The Cretaceous/Tertiary (K/Pg) mass extinction has long been viewed as a pivotal event in mammalian evolutionary history, in which the extinction of non-avian dinosaurs allowed mammals to rapidly expand from small-bodied, generalized insectivores to a wide array of body sizes and ecological specializations. Many studies have used global- or continental-scale taxonomic databases to analyze this event on coarse temporal scales, but few studies have documented morphological diversity of mammalian paleocommunities on fine spatiotemporal scales in order to examine ecomorphological selectivity and ecospace filling across this critical transition. Focusing on well-sampled and temporally well-constrained mammalian faunas across the K/Pg boundary in northeastern Montana, I quantified dental-shape disparity and morphospace occupancy via landmark- and semilandmark-based geometric morphometrics and mean body size, body-size disparity, and body-size structure via body-mass estimates.

My results reveal several key findings: (1) latest Cretaceous mammals, particularly metatherians and multituberculates, had a greater ecomorphological diversity than is generally appreciated, occupying regions of the morphospace that are interpreted as strict carnivory, plant-dominated omnivory, and herbivory; (2) the decline in dental-shape disparity and body-size disparity across the K/Pg boundary shows a pattern of constructive extinction selectivity against larger-bodied dietary specialists, particularly strict carnivores and taxa with plant-based diets, that suggests the kill mechanism was related to depressed primary productivity rather than a globally instantaneous event; (3) the ecomorphological recovery in the earliest Paleocene was fueled by immigrants, namely three multituberculate families (taeniolabidids, microcosmodontids, eucosmodontids) and to a lesser extent archaic ungulates; and (4) despite immediate increases in the taxonomic richness of eutherians, their much-celebrated post-K/Pg ecomorphological expansion had a slower start than is generally perceived and most likely only began 400,000 to 1 million years after the extinction event.

We report on the bone microstructure of the Late Cretaceous birds Patagopteryx deferrariisi and members of the Enantiornithes. These birds represent the most primitive birds ever studied histologically. The occurrence of growth rings indicating alternating periods of slowed and fast growth suggests that these basal birds had slower growth rates, and differed physiologically from their modern relatives. Our findings also call into question previous ideas suggesting that nonavian theropods developed a full avian degree of homeothermic endothermy, which was later inherited by birds. On the contrary, our findings suggest that birds developed classic endothermy relatively late in their phylogenetic history.

Phylogenetic patterns of trilobite clades were used to deduce biogeographic patterns during the Middle Devonian, a time of active plate collision between North America (Laurentia) and other plates, coincident with several major episodes of sea-level rise and fall. The mapping of biogeographic states onto phylogenies for asteropyginid and proetid trilobites indicated that during their history these trilobite clades often shifted the areas they occupied, and also underwent vicariant differentiation, followed by range expansion, followed by subsequent vicariance. Biogeographic patterns in these individual phylogenies were evaluated and synthesized using a modified version of Brooks Parsimony Analysis, which is discussed. This method makes it possible using cladistic methods to distinguish between episodes of vicariance and episodes of dispersal. Two types of dispersal are recognized herein: (1) the individualistic responses of certain taxa in a single clade that cannot be generalized, i.e., traditional ad hoc dispersal, and (2) those patterns of congruent range expansion that are replicated across several clades. The latter are not treated as true dispersal, expansion of a taxon's range over a barrier accompanied by diversification, but rather as a result of the temporary removal of barriers to marine taxa, due either to relative sea-level rise or to the collision of formerly disjunct plates. These are interpreted as changes in the structure of areas, and this type of dispersal is referred to as geo-dispersal. Geo-dispersal was found to have occurred in the Middle Devonian trilobite fauna of Eastern North America.

Biogeographic analysis indicated that Eastern North America is a strongly supported area, with the Appalachian and Michigan Basins as sister areas. Armorica and the Canadian Arctic are also sister areas. Congruence was found between area cladograms produced by vicariance and dispersal analyses for Middle Devonian trilobites, suggesting that in some cases the geological processes governing vicariance, such as sea-level changes, were the same as those that caused dispersal.

The evolution of scutes in thyreophoran dinosaurs, based on Scutellosaurus, Scelidosaurus, Stegosaurus, and several ankylosaurs, began with small rounded or ovoid structures that typically had slight, anteroposteriorly oriented keels. These scutes were elaborated in two general and overlapping ways: they could flare laterally and asymmetrically beneath the keels that mark the anteroposterior axis, and they could be hypertrophied in their distal growth to produce plates, spikes, and other kinds of ornamentation. Stegosaurus plates and spikes are thus primarily hypertrophied keels of primitive thyreophoran scutes, sometimes with elaboration of dermal bone around their pustulate bases. Histologically, most thyreophoran scute tissues comprise secondary trabecular medullary bone that is sandwiched between layers of compact primary bone. Some scutes partly or mostly comprise anatomically metaplastic bone, that is, ossified fibrous tissue that shows incremental growth.

The “plumbing” of Stegosaurus plates was not apparently built to support a “radiator” system of internal blood vessels that communicated with the outside of the plates and coursed along their external surfaces to return heated or cooled blood to the body core. Possibly a purely external system supported this function but there is no independent evidence for it. On the other hand, many of the vascular features in stegosaurian plates and spikes reflect bautechnisches artifacts of growth and production of bone. Surface vascular features likely supported bone growth and remodeling, as well as the blood supply to a keratinous covering. When the gross and microstructural features of the plates and spikes are viewed in phylogenetic context, no clear pattern of thermoregulatory function emerges, though an accessory role cannot be eliminated in certain individual species. It seems more likely, as in other groups of dinosaurs, that the variation of dermal armor form in stegosaurs was primarily linked to species individuation and recognition, perhaps secondarily to inter- and intraspecific display, and rarely to facultative thermoregulation.

Morphologic analysis of 281 species of ammonoids from Great Britain, the North American mid-continent, and the South Urals, at eight successive levels within the Namurian Series (ca. 18 Myr duration), using bivariate plots and principal-components analysis, permits definition of morphologic diversity and identification of morphotypic patterns in time and space. Namurian ammonoids exhibit the same general range of shell geometry that characterizes ammonoids as a whole; there were few post-Namurian innovations in the basic geometry of planispiral ammonoids. Within this overall range of geometry, there are eight preferred morphotypes: two were phylogenetically monopolized by long-ranging forms; three were generalized and reoccur in successive horizons; two others were homeomorphically utilized at different times by different lineages; and one represents morphologic innovation followed by radiation. Such patterns seem to represent combined effects of function, phylogeny, and ecology. Synchronous variations in isolated successions suggest global controls such as eustatic sea-level fluctuations, whereas provincial differences in diversity may be attributable to paleogeographic and ecologic factors. We predict that the Namurian record of ammonoid morphologic diversity and change will be found to be distinctive and differentiable from earlier and later intervals.

Consideration of the ways in which ontogenetic development may be modified to give morphological novelty provides a conceptual framework that can greatly assist in formulating and testing hypotheses of patterns and constraints in evolution. Previous attempts to identify distinct modes of ontogenetic modification have been inconsistent or ambiguous in definition, and incomprehensive in description of interspecific morphological differences. This has resulted in a situation whereby almost all morphological evolution is attributed to heterochrony, and the remainder is commonly either assigned to vague or potentially overly inclusive alternative classes, or overlooked altogether.

The present paper recognizes six distinct modes of ontogenetic change, each a unique modification to morphological development: (1) rate modification, (2) timing modification, (3) heterotopy, (4) heterotypy, (5) heterometry, and (6) allometric repatterning. Heterochrony, modeled in terms of shape /time /size ontogenetic parameters, relates to parallelism between ontogenetic and phylogenetic shape change and results from a rate or timing modification to the ancestral trajectory of ontogenetic shape change. Loss of a particular morphological feature may be described in terms of timing modification (extreme postdisplacement) or heterometry, depending on the temporal development of the feature in the ancestor. Testing hypotheses of the operation of each mode entails examining the morphological development of the ancestor and descendant by using trajectory-based studies of ontogenetically dynamic features and non-trajectory-based studies of ontogenetically static features.

The modes identified here unite cases based on commonalities of observed modification to the process of morphological development at the structural scale. They may be heterogeneous or partially overlapping with regard to changes to genetic and cellular processes guiding development, which therefore require separate treatment and terminology. Consideration of the modes outlined here will provide a balanced framework within which questions of evolutionary change and constraint within phylogenetic lineages can be addressed more meaningfully.

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.

One of the enduring puzzles to Stephen Jay Gould about life on Earth was the cause or causes of the fantastic diversity of animals that exploded in the fossil record starting around 530 Ma–the Cambrian explosion. In this contribution, we first review recent phylogenetic and molecular clock studies that estimate dates for high-level metazoan diversifications, in particular the origin of the major lineages of the bilaterally-symmetrical animals (Bilateria) including cnidarians. We next review possible “internal” triggers for the Cambrian explosion, and argue that pattern formation, those processes that delay the specification of cells and thereby allow for growth, was one major innovation that allowed for the evolution of distinct macroscopic body plans by the end of the Precambrian. Of potential “external” triggers there is no lack of candidates, including snowball earth episodes and a general increase in the oxygenation state of the world's oceans; the former could affect animal evolution by a mass extinction followed by ecological recovery, whereas the latter could affect the evolution of benthic animals through the transfer of reduced carbon from the pelagos to the benthos via fecal pellets. We argue that the most likely cause of the Cambrian explosion was the evolution of macrophagy, which resulted in the evolution of larger body sizes and eventually skeletons in response to increased benthic predation pressures. Benthic predation pressures also resulted in the evolution of mesozooplankton, which irrevocably linked the pelagos with the benthos, effectively establishing the Phanerozoic ocean. Hence, we suggest that the Cambrian explosion was the inevitable outcome of the evolution of macrophagy near the end of the Marinoan glacial interval.

Raup's (1976a) data on Phanerozoic species numbers are examined for species-area relationships, using published estimates of areas of continental seas. By means of multiple regression, species numbers are regressed on both estimated areas of seas and amounts of available rock for sampling, as measured by outcrop area and rock volume. Although the sampling effects apparently have the strongest influence on fossil species diversity, areas of seas substantially increase the total correlation, suggesting that Phanerozoic species numbers were in equilibrium with habitat area. This is further supported by the fact that estimated parameters in the regressions are fairly consistent with established island biogeographic theory. Much of the remaining residual variation can be explained by periods of disequilibrium.

Many areas of paleobiological research require reliable extinction metrics. Branching-and-extinction simulations and data on Phanerozoic marine families and genera are used to investigate the relationship between interval length and commonly used extinction metrics. Normalization of extinction metrics for interval length is problematic, even when interval length is known without error, because normalization implicitly assumes some model of variation in extinction risk within an interval. If extinction risk within an interval were constant, or if it varied but played no role in the definition of stratigraphic intervals, then Van Valen's time-normalized extinction metric would provide a measure of average extinction risk that is effectively unbiased by interval length. When extinction risk varies greatly within an interval and interval boundaries are drawn at times of heavy extinction, extinction metrics that normalize for interval length are negatively correlated with interval length. Despite its intuitive appeal, the per-taxon extinction rate (proportional extinction per million years) is biased by interval length under a wide range of extinction models.

Empirically, time-normalized extinction metrics for Phanerozoic families and genera are negatively correlated with interval length. This is consistent with an extinction model in which many times of very low risk are punctuated by a few times of very high risk which in turn determine stage boundaries. Origination and extinction patterns are similar, but origination intensity varies less among stages than extinction intensity. This observation has at least two plausible explanations: that origination episodes are more protracted than extinction episodes, and that biologic groups do not respond in unison to origination opportunities the way they seem to respond during extinction events. For families and genera, there is enough variation in extinction intensity among stages that stage length can be ignored when studying certain extinction patterns over the entire Phanerozoic.

For some higher taxa, species can be identified in the fossil record with a high degree of reliability. The great geological durations of species indicate that phyletic evolution is normally so slow that little change occurs within a lineage during 105–107 generations. Failure to recognize sibling species in the fossil record has no bearing on this conclusion because they embody virtually no morphological change. Although slowness is the rule, we have no more precise assessment of morphological rates of phyletic evolution for any major taxon. Morphological data that have been assembled to assess rates of phyletic evolution have been meager, unrepresentative, and predominantly reflective of nothing more than body size. Net selection pressures within long segments of phylogeny—even ones that embrace large amounts of evolution—are infinitesimal and seemingly unsustainable against random fluctuations. This suggests that natural selection operates in a highly episodic fashion.

Rates of adaptive radiation and extinction at the species level can be estimated for many taxa and, from them, rates of speciation in adaptive radiation. Species selection should universally tend to increase rate of speciation and decrease rate of extinction, yet these rates are positively correlated in the animal world, apparently because they are linked by common controls: both rate of speciation and rate of extinction seem to increase with level of stereotypical behavior and to decrease with dispersal ability. Only a few “supertaxa” have been able to combine high rates of speciation with moderate rates of extinction.

The sequences of skeletal disarticulation in a broad range of African mammals in a tropical savanna environment suggest that in general the process is very consistent. There is some variation among species, but this cannot yet be convincingly related to higher taxonomic categories or to body size. Disarticulation begins shortly after death, proceeds more or less continuously, and is almost complete after 5 yr. Because of the overall predictability of disarticulation, the stage of disarticulation reached by comparable fossil skeletons provides useful information about their taphonomic history prior to burial.

Pterosaurs were the first flying vertebrates and formed important components of terrestrial and marginal marine ecosystems during the Mesozoic. They became extinct during the latest Cretaceous (latest Maastrichtian), at, or near, the Cretaceous/Paleogene boundary, following an apparent decline in diversity in the Late Cretaceous. This reduction in species richness has been linked to the ecological radiation of birds in the Early Cretaceous and the proposal that birds competitively excluded pterosaurs from many key niches. However, although competition is often posited as a causal mechanism for many of the clade-clade replacements observed in the fossil record, these hypotheses are rarely tested. Here we present a detailed examination of pterosaur diversity through time, including both taxic and phylogenetically corrected diversity estimates and comparison of these estimates with a model describing temporal variation in the number of pterosaur-bearing formations (a proxy for rock availability). Both taxic and phylogenetic diversity curves are strongly correlated with numbers of pterosaur-bearing formations, suggesting that a significant part of the signal contained within pterosaur diversity patterns may be controlled by geological and taphonomic megabiases rather than macroevolutionary processes. There is no evidence for a long-term decline in pterosaur diversity through the Cretaceous, although a reduction in morphological, ecological, and phylogenetic diversity does appear to have occurred in the latest Cretaceous. Competitive replacement of pterosaurs by birds is difficult to support on the basis of diversity patterns.

Late Paleocene planktic foraminifera exhibit strong positive correlations between carbon isotopic values and test mass, but negative correlations between oxygen isotopic values and test mass. Based on analogy with modern taxa, these trends are probably ecotypic and may or may not apply to an ontogenetic series. Among Acarinina and Morozovella species, the magnitude and direction of these trends resemble those of modern planktic foraminifera with dinoflagellate photosymbionts. This is consistent with current models of photosymbiosis and calcification in planktic foraminifera and suggests that Acarinina and Morozovella relied heavily on photosymbionts for nutrition.

Acarinina and Morozovella species resemble modern, strongly photosymbiotic taxa in their association with low and mid latitude nearsurface water masses. However, their test morphologies differ greatly from those of extant taxa that bear obligate photosymbionts. Earliest Paleocene taxa that exhibit a similar paleohabitat association and similar size-related isotopic trends are characterized by still different test morphologies. These comparisons suggest that (1) throughout geologic time, strong reliance on photosymbiont activity has been closely linked to habitat, but not to test morphology; (2) photosymbiosis has been a common and convergently evolved strategy of planktic foraminifera over geologic time, and (3) modern relationships between planktic foraminiferal test morphology and photosymbiont dependence are largely an artifact of geologically recent phylogenetic relationships and shared ecologic strategies.

Intersite comparison suggests that the stable isotopic signals of narrowly constrained size fractions of a late Paleocene Acarinina or Morozovella species can be used to reconstruct the magnitude and direction of relative variation in equilibrium stable isotopic values throughout its geographic and temporal range. This is supported by analogy with extant photosymbiotic taxa. However, since photosynthetic depletion of 12C leaves 13C-enriched HCO3-for calcification, the carbon isotopic values of Acarinina and Morozovella tests may have been consistently greater than paleoseawater values. Failure to account for this effect could lead to overestimation of late Paleocene marine productivity.

Fossil bone microanalyses reveal the ontogenetic histories of extinct tetrapods, but incomplete fossil records often result in small sample sets lacking statistical strength. In contrast, a histological sample of 50 tibiae of the hadrosaurid dinosaur Maiasaura peeblesorum allows predictions of annual growth and ecological interpretations based on more histologic data than any previous large sample study. Tibia length correlates well (R2>0.9) with diaphyseal circumference, cortical area, and bone wall thickness, thereby allowing longitudinal predictions of annual body size increases based on growth mark circumference measurements. With an avian level apposition rate of 86.4 μm/day, Maiasaura achieved over half of asymptotic tibia diaphyseal circumference within its first year. Mortality rate for the first year was 89.9% but a seven year period of peak performance followed, when survivorship (mean mortality rate=12.7%) was highest. During the third year of life, Maiasaura attained 36% (x=1260 kg) of asymptotic body mass, growth rate was decelerating (18.2 μm/day), cortical vascular orientation changed, and mortality rate briefly increased. These transitions may indicate onset of sexual maturity and corresponding reallocation of resources to reproduction. Skeletal maturity and senescence occurred after 8 years, at which point the mean mortality rate increased to 44.4%. Compared with Alligator, an extant relative, Maiasaura exhibits rapid cortical increase early in ontogeny, while Alligator cortical growth is much lower and protracted throughout ontogeny. Our life history synthesis of Maiasaura utilizes the largest histological sample size for any extinct tetrapod species thus far, demonstrating how large sample microanalyses strengthen paleobiological interpretations.

Five generalizations derived from the shell form of prosobranch gastropods are developed. (1) A univalve of more than one volution whose aperture lies in a plane that passes through the axis of coiling does not live with the aperture parallel to the substrate. (2) A univalve of more than one volution whose aperture lies in a plane that is tangential to the body whorl does live with the plane of the aperture parallel to the substrate. (3) Gastropods with tangential apertures, when extended, support the shell so that the center of mass of the shell and its contents is over the midline of the cephalopedal mass; this balancing of the shell may be accomplished either by regulatory detorsion, by inclination or by a combination thereof, to keep the center of gravity of the shell as low as possible. (4) Angulations or re-entrants in the gastropod aperture are usually indicative of inhalent or exhalent areas; inhalent areas are directed as far anteriorly as possible. (5) Gastropods having elongated apertures possess only a single gill and develop a water current through the mantle cavity from anterior to posterior along the long axis of the aperture; this axis is subparallel to the anterior-posterior axis of the foot.

These generalizations are then used as the basis for some deductive interpretations of behavioral modes of Paleozoic Gastropoda.

The directionality of long-term trends can be strongly biased by forces intrinsic to a clade. Trends in body size and skeletal shape may be dictated more by variations in survivorship that reflect differences in ecology than by long-term directional changes in the environment. Hence, mass extinctions can help drive evolutionary trends by selectively eliminating some morphologies and permitting the survivors to found the next radiation.

Examples include repeated trends toward larger maximum body size and the evolution of keeled species from those with globose tests in planktonic foraminifera. Both the trends in size and shape develop because small species with globose tests are significantly more resistant to extinction than species that are large or have peripheral keels. Hence, the survivors of both the Cretaceous-Tertiary and Eocene-Oligocene extinction episodes are small, unkeeled taxa. Large species and species with keels evolved convergently after both mass extinctions as the founders radiated anew.

Comparison of three radiations of planktonic foraminifera suggest that the convergent evolution of similar test shapes and sizes is not due to synchronous changes in oceanography that track evolutionary trends. Instead, the reestablishment of habitat heterogeneity is needed to permit the ensuing radiation to unfold rather than to closely guide its progress. Similar evolutionary trends will develop in each radiation as long as the founders have similar morphology and the evolution of variants present in the previous radiation is not precluded by the environment.

The well-known decline of global background extinction intensity was caused by the sorting of higher taxonomic groups. Two factors were responsible. First, probabilities of familial origination and extinction in these groups (taxonomic orders) were highly correlated. Groups whose families had high probabilities of origination and extinction tended to have highly volatile diversity paths and, consequently, short life spans. Second, orders with high probabilities of familial origination and extinction were rarely replaced by new high-turnover orders. Thus, because high-turnover orders tended to become extinct without replacement, the global background extinction intensity declined. Since familial origination and extinction probabilities are correlated, global background origination intensity inevitably declined as well. As a consequence of these processes, virtually all groups of organisms now living have low probabilities of familial origination and extinction.

Simulations of branching evolution were used to obtain the expected relationships among probabilities (of origination and extinction), volatilities, and longevities for the entire range of possible probabilities, and these relationships were compared to those obtained from the empirical record. In the simulations, only the probabilities of origination and extinction were specified, so volatilities and clade longevities were determined entirely by the probabilities. The similarity between results obtained by simulation and those obtained by analysis of the empirical record further supports the inference that the observed decline of background extinction (and origination) intensity can be explained largely by the loss of high-probability groups to induced volatility.

How much of a living marine fauna would be reflected in the fossil record? In order to answer this, I investigated the probable fate of 169 megascopic genera of the intertidal fauna of the Friday Harbor, Washington region. Three methods were used and these give very similar results. (I) From morphologic examination, 30% of the mud fauna, 32% of the sand fauna, and 29% of the rock fauna are predicted to yield many identifiable fossils; 38% of the mud fauna, 42% of the sand fauna, and 41% of the rock fauna are predicted to yield few identifiable fossils; and the remainder are predicted to yield no fossils. (II) In actual fact, 44% of the mud fauna, 32% of the sand fauna, and 39% of the rock fauna have a fossil record (data from Treatise on Invertebrate Paleontology). (III) The 16 sediment samples which were examined yielded 29% of the total fauna. I conclude (1) that the fossilization potential for the Friday Harbor intertidal fauna is largely independent of habitat, and (2) that 40% of the present megascopic fauna would be (and has been!) preserved in the fossil record.

The fossil record would accurately (and preferentially) include the herbivore and filter feeding genera. The reason which I postulate for this is based on the suitability of heavily calcified exoskeletons to an essentially sessile mode of life, and the lack of suitability of such skeletons for readily mobile forms. 67% of the genera which essentially rest in one place are known as fossils. In contrast, only 16 to 27% of the burrowing detritus eaters (e.g., polychaetes) and roving carnivores (e.g., sea stars and crabs) are known as fossils. The percentage of herbivore and filter feeding genera in rocky environments is 39%, in sand 16% and in mud 34%. In the fossil record, deposits which were originally mud are likely to be most fossiliferous because (1) that environment has a high proportion of essentially sessile genera, (2) essentially sessile genera are far more likely to have a heavily calcified skeleton, and (3) few rock intertidal regions are buried.

We offer an alternative interpretation of the Kingdom Vendobionta as a monophyletic sister group to the Eumetazoa. We hypothesize that the Vendobionta are cnidarian-like organisms that lacked cnidae. Cnidarians are held to have arisen by acquisition of cnidae by symbiosis with a microsporidian. Our analysis differs from existing interpretations of the Ediacaran fossils as ancestors of extant cnidarians in that we do not regard any of these forms as either polypoid or medusoid. This interpretation requires the erection of a new metazoan phylum, the Vendobionta.