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

Подождите немного. Документ загружается.

328 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD

Box 13.1 Classiﬁ cation of Mollusca

Class CAUDOFOVEATA

• Worm-like, shell-less mollusks living inverted in burrows in the seabed

• Recent

Class APLACOPHORA

• Worm-like, spiculate mollusks

• Possibly Carboniferous (or older) to Recent

Class MONOPLACOPHORA

• Limpet-like, cap-shaped shells with segmented soft parts

• Cambrian (Lower) to Recent

Class DIPLACOPHORA

• Anterior and posterior shell separated by elongate zone of scale-like sclerites

• Cambrian (Lower)

Class POLYPLACOPHORA

• Segmented shell usually with eight plates, large muscular foot and a series of gill pairs

• Cambrian (Upper) to Recent

Class TERGOMYA

• Exogastrically coiled, univalved, bilaterally symmetric, often planispirally coiled or cap-shaped

mollusks

• Cambrian (Middle) to Recent

Class HELCIONELLOIDA

• Endogastrically coiled, univalved, untorted mollusks

• Cambrian (Lower) to Devonian (Pragian)

Class GASTROPODA

• Univalved, shell usually coiled, having head with eyes and other sense organs, muscular foot for

locomotion. Internal organs rotated through 180˚ during torsion early in ontogeny

• Cambrian (Upper) to Recent

Class BIVALVIA

• Twin-valved, joined along dorsal hinge line commonly with teeth and ligament; lacking head but

with well-developed muscular foot and often elaborate gill systems

• Cambrian (Lower) to Recent

Class ROSTROCONCHIA

• Superﬁ cially similar to bivalves but with shells fused along dorsal midline

• Cambrian (Lower) to Permian (Kazanian)

SPIRALIANS 2: MOLLUSKS 329

(a)

(b)

Solenogastres

Caudofoveata

Polyplacophora

Monoplacophora

Bivalvia

Scaphopoda

Gastropoda

Cephalopoda

Solenogastres

Caudofoveata

Polyplacophora

Monoplacophora

Bivalvia

Scaphopoda

Gastropoda

Cephalopoda

Aculifera

Conchifera

Testaria

Visceral mass

(guts, reproductive

organs, etc.)

Shell

Mouth with

radula

Gills Foot

Anus

Mantle

cavity

HAM

Visceral mass

(guts, reproductive

organs, etc.)

Shell

Mouth with

radula

Gills Foot

Anus

Mantle

cavity

HAM

Aplacophora

Mantle cavity

Mantle

cavity

Palps

Siphons

Enlarged

gills

Head

Siphuncle

Chambers

Funnel

Tentacles

surrounding

mouth

Mantle cavity

Mantle

cavity

Palps

Siphons

Enlarged

gills

Head

Siphuncle

Chambers

Funnel

Tentacles

surrounding

mouth

Figure 13.1 Pseudocladograms of molluskan evolution: hypothetical archemollusk (HAM) evolution

integrated with a cladistic-type framework. Model (a) demonstrates a split into the Aculifera and

Conchifera, whereas (b) indicates a division into the Aplacophora and Testaria. (Based on Sigwart &

Sutton 2007.)

Class SCAPHOPODA

• Long, cylindrical shell, open at both ends

• Devonian to Recent

Class CEPHALOPODA

• Most advanced mollusks with head and well-developed sensory organs together with tentacles

• Cambrian (Upper) to Recent

330 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD

Box 13.2 Kimberella and Odontogriphus join the mollusks

A modest-sized, disk-shaped fossil from the Late Precambrian, named Kimberella in 1959, has suf-

fered mixed fortunes. First described from the Ediacaran rocks of Australia as a jellyﬁ sh and later

a cubozoan, Mikhail Fedonkin and Ben Waggoner (1997) then reconstructed Kimberella as a bilater-

ally symmetric, benthic crawler with a non-mineralized, single shell, on the basis of new material

from the White Sea, Russia. Kimberella is linked with a variety of trace fossils suggesting mobility

and a feeding strategy that must have involved a radula. The body fossils and trace fossils place

Kimberella near the base of the molluskan clade and suggest a deep origin for the phylum (Fig. 13.2),

and for the bilateralians, signiﬁ cantly earlier than the Cambrian explosion. But who were its closest

relatives? A new investigation by Jean-Bernard Caron and his colleagues (2006) offers some clues.

They studied another enigmatic animal, Odontogriphus from the Burgess Shale. Odontogriphus had

previously been allied with the brachiopods, bryozoans, phoronids and even early vertebrates. The

new study shows that Odontogriphus possesses a radula, a broad foot and a stiffened dorsum, so

placing it ﬁ rmly within the mollusks, close to Kimberella, together with Wiwaxia (another enigmatic

soft-bodied organism covered with possible scierites), which also possesses a radula, and another

enigma, Halkieria (Box 13.3).

Annelida

Odontogriphus

Wiwaxia

Halkieriid

Neomeniomorpha

Polyplacophora

Other crown-group

Mollusca

Kimberella?

Firm Substrate Soft

Mat-based ecology

Vertical burrowers

Ediacaran

Early

Cambrian

Mid Late

Ordovician

N-D T A B/T

555 542 501 488 Time (Ma)

Cambrian substrate revolution:

crown-group Mollusca

and known fossil range

stem-group Mollusca

and known fossil range

putative range extension

total-group Mollusca

???

(c)

(a) (b)

Figure 13.2 The early mollusks (a) Kimberella, (b) Odontogriphus and (c) phylogeny and

stratigraphic ranges of early mollusks mapped onto some ecological changes. N-D, Nemakit-

Daldynian; T, Tommotian; A, Atdabanian; B/T, Botomian. (a, courtesy of Ben Waggoner; b, c,

courtesy of ten-Bernard Caron.)

SPIRALIANS 2: MOLLUSKS 331

Box 13.3 Halkieria: from stem-group brachiopod to new class of mollusk

Halkieria was ﬁ rst described on the basis of disarticulated shells from the Cambrian rocks of the

Danish island of Bornholm. But the discovery in the 1980s of articulated specimens from the Early

Cambrian Sirius Passet fauna from North Greenland (see p. 386) generated huge excitement. The

animal was in fact an elongate, worm-like creature with two mollusk-like shells at the front and the

back separated by an armor of sclerites between (Fig. 13.3), quite bizarre and quite different from

previous interpretations of the animal. Initial attempts to place it together with the mollusks were

superseded by its placement as a stem-group brachiopod; reasonable enough because both shells are

very similar to the dorsal and ventral valves of some non-articulated brachiopods. However, to

become a brachiopod, Halkieria would have had to lose its foot, develop a lophophore as a feeding

organ and convert its sclerites to chaetae. Jakob Vinther and Claus Nielsen (University of Copenha-

gen) in 2004 dissected the fossil in detail and compared it with a range of living mollusks. There

was a simpler solution. Halkieria is in fact a mollusk, possessing most of the features that deﬁ ne the

phylum, but a number of characters (such as the shells at the anterior and posterior of the animal)

have formed the basis for a new class of mollusk, the Diplacophora.

Figure 13.3 The mollusk Halkieria from Sirius Passet (natural size).

332 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD

Box 13.4 Computer-simulated growth of mollusks

Most valves of any shelled organism can be modeled as a coil and, in fact, the ontogeny of living

Nautilus was known to approximate to a logarithmic spiral in the 18th century. David Raup

(University of Chicago), in an inﬂ uential study, deﬁ ned and computer-simulated the ontogeny of

shells on the basis of a few parameters: (i) the shape of the generating curve or axial ratio of the

ellipse; (ii) the rate of whorl expansion after one revolution (W); (iii) the position of the generating

curve with respect to the axis (D); and (iv) the whorl translation rate (T). Shells are generated by

translating a revolving generating curve along a ﬁ xed axis (Fig. 13.4). For example, when T = 0,

shells lacking a vertical component such as bivalves and brachiopods, are simulated, whereas those

with a large value of T are typical of high-spired gastropods. Only a small variety of possible shell

shapes occur in nature. Raup’s (1966) original simulations were executed on a mainframe system.

Andrew Swan (1990) adapted the software for microcomputers and has simulated a wide variety of

shell shapes. More recent work has applied more complex techniques to simulate ammonite hetero-

morphs. Nevertheless only a relatively small percentage of the theoretically available morphospace

has actually been exploited by fossil and living mollusks. Clearly some ﬁ elds map out functionally

and mechanically improbable morphologies – perhaps the aperture is too small for the living animal

to feed from within the shell, or the shape would not allow the animal to move; other ﬁ elds have

yet to be tested in evolution. Raup’s morphospace is, however, non-orthogonal and it has been argued

that the mosaic of morphospace occupation is merely an artifact of presentation. Theoretical mor-

phospace has been explored for a range of other groups including bryozoans, echinoids, graptolites,

some ﬁ shes and some plants (Erwin 2007).

There have been many modiﬁ cations of Raup’s original algorithm and a number of web

interfaces that can generate shell shapes; one of the simplest may be accessed via http://www.

blackwellpublishing.com/paleobiology/.

the Lower Cambrian rocks of north Green-

land has promoted new discussion on the

identity of the earliest mollusks (Box 13.3).

The halkieriid not only displays the articula-

tion of a series of sclerites, or plates, com-

monly described in the past as discrete

organisms, but also two large mollusk-like

shells at the front and back of the worm-like

animal. The many, often bizarre but distinc-

tive, early mollusks formed the basis for sub-

sequent radiation of the phylum particularly

during the Late Cambrian and Early Ordovi-

cian. The shapes of these and other mollusk

shells have formed the basis numerical model-

ing, demonstrating that fossil and living

shell shapes, and indeed many unknown in

nature, can be generated by computers (Box

13.4).

The hyoliths – long, conical, calcareous

shells with an operculum-covered aperture –

have often been called mollusks. The group

ranges from the Cambrian to Permian with

some of the 40 known genera reaching lengths

of 200 mm. Current studies assign the group

to its own phylum, related to the mollusks

and the peanut worms, the Sipunculida.

CLASS BIVALVIA

Bivalves are among the commonest shelly

components of beach sands throughout the

world. Many taxa are farmed and harvested

for human consumption, and pearls are a

valuable by-product of bivalve growth. The

bivalves developed a spectacular variety of

shell shapes and life strategies, during a history

spanning the entire Phanerozoic, and all

are based on a simple bilaterally symmetric

exoskeleton. The ﬁ rst bivalves were

marine shallow burrowers; epifaunal, deep

SPIRALIANS 2: MOLLUSKS 333

(b)

Buccinum Epitonium Ceratomya

DactyliocerasGryphaea

expansion rate (W)

gastropods

02341

translation rate (T)

bivalves

brachiopods

distance (D) of generating

curve from axis

0.2

0.4

0.6

0.8

1.0

planispiral forms

coiled

cephalopods

(a)

Figure 13.4 Theoretical morphospace created by the computer simulation of shell growth (a) and

some computer simulations matched with reality (b). (a, based on Raup 1966; b, from Swan

1990.)

334 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD

burrowing and boring strategies together with

migrations to freshwater habitats were sec-

ondary innovations. There are over 4500

genera of living bivalves, with fewer than half

of that number described from the fossil

record. In view of the wide range of life strate-

gies and their relationships to particular sedi-

ments, the bivalves are good facies fossils.

Although non-marine bivalves have been used

extensively, in the absence of other groups, to

zone parts of the Upper Carboniferous and by

Charles Lyell in his classic work in the 1820s

and 1830s to subdivide the Tertiary (the

increasing proportion of living forms in fossil

faunas through the Tertiary was used to sub-

divide the system; see p. 29), their biostrati-

graphic precision is limited.

Basic morphology

Bivalves are twin-valved shellﬁ sh superﬁ cially

resembling the brachiopods and common in

modern seas (Fig. 13.5). In contrast to the

Brachiopoda, bivalve shells are always com-

posed of calcium carbonate, usually arago-

nite, and many have a plane of symmetry

parallel to the commissure separating the left

and right valves from each other, i.e. the two

valves are virtually mirror images of each

other. Bivalves have sometimes been termed

lamellibranchs or pelecypods, but they

were ﬁ rst named Bivalvia by Linnaeus in

1758.

In the bivalves the molluskan head is lost,

only the anterior mouth indicates its position.

Sensory organs are concentrated instead on

the mantle margins and include eye-spots,

chemoreceptors and statocysts. The bivalve

exoskeleton has two lateral valves, left and

right, essentially mirror images of each other,

united dorsally along the hinge line by an

elastic ligament and usually interlocking teeth

and sockets; the valves open ventrally. The

valves are secreted by mantle lobes. The

attachment of the mantle is marked by

the pallial line, which may be indented poste-

riorly with the extension of the siphons. The

earliest-formed parts of each shell, the beaks

or umbones, may be separated by the cardinal

area supporting the dorsal ligament. When

the valves are closed, a pair of adductor

muscles, situated anteriorly and posteriorly, is

in contraction. While the shells are closed, the

hinge ligament is constrained between the

dorsal parts of the shells; when the adductors

relax, the ligament expands and the shells

spring open. The scars of these shell-closing

muscles may be seen usually as clear rough-

ened and depressed areas inside both

valves.

Classiﬁ cation of the bivalves is based pri-

marily on gill structure (Fig. 13.6a). Dentition

is of secondary importance (Fig. 13.6b). Teeth

may be all along the hinge line or separated

into discrete cardinal (subumbonal) and

lateral (both anterior and posterior of the

hinge line) teeth. The three most important

tooth arrangements are: (i) taxodont – numer-

ous subequal teeth arranged in a subparallel

pattern; (ii) actinodont – teeth radiating out

from beneath the umbo; or (iii) heterodont – a

mixture of cardinal (beneath umbo) and

lateral teeth. Various other terms have been

employed in the past when the teeth are thick-

ened, modiﬁ ed or reduced, but are now less

commonly used.

In most cases the umbones of the valves

point or face obliquely anteriorly, the pallial

sinus (if present) is situated posteriorly and

the posterior adductor is usually the larger of

the two scars. In some forms the anterior

adductor is lost, together with the foot. When

the valves are held with the commissure

between the two valves vertically, the anterior

end pointing away from the observer and the

umbones at the top, then the right and left

valves are in the correct orientation.

Main bivalve groups

The Bivalvia are classiﬁ ed by zoologists mainly

on the basis of soft-part morphology such as

features of the digestive system and the gills;

paleontologists have usually attempted to use

details of the hinge structures. There are seven

basic features that are of use for classiﬁ cation

at various levels within the Bivalvia: gill

structure (subclass and infrasubclass levels),

dentition (all levels), ligament insertion (infra-

subclass down to ordinal levels), adductor

muscle scars (superfamily to orders), pallial

line (family level and below), shell shape (all

levels), and shell fabric (infrasubclass down to

superfamily level). Two subclasses are recog-

nized: (i) the Protobranchia with simple pro-

SPIRALIANS 2: MOLLUSKS 335

tobranch gills very like those of the archetype

mollusk, that are deposit feeders; and (ii) the

Autolamellibranchiata that mostly have large

leaf-like gills modiﬁ ed for food gathering as

well as for respiration (ﬁ libranch and eula-

mellibranch types), but some have lost their

gills altogether and use the mantle cavity for

respiration (septibranch) (Fig. 13.6a).

A number of taxa from the two subclasses,

the protobranchs and autolamellibranchs, are

illustrated in Fig. 13.7.

Protobranchs

The Nuculoida is the oldest and most

primitive infrasubclass, characterized by

shell teeth

gonad

ligament

style sac

kidney

(c)

heart

digestive diverticulum

mouth

palp

shell

adductor muscle

posterior

adductor muscle

anus

exhalent siphon

inhalent siphon

position of the gill

(shown in part)

foot

(a)

(b)

beak

dorsal

beak

dorsal

ligament area

hinge plate

cardinal teeth

socket

adductor

scar

pallial line

posterior

adductor

scar

ventral

pallial sinus

posterior

Figure 13.5 Bivalve morphology based on a living bivalve: (a) internal features of the right valve,

(b) external features of the left valve, and (c) reconstruction of the internal structures attached to the

right valve. (Based on Treatise on Invertebrate Paleontology, Part N. Geol. Soc. Am. and Univ. Kansas.)

336 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD

(a)

(b)

protobranch filibranch eulamellibranch septibranch

taxodont

dysodont

isodont

schizodont

heterodont

desmodont

Figure 13.6 (a) Main gill types in the bivalves. (b) Main types of bivalve dentition.

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

(e)

(f) (g)

Figure 13.7 Some bivalve genera: (a) Glycimeras (Miocene), (b) Trigonia (Jurassic), (c) Gryphaea

(Jurassic), (d) Chlamys (Jurassic), (e) Mya (Recent), (f) Pholas (Recent), and (g) Spondylus (Cretaceous).

Magniﬁ cation ×0.75 for all.

SPIRALIANS 2: MOLLUSKS 337

prismato-nacreous shells, taxodont dentition,

equivalved shells and protobranch gills. Most

are detritus-feeding infaunal marine animals,

such as Nucula, and most abundant today in

deeper-water environments. Ctenodonta has

a typical taxodont dentition, an elliptical shell

and an external ligament; it is principally

Ordovician in age.

Members of the infrasubclass Solemyoida

are specialized, infaunal burrowers with an

anteriorly elongate shell. Most have symbiotic

autochemotrophic bacteria allowing them to

live in fetid muds, ranging in age from Early

Ordovician to Recent.

Autolamellibranchs

Autolamellibranchs were derived from the

protobranchs by the earliest Ordovician, pos-

sibly via a group of nuculoids that developed

hinge-teeth allowing greater opening of the

valves. This is necessary to avoid sediment

inadvertently trapped by the gills during the

food-gathering process.

The pteriomorphs are mainly marine, ﬁ xed

benthos, attached by a byssus, or pad of sticky

threads, modiﬁ ed from the foot, or they may

be cemented. They are an important part of

bivalve faunas from the earliest Ordovician

and most had an outer mineralized shell layer

of calcite; the gills are of ﬁ libranch grade. The

group includes the mussels Modiolus and

Mytilus and the ark shells Arca and Anadara,

the scallops Chlamys and Pecten, and the

oysters Crassostrea and Ostrea.

The heteroconchs are a mixed bag of mainly

suspension feeders, important in bivalve

faunas from the earliest Ordovician and radi-

ating during the Mesozoic when mantle fusion

and the development of long siphons pro-

moted a deep-infaunal life mode. They are

and were very successful burrowers. Gill

grades are mainly eulamellibranch and many

have crossed-lamellar or complex crossed-

lamellar shell microstructures. This group

includes the typical clams such as the giant

clam Tridacna, the horse-hoof clam Hippopus

and the surf-clam Donax, together with the

razor shells Ensis and Tagelus, the ship-worm

Teredo and the cockle Cerastoderma.

The anomalodesmatans are predominantly

suspension-feeding marine forms with pris-

mato-nacreous shells and reduced dentitions,

such as Pholadomya. They have eulamelli-

branch or septibranch gill grades. These too

are found from the earliest Ordovician but

only form a minor part of bivalve faunas.

Lifestyles and morphology

There are seven main bivalve forms that relate

to their modes of life (Stanley 1970): infaunal

shallow burrowing, infaunal deep burrowing,

epifaunal attached by a byssus, epifaunal with

cementation, free lying, swimming, and borers

and cavity dwellers. Speciﬁ c assemblages of

morphological features are associated with

each life mode; these are summarized in

Fig. 13.8. Steven Stanley’s studies have been

adapted by a number of authors for similar

bivalve-dominated communities throughout

the Phanerozoic (Fig. 13.9). Most bizarre

were the rudists that built extensive reefs in

the Cretaceous (Box 13.5).

Bivalve evolution

The earliest known bivalves have been

reported from the basal Cambrian. Two Early

Cambrian genera are the praenuculid Pojetaia

from Australia and China and Fordilla from

Denmark, North America and Siberia. Both

genera have two valves separated by a working

hinge with a ligament, together with muscles

and teeth. These probably came about 10 myr

after the oldest rostroconch, Heraultipegma,

and so the bivalves might just have evolved

from rostroconchs (see p. 357) or something

like them. The class evolved rapidly in the

Early Ordovician to include basal forms of all

bivalve infrasubclasses. Not only were tax-

odont, actinodont and heterodont dentitions

established, but a variety of feeding types had

also developed following the Tremadocian

and Floian radiation.

Following this major diversiﬁ cation, the

group stabilized during the remaining part of

the Paleozoic, although some groups evolved

extensive siphons that aided deep-burrowing

life modes. This adaptation, together with the

mobility provided by the bivalve foot, were

important advantages over most brachiopods,

which simultaneously pursued a ﬁ xed epifau-

nal existence. The earliest autolamellibran-

chiate forms are known from the Early

Tremadocian. The early Mesozoic radiation

of the group featured siphonate forms

with desmodont and heterodont dentitions,