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

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318 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD

(a)

(c)

(d)

(b)

Figure 12.16 The modular construction of a colony using Lego blocks: complex forms are

generated by iteration of higher order modular units. (From Hageman 2003.)

SPIRALIANS 1: LOPHOPHORATES 319

Stenolaemata

Gymnolaemata

~50 genera

Paleozoic Mesozoic Ceno.

O S D C Pm Tr Jr K P N

Cryptostomata

Cyclostomata

Cystoporata

Fenestrata

Trepostomata

non-boring Ctenostomata

boring Ctenostomata

Cribri-

morpha

Anasca

Ascophora

Figure 12.17 Stratigraphic ranges and absolute abundances of the main bryozoan groups. Geological

period abbreviations are standard, running from Ordovician (O) to Neogene (N). (From Taylor, 1985.)

(a)

(b)

(c)

(d) (e) (f)

(g) (h)

Figure 12.18 Some bryozoan genera: (a) Rhabdomeson (Carboniferous cryptostome), (b)

Rectifenestella (Carboniferous fenestrate), (c) Fistulipora (Carboniferous cystopore), (d) Penniretepora

(Carboniferous fenestrate), (e) Archimedes (Carboniferous fenestrate), (f) Archaeofenestella (Silurian

fenestrate), (g) Lunulites (Cretaceous cheilostome), (h) Castanapora (Cretaceous cheilostome).

Magniﬁ cation approximately ×30 (a), ×15 (b, c), ×1 (d–f), ×5 (g), ×20 (h). (a–c, courtesy of Patrick

Wyse Jackson; d–h, from Taylor 1985.)

320 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD

Box 12.8 Competition and replacement in cyclostome and cheilostome

clades: what really happened at the KT boundary?

Perhaps one of the most obvious changes in bryozoan faunas through time involves the relative

decline of the cyclostomes and the diversiﬁ cation of the cheilostomes leading up to the Cretaceous–

Tertiary (KT) boundary. Since both groups occupied similar ecological niches and are comparable

morphologically, many workers have assumed that the cyclostomes, originating during the Ordovi-

cian and diversifying in the Cretaceous, were outcompeted by the cheilostomes at the end of the

Cretaceous. However Scott Lidgard (Field Museum of Natural History, Chicago) and his colleagues

have analyzed this transition in detail and the results are far from conclusive (Lidgard et al. 1993).

Both groups continued to participate together in bryozoan communities during the Cenozoic and

much of the apparent decline in the cyclostome numbers may be due to the greater diversiﬁ cation

or expansion of the cheilostomes that began to dominate these assemblages in the Cenozoic. Perhaps

this expansion had already been seeded in the Jurassic, when the poor and sporadic bryozoan fauna

provided the ecological space for the expansion of the cheilostomes. A detailed statistical study based

on generic-range data from Sepkoski’s database (McKinney & Taylor 2001) has conﬁ rmed that

origination within the cheilostome clades was the driving force behind the apparent takeover by this

group (Fig. 12.19).

See http://www.blackwellpublishing.com/paleobiology/.

and elsewhere. The trepostomes, however, lin-

gered on until the Late Triassic.

The cyclostomes have tube-shaped zooecia

and often grew as branching tree-like colonies

or alternatively encrusting sheets or ribbons.

The ﬁ rst representatives of the order are

known in Lower Ordovician rocks, but the

group peaked during the mid-Cretaceous in

spectacular style, with a diversity of over 70

genera. Many genera such as Stomatopora,

consisting of a series of bifurcating, encrust-

ing branches, have very long stratigraphic

ranges; moreover Stomatopora may have

pursued an opportunist life strategy, rapidly

spreading their zooids over hard surfaces.

The Gymnolaemata are represented in the

fossil record by two orders, the ctenostomes

and the cheilostomes. The ctenostomes ﬁ rst

appeared in the Early Ordovician and many

genera have since pursued boring and encrust-

ing life strategies. Penetrantia and Terebri-

pora are borers whereas the modern genus

Bowerbankia has an erect colony with semi-

spirally arranged zooecia clustered around a

central branch. The cheilostomes, however,

dominate the class and are most diverse of all

the bryozoan groups (Box 12.8). Cheilostomes

typically have polymorphic zooids, adapted

for different functions, which are usually

linked within the highly integrated colony.

This advanced group appeared during the

Late Jurassic; they are particularly common

in shallow-water environments of the Late

Cretaceous and Paleogene of the Baltic and

Denmark. Lunulites, for example, is discoidal

and free-living, whereas Aechmella is an

encrusting form often associated with sea

urchins.

Ecology and life modes

Virtually all bryozoans are part of the sessile

benthos, mainly occurring from the sublitto-

SPIRALIANS 1: LOPHOPHORATES 321

Time (Ma)

Genus extinction standing diversity (myr)Genus extinction standing diversity (myr)

encrusting genera

erect genera

0.08

0.06

0.04

0.02

Time (Ma)

(a)

(b)

125 100 75 50 25 0

0.05

0.04

0.03

0.02

0.01

Figure 12.19 Distribution of (a) cyclostome and (b) cheilostome bryozoans across the Mesozoic–

Cenozoic boundary: the cheilostomes suffered the heaviest losses while the erect genera of both

groups suffered more than the encrusters. (Replotted from McKinney & Taylor 2001.)

322 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD

Box 12.9 Bryozoans and environments

The majority of bryozoans grow as mounds, sheets or runners parallel to the substrate, many grow

erect colonies perpendicular to the seabed and some colonies are actually mobile. There have been

a number of growth–mode type classiﬁ cations, some associated with particular genera, construc-

tional geometry or based on autecology. A more comprehensive way at looking at these complex

colonies is to combine attachment modes, construction orientation and the geometry of the individual

zooids (Hageman et al. 1997). Such a hierarchical growth–mode classiﬁ cation can be used to describe

regional biotas and predict paleoenvironments on limited datasets. However, as in many ecological

studies, the most common species or growth forms can swamp the overall ecological signal; some

form of scaling is needed. We can ask a couple of questions: How important is D at locality 1 rela-

tive to other occurrences of D and how important is D relative to all the other localities? Firstly a

simple data table is set up with growth forms along the y-axis and localities along the x-axis (see

below). One method of standardizing the data is to: (i) divide the number of growth type D at local-

ity 1 by the product of all the different growth types and the total at this one locality [10/(45 * 22)];

and (ii) this is then multipled by 100

to scale values to roughly between 0 and 100. This equals

101; this growth is clearly important at this locality. The relative importance of each growth form

at each locality can be plotted in a histogram.

This type of study has been expanded to an analysis of the distribution of growth forms across

the shelf-slope transition on the Lacepede Platform, southern Australia. A distinct pattern emerged

with free-living forms most important on the inner shelf and rigid cone-disk forms most important

on the deep slope (Fig. 12.20).

Form A Form B Form C Form D Sum

Locality 1 20 10 5 10 45

Locality 2 20 40 10 5 75

Locality 3 20 40 40 5 105

Locality 4 20 120 60 2 202

Sum 80 210 115 22

ral zone to the edge of the continental shelf

at depths of about 200 m. Nevertheless a few

intertidal forms are known, while some bryo-

zoans have been dredged from depths of over

8 km in oceanic trenches; moreover numerous

species have been recorded from the hulls of

ships. Most species are sensitive to substrate

types, turbulence, water depth and tempera-

ture together with salinity. The shape of colo-

nies can be very plastic, adapting to

environmental conditions, with erect, tree-

like colonies varying their branch thickness

according to depth. In addition spines may be

induced by high current velocities or by the

presence of predators (Taylor 2005). Bryozo-

ans are thus typical facies fossils exhi-

biting marked ecophenotypic variation (Box

12.9).

Bryozoans have successfully pursued several

different life modes. Encrusting, erect, unat-

tached or rooted phenotypes all reﬂ ect adap-

tive strategies in response to ambient

environmental conditions. Shallow-water col-

onies, particularly in the subtidal zone, are

and were dominated by encrusting, erect,

rooted and free-living forms. But deeper-water

environments, over 1 km deep, are character-

ized by mainly attached and rooted forms.

Nevertheless bryozoan colonies have occa-

sionally formed reefs or bryoherms, par-

ticularly during the mid-Silurian and

Carboniferous.

SPIRALIANS 1: LOPHOPHORATES 323

Articulated cylindrical branches

Articulated unilaminar branches

Articulated zooids

Rooted encrusting sheet

Rooted cone-disk

Rooted branches

Rooted fenestrate sheet

Rooted bilaminar sheet

Free-living motile

Unilaminar solid substrate

Unilaminar flexible substrate

Unilaminar hollow cylinder

Multilaminar massive

Cemented cylindrical branches

Cemented unilaminar branches

Cemented fenestrate sheet

Inner shelf A

Inner shelf B

Outer shelf

Slope

Scaled index

Slope

Outer shelf

Inner shelf B

Inner shelf A

0.0

(a)

(b)

3.0

Distance

coefficient

107

Locality

Figure 12.20 (a) Cluster analysis of bryozoan growth forms across a shelf–slope transition,

showing an inner shelf A (clastic dominated), inner shelf B (carbonate dominated), outer shelf and

slope. The cluster analysis, using a distance coefﬁ cient (x-axis) and average group linkage,

indicates the presence of four distinctive assemblages. (b) Distribution of growth forms across the

onshore–offshore gradient within the assemblages identiﬁ ed by cluster analysis. (Based on

Hageman et al. 1997.)

324 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD

Review questions

1 Current brachiopod research suggests

that the phylum Brachiopoda can be split

into three subphyla: Linguliformea, Crani-

iformea and Rhynchonelliformea. What

sort of criteria can we use to discover how

each subphylum was related to each other

and the stem-group brachiopod?

2 Brachiopod shells store a huge amount of

data, not only about the secretion of the

shell, but also about its surrounding envi-

ronment. How have brachiopod shells,

particularly their stable isotopes, contrib-

uted to our understanding of climate

change?

3 Although the thick-shelled and ornate

productid brachiopods of the Late Paleo-

zoic were resistant to attack, why did bra-

chiopods apparently not feature much in

the Mesozoic marine revolution or Meso-

zoic arms race?

4 The “dawn of the Danian” witnessed a

marked change in bryozoan faunas with the

dominance of the cheilostomes over the

cyclostomes. Both are ecologically similar so

why were the cheilostomes relatively more

successful after the KT extinction event?

5 Brachiopods and bryozoans were both

conspicuous members of the ﬁ lter-feeding

Paleozoic evolutionary fauna. Why then

are brachiopods a relatively minor part of

the Recent marine fauna but bryozoans

continue to ﬂ ourish?

Further reading

Boardman, R.S. & Cheetham, A.H. 1987. Phylum

Bryozoa. In Boardman, R.S., Cheetham, A.H. &

Rowell, A.J. (eds) Fossil Invertebrates. Blackwell Sci-

entiﬁ c Publications, Oxford, UK, pp. 497–549. (A

comprehensive, more advanced text with emphasis

on taxonomy; extravagantly illustrated.)

Carlson, S.J. & Sandy, M.R. (eds) 2001. Brachiopods

Ancient and Modern. A tribute to G. Arthur Cooper.

Paleontological Society Papers No. 7. University of

Yale, New Haven, CT. (Diverse aspects of contem-

porary brachiopod research.)

Clarkson, E.N.K. 1998. Invertebrate Palaeontology and

Evolution, 4th edn. Chapman and Hall, London.

(An excellent, more advanced text; clearly written

and well illustrated.)

Cocks, L.R.M. 1985. Brachiopoda. In Murray, J.W.

(ed.) Atlas of Invertebrate Macrofossils. Longman,

London, pp. 53–78. (A useful, mainly photographic

review of the group.)

Harper, D.A.T., Long, S.L. & Nielsen, C. (eds) 2008.

Brachiopoda: Fossil and Recent. Fossils and Strata

54, 1–331. (Most recent proceedings from an inter-

national brachiopod congress.)

Kaesler, R.L. (ed.) 2000–2007. Treatise on Invertebrate

Paleontology. Part H, Brachiopoda (revised), vols

1–6. Geological Society of America and University

of Kansas, Boulder, CO/Lawrence, KS. (Up-to-date

compendium of most aspects of the phylum.)

McKinney, F.K. & Jackson, J.B.C. 1989. Bryozoan Evo-

lution. Unwin Hyman, London. (Evolutionary studies

of the phylum.)

Rowell, A.J. & Grant, R.E. 1987. Phylum Brachiopoda.

In Boardman, R.S., Cheetham, A.H. & Rowell, A.J.

(eds) Fossil Invertebrates. Blackwell Scientiﬁ c Publi-

cations, Oxford, UK, pp. 445–96. (A comprehensive,

more advanced text with emphasis on taxonomy;

extravagantly illustrated.)

Rudwick, M.J.S. 1970. Living and Fossil Brachiopods.

Hutchinson, London. (Landmark text.)

Ryland, J.S. 1970. Bryozoans. Hutchinson, London.

(Fundamental text.)

Taylor, P.D. 1985. Bryozoa. In Murray, J.W. (ed.) Atlas

of Invertebrate Macrofossils. Longman, London, pp.

47–52. (A useful, mainly photographic review of the

group.)

Taylor, P.D. 1999. Bryozoa. In Savazzi, E. (ed.) Func-

tional Morphology of the Invertebrate Skeleton.

Wiley, Chichester, UK, pp. 623–46. (Comprehensive

review of the functional morphology of the group.)

References

Clarkson, E.N.K. 1998. Invertebrate Palaeontology and

Evolution, 4th edn. Chapman and Hall, London.

Cohen, B.L., Holmer, L.E. & Luter, C. 2003. The bra-

chiopod fold: a neglected body plan hypothesis. Pal-

aeontology 46, 59–65.

Freeman, G. & Lundelius, J.W. 2005. The transition

from planktotrophy to lecithotrophy in larvae of

lower Palaeozoic Rynchoneliiform brachiopods.

Lethaia 38, 219–54.

Geldern, van, R., Joachimski, M.M., Day, J., Jansen, U.,

Alvarez, F., Yolkin, E.A. & Ma, X.-P. 2006. Carbon,

oxygen and strontium isotope records of Devonian

brachiopod shell calcite. Palaeogeography, Palaeocli-

matology, Palaeoecology 240, 47–67.

Hageman, S.J. 2003. Complexity generated by iteration

of hierarchical modules in Bryozoa. Integrated Com-

parative Biology 43, 87–98.

Hageman, S.J., Bone, Y., McGowran, B. & James, N.P.

1997. Bryozoan colonial growth-forms as palaeoen-

vironmental indicators: evaluation of methodology.

Palaios 12, 405–19.

Harper, D.A.T., Alsen, P., Owen, E.F. & Sandy, M.R.

2005. Early Cretaceous brachiopods from North-

SPIRALIANS 1: LOPHOPHORATES 325

East Greenland: biofacies and biogeography. Bulletin

of the Geological Society of Denmark 52, 213–

25.

Harper, E.M. 2006. Dissecting arms races. Palaeogeog-

raphy, Palaeoclimatology, Palaeoecology 232,

322–43.

Lidgard, S., McKinney, F.K. & Taylor, P.D. 1993. Com-

petition, clade replacement, and a history of cyclo-

stome and cheilostome bryozoan diversity.

Paleobiology 19, 352–71.

McKinney, F.K. & Taylor, P.D. 2001. Bryozoan genetric

extinctions and originations during the last 100

million years. Palaeontologia Electronica 4, 26

pp.

O’Dea, A. 2003. Seasonality and zooid size variation in

Panamanian encrusting bryozoans. Journal of the

Marine Biological Association 83, 1107–8.

Perez-Huerta, A., Cusack, M., Zhu, W.-Z., England, J.

& Hughes, J. 2007. Material properties of the bra-

chiopod ultrastructure by nanoindentation. Interface

4, 33–9.

Robinson, J.H. & Lee, D.E. 2008. Brachiopod pedicle

traces: recognition of three separate types of trace

and redeﬁ nition of Podichnus centrifugalis Bromley

& Surlyk, 1973. Fossils and Strata 54, 219–25.

Schumann, D. 1991. Hydrodynamic inﬂ uences in bra-

chiopod shell morphology of Terebratalia transversa

(Sowerby) from the San Juan Islands. In MacKinnon,

D.I., Lee, D.E. & Campbell, J.D. (eds) Brachiopods

through Time. A.A. Balkema, Rotterdam.

Taylor, P.D. 1985. Bryozoa. In Murray, J.W. (ed.) Atlas

of Invertebrate Macrofossils. Longman, London, pp.

47–52.

aylor

, P.D. 2005. Bryozoans and palaeoenvironmental

interpretation. Journal of the Palaeontological

Society of India 50, 1–11.

Taylor, P.D. & Vinn, O. 2006. Convergent morphology

in the small spiral worm tubes (“Spirobis”) and its

palaeoenvironmental implications. Journal of the

Geological Society, London 163, 225–8.

Williams, A., Carlson, S.J., Brunton, C.H.C., Holmer,

L.E. & Popov, L. 1996. A supra-ordinal classiﬁ cation

of the Brachiopoda. Philosophical Transactions of

the Royal Society B 351, 1171–93.

Zhang Zhifei, Shu Degan, Han Jian & Liu Jianni. 2006.

New data on the rare Chengjiang (Lower Cambrian,

South China) linguloid brachiopod Xianshanella

haikouensis. Journal of Paleontology 80, 203–

11.

Chapter 13

Spiralians 2: mollusks

Key points

• The Phylum Mollusca can be traced back to at least the Late Precambrian, when

Kimberella probably fed on algae in Ediacaran communities.

• Early mollusks were characterized by some short-lived, unusual forms but with the

molluskan features of a mantle, mineralized shell and radula; these were members of

the small shelly fauna.

• Mollusk shell shape and even ornament can be modeled by a variety of microcomputer-

based software packages; only a small percentage of theoretical morphospace is occu-

pied by living and fossil mollusks.

• Bivalves are characterized by a huge variety of shell shapes, dentitions and muscle

scars, adapted for a wide range of life strategies in marine and some freshwater

environments.

• Most gastropods undergo torsion in early life; they have a single shell, often coiled. The

group adapted to a wide range of environments from marine to terrestrial.

• Cephalopods are the most advanced mollusks, with a well-developed head, senses and

a nervous system; they include the nautiloids, ammonoids and the coleoids. The group

is carnivorous.

• During the Mesozoic many mollusks developed a number of protective strategies such

as robust armor or deep infaunal life modes. The group may also have relied on multi-

formity of shape and color to confuse predator search images.

• Annelid worms were a sister group to the mollusks; their jaws, the scolecodonts, are

relatively common in Paleozoic faunas.

She sells seashells on the seashore;

The shells that she sells are seashells I’m sure.

So if she sells seashells on the seashore,

I’m sure that the shells are seashore shells.

Old nursery rhyme

SPIRALIANS 2: MOLLUSKS 327

This famous tongue twister was ﬁ rst recited

over 200 years ago in England, and it is very

likely based on the exploits of Mary Anning,

the most famous fossil collector of her time.

She is best known for her spectacular discov-

eries of marine reptiles in the Lower Jurassic

rocks of her native Lyme Regis in southern

England; but she made most of her regular

income from selling fossil ammonites and

other mollusk fossils to visitors. Most of us

have found seashells while playing or walking

on the beach and have been amazed by their

colors, shapes and ornaments. Not only are

clams, oysters and scallops good to eat, but

their shells, throughout historic times, have

featured as ornaments, tools and even cur-

rency. The Mollusca is the second largest

animal phylum after the Arthropoda, with

records of over 130,000 living species and

a history extending back into the

Precambrian.

MOLLUSKS: INTRODUCTION

The Phylum Mollusca includes the slugs,

snails, squids, cuttleﬁ sh and octopuses in

addition to all manner of marine shellﬁ sh

such as clams, mussels and oysters (Box 13.1).

Although some mollusks are the size of sand

grains, the giant squid Architeuthis can grow

to over 20 m in length, the largest and possi-

bly the most frightening genus of all living

invertebrates. Mollusks are probably the most

common marine animals today, occupying a

very wide range of habitats, from the abyssal

depths of the oceans across the continental

shelves and intertidal mudﬂ ats to forests,

lakes and rivers. Mollusks are usually unseg-

mented, soft-bodied animals with a body plan

based on four features:

1 The head contains the sensory organs, and

a rasping feeding organ, the radula, com-

posed of chitin and designed to scrape and

in some cases drill.

2 The foot is primitively a sole-like structure

on which the animal crawls, but is consid-

erably modiﬁ ed in many mollusks.

3 The visceral mass of the digestive, excre-

tory, reproductive and circulatory organs

is enclosed in the celomic cavity.

4 The mantle is a sheet of tissue lying dor-

sally over the visceral mass that is respon-

sible for secreting the shell.

Molluskan shells are secreted as calcium

carbonate, mainly aragonite, with an organic

matrix and an outer organic layer. In the case

of the bivalves, a range of shell fabrics have

evolved from simple prismatic structures,

through nacreous and prismatic, to crossed-

lamellar aragonitic and prismatic and foliated

calcite fabrics. Shell structure has been used

in the higher classiﬁ cation of the group (as in

the brachiopods). Crossed-lamellar structures

evolved independently in some gastropods.

Beneath the mantle, the mantle cavity lies

behind the visceral mass and is the respiratory

chamber that houses the molluskan gills

(ctenidia); the openings of the excretory and

reproductive ducts and the anus open into the

mantle cavity and their products are carried

out on the exhalant current.

From simple beginnings as a limpet-like

crawler back in the Precambrian, mollusks

have evolved a spectacular range of shapes

and sizes, and their hard, calcareous shells are

readily fossilized.

A simple method of visualizing molluskan

evolution is to consider the hypothetical

ancestor, or archemollusk, with a minimal

molluskan morphology; this approach has

been modiﬁ ed and merged with a recent

cladogram for the phylum (Fig. 13.1). There

is still a great deal of uncertainty about the

identity of the ﬁ rst mollusks, and new ﬁ nds

constantly change the picture (Boxes 13.2,

13.3). The most recent common ancestor of

the mollusks probably had seven- to eight-

fold serial repetition, the presence of valves

and a foot, and had a crawling mode of life

(Sigwart & Sutton 2007).

EARLY MOLLUSKS

The Early Cambrian was a time of experimen-

tation, with a variety of short-lived, often

bizarre, molluskan groups, such as the helcio-

nelloids, dominating many faunas (Peel 1991).

Most workers now agree that the ﬁ rst mol-

lusks were descended from forms like living

ﬂ atworms – probably spiculate animals with

radula and gills situated posteriorly. These

mollusks were similar to modern soft-bodied

aplacophorans, a group of shell-less mollusks.

The aplacophorans and the shelled mollusks

shared a common ancestor probably during

the Late Precambrian. Signiﬁ cantly, the artic-

ulated remains of a halkieriid mollusk from