Moulting arthropod caught in the act

A Cambrian fossil confirms that early arthropods shed their coats just as they do today.

Until now, the existence of ecdysis (moulting) in early arthropods has been 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 specimen 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, onychophorans, nematodes, nematomorphs, kinorhynchs and priapulids1. It may be that creatures in other groups moult2, but evidence for this is anecdotal. Although ecdysis seems to have been a common feature of these phyla as far back as the Cambrian period3, evidence for moulting during the Cambrian is circumstantial.

For example, a few Cambrian trilobite specimens have been recorded as being preserved in an exuvial configuration4, indicating 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 arthropods is not questioned, and all arthropods living today moult during growth. Still, because the soft-bodied moult of the trilobite 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 provides visible proof that Cambrian arthropods did indeed moult.

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.
figure1

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).

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 similarly 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 explosion 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 morphology7. 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 Marrellomorpha, 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.

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Correspondence to Desmond H. Collins.

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García-Bellido, D., Collins, D. Moulting arthropod caught in the act. Nature 429, 40 (2004). https://doi.org/10.1038/429040a

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