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Jason Loxton


Ph. D. Candidate

Department of Earth Sciences

Email: Jason.Loxton@dal.ca
Phone: +1 902 494 2358
Fax: +1 902 494 6889
Mailing Address: 

Dalhousie University
1459 Oxford Street
PO BOX 15000
Halifax NS B3H 4R2




Michael Melchin

Thesis Title:

Evolutionary Responses to the Hirnantian Mass Extinction Event: Graptolite Biostratigraphy, Biodiversity, and Systematics in Northern Laurentia.


The latest Ordovician to earliest Silurian was a time of profound environmental upheaval. The onset of glaciation in the southern hemisphere produced dramatic oceanographic changes that eliminated upwards of 80% of all species-the second largest extinction in Earth's history. Since it was initiated by terrestrial processes (climate change), and because of its magnitude and profound impact on evolutionary dynamics, this extinction-the Hirnantian Extinction Event (HME)-has recently become the focus of an intense research effort. Unfortunately, the processes that produced the extinction also erased most of its record: with rare exceptions, glacial sea-level fall produced either a gap in the fossil record or an abrupt change in facies and fauna, making study problematic. South China is globally unique in hosting abundant, intact boundary sections. Recent studies have also shown it to be (apparently) unique in graptolite biodiversity, before, during, and after the extinction, perhaps acting as a refugium. To test this hypothesis against the null (that elevated Chinese diversity is an artefact of better preservation/more intense sampling) and to better understand the global biotic response to the HME, an international collaborative effort was launched to restudy key sections in Scotland, Arctic Canada, China, the Czech Republic, and the US. To avoid methodological issues common to previous global surveys, a standardized intensive sampling protocol and uniform taxonomic scheme were adopted. This proposal adds an important new data set to this effort. Reconnaissance sampling on the Blackstone River, northern Yukon, in 2006 by the applicant, and further collecting in 2007, identified the thickest, most biodiverse, and continuous boundary sequence in North America. Comprising ca. 500 kg of material, including large slabs with complete, mature specimens, this collection is world class in quality and stratigraphic resolution.


The 50 m sequence of shale and limestone containing the extinction episode and recovery (ca. 450-440 Ma) is measured and lithologically described at the cm scale. Forty large (min. 2,500 cm2) collections of uniform thickness (10 cm) are taken through the section, with additional sequential 5 cm samples taken through the critical 20 cm interval of peak extinction at glacial onset (completed 2007). From each sample, =300 graptolite specimens are identified, providing a 95% confidence that all taxa present in =1% abundance are counted, allowing relative abundances to be estimated to ±10%. The remaining sample is scanned for rare species present at < 1%. Laboratory observation of graptolites is by reflected light microscopy. Illustrations are produced by digital photomicroscopy and/or camera lucida. Distinction of species is based on qualitative description of form and quantitatively measured characters using univariate, bivariate, and multivariate analyses. Identification is done by comparison with published literature and study of type and reference material from other regions (in consultation with collaborators).


The Ordovician/Silurian time scale uses graptolite zones, defined by the first appearance of distinctive species, with average durations of >1 million years. Zonal binning, used in previous interregional studies, loses substantial information on within zone biodynamic changes. The quantitative biostratigraphic and collaborative systematic approach proposed produces data that can be combined with those from other regional sections using modern optimization algorithms, allowing the construction of robust global composite sections on which to test biodynamic hypotheses. The Yukon data, which spans the early Silurian, is particularly important in this respect: This interval is not preserved elsewhere in Canada or the US, and contains both the post-extinction rediversification and two additional episodes of elevated extinction observed in China. Finally, the size and quality of this collection-which includes species previously undescribed in Canada, as well as numerous species new to science-makes it, if properly described, an ideal reference for recently begun revision of Ord-Sil graptolite systematics. (Historical work was largely conducted by national groups working in isolation, often on small samples or single specimens, sometimes with unacknowledged deformation.) Although unglamorous and decidedly not "trendy," this basic taxonomic work is essential to producing a data set capable of supporting scientifically meaningful statements about Ordovician-Silurian biodynamics.