Carl
Brett
Evolutionary Ecology and Coordinated Stasis of Devonian Benthic Faunas in
the Appalachian Basin
(funded by NSF EAR 9219807, with Niles Eldredge)
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This is a large scale study of morphologic stasis and change
in marine benthic fossil assemblages. Current research involves detailed
study of numerous lineages of Devonian brachiopods, trilobites, and other
fossil invertebrates and problems of community level stability and change.
The basic hypothesis being tested in these studies are: 1) that a majority
of invertebrate lineages display little morphological change for extended
periods;2) that biofacies of which these species are a part are likewise
very stable in terms of diversity, guild structure, and species composition;
3) that extended blocks of stability (ecological- evolutionary sub-units)
are terminated by abrupt episodes of local extinction, rapid change in species,
immigration, and community restructuring.
Results of this research to date include the following:
A) Identification of broad ecological-evolutionary subunits in the Devonian:
Based on literature survey and field observations, Brett and Baird (1995)
have identified six ecological-evolutionary (E-E) subunits in the Lower
to Middle Devonian of New York and Pennsylvania; within the Eifelian-Givetian
interval, they have defined the "Onondaga", "Stony Hollow",
"Hamilton-Tully", and "Genesee" faunas, each bounded
by short-lived faunal crises and immigration events that appear to correlate
with global bioevents. Detailed documentation has focused on the Hamilton-Tully
E-E subunit as a test case (Brett and Baird, 1995; Brett et al., 1996).
B) Establishment of a detailed sequence stratigraphic framework: Middle
Devonian strata in Ontario, New York, and Pennsylvania have been correlated
as precisely as possible in terms of biostratigraphy and cyclostratigraphy
(see sequence stratigraphy of Middle Paleozoic Strata). This stratigraphic
framework has helped to define the boundaries and duration of the "Hamilton-Tully"
E-E subunit. Durations of depositional sequences (1-3 Ma), sub-sequences
(~100 Ka) and smaller scale cycles have been calibrated with a newly developed
absolute time scale for the Givetian Stage and its component conodont
zones (House, 1995). Working within this framework, the lower and upper
boundaries of the "Hamilton-Tully" fauna (or E-E subunits) have
been resolved within single small scale (~100 Ka) cycles, and are found
to correlate, respectively, with the previously recognized upper Kacak
and Pharciceras global bioevents (Brett and Baird, 1995); fossil data
obtained from beds adjacent to the faunal boundary indicate that very
few species were shared between the adjacent faunas.
C) Morphometric study of species lineages: The purpose of this ongoing
research is to test for an apparent pattern of shared stasis. Detailed
examination of 25 concurrent brachiopod and trilobite lineages indicates
near stasis for most species but with minor, non-directional anagenetic
change (Lieberman et al. 1994, 1995).
D) Documentation of ecological characteristics of biofacies within the
E-E-subunit: Approximately 500 bulk samples, with more than 100,000 fossils
belonging to about 150 taxa have been extracted, and counted. These samples
provide quantitative faunal data in eight biofacies for some 120 horizons
and intervals covering the full range of the Hamilton Group and Tully
Limestone, in western-central New York State and central Pennsylvania.
These data indicate the persistence of 60 to over 80% of species through
the total range of the Hamilton fauna; each biofacies retain nearly identical
species richness, species lists and rank abundance of abundant species.
These results document a pattern of stasis within biofacies and faunal
gradients for 5-6 My.
On the basis of these observations Brett and colleagues have proposed
the concept of coordinated stasis as a possibly general pattern in the
history of life (see Brett et al. 1996), and have considered possible
ecological-evolutionary mechanisms responsible for this pattern (Morris
et al. 1995; Brett et al. 1996). A next phase of our research will involve
systematic analysis of stratigraphic sedimentologic and geochemical (including
stable isotopic) patterns associated with major faunal turnovers.
In 2001 and 2002 work
with Gordon Baird and graduate students Jocelyn Sessa and James Bonneli
extended this research with discovery and documentation of the highest
known Hamilton beds in the area of Shedds, NY. These beds are virtually
conformable with the basal Tully units (this contact is disconformable
nearly everyhere else in the basin). Despite the conformable relationship
of the upper Hamilton with the Tully at Shedds, the faunal transition
remains sharp. The highest Hamilton “Shedds beds” carry a
diverse and typical Hamilton fauna, which gives way abruptly to the unusual
lower Tully (Emmanuella-Hypothyridina) fauna across a single bed.
Together with our colleagues Brad Sageman and Adam Murphy (Northwestern
University), Rex Crick (University of Texas at Arlington) and Brooks Elwood
(Louisiana State University), we sampled the uppermost Hamilton and complete
Tully interval in central New York, at decimeter intervals, for geochemical
and magnetic susceptibility (MS) analyses.. In conjunction with work already
completed on the AKZO drill cores, these analyses will provide a nearly
unbroken record of geochemical and MS fluctuations through the late Eifelian
through early Famennian in New York State. The intent is to correlate
this physical and geochemical record with the patterns of faunal stability
and change. Preliminary work shows that all Middle and Late Devonian faunal
overturns are associated with intervals of widespread deposition of organic
rich sediments associated with probable productivity events (Brett et
al., 1999). |
References
Brett, C. E. 1995. Stasis: Life in the Balance. Geotimes. 40 (3): 18-20.
Brett, C. E. and Baird, G. C. 1995. Coordinated stasis and evolutionary
ecology of Silurian-Devonian faunas in the Appalachian Basin. In Erwin,
D. H. and Anstey, R. L., eds., New Approaches to Speciation in the Fossil
Record. Columbia University Press, New York, p. 285-315.
Brett, C. E., Ivany, L. and Schopf, K. 1996. Coordinated stasis: An overview.
Palaeogeography, Palaeoclimatology, Palaeoecology, v. 125.
Brett, C. E., Miller, K. B., and Baird, G. C. 1990. A temporal hierarchy
of paleoecologic processes within a Middle Devonian epeiric sea. In Miller,
W., III, ed., Paleocommunity Temporal Dynamics: The Long-Term Development
of Multispecies Assemblages. Paleontological Society Spec. Pub. 5, 178-209.
Lafferty, A.G., Miller, A. I. and Brett, C. E. 1994. Comparative spatial
variability in faunal composition along two Middle Devonian paleoenvironmental
gradients. Palaios 9: 224-236.
Lieberman, B. S., Brett, C. E. and Eldredge, N. 1994. Patterns of stasis
in two species lineages of brachiopods from the Middle Devonian of New
York State. American Museum Novitates, no. 3114, 23 p.
Lieberman, B., Brett, C. E., and Eldredge, N. 1995. A study of stasis
and change in two lineages from the Middle Devonian of New York State.
Paleobiology 21: 15-27.
Miller, K. B., Brett, C. E. and Parsons, K. M., 1988, The paleoecologic
significance of storm-generated disturbance within a Middle Devonian muddy
epeiric sea. Palaios 3, 35-52.
Morris, P. M., Ivany, L., Schopf, K. and Brett, C. E. 1995. The challenge
of paleoecological stasis: reassessing sources of evolutionary stability.
Proceedings of the National Academy of Sciences, vol. 92, p. 11269-11273.
Morris, P. J. and Brett, C. E., 1996, Scale, biofacies stasis, and faunal
recurrence: comparative studies. In Repetski, J. E., ed., Sixth North
American Paleontological Convention, Abstracts of Papers, Paleontological
Society Special Publication no. 8, p. 283.
Ver Straeten, C.A. and Brett, C. E., 1996, The Kacak -otomari and associated
bioevents (late Eifelian-early Givetian) in the Appalachian basin: Relationship
to sequence and tectonic patterns. Geological Society of America, Abstracts
with Programs. v. 28, p. 107.
Brett, C.E., 1998, Sequence stratigraphy, paleoecology, and evolution:
biotic clues and responses to sea-level fluctuations. Palaios, v. 13,
p. 241-262.
Brett, C.E., and Baird, G.C. 1999, Flexural events and faunal changes
preceding the third Acadian Tectophase in the Late Givetian of the northern
Appalachian Basin region. Abstracts, Errachidia Meeting, SDS-IGCP 421,
p. 4.
Brett Sageman and Murphy (GSA Abstract-must find ref) |
