Marrella splendens: a many-segmented arthropod from Burgess Shale

Marrella splendens is a small arthropod from the Wuliuan Burgess Shale, about 505 million years ago. Its head shield carries two pairs of long spines, while a trunk of 26 segments bears repeated biramous appendages. Charles Doolittle Walcott first sketched the animal in 1909 and formally described it three years later as one of the “lace crabs.” The name is memorable; the animal's position in arthropod evolution is less settled. Its distinctive anatomy makes it a striking entry in the Cambrian animal catalogue.

Quick facts

From Walcott's notebook to the lectotype

Walcott's field notebook contains a sketch of Marrella dated 31 August 1909, among his earliest records of the Burgess Shale's soft-bodied fossils. In 1910 he and his son Stuart located the fossil-bearing beds that became the Walcott Quarry. Walcott published the species as Marrella splendens in 1912. The genus honours the Cambridge palaeontologist John Edward Marr, and the species name refers to its striking appearance.

The name is anchored by a lectotype, USNM 57674, held by the Smithsonian National Museum of Natural History. It preserves a small animal only about 17 millimetres long, with limbs and a dark stain near the posterior. Later Royal Ontario Museum collections added specimens with clearer appendages and a fossil caught during moulting. Those discoveries matter because a flattened arthropod can look deceptively simple when only its dorsal outline is visible.

The best-known locality is the Walcott Quarry on Fossil Ridge, but Marrella has also been recorded from the Raymond Quarry, Mount Field, the Tulip Beds on Mount Stephen, Mount Odaray and other nearby sites. Related material assigned to Marrella occurs in Cambrian formations in China, although the classic species M. splendens is tied to the Burgess Shale record.

Long spines frame a body of repeated parts

The head bears a small shield with two pairs of exceptionally long, backward-curving spines. A pair of antennae and short head appendages emerge beneath it. Behind the head are 26 rounded trunk segments. Each carries biramous limbs: an inner branch suited to movement and an outer branch with flattened elements that may have assisted respiration or swimming. The evidence is the repeated arrangement of limbs and segments, not a preserved sequence of strokes.

The paired branches are important because they show that the trunk was not just a flexible chain. Each segment repeated a set of functional structures, while the head concentrated sensory and feeding parts. The large spines make the outline unmistakable, but their precise purpose is unresolved. They may have increased the animal's apparent width or made it harder for a predator to seize; no fossil records a predator rejecting a Marrella because of them.

Several specimens show the cuticle separating in a way consistent with moulting. One fossil documents an individual in the act of shedding its exoskeleton. This provides direct evidence that Marrella moulted, as arthropods do, but it does not reveal how often moults occurred or how long the animal took to grow from one size to another.

A marrellomorph, but not a settled branch on the tree

Marrella is the best-known member of Marrellida, a group whose other representatives range from the Ordovician into the Devonian. They share features such as very long head spines, simple head limbs and a trunk with more than 25 segments. The broader relationships of marrellomorphs have been difficult to resolve because the relevant fossils combine characters that do not map neatly onto one living arthropod group.

Early interpretations allied Marrella with trilobites. Some later phylogenetic analyses have retained an arachnomorph affinity, while others place marrellomorphs on the arthropod stem or nearer mandibulate arthropods. More recent treatments have noted similarities to sea spiders, including aspects of the head appendages and the absence of compound eyes. These are competing hypotheses, not a settled conclusion that Marrella was a sea spider, crab or trilobite.

The disagreement reflects limits in both fossils and methods. Some characters are only visible in a handful of exceptionally preserved specimens; others can be coded differently when a spine or limb is incomplete. Analyses also depend on which fossil taxa and anatomical characters are compared. A secure family-level description can therefore coexist with uncertainty about where the group branches among arthropods.

Abundance tells us about preservation as well as life

Marrella is one of the most numerous animals in the classic Walcott Quarry collections. Its abundance shows that small, soft-bodied arthropods were not rare curiosities in the local ecosystem. The Burgess Shale preserves anatomical details that ordinarily vanish, so counts from this deposit represent both living communities and the unusual conditions that captured them.

Thousands of fossils do not imply that the animals formed schools. A shale bed can combine remains from many burial events, and fossils may be moved or concentrated before final burial. The evidence supports a common and repeatedly preserved animal, not synchronized behaviour. Likewise, the presence of sediment, filter-feeding organisms and predatory arthropods in the same broader fauna does not by itself reveal what Marrella ate.

Some specimens preserve a dark stain associated with decayed tissues. A 2014 synchrotron study interpreted copper enrichment in this stain as possible evidence of hemocyanin, a copper-bearing respiratory protein. A later taphonomic reassessment argued that chalcopyrite in and around some Burgess Shale fossils formed through much younger fluid flow. The blood-chemistry interpretation is therefore contested, not a settled anatomical fact. The fossils offer an unusually detailed view of the limbs and body outline, but colour, social habits and the exact feeding mechanism remain outside what they show.