In marsupial mammals and their extinct relatives—collectively, metatherians—only the last premolar is replaced, but the timing of dental eruption is variable within the group. Our knowledge of fossils metatherians is limited, but is critical to understanding several aspects of the evolution and morphological diversification of this clade. We analyzed the sequence of eruption of 76 specimens of metatherians, including Sparassodonta, an extinct clade of specialized carnivores from South America. In Sparassodonta (1) the P3/p3 erupt simultaneously, in common with some didelphids (in other didelphids, p3 erupts before P3, whereas in the remaining didelphids, some peramelids, one caenolestid, and Pucadelphys this order is reversed); (2) the upper and lower molars at the same locus erupt more in synchrony than in other carnivorous metatherians in which the lower molars clearly precede the upper equivalents; (3) the upper canine in thylacosmilids and proborhyaenids is hypselodont; (4) species with similar molar morphologies have different morphologies of the deciduous premolars, suggesting diverse diets among the juveniles of different taxa; (5) deciduous teeth are functional for a long period of time, with thylacosmilids even retaining a functional DP3 in the permanent dentition. The retention of the DP3 and the hypertrophied and hypselodont upper canine of thylacosmilids represent clear heterochronic shifts. Specializations in the timing of dental eruption and in the deciduous tooth shape of sparassodonts are evolutionary mechanisms that circumvent constraints imposed by the metatherian replacement pattern and increase morphological disparity during ontogeny.

Fourier analysis of percent extinction data for genera of fossil marine animals during the past 260 million years indicates the existence of a 26 m.y. extinction cycle. The first three harmonics, which account for 51.7 percent of the sum of squares, show the major extinction trends in the late Paleozoic, Mesozoic and Cenozoic. The first ten harmonics, which account for 89.5 percent of the sum of squares, are aligned with eight extinction peaks. The tenth harmonic, with a period of 26 million years, has an amplitude of 6.5 percent and accounts for 12.7 percent of the sum of squares.

Several different methods are used to test the validity of the apparent 26 m.y. extinction cycle. Analysis of variance indicated that the 26 m.y. extinction cycle is statistically significant at the 95 percent level. When the 260 m.y. sample interval is divided into equal 130 m.y. segments, the phases and amplitudes for the 26 m.y. cycle were almost equal within the two independent segments. When the 260 m.y. interval is divided into 10 shorter time segments of different lengths, the 26 m.y. cycle is represented in each time segment.

A series of experiments is run to test the influence of the Late Permian and Cretaceous peaks on the observed 26 m.y. cycle. When all peaks are smoothed out except for the two major peaks, a pseudo-cycle with a period of 26 m.y. is generated that accounts for 3.0 percent of the sum of squares. When the Permian and Cretaceous peaks are reduced to about half their observed height, the sum of squares accounted for by the 26 m.y. cycle is reduced from 12.7 to 11.0 percent. When the number of extinctions within each stage is used in the harmonic analysis in place of extinction percent, the 26 m.y. cycle accounts for 19.3 percent of the sum of squares.

Therefore, the evidence from harmonic analysis of fossil marine animals points toward a distinct and persistent 26 m.y. cycle in mass extinctions in the late Paleozoic, Mesozoic and Cenozoic.

Principal components analysis (PCA) of 21 shell parameters (geometry, sculpture, aperture shape, and suture complexity) in 597 L. Devonian to L. Triassic ammonoid genera (spanning ~166 Myr) shows that eight basic morphotypes appeared within ~20 Myr of the first appearance of ammonoids. With one exception, these morphotypes persisted throughout the Paleozoic, occurring in ~75% of the ~5-Myr time bins used in this study. Morphotypes were not exclusive to particular lineages. Their persistence was not just a product of phylogenetic constraints or longevity, and multiple iterations of the same morphotypes occurred at different times and in different groups. Although mass extinction events severely condensed the range of morphologic variation and taxonomic diversity, the effects were short lived and most extinct morphotypes were usually iterated within 5 Myr. The most important effect of mass extinctions on ammonoid evolutionary history seems to have been their role in large scale taxonomic turnovers; they effectively eliminated previously dominant orders at the Frasnian/Famennian (F/F) (Agoniatitida), the Devonian/Mississippian (D/M) (Clymeniida), and the Permian/Triassic (P/T) (Goniatitida and Prolecanitida) extinctions. Survivors varied from two (P/T) to four (D/M) and five genera (F/F). These events generated sharp reductions in morphologic disparity at the D/M (58%) and at the P/T (59%), but there was a net increase at the F/F (38%). There was no obvious survival bias for particular morphotypes, but 64% are interpreted to have been Nautilus-like nektobenthic. The recurrence of particular combinations of morphology and their strong independence of phylogeny are strong arguments for functional constraint. Intervals between mass extinctions seem to have been relatively static in terms of morphotype numbers, in contrast to numbers of genera. Significant decreases in genus diversity (54%) and morphologic disparity (33%) commenced in the mid-Permian (Wordian/Capitanian boundary), well before the final P/T event.

An early Pliocene fossil locality in the Canadian High Arctic preserves the remains of the extinct beaver Dipoides sp. (Castoridae, Rodentia) in association with an assemblage of fossil beaver-cut wood. The wood assemblage presents a first opportunity to investigate woodcutting behavior and ecological performance in an extinct castorid genus. This study compares woodcutting in Dipoides sp. with that of the modern beaver, Castor canadensis, using evidence from small-diameter cut sticks (i.e., sticks transected by parallel series of cut marks) in combination with behavioral observations of Castor woodcutting. During woodcutting both Castor and Dipoides used their incisors unilaterally; the upper incisor was pressed against the stick while the corresponding lower incisor cut. Cut marks were relatively larger for Castor than Dipoides (scaled to incisor size). Compared with Dipoides, Castor more frequently used a cutting strategy that minimized the number of cuts needed to transect a stick (e.g., clipping as opposed to chip removal). Taken together, the behavioral evidence suggests that ecological cutting performance was lower for Dipoides than Castor.

Examination of organismal characteristics which promote survivorship through both background and mass extinctions may reveal general ecological principles potentially critical to modern conservation efforts. This study explores survivorship of brachiopods, a highly diverse and abundant Paleozoic clade, through the mid-Permian to mid-Triassic interval, which includes the greatest mass extinction in the history of metazoan life. This interval of time separates two of the major Phanerozoic evolutionary faunas. In this regard, survivorship across any one extinction during the interval would not have been relevant if the survivor went extinct shortly after the extinction event; surviving background extinction is as important as surviving a mass extinction. Similarly, taxa that survived but failed to rediversify also were not major elements of the Mesozoic evolutionary fauna. Thus, the analysis aims to analyze survivorship not just across a single extinction but across the entire mid-Permian to mid-Triassic; only survivors through the entire interval can be the ancestors of the Mesozoic clades.

Fewer brachiopod genera survived the interval than did brachiopod clades, suggesting that pseudoextinction or insufficient sampling could be a problem in analyzing these extinctions; thus, survivorship analysis should be conducted at the clade level. Nine characteristics were examined for generic representatives of 20 North American brachiopod clades, five of which survived both Permian extinctions and the subsequent earliest Triassic transitional interval. Characteristics include both those that operate on global scales and those that operate on the higher-resolution scales of individuals and populations.

Survivors were significantly smaller and occurred less frequently than victims. Mean diversity of communities in which survivors were present was significantly greater. The finding that rare taxa belonging to high-diversity communities were more likely to survive runs counter to traditional predictions. However, these results are consistent with recent studies suggesting that higher diversity within a trophic level may create a buffer, as surviving taxa quickly occupy the vacant niche space of the victims. As size, abundance, and community diversity are all statistically related, the small size of survivors may be an artifact of reduced biovolume per taxon in a diverse community.

No significant relationship exists between global-scale processes and survivorship of brachiopods through the mid-Permian to mid-Triassic. The results suggest that ecological processes can strongly influence global extinction patterns.

Species abundance data are of vital importance in paleontology, but fossil accumulations invariably represent a biased subset of original source communities. Efforts to quantify taphonomic biases are typically prevented by a lack of independent data on the ecological composition of prehistoric faunas. However, analysis of the continental Holocene record can provide a rare opportunity for independent calibration of fossil abundance patterns. We analyzed a comprehensive data set available for the Holocene avifauna of Sweden to investigate the relationship between species abundance in the recent fossil and zooarchaeological records and in prehistoric source communities, and to characterize the importance of different ecological factors in determining terrestrial vertebrate fossil abundances. The number of assemblages in which species occurred was compared with modern-day species abundance, annual residence, body mass, and ecological realm. Modern-day abundance is only one of several significant predictors of fossil abundance; the strongest predictor is body mass, and Holocene species abundance can be interpreted as a measure of species abundance in source communities for a given size class only. Our study represents one of the only direct attempts to quantify species abundance biases between fossil faunas and source communities, and has general applicability for a wide range of terrestrial vertebrate faunas.

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

Analysis of relative changes in plate position during ontogeny was made on a number of Cenozoic spatangoid echinoids: species of Breynia, Lovenia, Protenaster and Echinocardium. Contrary to the generally held view that adjacent ambulacral and interambulacral columns in echinoids always remain in a fixed relative position during ontogeny, many spatangoids show great fluidity in plate growth, with adjacent columns ‘sliding past’ one another during ontogeny. Furthermore, in many genera plates from one column may undergo strong lateral growth and bisect pairs of plates in adjacent rows. This phenomenon of relative plate movement, both meridional and equatorial, is herein termed ‘plate translocation’. It is considered to occur by localized resorption and redeposition in a narrow zone along adjacent plate boundaries. Plate translocation has been of considerable phylogenetic significance to spatangoids and, combined with an increase in differential allometries between plates, has been one of the most important factors in the evolution of the Spatangoida. Furthermore, the initiation of plate translocation in the apical system in certain Jurassic echinoids may have been the trigger for the migration of the periproct out of the apical system, and a major factor in the evolution of irregular echinoids. Heterochrony in plate growth has been critical in controlling the course of evolution in many lineages of spatangoid echinoids.

In this paper we reconstruct the phylogeny of a clade of pachypleurosaurs (Reptilia: Sauropterygia) occurring in Triassic-age deposits in the Monte San Giorgio region, Switzerland. We also present the phylogeny of this clade as a case study for two paradigms of phylogeny reconstruction: cladistics and phenetic/stratigraphic methods. While this dichotomy is not held rigidly by all workers, its advancement by cladists leads us to retain it initially for rhetorical purposes. We review the philosophical bases of species, species concepts, and speciation, as well as cladograms and phylogenies, before introducing the experimental system.

Data are presented from cladistic analyses, phenetic analyses, and stratigraphic information. Phylogeny of the clade is interpreted from both paradigms, and the interpretations are found to be inconsistent. Resolution of the phylogeny rests on the emphasis of one type of data over another. An interpretation of cladogenesis within the genus Neusticosaurus entails rejection of suggestive phenetic and stratigraphic data, whereas an anagenetic interpretation entails reversal of autapomorphies in ancestral taxa. Anagenesis is deemed to be the more probable interpretation, based on the strength of the stratigraphic and phenetic data relative to the character data. Implications of the test case results for phylogeny reconstruction in general are discussed, ending with a call for pluralism in approach.

Stick-like bryozoans from 1 to 5 cm tall, and of varying degrees of colony robustness, are common in deposits from Ordovician through Cenozoic in age; their occurrence has been used to infer strength of bottom currents. Three species which are modern analogues of this mode of growth occur abundantly at subtidal depths near Friday Harbor, Washington; we studied one of these forms, the calcitic cyclostome Heteropora pacifica. Branch diameter was used as an indication of colony robustness because it can be measured in both living and fossil material. Branch diameter decreases significantly with depth (in collections from approximately 9, 15 and 30 m depth) at each of 4 transect localities.

At each locality, currents 6 to 10 cm above the bottom were measured continuously in water depth of ≃ 15 m, for several hours at a time. Under these conditions, the maximum sustained current through the several hour tidal cycle was 70 to 90 cm/sec at one locality, and 20 cm/sec or less at each of the other 3 localities. Suspended matter was determined at heights of 0.2, 0.5 and 1.0 m above the bottom. The amount of suspended matter 0.2 m above the bottom was 40 to 45 mg/cm2/day at the locality with the high current (depth of 15 and 30 m), and usually 5 to 10 mg/cm2/day at the localities with the low current (depth 15 and 30 m). Despite these several fold differences in current strength and suspended matter between localities, for any given depth bryozoan branch diameter does not vary. Branch breaking strength (23 MN/m2) and elastic modulus (30 GN/m2) were measured using small (.16 mm) strain gauges affixed to dry branches of Heteropora. These values are typical of carbonate skeletons of corals. Although Heteropora branch diameter (and colony robustness) decreases significantly with depth, this trend probably is unrelated to current strength.

The history of diversity, origination and extinction of Cambro-Ordovician nautiloid cephalopods is explored to determine if differences in evolutionary rates between nautiloid orders are sufficient to document significantly high or low rates of evolutionary turnover (taxotely of Raup and Marshall 1980). The stratigraphic ranges of 425 nautiloid genera are analyzed for this purpose.

Evolutionary rates for five of the seven time intervals analyzed fall within frequency distributions of rates which are thought to be characteristic for a given time interval (horotelic distribution of Simpson 1944). Sufficient heterogeneity is present among extinction rates of Arenigian orders and origination rates of Caradocian orders to reject the null hypotheses of horotely in favor of taxotely. The orders Ellesmerocerida and Tarphycerida, each with a significantly high rate of extinction (P ≥ 0.99), and the Actinocerida, with a significantly low rate of extinction (P ≥ 0.99), were responsible for taxotely during the Arenigian. The Oncocerida and Discosorida, each with a significantly high rate of origination (P ≥ 0.99), were responsible for taxotely during the Caradocian. In each case, taxotely is attributable to the influence of North American endemics. This effect is believed to be more the result of real biological aspects of nautiloid evolution than an artifact of the fossil record.

Ammonites have disparate adult morphologies indicative of diverse ecological niches, but ammonite hatchlings are small (~1 mm diameter), which raises questions about the similarity of egg incubation and hatchling life mode in ammonites. Modern Nautilus is sometimes used as a model organism for understanding ammonites, but despite their outward similarities, the groups are only distantly related. Trends in ammonite diversity and extinction vulnerability in the fossil record contrast starkly with those of nautilids, and embryonic shells from Late Cretaceous ammonites are two orders of magnitude smaller than nautilid embryonic shells. To investigate possible environmental changes experienced by ammonite hatchlings, we used secondary ion mass spectrometry to analyze the oxygen and carbon isotope composition of the embryonic shells and early postembryonic whorls of five juveniles of Hoploscaphites comprimus obtained from a single concretion in the Fox Hills Formation of South Dakota. Co-occurring bivalves and diagenetic calcite were also analyzed to provide a benthic baseline for comparison. The oxygen isotope ratios of embryonic shells are more like those of benthic bivalves, suggesting that ammonite eggs were laid on the bottom. Ammonite shell immediately after hatching has more negative δ18O, suggesting movement to more shallow water that is potentially warmer and/or fresher. After approximately one whorl of postembryonic growth, the values of δ18O become more positive in three of the five individuals, suggesting that these animals transitioned to a more demersal mode of life. Two other individuals transition to even lower δ18O values that could suggest movement to nearshore brackish water. These data suggest that ammonites, like many modern coleoids, may have spawned at different times of the year. Because scaphites were one of the short-term Cretaceous–Paleogene extinction survivors, it is possible that this characteristic allowed them to develop a broader geographic range and, consequently, a greater resistance to extinction.

We compare refined data sets for Atlantic benthic foraminiferal oxygen isotope ratios and for North American mammalian diversity, faunal turnover, and body mass distributions. Each data set spans the late Paleocene through Pleistocene and has temporal resolution of 1.0 m.y.; the mammal data are restricted to western North America. We use the isotope data to compute five separate time series: oxygen isotope ratios at the midpoint of each 1.0-m.y. bin; changes in these ratios across bins; absolute values of these changes (= isotopic volatility); standard deviations of multiple isotope measurements within each bin; and standard deviations that have been detrended and corrected for serial correlation. For the mammals, we compute 12 different variables: standing diversity at the start of each bin; per-lineage origination and extinction rates; total turnover; net diversification; the absolute value of net diversification (= diversification volatility); change in proportional representation of major orders, as measured by a simple index and by a G-statistic; and the mean, standard deviation, skewness, and kurtosis of body mass. Simple and liberal statistical analyses fail to show any consistent relationship between any two isotope and mammalian time series, other than some unavoidable correlations between a few untransformed, highly autocorrelated time series like the raw isotope and mean body mass curves. Standard methods of detrending and differencing remove these correlations. Some of the major climate shifts indicated by oxygen isotope records do correspond to major ecological and evolutionary transitions in the mammalian biota, but the nature of these correspondences is unpredictable, and several other such transitions occur at times of relatively little global climate change. We conclude that given currently available climate records, we cannot show that the impact of climate change on the broad patterns of mammalian evolution involves linear forcings; instead, we see only the relatively unpredictable effects of a few major events. Over the scale of the whole Cenozoic, intrinsic, biotic factors like logistic diversity dynamics and within-lineage evolutionary trends seem to be far more important.

A nonparametric analysis of the extinction patterns of 10 major marine invertebrate groups at the five most profound mass extinction events leads to five observations: (1) At each event some taxonomic groups were affected much more than others. (2) There is little consistency among events in terms of which taxonomic groups were most or least affected; however, adaptive groupings do exhibit consistency: benthic, mobile organisms suffered significantly fewer extinctions than sessile suspension feeders, while the pelagic organisms apparently suffered the most. (3) There are no convincing patterns of interrelated extinctions among taxonomic groups. (4) No group exhibits a persistent tendency through time for a relative increase or decrease in their extinction rate at the events. (5) Some relationships are seen between the extinction patterns of three pairs of events; the Late Ordovician and Late Devonian events exhibit a significantly similar pattern (the same taxonomic groups suffered the most extinction in both cases) as do the Late Triassic and Late Cretaceous events. The Late Permian and Late Cretaceous events show a significantly inverse pattern (the most affected groups in the former were among the least affected in the latter). Upon examination, these observations, notably 1, 2, and 5, are consonant with current scenarios of the effects of catastrophic bolide impacts on marine fauna.

Our knowledge of the diversity, ecology, and phylogeny of Mesozoic birds has increased significantly during recent decades, yet our understanding of their flight competence remains poor. Wing loading (WL) and aspect ratio (AR) are two aerodynamically relevant parameters, as they relate to energy costs of aerial locomotion and flight maneuverability. They can be calculated in living birds (i.e., Neornithes) from body mass (BM), wingspan (B), and lift surface (S L). However, the estimates for extinct birds can be subject to biases from statistical issues, phylogeny, locomotor adaptations, and diagenetic compaction. Here we develop a sequential approach for generating reliable multivariate models that allow estimation of measurements necessary to determine WL and AR in the main clades of non-neornithine Mesozoic birds. The strength of our predictions is supported by the use of those variables that show similar scaling patterns in modern and stem taxa (i.e., non-neornithine birds) and the similarity of our predictions with measurements obtained from fossils preserving wing outlines. In addition, although our WL and AR values are based on estimates (BM, B, and S L) that have an associated error, there is no cumulative error in their calculation, and both parameters show low prediction errors. Therefore, we present the first taxonomically broad, error-calibrated estimation of these two important aerodynamic parameters in non-neornithine birds. Such estimates show that the WL and AR of the non-neornithine birds here analyzed fall within the range of variation of modern birds (i.e., Neornithes). Our results indicate that most modern flight modes (e.g., continuous flapping, flap and gliding, flap and bounding, thermal soaring) were possible for the wide range of non-neornithine avian taxa; we found no evidence for the presence of dynamic soaring among these early birds.

Despite a global fossil record, Metatheria are now largely restricted to Australasia and South America. Most metatherian paleodiversity studies to date are limited to particular subclades, time intervals, and/or regions, and few consider uneven sampling. Here, we present a comprehensive new data set on metatherian fossil occurrences (Barremian to end Pliocene). These data are analyzed using standard rarefaction and shareholder quorum subsampling (including a new protocol for handling Lagerstätte-like localities).

Global metatherian diversity was lowest during the Cretaceous, and increased sharply in the Paleocene, when the South American record begins. Global and South American diversity rose in the early Eocene then fell in the late Eocene, in contrast to the North American pattern. In the Oligocene, diversity declined in the Americas, but this was more than offset by Oligocene radiations in Australia. Diversity continued to decrease in Laurasia, with final representatives in North America (excluding the later entry of Didelphis virginiana) and Europe in the early Miocene, and Asia in the middle Miocene. Global metatherian diversity appears to have peaked in the early Miocene, especially in Australia. Following a trough in the late Miocene, the Pliocene saw another increase in global diversity. By this time, metatherian biogeographic distribution had essentially contracted to that of today.

Comparison of the raw and sampling-corrected diversity estimates, coupled with evaluation of “coverage” and number of prolific sites, demonstrates that the metatherian fossil record is spatially and temporally extremely patchy. Therefore, assessments of macroevolutionary patterns based on the raw fossil record (as in most previous studies) are inadvisable.

A central premise of bioclimatic envelope modeling is the assumption of niche conservatism. Whereas such assumptions are testable in modern populations, it is unclear whether niche conservatism holds over deeper time spans and over very large geographic ranges. Hyaenids occupied a diversity of ecological niches over time and space, and until the end-Pleistocene they occurred in Europe and most of Asia, with Asian populations of Crocuta suggested as being genetically distinct from their closest living relatives. Further, little is known regarding whether and how the dietary ecology of extinct populations of Crocuta differed from those of their extant African counterparts. Here, we use a multiproxy approach to assess an assumption of conserved dietary ecology in late Pleistocene extant spotted hyenas via finite element analysis, dental microwear texture analysis, and a novel dental macrowear method (i.e., whether teeth are minimally, moderately, or extremely worn, as defined by degree of dentin exposure) proposed here. Results from finite element simulations of the masticatory apparatus of Chinese and African Crocuta demonstrate lower skull stiffness and higher stress in the orbital region of the former when biting with carnassial teeth, suggesting that Chinese Crocuta could not process prey with the same degree of efficiency as extant Crocuta crocuta. Dental microwear texture data further support this interpretation, as Chinese Crocuta have intermediate and indistinguishable complexity values (indicative of hard-object feeding) between the extant African lion (Panthera leo) and extant hyenas (C. crocuta, Hyaena hyaena, and Parahyaena brunnea), being most similar to the omnivorous P. brunnea. The use of dental macrowear to infer dietary behavior may also be possible in extinct taxa, as evinced by dietary correlations between extant African feliforms and dental macrowear assignments. Collectively, this multiproxy analysis suggests that Chinese Crocuta may have exhibited dietary behavior distinct from that of living C. crocuta, and assumptions of niche conservatism may mask significant dietary variation in species broadly distributed in time and space.

The Mesozoic marine revolution theory postulates a causal connection between the Cretaceous radiations of durophagous predators and the decline of suspension-feeding echinoderms in shallow-water habitats. In order to test the temporal distribution of dense ophiuroid populations for such a decline, I present a method of calculating the expected distribution of populations or communities in different geologic timespans. This statistical null hypothesis may then be compared with data from the fossil record to draw paleoecological inferences. The model takes into account the relative lengths of time blocks, the decay of sedimentary rock, and changes in shallow sea area through time. Although mass extinctions did not cause the immediate disappearance of shallow-water “brittlestar beds,” brittlestar beds show a significant decline in the Early Cretaceous. Results of several studies suggest that predators were at least partially responsible for reducing the distribution of dense ophiuroid populations.

Computer-generated searches through hypothetical fitness landscapes for progressively more fit variants were used to assess the effects of developmental barriers (mimicked by barring specific types of morphological transformations) and abruptly shifting environmental conditions (mimicked by sudden shifts in how fitness was defined) on the number and accessibility of optimal phenotypes. Relative fitness was defined in terms of maximizing light interception, mechanical stability, or reproductive success, or minimizing surface area, or optimizing the performance of various combinations of these tasks. Developmentally obstructed and unobstructed walks located, on average, equivalent numbers of phenotypic optima. The number of optima identified by both kinds of walks increased in proportion to the number of simultaneously performed tasks used to measure fitness. Walks passing from more complex to less complex fitness landscapes located more optima than walks passing through unchanging, stable landscapes. The model thus suggests that there are no a priori reasons to assume that (1) the morphological options available to adaptive evolution become more restrictive as biological complexity increases, (2) “developmental barriers” necessarily restrain a lineage from evolving well-adapted morphologies, and (3) generalist organisms are less successful than specialists. Also, because the number and accessibility of fitness peaks were proportional to the complexity of fitness landscapes, the model predicts that the probability of cladogenesis will increase as landscape complexity increases, while anagenesis will be encouraged when fitness is defined in terms of performing one or a few tasks simultaneously.