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

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368 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD

and Ceratopyge together with pelagic forms

such as Cyclopyge and Remopleurides, and

the stratigraphically important trinucleids

such as Onnia, Cryptolithus and Tretaspis.

The proetids were isopygous forms with

large glabellae and long hypostomes having

genal spines and large holochroal eyes. The

group ranged from the Lower Ordovician to

the Upper Permian. Proetus was a small form

with a relatively large, inﬂ ated and often

granular glabella, known from the Ordovi-

cian to the Devonian. Phillipsia, one of the

youngest members of the order, was a small

isopygous genus with large crescent-shaped

eyes and an opisthoparian suture.

The naraoids, including Naraoia itself and

Tegopelte, have often been included with the

trilobites. They were not calciﬁ ed and lacked

thoracic segments. The group was restricted

to the Middle Cambrian. Naraoia was ﬁ rst

described from the Burgess Shale as a bran-

chiopod crustacean, but it has only more

(a)

(b)

Figure 14.4 Vision in trilobites: (a) lateral view of a complete specimen of Cornuproetus,

Silurian, Bohemia (×4); (b) detail of the compound eye of Cornuproetus (×20); (c) holochroal

compound eye of Pricyclopyge, Ordovician, Bohemia (×6); (d) schizochroal compound eye of

Phacops, Devonian, Ohio (×4); and (e) schizochroal compound eye of Reedops, Devonian,

Bohemia (×5). (Courtesy of Euan Clarkson.)

proparian gonatoparian opisthoparian

Figure 14.5 Facial sutures: the tracks of the

proparian, gonatoparian and opisthoparian

sutures. The lateral suture (not illustrated)

follows the lateral margin of the cephalon.

ECDYSOZOA: ARTHROPODS 369

recently been reclassiﬁ ed as a soft-bodied tri-

lobite. It is now known from other Cambrian

Lagerstätten together with a number of related

taxa. The group is probably a sister group

to the trilobites + agnostids (Edgecombe &

Ramsköld 1999) and recent cladistic analyses

conﬁ rm this phylogenetic position, basal to

Trilobitomorpha, and within the larger clade

Arachnomorpha (Cotton & Braddy 2004).

Trilobite morphology is hugely variable,

presumably reﬂ ecting their broad range of

adaptations (Fig. 14.7). Most trilobites were

almost certainly benthic or nektobenthic,

leaving a variety of tracks and trails in the

marine sediments of the Paleozoic seas (see

Chapter 19). With the exception of the pha-

copids that may have hunted, the simple

mouthparts of the trilobites suggest a diet of

microscopic organisms and a detritus-feeding

strategy.

Many trilobites developed spinose exoskel-

etons. The spines reduce their weight : area

ratio and this suggested that these trilobites

adopted a ﬂ oating, planktonic life strategy,

supposedly backed up by the fact they occa-

sionally had inﬂ ated glabellae. More recently,

however, the suggestion that their glabella

was ﬁ lled with gas has been shown to be a

little fanciful, and it seems more likely that

these forms used their long spines to spread

the weight on a soft muddy substrate. Down-

ward-directed spines probably held the thorax

and pygidium well above the sediment–water

interface. In some forms, the spines probably

aided shallow burrowing when the body

ﬂ exed. Spines are most extravagantly devel-

oped in the odontopleurids.

Some trilobites such as Cybeloides and

Encrinurus evolved eyes on stalks or others,

for example Trinucleus, lost them altogether

in favor of possible sensory setae (stiff hair-

like structures). These specialized forms may

have periodically concealed themselves in the

sediment. Trimerus had a cephalon and pygid-

ium fashioned in the shape of a shovel that

might have helped it plow through the sedi-

ment. The cyclopygid Opipeuter, from the

Lower Ordovician of Spitzbergen, Ireland and

Utah, on the other hand, seems to have been

an active pelagic swimmer; it had a long,

slender body with a ﬂ exible exoskeleton and

large eyes, just like a modern shrimp-like

amphipod, together with a widespread

distribution.

Trilobites show extensive convergence: the

same broad morphotypes appear repeatedly

in different lineages, presumably reﬂ ecting

repeats of the same life strategies. Richard

Fortey and Robert Owens documented seven

ecomorphic groups ranging from the turber-

culate, mobile phacomorphs to the smooth,

infaunal illaenimorphs (Fig. 14.8) and these

were related to their wide variety of lifestyles

(Fig. 14.9).

Distribution and evolution: trilobites in space

and time

Trilobite faunas have formed the basis for

many paleogeographic reconstructions of the

Cambrian and Ordovician world. During

the Cambrian, biogeographic patterns were

complex, but some provinces have been

deﬁ ned, such as the high-latitude Atlantic

region (with redlichiids) and the low-latitude

Paciﬁ c region (with olenellids). Statistical

analysis of Ordovician trilobite faunas in the

early 1970s established a low-latitude bathy-

urid province (Laurentia), an intermediate to

high-latitude asaphid province (Baltica) and a

meraspidprotaspid 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.)

(a) (b)

(c)

(d)

(e)

(f)

(g)

(h)

(i) (j) (k)

Figure 14.7 Some common trilobite taxa: (a) Agnostus (×10), (b) Pagetia (×5), (c) Paradoxides (×0.5),

(d, e) Illaenus (×1), (f) Warburgella (×3), (g, h) Phacops (×0.75), (i) Spherexochus (×0.75), (j) Calymene

(×0.75), (k) Leonaspis (×2). Magniﬁ cations are approximate.

ECDYSOZOA: ARTHROPODS 371

high-latitude Selenopeltis province (Gond-

wana). Despite a number of modiﬁ cations,

this basic pattern is generally accepted (see

also Chapter 2).

Some Early Paleozoic trilobite communities

may also be interpreted as showing an

onshore–offshore spectrum, from shallow-

water illaenid–cheirurid associations to deep-

water olenid communities (Fig. 14.10). In

general terms, the shallow-water, pure car-

bonate, illaenid–cheirurid communities appar-

ently lasted the longest.

Trilobites (such as Choubertella and

Schmidtiellus) ﬁ rst appeared in the Early

Cambrian and the group survived until the

end of the Permian, when the last genera, such

as Pseudophillipsia, disappeared (Fig. 14.11).

In a history of 350 million years, the basic

body plan was essentially unchanged, but

many modiﬁ cations promoted trilobite

(a) (b) (c) (d)

(e) (f) (g) (h)

(i) (j) (k) (l)

Pelagic Illaenimorph

Marginal cephalic spines Olenimorph

Pitted fringe Miniaturization Atheloptic

Figure 14.8 Trilobite ecomorphs: pelagic (a, b), illaenomorph (c, d), marginal cephalic spines (e, f),

olenimorph (g, h), pitted fringe (i), miniature (j, k) and atheloptic (blind) (l) morphotypes. (Based on

Fortey & Owens 1990.)

372 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD

abundance and diversity. Not surprisingly the

Trilobitomorpha has been a major source of

evolutionary data and there have been many

studies on the functional morphology of the

group (e.g. Bruton & Haas 2003).

Trilobites have provided key evidence in

studies of macroevolution, especially in the

controversy over punctuated equilibrium and

phyletic gradualism (see Chapter 5). Trilo-

bites have complex morphologies that can be

easily measured and analyzed statistically

(Box 14.4). The studies of Niles Eldredge and

Stephen Jay Gould on the number of lens ﬁ les

of the Devonian trilobite Phacops rana formed

the basis for their punctuated equilibrium

model. On the other hand Peter Sheldon’s

investigation of over 15,000 specimens of

Mid Ordovician trilobites demonstrated

gradual changes in the number of pygidial

ribs, possibly a slower, adaptive, ﬁ ne-tuning

to more stable environments. Euan Clarkson’s

survey of microevolutionary change in Upper

Cambrian olenid trilobites from the Alum

Shales of Sweden provided evidence of similar

gradual change (Fig. 14.13). Macroevolution-

ary change in trilobites was effected by heter-

ochrony (see p. 145). Pedomorphosis during

ontogeny of the animal as a whole or applied

to particular organs such as the eyes gener-

ated new species and new biological

structures.

Trilobites show a number of evolutionary

trends. Through time, for example, those tri-

lobites that adopted enrolment as a defensive

strategy became better at it: the spines and

sockets around their exoskeletons came to ﬁ t

and lock better and better. Early trilobites

probably rolled up into a rough ball, but

could be prized apart by a persistent

predator; later enrolling trilobites were impen-

etrable. There was a reduction in the size of

the rostral plate and in some groups there was

an increase in spinosity and a trend from

micropygy to isopygy. The evolution of schizo-

chroal visual systems appeared, by pedomor-

phosis, during the Early Ordovician in the

phacopids.

Trilobite abnormalities and injuries

Trilobites have left a rich record of abnor-

malities and injuries, some evidence that they

faced problems during ecdysis and that they

were attacked by predators (Fig. 14.14). There

are three main types of abnormality (Owen

1985):

1 Injuries sustained during molting.

2 Pathological conditions resulting from

disease and parasitic infestations.

3 Teratological effects arising through some

embryological or genetic malfunctions.

pelagic – swimming

and floating

enrolled

resting

feeding

molting

molt stage

mobile

nektobenthos

infaunal – living in burrows

Figure 14.9 Lifestyles of the trilobites: a mosaic of selected Lower Paleozoic trilobites in various life

attitudes.

ECDYSOZOA: ARTHROPODS 373

pelagic fauna

illaenid–

cheirurid

association

nileid

association

olenid

association

graptolite

shales

Valhallfonna

Formation

(a)

(b)

(c)

shelf

faunas

“intermediate”

faunas

illaenid–cheirurid–

lichid association

Opsimasaphus–

Nankinolithus

association

platform edge

(or tectonically

elevated area

within basin)

Novaspis–cyclopygid

association

Crugan Mudstone fauna

Dwyfor Mudstone fauna

typical lithofacies

carbonate

calcareous

argillaceous

mudstone

graptolitic shales and

turbidites

sparry algal limestonesshale beltshelf limestonessandstone belt

Delops–Miraspis association

Radnoria–Cornuproetus

association

Dalmanites–Raphiophorus

association

Proetus–Warburgella

association

Acaste–Trimerus association

Acaste

Trimerus

Cornuproetus

Scutelluidae

Radnoria

Dalmanites

W Warburgella

P Proetus

Calymene

Harpidella

Raphiophorus

Decoroproetus

nodulosa-type calymenids

illaenimorphs

Decoroproetus

Delops and Struveria

Proromma

Miraspis

Figure 14.10 Trilobite communities: overview of (a) Early Ordovician (Arenig), (b) Late Ordovician

(Ashgill) and (c) Mid Silurian (Wenlock) trilobite associations in relation to water depth and

sedimentary facies. (a, from Fortey, R.A. 1975. Fossils and Strata 4; b, from Price, D. 1979. Geol. J.

16; c, from Thomas, A.T. 1979. Spec. Publ. Geol. Soc. Lond. 8.)

374 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD

SilurianOrders

Agnostida

Cambrian

Carboniferous

Agnostina

Eodiscina

Olenellina

Redlichiina

Illaenina

Asaphina

Ptychopariina

Trinucleina

Harpina

Calymenina

Phacopina

Cheirurina

PhacopidaPtychopariidaRedlichiida

Corynex-

ochida

Proetida

Lichida

Devonian

Ordovician

Permian

Odontopleurida

Figure 14.11 Stratigraphic distributon of the main trilobite groups. (From Clarkson 1998.)

Box 14.4 Landmarks: the Silurian trilobite Aulacopleura

Landmarks, as the name suggests, are recognizable geographic features. Such features can also be

deﬁ ned on fossil organisms and they form the basis for geometric morphometrics. The aim of these

statistical techniques is to deﬁ ne precisely how shapes differ from each other, and the landmarks are

the ﬁ xed points of comparison. Each landmark can be recorded as a set of coordinates or the dis-

tances between points, and they can be recorded from digital photographs or image analysis systems

and stored in spreadsheets. For example, 22 landmarks were necessary to deﬁ ne shape variations in

the exoskeletons of well-preserved Aulacopleura from the Silurian rocks of Bohemia (Fig. 14.12).

The data can be used in a variety of ways. For example it is relatively easy to see, visually, how the

trilobite actually grew; the most substantial growth took place in the thoracic region during ontogeny.

In some studies it is necessary to translate this into quantitative terms, and landmark analysis is the

key.

A large dataset is available at http://www.blackwellpublishing.com/paleobiology/. These data may

be analyzed and manipulated using a range of morphometric techniques such as principal component

analysis (see also Hammer & Harper 2005).

ECDYSOZOA: ARTHROPODS 375

In addition there are signs of predation that

often show an asymmetric distribution. Pre-

dation scars are three times as likely to be

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

already a lateralization of their nervous system

and other organs (Babcock 1993). However,

it can be argued that these specimens were the

survivors and that predators preferred to

attack on the left-hand side of the trilobite,

and we never see those victims.

SUBPHYLUM CHELICERATA

The chelicerates are a diverse and heteroge-

neous group including the mites, scorpions

and spiders. The familiar horseshoe crab,

FAW

PGW

FAL

GLL

PLL

THL

PYL

6.0

(a)

(d) (e)

(b) (c)

5.0

4.0

Frontal area length (mm)

3.0

2.0

1.0

0.0

0.0 1.0 2.0

Occipital-glabellar length (mm)

3.0 4.0 5.0 6.0

18.0

16.0

14.0

12.0

10.0

8.0

6.0

4.0

2.0

Thoracic length (mm)

0.0

0.0 1.0 2.0

Occipital-glabellar length (mm)

r = 0.9889

n = 152

p << 0.01

r = 0.9653

n = 152

p << 0.01

RMA: Y = 3.429X – 1.318RMA: Y = 0.976X – 0.229

3.0 4.0 5.0 6.0

PYW

PAW

OCW

EGW

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.)

376 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD

Number of

olenid specimens

Number of

agnostid specimens

Vanadium/(Vanadium+Nickel)

ratio

0 5 10 15 20 25 30 35 0 5 10 15 20 25 30 35 40

(m)

0.85 0.80 0.75 0.70

(cm)

230

210

220

200

190

180

170

160

150

140

130

120

110

100

–50

–40

–30

Cyclotron sp.

Faulted cut in drill core

Upper O. attenuatus

Lower O. attenuatus

Range of O. truncatus

to O. wahlenbergi

Range of O. gibbosus

O. transversus

No agnostic or

Olenus species

Limestone

Sole presence of

agnostids

Complete specimen

Cranidium, Olenus

Pygidium, Olenus

Pygidium, Homagnostus

Cephalon, Homagnostus

Cephalon, Glyptagnostus

Cyclotron sp. (bradoriid)Loose cheek, Olenus

No data at this level

–78.0

–78.1

–78.2

–78.3

–78.4

–78.5

–78.6

–78.7

–78.8

–78.9

–79.0

–79.1

–79.2

–79.3

–79.4

Material collected by Lauridsen and Nielsen (2005)

Material collected

by Lauridsen and

Nielsen (2005)

Material collected by Clarkson et al. (1998)

O. dentatus

O. attenuatusO. gibbosus O. transversus – O. truncatus – O. wahlenbergi

Limestone

interval

not sampled

Figure 14.13 Microevolution and faunal dynamics of olenids in the Swedish Alum Shales. Olenus

species evolve gradually up through the section. (Based on Clarkson et al. 1998.)

ECDYSOZOA: ARTHROPODS 377

Limulus, together with the extinct sea scorpi-

ons, the eurypterids, are also members of the

subphylum, deﬁ ned in terms of a prosoma

(head and thorax) with six segments (bearing

appendages), an opisthosoma (abdomen) with

at most 12 segments, and a pair of chelicerae

(pincers) attached to the ﬁ rst segment of the

prosoma (Fig. 14.15).

There have traditionally been two main

chelicerate groups: (i) the merostomes, includ-

ing the aquatic horseshoe crab Limulus and

the giant sea scorpions or eurypterids; and

(ii) the arachnids that mainly comprise the

terrestrial spiders and scorpions. But this

traditional split of the subphylum into marine

meristomes and non-marine arachnids has

been challenged; both groups probably had

marine and non-marine representatives. The

bizarre Sanctacaris from the Middle Cam-

brian Burgess Shale may be a basal outgroup

to the clade Chelicerata, whereas the so-called

“great appendage” arthropods such as Emer-

aldella and Sidneyia together with the aglas-

pids belong within the clade (Cotton & Braddy

2004).

The xiphosures, or horseshoe crabs, have a

relatively large, convex prosoma, approxi-

mately equal in length to the opisthosoma,

which usually contains less than 10 segments.

The telson, or tail spine, is commonly long

(a) (c)(b)

Figure 14.14 Pathological trilobites: (a) Onnia superba – the fringe in the lower part of the photograph

has an indentation and a smooth area, probably regeneration following an injury during molting (×4);

(b) Autoloxolichas – the deformed segments on the left-hand side may be either genetic or the result of

repair following injury (×3); and (c) Sphaerexochus – only two ribs are developed on the right-hand

side, probably a genetic abnormality (×25). (Courtesy of Alan Owen.)

(a) (b)

ocelli

pedipalp

prosoma

compound

eye

walking leg

for swimming

walking legs

tergite

postabdomen

preabdomen

telson

chelicerae

mouth

last gill

appendage

paddle

metastoma

genital

appendage

sternite

pretelson

telson

Figure 14.15 Chelicerate morphology displaying features of (a) dorsal and (b) ventral surfaces. (Based

on McKinney 1991.)