The Green River Formation - Fossil Ecosystems of North America: A Guide to the Sites and their Extraordinary Biotas - page 192

upper waters were smaller in size than in
the other lakes. The plant and insect beds,
alternating with the fish beds, suggest that
the lake repeatedly dried up and was
reflooded.
Fossil Lake was the smallest of the three
lakes, but was also the deepest and
supported the highest species diversity.
Bottom-dwelling fish are rare and benthic
crustaceans absent in the midlake 18-inch
layer, but rays and crayfish are common in
the near-shore split-fish layer (see section
on Stratigraphic setting and taphonomy,
p. 188), suggesting anoxic bottom waters
in the center of the lake, but well
oxygenated bottom waters near shore.
Those horizons, particularly below the
18-inch layer, containing abundant plants,
insects, and mollusks, suggest alternating
transgressive and regressive events
(Grande, 1984).
Finally, the lake margins and adjacent
rivers were home to frogs, turtles,
crocodiles, and alligators, while its shores
were visited by various waterfowl and
wading birds, often leaving their tracks in
the mud as they probed for food. On
land, a rich flora of palms and other large
trees and many smaller plants supported
a diverse insect and arachnid fauna,
which in turn formed the diet of lizards
and snakes, a variety of birds, and small
insectivorous mammals. A varied fauna of
larger mammals included hyena-like
carnivores, small ancestral horses, and
some enormous and bizarre forms such
as Uintatherium , a 6-horned, sabre-
toothed herbivore - evidence, for sure,
that the 'Age of the Mammals' had
arrived.

bounded basin within the Rhine Rift
Valley (see Selden and Nudds, 2004).
The mammal faunas correlate the
deposit to the Lutetian Stage of the
Lower Eocene and it is therefore coeval
with the Fossil Lake deposits of Green
River.
The major difference in the fauna
is that Grube Messel is best known
for its mammals, including rare
marsupials. Primitive placentals include
the insectivore-like mammals, early
hedgehog relatives, and early ungulates,
but more modern types are also
found such as rodents, horses, bats and
primates - comparable to those found at
Green River.
Messel birds all belong to modern
orders and include several found at Green
River, but strangely there are almost no
waterfowl, most being forest dwellers such
as owls, swifts, rollers, and woodpeckers.
Cranes, rails, and kingfishers also occur at
both sites, but peculiar to Messel is
Palaeotis , an ostrich-like paleognathan
which could represent an ancestor to
modern ratites.
The amphibian and reptile fauna
compares closely to Green River, with
frogs, crocodiles, and turtles (including
the soft-shelled Trionyx ) known from
both sites. Messel has also yielded rare
snakes including Boa and Python , in
contrast to the wood snake, Boavus , at
Green River.
Insects are much better preserved and
are dominated by beetles, often with
their elytra displaying iridescent colours
(Selden and Nudds, 2004, Figure 221).
Hymenoptera (ants, bees, and wasps)
and Heteroptera (bugs) are the next
most common, but most of the major
groups seen at Green River do occur,
although Diptera (true flies), Odonata
(dragonflies) and Lepidoptera (butterflies
and moths) are strangely rather rare.
Although nowhere near so numerous,
the Messel fish are just as diverse as at
Green River and include many identical
genera. Conversely, however, among the
most common fish at Messel are the more

C OMPARISON OF THE G REEN
R IVER F ORMATION WITH OTHER
E OCENE LAKE SITES
Grube Messel, Frankfurt am Main,
Germany
This World Heritage Site south of
Frankfurt in Germany was formerly a
brown coal open pit dug from 1875 until
the 1960s, and represents a lake system
formed in early Eocene times in a fault-