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

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258 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD

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ORIGIN OF THE METAZOANS 259

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Chapter 11

The basal metazoans:

sponges and corals

Key points

• Parazoans are a grade of organization within the metazoans composed of multicellular

complexes with few cell types and lacking variation in tissue or organs; the sponges

(Phylum Porifera) are typical parazoans that lack a gut.

• Sponges are almost entirely ﬁ lter-feeding members of the sessile benthos. The group

contains a variety of grades of functional organization that cut across the traditional

classiﬁ cation of the phylum.

• Sponge reefs were dominated, during most of the Phanerozoic, by calcareous grades

developed convergently across the phylum; siliceous sponges were important reef build-

ers mainly during the Mesozoic.

• Stromatoporoids are a grade of organization within the Porifera with a secondary cal-

careous skeleton, important in reefs during the mid-Paleozoic and mid-Mesozoic.

• Archaeocyaths are Cambrian organisms of sponge grade. They were mainly solitary but

developed a branching, modular growth mode and successfully built reefs in often

turbulent and unstable environments.

• Reef-type structures were already present in the Late Precambrian hosting large, robust,

colonial organisms.

• The cnidarians are the simplest of the higher metazoans with a radial diploblastic body

plan and stinging cells or cnidoblasts. The phylum includes sea anemones, jellyﬁ sh and

hydra together with the corals.

• The Paleozoic rugose and tabulate corals displayed a wide range of growth modes often

related to environments; neither group was a dominant reef builder.

• The scleractinians radiated during the Mesozoic with zooxanthellate forms dominating

biological reefs. Scleractinian-like morphs in Paleozoic faunas arose several times inde-

pendently from anemones with scleractinian-type polyps.

• Reef development through time has waxed and waned, dominated at different times by

different groups of reef-building organisms.

• Coloniality within the metazoans has evolved many times; one hypothesis suggests that

a Precambrian colonial organism may have been a source for the bilaterians.

THE BASAL METAZOANS: SPONGES AND CORALS 261

Until nearly the end of the Neoproterozoic,

some 80% of geological time, the oceans of

the world were mainly occupied by rather

simple, usually unicellular, organisms. By the

Ediacaran it was clear that this simple exis-

tence was not enough, and more complex

body plans were soon to develop their own

ecosystems. Two groups, the Porifera and the

Cnidaria, form the basal parts of the meta-

zoan tree, diverging during the Neoprotero-

zoic. Despite their origins in deep time and

their relative simplicity, both maintained

high diversity, notably as colonial organisms,

throughout the Phanerozoic, frequently

becoming important parts of the planet’s reef

ecosystems.

PORIFERA

So he dissected sea sponges by night,

winter night after winter night . . . adult

and embryo human body parts by day,

adult and larval sponge body parts by

night.

Rebbeca Stott (2003) Darwin and the

Barnacle, on the sponge doctor, Robert

Grant

Most of us have used a bath sponge, probably

a synthetic replica of the real thing. But ancient

peoples used sponge skeletons as an aid to

bathing and possibly exfoliation in some of

the world’s earliest and most exclusive health

farms. Most considered they were some form

of plant until proper biological study in the

mid and late 1700s suggested they were

animals – and at ﬁ rst they were classiﬁ ed as

corals. It was in fact Dr Robert Grant (1793–

1874), one time mentor to Charles Darwin,

who later established the Porifera as a unique

group in its own right. The poriferans or

sponges have a unique porous structure and

a body plan based at the cellular level of orga-

nization; they are said to lack true tissues.

Most lack symmetry, true differentiated

tissues, and organs, although their cells, like

those of the protists, can switch function.

They reproduce both asexually (by budding)

and sexually with different cells expelling

clouds of eggs and sperm out through an

opening; some are even viviparous, with the

eggs hatching within the parent sponge, and

larvae released into the water.

There are over 10,000 species of sponge.

All are aquatic, and most are marine. Sponges

are part of the sessile benthos, ﬁ xed to the

seabed, pumping large volumes of water – in

extreme cases over 1000 L per day – through

their ﬁ xed but commonly ﬂ exible bodies,

which act as ﬁ lters for nutrients. The group

has a remarkable range of morphologies; the

more specialized, stalked forms live in deep-

water environments and ﬂ attened, dumpy

forms prefer shallower-water, high-energy

environments. Despite the apparent simplicity

of the sponges, the classiﬁ cation of the phylum

has recently undergone considerable revision

(Box 11.1). Some well-established calciﬁ ed

groups, such as the “chaetetids” and “sphinc-

tozoans”, are probably polyphyletic, merely

representing convergence towards common

grades of organization. The well-established

and diverse Demospongea, the common

sponges, may too be polyphyletic. Despite

their relative simplicity, the complex relation-

ships of “sponge-grade” animals have yet to

be resolved.

Morphology: examining a typical sponge

A typical sponge individual is not particularly

complex or intellectually demanding to under-

stand; it is nonetheless a remarkable organ-

ism. It is sac-shaped with a central cavity or

paragaster, which opens externally at the top

through the osculum (Fig. 11.1). The sponge

is densely perforated by ostia, small holes

marking the entrances to minute canals

through which pass the inhalant currents. In

simple terms, there are three main cell types:

(i) ﬂ attened epithelial cells; (ii) collar cells or

choanocytes, which occupy the internal cham-

bers and move water along by beating their

ﬂ agella; and (iii) amoeboid cells, which have

digestive, reproductive and skeletal functions.

Amoeboid cells can actually irreversibly

change into other cell types with other func-

tions. Nutrient-laden water is thus sucked

through the ostia, ﬂ agellated by the choano-

cytes and processed by the amoeboids. Waste

products and spent water, together with repro-

ductive products when in season, are ejected

upwards through the paragaster into the

water column.

262 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD

osculum

ostium

bud

spongocel

canal

root tuft

Figure 11.1 Basic sponge morphology.

Ascon Sycon Leucon

Figure 11.2 Main grades of sponges.

Box 11.1 Classiﬁ cation and spicule morphology of the sponges

CLASSIFICATION OF THE SPONGES

The phylum Porifera was traditionally subdivided into four classes, the Demospongea, Calcarea,

Sclerospongea and Hexactinellida, based mainly on the composition of the skeleton and type of

spicules. Higher-level taxonomy is based exclusively on soft-tissue morphology. Some workers have

suggested the exclusion of the glass sponges from the Porifera but this is poorly supported; rather

they are closely related to the demosponges. However, the sclerosponges, with some additional cal-

careous skeletons, are now placed within the Demospongea. Thus three classes now comprise the

phylum (Fig. 11.3).

Class CALCAREA (calcareous sponges)

• Sponges with calcitic spicules, usually simple, and/or porous calcareous walls. Marine

environments

• Cambrian to Recent

Class DESMOSPONGEA (common sponges)

• Sponges with skeletons of spongin, a mix of spongin and siliceous spicules or only siliceous

spicules. The spicules may be of two different sizes and the larger are represented by monaxons

and tetraxons. Marine, brackish and freshwater environments. Living sponges previously assigned

to the Sclerospongiae (coralline sponges) – sponges with a compound skeleton of siliceous spic-

ules, spongin and an additional basal layer of laminated ﬁ brous aragonite or calcite – are now

also included here

• Cambrian to Recent

Class HEXACTINELLIDA (siliceous sponges)

• These are the glass sponges with complex siliceous spicules having six rays directed along three

mutually perpendicular axes. Deep-water marine environments

• Precambrian (?) and Cambrian to Recent

However, two form-groups of sponge, the sphinctozoans (with a segmented chambered skeleton)

and the chaetetids (with microscopic tubules) have representatives within the Calcarea and Demo-

spongea; both were important reef builders.

THE BASAL METAZOANS: SPONGES AND CORALS 263

HEXACTINELLIDA

Archaeoscyphia

(Ordovician)

Siphonia

(Cretaceous-Tertiary)

DEMOSPONGEA

CALCAREA

Protospongia

(Cambrian-Ordovician)

Hydnoceras

(Silurian-Carboniferous)

Prismodictya

(Devonian-Carboniferous)

Rhaphidonema

(Triassic-Cretaceous)

Corynella

(Triassic-Cretaceous)

Astraeospongium

(Silurian-Devonian)

Figure 11.3 Some examples of the main groups of sponges: Archaeoscyphia (×0.25), Siphonia

(×0.4 and 0.8), Protospongia (×0.4), Hydnoceras (×0.25), Prismodictya (×0.6), Rhaphidonema

(×0.8), Corynella (×0.8) and Astraeospongium (×0.4).

The relationships among the three major groups of Porifera are obscure. Analyses of poriferan

morphology and structure, cytology and molecular biology suggest that, ﬁ rst, sponges are a para-

phyletic grouping (Sperling et al. 2007) and, second, that the Calcarea and Hexactinellida form

monophyletic groups that are close to the base of the Eumetazoa. The Demospongea is more basal

(Fig. 11.4).

Continued

264 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD

Eumetazoans Demosponges Calcisponges Demosponges Calcisponges Eumetazoans

gut

(macrophagy)

gut

(macrophagy)

water-canal system

(microphagy)

water-canal system

(microphagy)

Sponge

(a) (b)

Figure 11.4 Sponge paraphyly. (a) The more traditional view presenting both the eumetazoans

and poriferans as monophyletic groups; feeding strategies cannot be polarized since all the

outgroups are non-metazoan. (b) If, however, poriferans are paraphyletic and calcisponges are

more closely related to eumetazoans then the water canal system is a primitive character and the

gut is more derived.

SPICULE MORPHOLOGY

Commonly the spongin skeletons decay and unfused spicular skeletons disintegrate shortly after

death leaving only a selection of hard parts, such as spicules (Fig. 11.5). Spicule morphology is thus

a fundamental means of identiﬁ cation of those spiculate forms. Spicules may be large (megascleres),

acting as part of the skeleton, or small (microscleres), scattered throughout the sponge and rarely

preserved. Five basic types of spicule have been recognized:

1 Monaxons: single axial forms that may grow in one (monactinal) or two (diactinal) directions.

2 Tetraxons (hexactines): four-rayed forms that may have axes of equal length (calthrop).

3 Triaxons: six-rayed forms that form regular networks within the Hexactinellida or glass sponges.

4 Desmas: irregular-shaped forms with ends modiﬁ ed to articulate with one another.

5 Polyaxons: multirayed forms including spherical or star-shaped spicules.

Monaxons

Triaxons

Tetraxons

Desmas

MicroscleresPolyaxons

Figure 11.5 Main categories of spicule morphology. Magniﬁ cation approximately ×75 for all,

except microscleres which are about ×750.

THE BASAL METAZOANS: SPONGES AND CORALS 265

Despite the ﬂ exibility of a typical bath

sponge, sponges are skeletal organisms. Skel-

etons are composed of a colloidal jelly or

spongin, a horny organic material; calcareous

or siliceous spicules may occur with or without

spongin. These structures support the body

shape and provide a framework for the rather

disparate cells of the sponge. In simple terms,

the sponge animal functions as a colonial,

loosely-integrated protist, but with a higher

degree of physiological integration.

Three basic levels of chamber organization

have been recognized among the sponges (Fig.

11.2), and these provide a useful guide to

their shape. The simple ascon sponges are sacs

with a single chamber lined by ﬂ agellate cells,

whereas the sycon grade has a number of

simple chambers with a single central para-

gaster. The leucon grade is the most common

where a series of sycon chambers access a

large central paragaster.

Autecology: life as a sponge

Sponges are part of the sedentary benthos,

with large exhalant openings, communicating

upwards with the water column. When not

resting, the sponge sucks in water through its

upward-facing ostia, forming inhalant cur-

rents; material is then pumped out of the

animal though the exhalant opening. The

group is entirely aquatic, living attached in a

range of environments from the abyssal depths

of oceans to the moist barks of trees in the

humid tropics. Most Paleozoic and early

Mesozoic forms have been collected from

shallow-water environments, although like

many other groups they expanded into deep-

water environments during the Ordovician

where they remained an important part of the

benthos.

Today, sponges occupy a wider range of

environments than in the past. Modern hexac-

tinellids prefer depths of 200–600 m, proba-

bly extending down onto the abyssal plains

and into submarine trenches, whereas the cal-

careous sponges are most common in depths

of less than 100 m. The modern calciﬁ ed

sponges are either deep-reef or, more often,

cave dwellers, lurking in the shadows of sub-

marine crevices at depths of 5–200 m, mainly

in the Caribbean although the group occurs

elsewhere, including the Mediterranean. The

meadows of Antarctic cold-water sponges can

comprise up to 75% of the living benthos in

seas under the ice sheets.

Sponges use a variety of substrates. Clionid

sponge borings, producing the trace fossil

Entobia (see p. 523) in mollusk shells, have a

long geological history and today Cliona is

commonly associated with many oyster beds.

Spicules themselves can form mat-like sub-

strates that when colonized form local pockets

of biodiversity. Although almost all sponges

are ﬁ xed ﬁ lter feeders, some deep-water forms

are carnivorous: their long barbed spicules

entangle ﬁ sh and arthropods, and the sponge

tissue rapidly grows over the prey to digest it.

Moreover some encrusting sponges can crawl

slowly over the surface in search of food. Few

predators attack sponges, although some

ﬁ shes, snails, starﬁ sh and turtles have been

observed eating their soft tissues in the tropics;

and some organisms have used sponges as a

refuge, including hermit crabs, while dolphins

sometimes use sponges to protect their snouts

when investigating crevices.

Synecology: sponges and

sponge reefs through time

Sponges and corals are the major components

of modern and ancient reefs (Wood 1990).

The ﬁ rst sponges probably appeared in the

Late Proterozoic as clusters of ﬂ agellate cells.

But the evolution of the main groups of fossil

sponges is intimately related to their partici-

pation in reef ecosystems (Fig. 11.6). Particu-

lar grades of organization were suited to

special environmental conditions and sponges

can possess a rigid, reef-building skeleton by

the fusion of strong spicules or by the devel-

opment of an additional basal calcareous

skeleton. The Cambrian sponge fauna, of

thin-walled and weakly-fused spiculate demo-

sponges and hexactinellids together with early

calcisponges, is mainly cosmopolitan, having

a wide geographic distribution. In contrast,

Ordovician sponge faunas are characterized

by the heavier, thick-walled demosponges that

continued to dominate Silurian faunas in car-

bonate environments; siliciclastic facies were

dominated by the hexactinellids. The demo-

sponges, however, became less important as

the stromatoporoids together with rugose and

tabulate corals began to sneak into these sorts

of niches. Hexactinellids were locally abun-

dant during the Late Devonian, and in the

266 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD

Late Carboniferous the chaetetid calciﬁ ed

sponges were important reef builders. In the

Permian and mid-Triassic, structures involv-

ing sphinctozoans were common and the Mid

to Late Jurassic was marked by bioherms of

lithistid demosponges, while the hexactinel-

lids migrated into deeper-water environments.

Jurassic sponge reefs dominated by hexacti-

nellids and lithistids have been documented

throughout the Alpine region. Cup-shaped

and discoidal morphotypes dominated hard

and soft substrates, respectively, and these

developed a substantial topography above the

seaﬂ oor, and modern analogs of these hexac-

tinellid reefs are now known from off the

coast of Canada.

As noted earlier, the acquisition of a calcar-

eous skeleton was not conﬁ ned to any one

class; the calcareous skeleton was developed

a number of times, convergently, across the

phylum, with a few basic plans superimposed

on pre-existing sponge morphology. Conse-

quently, various groups have been recognized

on the basis of the calcareous skeleton, but

components of each group arose indepen-

dently in different clades. In broad terms, the

chaetetids and sphinctozoans, together with

the archaeocyaths and stromatoporoids, were

the most important calcareous reef builders.

However, the decline of the calcareous sponges

in reef ecosystems during the Mesozoic is

often correlated with the rise of the scleractin-

ian corals, equipped with a superior nutrition-

gathering system, associated with symbiotic

zooxanthellae (see p. 285).

Stromatoporoidea

The stromatoporoids were mound and sheet-

like marine, modular organisms that appeared

in the Mid Ordovician. These animals were

common components of Late Ordovician,

Silurian and Early to Mid Devonian shallow-

water marine communities, forming irregular

mounds on the seabed, associated with calcar-

eous algae and corals. They have a superﬁ cial

resemblance to some tabulate corals. The

group reached an acme during the Mid Devo-

nian but declined during the later Paleozoic

and Mesozoic. Although stromatoporoids

have been understandably classiﬁ ed with the

cnidarians, their similarity to the modern cal-

ciﬁ ed sponges and the discovery of spicules

within the skeleton suggest that these, too, are

poriferans and may well be a grade of orga-

nization within the Demospongea. In common

with a number of other poriferans, the group

is polyphyletic, with stromatoporoid taxa

showing gross morphological convergence

towards a common body plan or grade of

organization. Because most stromatoporoids

Parazoans

Cnidarians Times of major

reef-building

100

200

300

400

500

600

Cenozoic

Cretaceous

Jurassic

Triassic

Permian

Carboniferous

Devonian

Silurian

Ordovician

Cambrian

Archaeocyathans

Sphinctozoans

Inozoans

Scleractinia

aclonal, solitary

clonal, modular

degree of integration

Stromatoporoids

Age (Ma)

Chaetetids

Figure 11.6 Stratigraphic distribution of reef-building sponges and related parazoans, together with the

scleractinian corals.

THE BASAL METAZOANS: SPONGES AND CORALS 267

look like solidiﬁ ed cow pats, and are all super-

ﬁ cially very similar, paleontologists must use

thin sections to describe the microstructure

and classify the species.

Morphology and classiﬁ cation

Typical stromatoporoids have a calcareous

skeleton with both horizontal and vertical

structures and often a ﬁ brous microstructure

(Fig. 11.7). The skeleton is constructed from

undulating layers of calcareous laminae punc-

tuated perpendicularly by vertical pillars. The

surfaces of some forms are modiﬁ ed by small

swellings or mamelons together with astrorhi-

zae, radiating stellate canals, which are the

traces of the exhalant current canal system.

Siliceous spicules have been identiﬁ ed in some

Carboniferous and Mesozoic taxa, suggesting

that the primary skeleton was in fact spicu-

late; the calcareous casing is secondary with

probably low magnesium calcite precipitated

within a framework of spongin.

Some authors have included the extinct

stromatoporoids within the sclerosponges, a

small group of enigmatic sponges with sili-

ceous spicules embedded in aragonite, com-

monly found today in cryptic environments in

the tropics. Others have classiﬁ ed them as

cyanobacteria, foraminiferans or even as a

separate phylum. But these assignments are

probably only of historical interest because

most morphological evidence places them

ﬁ rmly in the sponges.

Autecology and synecology:

stromatoporoid life and times

Stromatoporoids were marine organisms

usually associated with shallow-water car-

bonate sediments often deposited in turbulent

environments. Many genera were important

constituents of reefs, particularly during the

Silurian and Devonian. For example, the spec-

tacular Silurian reefs on the Swedish island of

Gotland are characterized by a variety of stro-

matoporoid growth forms (Kershaw 1990),

whereas throughout North America and

northern Europe Devonian reef complexes

and bioherms are dominated by stromatopo-

roids. These animals had complex water

systems and grew in a variety of different

ways: columnar, dendroid, encrusting and

hemispherical forms were associated

with speciﬁ c energy and turbulence levels

(Fig. 11.8).

Stromatoporoids were also associated with

their own diverse microecosystems; those pre-

served in the Silurian of Gotland provided

habitats for communities with over 30 epibi-

ont species (see p. 97) that lived attached to

the animals. Boring, encrusting and epifaunal

organisms made good use of the cavities and

substrates available in and on the stromato-

astrorhizal

canal

astrorhiza on

mamelon

latilamina

lamina

gallery

pillar

Figure 11.7 Stromatoporoid morphology.