Registered users receive a variety of benefits including the ability to customize email alerts, create favorite journals list, and save searches.
Please note that a BioOne web account does not automatically grant access to full-text content. An institutional or society member subscription is required to view non-Open Access content.
Contact email@example.com with any questions.
Individual egg size and clutch size of the largest of the dinosaurs (the sauropods) are both smaller than might be expected for such large oviparous organisms. We suggest that these effects can be understood in the light of likely incubation times of sauropod eggs. Using allometric relationships from extant birds and crocodilians, we estimate that time from laying to hatching was likely to have been 65–82 days. If total predation risk varies with length of incubation time, there may be egg sizes above which the advantages of larger initial hatchling size are outweighed by the increased risk of predation during the egg stage. Also, in seasonal environments there will often be a finite limit to the period over which environmental temperatures are high enough for egg development. Thus incubation time may have been an important constraint explaining the small individual size of sauropod eggs. We further suggest that for sauropods spatial dispersal of eggs in small clutches was an adaptive strategy to mitigate this high predation risk associated with long time of exposure in the egg stage. Such a dispersive strategy brings several benefits. Thus, incubation time may also be key to explaining the surprisingly small clutch sizes.
Phylogeny-based approaches can be used to infer diversification dynamics and the rate and pattern of trait change. Applying these analyses to fossil data often requires time-scaling a cladogram of morphotaxon relationships. Although several time-scaling methods have been developed for this purpose, the incomplete sampling of the fossil record can distort the apparent timing of branching. It is unclear how well different time-scaling methods reconstruct the true temporal relationships or how any such inaccuracy could affect tree-based evolutionary analyses. I developed process-based simulations of the fossil record that allow the comparison of approximated time-scaled trees to true time-scaled trees. I used this simulation framework to test the effect of time-scaling methods on the fidelity of several commonly applied tree-based analyses, across a range of simulation conditions. When the fidelity of time-scaling methods differed, the stochastic “cal3” time-scaling method with ancestral assignment produced preferable results. Estimating rates and models of continuous trait evolution was particularly sensitive to bias from scenarios that forced the insertion of many short branch lengths, a bias that is not solved by any of the considered time-scaling methods in all scenarios. The cal3 method of time-scaling can be recommended as the preferred time-scaling method among those tested, but caution must be exercised because tree-based analyses are prone to easily overlooked biases.
Despite the mounting evidence that taxonomic diversity dynamics are patterned environmentally and that taxonomic diversity and morphological disparity are decoupled both temporally and spatially in many clades, very little work has been done to assess whether disparity is also influenced by environment. Here I investigate whether trilobite disparity shows environmental patterning through time. I used the method developed by Simpson and Harnik (2009) for estimating latitudinal, substrate, and bathymetric affinities from fossil occurrence data, downloaded from the Paleobiology Database. This method has the advantages that the biological null hypothesis is explicitly separated from the expectation due to sampling, and that the posterior probability can be used to infer degree of preference for one habitat compared to another. To measure morphology, I used a data set of outlines of the trilobite cranidium from Foote (1993). Many of the species in this data set are not represented in the Paleobiology Database in sufficient numbers to assess species-level affinity for these taxa, but species-level affinity could be estimated with high fidelity by using genus-level affinities. Results show that cranidial morphological diversity was structured by environmental preferences of the taxa but the structure was complex and changed through time. In particular, there was little differentiation in morphospace around latitudinal, substrate, or bathymetric affinity during the Cambrian. In contrast, both diversification and expansion into previously unoccupied areas of morphospace during the Ordovician were dominated by tropical, deeper-water taxa.
Paleobiologists have used many different methods for estimating rates of origination and extinction. Unfortunately, all equations that consider entire age ranges are distorted by the Pull of the Recent, the Signor-Lipps effect, and simple edge effects. Attention has been paid recently to an equation of Foote's that considers counts of taxa either crossing the bottom and top of an interval or crossing one boundary but not the other. This generalized boundary-crosser (BC) method has important advantages but is still potentially subject to the major biases. The only published equation that circumvents all of them is the three-timer (3T) log ratio, which does so by focusing on a four-interval moving window. Although it is highly accurate it is noisy when turnover rates are very high or sampling is very poor. More precise values are yielded by a newly derived equation that uses the same counts. However, it also considers taxa sampled in a window's first and fourth intervals but missing from the third (i.e., gap-fillers). Simulations show that the 3T, gap-filler (GF), and BC equations yield identical values when sampling and turnover are uniform through time. When applied to Phanerozoic-scale marine animal data, 3T and GF agree well but the BC rates are systematically lower. The apparent reason is that (1) long-ranging but infrequently sampled genera are less likely to be split up by taxonomists and (2) the BC equation overweights taxa with long ranges. Thus, BC rates pertain more to rare genera that are likely to represent large clades whereas GF rates pertain more to actual species-level patterns. Given these results, all published turnover rates based either on genus-level data or on age ranges must be reconsidered because they may reflect taxonomic practices more strongly than the species-level dynamics of interest to biologists.
Climate changes are multivariate in nature, and disentangling the proximal drivers of biotic responses to paleoclimate events requires time series of multiple environmental proxies. We reconstruct a multivariate time series of local environmental change for the early Miocene Newport Member of the Astoria Formation (20.26–18 Ma), using proxies for temperature (δ18O), productivity (δ13C), organic carbon flux (Δδ13C), oxygenation (δ15N), and sedimentary grain size (% mud). Our data suggest increases in productivity and declines in oxygenation on the Oregon shelf during this interval of global warming. We evaluate the association of individual environmental factors, and combinations of factors, with changes in faunal composition observed in benthic foraminiferal and molluscan communities collected from the exact same sediments as the environmental data. The δ15N values are the most parsimonious correlates with major changes in foraminiferal composition, whereas molluscan composition is most closely related to δ13C values, suggesting that different components of the environment are influencing each group. When the proxies that have the best supported relationships with the faunal gradients are removed from the analyses to simulate the absence of those proxy data, significant relationships between the faunal gradients and the remaining environmental proxies can still be found. This suggests that environmental drivers can be incorrectly attributed to faunal changes when key proxy data are missing. Paleoecological studies of biotic response that test multiple environmental drivers for multiple taxonomic groups are powerful tools for identifying the ecological consequences of past warming events and the regional drivers of ecological changes.
The rise of durophagous predators during the Paleozoic represents an ecological constraint imposed on sessile marine fauna. In crinoids, it has been suggested that increasing predation pressure drove the spread of adaptations against predation. Damage to a crinoid's arms from nonlethal predation varies as a function of arm branching pattern. Here, using a metric for resilience to predation (“expected arm loss,” EAL), we test the hypothesis that the increase in predation led to more predation-resistant arm branching patterns (lower EAL) among Paleozoic crinoids. EAL was computed for 230 genera of Paleozoic crinoids and analyzed with respect to taxonomy and time. The results show significant variability among taxa. Camerates, especially monobathrids, display a pattern of increasingly convergent and predation-resistant arm morphologies from the Ordovician through the Devonian, with no significant change during the Mississippian. In contrast, the mean EAL among cladids follows no overall trend through the Paleozoic. Regenerating arms are known to be significantly more common in camerates than in other Paleozoic taxa; if regeneration is taken as a proxy for nonlethal interactions with durophagous predators, this indicates that nonlethal predation occurred more often among camerates throughout the Early and Middle Paleozoic. In addition, frequency of injury among camerates is inversely correlated with EAL and positively correlated with infestation by parasitic snails. From this we conclude that decreasing EAL signals a selective pressure in favor of resistance to grazing predation in camerates but not in other subclasses before the Mississippian, with an apparent relaxation in this constraint after the late Devonian extinctions.
In the assessment of Phanerozoic marine global biodiversity, there has been longstanding interest in quantifying compositional similarities among sampling points as a function of their distances from one another (geodisparity). Previous research has demonstrated that faunal similarity between any two locations tends to decrease significantly as the great circle distance (GCD) between the locations increases, but the rate of decrease begins to stabilize at transoceanic distances. The accuracy of these assessments, and comparisons among different temporal intervals, may suffer, however, because of intervening landmasses that are not accounted for when distance is calibrated simply as GCD. Here, we present a new method for determining the shortest overwater distance (WD) between two marine locations, and we use the method to recalibrate for several Phanerozoic intervals previous measures of global geodisparity in the taxonomic compositions of marine biotas. WD was determined by using a cost-distance approach in ArcGIS, modified to work on a spherical, as opposed to a planar, surface. Results demonstrate two notable effects of using WD. First, mean compositional similarity between locations tends to decrease more continuously as a function of distance with WD than with GCD. Second, pairs of locations with WDs that are at least 50% greater than their GCDs tend to have lower compositional similarity to one another than those with more closely matching WDs and GCDs. These differences are expected as WD better represents the “true” distance between locations; they diminish at GCDs of 5000 km or more when clear, transoceanic paths between locations become more common. Despite these effects, using WD does not alter fundamental temporal trends in global geodisparity through the Phanerozoic observed in previous research, but it is likely to have more significant ramifications for more confined paleobiogeographic investigations.
Here I test the hypothesis that temporal variation in geographic range size within genera is affected by the expansion and contraction of their preferred environments. Using occurrence data from the Paleobiology Database, I identify genera that have a significant affinity for carbonate or terrigenous clastic depositional environments that transcends the Database's representation of these environments during the stratigraphic range of each genus. These affinity assignments are not a matter of arbitrarily subdividing a continuum in preference; rather, genera form distinct, nonrandom subsets with respect to environmental preference. I tabulate the stage-by-stage transitions in range size within individual genera and the stage-by-stage changes in the extent of each environment. Comparing the two shows that genera with a preference for a given environment are more likely to increase in geographic range, and to show a larger average increase in range, when that environment increases in areal extent, and likewise for decreases in geographic range and environmental area. Similar results obtain for genera with preferences for reefal and non-reef settings. Simulations and subsampling experiments suggest that these results are not artifacts of methodology or sampling bias. Nor are they confined to particular higher taxa. Genera with roughly equal preference for carbonates and clastics do not have substantially broader geographic ranges than those with a distinct affinity, suggesting that, at this scale of analysis, spatial extent of preferred environment outweighs breadth of environmental preference in governing geographic range. These results pertain to changes over actual geologic time within individual genera, not overall average ranges. Recent work has documented a regular expansion and contraction when absolute time is ignored and genera are superimposed to form a composite average. Environmental preference may contribute to this pattern, but its role appears to be minor, limited mainly to the initial expansion and final contraction of relatively short-lived genera.
Reconstructing the tree of life requires deciphering major evolutionary transformations and the functional capacities of fossils with “transitional” morphologies. Some of the most iconic, well-studied fossils with transitional features are theropod dinosaurs, whose skeletons and feathered forelimbs record the origin and evolution of bird flight. However, in spite of over a century of discussion, the functions of forelimb feathers during the evolution of flight remain enigmatic. Both aerodynamic and non-aerodynamic roles have been proposed, but few of the form-function relationships assumed by these scenarios have been tested. Here, we use the developing wings of a typical extant ground bird (Chukar Partridge) as possible analogues/homologues of historical wing forms to provide the first empirical evaluation of aerodynamic potential in flapping theropod “protowings.” Immature ground birds with underdeveloped, rudimentary wings generate useful aerodynamic forces for a variety of locomotor tasks. Feather development in these birds resembles feather evolution in theropod dinosaurs, and reveals a predictable relationship between wing morphology and aerodynamic performance that can be used to infer performance in extinct theropods. By spinning an ontogenetic series of spread-wing preparations on a rotating propeller apparatus across a range of flow conditions and measuring aerodynamic force, we explored how changes in wing size, feather structure, and angular velocity might have affected aerodynamic performance in dinosaurs choosing to flap their incipient wings. At slow angular velocities, wings produced aerodynamic forces similar in magnitude to those produced by immature birds during behaviors like wing-assisted incline running. At fast angular velocities, wings produced forces sufficient to support body weight during flight. These findings provide a quantitative, biologically relevant bracket for theropod performance and suggest that protowings could have provided useful aerodynamic function early in maniraptoran history, with improvements in aerodynamic performance attending the evolution of larger wings, more effective feather morphologies, and faster angular velocities.
Within ancient ecosystems, it is generally difficult to determine the specific diets of species from higher trophic levels, which in turn hinders our understanding of trophic relationships and energy flow through these systems. To better understand the ecology of taxa at higher trophic levels, we used analysis of tooth enamel stable carbon isotope values to infer the dietary preferences of Canis edwardii and Smilodon gracilis from the Leisey Shell Pit 1A (LSP 1A) and Inglis 1A, two Pleistocene localities in Florida. The goals of the analyses were to (1) determine whether these carnivorans specialized in particular prey types or maintained a generalist diet; (2) ascertain whether carbon isotope values support what was previously suggested about the ecology of these species; and (3) establish what ecological details of ancient food webs can be discovered by carbon isotope analyses at higher trophic levels. Results show that the sampled carnivoran carbon isotope values are distributed among suspected prey isotope values, suggesting that varied prey were taken at the study localities. Prey compositions were modeled for each carnivoran species by using Stable Isotope Analysis in R (SIAR). The modeled diets indicate that each studied carnivoran had a generalist diet; however, there are differences in how these taxa achieved dietary generalization. At the glacial Inglis 1A locality, sampled individuals of C. edwardii and S. gracilis show similar isotope values and modeled dietary prey proportions, although both carnivorans do show a preference for grazing prey species. The similar isotopic values, and calculated prey proportions, observed between these species may imply greater interspecific competition for food. At the interglacial LSP 1A locality, C. edwardii shows values similar to those observed at Inglis 1A. In contrast, the data for S. gracilis shows a preference for consuming browsing prey species. Further, its restricted range of carbon isotope values suggests that S. gracilis may have concentrated its feeding within a particular habitat. Examination of stable carbon isotope values among species at higher trophic levels reveals that some intricacies of ancient food webs can be discerned.
This work shows the potential for applying three-dimensional biometry to studying cell growth in larger benthic foraminifera. The volume of each test chamber was measured from the three-dimensional model obtained by means of computed tomography. Analyses of cell growth based on the sequence of chamber volumes revealed constant and significant oscillations for all investigated specimens, characterized by periods of approximately 15, 30, 90, and 360 days. Possible explanations for these periods are connected to tides, lunar cycles, and seasonality. The potential to record environmental oscillations or fluctuations during the lifetime of larger foraminifera is pivotal for reconstructing short-term paleoenvironmental variations or for gaining insight into the influence of tides or tidal current on the shallow-water benthic fauna in both recent and fossil environments.