Page 377 - Introduction to Paleobiology and The Fossil Record

364 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD

appendages that have lateral branches devel- 14.3), with the facial or cephalic suture sepa-

oped from the walking limbs. The trilobito- rating the inner fixed and the outer free cheeks.
morphs include mainly the trilobites and over Although many trilobites lacked eyes, this
15,000 species are known. Trilobites were a may be a secondary condition; despite loss
unique and very successful arthropod group, of vision the cephalic sutures remained. The
common throughout the Paleozoic until their sutures themselves are very important for
extinction at the end of the Permian. There is understanding the functional morphology

no doubt that the trilobites are one of the and classification of the group. There are four
most attractive fossils groups, much prized by main types of suture (Fig. 14.5): the proparian
both amateur and professional collectors mode extends posteriorly in front of the genal
alike. Some of the earliest arthropods were angle, whereas the opisthoparian mode cuts
trilobites, and marine Cambrian strata (as the posterior margin of the cephalon behind
well as many later deposits) are usually cor- the genal angle, and the gonatoparian suture
related on the basis of trilobite assemblages. bisects the genal angle and lateral sutures
The group formed an important part of the follow the margin of the cephalon. In the
mobile benthos, although a few groups were rarer metaparian condition the suture extends
adapted to pelagic life modes. from near the genal angle on the posterior
margin, around the eye to fi nish farther along
the same margin.
Trilobite morphology
On the ventral surface, underneath the
The trilobite exoskeleton (Fig. 14.3), as the cephalon, three plates were associated with
name trilobite (“three-lobed”) suggests, is the anterior soft parts including the mouth.
divided longitudinally into three lobes; the The rostral plate is situated at the anterior
axial lobe protects the digestive system, margin. Posterior to the rostral plate, the
whereas the two pleural lobes cover append- hypostome, a plate of variable shape and size,
ages. In virtually all trilobites a well-defi ned is usually sited under the glabella. The shape
cephalon, thorax and pygidium are devel- and position of the hypostome is of great
oped; the trilobite exoskeleton is composed help in classifying the group. The small metas-
almost entirely of calcite. toma is known from only a few taxa and
The cephalon has a raised axial area, the apparently lay behind the mouth. The dorsal
glabella, with a series of glabellar furrows. margin was protected by a ventral fl ange or
Eyes are commonly developed laterally (Box doublure.

Box 14.2 Ediacaran arthropods?

Are they or aren’t they? Some paleontologists believe they can identify some of the Ediacaran animals
as arthropods or proto-arthropods; others dispute this. Parvancorina (Fig. 14.2), for example, is a
possible candidate, with its shield-shaped outline, strong axial ridge and arched anterior lobes,

together with a convex profile. It really looks like a juvenile trilobite molt stage, but did not have a
mineralized skeleton. Not convinced? Beautifully preserved fossils from a new Cambrian Lagerstätte,

the Chinese Kaili fauna seem to confirm it. Specimens of the genus Skania, first described from the
Burgess Shale, have many similarities to Parvancorina but this genus has an exoskeleton and a better-
defined cephalon and dorsal trunk (Lin et al. 2006). Skania together with Parvancorina and Primi-

caris may have formed a sister group to the Arachnomorpha (which includes the spiders). Moreover

this relationship establishes a Proterozoic root for the Arthropoda and is the first arthropod crown
group that demonstrably ranges through the Precambrian–Cambrian boundary. A Proterozoic origin
for the arthropods may help pin down more precisely their time of divergence from the last common
ancestor of the arthropods and priapulid worms.