The Palaeobiology of Avalonian (Ediacaran) Rangeomorphs
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The Earth has supported life for most of its 4.5 billion year history, but the first macroscopic organisms
only appeared some 600 million years ago, in the Ediacaran. Their world was fundamentally different
from the one we know today, and many aspects of their biology and ecology remain a mystery. The
late Ediacaran fossil assemblages of Avalonia represent some of the oldest evidence for complex
macroscopic life, and are dominated by rangeomorphs, a group characterised by their self-similar
branching architecture. In this thesis, I investigate several aspects of the preservation, classification
and ecology of these enigmatic deep marine organisms.
The biotas of Charnwood Forest host several taxa which are new to science. Five of these are described
here, and include two new genera, Orthiokaterna fordi gen. et sp. nov., and Undosyrus nemoralis gen.
et sp. nov., and three new species: Primocandelabrum anatonos sp. nov., P. boytoni sp. nov., and P.
katatonos sp. nov.. The Primocandelabrum species in particular encompass a great deal of variation in
both branching characters and overall morphology. By using a novel multivariate statistical approach
to analyse multiple characters in tandem, individual taxa can be discriminated from one another. Much
of the observed variation is interpreted as intra-specific. This level of variation within a single taxon
has not previously been recognised in rangeomorphs, and is likely attributable to (eco?)phenotypic
rather than ontogenetic variability. Orthiokaterna displays eccentric branches, interpreted as a growth
response to mechanical damage, reflecting a greater degree of growth plasticity than that recognised
in other rangeomorphs, while Undosyrus had an external sheath, interpreted as modified rangeomorph
elements serving a protective role.
Even without knowing the phylogenetic relationships of rangeomorphs, it is possible to resolve key
aspects of their palaeoecology. The response(s) of communities in Charnwood Forest and Newfoundland
to both ambient disturbance and to more substantial events is investigated by combining detailed
petrographic analysis of the host sediments with multivariate statistical techniques. I demonstrate that
higher taxonomic diversity is correlated with low–intermediate physical disturbance; that upright taxa
(e.g. Charnia) dominate surfaces which experienced small-scale, sub-lethal sedimentation events and
comparably high background sediment input; and that flat-lying forms (e.g. Fractofusus) preferentially
occur on surfaces with low sediment input. The population demographics of several taxa also show
evidence of multimodality: in some (including Charnia and Primocandelabrum), bimodality was induced by culling of part of an incumbent population by a substantial disturbance event, followed
by re-colonisation; in others (e.g. Fractofusus), overlapping cohorts reflect non-continuous or
pulsed reproduction. Disturbance (ambient and discrete events) demonstrably influenced community
succession, with early-colonising taxa dominating horizons with low overall levels of disturbance, and
those able to survive disturbance events dominating recovery populations and horizons with higher
levels of disturbance. Based on their inferred life history traits and their environmental preferences, I
propose a model of ecological succession for rangeomorph communities.Keywords:
Paleobiology
Variation (astronomy)
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During my graduate studies, I characterized patterns of geographical distribution and taxonomic differentiation in birds of the West Indies, which suggested that species undergo phases of expansion and contraction similar to the taxon cycles that E. O. Wilson had described for Melanesian ants. Fieldwork in the early 1970s with George Cox confirmed that these phases were associated with variation in habitat distribution and abundance on individual islands, tying together local ecology and biogeography. Because taxon-cycle stage was independent of taxonomic or ecological relationships among birds of the West Indies, George and I postulated that whether a species was in a phase of expansion or contraction reflected the outcome of coevolved relationships with antagonists, including pathogens. The taxon cycle concept had a cool reception initially, but subsequent phylogeographical analyses, beginning in the early 1990s with Eldredge Bermingham, provided a time scale that confirmed the relationship between taxon cycle stage and the relative age of the most recent population expansion. The discrete nature of islands allows one to visualize taxon cycles in island systems, but the principle should apply in a continental biota as well. The absence of strong phylogenetic effects in distribution and abundance is consistent with evolutionary lability caused by coevolutionary outcomes with specialized antagonists. Related species appear to compete for resources on a more-or-less equal footing across a broad range of environments, and their distribution at any particular time is likely to be determined primarily by their relationships with pathogens, among other antagonists. This model of distribution and abundance within a regional community is consistent with much of what we know about the interactions between pathogens and their host populations, but testing the model will require the development of a new research programme focused on endemic pathogen effects in natural communities.
Disjunct
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Abstract Bedding-plane assemblages of Ediacaran fossils from Mistaken Point, Newfoundland, are among the oldest known records of complex multicellular life on Earth (dated to ~565 Ma). The in situ preservation of these sessile but otherwise deeply enigmatic organisms means that statistical analyses of specimen positions can be used to illuminate their underlying ecological dynamics, including the interactions between taxa. Fossil assemblages on Mistaken Point D and E surfaces were mapped to millimeter accuracy using differentiated GPS. Spatial correlations between 10 well-defined taxa ( Bradgatia , Charniid, Charniodiscus , Fractofusus , Ivesheadiomorphs, Lobate Discs, Pectinifrons , Plumeropriscum , Hiemalora , and Thectardis ) were identified using Bayesian network inference (BNI), and then described and analyzed using spatial point-process analysis. BNI found that the E-surface community had a complex web of interactions and associations between taxa, with all but one taxon ( Thectardis ) interacting with at least one other. The unique spatial distribution of Thectardis supports previous, morphology-based arguments for its fundamentally distinct nature. BNI revealed that the D-surface community showed no interspecific interactions or associations, a pattern consistent with a homogeneous environment. On the E surface, all six of the abundant taxonomic groups ( Fractofusus , Bradgatia , Charniid, Charniodiscus , Thectardis , and Plumeropriscum ) were found to have a unique set of interactions with other taxa, reflecting a broad range of underlying ecological responses. Four instances of habitat associations were detected between taxa, of which two ( Charniodiscus – Plumeropriscum and Plumeropriscum – Fractofusus ) led to weak competition for resources. One case of preemptive competition between Charniid and Lobate Discs was detected. There were no instances of interspecific facilitation. Ivesheadiomorph interactions mirror those of Fractofusus and Charniodiscus , identifying them as a form-taxonomic grouping of degradationally homogenized taphomorphs. The absence of increased fossil abundance in proximity to these taphomorphs argues against scavenging or saprophytic behaviors dominating the E-surface community.
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The evolution and diversification of euthyneuran slugs and snails was likely strongly influenced by habitat transitions from marine to terrestrial and limnic systems. Well-supported euthyneuran phylogenies with detailed morphological data can provide information on the historical, biological and ecological background in which these habitat shifts took place allowing for comparison across taxa. Acochlidian slugs are "basal pulmonates" with uncertain relationships to other major panpulmonate clades. They present a unique evolutionary history with representatives in the marine mesopsammon, but also benthic lineages in brackish water, limnic habitats and (semi-)terrestrial environments. We present the first comprehensive molecular phylogeny on Acochlidia, based on a global sampling that covers nearly 85% of the described species diversity, and additionally, nearly doubles known diversity by undescribed taxa. Our phylogenetic hypotheses are highly congruent with previous morphological analyses and confirm all included recognized families and genera. We further establish an ancestral area chronogram for Acochlidia, document changes in diversification rates in their evolution via the birth-death-shift-model and reconstruct the ancestral states for major ecological traits. Based on our data, Acochlidia originated from a marine, mesopsammic ancestor adapted to tropical waters, in the mid Mesozoic Jurassic. We found that the two major subclades present a remarkably different evolutionary history. The Microhedylacea are morphologically highly-adapted to the marine mesopsammon. They show a circum-tropical distribution with several independent shifts to temperate and temperate cold-habitats, but remained in stunning morphological and ecological stasis since the late Mesozoic. Their evolutionary specialization, which includes a remarkable and potentially irreversible "meiofaunal syndrome", guaranteed long-term survival and locally high species densities but also resembles a dead-end road to morphological and ecological diversity. In contrast, the Hedylopsacea are characterized by morphological flexibility and ecologically by independent habitat shifts out of the marine mesopsammon, conquering (semi-)terrestrial and limnic habitats. Originating from interstitial ancestors with moderate adaptations to the mesopsammic world, they reestablished a benthic lifestyle and secondary "gigantism" in body size. The major radiations and habitat shifts in hedylopsacean families occured in the central Indo-West Pacific in the Paleogene. In the Western Atlantic only one enigmatic representative is known probably presenting a relict of a former pan-Tethys distribution of the clade. This study on acochlidian phylogeny and biogeography adds another facet of the yet complex panpulmonate evolution and shows the various parallel pathways in which these snails and slugs invaded non-marine habitats. Given the complex evolutionary history of Acochlidia, which represent only a small group of Panpulmonata, this study highlights the need to generate comprehensively-sampled species-level phylogenies to understand euthyneuran evolution.
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Author(s): Tomiya, Susumu | Advisor(s): Barnosky, Anthony D | Abstract: The dissertation research presented herein addresses two questions on possible ecological drivers of mammalian diversity dynamics at macroevolutionary time scales. The first question is whether key intrinsic biological traits that are tightly correlated with body size (e.g., reproductive rates) have strong influence on the extinction probability of mammalian taxa at the generic level. The second question is whether, within a regional mammalian fauna, the ecological composition of carnivores (as inferred from their dental morphology) responds in predictable manners to shifts in the ecological composition of non-carnivores that represent their potential prey.In preparation for the ecological analyses of carnivore and non-carnivore compositional changes through time, concentrated effort was made to advance the taxonomy of carnivorous mammals from the middle Eocene of southern California. As part of this effort, the first chapter describes a carnivoramorphan that sheds a new light on the origin and early evolution of crown-group carnivorans. The new taxon, Lycophocyon hutchisoni, exhibits stages of dental and basicranial evolution that are intermediate between earlier carnivoramorphans and the earliest representatives of canoid carnivorans. The evolutionary affinity of the new taxon was determined by a cladistic analysis of previously-published and newly-acquired morphological data for 30 Paleogene carnivoramorphans. The most-parsimonious trees identified L. hutchisoni as a basal caniform carnivoran, and placed (1) Tapocyon robustus, Quercygale angustidens, Miacis sylvestris, M. uintensis, and M. gracilis inside or outside the Carnivora, (2) nimravids within the Feliformia, and (3) the amphicyonid Daphoenus outside the crown-group Canoidea. Parsimony reconstructions of ancestral character states suggest that loss of the upper third molars and development of well-ossified entotympanics that are firmly fused to the basicranium (neither condition is observed in L. hutchisoni) are not associated with the origin of the Carnivora as traditionally thought, but instead occurred independently in the Caniformia and the Feliformia. A discriminant analysis of the estimated body weight and dental ecomorphology predicted a mesocarnivorous diet for L. hutchisoni, and the postcranial morphology suggests a scansorial habit. Thus, Lycophocyon hutchisoni illuminates the morphological evolution of early caniforms leading to the origin of crown-group canoids. Nevertheless, considerable uncertainty remains with respect to the phylogenetic origin of the Carnivora. The minimum date of caniform-feliform divergence is provisionally suggested to be either 47 million years ago or 38 million years ago, depending on the position of Miacis sylvestris within or outside the Carnivora, respectively.The second chapter investigates the relationship between body size as a proxy for various intrinsic biological traits of key importance and extinction probability as measured by durations of genera in the fossil record. Preservation of mammalian diversity requires a concentrated effort to identify biological correlates of vulnerability to environmental perturbations. Studies of living mammals and late-Quaternary extinctions frequently point to large body size as a correlate--if not necessarily a determinant--of elevated extinction risk in mammalian species, and this correlation is often attributed to slow reproductive rates and lower population densities of large taxa. At the same time, biological patterns of extinction risk above the level of species have received much less attention, despite their relevance to conservation of evolutionary history embedded in ecological types that are more inclusive than individual species. I examined the North American fossil record of modern and some extinct families of terrestrial mammals to test whether extinction probability (or, more precisely, inter-regional extirpation probability) of genera, as measured by their durations in geologic time, scales with body size across 7 orders of magnitude in body weight. After adjusting observed generic durations for significant paleontological sampling bias against small taxa, generalized least-squares regression analyses showed no correlation between estimated body weights and durations in 221 Oligo-Pleistocene genera ranging from shrews to rhinoceroses. The same lack of correlation was observed for subsets of the data that (a) approximated basic trophic divisions (small/large herbivores, insectivores, and carnivores) or (b) were grouped by the timing of extinctions, suggesting that the overall pattern is not clouded by trophic and temporal variations in the relationship between size and vulnerability. The only notable deviation from this pattern was the significantly shorter durations of carnivorans compared to other taxonomic/ecological groups. Qualitatively identical results were obtained by analyses of durations and inter-birth interval lengths expected from body weights. Thus, in general, the population-biological expectation of higher extinction risk for large and slow-reproducing mammals was not supported for the genera that lived prior to significant anthropogenic influence. Two non-exclusive hypotheses are offered to explain this apparent mismatch: (1) the size-biased extinctions since the late Quaternary and elevated extinction risk for living large mammals signify an abnormal state of diversity dynamics brought about by human-induced reduction of large-mammal populations to critical levels, below which demographic or environmental stochasticity alone can threaten slow-reproducing taxa of low population density; (2) large mammalian species indeed have higher probabilities of extinction, but replacement of lost species within genera compensates for this pattern, resulting in comparable durations of large and small taxa at the genus level. The corollary of the first hypothesis is that, in normal times, thriving large mammals are no more likely--and perhaps less likely--to reach the critical population size than small mammals. The second hypothesis, if true, would indicate that extinction processes are distinct across levels of phylogenetic hierarchy and that prediction of future extinctions at supraspecific levels should not simply rely on extrapolation of extinction risk for individual species, especially if some of the species constituting a genus of interest are poorly known.Building on the taxonomic work on the middle-Eocene carnivores of southern California, I investigate in the final chapter the matches and mismatches between shifts in the ecological compositions of mammalian carnivores and other mammals that constitute their potential prey at the macroevolutionary time scale of approximately 6-9 million years. The middle-Eocene fossil record of southern California, which includes a diverse array of carnivores and particularly rich record of small mammals, was analyzed. Appearance event ordination was used to estimate the relative ages of fossil-bearing localities and their associated assemblages. Using the predicted temporal ranges of carnivore taxa and locality-level occurrence data for non-carnivores (ultimately grouped into time bins), it was found that changes in the distribution of taxa, and possibly taxonomic abundance as well, across morphological categories (defined by estimated body weight, arboreal versus non-arboreal habit, and ecologically-informative dental morphology) are largely discordant between carnivores and non-carnivores in the study system, except for the overall increase in the number of taxa in both groups. Analysis of morphological-compositional variation and factors that correlate with taphonomic disparity lend support to the interpretation of observed diversity fluctuations through time in non-carnivores. The findings raise additional questions about the controls of carnivore diversity--for example, what promotes the appearance of new morphotypes--and predictability of their extinctions.
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Many studies have tried to identify factors that explain differences in numbers of species between clades against the background assumption that older clades contain more species because they have had more time for diversity to accumulate. The finding in several recent studies that species richness of clades is decoupled from stem age has been interpreted as evidence for ecological limits to species richness. Here we demonstrate that the absence of a positive age-diversity relationship, or even a negative relationship, may also occur when taxa are defined based on time or some correlate of time such as genetic distance or perhaps morphological distinctness. Thus, inferring underlying processes from distributions of species across higher taxa requires caution concerning the way in which higher taxa are defined. When this definition is unclear, crown age is superior to stem age as a measure of clade age.
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Abstract The origin of a new higher taxon is characterized by a long‐term phylogenetic trend, involving evolutionary changes in a large number of characters. At this phylogenetic level, the conflict between internal integration of the phenotype and its evolvability can be resolved by the correlated progression model, in which many disparate traits evolve by a sequence of small increments in loose correlation with one another, rather than by the modularity model. The trend leading to the new higher taxon implies the existence of a long ridge in an adaptive landscape. An evolutionary lineage tracking it requires adaptive changes in broad biological characteristics, involving many traits. Species selection is a possible additional driver of the trend. These conclusions are tested against the synapsid fossil record of the origin of mammals. The reconstructed sequence of acquisition of mammalian traits supports the correlated progression model. The adaptive ridge involved is postulated to have been a sequence of overlapping niches requiring increasing ability to remain active in daily and seasonally fluctuating environments by means of increasing internal regulation. An inferred speciation bias in favour of relatively small, relatively more progressive carnivores indicates that species selection was also involved in driving the trend. Palaeoenvironmental evidence indicates that ecological opportunity probably played a role at certain points along the lineage.
Lineage (genetic)
Evolvability
Macroevolution
Genetic algorithm
Modularity
Sequence (biology)
Adaptive Radiation
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Extinction (optical mineralogy)
Taxonomic rank
Global biodiversity
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Biota
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Research Article| April 01, 1974 Evolutionary Patterns in the Paleozoic Bivalvia: Documentation and Some Theoretical Considerations: Reply PETER W. BRETSKY PETER W. BRETSKY 1Department of Earth and Space Sciences, State University of New York at Stony Brook, Stony Brook, New York 11790 Search for other works by this author on: GSW Google Scholar Author and Article Information PETER W. BRETSKY 1Department of Earth and Space Sciences, State University of New York at Stony Brook, Stony Brook, New York 11790 Publisher: Geological Society of America First Online: 01 Jun 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Geological Society of America GSA Bulletin (1974) 85 (4): 667–668. https://doi.org/10.1130/0016-7606(1974)85<667:EPITPB>2.0.CO;2 Article history First Online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation PETER W. BRETSKY; Evolutionary Patterns in the Paleozoic Bivalvia: Documentation and Some Theoretical Considerations: Reply. GSA Bulletin 1974;; 85 (4): 667–668. doi: https://doi.org/10.1130/0016-7606(1974)85<667:EPITPB>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract No Abstract Available. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.
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Endemic mammalian species on islands are generally known to have followed a different evolutionary pathway than their mainland relatives. General patterns, such as body size trends, have been described regularly. However, most island mammal species are unique and each of them is adapted to a specific local niche as part of an equally specific ecological assemblage. Therefore, comparing island species across taxa, islands and time is inherently dangerous without understanding the adaptational value of the studied feature in the compared taxa and without taking the ecological setting of the taxa into account. In this contribution, general and recurring patterns are described per taxon. Some features, like body mass change and sturdy limbs, are relatively general, whereas most features, like bone fusions and change of orbital axis, occur only in a very few taxa. Some features are even contradictory, such as brain size and degree of hypsodonty, with each taxon having its own particular design. In conclusion, general patterns are more often than not just trends and need to be applied with caution.
Mammal
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