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

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338 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD

equipped to handle life deep in the sediments

of nearshore and intertidal zones where they

diversiﬁ ed.

CLASS GASTROPODA

The gastropods, the “belly-footed” mollusks,

are the most varied and abundant of the mol-

luskan classes today. The group includes the

snails and slugs, forms both with and without

a calcareous shell. During a history spanning

the entire Phanerozoic, gastropods evolved

creeping, ﬂ oating and swimming strategies

together with grazing, predatory and parasitic

trophic styles.

Most gastropods are characterized by

torsion in which the mantle cavity containing

the gills and anus, excretory and reproductive

openings comes to lie above the head (Fig.

13.11). The advantages of this arrangement

are unclear. In fact, torsion seems to be dis-

tinctly disadvantageous because it involves

the loss of one of the gills and/or development

of a peristomal slit allowing separation of

inhalant and exhalant currents. The ﬁ rst larval

stage, the trochophore, is usually ﬁ xed.

However, the second, veliger, phase is free-

swimming and unique to the mollusks. During

development, the head and foot remain ﬁ xed

but all the visceral mass, the mantle and the

larval shell are, in effect, rotated through

180˚. The process of torsion is characteristic

of the Gastropoda, although in some groups

there may be secondary reversal. The coiling

of the gastropod shell is unrelated to the rota-

tion of the soft parts. Following torsion, the

mantle cavity and anus are open anteriorly

and the shell is coiled posteriorly in an endo-

gastric position, in contrast to the exogastric

style of the “monoplacophoran” grade shell.

The gastropod shell is usually aragonitic,

usually conical with closure posteriorly at the

pointed apex, and open ventrally at the aper-

ture. Each revolution of the shell or whorl

meets adjacent whorls along a suture, and the

whorls together comprise the spire. Tight

coiling about the vertical axis generates a

central pillar or columella. The aperture is

commonly oval or subcircular and is circum-

scribed by an outer and inner lip. The head

emerges at the anterior margin of the aperture,

where the aperture may be notched or extended

as a siphonal canal supporting inhalant ﬂ ow

through the siphons. Material is ejected

Glycimeris

Mya

Mytilus

Ostrea

Gryphaea

Pecten

Teredo

1. Infaunal shallow burrowers

2. Infaunal deep burrowers

3. Epifaunal with byssus

4. Epifaunal with cementation

5. Unattached recumbents

6. Swimmers

7. Borers and cavity dwellers

equivalved, adductor muscles of equal

sizes and commonly with strong external

ornament.

elongated valves, often lacking teeth and

with permanent gape and a marked pallial

sinus.

elongate valves with flat ventral surface

and reduction of both the anterior part of

the valve and the anterior muscle scar.

Attached by thread-like byssus.

markedly differently shaped valves,

sometimes with crenulated commissures;

large single adductor muscle.

markedly differently shaped valves

sometimes with spines for anchorage or

to prevent submergence in soft sediment.

valves dissimilar in shape and size with

very large, single adductor muscle and

commonly with hinge line extended as

ears.

elongate, cylindrical shells with strong,

sharp external ornament; cavity dwellers

commonly grow in dimly lit conditions

following the contours of the cavity.

Figure 13.8 Morphology and adaptations of the main ecological groups of bivalve mollusk.

SPIRALIANS 2: MOLLUSKS 339

Ve nu s

Mya

Ensis

Soft to

firm

substrate

Shallow infaunal

Deep infaunal

Seawater

(a)

(b)

(c)

Pholas

Hiatella

Hard rock substrate

Seawater

Pecten

Mytilus

Crassostrea

Gryphaea

Firm to hard substrate

Soft to firm substrate

Seawater

Figure 13.9 Life modes of bivalve mollusks: (a) shallow and deep burrowers into soft to ﬁ rm

substrates, (b) epifaunal swimming, attached or resting on soft to ﬁ rm substrates, and (c) boring into

hard substrates. (From Milsom & Rigby 2004.)

340 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD

Box 13.5 Rudists: bivalves disguised as corals

The rudists were aberrant heteroconch bivalves that range in age from the Late Jurassic to the Late

Cretaceous and occupied the Tethyan region. During a relatively short interval they developed a

bizarre range of morphologies, and although many groups apparently mimicked corals, the rudists

were probably not reef-building organisms. The rudists were inequivalved with a large attached

valve, usually the right valve of conventional terminology, and a small cap-like free valve. Virtually

all rudists had a single tooth ﬂ anked by two sockets in the attached valve, and two corresponding

teeth and a socket in the free valve. The valves functioned with an external ligament and pairs of

adductors attached to internal plates or myophores. Three growth strategies have been identiﬁ ed

(Fig. 13.10). Elevators had tall conical shells with a commissure raised above the sediment–water

interface to free the animal from the risk of ingesting sediment. The elevators were thus similar to

solitary corals, suggesting a possible reef-building strategy. Clingers or encrusters were ﬂ at, bun-

shaped forms that usually adhered to hard substrates. The recumbents had large shells, extending

laterally extravagantly over the seaﬂ oor like large calciﬁ ed bananas. The rudists occupied carbonate

shelves throughout the Tethys region, with their larvae island hopping around the tropics, often

growing together in a gregarious habit; clusters or clumps probably trapped mud in molluskan-rich

structures. As noted above it now seems likely that the rudists were never true reef-building organ-

isms although they came close to fulﬁ lling that mode of life.

Thomas Steuber (University of Bochum) has developed a comprehensive database on rudist

bivalves together with spectacular pictures of rudist accumulations. Study of this comprehensive

database, and a smaller dataset that can be used to reconstruct ancient paleogeographic associations

at http://www.blackwellpublishing/paleobiology/, can be used for a variety of exercises. The small

dataset investigates the biogeography of Campanian rudists, emphasizing their relationship to the

paleotropics (Tethyan province) on http://www.blackwellpublishing.com/paleobiology/.

encrusters

recumbents

elevators

Figure 13.10 Rudist growth strategies: encrusters (A, B, H and I), elevators (C, D and E) and

recumbents (F, G). (From Skelton, P.W. 1985. Spec. Pap. Palaeont. 33.)

through the exhalant slit in the outer lip. During

ontogeny the inactive track of the slit is succes-

sively overgrown with shell material to form

the selenizone, the calciﬁ ed track of the slit

band separating the siphons from the mouth.

The gastropod shell is normally oriented

with the aperture facing forward and the apex

facing upwards. If the aperture is on the right-

hand side, the shell is coiled clockwise in a

so-called dextral mode; sinistral shells have

SPIRALIANS 2: MOLLUSKS 341

the opposite sense of coiling. The shell surface

is commonly modiﬁ ed by strong growth lines,

ribs, tubercles and projections. Many gastro-

pods have an operculum covering the

aperture.

Gastropods developed a variety of shell

shapes. Eight different morphologies ranging

from the simple patelliform to the complex

digitate shell are illustrated in Fig. 13.12 as a

sample of the large amount of exoskeletal

variation in the group.

Main gastropod groups and their ecology

Gastropods have been divided into three

classes largely based on information from

their soft parts. Three subclasses are tradi-

tionally deﬁ ned on the basis of the radula and

their respiratory and nervous systems,

although some of the groups may not be truly

monophyletic: (i) the Prosobranchia are fully

torted with one or two gills, an anterior

mantle cavity and cap-shaped or conispiral

shells; (ii) the Opisthobranchia are untorted

(having gone through torsion followed by

detorsion) with the shell reduced or absent,

and the mantle cavity posterior or absent; and

(iii) the Pulmonata are untorted with the

mantle cavity modiﬁ ed as a lung, and the

shells are usually conispiral. Fossil taxa are

usually assigned to these categories on the

basis of similarities in shell morphology with

their living representatives.

The prosobranchs are mainly part of the

marine benthos with a few freshwater and

terrestrial taxa. The primitive members of the

group, the Eogastropoda, are marine, mainly

head

siphonal canal

operculum

opercular lobe

foot

inhalent

siphon

mantle flap

penis

mantle flap

shell

eye

tentacle

proboscis

(a)

mouth

spire

body whorl

callus

over

umbilicus

whorl

varices

spiral ornament

inductura on

inner (=

columellar)

lip

aperture

siphonal

canal

apex

suture

growth line

operculum

selenizone

slit

umbilicus

growth line

axis of coiling

(b)

(c)

exhalent notch

aperture

denticle

inhalent

siphonal

notch

conisprial

sinistral

dextral

planispiral

hyperstrophic

Figure 13.11 Gastropod morphology:

(a) annotated reconstruction of a living

gastropod, (b) annotated shell morphology of

three gastropod shell morphotypes, and (c) main

types of gastropod coiling strategy.

patelliform

convolute

digitate

turbinate

turreted

pupiform

bi-conical

discoidal

Figure 13.12 Gastropod shell shapes.

342 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD

grazing herbivores with cap-shaped or low-

spired forms and include a diverse set of

superfamilies including the following groups.

Macluritines have large, thick shells lacking a

slit-band; for example Maclurites is planispi-

rally coiled, hyperstrophic with a robust oper-

culum and ranged from the Ordovician to the

Devonian. The pleurotomariines have vari-

ably shaped shells, usually conispiral. They

dominated shallow-water Paleozoic environ-

ments, although today the group is restricted

to deeper-water settings. Pleurotomaria had a

trochiform shell with a broad selenizone; the

older Ordovician-Silurian Lophospira had a

turbinate shell. The trochines are typical of

rocky coasts, grazing on algae; Paleozoic taxa,

for example the Ordovician-Silurian Cyclo-

nema, were probably scavengers, whereas

some, such as the Devonian Platyceras, are

commonly attached to the anal tubes of cri-

noids and were parasites. The patellines, such

as the limpets like Patella, have cap-like shells

and they graze on algae on rocks in the inter-

tidal zone. The euomphalines were mainly

discoidal, such as Euomphalus, which ranged

from the Silurian to the Permian.

The murchisoniines were a more advanced

group that ranged from the Ordovician to the

Triassic, possessing high-spired shells with a

siphonal notch. Murchisonia is a long-ranging

genus (Silurian-Permian).

Finally, the precise systematic position of

the bellerophontines is still unresolved; they

were planispirally-coiled shells with a well-

developed slit, ranging in age from the

Cambrian to the Triassic. The long-ranging

Bellerophon was very common in the Early

Carboniferous.

The order Mesogastropoda consists of pro-

sobranchs that have lost the right gill and

usually have conispiral shells with siphonal

notches. These taxa have diversiﬁ ed in marine,

freshwater and terrestrial environments.

Turritella is a high-spired, multiwhorled shell

with strong ribs and a simple aperture,

whereas Cypraea is involute with the earlier

whorls completely enclosed by the ﬁ nal

whorl.

The order Neogastropoda contains coni-

spiral, commonly fusiform, shells with a siph-

onal notch; most of the order is carnivorous

and members dominated marine environ-

ments from the Tertiary onwards. Neptunea

has a large body whorl and a short siphonal

canal whereas Conus is biconical with a

narrow aperture and a siphonal notch.

The subclass Opisthobranchia includes

marine gastropods with reversed torsion and

commonly lacking shells. Pteropods and sea

slugs are typical opisthobranchs.

The subclass Pulmonata contains detorted

gastropods, with the mantle cavity modiﬁ ed

as an air-breathing lung. The group probably

ranges in age from the Jurassic to the present,

and is characteristic of terrestrial environ-

ments. Planorbis has a smooth, planispiral

shell with a wide umbilicus whereas Helix is

smooth and conispiral and Pupilla has a

smooth pupiform shell.

The gastropods show a considerable diver-

sity of form across the entire class (Fig. 13.13).

It is difﬁ cult to relate given morphotypes to

particular life modes although the overall

morphology of the shell can reﬂ ect its trophic

function (Wagner 1995). In general terms,

however, gastropods occupying high-energy

environments have thick shells and are com-

monly cap-shaped or low-spired, whereas

shells with marked siphonal canals are adapted

to creeping across soft substrates. Carnivores

are usually siphonal whereas herbivores have

complete apertural margins and commonly

grazed on hard substrates. Thin-shelled

taxa are typical of freshwater and terrestrial

environments.

Gastropod evolution

There is no general agreement on the origin

of the gastropods. Currently the group is

thought to have been derived from a mono-

placophoran-type ancestor by torsion and

development of an exogastric condition,

where the shell is coiled away from the ani-

mal’s head. An origin from among coiled

forms such as Pelagiella may link the mono-

placophoran grade through the Tommotian

Aldanella to the gastropods.

The monophyly of the gastropods has been

questioned. It is possible that many of the

traditional groups, for example the archaeo-

gastropods, mesogastropods, opisthobranchs

and pulmonates may be grades of gastropod

organization, forming a series of parallel-

evolving clades. In particular the archaeogas-

tropods have been shown to be polyphyletic

and they are no longer considered to be a

natural grouping. Nevertheless, the neogas-

SPIRALIANS 2: MOLLUSKS 343

tropods appear to comprise a uniﬁ ed group

derived from either advanced eogastropods or

primitive mesogastropods during the Late

Mesozoic.

Most Paleozoic gastropods were probably

herbivores or detritus feeders. Drill holes in

brachiopod shells, however, suggest that a few

genera were carnivores and some, such as

Platyceras, were parasites. The class became

more important during the Late Paleozoic and

the Mesozoic when many more predatory

groups evolved. However, during the

Cenozoic, gastropods reached their acme with

the neogastropods in particular dominating

molluskan nektobenthos.

Gastropods are not particularly good

zone fossils, although nerineid gastropods are

stratigraphically useful in parts of the English

Middle Jurassic in the absence of ammonites.

Gastropods are generally associated with

particular facies and few rapidly evolving

lineages are known in detail. Nevertheless,

microevolutionary sequences in the genus

Poecilizontes from the Pleistocene of Bermuda,

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

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

Figure 13.13 Some gastropod genera: (a) Murchisonia (Devonian) (×1.25), (b) Euomphalus

(Carboniferous) (×0.5), (c) Lophospira (Silurian) (×0.5), (d) Patella (Recent) (×1), (e) Platyceras

(Silurian) (×1), (f) Neptunea (Plio-Pleistocene) (×0.6), (g) Viviparus (Oligocene) (×0.8), and (h) Turritella

(Oligocene) (×1). (Courtesy of John Peel.)

344 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD

described in detail by Stephen Jay Gould,

suggest that new subspecies evolving by allo-

patric speciation arose suddenly by pedomor-

phosis (see p. 145). These rapid speciation

events, separated by intervals of stasis, are

strong supportive evidence of the punctuated

equilibrium model of microevolutionary

change. Moreover, in a classic study of Late

Tertiary snails from Lake Turkana, Kenya,

Peter Williamson (1981) suggested there had

been punctuated changes in 14 separate lin-

eages (see also p. 123).

CLASS CEPHALOPODA

The cephalopods are the most highly orga-

nized of the mollusks, with the greatest com-

plexity of any of the spiralian groups. The

close association of a well-deﬁ ned head with

the foot modiﬁ ed into tentacles is the source

of their name, meaning “head-footed”. High

metabolic and mobility rates, a well-

developed nervous system, and sharp eyesight

associated with an advanced brain, are ideal

adaptations for a carnivorous predatory life

mode. The funnel or hyponome is also modi-

ﬁ ed from the foot, and squirts out water from

the mantle cavity providing the animal with

a form of jet propulsion.

Modern cephalopods belong to two groups.

Firstly, living Nautilus has an external coiled

shell with a thin internal mantle and nearly

100 tentacles. Only ﬁ ve species of this genus

are extant although it was once used as an

analog for the behavior of all extinct exter-

nally-shelled cephalopods such as the ammo-

noids. Secondly, the coleoids; these have

internal shells and thick external mantles.

They include the 10-tentacled extinct belem-

nites, the squids and cuttleﬁ sh; the octopods

have eight tentacles and have lost their skele-

ton. These living forms are most common

in shallow-water belts around the ocean

margins.

A tripartite division of the cephalopods

into three subclasses includes: (i) Nautiloidea,

with straight or coiled external shells with

simple sutures (Late Cambrian to Recent); (ii)

Ammonoidea, with coiled, commonly ribbed

external shells with complex sutures (Early

Devonian to latest Cretaceous, possibly earli-

est Paleogene); and (iii) Coleoidea, with

straight or coiled internal skeletons (Carbon-

iferous to Recent).

The origin of the cephalopods remains con-

troversial, although most agree the group was

derived from a monoplacophoran-like ances-

tor. John Peel (1991) suggested that the group

is derived from within the class Helcionel-

loida; both groups are characterized by endo-

gastric coiling and, moreover, the helcionelloids

predate the appearance of the cephalopods by

some 10 million years. Another group of gas-

tropod-like shells, the tergomyans, with apical

septa, might also have been ancestral, only

they lack perforate septa.

Nautiloidea

Most information about nautiloids comes

from studies of the behavior and morphology

of the living Nautilus that occurs mainly in

the southwest Paciﬁ c, normally at depths of

5–550 m (Box 13.6). It pursues a nocturnal,

nektobenthonic life mode as both a carnivore

and scavenger; however it is prey to animals

with powerful jaws such as the perch, marine

turtles and sperm whales.

Living Nautilus has its head, tentacles, foot

and hyponome concentrated near the aper-

ture of the body chamber; the visceral mass

containing other vital organs is situated to the

rear of the body chamber (Fig. 13.14). The

surrounding mantle extends posteriorly as the

siphuncular cord connecting all the previous,

now empty, chambers that together constitute

the phragmocone. Each chamber is parti-

tioned from those adjacent by a sheet of cal-

careous material, the septum; the suture is

formed where each septum is cemented to the

outer shell. The form of the suture, or the

suture pattern, is used in the classiﬁ cation of

externally-shelled cephalopods. The conch is

usually oriented as follows: anterior at the

aperture, posterior at the point furthest from

the aperture, the venter on the side with the

hyponome, usually the outside, and the

dorsum opposite. Despite the simplicity of

this arrangement, fossil nautiloids developed

a wide range of shell morphologies

(Fig. 13.15).

Ammonoidea

The ammonite usually had a planispirally

coiled shell comprising the protoconch, phrag-

mocone and body chamber (Fig. 13.16). The

protoconch or larval shell records the earliest

SPIRALIANS 2: MOLLUSKS 345

ontogeny of the animal. The phragmocone is

chambered, with each chamber marking suc-

cessive occupation by the animal, and sealed

off from previous chambers by a septum,

complex in structure at its margins, like a

sheet of corrugated iron. Where the septum is

welded to the shell, a suture is developed,

commonly with a complex pattern of frilled

lobes and saddles.

Five main sutural types are recognized

among cephalopods (Fig. 13.17). The ortho-

ceratitic pattern, with broad undulations or

orthoconic

cyrtoconic

longicones

brevicones

(c)

(a)

dorsal

nacreous layer

septal

line

mural part

of septum

siphuncle

color

bands

body chamber

growth lines

intestine

stomach

septum

chambers or camerae

body tissue and

cavities shaded

hood

eye

mouth

radula

tentacles

hyponome

oesophagus

gills

(b)

mantle cavity

siphuncular

cord

orthoconic

cyrtoconic

lituiticone

gyrocone

torticone

Figure 13.14 (a) Features of the shell and (b) internal morphology of a living Nautilus. (c) Shell shapes

of the nautiloids.

Box 13.6 Living Nautilus

Living Nautilus has allowed biologists and paleontologists to model the functions and life modes of

the ancient ammonites by using a modern analog. But despite the similarity of their respective shells,

coleoids are, in fact, more closely related to ammonites than modern nautiloids, and thus better

behavioral analogs may be found within the coleoids (Jacobs & Landman 1993). It is probable that

coleoid-type swimming mechanisms probably evolved prior to the loss of the body chamber in the

coleoids. Ammonoids thus probably had a coleoid-like mantle and thus may have operated quite

differently from living Nautilus. But how far could an empty ammonite shell travel? Ryoji Wani and

his colleagues (2005) have demonstrated that the phragmocone of living Nautilus pompilius becomes

waterlogged only after the mantle tissue decomposes. Water is then sucked into the shell because of

its lower internal gas pressures. This is actually more common for smaller shells, generally with

diameters less than 200 mm, and these ﬁ ll up with water more quickly. Only larger shells had the

ability to drift long distances. Since the ratio of volumes of the body chamber to the phragmocone

in nautiloids is similar to that of the ammonoids, they probably behaved similarly. The small shells

sank and the large shells drifted.

346 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD

rounded lobes and saddles, characterizes

mainly nautiloids ranging in age from Late

Cambrian to Late Triassic. Anarcestid and

agoniatitic patterns, however, have a narrow

mid-ventral lobe and a broad lateral lobe with

additional lobes and saddles, and range in age

from the Early to Mid Devonian. Goniatitic

sutures are characterized by sharp lobes and

rounded saddles, and are found in Late Devo-

nian-Permian ammonoids. Ceratitic sutures

show frilled lobes and undivided saddles, and

ammonitic patterns have both the lobes and

saddles ﬂ uted and frilled. Based on these

sutural patterns, three groups among the

ammonoids can be recognized in a general

way: the goniatites are typical of the Devo-

nian-Permian, the ceratites of the Triassic,

and the ammonites dominated the Jurassic

and Cretaceous. Nevertheless, these sutural

patterns may be cross-stratigraphic, with

Cretaceous taxa having both goniatitic and

ceratitic grades of suture in homeomorphs of

more typical Devonian and Triassic forms.

The siphuncle connects the outer body

chamber with the phragmocone that includes

all the empty, previous chambers. Septal necks

act like washers, guiding the passage of the

siphuncle through each septum. Excepting

the clymeniids, the siphuncle is situated along

the outer ventral margin of the shell. Seawater

may be pumped in or out of the chambers

through the siphuncle in order to alter the

buoyancy of the ammonite, similar to mecha-

nisms in the nautiloids and in submarines.

The body chamber contains the soft parts

of the ammonite. The aperture may be modi-

ﬁ ed laterally with lappets and ventrally with

the rostrum. In many taxa, aptychi sealed the

aperture externally, although these plates may

also have been part of the jaw apparatus.

Main ammonoid groups

The subclass Ammonoidea is currently split

into nine orders. The ﬁ rst three, the Anarces-

tida, the Clymeniida and the Goniatitida,

have goniatitic sutures and are included in the

order Goniatitida. The anarcestides charac-

terize Early to Mid Devonian faunas when

forms such as Anarcestes and Prolobites dis-

played tightly coiled shells together with a

ventral siphuncle. The clymeniids were the

only ammonoids with a dorsal siphuncle; they

radiated in Late Devonian faunas in Europe

and North Africa, where the group is impor-

tant for biostratigraphic correlation. The

order developed a variety of shell shapes: Pro-

gonioclymenia is evolute with simple ribs,

Soliclymenia evolved triangular whorls, and

Parawocklumeria is a globular involute form

with a trilobed appearance. As a whole, the

goniatitides ranged in age from the Mid Devo-

nian to Late Permian, with typical goniatitic

sutures consisting of eight lobes and ventral

siphuncles. Goniatites, for example, was a

spherical inﬂ ated form with spiral striations,

Figure 13.15 Life attitudes and external

morphologies of the nautiloids. (From Peel et al.

1985.)

SPIRALIANS 2: MOLLUSKS 347

(a)

dorsal

impressed

area

outside

umbilical

diameter

internal mould

whorl width

umbilical seam

peristome

test

protoconch

growth lines

body chamber

prong of ventral lobe

(b)

cadicone

sphaerocone platycone

ellipticone

serpenticone

oxycone

(c)

ventral lobe

1st lateral saddle

umbilical border

umbilical seam

1st lateral

lobe

secondary

ventral

saddle

phragmocone

height of whorl

dorso-

lateral

lobe

dorso-

lateral

saddle

umbilical

saddle

dorsal

lobe

Figure 13.16 Morphology and shape terminology of the ammonoids: (a) external morphology,

(b) suture pattern, and (c) shell shapes.