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

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308 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD

LIFESTYLE BRACHIOPOD TAXA ADAPTATIONS

Attached by pedicle

Epifaunal – hard substrate (1)

(plenipedunculate)

Epifaunal – soft substrate (2)

(rhizopedunculate)

Cryptic

Interstitial

Cemented

Encrusting (3)

Clasping spines (4)

Mantle fibers

Unattached

Cosupportive (5)

Coral-like (6)

Recumbent

Pseudofaunal (7) and inverted (8)

Free-living (9, 10)

Mobile

Infaunal (11)

Semi-infaunal (12)

Orthides, rhynchonellides,

spiriferides and terabratulides

Chlidonophora and Cryptopora

Argyrotheca and Terebratulina

Acrotretides and Gwynia

Craniops and Schuchertella

Craniids and disciniids

Linoproductus and Tenaspinus

Orthotetoids

Pentamerids and trimerellids

Gemmellaroids and

richthofeniids

Strophomenides

Waagenoconcha and Marginifera

Cyrtia, Chonetes, Neothyris

and Terebratella

Linguloids

Camerisma and Magadina

Figure 12.9 Brachiopod lifestyles. (Courtesy of David Harper and Roisin Moran.)

SPIRALIANS 1: LOPHOPHORATES 309

Box 12.3 Chinese lingulides

Did early lingulides live in burrows like many of their descendants? Xianshanella haikouensis from

the Lower Cambrian Chengjiang fauna, South China was a subcircular animal with horny setae and

a massive pedicle. Zhang Zhifei and his colleagues (2006) have shown that these earliest brachiopods

did not live in burrows, but actually attached themselves to the shells of other invertebrates – an

epibenthonic rather than infaunal mode of life (Fig. 12.10). Moreover the Chengjiang lingulide has

a lophophore, a U-shaped digestive tract and an anteriorly-located anus; these advanced features

were already present in the lingulate brachiopod lineage right from the start it seems.

number of quite different non-articulated and

articulated taxa were cemented to the sub-

strate, whereas some groups evolved clasping

spines to help stabilize their shells. In a number

of groups the pedicle atrophied during ontog-

eny. Many taxa thus developed strategies

involving inverted, pseudoinfaunal and recum-

bent life modes; a number lived in cosupport-

ive clusters and others mimicked corals. Not

all brachiopods were sessile; a few, such as

Lingula, adopted an infaunal lifestyle (Box

12.3), whereas the articulated forms Camer-

isma and Magadina were semi-infaunal.

Throughout the Phanerozoic the brachio-

pods have participated in a spectrum of level-

bottom, benthic paleocommunities. Pioneer

studies on Silurian brachiopods suggested that

their paleocommunities were depth related,

and a predictable succession of faunas, each

characterized by one or more key brachiopods,

has been identiﬁ ed (Fig. 12.11). The onshore–

offshore assemblages of the Lingula, Eocoelia,

Pentamerus, Stricklandia (or its close relative

Costistricklandia) and Clorinda paleocommu-

nities, ﬁ rst identiﬁ ed in the Silurian of Wales,

form the basis of benthic assemblage (BA)

zones 1–5, ranging from intertidal environ-

ments to the edge of the continental slope;

more basinal environments are included in

BA6. Parallel studies on Mesozoic brachio-

pods have, on the other hand, suggested that

brachiopod-dominated paleocommunities

were controlled by substrate rather than depth

(Fig. 12.12). Clearly, in reality, a combination

of these and other factors controlled the distri-

butions of the Brachiopoda in a complex

system of suspension-feeding guilds.

Brachiopods have also acted as substrates

for a variety of small epifaunal animals (see

p. 97). The progressive and sequential coloni-

zation of Devonian spiriferids, by Spirorbis,

itself a possible lophophorate (Taylor & Vinn

2006), Hederella, Paleschara and Aulopora

marked the development of an eventual climax

paleocommunity on the actual brachiopod

shell itself. Were they feeding on incoming

brachiopod food or just waste? The general

view is that these animals congregated beside

the inhalant currents on the median parts of

the anterior commissure, and beneﬁ ted from

the indrawn particles of food. An alternative

view, and it is hard to prove or disprove, is

that they were taking advantage of waste

being ejected from the brachiopod.

Brachiopods not only acted as suitable sub-

strates for an epifauna, they were also prone

to attack (Box 12.4) and drill holes suggest

predation and in some cases attachment of

other brachiopods themselves (Robinson &

Lee 2008).

Brachiopods, functional morphology

and paradigms

Martin Rudwick, an English brachiopod

expert just beginning his career in the 1960s

(he is now a distinguished historian of

geology), proposed the paradigm approach in

functional interpretation of fossils. His idea

was to create an engineering model for a func-

tion, such as water ﬂ ow in feeding. For

example, does the costation, the zig-zag

pattern of ridges and furrows, of the anterior

commissure of the brachiopod have a real

functional signiﬁ cance? In numerical terms it

can be shown that costation increases the

length of commissure and hence the intake

area that may be held open without increasing

Continued

310 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD

(a) (b)

(c)

(d)

Dva

Vva

Dva

Vva

Um?

Ct Ct

Figure 12.10 Chinese lingulides: Reconstruction of the Chengjiang lingulid Xianshanella. A, anal

opening; B, brachial arm; Co, cone-like organisms; Ct, cheek of trilobite; Dd, digestive tract; Dva,

dorsal visceral area; Pc, pedicle cavity; St, stomach; Um?, possible umbonal muscle; Vs, setae

fringing ventral valve; Vva, ventral visceral area. Scale bars, 2 mm. (From Zhang et al. 2006.)

SPIRALIANS 1: LOPHOPHORATES 311

Outcrop area

Graptolitic muds

TurbiditesT

Shelf margin

Current direction

Land area

Lingula community

Eocoelia community

Pentamerus community

Costistricklandia community

Clorinda community

050

Anglesey

Aberystwyth

Haverfordwest

Bristol Channel

Llandovery

Presteigne

Wenlock

Shropshire

Malvern

line

May Hill

LAND

Cardigan Bay

Rosemarket

Lingula sp.

Eocoelia curtisi

Pentamerus

oblongus

Costistricklandia

lirata alpha

Clorinda globosa

Figure 12.11 Lower Silurian depth-related paleocommunities developed across the Welsh and Anglo-

Welsh region. (Based on Clarkson 1998.)

Clastic

dykes

Rocky

sea-floor

Transported

shallow-water

material

Unstable coastlines Normal coastlines

Flysch-type

deposits

Floating algae

Dominantly

pelagic

sediments

Normal

muddy

sediments

Normal

sand-grade

sediments

Coarse

littoral

sediments

Coast without

reefs

Coast with

reefs

Lagoonal

sediments

Reef

detritus

Figure 12.12 Mesozoic palaeocommunities developed across Alpine Europe. Numbers 1 to 7 refer to

the seven different biotypes described on the ﬁ gure. (Based on Ager, D.V. 1965. Palaeogeogr.

Palaeoclimatol. Palaeoecol. 1.)

312 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD

Box 12.4 Brachiopod predation

Brachiopods were eaten by gastropods, arthropods and other predators, and the best evidence is

found in Paleozoic examples, especially in the Devonian. Many predatory gastropods feed by drilling

into the shells of their prey, and two types of drill hole are commonly present in Paleozoic brachio-

pods: small, cylindrical holes made by Oichnus simplex and larger, often beveled holes made by O.

paraboloides; these are, of course, ichnogenera (see p. 525) and not actual brachiopods (Fig. 12.13).

After the Devonian peak in drilling diversity, there was apparently a marked drop in the frequency

of drilled shells, particularly after the Mid Carboniferous. Many Carboniferous and Permian groups

such as the productides have thickened shells with an armor of frills, lamellae and spines, all perhaps

acting as defense against marauders. Maybe the prey had won this early arms race or perhaps the

introduction of mollusks into these communities provided fresh and preferable seafood for the preda-

tors. Nevertheless, if we use the Recent Antarctic benthos as a model for the Paleozoic fauna, there

is a lack of fast-moving durophagous predators (Harper 2006). Some authors have speculated that

the toxins within the ﬂ esh of some modern groups, such as the rhynchonellids, may have protected

them from attack.

Figure 12.13 Brachiopod predation: boring of Oichnus paraboloides in the conjoined valves of

Terebratulina from the Pleistocene rocks of Barbados. Scale bar is in millimeters. (Courtesy of

Stephen Donovan.)

SPIRALIANS 1: LOPHOPHORATES 313

the gape of the two shells. Thus an increased

volume of nutrient-laden ﬂ uid may ﬂ ow into

the mantle cavity while grains of sediment

with diameters exceeding the shell gape will

still be excluded. So far so good.

During the Permian, a group of aberrant

productoids, called the richthofeniids, mim-

icked corals and built biological frameworks

that may be found as fossils in the Salt Ranges

of Pakistan and the Glass Mountains of Texas.

These brachiopods have a cylindrical pedicle

valve attached to the substrate and a small,

cap-like brachial valve. It is difﬁ cult to under-

stand how these animals fed. A possible sce-

nario involves the ﬂ apping of the upper,

brachial valve to generate currents through the

brachiopod’s mantle cavity. Rudwick ﬁ lmed

the ﬂ ow of water through the cylindrical,

lower, pedicle valve as the upper valve was

moved up and down. Fluid did in fact move

efﬁ ciently through the animal, bringing in

nutrients and ﬂ ushing out waste. The para-

digm, however, failed the test of ﬁ eld-based

evidence. Specimens of the athyride Compos-

ita apparently in life position occur attached to

the upper valve of the richthofeniid. Vigorous

ﬂ apping of the valve was thus unlikely and it

would not have been an ideal attachment site

for an epifauna. Rather, these aberrant animals

may have developed lophophores with a ciliary

pump action to move currents through the

valves. One hypothesis has been rejected, and

another stands as a possibility – we cannot

prove how the richthofeniid brachiopods func-

tioned, but the paradigm approach offers a

reasonably objective way for paleontologists

to approach these problems.

Distribution in space: biogeography

The biogeographic patterns of the linguli-

formean brachiopods were quite different

from those of the craniiformeans and rhyn-

chonelliformeans. The former had planktotro-

phic larval phases (see p. 241) with a facility

for wide dispersal; in contrast the lecithotro-

phic larvae of the latter were short-lived and

thus individual species were less widely distrib-

uted. Cambrian brachiopods were organized

into tropical and polar realms. Linguliformeans

developed widespread distributions in shelf

and slope settings; rhynchonelliformeans were

more diverse in the tropics, preferring shallow-

water carbonate and mixed carbonate-

siliciclastic environments. In the Ordovician,

brachiopod provincialism generally decreased

during the period. Provinciality was most

marked during the Early Ordovician, when a

range of platform provinces associated with

the continents of Baltica, Gondwana, Lauren-

tia and Siberia (see Appendix 2) were supple-

mented by centers of endemism associated

with a range of microcontinents and volcanic

arcs and island complexes.

Provincialism was reduced during the Silu-

rian with the close proximity of many major

continents. By the Wenlock, however, two

broad provinces, the cool-water Clarkeia and

the mid-latitudinal Tuvaella faunas, empha-

sized an increasing endemism, climaxing

during the Ludlow and Prídolí epochs. Pro-

vinciality was particularly marked during the

Mid Devonian coincident with peak diversi-

ties in the phylum. Clear biogeographic pat-

terns continued into the Carboniferous, but

the Permian was characterized by higher

degrees of provinciality probably associated

with steep climatic gradients.

During the Triassic, brachiopod faunas, fol-

lowing an interval of cosmopolitan disaster

taxa, became organized into Boreal (high-lati-

tude) and Tethyan (low-latitude) realms (Box

12.5). This pattern continued throughout the

Mesozoic, but with centers of endemism and

occasional modiﬁ cations due to ecological

factors such as the circulation of ocean currents

and the local development of chemosynthetic

environments. Biogeographic patterns among

living forms reﬂ ect their Cenozoic roots: a

southern area, the northern Paciﬁ c, and a

northern area (Atlantic, Mediterranean, North

Sea and the circumpolar northern oceans) are

based on a variety of articulated brachiopod

associations. The linguliformeans have more

widespread, near-cosmopolitan distributions.

BRYOZOA

Besides these, there were the Bryozoa, a

small kind of Mollusk allied to the Clams,

and very busy then in the ancient Coral

work. They grew in communities, and

the separate individuals are so minute

that a Bryozoan stock looks like some

delicate moss. They still have their place

among the Reef-Building Corals, but

play an insigniﬁ cant part in comparison

with that of their predecessors.

Atlantic Monthly (April, 1863)

314 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD

Box 12.5 Tethyan brachiopods in Greenland: a Cretaceous Gulf Stream

current?

Brachiopods can give clues about ancient ocean currents. Today, the Gulf Stream runs out from the

Caribbean, sweeps up the eastern seaboard of North America, and then detaches from the coast just

north of New York and heads across the Atlantic to wrap the shores of Britain and western Europe

in warmer-than-expected waters. Has the Gulf Stream always ﬂ owed the same way? Some Cretaceous

brachiopods give us a clue. David Harper and colleagues (2005) showed how some Early Cretaceous

brachiopod faunas from East Greenland were a mix of animals from two ocean provinces, Tethyan

(low latitude) and Boreal (high latitude). The Boreal, shallow-water assemblage is dominated by

large terebratulids and ribbed rhynchonellids, and occurs adjacent to a fauna containing Tethyan

elements, more typical of deeper water, including Pygope (see p. 311). How did these exotic, tropical

visitors travel so far north? Harper and colleagues suggested that an Early Cretaceous out-of-Tethys

migration was helped by the early and persistent northward track of a proto-Gulf Stream current

(Fig. 12.14). These kinds of studies of changing patterns of paleobiogeography through time are

critical for understanding modern climate and ocean patterns.

Greenland

Gondwana

Russian

Platform

Mediterranean

Tethys

Land area

Present-day

coastline

Migration route

Figure 12.14 Tethyan brachiopods in East Greenland: Pygope and the proto-North Atlantic

current (arrows), one of its possible migration routes. The star indicates the Lower Cretaceous,

East Greenland locality.

Bryozoans are the only phylum in which all

species are colonial. Many skeletons are

exquisitely designed, but fragment very easily

after death. Although relatively common,

bryozoans are among the least well-known

invertebrates. There are about 6000 living

and 16,000 fossil species, and most are marine

(Box 12.6). Superﬁ cially resembling the corals

and hydroids, the bryozoans (“moss animals”)

are like minute colonial phoronids (see p.

298) with tiny individuals or zooids, com-

monly less than 1 mm in diameter. Each zooid

is celomate with a separate mouth and anus

together and a circular or horseshoe-shaped

SPIRALIANS 1: LOPHOPHORATES 315

lophophore equipped with a ring of 8–100

tentacles – a major organizational jump from

the cnidarians. The bryozoan lophophore is

constructed differently from those of the bra-

chiopods and phoronids and it may be a

mistake to think that all three groups are

closely related just because they possess cili-

ated feeding organs. Individual zooids are

enclosed by a gelatinous, leathery or calcare-

ous exoskeleton, usually in the form of slender

tubes or box-like chambers called zooecia.

The primary function of most zooids is the

capture of food, but some are specialists in

defense, reproduction or sediment removal;

the bryozoan colony thus functions as a well-

organized unit.

Morphology: Bowerbankia

The genus Bowerbankia is a relatively simple

bryozoan useful for illustrating the general

anatomy of bryozoan zooids (Fig. 12.15).

Each living zooid is enclosed by a body wall

or cystid. The lophophore, with its beating

cilia, extends outwards from the zooid and

comprises a ring of 10 tentacles, directing

food to a central mouth leading into a U-

shaped gut; the feces ﬁ nally exit out through

an anus. A funiculus extends along the stolon

connecting all the zooids. This is thought to

be a homolog of the blood vessels found in

other animals. The individual zooids are her-

maphrodites, developing eggs and sperm at

different times; the eggs are usually fertilized

in the tentacle sheath, developing later into

trochophore larva.

Ecology: feeding and colonial morphology

Feeding strategies of bryozoans have had a

major inﬂ uence on the style of colony growth.

Feeding behavior patterns are correlated with

the shape of the colony and the size of the

zooids. Bryozoan colonies can grow in a

variety of modes from encrusting runners,

uniserial or multiserial branches that split,

and sheets where growth occurs around the

entire margin, to more erect type forms that

have complex three-dimensional morpholo-

gies (Box 12.7). Many elegant forms have

evolved such as the bush- and tree-like trepo-

stomes of the Paleozoic, the spiral Archimedes

and vase-shaped Fenestrella, in both of which

the entire colony may have acted like a sponge.

But bryozoan colonies can also move. For

example, colonies of Selenaria can scuttle

across the seaﬂ oor. Stilt-like appendages or

setae project downwards from specialized

zooids and as the setae move in waves, the

colony is transported across the seabed. Such

a lifestyle can be traced back to the Late Cre-

taceous when free-living colonies, the so-

called lunulitiforms, evolved their regular

shape, without interference from adjacent

objects on the seaﬂ oor.

Zooid size can give important clues about

environment and particularly water tempera-

ture. Increased ranges of seasonal variation in

temperature seem to be correlated with an

increased amount of variation in the size of

zooids in the colony (O’Dea 2003). It is not

clear why there is this relationship, but nev-

ertheless zooid size may also be a useful envi-

ronmental proxy.

Evolution: main fossil bryozoan groups

The oldest bryozoans in the fossil record

occur in the Tremadocian Stage of the Lower

Ordovician, but it is very likely that primitive,

soft-bodied bryozoans existed during the

Cambrian but have not been fossilized; numer-

ous families of bryozoans are found in the

succeeding Floian Stage. The Stenolaemata

dominated Paleozoic bryozoan faunas (Fig.

12.17). The trepostomes or stony bryozoans

commonly had bush-like colonies with pris-

matic zooecia having polygonal apertures.

The group diversiﬁ ed during the Ordovician

to inﬁ ltrate the low-level benthos. Genera

such as Monticulipora, Prasopora and

Parvohallopora are typical of Ordovician

assemblages.

The cryptostomes, although originating

during the Early Ordovician, were more abun-

dant during the Mid and Late Paleozoic as the

trepostomes declined; in some respects the

group forms a link with the net-like fenes-

trates that were particularly common in the

Carboniferous (Fig. 12.18). Fenestella, itself,

may be in the form of a planar mesh, cone or

funnel. The branches of the colony are con-

nected by dissepiments; rectangular spaces or

fenestrules separate the branches that contain

the biserially-arranged zooids. Archimedes,

however, has a meshwork wound around a

screw-shaped central axis. Richard Cowen

and his colleagues (University of California)

316 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD

have modeled the feeding strategies of these

screw-shaped colonies and other fenestrates.

Carboniferous fenestrate colonies usually had

inward-facing zooids and probably drew

water in through the top of the colony and

ﬂ ushed it out through the fenestrules at the

sides. On the other hand, Silurian colonies

had outward-facing zooids and sucked in

water through the fenestrules, expelling it out

of the open top of the colony.

In general both the cryptostomes and fenes-

trates outstripped the trepostomes during the

Late Paleozoic, many of the fenestrates popu-

lating reef environments. Although both

groups disappeared at the end of the Permian

or soon after, they were still conspicuous

members of the Late Permian benthos; both

Fenestella and Synocladia form large, vase-

shaped colonies in the communities of the

Zechstein reef complex in the north of England

(a)

(b)

lophophore

collar

muscular

sphincter

metacel

retractor

muscle

stolon

tissue

cords

parietal

muscles

gut

transverse

parietal

muscles

ovary

frontal

membrane

retractor

muscles

operculum

anus

lophophore

operculum

closing

muscle

metacel

testis gut

lateral

pores

tissue

cord

parietal

muscles

Figure 12.15 Morphology of two living bryozoans: (a) a stenolaemate and (b) a gymnolaemate. (Based

on various sources.)

SPIRALIANS 1: LOPHOPHORATES 317

Box 12.6 Bryozoan classiﬁ cation

Class PHYLACTOLAEMATA

• Cylindrical zooids with horseshoe-shaped lophophore. Statoblasts arise as dormant buds. Fresh-

water with non-calciﬁ ed skeletons. Over 12 genera

• Triassic, possibly Permian to Recent

Class STENOLAEMATA

• Cylindrical zooids with calcareous skeleton. Membraneous sac surrounds each polypide; lopho-

phore protrudes through an opening at the end of the skeletal tube. Marine, with an extensive

fossil record. Contains the following orders: trepostomes (Ordovician–Triassic), cystoporates

(Ordovician–Triassic), cryptostomes (Ordovician–Triassic), cyclostomes (Ordovician–Recent)

and fenestrates (Ordovician–Permian). About 550 genera

• Ordovician (Tremadoc) to Recent

Class GYMNOLAEMATA

• Cylindrical or squat zooids of ﬁ xed size with circular lophophore, usually with a calcareous

skeleton. The majority are marine but some are found in brackish and freshwater environments.

Includes the cheilostomes (Jurassic–Recent). Over 650 genera

• Ordovician (Arenig) to Recent

Box 12.7 Module iteration: building a Lego bryozoan

Bryozoan colonies grow by iteration, repeating the same units again and again until the colony is

built. But is this process just a simple addition of individual units (zooids) within the colony? If so,

the opportunity for evolution and morphological complexity would be very limited. There may be

a whole hierarchy of types of modules that are in fact iterated (repeatedly re-evolved). For example,

much more variability will be generated if a branch rather than a zooid is duplicated and attached

to various parts of the colony in various different orientations. Steven Hageman of Appalachian

State University suggested just this in a paper published in 2003: there is a hierarchy of such modules

and those second-order blocks will have a much greater effect on morphological change and evolu-

tion of the colony than simply duplicating the zooids. This can be easily demonstrated by an analogy

with a Lego model. The individual blocks, if iterated, will form only fairly simple patterns, but build

a structure and iterate that and suddenly considerable morphological complexity can be generated

from relatively simple building blocks (Fig. 12.16).

Continued