Beechers Trilobite Bed - Fossil Ecosystems of North America: A Guide to the Sites and their Extraordinary Biotas - page 45

Beechers
Trilobite Bed

B ACKGROUND
Between the extraordinary biotas of the
Early - Middle Cambrian (such as Burgess
Shale, Chengjiang, Chapter 3) and the
beautifully preserved arthropods of the
late Silurian Bertie Waterlimes described
in Chapter 5, the fossil record shows few
major Lagerstätten. The Ordovician
Period is particularly barren of
exceptional biotas. However, one
Ordovician horizon, Beecher's Trilobite
Bed, part of the Upper Ordovician Utica
Basin shales of upper New York State ( 48 ),
has long been famous for its trilobites with
appendages exquisitely preserved in iron
pyrite (FeS 2 ). While there are many
occurences of fossils preserved in pyrite in
the fossil record, e.g. beautiful golden
goniatite shells in Coal Measure marine
bands of northern England, pyrite
preservation of soft-parts anatomy is
extremely rare. Another example of soft-
part preservation in pyrite is in the
Devonian Hunsrück Slate of Germany
(Bartels et al ., 1998; Selden and Nudds,
2004, Chapter 4).
As we saw in the last chapter, life in
Cambrian seas was remarkably different
from today. While probably most of the
animals in the Burgess Shale belong to
phyla which are still in existence, many
species did not extend past the end of the
Cambrian. The relative abundances of
phyla also differed greatly from a typical
marine scene today. The Cambrian sea

was rich in siliceous sponges (Porifera)
and inarticulate brachiopods with
phosphatic shells; priapulid worms
dominated the soft-bodied fauna - in
contrast, the polychaete annelids are the
most familiar worms today.
Jack Sepkoski of the University of
Chicago was the first scientist to identify a
distinct Cambrian Fauna (Sepkoski, 1979).
He noticed how after the end of the
Cambrian Period, many of the typical
animals of the Cambrian had either
became extinct or (mostly) persisted at
much lower diversity than before. Their
ecological niches had been taken over by
new forms. For example, inarticulate
brachiopods still persisted, even to this
day, but articulate brachiopods became
the dominant sea-floor shells; the sessile
fauna had been dominated by sponges,
but in the Ordovician Period corals and
echinoderms were outcompeting them.
Our main interest in this chapter is
with the trilobite arthropods. Two kinds
of trilobites were typical of Cambrian seas:
large, multisegmented redlichiids, and
tiny, two- or three-segmented agnostids.
The former lay flat on the sea bed while
the latter most likely belonged to the
plankton (floating forms). Other trilobite
orders emerged during the Cambrian,
but these two were present from the
beginning. Redlichiids did not extend
beyond the Cambrian, but the agnostids
continued to the end of the Ordovician