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

Подождите немного. Документ загружается.

278 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD

forms have been collected from the Late Juras-

sic Solnhofen Limestone of Bavaria. Living

members of the group include Aurelia, the

moon jellyﬁ sh, and the compass jellyﬁ sh,

Chrysaora. Although the anthozoans include

the sea anemones, sea fans, sea pens and sea

pansies, the class also includes the soft and

stony corals. Following a short, mobile,

planula larval phase, all members of the group

pursue a sessile life strategy as polyps.

Corals

Corals are probably best known for their

place in one of the planet’s most diverse but

most threatened ecosystems, the coral reef.

Shallow-water coral reefs form only in a zone

extending 30˚ degrees north and south of the

equator and reef-forming corals generally do

not grow at depths over 30 m or where the

water temperature falls below 18˚C, although

certain groups of corals can also form struc-

tures in deep-water environments. Corals are

not the only reef-forming organisms but

throughout geological time they have con-

structed three main types of reefs: fringing

reefs, barrier reefs and atolls. These structures

formed the basis for Charles Darwin’s then

cutting-edge analysis Coral Reefs published in

1842. Unfortunately, such structures are under

current threat, including damage from

increased bleaching, coastal development,

temperature change of seawater, tourism,

runoff containing agricultural chemicals,

abrasion by ships’ hulls and anchors, smoth-

ering by sediment, poisoning or dynamiting

during ﬁ shing, overﬁ shing of important her-

bivores and predators, and even harvesting

for jewelry. There seems little hope for this

spectacular habitat unless more attention is

paid to conservation.

The anthozoans are the most abundant

fossil cnidarians, pursuing a polypoid life-

style. The class Anthozoa contains two sub-

classes with calcareous skeletons. Whereas

the Octocorallia have calciﬁ ed spicules and

axes, the Zoantharia include the more famil-

iar fossil coral groups, the orders Rugosa,

Tabulata and Scleractinia (Table 11.1). The

Octocorallia, including the Alcyonaria, have

eight complete mesenteries and a ring of eight

hollow tentacles; the skeleton lacks calciﬁ ed

septa but calcareous or gorgonin spicules and

axes comprise solid structures in the skeleton.

Although the group is only sporadically rep-

resented in Silurian, Permian, Cretaceous and

Tertiary rocks, the octocorals are important

reef dwellers today. Some familiar genera

include Alcyonium (dead men’s ﬁ ngers), Gor-

gonia (sea pen) and Tubipora (organ-pipe

coral).

Morphology: general architecture

There are four main elements to the zoanthar-

ian coral skeleton: radial and longitudinal

structures, together with horizontal and axial

elements. Corals have planula larvae. Follow-

ing the planula larval stage the coral polyp

initially rests on a basal plate or disk termed

the holotheca and begins the secretion of a

series of vertical partitions or septa in a radial

Table 11.1 Features of the main coral groups.

Feature

Rugosa

Tabulata

Scleractinia

Growth mode Colonial and solitary Colonial Colonial and solitary

Septa 6 prosepta; later septa in

only 4 spaces

Septa weak or absent 6 prosepta; later septa in all 6 spaces

Tabulae Usual Well developed Absent

Skeletal material Calcite Calcite Aragonite

Stability Poor Poor Good with basal plate

Range Ordovician to Permian Ordovician to Permian Triassic to Recent

THE BASAL METAZOANS: SPONGES AND CORALS 279

grams (Fig. 11.20). Moreover colonies with

imperforate walls may exhibit phaceloid,

cateniform, cerioid and meandroid forms,

whereas those with perforate walls have only

phaceloid and cerioid growth modes together

with coenenchymal structures in some taxa,

such as the sarcinulids. These growth modes

are variably developed across the rugosans,

tabulates and scleractinians – but meandroid

and hydnophoroid modes were developed

during the Mesozoic and are thus restricted

to the scleractinians.

Colonial integration usually involves a loss

of individuality. Many organisms display a

transition from solitary growth modes,

through morphologies with asexually budded

modules, to a fully integrated colony with the

growth or astogeny of the compound struc-

ture showing little variation across the indi-

vidual corallites. The degree of integration of

a colony is usually measured by the amount

of cohesion between the individual skeletal

parts and soft tissues and by the range of form

observed between individual components.

Clearly there is a spectrum from phaceloid

modes with little or no integration to thamn-

asteroid and meandroid (and coenenchymal)

modes with high levels of integration. Indi-

vidual polyps are no longer separated by cor-

allite walls and may share a common enteron

and nervous system. This suggests a high

degree of integration where the colony

approaches the body plan of a typical meta-

zoan. These modes have varied through time

(Fig. 11.21).

arrangement. At the circumference, the septa

are joined to the theca or skeletal wall, which

extends longitudinally from the apex of the

corallum to the calice where the polyp is

attached. During growth the polyp may secrete

a series of horizontal sheets, or tabulae,

together with smaller curved or angled plates

or dissepiments. The columella, usually arising

from the fusion of the axial edges of the septa,

occupies the core region of the corallum. The

vertical walls or septa radiate outwards from

the columella and divide the corallite. Despite

the apparent simplicity of the coral skeleton,

there is a great deal of variation in both soli-

tary and colonial growth programs and the

end result is a remarkable array of shapes and

sizes of corals.

The three main subclasses of stony corals

have colonial or compound growth modes

whereas only the Scleractinia and Rugosa

have solitary skeletons. The solitary growth

forms include conical, ceratoid or horn-

shaped, calceoloid, cylindrical, discoid, patel-

late, scolecoid, trochoid and turbinate

skeletons (Fig. 11.19). Colonial corals with

corallites have adopted either fasciculate or

massive growth modes. Fasciculate styles

exhibit either dendroid or phaceloid strategies

with either no or poor integration. The hal-

ysitid or cateniform chain-like growth strat-

egy is a further variation on this pattern. The

massive colonies are much more varied, with

cerioid, astraeoid, aphroid, thamnasteroid,

meandroid and hydnophoroid together with

coenenchymal or coenostoid growth pro-

discoid

patellate

turbinate

trochoid

scolecoid

ceratoid

cylindrical

calceoloid

pyramidal

Figure 11.19 Terminology for the main modes of solitary growth in corals. (From Treatise on

Invertebrate Paleontology, Part F. Geol. Soc. Am. and Univ. Kansas Press.)

280 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD

Scleractinian corals may be highly integrated

because they have symbiotic zooxanthellae

(see p. 285). The relatively low levels of inte-

gration seen in the Rugosa and some Tabulata

colonies perhaps suggests a lack of algal sym-

bionts. There has been a great deal of argu-

ment about this. Some rugosans are in fact

quite highly integrated, and it is questionable

whether high integration should only be asso-

ciated with the presence of zooxanthellae.

Coral experts also use quantitative

approaches in describing colony shapes. Key

measurements are made on the colony and

these are plotted on a ternary diagram. A

series of ﬁ elds can be mapped out within the

triangle – for example, bulbous, columnar,

domal, tabular and branching colonies are

discriminated (Fig. 11.22). These different

growth strategies may be ecophenotypic (see

p. 123), commonly reﬂ ecting ambient envi-

ronmental conditions.

Rugose corals

Rugose corals are generally robust, calcitic

forms with both colonial and solitary life

modes, more varied than those of tabulates.

Rugosans have well-organized septal arrange-

ments with six cardinal or primary septa. Sec-

ondary septa are inserted in four spaces

around the corallum – between the cardinal

septa and the two alar septa and also between

the two counterlateral septa and lateral septa

(Fig. 11.23a). Horizontal structures such as

the tabulae, dissepiments and dissepimentaria

are also well developed across the order.

Undoubted rugosans, such as Streptelasma,

with short secondary septa and lacking a dis-

sepimentarium, are not recorded until the

Mid Ordovician. By the Late Ordovician,

rugose faunas were well established with the

development of a wide variety of morpholo-

gies (Fig. 11.23b; Box 11.5). For example, the

thamnasteroidastraeoidcerioid

hydnophoroidmeandroidaphroid

halysitidphaceloidcoenostoid

Figure 11.20 Terminology for the main modes of colonial growth in corals. (Redrawn from various

sources.)

THE BASAL METAZOANS: SPONGES AND CORALS 281

Silurian Goniophyllum was pyramidal with a

deep calyx, whereas the Devonian Calceola

was a slipper-shaped form with a semicircular

lid and the compound Phillipsastrea had a

massive, astraeoid growth mode (Fig.

11.25).

Diverse rugosan faunas occurred during

the Carboniferous Period. Solitary forms such

as the large horn-shaped to cylindrical

Caninia, the cylindrical Dibunophyllum with

a marked dissepimentarium, the long cylindri-

cal Palaeosmilia, and the smaller horn-shaped

Zaphrentis are often conspicuous members of

Carboniferous coral assemblages. The fascic-

ulate, phaceloid Lonsdaleia and Lithostrotion

with usually massive, cerioid growth modes

are locally common. The order declined during

the Permian until there were only 10 families

left, and these disappeared by the end-Permian

mass extinction (see p. 170).

Tabulate corals

As the name suggests, tabulate corals have

well-developed tabulae (Fig. 11.26). The septa

are usually very much reduced to short spines

or are absent, and dissepiments are variably

developed (Fig. 11.27). The group is varied,

with erect, massive, sheet-like and chain-like

colonies and branching forms; some authors

have suggested that some tabulates, such as

Cambrian

Number of genera

Cerioid

Phaceloid

Coenostoid

Meandroid

Cerioid

Astraeoid

Aphroid

Ordovician

Silurian

Devonian

Carboniferous

Permian

Triassic

Jurassic

Cretaceous

Tertiary

Figure 11.21 Schematic graph of the distribution

of colonial growth modes through the

Phanerozoic. (Based on data in Coates, A.G. &

Oliver, W.A. Jr. 1973. In Animal Colonies:

Development and function through time.

Dowden, Hutchinson and Ross.)

M, × 5

50%

perimeter

width

height

bulbous

domaltabular

width, × 3

50%

branching

columnar

(perimeter + height) × 2

Figure 11.22 Ternary plot of colonial growth modes based on the shape of the colonial coral. (Based

on data in Scrutton, C.T. 1993. Cour. Forsch. Inst. Senckenberg 164.)

282 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD

(a)

(b)

III

IIIIII

III

outer wall (epitheca)

dissepiment

tabula

tabularium

dissepimentarium

(ii)

(i)

septa

tabulae

rugose corallite

operculum

calicular boss

calice

fossula

epitheca

calicular platform

septal groove

calicular pit

interseptal ridge

transverse striation

rootlet

OmphymaPalaeosmiliaAmplexizaphrentis

Calceola

Figure 11.23 (a) Septal and tabular development in solitary rugose corals with (i) details of vertical

partitions, and (ii) details of horizontal structures. C, cardinal septa; K, counter-cardinal septa; KL,

counterlateral septa; L, alar septa. (b) Rugose coral morphology: external morphology of a variety of

solitary rugose corals. (Based on various sources.)

the heliolitids, may not even be cnidarians.

The occurrence of fossilized polyps in Silurian

tabulates clearly demonstrates that at least

some of them were corals. Only colonial or

compound growth forms evolved in this order,

usually with small, elongate corallites ranging

from 0.5 to 5 mm in diameter. Commonly, the

corallite walls are perforated by minute holes

or mural pores. Tabulate corals ﬁ rst appeared

in the Early Ordovician, probably predating

the ﬁ rst rugosans. Forms such as Lichenaria

have been recorded from Tremadocian rocks

in the United States, although more deﬁ nitive

reports of the same genus are from the

Darriwilian. Tabulates such as Catenipora,

Paleofavosites and Propora became wide-

spread during the later Ordovician.

Silurian tabulate coral faunas were domi-

nated by massive to domal Favosites with

cerioid corallites, Halysites, the chain coral

with a series of linked, long cylindrical coral-

lites of elliptical cross section, and Heliolites,

the sun coral, with short, stubby septa. Simi-

larly distinctive tabulates were characteristic

of the Devonian. Aulopora usually comprised

branching, encrusting colonies (Box 11.6),

similar to the bryozoan Stomatopora; the

extraordinary Pleurodictyum with large mural

pores and thorn-like septa was virtually

always associated with the commensal worm

Hicetes.

Carboniferous tabulates such as Michelinia,

with small colonies possessing large, massive,

thick-walled corallites, and the long-ranging

THE BASAL METAZOANS: SPONGES AND CORALS 283

Box 11.5 Rugosan life strategies

Despite the apparent simplicity of rugosan architecture, these corals may have pursued a number of

different life strategies (Fig. 11.24). A number of corals, for example Dokophyllum, probably sat

upright in the sediment rooted by ﬁ ne holdfasts extending from the epitheca. Other taxa, such as

Holophragma, were initially attached to a patch of hard substrate but subsequently toppled over to

rest on the seabed. Grewingkia was cemented to areas of hard substrate. The small discoidal Pal-

aeocyclus, however, may have been mobile, creeping over the substrate on its tentacles. A number

of strongly curved rugosans, for example Aulophyllum, probably lay within the substrate, concave

upwards. Successive increments of growth were directed more or less vertically giving the coral

exterior a stepped appearance. Many other solitary corals exhibit a similar terraced theca, which

may be due to changes in growth direction associated with adjustments following toppling of the

corallum during slight turbulence or storms.

Recumbent

Fixosessile

Rhizosessile

Attached to clast

Figure 11.24 Rugose solitary life strategies displaying attached, ﬁ xosessile, rhizosessile and

recumbent life modes. (Based on Neuman; B.E.E. 1988. Lethaia 21.)

phaceloid Syringopora, with long, thin, cylin-

drical corallites, characterize the coral faunas

of the period. By the Late Permian the group

was very much in decline following a long

period of deterioration after the Frasnian

extinctions; only ﬁ ve families survived to the

end of the period.

Scleractinian corals

The scleractinians are elegant zoantharian

corals with relatively light, porous skeletons

composed of aragonite (Fig. 11.29). Both soli-

tary and colonial modes exist with even more

varied architectures than those of the rugo-

sans. Secondary septa are inserted in all six

spaces between the primary or cardinal septa.

In further contrast to septal insertion in the

rugosans, each cycle of six is fully completed

before the next cycle of insertion commences.

Tabulae are absent, although dissepiments

and dissepimentaria are developed. Moreover

the scleractinian skeleton, although relatively

light and porous, has the stability of a basal

plate which aids anchorage in the substrate.

Additionally, the scleractinian polyp can

secrete aragonite on the exterior of the coral-

lite, often in the form of attachment struc-

tures. Both adaptations provided a much

greater potential for reef building than the less

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

(e) (f) (g)

Figure 11.25 Some rugose corals: (a, b) cross and longitudinal sections of Acervularia (Silurian);

(c, d) cross and longitudinal sections of Phillipsastrea (Devonian); (e) Amplexizaphrentis

(Carboniferous); and (f, g) cross and longitudinal sections of Palaeosmilia (Carboniferous).

Magniﬁ cation approximately ×2 (a–d), ×3 (e), ×1 (f, g). Note that here and elsewhere, age assignments

refer to the specimen ﬁ gured and not to the entire stratigraphic range of the taxon. (Courtesy of Colin

Scrutton.)

(a) (b) (c)

(e)

(d)

Figure 11.26 Some tabulate corals: (a, b) cross and longitudinal sections of Favosites (Silurian);

(c, d) cross and longitudinal sections of Syringopora (Carboniferous); and (e) Aulopora (Silurian).

Magniﬁ cation approximately ×2. (Courtesy of Colin Scrutton.)

THE BASAL METAZOANS: SPONGES AND CORALS 285

stable rugose and tabulate corals of the Paleo-

zoic. Finally, scleractinians have a distinctive

ultrastructure composed of aragonite and a

widespread development of coenosarc.

Although scleractiniomorph corals are

now known from both the Cambrian and

Ordovician (Box 11.7), the scleractinians

ﬁ rst appeared in the Mid Triassic with forms

such as Thamnasteria becoming quickly

widespread throughout Europe.

The scleractinians developed a wide range

of morphologies (Fig. 11.31). For example,

Montlivaltia is a small, cup-shaped coral

common from the Early Jurassic to the

Cretaceous. Thecosmilia is a small, dendroid

to phaceloid colonial form with similar coral-

lites that ranges from the Middle Jurassic to

the Cretaceous; the massive cerioid Isastraea

has a similar range. Scleractinians are now the

dominant reef-building animals in modern

seas and oceans where they form reef struc-

tures in a variety of settings, usually in the

tropics.

Synecology: corals and reefs

Virtually all fossil corals were benthic. Two

ecological groups have been recognized among

Recent scleractinians. Hermatypic corals are

associated with zooxanthellae (dinoﬂ agel-

lates) and are restricted to the photic zone to

maintain this symbiosis. Symbiosis between

the dinoﬂ agellates and cnidarians is wide-

spread across the living representatives of the

phylum, with algae associating not only with

corals but also anemones and gorgonians.

The zooxanthellae are endosymbionts living

in the tentacles and mouth of the cnidarian

where they recycle nutrients, accelerate the

rate of skeletal deposition and convey organic

carbon and nitrogen to the cnidarian in return

for support and protection from grazers.

Hermatypic corals are commonly multiserial

forms, with small corallites displaying a high

degree of integration. Ahermatypic corals,

lacking algal symbionts, are commonly soli-

tary or uniserial compounds with large, poorly

integrated corallites.

Some have suggested that coral morphol-

ogy may help predict the presence of symbi-

onts in fossil coral communities. It is probable

that many tabulates were zooxanthellate

whereas the rugosans were not. In broad

terms, there may be parallels between the

platform and basin associations of rugose and

tabulate corals of the Paleozoic and reef-

building and non-reef-building scleractinian

corals of the Mesozoic and Cenozoic.

Reefs are biological frameworks with sig-

niﬁ cant topography (Box 11.8). Three main

types of structure occur in tropical shallow

water: (i) fringing reefs develop directly adja-

cent to land areas; (ii) barrier reefs have an

intervening lagoon; and (iii) atolls completely

surround lagoons and are usually of volcanic

origin. The last will continue to grow as the

volcanic island subsides until eventually only

a barrier reef, enclosing a lagoon, remains.

Paleozoic corals were not particularly suc-

cessful reef builders; many preferred ﬁ rm sub-

strates and lacked structures that allowed

anchorage and aided stability; calcareous

(a) (b)

septal

spines

tabula

mural

pores

Figure 11.27 Tabulate morphology: (a) transverse and (b) longitudinal sections of Favosites. The insets

on (a) show the lateral and upper surfaces of the entire Favosites colony.

286 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD

Box 11.6 Computer reconstruction of colonies

The colonial tabulate Aulopora had a long geological history and mainly occupied an encrusting

niche, coating brachiopods, stromatoporoids and other, larger corals. Aulopora grew by dichoto-

mous branching, pursuing a creeping or reptant life mode, efﬁ ciently siting its corallites adjacent to

potential sources of food at, for example, the inhalant currents through brachiopod commissures.

Colin Scrutton (University of Durham) has reconstructed colonies of the free-living animals in three

dimensions using a computer-based technique (Fig. 11.28). Serial sections of the colony were digitized

and assembled on a micro-VAX mainframe with software routinely used for building up three-

dimensional views of diseased kidneys. Both the ontogeny of the procorallites and the astogeny of

the colony as a whole were established in considerable detail by these techniques. With the develop-

ment of desk and laptop microcomputers such modeling is now, more or less, routine.

Halysitids were tabulate corals that dominated some Ordovician and Silurian assemblages. As

each colony grew, budding chains were able to ﬁ nd their way back to the colony, instead of heading

off in random directions. Perhaps they could sense the gradient of a diffusive ﬁ eld of “pheromones”,

their waste products or the depletion of nutrients set up by the colony. In a simulation by Hammer

(1998), new protocorallites are introduced into random positions, simulating “polyplanulate” asto-

genesis and the diffusive zones are established by numerically solving the differential equation for

diffusion and decay.

Other fossil simulations are available at http://www.blackwellpublishing.com/paleobiology/.

(a)

(b)

(c)

(d)

Figure 11.28 Aulopora morphology: computer-generated reconstructions of (a) the plan, (b) the

lower side, and (c) the direction of the procorallite; (d) reconstruction of the colony. (Courtesy of

Colin Scrutton.)

THE BASAL METAZOANS: SPONGES AND CORALS 287

algae and stromatoporoids were usually more

important. Nevertheless, frameworks domi-

nated by colonial tabulates, and to a lesser

extent rugosans, do occur, particularly during

the Mid Paleozoic. Growth bands on the latter

have provided us with a Paleozoic calendar

(Box 11.9).

Pioneer and climax communities have been

described from a number of Silurian and

Devonian successions (Fig. 11.34). The scler-

actinians gradually became the dominant reef

builders during the Mesozoic and Cenozoic.

Modern coral reef associations have been

documented in detail from eastern Australia,

the eastern Paciﬁ c and the Caribbean.

The Great Barrier Reef on the continental

shelf of eastern Australia is the largest coral

structure on Earth, approaching 3000 km

mesentery

septum

epithecal wall

b'b

(a) (b) (c)

Figure 11.29 Scleractinian morphology: (a) longitudinal and (b) transverse sections, and (c) mode of

septal insertion.

Box 11.7 Kilbuchophyllida and iterative skeletalization

Did the scleractinian corals have a long cryptic history through the Paleozoic? When the coral Kil-

buchophyllum (Fig. 11.30a) was described from the Middle Ordovician rocks of southern Scotland,

it caused a sensation, at least amongst coral workers. Kilbuchophyllum seemed to have patterns of

septal insertion and a microstructure identical to those of modern scleractinians, and quite unlike

the contemporary rugosans and tabulates. At ﬁ rst, some paleontologists said this was an aberrant

local form, but specimens have been found in the Silurian too. It is unlikely that Kilbuchophyllum

was the stem group for the scleractinians; however, clearly other groups of soft-bodied anemones

with the potential of skeletalization were around early in the history of the group. Following the

end-Permian mass extinction, when the rugose and tabulate corals ﬁ nally disappeared, calciﬁ cation

of other scleractinian-type morphs during the Triassic marked a new start of another highly success-

ful calciﬁ ed coral group. Similarly calciﬁ ed, scleractinian-type polyps are known from the Permian,

implying that this skeletal type re-evolved iteratively, that is time and time again. But what did the

naked scleractinian-type polyps look like? Hou Xian-guang (Yunnan University) and his colleagues

(2005) have described the sea anemone-like Archisaccophyllia from the Early Cambrian Chengjiang

fauna (Fig. 11.30b; see p. 386). This organism may well have been one of a group of naked polyps

that generated various scleractiniomorph corals during the Paleozoic and probably were responsible

for seeding the Mesozoic radiation of the most successful reef builder in the oceans today.

Continued