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

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408 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD

long, thin, ﬂ exible arms. The mouth is situ-

ated centrally on the lower surface of the disk.

Most of the disk is ﬁ lled by the stomach and,

in the absence of an anus, waste products are

regurgitated through the mouth. The arms

consist of highly specialized ossicles or verte-

brae. Ophiuroids are common in modern seas

and oceans, preferring deeper-water environ-

ments below 500 m. Their basic architecture

differs little from some of the ﬁ rst members

of the group, for example Taeniaster from the

Middle Ordovician of the United States.

A few modern starﬁ sh are vicious and vora-

cious predators enjoying a diet of shellﬁ sh.

Asteroids can prize apart the shells of bivalves

with their sucker-armored tube feet far enough

to evert their stomachs through their mouths

and into the mantle cavity of the animal, where

digestion of the soft parts takes place. Stephen

Donovan and Andrew Gale (1990) suggested

that this predatory life mode signiﬁ cantly

inhibited the post-Permian diversiﬁ cation of

some brachiopod groups. The strophomenides,

the most diverse Permian brachiopods, largely

pursued a reclined, quasi-infaunal life strategy

and they may have presented an easy kill for

the predatory asteroids.

Carpoidea

The carpoids include some of the most bizarre

and controversial fossil animals ever described.

Variably described as carpoids, homalozoans

or calcichordates, depending on preference,

most authorities consider the group to be very

different to the radiate Echinodermata; indeed,

carpoids show some puzzling similarities to

the chordates.

The carpoids were marine animals ranging

in age from Mid Cambrian to possibly Late

Carboniferous, with a calcitic, echinoderm-

type skeleton lacking radial symmetry (Fig.

15.17). Two main types of carpoid are recog-

nized: the cornutes and the mitrates. The cor-

nutes were often boot-shaped and appear to

have a series of gill slits on the left side of the

roof of the head, whereas the mitrates, derived

from a cornute ancestor, were more bilaterally

symmetric with covered gill slits on both

sides.

It might seem unexpected, but the carpoids

have featured at the center of a long-running

and heated debate that has hit the headlines

over the past 50 years. After much careful

study, Richard Jefferies (1986) presented

detailed evidence that carpoids and chordates

share many characters, the so-called “cal-

cichordate hypothesis”. He based his conclu-

sion on painstaking studies of their anatomy

and the anatomy of embryos of modern echi-

noderms and chordates. A chordate-implied

reconstruction of carpoids suggests that the

body consists of a head and a tail used for

locomotion (Sutcliffe et al. 2000) (Fig. 15.18).

arm

disk

mouth

pedicellariae

ambulacral spines

ambulacral

groove

(a) (b)

anus

madreporite

Figure 15.16 Morphology of the asterozoans: (a) ventral and (b) dorsal surfaces. (Based on Treatise on

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

DEUTEROSTOMES: ECHINODERMS AND HEMICHORDATES 409

Moreover, Jefferies (1986) described struc-

tures indicating a ﬁ sh-like brain, cranial

nerves, gill slits and a ﬁ lter-feeding pharynx

similar to that in tunicates (often known as

the sea squirts). In the calcichordate hypoth-

esis, Hemichordata is identiﬁ ed as a sister

group to Echinodermata + Chordata, a clade

that Jefferies called Dexiothetica. Reapprais-

als of the anatomy of carpoids have shown,

however, that they may be interpreted rather

more convincingly as echinoderms, and that

the calcichordate hypothesis fails (Box 15.7).

Further, when these redescriptions of the fossil

material are combined with new molecular

evidence on phylogeny, the case is lost (Ruta

1999). Molecular phylogenetic analyses

(Winchell et al. 2002; Delsuc et al. 2006)

show that Hemichordata is the sister group of

Echinodermata, forming together the Ambu-

lacraria, and that Ambulacraria is the sister

group of Chordata. Dexiothetica does not

exist.

HEMICHORDATES

What was the character of the vegetation

that clothed this earliest prototype of

Europe is a question to which at present

no deﬁ nite answer is possible. We know,

however, that the shallow sea which

spread from the Atlantic southward and

eastward over most of Europe was ten-

anted by an abundant and characteristic

series of invertebrate animals – trilobites,

graptolites, cystideans, brachiopods, and

cephalopods, strangely unlike, on the

whole, to anything living in our waters

now, but which then migrated freely

along the shores of the arctic land

between what are now America and

Europe.

Sir Archibald Geikie from a lecture

delivered to the Royal Geographical

Society (1897)

The hemichordates form a small phylum of

only a few hundred species and are unfamiliar

to most people, but their importance for the

study of vertebrate evolution cannot be under-

estimated. Their most common fossil repre-

sentatives, the graptolites, were abundant in

gill slit

arm

hydropore

gonopore

ridge

anus

(a) (b)

periproct

hind tail

peripheral

hinge line

central

plate

fore tail

mid tail

Figure 15.17 Morphology of the carpoids:

(a) dorsal and (b) ventral surfaces. (From

Jefferies & Daley 1996.)

Figure 15.18 Reconstruction of a living carpoid: the Devonian Rhenocystis moving across and through

the sediment from left to right. (From Sutcliffe et al. 2000.)

410 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD

Box 15.7 The oldest stylophorans: echinoderms with a locomotory organ

How much further can we go with the debate on the afﬁ nities of the carpoids? New material and

new investigative techniques will always help. The oldest stylophoran carpoid is from the Middle

Cambrian rocks of Morocco. Sebastien Clausen and Andrew Smith (2005) have analyzed the mor-

phology of the animal in great detail, particularly its microstructure with the scanning electron

microscope. Ceratocystis in fact has a stereom microstructure typical of most echinoderms, but its

appendage was covered by articulating plates and ﬁ lled with muscle tissue and ligaments (Fig. 15.19).

It seems that this bizarre asymmetric animal has an echinoderm skeletal structure but also possessed

a muscular locomotory appendage rather similar to its sister taxon, the pterobranchs. This group

has all the features of a stem-group echinoderm prior to acquiring ﬁ ve-fold symmetry and presum-

ably a water vascular system, and it rather conclusively disproves a close phylogenetic association

between carpoids and chordates.

(a)

(b)

(c)

(d)

Gill slits?

Body

Appendage

Stylocone

Lines of section

Fixed plates

Moveable

plates

Body

Brain

Gut

Muscle somites

Muscle

Stylocone

Muscle

Stylocone

Metacoel

Nerve

Notochord

Mouth

Water vascular system

(with tube feet)

Figure 15.19 Ceratocystis from North Africa. (a) Basic anatomic features. (b–d) Three current

interpretations of the soft-tissue anatomy of the stylophoran appendage in proximal longitudinal

(left) and distal transverse (right) section: (b) primitive echinoderm model, (c) calcichordate model

and (d) crinozoan model. (Based on Clausen & Smith 2005.)

the ancient seas of the Early Paleozoic, in

communities and environments quite differ-

ent from those of today. Graptolites are widely

used for correlation because of their abun-

dance, widespread distribution and rapid evo-

lution. Although graptolites are extinct, and

their life styles are difﬁ cult to interpret, they

were hemichordates – a phylum containing

about 100 living species characterized by a

rod-like structure, the notochord. They were

small, soft-bodied animals with bilateral sym-

metry and a lack of segmentation. The phylum

contains two very different classes: ﬁ rst, the

tiny, mainly colonial, pterobranchs that lived

in the sessile benthos and, second, the larger

infaunal acorn or tongue worms, the entero-

DEUTEROSTOMES: ECHINODERMS AND HEMICHORDATES 411

pneusts that lived in burrows mainly in sub-

tidal environments.

The hemichordates have a mixture of char-

acters suggesting links with the lophopho-

rates, the echinoderms and the chordates.

They have been closely related to the cepha-

lochordates and urochordates or tunicates,

but molecular and other data suggest that the

latter two groups are more closely related to

the chordates than the hemichordates.

Although the notochord is now known to be

unrelated to a true backbone, the hemichor-

dates have, nevertheless, gill slits and a nerve

cord.

Modern hemichordate analogs

Pterobranchs superﬁ cially resemble the bryo-

zoans – both are colonial animals and the

individual zooids feed with tentaculate, cili-

ated arms (Fig. 15.20). The group has a long

geological history with early records such as

Rhabdotubus from the Middle Cambrian and

Graptovermis from the Tremadocian. The

living genera Cephalodiscus and Rhabdo-

pleura (Fig. 15.20) have been used as analogs

for many aspects of graptolite morphology,

ontogeny and paleoecology. These living

genera and the graptolites both have a peri-

derm, or skin, with fusellar tissue, while the

dendroid stolon, a tube that connects the

thecae to each other, may be related to the

pterobranch pectocaulus. Rhabdopleura is

known ﬁ rst from the Middle Cambrian and

occurs in oceans today mainly at depths of a

few thousand meters. The genus is minute

with a creeping colony hosting a series of

exoskeletal tubes, each containing a zooid

with its own lophophore-like feeding organ

comprising a pair of arms. The zooids are

budded from a stolon and interconnected by

a contractile stalk, the pectocaulus.

Cephalodiscus, however, is rather different,

being constructed from clusters of stalked

stomochord

lophophore

preoral disk

mouth

stomach

intestine

peduncle

bud 1

bud 2

bud 3

(b)

extended zooid

terminal bud

contractile stalk

retracted zooid

pectocaulus

bud 1

bud 4

bud 2

bud 3

(a) (c)

Figure 15.20 Rhabdopleurid morphology: (a, b) Rhabdopleura and (c) Cephalodiscus. (Based on

Treatise on Invertebrate Paleontology, Part V. Geol. Soc. Am. and Univ. Kansas Press.)

412 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD

tubes budded from a basal disk. Moreover, in

further contrast to Rhabdopleura, species of

Cephalodiscus usually have ﬁ ve pairs of cili-

ated feeding arms. Individual zooids in the

Cephalodiscus colony can actually crawl

outside the colony along its exterior and often

farther aﬁ eld onto adjacent surfaces. The

zooids of living Cephalodiscus, with their

considerable freedom of mobility, can even

construct external spines from outside the

skeleton.

Graptolites

The graptolites, or Graptolithina, are gener-

ally stick-like fossils, very common in many

Lower Paleozoic black shales. In fact the

group is so prevalent that it has proved to be

of key importance in correlating Lower Paleo-

zoic strata. The majority of Ordovician and

Silurian biozones are based on graptolite

species or assemblages. Graptolites, from the

Greek “stone writing”, usually occur in black

shales as ﬂ attened carbonized ﬁ lms resem-

bling hieroglyphics. Graptolite fossils often

show evidence of having been transported

by currents, although fortunately complete,

unﬂ attened specimens have been extracted

from cherts and limestone by acid-etching

techniques. The afﬁ nities of the group were

largely unknown until the 1940s, when the

Polish paleontologist Roman Kozłowski iden-

tiﬁ ed a notochord in three-dimensional mate-

rial isolated from limestones. There are several

groups of graptolites and graptolite-like

animals (Box 15.8).

Morphology: the graptolite colony

The basic graptolite architecture consists of a

probably collagenous skeleton characterized

by a growth pattern of half rings of periderm

interfaced by zigzag sutures, similar to the

construction of the pterobranchs (Fig. 15.21).

Each colony or rhabdosome grew from a

small cone, the sicula, as one or a series of

branches or stipes. The stipes may be isolated

or linked together by lateral struts to resemble

a reticulate lattice. A series of variably cylin-

drical tubes are developed along the stipes;

these thecae house the individual zooids of the

colony. Aggregates of rhabdosomes, synrhab-

dosomes, have been documented for some

species. These complex structures have gener-

ally been explained by asexual budding or

common attachment to a single ﬂ oat or patch

of substrate. A more recent taphonomic expla-

nation, however, suggests they formed by

entrapment of clusters of rhabdosomes using

dissepiment

stipe

sicula

autotheca

bitheca

dissepiment

lateral view

ventral view

proximal

(a)

(b)

(c)

distal

nema

growth lines

prosicula

theca

sicula

sicula aperture

virgula

initial bud

thecal aperture

Figure 15.21 Graptolite morphology: (a) dendroid morphology with a detail of the thecae (b), and

DEUTEROSTOMES: ECHINODERMS AND HEMICHORDATES 413

Box 15.8 Graptolite classiﬁ cation

A complete classiﬁ cation of the group is presented here although in practice it is only the dendroids

and graptoloids that have good fossil records. (The stolonoids, an encrusting or sessile group,

restricted to Poland, may be Pterobranchia.)

Class GRAPTOLITHINA

Order DENDROIDEA

• Multibranched colonies; stipes, commonly supported by dissepiments, have autothecae, bithecae

and a stolotheca. Anisograptids, intermediate between the dendroids and graptoloids, are retained

here

• Cambrian (Middle) to Carboniferous (Namurian)

Order TUBOIDEA

• Similar to dendroids but characterized by irregular branching and reduced stolothecae. Autothe-

cae and bithecae commonly form clusters

• Ordovician (Tremadocian) to Silurian (Wenlock)

Order CAMAROIDEA

• Encrusting life mode; endemic to Poland. Autothecae with expanded, sack-like bases. Bithecae

small and irregularly spaced. Stolotheca black and hard

• Ordovician (Tremadocian-Darriwilian)

Order CRUSTOIDEA

• Encrusting life mode; endemic to Poland. Autothecae with complex apertures

• Ordovician (Floian-Darriwilian)

Order DITHECOIDEA

• Sister group to the dendroids and graptoloids; central axis with a holdfast

• Cambrian (Middle) to Silurian (Lower)

Order GRAPTOLOIDEA

• The jury is still out on the detailed classiﬁ cation of this group. The position of the anisograptids

(included here with the dendroids) is uncertain, as is the status of the dichograptids; the retiolitids

are aberrant diplograptids. Colonies with few stipes (one to eight), a nema and sicula and a single

type of theca

• Ordovician (Tremadocian) to Devonian (Pragian)

Suborder DICHOGRAPTINA

• Basal graptoloids lacking both bithecae and virgellae

• Ordovician (Tremadocian-Katian)

Suborder VIRGELLINA

• Virgella always present

• Ordovician (Floian) to Devonian (Pragian)

414 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD

marine snow, a bonding material composed

of organic debris and mucus; this seems

less likely because the synrhabdosomes are

remarkably symmetric, which suggests they

grew that way. About six orders are now rec-

ognized in the class Graptolithina, but only

two, the Dendroidea and Graptoloidea have

important geological records. The patterns of

evolution linking these groups are uncertain

(Box 15.9).

Box 15.9 The ﬁ rst graptolites: a cryptic Cambrian dimension?

By the Ordovician, the graptolites were represented by a number of well-deﬁ ned groups including

the familiar dendroids and graptoloids and the less well-known camaroids, crustoids, dithecoids and

tuboids. It has long been a mystery where these diverse groups came from because the Cambrian

record was virtually non-existent. Barrie Rickards and Peter Durman (2006) have reassessed all the

possible ancestors, Cambrian specimens that have been variably assigned to graptolites, hydroids or

algae from the Middle and Upper Cambrian. They reassigned some of these cryptic Cambrian speci-

mens to the rhabdopleurids and excluded a number of them from the graptolites. The graptolites

and rhabdopleurids therefore probably shared a common ancestor in the Early Cambrian (Fig.

15.22). The rhabdopleurids are remarkable animals; Cambrian forms are virtually identical to

modern rhabdopleurids, making them true living fossils. The common ancestor to the graptolites

and rhabdopleurids was probably a solitary, worm-like animal, equipped with a lophophore, and

living in pseudocolonial ﬁ lter-feeding clumps on the seaﬂ oor. Thus the graptolites, which dominated

the Early Paleozoic water column, started out as rather anonymous benthic ﬁ lter feeders in the

shadow of the more obvious early arthropods, crinoids and mollusks of the Cambrian evolutionary

fauna.

Cambrian Ordovician Silurian Devonian Carboniferous

Rhabdopleurids

Camaroids

Crustoids

Tuboids

Dithecoids

Dendroids

Graptoloids

Figure 15.22 Generalized phylogenetic model for rhabdopleurid and graptolite evolution. (From

Rickards & Durman 2006.)

DEUTEROSTOMES: ECHINODERMS AND HEMICHORDATES 415

Dendroidea

The Dendroidea is the older of the two main

groups with important geological records, ﬁ rst

appearing in the Middle Cambrian and disap-

pearing during the Late Carboniferous. The

dendroid rhabdosome was multibranched, like

a bush, with its many stipes connected laterally

by struts or dissepiments. Two types of theca,

of different sizes, the autotheca and bitheca,

grew along the stipes. The earlier genera were

benthic, attached to the seaﬂ oor by a short

stalk and basal disk. Probably during the latest

Cambrian a few genera, including Rhabdino-

pora, detached themselves to evolve a new

lifestyle in the plankton; together with minute

brachiopods and the occasional trilobite, they

probably formed a major part of the preserved

Early Paleozoic plankton.

Dendroid taxa Dendrograptus was a benthic

genus, bush-like, erect and attached to the sea-

ﬂ oor by a rooting structure or holdfast. Dicty-

onema was also benthic and ranged in age

from the Late Cambrian to the Late Carbonif-

erous. The rhabdosome was conical to cylin-

drical in shape. Planktonic dendroids similar

to Dictyonema are placed in Rhabdinopora.

The following anisograptid genera are in

some ways intermediate between the typical

dendroids and graptoloids and may be classi-

ﬁ ed with either group. Here they are included

with the dendroids. Radiograptus, for

example, developed large spreading colonies.

Both Kiaerograptus and some early species of

Bryograptus had both auto- and bithecae, and

the latter had triradiate rhabdosomes with,

initially, three primary stipes. Clonograptus

had a horizontal, biserially symmetric rhab-

dosome with stipes generated by dichoto-

mous branching from an initial biradiate

conﬁ guration.

Graptoloidea

Compared with the dendroids, the graptoloid

rhabdosome is superﬁ cially simpler and con-

sists of an initial sicula, divided into an upper

prosicula and a lower metasicula, with at its

apex, distally, a long thin, spine, the nema.

The metasicula, like the rest of the rhabdo-

some, was composed of fusellar tissue, bundles

of short, branching ﬁ brils. The virgella pro-

jected below the secular aperture, proximally

and is characteristic of the suborder Virgel-

lina. The thecae grew out from the sicula and

subsequent thecae grew in sequence as the

rhabdosome developed.

Graptoloid taxa The architecture of the grap-

toloid skeleton depended on three sets of

structures: the number of stipes or branches,

their mutual attitudes and the shape of the

thecae. Morphology in this order is thus based

on permutations of these structures; the

following genera illustrate this variation

(Fig. 15.23).

Tetragraptus, common during the Floian

(later Early Ordovician), typically had four

stipes arranged in horizontal, pendent or

reclined attitudes with simple, overlapping

thecae. Didymograptus was twin-stiped or

biramous, commonly with the branches in

horizontal, pendent or reclined orientations;

thecae were simple. Isograptus, however, had

two relatively wide stipes, reclined with a

long, thread-like sicula. Nemagraptus had a

very distinctive rhabdosome consisting of two

sigmoidal stipes, initially diverging from the

sicula at about 180˚, with additional stipes,

curved, and arising at intervals along the main

branches. Thecae were long, thin and diverged

at small angles from the stipes. Dicellograptus

had a pair of stipes that adopted reclined atti-

tudes but often the branches were curved or

even coiled; the thecae were characterized by

extravagant sigmoidal shapes and incurved

apertures. Monograptus was a uniserial scan-

dent form with a straight or curved rhabdo-

some and a nema embedded in the dorsal wall

that projected distally. Rastrites possessed

long, straight, widely separated thecae, often

with hooked ends. Cyrtograptus had a spi-

rally coiled rhabdosome with secondary

branches or cladia oriented like the arms of a

spiral galaxy. Corynoides was minute, con-

sisting of a sicula and three to four thecae.

Retiolitids

The retiolitids are a spectacular group of

apparently scandent, diplograptid biserials

with a reduced, minimalist periderm consist-

ing of a network of bars or lists probably

surrounded by a net-like structure, termed the

ancora sleeve in Silurian forms (Fig. 15.24).

The group appeared in the Mid Ordovician

and continued successfully

, for almost 50

myr,

416 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD

(a) (b)

(e)

(f)

(g)

(h)

(i) (j) (k) (l)

(m) (n)

Figure 15.23 Some graptolite genera: (a) Rhabdinopora (×2), (b) Tetragraptus (×2), (c) Tetragraptus,

proximal end (×20), (d) Isograptus, proximal end (×20), (e) Xiphograptus (×20), (f) Isograptus (×10),

(g) Appendispinograptus (×2), (h) Dicranograptus (×2), (i) Dicellograptus (×2), (j) Orthograptus (×2),

(k) Undulograptus (2), (l) Nemagraptus (×2), (m) Didymograptus (Expansograptus) (×20) and (n)

Atavograptus (×2). (a) An Early Ordovician dendroid, (b–f, k, m) Early Ordovician graptoloids;

(g–j, l) Late Ordovician graptoloids; and (n) a Silurian monograptid. (Courtesy of Henry Williams.)

DEUTEROSTOMES: ECHINODERMS AND HEMICHORDATES 417

until the latest Silurian (Kozl

owski-Dawid-

ziuk 2004). The retiolitids probably represent

a polyphyletic grade of organization where

the rhabdosome of various groups may have

functioned like a sponge, drawing in ﬂ uid and

nutrients through the periderm and expelling

waste upwards.

Growth and ultrastructure of the graptolites

Detailed studies on the ultrastructure of the

graptolite using both scanning and transmis-

sion electron microscopes has identiﬁ ed two

types of skeletal tissue. Fusellar tissue occurs

together with cortical tissue in the form of

longer parallel ﬁ bers. Fusellar material was

secreted as a series of half rings with the corti-

cal tissue overlapping the fusellar layer both

inside and outside the rhabdosome (Fig.

15.25). The cortical tissue itself was secreted

as “bandages”, looking rather like multiple

overlapping band-aids. Secretion may have

been by mobile zooids, free to patrol the exte-

rior of the colony while still attached by a

ﬂ exible cord to the rest of the colony, rather

like an astronaut maintaining a space station,

or the entire rhabdosome may have been sur-

rounded by soft tissue.

Although graptolites are abundant and

important fossils in many Early Paleozoic

assemblages, it is notoriously difﬁ cult to dis-

cover what they were actually made of. Most

assemblages occur in black shales that have

been compacted, diagenetically altered and

often metamorphosed within or around oro-

genic belts. Moreover, graptolite periderm,

when actually preserved, consists mainly of

an aliphatic polymer, immune to base hydro-

lysis. It lacks protein even though both the

structure, as well as chemical analyses, of the

periderm of living Rhabdopleura suggest that

it was originally composed of collagen. Previ-

ous studies suggested that the collagen had

been replaced by macromolecular material

from the surrounding sediment. New analyses

suggest that the aliphatic composition of

graptolite periderm reﬂ ects direct incorpora-

tion of lipids from the organism itself by in

situ polymerization (Gupta et al. 2006). A

similar process may account for the preserva-

tion of many other groups of organic fossils

(see p. 60).

Colonial growth of the graptoloids The

growth of a colony lends itself to graphic and

mathematical simulations. A few authors have

devised computer models based on a set of

simple rules that dictate such growth modes.

These models are usually deterministic and

static. For example, Andrew Swan (1990)

generated a series of theoretical morphotypes

based on a model of dichotomous branching

at given stipe lengths; the orientation of the

bifurcation together with the stipe length and

width was varied. Additionally, soft tissue

could be added to the computer reconstruc-

tions. Swan showed that the shapes of most

graptolite colonies could be simulated using

variations in only a few parameters, and he

was able to test the efﬁ ciency of each colony

for particular functions. Swan targeted the

efﬁ ciency of the graptolite feeding strategy

and tested the efﬁ cacy of nutrient capture for

a sequence of computer-generated colonies,

and he showed that known graptolite colonies

pass the test as being the most efﬁ cient shapes

Figure 15.24 Retiolitid Phorograptus (Middle

Ordovician) (×30). (Courtesy of Denis Bates.)