Revealing rangeomorph species characters using spatial analyses
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Rangeomorphs dominate the Ediacaran Avalonian macrofossil assemblages of Charnwood Forest, UK (∼562 Ma). However, their unfamiliar fractal architecture makes distinguishing phylogenetically reliabl...Cite
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 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.
Paleobiology
Variation (astronomy)
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Rangeomorphs dominate the Ediacaran Avalonian macrofossil assemblages of Charnwood Forest, UK (∼562 Ma). However, their unfamiliar fractal architecture makes distinguishing phylogenetically reliable characters from intraspecific features difficult. Fortunately, spatial analysis of large in-situ populations offers an independent means of assessing their taxonomy. Populations of a single biological species are likely to exhibit similar spatial distributions due to their shared responses to the biological and ecological processes acting upon them. As such, spatial analyses can be used to interrogate which are the most taxonomically deductive characters in similar species. We used random labelling analyses to investigate the presence or absence of characters of Primocandelabrum boyntoni, P. aethelfalaedia, and P. aelfwynnia on the Bed ‘B’. The resultant spatial distributions were compared to observed characters using goodness-of-fit tests to determine which characters were associated with unique populations, and which were found across multiple populations. We found that P. boyntoni and P. aelfwynnia had statistically indistinguishable character distributions, suggesting that they represent a single biological species, and that they exhibited significantly different distributions to P. aethelfalaedia, suggesting that there are two (rather than three) species of Primocandelabrum present on the B surface. Furthermore, we found that the distribution of concealed versus unconcealed 1st order branches across all specimens exhibited significantly different density-dependant behaviour, with unconcealed branching occurring in areas of higher density populations and concealed branching occurring in the lower density areas of Primocandelabrum. We speculate that unconcealed branches may have been a response to the reduced availability of resources in higher density areas, implying rangeomorphs were capable of ecophenotypic responses.
Branching (polymer chemistry)
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