A crinoid from the Pennsylvanian Essex fauna of Illinois

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FIELDIANA . GEOLOGY

Volume 12, No. 9 March 24, 1969 Publication 1066

A Crinoid from the
Pennsylvanian Essex Fauna of Illinois

N. Gary Lane
AssocjATE Professor of Geology, University of California at Los Angeles

CRINOIDEA

The only crinoid crown so far discovered from the Middle Penn-
sylvanian concretions of the Francis Creek shale of the Mazon Creek
area, Illinois, is described and illustrated. The specimen is not as-
signed to family, genus or species because it is immature; its arms
branch on the third primibrach, which is unusual for a Pennsylvanian
inadunate crinoid; and uncertainty exists that the posterior side of
the dorsal cup is exposed. The pathway of the aboral nervous sys-
tem can be traced within some of the arm and cup plates. Long
slender cirri that are proximally directed toward the crown may in-
dicate an epiplanktonic mode of life.

Class Crinoidea Miller, 1821

Sub-class Inadunata Wachsmuth and Springer, 1897

Order Cladida Moore and Laudon, 1943

Sub-order Poteriocrinitina Jaekel, 1918 '

Family, Genus, and Species Unknown '

Figures 81- 83

Description. — Crown small, explanate; dorsal cup low cone-shaped,
wider than high, with straight sides. Infrabasals 5, low, relatively
large, visible in side view distally and flat proximally; basals spear-
shaped, small, wider than high, with narrow interbasal sutures; radials
large, wider than high, with distinct notches between distal edges of
adjacent radials; radial facets distinctly narrower than radials, slop-
ing slightly outward; either the posterior side of the cup is buried in
matrix or a minute anal plate is preserved almost out of the cup be-
tween distal edges of two radials. Arms uniserial, branching isotom-

Library of Congress Catalog Card Number: 68-59026

151

152

FIELDIANA: GEOLOGY, VOLUME 12

Fig. 81, Line drawing of crinoid specimen from Mazon Creek area, Illinois.
X? indicates questionable anal plate, and dotted lines indicate observed trace with-
in cup and arm plates of aboral nerves. The stem has been dislocated from its
attachment at the base of the cup.

ously at least one time on primibrach 3; three secundibrachs above
premaxil preserved in most complete branch. Column round, lumen
round, columnals high, with long slender, round, proximally directed
cirri preserved on every second columnal below cup.

The single known specimen, consisting of a split nodule with plate
and counterplate of the crinoid specimen, was collected from the Es-
sex nodule locality (Johnson and Richardson, 1966) by Mr. David R.
Cooper of Evergreen Park, Illinois. Mr. Cooper has generously de-
posited the specimen in Field Museum of Natural History. It is speci-
men No. PE 13946. I am grateful to both Mr. Cooper and Dr. E.
S. Richardson, Jr. of Field Museum for making the specimen available.

LANE: CRINOID OF PENNSYLVANIAN ESSEX FAUNA 153

Fig. 82. Crinoid specimen in concretion, X 2.

Measurements. — Height of dorsal cup, 2.5 mm.; width of dorsal
cup, 3.8 mm,; width of infrabasal circlet, 1.6 mm.; width of column
just below cup, 0.8 mm. ; distance from top of radial to tip of axillary
primibrach 3, 2.5 mm.; width of primibrach 1, 1.3 mm.; exposed
length of column, 8 mm.; length of longest exposed cirrus, 11 mm.

Remarks. — There are three reasons for hesitation in assigning this
crinoid to a family, genus or species. On the left side of the exposed
part of the dorsal cup there is a small, triangular portion of replaced
ossicle that is separated from adjacent radial plates on either side by
a narrow band of white mineral, like all other discrete plates of the
specimen (x? in Fig. 81). If this small piece is an anal plate, it has
been almost completely eliminated from the cup and records an evo-
lutionarily advanced arrangement of anal plates in the cup. There
is a possibility that this small piece is formed by a filled-in crack in
one of the radial plates, and is therefore fortuitous, and that the pos-
terior side of the cup is not exposed. The specimen is too small, the
replaced ossicles too soft and delicate, and the matrix too hard to
permit excavation completely around the cup.

The second uncertainty contributing to lack of systematic place-
ment of the specimen below suborder is the presence of three primi-
brachs in each of the exposed arms. The vast majority of Pennsyl-
vanian inadunate crinoids have either one or two primibrachs, or the

154

FIELDIANA: GEOLOGY, VOLUME 12

■i1

Fig. 83. Crinoid specimen in concretion, X 8.4.

arms are unbranched. Only groups like the relatively primitive Po-
teriocrinitidae and some genera of the Blothrocrinidae have more than
two primibrachs to a ray. These forms typically have three anal
plates in the cup and are much more common in, and characteristic
of, pre-Pennsylvanian crinoid faunas. Consequently, if the Essex
crinoid does have a single,^small anal plate, it could not be satisfac-
torily assigned to either of these families.

LANE: CRINOID OF PENNSYLVANIAN ESSEX FAUNA 155

Finally, the specimen may be immature and therefore cannot be
reliably assigned to an already-named genus based on adult charac-
ters. For instance, if the specimen is immature, the three primi-
brachs might fuse into one or two plates with continued growth,
which would result in an entirely different familial and generic as-
signment. For these reasons the specimen is not assigned to rank
below suborder Poteriocrinitina.

Preservation. — All ossicles of this crinoid are replaced by brown
ferruginous material internally, with narrow layers of soft white kao-
lin (E. S. Richardson, Jr., pers. comm., Jan. 13, 1968) along all plate
sutures (Fig. 83) . The external surfaces of all plates presumably were
also formed of this material that has been obliterated, so that pres-
ence or absence of external plate ornament cannot be determined.

The primibrachs exhibit a distinct longitudinal median line that
extends from the proximal edge of primibrach 1 to the center of axil-
lary primibrach 3, where the line divides and extends upward to
adjacent secundibrachs. One radial plate has a similar line extending
from the center of the radial-primibrach suture two-thirds of the way
down the radial plate where it divides, one branch extending to each
of the subjacent basals. These lines preserved in the interior of cup
and arm plates record the course of principal nerves of the aboral or
entoneural system of the crinoid (Hyman, 1955, p. 62).

Paleoecology. — One of the rather unusual aspects of this specimen
is the presence of long, proximally directed cirri just below the crown.
The majority of Paleozoic crinoids either have cirri confined to the
distal root system or have laterally- or distally-directed cirri close to
the crown on a pentagonal stem. A similar configuration of proxi-
mally-directed cirri has been observed in a few other unrelated cri-
noids, such as the Mississippian monobathrid camerate Dichocrinus
oblongus Wachsmuth and Springer (Springer, 1926), and the cladid in
adunate Goniocrinus harrisi (Miller) (Van Sant and Lane, 1964), and
has also been observed by this author in an undescribed Devonian
species of the inadunate family Gasterocomidae. In the latter two
examples the cirri are sufficiently long and abundant to have par-
tially hidden the crown, which may have had some advantage as
camouflage from predators.

All of these crinoids, including the Essex specimen, have relatively
small and lightly constructed crowns. It is possible that this con-
vergent pattern of cirral arrangement may be the result of a similar
life habit — these crinoids may have been epiplanktonic, attached by

156 FIELDIANA: GEOLOGY, VOLUME 12

distal cirri to floating seaweed or logs, and hanging upside down or
obliquely downward in the water. Cirral growth near the crown may
have then been geotropic in nature, resulting in proximally-directed
cirri.

REFERENCES

Hyman, L. H.

1955. The Invertebrates: Echinodermata. The coelomate Bilateria, 4, McGraw-
Hill Book Co., New York. 746 pp.

Johnson, R. G. and E. S. Richardson, Jr.

1966. A remarkable Pennsylvanian fauna from the Mazon Creek area, Illinois.
Jour. Geol., 74, pp. 626-631.

Springer, Frank

1926. Unusual fossil crinoids. Proc. U. S. Nat. Mus., 67, pp. 1-137, pis. 1-26.

Van Sant, J. F. and N. G. Lane

1964. Crawfordsville (Indiana) Crinoid Studies. Univ. Kansas Paleontol.
Contr., Echinodermata, Art. 7, pp. 1-136, pis. 1-8.

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