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^ rF^owards

  the end of the

XV III century, the Scottish

poet Rober t Burns was

ploughing when he dug up a tiny

harvest mouse that had made its

nest in his field. T h e harvest

mouse  Micromys minutus)  is the

smallest European rodent, a mere

7 cm in  body length and weighing

a mere 6 gr. So impressed was the

poet by his find that it inspired

him to write his famous poem .

"Wee, sleeckit, cowrin,  tim'rous  beastie,

O, what a panic's in thy breastie "

(To a Mouse, R. Burns 1785).

If we were to imagine Robert Burns

ploughing a field on another farm in

South America 10 million years (Ma)

ago,  his encounter with a rodent would

surely have taken a quite different turn .

Rodents make-up approximately 40%

of all living mamm als. We are all familiar

with rats, mice, hamsters, squirrels, mar

mots, castors, prairie dogs, etc., but ther e

The

  t i m ' r o u s g i a n t s

the largest rodents in the world

By M aria  Guiomar Vucetich

\ \

  i /•

  •

  / - I

  Of

,- //

are a many more rodents that because

their small size or their way of life

hardly known although routinely s tud

by biologists and m edical researchers

Rodents are a success s tory am

mammals for they have adapted to

broad range of environments: from lo

lying plains to high altitudes, to fore

of every kind, deserts, and continen

waters. Some can even glide over sh

distances. Only the oceans have yet to

colonized by these versatile mammals

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Rodents can be solitary or gregarious.

  worms, insects, larvae, and

rodents probably originated during the

Lower Eocene (about 55 million years

ago). N ot long after this , they migra t

ed to Europe and

N or th A mer ica ,

eventually reaching

Africa. Ultimately,

probably before

the end of the

Eocene (more than

34 million years ago), rodents invaded

South America, via Africa, crossing on

floating debris. Of course, the two con

tinents were much closer together 34

million years ago. Once settled in South

America, rodents proliferated spectacu

larly, giving rise to strange types, some

of which resemble other mammals. For

example, the capybara can be considered

as a small version of the African pygmy

hippopotamus while agouties remind us

of the mouse deer.

Rodents are classified into three main

groups. The Myomorpha includes rats ,

mice, hamsters and many other s imilar

Rodents

 are a

 success story

among; mamm als

species. T h e Sciurom orpha encompass

squirrels , marmots, gophers and beave

To the Hystricognathi, also known as t

C a v i o m o r p h a

belong many sp

cies endemic

South Americ

such as vizcacha

chinch illas, mara

cuises, capybara

coypos, porcupines and tuco tucos. T

Hystricognathi also include Old Wor

species like the p orcu pine s of Asia, Afri

and Europe, the cane rats of sub-Sahar

Africa, and the naked African mole r

that l ives in colonies underground.

Rodents are generally small in size, the

body mass ranging from a few grams

some mice (like the poor

 Micromys min

tus

  of Robert Burns ' poem) up to 70 k

the weight of a capybara

 {Hydrochoe

hydrochaeris)

  the giant of l iving rode

Mo dern rodents rare ly weigh more than

kilo and species with a body mass excee

ing 10 kilos are the exception. A goo

Figure 2.

r

People are sometimes scared or put-off

  fur

From Asian fossils we know  th t

Number 76 , Summer 2004

n tochoerus   (skull in ventral view)

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example is the castor (Sciuromorpha),

native to North America and Europe.

The hystricognath, also boast several

species that weigh ov er 10 kilos. M os t of

these are found in South America.

It's interesting that of all the mammal

orders rodents have the largest range in

size, because, although most rodents are

small, there was a t ime when a few reache d

size. This development of large sizes

occurred more or less simultaneously but

independently in different lineages. It

is a curious phenomenon, for which we

still lack a clear explanation.

But let's go back to Robert Burns living

on a hypothetical farm in South America

ten million years ago. What might he

have come across on his farm in Central

Figure 3.

a,

 Neochoerus

b, capybara c, rat

enormous size. According to recen t s tud

ies,

  the extinct South American cavio-

morph  Phoberomys,

  was ten times larger

than today's largest living rodent, the

capybara. This article is about this group

of gigantic caviomorphs.

The first rodents to appear in South

America were small as are most rodents

Rodents invaded

South America

worldwide. But around  15  million years

ago large sized rodent species began to

develop. Th is occurred inde pend ent

ly in three of the four m ain lineage s

that together make up the caviomorphs:

cavioids (capybaras, cavies and maras),

chinchilloids (chinchillas, vizcachas and

pacaranas) and in a lesser degree the

erethizontoids (porcupines). Over time

they reached, for a roden t, enorm ous

Argentina? For s tarters , surely there

were a lot of small and even tiny rodents

inhabi t ing the xerophyt ic woodland.

Also, he was sure to see some medium

size capybaras in surrounding ponds and

almost certainly some very large beasts,

known as eumegamyids (gigantic mice),

to palaeontologists . Today, the paca

ranas  {Dinomy s branickii ,  are the only

living close relatives of the eumegamy

ids.  Pacaranas are found in the valleys

and the low slopes of the Andes from

Colombia to Bolivia. Although  Dinomys

means "terrible mouse" the pacaranas

are, in fact, slow, peaceful, good-natured

anim als. Adult pacaranas weigh in at

about 12 kg. Their length [head and

body but not the tail] is roughly 70 cm.

During the Late Miocene  (11  to 5

million years ago) a sizeable population

of medium to enormous s ized eume

gamyids wandered across the Pampas.

Over the last  120  years paleontologists

have described mo re than 50 species of

eumegam yids, but this might be an exag

geration of their actual number. W

we know of Eumegamyids comes fr

studies of jaw remains, partial skulls

isolated teeth.

Eumegamyids had continuously gr

ing cheek teeth. These took the form

several parallel laminae of dentin

enamel arranged close together and

rounded by a thin layer of  cemen

(FIG. 2). This type of cheek tooth

typical of grazing mammals and is

dence eumegamyds were grass eat

Eumegamyids inhabited most of So

America, from the low slopes of

Andes to the Atlantic Ocean, and f

Venezuela to Patagonia. One of

eumegamyids that Robert Burns m

have come across near what is today

city of Santa Rosa in Central Argen

was   Tetrastylus.  Th is capybara-si

rodent is known from several jaws

isolated cheek teeth .

 Tetrastylus

 may

reached a body mass of 40 kg or so.

Had our poet wandered a li t t le fur

North into the present day provinces

Catamarca, San Luis and Cordo ba (FIG

he might have glimpsed   Neosteiro

bombifrons,  a gigantic porcupine. U

the much smaller South American

cupines of today this creature may h

preferred to live on the ground o

a life in the trees. Since we have

to find any post-cranial bones of

rodent, we cannot verify this hypo

esis, however. Still, given its relati

large size (about 20 kg, or more t

twice the weight of  Erethizon,  the

These huge

eumegamyids

 may ha

roamed

 in herds

ing North American porcupine) and

robust skull,

 Neosteiromys

 would cert

have had a hard time climbing a tree

protection or to find food, let alone m

aging to sleep on a branch. In any ev

Neosteiromys,  like oth er caviom or

could probably climb a tree if it had

Along with  Neosteiromys,  numer

small rodents similar to the living "sp

rats", "coruros" and "cavies" lived

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Capalmatherimn

Tctmstyhts

this region of Central Argentina. Surely,

some dug burrows for shelter or as nests

for their offspring, while others probably

lived in the trees . But this area was also

home to gigantic eumegamyids, such

as

  Telicomys

  giganteus,  tha t lived in w hat

is now the arid Catamarca province.

Telicomys

  was most certainly larger than

Tetrastylus

  and may well have weighed a

hopping 80 kg. Th ese huge eum ega

myids may have roamed in herds or, at

least, in small family groups.

And imagine if Rob ert Burns had inher

ited lands close to the Parana River, near

the city of Parana. Living near this large

river 6 million years ago was probably

even more exciting than the life led by

Aime

  Bonpland (1773-1858), the French

naturalist who spent his last 40 years

in this region putting-up with the sav

age environment and political upheavals

(actually, not so different from condi

tions today.) To survive, Robert Burns

would have to compete for food with a

whole slew of gigantic animals such as

the enormous capybaras, one of which

goes by the curious name

 Anatochoe

inusitatus  (unexpected duck-pig). H

came it by such a name? The skull

this capybara possessed a wide ro str

that resembles the beak of a duck.

Our knowledge of  Anatochoerus

based o n a single partial skull (on exh

in the Museo de La Plata, Argentin

Some 30 cm in length, the skull is

and a half times longer than the sk

of today's capybara. It would seem t

this rare find belonged to an animal

the late Miocene (about 6 million ye

ago). Sometime, during this period t

particular capybara became extinct.

Cardiatherium paranense  ("beast w

heart-like teeth from the Parana Rive

was another genus of capybara that liv

on the banks of the old Parana River

was somewhat larger than Anatochoe

but unlike  Anatochoerus,

 Cardiather

looked like the capybara of today,

least in respect to the form of its jaw

was likely an ancestor of the capyba

we see now.

And there were other giant rode

inhabiting Central Argentina at t

t ime. One of the largest was Eumega

paranenesis. A massive, nearly com p

skull of this species can be found in

Museo de La Plata together with

remains of the A natochoerus.  It meas

40 cm. A skull that large would m

Eumegamys paranes

was about the size

of a bear.

tha t  Eumegamys

 paranesis

  was about

size of

 a

 bear.

But the largest of these very la

rodents that roamed South Amer

in this period was a member of

Family Neoepiblemidae. Neoepiblem

are characterized by cheek teeth p

sessing two or three parallel lamin

These differ from the teeth of eu

gamyids. In neoepiblemids, the lami

are widely separated and have a th

layer of cement between them (FIG.

Neoepiblemids are known to have ex

ed during the Late Oligocene (about

million years ago) from remains found

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Phoberomys

—~̂ =w=s

  V \ \

  •

  \ V \

  I  iff

apahmtherium

southern Patagonia (FIG. 1). Until

the Middle Miocene (16 to 11 mil

lion years ago) their size remained

unremarkable. But, during the Late

Miocene they underwent a growth

spurt—becoming

  huge. These new

oversized huge neoepiblemids occu

pied a large expanse extending from

central east Argentina up to Venezuela

and into th e Acre region in western Brazil

(FIG. 1). Our knowledge is based pri

marily on isolated teeth, but also on jaw

fragments and skulls unearthed in Acre

and Venezuela. This family encompasses

several species. Some, like Neoepiblema

musteloides,

 increased in size but neve r

attained the more massive dimensi

of

  Phoberomys

  ("formidable mo use"),

generic name of the largest rodents t

ever lived. This King Kong of rode

was first described by Lucas Kragliev

from remains found in Argentina. B ut

most spectacular and significant rema

of this rodent come from northwest

Venezuela and comprise a fairly co

plete skeleton, also including part of

skull. Recent studies of this skeleton,

M. Sanchez

 Villagra

  and colleagues, p

tray the  Phoberomys  pattersoni  as a b

with a relatively gracile forelimb and d

This

 King

 Kong of

rodents

proportionately robust hind limb. Fr

analyses of the anterop osterior diamet

of humeral and femoral diaphyses th

concluded the

  Phoberomys

  weigh ed f

400 kg up to 700 kg. The Argent

species of  Phoberomys  are known ma

through isolated cheek teeth and a f

jaw remains. From a comparison of

last upper cheek tooth (M3) of both s

cies,

 it looks like the

 Phoberomys

 that l

near the Parana River, might have be

even larger than

  Phoberomys patterson

appears

 Phoberomys lozanoi

 (FIG . 2) h

M3 with an anteroposterior diameter

48 mm , making i t 7 mm longer than

Phoberomys pattersoni\

  For a compari

look at the

 Phoberomyss

 M 3 next to

of  a  living capybara (FIG. 2).

There's growing evidence that

Phoberomys

  species lived near large

ers,

  maybe in marginal lagoons. T

offspring of these rodents may well ha

been prey to the gigantic crocodiles t

abounded in these large rivers .

As the Late Miocene drew to a clo

many of these giant rodents died o

entire species becoming extinct. On

a few of the large capybaras and eum

gamyids  survived. One survivor was

Chapalmatherium

 perturbidus,

  nicknam

by palaeontologists as "the great ru

ner capybara" because of its unusu

ly long limbs. The long limbs of t

Chapalmatherium  strongly suggest it w

22 S o u t h A m e r i c an E x p l o re r

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m like today's capybaras. W it h its

  weighed some 100 kg.

  the various species

 oi

 C hapalmatherium

  Chapalmatherium  may have wan

Another giant rodent,

  Telicomys

  gigan-

  also lived on the coast of w hat

One of the last  tim'rous  giants ,

 lived only a m illion years

  Close relative to the living capy-

  was native to both South and

In addition to Argentina and Venezuela,

  Amb lyrhiza inundata,  one of

Time Chart

Holocene

  Pleistocene

Pliocene

Miocene

Oligocene

Eoce/ie

Paleocene

upper

m

 t A d J k

lower

upper

lower

tipper

middle

lower

upper

Last large

caviot/iorphs

Huge

 caviomorpbs

First large caviomorpbs

Caviomorpb arrival

in South America

First rodents

to move around. It's quite likely that

Ajnblyrhiza

  was related to the South

American eumegamyids and pacara-

nas.

  Ancestors of these species probably

arrived in the West Indies from Central

or South America sometime during the

Middle Miocene (15 million years ago).

Living on islands,  Amblyrhiza  popula

tions were probably smaller than those

of their South American relatives.

As we have seen above, gigantic rodents

thrived in different climates and environ

ments . During the upper Miocene, vast

plains spread across Central Argentina.

These expanses covered a range of envi

ronmental and c l imact ic condi t ions .

Towards the West and along the edges

of the incipient Cordillera, the plains

were s teppe like, patched with some

woodlands. Towards the East, along the

old Parana River, sub-tropical condition s

prevailed. Temperatures throughout this

region were surely higher than exist

today. In northwestern Venezuela, home

to the

  Phoberomys pattersoni,

  there is evi

dence that conditions that ranged from

savannas to littoral.

W ha t caused to extinction of these

large rodents? Several factors have been

put forward. Changes in climate led to

a cooler and drier environment. Then

too mammals invading from the north,

changed the rules of competition and

predation.

It's obvious that a world of great fecun

dity would be required to support such

a great variety of gigantic herbivorous

rodents 10 million years ago, especially

when one considers

that they were not

alone. T he region

teemed with many

other g iant mam

mals,

  l ike ungu

la tes , g lyptodonts

and ground s lo ths .

G i a n t r o d e n t s

belong to this splen

did menagerie. Oh,

what a magnificent

poem might Rober t

Burns have penned

could he have lived

surrounded by such

amazing beasties.

Reconstructing Extinct

Rodents

To reconstruct extinct rodents requir

extensive scientific research and artis

sensitivity. It further demands a care

study of bone size and structure , musc

insertions, and movement possibilit i

of joints.

Modern techniques of recons truct i

produce more accurate results , but

enjoy the old ones made with less po

erful techniques, but with a delight

naive inspiration. T h e illustrations he

show some older reconstructions ma

more than 40 years ago.

S U G G E S T E D R E A D I N G S :

Bikneviciuc, A.R., McFarlane, D.A. and

MacPh ee, R .D.E., 1993. Body size

in

 Amblyrhiza inundata

  (Rodentia:

Caviomorpha), an extinct

 megafaunal

rodent from the Anguilla Bank, West

Indies: estimates and implications.

American Museum Novitates, 3079: 1-2

Pascual, R., Vuc etich, M .G. y Scillato Yan

G.J., 1990. Extinct and Rece nt South

American and Caribbean Edentates and

Hystricognathi rodents: Outstanding

Examples of Isolation.  In Biogeographic

Asapects of

 Insularity.

 Accademia

Nazionale dei Lincei, Atti dei Convegni

Lincei 85:627-640.

Saiichez-Villagra,

 M.R., Aguilera, O. and

Horovitz, I. 2003. The anatomy of the

world's largest extinct rodents. Science,

01:1708-1710.

Simpson, G.G., 1980. Splendid Isolation.

The curious history of South American

Mam mals. Yale Unive rsity Press: i-ix, 1

266.

Vucetich, M.G., 1986. Historia de

 los

roedores y primates en Argentina: su

aporte

 al

 conocimiento de los cambios

ambientales durante el Cenozoico. IV

Congr Argent. Paleont y B ioestrat.

Actas2: 157-165.

Guiornar

  Vucetich   is   scientific researcher   of

CONICET and Professor  of  vertebrate   paleon

tology

  at

 the Facultad de Cienclas Maturates y

Museo.

 She is

 a

 specialist in the study of South

American fossil rodents.

x;;:^ -: --.: - --- - -

Number 76, Summer 2004

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