SCHECKLER 1, STEPHEN E.*, DONNA L. LINDSAY POSTNIKOFF 2, AND ERIC J. CHAMEROY 1. 1. Department of Biology, Virginia Polytechnic Institute & State University, Blacksburg, VA 24061-0406 and , 2. Department of Geological Sciences, University of Saskatchewan, Saskatoon, SK, CANADA S7N 5E2. - Late Devonian forests with the first large trees (Archaeopteris).
Aneurophyte progymnosperms are common shrubs of an early Late Devonian
(basal Frasnian Oneonta Fm.) flora from a quarry near Ashland, New
York. Floristic census of the marine bay-fill/prodelta deposits at the
lowest quarry bench shows that the regional flora was dominated by
aneurophytes (80-85%) with Tetraxylopteris the commonest of
these (plus Triloboxylon & Proteokalon). Lesser
components include cladoxylalean trees and vines
(Pseudosporochnus & Rhymokalon), the progymnosperm tree
Archaeopteris (Callixylon zalesskyi and A.
macilenta) and a few scarce lycopsids (Archaeosigillaria).
Delta progradation established vegetated floodplains with sandy
paleosols on this site within 4 m upsection. Farther upsection,
paleosols are commoner, thicker, and mud-dominated. The floor of the
uppermost quarry bench exposes about 1 ha of horizontal paleosol from
which we uncovered six small test plots that show stump bottom casts,
root penetration zones, and other depositional features of this
floodplain soil. Shrink/swell slickensides, many with root
penetrations, characterize this vertisol. XRD and SEM analyses show a
combined kaolinite/smectite signature, illite (derived from
smectite?), and an absence of caliche carbonate, which suggest a
savanna-like paleoclimate. Larger stumps are more widely spaced while
smaller root zones are closer and clustered farther away from the
larger stumps. The ratio of large to small stumps/root masses
corresponds to our gross floristic census and suggests that this
floodplain paleosol exposure is representative of the regional
vegetation of this time on the Catskill Delta. Except for
Archaeopteris, these were short-lived taxa, which implies
considerable successional turnover. This mixed community, was rapidly
re-organized, however, by the world-wide spread of
Archaeopteris, a large tree with a much branched, leafy, and
shade producing crown, and decline then extinction of aneurophytes,
small xeromorphic shrubs with high light tolerance. Nearby quarries of
slightly younger strata show greater domination by
Archaeopteris, which reaches nearly 95% of biomass by mid
Frasnian time.
Key words: Archaeopteris, Devonian, progymnosperm