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The pattern of variation in taxonomic turnover on short timescales is expected to leave detectable signals even when taxonomic data are compiled at coarser timescales. Global, stage-level data on first and last appearances of marine animal genera are analyzed to determine whether it is more likely that origination and extinction were spread throughout stages or that they were concentrated at a single episode per stage. The analysis takes incomplete and variable sampling of stratigraphic ranges into consideration, and it takes advantage of the fact that empirical sampling rates are within the range of values that allow the within-stage turnover models to be distinguished on the basis of stage-level data. The data strongly support the model of a single extinction pulse per stage over the alternative of continuous extinction within the stage. Pulsed origination is also supported over continuous origination, but the case is not as compelling as for extinction. Differential support for pulsed turnover is not confined to a few stages. Pulsed turnover therefore appears to be a general feature of the evolution of marine animals.
The traditional “taxon counting” method of estimating ancient biodiversity is open to many criticisms, not least of which are the problem of inconsistency in the preservation of fossil organisms and the associated error on first and last appearance times of taxa. Construction of phylogenetic trees provides a way of correcting the first appearance of a taxon based on the origination time of its sister group. Workers have suggested that biodiversity studies include such phylogenetically implied range extensions. Potential problems with this method, in particular the bias inherent in altering origination—but not extinction—times, and the potential for incorrect addition of ghost ranges if the ancestor of a taxon is defined as its sister, are investigated by using a new computer simulation. The program creates a phylogeny, samples it and then adds ghost lineages, with diversity counts being made at all three stages. Results show that under certain conditions, such as in the case of a taxonomic group with many extant representatives, the phylogenetic method is superior to the taxic at capturing diversity pattern. However, there are also important conditions where the taxic approach provides an equal or superior estimate of diversity, such as if the group is extinct or has few extant lineages. Use of the phylogenetic method has the effect of magnifying the Signor-Lipps sampling effect seen before mass extinction events, and if ancestral species within a phylogeny are misdiagnosed as the sister species of their descendants, the phylogenetic method also consistently overestimates diversity magnitudes.
The evolutionary history of a clade has traditionally been studied through phylogenetics, and taxonomic diversity has been used as a crude proxy for morphological diversity. However, morphological diversification—beyond counting taxa—can provide a very different view of a clade's evolutionary history and allows the investigation of patterns and timing of morphological evolution.
In this paper I use dentition to document the pattern of morphological and taxonomic diversification of Carnivoramorpha and mammalian meat eaters in North America. Using the dentition permits ecological inferences to be made, because teeth and diet are closely related. I present a method developed to describe the entire dentition of the Carnivoramorpha and other mammalian meat eaters (Creodonta). Morphological diversification is measured by dental disparity, using the mean pairwise dissimilarity among species.
I test the following hypotheses: (1) Morphological diversification was suppressed relative to taxonomic diversification, early in the evolutionary history of Carnivoramorpha; and (2) once an efficient system for consuming meat evolved, the dental system remained relatively unchanged.
The first hypothesis is rejected. Taxonomic and morphological diversity increase together through the clade's early evolution. There is no evidence of a morphological release in the carnivoramorphans with the demise of creodonts. The second hypothesis is supported. The ecological group “mammalian meat eaters” rapidly diversified morphologically and reached its maximum disparity early in its history, after which the dental system remained relatively unchanged.
Communal hunting allows some modern canids to catch large and powerful prey. As opposed to felids, for example, Recent canids have a limited ability to grapple and subdue prey by using their forelimbs. Instead, they engage in sustained pursuit predation and the success rate during this activity typically increases with the number of individuals participating in the hunt. Clearly, such behaviors do not fossilize directly and have to be inferred from anatomy. This paper focuses on how social pack-hunting in large-bodied fossil canids can be determined and the potential for it among Tertiary canids (Canidae, Carnivora). Craniodental adaptations for handling and killing large prey and forearm utility in running and grappling are investigated by principal components and canonical variates analyses. I also test whether fossil canids responded to predation of large prey by evolving the same morphological traits as their Recent pursuit-type relatives. The analyses show that small and large members of the Recent Caninae share similar craniodental morphologies. However, the same pattern is not present in the fossil subfamilies Borophaginae and Hesperocyoninae. In the latter, large representatives are characterized by being relatively short-faced with reduced anterior premolars and enlarged posterior premolars, thus approaching a “pantherine-like” configuration. These traits are interpreted as an adaptation for killing prey with felid-like canine bites. The elbow joints of large canids also do not converge on a single morphotype. All analyzed species of borophagines and hesperocyonines have retained the ability to supinate their forearms, unlike Recent large Caninae. It is therefore likely that manual manipulation was part of their hunting behavior, thus removing an essential part of the argument for social pack-hunting in these forms, as the benefits of such a strategy become less obvious. An association between the origin of pack-pursuit “wolf avatars” and the origin and evolution of grass-dominated ecosystems is hypothesized. The results presented here clearly suggest that Recent large canids are poor ecological, morphological, and behavioral analogs for their large fossil relatives.
During the Early Jurassic, cyst-forming dinoflagellates began a long-term radiation that would portend ecological importance of these taxa in the pelagic plankton community throughout the rest of the Mesozoic era. The factors that contributed to the evolutionary success of dinoflagellates are poorly understood. Here we examine the relationship between oceanographic and climatic conditions during the Hettangian–Toarcian interval in relation to the radiation of dinoflagellates and other organic-walled phytoplankton taxa in the Tethys Ocean. Our analysis is based on two data sets. The first includes δ13Ccarb, δ13Corg, total organic carbon (TOC), and quantitative palynological observations derived from the Mochras Core (Wales, U.K.), which spans the complete Early Jurassic. The second is a coupled Mg/Ca and δ18O record derived from analyses of belemnite calcite obtained from three sections in northern Spain, covering the upper Sinemurian to Toarcian. From these two data sets we reconstructed the influence of sea level, trophism, temperature, and salinity on dinoflagellate cyst abundance and diversity in northwest Europe. Our results suggest that organic-walled phytoplankton (acritarchs, prasinophytes, and dinoflagellates) diversity increased through the Early Jurassic. The radiation coincides with a long-term eustatic rise and overall increase in the areal extent of continental shelves, a factor critical to cyst germination. On shorter timescales, we observed short bursts of dinoflagellate diversification during the late Sinemurian and late Pliensbachian. The former diversification is consistent with the opening of the Hispanic Corridor during the late Sinemurian, which apparently allowed the pioneer dinoflagellate, Liasidium variabile, to invade the Tethys from the Paleo-Pacific. A true radiation pulse during the late Pliensbachian, with predominantly cold-water taxa, occurred during sea level fall, suggesting that climate change was critical to setting the evolutionary tempo. Our belemnite δ18O and Mg/Ca data indicate that late Pliensbachian water masses cooled (ΔT ≈ −6°C) and became more saline (ΔS ≈ 2 psu). Cooling episodes during generally warm and humid Early Jurassic climate conditions would have produced stronger winter monsoon northeast trade winds, resulting in hydrographic instability, increased vertical mixing, and ventilation of bottom waters. During the late Pliensbachian, dinoflagellates replaced green algae, including prasinophytes and acritarchs, as primary producers. By producing benthic resting cysts, dinoflagellates may have been better adapted to oxidized ocean regimes. This hypothesis is supported by palynological data from the early Toarcian ocean anoxic event, which was marked by highly stratified anoxic bottom water overlain by low-salinity, warm surface waters. These conditions were advantageous to green algae, while cyst-producing dinoflagellates temporarily disappeared. Our results suggest that the rise in dinoflagellate diversity later in the Jurassic appears to correspond to deep water ventilation as a result of the opening of the Atlantic seaway, conditions that appear to have simultaneously led to a loss of prasinophyte dominance in the global oceans.
Multivariate analysis of shell characters and quantification of morphological diversity (morphospace occupation and disparity) are used here to investigate the modes of morphological diversification of ammonites. We define five events in early cardioceratid history that connect geographical changes causing emigration or immigration phases with biodiversity dynamics: (1) the initial colonization of the Arctic Basin by the Cardioceratidae at the end of the Bajocian, Middle Jurassic; (2) the first appearance of the Kosmoceratidae clade in the Boreal Realm during the Bathonian; (3) the ensuing expansion phase of this clade in the Boreal Realm; (4) the first phase of migration of the Cardioceratidae (early Callovian) through Eastern Europe, Western Britain and the Yukon corridor; and (5) the second unrelated migration phase in the Western Interior only. Analysis of spatial occupation shows that acquisition of this field occurs essentially by replacement or subdivision of preexisting peaks of occupation. These replacements seem to follow different patterns: progressive trend, saturation, iteration, and apparent preferential extinction. We describe these patterns and suggest different factors that may have shaped them, including a morphological differentiation that has been interpreted by various authors as sexual dimorphism. Another factor that could cause disparity modification is fluctuations in the ammonites' proximal environment. The effect of immigrating faunas is a third (and preponderant) factor that is prominent in the studied example: immigration phases of the Cardioceratidae lead to increased morphological diversity, whereas the spread of nonindigenous species reduces it and is contemporaneous with a morphological shift in the native clade. We thus demonstrate here that geographical constraints play a significant role in the expression of innovation and may be seen as a major factor in macroevolutionary dynamics.
Global information on Paleozoic, Mesozoic, and extant non-angiosperm leaf morphologies has been gathered to investigate morphological diversity in leaves consistent with marginal growth and to identify likely departures from such development. Two patterns emerge from the principal coordinates analysis of this data set: (1) the loss of morphological diversity associated with marginal leaf growth among seed plants after sharing the complete Paleozoic range of such morphologies with ferns and (2) the repeated evolution of more complex, angiosperm-like leaf traits among both ferns and seed plants. With regard to the first pattern, morphological divergence of fern and seed plant leaf morphologies, indirectly recognized as part of the Paleophytic-Mesophytic transition, likely reflects reproductive and ecological divergence. The leaf-borne reproductive structures that are common to the ferns and Paleozoic seed plants may promote leaf morphological diversity, whereas the separation of vegetative and reproductive roles into distinct organs in later seed plant groups may have allowed greater functional specialization—and thereby morphological simplification—as the seed plants came to be dominated by groups originating in more arid environments. With regard to the second pattern, the environmental and ecological distribution of angiosperm-like leaf traits among fossil and extant plants suggests that these traits preferentially evolve in herbaceous to understory plants of warm, humid environments, thus supporting inferences concerning angiosperm origins based upon the ecophysiology of basal extant taxa.
Polar deciduous forests were an important biome during much of the Mesozoic and Paleogene, occupying upwards of 40% of the total land surface. Little is known about their physiological ecology, however, because these types of forests do not exist for study today. Furthermore, the role of high atmospheric CO2 levels in modulating the physiological response of ancient polar forests is poorly known. Here we report detailed measurements of whole-tree net carbon uptake over a full annual cycle for five tree species whose close ancestors were components of Cretaceous and Paleogene polar forests. Measurements were made on both evergreen and deciduous species after two years growth in a simulated Mesozoic polar (69°N) environment at either ambient (400 ppmv) or elevated (800 ppmv) levels of CO2. The deciduous species exhibited a significant pulse in carbon uptake during the late summer and early autumn (August to mid-October) that enabled them to achieve annual carbon budgets similar to those of evergreen trees, despite incurring higher carbon losses through annual leaf shedding. Area-based photosynthetic rates dropped progressively in all species during the polar summer (June to mid-July), resulting in decreases in whole-tree carbon uptake late in the polar summer. The high-CO2-grown trees were more strongly affected by this polar summer depression than the low-CO2-grown trees. Our results indicate that, from a carbon balance perspective, deciduous taxa have no clear advantage over evergreens. Moreover, the seasonal patterns reported here suggest that at latitudes poleward of 69°, evergreens will be even more strongly favored. The consideration of factors not directly related to carbon budgeting is probably therefore required to fully understand the adaptive significance of the deciduous leaf habit in ancient polar forests.
Although direct predator-prey interactions are unobservable in the fossil record, predation has been used to explain many evolutionary trends. Evidence of predation supporting such hypotheses is often presented as isolated instances of preserved sublethal damage, and less commonly, as the frequency of such injuries. For instance, numerous morphological and ecological trends and innovations observed in Phanerozoic crinoids have been causally linked to predation, and whereas the high frequency of arm regeneration in living crinoids is generally assumed to represent intense predation, attempts to assess regeneration frequency and patterns in paleontological samples are few. Can the frequency of fossil injuries be assessed to test hypothesized predation-driven trends, or are such data unavailable?
To address this question, we analyzed regeneration in crinoids from the lower Mississippian (Kinderhookian) Maynes Creek Formation near Le Grand, Iowa, a locality renowned for the preservation of thousands of crinoids in tangled masses of crowns, stalks, and holdfasts. Nine percent of the specimens that we examined contained at least one regenerating arm; however, whereas some species lacked evidence of regeneration, others preserved up to 27% arm regeneration. Furthermore, we observed specimens with all arms regenerating, multiple adjacent arms regenerating from the same place along the arm, and a specimen with a damaged and regenerated primaxil and anal sac.
The highest regeneration frequency was observed in the most abundant species, Rhodocrinites kirbyi, a significantly higher value than expected under a model of no taxonomic selectivity (binomial: p < 0.05). Furthermore, bootstrapped simulations of the probable number of regenerated individuals suggest that the number of regenerated arms observed in our sample is two to three times less than what existed in the living population. Rhodocrinites kirbyi constituted over 40% of the individuals in the Le Grand crinoid fauna and had the longest stalk of the studied species. In addition, regeneration in R. kirbyi is size related, with individuals above median dorsal cup height (7 mm) displaying nearly 50% regeneration, and smaller individuals only 2% (a statistically significant difference; χ2 test: p < 0.001). The regeneration patterns in R. kirbyi are consistent with predatory attacks that target the most apparent prey. Moreover, this study suggests that predation is the most likely explanation for the regeneration patterns observed in Le Grand crinoids, and that the fossil record potentially provides a valuable, yet overlooked, data source for testing hypotheses pertinent to the role of predation in the evolution of Phanerozoic marine life.
A large, morphologically heterogeneous population of acanthomorphic acritarchs from the early Neoproterozoic Wynniatt Formation, Victoria Island, northwestern Canada, is ascribed to two form-genera, Tappania and Germinosphaera, but just a single natural taxon, Tappania. Analysis of Tappania morphology shows it to have been an actively growing, benthic, multicellular organism capable of substantial differentiation. Most notably, its septate, branching, filamentous processes were capable of secondary fusion, a synapomorphy of the “higher fungi.” Combined with phylogenetic, taphonomic and functional morphologic evidence, such “hyphal fusion” identifies Tappania reliably, if not conclusively, as a fungus, probably a sister group to the “higher fungi,” but more derived than the zygomycetes.
The presence of Tappania in the Mesoproterozoic Roper Group of Australia extends the record of putative fungi to 1430 Ma. Along with other Proterozoic acritarchs exhibiting fungus-like characteristics (e.g., Trachyhystrichosphaera, Shuiyousphaeridium, Dictyosphaera, Foliomorpha), there is a case to be made for an extended and relatively diverse record of Proterozoic fungi.