Early Mississippian Crinoids and the Flowering of a
Paleontologist
Forest Gahn
(Department of Geology, Brigham Young University at Idaho, Rexburg, Idaho, USA)
Dry Dredgers meeting (Cincinnati, Ohio, USA)
20 November 1998
Crinoids
- there are ~615 living species today, and only ~100 of which are
stalked. The rest are stalkless (the comatulids). Comatulid
crinoids do have stalks as juveniles, but they are lost with ontogeny.
Wachsmuth
& Springer - North American Crinoidea Camerata - a monumental work
with 1000 pages and 87 plates.
A
diverse set of crinoids exists in the Early Mississippian Burlington Limestone
of Iowa - it is the immensest deposit for crinoid diversity in the whole
geologic record.
Examples of crinoids from the Burlington Limestone:
1)
Eretmocrinus
- has flared/flattened arm tips - these are older forms than those from the Ft.
Payne Formation of Kentucky;
2) Actinocrinites - with deeply lobed top of calyx;
3)
Scrotocrinus - with a flared, umbrella-like structure at top of calyx;
4)
Platycrinites
(and
see) - with a twisted stem and large radial plates;
5)
Eucladocrinus
- also with a twisted stem, but columnals are very narrow;
6) Agaricocrinus.
In
the modern, fish like to nip off crinoid arms. In the past, this may also
have happened with shell-crushing sharks eating off crowns. Crinoids
started to evolve spines to thwart such predation (as the “fish-hook” crown of Dorycrinus). There
has been a suggestion that there is a correlation through time between an
increase in predation and the development in crinoids of anti-predator
morphological structures.
We’ve
been looking at camerate crinoids up to now.
Now
the inadunates, which has 2 groups - the cladids (has 2 circlets of plates
below the radials) and the disparids (has 1 circlet of plates below the
radials).
The
Burlington Limestone has species of the disparid Synbathrocrinus
and Halysiocrinus
(and see) the
latter had a stem parallel to the seafloor, with arms bent upward into the
water.
The
Burlington also has species of the cladids Celiocrinus, Springercrinus,
Hypsilocrinus, etc.
Springer
(1920) - The Crinoidea Flexibilia.
The
flexibles have tightly-plated, thin-plated calices and divets or grooves at the
plate margins. Burlington flexibles include Taxocrinus,
Wachsmuthocrinus, and Perithiocrinus.
Gahn
has also looked at Burlington-aged rocks in northwestern Montana, just south of
Glacier National Park, and collected nice crinoids and blastoids. Gahn
also looked at Ireland, especially Hook Head, which has the same groups of
crinoids.
Current research -
looking at the Wassonville Formation, which occurs stratigraphically right
below the Burlington Limestone in Iowa. The Burlington contains ~300
species of crinoids. Why was there such diversity then? Answering
this question was the reason for looking below the Burlington at the
Wassonville in southeastern Iowa. The Wassonville is the equivalent of a
crinoid-rich horizon from central Iowa (the Legrande beds).
Collected
Wassonville Formation crinoids from a quarry, and had sparse luck. Then,
got to collect in a new quarry and found 150 crinoids in 1 day. Since,
lots more crinoids have been found. There are two productive crinoid
zones in the Wassonville - a lower zone near the base of the formation, and an
upper zone near the middle of the formation.
The
lower zone was deposited in a high-energy, near-shore environment. The
upper zone was deposited in quieter water, near the lower limit of storm wave
base. This deeper water environment has very finely laminated mudstones,
with crinoids in it. These crinoids were deposited very rapidly, by
storms. They preservation is so high that cirri are preserved intact on
the stem. Also found echinoids with in-place spines. This deeper
water fauna is dominated by cladids with delicate morphology. There are
also some camerates and ophiuroids and asteroids.
The
lower zone was a higher-energy environment, with cross-bedding and mud clasts -
all storm sedimentation. There is a distinct crinoid fauna copmared with
the deeper fauna. Still, the preservation is good. Crinoid fauna is
dominated by cladids. The cladids from the deeper water zone are more
delicate than in the shallow water environments. Also finding flexibles
in the high-energy environment, but they are absent from the low-energy
environment.
Southeastern
Iowa - a middle shelf setting. Legrande, Iowa (central Iowa) - an inner
shelf setting. The Legrande crinoids are truly phenomenal there.
Species can be consistently identified based on color patterns alone (including
varying coloration - dark stem & basal calyx and whitish upper calyx in one
species). Legrande crinoids are on display at Beloit College, in
Wisconsin.
In
terms of species and generic diversity, the higher energy, middle shelf
environment shows the highest diversity. The low energy middle shelf
environment has less diversity, and the high energy inner shelf environment
shows the lowest diversity, probably due to fluctuating temperature and
salinity conditions. Can see onshore-offshore trends in abundance and
diversity data for the different crinoid groups.
Crinoids
with the most densely pinnulated arms are most abundant in high-energy inner
shelf environments. Paired pores (interpreted as sensory) next to arm
facets developed in many camerates in the Mississippian.
Belemnocrinus -
previously only known from the Burlington Limestone, and was very rare there
(only about 7 specimens). Now, this crinoid is known (rather commonly) in
the older Wassonville Formation.
There
are no encrusters on Wassonville Formation crinoids. There are a few on
Burlington Limestone crinoids. One platycerid gastropod has been found on
a crinoid by Gahn in all the years of collecting Burlington Limestone crinoids.
There is a specimen known
of a crinoid stem wrapped around 2 blastoids, like a twisty tie.
