Florissant - Fossil Ecosystems of North America: A Guide to the Sites and their Extraordinary Biotas - page 219

C OMPARISON OF F LORISSANT
WITH OTHER E OCENE LAKE SITES
Green River (Chapter 11), of middle
Eocene age ( c . 50 Ma), is older than
Florissant which is late Eocene ( c . 34 Ma)
in age. However, they share some
similarites in flora. About 41% of the plant
species are loosely related at the two sites
(MacGinitie, 1969); e.g. Sequoia , Populus ,
Quercus , Cedrelospermum , Ailanthus , and
Koelreuteria . However, there are tropical
species at Green River which do not occur
at Florissant. Since both are in the same,
Rocky Mountain, area, it is quite likely that
the Florissant biota was derived from
Green River biota.
The John Day Fossil Beds National
Monument in Oregon represents a
lacustrine deposit which formed in the
early Oligocene ( c . 32-33 Ma), at a much
lower elevation than Florissant. At the
Eocene-Oligocene transition the world's
climate cooled, possibly as a result of
changing ocean current circulations

triggered by plate tectonic movements.
Insects are rare in the John Day beds, and
the Bridge Creek flora of the John Day
Formation shows similarities with a
temperate broad-leaved forest. Similarities
with Florissant include To r r e y a , Abies ,
Pinus , Sequoia , Mahonia , Quercus ,
Florissantia , and Cedrelospermum . The
Creede flora of Colorado, late Oligocene
( c . 27 Ma) was formed in a caldera lake. By
this time, elevation of the Rockies was well
under way which, in conjunction with the
Oligocene cooling, produced a cool
temperate forest ecosystem at Creede.
Abies , Picea , Pinus , Cercocarpus , and
Mahonia occur, as they do in this region
today, together with Juniperus . Most of
these were present at Florissant, so it is
possible they evolved to withstand the
cooling climate. Some genera are able to
cope with changing environments while
others, such as the giant brontotheres or
the widespread Eocene beech genus
Fagopsis , could not.

F URTHER R EADING
Brues, C. T. 1906. Fossil parasitic and
phytophagous Hymenoptera from
Florissant, Colorado. Bulletin of the
American Museum of Natural History
22 , 491-498.
Brues, C. T. 1908. New phytophagous
Hymenoptera from the Tertiary of
Florissant, Colorado. Bulletin of
the Museum of Comparative Zoology
at Harvard College 51 , 259-276.
Brues, C. T. 1910. The parasitic
Hymenoptera of the Tertiary of
Florissant, Colorado. Bulletin of the
Museum of Comparative Zoology at
Harvard College 54 , 1-125.
Carpenter, F. M. 1930. The fossil ants of
North America. Bulletin of the
Museum of Comparative Zoology at
Harvard College 70 , 1-66.

Carpenter, F. M. 1992. Superclass
Hexapoda. In: Treatise on Invertebrate
Paleontology. Part R, Arthropoda 4,
volumes 3 and 4 . Kaesler, R. L. (ed.).
Geological Society of America, and
University of Kansas, Boulder, Colorado
and Lawrence, Kansas.
Cope, E. D. 1875. On the fishes of the
Tertiary shales of the South Park.
Bulletin of the United States
Geological and Geographical Survey
of the Territories 1 , 3-5.
Cope, E. D. 1878. Descriptions of fishes
from the Cretaceous and Tertiary
deposits west of the Mississippi river.
Bulletin of the United States
Geological and Geographical Survey
of the Territories 4 , 67-77.