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Moulting Arthropod Caught in the Act a Cambrian Fossil Confirms That Early Arthropods Shed Their Coats Just As They Do Today

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Moulting Arthropod Caught in the Act a Cambrian Fossil Confirms That Early Arthropods Shed Their Coats Just As They Do Today brief communications Moulting arthropod caught in the act A Cambrian fossil confirms that early arthropods shed their coats just as they do today. ntil now, the existence of ecdysis (moulting) in early arthropods has Ubeen 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 Figure 1 The oldest known fossil of an arthropod in the act of moulting: Marrella splendens, from the Middle Cambrian Burgess Shale of British have been interpreted as a new exoskeleton Columbia, Canada. a, Specimen of M. splendens (ROM 56781) emerging and pulling out the flexible lateral spines from the old exoskeleton emerging from the old one, the exuvia. This (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). interpretation is accepted because, even though trilobites have been extinct for 250 its exuvia; the duration would have been splendens,preserved in the act of moulting million years, their classification as arthro- roughly comparable to the time that a simi- 505 million years ago, confirms that ecdysis pods is not questioned, and all arthropods larly sized, non-mineralized lobster larva was occurring early in arthropod evolution. living today moult during growth. Still, takes to moult (1–10 minutes; ref. 5) or to Diego C. García-Bellido*†, because the soft-bodied moult of the trilo- the 20 minutes that some cockroaches take Desmond H. Collins* bite is not preserved, the interpretation to emerge from their exuviae6. *Department of Palaeobiology, Royal Ontario remains only an inference. Why did we find evidence of this act in a Museum, Toronto, Ontario M5S 2C6, Canada Direct evidence of moulting is provided Burgess Shale M. splendens rather than any e-mail: [email protected] by a specimen of the arthropod M. splendens other specimen? The Burgess Shale is justly †Departamento de Paleontología, Facultad de (Walcott 1912) from the Middle Cambrian famous for its exquisitely preserved fossils, Ciencias Geológicas, Universidad Complutense de Burgess Shale of British Columbia. This which provide the best view of animals Madrid, 28040 Madrid, Spain specimen is preserved halfway through the following the Cambrian evolutionary explo- 1. Aguinaldo, A. M. A. et al. Nature 387, 489–493 (1997). act of moulting, with its cephalic shield and sion of life roughly 520 million years ago. 2. Nielsen, C. Zool. Scripta 32, 475–482 (2003). 3. Valentine, J. W. & Collins, A. G. Evol. Dev. 2, 152–156 (2000). lateral spines still flexible, squeezing out M.splendens is the most numerous arthropod 4. Brandt, D. S. Alcheringa 26, 399–421 (2002). through an ecdysial opening at the front of in the Burgess Shale — more than 25,000 5. Matsuda, H., Takenouchi, T. & Yamakawa, T. Fish. Sci. 69, the head shield of the old exoskeleton. The specimens have already been collected. If any 124–130 (2003). antennae are already freed; the distal ends of Cambrian, soft-bodied arthropod is going to 6. Kunkel,J.G.Biol. Bull. 148, 259–273 (1975). 7. Edgecombe, G. D. & Ramsköld, L. J. Paleontol. 73, 263–287 the lateral spines, and the rest of the body, be preserved in the act of moulting, it is most (1999). have yet to emerge from the stiff exuvia likely to be M.splendens in the Burgess Shale. 8. Whittington, H. B. Geol. Surv. Can. Bull. 209, 1–24 (1971). (Fig. 1a, b). This remarkable specimen pro- Marrella is considered to be a basal 9. Wills, M. A. et al. in Arthropod Fossils and Phylogeny vides visible proof that Cambrian arthro- arthropod because of its generalized mor- (ed. Edgecombe, G. D.) 33–105 (Columbia Univ. Press, New 7 York, 1998). pods did indeed moult. phology .It has a head with two pairs of Competing financial interests: declared none. Why have Cambrian arthropods not spines and two pairs of appendages; a trunk been caught in the act of moulting before? with up to 25 segments,each bearing a pair of The answer is twofold. First, a moulting biramous (two-branched) appendages; and brief communications arising online event like this would only be recorded in a tiny last segment, or telson (Fig. 1c; ref. 8). ➧ www.nature.com/bca taphonomic conditions that preserved soft The Marrella genus is included in its own tissues, and most fossil faunas did not small arthropod group, the Marrellomor- Optical media: Superluminal speed of information? have such an environment. Second, a non- pha, at the base of the cluster that includes G. Nimtz (doi:10.1038/nature02586) mineralized arthropod would have taken a crustaceans, trilobites and chelicerates9.So Reply: M. D. Stenner, D. J. Gauthier & M. A. Neifeld very short time to emerge completely from this instance of the early arthropod Marrella (doi:10.1038/nature02587) 40 NATURE | VOL 429 | 6 MAY 2004 | www.nature.com/nature © 2004 Nature Publishing Group.
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