Marrella is an extinct animal that
lived during the
Cambrian period. It is the most common
fossil
found in the Burgess Shale in the Canadian Rockies in British Columbia,
Canada.
Marrella was a small bottom-dwelling ("benthic") animal, about ¾ inch (2 centimeters) or less
in length. It had a body consisting of 24 to 26 segments each with a
pair of appendages - the appendages each have two branches, a lower
branch for walking, and a upper branch with a feathery gill. The animal
also a head with two long backward pointing spikes, and eyes beneath
the head.
There has been some debate about how Marrella
should be classified.
The first
fossil
was described by its discoverer, Charles Walcott,
informally as a "lace crab", and more formally as a odd
Trilobite.
However, while it seems clear that Marrella is a member of the Arthropod
phylum (the group containing insects, crustaceans, millipedes, etc.), it
does not easily fit into any of the major subdivisions,
Marrella has major differences (too many antennae, too few segments per leg,
and too few legs) to be a
Trilobite,
and does not seem to be a Crustacean (a member of the group containing crabs, lobsters, shrimp, etc.),
and nor does it seem to be a Chelicerate (a member of the group containing
scorpions, spiders, horseshoe crabs, etc.).
Today, the general thinking is that Marrella must be a stem group Arthropod -
that is to say that it is descended from an ancestor of most/all other
Arthropod groups.
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