The North African murine rodent Paraethomys evolved as an anagenetic lineage from the late Miocene until its extinction in the late—middle Pleistocene. A Fourier analysis of the outlines of the first upper and lower molars of this rodent was used to quantify the evolutionary patterns of this lineage and to compare evolutionary patterns to the climatic record. Morphological evolution and long-term environmental variations are strongly correlated. A change in molar shape, which may be related to the development of a more grass-eating diet, corresponds to the global cooling beginning around 3 Ma and the subsequent increase in aridity in North Africa. Concurrently, size increased, which may be related to increased masticatory efficiency or to metabolic adaptation to the cooler environmental conditions according to Bergmann's rule. This adaptive response to changing environmental conditions corresponds to an acceleration of evolutionary rates in the lineage. The modalities of the evolutionary response in size and shape are probably controlled by intrinsic factors such as different genetic determinisms for both characters.

Macerations of fragmented plant compressions of Silurian (Llandoverian) age yield sheets of organic material bearing ridges and depressions (interpreted as corresponding to surficial dimensions of cells), fragments of smooth walled tubular elements, and fragments of tubular elements with differentially thickened walls (“banded tubes”). A fragment of tissue (1.2 mm long), consisting of smooth walled tubes (17 ± 6.9 μm in diameter), surrounding 2–3 larger (18.8 ± 2.1 μm in diameter) banded tubes, was isolated from an irregularly shaped, relatively large (1 × 3 mm) compression. Comparisons between (1) fragments of tubular cell types, not organized into strands, isolated from 122 compressions, and (2) dispersed tubular cell types previously reported from the Massanutten Formation and other Silurian formations (Tuscarora and Clinton), reveal no significant morphologic differences. Comparisons between the organization of smooth-walled and banded tubular cell types found in the tissue strand and the organization of cell types in nematophytic plants (Nematothallus, Nematoplexus, Prototaxites) indicate a similarity in construction (tubular) but a lack of correspondence in organization. The strand of tissue is interpreted as representing part of the internal anatomy of a nonvascular land plant of unknown taxonomic affinity. On the basis of analogy with present-day embryophytes, the strand of tubular cell types is inferred to have functioned as a conductive tissue. The significance of “banded tubes” in Silurian strata is discussed, and it is concluded that, until more is known about the anatomy, morphology, and biochemistry of the parent plant(s), the habitat and systematic affinity of these organisms are conjectural.

Recent observations indicate that shell fragmentation can be a useful tool in assessing crushing predation in marine communities. However, criteria for recognizing shell breakage caused by durophagous predators versus physical factors are still not well established. Here, we provide data from tumbling and aquarium experiments to argue that physical and biotic processes lead to different patterns of shell damage, specifically that angular shell fragments are good indicators of durophagous predation. Using such angular shell fragments as a predation proxy, we analyze data from 57 European Paleozoic localities spanning the Ordovician through the Mississippian. Our results reveal a significant increase in angular shell fragments (either occurring as isolated valves or present in regurgitalites) in the Mississippian. The timing of this increase is coincident with the increased diversity of crushing predators as well as marked anti-predatory changes in the architecture and mode of life of invertebrate prey observed after the end-Devonian Hangenberg extinction (359 Ma). More specifically, the observed trend in shell fragmentation constitutes strong and independent confirmation of a recently suggested end-Devonian changeover in the primary method of fish predation from shearing to crushing. These results also highlight the important effect of extinction events, not only on taxonomic diversity, but also on the nature of predator-prey interactions.

Abundance is one of the primary factors believed to influence extinction yet little is known about its relationship to extinction rates over geologic time. Using data from the Paleobiology Database we show that abundance was an important factor in the extinction dynamics of marine bivalve genera over the post-Paleozoic. Contrary to expectations, our analyses reveal a nonlinear relationship between abundance and extinction rates, with rare and abundant genera exhibiting rates elevated over those of genera of moderate abundance. This U-shaped pattern is a persistent feature of the post-Paleozoic history of marine bivalves and provides one possible explanation for why we find strong support for heterogeneous extinction rates among genera grouped by similarity in abundance yet effectively no net relationship among these rates when using models of directional selection on abundance.

The 2.0–1.8-billion-year-old Stirling Range Formation in southwestern Australia preserves the deposits of a siliciclastic shoreline formed under the influence of storms, longshore currents, and tidal currents. Sandstones contain a megascopic fossil biota represented by discoidal fossils similar to the Ediacaran Aspidella Billings, 1872, as well as ridge pairs preserved in positive hyporelief on the soles of channel-fill sandstones bounded by mud drapes. The ridges run parallel or nearly parallel for most of their length, meeting in a closed loop at one end and opening with a slight divergence at the opposite end. The ridges are interpreted as casts of sediment-laden mucus strings formed by the movement of multicellular or syncytial organisms along a muddy surface. The taxa Myxomitodes stirlingensis n. igen., n. isp., are introduced for these traces. The Stirling biota was roughly coeval with other presumed multicellular eukaryotes appearing after a long period of profound environmental changes involving a rise in ambient oxygen levels, similar to that which preceded the Cambrian explosion. The failure of multicellular life to diversify during most of the Proterozoic may be due to environmental constraints related to the comparatively low level of oxidation of the world oceans.

Use of quantitative statistical tests can show that there is generally good congruence between estimated cladistic hypotheses of relationship and observed stratigraphy. A data set of 376 cladograms of fishes, continental tetrapods, and echinoderms was tested using three metrics, Spearman Rank Correlation (SRC), Relative Completeness Index (RCI), and Stratigraphic Consistency Index (SCI), to explore the relationships between the indices and differences in results among the three groups of organisms.

There is a strong relationship between SCI and SRC, since both tests measure the same aspect of the fossil record. There is no relationship between RCI and either SCI or SRC. There is a highly significant relationship, as expected, between SRC coefficients and the number of taxa in a cladogram, but no such relationship for RCI or SCI (except in fishes). There is no significant relationship between any of the indices and either the number of internal nodes or tree balance.

Echinoderms show the best stratigraphic consistency of nodes, while continental tetrapods have the best matching of stratigraphic age and cladistic node order. Fishes have the worst match of age and clade ranks, but they do have the most complete fossil record as measured by the RCI. They are followed by echinoderms, and then continental tetrapods, which have the least complete record. This seems to show that life in an aquatic environment leads, in general, to a more complete fossil record.

Formulae are developed for the fraction of the population that must be eliminated by selection in each generation in order to account for a given rate of evolution in a metrical trait. A combination of directional and stabilizing selection is assumed. The effects of competition based on phenotypic value are also considered. The formulae are applied to data on evolutionary rates and their variances. It is concluded that most selective elimination is due to stabilizing selection, and that even very rapid evolutionary change in a single character usually involves low levels of additional elimination. It is suggested that long-sustained evolutionary trends are unlikely to be caused either by gradual change of the optimum under stabilizing selection or by the effects of competitive selection in favor of extreme individuals. It is possible that genetic correlations between traits under selection may limit the response of a single trait to directional selection and hence produce very gradual change.

Zooids of cheilostome bryozoans are on average substantially more robust than are zooids of cyclostome bryozoans. The differences include greater number, length, and cross-sectional area of tentacles, plus a more extensively developed funiculus. Median values for mouth size and cilia-generated feeding current velocity are greater for cheilostomes than for cyclostomes so that cheilostomes have the potential for greater intake of nutrient energy per unit time, which may explain their apparently higher growth rates. For unit area of substrate occupied, the Cheilostomata (Class Gymnolaemata; members of the post-Paleozoic fauna) contain greater biomass and apparently generate greater energy flow than do the Cyclostomata, which are the only extant order of the Class Stenolaemata (characteristic of the Paleozoic fauna).

Quantifying the effects of taphonomic processes on species abundances in time-averaged death assemblages (DAs) is pivotal for paleoecological inference. However, fidelity estimates based on conventional “live-dead” comparisons are fundamentally ambiguous: (1) data on living assemblages (LAs) are based on a very short period of sampling and thus do not account for biological variability in the LA, (2) LAs are sampled at the same time as the DA and thus do not necessarily reflect past LAs that contributed to the DA, (3) compositions of LAs and DAs can be autocorrelated owing to shared cohorts, and (4) fidelity estimates are cross-scale estimates because DAs are time-averaged and LAs are not. Some portion of raw (total) live-dead (LD) variation in species composition thus arises from incomplete sampling of LAs and from biological temporal variation among LAs (together = premortem component of LD variation), as contrasted with new variation created by interspecific variation in population turnover and preservation rates and by the time-averaging of skeletal input (together = postmortem component of LD variation). To tackle these problems, we introduce a modified test for homogeneity of multivariate dispersions (HMD) in order to (1) account for temporal autocorrelation in composition between LAs and DAs and (2) decompose total LD compositional variation into premortem and postmortem components, and we use simulations to evaluate the contribution of within-habitat time-averaging on the postmortem component. Applying this approach to 31 marine molluscan data sets, each consisting of spatial replicates of LAs and DAs in a single habitat, we find that total LD variation is driven largely by variation among LAs. However, genuinely postmortem processes have significant effects on composition in 25–65% of data sets (depending on the metric) when the effects of temporal autocorrelation are taken into account using HMD. Had we ignored the effects of autocorrelation, the effects of postmortem processes would have been negligible, inflating the similarity between LAs and DAs. Simulations show that within-habitat time-averaging does not increase total LD variation to a large degree—it increases total LD variation mainly via increasing species richness, and decreases total LD variation by reducing dispersion among DAs. The postmortem component of LD variation thus arises from differential turnover and preservation and multi-habitat time-averaging. Moreover, postmortem processes have less effect on the compositions of DAs in habitats characterized by high variability among LAs than they have on DAs in temporally stable habitats, a previously unrecognized first-order factor in estimating postmortem sources of compositional variation in DAs.

Small mammal species distribution, density, and composition of modern and Pleistocene faunas from the eastern United States provide information about environmental gradients. Higher summer temperatures and less effective moisture along a transect from the northeast to the southwest can be correlated with the distribution of small mammals as well as decreases in the total number of shrew and vole species. Relative frequencies of “boreal”, “deciduous”, and “steppe” species are more equal in late Wisconsin than modern faunas which are predominantly composed of only one group. A higher vole species density is prevalent in each late Wisconsin fauna and eleven of twelve of these faunas have a higher shrew species density than the corresponding modern fauna. Many shrew and vole species that are today endemic to the boreal provinces were cosmopolitan during the late Wisconsin. Moderate environmental gradients of the equable late Wisconsin climates allowed integration of boreal species with resident species in the south.

The European Plio-Pleistocene Mimomys (Hintonia) occitanus-ostramosensis lineage of arvicolid rodents appears to present a genuine case of gradual evolution. Analysis of numerous dental samples recorded from 3 to 1.5 ma by several biometrical methods led to the following conclusions: samples from the whole geographical area order into a morphological continuum (chronomorphocline); the successive populations in the trend show important overlap of their variation; and the absence of stasis or any interruption in the trend, combined with its unidirectionality, suggests genuine phyletic gradualism. The chronomorphocline is characterized by an important increase in tooth hypsodonty, by the progressive and continuous development of lateral enamel tracks. Four phases of evolution can be recognized in the lineage, the rates of morphological changes being variable and related to climatic fluctuations (Reuverian, Pretiglian, Tiglian, Eburonian). Morphological changes are initiated by environmental fluctuations, although the polarity of climatic change is not a determining factor. Environmental changes act as stimulus and release drift processes. Phyletic gradualism occurring over such a wide area implies extensive gene flow, and suggests that only general, not local, environmental components act to produce the observed morphological changes.

Reconstructing the tree of life involves more than identifying relationships among lineages; it also entails accurately estimating when lineages diverged. Paleontologists typically scale cladograms to time a posteriori by direct reference to first appearances of taxa in the stratigraphic record. Some approaches use probabilistic models of branching, extinction, and sampling processes to date samples of trees, such as the recently developed cal3 method, which stochastically draws divergence dates given a set of rates for those processes. However, these models require estimates of the rates of those processes, which may be hard to obtain, particularly for sampling. Here, we contrast the use of cal3 and other a posteriori time-scaling approaches by examining a previous study that documented a decelerating rate of morphological evolution in pterocephaliid trilobites. Although aspects of the data set make estimation of branching, extinction, and sampling rates difficult, we use a multifaceted approach to calculate and evaluate the rate estimates needed for applying cal3. In agreement with previous simulation studies, we find that the choice of phylogenetic dating method impacts downstream macroevolutionary conclusions. We also find contradictory evolutionary inferences between analyses on ancestor–descendant contrasts (based on ancestor trait reconstruction methods) and maximum-likelihood parameter estimates. Ancestral taxon inference in cal3 corroborates previously hypothesized ancestor–descendant sequences, but cal3 suggests greater support for budding cladogenesis than anagenesis. This case study demonstrates the potential and wide applicability of the cal3 method and the benefits afforded by choosing cal3 over simpler a posteriori time-scaling approaches.

Age-frequency distributions of dead skeletal material on the landscape or seabed—information on the time that has elapsed since the death of individuals—provide decadal- to millennial-scale perspectives both on the history of production and on the processes that lead to skeletal disintegration and burial. So far, however, models quantifying the dynamics of skeletal loss have assumed that skeletal production is constant during time-averaged accumulation. Here, to improve inferences in conservation paleobiology and historical ecology, we evaluate the joint effects of temporally variable production and skeletal loss on postmortem age-frequency distributions (AFDs) to determine how to detect fluctuations in production over the recent past from AFDs. We show that, relative to the true timing of past production pulses, the modes of AFDs will be shifted to younger age cohorts, causing the true age of past pulses to be underestimated. This shift in the apparent timing of a past pulse in production will be stronger where loss rates are high and/or the rate of decline in production is slow; also, a single pulse coupled with a declining loss rate can, under some circumstances, generate a bimodal distribution. We apply these models to death assemblages of the bivalve Nuculana taphria from the Southern California continental shelf, finding that: (1) an onshore-offshore gradient in time averaging is dominated by a gradient in the timing of production, reflecting the tracking of shallow-water habitats under a sea-level rise, rather than by a gradient in disintegration and sequestration rates, which remain constant with water depth; and (2) loss-corrected model-based estimates of the timing of past production are in good agreement with likely past changes in local production based on an independent sea-level curve.

Seven endemic species of skates (Chondrichthyes: Rajidae) represent the only family of elasmobranchs currently known to live in Antarctic continental waters. Many previous authors believed skates colonized Antarctic waters from Patagonia during interglacial periods in the Quaternary. However, recent fossil material collected from the middle Eocene La Meseta Formation of Seymour Island, Antarctic Peninsula, indicates that they may have persisted in Antarctic waters since the Paleogene. Additionally, oceanographic barriers present in the Neogene and Quaternary would have prevented dispersal from southern continents to Antarctica. A revised dispersal scenario, based on skate fossils, biology, paleogeography, and present centers of skate diversity, suggests that skates evolved in the western Tethys and North Boreal seas of western Europe in the Late Cretaceous and early Paleogene and emigrated into Antarctica during the early to middle Eocene via a dispersal corridor along the continental margins of the western Atlantic Ocean. Skates probably populated the Pacific Basin by passing from this dispersal corridor through the Arctic Ocean. Vicariant events, such as opening of the Drake Passage, the development of the Circum-Antarctic Current, and formation of deep and wide basins around Antarctica in the late Paleogene, created barriers that isolated some species of skates in Antarctica and prevented movement of other species of skates into Antarctica from northern areas. Skates are the only group of fishes known to have survived the Oligocene cooling of Antarctica that killed or extirpated the Paleogene ichthyofauna; they persisted by a combination of cold-tolerance, generalized diet, and unspecialized bathymetric and habitat preferences.

Colonies of the fossil bryozoan Archimedes and erect, spiralled species of the living bryozoan Bugula consist of wedge-shaped systems of radially diverging, bifurcated branches that extend from a helical axial margin. Morphology of these colonies may be simulated using few growth rules. These include 1) radius of the central helical margin (RAD), 2) rate of climb of central helical margin (ELEV), 3) radial angle between successive branches that originate from the central helical margin (ANG), 4) radial growth of all branches, 5) angle between branches and axis of central helical margin (BWANG), 6) distance between three adjacent, radially diverging branches at which the central branch bifurcates into two branches equally spaced between the two side branches (XMIN), and 7) placement of a spacing bar at base of newly bifurcated branches. In addition, size constraints on the simulations must be stipulated.

Simulations are begun at a proximal point along the central helix where a radial branch originates and are “grown” in repetitive steps by extending the central helical margin a distance distally, determined by ANG, then censusing established branches for XMIN in order to bifurcate appropriate branches and extend others in several short growth increments, etc. Growth of branches ceases at stipulated maximum width, and growth of the entire simulation ceases at stipulated maximum height.

The presence of a helical inner margin marked by uniformly spaced bifurcations generates the spiralled shape, i.e. ELEV must be a positive number. Values of RAD, ELEV (not zero), ANG and BWANG determine form of the spiral; the other growth rules apply to bifurcated unilaminate branch systems in general.

The range of observed colony forms and hypothetical morphospace of Archimedes may be simulated by varying BWANG, XMIN, and ELEV. RAD was kept constant, as its variation would be redundant with XMIN and ELEV. Variation in ANG affects near-helix morphology, but its influence is undetectable beyond this zone. Variability within colonies may be simulated by assigning to each variable a standard deviation with mean-centered randomly chosen values for each decision.

Chitin occurs in a variety of invertebrates, especially in arthropod cuticles, but is rarely reported in the fossil record. Although it has been detected in fossils as old as Middle Cambrian and Silurian, the majority of records come from much younger, Cenozoic deposits. In this paper, we report the preservation of chitin in Early Jurassic neritimorph gastropod egg capsules deposited in bivalve shells from prodelta-deltafront and nearshore paleoenvironments of the Holy Cross Mountains, Poland. We used a number of analytical methods to confirm the presence of chitin preserved in these ancient fossils. This is the first record of chitin preservation in Mesozoic deposits that, interestingly, do not follow the conventional Konservat-Lagerstätten manner of preserving soft-bodied and non-biomineralized organisms. We believe that deltaic settings characterized by episodic, high input of fluvial deposits, oligohaline conditions, and oxygen-poor microenvironment within the sediment—as well as early cementation of sediment infilling the shells—were crucial for chitin preservation. The preservation of chitin in such recalcitrant structures as egg capsules and deposits that formed outside conventional Konservat-Lagerstätten conditions renders it likely similar deposits may yield promise for discoveries of similar biological macromolecules.

The EDTA-soluble fractions extracted from rostra of two late Cretaceous belemnites (Cephalopoda), Gonioteuthis and Belemnitella, were divided into an humic acid and a fulvic acid fraction (the latter was absent in Belemnitella). The extracts are compared with preparations from shells of two recent cephalopods, Nautilus pompilius and Sepia officinalis. Use was made of immunology, amino acid analysis, pyrolysis mass spectrometry and some other techniques.

The fulvic acid fraction of Gonioteuthis, a mixed peptide-like saccharide-like substance, produced confluent immunodiffusion patterns with an EDTA-soluble Nautilus extract against anti-Nautilus rabbit serum. The humic acid of Gonioteuthis did not contain D-alloisoleucine and its amino acid composition was very similar to that of the EDTA-insoluble fraction of Nautilus. This humic acid was enriched in polyphenol, which may be due to chemical reaction of peptides and carbohydrates during diagenesis. It is concluded that both fractions of Gonioteuthis are original belemnite materials that have undergone only minor alterations during diagenesis.

This is an exploratory study of biochemical compounds derived from fossils, with particular emphasis on immunological methods.

“It's not all heterochrony.” Raff (1996)

“Heterochrony … explains everything.” McNamara (1997)

Few evolutionary topics have generated more confusion and controversy than heterochrony. Commonly defined as “evolution via change in rate or timing of development,” heterchrony has historically become associated with genocidal ideologies, simple-minded theories of evolution, and a bloated, baroque jargon describing patterns produced by largely unknown mechanisms. With a track record like that, perhaps the most surprising aspect of heterochrony is its continued, even rapid, growth as an area of productive scientific inquiry. For example, the number of papers devoted to heterochronic topics continues to increase in many evolutionary journals and books (reviews in Reilly et al. 1997; Klingenberg 1998), including those devoted to human development (Bogin 1997) and evolution (Vrba 1998; McKinney 1998; Parker and McKinney 1999).

The small size of Early Triassic marine organisms has important implications for the ecological and environmental pressures operating during and after the end-Permian mass extinction. However, this “Lilliput Effect” has only been documented quantitatively in a few invertebrate clades. Moreover, the discovery of Early Triassic gastropod specimens larger than any previously known has called the extent and duration of the Early Triassic size reduction into question. Here, we document and compare Permian-Triassic body size trends globally in eight marine clades (gastropods, bivalves, calcitic and phosphatic brachiopods, ammonoids, ostracods, conodonts, and foraminiferans). Our database contains maximum size measurements for 11,224 specimens and 2,743 species spanning the Late Permian through the Middle to Late Triassic. The Permian/Triassic boundary (PTB) shows more size reduction among species than any other interval. For most higher taxa, maximum and median size among species decreased dramatically from the latest Permian (Changhsingian) to the earliest Triassic (Induan), and then increased during Olenekian (late Early Triassic) and Anisian (early Middle Triassic) time. During the Induan, the only higher taxon much larger than its long-term mean size was the ammonoids; they increased significantly in median size across the PTB, a response perhaps related to their comparatively rapid diversity recovery after the end-Permian extinction. The loss of large species in multiple clades across the PTB resulted from both selective extinction of larger species and evolution of surviving lineages toward smaller sizes. The within-lineage component of size decrease suggests that only part of the size decrease can be related to the end-Permian kill mechanism; in addition, Early Triassic environmental conditions or ecological pressures must have continued to favor small body size as well. After the end-Permian extinction, size decrease occurred across ecologically and physiologically disparate clades, but this size reduction was limited to the first part of the Early Triassic (Induan). Nektonic habitat or physiological buffering capacity may explain the contrast of Early Triassic size increase and diversification in ammonoids versus size reduction and slow recovery in benthic clades.