TAXIC AND PHYLOGENETIC APPROACHES TO UNDERSTANDING THE LATE ORDOVICIAN MASS EXTINCTION AND EARLY SILURIAN RECOVERY
Open Access
- Author:
- Krug, Andrew Zachary
- Graduate Program:
- Geosciences
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- March 16, 2006
- Committee Members:
- Mark E Patzkowsky, Committee Chair/Co-Chair
Timothy Bralower, Committee Member
Christopher Howard House, Committee Member
Peter Daniel Wilf, Committee Member
Alan Walker, Committee Member - Keywords:
- recovery
mass extinction
Late Ordovician
diversity - Abstract:
- The rules governing the accumulation and depletion of diversity vary at different geographic scales. Because of the spatially complex nature of the global ecosystem, understanding major macroevolutionary events requires an understanding of the processes that control diversity and turnover at a variety of geographic and temporal scales. This is of particular importance in the study of mass extinction events, which can eliminate established evolutionary lineages and ecologically dominant taxa, setting the stage for post-extinction radiation of previously obscure or minor lineages. If the macroevolutionary consequences of extinctions and recoveries are to be understood and predicted, the spatial and temporal variations in diversity and turnover must be quantified and the processes underlying these patterns dissected. Here, I analyze regional diversity and turnover patterns spanning the Late Ordovician mass extinction and Early Silurian recovery using a database of genus occurrences for inarticulate and articulate brachiopods, bivalves, anthozoans and trilobites. Chapter 2 compares sampling standardized diversity and turnover trends for the paleocontinent of Laurentia to the global pattern derived from genus first and last appearances. After accounting for variation in sampling intensity, we find that marine benthic diversity in Laurentia recovered to pre-extinction levels within 5 Myr, which is nearly 15 Myr sooner than suggested by global compilations. The rapid turnover in Laurentia suggests that processes such as immigration may be particularly important in the recovery of regional ecosystems from environmental perturbations. Chapter 3 explores variability in the dynamics of recovery at the regional scale, by expanding the database both by doubling the number of occurrences for Laurentia and including data from Baltica and Avalonia. These data show that sampling standardized diversity trends for the three regions are variable. Despite the expansion of the database, diversity continues to rebound to pre-extinction levels within 5 Myr of the extinction event in the paleocontinent of Laurentia. However, diversity in Baltica and Avalonia requires 15 Myr or longer to reach pre-extinction levels. This increased rate of recovery in Laurentia is due to both lower Late Ordovician extinction intensities and higher Early Silurian origination rates relative to the other continents. Using brachiopod data, the Rhuddanian recovery was dissected into genus origination and invasion. This analysis reveals that standing diversity in the Rhuddanian of Laurentia consists of a higher proportion of invading taxa than in either Baltica or Avalonia, indicating invading taxa were prominent in driving the rapid Laurentian rebound. However, when invading taxa are excluded from diversity counts, Laurentian diversity still rebounds to pre-extinction levels within 10 Myr of the extinction event, suggesting genus origination rates were also higher in Laurentia than in either Baltica or Avalonia. Higher rates of origination in Laurentia may be expected due to its large size, paleogeographic location, and environmental make-up. Hypotheses explaining the increased levels of invasion into Laurentia remain largely untested and require further scrutiny. In chapter 4, a phylogenetic analysis of strophomenid brachiopods was performed to explore the decoupling of ecological, evolutionary, and taxonomic severity resulting from the Late Ordovician mass extinction. After calibrating the cladogram to the fossil record of strophomenids, the stratigraphic ranges of genera were adjusted and ghost lineages were added where applicable. Many genera sampled only from the Silurian had their ranges extended into the Ordovician after phylogenetic correction. Using lineages instead of genera resulted in only minor changes to counts of diversity and origination rates. Lineage terminations were accurately recorded using genera, though proportional extinction patterns were altered due to the addition of Ashgillian lineages through the backwards extension of Silurian lineage ranges. No monophyla were eliminated in the Ashgill, and surviving lineages were derived from throughout the phylogenetic tree. This enabled strophomenids to diversify and fill many of the same ecological niches as they had prior to the extinction event.