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ECDYSOZOA: ARTHROPODS 369
protaspid meraspid holaspid
Figure 14.6 Molt phases of the Bohemian trilobite Sao hirsuta Barrande. Magnifi cations: protaspid
stages approximately ×9, meraspid stages approximately ×7.5 and the holaspid stages approximately
×0.5. (Based on Barrande 1852.)
recently been reclassified as a soft-bodied tri- in favor of possible sensory setae (stiff hair-
lobite. It is now known from other Cambrian like structures). These specialized forms may
Lagerstätten together with a number of related have periodically concealed themselves in the
taxa. The group is probably a sister group sediment. Trimerus had a cephalon and pygid-
to the trilobites + agnostids (Edgecombe & ium fashioned in the shape of a shovel that
Ramsköld 1999) and recent cladistic analyses might have helped it plow through the sedi-
confirm this phylogenetic position, basal to ment. The cyclopygid Opipeuter, from the
Trilobitomorpha, and within the larger clade Lower Ordovician of Spitzbergen, Ireland and
Arachnomorpha (Cotton & Braddy 2004). Utah, on the other hand, seems to have been
Trilobite morphology is hugely variable, an active pelagic swimmer; it had a long,
presumably reflecting their broad range of slender body with a fl exible exoskeleton and
adaptations (Fig. 14.7). Most trilobites were large eyes, just like a modern shrimp-like
almost certainly benthic or nektobenthic, amphipod, together with a widespread
leaving a variety of tracks and trails in the distribution.
marine sediments of the Paleozoic seas (see Trilobites show extensive convergence: the
Chapter 19). With the exception of the pha- same broad morphotypes appear repeatedly
copids that may have hunted, the simple in different lineages, presumably refl ecting
mouthparts of the trilobites suggest a diet of repeats of the same life strategies. Richard
microscopic organisms and a detritus-feeding Fortey and Robert Owens documented seven
strategy. ecomorphic groups ranging from the turber-
Many trilobites developed spinose exoskel- culate, mobile phacomorphs to the smooth,
etons. The spines reduce their weight : area infaunal illaenimorphs (Fig. 14.8) and these
ratio and this suggested that these trilobites were related to their wide variety of lifestyles
adopted a floating, planktonic life strategy, (Fig. 14.9).
supposedly backed up by the fact they occa-
sionally had inflated glabellae. More recently,
however, the suggestion that their glabella Distribution and evolution: trilobites in space
was filled with gas has been shown to be a and time
little fanciful, and it seems more likely that Trilobite faunas have formed the basis for
these forms used their long spines to spread many paleogeographic reconstructions of the
the weight on a soft muddy substrate. Down- Cambrian and Ordovician world. During
ward-directed spines probably held the thorax the Cambrian, biogeographic patterns were
and pygidium well above the sediment–water complex, but some provinces have been
interface. In some forms, the spines probably defined, such as the high-latitude Atlantic
aided shallow burrowing when the body region (with redlichiids) and the low-latitude
flexed. Spines are most extravagantly devel- Pacific region (with olenellids). Statistical
oped in the odontopleurids. analysis of Ordovician trilobite faunas in the
Some trilobites such as Cybeloides and early 1970s established a low-latitude bathy-
Encrinurus evolved eyes on stalks or others, urid province (Laurentia), an intermediate to
for example Trinucleus, lost them altogether high-latitude asaphid province (Baltica) and a
