Philip W. Signor

The mid-Paleozoic precursor to the Mesozoic marine revolution

The mid-Paleozoic was punctuated by a rapid radiation of durophagous (shell-crushing) pred- ators. These new predators were primarily placoderm and chondrichthyan fishes but probably also included phyllocarid and eumalacostracan arthropods. Coincident with the radiation of these durophages, beginning in the mid-Devonian, there was an increase in the frequency of predation-resistant morphologies in a variety of marine invertebrate taxa. Among bellerophontid molluscs, disjunct coiling disappeared and umbilici became less common while the frequency of genera with sculpture increased. The abundance of brachiopod genera with spines on one or both valves increased dramatically. Sculpture became more pronounced and common among genera of coiled nautiloids. Inadunate and camerate crinoids showed a marked increase in spinosity, and all three crinoid subclasses tended to develop thicker thecal plates. Trends toward increasing relative frequencies of predation-resistant features were formed in different ways. Bellerophontid genera lacking predation-resistant features tended to go extinct, leaving the sculp- tured, tightly coiled forms as the predominant forms. Among Brachiopoda, the radiation of productids provided the tremendous increase in numbers of spinose genera. Among crinoids, predation-resistant features were acquired through evolution within established clades. These observations suggest that predation by shell-crushing predators has been an important control on the morphology and composition of the marine invertebrate fauna since at least the Middle Devonian. The mid-Paleozoic radiation of durophages and response of the marine fauna was in many respects similar to events of the Mesozoic Marine Revolution, in effect, the Paleozoic precursor to that event.

The plankton and the benthos: origins and early history of an evolving relationship

Modern marine plankton communities include a broad diversity of metazoans that are suspension-feeding or micropredatory as adults. Many benthic marine species have larval stages that reside, and often feed, in the plankton for brief to very long periods of time, and most marine benthic communities include large numbers of suspension-feeders. This has not always been the case. Cambrian benthic communities included relatively few suspension-feeders. Similarly, there were few metazoan clades represented in the plankton, either as adult suspension-feeders or as larvae. Review of the fossil record suggests that the diversification of the plankton and suspension- feeding marine animals began in the Late Cambrian and continued into the Ordovician. These changes were accompanied by, and probably influenced, concurrent major changes in the marine realm, including an increase in tiering within benthic communities, the replacement of the Cambrian fauna by the Paleozoic fauna, and a general taxonomic diversification. The ultimate cause of these changes is uncertain, but it appears likely that the plankton was and is a refuge from predation and bioturbation for adults and larvae alike. The expansion in plankton biomass thus provided increased ecological opportunities for suspension-feeders in the plankton and benthos.

Natural clades differ from "random" clades; simulations and analyses

Using computer simulations and analytic calculations, we have evaluated whether conspic- uous expansions and contractions of natural clades may have represented chance fluctuations that oc- curred while probabilities of speciation and extinction remained equal and constant. Our results differ from those of previous workers, who have not scaled generating parameters empirically at the species level. We have found that the waxing and waning of many real clades have almost certainly resulted from changes in probabilities of speciation and extinction. For some of these changes, likely explanations are evident. The emplacement of adaptive innovations, for example, has at times elevated probability of speciation. We conclude that chance factors have not played a dominant role in producing dramatic changes in standing diversity within speciose higher taxa.

Evolutionary tempo in jurassic and cretaceous ammonites

Stage-level range data for 983 Jurassic and Cretaceous ammonite genera, distributed within 83 families, were analyzed by assigning absolute ages to stages or substages. We found ammonite genera to have a mean generic range of 7.3 Myr/ammonite genus. Using a similar methodology, mean generic range per family was also computed. The distribution of long-ranging genera (arbitrarily chosen as those ammonite genera ranging for 12 Myr or more) among families was found to be nonrandom. Instead, long-ranging genera were found to be concentrated in a few families, resulting in significant heterogeneity in the distribution of generic longevities within families (taxotely sensu Raup and Marshall 1980). Al- though some of the long-ranging genera were found to be morphologically simpler than shorter-ranging genera, others were equally or even more complex, indicating that longevity among ammonite genera is not merely a taxonomic artifact, dependent on degree of differentiable conch morphology. Those Cre- taceous families composed of a large number of long-ranging genera were also among the leaders in mean species longevity per family, based on species-level range data for Cretaceous ammonites of the Great Valley Sequence of California. Many of the long-ranging genera and species possess a similar morphologic attribute: siphuncular tubes (connecting rings) of small diameter but high wall thickness.

A pre-trilobite shelly fauna from the White–Inyo region of eastern California and western Nevada

A low-diversity shelly fauna occurs in the Deep Spring Formation of the White–Inyo Mountains of eastern California and in Esmeralda County, Nevada. Although poorly preserved, specimens can be recovered through acid digestion of the limestone matrix. The fauna is composed of three tubes of uncertain affinities and a hyolith. Nevadatubulus dunfeei n. gen. and sp., a distinctive, randomly curved and annulated tube, is abundant and far outnumbers the remaining three elements: Coleoloides inyoensis n. sp., Sinotubulites cienegensis McMenamin, and the hyolith Salanytheca sp. The original composition of the faunal elements appears to have been calcite or aragonite, but recrystallization has destroyed any ultrastructure. No phosphatic elements occur with the fauna nor have phosphatic fossils been recovered from the underlying Wyman and Reed Formations or the other members of the Deep Spring Formation. The fauna occurs 1,500 meters below the first trilobite body fossils and may be coeval with faunas from the basal Cambrian Tommotian Stage of the Siberian Platform. Wyattia , the only previously described pre-trilobite shelly fossil from the region, occurs in approximately the same stratigraphic interval but was not recovered in our samples.

The Early Cambrian worm tube Onuphionella from California and Nevada

Two new species of worm tubes referable to the genus Onuphionella occur in Lower Cambrian strata in eastern California and western Nevada. Onuphionella durhami n. sp. is found in the Campito Formation (in pre-trilobite strata, in the Fallotaspis and, possibly, the Nevadella Zones) and O. claytonensis n. sp. occurs in the Middle Member of the Poleta Formation ( Nevadella Zone). The unusual tubes are armored with an imbricated coat of mica flakes, reminiscent of the modern genus Owenia. The lowest occurrence of Onuphionella in western North America corresponds closely with the lowest occurrence of the genus in the Baltic region, indicating that correlations between the two regions are not greatly in error.

Species richness in the Phanerozoic; an investigation of sampling effects

-Given estimates of the variation in total standing species richness through the periods of the Phanerozoic, mean species duration, and the relative intensity of the sampling of the fauna from each of the periods, the expected number of described species can be predicted for each period of the Phanerozoic using an analytic sampling model. This model is based on the assumption that the relative abundances of species in any geologic period can be approximated by the canonical (lognormal) speciesabundance distribution. Three commonly cited models of standing species richness (Valentine, 1973; Gould et al., 1977; Bambach, 1977) each suggest different patterns of species richness in the Phanerozoic. By assuming that sampling of the fossil record is proportionate to sediment volume, it can be shown with the sampling model that the Empirical, Equilibrium, and Species-Richness Models each predict that the number of described species will be strongly correlated with sediment volume. Equally high correlations are predicted if it is assumed that sampling is proportionate to sediment area or to paleontological interest. The correlations predicted for each of the three models are remarkably similar. The impact of sampling effects is so strong that the variations in species richness postulated by these three models are almost completely obscured. Preservational biases will probably only further obscure the relationship between the number of described species and total species richness. Therefore, it seems likely that analysis of trends in the total number of described species will be of little use in determining trends in worldwide species richness in the Phanerozoic. Comparison of the actual patterns of variation in the number of described species and the expected numbers of described species predicted by the sampling model reveals that more species are known from the Cenozoic than would be predicted from the abundance of Cenozoic sediments or from the amount of paleontological interest in the Cenozoic. This might have resulted from the Cenozoic sediments remaining relatively free of diagenetic effects which might have destroyed the fossils entombed in the sediments. Philip W. Signor, III. Department of Earth and Planetary Sciences, The Johns Hopkins University, Baltimore, Maryland 21218 Accepted: July 14, 1978

Species richness in the Phanerozoic: Compensating for sampling bias

Sampling biases are the greatest impediment to resolving the history of species richness of fossilizable marine invertebrates in the Phanerozoic. Actual patterns of species richness have remained uncertain because no method is available to compensate for variations in sampling intensity. Data are not obtainable which would permit application of techniques that allow direct compensation for sampling intensity, such as rarefaction, but actual patterns can be estimated with a sampling model designed to account for sampling bias. One can estimate the total species richness of a geologic period if one knows the relative sampling intensity devoted to that period, the original species-abundance distribution of all species that existed during the interval, and the number of species that existed during the Cenozoic. The model presented here is based on the assumption that the species-abundance distributions of fossilizable marine invertebrates were lognormal and that sampling was proportional to sediment area, volume, or paleontologist interest units. The model produces consistent results with different estimates of total Cenozoic species richness and sampling intensity and strongly suggests low diversity during the Paleozoic and Mesozoic, followed by a dramatic early Cenozoic increase in standing species richness. These results are consistent with Valentine9s (1970) Empirical model of species richness.

Resolution of life habits using multiple morphologic criteria: Shell form and life-mode in turritelliform gastropods

Shell form is not strictly linked to life habits in modern marine turritelliform gastropods. To test the usefulness of various morphological characters in determining life-mode, I present a set of predictions giving the expected distribution of characters occurring in turritelliform snails with three different life-modes. Burrowing species should lack sculpture, possess columellar folds and a flat whorl profile, and have an orthocline or prosocline aperture. Mobile epifaunal forms should have sculpture, a rounded whorl profile, a displaced tangential aperture and a smooth columella. Sedentary forms should resemble epifaunal forms but have non-tangential apertures. These predictions were tested with a sample of 105 Recent marine species. Each hypothesis was found to be a statistically valid generalization and in 92 of the species (88%) the life habits were correctly predicted. Accuracy may be further improved by considering additional features such as ratchet sculpture and disjunct or open coiling. These patterns of shell form can be used to interpret fossil species as burrowers, or as sedentary or active epifaunal forms. For example, the unusual Devonian murchisoniid gastropod Ptychocaulus verneuili is interpreted as an active burrower. The relatively imperfect relationship between shell form and life-mode in turritelliform gastropods, as compared to the Bivalvia, apparently results in part from the behavioral complexity of the Gastropoda. Gastropods have a repertoire of activities which would place them in different life-modes at different times; snail morphology reflects this complexity.

Lower Cambrian fossil Volborthella: The whole truth or just a piece of the beast?

Early Cambrian faunas are rich in strange and distinctive fossils that are difficult to interpret or to classify. The small, conical fossils assigned to the extinct phylum Agmata, and the arguments surrounding their affinities and paleoecology, are a classic example of this problem. Volborthella are commonly found in Lower Cambrian strata of North America and in coeval units on the East European platform. These agglutinated fossils are traditionally interpreted as the complete skeleton of individual animals. However, a newly discovered fossil from the White-Inyo Mountains of eastern California demonstrates that Volborthella was a bilaterally symmetrical animal bearing multiple pairs of conical agglutinated sclerites. Volborthella, as traditionally defined, was one of many sclerites covering a relatively large metazoan, an Early Cambrian armored worm or mollusklike animal, and is the only known metazoan with a scleritome composed of agglutinated elements. This discovery ends more than a century of misinterpretation of this enigmatic Early Cambrian fossil.