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

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268 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD

poroid skeleton; both bivalves and brachio-

pods have been seen in borings within

stromatoporoids that may have provided

some of the ﬁ rst cryptic habitats for Phanero-

zoic biotas.

Animals with a stromatoporoid grade of

organization have been identiﬁ ed from rocks

of Botomian age; however these forms were

apparently short lived. Pseudostylodictyon

from the Middle Ordovician of New York

and Vermont may be the oldest true stromato-

poroid, derived from a soft-bodied, sponge-

like ancestor in the Early Ordovician.

Stromatoporoids formed the basis for reef

ecosystems during the Silurian and Devonian,

becoming largely extinct during the end-

Frasnian (Late Devonian) event. The group

revived in the Mid and Late Jurassic when

stromatoporoids again participated in reef

frameworks. Nevertheless, most groups dis-

appeared at the end-Cretaceous mass extinc-

tion. However, some living sponges have a

stromatoporoid grade of organization;

Astrosclera and Calciﬁ mbrospongia are both

calciﬁ ed demosponges with a stromatoporoid

architecture.

Archaeocyatha

The Archaeocyatha or “ancient cups” are one

of only a few major animal groups that are

entirely extinct. They appear to have been an

evolutionary dead end. The group exploited

calcium carbonate during the early part of the

Cambrian radiation to construct porous cup-

or cone-like skeletons, usually growing

together in clumps and often living with stro-

matolites to form reefs. The Archaeocyatha

dominated shallow-water marine environ-

ments, usually in tropical paleolatitudes. From

an Early Cambrian origin on the Siberian

Platform, the group spread throughout the

tropics, forming the ﬁ rst Paleozoic reefs.

However, by the end of the Early Cambrian

and the start of the Middle Cambrian,

archaeocyaths are known only from Austra-

lia, the Urals and Siberia. They disappeared

at the end of the Cambrian.

Current studies suggest that the Archaeo-

cyatha have a grade of organization similar

to poriferans; in fact most authorities

would place the group ﬁ rmly within the

sponges as a separate class. Because no living

laminar low domical low domical low domical

high domical

extended domical

bulbous

side

view

ragged

Mamelons

Terminology

Morphotype classes:

smooth

non-enveloping

enveloping

Laminar: V/B up to 0.1

Low domical: V/B 0.1–0.5

High domical: V/B 0.5–1

Extended domical: V/B > 1

Stromatoporoid morphotypes

Figure 11.8 Stromatoporoid growth modes. (Based on Kershaw, S. 1984. Palaeontology 27.)

THE BASAL METAZOANS: SPONGES AND CORALS 269

representatives of the group exist there has

been, in the past, considerable speculation

about the taxonomic afﬁ nities of the archaeo-

cyaths: they have been classiﬁ ed with algae,

calciﬁ ed protozoans, poriferan-grade metazo-

ans, animals with a grade of organization

intermediate between protozoans and meta-

zoans, and cnidarians – none of which now

seems likely.

Morphology and classiﬁ cation:

archaeocyath individuals and modules

Archaeocyaths are most commonly found in

carbonates, and details of their morphology

are usually reconstructed from thin sections.

Unfortunately many Cambrian carbonates

have been recrystalized, often destroying the

details of skeletal morphology. The exoskele-

ton of the archaeocyathan animal is aspiculate

and usually composed of a very porous,

inverted cone composed of two nested concen-

tric walls separated from each other by radi-

ally arranged, vertical septa (Fig. 11.9). Both

the inner and outer walls are densely perfo-

rated and together deﬁ ne the intervallum, or

central cavity, partitioned into a number of

segments (loculi) by the radial septa, which

are often less porous than the walls or some-

times aporous. The inner wall circumscribes

the central cavity, open at the top and closed

at its base to form a tip. The apex of the

intervallum

septum

aporous septum

outer wall

inner wall

porous septum

sole

(a)

(b)

branching

solitary

catenulate

Ajacicyathida

Monocyathida

pseudo-

cerioid

Archaeocyathida

Tabulacyathida

massive

Kazachsetanicyathida

encrusting

Direction of water flow:

Inhalent

Exhalent

Inferred soft tissue distribution

Figure 11.9 The Archaeocyatha: (a) morphology and (b) classiﬁ cation, function and growth modes of

the main groups. (Based on Wood et al. 1992.)

270 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD

skeleton is usually buried in the sediment with

a basal ﬂ ange and roots or holdfasts adding

anchorage and stability. In some taxa, the

intervallum is partitioned horizontally ﬁ rstly

by porous shelves or tabulae or secondly with

aporous, convex dissepiments, often extend-

ing into the adjacent central cavity.

Two main subdivisions have been deﬁ ned

within the group: the “Regulares” and the

“Irregulares”. The regular forms have an

initial aporous, single-walled stage lacking

dissepiments; soft tissue ﬁ lled the entire body.

The inner and outer walls are punctuated by

septa and tabulae developed either singly or

together. The irregular forms have initial

aporous, single-walled stages with dissepi-

ments. The twin walls have irregular pore

structures, always dissepiments, and the skel-

eton is asymmetric; soft tissue was restricted

by the development of secondary skeletal

material. These groupings have now been

shown to have little taxonomic value,

reﬂ ecting rather ecological preferences

(Debrenne 2007). Most archaeocyaths are

“Regulares”, including the orders Ajacicy-

athida and Coscinocyathida; however the

apparent abundance of regular genera may be

due to excessive taxonomic splitting. There

are fewer “Irregulares” but this ecogroup

includes the orders Archaeocyathida and

Kazachsetanicyathida.

Synecology: archaeocyath reefs

The archaeocyaths were exclusively marine,

probably living at depths of 20–30 m on car-

bonate substrates. The phylum developed an

innovative style of growth based on modular

organization (Fig. 11.10). Such modularity

permitted encrusting abilities and the possi-

(a) (b)

Figure 11.10 Some archaeocyaths from the Lower Cambrian of Western Mongolia, in thin section:

(a) cryptic, solitary individual of Cambrocyathellus showing holdfast structures (×7.5), and (b)

branching Cambrocyathellus tuberculatus with skeletal thickening between individuals associated with

transverse sections of Rotundocyathus lavigatus (×5). (Courtesy of Rachel Wood.)

THE BASAL METAZOANS: SPONGES AND CORALS 271

bilities of secure attachment on a soft sub-

strate; moreover growth to large size was

enabled, together with a greater facility for

regeneration (Wood et al. 1992). The archaeo-

cyaths were thus key elements of the ﬁ rst reef-

type structures of the Early Cambrian (Fig.

11.11), in intervals of high turbulence and

rates of sedimentation. However, although

archaeocyathan reefs were probably not par-

ticularly impressive, usually up to 3 m thick

and between 10 and 30 m in diameter, they

were nevertheless amongst the ﬁ rst animals to

establish complex biological frameworks,

processing large amounts of seawater through

their bodies (Box 11.2). Archaeocyathan reefs

were always associated with calcimicrobes

that may have been the main frame builders.

There are also some examples of cryptic

organisms living within the reef cavities,

including other sponges.

Distribution: Cambrian world of

the archaeocyaths

The ﬁ rst archaeocyaths are known from the

lowest Cambrian (Tommotian) rocks of the

Siberian Platform and are represented by

mainly solitary regulars. During the Early

Cambrian, the phylum diversiﬁ ed, migrating

into areas of North Africa, the Altai Moun-

tains of the former Soviet Union, North

America and South Australia (Fig. 11.13).

Archaeocyaths were most common in the Mid

to Early Cambrian (Botomian) when a number

of distinct biogeographic provinces can be

deﬁ ned, but by the Lenian Stage the group

was very much in decline. Few genera have

been recorded from the Middle Cambrian and

only one is known from Upper Cambrian

strata. Archaeocyath history demonstrates a

progressive move towards a more modular

architecture in response to conditions of high

turbulence. In general, solitary taxa domi-

nated the Early Cambrian; but following the

late Botomian, modular morphotypes contin-

ued after the extinction of most solitary forms

(Fig. 11.14; Box 11.3). One advantage is that

the abundance and diversity of the group in

some parts of the world, particularly in Lower

Cambrian rocks, has allowed its effective use

in biostratigraphic correlation when there

were few other organisms around that could

act as zone fossils (see p. 28).

CNIDARIA

The bottom was absolutely hidden by a

continuous series of corals, sponges,

actiniæ [sea anemones] and other marine

productions, of magniﬁ cent dimensions,

(a) (c)

(b) (d)

Figure 11.11 Archaeocyathan reef structures which, when preserved, become (a) boundstones, (b)

bafﬂ estones, (c) bindstones or (d) bioherms. (Based on Wood et al. 1992.)

272 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD

Box 11.2 Plugging the leaks: experimental morphology of archaeocyaths

It is often extremely difﬁ cult to reconstruct the life modes of long-extinct organisms that apparently

lack modern analogs (see p. 150), particularly when the entire phylum is extinct. In an innovative

experimental biomechanical study Michael Savarese (then at Indiana University) constructed models

of the three main archaeocyathan morphotypes (aseptate, porous septate and aporous septate), and

subjected each to currents of colored liquid in a ﬂ ume (Fig. 11.12). The ﬁ rst morphotype, a theoreti-

cal reconstruction, performed badly with ﬂ uid escaping through the intervallum while also leaking

through the outer wall. The porous septate form, however, suffered some slight leakage through the

outer wall but no ﬂ uid passed through the intervallum. The aporous septate form was most efﬁ cient

with no leakage through the outer walls and no ﬂ ow through the intervallum. Signiﬁ cantly, ontoge-

netic series of the fossils show that an initially porous septate morphotype become aporous in later

life, perhaps to avoid leakage through the outer wall (Savarese 1992). This was clearly a

great advantage to an organism that survived by pumping huge volumes of seawater through its

system!

aseptate

condition

current flow

porous septate

condition

aporous septate

condition

Figure 11.12 Modeling the functional morphology of the archaeocyaths. (From Savarese 1992.)

varied forms, and brilliant colours. . . . In

and out among [the rocks and living

corals] moved numbers of blue and red

and yellow ﬁ shes, spotted and banded

and striped in the most striking manner,

while great orange or rosy transparent

medusæ [jellyﬁ sh] ﬂ oated along near the

surface. It was a sight to gaze at for

hours, and no description can do justice

to its surpassing beauty and interest. For

once, the reality exceeded the most

glowing accounts I had ever read of the

wonders of a coral sea.

Alfred R. Wallace (1869)

The Malay Archipelago

The cnidarians (or “nettle-bearers”) include

the sea anemones, jellyﬁ sh and corals and are

the least complex of the true metazoans

(eumetazoans), having cells organized into a

THE BASAL METAZOANS: SPONGES AND CORALS 273

Mountains Land Continental shelf Reefs

30°N

0°

66°N

Figure 11.13 Paleogeographic range of Early Cambrian archaeocyathid reefs. (Replotted from

Debrenne 2007.)

IRREGULARESREGULARES

Erbocyathoidea

Erismacoscinina

Ajacicyathida

Dokidocyathina

Monocyathida

Syringocnemidina

Kazachsetanicyathida

Archaeocyathida

Figure 11.14 Evolutionary trends within the archaeocyaths; modular forms, appearing iteratively, are

indicated by M. (Based on Wood et al. 1992.)

274 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD

Box 11.3 Neoproterozoic colonies

When was the transition complete from the isolated protist way of life to the loosely integrated

colonies of cells in the earliest poriferans? The Neoproterozoic rocks of Namibia yield some clues.

Rachel Wood and her colleagues (2002) have described a giant, fully-mineralized, complex colonial

skeleton, Namapoikea, from the Northern Nama Group, dated at about 550 Ma (Fig. 11.15). This

postdates some of the earliest putative cnidarians and sponges in the Ediacara biota, but predates

currently known metazoan reef-type ecosystems. Namapoikea is huge (up to 1 m in diameter),

robust, with an irregular structure in transverse section but apparently lacking any internal features.

It is uncertain whether this is a sponge or a coral but clearly large, modular, skeletal metazoans

were already around in the Late Neoproterozoic, providing a hitherto unexpected complexity to

terminal Proterozoic reefs and with the potential to provide both open surface and cryptic habitats.

Perhaps these encrusting sheets provided shelter for some of the ﬁ rst micromorphic skeletal

metazoans?

(a)

(b)

50 mm

Figure 11.15 Namapoikea: (a) nodular individual perpendicular to a ﬁ ssure wall, and (b) section

showing tubular construction. (Courtesy of Rachel Wood.)

THE BASAL METAZOANS: SPONGES AND CORALS 275

relatively few different tissue types in a radial

plan. They are typiﬁ ed by the well-known

hydra (Fig. 11.16). Although there are no spe-

cialized organs and only a few tissue types,

they are more complex than the parazoans.

The group was, in the past, referred to as the

Coelenterata, but because that phylum also

included the sponges and the gelatinous cteno-

phores or comb-jellies, the more restricted

term Cnidaria is now generally preferred.

Two basic life strategies occur (Fig. 11.17):

polyps are usually sessile or attached, although

some can jump and somersault, while medusae

swim, trailing their tentacles like the deadly

and vicious snakes that adorned the head of

the mythical Medusa. Although medusoids

and polyps appear different, they are essen-

tially the same structures but inverted. Many

cnidarians exhibit both forms through their

life cycles, others only one. The Portuguese

man-of-war, for example, is a spectacular and

scary colonial form with a medusoid module

for ﬂ oatation and various types of polyps that

help feeding, locomotion and reproduction.

As a whole the group is carnivorous, attack-

ing crustaceans, ﬁ shes, worms and even

microscopic diatoms, with their poisonous

stinging cells (cnidoblasts) – the reason they

are called “nettle-bearers”.

Morphology: the basic cnidarian

The cnidarians are multicellular, having a

single body cavity or enteron; the opening at

the top (or bottom in most medusae), sur-

mouth

tentacle

endoderm

mesoglea

ectoderm

(a)

(b)

mesentery

nerve net

enteron

musculo-epithelial cell

sensory cell

nematocyst

Figure 11.16 Morphology of Hydra: (a) general body plan, and (b) detail of the body wall.

medusa

polyp

Figure 11.17 Cnidarian life cycles: generalized view of the life of the hydrozoan Obelia, alternating

between the conspicuous polyp and medusa stages.

276 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD

rounded by tentacles with stinging cells or

nematocysts, functions both as a mouth and

an anus. There is thus no head or tail, and

nutrients and waste pass through the same

opening. The body itself, although diploblas-

tic, is, in fact, composed of three layers; the

inner endoderm and the outer ectoderm both

consist of living cells while the intervening

mesoglea is a gelatinous, acellular substance

containing rare cells. The outer layer of the

body wall contains cnidoblast cells that

contain the primed stings or nematocysts that

are usually conﬁ ned to the tentacles. A primi-

tive nerve net is embedded in the mesoglea.

Fingers of endoderm commonly poke into the

enteron, forming radial partitions that increase

the area of absorption of nutrients. These

mesenteries can, in the case of the corals,

secrete calcium carbonate to form solid, calci-

ﬁ ed partitions or septa. Most species are

found in marine environments although

hydrozoans can be very abundant in fresh-

water habitats.

Classiﬁ cation: design and relationships of

the main groups

The phylum Cnidaria is usually split into

three classes: hydrozoans, scyphozoans and

anthozoans (Box 11.4). The hydrozoans

Box 11.4 Classiﬁ cation of Cnidaria

The phylum is characterized by radial symmetry, with the ectoderm and endoderm separated by the

mesoglea; the enteron has a mouth surrounded by tentacles with stinging cells. The phylum ranges

from Upper Precambrian to Recent. The putative medusoid Brooksella, which predates the Ediacara

fauna may, in fact, be a trace fossil. The group has a wide range of body plans (Fig. 11.18).

Class HYDROZOA

• This includes six main orders of small, usually polymorphic forms. Each has an undivided enteron

and solid tentacles, and may form colonies. There are six main orders; the Chondrophora con-

tains some of the oldest cnidarians

• Ediacaran to Recent

Class SCYPHOZOA

• Mainly jellyﬁ sh, contained in the Scyphomedusae, which are only preserved in Lagerstätten. The

extinct Conulata is often included here since the group has a tetrameral symmetry and apparently

has tentacles. Their long conical shells, for example Conularia, are composed of chitinophos-

phate; conulates appeared in the Cambrian and were extinct by the Mid Triassic

• Ediacaran to Recent

Class ANTHOZOA

• These are exclusively marine, and most are sessile, colonial forms (though they have mobile

planula larvae). The three subclasses, Ceriantipatharia, Octocorallia and Zoantharia (including

the orders Rugosa, Tabulata and Scleractinia), all lack medusoid stages, possess hollow tentacles

and have the enteron divided, longitudinally, by vertical septa. Both solitary and colonial forms

occur. The class includes corals, sea anemones and sea pens. Octocorals often produce spicules

that occur as microfossils

• Ediacaran to Recent

Class CUBOZOA

• The sea wasps and box jellyﬁ sh have both medusae and polyps and are mainly restricted to

tropical and subtropical latitudes

• Carboniferous to Recent

THE BASAL METAZOANS: SPONGES AND CORALS 277

mouth and gullet

tentacles

polyp wall

mesentery

basal

infolding

over

septum

basal plate

(a)

septum or

scleroseptum

mesentery

tentacles

abandoned

corallite

(b)

(c)

(d)

Figure 11.18 Main cnidarian body plans: (a) generalized scleractinian polyp, (b) generalized part

of scleractinian coral colony, (c) living anemone, and (d) living jellyﬁ sh. (From various sources.)

include freshwater and colonial forms together

with the ﬁ re corals and most kinds of “jelly-

ﬁ sh”. There are over 3000 living species

inhabiting water depths up to 8000 m, mainly

in marine environments. Supposed hydrozo-

ans have been recorded from the Late Precam-

brian Ediacara fauna (see p. 242), where

genera such as Eoporpita and Ovatoscutum

may be the oldest sessile members of the

phylum. Hydrozoans reproduce either sexu-

ally or by asexual budding; the polyp stage is

asexual and the medusoid normally sexual.

The scyphozoans are mainly free-swimming

medusae or jellyﬁ sh often inhabiting open-

ocean environments. Some elements of the

Ediacara fauna may be scyphozoans, for

example Conomedusites and Corumbella;

however many of the best-preserved fossil