Florissant - Fossil Ecosystems of North America: A Guide to the Sites and their Extraordinary Biotas - page 203

Mountain region, causing the Colorado
area to rise some 1500 m (5000 ft) above
their former levels. So mountains which
were 2750 m (9000 ft) high eventually
became the 'fourteeners' such as Pikes
Peak. Conversely, the fault-bounded
Arkansas River-San Luis-Rio Grande rift
valley, south of South Park ( 220 ), dropped
down and is now surrounded by immense
mountain ranges.

about 60 m (200 ft), so the fossils compare
well with the average size of modern
redwood trees. A spectacular stump, the
Redwood Trio ( 224 ), consists of three
interconnected trunks, leaving little doubt
that they represent a clone - effectively a
single tree with three trunks. Such trees
occur in redwood forests today.
Not all of the stumps belong to
redwoods; there are angiosperm tree
remains as well. They include woods similar
to the modern locust tree ( Robinia ), the
golden-rain tree ( Koelreuteria ), the
Caucasian elm ( Zelkova ), and a fossil genus
Chadronoxylon which shows affinities to a
number of modern families. The growth
rings of these trees are preserved. The
Redwood Trio has 500-700 rings, and the
angiosperm wood rings show features
which indicate they grew in a strongly
seasonal climate, which accords with the
evidence of annual algal blooms.
Fossils of a few more primitive plants
such as mosses, ferns and horsetails (also
known as cryptogams) have been found at
Florissant. Mosses are rarely preserved as
fossils, but one remarkably preserved
specimen of Plagiopodopsis cockerelliae
looks as though it has just fallen off a tree
branch into the lake. The modern fern
genus

D ESCRIPTION OF THE F LORISSANT
BIOTA
Plants
The diatoms have already been discussed.
These algae are common today in marine
and freshwaters, and whereas marine
diatoms are known from Jurassic times,
Florissant preserves some of the oldest
known freshwater diatoms. At the other
end of the size scale, the giant stumps at
Florissant have been identified as
belonging to the family Taxodiaceae
(now sometimes included in the
Cupressaceae), and the wood is similar to
that of the modern Sequoia , the coast
redwood of California. There is some
difference, however, so the fossil wood has
been given the name Sequoioxylon . The
largest stump measures 4.1 m (13.5 ft) in
diameter about 1 m (3.3 ft) above the
base. This size suggests a canopy height of

Dryopteris is represented, as is

the modern horsetail genus Equisetum

224

224 The Redwood Trio: three
separate trunks of Sequoia
arising from the same
rootstock. Such triplets occur
commonly today when new
trunks emerge after the death
of a single parent trunk.