Page 391 - Introduction to Paleobiology and The Fossil Record

378 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD
Baltoeurypterus
(a)
Parastylonurus
(c)
Figure 14.16 Eurypterid functional morphology showing (a) swimming and (b, c) walking life modes.
(From Clarkson 1998.)
and spiny, and the ophthalmic ridges and
cardiac lobe are usually well preserved.
Although some enigmatic xiphosure-like taxa,
such as Eolimulus, have been described from
the Lower Cambrian, the first are probably of
Early Ordovician age. A trend towards larger
size and a shorter fused abdomen is seen in
most groups. Carboniferous taxa, for example
Belinurus and Euproops, have well-developed
cardiac lobes and ophthalmic ridges together
with fused abdomen. Mesolimulus from the
Upper Jurassic Solnhofen Limestone, however,
is smaller than living taxa but it too was
marine and left clear evidence of its append-
ages in a trackway in the lagoonal muds of
Bavaria.
The eurypterids include the largest known
and most scary of the arthropods, some
approaching 2 m in length, that range in age
from Ordovician to Permian. They occupied
a variety of environments from marine to
freshwater and some may have been amphibi-
ous. Much of our knowledge of eurypterid
morphology has been derived from superbly
preserved specimens from Silurian dolomites
on the island of Saaremaa that were acid-
etched from the rock by the Swedish paleon-
tologist Gerhard Holm at the end of the
1800s. The exoskeleton is long and relatively
narrow. The subrectangular prosoma bears a
variety of appendages; the first pair of append- ably over 100,000 known species of arach-
ages were chelicerae adapted for grasping
while others were modifi ed for movement,
with the last pair of large, paddle-like append- feeding pedipalps and four pairs of walking
Mixopterus
(b)
ages probably adapted for swimming. The
opisthosoma, comprising the pre- and posta-
bdomen, consists of 12 visible segments. The
telson was variably developed as a long spine
or a fl attened paddle.
With the exception of generalist feeders
such as Baltoeurypterus, most eurypterids
were predators, attacking fi shes and other
arthropods. Moreover, where relatively
common, a number of eurypterid-dominated
communities have been described, emphasiz-
ing the range of habitats occupied by these
large, versatile animals. In the Silurian rocks
of the Anglo-Welsh area, Pterygotus and its
allies are associated with normal marine
faunas, whereas Eurypterus itself preferred
inshore environments. Hughmilleria and
related forms dominated brackish to fresh-
water communities.
Many more than 50 genera of eurypterids
have been described. The group was most
abundant during the Silurian and Devonian,
but only two families, the adeloopthalmids
and the hibbertopterids, survived into the
Permian. The varied styles of locomotion of
the group suggest a diversity in lifestyles
(Fig. 14.16).
The arachnids are a huge group of terres-
trial carnivores containing mites, scorpions,
spiders (Box 14.5) and ticks. There are prob-
nids. The prosoma consists of six segments
with a pair of chelicerae, a pair of sensory or