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

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208 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD

bonates of the Hunting Formation, eastern

Canada, dated at 1.2 Ga (see p. 200). After

1 Ga, algae are reported from a range of locali-

ties around the world. A range of protists such

as the acritarchs, chitinozoans, coccolitho-

phores and diatoms dominated the phyto-

plankton at various stages from the Late

Precambrian to the present, whereas the fora-

miniferans and radiolarians were important

parts of the zooplankton (Fig. 9.2). Apart from

a role as a primary food source, the marine

phytoplankton function as a major carbon

sink, initially removing CO

from the atmo-

sphere as carbonate ions. These cycles may

already have been in place throughout much of

the later Proterozoic, anticipating more modern

oceanic biological and chemical systems.

PROTOZOA

Protozoans are neither animal nor plant, but

single-celled eukaryotes that commonly show

animal characteristics such as motility and

heterotrophy; some groups are able to form

cysts. Most are about 50–100 μm in size and

are very common in aquatic environments

and in the soil. They can occupy various levels

in the food chain ranging from primary pro-

ducers to predators and some groups function

as parasites and symbionts.

calcareous

phosphatic

siliceous

agglutinated

organic-walled

Plankton

Neogene

Paleogene

Cretaceous

Jurassic

Triassic

Permian

Carboniferous

Devonian

Silurian

Ordovician

Cambrian

Ediacaran

Cryogenian

700

600

500

400

300

200

100

Benthic

foraminiferans

Neoproterozoic Paleozoic Mesozoic

Ceno-

zoic

Acritarchs

Dinoflagellates

Radiolarians

Diatoms

Silicoflagellates

Calcareous nanoplankton

Globigerinine foraminiferans

Allogromiina

Textulariina

Fusulinina

Miliolina

Lagenina

Rotaliina

Ostracoda

Chitinozoans

Conodonts

Tintinnids

}

Age (Ma)

Figure 9.2 Stratigraphic ranges of the main protist groups. (From Armstrong & Brasier 2005.)

PROTISTS 209

Foraminifera

Foraminifera are shelled, heterotrophic proto-

zoans, common in a wide variety of Phanero-

zoic sedimentary rocks and of considerable

biostratigraphic and paleoenvironmental

value. The foraminiferans are characterized

by a complex network of granular pseudopo-

dia. The foraminiferans were traditionally

included in the phylum Sarcodina together

with the Radiolaria and a range of other non-

ﬂ agellate protozoans. In modern classiﬁ ca-

tions the foraminiferans are usually regarded

as a discrete phylum, the Granuloreticulosa.

Cavalier-Smith (2002), for example, regarded

the Foraminifera as a member of the infrak-

ingdom Rhizaria and placed them within the

phylum Retaria together with the Radiolaria.

Foraminifera are easily the most abundant

of microfossils and can be studied with simple

preparation techniques and low-power micro-

scopes. Consequently, pioneer studies in

micropaleontology were based on the forami-

niferans and techniques established for the

study of this group were extended to many

other microfossil taxa. Foraminifera have

proved extremely useful in the petroleum

industry, where detailed biostratigraphic

schemes, particularly for Cenozoic rocks,

have helped correlate oil ﬁ eld data. Moreover

stable isotopes extracted from foraminiferan

tests have provided valuable data on ancient

sea temperatures through the Mesozoic and

Cenozoic.

Morphology and classiﬁ cation

Although many different classiﬁ cations have

been published, shell morphology and miner-

alogy form the prime basis for identiﬁ cation

of species and higher categories of Foramin-

ifera. Most have a shell or test comprising

chambers, interconnected through holes or

foramina. The test may be composed of a

number of materials and three main catego-

ries have been documented, organic, aggluti-

nated and secreted calcareous:

1 Organic tests consist of tectin, which

is a protinaceous or pseudochitinous

substance.

2 Agglutinated (“glued”) tests comprise

fragments of extraneous material bound

together by a variety of cements. The

debris may be siliciclastic, such as quartz,

mica grains or sponge spicules, or calcare-

ous, recycling fragments of coccoliths or

other forams.

3 Secreted calcareous tests may be subdi-

vided into three categories: porcellaneous,

hyaline and microgranular (Fig. 9.3a).

Porcellaneous tests are formed of small,

randomly oriented crystals of high-

magnesium calcite giving a smooth white

shell. Hyaline tests are formed of larger

crystals of low-magnesium calcite and

have a glassy appearance when well pre-

served. Hyaline tests have two main

modes: the radial tests are made up of

minute calcite crystals with their c-axes

normal to the test surface, whereas granu-

lar forms consist of microcrystals of calcite

with variable orientations. Both modes

usually have a multilayered structure (Fig.

9.3b) and perforations. Hyaline aragonitic

tests occur but are much rarer than calcitic

tests. Finally, microgranular tests consist

of tightly packed, similar-sized grains

of crystalline calcite. Most members of

this group are known from the Upper

Paleozoic.

The gross morphology of a foraminiferan

test is governed by the shape and arrangement

of the chambers. The group has evolved a

wide range of test symmetries (Fig. 9.4) from

simple uniserial and biserial forms to more

complex planispiral and trochospiral shapes

(Fig. 9.5). Chambers also come in a wide

(a)

(b)

microgranular

compound

porcellaneous

hyaline radial

non-lamellar monolamellar multilamellar bilamellar

Figure 9.3 Main types of foraminiferan test

walls: (a) the composition and structure of test

walls and (b) lamellar construction.

uniserial biserial triserial trochospiral planispiral quinqueloculine

Figure 9.4 Main types of foraminiferan chamber construction.

(a)

(b)

(c)

(d)

(e)

(f)

(g)

(h)

(i)

(j)

(k)

(l)

Figure 9.5 Some genera of foraminiferans: (a) Textularia, (b) Cribrostomoides, (c) Milionella,

(d) Sprirolina, (e) Brizalina, (f) Pyrgo, (g) Elphidium, (h) Nonion, (i) Cibicides, (j) Globigerina,

(k) Globorotalia, and (l) Elphidium (another species). Magniﬁ cation ×50–100 for all. (Courtesy of John

Murray (b, d, e, g, h, j, k) and Euan Clarkson (a, c, f, i, l).)

PROTISTS 211

spectrum of shapes, from simple spherical

compartments through tubular to clavate

forms. Moreover, the shape and position of

the aperture may vary. Surface ornament may

include ribs and spines or be merely punctate

or rugose. Foraminifera are classiﬁ ed

according to test type and ornamentation

(Box 9.2).

Life modes

The foraminiferans have adopted two main

life modes, benthic and planktonic. The

majority are benthic, epifaunal organisms;

they are either attached or cling to the sub-

strate or crawl slowly over the seabed by

extending their protoplasmic pseudopodia.

Infaunal types live within the top 15 cm of

sediment. Most benthic forms have a restricted

geographic range. Planktonic foraminiferans

are most diverse in tropical, equatorial regions

and may be extremely abundant in fertile

areas of the oceans, particularly where upwell-

ing occurs.

The functional morphology of these groups

can now be modeled mathematically (Box

9.3) and potentially can be related to different

life modes in the group. Moreover their rela-

tionships to different environments, past and

present, are well established (Box 9.4).

Evolution and geological history

The earliest foraminiferans are known from

the Lower Cambrian, represented by simple

agglutinated tubes assigned to Bathysiphon, a

living benthic genus (Fig. 9.8). More diverse

agglutinated forms appeared during the Ordo-

vician while microgranular tests evolved

during the Silurian; however, it was not until

the Devonian that multichambered tests prob-

ably developed. Nevertheless, Carboniferous

assemblages have a variety of uniserial, bise-

rial, triserial and trochospiral agglutinated

tests. Around the Devonian–Carboniferous

boundary the ﬁ rst partitioned tests displaying

multilocular growth modes (the addition of

new chambers in series) appeared. Two fami-

lies, the Endothyridae and Fusulinidae, domi-

nated Carboniferous assemblages and the

porcellaneous Miliolinidae achieved impor-

tance in the Permian. The Fusulinidae were

generally large, specialized foraminiferans,

adapted to carbonate and reef-type facies

during the Late Carboniferous and Permian.

Despite a high diversity during the Late

Permian, they became extinct at the end of the

Paleozoic, and the Endothyridae and the

Miliolinidae were very much reduced in

diversity.

Although Triassic assemblages were gener-

ally impoverished, the stage was set for a con-

siderable radiation during the Jurassic. Two

hyaline groups, the benthic Nodosariidae and

planktonic Globigerinidae, diversiﬁ ed, while

the agglutinates, Lituolitidae and Orbitolini-

dae, continued. The planktonic foraminifer-

ans diversiﬁ ed in the Cretaceous, culminating

in the near extinction of the group during the

Cretaceous–Tertiary (KT) mass extinction.

Two further periods of diversiﬁ cation took

place during the Paleocene-Eocene and the

Miocene.

Radiolaria

The radiolarians are marine, unicellular,

planktonic protists with delicate skeletons

usually composed of a framework of opaline

silica (Fig. 9.9). Their name is derived from

the radial symmetry, commonly marked by

radial skeletal spines, characteristic of many

forms. Many others, however, lack radial

symmetry. Most radiolarians feed on bacteria

and phytoplankton, but also on copepods and

crustacean larvae and occupy levels in the

water column from the surface to the abyssal

depths, although most live in the photic zone

commonly associated with symbiotic algae.

The radiolarian ectoplasm covers the test and

holds symbiotic zooxanthellae, microorgan-

isms enclosed within the cell mass, and perfo-

rations, providing some nourishment. The

radiolarian endoplasm (surrounded by the

capsular membrane) contains the nucleus and

other inclusions. The group has two types of

pseudopodia: the axopodia are rigid and not

ramiﬁ ed, whereas the ﬁ lipodia are thin, rami-

ﬁ ed extensions of the ectoplasm.

Morphology and classiﬁ cation

The radiolarian skeleton or test consists of

isolated or networked spicules, composed of

opaline silica and forming sponge-like struc-

tures or trabeculae. Three of the main

groups are recognized (Box 9.5) on the basis

of skeletal structure and arrangement of

212 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD

Box 9.2 Classiﬁ cation of Foraminifera

Suborder ALLOGROMIINA

• Organic tests, usually unilocular, occurring in fresh, brackish and marine conditions. Not usually

fossilized

• Cambrian (Lower) to Recent

Suborder TEXTULARIINA

• Agglutinated tests consist of debris bound together with cement; both septate and non-septate

• Cambrian (Lower) to Recent

Suborder FUSULININA

• Microgranular tests, some with two or more laminae; septate and non-septate forms

• Ordovician (Llandeilo) to Permian (Changhsingian)

Suborder INVOLUTININA

• Aragonitic hyaline tests

• Permian (Rotliegendes) to Cretaceous (Cenomanian)

Suborder SPIRILLININA

• Calcitic hyaline tests, planispiral to conical

• Triassic (Rhaetic) to Recent

Suborder CARTERININA

• Tests comprise calcareous spicules in calcareous cement

• Tertiary (Priabonian) to Recent

Suborder MILIOLINA

• Porcellaneous tests, imperforate, both septate and non-septate, which are often large and

complex

• Carboniferous (Viséan) to Recent

Suborder SILICOLOCULININA

• Imperforate tests of opaline silica

• Tertiary (Miocene) to Recent

Suborder LAGENINA

• Calcitic monolamellar tests, hyaline radial

• Silurian (Prídolí) to Recent

Suborder ROBERTININA

• Aragonitic, hyaline radial tests; both septate and ﬁ nely perforate

• Triassic (Anisian) to Recent

Suborder GLOBIGERININA

• Calcitic, hyaline tests; ﬁ nely perforate planktonic forms

• Jurassic (Bajocian) to Recent

Suborder ROTALIINA

• Calcitic, hyaline radial; perforate multilocular forms

• Jurassic (Aalenian) to Recent

PROTISTS 213

Box 9.3 Modeling of foram tests

David Raup’s theoretical work on the modeling of mollusk morphospace created a paradigm shift

in our understanding of shell ontogeny (see p. 332). The skeletons of many groups of organisms can

now be generated, mathematically, according to a simple set of equations in each case. The shapes

of microfossils can also be modeled in this way, with a set of rules based on the angle of deviation,

a translation factor and a growth factor (Tyszka 2006). By varying these, a huge range of possible

and impossible tests can be homegrown on the computer (Fig. 9.6). The forms illustrated here are

only a subset of the total number of possibilities. Interestingly, these sorts of computer models always

generate some bizarre forms. The dysfunctional forms, for example, are geometrically possible but

the shapes and volumes of the chambers could simply not function; vacant ranges on the other hand

contain fully functional morphologies but these forms have not yet been found in the fossil record.

Why not?

0° 30° 60° 90° 120° 150° 179.99°

0.90.60.30.001–0.3–0.6–0.9

–0.9–0.6–0.30.0010.30.60.9

1.3

1.2

1.1

1.3

1.2

1.1

Δφ

Figure 9.6 Modeling foraminiferan tests: part of a theoretical three-dimensional morphospace for

foraminiferans. GF, growth factor; TF, translation factor; Δφ, deviation factor. (From Tyszka

2006.)

214 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD

Box 9.4 Forams and environments

The ratio of agglutinated : hyaline : porcellaneous foram tests has been used extensively to differenti-

ate among a range of modern environments. Ternary plots of the relative frequencies of test type

distinguish ﬁ elds for hypersaline and marine lagoons, estuaries and open shelf seas (Fig. 9.7). Fossil

faunas may be plotted on these templates, and these allow paleontologists to estimate the salinity

of ancient environments.

The ratio of infaunal : epifaunal benthic foraminiferans has also been widely used to determine

the relative content of dissolved oxygen and/or organic carbon on the seaﬂ oor. Epifaunal and infaunal

foraminiferans can be distinguished by their test morphologies, where epifaunal forms occur mainly

in aerobic conditions with low amounts of organic carbon, and infaunal forms occur in more oxygen-

deﬁ cient conditions with higher organic carbon content.

Measures of the ratio of benthic : planktonic foraminiferans are also useful in environmental

studies. In general terms, the percentage of benthic taxa declines rapidly below depths of about

500 m in modern seas and oceans. Data from living assemblages have been used to interpret paleoen-

vironments with diverse fossil foraminiferan faunas. For example, microfossil analysis of the upper

part of the Late Cretaceous chalk of the Anglo-Paris basin has suggested water depths of between

600 and 800 m during the Turonian on the basis of the high proportions of planktonic foraminifer-

ans; however, by the Campanian, water depths of about 100 m are suggested by the rich benthic

fauna.

perforations: the nassellarians (Fig. 9.10) and

entactinarians develop a lattice from bar-like

spicules, each end having a bundle of spicules.

The initial nassellarian spicule is enclosed in

the cephalis, and the skeleton develops further

by the addition of segments following axial

symmetry. By contrast, the initial entactinar-

ian spicule is enclosed in a latticed or spongy

test with radial symmetry based on a spherical

body plan; this is similar to those of the

spumellarians (Fig. 9.10), which however

have a microsphere (instead of a spicule)

internally.

Evolution and geological history

Although some records suggest an origin in

the Mid Cambrian or earlier, the radiolarians

became common in the Ordovician, and they

are often found in deep-sea cherts associated

with major subduction zones. The albaillellar-

ians together with the entactinarians were the

dominant forms, although after the Devonian,

spumellarians with sponge-like tests were

more prominent (Fig. 9.10).

Spumellarians remained important during

the Triassic, with genera such as Capnuchos-

phaera, although the nassellarians had

appeared; they continued as the major group

through the Jurassic, Cretaceous and Early

Tertiary. Late Tertiary forms evolved thinner

skeletons, perhaps because of increased

competition with the diatoms for mineral

resources.

Radiolarian oozes cover about 2.5% of the

ocean ﬂ oors, accumulating at rates of 4–5 mm

per 1000 years. Radiolarians are useful in

paleo-oceanographic investigations, and they

are particularly useful in dating the formation

of deep-water sediments accumulating beneath

the carbonate compensation depth (CCD),

where carbonate-shelled organisms such as

foraminiferans cannot survive. Radiolarian

cherts and radiolarites commonly occur in

oceanic facies preserved in mountain belts

and are commonly associated with ophiolites,

sections of the ancient ocean crust and upper

mantle that have been uplifted (see p. 48),

so they are very important in deciphering

the origins and destruction of ancient ocean

systems such as Tethys.

But the beauty of the radiolarian skeleton

has also assured the group’s place in the

history of art (Box 9.6).

PROTISTS 215

Allogromia

Cibicides

Ammonia

Quinqueloculina

Triloculina

Miliolina

Reophax

Discoris

Rotaliina

Textulariina

Globigerinoides

Globorotalia

freshwater

hypersaline lagoons

normal marine

lagoons and

carbonate platforms

brackish lagoons

and estuaries

most shelf seas

100%

spinose planktonics

100 m

3000 m

shelf (neritic)

bathyal

abyssal

keeled planktonics

relative abundance of benthic Foraminifera on the sea floor

3000 m

200 m

% planktonics (Globigerinacea)

maximum diversity of calcareous

benthic forms

maximum diversity of

agglutinated forms

% agglutinated forms

% calcareous benthic forms

0 m

50 m

100 m

relative abundance of

planktonic foraminifera

Figure 9.7 Foram test and environments: distribution of test types and genera of Foraminifera against environmental gradients.

(From Armstrong & Brasier 2005.)

216 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD

Acritarchs

The acritarchs are a mixed bag of entirely

fossil, hollow, organic-walled microfossils that

are impossible to classify. The acritarchs are

probably polyphyletic; they include a wide

range of forms, probably representing the cyst

stages or resting phases in the life cycles of

various groups of planktonic algae. Funda-

mental work on the group by Alfred Eisenack

(1891–1982) initially suggested that these tiny

fossils were the eggs of planktonic inverte-

brates; however, later he considered the group

to be fossil members of the phytoplankton,

plants rather than animals. William Evitt of

Stanford University, in establishing the scope

of the group in the early 1960s, noted that his

term “acritarch” (meaning “uncertain origin”)

Classification

Agglutinates

Micro-

granular

Porcel-

aneous

Hyaline

Benthic

Plank-

tonic

Stratigraphy

Families of

Foraminifera

Ammodiscidae

Lituolidae

Orbitolinidae

Endothyridae

Fusilinidae

Miliolidae

Alveolinidae

Nodosariidae

Buliminidae

Discorbidae

Rotaliidae

Orbitoididae

Nummulitidae

Miogypsinidae

Globigerinidae

Paleozoic Mesozoic

Ceno-

zoic

Cretaceous

Tertiary

Jurassic

Triassic

Permian

Carboniferous

Devonian

Silurian

Ordovician

Cambrian

Figure 9.8 Stratigraphic ranges of the main foraminiferan groups. (Based on various sources.)

spine

cortical shell

pore

apical horn

cephalis

joint

medullary

shells

bar

chambered

lattice shell

tripod

NassellariaSpumellaria

Figure 9.9 Descriptive morphology of the radiolarians.

PROTISTS 217

SPUMELLARIA

Lenosphaera

Actinomma

Alievium

Anthocyrtidium

Calocyclas

Peripyramis

NASSELLARIA

Figure 9.10 Some radiolarian morphotypes: Lenosphaera (×100), Actinomma (×240), Alievium (×180),

Anthocyrtidium (×250), Calocyclas (×150) and Peripyramis (×150).

Box 9.5 Classiﬁ cation of the radiolarians

The classiﬁ cation of the radiolarians is currently in a state of ﬂ ux. Six orders are recognized (De

Wever et al. 2001).

Order ARCHAEOSPICULARIA

• Mid Cambrian to Silurian

Order ALBAILLELLARIA

• Late Ordovician to Late Silurian or ?Devonian

Order LATENTIFISTULARIA

• Early Carboniferous (or earlier?) to Permian

Order SPUMELLARIA

• Paleozoic (precise age uncertain) to Recent

Order ENTACTINARIA

• (?Cambrian), Ordovician to Recent

Order NASSELLARIA

• (?Late Paleozoic), Triassic to Recent