Ordivician

aragonite/calcite

two forms of calium carbonate, the ocean has oscillated between calcite and aragonite (aragonite shells rapidly dissolve after death)

arandapsis

one of the earliest crown-group fish

filter feeding

feeding by filtering out planktons or nutrients suspended in the water

fish

parphyletic group of vertabrates that lack limbs with digits. Most breathe through gills.

mass extinctions

widespread and rapid decrease in the biodiversity on Earth.

paraphyletic clade

a group made up of a common ancestor and some, but not all of its descendents.