Page 380 - Introduction to Paleobiology and The Fossil Record

ECDYSOZOA: ARTHROPODS 367

doublure. Natant hypostomes were not large smooth forms such as Bumastus and
attached to the skeleton, whereas the impen- Illaenus, having impendent hypostomes.
dent hypostome was attached to the doublure, The lichids contain mainly spiny forms
but its shape was quite different from the with conterminant hypostomes. Apart from
glabella above. Lichas itself the order also includes the spiny
Some authorities have excluded the distinc- odontopleurids such as Leonaspis.
tive agnostids from the Trilobitomorpha and Phacopids were mainly proparian trilobites
there is now strong evidence to suggest they with schizochroal eyes (Box 14.3) and lacking
were crustaceans. Agnostids were small to rostral plates that ranged from the Lower
minute, usually blind animals with subequal Ordovician to Upper Devonian. The order
cephala and pygidia and only two thoracic includes the large tuberculate Cheirurus,
segments; they were probably planktonic, Calymene with a marked gonatoparian suture,
which may account for their very wide and Dalmanites with long genal spines,
distribution. kidney-shaped eyes, spinose thoracic segments
The redlichiids include Olenellus, with 18– and the pygidium extended as a long spine.
44 spiny thoracic segments and typical of the The ptychopariids are all characterized by
Atlantic province, Redlichia itself, more natant hypostomes and include some special-
typical of the Pacific province, and the large, ized groups. For example Triarthrus was

spiny, micropygous Paradoxides, common in modifi ed for burrowing, Conocoryphe was
the high latitudes of the Mid Cambrian. blind and Harpes had a sensory fringe round
Corynexochid trilobites were a mixed bag the cephalon.
of taxa; the order includes genera with con- Asaphids had either conterminant or
terminant hypostomes such as Olenoides and impendent hypostomes and include Asaphus

Box 14.3 Vision in trilobites: from corrective lenses to sunshades

Trilobites have the oldest known visual system based on eyes: paleontologists can even look through
the ancient lenses and see the world as trilobites saw it! Trilobite eyes are compound, consisting of
many lenses, just like those of the crustaceans and insects. Euan Clarkson’s classic studies (1979)
emphasized the functions of the two main types of lens arrangement found in trilobites (Fig. 14.4).
The trilobite eye generally consists of many lenses of calcite with the c-axis (the main optical axis)
perpendicular to the surface of the eye. The more primitive and widespread holochroal eye has many
close-packed lenses, all about the same size, covered by a single membrane. The more advanced and
complex schizochroal condition has no modern analog and has larger, discrete lenses arranged in

rows or files. It is uncertain how this system operated in detail; presumably it offered higher-quality
images than those of the holochroal systems. Moreover both mature holochroal and schizochroal

configurations apparently developed from immature schizochroal conditions. Thus early growth
stages of holochroal eyes in quite different groups such as those of the Cambrian eodiscid Shizhu-
discus with the oldest visual system in the world and Phacops from the Devonian with a schizochroal
system have broadly similar arrangements, suggesting that the latter system developed by pedomor-
phosis (see p. 145). A third, less well known optical system, abathochroal, is confined to a short-lived

Cambrian group, the eodiscids (most of which were blind). Less is known about them than other
visual systems and their origins remain obscure.
But could such visual systems cope with bright sunlight? Probably not, and this suggests that
many groups were nocturnal. But not all. A remarkable Devonian phacopid trilobite, Erbenochile,
from Morocco, actually has a type of sunshade covering the top of a column of lenses (Fortey &

Chatterton 2003). The animal could scan the seafloor for potential prey without the distraction of
direct sunlight.
Continued