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418 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD

for capturing most food in the shortest time

from a given water volume.

Ecology: modes of life and feeding strategy

There is little doubt that the earliest bush-like

dendroids were attached to the seabed and

functioned as part of the sessile benthos.

Detachment in various benthic genera occurred

at the beginning of the Ordovician with genera

such as Rhabdinopora entering the plankton.

More controversial is the mode of life of the

various graptoloid groups (Fig. 15.26). Con-

ventionally the graptoloids were considered

to be passive drifters, their ﬂ otation being

aided by fat and gas bubbles in their tissues

or even by vane-like extensions to the nema.

But they clearly occupied different levels in

the water column (Underwood 1994).

The suggestion by Nancy Kirk (1969) that

far from being passive members of the plank-

ton, the graptolites were automobile, moving

up and down in the water column, has stimu-

lated considerable and continued interest and

research on the life habits of these extinct

organisms. During intervals of intense feeding,

a reactive upward movement of the colony in

the water column would have occurred. At

night the colony could move vertically into

the nutrient-rich photic zone and later, when

replete, the rhabdosomes would sink to posi-

tions in the water column where the speciﬁ c

(b)

(c)

(a)

Figure 15.25 Graptolite ultrastructure: (a) collage of Geniculograptus rhabdosome showing banded

fusellar tissue (×50); (b) detailed section through part of a rhabdosome showing relatively thin, parallel

sheet fabric (top) and criss-cross fusellar fabric (below) (×1000); and (c) detail of aperture exterior of

Geniculograptus showing the development of bandages (×500). (Courtesy of Denis Bates.)

DEUTEROSTOMES: ECHINODERMS AND HEMICHORDATES 419

gravity of the colony matched that of the sur-

rounding seawater.

Computer models and physical models,

including exposure to wind-tunnel conditions

that mimic the effects of water currents, have

emphasized the importance of harvesting

strategies for the success of the colony. These

probably exerted an important inﬂ uence on

the evolutionary pathways that the grapto-

loids followed.

Evolution: graptolite stipes and thecae

Graptolite evolution has been described in

terms of four main stages of morphological

development:

1 The transition from sessile to planktonic

strategies in the dendroids during the Late

Cambrian and Early Ordovician.

2 At the end of the Tremadocian (early Early

Ordovician), the appearance of the single-

type thecae of the graptoloids.

3 The development of the biserial rhabdo-

some in the Floian (late Early

Ordoviaian).

4 Finally, the origin of the uniserial

monograptids.

The small, stick-like benthic organisms

reported from Middle Cambrian rocks on

the Siberian platform and ascribed to the

graptolites may be better assigned to the

321

Figure 15.26 Graptolite life modes: 1, conical forms with spiral motion; 2, ﬂ at or slightly conical forms

with slow, slightly spiral velocities; 3, mono- or biramous forms with spiral movement due to

asymmetry; 4, forms with high angles between stipes having linear movement; 5, straight forms with

mainly linear descent. (Based on Underwood 1994.)

420 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD

rhabdopleurids. The ﬁ rst undoubted grapto-

lites include the dendroids Callograptus, Den-

drograptus and Dictyonema occurring in

Middle Cambrian rocks of North America.

But by the Late Cambrian, the diversity of the

dendroid fauna had markedly increased. The

fauna included genera such as Aspidograptus

and Dictyonema, which resembled small

shrubs and were attached to the substrate by

holdfasts or more complex root-like struc-

tures. During the Late Cambrian and Early

Ordovician, some dendroids made the jump

from the sessile benthos to the plankton;

attachment disks continuous with the nema

suggest these genera may have hung suspended

in the surface waters and pursued an epi-

planktonic life strategy. Both Radiograptus

and Dictyonema have been cited as possible

ancestors for the planktonic graptolites, and

perhaps Staurograptus was in fact the ﬁ rst

planktonic graptolite. The Tremadocian seas

witnessed the radiation of the anisograptids.

The explosion of dichograptid genera

during the Floian introduced a variety of sym-

metric graptolites with from about eight to

two stipes oriented in declined, pendent and

scandent attitudes (Fig. 15.27). A twin-stiped

dichograptid was probably ancestral to the

next wave of graptolites, the diplograptids,

which radiated in the Mid Darriwilian (Middle

Ordovician).

The single-stiped monograptids dominated

Silurian graptolite faunas and, despite their

apparent simplicity, the group developed a

huge variety of forms (Fig. 15.28). The last

graptolites, species of Monograptus, disap-

Climacograptus

Dicellograptus

Leptograptus

Didymograptus extensus

Didymograptus 'bifidus'

Didymograptus deflexus

scandent

reclined

declined

horizontal

pendent

Figure 15.27 Evolution of stipes.

DEUTEROSTOMES: ECHINODERMS AND HEMICHORDATES 421

peared during the Early Devonian (Pragian)

in China, Eurasia and North America. Nev-

ertheless this uniserial morphology had sur-

vived for over 30 million years and may have

continued after the Early Devonian in lineages

that did not secrete a preservable skeleton.

Why should a trend towards a reduction in

stipes be such an advantage? Perhaps the

simpler stipe conﬁ guration was hydrodynami-

cally more stable, better adapted to turbu-

lence and aided the motion of the graptolites

through the water column on feeding

forays. It may also have prevented the inter-

ference between thecae on adjacent stipes,

providing a simpler, more efﬁ cient colony

structure.

Graptolite morphology and stratigraphy

have formed the basis for the deﬁ nition of

evolutionary faunas within Ordovician assem-

blages that include anisograptid, dichograptid

Leptograptus

(M. triangulatus)

(Cucullograptus)

hooked lobatetriangulate

with lappets

isolate

(Rastrites)(M. lobiferus)(M. priodon)

GlyptograptusDicranograptusClimacograptus

Figure 15.28 Evolution of thecae. M, Monograptus.

Ordovician

Number of genera

Dichograptid

fauna

I II III

Anisograptid

fauna

Diplograptid

fauna

Middle Ordovician

Upper Ordovician

Lower Ordovician

1a 1b 1c 1d 2a 2b 2c 3a 4a3b 4b 5b5a 6a 6b 6c4c 5c 5d

Figure 15.29 Graptolite biostratigraphy and graptolite evolutionary faunas. I–III indicate the three

main radiations: anisograptid, dichograptid and diplograptid; 1a–6c represent 19 time slices through

the Ordovician Period. (Based on Chen et al. 2006.)

422 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD

and diplograptid evolutionary faunas (Chen

et al. 2006). These faunas not only help us

understand better the Ordovician radiation

of the graptolites but add an additional

dimension to the zonal framework of the

Ordovician System.

Biostratigraphy: graptolites and time

Graptolites are among the best zone fossils

(see p. 28) and are excellent for biostrati-

graphic correlation. Traditionally, four

sequential graptolite faunas have been recog-

nized through the Early Ordovician to Early

Devonian interval (Fig. 15.29). The anisograp-

tid fauna, with Rhabdinopora and allied

genera, characterizes the Tremadocian;

although Upper Tremadocian graptolite

faunas are rare, the genera Bryograptus,

Kiaerograptus and Aorograptus are impor-

tant and have been described in detail from

western Newfoundland (Williams & Stevens

1991). The appearance of the Floian dichograp-

tid fauna is signaled by Tetragraptus, associ-

ated with didymograptids and some relict

anisograptids. The later diplograptid

fauna contains four smaller units: the

Glyptograptus-Amplexograptus (Darriwil-

ian), Nemagraptus-Dicellograptus (Sand-

bian), Orthograptus-Dicellograptus (Katian)

and Orthograptus-Climacograptus (Hirnan-

tian) subfaunas. The monograptid fauna con-

tains a variety of evolving single-stiped forms.

The last graptoloids disappeared in the

Pragian (Lower Devonian).

In some parts of the world graptolites have

provided the basis for some high-resolution

stratigraphy. The Upper Ordovician–Lower

Silurian clastic succession in the Barrandian

basin in the Czech Republic was deposited on

an outer shelf, inﬂ uenced by the end-Ordovi-

cian Gondwanan glaciation and an aftermath

that included a persistent post-glacial anoxia

related to upwelling systems. High-resolution

graptolite stratigraphy based on some 19

biozones has provided a framework to link

sedimentary environments with graptoloid

faunal dynamics and ﬂ uctuations in organic

content (S

torch 2006). The resulting analyses

have provided an accurate time line through

four major transgressive cycles. On these are

superimposed glacial and interglacial events,

intervals of upwelling and oceanic perturba-

tions (Fig. 15.30). Moreover these new data

suggest that, far from being a quiet period,

the Silurian was punctuated by a number of

signiﬁ cant extinction events associated with

large climatic and environmental ﬂ uctuations.

Critical is the accurate correlation between

sections in the Barrandian basin with sections

elsewhere, otherwise we cannot show these

changes were indeed global. Higher in the

Silurian, Lennart Jeppsson and Mikel Calner

(2003) have reported that the Mulde Secundo-

Secundo Event (Wenlock), ﬁ rst identiﬁ ed in

the Silurian platform carbonates of the

Swedish island of Gotland, includes three

extinctions, widespread deposition of carbon-

rich sediments, and wild sea-level ﬂ uctuations

together with a glaciation event. These extinc-

tions are related to a severe reduction in

primary plankton productivity. Amazingly,

such precision and the recognition of these

important events was only made possible by

the accurate graphic correlations of sections

based on the rapid evolution of these small,

beautiful creatures.

Biogeography: graptolites in space

Since the majority of graptolites lived either in

the water column or within the plankton, quite

different factors inﬂ uenced their distribution

in contrast to, say, that of the coeval benthos.

Provinciality was most marked in the earlier

Ordovician (Darriwilian) when two main

provinces, the Atlantic and Paciﬁ c, were recog-

nized. The Atlantic province, including the

then high-latitude regions of Avalonia and

Gondwana, was characterized by pendent

Didymograptus species. The Paciﬁ c province,

including low latitude, tropical regions such as

the Laurentian margins, was more diverse with

isograptids, cardiograptids and oncograptids.

The isograptid biofacies, itself, was more pan-

demic, occupying deeper water and associated

with the world’s continental margins. During

the end-Ordovician extinction events (see p.

169), the Paciﬁ c province graptolites suffered

particularly badly, and although graptolites

again diversiﬁ ed during the Silurian their pro-

vinciality developed a different if less obvious

pattern. During the Early Silurian, the Gond-

wanan province was characterized by endemic

taxa while later, in the Mid Silurian, the

equatorial region hosted taxa not known from

elsewhere.

DEUTEROSTOMES: ECHINODERMS AND HEMICHORDATES 423

shale, mudstone

black shale

sandy/ silty-micaceous

laminite

storm sandstones,

siltstones and shale

interbeds

graded sandstone

and conglomerate

glaciomarine

diamictite

limestone nodules

and lenticular beds

alternating pale mudstone

and black shale

pre-Hirnantian shelly

fauna

Laticeps

Persculptus

shelly

Hirnantia

fauna

Cyphus

Vesiculosus

Acuminatus

Ascensus

Sedgwickii

Convolutus

Leptotheca

Simulans

Triangulatus-Pectinatus

atavus

richteri

paradenticul.

tenuis

hispanicus

palmeus

grandis

Centrifugus

Insectus

Lapworthi

Spiralis

Tullbergi

Griestoniensis

Crispus

Turriculatus

Linnaei

Upwelling

possibly

weakened

HST

TST

(LST)

HST

TST

LST

HST

TST

HST

TST

LST

(LST)

UNIT 4

UNIT 3UNIT 1

UNIT 2

? sb

Oceanic

perturbations

Maximum

upwelling

Anoxic

turnover

deglaciation

2nd glacial

maximum

interglacial

1st glacial

maximum

Glaciation

Climatically

warm

period

Graptolite

biozones and subzones

Lithology

and gaps in

sedimen-

tation

Relative

sea level

fall rise

Sequence

-stratigraphic

units and

system tracts

Environmental

features

Formations

Systems

series

stages

Silurian

Llandovery

TelychianAeronianRhuddanian

Ordovician

Hirnantian

Ojsuensis

KR. DVUR

KOSOV

ŽELKOVICE

LITOHLAVY

MOTOL

Figure 15.30 Graptolite biostratigraphy of the Upper Ordovician–Lower Silurian strata of the

Barrandian basin. HST, highstand systems tract; TST, transgressive systems tract; LST, lowstand systems

tract. (Based on S

torch 2006.)

424 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD

Box 15.10 The Vetulicolians: protostomes, deuterostomes or phylogenetic

orphans?

Sometimes the fossil record throws out a weird animal that it is just impossible to classify. The

material may be common, distinctive and well preserved but there are simply not enough key

characters to link it with other groups. The vetulicolians have been characterized as unusual

arthropods, stem-group deuterostomes and even tunicates (Aldridge et al. 2007). They have been

reported from a number of Cambrian Lagerstätten and two classes have been recognized, the

Vetulicolida and Banffozoa. They were probably active, nektobenthic animals with the facility to

both deposit and ﬁ lter feed. But what were they? In simple terms they lack limbs, making

assignment to the arthropods difﬁ cult, whereas they have gills similar to those of the deuterostomes

(Fig. 15.31). If they were, in fact, deuterostomes they probably lay close to the tunicates as stem

vertebrates. But despite well-preserved material from the Chengjiang fauna and careful phylogenetic

analyses, it remains impossible to classify them. Their unique combination of characters is thus still

an enigma awaiting the discovery of new animals that could link the vetulicolians to a crown

group.

(a)

(b)

Figure 15.31 (a) Photograph (scale bar, 5 mm) and (b) reconstruction of Vetulicola. (Courtesy of

Dick Aldridge.)

DEUTEROSTOMES: ECHINODERMS AND HEMICHORDATES 425

Review questions

1 The deuterostomes include two appar-

ently morphologically different groups,

the echinoderms and hemichordates. What

sort of characters could be used to unite

them?

2 Crinoids are most common in deep-water

environments but probably exploited

much shallower-water environments

during the Paleozoic. When and why did

they move to deeper water?

3 Echinoids have a long history. Why did it

take over 250 myr to develop the buried

(sand dollars) and burrowing (sea urchin)

life strategies?

4 Graptolites evolved through time by reduc-

ing their numbers of stipes and developing

more complex thecae. What were the eco-

logical advantages of this more stream-

lined body plan with more elaborate zooid

openings?

5 The vetulicolians highlight one of the dif-

ﬁ culties of the fossil record, identifying

deﬁ nitive characters of phylogentic signiﬁ -

cance in bizarre taxa. Should new higher

taxa, for example classes of phyla, be

established to accommodate such material

or should it be shoehorned into existing

taxa?

Further reading

Berry, W.B.N. 1987. Phylum Hemichordata (including

Graptolithina). In Boardman, R.S., Cheetham, A.H.

& Rowell, A.J. (eds) Fossil Invertebrates. Blackwell

Scientiﬁ c Publications, Oxford, pp. 612–35. (A com-

prehensive, more advanced text with emphasis on

taxonomy; well illustrated.)

Clarkson, E.N.K. 1998. Invertebrate Palaeontology and

Evolution, 4th edn. Chapman and Hall, London.

(An excellent, more advanced text; clearly written

and well illustrated.)

Rickards, R.B. 1985. Graptolithina. In Murray, J.W.

(ed.) Atlas of Invertebrate Macrofossils. Longman,

Harlow, Essex, pp. 191–8. (A useful, mainly photo-

graphic review of the group.)

Smith, A.B. & Murray, J.W. 1985. Echinodermata. In

Murray, J.W. (ed.) Atlas of Invertebrate Macrofos-

sils. Longman, Harlow, Essex, pp. 153–90. (A useful,

mainly photographic review of the group.)

Sprinkle, J. & Kier, P.M. 1987. Phylum Echinodermata.

In Boardman, R.S., Cheetham, A.H. & Rowell, A.J.

(eds) Fossil Invertebrates. Blackwell Scientiﬁ c Publi-

cations, Oxford, pp. 550–611. (A comprehensive,

more advanced text with emphasis on taxonomy;

extravagantly illustrated.)

References

Aldridge, R.J., Hou Xian-Guang, Siveter, D.J., Siveter,

D.J. & Gabbott, S.E. 2007. The systematics and

phylogenetic relationships of vetulicolians. Palaeon-

tology 50, 131–68.

Bottjer, D.J., Hagadorn, J.W. & Dornbos, S.Q. 2000.

The Cambrian substrate revolution. GSA Today 10,

1–7.

Chen Xu, Zhang Yuan-Dong & Fan Jun-Xuan. 2006.

Ordovician graptolite evolutionary radiation: a

review. Geological Journal 41, 289–301.

Clausen, S. & Smith, A.B. 2005. Palaeoanatomy and

biological afﬁ nities of a Cambrian deuterostome

(Stylophora). Nature 438, 351–4.

Delsuc, F., Brinkmann, H., Chourrout, D. & Philippe,

H. 2006. Tunicates and not cephalochordates are the

closest living relatives of vertebrates. Nature 444,

85–8.

Donovan, S.K. & Gale, A.S. 1990. Predatory asteroids

and the decline of the articulate brachiopod. Lethaia

23, 77–86.

Gupta, N.S., Briggs, D.E.G. & Pancost, R.D. 2006.

Molecular taphonomy of graptolites. Journal of the

Geological Society, London 163, 897–900.

Jefferies, R.P.S. 1986. The Ancestry of the Vertebrates.

British Museum (Natural History), London.

Jefferies, R.P.S. & Daley, P. 1996. In Harper, D.A.T. &

Owen, A.W. (eds) Fossils of the Upper Ordovician.

Field Guide to Fossils No. 7. Palaeontological Asso-

ciation, London.

Jeppsson, L. & Calner, M. 2003. The Silurian Mulde

event and a scenario for secundo-secundo events.

Transactions of the Royal Society of Edinburgh:

Earth Sciences 93, 135–54.

Kammer, T.W. & Ausich, W.I. 2006. The “Age of

crinoids”: a Mississippian biodiversity spike coinci-

dent with widespread carbonate ramps. Palaios 21,

238–48.

Kirk, N. 1969. Some thoughts on the ecology, mode of

life, and evolution of the Graptolithina. Proceedings

of the Geological Society of London 1659,

273–93.

Kozl

owska-Dawidziuk, A. 2004. Evolution of retiolitid

graptolites – a synopsis. Acta Palaeontologica

Polonica 49, 505–18.

Paul, C.R.C. & Smith, A.B. 1984. The early radiation

and phylogeny of echinoderms. Biological Reviews

59, 443–81.

Rickards, R.B. & Durman, P.N. 2006. Evolution of the

earliest graptolites and other hemichordates. In

Bassett, M.G. & Diesler, V.K. (eds) Studies in Pal-

aeozoic Palaeontology

. Geological Series No. 25.

National Museum of W

ales, Cardiff, pp. 5–92.

Ruta, M. 1999. Brief review of the stylophoran debate.

Evolution and Development

1, 123–35.

426 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD

Smith, A.B. 1984. Echinoid Palaeobiology. Special

Topics in Palaeontology No. 1. George Allen and

Unwin, London.

Smith, A.B. 2005. The pre-radial history of the echino-

derms. Geological Journal 40, 255–80.

Smith, A.B. & Murray, J.W. 1985. Echinodermata. In

Murray, J.W. (ed.) Atlas of Invertebrate Macrofos-

sils. Longman, Harlow, Essex, pp. 153–90.

Smith, A.B. & Stockley, B. 2005. The geological history

of deep-sea colonization by echinoids: roles of surface

productivity and deep-water ventilation. Proceedings

of the Royal Society B 272, 865–9.

Sprinkle, J. 1980. Echinoderms: Notes for a short

course. Studies in Geology No. 3. University of

Tennessee.

torch, P. 2006. Facies development, depositional set-

tings and sequence stratigraphy across the Ordovi-

cian-Silurian boundary: a new perspective from the

Barrandian area of the Czech Republic. Geological

Journal 41, 163–92.

Sutcliffe, O.E., Südkamp, W.H. & Jefferies, R.P.S. 2000.

Ichnological evidence on the behaviour of mitrates:

two trails associated with the Devonian mitrate Rhe-

nocystis. Lethaia 33, 1–12.

Swan, A.H.R. 1990. A computer simulation of evolu-

tion by natural selection. Journal of the Geological

Society, London 147, 223–8.

Underwood, C.J. 1994. The position of graptolites

within Lower Paleozoic planktic ecosystems. Lethaia

26, 198–202.

Williams, S.H. & Stevens, R.K. 1991. Late Tremadoc

graptolites from western Newfoundland. Palaeontol-

ogy 34, 1–47.

Winchell, C.J., Sullivan, J., Cameron, C.B., Swalla, B.J.

& Mallatt, J. 2002 Evaluating hypotheses of deu-

terostome phylogeny and chordate evolution with

new LSU and SSU ribosomal DNA data. Molecular

Biology and Evolution 19, 762–76.

Chapter 16

Fishes and basal tetrapods

Key points

• Vertebrates are characterized by a skeleton made from bone (apatite).

• The oldest vertebrates are small ﬁ sh-like creatures from the Early Cambrian of China.

• Armored ﬁ shes were abundant in Devonian seas and lakes.

• After the Devonian, the cartilaginous and bony ﬁ shes radiated in several phases.

• Conodonts commonly occur as tooth-like elements that are useful in biostratigraphy,

as are some other ﬁ sh teeth and scales (ichthyoliths).

• Tetrapods arose during the Devonian from lobe-ﬁ nned ﬁ sh ancestors, and ﬁ sh-eating

amphibians diversiﬁ ed in the Carboniferous.

• The ﬁ rst reptiles were small insect eaters.

• Synapsids dominated ecosystems on land during the Permian and Triassic.

• These groups were heavily hit by the end-Permian mass extinction event, and diapsid

reptiles, most notably the dinosaurs, were key forms through the Mesozoic.

Most species do their own evolving, making it up as they go along, which is the way

Nature intended. And this is all very natural and organic and in tune with mysterious

cycles of the cosmos, which believes that there’s nothing like millions of years of really

frustrating trial and error to give a species moral ﬁ ber and, in some cases,

backbone.

Terry Pratchett (1991) Reaper Man