ws
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Royal Ontario Museum

Taxonomy and Palaeoecology
of the Silurian Myelodactylid Crinoid
Crinobrachiatus brachiatus (Hall)

James D. Eckert
and :
Carlton E. Brett | ier

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LIFE SCIENCES CONTRIBUTIONS 141

Taxonomy and Palaeoecology
of the Silurian Myelodactylid Crinoid
Crinobrachiatus brachiatus (Hall)

James D. Eckert
and

Carlton E. Brett

is
ROM

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James D. Eckert graduated from the University of Toronto with an M.Sc. degree in
Geology in 1981. He is currently continuing graduate studies in the Department of
Geological Sciences, University of Rochester, Rochester, New York 14627.

Carlton E. Brett is Associate Professor in the Department of Geological Sciences,
University of Rochester.

Canadian Cataloguing in Publication Data
Eckert, James D., date
Taxonomy and palaeoecology of the Silurian
myelodactylid crinoid Crinobrachiatus
brachiatus (Hall)

(Life sciences contributions, ISSN 0384-8159 ;
no. 141)

Includes bibliographical references.

ISBN 0-88854-315-8

1. Crinoidea, Fossil. 2. Paleontology -
Silurian. 3. Paleontology - New York (State) -
Niagara County. 4. Paleontology - Ontario -
Lincoln (County). I. Brett, Carlton Elliot.

II. Royal Ontario Museum. III. Title.

IV. Series.

QE782.E27 1985 563’ .910974798 C85-098501-3

Publication date: 15 June 1985

ISBN 0-88854-315-8

ISSN 0384-8159

© The Royal Ontario Museum, 1985

100 Queen’s Park, Toronto, Canada, MSS 2C6
PRINTED AND BOUND IN CANADA AT THE ALGER PRESS

Taxonomy and Palaeoecology

of the Silurian Myelodactylid Crinoid

Crinobrachiatus brachiatus (Hall)

Abstract

Discovery of well-preserved specimens of the fossil myelodactylid crinoid Crino-
brachiatus brachiatus (Hall) permits detailed description of this genus for the first time.
The presence of an aniradial plate, possibly derived from fusion of C-ray inferradial and
superradial plates, may indicate derivation from the locrinidae. In life, the specialized
column could be tightly coiled to partly enclose the crown in a loose baffle formed by
unique, branching cirri. These cirri may have served as props to support the column in a

semi-recumbent position on the substrate.

Introduction

The Upper Silurian Rochester Formation of western New
York and southern Ontario contains a diverse fauna of
pelmatozoan echinoderms (Hall, 1852; Springer, 1920,
1926; Brett, 1978a, b). Among these is the myelodactylid
crinoid Crinobrachiatus, a disparid inadunate charac-
terized by a doubly recurved and coiled stem that bears
specialized branching cirri. Occurrence of Crino-
brachiatus is restricted to the Rochester Formation, where
it is represented solely by the type-species C. brachiatus
(Hall). The distinctive columnals and pluricolumnals of
this species are common fossils in the Rochester Forma-
tion. The crown of Crinobrachiatus, however, is rarely
preserved intact. The original description of Crino-
brachiatus was based on specimens without preserved
crowns (Hall, 1852). A collection of 35 specimens
obtained at Lockport, New York, possesses only one

individual with an intact crown (Springer, 1926). Springer
noted that the crown had a general resemblance to
Tocrinus, but was unable to make out details of its
structure. Consequently, taxonomy of Crinobrachiatus
has been based solely on features of the column and cirri.
In this paper, the morphology of the crown of Crino-
brachiatus is documented for the first time, based on a
well-preserved specimen obtained at Lockport. Crino-
brachiatus is shown to possess features that suggest
derivation from the Iocrinidae.

We recommend that classification of the Myelodac-
tylidae be based primarily on morphology of the crown, as
the specialized stem and cirri of these genera are
potentially homeomorphic features.

Crinobrachiatus brachiatus was a sessile pelmatozoan
that was adapted primarily to rather quiet water settings.

Occurrence

Crinobrachiatus is restricted in occurrence to mudstone,
calcareous shale, and argillaceous limestone of the lower
Rochester Formation in western New York and southern
Ontario (Lewiston Member of Brett, 1983). It is com-

monly associated with small biostromes of ramose
bryozoans dominated by Chilotrypa, Hallopora, and
Trematopora. In these biostromes, Crinobrachiatus is a
minor constituent of a diverse echinoderm fauna typically

dominated by Caryocrinites and Stephanocrinus.
Recently, a specimen of Crinobrachiatus with intact
crown was collected from a small outcrop in the west
branch of Eighteen-Mile Creek at Lockport, New York.
This individual (ROM 43049) was derived from a thin
biostratigraphic interval designated as the Homocrinus
beds (Ringueberg, 1888). Frederick Braun, employed by
Frank Springer, quarried this layer in 1910, 1911, and
1914, and obtained the only other specimen of Crino-

brachiatus with an intact crown (USNM S2101). This
occurrence of Crinobrachiatus is atypical in that it is
dominated by the brachiopod Striispirifer niagarensis and
the small inadunate crinoid Homocrinus parvus.

Additional specimens of Crinobrachiatus were obtained
at Middleport, New York, and two localities in the
Niagara Peninsula in southern Ontario. Precise locality
information is given in the Appendix.

Systematic Palaeontology

REPOSITORIES

The illustrated specimens are in the collections of the
Department of Invertebrate Palaeontology, Royal Ontario
Museum (ROM) and the United States National Museum
(USNM).

Subclass Inadunata Wachsmuth and Springer, 1885
Order Disparida Moore and Laudon, 1943
Superfamily Myelodactyloidea S. A. Miller, 1883
Family Myelodactylidae S. A. Miller, 1883

DISCUSSION

Classification of the Myelodactylidae differs from that of
most crinoids in that this family is characterized by a
specialized stem considered to be an important taxobase.
The Myelodactylidae constitute the only family of crinoids
originally defined on characteristics of the column alone
(Miller, 1883). However, the Mississippian camerate
genus Camptocrinus possesses a specialized stem re-
sembling that of the Myelodactylidae (Springer, 1926;
Van Sant and Lane, 1964). Apparently, selection pressure
favoured convergent evolution in unrelated crinoids,
resulting in homeomorphy of the stem. Homeomorphic
tendencies of pelmatozoan stems suggests that classifica-
tion of the Myelodactylidae should not be based primarily
on characteristics of the stem and cirri. Unfortunately,
because of nonpreservation of calyces, this is the case in
several supposed myelodactylid crinoids including
Brachiocrinus and Eomyelodactylus. Until the crowns of
these genera are known, their assignment to the
Myelodactylidae must remain tentative. Despite this
potential taxonomic pitfall, discovery of calyces in several
myelodactylids (Crinobrachiatus, Herpetocrinus,
Myelodactylus) suggests that the Myelodactylidae com-
prise a monophyletic lineage.

Genus Crinobrachiatus Moore, 1962

TYPE-SPECIES
Myelodactylus brachiatus Hall, 1852.

DIAGNOSIS

A monotypic genus of Myelodactylidae characterized by a
small, conical cup, five pentagonal basals, and five
radials, including an undivided C-ray radial (aniradial)
supporting an elongate anal tube. Arms five, nonpinnu-
late, branching isotomously several times. Stem excep-
tionally varied in diameter, bent in an open S-shaped
curve, and bearing numerous stout, branching cirri.

DISCUSSION

Crinobrachiatus is differentiated from other myelodac-
tylids by several characteristics. Crinobrachiatus has an
undivided rather than a compound C-ray radial, a stem
displaying exceptional variation in diameter, and stout,
branching cirri instead of the elongate, unbranched cirri
characteristic of most myelodactylids.

The lack of intermediate forms inhibits phylogenetic
interpretation of Crinobrachiatus. Myelodactylids were
possibly derived from an iocrinid stock during the Middle
or Late Ordovician. Potential ancestors include Caleido-
crinus, locrinus, and Peltacrinus. However, with the
exception of Peltacrinus, these genera lack the pentapar-
tite stem characteristic of the proxistele of Crino-
brachiatus. Eomyelodactylus Foerste is presently the
oldest known presumed myelodactylid. It occurs in the
Lower Silurian Brassfield Formation of Ohio and is of
Middle Llandoverian age. Eomyelodactylus may represent
a root-stock that gave rise to North American myelodac-
tylids.

Whatever the affinities of Crinobrachiatus, its unique
branching cirri and highly xenomorphic stem are clearly

specialized characteristics not present in other myelodac-
tylids and suggest that the lineage that led to Crino-
brachiatus was an offshoot that became extinct after the
Wenlockian.

Crinobrachiatus brachiatus (Hall, 1852)
Pils ele 2; Wext-tigss 1-3

DESCRIPTION

Cup of small size, height and width about 3 mm (ROM
43049), conical with slightly curved walls. Plates of cup
smooth, without ornament. Basals five, pentagonal,
subequal. Height of basal circlet approximately one-half
cup height. A- and E-ray radials larger than basals,
five-sided, expanding adorally, height/width ratio ap-
proximately 0.8. Posterolateral borders of B- and D-ray
radials slightly truncated to accommodate C-ray
brachitaxis and anitaxis respectively (Text-fig. 1). C-ray
aniradial overlying BC- and CD-interray basals, undi-
vided, hexagonal, slightly wider than high, supporting
anal X on oblique, left distal margin and C-ray arm on
right facet (Text-fig. 1; Pl. 1, fig. 5). Remaining radial
facets plenary, planate.

Arms five, apinnulate, branching isotomously several
times. Primibrachials four per ray. First primibrachial of
C-ray four-sided, equidimensional, smaller than other first
primibrachials. Remaining first primibrachials slightly
smaller than underlying radials, trapezoidal, tapering
adorally, height/width ratio about 0.6. Second and third
primibrachials equidimensional, much narrower than first
primibrachials (Pl. 1, fig. 6). Fourth primibrachial slightly
elongate, axillary. Secundibrachials five or six per taxis in
E-ray arm of ROM 43049. Arms branch again on sixth
tertibrachial (Text-fig. 1). Distal portions of arms not
preserved. Preserved length of arms 17 mm in ROM
43049. Anal tube narrow, long (19 mm in ROM 43049).
Anal X area obscured by pyrite coating in type-specimens.
Plates not visible in any portion of anal tube (PI. 1, figs. 2,
4-6).

Column bilaterally symmetrical and highly xenomor-
phic with exceptional variation in diameter, ranging from
1 mm immediately below the cup to 6 mm in cirriferous
zone; bent in a recurved S-shaped coil that could partly
enclose crown (Text-figs. 2, 3; Pl. 1, figs. 1-3). Proxistele
comprising one-quarter to one-third of column length,
very slender, cylindrical proximally, becoming flattened
and ribbonlike distally in plane of coiling (Pl. 2, fig. 1).
Proximal columnals low, height/width ratio about 0.2.
Distal columnals of proxistele higher, height/width ratio in
lateral view about 0.5. Distal columnals of proxistele
flattened and elliptical in cross-section, major diameter
twice that of minor diameter (PI. 2, fig. 4). Columnals
near U-bend at proxistele-mesistele junction slightly

cuneiform. Stem diameter increasing rapidly in mesistele,
attaining diameter five or six times that of proxistele.
Mesistele curved in broad, open loop, composed of
uniformly low, broad columnals, trapezoidal in cross-
section. Mesistele columnals flattened to slightly concave
on inner surface of curve, strongly convex on outer side
(Pl. 2, figs. 2, 4). Articular surfaces of mesistele

Text-fig. | Plate diagram of Crinobrachiatus brachiatus. Note
undivided aniradial in C-ray. Block shading indicates radial
plates; stippling indicates anal tube.

wn

columnals bilaterally symmetrical with broad, elliptical
lumen surrounded by five shallow, depressed petaloid
areas possibly representing ligament fossae (Pl. 2, fig. 4).
Three fossae above lumen towards outside of curve;
remaining two below and directed towards lower corners
of columnal. Crenularium ill defined. Area between the
two lower ligament fossae possibly bearing vague
vermicular culminae. Dististele composed of trapezoidal
columnals proximally and subpentagonal columnals near
terminus. Dististele bearing two rows of alternating cirri,
one on each side of plane of coiling, located beyond
closely coiled portion of column (PI. 2, fig. 3). Three to
nine large, branching cirri present in each row in adult
individuals, becoming progressively smaller and more
closely spaced distally. Cirri branching several times,
giving rise to anteriorly directed spurlike projections (PI.
1, fig. 7; Pl. 2, fig. 11). Proximal cirri stout, directed
obliquely upwards and inwards, covering coiled portion of
column (Text-figs. 2, 3), each typically borne by single
columnal on lobate, spear-shaped extension (Pl. 2, fig.
13). Adjoining columnals fused together forming this
platform in some individuals (ROM 43049; Pl. 1, fig. 6).
Distal columnals with cirri lacking these extensions. Distal
cirri oriented at nearly right angles to stem. Cirrals stout,
blocky, with symplexial sutures (Pl. 2, figs. 5, 9).
Dististele tapering gradually, terminating in discoidal or
digitate holdfast in some specimens (Pl. 2, figs. 6-8).

DISCUSSION
Although admitting he could not fully determine details of

its structure, Springer (1926, pl. 4, fig. 1A; Pl. 1, fig. 1
herein) provided a diagram of the crown of Crino-
brachiatus brachiatus based on the single specimen then
available for study. Examination of this specimen (USNM
$2101) by us indicates that Springer’s interpretation is in
error. Apparently, Springer did not recognize the CD-
interray of his specimen and interpreted the poorly
preserved anal tube as an arm. This misinterpretation is
understandable as the structure is partially obscured by
pyrite. However, the undivided C-ray radial (aniradial) is
visible when this specimen is immersed in ethanol. We
initially thought that the undivided aniradial might be an
abnormality since a divided C-ray radial is characteristic
of other myelodactylids. However, partial dissection of
ROM 43049 revealed that this individual also possesses an
undivided C-ray aniradial. This specimen was_ sub-
sequently reassembled without damage. We consider an
undivided aniradial to be characteristic of Crino-
brachiatus . This plate was possibly derived from fusion of
separate C-ray inferradial and superradial ossicles typical
of the myelodactylids Myelodactylus and Herpetocrinus.
Why fusion of these plates may have occurred in
Crinobrachiatus is open to speculation. It may represent a
response to the problem of support of the relatively long
anal tube. Fusion of C-ray radial plates, also observed in
certain calceocrinids, would produce a single larger plate
that would strengthen the small cup by spreading stress
over a wider area (Brett, 1981a). Alternatively, the C-ray
of Crinobrachiatus may have been simplified by expulsion
of an overlying brachianal from the cup.

Biostratinomy of Crinobrachiatus

The Homocrinus beds of the Rochester Formation at
Lockport, New York, are noted for well-preserved fossils.
Ringueberg (1888:269) observed that “‘the fine state of
preservation of many fossils occurring in this band and the
layers immediately above and below is noteworthy. This is
especially true of species as a rule found only in
disarticulated condition.’’ Asaphocrinus ornatus, Homo-
crinus parvus, Caryocrinites ornatus, and _ other
echinoderms associated with Crinobrachiatus in this unit
commonly are completely articulated and possess the
entire column and holdfast. Thus, the rarity of Crino-
brachiatus individuals preserved with the crown intact is
somewhat enigmatic. Autotomization is a possible expla-
nation. Springer (1926:18) judged that ‘‘this form must
have been peculiarily sensistive to disturbance or change
of conditions, causing it to cast off the crown, as certain
existing crinoids cast off their arms on being brought to the
surface.”’

One specimen of Crinobrachiatus with intact crown
(ROM 43049, PI. 1, figs. 3-6) was discovered by splitting
a slab of shale. The posterior side of this individual
exhibits better preservation than the anterior side. This is
indicated by the arms, which are preserved almost to their
extremities on the posterior side of the crown (C-, D-, and
E-rays), but only to the first secundibrachials on the
anterior side (A- and B-rays) (PI. 1, fig. 6). Furthermore,
the stout, branching cirri are represented only by their
attachment scars on the anterior side, yet well-preserved
cirri were observed on the posterior side during partial
dissection of the crown. The tightly coiled stem also
shows slight separation of columnals in several places.
Associated crinoids in the Homocrinus beds show similar
features. When this mode of preservation is present, the
lower surface of the crown—the side that rested on the
seafloor—is invariably better preserved than the upper
surface. This suggests that, following dislodgement by

currents, the echinoderms lay partly exposed on the
seafloor, where decay aided disarticulation and scattering
of ossicles. This process was restricted to exposed upper
surfaces as underlying sediment prevented disarticulation
of the lower surfaces. Only a brief period of time was
involved. Recent experiments show that modern crinoids

decay rapidly and disarticulate completely within five to
ten days of death (Meyer, 1971; Liddell, 1975). In
individuals from the Homocrinus beds, the decay process
had barely begun before the crinoids were completely
buried by an influx of sediment that inhibited further
disarticulation.

Functional Morphology

The family Myelodactylidae is characterized by develop-
ment of specialized cirri on each side of a bilaterally
symmetrical, coiled, and recurved column (Springer,
1926; Moore, 1962). Most myelodactylids, including
Herpetocrinus and Myelodactylus , possess closely spaced
cirri that are long, slender, and unbranched. In contrast to
these crinoids, Crinobrachiatus possesses stout, widely
spaced, branching cirri. These cirri are unique; in all other
pelmatozoans branching cirri, if present, are restricted to
distally developed holdfasts. The branching cirri and
strongly xenomorphic column of Crinobrachiatus suggest
that this crinoid had a mode of life unlike that of other
myelodactylids. However, the functional morphology and
palaeoautecology of Crinobrachiatus have not previously
been considered in detail (but see Ehrenberg, 1929,
1930a, b; Brett 1981b).

Profuse development of long, slender, unbranched cirri
in most myelodactylids presumably had a defensive
function, providing a well-developed baffle that could
completely enclose the delicate crown. This adaptation
may have permitted these crinoids to inhabit higher-energy
environments and may account for their widespread
distribution in calcarenite and mudstone facies. The
unique cirri of Crinobrachiatus, in contrast, formed a
loose baffle that could only partly conceal the crown.
During periods of intense storm activity, the partly
exposed crown may have been susceptible to damage. We
postulate that Crinobrachiatus was primarily adapted to
rather quiet-water, muddy settings. At the Lockport
occurrence, Crinobrachiatus is associated with the mi-
nute, fragile inadunate crinoid Homocrinus parvus. This
occurrence is interpreted to represent a normally quiet-
water, mud-bottom community that was disrupted by
storm waves or currents (Brett, 1978a).

Previous workers (Kirk, 1911; Springer, 1926, pl. 4,
fig. 1; Ehrenberg, 1929, 1930a, b) believed that Crino-
brachiatus lacked a distal holdfast. However, we have
discovered both juvenile and adult Crinobrachiatus
individuals with lobate or digitate holdfasts typically
cemented to bryozoans (PI. 2, figs. 6-8). Common
association of Crinobrachiatus with bryozoan patches in
the Rochester Formation suggests that this crinoid may
have selectively colonized these biostromes. This is

particularly evident in the Lockport occurrence, where
Crinobrachiatus occurs selectively with small localized
bryozoan patches but not commonly in intervening areas
of the same bedding plane.

During early ontogenetic stages, Crinobrachiatus prob-
ably grew upright as in “‘normal’’ crinoids, as indicated by
the development of a roughly circular holdfast and radially
symmetrical distal-most columnals. Although evidence
was lacking, previous investigators have also depicted
mature individuals as growing upright (Fenton and
Fenton, 1958:130; Text-fig. 2). A recently discovered
large Crinobrachiatus specimen with a stout, well-
developed holdfast may have retained an upright orienta-
tion throughout its life (ROM 43056, Pl. 2, fig. 8). This
individual is unusual in that it possesses only two or three
robust, branching cirri located immediately above the
holdfast and directed upwards towards the crown. These
cirri may have formed a loose baffle enclosing the crown.
Alternatively, distal portions of the cirri may have grasped
other objects, providing additional support. However,
Crinobrachiatus cirri are rather stout and have not been
observed coiled around other objects.

Digitate or lobate holdfasts were apparently absent in
many mature Crinobrachiatus individuals. The stems of
these individuals are lined with branching cirri that
become shorter and more closely spaced distally (ROM
43053, Pl. 2, fig. 10). The absence of digitate or lobate
holdfasts suggests that the lateral cirri had supportive
functions. These and other morphological features dis-
cussed below lead to the interpretation of a unique,
semi-reclined mode of life for many adult individuals.

There are several peculiarities of the Crinobrachiatus
column and cirri that must be considered in any functional
reconstruction: a) medial and distal portions of the column
are coiled and bilaterally symmetrical, b) cirri occur in two
rows, one on each side of the column, c) cirri are branched
and stout, d) cirri length increases progressively towards
proximal part of column, and e) proximal column is
recurved and lacks cirri.

An evolutionary trend existed within primitive pelmato-
zoans towards perfection of radially symmetrical columns
from irregular polyplated steles or pentameric stalks.
Crinoid columns that were vertically oriented in life are

a

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Text-fig. 2 Restoration of Crinobrachiatus brachiatus in
upright orientation with column partly uncoiled (A), and
column fully retracted (B). Cirri are depicted on one side of
the column only. Crinoid is shown attached to bryozoan.
Scale is 1 cm.

typically composed of radially symmetrical columnals.
Lateral cirri, if present, are also radially disposed around
the column and are invariably unbranched. This may be
adaptively advantageous in providing a_ streamlined
support rod for the crown; water will flow evenly around
the symmetrical column, generating a minimum of torque

which might tend to dislodge the holdfast. The cirri were
probably functional in maintaining balance in upright
crinoid columns (Lane, 1968; Haugh, 1978).

In contrast, distal portions of columns that acted as
horizontal creeping holdfasts or stolons are commonly
bilaterally symmetrical in cross-section, with the flattened
to slightly concave lower surface resting on or directly
cemented to the substrate (Franzén, 1977; Brett, 1981b).
Stoloniferous holdfasts also commonly possess rows of
pseudocirri or radicular cirri, which are symmetrically
disposed on either side of the column and are directed
downwards and attached to the substrate (Brett, 1981b).
The dististele of Crinobrachiatus resembles stoloniferous
holdfasts in that it is bilaterally symmetrical in cross-
section and possesses two rows of cirri. By analogy, we
postulate that Crinobrachiatus cirri were typically directed
downwards towards the seafloor in life orientation, with
the dististele directed at an oblique angle to the substrate
(Text-fig. 3).

A semi-recumbent orientation for Crinobrachiatus is
also supported by the morphology of the cirri. The cirri are
stout and branched, with most branches directed away
from the dististele. Branching cirri are common in
pelmatozoans but they are typically associated with distal
holdfast areas. Stout, branched, radicular cirri are
generally directed downwards at an oblique angle to the
main column. They invariably bifurcate towards the
substrate rather than away from it. In Dolatocrinus,
Gennaeocrinus, and many other crinoids, branched
radicles apparently acted as a prop for the main column. A
similar interpretation is made here for Crinobrachiatus .

Crinobrachiatus cirri show a regular, progressive
increase in length towards the coiled portion of the
mesistele. The increase in cirral length is readily
interpretable if this crinoid had a semi-recumbent orienta-
tion as an adult: such an array of cirri would raise the
crown and proxistele above the substrate. Intergrowth of a
cirrus of Crinobrachiatus with roots of Caryocrinites (PI.
2, fig. 12) indicates that the tips of the cirri rested on the
substrate. Short distal cirri would function primarily in
anchorage, whereas proximal cirri would serve mainly as
struts or stilts (Text-fig. 3).

Extensive development of Crinobrachiatus cirri in a
column with an upright orientation may have produced a
‘“‘top-heavy’’ condition which would have tended to cause
the crinoid to topple towards the cirrus-weighted side.
Indeed, this may have been the common ontogenetic
pattern, assuming that immature Crinobrachiatus indi-
viduals had an upright orientation.

In order for the crown to be retracted into the coiled
mesistele with the semi-recumbent model, it would have
to have been rotated to an inverted position obliquely
parallel to the substrate (Text-fig. 3). To test whether or
not the proposed orientation provides sufficient clearance

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Text-fig. 3. Restoration of Crinobrachiatus brachiatus in semi-recumbent orientation with

column partly uncoiled (A), and column fully retracted (B). Cirri are depicted on one side of
column only. Scale is | cm.

for the crown to be retracted, we constructed a wire scale
model of Crinobrachiatus. The model demonstrates that
the long proximal cirri would have raised the proximal
column sufficiently above the seafloor to allow the crown
to clear the substrate during coiling. It is still possible that
distal tips of the arms would have dragged across the
substrate; however, we do not believe that this would
preclude the interpretation of a recumbent orientation. The
ambulacra of Crinobrachiatus and other crinoids pos-
sessed cover plates which could be closed to prevent
damage or fouling of the tube feet. Moreover, in
calceocrinids, for which a recumbent life-mode is known,
closing of the hinged crown brought the arms virtually into
contact with the substrate without ill effect.

The coiled column of Crinobrachiatus is commonly
considered to have been a defensive adaptation coordi-
nated with the evolution of a small, fragile crown. This
interpretation appears plausible. However, as indicated
previously (p. 7), the cirri of Crinobrachiatus could not
enclose the crown as effectively as those of other

myelodactyiids. We believe that the coiled column also
served to reduce stresses associated with life in a
near-substrate zone. Organisms that live at or close to the
sediment-water interface are subject to certain common
stresses not shared with high-level suspension-feeding
pelmatozoans, most notably burial and fouling by resus-
pended fine-grained sediments. Most near-substrate or-
ganisms possess mechanisms for closure during periods of
high turbidity and rapid sedimentation. Examples include
bivalved shells and the operculum in the coral Calceola.
Brett (198la) postulated that a major function of the
calceocrinid hinge was protection from turbidity and
burial. Calceocrinids could disinter themselves from thin
layers of sediment simply by opening the hinge. Uncoiling
of the column in Crinobrachiatus could have performed a
similar function.
Mesistele columnals of Crinobrachiatus possess well-
developed articular surfaces and muscle-attachment areas
that permitted the column to be readily coiled (PI. 2, fig.
4). Each mesistele columnal has a trapezoidal cross-

section that reduces its effective radius in the plane of
coiling. This characteristic permitted the mesistele to be
tightly coiled. Furthermore, the recurved portion of the
column is characterized by flattened, ribbonlike columnals
that permitted an abrupt change in curvature so that the
crown could be partly enclosed within the coil.

In summary, we infer that Crinobrachiatus had both
upright and semi-recumbent modes of life. Columns of

juvenile individuals were probably oriented approximately
perpendicular to the substrate. This orientation was
retained in some adults with limited development of
branching cirri (Text-fig. 2). In many mature individuals,
extensive development of cirri shifted the centre of gravity
to one side of the column, causing the crinoids to
gradually assume a semi-recumbent orientation (Text-fig.
3):

Acknowledgements

Dr Porter M. Kier of the United States National Museum
kindly loaned a specimen of Crinobrachiatus from the
Springer collection. The specimens illustrated on Plate 2
(figs. 7 and 8) were collected by Denis Tetreault and
Bea- Yeh Lin, respectively, of the University of Rochester.
Preparation facilities and photographic equipment were
provided by the Royal Ontario Museum and the University

of Rochester. This study benefited from discussion and
curatorial services provided by Dr Peter von Bitter, David
Rudkin, and Janet Waddington of the Department of
Invertebrate Palaeontology at the Royal Ontario Museum.
James Brower and James Sprinkle critically reviewed the
manuscript.

Appendix

Register of Localities

1. United States Geological Survey (USGS) Medina 7.5’
Quadrangle. Exposure of Rochester Formation in bed
of tributary of Jeddo Creek, 0.7 km south of Route 31
and 0.4 km east of Telegraph Road, Niagara County,
New York. 43°11'5’’ N. Lat., 78°28'19’’ W. Long.

2. USGS Lockport 7.5’ Quadrangle. Highly weathered
and largely overgrown exposure of Rochester Forma-
tion in hill slope just west of Scovell Street and 0.2 km
north of intersection of Scovell and Gooding streets,
Niagara County, New York. 43°10'38’’ N. Lat.,
78°41'29'' W. Long.

3. USGS Lockport 7.5’ Quadrangle. Exposure of Homo-
crinus beds of the Rochester Formation in west branch

10

of Eighteen-Mile Creek, 0.25 km east of Route 93 and
1 km north of Route 31, Niagara County, New York.
43°10'15'' N. Lat., 78°43'38'’ W. Long.

4. National Topographic Series (NTS) Niagara 30M/3.
Rochester Formation formerly exposed in tributary of
Dick Creek, 200m east of Highway 406, St
Catharines, Lincoln County, Ontario. Exposure was
covered by construction of erosion-resistant barrier in
1984 and is no longer accessible. 43°07'56"’ N. Lat.,
78°13'26'' W. Long.

5. NTS Niagara 30M/3. Rochester Formation exposed in
west bank of Twenty-Mile Creek, 0.6 km south of
lower falls at Ball’s Falls, Jordan, Lincoln County,
Ontario, 43°07'57'’ N. Lat., 79°23'04’’ W. Long.

Literature Cited

BRETT, C.E.

1978a Systematics and paleoecology of Late Silurian
(Wenlockian) pelmatozoan echinoderms from
western New York and Ontario. Ph.D. Thesis,
University of Michigan. 603 pp.

1978b _—Host-specific pit-forming epizoans on Silurian
crinoids. Lethaia 11:217—232.

198la Systematics and paleoecology of Late Silurian
(Wenlockian) calceocrinid crinoids from western
New York and southern Ontario. Journal of
Paleontology 55:145—-175.

1981b Terminology and functional morphology of at-
tachment structures in pelmatozoan echinoderms.
Lethaia 4:343-370.

1983 Sedimentology, facies and depositional environ-
ments of the Rochester Shale (Silurian: Wenlock-
ian) in western New York and Ontario. Journal of
Sedimentary Petrology 53:947-971.

EHRENBERG, K.

1929 Pelmatozoan root-forms (fixation). Bulletin of the
American Museum of Natural History 59: 1—76.

1930a Die “‘Nebenformen’’ der Crinoiden, ihre stam-
mesgeschichtliche Entwicklung und Bedeutung.
Palaeobiologica 3:257—324.

1930b Form und Funktion bei der ‘‘Nebenformen’’ der
Crinoiden. Palaontologische Zeitschrift 12:170—
177.

FENTON, C.L. and M.A. FENTON
1958 The fossil book. Garden City, N.Y., Doubleday.
482 pp.
FRANZEN, C.
1977 Crinoid holdfasts from the Silurian of Gotland.
Lethaia 10:219-234.

HALL, J.

1852 Palaeontology of New York. Volume 2, contain-
ing descriptions of the organic remains of the
lower Middle division of the New York System.
Albany, printed by C. Van Benthuysen. 362 pp.

HAUGH, B.N.
1978 Biodynamic and phyletic paradigms for sensory
organs in camerate crinoids. Lethaia 11: 145-173.

KIRK, E.R.
1911 The structure and relationships of certain
eleutherozoic pelmatozoa. Proceedings of the
United States National Museum 41: 1—137.

LANE, N.G.
1968 The adaptive significance of balance in Paleozoic
stalked crinoids. Geological Society of America,
Abstracts for 1968 (GSA Special Paper 121):169.

[Abstract]
LIDDELL, W.D.
1975 Recent crinoid biostratinomy. Geological Society
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[Abstract |
MEYER, D.L.
197] Post-mortem disarticulation of recent crinoids and
ophiuroids. Geological Society of America,
Abstracts with Programs 3:645. [Abstract]
MILLER, S.A.
1883 Echinodermata. In The American Palaeozoic
fossils, Cincinnati, private publication, S.A.
Miller, pp. 276-288.
MOORE, R.C.
1962 Ray structure of some inadunate crinoids. Univer-

sity of Kansas Paleontological Contributions,
Echinodermata, Article 5:1—47.

MOORE, R.C. and L.R. LAUDON

1943 Evolution and classification of Paleozoic crinoids.
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46: 1-153.

RINGUEBERG, E.N.S.
1888 The Niagara Shales of western New York.
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SPRINGER, F.
1920 The Crinoidea Flexibilia. Smithsonian Institution
Publication 2501: 1—443.

1926 Unusual forms of fossil crinoids. Proceedings of
the United States National Museum 67, Article
9: 1-137.

VAN SANT, J.F. and N. LANE
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Platten eatiesn 7,

Crinobrachiatus brachiatus (Hall). Rochester Formation, western New York and southern Ontario.

1. Partly coiled individual with crown and most of the highly
xenomorphic stem. This specimen was figured by Springer
(1926, pl. 4, fig. 1) and Moore (1962, pl. 3, fig. 2a). USNM
$2101, x 2.4, locality 3.

2. CD-interray of crown of above individual. Proximal portion
of the anal tube is visible. USNM $2101, x 10.5.

3. Specimen with well-preserved crown, showing column in
fully retracted position. In life, the crown was partly concealed
by branching cirri, not shown in this individual. ROM 43049, x
3.3, locality 3.

4. Distal portion of anal tube preserved on counterpart of above
individual. ROM 43049a, x 6.1.

5. CD-interradius of ROM 43049, x 6.7. Note undivided
aniradial in C-ray.

6. Crown of ROM 43049, x 6.7, centred on A-ray. The
isotomously branching arms are preserved nearly to their
extremities in the D-ray. Proximal portion of the anal tube is
visible. Attachment sites for three cirri are preserved on the
column. This specimen displays the exceptional variation in
diameter of the stem that is typical of this species.

7. Large individual with missing crown, showing typical
preservation. The stout, branching cirri are well preserved. ROM
43050, x 1.8, locality 5.

Plate 2, figs. 1-13

Crinobrachiatus brachiatus (Hall). Rochester Formation, western New York and southern Ontario.

1. Pluricolumnal seen from inner side of coil. The narrow,
proximal portion of the proxistele is resting on the much larger
mesistele. (See also Pl. 2, fig. 4.) ROM 43049, x 3, locality 3.
2. Pluricolumnal seen from outer side of coil, showing gaps
between columnals. ROM 43050, x 3.3, locality 5.

3. Pluricolumnal seen from outer side of coil, showing alternate
arrangement of cirri. ROM 43050, x 3.8, locality S.

4. Transverse section through mesistele (top) and proxistele.
Individual pentameres can be seen in the proxistele. A narrow,
elliptical lumen is visible in the mesistele and is apparently
bisected by a ligament fossa. ROM 43049, x 6.5, locality 3.

5. Articular surface of cirral. ROM 43055, x 12, locality 2.

6. Nearly complete juvenile individual with holdfast and
disarticulated calyx. ROM 43051, X 6.8, locality 3.

7. Nearly complete juvenile individual attached to fragment of
trepostome bryozoan. ROM 43052, x 5.5, locality 1.

8. Large individual with well-developed holdfast attached to
fragment of trepostome bryozoan. ROM 43056,x 1.7, locality 1.
9. Part of cirrus with slightly depressed cirral sutures. ROM
43050, x 8, locality 5.

10. Specimen attached to foliate zoarium of Lichenalia. A tre-
postome bryozoan is also present. ROM 43053, x 3.2, locality 1.
11. Typical specimen with partial preservation of branching
cirri. ROM 43054, x 1.7, locality 4.

12. Lower right area of above with intergrowth of Caryocrinites
radicle and cirrus of Crinobrachiatus. ROM 43054, X 6.3.

13. Pluricolumnal from mesistele with attachment areas for two
citi. Note partial fusion of columnals. ROM 43054, x 6.4,
locality 4.

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