There
are more than 60 species of crinoids in 40-plus genera found
in the Crawfordsville area, with all major groups of Lower
Mississippian crinoids
represented: Cladids, Camerates, Disparids, and Flexibles.
What is most noteworthy about the Crawfordsville crinoids
is the size of the calices, with some approaching and even
exceeding 10 cm in length, rendering them most appealing to
collectors. The most important aspect of this fauna is the
amazing diversity of well-preserved specimens found in exquisite
3-D relief. While the sheer diversity alone is astonishing,
the fact that many complete crowns with attached stems are
found is a further attraction. Nearly half of all species
are known from complete specimens. This Lagerstatt site provides
rich opportunity for researchers to formulate observations
about the morphology, ecology, and behavior of the crinoids
of Late Mississippian age.
The
first crinoid calyx collected from the Crawfordsville, Indiana
area was by 9 year old Horace Hovey in 1842, who was collecting
"encrinites" along the banks of Sugar Creek for
sale. The magnificently-preserved specimens have been sought
after ever since by scholars and collectors alike.
While crinoids are found in a number of locations in the Crawfordsville
area, the most abundant beds are those of Corey's Bluff and
along Indian Creek, both of which are currently thought to
be within the Edwardsville Formation (approximately 340 million
years old). The Edwardsville Formation is dated as Late Osagean
Stage in central and southern Indiana.
The
crinoids are found with differing stem lengths, allowing each
to find its own feeding niche in the water column, using its
filter-feeding apparatus to strain the food contained within
its target stratum. Additionally, some crinoids with similar
length stems were further subdivided according to the size of
prey that the species targeted. In these ways, the amazing diversity
could be maintained while minimizing competition between species,
a good example of the workings of evolutional adaptation. Such
a method of tiering has subsequently been found to be an important
aspect of many marine ecosystems.
Seafloor
conditions in the Crawfordsville area were obviously vary favorable
to the proliferation of crinoids, an environment that had both
shallow water conditions and an influx of silt from a neighboring
delta. The crinoids, living in high densities in their tired
habitat were periodically buried alive by storm-generated slumping
or silt flows. Such tempestites or turbidites must be of sufficient
depth to prevent later re-excavation. Collectors are fortunate
that the resultant siltstone deposits of Crawfordville are sufficiently
soft to allow microabrasive preparation techniques to expose
the crinoids in all their past exquisite details, affording
the remarkable specimens such as shown in this fossil
images gallery.