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

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458 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD

Box 17.1 Paleobiology of the largest animals ever

When the monster sauropods of the Late Jurassic were ﬁ rst discovered in the 19th century,

many paleontologists thought that they were too big to have lived fully on land. It was

assumed that the sauropods lived in lakes, supporting their bulk in the water, and feeding on

waterside plants. New evidence shows, however, that life on land was quite possible, and sauropods,

like elephants today, could move freely over vast plains, and in and out of the water at times as

well.

Sauropods may have divided their feeding preferences by height. Modern herbivores show such

niche partitioning, where each animal has its preferred food and feeding mode. Some sauropods

such as Brachiosaurus (Fig. 17.3b) may have been like super giraffes, feeding on leaves from very

tall trees. Most other sauropods, however, were designed for lower-level browsing, and were proba-

bly not able to raise their necks much above horizontal. This allowed several species of sauropods

to live side by side, some feeding on low plants, others on mid-height shrubs, and yet others on the

leaves of trees.

But how did sauropods get to be so big? Brachiosaurus and relatives reached lengths of

20 m or more, and some weighed as much as 50 tonnes. Did they take 70 or 100 years

to reach sexual maturity, growing at the same rate as a modern crocodile, as some

paleobiologists have suggested? Or did they grow fast, like modern mammals? An elephant

reaches sexual maturity at about 15 years old, while a blue whale grows even faster, reaching

sexual maturity at 5–10 years. Blue whales can put on as much as 90 kg per day (equivalent to

30 tonnes per year) during their fastest juvenile growth. How can you tell how fast a dinosaur

grew?

The secret is in the bones. Modern reptiles grow in ﬁ ts and starts – bursts of fast growth when

food is plentiful, and very slow growth when they are starved. Typically, there is one good

season and one poor season each year, and this is shown in growth rings in the bone. Greg

Erickson, a paleontologist at Florida State University in Tallahassee, and Martin Sander, a

paleontologist from the University of Bonn in Germany, have sectioned the ribs and leg

bones of many dinosaurs, and they have counted the growth lines or “lines of arrested growth”

(LAGs). The bones are cut through, and thin sections are glued on to large glass slides, and

then ground down to a uniform thickness, so light can pass through. Erickson and Sander

have examined these thin sections under a polarizing light microscope that highlights the

crystalline apatite components of the fossil bone and allows them to count the LAGs easily

(Fig. 17.4a).

Bone histologists, people who study the microscopic structure of bone, argue that each

LAG represents a year because this is the case in living reptiles and other animals that have

them. Paleontologists can count the LAGs to give the age when a dinosaur died, and then

compare these ages with estimates of body masses assessed from the size of the bones. Erickson

has looked at a growth series of bones, from tiny (well actually quite big) baby Apatosaurus

through juveniles to fully adult specimens. These suggest that Apatosaurus juveniles stayed

pretty small for the ﬁ rst 5 years of their life, and then there was a burst of growth from age 5 to

12, when they put on up to 5 tonnes a year, to reach a young adult body mass of 26 tonnes, pre-

sumably the age of sexual maturity (Fig. 17.4b). So the LAGs indicate stop–start seasonal patterns

of growth, just like a modern reptile, but the rate of juvenile growth is much more like a bird or

mammal. Sauropods did not have to wait until they were 100 years old before they could have

sex.

Read more about sauropod bone histology in Erickson et al. (2001) and Sander and Klein

(2005) and Sander et al. (2006) and ﬁ nd a selection of the best dinosaur web sites at http://www.

blackwellpublishing.com/paleobiology/.

DINOSAURS AND MAMMALS 459

(a)

(b) Age (years)

Mass (kg)

5000

10,000

15,000

20,000

25,000

30,000

10 15

Maximum growth

rate = 5466 kg yr

–1

Figure 17.4 Measuring the growth rate of a sauropod dinosaur. (a) Cross-section through the

bone wall of the femur of the sauropod Janenschia from the Late Jurassic of Tanzania; the animal

was full grown and the femur was 1.27 m long. The section was made by drilling into the bone

and extracting a core that was then cut through; the center of the bone is to the left, the outside

to the right. Lines of arrested growth are the darker bands, where the bone structure is tighter,

indicating a slow-down in growth. These are marked off with tick marks on the side of the slide.

(b) Growth curve for the sauropod Apatosaurus based on sections from the limb bones and ribs

of several individuals, juveniles and adults, showing how the animal reached adult size with a

spurt of growth from years 5 to 12. (Courtesy of Martin Sander and Greg Erickson.)

200 mm

stiff tail

huge slashing

claw

1 m

square-fronted ilium

tiny

arm

(a)

(b)

Figure 17.5 Cretaceous theropod dinosaurs:

(a) Deinonychus, and (b) Tyrannosaurus. (a, based

on Ostrom 1969; b, based on Newman 1970.)

ichthyosaurs (Fig. 17.9a) were ﬁ sh-shaped

animals, entirely adapted to life in the sea,

and they evolved from land-living diapsids.

Ichthyosaurs had long, thin, snouts lined with

sharp teeth, and they fed on ammonites, bel-

emnites and ﬁ shes. Exquisite preservation of

many specimens shows the tail ﬁ n, dorsal ﬁ n

and the paddle outlines. Ichthyosaurs swam

by beating the body and tail from side to side,

and they used the front paddles for steering.

There are even some remarkable specimens of

mothers with developing embryos inside their

bellies: like whales and dolphins, ichthyosaurs

could not ﬂ op up on to land to lay eggs, and

they gave birth to live young while at sea.

The second major marine reptile group was

the plesiosaurs. Most plesiosaurs had long

necks and small heads (Fig. 17.9b), but the

pliosaurs were larger and had short necks and

large heads. Plesiosaurs fed mainly on ﬁ shes,

using their long neck like a snake to dart after

460 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD

fast-moving prey, and they swam by beating

their paddles in a kind of “ﬂ ying” motion.

The extraordinary diversity of tetrapod pred-

ators in the sea came to an end 65 Ma during

the great Cretaceous-Tertiary mass extinction

(see pp. 174–7) that saw the end of the dino-

saurs and pterosaurs too.

BIRD EVOLUTION

One of the most famous fossils is Archaeop-

teryx, the oldest known bird (Fig. 17.10). The

ﬁ rst specimen was found in Upper Jurassic

sediments in southern Germany in 1861, and

was hailed as the ideal “missing link” or proof

of evolution in action. Here was an animal

with a beak, wings and feathers, so it was

clearly a bird, but it still had a reptilian bony

tail, claws on the hand, and teeth. Since 1861,

nine more skeletons have come to light, the

last two in 1992 and 2005.

Archaeopteryx was about the size of a

magpie, and it fed on insects. The claws on

its feet and hands suggest that Archaeopteryx

could climb trees, and the wings are clearly

those of an active ﬂ ying animal. This bird

could ﬂ y as well as most modern birds, and

ﬂ ying allowed it to catch prey that were not

available to land-living relatives. The skeleton

of Archaeopteryx is very like that of Deinony-

chus (see Fig. 17.5a), especially in the details

of the arm and hindlimb, showing that birds

are small ﬂ ying theropod dinosaurs.

Until recently, birds remained rare until the

Late Cretaceous, but now numerous spectac-

ular fossils of birds and dinosaurs, with feath-

ers preserved, have been reported from China,

and astonishing new specimens are announced

every month (Box 17.3). In the Late Creta-

ceous, new sea birds radiated. They still had

teeth, but the bony tail was reduced to a short

knob as in modern birds, and they had other

modern features. Modern groups of birds

appeared in the latest Cretaceous and Early

Tertiary, including ﬂ ightless ratites and ances-

tors of water birds, penguins and birds of

prey. The perching birds or songbirds, consist-

ing of 5000 species today, radiated in the

Miocene.

0.5 m

ossified tendons

tall neural spines

(a)

(b)

(c)

Figure 17.6 Armored ornithischian dinosaurs

from the Jurassic (a) and Cretaceous (b, c):

(a) Stegosaurus, (b) Euoplocephalus, and (c)

Centrosaurus. (a, c, based on Gregory 1951; b,

based on Carpenter 1982.)

Figure 17.7 Skull of the Late Cretaceous

hadrosaur Edmontosaurus.

DINOSAURS AND MAMMALS 461

Box 17.2 Oldest dinosaur embryos

Dinosaur eggs and nests have been known since the 1860s when the ﬁ rst ﬁ nds were made in France.

The most famous ﬁ nds were made by American expeditions to the Late Cretaceous of Mongolia in

the 1920s, when whole nests with eggs were brought back to the American Museum of Natural

History. Since then, hundreds of dinosaur nest sites have been found. There is no question that

dinosaurs laid eggs, and they placed them in nests they kicked out of the dirt on the ground. Dino-

saurs did not build nests in trees, for rather obvious reasons!

Maybe dinosaurs covered their nests with sand, and left them to develop, or maybe they covered

them with plant debris, which formed a kind of compost to keep the babies warm, as some crocodil-

ians do today. Some dinosaur mothers even incubated their eggs: a mother Oviraptor was found in

the 1990s sitting on a nest in Mongolia.

Most spectacular of all are unhatched eggs containing embryos inside. The oldest examples were

reported in 2005, when Robert Reisz and colleagues announced some eggs laid by the prosauropod

Massospondylus in the Early Jurassic of South Africa. The researchers X-rayed the eggs, and saw

the little bones inside, so Diane Scott, a skilled preparator, took a year of painstaking work to remove

the eggshell on one side, and pick the mudstone off the bones with a ﬁ ne needle, to reveal – a com-

plete little baby, just about to hatch (Fig. 17.8). Reisz and colleagues believe the baby Massospon-

dylus would have walked on all fours when it was born, but it had to be looked after. Its teeth were

just too small for it to be able to tear up plants by itself. Is this the oldest evidence for parental care

in the fossil record?

Read more about dinosaur eggs and nests at http://www.blackwellpublishing.com/paleobiology/.

The full description is in Reisz et al. (2005).

Figure 17.8 Photograph of one of the Massospondylus eggs with a complete embryo skeleton

inside, measuring some 15 cm in total length. It died just before hatching. As an adult, it would

have grown to a length of 5 m. (Courtesy of Robert Reisz.)

462 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD

RISE OF THE MAMMALS

Primitive forms, and then success

The ﬁ rst mammals, small insect eaters in the

Late Triassic and Early Jurassic (see p. 450),

probably hunted at night. Mammals remained

small through most of the Mesozoic and they

did not achieve high diversity, perhaps held in

check in some way by the dinosaurs. Several

lines of insectivorous, carnivorous and her-

bivorous forms appeared, some of them

adapted to climbing trees. Most Mesozoic

mammals were small, and a recent ﬁ nd from

China proved to be an exception (Box 17.4).

Nonetheless, most basal mammal groups did

not outlive the Cretaceous-Tertiary mass

extinction (see pp. 174–7). Three of the clades

that did survive into the Tertiary were the

monotremes, marsupials and placentals, the

modern groups (Box 17.5).

Monotremes today are restricted to Aus-

tralasia, being represented by the platypus

and the echidnas. These mammals are unique

in still laying eggs, as the cynodont ancestors

of mammals presumably did. The young hatch

out as tiny helpless creatures, and feed on

their mother’s milk until they are large enough

to live independently.

It is often said that mammals owe every-

thing to their teeth. The marsupials, and espe-

cially the placentals, radiated dramatically in

the Tertiary, and this is often taken as a classic

example of an adaptive radiation (see p. 544).

It is notable that previously rather small,

similar-looking animals had diversiﬁ ed within

10 myr to forms as disparate as bats and rats,

monkeys and whales. Mammals uniquely

have differentiated teeth, with incisors, canines

and cheek teeth. Fishes, amphibians and rep-

tiles have undifferentiated teeth – their teeth

are pretty much identical from front to back.

Differentiated teeth allow mammals to adopt

a huge array of diets, and to become super-

efﬁ cient at biting, and especially chewing.

High metabolic rates need lots of nutritious

food, and chewing, moving the cheek teeth

round and across the food, allows mammals

to improve the efﬁ ciency of their digestive

systems. Most dinosaurs (except ornithopods;

see p. 457), like reptiles in general, could not

chew, or at least not well – so they swallowed

their food whole and probably failed to digest

much of it. One thing is for sure: don’t stand

downwind of a dinosaur!

Marsupials: the pouched mammals

Marsupial young are also born tiny and help-

less, and have to feed on maternal milk in a

(a)

(b)

Figure 17.9 Jurassic marine reptiles: (a) the ichthyosaur Stenopterygius and (b) the plesiosaur

Cryptoclidus. (Courtesy of Rupert Wild.)

long bony tail

three

fingers

on hand

20 mm

Figure 17.10 The oldest bird, Archaeopteryx,

from the Late Jurassic. (Courtesy of Andrzej

Elzanowski.)

DINOSAURS AND MAMMALS 463

Box 17.3 The spectacular birds of Liaoning

Back in 1984, local farmers in Liaoning Province, China, began to send fossils they had found in

their ﬁ elds to paleontologists in Beijing and Nanjing. The fossils were all fantastically well preserved:

ﬁ shes with skin, insects with color patterns, birds and dinosaurs with feathers, and early mammals

with hair. The Chinese paleontologists began to publish accounts of the specimens, and they mounted

organized digs to recover many tonnes of fossiliferous sediments. It seems all these ancient creatures

had swum, or fallen, into ponds where ﬁ ne lime muds were accumulating and locking in the cadav-

ers before they could decay.

So far, about 20 species of birds have been named from the Liaoning sites (Zhou et al. 2003),

including Confuciusornis, perhaps the best-known new genus. The most amazing specimen of Con-

fuciusornis shows a male and female bird of the same species sitting side by side on the slab (Fig.

17.11). The male has long, streamer-like tail feathers, almost certainly brightly colored in life and

used in sexual displays. Confuciusornis was about the size of a rook, and it is advanced over Archae-

opteryx in that it has no teeth in the jaws, and its bony tail has been reduced to a nubbin of bone,

or pygostyle. But the Chinese bird is still primitive in having powerful ﬁ ngers and claws on its wings.

Nonetheless, its feathers are like those of any modern bird, and the confuciusornithids almost cer-

tainly ﬂ apped in and out of the trees in China 125 Ma, swooping after prey and landing gracefully

on the branches, just like any modern bird.

Not only birds with feathers, but dinosaurs too! Since 1995, a string of reports of small theropod

dinosaurs from Liaoning have shown that many ﬂ esh-eating dinosaurs also had feathers, even though

they did not ﬂ y. The feathers were presumably initially for insulation, so the theropod dinosaurs at

least must have been warm-blooded. So, in the evolution of birds, feathers came ﬁ rst, perhaps as

early as the Early Jurassic and then wings and ﬂ ight came in the Late Jurassic when Archaeopteryx

evolved.

Read more about the Liaoning birds and dinosaurs in Zhou et al. (2003), and at web sites linked

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

Figure 17.11 Two examples of the Early Cretaceous bird Confuciusornis from Liaoning, China,

showing a male (below, with long tail streamers) and a female. (Courtsey of Zhou Zhonghe.)

464 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD

Box 17.4 Mammal eats dinosaur shock!

A common view about the mammals of the Mesozoic is that they were all rather small, and that

this was because the dinosaurs preyed on them and prevented any becoming large. Two new

mammals from the mid-Cretaceous of China have turned this idea over: one was as big as a dog,

the other as big as a cat, and one of them had just eaten a dinosaur – admittedly a baby dinosaur

only 140 mm long.

The new fossils come from the classic localities around Liaoning, China, that have produced so

many spectacular fossils of dinosaurs, birds (see Box 17.3), salamanders, ﬁ shes, insects and plants.

Yaoming Hu, a graduate student at the American Museum of Natural History in New York and his

colleagues from China described two new species, Repenomamus giganticus and R. robustus, both

based on excellent skeletons (Hu et al. 2005), 1.0 and 0.4 m in length, respectively. Repenomamus

is a triconodont, a group known otherwise mainly from isolated jawbones, and thought before to

have specialized in eating insects. One specimen of R. robustus had the remains of a baby Psittaco-

surus, a ceratopsian dinosaur (see p. 457), torn into chunks inside its rib cage, in the region of the

stomach (Fig. 17.12).

“This is a good man-bites-dog story”, commented paleontologist Kevin Padian, when the discov-

ery was announced. Read a review of Mesozoic mammals in Luo (2007), and ﬁ nd out more about

Repenomamus, and see color images, at http://www.blackwellpublishing.com/paleobiology/.

bones of juvenile

Psittacosaurus in stomach

(b)

(a)

50 mm

Figure 17.12 The dog-sized triconodont mammal, Repenomamus, from the mid-Cretaceous

of Liaoning, China: (a) reconstruction of this mammal eating a small Psittacosaurus, and

(b) specimen showing Psittacosaurus bones inside the rib cage. (Courtesy of Hu Yaoming.)

DINOSAURS AND MAMMALS 465

Box 17.5 Classiﬁ cation of mammals

Modern mammals fall into three groups – the monotremes, marsupials and placentals – characterized

by their breeding modes. Various primitive groups are omitted.

Class MAMMALIA

Subclass MONOTREMATA

• Females lay eggs, and newborn young grow in pouch

• Early Cretaceous to Recent

Subclass METATHERIA (marsupials and extinct relatives)

• Young are born live, but continue development in pouch

• Late Cretaceous to Recent

Subclass EUTHERIA (placentals and extinct relatives)

• Young are born live at an advanced stage, having been nourished by a placenta while in the

womb. Main orders only are listed

• Mid-Cretaceous to Recent

Infraclass AFROTHERIA

Order PROBOSCIDEA (elephants)

• Eocene to Recent

Infraclass XENARTHRA

Order EDENTATA (armadillos, tree sloths, anteaters)

• Paleocene to Recent

Infraclass LAURASIATHERIA

Superorder BOREOEUTHERIA

Order LIPOTYPHLA (“insectivores”: hedgehogs, moles, shrews)

• Paleocene to Recent

Order CHIROPTERA (bats)

• Eocene to Recent

Order ARTIODACTYLA (pigs, hippos, camels, cattle, deer, giraffes, antelopes)

• Eocene to Recent

Order CETACEA (whales and dolphins)

• Eocene to Recent

Order PERISSODACTYLA (horses, rhinos, tapirs)

• Eocene to Recent.

Order CARNIVORA (dogs, bears, cats, hyaenas, seals)

• Paleocene to Recent

Superorder EUARCHONTOGLIRES

Order PRIMATES (monkeys, apes, humans)

• Paleocene to Recent

Order RODENTIA (mice, rats, squirrels, porcupines, beavers)

• Paleocene to Recent

Order LAGOMORPHA (rabbits and hares)

• Eocene to Recent

466 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD

0.5 m

(a)

(b)

20 mm

Figure 17.13 Extinct marsupials: (a) the

sabretooth Thylacosmilus from South America,

and (b) the giant herbivore Diprotodon from

Australia. (Based on Gregory 1951.)

pouch for many months, but egg laying has

been abandoned. The oldest marsupial fossils

come from the mid-Cretaceous of North

America. The group radiated successfully in

South America during the Tertiary, and

included several lines of insectivores, carni-

vores and herbivores, many of which were

remarkably like unrelated placental mammals

elsewhere. Some forms were dog-like, and

Thylacosmilus (Fig. 17.13a) independently

evolved all the characters of the placental

saber-toothed cats of Europe and North

America.

In Australia, the marsupials diversiﬁ ed even

more, after reaching that continent in the

Eocene by traveling across a much warmer

Antarctica, which then linked the southern tip

of South America with Australia. Once there,

the Australian marsupials radiated to parallel

placental mammals in functions and body

forms, except of course for the unique kanga-

roos. In the Pleistocene, there were abundant

and diverse faunas of large marsupials,

including giant kangaroos and the hippopota-

mus-sized herbivorous Diprotodon (Fig.

17.13b).

Palaeogeography and diversiﬁ cation of

the placentals

Placental mammals produce young that are

retained in the mother’s womb much longer

than is the case in marsupials, and they are

nourished by blood passed through the pla-

centa. The oldest is the mid-Cretaceous

Eomaia from Liaoning in China. Many fossil

placental mammals have been reported from

the Late Cretaceous, but most are rather

incomplete, and sometimes their classiﬁ cation

has been controversial.

Mammalogists have struggled for two or

more centuries to understand the relation-

ships of the major groups of living placentals

– are cattle related to horses, bats to monkeys,

whales to seals? Some morphological evidence

was found to show that, for example, rabbits

and rodents are sister groups, elephants are

closely related to the enigmatic African

hyraxes and the aquatic sirenians, but many

other supposed relationships were hotly dis-

puted. Now, however, everything seems to

have been resolved (Box 17.6).

Some time early in the Late Cretaceous, the

placental mammal clade split into four. First

to split off were the Afrotheria, and that clade

continued to evolve in Africa. Then the Xen-

arthra became isolated in South America. The

Boreoeutheria remained in the northern hemi-

sphere and split there into Laurasiatheria and

Euarchontoglires. So the split into placentals

in Africa, South America and Laurasia (North

America–Europe–Asia) seems to have been

central to the diversiﬁ cation of the group, and

it ties perhaps with the split of major conti-

nents through the mid-Cretaceous, with the

South Atlantic splitting South America from

Africa, and with other oceans separating those

southern continents from North America,

Europe and Asia.

Placentals in southern continents

The Afrotheria (“African mammals”) are

known best by the elephants. The African and

Indian elephants of today (Proboscidea) are a

sorry remnant of a once-diverse group. Early

elephant relatives such as Moeritherium (Fig.

17.15a) were small hippo-like animals that

DINOSAURS AND MAMMALS 467

Box 17.6 Mammals, morphology and molecules

The classiﬁ cation of living placental mammals has long been mysterious. We know what a whale or

a bat or a primate is, but how do these major orders relate to each other? The techniques of molecular

phylogeny estimation (see pp. 133–4) have revealed the answer.

The story began when Mark Springer and colleagues (1997) discovered the Afrotheria, a clade

consisting of African animals, linking the elephants (Proboscidea), hyraxes and sirenians with the

aardvarks (Tubulidentata), tenrecs and golden moles. The last three groups had all been assigned

various positions in the classiﬁ cation of mammals, but their genes show they shared a common

ancestor with the elephant – hyrax – sirenian group. After 1997, everything else fell into place (Fig.

17.14). The South American placentals, the edentates, formed a second major group, the Xenarthra.

And the remaining mammalian orders formed a third major clade, the Boreoeutheria (“northern

mammals”), split into Laurasiatheria (insectivores, bats, artiodactyls, whales, perissodactyls, carni-

vores) and Euarchontoglires (primates, rodents, rabbits). So, in the course of 2 or 3 years, several

independent teams of molecular biologists solved one of the outstanding puzzles in the tree of life

(e.g. Springer et al. 2003; Asher 2007).

But why had this proved to be such a phylogenetic puzzle? Some suggest that the major splits

among placental mammals happened very rapidly, and there was no time for shared morphological

characters to become ﬁ xed. But the morphologists are ﬁ red up to ﬁ nd such characters: if the Afroth-

eria is really a clade, then there must be some obscure anatomic feature shared among them all! The

hunt goes on.

Read a review of Afrotheria in Tabuce et al. (2008), and ﬁ nd out more about the search for

morphological characters of the clade at http://www.blackwellpublishing.com/paleobiology/.

Tubulidentata

Proboscidea

Hyracoidea

Sirenia

XENARTHRA

Lipotyphla

Chiroptera

Artiodactyla

Cetacea

Perissodactyla

Carnivora

Primates

PLACENTALIA

AFROTHERIA

BOREOEUTHERIA

Lagomorpha

Rodentia

EUARCHONTOGLIRES

LAURASIATHERIA

Figure 17.14 Cladogram of the major orders of placental mammals based on molecular evidence.

The four deep splits among modern orders happened in the Late Cretaceous, but modern

placentals did not become diverse until after the extinction of the dinosaurs.