The pattern of differential species longevities among five Osagean–Meramecian crinoid clades is analyzed for its evolutionary significance. Differences in mean species longevity between clades may have resulted from species sorting based on eurytopy (niche breadth). In order to test the relationship between longevity and eurytopy it was first necessary to recognize generalists (eurytopes) vs. specialists (stenotopes) objectively. Three different approaches were used: (1) the “Eurytopy Index” (EI), which is a measure of mean number of facies per species; (2) analysis of crinoid functional morphology; and (3) use of canonical discriminant analysis to analyze species distributions between facies in order to separate generalists from specialists. Mean species longevity for each clade was evaluated by four different approaches: (1) rarefaction was used to control for differences in sample size, including both species richness and number of occurrences, between clades; (2) potential facies control of species longevity was evaluated by a bootstrap that compared the observed data to a null model where species longevity was limited only by the actual occurrences of each species known facies through time; (3) uniformity of clade species richness through time was evaluated by the “Index of Uniformity for Species Richness” based on the standard deviation of clade species richness across the time intervals; and (4) potential species range truncations were evaluated by a biostratigraphic gap analysis based on the binomial distribution.

The general order of increasing longevity and eurytopy is (from least to most): flexibles, advanced cladids, camerates, disparids, and primitive cladids. In general the pinnulate crinoids (advanced cladids and camerates) were specialists with lower mean species longevity, and the non-pinnulate crinoids (disparids and primitive cladids) were generalists with higher mean species longevity. Pinnulate crinoids were specialized for feeding in high-energy currents and, thus, were limited in their facies distribution and presumably more extinction-prone. The non-pinnulates could feed in both low- and high-energy currents and, thus, were less limited in their facies distribution and presumably less extinction-prone. The flexibles were the exception in that they were non-pinnulate but had the lowest mean species longevity, apparently because they were specialized for deeper-water clastic environments.

On average, generalist clades have mean species longevities that at a minimum are up to 45% (≈1.0 ± 0.7 m.y.) longer than specialist clades. However, greater mean species longevity did not necessarily confer long-term advantages to a clade. The specialist advanced cladids became the dominant crinoid clade of the late Paleozoic and gave rise to the articulate crinoids of the post-Paleozoic. This may have resulted from the more rapid species turnover of stenotopes creating adaptive evolutionary novelties for their clade. Alternatively, it could simply be the result of stochastic processes.

The finer subdivision of niche space by specialists has led previous workers to predict that specialist clades should have higher species richness than generalist clades. The present study supports this prediction.

Developmental constraints presumably operate by influencing patterns of variability: when development causes some features to vary more than others and when the level of variability is correlated with evolutionary change, then development can be said to constrain evolution. This idea was tested by examining the relationship between tooth variation and three other factors: developmental processes, tooth function, and evolutionary change. Data came from two lineages of viverravid carnivorans (Viverravidae, Carnivora) from the Paleogene of North America. Variability in cusp position was significantly correlated with position in the developmental cascade, with the amount of intercusp growth (when growth is relatively greater in some cusps than others), and with amount of evolutionary change. This indicates that tooth development exerts a local constraint on phenotypic variability and on the evolutionary response to functional selection, but comparative data suggest that the developmental constraint itself may evolve. Intense directional or stabilizing selection may modify the developmental cascade so that the constraint is either removed or modified to permit new evolutionary patterns. Thus development does not constrain evolution in an absolute sense, but rather introduces modifiable patterns of covariance among crown features. Both development and function seem to play important, intertwined roles in coordinating evolutionary change in mammalian molars.

This study quantifies the fine structure of time-averaging by using large samples of dated shells collected from within individual strata. Time-averaging results in both good and bad news for interpreting bioclastic deposits.

Nine samples of shells were collected from four Holocene cheniers on the Colorado Delta (Gulf of California) and 165 shells of the bivalve Chione fluctifraga were dated using 14C-calibrated amino acid racemization (D-alloisoleucine/L-isoleucine). The age range of shells within samples averages 661 years and, in seven out of nine samples, exceeds 500 years. The sample standard deviation ranges from 73 to 294 years and averages 203 years, far exceeding the dating errors (≪100 years) and potential variation in the life span of Chione (<10 years). Time-averaging is homogeneous among strata within cheniers but varies significantly among cheniers. Age-distributions of dated shells indicate that at 50-year resolution, the samples provide a continuous and uniform record for the entire interval. The actual sample completeness (63.6%) is very close to that predicted by simulations of sampling a 100% complete, uniform record (67.3%).

The bad news is that, no matter how carefully collected, data from shell beds may not be suitable for studying processes on timescales shorter than 102 to 103 years; explanations for faunal change that invoke reasoning or models derived from a strictly ecological point of view may rarely be justifiable. Also, notable differences in temporal resolution between the shell beds of seemingly identical origin imply that paleontological patterns (e.g., species diversity) may be affected by cryptic variation in time-averaging. The comparison of our data with time-averaging estimates obtained from other cheniers at coarser sampling resolutions indicates that pooling of samples (analytical time-averaging) can significantly reduce the temporal resolution of paleontological data.

The good news is that shell beds can record the optimal type of time-averaging: where paleobiological data are a time-weighted average of the faunal composition from the spectrum of environments that existed during the entire interval of time. Samples from single strata provide a long-term record that is representative of the predominating environments. Within the range of 14C dating, shell beds can provide a complete, high-resolution record, and thus may offer exceptional insights into the environmental and climatic changes of the last 40 thousand years.

The Ordovician saw the transformation of marine benthic communities from the trilobite-based Cambrian Fauna to the brachiopod-dominated Paleozoic Fauna. An evaluation of the changing importance of trilobites during the Ordovician can be made from accurate assessments of taxonomic richness in various habitats. Here we present a new compilation of trilobite alpha diversity based on field collections and survey of the literature. The data indicate that trilobite species richness within nearshore, shallow subtidal, carbonate buildup and deep subtidal shelf environments was essentially constant between the Late Cambrian and the Late Ordovician. The alpha diversity patterns do not support the notion that trilobites became displaced from inner shelf environments during the Ordovician. Rather, the data are consistent with a decline in relative importance of the group through dilution as newly radiating invertebrate groups entered Ordovician paleocommunities. They also imply that direct interactions between elements of the Cambrian and Paleozoic faunas were not involved in the Ordovician reorganization of paleocommunities. Like many other major faunal transitions during the Phanerozoic, the Ordovician radiations appear to have been essentially non-competitive in nature.

An understanding of several macroevolutionary trends has been greatly advanced in recent years by a focus on disparity (morphological variety) rather than taxic diversity. A seminal issue has been the nature of the Cambrian Radiation, and the question of whether problematical Cambrian fossils embody a range of anatomical design far exceeding that observed thereafter. Arthropods have hitherto furnished the only case study, revealing comparable levels of Cambrian and Recent disparity. The generality of this observation needs to be tested in other groups, and the priapulid worms provide a well-documented example. Cladistic analysis of morphological characters for priapulids reveals a paraphyletic series of Cambrian taxa below a crown-group of post-Cambrian genera. However, one extant family (the Tubiluchidae) may be more closely related to Cambrian forms or resolve basally. Character-based morphospace analysis demonstrates greater disparity amongst Recent taxa than amongst their Cambrian counterparts. There is relatively little overlap between the regions of morphospace occupied by Cambrian and Recent genera (contrasting with the situation in arthropods). The Tubiluchidae are morphologically intermediate between Cambrian and other Recent families using several measures of phenetic proximity, and they inhabit environments more comparable with their Cambrian cousins. This work confirms the extensive morphological diversification of major clades by the Cambrian but lends no support to models of a post-Cambrian “decimation” of disparity.

Phenotypic variation within species provides the raw material acted upon by natural selection and other evolutionary mechanisms. As such, the range and variation of morphology within a species can play an important role in determining the tempo of evolution. The range and variance of aspects of cranidial morphology for nine lower Paleozoic trilobites were measured to identify microevolutionary correlates of macroevolutionary patterns. Comparisons were made among sets of homologous landmarks or upon partial warp vector matrices containing similar proportions of variance. Rarefaction and bootstrap analyses helped estimate the effects of sampling. Levels of variance and range of morphology differed considerably within and among time periods. There is no significant temporal decline in the variance or range of morphology, suggesting that developmental or genomic constraints may not have been the primary factors controlling the tempo of trilobite macroevolution. The spatial distribution of cranidial variance differed considerably among taxa, suggesting that a complex set of developmental processes governed the morphogenesis of cranidia within trilobites.

This paper concerns paleontological phylogenies that have a “budding” configuration, wherein “ancestral” species persist through branching events to coexist with their “descendants.” Two principal tests are proposed for detecting patterns within such trees. The first test, called the “ancestor-descendant extinction test,” compares the number of cases in which, after a split, the ancestral species became extinct before its descendant with the number of cases in which the descendant became extinct before its ancestor. The second test, called the “ancestor-descendant speciation test,” compares the number of cases in which, after a split, the ancestral species gave rise to a further species with the number of cases in which the descendant species gave rise to a further species. The null hypothesis in each case is that the frequencies are equal, as predicted by a random Markovian branching model of evolution.

Five stratophenetic species-level phylogenies of three taxonomic groups, planktonic foraminifera, nannofossils, and graptoloids, are examined using these tests, including one (Paleogene planktonic foraminifera) that is presented for the first time. In all cases, the phylogenetic trees are found to be strongly nonrandom. The general pattern, although by no means expressed perfectly in every case, corresponds to a Simpsonian “step-series,” in which ancestor taxa are simultaneously more likely to become extinct and less likely to speciate than their coexisting descendants. It is shown that this pattern cannot simply be the result of simple age-dependent factors such as an increasing extinction risk in older taxa. Instead, the very fact that a species has given rise to another appears to increase its future extinction risk and decrease its likelihood of further speciation.

Many possible biases may affect the shape of paleontological phylogenies, which are as yet poorly understood and unquantified. One potentially important effect follows from the taxonomic subdivision of gradual chronoclines into artificial morphospecies, such as might conceivably induce a step-series pattern in the phylogeny. Even if this is the partial or entire reason for the observed patterns, it would appear to imply directional evolution in phyletic gradualism. Other possible artifacts are discussed, but they are regarded as probably too weak to produce the observed patterns.

If the pattern is not artificial, the fact that three of the best known fossil groups exhibit substantial asymmetries in speciation and extinction argues against the currently popular “nonprogressive” view of evolution. Instead, the evolutionary step-series pattern is consistent with the classical Darwinian concept of the general competitive superiority of newly evolved species over their ancestors and supports the idea of evolutionary progress.

Estimates of phylogenetic relationships among fossil taxa implicitly provide hypotheses about the quality of the fossil record. Phylogenetic inferences also provide hypotheses about character evolution. The likelihood of any hypothesis that makes predictions about two data sets is simply the likelihood of the hypothesis given the first data set times the likelihood of the same hypothesis given the second data set. In this case, data set 1 represents stratigraphy and data set 2 represents morphology. Statistical methods exist for determining the likelihood of hypothesized levels of sampling. The likelihood of a hypothesized amount of character change yielding a particular most-parsimonious solution (i.e, L[hypothesized length | parsimony length] can be evaluated with simulations. A reanalysis of hyaenid phylogeny based on published character and stratigraphic data is presented here, using the maximum likelihood method. Two trees are found, depending on assumptions about ambiguous species, which are 11 and 10 steps longer than the most parsimonious tree (61 or 60 vs. 50 steps). However, the trees invoke far less stratigraphic debt (9 or 12 units vs. 47 units as measured in Mammal Zones). An important feature of the results is that the most likely tree length given hyaenid character data is estimated to be 56 to 62 steps (depending on the model of character evolution) rather than 50 steps. The likelihood tree suggests stronger trends toward bone-crushing specializations than does the parsimony tree and further suggests that high levels of homoplasy caused parsimony to underestimate the true extent of those trends. Simulations based on the character data and fossil record of hyaenids suggest that the maximum likelihood method is better able to estimate correct trees than is parsimony and somewhat better able to do so than previously proposed phylogenetic methods incorporating stratigraphy.

A new hypothesis concerning the evolutionary origin of pentameral symmetry in echinoderms is presented. The BA-A-BA pattern of Lovén's law in echinoids and the 2-1-2 symmetry of the edrioasteroid Stromatocystites appear to be morphogenetically related, and this pattern appears to be locked into development. This pattern may have originated from a unirayed 0-1-0, —A—, ancestor. I propose that a duplication of the uniray occurred and resulted in the addition of a pair of rays that followed Bateson's rules of symmetry to form a three-rayed 0-1-2, —A-BA, construction. This change occurred on the side that corresponds to the left side of the organism. This event made the individual asymmetric with respect to its anterior-posterior axis. Therefore I propose that morphogenetic regulation of bilaterality of the organism then led to homeotic expression of a mirror-image pair of rays on the opposite side. These two morphogenetic steps achieved the 2-1-2, BA-A-BA pattern. “Appendage status” of rays is assumed necessary to invoke the Batesonian mirror-image duplications of the model.

Three robust morphological characters emerge from the “rays-as-appendages” model: (1) 2-1-2, BAo-A-BA, organization; (2) “locked-in” pentamerism; and (3) a 2-3 pattern of right and left rays. Results based on ray homology research are presented for echinoids, asteroids, ophiuroids, edrioasteroids, ophiocistioids, and holothurians. I speculate that helicoplacoids may have 0-1-2 triradiate construction, and that solutes may have 0-1-0 uniray construction. The model has limits; it does not explain 1-1, or 2-1, or 2-2 or 1-1-1 organization of ambulacra.

Synrhabdosomes, structures made of mostly radially arrayed graptolite rhabdosomes attached by distal regions of their virgulae, have been considered as biological entities by some in the past, but as taphonomic structures by others more recently. Our survey of at least 200 synrhabdosomes, more than 90 of which derive from one locality in northwestern Spain, shows that all synrhabdosomes are most probably entirely monospecific assemblages composed of a more or less finite number of rhabdosomes, all of which are consistently straight or only weakly curved, and all of which bear simple, or relatively simple, thecae. We conclude, therefore, that synrhabdosomes are biological entities, and we suggest that they formed infrequently as, perhaps, temporary structures to increase food-gathering efficiency during times of lower plankton supply in local, relatively restricted water masses.

Analyses of non-avian dinosaur locomotion have been hampered by the lack of an appropriate locomotor analog among extant taxa. Birds, though members of the clade Dinosauria, have undergone significant modifications in hindlimb osteology and musculature. These changes have resulted in a uniquely developed system of limb kinematics (involving a more horizontal femoral posture and knee-dominated limb motion), which precludes the direct use of extant birds as models for non-avian dinosaur locomotion. Analyses of locomotor data from extant birds and mammals suggest a causal link between general hindlimb kinematics, bone strains, and limb bone morphology among these taxa. A model is proposed that relates the amount of torsional loading in femora to bone orientation, such that torsion is maximal in horizontal femora and minimal in vertical femora. Since bone safety factors are lower for torsional shear strains than for longitudinal axial strains, an increase in torsion can potentially affect bone morphology dramatically over evolutionary time. Interpreting the nearly identical limb bone dimensions and limb element proportions of non-avian dinosaurs and mammals in the light of this relationship supports the prediction of similar vertical femoral postures and hip-driven limb kinematics in these two groups.

This information can be used to interpret patterns of locomotor evolution within Dinosauria. The evolution of quadrupedalism with large body size and the acquisition of cursorial or graviportal limb morphologies occurred repeatedly but did not affect the underlying uniformity of dinosaur locomotor morphology. Only derived coelurosaurian theropods (paravians) developed significant modifications of the basic dinosaurian patterns of limb use. Changes in theropod hindlimb kinematics and posture apparently began shortly prior to the origin of flight, but did not acquire a characteristically modern avian aspect until after the later acquisition of derived flight characteristics.

A simple mathematical model to examine the relationships between environmental instability and long-term macroevolutionary trends is presented. The model investigates the evolutionary changes that occur in certain population characters in an environment with physical disturbance. These quantitative genetic characters are related to intrinsic growth rates and mean carrying capacity. The model assumes that individual fitness is determined by these characters. I examine the likelihood of extinction under different degrees of environmental instability and for rapid change of environmental instability. The model suggests that characters that promote a high intrinsic growth rate and a low carrying capacity tend to evolve in the most unstable environments. This suggests that small body size, high fecundity, and simple forms evolve in unstable environments. The extinction probability of a population is the lowest for taxa possessing K-selected characters in the most stable environment. However, the extinction probability of a species (metapopulation) becomes lowest for r-selected species living in the most unstable environment and for the K-selected species living in the most stable environment, and it becomes the highest for taxa living in a moderately unstable environment. Increasing environmental instability changes the extinction probabilities of different taxa in different ways, due to differences in phenotypes and environments. The effect of environmental change is most serious for the K-selected taxa in the most stable environment. This also suggests that a continuously stable environment increases the extinction probability of taxa when environmental change occurs. Although catastrophic changes in environments are not presumed, these results are consistent with the existence of two “macroevolutionary regimes” in which a taxon's extinction rate and its characters differ for mass extinction and normal extinction. Mass extinction can occur as a result of long-term adaptation to a stable environment following a minor change of environment without catastrophes.

To a first approximation, the logarithmic spiral is a close fit to the biconvex lateral outline of three terebratellid genera from Australasia. Investigation of spiral parameters confirms the value of the biconvex morphospace of McGhee (1980), but not the method for estimating the spiral itself. Deviations from the estimated spiral are episodic and consistent with Rudwick's (1959) observations of allometry in brachiopod growth. Morphospace location is postulated to be indicative of a species' substrate and lifestyle. The logarithmic spiral offers much opportunity for gaining better understanding of external shape and its cause in articulate brachiopods.

A fractal geometry of clast size within the test wall in the Antarctic agglutinated foraminifera Hormosina mortenseni Cushman, 1910 and Cyclammina cancellata Brady, 1879 has been identified with the use of Scanning Electron Microscopic techniques. External surface and internal clast distributions in H. mortenseni display a self-similar distribution. C. cancellata has an internal self-similar grain arrangement, whereas the exterior surface shows an alternative grain distribution. Power law relationships between particle density and grain diameter enable values of fractal dimension (D) to be calculated; these “D-values” represent the absolute gradient of the power law relationship. The dimensions acquired from the foraminiferal study correspond well with those previously obtained from natural fractal geological structures and ideal fractals. The self-similar grain arrangement within walls of the foraminifera exists over three orders of magnitude, after which alternative methods of test wall construction are evident. This suggests that a limit exists where grain selection terminates. A self-similar grain distribution limits the amount of biologically produced adhesive material required by the foraminifera for constructing their tests.

Morphological evolution in the Late Cretaceous (Maastrichtian) Contusotruncana lineage of planktonic foraminifera was studied at DSDP Sites 525 (South Atlantic) and 384 (North Atlantic). A multivariable approach was used to separate aspects of form controlled by geographical variation (size, spiral roundness of the test, percentage of kummerform specimens) from those due to changes that occurred simultaneously in geographically distant populations of the lineage (shell conicity, number of chambers in the last whorl).

A gradual increase in mean shell conicity was observed over the last 3 million years of the Cretaceous. It arose from the combination of a rapid development of highly conical shells after 68.5 Ma and a long-term trend of progressive disappearance of the ancestral morphotype. Therefore, despite the gradual change in “mean form,” the morphological evolution in the Contusotruncana lineage differs from the classical image of phyletic gradualism. The gradual increase in mean shell conicity in the lineage was accompanied by a remarkable decrease in its absolute abundance (shell accumulation rate), suggesting that the changes in shell morphology might not have been neutral with respect to natural selection. Apparently, gradual change in “mean form” of fossil lineages does not require an equally gradual development of morphological novelties. It may be caused by natural selection operating on a constant range of variation in populations living in environments without geographical barriers.

The taxonomic diversity of crocodilians (Crocodylia) through the last 100 million years shows a general decline in the number of genera and species to the present day. But this masks a more complex pattern. This is investigated here using a comprehensive database of fossil crocodilians that provides the opportunity to examine spatial and temporal trends, the influence of sampling, and the role of climate in regulating biodiversity.

Crown-group crocodilians, comprising the extant families Alligatoridae, Crocodylidae, and Gavialidae, show the following trend: an initial exponential diversification through the Late Cretaceous and Paleocene that is restricted to the Northern Hemisphere until after the K/T boundary; relatively constant diversity from the Paleocene into the middle Eocene that may be an artifact of sampling, which might mask an actual decline in numbers; low diversity during the late Eocene and Oligocene; a second exponential diversification during the Miocene and leveling off in the late Miocene and Pliocene; and a precipitous drop in the Pleistocene and Recent. The coincidence of drops in diversity with global cooling is suggestive of a causal link—during the initial glaciation of Antarctica in the Eocene and Oligocene and the Northern Hemisphere glaciation at the end of the Pliocene. However, matters are complicated in the Northern Hemisphere by the climatic effects of regional uplift.

Although the global trend of diversification is unperturbed at the K/T boundary, this is largely due to the exceptionally high rate of origination in the early Paleocene. Nonetheless, the survival of such a demonstrably climate-sensitive group strongly suggests that a climatic explanation for the K/T mass extinctions, especially the demise of the dinosaurs, must be reconsidered.

A hierarchical birth-death process is often used as a model for the diversification of species and higher taxa. We evaluate the validity of this null hypothesis by introducing a new measure: the distribution of species within higher taxa. The proportion of monotypic taxa and the number of species in the largest taxon generated by 1000 simulations are compared with extant mammalian data. The initial model is amended to include two sampling patterns, polyphyletic origins as well as ecological and genetic constraints on higher taxon origination rates. Simulated results are extremely variable. For genera, they are found to predict the true distribution of species quite well. For families, however, the null hypothesis is rejected in all its forms. Simulated distributions have both too many monotypic taxa and too large a dominant taxon. The latter finding stands in contrast to previous claims. Interestingly, although polyphyly and constraints slightly improve the fit of the simulations to the data, sampling does not. Finally, Smith and Patterson's dismissal of monotypic taxa is reviewed in light of this model. We argue that while some apparently monotypic taxa are sampling artifacts, this observation has no bearing on the true proportion of monotypic taxa.

Antarctic Hoploparia exhibit morphologic changes upsection in a stratigraphic record considered to be long (approximately 15 m.y.) and free of major hiatuses. Five characters exhibit change upsection, and the overall morphologies of the geologically oldest and youngest lobsters are different. The observed patterns could be the result of either phyletic evolution or gradual invasion of one or more species into the range of the original species. The most parsimonious interpretation of the data argues against the invasion hypothesis but supports the phyletic evolution hypothesis.

The Scleractinia, which are one of the most important builders of modern reefs, have been considered to have first appeared in the Middle Triassic. Recently, Paleozoic scleractiniamorphs have been reported from both the Ordovician and the Permian, suggesting that the scleractinian-like body plan was already established in the Paleozoic. Those Paleozoic scleractiniamorphs are considered either unsuccessful skeletonized offshoots (extinct experiments) or Paleozoic progenitors of the post-Paleozoic Scleractinia. Permian scleractiniamorphs are characterized by “ancestral” features and have no specific morphologies that deny scleractinian affinities. Molecular phylogenetics also indicate that extant scleractinians are monophyletic and originated long before their Triassic appearance. A Paleozoic origin for the Scleractinia is supported by morphological and molecular phylogenetic data. On the other hand, there is no positive evidence to show that different groups of scleractinians had separate soft-bodied precursors.

The Paleozoic scleractinians evolved within the framework of their basic body plan, and a direct derivation of the Scleractinia from the Rugosa is not probable. The Anthozoa are characterized by a bilaterally symmetrical body plan, which is traditionally considered to have been derived from other radially symmetrical Cnidaria. The problem of the origin of scleractinian body plan may provide a key for deciphering the early anthozoan radiation within the Bilateria. Other examples of Paleozoic Scleractinia and scleractiniamorphs will be found, probably in shallow-water reefal facies or deeper-water communities, bridging the stratigraphic gaps in occurrence and elucidating the origin of the Scleractinia and their body plan.

A detailed morphometric approach based on size and on outline analyses has been used on an exceptionally well-preserved assemblage of silicified trilobite exuvia, recovered from a Late Devonian limestone from southeastern Morocco. The material comprises a series of late larval to postlarval growth stages belonging to a single phacopine species, Trimerocephalus lelievrei Crônier and Feist, 1997.

Plurimodality of size distribution has allowed us to discriminate postlarval instars. Distinct dimensional classes of isolated parts are obtained using the intertooth distances on the posterior pygidial margin and the internotch distances in the cephalic vincular furrow, which are functionally linked during trilobite enrollment. Morphometric analysis of development permitted demonstration of progressive shape change in agreement with ontogenetic ordination and a comparison of the timing of size and shape changes. The main shape changes appear to occur early in development, and once the “adult” morphology is obtained, size increases significantly. The growth rate during ontogeny is estimated by analogy with extant deep-sea crustaceans. Exponential size increase resulting from constant duration of intermolt periods may be regarded as a life history strategy to compete in a nutrient-impoverished offshore environment. The particular phacopine mode of molting, which involves the opening of the neck joint after ankylosis of the facial sutures, occurred in Trimerocephalus lelievrei between the first two postlarval instars, later than in its ancestor. Trimerocephalus lelievrei occupies an intermediate position within the phacopine paedomorphocline as indicated by the delayed onset of ankylosis.