Approximate periodicity for peak rates of global extinction during the past 250 m.y. may have resulted from delayed recovery following major extinction events. Two components can be envisioned for such delays: persistence of inimical environmental conditions for some time after the onset of the crisis, and slow restoration of vulnerable taxa. This general hypothesis is consistent with statistical evidence of linkage between measured rates of extinction of marine invertebrate genera for contiguous stages and substages of the geologic column. The nine broad valleys between the “periodic” peak rates for the past 250 m.y. exhibit only three trivial secondary peaks, indicating that, if the pattern is not artifactual, trends in global rates of extinction have not readily been abruptly reversed. Moreover, the smooth observed trends reflect the fact that regional crises tend to remove many species but few genera. To some degree, high rates of extinction that precede peak rates must represent bias of the incomplete fossil record (the Signor-Lipps effect). High rates that immediately follow peak rates also may, to a degree, reflect biological legacy: (1) final extinction of weakened genera or (2) extinction of new genera that contain few species or represent failed evolutionary “experiments.” Nonetheless, there is evidence that protracted intervals of stressful environmental conditions contributed to high rates of extinction preceding or following certain peak intervals, including the Scythian, Cenomanian, Early Paleocene, and Early Oligocene. The reef-building rudists, for example, suffered heavy extinction during both Cenomanian and Turonian time and then failed to recover quickly.

The late Neogene record of bivalve molluscs in the Western Atlantic offers a more detailed picture of delayed recovery. Here early intervals of glacial expansion caused heavy extinction, leaving an impoverished, eurythermal fauna that was virtually unaffected by late Pleistocene glacial episodes. The episode of heavy extinction in Late Eocene time exhibits a similar phenomenon on a worldwide scale. Among the planktonic foraminifera, warm-adapted stenothermal species died out, and eurythermal forms predominated throughout Oligocene time; restoration of vulnerable, stenothermal species proceeded gradually during the Miocene Epoch. This example of delayed recovery and others like it following earlier global crises may have prevented such crises from following one another in rapid succession, yielding an appearance of periodicity.

A large body of paleontological literature concerns the importance of ontogeny as a source of morphological variation for evolution; morphologies that appear during one stage of an organism's development are made available for use in another simply by modifying the developmental program. Paleontologists need to know why this occurs, so they can study the process of evolution in extinct animals and so they can discuss the fossil record in terms that are applicable to modern forms. If most cases of heterochrony can be attributed to life-history evolution then the fossil record provides evidence of the nature of selection (in particular the age-specific mortality) that extinct animals experienced. The hypothesis of interest here is that species in which maturity is accelerated will also show generalized morphology and small size, while those with delayed maturity will have more specialized morphology and large size.

Four species of the ostracode genus Cyprideis were studied to determine whether differences in age at maturity are correlated with heterochrony in the expected manner. For each species the changes in size and shape through geological time were evaluated in the statistical context of modern geographic and seasonal variation. Living populations were sampled regularly to detect differences in seasonality and to estimate the duration of development.

Evolution of ontogeny is apparent at the level of species in this group, but it is not simply related to differences in life-history. In comparisons among species, we find evidence of heterochrony where there is no difference in the age at maturity, and a difference in age at maturity where there is no heterochrony. Similarly, three of the four species show the expected positive correlation between size and age at maturity, yet the fourth species is relatively large and matures rapidly. Cyprideis does not support the generalization that life-history evolution causes heterochrony, and casts doubt on the inference of life-history evolution from heterochrony where the data are drawn exclusively from extinct forms.

Laboratory experiments on regular echinoids indicate that low water temperatures retard organic decomposition far more effectively than anoxia, and that the primary role of anoxia in the preservation of articulated multi-element calcareous skeletons may be in excluding scavenging organisms. When tumbled at 20 rpm, specimens that were first allowed to decay for two days in warm seawater (30°C) disintegrated more than six times faster than specimens treated at room temperature (23°C) and more than an order of magnitude faster than specimens treated in cool water (11°C). In contrast, the effects of aerobic versus anerobic decay on disintegration rates were insignificant. The longer the period that specimens were allowed to decay before tumbling, the greater the rate at which specimens disintegrated, until a threshold time that appears to mark the decomposition of collagenous ligaments. This required a few days at 30°C, about two weeks at 23°C, and more than 4 weeks at 11°C for Strongylocentrotus. Up until this threshold, coronas disintegrate by a combination of cross-plate fractures and separation along plate sutures; cross-plate fractures thus can be taphonomic in origin and are not necessarily related to predation. Specimens decayed for longer-than-threshold periods of time disintegrate virtually instantaneously upon tumbling by sutural separation only. Undisturbed coronas can remain intact for months, sufficient time for epibiont occupation. Rates of disintegration were documented semi-quantitatively by recognizing seven stages of test disarticulation, and quantitatively by tensometer measures of test strength and toughness. The effects of temperature and oxygen on decay and the existence of a decay threshold in disintegration should apply at least in a qualitative sense to many other animals whose skeletons consist of multiple, collagen-bound elements.

Regular echinoids should still be perceived as taphonomically fragile organisms, but our results suggest the potential for latitudinal as well as bathymetric gradients in the preservation of fossil echinoid faunas. Echinoid preservation under any given set of conditions should also be a function of taxonomic differences in test construction (particularly stereom interlocking along plate sutures) as suggested by previous workers, although our experiments indicate that these effects should only be significant among post-threshold specimens. A survey of regular echinoids from Upper Cretaceous white chalk facies of Britain substantiates the basic experimental patterns, yielding examples of all disarticulation stages and significant taxonomic differences in quality of preservation. A diverse array of borers and encrusters on fossil coronas also corroborates the post-mortem persistence of some tests on mid-latitude seafloors.

Observed stratigraphic ranges almost always underestimate true longevities. Strauss and Sadler (1987, 1989) provide a method for calculating confidence intervals on the endpoints of local stratigraphic ranges. Their method can also be applied to composite sections; confidence intervals may be placed on times of origin and extinction for entire species or lineages. Confidence interval sizes depend only on the length of the stratigraphic range and the number of fossil horizons. The technique's most important assumptions are that fossil horizons are distributed randomly and that collecting intensity has been uniform over the stratigraphic range. These assumptions are more difficult to test and less likely to be fulfilled for composite sections than for local sections.

Confidence intervals give useful baseline estimates of the incompleteness of the fossil record, even if the underlying assumptions cannot be tested. Confidence intervals, which can be very large, should be calculated when the fossil record is used to assess absolute rates of molecular or morphological evolution, especially for poorly preserved groups. Confidence intervals have other functions: to determine how rich the fossil record has to be before radiometric dating errors become the dominant source of error in estimated times of origin or extinction; to predict future fossil finds; to predict which species with fossil records should be extant; and to assess phylogenetic hypotheses and taxonomic assignments.

Organic preservation of non-mineralizing animals constitutes an important part of the paleontological record, yet the processes involved have not been investigated in detail. Organic-walled fossils are generally explicable as a coincidence of original, relatively recalcitrant, extra-cellular materials and more or less anti-biotic depositional circumstances. One of the most pervasive natural inhibitors of biodegradation results from substrate and enzyme adsorption onto, and within, clay minerals; such interactions are likely responsible for many of the organic-walled fossils preserved in clastic sediments. Close examination of the fossil Lagerstätte of the Burgess Shale (Middle Cambrian, British Columbia) reveals that most of its so-called soft-bodied fossils are composed of primary (although kerogenized) organic carbon. Their preservation can be attributed to pervasive clay-organic interactions as the organisms were transported in a moving sediment cloud and buried with all cavities and spaces permeated with fine grained clays. The organic-walled Burgess Shale fossils were studied both in petrographic thin section and isolated from the rock matrix, following careful acid maceration. Isotopic analysis of bulk organic and carbonate carbon yielded values consistent with a normal marine paleoenvironment. Anatomical and histological consideration of the enigmatic Burgess worm Amiskwia suggest that it may in fact be a chaetognath, while the putative chordate Pikaia appears not to be related to modern cephalochordates.

The Quaternary ice ages were paced by astronomical cycles with periodicities of 20–100 k.y. (Milankovitch cycles). These cycles have been present throughout earth history. The Quaternary fossil record, marine and terrestrial, near to and remote from centers of glaciation, shows that communities of plants and animals are temporary, lasting only a few thousand years at the most. Response of populations to the climatic changes of Quaternary Milankovitch cycles can be taken as typical of the way populations have behaved throughout earth history. Milankovitch cycles thus force an instability of climate and other aspects of the biotic and abiotic environment on time scales much less than typical species durations (1–30 m.y.). Any microevolutionary change that accumulates on a time scale of thousands of years is likely to be lost as communities are reorganized following climatic changes. A four-tier hierarchy of time scales for evolutionary processes can be constructed as follows: ecological time (thousands of years), Milankovitch cycles (20–100 k.y.), geological time (millions of years), mass extinctions (approximately 26 m.y.). “Ecological time” and “geological time” are defined temporally as the intervals between events of the second and fourth tiers, respectively. Gould's (1985) “paradox of the first tier” can be resolved, at least in part, through the undoing of Darwinian natural selection at the first tier by Milankovitch cycles at the second tier.

The frequent late Cenozoic glacial ages were accompanied by sea-level falls of 100–150 m amplitude. These falls stranded the complex inner-reef platforms and lagoons of tropical Pacific islands, while outer-reef-slope habitats persisted, although displaced downslope. The effects of glacial regressions on bivalves were studied by examining the zonation of species across reef systems and species composition on tectonically uplifted islands, islands with, effectively, local low sea stands. I show that qualitative habitat loss (the stranding of inner-reef habitats) was responsible for the local extinction of about one-third of the bivalve species that inhabit central Pacific islands during high sea stands, whereas quantitative loss of habitat area and climatic effects were inconsequential. Soft-sediment habitats, and consequently soft-bottom bivalves, were more drastically affected by sea-level fluctuations than were hard-bottom habitats and bivalves.

Although many bivalve species were extirpated in the central Pacific during low sea stands, they survived in the western Pacific, where the different geomorphology of many marine systems provided refugia for lagoonal species. Thus, a large proportion of Pacific bivalve species has very dynamic distributions, undergoing great range reductions and expansions with falls and rises of sea level, and much of the present central Pacific lagoonal fauna is of Holocene age. Several implications of these findings are discussed.

Geographic range appears to be an important aspect of the biology of species, but ranges cannot be unambiguously determined from the fossil record: ancient species can rarely be traced to the full extent of their original geographic ranges, and some ancient provinces are far better characterized than others. Here we test the degree to which paleogeographic range data for species within ancient provinces can be used to estimate the relative magnitudes of total geographic ranges, focusing on the 212 living bivalve species of the Oregonian Province, a north-south (N-S) province of the northeastern Pacific shelf.

We break the total range of each species into its within-province and extraprovincial components. Nonparametric rank correlation tests and simple linear regressions yield highly significant correlations, both between within-province and total ranges (which are not independent variables) and between within-province and extraprovincial ranges. The strongest correlations are obtained when north-ranging species are excluded. These north-ranging species are thermally tolerant and have access to broad east-west (E-W) provinces, thus weakening the relation between their within-province and total ranges. On a coarser scale, we also found that the extraprovincial ranges of species with small within-province ranges (<650 km, ca. 25% of focal province) are significantly less than those for species with large within-province ranges (> 1950 km, ca. 75% of focal province). The significant correlations of within-province range with both total and extraprovincial ranges hold for species with within-province ranges >650 km, so caution is needed regarding species that extend into a focal province only a short distance from its edges. We conclude that within-province geographic range can generally be used to rank species by total geographic range and thus is an adequate proxy in most comparative studies of the paleobiologic consequences of range magnitudes.

The periosteal cortex in the shaft of limb bones is described histologically in three ichthyosaurian genera, Omphalosaurus, Stenopterygius, and Ichthyosaurus. The primary periosteal deposits are composed of typical woven-fibered tissue that was accreted as spongy bone in young individuals, and more or less compact bone in older individuals. During growth, the bone tissue was extensively remodeled with a quantitative imbalance between resorption and redeposition. As a result, the cortex was made cancellous, if previously compact, or still more spongy, if already cancellous. This pattern of remodeling explains why compact cortices are generally lacking in the long bones of ichthyosaurs. The presence of woven-fibered tissue strongly suggests that the limb bones, and probably also the body as a whole, had a rapid postnatal growth in ichthyosaurs, that might have been related to a high, “endotherm-like” metabolic rate. This hypothesis bears on the ecological interpretation of the ichthyosaurs: they could have been capable of sustained, fast swimming and long-range movements, rather than being slow-moving creatures as commonly supposed.

The enigmatic fossil Wiwaxia corrugata is organically preserved in the Burgess Shale (Middle Cambrian, British Columbia) and is therefore extractable by careful acid maceration of the mineralic matrix. High magnification transmitted light microscopy and SEM of macerated Wiwaxia sclerites reveal a substantial amount of previously undescribed structural and microstructural detail. Anatomical and histological comparison with modern organisms indicates that Wiwaxia sclerites are polychaete paleae (flattened setae) and that Wiwaxia was a jawed annelid broadly related to the extant polychaete families Chrysopetalidae and/or Aprhoditidae (Palmyra). Canadia spinosa, an uncontested fossil polychaete from the same beds, shows a paleal microstructure identical to that of Wiwaxia, as well as a closely comparable gross anatomy and taphonomic grade. The unique combination of taxonomically significant characters shared by Wiwaxia and Canadia suggest that they are more closely related to each other than either is to any other fossil or extant polychaete. Thus they constitute a separate superfamily, Canadiacea superfam. nov., in the order Phyllodocida.

Attempts at understanding evolutionary relationships among Paleozoic Dipnoi (lungfish) using cladistic methodology have proved totally unsatisfactory (Miles 1977; Marshall 1987). We attempt to reconstruct the relationships between the better known genera using a method that involves the recognition of lineages based on evolving functional complexes, particularly those involved with food reduction and respiration. Within these broadly defined lineages, we have defined sub-lineages based on evolutionary patterns shown by structures that have been stratigraphically dated; such patterns are found inter alia in the roofing bones and the external dermal bones of the mandible. A number of new suborders and families are recognised; genera for which further morphological data are required before they can be assigned to a higher taxon are indicated; two generic synonyms are recognised.

In appendices, short descriptions are given of two new genera—Pillararhynchus from the Gogo Formation (Upper Devonian) of Western Australia, and Sorbitorhynchus from the Emsian of Guangxi, China.

Differences in zooid skeletal ontogeny are strongly linked to the distribution of bryozoan species, even along such large-scale environmental gradients as depth and latitude. Three growth patterns broadly characterize zooid formation for encrusting cheilostome bryozoans: zooidal and intrazooidal budding, which facilitate growth in two dimensions at colony margins; and frontal budding, which also permits upward growth in the third dimension. Analyses of skeletal growth patterns of encrusting species present in 230 Recent assemblages from the North Atlantic, Mediterranean, North Sea, and Gulf of Mexico show that species with zooidal budding dominate overwhelmingly at lower latitudes and in shallower water, as assessed by percentages of species within assemblages. In contrast, patterns within moderately diverse fossil assemblages from North America indicate that within the geographic and environmental limits sampled, species with intrazooidal budding once dominated and probably originated in shallow, warm water environments. Intrazooidal budding now occurs with greater relative frequency among species from deeper and higher latitude assemblages. Patterns for frontal budding suggest a slight decrease with increasing depth, but no consistent relationship with latitude. These paleoenvironmental trends occur withn a putatively monophyletic clade and are markedly similar to onshore/offshore trends recognized in other groups of marine benthos, based on patterns of occurrence of supra-specific taxa and novel morphotypes. Results presented here are based on relative dominance of species within assemblages and thus provide a novel comparative test of these previously reported trends.

Prominent extinction “events” have been recognized from statistical analyses of marine invertebrate genera represented in Mesozoic and Cenozoic assemblages, contrasting with relatively low “background” extinction intensities measured in terms of a “percentage extinction” index. On a logarithmic scale, the slope of the relationship between time and extinction intensity for background extinctions is shown to be parallel to the slope obtained for most extinction events, characterized by intensities 100.35 above prevailing background levels. Although extinction intensities are variable, this study suggests that the magnitude of the factor(s) primarily associated with most mass extinctions in a 260-m.y. period (N = 9) need not necessarily have been very different from one event to another, an exception being the mass extinction at the end of the Cretaceous.

The spinal quotient (S.Q.) is an osteologically defined estimate of the enlargement of the spinal cord at limb levels over that at interlimb levels. It is an efficient predictor of limb use in living reptiles and birds and may be used to predict limb function in fossil vertebrates. Among living reptiles, this ratio of limb to interlimb innervation is greatest in arboreal genera, followed by terrestrial sprawlers, aquatic forms, and undulatory forms. Birds show a wide range of brachial S.Q. values that are roughly commensurate with flight ability. S.Q. values for the manipulative forelimbs of some dinosaurs fall well above those of locomotory limbs. Dinosaur hind-limb values are either well within ranges predicted by living reptiles and birds (most taxa), or highly inflated (stegosaurs, sauropods). This inflation may be the result of presence of a glycogen body similar to that of birds. In no case does the lumbosacral S.Q. support the presence of a “sacral brain.”

Living crocodilians and limbed lepidosaurs have a large caudofemoralis longus muscle passing from tail to femur. Anatomical and electromyographic data support the conclusion that the caudofemoralis is the principal femoral retractor and thus serves as the primary propulsive muscle of the hind limb. Osteological evidence of both origin and insertion indicates that a substantial caudofemoralis longus was present in archosaurs primitively and was retained in the clades Dinosauria and Theropoda. Derived theropods (e.g., ornithomimids, deinonychosaurs, Archaeopteryx and birds) exhibit features that indicate a reduction in caudofemoral musculature, including fewer caudal vertebrae, diminished caudal transverse processes, distal specialization of the tail, and loss of the fourth trochanter. This trend culminates in ornithurine birds, which have greatly reduced tails and either have a minute caudofemoralis longus or lack the muscle entirely.

As derived theropod dinosaurs, birds represent the best living model for reconstructing extinct nonavian theropods. Bipedal, digitigrade locomotion on fully erect limbs is an avian feature inherited from theropod ancestors. However, the primitive saurian mechanisms of balancing the body (with a large tail) and retracting the limb (with the caudofemoralis longus) were abandoned in the course of avian evolution. This strongly suggests that details of the orientation (subhorizontal femur) and movement (primarily knee flexion) of the hind limb in extant birds are more properly viewed as derived, uniquely avian conditions, rather than as retentions of an ancestral dinosaurian pattern. Although many characters often associated with extant birds appeared much earlier in theropod evolution, reconstructing the locomotion of all theropods as completely birdlike ignores a wealth of differences that characterize birds.

Entelodonts are medium to large (perhaps 150–750 kg) Oligocene–Miocene bunodont artiodactyls with unique crania but typical artiodactyl postcrania. Functional and ecological interpretations are difficult because there is no clear modern analog to an entelodont in size, dentition, and cranial morphology. Entelodont crania combine primitive and derived features, including laterally expanded zygomatic arch/large temporal fossa (suggesting a large temporalis muscle), unreduced dental formula, long premolar row, fused mandibular symphysis, isognathy, and subcylindrical dentary condyles. Furthermore, the cranium has unique architectural and ontogenetic aspects unparalleled in extant mammals. Canines show heavy, carnivoran-like, apical wear in old individuals, suggesting regular contact with food. Conical premolars and associated diastemata dominate the tooth row. Premolars are often apically worn, in a fashion somewhat like those of carnivorans (e.g., Crocuta, Borophagus). Molars are low-cusped crushing teeth. Wide gape, indicated by the form of the coronoid process and temporal fossa, facilitated canine and premolar use, probably both in feeding and in social behavior. Jaw mechanics, tooth morphology, and tooth wear are compatible with omnivory and probable scavenging, an intriguing proposition for the huge Dinohyus hollandi.

Six compilations of fossil tetrapod families, spanning 100 years, each contain a broadly similar diversity pattern since the Upper Devonian. Comparison of four recent data bases, one of which is derived from a strict cladistic treatment, reveals widespread taxonomic and stratigraphic inaccuracies in three earlier data bases. Improvement of our interpretation of the tetrapod fossil record will come through continued taxonomic and stratigraphic revision as well as discovery of new fossils.

Convincing hypotheses of the origin of major invertebrate groups are difficult to make in the absence of phylogenetic analyses. In spite of this, several scenarios exist for the origin of the unusual echinoid order Clypeasteroida. I expand upon the most probable of these models by performing a phylogenetic analysis on three clypeasteroid suborders, the enigmatic fossil genus Togocyamus, and the extinct Oligopygoida. This analysis shows that the oligopygoids are the sister group of the Clypeasteroida plus Togocyamus. The latter is here considered a plesion (extinct sister group) to the crown group Clypeasteroida. Within that order, the suborder Clypeasterina is the sister group to the Laganina plus Scutellina. A new classification of all these taxa is presented. The phylogeny is based on 47 characters and incorporates data on external appendages, Aristotle's lantern anatomy, and test structure of irregular echinoids, as well as new information on the morphology of Togocyamus. The earliest clypeasteroids had a lantern similar to that of adult oligopygoids, which in turn inherited their lantern from a cassiduloid-like ancestor that retained the lantern into adulthood. This lantern is absent in adult cassiduloids. Subsequent changes, including modification of the lantern into a crushing mill, extreme flattening of the test, and proliferation of food-gathering tube feet have allowed clypeasteroids to become epifaunal inhabitants of environments characterized by fine, shifting substrates, a habitat previously inaccessible to most other irregular echinoids.

Paleozoic and post-Paleozoic marine faunas are strikingly different in composition. Paleozoic marine gastropods may be divided into archaic and modern groups based on taxonomic composition, ecological role, and morphology. Paleozoic assemblages were dominated by pleurotomariids (Eotomariidae and Phymatopleuridae), the Pseudozygopleuridae, and, to a lesser extent, the Euomphalidae, while Triassic assemblages were dominated by the Trochiina, Amberleyacea, and new groups of Loxonematoidea and Pleurotomariina. Several new groups of caenogastropods appeared as well. Yet the importance of the end-Permian mass extinction in generating these changes has been questioned. As part of a study of the diversity history of upper Paleozoic and Triassic gastropods, to test the extent to which taxonomic and morphologic trends established in the late Paleozoic are continued after the extinction, and to determine the patterns of selectivity operating during the extinction, I assembled generic and morphologic diversity data for 396 genera in 75 families from the Famennian through the Norian stages. Within this interval, gastropod genera underwent an adaptive radiation during the Visean and Namurian, largely of pleurotomariids, a subsequent period of dynamic stability through the Leonardian, a broad-based decline during the end-Permian mass extinction, and a two-phase post-extinction rebound during the Triassic. The patterns of generic diversity within superfamily-level clades were analyzed using Q-mode factor analysis and detrended correspondence analysis.

The results demonstrate that taxonomic affinity, previous clade history, generic age, and gross morphology did not determine survival probability of genera during the end-Permian extinction, with the exception of the bellerophontids, nor did increasing diversity within clades or expansion of particular morphologies prior to the extinction facilitate survival during the extinction or success after it. The pleurotomariids diversified during the Lower Permian, but were heavily hit by the extinction. Similarly, trochiform and turriculate morphologies, among those which Vermeij (1987) has identified as having increased predation resistance, were expanding in the late Paleozoic, but suffered similar extinction rates to other nondiversifying clades. Survival was a consequence of broad geographic and environmental distribution, as was the case during background periods.