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Establishing tephrostratigraphic frameworks to aid the study of abrupt climatic and glacial transitions: a case study of the Last Glacial-Interglacial Transition in the British Isles (c. 16-8 ka BP)
Distally dispersed tephra layers have become an important tool in the investigation of palaeoenvironmental and
archaeological records across the globe. They offer possibilities for the synchronisation and improved chronological
control in those records to which they can be traced and hence contribute to an improved understanding of the pattern
and timing of environmental and archaeological change during periods of rapid climatic adjustment. However, their use
as robust isochronous markers for synchronising records is frequently compromised by uncertainties relating to
stratigraphical context, precise chronology and chemical composition. Here we collate and review the
tephrostratigraphical information dating to the Last Glacial-Interglacial Transition (LGIT; c. 16-8 ka BP) in the British
Isles based on published and unpublished records obtained from 54 sites. Based on details of their stratigraphic
position, chronology and chemical composition, we propose that 26 individual eruption events may be represented in
this collective record which spans the LGIT. The great majority of these eruptives can be traced in origin to Iceland, but
we also report on the recent discoveries of ultra-distal tephra from the North American Cascades range, including for
the first time the Mount St Helens J Tephra at a site in southern Ireland. These particular ultra-distal discoveries have
resulted from a reinterpretation of older data, demonstrating the potential importance of ‘unknown’ analyses in older
tephra datasets. The outcome of this review is a comprehensive but provisional tephrostratigraphic framework for the
LGIT in the British Isles, which helps to focus future research on parts of the scheme that are in need of further
development or testing. The results, therefore, make an important contribution to the wider European
tephrostratigraphic framework, while adding new discoveries of transcontinental isochronous tephra markers
MESOGLOEAL ULTRASTRUCTURE AND REVERSIBLE OPACITY IN A TRANSPARENT SIPHONOPHORE
International audienc
MESOGLOEAL ULTRASTRUCTURE AND REVERSIBLE OPACITY IN A TRANSPARENT SIPHONOPHORE
International audienc
Phylogenetic analysis of phenotypic characters of Tunicata supports basal Appendicularia and monophyletic Ascidiacea
With approximately 3000 marine species, Tunicata represents the most disparate subtaxon of Chordata. Molecular phylogenetic studies support Tunicata as sister taxon to Craniota, rendering it pivotal to understanding craniate evolution. Although successively more molecular data have become available to resolve internal tunicate phylogenetic relationships, phenotypic data have not been utilized consistently. Herein these shortcomings are addressed by cladistically analyzing 117 phenotypic characters for 49 tunicate species comprising all higher tunicate taxa, and five craniate and cephalochordate outgroup species. In addition, a combined analysis of the phenotypic characters with 18S rDNA ‐sequence data is performed in 32 OTU s. The analysis of the combined data is congruent with published molecular analyses. Successively up‐weighting phenotypic characters indicates that phenotypic data contribute disproportionally more to the resulting phylogenetic hypothesis. The strict consensus tree from the analysis of the phenotypic characters as well as the single most parsimonious tree found in the analysis of the combined dataset recover monophyletic Appendicularia as sister taxon to the remaining tunicate taxa. Thus, both datasets support the hypothesis that the last common ancestor of Tunicata was free‐living and that ascidian sessility is a derived trait within Tunicata. “Thaliacea” is found to be paraphyletic with Pyrosomatida as sister taxon to monophyletic Ascidiacea and the relationship between Doliolida and Salpida is unresolved in the analysis of morphological characters; however, the analysis of the combined data reconstructs Thaliacea as monophyletic nested within paraphyletic “Ascidiacea”. Therefore, both datasets differ in the interpretation of the evolution of the complex holoplanktonic life history of thaliacean taxa. According to the phenotypic data, this evolution occurred in the plankton, whereas from the combined dataset a secondary transition into the plankton from a sessile ascidian is inferred. Besides these major differences, both analyses are in accord on many phylogenetic groupings, although both phylogenetic reconstructions invoke a high degree of homoplasy. In conclusion, this study represents the first serious attempt to utilize the potential phylogenetic information present in phenotypic characters to elucidate the inter‐relationships of this diverse marine taxon in a consistent cladistic framework.Peer Reviewe