Marrella is an extinct group of ancient sea creatures called marrellomorph arthropods. These creatures lived during the Middle Cambrian period in North America and Asia. Marrella is the most frequently found animal in the Burgess Shale, a famous fossil site in British Columbia, Canada, where thousands of its remains have been discovered. Fewer remains of Marrella have also been found in rock layers in China.

History

Marrella was the first fossil collected by Charles Doolittle Walcott from the Burgess Shale in 1909. The name honors Dr. John Edward Marr (1857–1933), a British geologist and professor at Cambridge University. Walcott described Marrella informally as a "lace crab" and formally as an unusual trilobite. Later, it was placed in the now defunct class Trilobitoidea in the Treatise on Invertebrate Paleontology. In 1971, Whittington conducted a detailed redescription of the animal. Based on its legs, gills, and head appendages, he concluded that Marrella was not a trilobite, chelicerate, or crustacean.

Marrella is one of several unique arthropod-like organisms found in the Burgess Shale. Other examples include Opabinia and Yohoia. The unusual and varied features of these creatures were surprising when they were discovered. The fossils helped scientists understand that the soft-bodied Burgess fauna was more complex and diverse than previously thought.

Morphology

Marrella specimens can be as small as 2.4 millimetres or as large as 24.5 millimetres in length. The head shield had two pairs of long, curved spines, with the back pair having a saw-like edge. No eyes were found on the head. Underneath the head, a pair of long, flexible antennae extended forward at an angle of 15 to 30 degrees. These antennae were made up of about 30 segments, and some joints between segments had tiny hair-like structures. Behind the antennae, a pair of short, paddle-like swimming appendages attached. Each appendage had one long base segment and five shorter segments, with the edges of the shorter segments covered in tiny hairs.

The body had at least 17 segments, with larger specimens having over 26 segments. Each segment had a pair of two-branched appendages. The lower branch of each appendage was long and leg-like, with five segments, and the last segment had a claw. These lower branches became smaller as they moved toward the back of the body, with the size decreasing more rapidly after the 9th pair. The upper branch, which acted as a gill, had thin, thread-like structures. At the end of the thorax, there was a tiny, button-like structure called a telson.

A 1998 study suggested that lines on the front spines of well-preserved Marrella specimens might have formed a pattern that could have made the creature appear shiny. However, other scientists have questioned whether similar patterns in other animals also produced shine. Dark spots are often found on the back of specimens, which may indicate waste or a blood-like fluid. One specimen was found in the process of shedding its exoskeleton, showing that the shell split at the front of the shield.

Ecology

Marrella was probably an active swimmer that moved near the seafloor. Its limbs moved in a way similar to the backstroke, and the large spines helped keep it steady and may have protected it from predators. Scientists think Marrella may have been a filter feeder. It used its back limbs to filter tiny food particles from the water while swimming, then moved the particles forward to its mouth.

Taxonomy

Marrella belongs to the Marrellida group within the Marrellomorpha, a category of arthropods with unknown relationships that lived from the Cambrian to Devonian periods. In the Marrellida group, Marrella is considered the earliest known member. A cladogram showing the Marrellida group, as described by Moysiuk et al. in 2022.

"Mimetaster florestaensis"
"Mimetaster hexagonalis"
"Moroccan marrellid ("Furca mauretanica")"

Occurrence

Marrella is the most common type of organism found in the Burgess Shale. Most Marrella fossils come from a thin layer called the "Marrella bed," but they are also found in many other areas where the Burgess Shale is exposed. More than 25,000 Marrella fossils have been discovered. Of these, 5,028 were found in the Greater Phyllopod bed, where they make up 9.56% of the fossils in that area.

A small number of fossils from an unknown species of Marrella have been found in the Kaili Formation in Yunnan, China. These fossils are from the Wuliuan stage of the Cambrian period. One incomplete fossil from another unknown species of Marrella has also been found in the Balang Formation in Yunnan, China, dating to Cambrian Stage 4. Both of these fossil sites are older than the Burgess Shale.