PRE-BURIAL ABRASION OF
ARTICULATED ASTEROID  SKELETONS
BY D, BRYAN BLAKE
Abstract
Although asteroids usually become disarticulated
rapidly after death, Recent forms collected on the beach of
Baja California show noteworthy transport abrasion but
little tendency for disarticulation.  A relatively complete
fossil starfish therefore cannot be, by itself, considered
proof of rapid deposition of the containing sediment; in
evaluating ossicle morphology of such specimens, the
possibility of pre-depositional abrasion should be con-
sidered.
The individual ossicles of an asteroid skeleton are
not sutured to one another but are bound together by
muscle and dermal tissue.  Unless an asteroid is quickly
buried upon death, the tissues usually decompose and the
skeleton disarticulates.  Therefore, discrete ossicles or
fragments are more common as fossils than entire tests.
Most discrete fossil ossicles are abraded evenly on all
surfaces indicating that this wear usually occurs after
disarticulation.  The observations recorded here indicate
that this is not always the sequence of events.
Complete fossil starfish are rarely discovered and
generally have been considered indicative of rapid burial.
However, a small sample of dead Recent asteroids collected
on the beach on Cerralvo Island, near La Paz, Baja
California shows that, at least under certain conditions,
the dermal tissue may be resistant to decomposition and
permit transportation and abrasion of the skeleton prior
to burial.  This sample contains the following species:
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Nidorellia armata (Gray) (3 specimens)
Pentaceraster cumingi (Gray) (5 specimens)
Pharia pyramidata (Gray) (2 specimens)
Phataria unifascialis (Gray) (3 specimens)
Asteropsis carinifera (Lamarck) (1 specimen)
The actinal surfaces of the arms and the infra-
marginal ossicles of the disc are highly abraded on all
specimens of Pentaceraster (Fig. la-d).  There is no
surface ornamentation remaining on the ossicles near the
arm tip and little on those in the interbrachial position.
The basic outline of these ossicles has been changed.  The
minor amount of differential weathering between the marginal
ossicles and the enclosing dermis illustrates the resistant
nature of this tissue and its physical resistance to
abrasion.  The oral surfaces of the discs are arched and
were not in contact with the substrate.  Most granules and
pedicellaria remain on the oral intermediate ossicles.  The
spines are present in the ambulacral furrows nearly to the
arm tip, protected by the relatively prominent oral inter-
mediate and inframarginal ossicles.  The abactinal surface
is somewhat abraded; the most prominent spines and
projecting portions of the ossicles are lost, while the
fine granules of the sides of the ossicles and the papular
area remain intact.  The arm tips of several specimens are
in the process of breaking away, perhaps by shrinkage and
breakage of the dermis during desiccation.
The specimens of Nidorellia are similar to
Pentaceraster in the abrasion suffered.  However, one
specimen is more worn on the oral surface, another on the
aboral surface indicating that this flat form may be un-
stable in the surf regardless of the surface toward the
substrate.
In contrast to Pentaceraster and Nidorellia, which
have heavy tests, those of Phataria and Pharia are light.
The surficial tissue has been largely removed in specimens
of these genera, but the individual ossicles show only
moderate wear.
In life, the plates of Asteropsis are covered by a
thick fleshy tissue.  In the specimen of this genus in the
collection, the fleshy layer has served to protect most
ossicles but projecting spines have been abraded.
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Figure 1 - a-d, Pentaceraster cumingi.  a-b, UCMP 30290,
lateral and oral views of abraded arm.  c-d, UCMP 35087,
lateral and oral views of unabraded arm.  e, Asteropsis
carinifera, UCMP 12106, aboral view of arm. Natural size.
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In another specimen of Asteropsis, collected from the
west coast of Baja California, the soft tissues have been
almost entirely lost, yet the skeleton remains articulated
(Fig. le).  There is no evidence of physical abrasion on
this specimen.
Steinbeck and Ricketts (1941) report Pentaceraster
from between 3 and 40 fathoms, and Nidorellia, Pharia, and
Phataria from the intertidal zone.  Sladen (1889) reports
Asteropsis from shallow water.  Therefore, all the present
species are from shallow water or are intertidal; none need
have been transported far prior to arrival on the beach.
The climate of the area is arid, and certainly contributed
to the preservation once the specimens were washed up on
the beach.  The considerable wear of the ossicles shows
that the specimens were transported and abraded by surf
action, however.  They were in water, wet, and presumably
subject to bacterial decomposition, yet the dermis had not
yet decomposed and the ossicles had not disarticulated.
These observations indicate that a relatively complete
fossil starfish is not necessarily proof of rapid deposi-
tion of the entombing sediment; the test must be examined
for evidences of pre-depositional transportation.  Further,
in evaluating any smooth surface of the ossicles of a
fossil starfish, the possibility of transportation and
abrasion prior to disarticulation must be considered.
LITERATURE CITED
Sladen, W. P.  1889.  Report on the scientific results of
the voyage of H. M. S. Challenger during the years
1873-76:  Zoology - vol. 30, Report on the Asteroidea.
H. M. Stationery Office (London).  893 p.
Steinbeck, J. & Ricketts, E. F.  1941.  Sea of Cortez.  The
Viking Press (New York).  598 p.
PALEOBIOS, No. 2, 1967.
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