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An intriguing phenomenon in the study of evolutionary rates of morphological change measured from fossil lineages has been the dependence of these rates on the inverse of the measurement interval. This effect has been reported across wide ranges of species as well as within single lineages, and has been interpreted as representing a smooth extension of evolutionary rate from generational timescales to paleontological timescales, suggesting that macroevolution may be simply microevolution extended over longer intervals. There has been some debate about whether this inverse dependence is a real feature of evolutionary change, or a mathematical or psychological artifact associated with the interpretation of data.
Our analysis indicates that the strong inverse dependence of rate on interval is an artifact produced by the phenomenon of spurious self-correlation. Spurious self-correlation can appear in any calculation when a ratio is plotted against its denominator, as is done in plotting rate versus interval, and when these two quantities are not well correlated with one another. We demonstrate that the effect of spurious self-correlation appears in seven published data sets of evolutionary rate that range from taxonomically broad compendia to studies of single families. The effect obscures the underlying information about the dependence of evolutionary change on interval that is present in the data sets. In five of the seven data sets examined there is no significant correlation between the extent of evolutionary change and elapsed time. Where such a correlation does exist, the inverse dependence of rate on interval length is weakened. We describe the role played by taxonomic, dynamic, and character inhomogeneity in producing the lack of correlation of change with interval in each of these data sets. This lack of correlation of change with interval, and the accompanying inverse correlation of rate with interval, most likely arises from discontinuous modes of evolutionary change in which a distinct long-term dynamic dominates net change over geological time spans. It is poorly explained by the extrapolationary microevolutionary models that have been said to account for this phenomenon.
The incorporation of the random walk model into stratophenetic analysis marked a turning point by presenting a potential null model for microevolutionary patterns. Random walks are derived from a family of statistical fractals, and their statistics can be reconstructed using appropriate techniques. This paper lays the foundation for the explicit and uniform description of evolutionary mode in stratophenetic series using random walk null models and the information contained within incompletely preserved time series.
The method relies upon the iterative analysis of subseries of an original stratophenetic series by measuring the presence of deviations from statistical randomness as the lineage evolves. This measure, and its probability of significance (evaluated using a randomization test), forms the dimensions of a descriptive space for microevolutionary modes. Each stratophenetic series can then be viewed as a journey through this space. Computer simulation of various evolutionary modes demonstrates that different modes, for example stasis and gradualism, have differing trajectories and occupy different regions of the microevolutionary space. The method is applied to two published foraminiferal stratophenetic series, the Mio-Pliocene Globorotalia plesiotumida-tumida punctuated transition and an anagenetic trend in the Late Cretaceous Contusotruncana fornicata-contusa lineage. An anagenetic trend is strongly supported in the latter example, whereas transformation of the Globorotalia species seems to result from the fluctuating effectiveness of constraining processes.
The ontogeny of arthropod exoskeletons is punctuated by short periods of growth following each molt, separated by longer stages of unchanging morphology called instars. The recognition of instar clusters in size distributions has been important in understanding the growth and evolution of fossil arthropods. Generally, these clusters have been identified by inspection, but this approach has been criticized for its subjectivity. In this paper, we describe a statistical framework for evaluating hypotheses of clustering based on maximum likelihood analysis of mixture models. The approach assumes that individuals are normally distributed within instars; thus an arthropod size distribution can be considered a mixture of normal distributions. This methodology provides an objective framework to compare various plausible hypotheses of grouping, including the possibility that there is no significant grouping at all.
We apply this method to evaluate clustering in two trilobite species, Ampyxina bellatula and Piochaspis sellata. Both of these data sets show statistically significant evidence of clustering, a phenomenon rarely documented for holaspid-stage trilobites. After consideration of alternative causes of clustering, we argue that the observed groupings are best explained as instar groups. In these two species, growth increments between molts were similar throughout the observed portion of ontogeny, although subtle yet significant variation can be seen within the ontogeny of Ampyxina bellatula.
No egg of any fossil nautiloid has yet been discovered. However, anomalies of embryonic shell growth, described for the first time in several Mesozoic Nautilida, provide important clues on morphology, structure, and size of their egg capsules; on the physical characteristics where egg laying occurred; and on the hatching processes. Roughness inside the inner egg capsule—caused by hard and uneven egg-laying substrate, locally and temporarily slowing down or stopping the apertural shell growth—could cause temporary deformations of growth lines. Such roughness, caused by stone, is described inside an egg capsule of Nautilus, which was fixed obliquely relative to the egg-laying substrate. This reduced the space between the inner and outer capsules, which locally fused together. The lateral-umbilical grooves, furrows, and deformations of growth lines were probably caused by the inner egg capsule during the prehatching stage. In fossil Nautilida, as in Nautilus, the size of this capsule was relatively small compared with the shell diameter at hatching. During the last stages of embryonic development, the shell extended backwards outside the egg capsule before hatching. This prehatching stage, during which the egg capsule continued to press against the shell, can be marked by a prehatching constriction. In fossil species, as in Nautilus, the inner capsule constituted a kind of “straitjacket” during the last stages of embryonic development. The expansion in whorl width at hatching, in normal as well as in abnormal shells, marks release of this straitjacket. Important deformations of the whorl section probably result from an abnormal form and size of the egg capsules mainly caused by the manipulations by the female during the egg laying on a hard and hollow substrate, increasing the straitjacket effect. An alternative explanation could be that the chorion did not expand adequately. From relatively early embryonic stages (approximately 180° adapical of the nepionic constriction) to hatching, both flaps of the hyponome could be turned backward under the shell, jammed between the inner wall of the egg capsule and the mantle margin, resulting in the formation of paired ventral parallel grooves. Many normal features of the embryonic development of nautiloids can be clarified through the study of the anomalies of embryonic shell growth.
Two models of faunal turnover patterns, one with constant turnover and another with climatically induced turnover pulses, were tested against the empirical fossil data of first and last appearances of large mammals from the late Pliocene and Pleistocene of East Africa. Computer simulations of each model were generated by first creating change in hypothetical faunal communities and then sampling the evolving communities in a manner scaled to the specific contingencies of the East African fossil record. Predictions of the two turnover models were compared with the empirical data. Neither model yielded predictions that deviated significantly from the observed patterns of first and last appearances of species, and both models produced extremely similar results. The implication is that the fossil data of East Africa are not refined enough to detect variations in the pace of turnover; the patterns of first- and last-appearance frequencies are determined more by the contingencies of the fossil record than by the underlying evolutionary and migrational patterns. Whereas these results undermine the primary basis of empirical support for the turnover-pulse hypothesis, they do not imply that other models are more likely. However, the simulation results were highly suggestive of significant reduction in species biodiversity of large mammals during the past 2 Myr.
The processes of fossilization have usually been perceived by paleontologists as destructive ones, leading to consecutive (and in most cases irretrievable) losses of paleobiological information. However, recent developments of conceptual issues and methodological approaches have revealed that the decrease in paleobiological information runs parallel to the gain of taphonomic information. This taphonomic imprinting often makes it possible to decode the fraction of paleobiological information that was lost during fossilization, and may also contribute new data for deciphering paleobiological information that was not originally preserved in the assemblage, such as paleoethology. A good example is the study of the macrovertebrate assemblage from the lower Pleistocene site at Venta Micena (Orce, southeastern Spain). Taphonomic analysis showed that the giant, short-faced hyenas (Pachycrocuta brevirostris) selectively transported ungulate carcasses and body parts to their maternity dens as a function of the mass of the ungulates scavenged. The fracturing of major limb bones in the dens was also highly selective, correlating with marrow content and mineral density. Important differences in bone-cracking intensity were related to which species the bones came from, which in turn biased the composition of the bone assemblage. The analysis of mortality patterns deduced for ungulate species from juvenile/adult proportions revealed that most skeletal remains were scavenged by the hyenas from carcasses of animals hunted by hypercarnivores, such as saber-tooths and wild dogs. Analytical study of the Venta Micena assemblage has unlocked paleobiological information that was lost during its taphonomic history, and has even provided paleobiological information that was not preserved in the original bone assemblage, such as the paleoethology of P. brevirostris, which differed substantially from modern hyenas in being a strict scavenger of the prey hunted by other carnivores.
Growth rings of Mesozoic fossil woods have often been used for paleoclimatological inferences. Most of the studies, however, rest upon uniformitarian deductions based on the observation of conifers from the present boreal temperate realm, whereas warm climates dominated during the Mesozoic. We propose a new approach, based on the study of the distribution of growth ring types among 643 samples from the Jurassic–Cretaceous interval. A clear picture emerges from analysis, consistent with what is known of Mesozoic climates from other sources. Woods with no rings are encountered in a wide latitudinal zone, extending up to 75°N and 65°S during the Late Cretaceous. Woods with well-developed latewood do not occur at low latitude and disappeared from the Northern Hemisphere during the Late Cretaceous. Our data set also shows that the taxonomic distribution of growth ring types is not regular. Among the genera encountered, 40% can build only one type of ring. The genus Agathoxylon never displays thick latewood, although it ranges from 75°S to 70°N. This demonstrates that growth ring studies must include a taxonomic analysis.
During the first 10–20 Kyr of the Eocene temperatures warmed by 4–8°C in middle and high latitudes, then cooled again over the succeeding ∼200 Kyr. Major changes in the composition of marine and terrestrial faunas, including one of the largest mammalian turnover events of the Cenozoic, occurred during this temperature excursion. To better understand the effects of rapid climatic change on continental biotas, we studied 60 fossil pollen samples collected from 900 m of section spanning approximately three million years of the late Paleocene and early Eocene; the samples come from the Fort Union Formation and Willwood Formation in the Bighorn Basin of northwestern Wyoming, paleolatitude approximately 47°N. There are 40 samples from the 500 m of rock deposited during the one million year interval centered on the Paleocene/Eocene boundary, although pollen was not preserved well in rocks representing the short warm interval at the base of the Eocene.
Overall, the palynoflora shows moderate change in composition and diversity. Two pollen taxa clearly expanded their ranges to include North America in the first 400 Kyr of the Eocene, Platycarya (Juglandaceae), and Intratriporopollenites instructus (cf. Tilia), but they account for less than 5% of pollen grains in the early Eocene. There are no last appearances of common taxa associated with the Paleocene/Eocene boundary. The most noticeable palynological changes are the decrease in abundance of Caryapollenites spp. and Polyatriopollenites vermontensis (Juglandaceae), and the increase in abundance of Taxodiaceae (bald cypress family), Ulmaceae (elm family), and Betulaceae (birch family), particularly Alnipollenites spp. (alder). There are 22% more species in the Eocene samples than in the Paleocene samples; mean richness of Eocene samples is 17% higher than the mean of Paleocene samples. The mean evenness of Eocene samples is higher than that of Paleocene samples, but the difference is not significant.
The modest level of floral change during the late Paleocene and early Eocene contrasts with the major taxonomic turnover and ecological rearrangement of North American mammalian faunas observed at the same time. Faunal change probably resulted from intercontinental range expansion across Arctic land bridges that became habitable as a result of high-latitude warming, so it is surprising that climatically sensitive plants did not also experience a major episode of interchange. The absence of fossil plants from the temperature excursion interval itself could prevent us from recognizing a transient shift in floral composition, but it is clear that the flora did not undergo a major and permanent restructuring like that seen in the mammals. The contrast between the moderate floral response to warming and the strong faunal response is consistent with the idea that interactions between immigrant and native taxa, rather than climate directly, were the primary cause of terrestrial biotic change across the Paleocene/Eocene boundary.
Vertebrate tracks are a unique, abundant source of fossil data that supplements the skeletal record in many ways. However, the utility of ichnofossil data depends on how specifically the authors of tracks can be identified. Despite this fact, there is little consensus about how to identify potential trackmakers, and existing methods differ in their bases, assumptions, and corresponding implications.
In this paper we support the proposal that trackmakers should be identified primarily by skeletal structures that are both preserved in the ichnofossils and synapomorphies of some body-fossil clade. This synapomorphy-based technique enables certain taxa to be positively identified as candidate trackmakers and others to be excluded from consideration. In addition, the diagnostic level of the synapomorphy (i.e., to a higher or lower level) corresponds to that of the trackmaker. Additional features, such as body size and provenance, can be used in association with synapomorphies as additional differentiae of trackmaker identity.
Trackway analyses are dependent on the level of trackmaker diagnosis, but not all analyses require the same diagnostic specificity. Palichnostratigraphic correlations to the stage level are shown to require at least a genus-level identification of a trackmaker, whereas studies of vertebrate distributions (i.e., origins, extinctions, ranges) accommodate much coarser designations. Anachronistic occurrences of trace and body fossils result in range extensions for either the skeletal taxon or the feature in question. For example, the temporal distribution of theropods can be extended on the basis of the footprint record, resulting in an earlier estimated divergence time for Dinosauria.