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

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378 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD

and spiny, and the ophthalmic ridges and

cardiac lobe are usually well preserved.

Although some enigmatic xiphosure-like taxa,

such as Eolimulus, have been described from

the Lower Cambrian, the ﬁ rst are probably of

Early Ordovician age. A trend towards larger

size and a shorter fused abdomen is seen in

most groups. Carboniferous taxa, for example

Belinurus and Euproops, have well-developed

cardiac lobes and ophthalmic ridges together

with fused abdomen. Mesolimulus from the

Upper Jurassic Solnhofen Limestone, however,

is smaller than living taxa but it too was

marine and left clear evidence of its append-

ages in a trackway in the lagoonal muds of

Bavaria.

The eurypterids include the largest known

and most scary of the arthropods, some

approaching 2 m in length, that range in age

from Ordovician to Permian. They occupied

a variety of environments from marine to

freshwater and some may have been amphibi-

ous. Much of our knowledge of eurypterid

morphology has been derived from superbly

preserved specimens from Silurian dolomites

on the island of Saaremaa that were acid-

etched from the rock by the Swedish paleon-

tologist Gerhard Holm at the end of the

1800s. The exoskeleton is long and relatively

narrow. The subrectangular prosoma bears a

variety of appendages; the ﬁ rst pair of append-

ages were chelicerae adapted for grasping

while others were modiﬁ ed for movement,

with the last pair of large, paddle-like append-

ages probably adapted for swimming. The

opisthosoma, comprising the pre- and posta-

bdomen, consists of 12 visible segments. The

telson was variably developed as a long spine

or a ﬂ attened paddle.

With the exception of generalist feeders

such as Baltoeurypterus, most eurypterids

were predators, attacking ﬁ shes and other

arthropods. Moreover, where relatively

common, a number of eurypterid-dominated

communities have been described, emphasiz-

ing the range of habitats occupied by these

large, versatile animals. In the Silurian rocks

of the Anglo-Welsh area, Pterygotus and its

allies are associated with normal marine

faunas, whereas Eurypterus itself preferred

inshore environments. Hughmilleria and

related forms dominated brackish to fresh-

water communities.

Many more than 50 genera of eurypterids

have been described. The group was most

abundant during the Silurian and Devonian,

but only two families, the adeloopthalmids

and the hibbertopterids, survived into the

Permian. The varied styles of locomotion of

the group suggest a diversity in lifestyles

(Fig. 14.16).

The arachnids are a huge group of terres-

trial carnivores containing mites, scorpions,

spiders (Box 14.5) and ticks. There are prob-

ably over 100,000 known species of arach-

nids. The prosoma consists of six segments

with a pair of chelicerae, a pair of sensory or

feeding pedipalps and four pairs of walking

Baltoeurypterus

Mixopterus

Parastylonurus

(a)

(b)

(c)

Figure 14.16 Eurypterid functional morphology showing (a) swimming and (b, c) walking life modes.

(From Clarkson 1998.)

ECDYSOZOA: ARTHROPODS 379

Box 14.5 Fossil spider webs

Beautifully intricate but lethal spider webs are an integral part of terrestrial arthropod ecosystems.

But how old is this specialized and unique mode of predation? Arthropods trapped in spider silk

webs have, in fact, been recovered from Early Cretaceous amber (Fig. 14.17). Enrique Peñalver and

his colleagues (2006) have illustrated a remarkable mosaic of silk strands, some with sticky droplets,

which ambushed a ﬂ y and a mite, whereas another piece of amber shows a trapped wasp. The webs

were elastic and armed with glue droplets – no match for stray insects. Preservation of spider silk

in Lower Cretaceous amber from the Lebanon and from Spain suggests this form of predation was

already well established by the Cretaceous. The diversiﬁ cation of the insects during the later Meso-

zoic was tracked by a similar diversiﬁ cation in spiders; perhaps the evolution of aerial webs and

winged insects was linked, evidence of an arms race in the airways of the Cretaceous.

1 mm

(a)

(b)

Figure 14.17 Insects trapped in a Cretaceous spider’s web: (a) reconstruction and (b) actual

specimen. Strands of the web have been emphasized on the reconstruction together with droplets;

a ﬂ y (center left) and mite (top right) were both caught in the web. (Courtesy of Enrique

Peñalver.)

legs. The opisthosoma usually has 12 seg-

ments. Arachnids breathe mainly through so-

called book lungs or tracheae or both.

SUBPHYLUM MYRIAPODA

The myriapods are a varied group comprising

the millipedes, centipedes, symphylans and

pauropods. They ﬁ rst appeared during the

Mid Silurian, when Kampecaris-like forms

were responsible for a variety of terrestrial

trails (Box 14.6). Some of the largest forms,

for example the giant Arthropleura, nearly

2 m long, hoovered their way through the

lush, green vegetation of the Late Carbonifer-

ous forests.

380 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD

Box 14.6 Invasion of the land

The myriapods were the ﬁ rst animals to colonize the land. Heather Wilson and Lyall Anderson

(2004) have described the few Silurian and Devonian taxa from Scotland (Fig. 14.18). One of the

oldest genera from the Middle Silurian, Cowiedesmus, is named after Cowie Harbour near

Stonehaven, near Aberdeen and occurs together with Pneumodesmus, which shows clear evidence

of a respiratory system. Cowiedesmus is so distinctive and different from other millipedes

that it forms the basis for a new order, the Cowiedesmeda. These animals suggest that terrestrializa-

tion amongst the arthropods had already begun by the Mid Silurian and millipedes were breathing

and scuttling across the emerging new landscapes of the Caledonian mountain belt in Scotland. But

there may be older indirect evidence. Trackways from Middle Ordovician rocks in the English

Lake District (Johnson et al. 1994) suggests that arthropods were on land about 50 myr earlier,

while trace fossils from Cambrian rocks in Ontario push arthropod life on land back even

further into the Cambrian (MacNaughton et al. 2002), suggesting the presence then of large,

amphibious euthycarcinoids. Euthycarcinoids are an enigmatic group of arthropods with antennae

and mandibles, that have been placed in phylogeny somewhere near the origin of myriapods and

insects.

(a)

(b)

(c)

Figure 14.18 The millipedes: (a) Archidesmus (Lower Devonian), (b) Cowiedesmus (Middle

Silurian) and (c) Pneumodesmus (Middle Silurian), from Scotland. Scale bars, 2 mm. (Courtesy of

Lyall Anderson.)

ECDYSOZOA: ARTHROPODS 381

SUBPHYLUM HEXAPODA

The Hexapoda, essentially the insects, can be

divided into pterygotes (with wings) and

apterygotes (without wings) and include the

springtails, dragonﬂ ies, cockroaches and

locusts. The group may prove to have as many

as 10 million living species when the rich

faunas of the tropics have been completely

described. The subphylum also includes the

onychophorans, with ﬂ exible segmented

bodies and unjointed limbs propelled by

changes in blood pressure analogous to the

water vascular system of the sea urchins. The

hexapods have unbranched or uniramous

appendages, a simple gut, a single pair of

antennae and a pair of mandibles, together

with a toughened head capsule. Insects have

six limbs.

The oldest insect is probably the springtail

Rhyniella praecursor from the Lower Devo-

nian Rhynie Chert of the Orcadian Basin

of northeast Scotland (Fig. 14.19). Conrad

Labandeira (Smithsonian Institution, Wash-

ington) and his colleagues have shown that

insects diversiﬁ ed earlier than had been

thought (Labandeira 2006), and the group

probably originated in freshwater during the

Late Silurian, which may account for the poor

fossil record of the group before the Devonian

(Glenner et al. 2006). Early and Mid Devo-

nian faunas are now well known from Rhynie,

Gaspé, Québec and Gilboa, New York State

and these probably coincided with the diver-

siﬁ cation of land plants. And by the Late

Carboniferous a very diverse insect fauna had

evolved, with forms such as the dragonﬂ ies

and mayﬂ ies capable of powered ﬂ ight (Box

14.7). By the end of the Permian, most of the

familiar insect orders had appeared. During

the later Mesozoic and Cenozoic, signiﬁ cant

coevolutionary relationships were established

between plants and insects, particularly

between ﬂ owering plants and insect pollina-

tors, and possibly even between spiders and

ﬂ ies (see Box 14.6). Moreover, by the Miocene,

fossil hair trapped in amber together with the

sand ﬂ y Lutzomyia suggests that these blood

suckers were already feeding on mammals

in arboreal nests during the Mid Tertiary

(Peñalver & Grimaldi 2006).

SUBPHYLUM CRUSTACEA

As the name suggests, the crustaceans have a

hard, crusty carapace. The group, which ﬁ rst

appeared in the Cambrian, is aquatic, mainly

marine, with gills, mandibles, two pairs of

antennae and stalked compound eyes. The

heavily armored crabs and lobsters typify this

diverse subphylum; but the barnacles and

ostracodes are also crustaceans with a notable

geological record.

There are at least eight main classes of crus-

tacean, but with the exception of the ostra-

codes, which are usually considered part of

the microfauna, only two groups, the Cirripe-

dia and the Malacostraca have signiﬁ cant

geological records.

Origination Extinction Diversity

S4 D1 D2 D3 C1 C2 P1 P2 Tr1 Tr2 Tr3 J1 J2 J3 K1 K2 Pal E O M Pli Q R

Number of orders

Figure 14.19 Ranges of selected insect orders. Geological period abbreviations are standard, running

from Silurian (S) to Recent (R). (Based on Jarzembowski, E.A. & Ross, A.J. 1996. Geol. Soc. Spec.

Publ. 102.)

382 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD

Box 14.7 Insects take to the airways

Fossil insects with functional wings are ﬁ rst reported from Mid Carboniferous strata. These insects

were extraordinary (Fig. 14.20); the dragonﬂ y Meganeura had an incredible wingspan of 70 cm.

Intense aerobic activity such as powered ﬂ ight suggests that atmospheric oxygen levels at the time

were unusually high. But how effective was Meganeura as a ﬂ yer? Robin Wootton and his colleagues

(1998) have identiﬁ ed so-called smart features that capitalized on both upstroke and downstroke

movements of the animal’s large wings. This form of smart engineering helps depress the trailing

edges of the wings, rather like an aircraft’s ﬂ aps during takeoff and landing, and helps wing twisting.

It is unlikely that the giant Mid Carboniferous dragonﬂ ies could actually hover like modern forms

but they had good maneuvrability. These winged giants had already developed a predatory lifestyle

and, being about the size of a seagull, would have made a highly visible addition to terrestrial life

in the forests of the Carboniferous (see p. 488).

Figure 14.20 Giant Carboniferous dragonﬂ ies from Ayr, Scotland. p, prothoracic lobe; r, rostrum.

Scale bar is in millimeters. (Courtesy of Ed Jarzembowski.)

The cirripedes or barnacles have shells, or

capitula (singular, capitulum), consisting of

several plates and these animals are adapted

to an encrusting lifestyle. Two groups, the

acorn barnacles and goose barnacles, have

contrasting life strategies. The acorn barna-

cles, such as Balanus, have capitula consisting

of overlapping plates and they are attached to

rocks and other shells. The group rapidly

diversiﬁ ed from an origin during the Late Cre-

taceous and are locally common. The goose

barnacles are pseudoplankton, living attached

to ﬂ oating debris, that have a relatively poor

fossil record.

The malacostracans include two subclasses,

the phyllocarids and the eumalacostracans.

The phyllocarids have large bivalved cara-

paces, seven abdominal somites and a telson

with a pair of furcae (forked extensions; sin-

gular, furca), extending posteriorly. Canadas-

pis from the Burgess Shale may be one of

the ﬁ rst crustaceans. Living phyllocarids are

usually minute, in contrast to their larger

Paleozoic ancestors. The eumalacostracans

include decapods – shrimps, lobsters and

crabs – together with the less common

branchiopods. Some of the most spectacular

Carboniferous eumalacostracans have been

ECDYSOZOA: ARTHROPODS 383

described from the Granton Shrimp Bed by

Euan Clarkson (University of Edinburgh) and

his colleagues (Fig. 14.21).

Ostracodes

Ostracodes are crustacean arthropods, abun-

dant and widespread in aquatic environments.

They have small bivalved carapaces, hinged

along the dorsal margin (Fig. 14.22a). The

carapace is perforate and completely covers

the entire animal when closed. Most ostra-

codes are benthic, swimming, crawling or

burrowing at the sediment–water interface in

muds or silts with abundant organic material.

A few, such as the myodocopids, are plank-

tonic and some are commensal or parasitic.

They are very useful for environmental recon-

structions and, at some levels in the strati-

graphic record, have been used for correlation.

Ostracodes have weak segmentation with a

poorly deﬁ ned head, thorax and abdomen;

the animal is contained within the two shells,

with the carapace united dorsally by an elastic

ligament and a variably developed hinge.

Growth is by periodic ecdysis or molting. Fol-

lowing each molt phase the carapace initially

develops as a pair of chitinous valves enclos-

ing the animal; most of the carapace is

then calciﬁ ed, except the dorsal margin that

remains as a chitinous ligament forcing the

valves apart when the internal adductor

muscles relax. The central muscle scars vary

across the class (Fig. 14.22b), from complex

patterns in the Leperditicopida to a single scar

in some members of the Palaeocopida.

(c)

(b)

(a)

Figure 14.21 Carboniferous shrimps: (a) Tealliocaris woodwardi from the Gullane Shrimp Bed, near

Edinburgh (×4); (b) Waterstonella grantonensis from the Granton Shrimp Bed, near Edinburgh (×2);

Euan Clarkson.)

384 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD

(b)

(c)

Dorsal

Ventral

length

Height

Posterior

dorsal

muscle

scars

central

muscle scar

normal

pores

hinge

marginal-pore

canals

marginal zone (fused

part of inner lamella)

free part of

inner lamella

selvage

line of

concrescence

outer margin

inner margin

Adont hinge

Merodont hinge

Amphidont hinge

posterior element

median element

anterior element

muscle scar

third thoracic leg

mandible

eye

antennule

antenna

(a)

POSTERIOR

mandible beside mouth

maxilla

first thoracic leg

second thoracic leg

claw

furca

carapace

egg

ventral

dorsal

Figure 14.22 Descriptive terminology of the ostracode animal (a), including muscle scars (b) and hinge

structures (c). (Based on Armstrong & Brasier 2005.)

ECDYSOZOA: ARTHROPODS 385

Articulatory structures are variably devel-

oped along the hinge line. Three main types

of hinge are known (Fig. 14.22c). Adont

hinges lack teeth but have a long median

element on the right valve that ﬁ ts a socket

on the left valve. The merodont hinge has

long striated terminal elements on the right

valve ﬁ tting respective sockets on the left

valve. Amphidont hinges have short terminal

elements with well-developed teeth on the

right valve.

The carapace is perforated by canals

holding setae that communicate with the exte-

rior. The body is suspended within the cara-

pace, attached by muscles. It is equipped with

seven pairs of appendages, three in front of

the mouth and four behind. The appendages

are specialized, acting as sensory organs, limbs

for the capture and processing of food; more-

over they allow locomotion and general clean-

ing and housekeeping within the carapace.

The animal has a digestive system, sophisti-

cated genitalia and a nervous system; com-

monly a median eye is located behind a

tubercle.

Sexual dimorphism is common and often

reﬂ ected in the ostracode carapace (Fig.

14.23). Males commonly have a greater

length : height ratio than the females, whereas

in some benthic Paleozoic ostracodes the

female had a brood pouch in the carapace

wall. Females are often called heteromorphs

while the males, lacking the brood pouch, are

the tecnomorphs.

Ostracodes appeared ﬁ rst during the Early

Cambrian. The archaeocopids were a bizarre

group of large taxa with distinctive append-

ages quite different from more typical ostra-

codes. The group was short lived, disappearing

during the latest Cambrian to earliest Ordovi-

cian. The later history of the group shows a

number of clear trends: evolution of small

size, simpler muscle systems and shorter hinge

lines; the functional signiﬁ cance of these

changes is not immediately obvious.

Large Leperditicopida and Palaeocopida

appeared during the Ordovician, dominating

ostracode faunas until the Devonian, when

deep-water limestones were locally character-

ized by the small spiny myocopids. Many new

groups appeared near the end of the Paleo-

zoic, but hitherto important groups such as

the palaeocopids eventually disappeared in

the Triassic after a decline during the Permian.

Although Early Jurassic ostracode assem-

blages are of low diversity the platycopines,

cypridaceans and cytheraceans radiated

steadily during the Jurassic. By the Cenozoic,

the cypridaceans dominated lake environ-

ments whereas the cytheraceans were estab-

lished in marine settings.

Any doubts that real ostracodes did not

actually exist in the Paleozoic have been

dispelled by some remarkable soft-part

preservation, digitally reconstructed from

material from the Silurian Lagerstätten at

Hereford, England (Siveter et al. 2003). The

precise details of the animal’s morphology,

down to the enormous male copulatory organ,

conﬁ rm the ostracode identity of the speci-

men; it seems even very similar to living

myodocopids. Lagerstätten, such as the Her-

eford biota, have provided a remarkable series

of windows on arthropod evolution through

time, right back to the Cambrian (Box

14.8).

(a) (b)

(e) (f)

Figure 14.23 Some ostracode genera: (a) left

valve of a male living Limnocythene showing

details of appendages (×30); (b, d) left valves of

female and male heteromorphs of Beyrichia

(Silurian) (×18); (c, e) external and internal views

of the left valve of living Patagonacythene (×30);

(f) palaeocopid Kelletina (Carboniferous) (×30).

(Courtesy of David Siveter.)

386 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD

Box 14.8 Exceptional arthropod-dominated faunas

Arthropods are common in a number of Lagerstätten deposits, suggesting that they were much more

diverse in the past than the regular fossil record suggests. More than 40% of the animals described

from the Mid Cambrian Burgess Shale are arthropods. Apart from typical trilobites such as Olenoi-

des there are also soft-bodied taxa, for example Naraoia and the larger Tegopelte. However the

commonest and ﬁ rst discovered Burgess arthropod is the elegant, trilobitomorph Marrella. The fauna

contains many other arthropods such as Canadaspis, probably the ﬁ rst phyllocariid crustacean. There

are many unique arthropods in the fauna that are difﬁ cult to classify: Anomalocaris, Emeraldella,

Leanchoilia, Odaraia, Sidneyia and Yohoia are not easily aligned with established groups. The small

and bizarre Hallucigenia was probably an onychophoran, while Sanctacaris was a stem-group che-

licerate. The slightly older faunas at Chengjiang, South China, and Sirius Passet, North Greenland,

have also yielded a spectacular array of enigmatic arthropod faunas, further contributing to our

knowledge of the Cambrian explosion.

Calcareous concretions (or orsten) from the Upper Cambrian of the Baltic area have yielded a

phosphatized fauna dominated by stem- and crown-group crustaceans and ostracodes together with

agnostid trilobites. Many of these diverse forms were minute, living in microhabitats within or on

the muds of the Cambrian seas (Fig. 14.24). These faunas are quite distinct from the earlier Burgess

Shale-type faunas and provide a window on a habitat occupied by a wide range of body plans on

a microscopic scale, possibly adapted to life below the sediment–water interface. Recent work by

Dieter Walossek (Ulm Universität) on, for example, remarkably preserved complete ontogenetic series

of Rehbachiella from orsten has helped elucidate the life cycle, habits and functional morphology

of these animals. Moreover some of the most remarkable of all the arthropods, the pycnogonids, or

sea spiders, are now known from the Cambrian orsten banks, the Silurian Herefordshire fauna and

the Devonian Hunsrückschiefer (Budd & Telford 2005).

The Early Devonian faunas of the Hunsrückschiefer of the German Rhineland contain beautifully

preserved phyllocariid crustaceans such as Nahecaris, together with a number of other arthropods

apparently lacking living counterparts such as Cheloniellon (a large, ovoid creature with a pair of

antennae, nine segments and conical telson) or Mimetaster, which is similar to Marrella from the

Burgess Shale.

Anoxic zone

a+A

BCd

Figure 14.24 Composite of Mid Cambrian and Late Cambrian forms and reconstructions. Lower

case letters (a–d), larvae; upper case letters (A–D), adult stages. Distance of sinking into the zone

of preservation: 1, short distance; 2, long distance. (Redrawn from Walossek, D. 1993. Fossils

and Strata 32.)

ECDYSOZOA: ARTHROPODS 387

Review questions

1 The spectacular arthropod faunas of

Burgess, Chengjiang and Sirius Passet

suggest an early diversiﬁ cation of these

ecdysozoan taxa. Was this really evolu-

tion’s “big bang” or are these arthropods

just too weird to comprehend when com-

pared to modern faunas?

2 Trilobites were an integral part of the

Paleozoic fauna for over 200 million years

yet they ﬁ nally became extinct at the end

of the Permian. What sorts of animals

ﬁ lled their niches in the Modern evolu-

tionary fauna?

3 Trilobites have featured in a number of

evolutionary schemes, some showing

gradualistic trends and others showing

punctuated trends. Are these different

patterns correlated with different groups

of trilobite or perhaps to their life

environments?

4 Insects are and probably were the most

numerically dominant life of Earth. Why

have they a relatively poor fossil record?

5 Exceptionally-preserved biotas occur spo-

radically throughout the Phanerozoic.

Arthropods are usually well represented.

Why?