Page 388 - Introduction to Paleobiology and The Fossil Record

ECDYSOZOA: ARTHROPODS 375

FAW
PGW
OCW
EGW
FAL
PLL
GLL

THL

PYL
PAW
PYW
(a) (b) (c)

6.0 18.0
RMA: Y = 0.976X – 0.229 RMA: Y = 3.429X – 1.318
16.0
5.0
14.0
Frontal area length (mm) 3.0 Thoracic length (mm) 10.0
12.0
4.0

8.0

6.0
2.0
n = 152
n = 152
1.0 r = 0.9653 4.0 r = 0.9889
p << 0.01 2.0 p << 0.01
0.0 0.0
0.0 1.0 2.0 3.0 4.0 5.0 6.0 0.0 1.0 2.0 3.0 4.0 5.0 6.0
Occipital-glabellar length (mm) Occipital-glabellar length (mm)
(d) (e)
Figure 14.12 Landmark analysis of Aulacopleura. (a) Measurements, (b) landmarks, (c) plot of
landmarks, (d) bivariate plot of occipital–glabellar length versus frontal area length, and (e)
bivariate plot of occipital–glabellar length versus thoracic length. FAW, width of frontal area;
PGW, OCW, width of occipital glabella; EGW, FAL, length of frontal area; PLL, GLL, THL,
length of thorax; PYL, length of pygidium; PAW, PYW, width of pygidium; RMA, reduced major
axis. (Courtesy of Nigel Hughes.)

In addition there are signs of predation that survivors and that predators preferred to
often show an asymmetric distribution. Pre- attack on the left-hand side of the trilobite,
dation scars are three times as likely to be and we never see those victims.
present on the right-hand side of the exoskel-
eton as the left. If predators preferred to
attack from the right then perhaps there was SUBPHYLUM CHELICERATA
already a lateralization of their nervous system The chelicerates are a diverse and heteroge-
and other organs (Babcock 1993). However, neous group including the mites, scorpions
it can be argued that these specimens were the and spiders. The familiar horseshoe crab,