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The extent of morphologic innovation during the Ediacaran—Cambrian diversification of animals was unique in the history of metazoan life. This episode was also associated with extensive changes in the redox state of the oceans, in the structure of benthic and pelagic marine ecosystems, in the nature of marine sediments, and in the complexity of developmental interactions in Eumetazoa. But did the phylogenetic and morphologic breadth of this episode simply reflect the unusual outcome of recurrent evolutionary processes, or was it the unique result of circumstances, whether in the physical environment, in developmental mechanisms, or in ecological interactions? To better characterize the uniqueness of the events, I distinguish among these components on the basis of the extent of sensitivity to initial conditions and unpredictability, which generates a matrix of possibilities from fully contingent to fully deterministic. Discriminating between these differences is important for informing debates over determinism versus the contingency in the history of life, for understanding the nature of evolutionary theory, and for interpreting historically unique events.
Insect taphonomy is a topic that has drawn interest because of its potential biases on diversity patterns and the ecological information recorded by ancient insect faunas. Other than the onset of common amber fossilization in the Cretaceous, very little is known about long-term trends in the nature and quality of insect preservation and, as a result, the effects of taphonomic biases are poorly constrained. We assembled a database of nearly 7000 Carboniferous—Pliocene insect adpression (compression and impression) species from the primary literature to assess changes in insect taphonomy over time and test biotic and environmental controls on preservation. We grouped the fossils into 10-Myr bins and scored preservation of holotype specimens as either articulated bodies or isolated wings; articulated specimens with a body implied a generally higher quality of preservation. Paleozoic and Triassic insect holotypes are known overwhelmingly from isolated wings (only 12% articulated bodies), but our database shows a significant increase in the percentage preserved as articulated bodies, to more than 70%, beginning about 160 Myr ago in the Late Jurassic. This transition could reflect variations in the robustness of different insect orders and shifts in the taxonomic composition of insect faunas, but all the major orders in the database exhibit significant increases in articulation. Instead, a shift to increased preservation in lacustrine paleoenvironments, which contain a greater proportion of articulated body fossils, explains most of the trend. The pronounced Late Jurassic increase in articulation has implications for evolutionary and ecological reconstructions, for example, suggesting that preserved insect diversity may be biased downward in the earlier part of their history when articulation was poor.
We use a Gaussian logistic regression model to characterize epoch-to-epoch and stage-to-stage changes in the latitudinal response curves of Cenozoic marine bivalve and gastropod genera along the global latitudinal gradient, and analyze these changes to understand the mode and tempo of changes in latitudinal distribution. A ubiquitous “hollow curve” pattern is apparent, wherein smaller changes in response-curve parameters are much more common than larger changes. Curves are strikingly consistent in terms of the average level of change exhibited, despite the many unique environmental and biological changes documented between each of these intervals. This implies that the pace and magnitude of changes in the latitudinal distribution of marine mollusks are not controlled, in aggregate, by time-period-specific conditions. Additionally, we find no evidence for long-term migration from tropical to extratropical latitudes. Our results instead favor a model of either equatorward migration or no general trend. This likely reflects the tendency of genera to maintain their highest concentrations in the tropics even if their ranges become extended out of the tropics over time.
Both molecular clocks and the first appearances of major groups in the fossil record suggest that most of the range of diatom morphologies observed today had evolved by the end of the Cretaceous Period. Despite this, a canonical reading of the Cenozoic fossil record suggests a dramatic rise in taxonomic diversity that can be interpreted as an explosion of morphological variety. We investigated this apparent discrepancy by using a discrete-character-based, empirical diatom morphospace, resolved by molecular phylogeny and by fossil occurrences through time. The morphospace shows little correspondence to phylogeny and little Cenozoic change in disparity as measured by mean pairwise distance. There is, however, an increase in the total volume of morphospace occupied. Although the increase in occupied volume through time ostensibly supports a conclusion of increasing morphological variety, sampling biases and other data suggest an underlying stationary pattern more consistent with molecular clock data.
Morphospace occupation through time provides a view of diversification distinct from the more familiar taxonomic tabulations. However, this view is subject to the same geological biases long recognized in studies of taxonomic diversification, where techniques for correcting secular bias in sampling have become standard practice. In this study, we apply sampling standardization techniques to a morphospace investigation to test whether observed stratigraphic trends in morphospace occupation are artifacts of trends in sampling. When sampling bias is corrected by randomized subsampling, all disparity metrics show stationary patterns, or at most directional changes of small magnitude. Metrics describing the average dispersion of taxa in morphospace are less subject to sampling bias than those describing the total extent of morphospace occupied. We also investigate a measure of disparity that is insensitive to sampling intensity, introducing a geographic component of morphological disparity. By analogy to α and β components of taxonomic diversity, we suggest the notions of α and β disparity, and find that α disparity remains roughly constant through time. Our analysis also allows us to present the first taxonomic diversity curve of diatoms under shareholder quorum subsampling (SQS), showing similar results to previously published subsampling methods: a roughly twofold rise over the Cenozoic, with peak diversity around the Eocene/Oligocene boundary. Tests for methodological bias from choices in ordination method and data culling during morphospace construction indicate that our results are relatively insensitive to both factors: Cenozoic occupation of planktonic diatom morphospace is largely unchanging. We find a similarly stationary pattern when we directly analyze the morphological data, seeing no change in the prevalence of taxa with different sets of morphological characters. More broadly, our results make clear that a complete view of morphological disparity must consider sampling biases, which can be addressed with wellestablished, quantitative methods in morphospaces populated using occurrence-level data.
Crinoids were relatively unaffected by the end-Devonian Hangenberg mass extinction event. Major clades of Devonian durophagous fishes suffered significant extinctions, however, and the dominant surviving clades were biting or nipping predators. In part as a response to the Hangenberg event, early Mississippian crinoids underwent an adaptive radiation, while fish clades with a shell-crushing durophagous strategy diversified. Durophagous predators are inferred to have been more effective predators on camerate crinoids; and it is hypothesized, following the predictions of escalation, that through the early Mississippian, camerate crinoids evolved more effective anti-predatory strategies in response. We test this hypothesis of escalation by examining the changes in spinosity and plate convexity among camerate crinoids throughout this interval. A new method was formulated to test for an increase in convexity of the tegmen plates. Traits in Agaricocrinus, Aorocrinus, and Dorycrinus (Family Coelocrinidae) were tested for congruence to the escalation hypothesis, and results were mixed. Convexity of tegmen plates in Agaricocrinus, spine length/calyx diameter in Aorocrinus, calyx size in Aorocrinus, central spine length in Dorycrinus, and spine width in Dorycrinus did not have size increase trends supporting escalation. Rather than an increase in convexity, the variance of convexity in Agaricocrinus tegmen plates narrowed, which could reflect an optimum. Alternatively, morphological change consistent with the escalation hypothesis occurred in calyx size of Agaricocrinus and in lateral spine length and calyx size in Dorycrinus. Furthermore, central and lateral spine length, parameters of the spine width, and size trends support escalation when Aorocrinus and Dorycrinus are treated as a lineage. Thus, inferred escalation acted on traits differently within a single lineage and was relevant for both speciation and the diversification of a new genus.
Evolution of photosymbiotic ecology is an important adaptation for planktic foraminifers that enhances the ecological advantage of living in oligotrophic oceans. Therefore, detecting photosymbiotic ecology in fossil species is one of the keys to understanding the paleobiodiversity dynamics of planktic foraminifers. Because foraminiferal tests record the ontogenetic history of ecological information in geochemical signatures, analyzing individual geochemical profiles with growth can reveal a species' ecology. This study examined chamber-by-chamber stable isotopes (δ13C and δ18O) of foraminiferal individuals to identify photosymbiotic signals. We observed an ontogenetic δ13C increase of up to 2.4‰, accompanied by relatively stable, negative δ18O, in the symbiotic species Globigerinoides conglobatus and Globigerinoides sacculifer. In contrast, δ13C and δ18O showed significant positive correlation during ontogeny in the asymbiotic species Globorotalia truncatulinoides. These two ecological groups produce contrasting isotopic profiles, thereby allowing us to use our ontogenetic isotopic analyses of individual specimens to identify algal photosymbiosis in fossil foraminifers. The chamber-by-chamber isotope analyses with individual ontogeny have great advantages in analyzing rare species because only one individual is required to describe ontogenetic isotopic history. In addition to photosymbiotic identification, our methods hold great potential to provide new insight into species paleoecological studies such as the ontogenetic history of calcification depth.
Understanding the life orientation of fossil organisms, such as brachiopods, is not only important for understanding the biology of the organism in question, but it also can be used to interpret paleoecological information about the assemblages from which the specimens were derived. The dorsibiconvex brachiopod morphology is particularly common, especially among the Order Atrypida, yet there have been few independent, biomechanical studies to assess the life orientation of these brachiopods. In this study, we assess potential orientations of two end-member morphologies of a dorsibiconvex brachiopod, Pseudoatrypa lineata, from the Mid-Late Devonian of North America by placing realistic models in a flume.
Using materials with the specific gravity of calcium carbonate, we modeled two well-preserved Pseudoatrypa lineata from the Waterways Formation (Givetian—Frasnian, Alberta, Canada) to represent the original shell. The hydrodynamic stability of the models was assessed by placing the models in a recirculating flume in one of three initial orientations: (1) anterior commissure upstream, (2) umbo upstream, and (3) lateral (specimen perpendicular to flow), each with the dorsal and ventral valve topmost. The entire process was conducted both on a Plexiglas substrate and on well-sorted, medium-grained sand. All scenarios were repeated five times for a total of sixty trials per specimen (120 total).
Flume trials indicate that neither brachiopod had a true hydrodynamically stable orientation. Reorientations occurred at low velocities (∼0.2 m/s), with transport occurring soon after (∼0.3 m/s). Assuming that a juvenile, pedunculate, dorsibiconvex brachiopod would initially have been oriented with its ventral valve topmost, our results suggest two outcomes: the brachiopods either (1) were attached via pedicles throughout their lives or (2) lived in quiet, undisturbed waters. Given the abundance of dorsibiconvex brachiopods in observed high-energy environments, our results indicate it is more conservative to assume dorsibiconvex brachiopods retained pedicles throughout their lives.
Parasitic trematode worms leave characteristic pits in their bivalve mollusk hosts and represent an ideal system for analyzing parasite-host interactions through space and time with statistically meaningful sample sizes. Previous work in Late Pleistocene—Holocene sequences from the Po plain revealed significant long-term fluctuations in trematode prevalence values: higher prevalence in retrogradational environments (TST) and negligible prevalence in progradational environments (HST). Here we expand upon this work by investigating traces of parasitism, kleptoparasitism, and predation on mollusk death assemblages from two domains along the northern Adriatic coastline. The domain north of the Po delta (TST-like) and the southern domain (including the Po delta; HST-like) comprise environments comparable to those recovered in late Holocene (<6 Kyr) subsurface progradational deposits. We collected 17,299 specimens representing 111 species from 11 locations on the northern Adriatic coast of Italy. Our results reveal high predation pressure, a high diversity of host taxa, and widespread presence of trematode infestation in starved, oligotrophic, environmentally more stable (i.e., TST-like) settings north of the Po delta. Immediately south of the Po delta, in settings with strong and variable sedimentary input, almost no infestation is recorded. The reappearance of infestation is evident in the southern portion of the study area (i.e., Cattolica-Montemarciano), relatively far from the highly stressed environments south of the Po River. There is no significant difference in trematode prevalence values between fossil and modern samples. The distribution of spionid traces (an indicator of stressed environments) was nearly the opposite of that displayed by trematodes. Drilling frequency is highest in TST-like environments and is not correlated with diversity indices. These results suggest that temporal trends of trematode prevalence (and possibly also other biotic interactions) in sedimentary successions are controlled by environmental changes driven by glacio-eustatic dynamics, and reaffirm the importance of interpreting temporal trends in the context of spatial variation.
Comparisons between death assemblages and their source living communities are among the most common actualistic methods of evaluating the preservation of compositional and environmental information in fossil assemblages. Although live-dead studies have commonly focused on marine mollusks, the potential of diatoms to preserve ecological information in continental settings has been overlooked. Thus, little is known about the nature and magnitude of the taphonomic biases affecting live-dead agreement of diatom assemblages, despite their extensive application as modern and fossil bioindicators in paleoecological and paleoenvironmental reconstructions. In this study, I analyzed three live-dead data sets in order to evaluate the compositional and environmental fidelity exhibited by diatom death assemblages in shallow lakes. I find that diatom death assemblages (DAs) do differ significantly in their taxonomic composition from living assemblages (LAs), mainly as a consequence of (1) differences in the temporal resolution between time-averaged DAs and non-averaged LAs, and (2) differential preservation of diatom taxa related to the intrinsic properties of their valves. Despite compositional dissimilarities, DAs were able to capture the same environmental gradients as LAs, with high significance. This decoupling between live-dead agreement in community composition and community response to gradients can be related to the existence of at least two mutually exclusive subsets of species that significantly captured compositional dissimilarities based on the full set of the species in the three lakes. This functional redundancy implies that the between-sample relationships of living assemblages can be significantly preserved by DAs even if some taxa are removed by taphonomic processes. The preservation of environmental gradients thus does not require good preservation of all living taxa. Structural redundancy compensates for the loss of compositional fidelity caused by postmortem processes in the diatom data set.
Plant-insect interactions are vital for structuring terrestrial ecosystems. It is still unclear how climate change in geological time might have shaped plant-insect interactions leading to modern ecosystems. We investigated the effect of Quaternary climate change on plant-insect interactions by observing insect herbivory on leaves of an evergreen sclerophyllous oak lineage (Quercus section Heterobalanus, HET) from a late Pliocene flora and eight living forests in southwestern China. Among the modern HET populations investigated, the damage diversity tends to be higher in warmer and wetter climates. Even though the climate of the fossil flora was warmer and wetter than modern sample sites, the damage diversity is lower in the fossil flora than in modern HET populations. Eleven out of 18 damage types in modern HET populations are observed in the fossil flora. All damage types in the fossil flora, except for one distinctive gall type, are found in modern HET populations. These results indicate that Quaternary climate change did not cause extensive extinction of insect herbivores in HET forests. The accumulation of a more diverse herbivore fauna over time supports the view of plant species as evolutionary “islands” for colonization and turnover of insect species.
Predation is an important process in modern oceans and in the evolutionary history of marine ecosystems. Consequently, it has been hypothesized that shelled prey modified their ornamentation in response to predation. However, bivalve ornamentation has also been argued to be important in maintaining a stable life position in the sediment and in burrowing. To test whether concentric ribs were effective against drilling by carnivorous gastropods, we examined drill hole position and completeness for four Cenozoic bivalve species that differ in rib strength (Astarte radiata, A. goldfussi, Lirophora glyptocyma, and L. latilirata). The percentage of drill holes located between the ribs increases with increasing rib strength, whereas the percentage of drill holes on top of ribs decreases. This result suggests that gastropods select the drill hole site more effectively as rib strength increases, thereby saving time and energy, and that natural selection favors gastropods that select drill hole sites between ribs. Because of this greater stereotypy, the percentage of drill holes that are incomplete is generally lower in strongly ribbed species. The proportion of drill holes located on top of ribs is greater for incomplete than complete holes, implying that ribs can be effective against predators, but only when selected as the drilling location. We show that ribs are most effective against drilling predation for bivalves with moderately sized ribs, between which gastropods have difficulty siting drill holes. Concentric ribs are unlikely to have evolved as an adaptation against drilling predation because concentric ribs evolved in the Paleozoic and were already common in the Mesozoic, whereas drilling frequency increased later, in the Late Cretaceous—Paleogene. Moreover, rib strength of North American Astarte did not change through this time interval. Thus, the ribs considered here are a likely exaptation to drilling given their effectiveness at deterring drilling predation on bivalves with moderate ribs.