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FIELDIANA - GEOLOGY

Published by
FIELD MUSEUM OF NATURAL HISTORY

Volume 16 May 26, 1970 No. 18

The Aboral Nervous System of
Marsupiocrinus Morris

Cane Cae AuIs

ASSISTANT PROFESSOR OF GEOLOGY, INDIANA UNIVERSITY NORTHWEST

Almost nothing is known about the nerve systems of fossil cri-
noids and little more about those of recent species. Hence any new
information, particularly from fossil crinoids, is of special interest.
This note describes an unusual structure which may have been part
of the aboral nerve system of a Silurian crinoid, Marsupiocrinus.
The crinoid specimen in question (FMNH P11945) comes from Ni-
agaran strata (probably the Brownsport Formation) of Decatur
County, Tennessee. The calyx is fairly complete although silicified.
Silicification has not completely destroyed the original meshwork
structure of the plates which can be detected in some areas when the
plates are wet. The plates are either deeply weathered or were split
irregularly when the specimen broke free of its matrix. In either
case the effect has been to reveal an unusual branching structure
which was entirely within the calyeal plates in life. The aboral posi-
tion of the structure within the calyeal plates suggests that it may
well have been part of the aboral nerve system. However, the struc-
ture is radically different from that of the aboral nerve systems of
recent crinoids.

I am grateful to Dr. Eugene S. Richardson, Jr. for the loan of the
specimen of Marswpiocrinus and to Drs. Bernd-Dietrich Erdtmann
and Thomas G. Perry for their opinions on the possible graptolite or
bryozoan affinities of the structure described here.

Description.— The calyx is bowl-shaped, about 40 mm. in diam-
eter and, despite the fracturing or weathering, outlines of three basal,
five radial, two inter-radial, and several brachial plates can be dis-
tinguished (cf. figs. 1, 4). The position of the azygous basal allows
the calyx to be oriented. In the A and B radii there is a delicate

Library of Congress Catalog Card Number: 78-122686

Publication 1098 461

GEOLOGY LIBRARY

Fic. 1. Stereo-photographs of Marsupiocrinus sp. (FMNH P11945) to show
supposed aboral nerve system, x 2. Lightly coated with ammonium chloride
sublimate.

162

PAUL: ABORAL NERVOUS SYSTEM OF MARSUPIOCRINUS 463

Fics. 2-3. Details of supposed aboral nerve system in Marsupiocrinus sp.
(FMNH P11945). Fig. 2, « 17.5; fig. 3, * 35. Both lightly coated with ammo-
nium chloride sublimate.

branching structure. It is incompletely exposed and cannot be de-
tected in other radii. As far as can be determined from the portion
preserved, there is a zig-zag main branch within the basal plate in
an inter-radial position. This zig-zag branch gives off lateral branches
to the A and B radii alternately. Within each radius the branches are
approximately parallel and they bifurcate a short distance from the
inter-radial branch. Each branch has a median ridge or keel and is
crenulate with short curved ridges or processes which alternate on
either side (figs. 2-8, 5). In several places narrow cross members
are preserved which connect adjacent branches and have narrow keels
(figs. 2, 5). There is a cross member developed for each five or six
pairs of curved processes along a branch and this gives the structure
a superficial resemblance to a fenestrate bryozoan or a dendroid grap-
tolite. Near the radial-basal sutures the structure is still buried
within the plates. In one place the structure can be traced within
the plates when the calyx is wet and there is no doubt that it is en-
tirely surrounded by the meshwork of the plates. In the radial plates
of the A and B radii further portions of the structure are exposed.
In the A radius there are several parallel branches all directed radi-
ally and five of which bifurcate before reaching the distal edge of the
preserved portion of the structure. Proximally, the branches are
broken off and there is no indication of the aboral nerve center nor
of the chambered organ.

464 FIELDIANA: GEOLOGY, VOLUME 16

Fic. 4. Outline drawing of plates of Marswpiocrinus sp. (FMNH P11945) to
show relationship of supposed aboral nerve system to plates. A-—E: radii.

The main branches of the structure reach 0.40-0.43 mm. in width
(including the curved processes); and the cross members 0.10-0.13
mm. There are 10-12 pairs of curved processes in 5 mm. along a
main branch. The preserved portion of the structure is 15 mm. long
from proximal to distal end in the A radius and 17 mm. in the B
radius.

Remarks.—-Despite its superficial resemblance to a dendroid grap-
tolite rhabdosome or a fenestrate bryozoan zoarium, the fossil struc-
ture in Marsupiocrinus differs in important details from either. In
particular, the mode of branching is unlike that of any fenestrate
bryozoan or dendroid graptolite. Among graptolites proper, only
Goniograptus has a similar structure of alternating branches but this
genus lacks anything to correspond to the cross members of the cri-
noid structure. The curved processes are not apertures such as
would appear on a bryozoan or graptolite and Goniograptus has aper-

PAUL: ABORAL NERVOUS SYSTEM OF MARSUPIOCRINUS 465

Fic. 5. Camera lucida drawing of a small area of supposed aboral nerve sys-
tem in Marsupiocrinus sp. A, B directions of radii A, B; C, cross member;
IR, zigzag inter-radial branch; K, keel; L, lateral branches; P, curved processes.

tures on one side of each stipe whereas the curved processes are devel-
oped on both sides of a branch. Quite apart from these considerations
it is impossible to imagine how a graptolite or bryozoan could become
preserved within the calycal plates of a crinoid and there is no doubt
that the structure is within the plates of Marsupiocrinus. The struc-
ture is much too regular to have been produced by a burrowing organ-
ism. Silicification has not destroyed the original calcite cleavage and

466 FIELDIANA: GEOLOGY, VOLUME 16

Fic. 6. Diagrammatic representation of aboral nerve system in Antedon as
seen in plan view (after Cuénot, 1948, fig. 53). C, chiasmata; N, nerve strands;
R, radial plates.

both the fossil structure and the surrounding plates “‘flash’’ when
held at a suitable angle to a light source. Undoubtedly, the structure
is part of the crinoid.

Among recent crinoids the only organ system developed within
the plates of the calyx is the aboral nerve system. Crinoids have
three more or less separate nerve systems of which the aboral is cen-
tered on the base of the calyeal cavity at the top of the stem and is
the main motor system concerned with orientation and movement.
Most knowledge of the aboral nerve system in recent crinoids is based
on studies of Antedon, a free-swimming comatulid crinoid. This is
very unfortunate from a paleontological point of view since Antedon
is a highly specialized crinoid with an atypical calyeal structure. The
aboral nerve system of Antedon was described by Hamann (1889)
and has been refigured recently by Cuénot (1948), Hyman (1955),
and Nichols (1962). It consists of an aboral center which gives off
branches to the cirri aborally. Laterally, five main branches pass
outward, immediately bifureate, and join a pentagonal ring of nerve
tissue within the radial plates (fig. 6). The five main branches are
radial. Distal to the pentagonal ring the main branches pass into
the arm plates and divide in a chiasma with each division of the arms.
This structure is very much simpler than the fossil structure. There
is nothing to correspond to the curved processes nor to the cross

PAUL: ABORAL NERVOUS SYSTEM OF MARSUPIOCRINUS 467

members, the five main branches are radial not inter-radial, and no
plate has more than two nerve trunks within it.

Antedon is a highly specialized, free-swimming crinoid which lacks
a stem in the adult form and has a modified calyx. The calyx plates
are reduced and, with the centro-dorsal, form a capsule which en-
tirely surrounds the aboral nerve center and chambered organ. The
basal plates are fused into a rosette which is adoral to the aboral
nerve center—-a most unexpected position. In view of these spe-
cializations it seems unlikely that the aboral nerve system of Ante-
don, or other comatulids, is typical of crinoids in general. In par-
ticular, the free-swimming mode of life requires considerable ¢o-ordi-
nation ability and modifications to the main motor nerve system
might be expected in such a case. However, no greater similarity
is revealed by comparing the fossil structure with published descrip-
tions of the aboral nerve system in recent stalked crinoids. The
figures given by Bather (1900) and Reichensperger (1905) indicate
a very simple structure usually with a single nerve trunk passing
from plate center to plate center within the plates (figs. 7-9). Reich-
ensperger studied Neocrinus decorus which has an aboral nerve system
basically the same as that of Antedon but which differs in having a

Fic. 7. Diagrammatic representation of aboral nerve system in Neocrinus
decorus in plan view (after Reichensperger, 1905, p. 26). B, basal plate; C, chias-
mata; CO, chambered organ; N, nerve strands; R, radial plates.

468 FIELDIANA: GEOLOGY, VOLUME 16

Fics. 8-9. Diagrammatic lateral views of aboral nerve systems in mono- and
dicyclic crinoids (after Bather, 1900, fig. 12). B, basal plates; IB, infra-basal
plates; N, nerve strands; R, radial plates.

partial ring of nerve tissue within the basal plates and slightly differ-
ent chiasmata (ef. figs. 6-7). Measurements given by Reichensperger
indicate that the nerve strands of Neocrinus decorus are approxi-
mately half the diameter or less of the branches in the fossil structure.
Unfortunately, Bather gives no source for his diagrams.

Marsupiocrinus belongs to an extinct order, the Camerata, while
all living crinoids are articulates. Camerates generally have a large,
well-developed calyx into which some arm plates are usually incor-
porated. It is conceivable that the large calyx plates of camerates
required a more complex nervous system within them. In articulates
the calyx is frequently reduced and individual plates are small. Such
reduction in size may account for the simplicity in the structure of
the aboral nerve system of articulates. The fact that the aboral
nerve system controls posture and movement by innervating muscles
—rather than being primarily sensory—-argues against this sugges-
tion since there are no muscles within the calyeal plates of either
order. Ramifying, anastomosing structures like that of Marsupio-
crinus are better adapted to a general sensory function. Perhaps
the aboral nerve system in camerates such as Marsupiocrinus also
had a sensory function.

To conclude, the fossil structure preserved in Marsupiocrinus is
an integral part of the crinoid and, since it is within the calyx plates,
it is tentatively interpreted as part of the aboral nerve system. The
structure in Marsupiocrinus is so different from that of the aboral
nerve systems of recent crinoids as to suggest either 7) that it repre-
sents some other organ system not found in recent crinoids or 2) that

PAUL: ABORAL NERVOUS SYSTEM OF MARSUPIOCRINUS 469

the function of the aboral nerve system was not exactly the same in
fossil camerates as it is in recent articulate crinoids.

LITERATURE CITED

BATHER, F. A.
1900. The echinoderms. Jn Lankester, E. R., Ed. A treatise on Zoology.
Vol. 3, VI + 344 pp., illust. London.
CUENOT, L.
1948. Classe des Crinoides. Jn Grassé, P., ed. Traité de Zoologie. Anatomie,
systématique, biologie. Vol.11. Echinodermes, Stomochordés, Protochordés.
Pp. 30-74, figs. 38-96. Paris.
HAMANN, O.
1889. Beitrage zur Histologie der Echinodermen. Heft 4, Ophiuren und Cri-
noiden. Jena.
HyMav\, L. H.
1955. The invertebrates. Vol. 4. Echinodermata. The coelomate Bilateria.
VI + 763 pp., 280 figs. New York, Toronto and London.
NICHOLS, D.
1962. Echinoderms. 200 pp., 26 figs. London.

REICHENSPERGER, A.

1905. Zur Anatomie von Pentacrinus decorus Wy. Th. Zeit. Wiss. Zool., 80:
pp. 22-55, pls. 3—5, 1 fig.

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