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This page lists all time most cited articles for this title. Please use the publication date filters on the left if you would like to restrict this list to recently published content, for example to articles published in the last three years. The number of times each article was cited is displayed to the right of its title and can be clicked to access a list of all titles this article has been cited by.
- Cited by 125
The Proterozoic Record of Eukaryotes
- Phoebe A. Cohen, Francis A. Macdonald
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- Published online by Cambridge University Press:
- 10 September 2015, pp. 610-632
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Proterozoic strata host evidence of global “Snowball Earth” glaciations, large perturbations to the carbon cycle, proposed changes in the redox state of oceans, the diversification of microscopic eukaryotes, and the rise of metazoans. Over the past half century, the number of fossils described from Proterozoic rocks has increased exponentially. These discoveries have occurred alongside an increased understanding of the Proterozoic Earth system and the geological context of fossil occurrences, including improved age constraints. However, the evaluation of relationships between Proterozoic environmental change and fossil diversity has been hampered by several factors, particularly lithological and taphonomic biases. Here we compile and analyze the current record of eukaryotic fossils in Proterozoic strata to assess the effect of biases and better constrain diversity through time. Our results show that mean within assemblage diversity increases through the Proterozoic Eon due to an increase in high diversity assemblages, and that this trend is robust to various external factors including lithology and paleogeographic location. In addition, assemblage composition changes dramatically through time. Most notably, robust recalcitrant taxa appear in the early Neoproterozoic Era, only to disappear by the beginning of the Ediacaran Period. Within assemblage diversity is significantly lower in the Cryogenian Period than in the preceding and following intervals, but the short duration of the nonglacial interlude and unusual depositional conditions may present additional biases. In general, large scale patterns of diversity are robust while smaller scale patterns are difficult to discern through the lens of lithological, taphonomic, and geographic variability.
- Cited by 123
Evolution of hypsodonty in equids: testing a hypothesis of adaptation
- Caroline A. E. Strömberg
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- 08 April 2016, pp. 236-258
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The independent acquisition of high-crowned cheek teeth (hypsodonty) in several ungulate lineages (e.g., camels, equids, rhinoceroses) in the early to middle Miocene of North America has classically been used as an indication that savanna vegetation spread during this time. Implicit in this interpretation is the untested assumption that hypsodonty was an evolutionary response to feeding in open habitats, either due to a change in food source (from browse to graze) or to increased incorporation of airborne grit in the diet. I examined the adaptive explanation for hypsodonty in equids using criteria pertaining to process and pattern of adaptations set up in the comparative-methods literature. Specifically, I tested whether hypsodonty appeared coincident with or just after the spread of open, grass-dominated habitats in the Great Plains of North America.
Phytolith (plant opal) analysis of 99 phytolith assemblages extracted from sediment samples from Montana/Idaho, Nebraska/Wyoming, and Colorado were used to establish the first continuous record of middle Eocene-late Miocene vegetation change in the northern to Central Great Plains. This record was compared with the fossil record of equids from the same area in a phylogenetic framework.
The study showed that habitats dominated by C3 grasses were established in the Central Great Plains by early late Arikareean (≥21.9 Ma), at least 4 Myr prior to the emergence of hypsodont equids (Equinae). Nevertheless, the adaptive hypothesis for hypsodonty in equids could not be rejected, because the earliest savanna-woodlands roughly co-occurred with members of the grade constituting the closest outgroups to Equinae (“Parahippus”) showing mesodont dentition. Explanations for the slow evolution of full hypsodonty may include weak and changing selection pressures and/or phylogenetic inertia. These results suggest that care should be taken when using functional morphology alone to reconstruct habitat change.
- Cited by 123
The Theory of Evolution by Natural Selection: A Hierarchical Expansion
- Anthony J. Arnold, Kurt Fristrup
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- 14 July 2015, pp. 113-129
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Alleles, individuals, and species are all examples of entities possessing variation in the properties that underlie natural selection: branching (reproduction), persistence (survivorship), and heritability of characters. This suggests that the logic embodied in the theory of natural selection can be abstracted from its usual application to the level of individuals to encompass selection operating among any biological entities for which these essential properties can be meaningfully defined. This approach leads to a unified perspective of adaptation, selection, and fitness at all levels. Expanded versions of the Price covariance selection equations provide a convenient and useful conceptual vehicle for this discussion. The advantages of a hierarchical approach are twofold: it permits exploration of concepts and ideas across levels by analogy, and it focuses attention upon the mechanisms that account for different evolutionary dynamics at each level rather than obscuring these biologically unique properties with argument by extension from a single “special” level.
We point out that the choice of a single measure of evolutionary change restricts the context in which “other level” processes will be perceived. We illustrate the limited forms in which higher and lower level selection can be recognized from the unique perspective provided by any given level through extensions of Price's formula.
An exploration of the implications of such an approach leads us to the assertion that the development of a unified theory of evolution demands the recognition and incorporation of hierarchical structure as a conceptual foundation.
- Cited by 123
Microstratigraphic sampling and the limits of paleontologic resolution
- David E. Schindelz
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- 08 February 2016, pp. 408-426
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Rates of modern sedimentation in various depositional settings have been compiled and adjusted for compaction for the purpose of estimating the spans of time represented in thin stratigraphic samples. Owing to discontinuous or low rates of sedimentation, it is either impossible or impractical to recover a continuous series of discrete life assemblages of fossil populations by collecting microstratigraphic samples from continuously fossiliferous intervals. Assemblages that are less time-averaged may be recovered from discontinuous records formed in depositional environments with higher rates of intermittent sedimentation, but these are isolated in time and cannot be used to observe the dynamics of local faunal history. Patterns of fossil distribution may appear similar to patterns of living populations explained by neontologic processes, but the longer time scale of even the best fossil sequences suggests the action of uniquely paleontologic processes that may be driven by environmental changes.
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Changes in theoretical ecospace utilization in marine fossil assemblages between the mid-Paleozoic and late Cenozoic
- Andrew M. Bush, Richard K. Bambach, Gwen M. Daley
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- 08 April 2016, pp. 76-97
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We present a new three-dimensional theoretical ecospace for the ecological classification of marine animals based on vertical tiering, motility level, and feeding mechanism. In this context, analyses of a database of level-bottom fossil assemblages with abundance counts demonstrate fundamental changes in marine animal ecosystems between the mid-Paleozoic (461–359 Ma) and late Cenozoic (23–0.01 Ma). The average local relative abundance of infaunal burrowers, facultatively motile animals, and predators increased, whereas surface dwellers and completely non-motile animals decreased in abundance. Considering tiering, motility, and feeding together, more modes of life had high to moderate average relative abundance in the Cenozoic than in the Paleozoic. These results are robust to the biasing effects of aragonite dissolution in Paleozoic sediments and to heterogeneities in the latitudinal and environmental distributions of collections. Theoretical ecospace provides a unified system for future analyses of the utilization of ecologic opportunities by marine metazoa.
- Cited by 122
Stasis, biological disturbance, and community structure of a Holocene coral reef
- Richard B. Aronson, William F. Precht
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- 08 February 2016, pp. 326-346
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Disturbances have drastically altered Caribbean coral reefs over the past two decades. Acropora cervicornis (staghorn coral), which predominated at intermediate depths (5—25 m) from the 1950s through the 1970s, has virtually disappeared from most reef environments. Other coral species have declined as well, and the cover of macroalgae has increased. In apparent contrast, fossil reef sequences suggest that the species composition and zonation of coral assemblages did not change during the Pleistocene and Holocene. One interpretation of these observations is that coral species persisted on Caribbean reefs for hundreds of thousands of years as components of tightly integrated communities, and that a rare or unique combination of disturbances led to the synchronous decline of A. cervicornis and other corals throughout the region. The hypotheses of (1) community integration and (2) a unique, recent community transition, were tested by ecological and paleoecological observations in the shelf lagoon of the Belizean Barrier Reef.
The reef growing along the flanks of Channel Cay, a lagoonal shoal, was monitored by point counts along transects over a ten-year period (1986—95). This reef was covered primarily by A. cervicornis at 3—15 m depth until the late 1980s. After 1986, A. cervicornis experienced a mass mortality from White Band Disease, an epizootic of presumed bacterial origin. The cover of A. cervicornis dropped from ~70% in 1986 to nearly 0% in 1993. Agaricia spp. (lettuce corals) responded opportunistically to the availability of free space in the form of A. cervicornis skeletal rubble. Agaricia, which had been a minor constituent of the sessile biota (10% cover in 1986), replaced A. cervicornis as the most common occupant of space on the reef (56% cover in 1995). The percent cover of other coral species and macroalgae remained low throughout the ten-year period. Similar changes were observed on other reefs over an area of at least 250 km2.
The Acropora-to-Agaricia transition left a clear signature in the sedimentary record. Trenches dug into the reef at Channel Cay revealed the accretion of a layer of Agaricia rubble with a mean thickness of 22 cm in the decade after 1986. Due to the unconsolidated, uncompacted nature of the reef sediments, evidence of previous Acropora-to-Agaricia transitions should have been visible in the fossil record as vertical accumulations of A. cervicornis branches interrupted by layers of imbricated Agaricia rubble. Coring studies at Channel Cay revealed that no other Agaricia layers were deposited during at least the past 3800 years; the recent transition was unique on a time scale of millennia. This result supports the contention that excursions from the Acropora-dominated situation are unusual in the history of Channel Cay and nearby reefs. However, the dynamics of the transition do not support the community integration hypothesis for the Channel Cay reef, indicating instead that different coral taxa in this assemblage responded differently, or not at all, to a large-scale biotic disturbance. The community transition also underscores the potential for biological factors in general, and disease in particular, to alter the composition of ecological communities and their sedimentary remains.
- Cited by 121
Crocodilian scatology, microvertebrate concentrations, and enamel-less teeth
- Daniel C. Fisher
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- 08 February 2016, pp. 262-275
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It has been suggested that certain fossil assemblages consisting of disarticulated and broken remains of small to medium-sized vertebrates (“microvertebrate concentrations”) may be accumulations of incompletely digested material defecated by crocodilians. Experiments on crocodilian digestion show, however, that these reptiles demineralize calcified tissues, frequently leaving intact organic matrices of dentine, cementum, and bones in their feces. Such matrices, even if preserved as fossils, would not resemble most specimens in microvertebrate concentrations. Therefore, crocodilian digestion does not appear to have been an important factor in the formation of these fossil assemblages. Teeth similar to those defecated by crocodilians nevertheless do occur in the fossil record. Such teeth, lacking enamel but often complete in other respects, are interpreted here as having been digested by crocodilians, defecated as demineralized organic matrices, and subsequently remineralized. Enamel, with its extremely low organic content, does not yield a demineralized matrix susceptible to remineralization. A number of recently recognized occurrences of enamel-less teeth attest to the significance of crocodilian digestion as a factor in the taphonomic history of many Mesozoic and Cenozoic fossil vertebrate assemblages.
- Cited by 120
Testing evolutionary constraint hypotheses with early Paleozoic gastropods
- Peter J. Wagner
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- 08 February 2016, pp. 248-272
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The evolution of higher taxa among early Paleozoic gastropods is similar to that among early metazoans as a whole, as higher taxa diversified rapidly and early. There are two issues pertinent to this pattern. First, were greater morphologic changes concentrated in the early phases of evolution? Second, does the pattern better fit models of increasing phylogenetic constraints or increasing ecologic restrictions? This paper presents a phylogeny-based method designed to test whether amounts of morphologic evolution decreased over time. It also explores whether the data better fits models of increasing phylogenetic (i.e., developmental or genetic) constraint or increasing ecologic restriction. Two metrics of morphologic separation (i.e., the morphologic difference between sister-species) are used: (1) Euclidean distance in morphospace and (2) transition magnitude. The latter metric is calculated by a multivariate analysis of sister-species contrasts, which determines both types and magnitudes of morphologic transitions. The advantage of using transition magnitudes is that it balances the effects of transitions that either affect more morphometric characters or occur more frequently. Both metrics indicate that larger morphologic separations between sister-species were concentrated early in gastropod evolution. Among gastropods, gross shell morphology often reflects basic trophic strategy and function whereas basic internal anatomy does not. Transition magnitudes can be broken down into transitions associated with differences in basic trophic strategies and shell functional biology (“external”), and those associated with differences in basic internal anatomy (“internal”). Internal transition magnitudes show a highly significant decrease over time (p < 10–04) whereas external transition magnitudes show a much less significant decrease over time (p < 0.10) and no significant decrease after the earliest Ordovician (p ≅ 0.50). The results therefore suggest that increasing phylogenetic constraints played a greater role in the early evolution of gastropods than did increasing ecologic ones.
- Cited by 120
Functional and phylogenetic interpretation of enamel microstructure in rhinoceroses
- John M. Rensberger, Wighart v. Koenigswald
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- 08 February 2016, pp. 477-495
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Worn enamel surfaces of the cheek teeth in fossil and recent rhinoceroses are characterized by fine, parallel ridges aligned perpendicular to the enamel-dentin interface. We show that these ridges result from an unusual enamel ultrastructure in which a primitively horizontal layering of the prisms has become vertical. The new structure apparently appeared between early and middle Eocene, at the time when the superfamilies of perissodactyls were rapidly diverging. Similar modifications of the enamel structure occurred in certain parts of the cheek teeth in tapiroids, chalicotherioids and brontotherioids, but hardly at all in the equids. The modified enamel structure, where it occurs in groups other than rhinocerotoids, is associated with lophs but not cusps. Experimental evidence shows that the modified enamel is more resistant to wear than the unmodified enamel. The consistent association with thin lophs rather than cusps suggests that the modified enamel evolved to prolong the life of the lophs, where occlusal pressures are highest and attrition greatest. The dominance of modified enamel in rhinocerotoids correlates with the higher degree of compression of the cusps and extreme lophodonty in this group. The absence of modified enamel structure in the equids, even in the ectoloph, correlates with the lesser importance of the ectoloph in equids relative to brontotherioids, chalicotherioids and rhinocerotoids.
- Cited by 119
The dynamics of peak shifts and the pattern of morphological evolution
- Russell Lande
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- 08 April 2016, pp. 343-354
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Recent theoretical results demonstrate that a phenotypic version of Wright's shifting balance theory generates the dynamical pattern of punctuated equilibria. Thus, classical mechanisms of random genetic drift and selection for multiple adaptive peaks produce geologically long periods of relative stasis interrupted occasionally by very brief intervals of rapid change. A simple extension of this theory is made here to encompass developmental constraints between quantitative characters, manifested as phenotypic and genetic correlations between characters. Developmental constraints do not qualitatively alter the dynamical pattern of phenotypic evolution produced by selection and random genetic drift. A quantitative definition of stasis is proposed, based on a common taxonomic practice for recognizing subspecies. From this it is concluded that stasis is not the rule for quantitative measurements of detailed sequences for fossil species throughout most of their existence. Instead, periods of relative stasis are interspersed with gradual fluctuating trends, short intervals of rapid change, and discontinuities of subspecific magnitude.
- Cited by 117
Biogenic and diagenetic Sr/Ca in Plio-Pleistocene fossils of the Omo Shungura Formation
- Andrew Sillen
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- 08 April 2016, pp. 311-323
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Under conditions of normal calcium metabolism, strontium/calcium ratios (Sr/Ca) have been shown to reflect the trophic level of contemporary and recent terrestrial fauna. These ratios therefore offer a potential means of studying fossil ecosystems and the diet of prehistoric humans. In cases in which suitable controls have demonstrated the preservation of biogenic Sr/Ca, it has been possible to investigate the proportionate importance of meat vs. vegetable foods in the diets of prehistoric humans. However, diagenetic change after interment has made it impossible to discern biogenic Sr/Ca in faunal and human skeletons over 15,000 y b.p. A procedure is investigated for the analysis of biogenic and diagenetic apatite in vertebrate fossils, on the basis of solubility differences among carbonate, hydroxy-, and fluorapatites. When applied to the 2 ma b.p. fauna of the Omo Basin (Ethiopia), distinct characterization of the herbivore, omnivore, and carnivore fauna in conformity with trophism was discerned, in spite of anomalous Sr/Ca of one highly specialized carnivore, Homotherium. Possible metabolic and/or taphonomic explanations of this anomaly are discussed, and future basic research into the solubility profile procedure is outlined.
- Cited by 117
Age (Mortality) Profiles as a Means of Distinguishing Hunted Species from Scavenged Ones in Stone Age Archeological Sites
- Richard G. Klein
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- 14 July 2015, pp. 151-158
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Some ungulate species at Upper Pleistocene and Holocene archeological sites in South Africa exhibit catastrophic mortality profiles, while others exhibit attritional ones. The awareness by Stone Age people that some species are especially amenable to driving or snaring probably accounts for the catastrophic profiles. Natural catastrophic death immediately followed by human scavenging is a much less likely explanation because the species samples comprise material lumped from deposits that accumulated more or less continuously over hundreds or even thousands of years during which period there is no reason to suppose the repeated occurrence of natural catastrophes nearby.
The inability of Stone Age people to obtain prime-age adults in species that are not particularly amenable to driving or snaring presumably accounts for the attritional mortality profiles. Although the species that display attritional profiles conceivably were scavenged, the high proportion of very young individuals in the profiles suggests active hunting. Very young individuals are much less abundant in attritional profiles from local non-archeological sites, probably because their carcasses were removed from the record before burial, primarily by carnivore or scavenger feeding. Scavenging would account for the abundance of very young individuals in the archeological sites only in the unlikely event that people could regularly locate carcasses before other predators did.
In general, geomorphic/sedimentologic context is probably the best criterion for determining whether a species characterized by a catastrophic profile in an archeological site was hunted or scavenged. At the majority of known sites, active hunting is suggested. In the case of a species characterized by an attritional profile in an archeological site, the proportion of very young individuals in the sample probably provides the best criterion for distinguishing hunting from scavenging. A relatively high proportion of very young individuals suggests active hunting.
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The ontogeny of trilobite segmentation: a comparative approach
- Nigel C. Hughes, Alessandro Minelli, Giuseppe Fusco
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- 08 April 2016, pp. 602-627
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Ontogenetic stages of trilobites have traditionally been recognized on the basis of the development of exoskeletal segmentation. The established protaspid, meraspid, and holaspid phases relate specifically to the development of articulated joints between exoskeletal elements. Transitions between these phases were marked by the first and last appearances of new trunk segment articulations. Here we propose an additional and complementary ontogenetic scheme based on the generation of new trunk segments. It includes an anamorphic phase during which new trunk segments appeared, and an epimorphic phase during which the number of segments in the trunk remained constant. In some trilobites an ontogenetic boundary can also be recognized at the first appearance of morphologically distinct posterior trunk segments. Comparison of the phase boundaries of these different aspects of segment ontogeny highlights rich variation in the segmentation process among Trilobita. Cases in which the onset of the holaspid phase preceded onset of the epimorphic phase are here termed protarthrous, synchronous onset of both phases is termed synarthromeric, and onset of the epimorphic phase before onset of the holaspid phase is termed protomeric. Although these conditions varied among close relatives and perhaps even intraspecifically in some cases, particular conditions may have been prevalent within some clades.
Trilobites displayed hemianamorphic development that was accomplished over an extended series of juvenile and mature free-living instars. Although developmental schedules varied markedly among species, morphological transitions during trilobite development were generally regular, limited in scope, and extended over a large number of instars when compared with those of many living arthropods. Hemianamorphic, direct development with modest change between instars is also seen among basal members of the Crustacea, basal myriapods, pycnogonids, and in some fossil chelicerates. This mode may represent the ancestral condition of euarthropod development.
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The evolution of nasal turbinates and mammalian endothermy
- Willem J. Hillenius
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- 08 February 2016, pp. 17-29
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Complex nasal turbinal bones are associated with reduction of respiratory water loss in desert mammals and have previously been described as an adaptation to xeric conditions. However, complex turbinates are found in virtually all mammals. Experimental data presented here indicate that turbinates also substantially reduce respiratory water loss in five species of small mammals from relatively mesic environments. The data support the conclusion that turbinates did not evolve primarily as an adaptation to particular environmental conditions, but in relation to high ventilation rates, typical of all mammals. Complex turbinates appear to be an ancient attribute of mammals and may have originated among the therapsid ancestors of mammals, in relation to elevated ventilation rates and the evolution of endothermy.
- Cited by 115
Confidence intervals on stratigraphic ranges: partial relaxation of the assumption of randomly distributed fossil horizons
- Charles R. Marshall
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- 08 February 2016, pp. 459-469
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The equations for calculating classical confidence intervals on the end points of stratigraphic ranges are based on the restrictive assumption of randomly distributed fossil finds. Herein, a method is presented for calculating confidence intervals on the end-points of stratigraphic ranges that partially relaxes this assumption: the method will work for any continuous distribution of gap sizes, not just those generated by random processes. The price paid for the generality of the new approach is twofold: (1) there are uncertainties associated with the sizes of the confidence intervals, and (2) for large confidence values (e.g., 95%) a rich fossil record is required to place upper bounds on the corresponding confidence intervals. This new method is not universal; like the method for calculating classical confidence intervals it is based on the assumption that there is no correlation between gap size and stratigraphic position. The fossil record of the Neogene Caribbean bryozoan Metrarabdotos is analyzed with the new approach. The equations developed here, like those for classical confidence intervals, should not be applied to stratigraphic ranges based on discrete sampling regimes, such as those typically established from deep-sea drilling cores, though there are exceptions to this rule.
- Cited by 114
Estimating taxonomic durations and preservation probability
- Mike Foote
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- 08 February 2016, pp. 278-300
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Paleontological completeness and stratigraphic ranges depend on extinction rate, origination rate, preservation rate, and the length of the interval of time over which observations can be made. I derive expressions for completeness and the distribution of durations and ranges as functions of these parameters, considering both continuous- and discrete-time models.
Previous work has shown that, if stratigraphic ranges can be followed indefinitely forward, and if extinction and preservation occur at stochastically constant rates, then extinction rate and preservability can be estimated from (1) discrete (binned) stratigraphic ranges even if data on occurrences within ranges are unknown, and (2) continuous ranges if the number of occurrences within each range is known. I show that, regardless of whether the window of observation is finite or infinite, extinction and preservation rates can also be estimated from (3) continuous ranges when the number of occurrences is not known, and (4) discrete ranges when the number of occurrences is not known. One previous estimation method for binned data involves a sample-size bias. This is circumvented by using maximum likelihood parameter estimation. It is worth exploiting data on occurrences within ranges when these are available, since they allow preservation rate to be estimated with less variance. The various methods yield comparable parameter estimates when applied to Cambro-Ordovician trilobite species and Cenozoic mammal species.
Stratigraphic gaps and variable preservation affect stratigraphic ranges predictably. In many cases, accurate parameter estimation is possible even in the face of these complications. The distribution of stratigraphic ranges can be used to estimate the sizes of gaps if their existence is known.
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Heterochrony and Phylogenetic Trends
- Kenneth J. McNamara
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- 14 July 2015, pp. 130-142
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A model is proposed, based on examples that have been interpreted as phylogenetic trends, to explain how directional morphological evolution at the species level can arise by heterochrony. The examples illustrated are of Tertiary to Recent rhynchonellide brachiopods, Cambrian olenellid trilobites, living spatangoid echinoids, Tertiary to Recent schizasterid echinoids, Cenomanian ammonites and Silurian monograptids. Morphological discontinuities between species along morphological gradients (which can be recognised both spatially and/or temporally), and temporal morphological stasis within species, are both consistent with the punctuated equilibria model of macroevolution. It is argued that morphological discontinuities have arisen by selection of morphological novelties produced by heterochronic processes. These novelties are preadaptations which allow ecological and, consequently, genetic isolation from ancestral species. Establishment of a heterochronic morphological gradient is only possible given a suitable environmental gradient. The terms “paedomorphocline” and “peramorphocline” are proposed for these heterochronic morphological gradients. Paedomorphoclines and peramorphoclines each comprise a number of species occupying a series of adaptive peaks, which have evolved sequentially through time by selection along an environmental gradient.
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Jurassic bivalve biogeography
- A. Hallam
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- 08 April 2016, pp. 58-73
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An analysis of the geographic and stratigraphic distribution of nearly 200 Jurassic bivalve genera leads to a number of new discoveries. Similarities between regions reached a maximum in the middle of the period, while the percentage of endemism correspondingly decreased. Diversity increased through the Lower Jurassic to a level which remained more or less stable from Middle Jurassic times onwards, while the origination rate shows the opposite trend. Extinction rate increased early in the period to a maximum in the Pliensbachian and fell thereafter to a low value until the Tithonian, which is marked by a sharp rise. The overall taxonomic composition of the fauna in terms of orders remained substantially stable throughout the period. The relationship with facies is discussed and three major ecological groups distinguished: marginal marine (euryhaline), shallow neritic and deep neritic. Certain pterioids have a very wide distribution and the order as a whole has a significantly higher proportion of cosmopolitan to endemic genera than any other order; the hippuritoids and trigonioids have the highest proportion of endemics. Five faunal provinces are distinguished, and the dominant control on distribution considered to be sea level. Times of high sea level were marked by widespread distribution of taxa and low endemism. High extinction rates were provoked both by regression (in the Tithonian) and by a sharp rise of sea level in the Toarcian, marked by the widespread onset of anaerobic or near-anaerobic conditions in many epicontinental seas. Some latitudinal control is recognised, notably for the hippuritoids and other stenotopic thick-shelled genera, which are confined to low latitudes.
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Jaw movement, dental function, and diet in the Paleocene multituberculate Ptilodus
- David W. Krause
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- 08 February 2016, pp. 265-281
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The shape and arrangement of cusps and crests and the orientation of wear striations on the cheek teeth of fossil mammals can be used to reconstruct occlusal patterns. Occlusal patterns have been documented previously in a variety of therian mammals and also in triconodonts and docodonts among nontherians. This is the first detailed analysis of patterns of jaw movement and dental function in a member of the highly specialized nontherian order Multituberculata (Allotheria). Ptilodus, a Paleocene multituberculate, appears to have had two cycles of mastication that followed different paths of movement and utilized different sets of teeth. The first cycle, the slicing-crushing cycle, occurred as the large, laterally-compressed fourth lower premolar (P4) sliced orthally into food items held primarily against the fourth upper premolar (P4). Food items sliced in this manner passed down both the labial and lingual sides of P4, forming subparallel striations in valleys between the nearly vertical enamel ridges. The second cycle is the grinding cycle in which the mandible was retracted while the molars were in tight occlusion, thus producing longitudinal striations on the molars. Unlike the pattern in therians, triconodonts, and docodonts, there is no transversely triangular masticatory orbit in the grinding cycle of multituberculates. The generally accepted idea that the labial aspect of P4 in ptilodontoid multituberculates sheared orthally against the lingual aspect of P4 is not supported. Instead, predominantly horizontal striations developed on the posterolabial wear facet of P4, and on a conjoined facet posterolingually on P4 and anterolingually on the first upper molar (M1), indicate that relative movement between these surfaces was largely palinal (i.e., the jaw moved from front to back), rather than orthal and occurred during the grinding cycle of mastication.
In considering the dietary preferences of ptilodontoid multituberculates, it appears that most members were not folivorous. The small size of many species of Ptilodontoidea suggests that they could not have subsisted on a folivorous diet, which is rich in structural carbohydrates. The length of striations on the sides of P4 of Ptilodus, one of the largest of the Ptilodontoidea, indicates that large, hard food items were ingested. The presence of both smooth and highly-striated enamel on homologous dental wear facets in different individuals of Ptilodus mediaevus from a single quarry sample suggests a varied diet. The recent suggestion that ptilodontoids were omnivorous is supported.
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Taking advantage of time-averaging
- Thomas Olszewski
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- 20 May 2016, pp. 226-238
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One of the major obstacles in dealing with any form of data derived from fossils is the effects of time-averaging, which are the result of mixing the remains of organisms that did not live contemporaneously. Although this process results in loss of temporal resolution, it also serves to filter out short-term variations. Temporal resolution of a collection depends not only on the range of fossil ages, but also on their frequency distribution. Previous studies of marine molluscs indicate that most shells in an accumulation are relatively young. Such a distribution of shell ages can be fit by an exponential curve (assuming both a constant probability of shell loss and a constant rate of shell addition), which implies that 90% of the shells were added during the last 50% of the time interval represented by the collection. That is to say, differences between two collections can be discerned even if they overlap 50% in time, because the proportion of shells with shared ages is only 10%. Applying the exponential model to previously published data suggests that long-term rates of destruction are controlled by how frequently shells from the taphonomically active zone are re-exposed to rapid destruction. To take advantage of the “noise-filtering” property of time-averaging, samples need to be large enough to catch the full range of environmental variation recorded by an accumulation. A simple probability formula indicates that samples of easily achievable size can give satisfactory time-averaged results depending on the level of confidence and sampling density defined by the researcher.