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

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218 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD

was a monument to our ignorance. Although

many more taxa have been described since,

and their value in biostratigraphic correlation

has been proved, uncertainty still surrounds

the origin and afﬁ nities of the group. Similar-

ity, however, with the cyst stages of modern

prasinophytes and dinoﬂ agellates suggests a

relationship to primitive green algae. However,

because they are very useful in hydrocarbon

exploration, perhaps the minor issue of their

identity can be left for future generations!

Morphology and classiﬁ cation

The composition and broad morphology of

the acritarchs suggest similarities with the

dinocysts; like the dinocysts, acritarchs are

also often found in clusters. The group prob-

ably had a similar life cycle to that of the

dinoﬂ agellates, single-celled protists that

mainly live in the marine plankton today.

Acritarchs seem to show encystment struc-

tures, or cysts – protective devices similar to

those of modern dinoﬂ agellates, in which the

organism can survive drying out or lack of

food for long periods. When conditions return

to normal, usually when the cyst is covered

with water again, the organism “escapes” by

bursting through the watertight skin of the

cyst, and resumes feeding and reproducing.

A number of escape structures have been

described including median splits, pylomes

and cryptopylomes, that would have allowed

material to seep out.

Acritarchs consist of vesicles composed of

various polymers combined to form sporopol-

lenin (Fig. 9.12). They range in shape from

spherical to cubic and in size from usually 50

to 100 μm, although some specimens from

the Triassic and Jurassic are as small as 15–

20 μm. Many lose these morphological details

when preserved as ﬂ attened ﬁ lms in black

shales. There is a huge variety of basic shapes

(Fig. 9.13). Acritarchs can have single- or

double-layered walls; the wall structure is

often useful taxonomically. The central cavity

or chamber can be closed or open externally

through a pore or slit called the pylome. The

opening or epityche presumably allowed the

escape of the motile stage and may be modi-

ﬁ ed with a hinged ﬂ ap.

On the outside the acritarch may be smooth

or, for example, have granulate or microgran-

ulate ornament. Moreover, the vesicle may be

modiﬁ ed by various extensions or processes

projecting outwards from the vesicle wall. If

an acritarch has a set of similar processes,

they are termed homomorphic, and if it

has a variety of different projections it is

heteromorphic.

Over 1000 genera of acritarchs are known,

deﬁ ned mainly on shape characteristics (Box

9.7). All acritarchs were aquatic with the vast

majority found in marine environments. The

classiﬁ cation of the group is based on the wall

structure, the shape of the body vesicle,

pylome type and the nature of the extensions

and processes.

Box 9.6 Ernst Haeckel, art and the radiolarians

The link between art and paleontology has always been strong, with many images ﬁ nding their

inspiration in the beauty of the fossil form. Ernst Haeckel (1834–1919), the German evolutionary

biologist, responsible for such terms as “Darwinism” and “ecology”, the phrase “ontogeny recapitu-

lates phylogeny” and the ﬁ rst detailed tree of life (see p. 128) was also an accomplished artist; he

believed in the esthetic dimension of morphology (Fig. 9.11). His giant opus Art Forms in Nature

(1899–1904) is considered to be one of the most elegant, artistic works of the 19th century, his

illustrations being a paleontological precursor to the Art Nouveau movement. His style is nowhere

better presented than in his monograph on the Radiolaria (Haeckel 1862). Unfortunately his attempts

to associate science with art may have damaged his career, but current interest in the tree of life has

generated a Haeckel renaissance. His illustrations are even available now as an attractive

screensaver!

You can see these beautiful images at http://www.blackwellpublishing.com/paleobiology/.

Figure 9.11 Haeckel’s radiolarians: plate 12 from Die Radiolarien (Rhizopoda Radiaria) by Ernst

Haeckel (1862).

220 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD

Box 9.7 Classiﬁ cation of the main organic-walled groups: acritarch form

classiﬁ cation

The classiﬁ cation of acritarchs is based entirely on morphology and as such is merely a set of shape

and ornament categories with no phylogenetic status. The names of the main groups thus are also

used as morphological terms to deﬁ ne the variation in shape (see Fig. 9.12). Clearly such a classiﬁ ca-

tion is rife with convergent morphotypes that may never be properly classiﬁ ed. Recent studies,

however, suggest that understanding the mode of encystment may be a step towards the development

of a more phylogenetic classiﬁ cation.

ACRITARCHS WITHOUT PROCESSES OR FLANGES

Sphaeromorphs

• Spherical forms lacking processes but with ornamented walls. These morphs are often variably

ornamented

• Precambrian (Animikean) to Recent

ACRITARCHS WITH FLANGES BUT LACKING PROCESSES

Herkomorphs

• Subpolygonal or spherical with polygonal ornament deﬁ ned by crests

• Cambrian (Lower) to Recent

Pteromorphs

• Forms equipped with an equatorial ﬂ ange

• Ordovician (Caradoc) to Recent

ACRITARCHS WITH PROCESSES BUT WITH FLANGES

Acanthomorphs

• Spherical forms lacking an inner body and crests, with simple or branching processes

Polygonomorphs

• Polygonal forms with simple processes

process

periphragm

excystment

opening

vesicle

endophragm

Figure 9.12 Descriptive morphology of the acritarchs.

PROTISTS 221

Evolution and geological history

Acritarchs had a wide geographic range,

apparently mainly controlled by latitude; the

entire group ranged from the poles through

the tropics. The wide distribution of the group

is similar to that of the dinoﬂ agellates and

strongly suggests that acritarchs were also

members of the phytoplankton. Biogeographic

provinces have been established for the Ordo-

vician, Silurian and Devonian periods and

have helped reconstruct ancient climate belts

and oceanic currents. Acritarchs have also

been of considerable value in regional correla-

tions, particularly during the Ordovician and

Silurian.

The acritarchs are some of the oldest docu-

mented fossils with a history of over 3000 myr,

although the group was not common until

some 1 Ga, when the ﬁ rst major diversiﬁ ca-

tion of the group, predating the Ediacara

biota (p. 242), was marked by large sphero-

morphs, acanthomorphs and polygono-

morphs. During the important Early Cambrian

radiation of the group, spinose morphs such

as Baltisphaeridium and Micrhystridium,

together with the crested Cymatiosphaera,

appeared. Signiﬁ cantly these armored vesicles

evolved during the expansion of marine pred-

ators: Was this a form of arms race or merely

a coincidence? By the Late Cambrian to Early

Ordovician, acritarch palynofacies (pollen

and spore assemblages) were dominated by

three main groupings: the Acanthodiacro-

dium, Cymatiogalea and Leiofusa groups

(Box 9.8). The acritarchs declined during the

Devonian, and are rare in Carboniferous-

Triassic rocks. Nevertheless the group staged

a weak recovery during the Jurassic and con-

tinued through the Cretaceous and Tertiary.

Dinoﬂ agellates

The dinoﬂ agellates, or “whirling whips”,

comprise a group of microscopic algae with

organic-walled cysts. The life history of these

organisms thus oscillates between a motile

(swimming) and a cyst (resting) stage; the

cysts usually range in size from 40 to 150 μm.

The motile phase is either ﬂ exible and unar-

mored, or rigid and armored with a network

of plates, the theca; the arrangement of the

Netromorphs

• Elongate, commonly fusiform morphs with poles variably developed as processes or spines

Diacromorphs

• Spherical to ellipsoidal, with ornament restricted to around the poles

Prismatomorphs

• Polygonal or prismatic, with edges commonly extended as ﬂ anges

Oomorphs

• Egg-shaped forms, one end smooth and the other highly ornamented

ultiplicisphaeridium

Villosacapsula

Baiomeniscus

Leiofusa

Figure 9.13 Some acritarch morphotypes:

Multiplicisphaeridium (×800), Baiomeniscus

(×200), Leiofusa (×400) and Villosacapsula

(×400).

222 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD

Box 9.8 Acritarchs and the food chain

Groups such as the acritarchs formed a prominent base to the relatively short, suspension-feeding

Early Paleozoic food chains, yet it is virtually impossible to quantify the abundance of microfossils

in sediments because many factors such as cyst production, hydrodynamic sorting and taphonomy

come into play. Unfortunately, diversity cannot be used as a proxy for abundance, so there is no

direct evidence in the fossil record of just how densely packed the water column was with phyto-

plankton, say during the Ordovician. However, it may be possible to speculate that primary produc-

tion increased rapidly during the Ordovician: This period was marked by the appearance and

radiation of the graptolites, phyllocarids, some groups of echinoderms and the radiolarians. Huge

bursts in diversity are seen among the brachiopods, mollusks and trilobites, while there was increas-

ing complexity in benthic and reef communities. Yet little is known about the cause of this phenom-

enal diversiﬁ cation. Marco Vecoli and his colleagues (2005) have now suggested that these massive

metazoan radiations probably signal a cryptic explosion in primary production in the world’s oceans

(Servais et al. 2008) that may have been one of the main triggers for the great Ordovician biodiver-

siﬁ cation (see p. 253). The diversity curves of these protistan groups appear to match perfectly those

of the metazoans (Fig. 9.14).

Figure 9.14 Acritarch and invertebrate diversity through Ordovician Period. (Courtesy of

Thomas Servais.)

PROTISTS 223

thecal plates comprises the dinoﬂ agellate

tabulation.

Morphology and classiﬁ cation

The plates of a dinoﬂ agellate theca are

arranged from the apex to antapex as follows:

apical, precingular, cingular, postcingular and

antapical; the ﬁ rst two are part of the eipi-

theca and the last two, the hypotheca (Fig.

9.15). There are a number of other plates with

further specialized terms and together the

plates are commonly labeled and numbered

in sequence. The motile phase is rarely fossil-

ized. In contrast, the cysts are chemically

resistant and relatively common. The mor-

phology of a motile dinoﬂ agellate is crudely

similar to its theca and comparable structures

in the motile form are preﬁ xed by the term

“para”.

Cysts have a paratabulation that is useful

taxonomically (Box 9.9): for example, the

cysts of peridiniaceans have seven precingular

and ﬁ ve postcingular paraplates, whereas the

gonyaulacaceans have six precingular and six

postcingular paraplates.

Dinoﬂ agellates are abundant and diverse

members of the living and more recent fossil

phytoplankton (Fig. 9.16), forming an impor-

tant part of the base of the food chain of the

oceans; they may in fact be second only in

abundance to diatoms as primary producers.

However, dinoﬂ agellate blooms or red tides,

when there is huge population explosion, can

lead to asphyxiation of other marine groups.

Mass mortalities of Cretaceous bivalves in

Denmark and of Oligocene ﬁ shes in Romania

have been blamed on fossil red tides.

There are three main cyst types. The proxi-

mate cyst is developed directly against the

theca itself and has a similar conﬁ guration. A

longitudinal flagellum

(b)

(a)

apex or anterior end

apical

horn

flagellar

pore(s)

antapex or posterior end

epitheca

plates

sutures

cingulum

hypotheca

antapical horns

ventral surface dorsal surface

rightleftright

transverse

flagellum

tabulation

epitheca

cingulum

hypotheca

sulcus

cysttheca

paratabulation

epicyst

paracingulum

hypocyst

parasulcus

Figure 9.15 Descriptive morphology of (a) a dinoﬂ agellate, and (b) a dinoﬂ agellate theca (left),

unpeeled (middle) to reveal the corresponding cyst (right).

224 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD

Box 9.9 Classiﬁ cation of the main organic-walled groups: dinoﬂ agellate

form classiﬁ cation

Class DINOPHYCEAE

Most dinoﬂ agellates belong to this class, which includes fossil representatives. They are free-living

cells with a large nucleus and numerous chromosomes; some are parasites and symbionts.

Order GYMNODINIALES

• Cretaceous to Recent

Order PTYCHODISCALES

• Cretaceous

Order SUESSIALES

• Triassic to Recent

Order NANNOCERATOPSIALES

• Jurassic

Order DINOPHYSIALES

• Jurassic?

Order DESMOCAPSALES

• Recent

Order PHYTODINIALES

• Recent

Order GONYAULACALES

• Jurassic to Recent

Order PERIDINIALES

• Triassic to Recent

Order THORACOSPHAERALES

• Triassic to Recent

Class BLASTODINIPHYCEAE

Parasites on or in copepods and other animals.

Order BLASTODINIALES

• Recent

Class NOCTILUCIPHYCEAE

Very large, naked cells lacking chloroplasts.

Order NOCTILUCALES

• Recent

Class SYNDINIOPHYCEAE

Symbionts or endoparasites lacking chloroplasts

Order SYNDINIALES

PROTISTS 225

chorate cyst is smaller than the theca and the

cysts are contained within the theca, intercon-

nected by various appendages and spines,

which are related to the external tabulation

of the theca. In cavate morphs there is a gap

between the cyst and the theca at the two

poles.

Evolution and geological history

Dinoﬂ agellate biomarkers have been identi-

ﬁ ed in Upper Proterozoic and Cambrian

rocks. Moreover the Late Precambrian and

Paleozoic diversiﬁ cations of the acritarchs

may mark an early phase in dinoﬂ agellate

radiation, involving non-tabulate forms. To

date, however, the oldest dinoﬂ agellate cyst is

probably Arpylorus from the Ludlow (Upper

Silurian) rocks of Tunisia; the cyst has feeble

paratabulation and a precingular archeopyle.

Oddly, there is a long gap after this record

until the Early Triassic, when Sahulidinium

appears off northwest Australia. Some authors

have suggested that a number of Paleozoic

acritarch taxa may in fact be dinoﬂ agellates.

Multiplated forms such as Rhaetogonyaulax

and Suessia appearing in the Late Triassic

characterize dinocyst ﬂ oras ranging from

Australia to Europe. Nannoceratopsis cysts

with characteristic archeopyles and tabula-

tion are common in Early Jurassic ﬂ oras,

while Ceratium-like forms appeared ﬁ rst

during the Late Jurassic and diversiﬁ ed in the

Cretaceous. Many precise zonation schemes

for Mesozoic and Cenozoic strata are based

on dinocyst distributions. However, during

the Eocene the global biodiversity of the group

began a steady decline.

Ciliophora

The Ciliophora today consist of some 8000

species of single-celled organisms that swim

by beating their cilia, minute hair-like organs.

Two fossil groups, the calpionellids and tin-

tinnids, may belong here. Calpionellids are a

(a)

(e) (f) (g) (h)

(b)

(c)

(d)

Figure 9.16 A prasinophyte (a) and some dinoﬂ agellate taxa (b–h): (a) Tasmanites (Jurassic), (b)

Cribroperidinium (Cretaceous), (c) Spiniferites (Cretaceous), (d) Deﬂ andrea (Eocene), (e) Wetzeliella

(Eocene), (f) Lejeunecysta (Eocene), (g) Homotryblium (Eocene), and (h) Muderongia (Cretaceous).

Magniﬁ cation ×250 (a, d, e), ×425 (b, c, f, g, h). (Courtesy of Jim Smith.)

226 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD

group of extinct, cup-shaped, calcareous

microfossils that were abundant in Late Juras-

sic and Early Cretaceous pelagic sediments,

especially in the Tethyan realm. As an extinct

group with no complex characters, no deﬁ ni-

tive evidence of their afﬁ nities has been found;

however, they are strikingly similar in shape

and size to an important group of ciliates, the

tintinnids.

Tintinnids are part of the zooplankton,

grazing on phytoplankton and providing a

food source for larger members of the plank-

ton. The cell is enclosed within a cup-shaped

test or lorica, often 10 times larger than the

cell itself. Modern tintinnids have an organic

lorica with, in some cases agglutinated mineral

grains or coccoliths, but without biomineral-

ization, whereas the fossil calpionellids had a

primary calcareous test (Fig. 9.17).

Two families of fossil tintinnid have been

recorded, together ranging in age from the

Tithonian (Upper Jurassic) to the Albian

(Middle Cretaceous).

CHROMISTA

The chromistans are probably a paraphyletic

group of eukaryotes that usually contains

chloroplasts with chlorophyll c, which is

absent from all known plant groups. The

group includes various algae, the coccolitho-

phores and the diatoms and the majority are

primary producers, functioning as part of the

phytoplankton.

Coccolithophores

Nannoplankton, are deﬁ ned as plankton less

than 63 μm across, the smallest standard

mesh size for sieves. Although the nanno-

plankton includes organic-walled and sili-

ceous forms, the calcareous groups are most

prominent in living ﬂ oras and dominate the

fossil record. Coccolithophores are the domi-

nant members of the fossil calcareous nanno-

plankton, and the calcareous plates they

produce, coccoliths, dominate nannofossil

assemblages. Many calcareous nannofossils

lack obvious shared characters with cocco-

liths and so are excluded from the coccolitho-

phores and instead are termed nannoliths.

These nannoliths may be related to coccolith-

bearing organisms, but in view of their diver-

sity in form, the group may contain calcareous

structures produced by quite unrelated

microbes. As a whole, calcareous nanno-

plankton ﬁ rst appeared during the Late Trias-

sic, increased in abundance and diversity

through the Jurassic and Cretaceous, reaching

an acme of diversity in the Late Cretaceous.

They were severely affected by the KT mass

extinction, but subsequently radiated in the

Early Paleogene and remained a major com-

ponent of the calcifying plankton throughout

the Cenozoic. They are extremely abundant

in the surface waters of modern oceans.

Morphology and classiﬁ cation

Coccolithophores are unicellular algae, pre-

dominantly autotrophic in dietary mode,

usually ranging in size from 5 to 50 μm, and

globular, fusiform or pyriform in shape. The

group constitutes the Phylum Haptophyta,

within the Kingdom Chromista, together with

various closely related non-calcifying algae;

they have golden-brown photosynthetic pig-

ments and, in motile phases, two smooth

ﬂ agella together with a third ﬂ agellum-like

structure, the haptonema. Coccolithophores

are almost exclusively marine (there is just

one, rather rare, freshwater species), usually

open marine, occupying the photic zone where

they photosynthesize. The group today is

most diverse and has its highest relative abun-

dances in the tropics although coccolitho-

phores occur at all latitudes. The shell is

composed of distinctive calcitic platelets or

coccoliths. These are produced intracellularly;

Tintinnopsella

Calpionella

Calpionellites

Coxlielina

Salpingellina

Deflandronella

Figure 9.17 Morphology of some tintinnids in

cross-section from limestones (×100–200).

PROTISTS 227

they then migrate to the cell surface and are

expelled to form a composite exoskeleton, the

coccosphere. Commonly the coccosphere

consists of 10–30 discrete coccoliths, although

some forms have many more (Box 9.10).

Many taxa produce coccospheres formed of

only one type of coccolith, but others show a

variety of coccolith morphologies (Fig. 9.18);

in particular there are often specialized coc-

coliths around the ﬂ agellar pole of the cell.

There are two fundamentally different types

of coccoliths: heterococcoliths have a radial

array of relatively few (typically 20–50)

complex-shaped crystal units, whereas holo-

coccoliths are formed of planar arrays of hun-

dreds of minute uniform-sized (typically c.

0.1 μm) rhombohedral crystallites.

Haptophyte life cycles were very poorly

known until recently; research has now shown

that cocolithophores, and possibly most hap-

tophtes, typically have alternating haploid

and diploid stages that are both capable

of asexual reproduction. Coccolithophores

usually have life cycles consisting of two main

phases producing radically different cocco-

liths that were often described initially as two

different species. The haploid phase (with half

the complement of chromosomes) is always

ﬂ agellate, and is usually coated by minute

holococcoliths; the diploid phase (with full

complement of chromosomes) is usually non-

ﬂ agellate, and is coated by heterococcoliths.

Both phases are capable of indeﬁ nite asexual

reproduction and it appears likely that the

two-phase life cycle is an adaptation allowing

coccolithophores to survive challenging eco-

logical conditions. The haploid (holoccolith-

producing) phase is thought to be adapted to

oligotrophic conditions (when nutrients are

scarce) whilst the diploid (heterococcolith-

producing) phase is thought to be adapted to

more eutrophic conditions (when nutrients

are abundant).

The classiﬁ cation of extant coccolitho-

phores is based largely on coccosphere mor-

phology and coccolith structure because the

intricate and distinctive form of coccoliths

makes them ideal for morphological classiﬁ -

cation. Cell characters can only be studied

with transmission electron microscopy and

have generally proved rather invariant. Data

from cytology and molecular genetics have

strongly supported the classiﬁ cation based on

morphological criteria. The reliance on coc-

coliths in the extant classiﬁ cation also means

that there are relatively few problems in align-

Box 9.10 Atomic force microscopy of coccolithophores

Coccolithophores, despite their small size, are attractive and sophisticated organisms. A number of

plate morphs, emphasizing the diversity of form within the group, have been described (Fig. 9.18c):

asterolith, star-shaped plates; cyclolith, open rings; lopadolith, vase-shaped morphs with elevated

edges; placolith, two disks fused by the median tube; stetolith, column-shaped plates; zygolith, ellipti-

cal ring with arches applied to holococcoliths. Apart from the term placolith most are not in routine

use. Additionally, helioliths, composed of a large number of small radially arranged crystals, and

ortholiths, with only a few crystals, have been recognized.

The coccolithophore is precipitated within the cell from the coccolith vesicle or Golgi body with

tightly regulated crystal growth, allowing the crystals to integrate as the complex and exquisite net-

works that comprise a complete skeleton. Karen Henriksen, a former graduate student at the Uni-

versity of Copenhagen, applied atomic force microscopy (AFM) to the surface of three coccolith

species, a technique that allows investigation at higher orders of magnitude than even scanning

electron microscopy (SEM) and transmission electron microscopy (TEM) equipment. Henriksen and

colleagues (2004) established key differences among these taxa suggesting that subtle changes in the

mechanisms of biomineralization can drive signiﬁ cant changes in morphology that have knock-on

effects for the adaptability, lifestyle and distribution of the coccolith species. The large morphological

disparity seen in this remarkable group is thus a function of the mode and orientation of crystal

growth at the atomic level and where the organism ultimately lived depended on the whims of a

crystal lattice.