Geoscience Reference
In-Depth Information
D
ESCRIPTION OF
B
EECHER
S
T
RILOBITE
B
ED BIOTA
Triarthrus
By far the commonest trilobite in Beecher's
Bed is
Triarthrus eatoni
(Hall, 1838) (
55,
56
). The Beecher's Bed specimens were
originally identified as
Triarthrus beckii
Green, 1832 by Matthew, Walcott, Beecher,
and Raymond, but New York State
Palaeontologist Rudolf Ruedemann (1926)
and later authors agreed it is really
T.
eatoni
.
T. eatoni
and
T. beckii
can be
differentiated by the shape of the palpebral
lobe and possibly the glabella (
50
). Also,
T.
beckii
occurs mainly in the lowest part of the
Upper Ordovician, but is replaced by
T.
eatoni
in higher strata of the Upper
Ordovician. Indeed, they have been used in
biostratigraphy: geologists who encounter
Ordovician shales in New York can
determine their stratigraphic position by
identifying the species of
Triarthrus
which
55
55 Pyritized Triarthrusseen from the ventral side,
showing the hypostome between the bases of the
antennae, biramous appendages on cephalic,
thoracic, and pygidial segments, and note the little,
stout spines on some podomeres YPM. Animal is
about 3 cm 1.2 in long excluding antennae.
56
56 Triarthrusin an
apparent side view, showing
the biramous appendages
emerging from beneath the
dorsal exoskeleton YPM.
Animal is about 3 cm 1.2 in
long excluding antennae.
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