Terrestrial mass extinctions have been attributed to a wide range of causes. Some of them are external to Earth, such as bolide impacts (as widely discussed for the K/T boundary) and radiation events. Among radiation events, there are possible large solar flares, nearby supernovae, gamma-ray bursts (GRBs), and others. These have variable intensity, duration, and probability of occurrence, although some generalizations are possible in understanding their effects (Ejzak et al. 2007). Here we focus on gamma-ray bursts (Thorsett 1995; Scalo and Wheeler 2002), a proposed causal agent for the end-Ordovician extinction. These are the most remote and infrequent of events, but by virtue of their power, a threat approximately competitive with, for example, that of nearby supernovae. A GRB of the most powerful type (Woosley and Bloom 2006) is thought to result from a supernova at the end of stellar evolution for very massive stars with high rotational speed. Much of their energy is channeled into beams, or jets, which include very high energy electromagnetic energy, i.e., gamma-rays and X-rays. It is a testament to the power of these events, far across the observable universe, that they were first detected in the 1969–1970 results from monitoring satellites designed to detect nuclear explosions on Earth's surface. It was not until the 1990s, when the distance to the events became known, that their power became apparent. Several such events occur every day in the observable universe. Other kinds of events are also potentially damaging, such as so-called short bursts and solar flares, but rate information is only now beginning to clarify how much threat is likely from such sources.

A global database of middle–upper Permian foraminiferal genera has been compiled from the literature for 75 Guadalupian and 62 Lopingian localities, grouped into 32 and 19 operational geographical units respectively. Cluster analysis reveals that five distinct Guadalupian provinces were reduced to four in the Lopingian, following the disappearance of the Eastern Panthalassa Province. Extinction magnitudes across the Guadalupian/Lopingian (G/L) boundary reveal that, in the remaining provinces, there is a strong regional variation to the losses at low paleolatitudes. The Central and Western Tethys Province experienced a markedly lower extinction magnitude, at both provincial and global levels, than the Eastern and Northern Tethys Province. Panthalassa experienced a high extinction magnitude of endemics, but a global extinction magnitude similar to that recorded in Central and Western Tethys. This regional bias is seen in both the fusulinacean and non-fusulinacean foraminifera, although fusulinaceans suffered much higher magnitudes of extinction. The regional selectivity also persisted during the subsequent Lopingian radiations, with the Central and Western Tethys Province recording the greatest magnitudes. Thus, of 35 new genera recorded globally from the Lopingian, 27 of these are recorded in Central and Western Tethys, compared to five and 12 genera respectively in Panthalassa and in Eastern and Northern Tethys. The Emeishan large igneous province erupted within the Eastern and Northern Tethys Province and may have been a factor in the high extinction–low radiation regime of this region. Regression (and consequent shallow-marine habitat loss) also appears to have been a significant factor. A major, but brief, late Guadalupian regression is best seen in those areas that suffered the greatest extinction losses.

The evolution of powered flight has traditionally been associated with the origin of birds, the most successful clade of modern tetrapods, as exemplified by the nearly 10,000 species alive today. Flight requires a suite of morphological changes to skeletal anatomy to create a light yet resistant framework for an airfoil and advanced nervous motor control. Given the level of morphological integration necessary to create a suitable aerofoil, the origin of flight may be intuitively assumed to be coupled with high evolutionary rates of wing-related morphologies. Here we show that the origin of birds is associated with little or no evolutionary change to the skeletal anatomy of the forelimb, and thus Archaeopteryx is unlikely to be the “Rosetta Stone” for the origin of flight it was once believed to be. Using comparative statistics and time-series analyses on a data set constructed from all known forelimb skeletal anatomy of non-avian theropod dinosaurs and a diverse assemblage of early birds, we demonstrate three focused peaks of rapid forelimb evolution at Tetanurae, Eumaniraptora, and Ornithothoraces. The peaks are not associated with missing data and remain stable under multiple perturbations to the phylogenetic arrangements. Different regions of the forelimbs are demonstrated to have undergone asynchronous periods of evolutionary peaks and stasis. Our results evince a more complicated stepwise mode of forelimb evolution before and after the origin of Aves than previously supposed.

Many studies have examined temporal changes in insect feeding on angiosperm leaves, but none have considered variability within a single stratigraphic level. If spatial variability within a level is high, a single sample will not adequately represent the level and may either mask true temporal changes or create spurious ones. In order to measure the spatial variability in fossil insect feeding damage, I collected 12 replicate samples from two laterally extensive carbonaceous shale beds (55.2 and 52.6 Ma) from the early Eocene of the Bighorn Basin, Wyoming. Over 2800 fossil angiosperm leaves were scored for presence or absence of 50 insect damage morphotypes. Damage frequency, diversity, and composition were computed for both the bulk flora and individual plant species in each sample, and variation within a bed was compared with differences between the two beds. Differences in diversity and composition between beds were significantly greater than variations within a bed, and intra-bed variation was primarily due to differing floral composition. Damage frequency within a bed, however, was more variable than diversity. Damage diversity and composition reflect the number of insect species present, whereas damage frequency also depends on the number of insects present, which may be much more variable over small distances.

Understanding the relationships between morphological disparity and environment, geography, and scale require examination at the local level. Even with disparity metrics that are inherently sample size independent, the nature of rare species and the segregation of common and rare species within morphospace can create substantial sampling issues. Eight well-sampled, Late Ordovician crinoid assemblages were examined for potential biases in the study of local disparity. Disparity is based on the ordination of discrete characters. The rare and common species within these assemblages contributed equally to disparity. In spite of this pattern, rare species in some localities occupy a different area of morphospace, causing disparity to vary greatly with sampling intensity. Morphological rarefaction based on the number of specimens shows that disparity weighted by abundance is constant past a sample size of approximately 30 individuals. This metric is dependent on the evenness within an assemblage as well as the abundance within subgroups in morphospace. Disparity weighted according to abundance gives a view of the functional disparity of an assemblage, which is more applicable in studies of local disparity, though unweighted disparity is still preferred in regional-scale studies and in investigations of morphospace filling through a clade's history.

The mammal fauna of the Willwood Formation, central Bighorn Basin, Wyoming, is ideal for paleoecological analysis because it is extensive, well studied, and continuously distributed over sediments representing the first 3 Myr of the early Eocene. The geology of the Bighorn Basin is also well known, providing a precise temporal framework and climatic context for the Willwood mammals. Previous analysis identified three “biohorizons,” based on simple counts of the first and last appearances of species. This study uses species diversity and appearance rates calculated from more extensive collections to approximate the ecological dynamic of the ancient fauna and assess whether the biohorizons were significant turnover events related to recently described climatic variation. Diversity and appearance data collected for this project are extensively corrected for uneven sampling, which varies by two orders of magnitude. Observed, standardized appearance and diversity estimates are subsequently compared with predicted background frequencies to identify significant variation. Important coincident shifts in the biotic parameters demonstrate that ecological change was concentrated in two discrete intervals ≤300 Kyr each that correspond with two of the original biohorizons. The intervals coincide with the onset and reversal of an episode of climate cooling identified directly from Bighorn Basin floras and sediments. Ecological changes inferred from the diversity and turnover patterns at and following the two biohorizons suggest short- and long-term faunal response to shifts in mean annual temperature on the order of 5–8°C.

Studies of ecological structure and diversity over time in extinct groups have always been challenged by the inability to observe the behavior of fossil taxa directly. The only available evidence for function, behavior, and interactions between taxa is the morphological characteristics of the preserved fossils. Recent studies on modern groups have shown that morphological analyses may give misleading results in terms of ecological pattern and diversity. An alternative approach is to focus on functionally relevant aspects of morphology through a paleobiomechanical paradigm. The purpose of this research is to examine variation in the lower jaw morphology in Late Devonian arthrodire placoderms and develop biomechanical metrics that can be used to quantify functional diversity among this fossil group. Nine functionally relevant morphological characters were collected for 94 isolated arthrodire inferognathals from the Gogo Formation in Western Australia and the Cleveland Shale in Ohio. These data were used to address aspects of functional morphology, biomechanical disparity, and ecological structure in arthrodire placoderms from the Late Devonian. Results were compared with results from previous morphometric work on the same set of jaws.

Statistical tests show a significant difference in functional characters between the two faunas. The differences may be related to phylogenetic differences between faunas, as the two major clades of arthrodire taxa included in this study are almost completely segregated between faunas. Average pairwise disparity analyses of the mechanical characters indicate that there is no significant difference in overall functional diversity between the Cleveland Shale and Gogo Reef arthrodire faunas. This result is at odds with previous results that show overall morphological disparity to be much higher in the Cleveland Shale. Clustering patterns within a multivariate function-space show tightly constrained functional groups of taxa independent of phylogenetic or shape-based morphological similarity. These functional groups illustrate a level of ecological diversity in Late Devonian arthrodires that is comparable to that in certain modern faunas.

Further statistical analysis of the morphological and functional disparity of these Late Devonian taxa shows a disjoint between the two measures. Model I regression analysis of and Spearman rank-correlation analysis of average pairwise morphological and functional disparity measures indicate no significant relationship between morphological and functional disparity among the jaws used in this study. Although function is obviously derived from morphology, these results show that morphological shape analysis is not necessarily a good proxy for eco-functional diversity.

The concept of coordinated stasis, manifest as a pattern of long intervals of concurrent taxonomic and ecologic persistence separated by comparatively abrupt periods of biotic change, has been challenged in recent studies that claim a lack of prolonged persistence of taxa and associations. A key problem has been the difficulty of distinguishing faunal change owing to localized, short-term environmental fluctuation or patchiness from that indicating regionally pervasive, long-term evolutionary or ecological change. Here, we use an extensive database from the Middle Devonian Hamilton Group of the Appalachian Basin to test for taxonomic and ecologic persistence within this ecological-evolutionary subunit, a succession of purported relative stability. Replicate samples collected from many localities and stratigraphic horizons over a wide geographic area allow us to address the effects of small-scale environmental variation and localized faunal patchiness while exploring basin-scale variation in faunal composition within and between the formations of the Hamilton Group.

Observed stratigraphic distributions of fossils are consistent with a scenario in which all taxa are present from bottom to top of the Hamilton Group, and absences result only from sampling failure. Although small-scale variation in faunal composition indeed does occur, there is no more variation among formations than occurs within them. Assemblages from different formations, whether they are defined by taxonomic or ecologic composition, are statistically indistinguishable according to several independent metrics, including ANOSIM and a maximum likelihood estimation that evaluates stratigraphic turnover using Bayesian “Information Criterion.” Simulated data sets indicate that test results are most consistent with species-level extinction of 2.6% per Myr within the Hamilton Group, far lower than the Givetian rate of 11.5% per Myr generic extinction derived from a global database. Such faunal persistence over the ~5.5 Myr encompassed by this unit is consistent with the pattern of coordinated stasis. Earlier studies showing greater amounts of temporal turnover in Hamilton Group faunas are likely influenced by their smaller geographic scale of analysis, suggesting that regional studies done elsewhere may yield similar results.

The end-Guadalupian extinction, at the end of the Middle Permian, is thought to have been one of the largest biotic crises in the Phanerozoic. Previous estimates suggest that the crisis eliminated 58% of marine invertebrate genera during the Capitanian stage and that its selectivity helped the Modern evolutionary fauna become more diverse than the Paleozoic fauna before the end-Permian mass extinction. However, a new sampling-standardized analysis of Permian diversity trends, based on 53731 marine invertebrate fossil occurrences from 9790 collections, indicates that the end-Guadalupian “extinction” was actually a prolonged but gradual decrease in diversity from the Wordian to the end of the Permian. There was no peak in extinction rates; reduced genus richness exhibited by all studied invertebrate groups and ecological guilds, and in different latitudinal belts, was instead driven by a sharp decrease in origination rates during the Capitanian and Wuchiapingian. The global diversity decrease was exacerbated by changes in beta diversity, most notably a reduction in provinciality due to the loss of marine habitat area and a pronounced decrease in geographic disparity over small distances. Disparity over moderate to large distances was unchanged, suggesting that small-scale beta diversity changes may have resulted from compression of bathymetric ranges and homogenization of onshore-offshore faunal gradients stemming from the spread of deep-water anoxia around the Guadalupian/Lopingian boundary. Although tropical invertebrate genera were no more likely than extratropical ones to become extinct, the marked reduction in origination rates during the Capitanian and Wuchiapingian is consistent with the effects of global cooling (the Kamura Event), but may also be consistent with other environmental stresses such as anoxia. However, a gradual reduction in diversity, rather than a sharp end-Guadalupian extinction, precludes the need to invoke drastic extinction mechanisms and indicates that taxonomic loss at the end of the Paleozoic was concentrated in the traditional end-Permian (end-Changhsingian) extinction, which eliminated 78% of all marine invertebrate genera.

A combination of historical, functional, and biomechanical constraints has shaped the masticatory apparatus of fossil and extant xenarthrans. Among the more notable features are the teeth: hypselodont; commonly reduced in size, complexity, and number; separated by short diastema; and composed of osteodentine. Enamel is absent, as are the cuspal patterns of other mammals. A comprehensive revision of teeth and other features of the masticatory apparatus of xenarthrans reveals that previous generalizations underestimate the morphological diversity and adaptive possibilities developed within the clade. The great diversity of forms suggests several such possibilities ranging from specialized myrmecophagous species to carrion feeders or predators among animalivores; selective browsers to bulk grazers among herbivores; and omnivores. In some cases xenarthrans represent less extreme versions of patterns developed in other major clades of mammals (marsupials, afrotheres, euarchontoglires, and laurasiatheres) clearly predetermined by a tribosphenic dental morphology, whereas in others they represent unique novelties indicative of particular biological roles. The combination of tooth features that characterize xenarthrans might be seen as the key innovation for the ecologic diversity developed at least since the Oligocene, breaking the mold of the tribosphenic condition that constrained the evolution of the other major clades of mammals.

Estimates of taxonomic richness and abundance are complicated by sampling biases. The failure to sample rare taxa is most often attributed to inadequate sampling and to removal during the process of sample-size standardization. Here I present two methods for unveiling rare diversity by integrating species presence/absence data from museum collections and the literature with quantitative estimates of species richness and abundance gathered from field-based bulk samples. Combining museum, literature, and field data can provide a more comprehensive estimate of taxonomic richness and abundance without substantial increase in current sampling effort. First, in a given bulk sample, the lowest proportional abundance value observed can be used to estimate the maximum abundance of rare species known to have occurred at the locality at least once but not recorded in the current sample. Second, a model-selection approach can be used, in which a set of relative abundance distribution models are fit to the bulk-sample abundance data and the parameter estimates for the best model used to calculate the abundance distribution for all species known from the locality. The Paleogene marine fossil record of the U.S. Gulf Coastal Plain is suitable for applying these methods, because (1) the molluscan fauna is well represented in museum collections and the literature, (2) the molluscan fauna has been taxonomically standardized, and (3) many classic localities remain accessible for standardized bulk sampling. I introduce these methods by applying them to a single locality and then, using the faunas of the Gosport, Moodys Branch, and Red Bluff Formations, I demonstrate how the model-fitting approach can be used to compare taxonomic richness among multiple localities. A substantial fraction of the molluscan richness known from each locality is not captured in bulk samples and much of this unobserved richness may be attributed to the rarity of species. The multiple-locality comparison suggests that the greatest Paleogene decline in standing richness occurred in the middle Eocene and that the recovery of richness following the Eocene-Oligocene extinction was quite rapid despite substantial loss of taxa. These analyses underscore the magnitude of veiled diversity in marine fossil assemblages and the potential of existing sources of data to unveil rare taxa, allowing them to be incorporated into quantitative diversity studies.

Most functional interpretations of ziphodont dentition are based on limited morphometric, behavioral, and taphonomic studies, but few are based on controlled observations of a modern ziphodont consumer. The purpose of this study is to determine through controlled feeding observations if the behaviors indicative of a ziphodont consumer are reflected by tooth marks left on bone surfaces by Varanus komodoensis, the Komodo monitor. We document feeding behavior, expand upon dental function, and correlate these aspects with tooth mark production. We also discuss the significance and limits of applying these data to fossil assemblages.

Goat carcasses were fed to 11 captive individuals. V. komodoensis modifies bone surfaces extensively. Individuals exhibit a “medial-caudal arc'7 when defleshing, followed by inertial swallowing. Bone crushing was not observed. The vast majority of tooth marks are scores, with pits being significantly less common. Tooth furrows and punctures are rare. “Edge marks” are produced on flat elements. Marks are elongate and narrow, with variable lengths and curvature. Over one-third of the marks occur within parallel clusters. Striations are evident on 5% of all marks.

Both feeding behavior and tooth marks indicate that ziphodont crowns are ideal for defleshing by being drawn distally through a carcass. Crowns are poorly built for crushing, and within-bone nutrients are acquired through swallowing. Mark production is a by-product of the distal crown movement during the flesh removal process. Scores are the consequence of apical dragging. Edge marks and striated scores result respectively from distal and mesial carinae contact. Mark curvature is the consequence of arcing motions. Parallel clusters may result from repetitive defleshing strokes and/or from multiple crown contacts during a stroke.

These observations can be used to draw functional, behavioral, and taphonomic interpretations from fossil assemblages. When they are provisionally applied to theropod tooth marks, similar crown function and defleshing behavior with little bone crushing is apparent. Differences occur concerning mark frequency and curvature, relating potentially to taphonomic biases and rostral motion, respectively.

A record of late Paleozoic foraminiferal diversity, origination and extinction frequencies, and provincialism at million-year temporal resolution and species-level taxonomic resolution has been achieved by analyzing composite standard databases. Foraminiferal species diversity increased throughout Mississippian and Pennsylvanian time leading up to its peak at the Pennsylvanian/Permian boundary. Foraminifers then experienced a steep decline in diversity during the Early Permian. Frequencies of origination and extinction broadly tracked changes in global diversity. From Late Mississippian time onward, patterns in total foraminiferal diversity were dominated by fusulinoideans. There is no clear relationship between rates of foraminiferal evolution and the alternating glacial and nonglacial intervals that characterized the late Paleozoic ice age. Rather, high rates of origination and extinction might reflect instability of neritic environments as a consequence of high-frequency, high-amplitude base-level fluctuations (cyclothemic deposition). Further, the advent of algal symbiosis in fusulinoideans was a physiologic innovation that promoted diversification as these symbiont-bearing taxa experimented with morphologic adaptations for partitioning the low-nutrient environments to which they were specialized. Growth to large size and delayed maturation in fusulinoideans might have been enabled by the late Paleozoic hyperoxic atmosphere and the widespread development of oligotrophic, carbonate platform and shelf environments. The late Paleozoic history of foraminiferal diversification was influenced also by closure of the Rheic Ocean beginning in Late Mississippian time. Foraminiferal associations on opposite sides of Pangea exhibited relatively high similarity prior to the closure, but then similarity decreased steadily after destruction of the subequatorial marine corridor. Arctic-Eurasian and North American associations were nearly isolated from one another throughout the main burst of fusulinoidean diversification, so that parallel lineages developed independently in the two regions, resulting in many instances of convergence.

Recent research has corroborated the long-held view that the diversity of genera within benthic marine communities has increased from the Paleozoic to the Cenozoic as much as three- to fourfold, after mitigating for such biasing influences as secular variation in time-averaging and environmental coverage. However, these efforts have not accounted for the considerable increase in the availability of unlithified fossiliferous sediments in strata of late Mesozoic and Cenozoic age. Analyses presented here on the Cenozoic fossil record of New Zealand demonstrate that unlithified sediments not only increase the amount of fossil material and hence the observed diversity therein, but they also preserve a pool of taxa that is compositionally distinct from lithified sediments. The implication is that a large component of the difference in estimates of within-community diversity between Paleozoic and Cenozoic assemblages may relate to the increased availability of unlithified sediments in the Cenozoic.

Recent analytical advances have permitted quantitative evaluations of evolutionary mode in populations of fossil organisms by providing tests of the null hypothesis that patterns of stratigraphic character variation do not differ from the expectations of a random walk. If the hypothesis can be rejected, then stasis and anagenesis represent alternative evolutionary modes discernable using values of the Hurst estimate. We used this approach to evaluate evolutionary mode in the bryozoan genus Peronopora across 34 characters in eight unbranched, cladistically defined, evolutionary sequences. Eight monophyletic crown species and eight paraphyletic (phenetically distinct) metaspecies constitute 16 species-rank taxa within the genus.

In seven of 15 species-rank transitions that had adequate sample sizes, significant character state changes—both phyletic gradualism and punctuated equilibrium—coincided with speciation events 11% of the time and were limited to more derived, crownward, ancestor-descendant pairs. Each of the 34 measured characters exhibited instances of transpecific stasis or anagenesis. Anagenesis of some characters persisted across unbranched lineages over 13 species (i.e., across 12 speciation events), whereas character stasis continued through unbranched lineages in up to 16 species (i.e., persisted unchanged across all 15 speciation events). Transpecific stasis and anagenesis were recognizable in over one-half of the data set, with stasis being approximately twice as common as anagenesis.

Across all character state transitions, approximately one-half reflect stasis, 30% anagenesis, and 20% could not be differentiated from a random walk. Similarly, across species and metaspecies characterized by a single intraspecific mode, stasis was twice as common as anagenesis and three times more common than undifferentiated random walks. The remaining instances of multiple intraspecific evolutionary modes occurred more commonly within metaspecies than within species. This difference might reflect the more frequent presence of unrecognized cryptic species or subspecies within metaspecies of Peronopora. Instantaneous rates of evolution can be estimated both within and between species of Peronopora for characters displaying anagenesis, potentially providing quantitative insights into evolutionary changes within the lineage.

Several lines of theoretical and empirical evidence suggest that there can be a positive correlation between alpha diversity (genus richness) of marine communities (D) and average longevity of marine genera included in these communities (L). One possible reason for such a correlation is that diversity can be expected to give rise to ecosystem stability, which, in turn, may slow down the extinction of taxa. However, this hypothesis has not been verified on the global scale. The analysis of two large data sets (Sepkoski's compendium of fossil marine genera and the Paleobiology Database) shows that the correlation (1) actually exists and (2) is robust to some possible sources of errors in L and D estimation. Further analysis reveals that the correlation is not a secondary pattern caused by any of the following factors: (1) encounter probability of taxa, which is greatly influenced by differential incompleteness of the fossil record; (2) degree of sediment lithification, which is one of the major factors affecting the preservation of fossils; (3) onshore-offshore gradient; (4) parallel growth of both L and D through the Phanerozoic; (5) paleolatitudinal gradient. Although there may be other factors that influence both L and D in a similar way, the results generally confirm the hypothesis that higher alpha diversity enhances longevity of genera.

The middle Carboniferous was an interval of global change when the climate was transitioning from greenhouse to icehouse conditions. Field collections of paleotropical brachiopod assemblages across the Mississippian/Pennsylvanian boundary reveal a taxonomic turnover event in which the overall diversity structure is conserved, despite an apparent regional extinction of 63% of latest Mississippian genera and an apparent regional origination of 50% of earliest Pennsylvanian. An analysis of the global ranges of the brachiopods encountered in the field reveals that turnover was driven primarily by extirpation and immigration rather than true extinctions and originations. Taxonomic richness and evenness are indistinguishable between the latest Mississippian and earliest Pennsylvanian stages. Additive diversity partitioning shows that the within-collection, between-collections (i.e., within-bed), and between-bed diversity components do not change across the Mississippian/Pennsylvanian boundary for richness or evenness. Rank-abundance plots of genera show the same distribution for both stages, but with no correlation between the Mississippian abundances of range-through genera and their abundance in the Pennsylvanian. Detrended correspondence analysis shows a major change in taxonomic composition across that Mississippian/Pennsylvanian boundary and consistency in the general gradient along which genera were distributed. An estimation of spatio-temporal heterogeneity of taxonomic composition within each stage reveals that the earliest Pennsylvanian was significantly more homogeneous. These results suggest that middle Carboniferous brachiopod assemblages from tropical shallow-water carbonate platform settings were organized by some factor that was independent of the specific taxa present. Furthermore, the increased homogeneity in taxonomic composition in the Morrowan did not affect the overall diversity structure. Strong competitive interactions among taxa do not appear to be important in determining the taxonomic compositions and abundances of brachiopod stage-level assemblages.

Biophysical modeling and morphologic data from the fossil record were used to investigate the functional significance of changes in thallus morphology during the early evolutionary history of dasycladalean algae (dasyclads), a clade of benthic marine macroalgae. Modeling results indicate that the addition of cylindrical appendages (laterals) to an upright main axis, a key morphological innovation in the evolution of dasyclad thallus form, can provide for large gains in light interception efficiency, near-maximum gains in this regard being achieved when the ratio of total lateral surface area to main axis surface area is 4 or greater. Among the 13 early Paleozoic study taxa, all but one was found to exceed this value. Modeling of surface area to volume ratios for early Paleozoic dasyclads indicates that laterals for these forms conveyed only modest gains in this regard and, therefore, likely played little role in improving nutrient uptake. For survivorship, it appears that increasing thallus complexity by developing laterals conveyed important benefits by imparting both compartmentalization and redundancy, thereby increasing the likelihood that lateral-bearing forms would survive attacks by early mesograzers. Trends and patterns in the fossil record support such a survival-enhancing role for laterals and are consistent with the initial evolution of these structures as an early manifestation of an evolutionary arms race between macroalgae and herbivores initiated near the Proterozoic/Phanerozoic boundary.

The fossil record has been used to show that in some geologic intervals certain traits of taxa may increase their survivability, and therefore that the risk of extinction is not randomly distributed among taxa. It has also been suggested that traits that buffer against extinction in background times do not confer the same resistance during mass extinction events. An open question is whether at any time in geologic history extinction probabilities were randomly distributed among taxa. Here we use a method for detecting random extinction to demonstrate that during both background and mass extinction times, extinction of marine invertebrate genera has been nonrandom with respect to species richness categories of genera. A possible cause for this nonrandom extinction is selective clustering of extinctions in genera consisting of species which possess extinction-biasing traits. Other potential causes considered here include geographic selectivity, increased extinction susceptibility for species in species-rich genera, or biases related to taxonomic practice and/or sampling heterogeneity. An important theoretical result is that extinction selectivity at the species level cannot be smoothly extrapolated upward to genera; the appearance of random genus extinction with respect to species richness of genera results when extinction has been highly selective at the species level.