In recent years several authors have questioned the reality of a widely accepted and apparently large increase in marine biodiversity through the Cenozoic. Here we use collection-level occurrence data from the rich and uniquely well documented New Zealand (NZ) shelfal marine mollusc fauna to test this question at a regional scale. Because the NZ data were generated by a small number of workers and have been databased over many decades, we have been able to either avoid or quantify many of the biases inherent in analyses of past biodiversity. In particular, our major conclusions are robust to several potential taphonomic and systematic biases and methodological uncertainties, namely non-uniform loss of aragonitic faunas, biostratigraphic range errors, taxonomic errors, choice of time bins, choice of analytical protocols, and taxonomic rank of analysis.

The number of taxa sampled increases through the Cenozoic. Once diversity estimates are standardized for sampling biases, however, we see no evidence for an increase in marine mollusc diversity in the NZ region through the middle and late Cenozoic. Instead, diversity has been approximately constant for much of the past 40 Myr and, at the species and genus levels, has declined over the past ~5 Myr. Assuming that the result for NZ shelfal molluscs is representative of other taxonomic groups and other temperate faunal provinces, then this suggests that the postulated global increase in diversity is either an artifact of sampling bias or analytical methods, resulted from increasing provinciality, or was driven by large increases in diversity in tropical regions. We see no evidence for a species-area effect on diversity. Likewise, we are unable to demonstrate a relationship between marine temperature and diversity, although this question should be re-examined once refined shallow marine temperature estimates become available.

Where did the modern lineages of placental mammals originate? Recent molecular data seemingly have overturned not only schemata of placental relationships based on morphological data, but also hypotheses about the time and place of origin of the modern lineages. The original hypothesis of Northern Hemisphere origin, based on the fossil record, has been replaced by a “Garden of Eden” hypothesis of origins on a southern continent, based on molecular phylogenies. But, do the molecular data really support this new view of placental mammal origins?

A model is developed to explore the potential responses of paleocommunities to disruptions of primary production during times of mass extinction and ecological crisis. Disruptions of primary production are expected to generate bottom-up cascades of secondary extinction, and these are predictable given species richnesses, functional diversity, and trophic link distributions. If, however, consumers are permitted to compensate for the loss of trophic resources by increasing the intensities of their remaining biotic interactions, top-down driven catastrophic increases of secondary extinction emerge from the model. Both bottom-up and top-down effects are themselves controlled by the geometry of the food webs. The general Phanerozoic trends of increasing taxonomic and ecological diversities, as well as the varying strengths of biotic interactions, have led to food webs of increasing complexity. The frequency of catastrophic secondary extinction increases as food web complexity increases, but increased complexity also serves to dampen the magnitude of the secondary extinctions. When intraguild competitive interactions are included in the model, competitively inferior taxa are observed to possess greater probabilities of survival if the guilds are embedded in simple subnetworks of the overall food web. The result is the emergence of post-extinction guilds dominated by those inferior taxa. These results are congruent with empirical observations of “disaster taxa” dominance after some mass extinction events, and provide a mechanism for the reorganization of ecosystems that is observed after those events. The model makes the testable prediction that dominance by disaster taxa, however, should be observed only when bottom-up disruptions have caused ecosystems to collapse catastrophically.

Where do species that become important players in ecosystems evolve? This simple yet crucial question must be answered if we want to understand how the biosphere is rejuvenated following a crisis. We cannot simply assume that the environments in which we find fossil remains of a given species, or living populations of a species, are the environments in which that species evolved. Take the most obvious example: Fossil human skeletons have been unearthed by the hundreds in North America, but all available evidence points to a human origin in Africa. We can often identify the general geographic origins of species and clades thanks to fossil occurrences and the application of phylogenetic techniques; but can we do likewise for more ecological aspects of the environment? Advances in population biology and in paleobiology now permit us to outline a hypothesis of the circumstances most favorable to the evolution of abundant, widespread, or ecologically powerful species, those with adaptations that are selectively advantageous across many environments, and large short-term and long-term effects in ecosystems.

Short-term fluctuations in the diversification rate of Paleozoic marine animal genera are more strongly correlated with extinction-rate variation than with origination-rate variation. Diversity dynamics are strikingly different in the Mesozoic and Cenozoic, when variation in origination is more important than extinction. Data on the lithologic context of taxonomic occurrences in the Paleobiology Database are used to assess the substrate affinities of Paleozoic genera. The greater role of extinction-rate variation in the Paleozoic is found to characterize genera with an affinity for carbonate substrates but not those that prefer terrigenous clastic substrates. It is therefore plausible that the Paleozoic to post-Paleozoic shift in diversity dynamics is underlain in part by the secular decline in the relative areal extent of carbonate environments, and the concomitant decline in the relative diversity of carbonate-versus clastic-loving taxa.

Recent molecular phylogenetic and molecular clock data both suggest a pre-Mesozoic age for the divergence of the angiosperm lineage from other seed plants, greatly predating the confirmed fossil record of the angiosperm crown group. In addition, molecular phylogenetic studies have not supported the morphologically based conclusion that gnetophytes are the extant sister group to angiosperms. We examine these relationships and divergence ages by using a novel approach of examining the presence of oleanane. This includes the development of methods using zeolites to preferentially reduce hopanes that can co-elute with oleanane. The presence of this molecular fossil strongly correlates with angiosperm diversification; in its functionalized form, along with its triterpenoid precursors, it is found in many living angiosperms. Our data show that among non-angiosperm seed plants examined thus far, oleanane is found only in fossil Cretaceous Bennettitales and Permian Gigantopteridales, both of which share characteristics with angiosperms. Previous morphological phylogenetic results indicate Bennettitales could be a sister group to or member of the angiosperm stem lineage, and results of our preliminary phylogenetic analysis including the Gigantopteridales suggests the same. Our data, based on a new pyrolysis method to treat living species, support previous research indicating that oleanane and its precursors are absent in living gnetophytes. If oleanane originated once in seed plants then the angiosperm stem lineage would have diverged from other seed plant lineages by the late Paleozoic.

Biological veracity of the sharp diversity increase observed in many analyses of the post-Paleozoic marine fossil record has been debated vigorously in recent years. To assess this question for sample-level (“alpha”) diversity, we used bulk samples of shelly invertebrates, representing three major fossil groups (brachiopods, bivalves, and gastropods), to compare the Jurassic and late Cenozoic sample-level diversity of marine benthos. After restricting the data set to single-bed, whole-fauna, bulk samples (n ≥ 30 specimens) from comparable open marine siliciclastic facies, we were able to retain 427 samples (255 Jurassic and 172 late Cenozoic), with most of those samples originating from our own empirical work.

Regardless of the diversity metric applied, the initial results suggest that standardized sample-level species (or genus) diversity, driven by evenness and/or richness of the most common taxa, increased between the Jurassic and late Cenozoic by at least a factor of 1.6. When the data are partitioned into the three dominant higher taxa, it becomes clear that (1) the bivalves, which dominated the samples for both time intervals, increased in sample-level diversity between the Jurassic and the late Cenozoic by a much smaller factor than the total fauna; (2) the removal of brachiopods, which were a noticeable component of the Jurassic samples, did not significantly affect standardized sample-level diversity estimates; and (3) the gastropods, which were rare in the Jurassic but common in many late Cenozoic samples, contributed notably to the increase in sample-level diversity observed between the two time intervals. Parallel to these changes, the samples revealed secular trends in ecological structure, including Jurassic to late Cenozoic increases in proportion of (1) infauna, (2) mobile forms, and (3) non-suspension-feeding organisms. These trends mostly persist when data are restricted to bivalves.

Supplementary analyses indicate that these patterns cannot be attributed to sampling heterogeneities in paleolatitudinal range, lithology, or paleoenvironment of deposition. Likewise, when data are restricted to samples dominated by species with originally aragonitic shells, the observed temporal changes persist at a comparable magnitude, suggesting that the pervasive loss of aragonite in the older fossil record is unlikely to have been the primary cause of the observed patterns. The comparable ratio of identified to unidentified species and genera, observed when comparing the Jurassic and late Cenozoic samples, indicates that the relatively poorer (mold/cast) preservation of Jurassic aragonite species also is unlikely to have been responsible for the observed patterns. However, the diagenesis-related taphonomic and methodological artifacts cannot be ruled out as an at least partial contributor to the observed post-Paleozoic changes in diversity, taxonomic composition, and ecology (the outcomes of the three tests of the diagenetic bias available to us are incongruent).

The study demonstrates that the post-Paleozoic trends in the sample-level diversity, ecology, and taxonomic structure of common taxa can be replicated across multiple studies. However, the diversity increase estimated here is much less prominent than suggested by many previous analyses. The results also narrow the list of causative explanations down to two testable hypotheses. The first is diagenetic bias—a spurious trend driven by either (a) increasing taphonomic loss of small specimens in the older fossil record or (b) a shift in sampling procedures between predominantly lithified rocks of the Mesozoic and predominately unlithified, and therefore sievable, sediments of the late Cenozoic. The second hypothesis is genuine biological changes—macroevolutionary trends in the structure of marine benthic associations through time, consistent with predictions of several related models such as evolutionary escalation, increased ecospace utilization, and the Mesozoic marine revolution. Future studies should focus on testing these two rival models, a key remaining challenge for identifying the primary causative mechanism for the long-term changes in sample-level diversity, ecology, and taxonomic structure observed in the Phanerozoic marine fossil record.

Previous observations about the stable nature of coral-rich assemblages from the Middle Devonian Hamilton Group have led some researchers to invoke the primacy of ecological controls in maintaining biofacies structure through time. However, few analyses have examined the degree to which recurring biofacies vary quantitatively, and none have assessed lateral variability as a benchmark for testing the significance of temporal variability. Thus, the extent to which Hamilton biofacies persist and the mechanism(s) responsible for their hypothesized stability remain contentious. In this study, recurring coral-rich biofacies were evaluated from two stratigraphic horizons within the Middle Devonian Appalachian Basin to examine (1) the extent to which species assemblages persisted within the basin through space and time, and (2) whether ecological interactions may be a plausible mechanism for generating the degree of stasis observed in this case.

Variations in species composition and abundance were examined across multiple spatial scales within both sampled coral-rich horizons. This permitted the establishment of a baseline against which temporal differences in biofacies composition and structure could be evaluated. Although successive coral-rich horizons remained taxonomically stable, their dominance structures changed significantly through the 1.5 Myr study interval. Moreover, additional comparisons among older Hamilton coral-rich horizons corroborate our primary results. These findings support a model in which species respond individually to fluctuations in the physical environment, as indicated by the fluidity of their relative abundances geographically and temporally.

The centric diatom Stephanodiscus yellowstonensis Theriot and Stoermer is endemic to Yellowstone Lake, where it can be an important component of the summer phytoplankton assemblage. Its close relative, Stephanodiscus niagarae Ehrenberg, is abundant in nearby lakes and regional reservoirs. We used the stratigraphic record of Yellowstone Lake to investigate the evolution of S. niagarae to S. yellowstonensis and to describe the limnologic and climatic conditions associated with its evolution. A dramatic morphological shift took place between about 13.7 and 10.0 Ka, but morphology remained stable from 10 Ka to the present. Coincident with morphological change in the S. niagarae/S. yellowstonensis complex were changes in the diatom species assemblage, biogenic silica concentrations, sediment lithology, and regional vegetation. These changes suggest an environment that experienced progressive warming following the retreat of continental glaciers. We could not identify a specific selective factor driving evolution. Nevertheless, nonrandom morphological evolution strongly associated with continuous environmental change suggests that directional selection is a reasonable hypothesis to account for evolution of S. yellowstonensis. Protists are presumed to evolve gradually after speciation events because of large population size, high dispersal capacity, and low reproductive barriers. However, published diatom examples and the evolution of S. yellowstonensis suggest that it is premature to generalize about rates of evolution in protists, or at least to include diatoms in this generalization.

Inferring the causes for change in the fossil record has been a persistent problem in evolutionary biology. Three independent lines of evidence indicate that a lineage of the fossil stickleback fish Gasterosteus doryssus experienced directional natural selection for reduction of armor. Nonetheless, application to this lineage of three methods to infer natural selection in the fossil record could not exclude random process as the cause for armor change. Excluding stabilizing selection and genetic drift as the mechanisms for biostratigraphic patterns in the fossil record when directional natural selection was the actual cause is very difficult. Biostratigraphic sequences with extremely fine temporal resolution among samples and other favorable properties must be used to infer directional selection in the fossil record.

Studies of taxonomic diversity over time commonly count and compare first- and last-appearance data (FADs and LADs) over a succession of temporal intervals, and interpret them with respect to taxon origination and extinction. Singleton taxa, which first appear and last appear in the same temporal interval, are often removed from analyses because they might result from preservational biases rather than evolutionary processes, or they might represent non-independent FADs and LADs. Should singleton taxa always be excluded? We argue that in the case of Paleozoic terebratulide brachiopods, although they may be sensitive to biases in sampling intensity, singleton genera should be included in diversity studies because they do not appear to result from more typical biases, such as Lagerstätten and temporal interval length, that arguably can result in artificially high numbers of singleton genera.

Singleton genera can be critical and effective when used to test hypotheses regarding the existence and generation of latitudinal diversity gradients. Contrary to the anti-tropical diversity pattern of modern articulated brachiopods, Paleozoic terebratulides show a latitudinal diversity gradient that peaks in the Tropics. The hypothesis that the Tropics are either a diversity source or sink can be tested by comparing FAD and LAD latitudes. For singleton genera, FAD and LAD latitudes are taken from the same data points and must be removed for statistical comparisons to be valid. We suggest that taxon age distributions can accommodate singleton data, as the taxon age metric considers origination and extinction simultaneously. We generated taxon age distributions to test the hypothesis that the observed Paleozoic diversity gradient results from a latitudinal bias in generic turnover rate. We discovered that singletons are not randomly distributed over latitude, with proportionally more singleton genera occurring in the Tropics. In this case, singleton genera may reflect rapid evolutionary turnover of taxa, rather than simply preservational bias. Methods that can accommodate singleton taxa should be used to study the diversity of Paleozoic terebratulides and possibly other well-skeletonized marine metazoans.

Stable isotope analysis of mammalian tooth enamel is a valuable method for examining resource partitioning in modern and ancient environments where there is a mixture of C3 and C4 plants. However, before 7 Ma North American ecosystems were composed predominantly of C3 plants, complicating isotopic assessment of resource partitioning. Study of modern African and North American ecosystems has shown that niche partitioning among mammals may be discerned in communities dominated by C3 plants, suggesting that a similar approach may work for ancient C3 ecosystems. Here, such analyses are applied to explore resource use and niche partitioning in two ancient C3-dominated communities, one from California and one from Florida. Each locality, Black Hawk Ranch (California) and the Love Bone Bed (Florida), occurs in Miocene deposits that accumulated prior to the rapid increase in C4 ecosystems 7 Myr ago. δ13C and δ18O values were obtained from the tooth enamel of eight species from Black Hawk Ranch, and 15 species from the Love Bone Bed. Results from the 197 bulk isotope samples showed significant differences in δ13C among taxa at the Love Bone Bed, but no significant differences were observed among taxa at Black Hawk Ranch. At both localities, equids generally have more positive δ13C values than co-occurring taxa, suggesting that equids occupied more open habitats, whereas antilocaprids, camelids, and proboscideans have more negative values, implying utilization of more closed communities. One result of note is the positive δ13C values of Pediomeryx (Yumaceras) hamiltoni from the Love Bone Bed, which suggests that P. (Y.) hamiltoni incorporated abundant fiber, possibly grass, in the diet similar to the horses from this locality. The lack of significant differences among taxa at Black Hawk Ranch may indicate a relatively homogeneous flora, or presence of abundant resources permitting niche overlap, whereas the opposite is implied by the presence of significantly different isotope values among taxa at the Love Bone Bed. The results from this study highlight the utility of isotopic techniques allowing discernment of resource partitioning in C3-dominated landscapes such as those that persisted for the millions of years before the rapid increase in C4 ecosystems that occurred during the late Miocene.

Spectral algorithms have been shown to work well in a wide range of situations that involve the task of ordering. When applied to the localities of a set of European Neogene land mammal taxa, spectral ordering relies almost entirely on the most common genera, depends on connectivity more than on length of taxon lists, and is robust to noise from rarer and less connected taxa. The spectral coefficients for localities are highly correlated with known geochronological ages. Although elementary compared with more sophisticated biochronological tools, spectral ordering allows a fast and standardized way to generate biochronological ordering of localities when other information than faunal lists is lacking. Compared with the conventional mammal Neogene (MN) units, spectral ordering of localities appears to lack distinct temporal boundaries in taxon content and render a much lower count of Lazarus events. If, as seems to be the case, biochronology depends mainly on the most common taxa and if evolutionary change is also most clearly reflected in them, then the main evolutionary patterns should be detectable at a modest level of sampling.

Persistent fossil taxa contravene paradigms of evolution: pervasive morphological change and taxic turnover. Comparative studies of taxic duration have often been approached from biogeographic, climatic, and ecological perspectives, with a focus on process. Here I use a morphological approach to study the pattern of longevity of a large family of marine living and fossil podocopid ostracodes, Trachyleberididae sensu lato. I test if geologically longer-lived genera are collectively morphologically more deviant from a group mean than their shorter-lived relatives by using both discrete morphological data and outline data. I discovered that long-lived genera are in general not significantly more or less morphologically deviant from the average morphology than their shorter-lived relatives. However, I found that contemporaneous subsets of long-lived trachyleberidids are often at least marginally significantly more deviant in discrete morphology than shorter-lived ones, especially in external morphology. No significant patterns of association between morphological deviation and durations in other subdivisions of the data emerged (i.e., whole data set, birth cohorts, groups of morphological characters, and outline data using both Fourier analysis and eigenshape analysis). This is in contrast to a previous finding that long-lived genera of crinoids within orders are often morphologically less deviant than their shorter-lived relatives than expected by chance.

For almost 30 years, paleontologists have analyzed evolutionary sequences in terms of simple null models, most commonly random walks. Despite this long history, there has been little discussion of how model parameters may be estimated from real paleontological data. In this paper, I outline a likelihood-based framework for fitting and comparing models of phyletic evolution. Because of its usefulness and historical importance, I focus on a general form of the random walk model. The long-term dynamics of this model depend on just two parameters: the mean (μstep) and variance (σ2step) of the distribution of evolutionary transitions (or “steps”). The value of μstep determines the directionality of a sequence, and σ2step governs its volatility. Simulations show that these two parameters can be inferred reliably from paleontological data regardless of how completely the evolving lineage is sampled.

In addition to random walk models, suitable modification of the likelihood function permits consideration of a wide range of alternative evolutionary models. Candidate evolutionary models may be compared on equal footing using information statistics such as the Akaike Information Criterion (AIC). Two extensions to this method are developed: modeling stasis as an evolutionary mode, and assessing the homogeneity of dynamics across multiple evolutionary sequences. Within this framework, I reanalyze two well-known published data sets: tooth measurements from the Eocene mammal Cantius, and shell shape in the planktonic foraminifera Contusotruncana. These analyses support previous interpretations about evolutionary mode in size and shape variables in Cantius, and confirm the significantly directional nature of shell shape evolution in Contusotruncana. In addition, this model-fitting approach leads to a further insight about the geographic structure of evolutionary change in this foraminiferan lineage.

Short-term variations in rates of taxonomic extinction and origination in the fossil record may be the result of true changes in rates of turnover, variable rates of fossil preservation, or some combination of the two. Here, positive extinction and origination rate excursions among Phanerozoic marine animal genera are reexpressed in terms of the amount of normal, background time they represent. In addition to providing a background-adjusted calibration of rate intensities, this reexpression determines the durations of sampling gaps that would be required to explain entirely all observed rate excursions as sampling artifacts. This possibility is explored by analyzing a new compilation of the timing and duration of sedimentary hiatuses in North America.

Hiatuses spanning more than approximately one million years (Myr) in the marine sedimentary rock record have a mean duration of 73 Myr. There are two major hiatus types—those that form in response to long-duration tectonic cycles and that bound the major Sloss-scale sequences (mean duration 200 Myr), and those that form in response to shorter-duration changes in sediment accommodation space and that occur within major Sloss-scale sequences (mean duration less than 23 Myr). The latter are approximately exponentially distributed and have a mean duration that is comparable to the mean duration of intervening sedimentary rock packages.

Although sedimentary hiatuses are generally long enough in duration to accommodate the hypothesis that short-term variations in rates of genus origination and extinction are artifacts of sampling failures at major unconformities (“Unconformity Bias” hypothesis), the observed evolutionary rates are not correlated with hiatus durations. Moreover, hiatuses that follow the major mass extinctions are not long in comparison to most other non-mass extinction intervals. These results do not support the hypothesis that hiatuses at major unconformities alone have artificially clustered genus first and last occurrences, thereby causing many of the documented statistical similarities between the temporal structure of the sedimentary rock record and macroevolutionary patterns. Instead, environmental changes related to the expansion and contraction of marine environments may have been the primary forcers of both biological turnover and the spatio-temporal pattern of sediment accumulation. Fully testing this “Common Cause” hypothesis requires quantifying and overcoming lingering taxonomic, biostratigraphic, facies, and numerous other biases that are both inherent in geologic data and imposed by imperfect knowledge of the fossil record.

The species-characteristic combination of morphological characters that depend on an environmental gradient can be used to determine the frequency distribution of the species along the gradient. All functional characters of a phylogenetically closely related species group demonstrate overlapping intervals along an environmental gradient. The gradual change in character composition along the gradient is called a morphocoenocline. Based on transfer functions, a morphocoenocline can be used for gradient values estimation (proxies) in the historical or geological past, similar to a coenocline based on species (democoenocline).

Transforming the empirical frequency distributions of characters and character states along the gradient to probability distributions enables calculating a probability density function of any subset of characters of the morphocoenocline. Because a species is distinguished by a specific combination of characters that are functionally related to the gradient, the distribution of this species along the gradient can be estimated using the probability density functions of combined characters. Assuming “functional uniformitarianism” this estimation can be extended into the geologic past for all fossil species, as long as their functional characters are homologous or analogous to those found among Recent forms. When a morphocoenocline is based on a compound environmental gradient, such as depth, which represents a combination of single environmental factors, the gradient estimation reflects only a specific combination of single factors.

A morphocoenocline for test characters of symbiont-bearing benthic foraminifera from the West Pacific was established for water depth. This compound environmental gradient represents tropical open sea conditions at a slope where the water is highly transparent (low inorganic nutrients and sediment input). Depth distributions based on probability density functions were compared with empirical distributions to prove the accuracy of this method, and were used to estimate the depth distribution of other living species that had not been included in the determination of the morphocoenocline because they live in other regions. The method was also applied to fossil species that are closely related to Recent forms (Nummulites, Assilina from the Eocene) and to fossil species that are more distantly related to the living species (Orbitoides from the Upper Cretaceous).

Ontogenetic stages of trilobites have traditionally been recognized on the basis of the development of exoskeletal segmentation. The established protaspid, meraspid, and holaspid phases relate specifically to the development of articulated joints between exoskeletal elements. Transitions between these phases were marked by the first and last appearances of new trunk segment articulations. Here we propose an additional and complementary ontogenetic scheme based on the generation of new trunk segments. It includes an anamorphic phase during which new trunk segments appeared, and an epimorphic phase during which the number of segments in the trunk remained constant. In some trilobites an ontogenetic boundary can also be recognized at the first appearance of morphologically distinct posterior trunk segments. Comparison of the phase boundaries of these different aspects of segment ontogeny highlights rich variation in the segmentation process among Trilobita. Cases in which the onset of the holaspid phase preceded onset of the epimorphic phase are here termed protarthrous, synchronous onset of both phases is termed synarthromeric, and onset of the epimorphic phase before onset of the holaspid phase is termed protomeric. Although these conditions varied among close relatives and perhaps even intraspecifically in some cases, particular conditions may have been prevalent within some clades.

Trilobites displayed hemianamorphic development that was accomplished over an extended series of juvenile and mature free-living instars. Although developmental schedules varied markedly among species, morphological transitions during trilobite development were generally regular, limited in scope, and extended over a large number of instars when compared with those of many living arthropods. Hemianamorphic, direct development with modest change between instars is also seen among basal members of the Crustacea, basal myriapods, pycnogonids, and in some fossil chelicerates. This mode may represent the ancestral condition of euarthropod development.

The Neoselachii are a monophyletic group including all of the extant sharks and rays. They underwent rapid diversification throughout the Jurassic and Cretaceous, going from low-diversity assemblages of members of extinct orders in the Late Triassic to diverse assemblages containing representatives of most extant clades by the end of the Cretaceous. The known fossil record of Mesozoic neoselachians is composed largely of isolated teeth, with articulated skeletal remains being known from a limited number of sites. The small tooth size of a large proportion of neoselachians, including almost all taxa in existence prior to the mid Cretaceous, led to very poor representation in older publications. Their state of knowledge has improved dramatically since 1970 with the increased use of bulk sampling for isolated dental remains. Despite this, the high proportion of Lazarus taxa from some stages suggests that the state of knowledge is still intermittent. Increase in assemblage diversity throughout the Jurassic and Cretaceous suggests that radiation events resulted in real and dramatic increases in diversity, and that the perceived diversification is not an artifact of poor knowledge. Cladogenesis inferred from the fossil record typically compares more favorably with divergence predicted from molecular analysis, where Batoidea form a discrete basal clade, than with divergence predicted from morphological analysis, where Batoidea are considered a derived crown group within the Squalea. The timing of diversification events is discussed in light of the known fossil record, cladistically generated divergence times, and the paleoenvironmental distribution of faunas.

Most studies of extinction selectivity have focused on mass extinctions. Here we analyze the patterns of susceptibility to extinction during the late Pliocene and Pleistocene of California in the family Pectinidae. The Pectinidae declined in diversity from a high of 32 species in the late Pliocene to the current level of 11 species living in the California region, indicating that the composition of the living fauna has been shaped primarily by extinction in the last several million years. Extinction appears to have occurred in one large pulse, but because of uneven sampling we were unable to resolve the timing further and have analyzed the patterns of extinction treating the late Pliocene through middle Pleistocene as a single period of elevated extinction. Extinctions were not random with regard to taxonomic relationships. Species-level extinctions were higher in more speciose genera, but these genera were buffered against genus-level extinctions. This resulted in a disproportionately large number of monospecific genera in the living fauna. In addition, extinctions were not random with regard to body size, with large species preferentially surviving. This selectivity pattern is evident only when clade membership is taken into account; when analyzed across the entire family, no pattern of size selectivity was apparent. Our results suggest that (1) patterns of extinction selectivity at the genus level may be a poor proxy for species-level patterns, and (2) whole-fauna analyses may not uncover strong selectivity within lineages.