Hemichordata; Class Graptolithina
Time: Lower Ordovician
are 17 to 30 mm
Site: Fillmore Formation, Millard County, Utah
is a fine assemblage of graptolites known as Phyllograptus archaios.
Graptolites are colonial animals belonging to the hemichordates.
The term originates from the patronymic genus Graptolithus. The
graptolites have a cosmopolitan distribution, and so serve as index
fossils in many locations. The Graptoloidea were pelagic, drifting
with the currents, and were the most important members of the plankton
before dying out in the early Devonian. This genus draws its name
from its leaf-like shape. There are a dozen example sof this large
species, along with a few thoracic segments of a trilobite at the
fossil pictures to enlarge