Page 398 - Introduction to Paleobiology and The Fossil Record

ECDYSOZOA: ARTHROPODS 385
Articulatory structures are variably devel-
oped along the hinge line. Three main types
of hinge are known (Fig. 14.22c). Adont
hinges lack teeth but have a long median
element on the right valve that fi ts a socket
on the left valve. The merodont hinge has
long striated terminal elements on the right
valve fitting respective sockets on the left
valve. Amphidont hinges have short terminal
elements with well-developed teeth on the
right valve.
The carapace is perforated by canals
holding setae that communicate with the exte-
rior. The body is suspended within the cara-
pace, attached by muscles. It is equipped with
seven pairs of appendages, three in front of
the mouth and four behind. The appendages
are specialized, acting as sensory organs, limbs
for the capture and processing of food; more-
over they allow locomotion and general clean-
ing and housekeeping within the carapace.
The animal has a digestive system, sophisti-
cated genitalia and a nervous system; com-
monly a median eye is located behind a
tubercle.
Sexual dimorphism is common and often
reflected in the ostracode carapace (Fig.
14.23). Males commonly have a greater
length : height ratio than the females, whereas
in some benthic Paleozoic ostracodes the
female had a brood pouch in the carapace
wall. Females are often called heteromorphs
while the males, lacking the brood pouch, are
the tecnomorphs.
Ostracodes appeared first during the Early
Cambrian. The archaeocopids were a bizarre
group of large taxa with distinctive append-
ages quite different from more typical ostra-
codes. The group was short lived, disappearing
during the latest Cambrian to earliest Ordovi-
cian. The later history of the group shows a
number of clear trends: evolution of small
size, simpler muscle systems and shorter hinge
lines; the functional signifi cance of these
changes is not immediately obvious.
Large Leperditicopida and Palaeocopida
appeared during the Ordovician, dominating
ostracode faunas until the Devonian, when
deep-water limestones were locally character-
ized by the small spiny myocopids. Many new
groups appeared near the end of the Paleo-
zoic, but hitherto important groups such as
the palaeocopids eventually disappeared in
the Triassic after a decline during the Permian.
(a) (b)
(c) (d)
(e) (f)
Figure 14.23 Some ostracode genera: (a) left
valve of a male living Limnocythene showing
details of appendages (×30); (b, d) left valves of
female and male heteromorphs of Beyrichia
(Silurian) (×18); (c, e) external and internal views
of the left valve of living Patagonacythene (×30);
(f) palaeocopid Kelletina (Carboniferous) (×30).
(Courtesy of David Siveter.)
Although Early Jurassic ostracode assem-
blages are of low diversity the platycopines,
cypridaceans and cytheraceans radiated
steadily during the Jurassic. By the Cenozoic,
the cypridaceans dominated lake environ-
ments whereas the cytheraceans were estab-
lished in marine settings.
Any doubts that real ostracodes did not
actually exist in the Paleozoic have been
dispelled by some remarkable soft-part
preservation, digitally reconstructed from
material from the Silurian Lagerstätten at
Hereford, England (Siveter et al. 2003). The
precise details of the animal’s morphology,
down to the enormous male copulatory organ,
confirm the ostracode identity of the speci-
men; it seems even very similar to living
myodocopids. Lagerstätten, such as the Her-
eford biota, have provided a remarkable series
of windows on arthropod evolution through
time, right back to the Cambrian (Box
14.8).