Marella
Encyclopedia
Marrella splendens is an arthropod
known from the middle Cambrian
Burgess Shale
of British Columbia
. It is the most common animal in the Burgess Shale.
from the Burgess Shale. Walcott described Marrella informally as a "lace crab" and described it more formally as an odd trilobite. It was later reassigned to the now defunct class
Trilobitoidea in the Treatise on Invertebrate Paleontology
. In 1971, Whittington
did a thorough redescription of the animal and, on the basis of its legs, gill
s and head appendages, concluded that it was neither a trilobite, nor a chelicerate
, nor a crustacean
.
Taken with two other unexpectedly unique arthropod-like organisms, Opabinia
and Yohoia
, Marrella helped to demonstrate that the soft-bodied Burgess fauna was more complex and diverse than had previously been suspected.
, one long and sweeping, the second shorter and stouter. Marrella has a body composed of 24–26 body segments, each with a pair of branched appendages. The lower branch of each appendage is a leg for walking, while the upper branch is a long, feathery gill. There is a tiny, button-like telson
at the end of the thorax
. It is unclear how the unmineralized head and spines
were stiffened. Marrella has too many antennae, too few cephalic legs, and too few segments per leg to be a trilobite. It lacks the three pairs of legs behind the mouth that are characteristic of crustacea. The legs are also quite different from those of crustaceans. The identification of a diffraction grating
pattern on well-preserved Marrella specimens proves that it would have harboured an iridescent sheen—and thus would have appeared colourful. A dark stain
is often present at the rear of the organism, probably representing decay fluids.
(bottom-dwelling) marine scavenger
living on detrital
and particulate material. One exceptional specimen shows the organism fossilized in the act of moulting
.
s, it is no more closely related to this group than any other arthropod.
. Most Marrella specimens herald from the 'Marrella bed', a thin horizon, but it is common in most other outcrops of the shale. Over 25 000 specimens have been collected;
Marrellomorph
s—that is, Marrella-like organisms—are well distributed in other Cambrian deposits, and are indeed known from sediments as late as the Devonian
.
Arthropod
An arthropod is an invertebrate animal having an exoskeleton , a segmented body, and jointed appendages. Arthropods are members of the phylum Arthropoda , and include the insects, arachnids, crustaceans, and others...
known from the middle Cambrian
Cambrian
The Cambrian is the first geological period of the Paleozoic Era, lasting from Mya ; it is succeeded by the Ordovician. Its subdivisions, and indeed its base, are somewhat in flux. The period was established by Adam Sedgwick, who named it after Cambria, the Latin name for Wales, where Britain's...
Burgess Shale
Burgess Shale
The Burgess Shale Formation, located in the Canadian Rockies of British Columbia, is one of the world's most celebrated fossil fields, and the best of its kind. It is famous for the exceptional preservation of the soft parts of its fossils...
of British Columbia
British Columbia
British Columbia is the westernmost of Canada's provinces and is known for its natural beauty, as reflected in its Latin motto, Splendor sine occasu . Its name was chosen by Queen Victoria in 1858...
. It is the most common animal in the Burgess Shale.
History
Marrella was the first fossil collected by Charles Doolittle WalcottCharles Doolittle Walcott
Charles Doolittle Walcott was an American invertebrate paleontologist. He became known for his discovery in 1909 of well-preserved fossils in the Burgess Shale of British Columbia, Canada.-Early life:...
from the Burgess Shale. Walcott described Marrella informally as a "lace crab" and described it more formally as an odd trilobite. It was later reassigned to the now defunct class
Class (biology)
In biological classification, class is* a taxonomic rank. Other well-known ranks are life, domain, kingdom, phylum, order, family, genus, and species, with class fitting between phylum and order...
Trilobitoidea in the Treatise on Invertebrate Paleontology
Treatise on Invertebrate Paleontology
The Treatise on Invertebrate Paleontology published by the Geological Society of America and the University of Kansas Press, is a definitive multi-authored work of some 50 volumes, written by more than 300 paleontologists, and covering every phylum, class, order, family, and genus of fossil and...
. In 1971, Whittington
Harry B. Whittington
Harry Blackmore Whittington FRS was a British paleontologist based at the Department of Earth Sciences, Cambridge, and was affiliated to Sidney Sussex College. He attended Handsworth Grammar School in Birmingham, followed by a degree and Ph.D in geology from the University of Birmingham...
did a thorough redescription of the animal and, on the basis of its legs, gill
Gill
A gill is a respiratory organ found in many aquatic organisms that extracts dissolved oxygen from water, afterward excreting carbon dioxide. The gills of some species such as hermit crabs have adapted to allow respiration on land provided they are kept moist...
s and head appendages, concluded that it was neither a trilobite, nor a chelicerate
Chelicerata
The subphylum Chelicerata constitutes one of the major subdivisions of the phylum Arthropoda, and includes horseshoe crabs, scorpions, spiders and mites...
, nor a crustacean
Crustacean
Crustaceans form a very large group of arthropods, usually treated as a subphylum, which includes such familiar animals as crabs, lobsters, crayfish, shrimp, krill and barnacles. The 50,000 described species range in size from Stygotantulus stocki at , to the Japanese spider crab with a leg span...
.
Taken with two other unexpectedly unique arthropod-like organisms, Opabinia
Opabinia
Opabinia is an animal genus found in Cambrian fossil deposits. Its sole species, Opabinia regalis, is known from the Middle Cambrian Burgess Shale of British Columbia, Canada. Fewer than twenty good specimens have been described; 3 specimens of Opabinia are known from the Greater Phyllopod bed,...
and Yohoia
Yohoia
Yohoia is a tiny, extinct animal from the Cambrian period that has been found as fossils in the Burgess Shale formation of British Columbia, Canada. It has been placed among the arachnomorphs, a group of arthropods that includes the chelicerates and trilobites. Their sizes range from 7 to 23 mm...
, Marrella helped to demonstrate that the soft-bodied Burgess fauna was more complex and diverse than had previously been suspected.
Morphology
Marrella itself is a small animal, 2 cm or less in length. The head shield has two pairs of long rearward spikes. On the underside of the head are two pairs of antennaeAntenna (biology)
Antennae in biology have historically been paired appendages used for sensing in arthropods. More recently, the term has also been applied to cilium structures present in most cell types of eukaryotes....
, one long and sweeping, the second shorter and stouter. Marrella has a body composed of 24–26 body segments, each with a pair of branched appendages. The lower branch of each appendage is a leg for walking, while the upper branch is a long, feathery gill. There is a tiny, button-like telson
Telson
The telson is the last division of the body of a crustacean. It is not considered a true segment because it does not arise in the embryo from teloblast areas as do real segments. It never carries any appendages, but a forked "tail" called the caudal furca is often present. Together with the...
at the end of the thorax
Thorax
The thorax is a division of an animal's body that lies between the head and the abdomen.-In tetrapods:...
. It is unclear how the unmineralized head and spines
Spine (zoology)
A spine is a hard, thorny or needle-like structure which occurs on various animals. Animals such as porcupines and sea urchins grow spines as a self-defense mechanism. Spines are often formed of keratin...
were stiffened. Marrella has too many antennae, too few cephalic legs, and too few segments per leg to be a trilobite. It lacks the three pairs of legs behind the mouth that are characteristic of crustacea. The legs are also quite different from those of crustaceans. The identification of a diffraction grating
Diffraction grating
In optics, a diffraction grating is an optical component with a periodic structure, which splits and diffracts light into several beams travelling in different directions. The directions of these beams depend on the spacing of the grating and the wavelength of the light so that the grating acts as...
pattern on well-preserved Marrella specimens proves that it would have harboured an iridescent sheen—and thus would have appeared colourful. A dark stain
Dark stain
A dark stain is often associated with fossils of the Burgess Shale, representing decay fluids that were squashed out of the organism during the taphonomic process.-Occurrence:...
is often present at the rear of the organism, probably representing decay fluids.
Ecology
Marrella is thought to have been a benthicBenthic zone
The benthic zone is the ecological region at the lowest level of a body of water such as an ocean or a lake, including the sediment surface and some sub-surface layers. Organisms living in this zone are called benthos. They generally live in close relationship with the substrate bottom; many such...
(bottom-dwelling) marine scavenger
Scavenger
Scavenging is both a carnivorous and herbivorous feeding behavior in which individual scavengers search out dead animal and dead plant biomass on which to feed. The eating of carrion from the same species is referred to as cannibalism. Scavengers play an important role in the ecosystem by...
living on detrital
Detritus
Detritus is a biological term used to describe dead or waste organic material.Detritus may also refer to:* Detritus , a geological term used to describe the particles of rock produced by weathering...
and particulate material. One exceptional specimen shows the organism fossilized in the act of moulting
Ecdysis
Ecdysis is the moulting of the cuticula in many invertebrates. This process of moulting is the defining feature of the clade Ecdysozoa, comprising the arthropods, nematodes, velvet worms, horsehair worms, rotifers, tardigrades and Cephalorhyncha...
.
Taxonomy
It is currently accepted that Marrella is a stem group arthropod—in other words, it is descended from an ancestor common to it and most or all of the later major arthropod groups. Despite its similarity to the trilobiteTrilobite
Trilobites are a well-known fossil group of extinct marine arthropods that form the class Trilobita. The first appearance of trilobites in the fossil record defines the base of the Atdabanian stage of the Early Cambrian period , and they flourished throughout the lower Paleozoic era before...
s, it is no more closely related to this group than any other arthropod.
Occurrence
Marrella is the most abundant genus in the Burgess ShaleBurgess Shale
The Burgess Shale Formation, located in the Canadian Rockies of British Columbia, is one of the world's most celebrated fossil fields, and the best of its kind. It is famous for the exceptional preservation of the soft parts of its fossils...
. Most Marrella specimens herald from the 'Marrella bed', a thin horizon, but it is common in most other outcrops of the shale. Over 25 000 specimens have been collected;
Marrellomorph
Marrellomorph
The Marrellomorphs are a clade of stem-group arthropods known from the Cambrian to the early Devonian. They lacked mineralised hard parts, so are only known from areas of exceptional preservation, limiting their fossil distribution....
s—that is, Marrella-like organisms—are well distributed in other Cambrian deposits, and are indeed known from sediments as late as the Devonian
Devonian
The Devonian is a geologic period and system of the Paleozoic Era spanning from the end of the Silurian Period, about 416.0 ± 2.8 Mya , to the beginning of the Carboniferous Period, about 359.2 ± 2.5 Mya...
.