effort to collate the information now accumulating in this important field. It will be a standard reference on which future study of one of the central dynamics of ecology as seen in the fossil record will be built.qn (Richard K. Bambach, Professor Emeritus, Virginia Tech, Associate of n the Botanical Museum, Harvard University)

Influence of larval dispersal and geographic distribution on species longevity in neogastropods

The Lower Tertiary Gulf Coast was subjected to repeated transgressive-regressive cycles of varying rate and extent which had a marked effect on species longevities. In some neogastropods dispersal and geographic distribution played important roles in species longevities. Species with planktic larval stages that inhabited broad areas of the Gulf Coast could maintain their distributions even during regressive phases when delta building was strong. Wide distributions, free gene flow, and resistance to isolation gave planktic species an evolutionary stability that resulted in greater species longevity. Species with nonplanktic development could attain wide geographic ranges when barriers were minimized, but they were unable to maintain their distributions in the face of regressive progradation. Thus their pop- ulations were isolated, and extinction and speciation accelerated, giving them lesser evolutionary lon- gevity. Although planktic volutids tend to be widespread, planktic fasciolariids and mitrids were often restricted in distribution and therefore probably stenotopic. If dispersal and environmental tolerance are considered as separate forces, they seem to act in combination to produce very long, very short or intermediate species longevities.

The Fossil Record of Drilling Predation on Bivalves and Gastropods

The fossil record yields abundant data on the interaction between drilling predators and their shelled prey. Predatory drill holes may date to the late Precambrian (Bengtson and Zhao, 1992) and have been reported from various Paleozoic assemblages (e.g., Sheehan and Lesperance, 1978; Smith et al., 1985; Conway Morris and Bengtson, 1994). Paleozoic drill holes have been reported primarily from brachiopods, although gastropods (Rohr, 1991) and bivalves (Kowalewski et al., 2000; Hoffmeister et al., 2001) also exhibit apparent predatory drill holes. In most cases, the identities of the Paleozoic drilling predators are unknown; platyceratid gastropods documented in association with drilled Paleozoic crinoids and blastoids appear to have been parasitic (Baumiller, 1990, 1996, 2001).

Evolution of the Naticid Gastropod Predator-Prey System: An Evaluation of the Hypothesis of Escalation

Previous work has suggested that escalation may have characterized the history of the naticid gastropod predator-prey system, based on apparent increases in drilling frequencies and the occurrence of antipredatory aptations among prey. We evaluate this hypothesis based on a comprehensive survey (over 40,000 specimens) of predation on molluscs from the Upper Cretaceous through lower Oligocene formations within the U.S. Gulf and Atlantic Coastal Plain. Patterns in drilling of both bivalve and gastropod prey are complex. Drilling frequencies were relatively low in the Cretaceous but increased sharply above the Cretaceous-Tertiary boundary, remaining high until the late Eocene

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.

Detritus feeding as a buffer to extinction at the end of the Cretaceous

At the end of the Cretaceous the principal animals that became extinct, such as dinosaurs, marine animals that lived in the water column, and benthic filter feeders, were in food chains tied directly to living plant matter. Animal groups less affected by extinction, including marine benthic scavengers and deposit feeders, small insectivorous mammals, and members of stream communities, were in food chains dependent on dead plant material. The proposal that an asteroid or comet impact at the end of the Cretaceous produced a dust cloud that cut off photosynthesis for several months is consistent with this pattern of extinction. Food chains dependent on living plant matter crashed, while food chains based on detritus were buffered from extinction because there was a food supply adequate for the interval when photosynthesis was halted.

Naticid Gastropod Prey Selectivity Through Time and the Hypothesis of Escalation

The hypothesis of escalation posits that biologic hazards such as predation have increased during the Phanerozoic. Previously, a survey of drilling frequencies in the Cretaceous and Paleogene of the North American Coastal Plain suggested an episodic pattern of escalation within the naticid gastropod predator-prey system. This study examines escalation from the perspective of naticid prey selectivity. If escalation occurred within the system, less selectivity of prey may be apparent in the Paleogene compared to younger assemblages. We test this hypothesis for four Eocene Coastal Plain assemblages. Contrary to predictions, intraspecific prey size selectivity was well developed for nine of eleven bivalve prey species. Drillhole size (indicating predator size) correlated significantly with prey size, especially for successful drillholes. Few incomplete or nonfunctional drillholes occurred, except within corbulid species. Interspecific prey selectivity was less developed than for Neogene or Recent assemblages. Naticid prey preferences predicted by cost-benefit analysis were consistent with actual drilling frequencies only for the Bashi Marl Member of the Hatchetigbee Formation (Alabama). In the Piney Point Formation of Virginia, all prey items were drilled at equivalent frequencies, despite their different cost-benefit rankings. Upper Lisbon (Alabama) and Moodys Branch (Mississippi and Louisiana) assemblages showed limited agreement with preferences predicted by cost-benefit analysis. Prey selectivity thus appears less developed in the Paleogene compared to the Neogene and Recent, in accordance with the hypothesis of escalation.

The first 2 million years after the Cretaceous-Tertiary boundary in east Texas: rate and paleoecology of the molluscan recovery

Analysis of molluscan collections from a 3+ m.y. interval around the Cretaceous-Tertiary (K-T) interval in east Texas suggests that molluscs suffered an extinction at or near the K-T boundary, followed by a prolonged period of stress which lasted through the PO and Pla planktic foraminiferal zones. The stressed period was characterized by low species richness, low abundances of individuals, high species turnover and a dominance of deposit feeders. Species richness and the relative abun- dance of deposit feeders generally track the 13C depletion curve suggesting that the stress was caused by a lack of primary production. A stable, relatively diverse, suspension feeding molluscan com- munity was reestablished less than two million years after the K-T boundary. The total number of species within the habitat did not recover to Cretaceous levels within the study interval.