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

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398 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD

wide range of strategies from a ﬁ xed sessile

mode to free-living recumbent styles. The dip-

loporites were very widespread from the Early

Ordovician to the Early Devonian and prob-

ably evolved from a Late Cambrian blasto-

zoan ancestor.

Rhombifera The rhombiferans appeared

during the Late Cambrian equipped with bra-

chioles and distinctive rhombic patterns of

respiratory pores crossing thecal plate sutures

(Fig. 15.7). They are classiﬁ ed according to

the pattern and shape of their pores; these

separate the order Dichoporita from the Fis-

tulipora. The rhombiferans became common

during the Early Ordovician and continued

with a near cosmopolitan distribution until

the Late Devonian, and were probably

replaced by the better adapted blastoids during

the Silurian and Devonian.

Blastoids

The extinct blastoids were small, pentamer-

ally symmetric animals with short stems

and hydrospires adapted for respiration (Fig.

15.8). They are represented by over 80 genera

in rocks of Silurian to Permian age. The

blastoid cup or theca is usually globular and

composed of a ring of three basal plates, sur-

mounted by a circle of ﬁ ve larger radial plates.

The mouth is often surrounded by ﬁ ve large

openings or spiracles associated with the

respiratory system. Although relatively rare,

Box 15.4 The age of crinoids: an Early Carboniferous diversity spike

Early Carboniferous (Mississippian) crinoids were abundant and diverse, so much so that this inter-

val is often called the “Age of crinoids”. Limestones of this age often consist of over 50% pelmato-

zoan debris, and are known as encrinites. Why then were crinoids so abundant at this time? Two

factors seem to have contributed to these extensive shoals of crinoids (Kammer & Ausich 2006).

Firstly, ﬁ ve major groups were in various states of recovery after the Frasnian-Famennian extinction

event, particularly the advanced cladids (Fig. 15.6). Secondly, with the disappearance of the shelf-

edge coral–stromatoporoid buildups at the end of the Devonian, platform geometries were quite

different. There was improved and unimpeded water circulation, which promoted stenohaline condi-

tions that encouraged the growth of crinoid communities. With new ecospace and a lack of predation

pressures, crinoid diversity exploded. Sadly, the good times came to an end with regression and the

cooler-water conditions associated with the Late Carboniferous glaciation. Crinoids were never again

so diverse.

120

160

2005

2002

Loch

Prag

Emsi

Eife

Give

Fras

Fame

Tour

Vise

Serp

Bash

Mosc

Step

200

Number of genera

Figure 15.6 Diversity of Early Carboniferous crinoids. (From Kammer & Ausich 2006.)

DEUTEROSTOMES: ECHINODERMS AND HEMICHORDATES 399

a few horizons are packed with blastoids, par-

ticularly when the diversity of the group

peaked in the Early Carboniferous. Viséan

reefal facies in northern England yield abun-

dant blastoids, as do the Permian limestones

on the island of Timor where, for example,

Timoroblastus and Schizoblastus occur.

The blastoids ﬁ rst appeared during the

Silurian, probably evolving from an Ordovi-

cian ancestor with brachioles and a reduced

number of plates. They initially competed,

ecologically, with the rhombiferan cystoids.

The evolutionary history of the group was

marked by changes in the shape of the theca

and variations in the length of the ambulacra.

Two main groups are recognized: the more

basal Fissiculata characterized by hydrospire

folds, and the Spiraculata with, as the name

suggests, well-developed spiracles.

Eocrinoids

The eocrinoids were the earliest of the brachi-

ole-bearing echinoderms. They had a huge

range of thecal shapes with primitive hold-

fasts and an irregular to regular arrangement

of plates (Fig. 15.9a). Sutural pores rather

than thecal pores, along the joins between the

plates, were characteristic of the earliest eocri-

noids; in others there is a total lack of respira-

tory structures. Eocrinoids differ from the

crinoid groups in having biserial brachial

Echinosphaerites

rhombiferan

Sphaeronites

diploporite

Haplosphaeronis

diploporite

Pleurocystites

rhombiferan

Figure 15.7 Some Ordovician cystoid genera:

Echinosphaerites and Sphaeronites, (×0.75),

Haplosphaeronis and Pleurocystites (×1.5).

(Based on Treatise on Invertebrate Paleontology,

Part S. Geol. Soc. Am. and Univ. Kansas Press.)

lateral view aboral view

lateral view lateral vieworal view

oral view

Timoroblastus

Pentremites Schizoblastus

Permian

Carboniferous

Carboniferous-Permian

Figure 15.8 Some blastoid genera. Magniﬁ cation

×0.6 for all. (Redrawn from various sources.)

(a) (b)

Figure 15.9 (a) An eocrinoid, and (b) a

paracrinoid. (Based on Treatise on Invertebrate

Paleontology, Part S. Geol. Soc. Am. and Univ.

Kansas Press.)

400 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD

appendages. Over 30 genera have been

described from rocks of Early Cambrian to

Late Silurian age. The origins of the other

blastozoan classes are probably to be found

within this heterogenous group; for example

the aberrant Late Cambrian eocrinoid Cam-

brocrinus has been cited as an ancestor for the

rhombiferan cystoids. Whereas many eocri-

noids were high-level suspension feeders with

the ﬁ rst columnal-constructed stems, some lay

reclined or recumbent on the seabed. The

Ordovician Cryptocrinus, for example, has a

globular theca with a more irregular arrange-

ment of plates.

Paracrinoids

The paracrinoids (Fig. 15.9b) are a small, odd

group of arm-bearing echinoderms that have

globular thecae and numerous irregularly-

arranged plates together with two to ﬁ ve arm-

like, food-gathering structures. They are so

different that some scientists have suggested

that they represent a separate subphylum.

The group is restricted to North America,

where they are common in the Middle

Ordovician.

Echinoidea

Echinoids, the well-known sea urchins and

sand dollars, have robust, rigid endoskeletons,

or tests, composed of plates of calcite coated

by an outer skin covered by spines. The tests

are usually either globular or discoidal to

heart-shaped (Smith 1984). Echinoids are

most common in shallow-water marine envi-

ronments where they congregate in groups as

part of the nektobenthos. Their classiﬁ cation

(Box 15.5) is based on the arrangement of

plates and their mouth structures.

Echinoids have a long history from their

ﬁ rst radiation in the Ordovician (Paul &

Smith 1984). Two of the most signiﬁ cant evo-

lutionary events in the history of the subphy-

lum were marked by sudden divergences from

the regular morphology to generate irregular

burrowing echinoids. The ﬁ rst, in the Jurassic,

led subsequently to a range of irregular bur-

rowers, and the second, during the Paleocene,

to the quasi-infaunal sand dollars. Both events

were probably rapid and permitted major

adaptive radiations of parts of the group into

new ecological niches.

Basic morphology

The exoskeleton or test of most regular echi-

noids, for example the common sea urchin

Echinus esculentus, is hemispherical and dis-

plays all the main features of the group (Fig.

15.11). The lower, adapical or oral, surface is

perforated by the mouth whereas the upper,

apical or aboral, surface has the anal opening.

The sea urchin is part of the active mobile

benthos, in contrast to the sand dollars which

were quasi-infaunal.

The test is built of a network of many hun-

dreds of interlocking calcite plates organized

into 10 segments, radiating from the oral

surface and converging on the aboral surface.

Five narrower segments or ambulacral areas

(ambs) carry the animal’s tube feet and are in

contact with the ocular plates. The ambs

alternate with the wider interambulacral areas

(interambs), are armed with spines and abut

against the genital plates. Together the ambs

and interambs comprise in total 10 areas and

20 columns, which make up the corona – the

majority of the test.

The central part of the aboral surface has

a ring of ﬁ ve genital plates, each perforated

by a hole to allow the release of gametes; the

madreporite is commonly larger than the

other genital plates and has numerous minute

pores interfacing, beneath, with the water

vascular system. These alternate with the

ocular plates, terminating the ambulacral

areas, and each houses further outlet holes for

the water vascular system. This part of the

apical system surrounds the periproct, or anal

opening, which is partially covered by a

number of smaller plates attached to a mem-

brane. On the underside of the test, the peri-

stome, containing the mouthparts, is also

covered by a membrane coated with small

plates. The mouth holds a relatively sophisti-

cated jaw apparatus comprising ﬁ ve individ-

ual jaws each with a single, curved, saber-like

tooth, operating like a mechanical grab and

forcing particles into the animal’s digestive

system. The great ancient Greek naturalist

Aristotle, who described the structure ﬁ rst,

compared it to a “horn lantern with the panes

of horn left out”, and the echinoid jaw is

often called Aristotle’s lantern. In crown-

group forms, muscles attached to the lantern

are anchored to the perignathic girdle, devel-

oped around the edge of the peristome.

DEUTEROSTOMES: ECHINODERMS AND HEMICHORDATES 401

Box 15.5 Echinoid classiﬁ cation

The traditional split of the class into regular and irregular forms is no longer considered to reﬂ ect

the true phylogeny of the echinoids. Whereas the irregular echinoids are probably monophyletic,

arising only once, the regular echinoids do not form a clade. The group was traditionally subdivided

into three subclasses (Fig. 15.10) – the Perischoechinoidea, Cidaroidea and Euechinoidea – the ﬁ rst,

however, has been shown to be polyphyletic and the term stem-group echinoids is preferred.

Stem-group ECHINOIDEA

• Regulars with ambulacra in more than two columns, interambulacra with many columns; in total

the test is composed of over 20 columns. Lantern with simple grooved teeth and lacking a peri-

gnathic girdle

• Upper Ordovician to Permian

Crown-group ECHINOIDEA

Subclass CIDAROIDEA

• Regulars with test consisting of 20 columns of plates; two columns in each ambulacra and inter-

ambulacral areas. Interambulacral plates have large tubercle. Teeth are crescentic to U-shaped

and the perignathic girdle includes only interambulacral elements

• Lower Permian to Recent

Subclass EUECHINOIDEA

• Post-Paleozoic taxa, both regular and irregular. Both ambulacra and interambulacra with twin

columns. Perignathic girdle composed on ambulacral projections

• Middle Triassic to Recent

crown-group echinoids

Euechinoidea

Acroechinoidea

876

Perischoechinoidea

Cidaroidea

Echinothuriacea

Diadematacea

Irregularia

Echinacea

Figure 15.10 Echinoid classiﬁ cation based mainly on cladistic analysis: 1, 10 ambulacral and 10

interambulacral areas; 2, upright lantern without foramen magnum; 3, distinctive perignathic

girdle; 4, distinctive ambulacral areas; 5, upright lantern with deep foramen magnum; 6, grooved

teeth; 7, stout teeth; 8, keeled teeth.

402 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD

Key

(a)

(b) (c)

DORSAL

VENTRAL

perignathic girdle

lantern protractor/retractor

muscles

tooth

large intestine

small intestine

axial vessel

stone canal

circumoral ring

esophagus

radial water vessel

ampulla

ocular pore

periproct

madreporite

gonopore

gonad

spine

corona

peristome

buccal tube foot

tube foot

peristome

tubercle

periproct

apical

system

ambulacral

pores

interambulacral

area

ambulacral

area

Figure 15.11 Echinoid morphology: (a) internal anatomy in cross-section; (b) dorsal and (c) ventral

views of Echinus. (Based on Smith 1984.)

The echinoid’s various organs are sus-

pended within the test and supported by ﬂ uid.

The water vascular system copes with a

number of functions. The stone canal rises

vertically, from the central ring around the

esophagus, to unite with the madreporite.

Five radial water vessels depart from the

central ring to service the ambulacral areas;

smaller vessels are attached to each tube foot

and its ampulla where variations in water

pressure drive the animal’s locomotory

system.

The echinoid digestive system lacks a

stomach and operates through the esophagus

together with a large and small intestine;

waste material is expelled through the rectum

into the anus and externally by way of the

periproct. An unsophisticated nervous system

comprising a nerve ring and ﬁ ve radial nerves

connects with the ambulacral pores where the

DEUTEROSTOMES: ECHINODERMS AND HEMICHORDATES 403

nerve ends divide externally to form a sensory

net.

The regular to irregular transition

Regular echinoids, the sea urchins, with a

compact, symmetric morphology are most

basal. Their shape contrasts with the irregu-

lars, with marked bilateral symmetries and

specialized for forward movement. The irregu-

lar echinoid morphotype evolved rapidly and

apparently involved some large architectural

changes to adapt the animal to burrowing.

Plesiechinus hawkinsi is one of the ﬁ rst irregu-

lar echinoids, appearing early in the Early

Jurassic (Sinemurian) with an asymmetric test,

short numerous spines, large adapical pores

and a posteriorly placed periproct together

with presumed keeled teeth. Ten million years

later, by the Toarcian, much of the “toolkit” of

adaptations had evolved for a burrowing life

mode. Secondary bilateral symmetry was

superimposed on the existing pentameral sym-

metry to form a heart-shaped or ﬂ attened ellip-

soidal test. The periproct migrated from a

position on the apical surface to the posterior

side of the test to eject waste laterally. By the

Early Cretaceous, one of the ambulacral areas

had become modiﬁ ed to form a food groove

and a series of tube feet were extendable with

ﬂ attened ends to assist respiration.

One of the earliest sand dollars, the

clypeasteroid Togocyamus, appeared during

the Paleocene, and some 20 million years later

in the Eocene more typical sand dollars had

evolved to command a cosmopolitan distribu-

tion. The ﬂ attened test was adapted for bur-

rowing, whereas the accessory tube feet could

encourage food along the food grooves and

draped the test with sand. The highly accentu-

ated petals helped respiration by providing an

increased surface area for the tube feet, and

the development of a low lantern with hori-

zontal teeth signaled changes in feeding

patterns.

Ecology: modes of life

The regular Echinus and the irregular Echino-

cardium probably mark the ends of a spectrum

of life modes from epifaunal mobile behavior

to a number of infaunal burrowing strategies

(Fig. 15.12). Mobile regular forms such as

Echinus grazed on both hard and soft sub-

strates and in caves and crevices on the sea-

ﬂ oor; these sea urchins may have been

omnivores, carnivores or herbivores. Irregular

forms display a range of adaptations appropri-

ate to an infaunal mode of life where burrows

were carefully constructed in low-energy envi-

ronments. Extreme morphologies were devel-

oped in the sand dollars or Clypeasteroidea,

permitting rapid burial just below the sedi-

ment–water interface in shifting sands. Echi-

noids generally lived in shallow seawaters, but

some went deeper; the timing of this move off-

shore has been controversial (Box 15.6).

Life modes and evolution: microevolution of

Micraster One of the classic case studies of

evolutionary patterns in fossils is seen in

Micraster, an infaunal, irregular echinoid.

Paleobiologists have repeatedly used this

example to test phyletic gradualist and punctu-

ated equilibria models (see p. 121) and as the

raw material for the rigorous statistical analy-

sis of both ontogenetic and phylogenetic

change. In the best-known lineage, M. leskei–

M. decipiens–M. coranguinum, the following

morphological changes occurred (Fig. 15.14):

1 The development of a higher, broader

(heart-shaped) form associated with an

increase in size and thickness of the test.

2 The peristome (mouth) moved anteriorly

and the posteriorly situated periproct

(anus) had a lower position on the side of

the test with a broader subanal fasciole.

3 The madreporite increased in size at the

expense of the adjacent specialized plates.

4 More tuberculate and deeper anterior

ambulacra evolved.

5 More granulated periplastronal areas

developed.

These morphological changes are associated

with life in progressively deeper burrows. But

there is a lack of associated trace fossils that

might prove this. On the other hand, the

adaptations may have been geared to greater

burrowing efﬁ ciency, probably in shallow

depths in the chalk where such traces were

destroyed by reworking of the sediments.

Microevolutionary trends have been tested

in other echinoid lineages. The irregular Dis-

coides occurs abundantly through an Upper

Cretaceous section at Wilmington, south

Devon, England. The height and diameter of

404 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD

the echinoids change through the section, but

this seems to be related to the grain size of the

sediment, where high, narrow forms favor ﬁ ne

sediment. The case history is available at http://

www.blackwellpublishing.com/paleobiology/.

Evolution

The ﬁ rst echinoids had appeared by the Mid

Ordovician but it is only in Lower Carbonif-

erous rocks that echinoids become relatively

(a)

(b)

Hettangian

anus

sea urchins

Lower TertiaryLower Jurassic

Sinemurian

heart urchins

water

sediment

sand dollars

flattening of test

elongation of test

decrease in size of tubercles and spines

increase in number of tubercles and spines

shifting of anus posteriorly

shifting of peristome anteriorly

decrease in size of peristome

regular

echinoids

debris-covered

regular echinoids

irregular, flat, shallow,

rapidly burrowing echinoids

lagoon

10–30 m

deep-burrowing

irregular echinoids

shallow-burrowing

irregular echinoids

offshore

high-energy bar

Paleocene

Toarcian

Figure 15.12 Echinoid life modes: (a) transition from the sea urchins through the heart urchins to the

sand dollars; (b) habits and modes of life of echinoids. (a, based on Kier, P. 1982. Palaeontology 25;

b, based on Kier, P. 1982. Smithson. Contr. Paleobiol. 13.)

DEUTEROSTOMES: ECHINODERMS AND HEMICHORDATES 405

Box 15.6 Into the deep: but not until the Late Cretaceous?

A number of animal groups common in the Paleozoic evolutionary fauna, such as the brachiopods

and crinoids, are common in deep-water environments. This reinforces the view that the deep sea is

some sort of refuge for archaic taxa that have been forced down the continental slope by predation

or unsuccessful competition on the shelf. Andrew Smith and Bruce Stockley (2005) have developed

a quite different model, however, based on molecular clock estimates and the phylogeny of echinoid

taxa. Results show that the modern deep-sea omnivore fauna appeared gradually over the last 150–

200 myr; detritivores, however, were in place during a much shorter time span between 75 and

50 Ma (Fig. 15.13). This 25 myr window of seaward migration appears to be associated with marked

increases in seasonality, continental sediment discharge and surface productivity. The increased avail-

ability of organic carbon and nutrients in the deep sea provided the means for habitat expansion

rather than an escape from competition and predation on the shelf.

Paleoproductivity

(gCm

–2

–1

)

K/T

160

140

120

100

Number of deep-sea lineages

OAEs

detritivores

carnivores

200 150 100 50

Time (Ma)

Figure 15.13 Events in the deep sea: cumulative frequency polygons for maximum and minimum

times of origin of 38 clades of extant, carnivore and detritivore deep-sea echinoids (Smith &

Stockley 2005). K/T, Cretaceous–Tertiary boundary; OAEs, oceanic anoxic events.

time

Coniacian – SantonianTuronian

sea bottom

M. gibbus

M. decipiens

M. leskei

M. coranguinum

inferred depth of burrowing

Figure 15.14 Evolution of the Late Cretaceous

heart urchin, Micraster. (Based on Rose, E.P.F. &

Cross, N.E. 1994. Geol. Today 9.)

abundant. The sparse early record of the

group might reﬂ ect a relatively fragile skele-

ton that quickly disintegrated after death; on

the other hand the echinoids were probably

not a common element of the Paleozoic

benthos. The enigmatic Bothriocidaris,

described from the Ordovician of Estonia and

from southwest Scotland, has been variously

classiﬁ ed as an echinoid, cystoid or holothu-

rian. Some authorities consider that Bothrio-

cidaris and Eothuria might be unclassiﬁ able

echinoids, hopeful monsters that arose during

the rapid Ordovician radiation of the group.

406 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD

Aulechinus from the Upper Ordovician of

southwest Scotland is one of the most primi-

tive echinoids and the ﬁ rst with only two plate

columns in the ambulacral areas. During the

Paleozoic there was generally an increase in

the number and size of ambulacral areas and

the sophistication of Aristotle’s lantern,

although most genera remained relatively

small (Fig. 15.15).

There was a signiﬁ cant decline in echinoid

diversity during the Late Carboniferous. By

the Permian only half a dozen species are

known, and they belonged to two primary

groups: detritus feeders and opportunists.

Large proterocidarids were highly specialized

detritus feeders, and the small omnivorous

Miocidaris and Xenechinus were opportun-

ists. Two lineages, including Miocidaris, sur-

vived the end-Permian extinction event to

radiate extravagantly during the early Meso-

zoic, thus ensuring the survival of the echi-

noids. Following the end-Permian extinctions

the regular echinoids diversiﬁ ed during the

Late Triassic and Early Jurassic with more

advanced regulars dominating the early Meso-

zoic record. The irregulars appeared during

the Early Jurassic and substantially increased

in numbers during the period. Diversity was

severely reduced by the Cretaceous-Tertiary

extinction event but both the regulars and

irregulars recovered rapidly during the early

Cenozoic.

Asteroidea

Starﬁ sh are common on beaches today, and

their biology has made them hugely success-

ful. Some feed by preying on shellﬁ sh and

other slow-moving shore and shallow-marine

animals. Their feeding mode is unusual but

deadly: they simply sit on top of their chosen

snack, turn their stomachs inside out and

absorb the ﬂ esh of their victim. The majority

are benthic deposit feeders that ingest prey or

ﬁ lter feed. Starﬁ sh are also unusual in that

they have eyes at the ends of their arms – these

are actually light-detecting cells, not true eyes,

but the adaptation is novel nonetheless.

Asteroids appeared ﬁ rst during the Early

Ordovician. The subphylum contains two

main groups: the asteroids or starﬁ sh and the

ophiuroids or brittle stars. These animals have

a star-shaped outline with usually ﬁ ve arms

radiating outwards from the central body or

disk. The water vascular system is open. The

mouth is situated centrally on the underside

of the animal on the oral or dorsal surface

whereas the anus, if present, opens ventrally

on the adoral surface. The asterozoans are

characterized by a mobile lifestyle within the

benthos, where many are carnivores. Astero-

zoan skeletons disintegrate rapidly after death

due to feeble cohesion between the skeletal

plates. Thus, recognizable fossils are relatively

rare. Nevertheless there are a number of star-

ﬁ sh Lagerstätten deposits where asterozoans

are extremely abundant and well preserved.

Distribution and ecology of the main groups

Three classes of asterozoans have been recog-

nized: the basal Somasteroidea, the Asteroidea

or starﬁ sh and the Ophiuroidea or brittle stars.

The Somasteroidea include some of the earliest

starﬁ sh-like animals, described from the

Tremadocian of Gondwana. These echino-

derms have pentagonal-shaped bodies with the

arms initially differentiated from around the

oral surface. In some respects this short-lived

group, which probably disappeared during the

Mid Ordovician, displays primitive starﬁ sh

characters intermediate between a pelmato-

zoan ancestor and a typical asterozoan descen-

dant. Typical asteroids have ﬁ ve arms radiating

from the disk, which is coated by loosely ﬁ tting

plates permitting considerable ﬂ exibility of

movement (Fig. 15.16). Additional respiratory

structures, called papulae, project from the

celom through the plates of the upper surface.

This backup system aids the high metabolic

rates of these active starﬁ sh.

The ﬁ rst true starﬁ sh were probably derived

from the somasteroids during the Early Ordo-

vician and were relatively immobile, infaunal

sediment shovelers. Some of the ﬁ rst starﬁ sh,

for example Hudsonaster, from the Middle

Ordovician, have similar plate conﬁ gurations

to the young growth stages of living forms such

as Asterias. Although relatively uncommon in

Paleozoic rocks, the group was important

during the Mesozoic and Cenozoic and is now

one of the most common echinoderm classes.

The ophiuroids ﬁ rst appeared during the

Early Ordovician (Arenig), but the group, as

presently deﬁ ned, may be paraphyletic. Clas-

siﬁ cation is based on arm structure and disk

plating. The ophiuroid body plan is distinc-

tive, with a subcircular central disk and ﬁ ve

DEUTEROSTOMES: ECHINODERMS AND HEMICHORDATES 407

(b) (c)(a)

(d)

(g)

(j) (k)

(l)

(h)

(i)

(e) (f)

Figure 15.15 Aboral, oral and lateral views of some echinoid genera: (a–c) Cidaris (Recent; regular),

(d–f) Conulus (Cretaceous; irregular), (g–i) Laganum (Recent; sand dollar) and (j–l) Spatangus (Recent;

heart urchin). All approximately natural size. (From Smith & Murray 1985.)