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they contain. However, Cisne
et al
. (1980)
showed, in a detailed morphometric study
of 639 specimens of
Triarthrus
through the
Trenton Group, that the two species
actually grade into one another. The fossils
show evidence of an evolutionary cline: the
two end-members of the cline are
demonstrably different species, but any
attempt to identify an intermediate is
difficult.
For this reason, Cisne
et al
. (1980)
considered the two morphs really to be
one species, called
Triarthrus beckii
because this was the earlier of the two
names to be given to the fossil, by Green
(1832). Ludvigsen and Tuffnell (1983)
re-examined the
Triarthrus
problem and
came to the conclusion that the two really
are distinct species, distinguishable on the
basis of the shape of the palpebral lobe.
There is some evidence that
T. eatoni
preferred deep-water muddy environ-
ments while
T. beckii
occurs more
commonly in rocks which represent
shallower water environments (Cisne
et al
.,
1980; Whittington and Almond, 1987).
Trilobites have three tagmata (main
body parts) called, from front to back:
cephalon, thorax, and pygidium (
50
), but
note that the three lobes which give these
animals their name - trilobite - refer to
the median axial and lateral, pleural lobes
(
50
). The single pair of antennae are long
and consist of many segments which taper
gradually to the distal end. The early
descriptions by Beecher (1895a, 1896)
showed four pairs of biramous (two-
branched) limbs beneath the cephalon
(
57
). This leg count was followed by
Raymond (1920) and Størmer (1951), but
Cisne (1974) questioned this number, and
his X-ray investigations (Cisne, 1981;
57
)
showed only three. The study by
Whittington and Almond (1987)
confirmed that three was the correct
number. Each thoracic segment bears a
pair of appendages underneath. Beecher's
studies showed the thoracic appendages to
consist of two branches, like those of the
cephalon. The basal segment (podomere)
is the coxa; this segment is attached to the
body, though its connection is not clear in
any specimen studied, and from the coxa
arise the two limb branches. Raymond
(1920, Figure 10) suggested that the coxae
were shown in this diagram to be too far
apart, and this has been confirmed by the
studies of Whittington and Almond
(1987). The coxae bear spines on their
inner side, which, by acting one against the
opposite coxa as a pair, are able to push
food forwards towards the mouth. In front
of the mouth is a plate called the
hypostome, which serves to prevent food
passing beyond the mouth, though this
structure is poorly known in
Triarthrus
.
The inner branch (leg branch or
endopodite) consists of six podomeres
following the coxa; the proximal ones bear
downward-pointing spines, and there are
5 7
5 7 X-ray photograph of a
Triarthrustaken by John
L Cisne, preserved in a similar
position to the one in 56. Note
the biramous appendages; the
appendages on the far side can
be seen also YPM. Animal is
about 3 cm 1.2 in long
excluding antennae.
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