Daniel Goldman, University of Dayton
This is a final report for this project. The principal results are listed below:
1) Bedding plane co-occurrence of graptolites and conodonts in Ordovician dark shale sequences:
Bedding
plane co-occurrence of biostratigraphically useful conodonts and graptolites in Ordovician shale sequences
enhances the overall correlation precision between platform and deep water
successions. Darriwilian shale successions in Tarim, western China, and Alabama and Idaho in North
America contain the key conodont zonal indicator
species Pygodus anitae,
P. serra, and P. anserinus
(as well as more long-ranging taxa) on bedding planes
with Pterograptus elegans
to Nemagraptus gracilis
Zone graptolites. Three of the Pygodus
bedding plane associations appear to be partial conodont
apparatuses. The occurrence of bedding plane conodonts
with graptolites across the Sandbian-Katian boundary
at Black Knob Ridge ( New collections across the Sandbian-Katian succession at the Hartfell
Score section near 2) High-resolution stratigraphic
correlation and Biodiversity Dynamicsin Middle and Late
Ordovician Marine Fossils From Baltoscandia: During
the early Late Ordovician there was a significant decline in marine
biodiversity that has been variously attributed to sea level, facies, and climatic changes. In the East Baltic area
several workers have described such a marked diversity decline and faunal
turnover in microfossils at the Keila - Oandu Stage boundary, an event
called the Oandu Crisis. To get a better
understanding of microfossil diversity dynamics in the Baltoscandian
Middle and Upper Ordovician succession we used constrained optimization
(CONOP9) to construct a composite range chart from the stratigraphic
data of 505 chitinozoan, conodont,
ostracod, and graptolite species from 20 boreholes
and 8 outcrops. We employed the CONOP composite as a timescale in which to
calculate biodiversity, extinction, and origination rates through the Middle
and Late Ordovician. Traditional
biodiversity metrics, and more recent probabilistic methods based on
capture-mark-recapture analysis, were used to estimate biodiversity and fossil
recovery patterns. We divided the CONOP composite into 860 Kyr
intervals spanning the Lasnamägi through Porkuni stages. Our data show that overall biodiversity
increased steadily from the beginning of the Keila to
the middle Rakvere stages, mainly due to an increase
in ostracod diversity. Chitinozoan
diversity reached a zenith in the late Keila, dropped
through the Oandu Stage, and then gradually declined
during the rest of the Ordovician. Chitinozoans
exhibited constant origination but variable extinction rates and underwent a
dramatic diversity decline associated with the d13C isotope excursion
known as the GICE event. Conodonts had diversity
peaks in the early Uhaku and early Kukruse stages, and then declined gradually through the
Late Ordovician. In this interval conodonts exhibited
constant extinction and origination rates and their diversity decline is
attributable to depressed origination rates. Interestingly, the fossil
preservation and recovery rate was highly variable and appears to exert a
strong influence on the observed biodiversity pattern. 3) A Composite Taxon Range
Chart and Conodont Biodiversity Dynamics from the
Ordovician of Baltoscandia Both CONOP 9 and a new method called Horizon Annealing were used to
construct composite range charts from the stratigraphic
range data of 159 Ordovician conodont species in 24
boreholes and outcrops in Baltoscandia. We converted
the composite sections to timescales in which to calculate biodiversity,
extinction, and origination rates through the Early, Middle and Late
Ordovician. The two methods produced broadly similar range charts and diversity
curves that differed in small but interesting ways. We divided the composites
into 1.15 my intervals (a temporal resolution twice that of the median zone
duration) spanning the Paltodus
deltifer through
Amorphognathus
ordovicicus
conodont zones Our data show that overall biodiversity increases steadily from the base of the P. deltifer Zone to the base of the E. suecicus Zone, and then steeply
declines throughout the remainder of the Ordovician. Interestingly, the start
of this decline is coincident with the mid-Darriwilian
(Kunda) regression and d13C isotope excursion.
Extinction rates climb steadily through the Early Ordovician, fluctuate around
higher values during much of the Middle Ordovician, before reaching a peak low
in the E. suecicus Zone. Extinction rates
then drop again to pre-E. suecicus
Zone values for the remainder of the Ordovician. Origination rates are very low
across the Billingen-Volkhov boundary (base of the Dapingian) and climb to a peak in the late Lenodus variabilis Zone.
Origination rates crash in the E. suecicus Zone and
remain low until the late A. tvaerensis Zone
when they begin to slowly rise again. Thus, the dramatic late Middle and Late
Ordovician drop in conodont diversity in Baltoscandia appears to be attributable to depressed
origination. Uniting these three projects is the fact that having direct
co-existences of graptolites and conodonts allows the
CONOP program to directly integrate sections that represent different biofacies. Thus, the bedding plane assemblages of taxa that generally occur in different biofacies
provide firm ties that CONOP can use to help solve correlation problems.
Copyright © American Chemical Society