Benton M.J. Introduction to paleobiology and the fossil record

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358 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD

develop a particular search image when

seeking their favored prey. Living terrestrial

snails show a wide range of color patterns and

the purpose of this variability may be to

confuse predators like the song thrush by pre-

senting a wide range of images. If a predator

targets as prey one particular variant in the

population, then other variants would be free

Box 13.9 Mesozoic marine revolution

The post-Paleozoic seas and oceans were probably different in many ways from those before. One

key difference is the more intense predator–prey relationships, signaled by the Mesozoic marine

revolution (MMR). During this interval, shell predation by, for example, crushing and drilling,

became commoner. A Mesozoic arms race, with predators evolving more highly developed weapons

of attack, was balanced by prey evolving better defensive mechanisms and structures. Thus whereas

crustaceans developed the efﬁ ciency of their claws, jaws and pincers, mollusks grew thicker, more

highly-ornamented shells and perhaps burrowed deeper and faster into the sediment. This form of

escalation is somewhat different from the mechanism of coevolution; organisms adapt to each other

rather than merely change together. In this system, predators will always be one step ahead of their

prey. Liz Harper (2006) has reviewed the evidence for post-Paleozoic escalation, plotting the ranges

of durophagous body and trace fossils that may have been predatory together with evidence for

crushing and drilling of shells (Fig. 13.27). The MMR may have been a complex series of events:

(i) a Triassic radiation of decapods, sharks and bony ﬁ shes; (ii) Jurassic-Cretaceous radiations of

malacostracans and marine reptiles; (iii) a Paleogene explosion of neogastropods, teleosts and sharks;

and (iv) the Neogene appearance of mammals and birds.

Tertiary

Cretaceous

Jurassic

Triassic

100

120

140

160

180

200

220

240

∗

∗∗ ∗∗

∗

Taphonomic

void?

marine reptiles?

Drillers

Crushers

Nautiloids

Ammonoids

Brachyuran crabs

Stomatopods

Sharks

Rays

Bssal actinopterygii

Teleosts

Placodonts

Marine reptile

Sea mammal

Shore birds

Naticid gastropods

Muricid gastropods

Octopods

Body fossil record Trace fossil record

drill holes repaired crushes

St Cassian

Sporadic evidence

of crushing

Figure 13.27 Stratigraphic relationships between predators and prey during the Mesozoic marine

revolution. The St. Cassian Formation, Italy has excellent preservation of aragonitic gastropods.

Double asterisks show the level of the St. Cassian Formation, while single asterisks indicate

sporadic evidence of crushing. (From Harper 2006.)

SPIRALIANS 2: MOLLUSKS 359

Box 13.10 Fossil annelids and their jaws

The annelids are segmented protostomes that are represented today by animals such as the

earthworms and leaches. Recent species are important, widely distributed, benthic predators and

occur from intertidal to abyssal depths. Modern molecular studies suggest they form a sister group

to the mollusks and, in fact, share a number of morphological characters such as the possession of

chaetae. In general the group has a fairly sparse fossil record, appearing ﬂ eetingly in Lagerstätte

deposits such as the Burgess Shale and Mazon Creek fauna. However many residues of acid-etched

Paleozoic limestones contain scolecodonts (Fig. 13.28). These were the jaws of ancient annelids and

are abundant and diverse at many horizons. They were similar to conodonts (see p. 429), forming

multielement apparatuses with similar functions but were composed of collagen ﬁ bers and various

minerals such as zinc. The group ﬁ rst appeared in the Lower Ordovician and diversiﬁ ed rapidly to

become common in Upper Ordovician-Devonian carbonate facies. Scolecodonts were relatively rare

after the Permian, but nevertheless have proved useful in biostratigraphic and thermal maturation

studies.

Figure 13.28 Scolecodont morphology. Reconstruction of the polychaete jaw apparatus of the

Ordovician Ramp hoprion Kielan-Jaworowska. (Courtesy of Olle Hints.)

to recover until a switch in images was pro-

duced. Although such relationships are docu-

mented for some Mesozoic (Box 13.9) and

Cenozoic faunas, data are sparse for the

Paleozoic. On the other hand, close relatives

of the mollusks, the annelids, may have been

important predators equipped with an efﬁ -

cient jaw apparatus (Box 13.10).

360 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD

Review questions

1 There has been some difﬁ culty identifying

the ﬁ rst mollusk. What are the key fea-

tures of the phylum and how would they

be recognized in the ﬁ rst mollusk?

2 Many taxa that form part of the Early

Cambrian biota are undoubtedly mol-

lusks. Which mollusk groups are already

present in the small shelly fauna?

3 Theoretical morphospace is a useful tool

to investigate shell morphology. Some

groups are more constrained in their

developmental opportunities than others.

What advantages should univalved mol-

lusks have over bivalved mollusks in a

quest to generate extreme morphotypes?

4 Belemnites seem an unlikely group to test

models for microevolution. What condi-

tions should be met in such tests of micro-

evolutionary hypotheses?

5 The Mesozoic marine revolution (or arms

race) was a complex ecological event that

set the agenda for marine life in the

Modern evolutionary fauna. How did

mollusks react to predation pressures?

Further reading

Clarkson, E.N.K. 1998. Invertebrate Palaeontology and

Evolution, 4th edn. Chapman and Hall, London.

(An excellent, more advanced text; clearly written

and well illustrated.)

Lehmann, U. 1981. The Ammonites – their Life and their

World. Cambridge University Press, Cambridge, UK.

Morton, J.E. 1967. Molluscs. Hutchinson, London.

Peel, J.S., Skelton, P.W. & House, M.R. 1985. Mollusca.

In Murray, J.W. (ed.) Atlas of Invertebrate Macrofos-

sils. Longman, London. (A useful, mainly photo-

graphic review of the group.)

Pojeta, J. Jr., Runnegar, B., Peel, J.S. & Gordon, M. Jr.

1987. Phylum Mollusca. In Boardman, R.S.,

Cheetham, A.H. & Rowell, A.J. (eds) Fossil Inverte-

brates. Blackwell Scientiﬁ c Publications, Oxford,

UK, pp. 270–435. (A comprehensive, more advanced

text with emphasis on taxonomy; extravagantly

illustrated.)

Vermeij, G.J. 1987. Evolution and Escalation. An Eco-

logical History of Life. Princeton University Press,

Princeton, NJ. (Visionary text.)

References

Batt, R. 1993. Ammonite morphotypes as indicators of

oxygenation in a Cretaceous epicontinental sea.

Lethaia 26, 49–63.

Caron, J.-B., Acheltema, A., Schander, A. & Rudkin, D.

2006. A soft-bodied mollusc with radula from the

Middle Cambrian Burgess Shale. Nature 442,

159–163.

Christensen, W.K. 2000. Gradualistic evolution in

Belemnitella from the middle Campanian of Lower

Saxony, NW Germany. Bulletin of the Geological

Society of Denmark 47, 135–63.

Doyle, P. & MacDonald, D.I.M. 1993. Belemnite battle-

ﬁ elds. Lethaia 26, 65–80.

Erwin, D.H. 2007. Disparity: morphological pattern

and developmental context. Palaeontology 50,

57–73.

Fedonkin, M. & Waggoner, B.M. 1997. The Late Pre-

cambrian fossil Kimberella is a mollusk-like bilate-

rian organism. Nature 388, 868–71.

Harper, E.M. 2006. Disecting post-Palaeozoic arms

races. Palaeogeography, Palaeoclimatology, Palaeo-

ecology 232, 322–43.

Jacobs, D.K. & Landman, N.H. 1993. Nautilus – a

poor model for the function and behavior of ammo-

noids. Lethaia 26, 101–11.

Milsom, C. & Rigby, S. 2004. Fossils at a Glance.

Blackwell Publishing, Oxford.

Peel, J.S. 1991. Functional morphology, evolution and

systematics of early Palaeozoic univalved molluscs.

Grønlands Geologiske Undersøgelse 161, 116 pp.

Raup, D.M. 1966. Geometric analysis of shell coiling:

general problems. Journal of Paleontology 40,

1178–90.

Reboulet, S., Giraud, F. & Proux, O. 2005. Ammonoid

abundance variations related to changes in trophic

conditions across the Oceanic Anoxic Event 1d

(Latest Albian, SE France). Palaios 20, 121–41.

Runnegar, B. & Pojeta, J. 1974. Molluscan phylogeny:

the palaeontological viewpoint. Science 186, 311–17.

Sigwart, J.W. & Sutton, M.D. 2007. Deep molluscan

phylogeny: synthesis of palaeontological and neon-

tological data. Proceedings of the Royal Society B

274, 2413–19.

Stanley, S.M. 1970. Relation of shell form to life habits

of Bivalvia. Geological Society of America Memoir

125

, 296 pp.

Swan, A.R.H. 1990. Computer simulations of inverte-

brate morphology

In Bruton, D.L. & Harper, D.A.T.

(eds) Microcomputers in Palaeontology. Contribu-

tions from the Palaeontological Museum, University

of Oslo, Vol. 370. pp. 32–45. University of Oslo,

Oslo.

Vinther, J. & Nielsen, C. 2004. The Early Cambrian

Halkieria is a mollusc. Zoologica Scripta 34, 81–9.

Wagner, P.J. 1995. Diversity patterns among early gas-

tropods: contrasting taxonomic and phylogenetic

descriptions. Paleobiology 21, 410–39.

Wani, R., Kase, T., Shigeta, Y. & De Ocampo, R. 2005.

New look at ammonoid taphonomy, based on ﬁ eld

experiments with modern chambered nautilus.

Geology 33, 849–52.

Williamson, P.G. 1981. Palaeontological documentation

of speciation in Cenozoic molluscs from Turkana

basin. Nature 293, 140–2.

Chapter 14

Ecdysozoa: arthropods

Key points

• Arthropods – such as lobsters, spiders, beetles and trilobites – have legs, a segmented

body plan with jointed appendages and the ability to molt.

• The ﬁ rst major arthropod faunas of the Early Cambrian appear bizarre by modern

standards but probably were no more morphologically different to each other than are

living faunas.

• A number of arthropod-like animals in the Ediacara biota suggest an ancient origin for

the phylum.

• Trilobites appeared in the Early Cambrian and during the Paleozoic evolved advanced

visual systems and enrolment structures while pursuing a variety of benthic and pelagic

life styles.

• The largest arthropods were the chelicerates and included the giant eurypterids that

patrolled marine marginal environments during the Silurian and Devonian.

• Myriapods represent the earliest terrestrial body fossils in the Mid Ordovician, but

trackways indicate euthycarcinoids (i.e. stem-group mandibulates) moved onto land

even earlier, in the Late Cambrian.

• Insects ﬁ rst appeared during the Early Devonian and diversiﬁ ed rapidly; there are prob-

ably 10 million species of living insects.

• Insects had probably already evolved ﬂ ight before the Mid Carboniferous, when giant

dragonﬂ ies patrolled the forests.

• The crustaceans include many familiar groups such as crabs, lobsters and shrimps,

together with the barnacles and ostracodes.

• Much of our knowledge of the early history of the phylum has come from exceptionally

preserved fossils from the Cambrian Burgess Shale, Chengjiang and Sirius Passet

faunas.

Whence we see spiders, ﬂ ies, or ants entombed and preserved forever in amber, a more

than royal tomb.

Francis Bacon, English philosopher (1561–1626)

362 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD

ARTHROPODS: INTRODUCTION

Arthropods are a very common and spectacu-

larly diverse group of legged invertebrates

accounting for about three-quarters of all

species living on the planet today, largely

because of the phenomenal abundance of the

insects. The basic body plan – conspicuously

segmented, with jointed appendages adapted

for feeding, locomotion and respiration –

together with a tough exoskeleton, ﬁ rst

appeared during the Early Cambrian and has

since been exploited by a huge variety of living

and fossil arthropods that pursue many life-

styles. All members of this phylum have both

segmented bodies and appendages (Fig. 14.1);

moreover the animal is differentiated into a

head, thorax and abdomen, with often the

head and thorax fused to form the cephalo-

thorax. The possession of mandibles, or hard

mouthparts, equipped many arthropods with

the ability to process a wide variety of

foods.

The arthropod exoskeleton is constructed

mainly from the organic substance chitin.

This is often hardened or sclerotized by

calcium carbonate or calcium phosphate, so

the potential for preservation is excellent

across the group. The exoskeleton acts as a

base for the attachment of locomotory

muscles, permitting rapid movement, and is

not usually mineralized. Although many

arthropods undergo metamorphosis, virtually

all the main groups grow by molting or

ecdysis; ﬁ rst the endoskeleton is dissolved and

second the old exoskeleton is detached along

sutures while the new exoskeleton is gener-

ated. Exuviae, or cast-off coverings, are all

that remain of the previous skeleton or cuticle

of the animal: one arthropod can thus produce

many potential skeletal fossils in its lifetime.

During a geological history of at least 540

million years, the ﬁ ve subphyla of arthropods

(Box 14.1) have adapted to life in marine,

freshwater and terrestrial environments. For

a long time their closest living relatives were

thought to be the segmented annelid worms,

but new studies show that the closest sister

group of arthropods is a clade of unsegmented

worms that includes the priapulids and the

nematodes or round worms. Their segmenta-

tion may thus either have arisen independently

to that of the annelids, or may have been

inherited from a very deep ancestor to both

groups.

EARLY ARTHROPOD FAUNAS

A huge variety of bizarre arthropod types

formed much of the basis for the Cambrian

explosion (see p. 249). Over 20 groups of

arthropod have been described from the Mid

Trilobitomorph:

trilobite

Chelicerate:

scorpion

Crustacean:

decapod

Crustacean:

decapod

Crustacean:

ostracode

Crustacean:

barnacle

Insect:

palaeopteran

Insect:

coleopteran

Figure 14.1 Some of the main arthropod groups: a variety of forms based on a simple body plan of a

tough exoskeleton and jointed limbs.

ECDYSOZOA: ARTHROPODS 363

Box 14.1 Classiﬁ cation of arthropods

There are currently differences in the status given to the main arthropod groups. If the Arthropoda

is in fact a superphylum, the following groupings are phyla. However, some authorities have assigned

the following superclass status within the phylum Arthropoda. The classiﬁ cation here is a compro-

mise. Basal to the phylum are a number of minor but evolutionarily important groups, such as the

tardigrades (water bears), that are now known from the Cambrian.

Subphylum TRILOBITOMORPHA

• Trilobites and their relatives; animals with a cephalon, thorax and pygidium; the body, length-

wise, has an axial lobe and two lateral pleural lobes

• Cambrian to Permian

Subphylum CHELICERATA

• Large group with a body divided into two tagmata; the prosoma (which bears six pairs of append-

ages, the ﬁ rst being the chelicerae or pincer-like appendages, giving the group its name), and the

opisthosoma with an extended tail or telson

• ?Cambrian to Recent

Subphylum MYRIAPODA

• Includes the ﬂ exible centipedes together with the millipedes

• ?Ordovician, Silurian to Recent

Subphylum HEXAPODA

• Highly-diverse group, with a head, thorax and abdomen and six legs; includes the ants, beetles,

dragonﬂ ies, ﬂ ies and wasps

• Devonian to Recent

Subphylum CRUSTACEA

• Includes the bivalved phyllocarids; Early Paleozoic taxa were ancestral to the crabs, shrimps and

lobsters

• Cambrian to Recent

Cambrian Burgess Shale and related deposits

(see Box 14.8); some have even been assigned

to new phyla, emphasizing the expansive

nature of the explosion, truly evolution’s “big

bang”. Stephen Jay Gould, in his bestseller

Wonderful Life argued that morphological

disparity during the Cambrian was greater

than at any time since. Nevertheless, cladistic,

and phenetic analyses of both morphological

and taxonomic criteria suggested otherwise

(Briggs et al. 1993). Rather, the morphologi-

cal disparity among the Cambrian arthropods

is not markedly different from that seen across

living taxa, they just look stranger to us. But

it is nonetheless remarkable that very early in

their history arthropods attained high levels

of morphological disparity not really exceeded

during the next 500 million years of evolu-

tion. Moreover, our knowledge of the Cam-

brian arthropod record, particularly that of

soft-bodied organisms, is probably not nearly

as complete as that of the modern fauna and

we should expect further surprises as more

Cambrian Lagerstätten are investigated (Box

14.2).

SUBPHYLUM TRILOBITOMORPHA

The trilobitomorphs are highly derived arthro-

pods lacking specialized mouthparts, and

with tagmata comprising the cephalon, thorax

and pygidium, together with trilobitomorph

364 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD

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 proﬁ le. 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 conﬁ rm it. Specimens of the genus Skania, ﬁ rst described from the

Burgess Shale, have many similarities to Parvancorina but this genus has an exoskeleton and a better-

deﬁ ned 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 ﬁ rst 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.

appendages that have lateral branches devel-

oped from the walking limbs. The trilobito-

morphs include mainly the trilobites and over

15,000 species are known. Trilobites were a

unique and very successful arthropod group,

common throughout the Paleozoic until their

extinction at the end of the Permian. There is

no doubt that the trilobites are one of the

most attractive fossils groups, much prized by

both amateur and professional collectors

alike. Some of the earliest arthropods were

trilobites, and marine Cambrian strata (as

well as many later deposits) are usually cor-

related on the basis of trilobite assemblages.

The group formed an important part of the

mobile benthos, although a few groups were

adapted to pelagic life modes.

Trilobite morphology

The trilobite exoskeleton (Fig. 14.3), as the

name trilobite (“three-lobed”) suggests, is

divided longitudinally into three lobes; the

axial lobe protects the digestive system,

whereas the two pleural lobes cover append-

ages. In virtually all trilobites a well-deﬁ ned

cephalon, thorax and pygidium are devel-

oped; the trilobite exoskeleton is composed

almost entirely of calcite.

The cephalon has a raised axial area, the

glabella, with a series of glabellar furrows.

Eyes are commonly developed laterally (Box

14.3), with the facial or cephalic suture sepa-

rating the inner ﬁ xed and the outer free cheeks.

Although many trilobites lacked eyes, this

may be a secondary condition; despite loss

of vision the cephalic sutures remained. The

sutures themselves are very important for

understanding the functional morphology

and classiﬁ cation of the group. There are four

main types of suture (Fig. 14.5): the proparian

mode extends posteriorly in front of the genal

angle, whereas the opisthoparian mode cuts

the posterior margin of the cephalon behind

the genal angle, and the gonatoparian suture

bisects the genal angle and lateral sutures

follow the margin of the cephalon. In the

rarer metaparian condition the suture extends

from near the genal angle on the posterior

margin, around the eye to ﬁ nish farther along

the same margin.

On the ventral surface, underneath the

cephalon, three plates were associated with

the anterior soft parts including the mouth.

The rostral plate is situated at the anterior

margin. Posterior to the rostral plate, the

hypostome, a plate of variable shape and size,

is usually sited under the glabella. The shape

and position of the hypostome is of great

help in classifying the group. The small metas-

toma is known from only a few taxa and

apparently lay behind the mouth. The dorsal

margin was protected by a ventral ﬂ ange or

doublure.

ECDYSOZOA: ARTHROPODS 365

With the exception of the agnostids and

eodiscids, which have two and two to three

thoracic segments, respectively, trilobites are

polymeric, that is usually having up to about

40 thoracic segments. The trilobite pygidium

is usually a plate of between one and 30 fused

segments. Most Cambrian trilobites have

small, micropygous pygidia, whereas later

forms are either heteropygous, where the

pygidium is smaller than the cephalon,

(a) (b)

(e) (f)

Figure 14.2 (a–d) Parvancoria from the Ediacara biota, Flinders Ranges, South Australia;

(e, f) Skania from the Middle Cambrian of Guizhou Province, South China. Scale bar: 3.5 mm

(a), 4 mm (b), 10 mm (c, d), 2 mm (e, f). (Courtesy of Jih-Pai (Alex) Lin.)

366 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD

or macropygous, where the pygidium is

larger.

Like virtually all arthropods the trilobites

grew by ecdysis or molting (Fig. 14.6). Ontog-

eny involved the periodic discarding of spent

exoskeletons or exuviae. Initial molt stages

were quite different from those of adults.

After a phaselus larval stage that swam freely

in the plankton, the protaspis stage is a minute

disk with a segmented median lobe destined

to become the glabella. The next, meraspis,

stage has a discrete, transitory pygidium

where thoracic segments form at its anterior

margin and are released at successive molts to

form the thorax. The holaspis stage has a full

complement of thoracic segments for the

species but growth continues through further

molts and maturity may not be reached until

some time after the holaspis stage was reached.

Clearly in many trilobite-dominated faunas,

counts of skeletal remains will signiﬁ cantly

over-represent the relative numbers of living

animals in the community. Many researchers

divide the number of exuviae by about six to

eight to obtain a more realistic census of the

trilobite population in a typical community

(see p. 93).

During times of stress, to avoid unpleasant

environmental conditions or perhaps an atten-

tive predator, most trilobites could roll up like

a carpet. During the Paleozoic, a number of

groups, including asaphids, calymenids, pha-

copids and trinucleids (see p. 374), evolved a

variety of sophisticated structures to enhance

this behavior, although Cambrian taxa prob-

ably had a limited ability to curl up. Spheroi-

dal enrolment involved articulation of all the

thoracic segments to form a ball, whereas in

the less common discoidal mode of enrolment

the thorax and pygidium were merely folded

over the cephalon. Cambrian trilobites could

certainly enrol, but it was not until the Ordo-

vician that true coaptative structures, locking

parts of the skeleton against each other, ﬁ rst

appeared. For example, in the phacopids,

tooth and socket pairs were developed on the

cephalic and pygidial doublure, respectively;

these opposing structures clicked together to

hold the trilobite in a tight ball, presenting

only the exoskeleton to the world outside

(Bruton & Haas 2003).

Main trilobite groups and lifestyles

Although some workers have split the trilo-

bites into two orders, the Agnostida and the

Polymerida, most currently recognize about

nine orders of trilobite based on a spectrum

of characters, including the anatomy of their

ontogenetic stages and more recently the loca-

tion and morphology of the hypostome. In the

most primitive conterminant condition, the

hypostome is similar in shape to the glabella

and is attached to the anterior part of the

(b)

(c)

glabella

eye

anterior border

glabellar furrow

free

cheek

facial

suture

occipital

ring

genal

spine

axial ring

pleuron

articulating

facet

fixed

cheek

(a)

cephalon

thorax

pygidium

doublure

free

cheek

rostral

plate

hypostoma

axial furrow

gill-bearing limb

walking limb

Figure 14.3 Trilobite morphology: (a) external

morphology of the Ordovician trilobite

Hemiarges; (b) generalized view of the anterior

of the Silurian trilobite Calymene revealing

details of the underside of the exoskeleton; and

segment of the exoskeleton.

ECDYSOZOA: ARTHROPODS 367

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 ﬁ les. 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

conﬁ gurations 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 conﬁ ned 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 seaﬂ oor for potential prey without the distraction of

direct sunlight.

doublure. Natant hypostomes were not

attached to the skeleton, whereas the impen-

dent hypostome was attached to the doublure,

but its shape was quite different from the

glabella above.

Some authorities have excluded the distinc-

tive agnostids from the Trilobitomorpha and

there is now strong evidence to suggest they

were crustaceans. Agnostids were small to

minute, usually blind animals with subequal

cephala and pygidia and only two thoracic

segments; they were probably planktonic,

which may account for their very wide

distribution.

The redlichiids include Olenellus, with 18–

44 spiny thoracic segments and typical of the

Atlantic province, Redlichia itself, more

typical of the Paciﬁ c province, and the large,

spiny, micropygous Paradoxides, common in

the high latitudes of the Mid Cambrian.

Corynexochid trilobites were a mixed bag

of taxa; the order includes genera with con-

terminant hypostomes such as Olenoides and

large smooth forms such as Bumastus and

Illaenus, having impendent hypostomes.

The lichids contain mainly spiny forms

with conterminant hypostomes. Apart from

Lichas itself the order also includes the spiny

odontopleurids such as Leonaspis.

Phacopids were mainly proparian trilobites

with schizochroal eyes (Box 14.3) and lacking

rostral plates that ranged from the Lower

Ordovician to Upper Devonian. The order

includes the large tuberculate Cheirurus,

Calymene with a marked gonatoparian suture,

and Dalmanites with long genal spines,

kidney-shaped eyes, spinose thoracic segments

and the pygidium extended as a long spine.

The ptychopariids are all characterized by

natant hypostomes and include some special-

ized groups. For example Triarthrus was

modiﬁ ed for burrowing, Conocoryphe was

blind and Harpes had a sensory fringe round

the cephalon.

Asaphids had either conterminant or

impendent hypostomes and include Asaphus

Continued