Exuvia_Marrela_2004

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40 NATURE | VOL 429 | 6 MAY 2004 | www.nature.com/nature
Until now, the existence of ecdysis(moulting) in early arthropods hasbeen based solely on inference. Here we
describe a 505-million-year-old specimen of
the Cambrian soft-bodied arthropod Marrella
splendens that has been visibly preserved in
the middle of the act of moulting. This speci-
men confirms that early arthropods moulted
during growth, just as they do today.
Ecdysis is a fundamental process that is
thought to characterize the clade Ecdysozoa,
which encompasses all moulting animals,
including arthropods, tardigrades, ony-
chophorans, nematodes, nematomorphs,
kinorhynchs and priapulids1. It may be that
creatures in other groups moult2, but evi-
dence for this is anecdotal. Although ecdysis
seems to have been a common feature of these
phyla as far back as the Cambrian period3,evi-
dence for moulting during the Cambrian is
circumstantial.
For example, a few Cambrian trilobite
specimens have been recorded as being pre-
served in an exuvial configuration4, indicat-
ing that they had just moulted. Mineralized
hard parts of the trilobite exoskeleton found
next to similar, less well mineralized parts
have been interpreted as a new exoskeleton
emerging from the old one, the exuvia. This
interpretation is accepted because, even
though trilobites have been extinct for 250
million years, their classification as arthro-
pods is not questioned, and all arthropods
living today moult during growth. Still,
because the soft-bodied moult of the trilo-
bite is not preserved, the interpretation
remains only an inference.
Direct evidence of moulting is provided
by a specimen of the arthropod M. splendens
(Walcott 1912) from the Middle Cambrian
Burgess Shale of British Columbia. This
specimen is preserved halfway through the
act of moulting, with its cephalic shield and
lateral spines still flexible, squeezing out
through an ecdysial opening at the front of
the head shield of the old exoskeleton. The
antennae are already freed; the distal ends of
the lateral spines, and the rest of the body,
have yet to emerge from the stiff exuvia
(Fig. 1a, b). This remarkable specimen pro-
vides visible proof that Cambrian arthro-
pods did indeed moult.
Why have Cambrian arthropods not
been caught in the act of moulting before?
The answer is twofold. First, a moulting
event like this would only be recorded in
taphonomic conditions that preserved soft
tissues, and most fossil faunas did not 
have such an environment. Second, a non-
mineralized arthropod would have taken a
very short time to emerge completely from
its exuvia; the duration would have been
roughly comparable to the time that a simi-
larly sized, non-mineralized lobster larva
takes to moult (1–10 minutes; ref. 5) or to
the 20 minutes that some cockroaches take
to emerge from their exuviae6.
Why did we find evidence of this act in a
Burgess Shale M. splendens rather than any
other specimen? The Burgess Shale is justly
famous for its exquisitely preserved fossils,
which provide the best view of animals 
following the Cambrian evolutionary explo-
sion of life roughly 520 million years ago.
M.splendens is the most numerous arthropod
in the Burgess Shale — more than 25,000
specimens have already been collected. If any
Cambrian, soft-bodied arthropod is going to
be preserved in the act of moulting, it is most
likely to be M.splendens in the Burgess Shale.
Marrella is considered to be a basal
arthropod because of its generalized mor-
phology7. It has a head with two pairs of
spines and two pairs of appendages; a trunk
with up to 25 segments,each bearing a pair of
biramous (two-branched) appendages; and
a tiny last segment, or telson (Fig. 1c; ref. 8).
The Marrella genus is included in its own
small arthropod group, the Marrellomor-
pha, at the base of the cluster that includes
crustaceans, trilobites and chelicerates9. So
this instance of the early arthropod Marrella
splendens, preserved in the act of moulting
505 million years ago, confirms that ecdysis
was occurring early in arthropod evolution.
Diego C. García-Bellido*†,
Desmond H. Collins*
*Department of Palaeobiology, Royal Ontario
Museum, Toronto, Ontario M5S 2C6, Canada
e-mail: desc@rom.on.ca 
†Departamento de Paleontología, Facultad de
Ciencias Geológicas, Universidad Complutense de
Madrid, 28040 Madrid, Spain
1. Aguinaldo, A. M. A. et al. Nature 387, 489–493 (1997).
2. Nielsen, C. Zool. Scripta 32, 475–482 (2003).
3. Valentine, J. W. & Collins, A. G. Evol. Dev. 2, 152–156 (2000).
4. Brandt, D. S. Alcheringa 26, 399–421 (2002).
5. Matsuda, H., Takenouchi, T. & Yamakawa, T. Fish. Sci. 69,
124–130 (2003).
6. Kunkel, J. G. Biol. Bull. 148, 259–273 (1975).
7. Edgecombe, G. D. & Ramsköld, L. J. Paleontol. 73, 263–287
(1999).
8. Whittington, H. B. Geol. Surv. Can. Bull. 209, 1–24 (1971).
9. Wills, M. A. et al. in Arthropod Fossils and Phylogeny
(ed. Edgecombe, G. D.) 33–105 (Columbia Univ. Press, New
York, 1998).
Competing financial interests: declared none.
brief communications arising online
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Optical media: Superluminal speed of information?
G. Nimtz (doi:10.1038/nature02586)
Reply: M. D. Stenner, D. J. Gauthier & M. A. Neifeld
(doi:10.1038/nature02587)
Moulting arthropod caught in the act
A Cambrian fossil confirms that early arthropods shed their coats just as they do today.
Figure 1 The oldest known fossil of an arthropod in the act of moulting: Marrella splendens, from the Middle Cambrian Burgess Shale of British
Columbia, Canada. a, Specimen of M. splendens (ROM 56781) emerging and pulling out the flexible lateral spines from the old exoskeleton
(exuvia). b, Camera lucida drawing of the same specimen. Scale bar for a and b, 5 mm. c, Reconstruction of Marrella (modified from ref. 8).
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