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

Подождите немного. Документ загружается.

438 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD

The lungﬁ sh Dipterus (Fig. 16.8d) was a long,

slender ﬁ sh that hunted invertebrates and

ﬁ shes, and crushed them with broad grinding

tooth plates. The “rhipidistian” Osteolepis

(Fig. 16.8e) was also long and slender, and

was an active predator. These lobeﬁ ns had

muscular front ﬁ ns, and could have used these

to haul themselves over mud from pond to

pond. Specimens of these ﬁ shes are known

from the Devonian of many parts of the world

(Box 16.5).

After the Devonian, the actinopterygians

seem to have radiated three times. The ﬁ rst

radiation (Devonian-Permian) consisted of

the palaeonisciforms (Fig. 16.10a), a para-

phyletic group of bony ﬁ shes with large bony

scales and heavy skull bones. The second

radiation of bony ﬁ shes, an assemblage termed

the “holosteans”, occurred in the Late Trias-

sic and Jurassic. Semionotus (Fig. 16.10b), a

small form that has been found in vast shoals,

had more delicate scales than the palaeonisci-

forms, and a jaw apparatus that could be

partly protruded, hence providing a wider

gape.

The third and largest radiation of actinop-

terygian ﬁ shes, occurred in the Late Jurassic

and Cretaceous (Fig. 16.10c), with the diver-

siﬁ cation of the teleosts. Teleosts are the most

diverse and abundant ﬁ shes today, including

23,000 living species, such as eel, herring,

salmon, carp, cod, anglerﬁ sh, ﬂ ying ﬁ sh, ﬂ at-

ﬁ sh, seahorse and tuna. The huge success of

this radiation may be the result of their

remarkable jaws. Palaeonisciforms opened

their jaws like a simple trapdoor, holosteans

could enlarge their gape a little, but teleosts

can project the whole jaw apparatus like an

extendable tube (Fig. 16.10d). This came

about because of great loosening of the ele-

ments of the skull: as the lower jaw drops, the

tooth-bearing bones of the upper jaw (the

maxilla and premaxilla) move up and for-

wards. Rapid projection of a tube-like mouth

allows many teleosts to suck in their prey,

while others use the system to vacuum up

food particles from the seaﬂ oor, or to snip

precisely at ﬂ esh or coral.

The evolution of sharks:

an arms race with their prey?

During the Carboniferous, numerous extraor-

dinary shark-like ﬁ shes arose, and these were

clearly important marine predators. A second

shark radiation took place in the Triassic and

Jurassic. Hybodus (Fig. 16.11a) was a fast-

swimming ﬁ sh, capable of accurate steering

using its large pectoral (front) ﬁ ns. The

hybodontiforms had a range of tooth types,

from triangular pointed ﬂ esh-tearing teeth to

broad button-shaped crushers, adapted for

dealing with mollusks. It is rare to ﬁ nd whole

dorsal fin spines

anal

fin

spine

10 mm

pelvic

spine

intermediate

spines

pectoral fin

spine

head shield

pectoral finpelvic finanal fin

100 mm

trunk shield

(a)

(b)

(d)

(c)

(e)

10 mm

25 mm

lateral line canal

lower

lobe

upper lobe

pectoral fin

pelvic fin

anal fin

10 mm

dorsal fins

Figure 16.8 Jawed ﬁ shes of the Devonian: (a)

the placoderm Coccosteus; (b) the acanthodian

Climatius; (c) the actinopterygian bony ﬁ sh

Cheirolepis; (d) the lungﬁ sh Dipterus; and (e) the

lobeﬁ n Osteolepis. (Based on Moy-Thomas &

Miles 1971.)

FISHES AND BASAL TETRAPODS 439

shark fossils because the bulk of the skeleton

is cartilaginous and rots away before fossiliza-

tion. The apatite teeth and scales are more

commonly found isolated, and these and other

ﬁ sh teeth and scales, sometimes called ich-

thyoliths, have proved useful in biostratigra-

phy (Box. 16.6).

Modern sharks, called collectively neosela-

chians, are faster swimmers and more fero-

cious ﬂ esh eaters than their precursors.

Box 16.5 The Scottish Old Red ﬁ shes

The Old Red Sandstone Continent of northern Europe and Canada lay close to the equator, in hot

tropical conditions, during the Devonian. Fishes lived in the shallow seas and in landlocked lakes

around this continent. One of the best collecting areas is in the north of Scotland, where the ﬁ rst

specimens came to light nearly 200 years ago.

The ﬁ sh beds were laid down in large deep lakes (Trewin 1986). Bulky armored ostracoderms

and placoderms fed on the bottom in shallow waters, while shoals of silvery acanthodians darted

and swirled near the surface. Bony ﬁ shes, such as the actinopterygian Cheirolepis, the “rhipidistian”

Osteolepis and the lungﬁ sh Dipterus, moved rapidly through the plants near the water’s edge seeking

prey, and sometimes swam out through the deeper waters to new feeding grounds.

The ﬁ shes are usually found, beautifully preserved and nearly complete (Fig. 16.9a), in dark-

colored siltstones and ﬁ ne sandstones. These rocks were deposited in the deepest parts of the lake,

probably in anoxic (low oxygen) conditions (Fig. 16.9b). There were repeated cycles of deposition,

perhaps controlled by the climate. During times of high rainfall, great quantities of sand were washed

into the lakes from the surrounding Scottish Highlands. Lake levels then fell during times of aridity,

and in places the lakes dried out, leaving mud cracks and soils. Then ﬂ ooding occurred, together

with mass kills of ﬁ shes, perhaps as a result of eutrophication (oxygen starvation caused by decaying

algae after an algal bloom) or following storms. In all, a pile of lake deposits some 2–4 km thick

accumulated over the 40–50 myr of the Devonian, and there are dozens of ﬁ sh beds throughout this

thickness.

Read more through http://www.blackwellpublishing.com/paleobiology/.

(a)

thermocline

(b)

Figure 16.9 The Old Red Sandstone lake in northern Scotland: (a) typical preservation of two

specimens of Dipterus; and (b) model of environmental cycles in the lake. Sediment is fed in from

the surrounding uplands during times of heavy rainfall. Fishes inhabit shallow and surface waters,

but carcasses may sink below the thermocline into cold, relatively anoxic waters, where they sink

to the bottom and are preserved in undisturbed condition in dark grey laminated muds. (Courtesy

of Nigel Trewin.)

440 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD

Neoselachians radiated dramatically during

the Jurassic and Cretaceous to reach their

modern diversity of 42 families. These sharks

can open their mouths wider than their pre-

cursors, and they have adaptations for gouging

masses of ﬂ esh from their prey. The body

shape (Fig. 16.11b) is more bullet-like than in

their ancestors, and the pectoral ﬁ ns are wider

and more ﬂ exible. Neoselachians range in size

from common dogﬁ shes (0.2–1 m long) to

basking and whale sharks (16 m long), but

the monster ones are not predators: they feed

on krill, which they ﬁ lter from the water. The

skates and rays, unusual neoselachians, are

specialized for life on the seaﬂ oor, having ﬂ at-

tened bodies and broad pectoral ﬁ ns for swim-

ming by sending waves of up-and-down

motion from front to back (Fig. 16.11c).

Sharks and their kin radiated three times

during the Paleozoic, Mesozoic and Cenozoic,

and this seems to match the three-phase radia-

tion of bony ﬁ shes, palaeonisciforms, holos-

teans and teleosts. It is impossible to say

which set of evolutionary radiations came

ﬁ rst: the bony ﬁ shes had to swim faster to

escape their sharky predators, and the sharks

had to swim faster to catch their bony ﬁ sh

prey. This is a classic example of an arms race,

where predator and prey keep upping the

ante, but neither side wins.

dorsal fin

20 mm

anal fin

jaws projected

well forwards

10 mm

50 mm

maxilla

premaxilla

(a)

(b)

(c)

(d)

Figure 16.10 Evolution of the ray-ﬁ nned bony

ﬁ shes: (a) the Carboniferous palaeonisciform

Cheirodus, a deep-bodied form; (b) the Triassic

“holostean” Semionotus; (c) the Cretaceous

teleost Mcconichthys; (d) evolution of

actinopterygian jaws from the simple hinge of a

palaeonisciform (left) to the more complex jaws

of a holostean (middle) and the fully pouting

jaws of a teleost (right). (a, b, based on Moy-

Thomas & Miles 1971; c, based on Grande

1988; d, based on Alexander 1975.)

heterocercal

tail

50 mm

calcified vertebrae

highly

mobile

jaws

50 mm

pectoral fin

(a)

(b)

(c)

Figure 16.11 Sharks and rays, ancient and

modern: (a) the Jurassic shark Hybodus; (b) the

modern shark Squalus; and (c) the modern ray

Raja. (Based on various sources.)

FISHES AND BASAL TETRAPODS 441

Box 16.6 Fish teeth and scales

Isolated teeth and scale, commonly grouped as ichthyoliths (“ﬁ sh stones”), can usually be assigned

to their ﬁ shy originator (Fig. 16.12). However, there is great debate over the precise identity of many

shark teeth, and especially over the amount of variation that may occur among the teeth from a

single species – are they all the same, or do they vary in shape around the jaws? Also, many Cam-

brian and Ordovician ichthyoliths are a mystery – the original host animal is generally unknown.

The thelodonts, an ostracoderm group, were known from a rich diversity of scales from the Ordovi-

cian to Devonian, but there are only a handful of partial or complete specimens of the whole

ﬁ shes.

Ichthyoliths have been used to establish stratigraphic schemes in the Silurian, Carboniferous, Tri-

assic, Cretaceous and Tertiary. In some Paleozoic strata, teeth and scales sometimes occur in associa-

tion with conodonts, and sometimes in situations where there are no other biostratigraphically useful

fossils. It is hard work to generate workable dating schemes using such ichthyoliths, not least because

their morphology can be so intricate; but it is also frustrating that it is often impossible to relate the

ichthyoliths to the complete ﬁ shes that might have produced them.

(a)

(b)

(c)

(d)

(e)

(f)

Figure 16.12 Some microvertebrate specimens: (a) thelodont scale (Devonian); (b) thelodont body

scale (Devonian); (c) protacrodont shark tooth (Late Devonian to Early Carboniferous);

(d) acanthodian scale (Devonian); (e) shark tooth-like scale (Triassic); and (f) shark scale

(Triassic). (Courtesy of Sue Turner.)

442 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD

Figure 16.13 Skull of the Late Devonian amphibian

Acanthostega, showing the streamlined shape,

deeply-sculpted bones and small teeth, all inherited from its ﬁ sh ancestor. (Courtesy of Jenny Clack.)

TETRAPODS

The origin of tetrapods: ﬁ ns to limbs

When a ﬁ sh became a land animal, surely the

key problem was breathing air? Not so. Early

bony ﬁ shes almost certainly had both lungs

and gills, and could already breathe air when

necessary. The main problem for the ﬁ rst tet-

rapods (“four feet”), the four-legged land ver-

tebrates, was support – in water, an animal

“weighs” virtually nothing, but on land the

body has to be held up from the ground, and

the internal organs have to be supported in

some way within a strong rib cage to prevent

them from collapsing. In addition, reproduc-

tive, osmotic (water balance) and sensory

systems had to adapt, but here the changes

did not happen all at once.

Tetrapods arose from ﬁ shes during the

Devonian. But what were the closest ﬁ shy

relatives of the tetrapods? Most attention has

focused on the lobeﬁ ns, because of their

complex bony and muscular pectoral (front)

and pelvic (back) paired ﬁ ns (Fig. 16.8d, e).

Close study of lobeﬁ ns such as Osteolepis and

Eusthenopteron shows that they share many

characters of the limbs and skull with the

earliest tetrapods.

The ﬁ rst good evidence of tetrapods comes

from the Late Devonian, and they have been

studied intensively in recent years. The

best-known forms are Acanthostega and

Ichthyostega, which were 0.6 and 1 m long,

respectively. The head is still very ﬁ sh-like in

shape and sculpturing (Fig. 16.13), and the

limbs and tail are clearly still adapted for

swimming. The limbs though are remarkable.

We have always thought that our ﬁ ve ﬁ ngers

and toes are a fundamental feature of tetra-

pods, but new work shows that this was not

the case (Box 16.7).

The amphibians: half-way land animals

Amphibians today are a minor group, consist-

ing of 4000 species of mainly small animals

that live in or close to water. They show many

adaptations to life on land, but they still rely

on the water for breeding and water

balance.

Frogs and toads (anurans), known since the

Triassic, have specialized in jumping: the

hindlimbs are long and the hip bones rein-

forced to withstand the impact of landing.

The head is broad, the jaws are lined with

small teeth, and most feed by ﬂ icking a long

sticky tongue out and trapping insects. The

urodeles – salamanders and newts – date from

the Jurassic, and consist of modest-sized,

long-bodied swimming predators. The third

living amphibian group, the caecilians, are

small limbless animals that look rather like

FISHES AND BASAL TETRAPODS 443

Box 16.7 The ﬁ rst tetrapods had seven or eight toes

New studies of Ichthyostega and Acanthostega from the Late Devonian of Greenland (Coates et al.

2002), and of other animals of the same age, including the “limbed ﬁ sh” Tiktaalik from Arctic

Canada (Daeschler et al. 2006; Shubin et al. 2006), show that the ﬁ rst tetrapods had more than

ﬁ ve ﬁ ngers and toes, indeed as many as seven or eight (Fig. 16.14b, c). It is possible to draw com-

parisons between the bones of the pectoral ﬁ n of a sarcopterygian (Fig. 16.4a) and those of the

forelimb of an early amphibian (Fig. 16.14b). This caused a major rethink of the classic story of the

evolution of vertebrate limbs: ﬁ ve digits must have become standard only after the origin of tetra-

pods. What if tetrapods had settled on seven, rather than ﬁ ve, ﬁ ngers? Probably we would not use

the decimal counting system, and imagine the changes to musical instruments and computer

keyboards!

The implications are wider, because the new evidence suggests that particular features of an organ-

ism may not all be preprogrammed in the genetic code of the developing embryo. In other words,

there is not a single gene that codes for each ﬁ nger and toe. It seems that aspects of the developmental

environment, rather than genetic programming, determine some details of adult structure: as a limb

develops in the embryo, at ﬁ rst it has no ﬁ ngers or toes, and then a pulse of information triggers

the sprouting of digits at a particular time. In rare cases, humans may be born with a sixth ﬁ nger,

perhaps a genetic memory of our condition 400 Myr ago. Also, many tetrapods have only four

(frogs), three (rhinos), two (cows) or one (horses) ﬁ nger – perhaps losing digits is associated with

the “switching” on or off of particular controlling genes.

Read more about the basal tetrapods and the ﬁ n to limb transition in Zimmer (1999), Clack

(2002) and Shubin (2008) and at http://www.blackwellpublishing.com/paleobiology/.

earthworms, and live largely in soil and leaf

litter in tropical lands. The oldest fossil form,

with reduced limbs, is Jurassic in age. All

living amphibians appear to be closely related,

forming a clade, the Lissamphibia (Box 16.8),

characterized by the structure of their tiny

teeth.

The lissamphibians form part of a larger

clade, the batrachomorphs. The most impor-

tant fossil batrachomorphs are the “temno-

spondyls”, a paraphyletic group that was

important in Carboniferous communities, and

continued with reasonable success through

the Permian and Triassic, ﬁ nally dying out in

the Early Cretaceous. Temnospondyls have a

low round-snouted skull (Fig. 16.15a, b), and

most of them appear to have operated like

sluggish crocodiles, living in or near freshwa-

ters and feeding on ﬁ shes. Some temnospon-

dyls became fully terrestrial, and others

evolved elongate gavial-like snouts for catch-

ing rapidly swimming ﬁ shes. Some Carbonif-

erous temnospondyls had tadpole young, just

as modern amphibians do. This proves that

they had a similar developmental pattern to

modern amphibians, with an aquatic larval

stage, the tadpole, that metamorphoses into

the adult land-living form. Relatives of the

temnospondyls included small forms, the

aquatic nectrideans and the aquatic and ter-

restrial microsaurs.

The second amphibian lineage, the reptili-

omorphs (see Box 16.8), included important

groups in the Carboniferous and Permian, as

well as the ancestors of reptiles, birds and

mammals. Anthracosaurus had a longer, nar-

rower skull than the temnospondyls, but may

have had similar lifestyles – hunting prey on

land and in freshwaters. Some Permian rep-

tiliomorphs, such as Seymouria (Fig. 16.15c)

were seemingly adapted to a fully terrestrial

life. Seymouria has long limbs and a relatively

small skull, and probably hunted microsaurs

and other small tetrapods.

REIGN OF THE REPTILES

Making the break: the origin of the reptiles

Amphibians only made it halfway on to land,

and they still produce swimming tadpoles.

Continued

444 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD

(c)

(d)

(b)

(a)

humerus

radius

lepridotrichia

ulna

humerus

radius

ulna

8 fingers

tibia

fibula

femur

7 toes

Figure 16.14 Matching ﬁ ns and legs of the ﬁ rst tetrapods: the pectoral ﬁ n of the Devonian

sarcopterygian ﬁ sh Eusthenopteron (a) shows bones that are probable homologs of tetrapod arm

bones, such as in the Devonian amphibian Acanthostega (b). Acanthostega had eight ﬁ ngers and

Ichthyostega had seven toes on its hindlimb (c). (d) The early tetrapod Acanthostega. (Courtesy

of Mike Coates.)

Reptiles and their descendants made a clean

break from the water by producing a kind of

egg that did not have to be laid in water.

The cleidoic (“closed”) egg, sometimes

called the amniotic egg, is enclosed within a

tough semipermeable shell, hence its name.

This egg type is seen in all members of the

clade Amniota (reptiles, birds, mammals): it

is the familiar hen’s egg that we eat for break-

fast. Primitive mammals, such as the platypus,

still lay eggs, but most mammals have sup-

pressed the egg and it “hatches” inside the

FISHES AND BASAL TETRAPODS 445

Box 16.8 Classiﬁ cation of amphibians

The amphibians are a paraphyletic group as they exclude their descendants, the reptiles. Modern

amphibians are clearly distinguishable from the diverse fossil groups.

Superclass TETRAPODA

Subclass BATRACHOMORPHA (Amphibia)

Order “TEMNOSPONDYLI”

• Broad-snouted, low-skulled amphibians, showing a range of sizes

• Early Carboniferous to Early Cretaceous

Infraclass LISSAMPHIBIA

• Frogs, salamanders (newts) and caecilians (gymnophionans)

• Early Triassic to Recent

Order NECTRIDEA

• Small slender aquatic forms

• Early Carboniferous to Late Permian

Order MICROSAURIA

• Terrestrial and aquatic long-bodied forms with deep skulls

• Early Carboniferous to Early Permian

Subclass REPTILIOMORPHA

Order ANTHRACOSAURIA

• Narrow-skulled, ﬁ sh-eating amphibians

• Early Carboniferous to Late Permian

Order SEYMOURIAMORPHA

• High-skulled terrestrial amphibians

• Late Carboniferous to Late Permian

(a)

250 mm

10 mm

(c)

(b)

Figure 16.15 Fossil amphibians: (a) skull of the Early Triassic temnospondyl Benthosuchus;

(b) skeleton of the Early Permian temnospondyl Eryops; and (c) skeleton of the Early Permian

reptiliomorph Seymouria. (a, courtesy of Mikhail Shishkin; b, c, based on Gregory 1951/1957.)

446 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD

mother’s womb. The amniotic eggshell is

usually hard and made from calcite, but some

lizards and snakes have leathery eggshells.

The shell retains water, preventing evapora-

tion, but allows the passage of gases, oxygen

in and carbon dioxide out. The developing

embryo is protected from the outside world,

and there is no need to lay the eggs in water,

nor is there a larval stage in development.

Inside the eggshell is a set of membranes that

enclose the embryo (the amnion), that collect

waste (the allantois) and that line the eggshell

(the chorion) (Fig. 16.16). The chorion is the

thin papery tissue just inside the eggshell,

which you peel off a hard-boiled egg. Food is

in the form of yolk, a yellow material rich in

protein.

The oldest-known amniote, Hylonomus

from the mid-Carboniferous of Canada (Fig.

16.17a, b) is known only from its skeleton;

no amniotic eggs are known from the Car-

boniferous. Hylonomus has been superbly

well preserved inside ancient tree stumps, into

which it crawled in pursuit of insects and

worms, and then was overwhelmed by ﬂ ood-

waters. Hylonomus looks little different from

some amphibians of the time, such as the

microsaurs, but it shows several clearly

amniote characters, a high skull, evidence for

allantois

yolk sac

chorion

egg

shell

amnion

Figure 16.16 The cleidoic egg of amniotes in

cross-section, showing the eggshell and extra-

embryonic membranes.

orbit

nostril

astragalus

10 mm

high skull

(a)

Anapsid

DiapsidSynapsid

(b)

(c)

(d) (e)

Figure 16.17 The earliest reptile, and early reptile evolution: (a, b) the mid-Carboniferous reptile

Hylonomus, skeleton and skull; (c-e) the three major skull patterns seen in amniotes: anapsid, diapsid

and synapsid. (Based on Carroll 1987.)

FISHES AND BASAL TETRAPODS 447

additional jaw muscles and an astragalus bone

in the ankle. We know Hylonomus laid clei-

doic eggs because of the shape of the evolu-

tionary tree: how can that be?

Amniotes radiated during the Late Carbon-

iferous, giving rise to three main clades.

These are distinguished by the pattern of

openings in the side of the skull, especially

the temporal openings behind the eye socket

(Fig. 16.17c–e). The primitive state is termed

the anapsid (“no arch”) skull pattern, since

there are no temporal openings. The two

other skull patterns seen in amniotes are

the synapsid (“same arch”), where there is a

lower temporal opening, and the diapsid

(“two arch”) pattern, where there are two

temporal openings. These temporal openings

correspond to low-stress areas of the skull,

and the edges serve as attachment sites for jaw

muscles.

These three skull patterns diagnose the key

clades among amniotes (Fig. 16.18; Box 16.9).

The Anapsida include various early forms

such as Hylonomus, as well as some Permian

and Triassic reptiles, and the turtles. The Syn-

apsida include the “mammal-like reptiles”

and the mammals, and the Diapsida includes

a number of early groups, as well as the lizards

and snakes and the crocodiles, pterosaurs,

dinosaurs and birds. All modern amniotes

produce the cleidoic egg, and the structure is

so similar that we can be sure this egg arose

at the base of clade Amniota; so, Hylonomus,

clearly located within the evolutionary tree

of Amniota, must have shared the cleidoic

egg.

550 500 450 400 350 300 250 200 150 100 520

Paleozoic Mesozoic Cenozoic

Cambrian Ordovician

Carboniferous

TriassicPermianDevonianSilurian CretaceousJurassic Tertiary

Time (Ma)

Armored agnathans

Conodonts

Quaternary

III

VERTEBRATA

GNATHOSTOMATA

TETRAPODA

BATRACHOMORPHA

REPTILIOMORPHA

AMNIOTA

SYNAPSIDA

ANAPSIDA

DIAPSIDA

ARCHOSAURIA

Acanthodians

Osteichthyes

Ichthyostega

Nectridea

Microsauria

Temnospondyli

Anthracosauria

Seymouriamorpha

Hylonomus

Pelycosauria

Therapsida

Mammalia

Procolophonidae

Erythrosuchus

Crocodylia

Dinosauria

Aves

Ichthyosauria

Plesiosauria

Pterosauria

Lepidosauria

Testudines

145

205

250

286

360

408

438

505

550

Chondrichthyes

lampreys, hagfishes

Lissamphibia

Figure 16.18 Phylogeny of the major groups of ﬁ shes and tetrapods.