Gilboa - Fossil Ecosystems of North America: A Guide to the Sites and their Extraordinary Biotas - page 91

eventually killed the trees, their trunks
gradually died and snapped off, then
further influxes of sand filled the empty
moulds left by the decayed tree stumps.
The Brown Mountain strata belong to
the Panther Mountain Formation of the
Hamilton Group, of Givetian, upper
Middle Devonian, in age ( 91 ) (Sevon and
Woodrow, 1985). The Riverside Quarry
and Manorkill Falls fossil forests belong to
the slightly younger Moscow Formation of
the Hamilton Group (Banks et al ., 1985).
The still younger deposit at South
Mountain, belonging to the Oneonta
Formation, Genesee Group (latest
Givetian-earliest Frasnian) has also yielded
fossil fauna (Shear and Selden, 1995, 2001)
as well as abundant flora (Hueber, 1960;
Carluccio et al ., 1966; Hueber and Banks,
1979). In addition, there are many, many
small quarries and roadcuts around the
Catskills, mainly for shale to mend the dirt
roads in this area, which yields fossil plants.
The Hamilton Group sediments in
eastern New York consist of sandstones and
shales derived from erosion of a chain of
mountains to the east. The mountains were
rising and, simultaneously, being eroded,
due to the action of the Acadian Orogeny.
In Chapter 5, 67 , the top of the Silurian
sequence shows a marked unconformity,
with the oldest Devonian beds being laid
down a long time after the end of the
Silurian Period. At the end of the Silurian
Period, the microcontinent Avalonia
collided with the edge of the continent
Laurasia on which the present-day New
York lay. As subduction continued, so a
chain of mountains was created parallel to
the continental edge. The situation would
have resembled that which we see today
where the Indian tectonic plate continues
to attempt to subduct beneath the
Himalaya mountain belt, which is still
rising but also being deeply eroded at the
same time, producing a mass of sediment
which is pouring down into the Indian
Ocean, forming the mighty Ganges and
Brahmaputra river deltas. Similarly, as the
Acadian mountain chain was eroded, so
rivers draining to the west poured their

sediments into the marine basins of New
York, building up the vast Catskill delta.
Uplift periods were episodic, resulting in
cycles of shallow-marine limestones (e.g.
the Onondaga limestones, 91 ), followed by
deeper marine shales (e.g. the Marcellus
shales), then coarse clastics (e.g. the
Hamilton Group) as a delta lobe pushed
westward into the marine basin. Each
cycle was then interrupted by uplift
(unconformity) and then repeated (e.g.
Tully Limestones).

D ESCRIPTION OF THE G ILBOA
BIOTA
Plants
Zosterophyllopsida
These are the most primitive vascular plants
to be found in the New York Devonian, and
are characterized by lateral sporangia (i.e.
sporangia borne on the sides of the stem
(axis) rather than the tip, as in even more
primitive plants), and an elliptical vascular
strand (the cross-sectional shape of the
water-conducting tissue in the axis). By
Middle Devonian times, such forms were
beginning to become extinct as they were
outcompeted by more modern groups. The
best known zosterophyll in the New York
Devonian is Serrulacaulis (Hueber and
Banks, 1979), which occurs at South
Mountain. Specimens of Sawdonia
(formerly called Psilophyton ) have been
found at the same locality, but only sterile
axes, i.e. lacking the characteristic
sporangia (Hueber and Grierson, 1961).

Lycopsida
The commonest group of plants in the
Gilboa beds are the club-mosses or
lycopods (Lycopsida); together with
zosterophylls, they form the sister-group to
most other land plants. At the present day,
club-mosses are small, creeping forms with
erect fertile branches maximally 30-60 cm
(1-2 ft) high, e.g. the ground pines
( Lycopodium , Selaginella ) and ground
cedars ( Diphasiastrum , 92 ). Many of the
Gilboa lycopods were no bigger, but some
reached tree-sized proportions, and by
the Pennsylvanian Period they were the