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

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388 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD

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Chapter 15

Deuterostomes: echinoderms

and hemichordates

Key points

• Echinoderms today include sea urchins, starﬁ sh and sea cucumbers. They are all equipped

with a water vascular system, a mesodermal skeleton of calcitic plates with a stereom

structure, pentameral symmetry and tube feet.

• During the Cambrian radiation many bizarre forms evolved. The spindle-shaped

Helicoplacus may be part of the stem group for the entire phylum but did not survive

the Cambrian substrate revolution.

• Pelmatozoans were mainly ﬁ xed echinoderms and include the blastoids, crinoids and

cystoids; the crinoids include four classes: the Inadunata, Flexibilia, Camerata and

Articulata.

• The echinoids were part of the mobile benthos. During the Mesozoic irregular groups,

adapted for burrowing, evolved from the more regular forms that characterized the

Paleozoic.

• Asteroids (starﬁ sh) were more important in post-Paleozoic rocks; their Triassic radiation

may have inhibited the re-radiation of some key groups of brachiopod.

• Carpoids are traditionally classed with the echinoderms, although some have argued

they were ancestral to chordates; they were probably stem-group echinoderms.

• Graptolites are hemichordates closely related to the living rhabdopleurids with similarly

constructed rhabdosomes and ultrastructure.

• Dendroids, with autothecae and bithecae together with many stipes, and graptoloids,

with generally fewer stipes and only one type of theca, are the two most common grap-

tolite orders.

• Graptolites probably pursued benthic (dendroids), planktic (dendroids and graptoloids)

and automobile (graptoloids) lifestyles.

• Graptoloids evolved rapidly and were widespread, the ideal zone fossils in rocks of

Ordovician-Silurian and Early Devonian age.

390 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD

The echinoderms and hemichordates appear

to be two very different groups of animals,

one characterized by ﬁ ve-fold symmetry and

a water vascular system, the other a group of

odd stick-like colonial organisms. Surpris-

ingly, both are closely related to each other

and, moreover, are not so distant from our-

selves, the chordates. Both groups are deu-

terostomes; the ﬁ rst opening to develop in the

embryo is the anus and a second forms the

mouth. The group has a dipleurula larva and

a body cavity that developed from an exten-

sions of the embryonic gut (see p. 240).

Modern morphological and molecular analy-

ses indicate that the echinoderms and hemi-

chordates are in fact sister groups (Smith

2005). A small, extinct group – the Vetulicolia

– so far known only from the Cambrian, has

also been related to the deuterostomes because

of similar gill structures and the absence of

limbs. But although recent ﬁ nds from Utah

have suggested that this group has more in

common with the arthropods and probably

belongs to the ecdysozoans (see p. 361), the

group remains an enigma (see Box 15.10).

ECHINODERMS

Clearly we stood among the ruins of

some latter-day South Kensington! Here,

apparently, was the Palæontological

Section, and a very splendid array of

fossils it must have been. . . . The place

was very silent. The thick dust deadened

our footsteps. Weena, who had been

rolling a sea urchin down the sloping

glass of a case, presently came, as I stared

about me, and very quietly took my hand

and stood beside me. And at ﬁ rst I was

so much surprised by this ancient monu-

ment of an intellectual age, that I gave

no thought to the possibilities it pre-

sented. Even my preoccupation about the

Time Machine receded a little from my

mind.

H. G. Wells (1898) The Time Machine

Echinoderms today are one of the most abun-

dant marine animal groups, and as fossils they

can sometimes be rather robust, as Weena

from The Time Machine found. Sea urchins

are common in many intertidal environments,

and out in the deep sea the ocean ﬂ oors are

covered by brittle stars and sea cucumbers.

The phylum Echinodermata has an unusual

ﬁ ve-fold symmetry and is uniquely equipped

with a water vascular system in which water

is forced around the plumbing by muscular

action, while tube feet, extending from the

system, are often modiﬁ ed for food process-

ing, locomotion and respiration. The 6000 or

so living echinoderm species include familiar

forms such as sea lilies, sea urchins, sand

dollars, starﬁ sh and sea cucumbers (Fig. 15.1).

Although many species today live in the inter-

tidal or subtidal zones, the group is most

diverse in the deep sea. Echinoderms also

occupied a wide range of marine environ-

ments and pursued a variety of life strategies

in the geological past. Fossil echinoderms

are relatively common, and because many

echinoderm skeletons disintegrate rapidly

after death, many limestones are packed with

the distinctive skeletal debris of calcitic

plates.

Apart from the water vascular system, echi-

noderms have a number of other distinctive

features. All members of the phylum have a

mesodermal skeleton constructed from porous

plates of calcite; each plate is usually a single

crystal of calcite and easy to recognize in thin

sections. In addition, the plates have a unique

ultrastructure of rods linked to form a three-

dimensional lattice. This network, or stereom,

is permeated by ﬁ nger-like pieces of soft tissue

that occupy the spaces, or stroma, in the

lattice. Finally, ﬁ ve-rayed or pentameral sym-

metry, occasionally modiﬁ ed by a secondary

bilateral symmetry, is typical of the echino-

derms. The phylum is generally split into the

mobile, non-stalked eleutherozoans and the

mainly ﬁ xed, stalked pelmatozoans (Box

15.1), but the earliest forms are hard to

classify (Box 15.2).

The multiplated echinoderm skeleton disin-

tegrates very rapidly after death; although

individual plates or ossicles have high preser-

vation potential, the complete skeletons do

not. Nevertheless, occasionally rapid burial or

transportation into anoxic conditions may

result in the preservation of complete echino-

derm skeletons. Starﬁ sh beds, usually charac-

terized by accumulations of complete

echinoderms, occur sporadically throughout

the fossil record. The Leintwardine Starﬁ sh

Bed of the England–Wales border area con-

DEUTEROSTOMES: ECHINODERMS AND HEMICHORDATES 391

tains Late Silurian echinoderms within ﬁ ne-

grained turbidites, and the Lower Jurassic

Starﬁ sh Bed of South Dorset, England is dom-

inated by ophiuroids suddenly buried by a

thick layer of sandstone. However, one of the

most remarkable echinoderm Lagerstätte

occurs in the Upper Ordovician succession of

the Craighead inlier, north of the Girvan

valley, southwest Scotland. Here, the Lady

Burn Starﬁ sh Bed is one of several sandstone

Edrioasteroids

Low-level epifaunal

High-level epifaunal

Mobile epifaunal

Mobile infaunal and epifaunal

Helicoplacoid

Holothurian

Blastoid

Crinoid

Calcichordate

Rhombiferan

Echinoid

Asteroid

Ophiocistioid

Figure 15.1 Life modes of the main echinoderm body plans. (Based on Sprinkle 1980.)

Box 15.1 Echinoderm classiﬁ cation

In broad terms, the Echinodermata may be divided into two main sister groups – the stalked pel-

matozoans and the mobile eleutherozoans. But there are a number of more bizarre Lower Paleozoic

forms, known from only a few specimens at single localities, which are difﬁ cult to classify at present.

The classiﬁ cation is based on a number of key features: the main body of the animal, enclosed by

plates (the theca or test), areas bearing tube feet (ambulacra) with perforations or holes (brachioles)

and, in the case of the pelmatozoans, the possession of a cup (calyx) and arms (brachia).

Subphylum PELMATOZOA

Class EOCRINOIDEA

• Globular or ﬂ at theca with 2–5 ambulacra bearing brachioles

• Cambrian (Lower) to Silurian

Continued

392 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD

Class PARACRINOIDEA

• Irregularly arranged plates comprising globular to lenticular theca; 2–5 ambulacra commonly

with pinnules. Hydropore adjacent to mouth. Stem attached to three basal plates

• Ordovician (Darriwilian to Hirnantian)

Class BLASTOIDEA

• Flask-shaped theca commonly with three basal plates; ambulacra with elongate lancet plate and

rows of side plates

• Ordovician (Katian) to Permian (Tatarian)

Class DIPLOPORITA

• Globular theca with many plates in an irregular to regular pattern; 3–5 food grooves with

brachioles. Diplopores perforate thecal plates

• Cambrian (Middle) to Devonian (Eifelian)

Class RHOMBIFERA

• Globular theca with 2–5 ambulacra extending from the mouth to, commonly, the edge of the

upper surface. Pore structures cross adjacent thecal plates arranged in a rhomboid pattern, and

comprise the respiratory system

• Cambrian (Upper) to Devonian (Frasnian)

Class CRINOIDEA

• Calyx with lower cup and upper tegmen. Sea lilies and feather stars

• Ordovician (Tremadocian) to Recent.

Subphylum ELEUTHEROZOA

Class EDRIOASTEROIDEA

• Disk-shaped thecae with straight or curved ambulacra with the mouth situated centrally and the

anus sited on the interambulacra

• ?Precambrian (Ediacaran), Cambrian (Lower) to Carboniferous (Gzelian)

Class ASTEROIDEA

• Between 5 and 25 arms with large tube feet extend from a central disk. Starﬁ shes or sea stars

• Ordovician (Floian) to Recent

Class OPHIUROIDEA

• Five long, thin, ﬂ exible arms, consisting of vertebrae and with small tube feet, extend from large,

circular central disk. Brittle stars or basket stars

• Ordovician (Floian) to Recent

Class ECHINOIDEA

• Test is usually globular with plates differentiated into ambulacral and interambulacral areas.

Mouth on underside, anus on upperside or sited posteriorly. Sea urchins, heart urchins and sand

dollars

• Ordovician (Katian) to Recent

Class HOLOTHUROIDEA

• Body is cucumber-shaped with leathery skin with muscular mesoderm and spicules. A ring of

modiﬁ ed tube feet surround the mouth. Sea cucumbers

• Ordovician (Floian) to Recent

DEUTEROSTOMES: ECHINODERMS AND HEMICHORDATES 393

Box 15.2 Origin of the echinoderms and the status of the helicoplacoids

During the major Early Cambrian radiation of echinoderms, many rather bizarre forms appeared

suddenly with very different morphologies. At least nine genera were present, of which about half

had pentameral symmetry, but the others were not pentameral at all. One such non-pentameral

group, the helicoplacoids (Fig. 15.2), is unique in having only three ambulacral areas with tube feet

wrapped around their spindle-shaped bodies. Moreover, the group lacked appendages and individu-

als probably lived with their shorter ends anchored to the sediment. However, helicoplacoids have

many plates with the distinctive stereom structure, ambulacra and a mouth sited laterally together

with an apical anus. The helicoplacoids have thus been interpreted as primitive echinoderms, surviv-

ing by suspension feeding in the sessile benthos. Helicoplacus may be very close to the stem group

of all subsequent Echinodermata, and something like this animal might have given rise to the pel-

matozoan and eleutherozoan body plans. Other groups of echinoderms were already diverse and

widespread during the Early Cambrian, but the helicoplacoids were apparently restricted to western

North America, where they were very abundant during only the Early Cambrian. Their extinction

may have been a very important ecological signal. Such groups of unattached “sediment stickers”

were well adapted to the algal mat substrates of the Neoproterozoic. Perhaps they could not cope

with the increased bioerosion and bioturbation of soft substrates that were part of the move away

from seaﬂ oors covered by microbial mats that prompted the Cambrian substrate revolution (Bottjer

et al. 2000).

Figure 15.2 Helicoplacus from the Lower Cambrian (×10). (Based on Treatise on Invertebrate

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

394 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD

units within a deep-water mudstone sequence.

Entire crinoids, cystoids, echinoids and cal-

cichordates were carried downslope and

rapidly buried on the unstable slopes of a

submarine fan system.

Crinoidea

Although the crinoids, famously called “sea

lilies”, look more like plants than animals,

there is no doubt they are animals, and echi-

Box 15.3 Columnal classiﬁ cation

The majority of crinoid assemblages are represented by disarticulated ossicles. Conventional taxon-

omy based on a description of complete, articulated specimens is thus not possible. Nevertheless,

ossicles have many distinctive features, arguably with more well-deﬁ ned characteristics than many

groups of macrofossils (Fig. 15.3). Single stems consist of many ossicles with a central canal or lumen

usually carrying nerve ﬁ bers. Both the ossicles and lumens have distinctive shapes that are the basis

of a form taxonomy of the group. Form taxonomy helps us classify the shapes of fossils, in the same

way that we can classify nuts and bolts. It is a useful method of organizing our data, but since it is

not biologically meaningful, cannot be used in phylogenetic analyses. Stems may be either homeo-

morphic, composed of similarly shaped ossicles, or heteromorphic with a variety of different-shaped

ossicles. Moreover stems may be subdivided into zones that may be internally homeomorphic or

heteromorphic. Columnal taxonomy has proved useful in describing taxa (so-called col. taxa) of

pelmatozoan, particularly crinoid, ossicles of stratigraphic signiﬁ cance.

(a) (b)

(c)

Figure 15.3 Some crinoid ossicle types. (a) Articular facet of a columnal of the bourgueticrinid

Democrinus (?) sp., with a fulcral ridge of the synarthrial articulation; the lumen opens at the

bottom of the “8”-shaped depression (×15). (b) Cirral scar on a nodal of the isocrinoid Neocrinus

with well-preserved stereom microstructure and knob-like synarthrial fulcrum (×18). (c) Articular

facet of a columnal of the isocrinoid Neocrinus with symplectial articulation around the ﬁ ve

petal-like areola areas (×9). (Courtesy of Stephen Donovan.)

DEUTEROSTOMES: ECHINODERMS AND HEMICHORDATES 395

noderms at that. They are usually sessile, with

characteristic echinoderm pentameral symme-

try, rooted by a stalk, for at least part of their

life cycle, to the seabed; but some forms after

a short ﬁ xed stage are entirely free living.

Modern forms live in dense clusters or

“forests” ranging from the warm waters of

the tropics to the icy conditions of polar lati-

tudes. The “feather stars” prefer the clear-

water conditions of the continental shelf,

living in nooks and crevices, and emerging at

night to perch on ridges. The ﬁ xed sea lilies

occupy the deep-water environments of the

continental slope. The majority of fossil forms

were almost certainly part of the shallow-

water sessile benthos. The success of the cri-

noids may be measured by the fact we know

over 6000 fossil species and an age range

from the Early Ordovician to the present

day.

Morphology and life modes

The crinoids consist of a segmented stalk or

stem composed of columnals or ossicles (Box

15.3) ﬁ xed to the seabed by root-like struc-

tures or holdfasts. Attached to the top of the

stalk is the case containing the main func-

tional part of the animal called alternatively

the calyx, aboral cup or theca. The calyx is

built of two rings of calcitic plates – the basals

and the overlying radials in a monocyclic con-

ﬁ guration. In a number of taxa, the dicyclic

forms, a second circle of smaller plates, the

infrabasals, interface between the basals and

the stem, providing further articulation. The

upper, oral surface of the calyx is covered by

a ﬂ exible membrane or tegmen and houses a

number of important structures. These are the

mouth, which is usually situated centrally at

the convergence of ﬁ ve radially arranged

feeding grooves; the anus, which is sited pos-

teriorly with the outlet often modiﬁ ed by an

anal tube enhancing the efﬁ ciency of waste

disposal; and the arms or brachials, which

extend upwards from the calyx and together

form the crown.

As already noted, two main life strategies

were pursued by the crinoids (Fig. 15.4). The

majority of fossil crinoids and about 25 Recent

basals

infrabasals

stem

pinnules

holdfast

calyx

brachials

radials

(a) (b)

Freely mobile

Stalked attached

Florometra

Antedon

anal tube

Figure 15.4 (a) Morphology of the Ordovician Dictenocrinus. (b) Two main crinoid life strategies,

ﬁ xed and mobile. (Redrawn from various sources.)

396 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD

genera are stalked forms, attached to the

seabed. Modern oceans, however, are domi-

nated by mobile comatulids that move about

like pneumatic umbrellas, pumping their long

arms in unison. Antedon is one of nearly 100

non-stalked genera that, after a short ﬁ xed

stage, are free to crawl and swim with the aid

of ﬂ exible arms and cirri.

Classiﬁ cation and evolution

The oldest reported crinoid, Echmatocrinus

brachiatus from the Middle Cambrian Burgess

Shale, has uniserial or single brachials, in con-

trast to the biserial arms of contemporary

eocrinoids. Echmatocrinus has few other

unequivocal crinoid characters and there is

wide agreement that it is actually an octocoral.

More recognizable crinoids with more typi-

cally constructed cups and columnal-bearing

stems, such as Dendrocrinus, appear some time

later during the Tremadocian. A major expan-

sion in the Early Ordovician tropics marked a

period of intense morphological experimenta-

tion and many adaptive radiations.

Virtually all Paleozoic crinoids were stalked,

and traditionally have been grouped into three

subclasses, the Inadunata, Flexibilia and Cam-

erata (Fig. 15.5). Inadunate crinoids comprise

a large and varied group, originating in the

Early Ordovician and continuing until the

Triassic. They have a rigid calyx with either

free or loosely attached brachials, and mono-

cyclic or dicyclic calyx bases. Camerate cri-

noids are characterized by large cups with

both monocyclic and dicyclic plate conﬁ gura-

tions. The uniserial or biserial brachials, deco-

rated with pinnules, are ﬁ rmly attached to the

cup and the tegmen is heavily plated, obscur-

ing the food grooves and mouth, but devel-

oped laterally with an anal tube. The Flexibilia,

comprising some 60 genera, have a dicyclic

plate conﬁ guration comprising three infra-

basals. The brachials are uniserial and lacking

pinnules, and the tegmen is ﬂ exible with a

mosaic of small plates. Their stems have

circular cross-sections and lack cirri. These

groups are especially well known in the

Carboniferous (Box 15.4).

The fourth subclass of crinoids, the Articu-

lata, with the exception of some Triassic ina-

dunates, includes all post-Paleozoic crinoids.

A few Paleozoic forms with articulate simi-

larities such as Ampelocrinus and Cymbiocri-

nus may be stem-group articulates. Over 250

genera are recognized with almost two-thirds

of known genera extant. Microcrinoids are a

highly specialized crinoid morphotype devel-

oped within both the Inadunata, during the

Paleozoic, and the Articulata, during the

Mesozoic. Microcrinoids are minute, never

more than 2 mm in size; they may be pedo-

morphic forms living together with more

typical crinoid communities.

Blastozoans

Blastozoans are an informal grouping that

includes three of the more minor, yet never-

theless important, echinoderm groups that are

all extinct: the cystoids, blastoids and eocri-

noids. These pelmatozoans were usually

equipped with a short stem but often lacked

brachia or arms. Blastomorphs were probably

high-level ﬁ lter feeders, particularly charac-

terized by pores or brachioles punctuating the

thecal plates. Eocrinoids are included by some

authors in the cystoids, appearing near the

base of the Cambrian and becoming extinct

during the Silurian. The eocrinoids, however,

probably included ancestors to both the cys-

toids and the crinoids.

Cystoids

Mid Paleozoic blastozoans with respiratory

pore structures modifying the thecal plates

have been traditionally placed within the Cys-

toidea. This mixed bag includes two classes,

the Diploporita and the Rhombifera, that

became very widespread during the Mid

Paleozoic. They had spherical or sac-like

thecae, commonly with 1000 or more irregu-

larly arranged plates. Moreover the group has

brachioles lacking pinnules and characteristi-

cally the plates are usually equipped with dis-

tinctive pore structures. A variety of such pore

structures have been recognized in the cys-

toids (Fig. 15.7), and they are fundamental in

the higher-level classiﬁ cation of the group.

Diploporita The diploporites had thecal

plates punctuated by pairs of pores either

covered with soft tissue (diplopores) or a layer

of stereom with the pore pairs joined by a

network of minor canals (humatipores). These

pores probably held a bulbous respiratory bag

and allowed for the efﬁ cient entry and exit of

celomic ﬂ uid. Both stalked and non-stalked

forms are present in this group, suggesting a

(a)

(b)

(d)

Figure 15.5 Some crinoid genera: (a) Dimerocrinites (Silurian; Camarata), (b) Cupalocrinus

(Ordovician; Indunata), (c) Sagenocrinites (Silurian; Flexibilia), (d) Chladocrinus (Jurassic; Articulata)

and (e) Paracomatula (Jurassic; Articulata comatulide). Magniﬁ cation approximately ×1 (a, c), ×2

(b, d, e). (From Smith & Murray 1985.)