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American Society of Mammalogists

Dental Age Classes in Marmosa incana and Other DidelphoidsAuthor(s): Christopher J. TribeSource: Journal of Mammalogy, Vol. 71, No. 4 (Nov., 1990), pp. 566-569Published by: American Society of Mammalogists

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DENTAL

AGE CLASSES

IN MARMOSA

INCANA AND

OTHER DIDELPHOIDS

CHRISTOPHER

J.

TRIBE

Department

f Biology

MedawarBuilding),

UniversityCollege

London,

Gower

Street,

LondonWC1E

6BT,

England

ABSTRACT.-The

ooth

eruption

nd

replacement

equence

n Marmosancana

does

not follow

that described

y Tyndale-Biscoe

nd Mackenzie

1976)

or

Didelphismarsupialis;

herefore,

modified

ystem

of dental

age

classes s

proposed.

Whereas,

n

Didelphis

he firstor deciduous

molar

herein

eferred o as

dP3)

s

shedand

P3

erupts

before he

appearance

f

M4,

n M. incana

dP3 is retaineduntil

M4

is

functional;

our molariform

eeth,

therefore,

re

present

t all times

fromclass

4

onward.

The

same

equence

ccurs n

many

pecies

f Marmosa

sensuato),

Monodel-

phis,

and

Caluromys,

whereas he

Didelphis

pattern

s

followed

by

species

of

Philander,

Chi-

ronectes,Lutreolina, nd themicrobiotheriidromiciops. hesequencen Metachiruss some-

what

ambiguous.

possible

nference s

that

Marmosa nd

Monodelphis elong

with

Caluromys

in

a

monophyletic

roup,

rather hanwith the

other

didelphids.

Systematic

work on

mammal

specimens

in

museum collections

often entails determination of

age

classes based on cranial or dental

features.

One

system

commonly

adopted

for

didelphoid

marsupials

s

that

of

Tyndale-Biscoe

and Mackenzie

(1976),

based on the

presence

and functional

state of

maxillary

cheek teeth from the third

premolar

and its deciduous

predecessor

to the

fourth molar.

Herein,

I refer to the deciduous

molariform tooth as

dP3,

partly

to facilitate

comparison

between the

new

system

of

age

classes

and that of

Tyndale-Biscoe

and

Mackenzie

(1976) and partlybecause it physicallyoccupies the place of the third premolar.Ontogenetically,

however,

it would be

termed more

correctly

the first or deciduous

molar,

the

permanent

molars

herein

referred

to as

M1-4

being

in

reality

M2-5

(Archer,

1978).

Their

system

is based on

the

eruption

sequence

and

cusp

wear

of

these teeth

in

Didelphis marsupialis

from

Colombia;

seven

age

classes

from

1

(dP3

present

and

M1

erupting)

to

7

(P3

and

M1-4

with

cusp

wear)

are

recognized.

These

agree

closely

with the

six

numbered

groups

established

by

Gardner

(1973)

for

D.

marsupialis

and

D.

virginianus

in

North and

Middle

America,

with the insertion of

an

additional

class,

4.

Cerqueira

(1980,

1984)

confirmed

the

validity

of

the

sequence

for

D.

albiventris

and

D.

auritus from eastern

Brazil,

and Motta

(1988)

related it to

chronological age

for

captive

D.

auritus.

During a study of the marsupialsof Rio de Janeirostate, Brazil (Tribe, 1987), I used Tyndale-

Biscoe

and Mackenzie s

(1976)

system

with

several

genera

of

didelphids

and found

that it worked

well with the

larger

species: Didelphis

auritus

(n

=

66),

Philander

opossum

(n

=

75),

and

Metachirus nudicaudatus

(n

=

118).

For Marmosa incana

(n

=

86),

however,

the

sequence

of

tooth

eruption

in

young specimens

did not

agree

with

the

classes as defined

by Tyndale-Biscoe

and Mackenzie

(1976).

A

modified

system

of dental

age

classes,

therefore,

was

required

for

Marmosa incana.

The

question

of whether

any

other

didelphoid species

followed

the M.

incana

pattern

of tooth

eruption

and

replacement

also was

investigated.

METHODS

The

proposed system

of

age

classes is based on examination of a series of 69

specimens

of Marmosa incana

held

by

the

Museu

Nacional,

Rio

de

Janeiro,

all

collected in the

vicinity

of Alm

Paraiba,

southern

Minas

Gerais,

Brazil.

Twelve

of

the

skulls

belong

to classes 4 and

5,

in which

the

greatest divergence

from

the

Didelphis pattern

can be seen. A

binocular

microscope

was used

under low to medium

power

for assessments

of tooth

wear.

To

investigate

whether the

same

sequence

of tooth

eruption

occurredin M. incana from

other

localities,

and in other

species

of

didelphoids,

I

examined

young

specimens

in collections of

the Museu

Nacional,

Rio

de

Janeiro

(MN),

and the Natural

History

Museum,

London

(BMNH).

Because the

fundamental

difference

between the M.

incana and

Didelphis

sequences

is the order of

appearance

of P3

and

M4,

a

species

was

J.

Mamm.,

71(4):566-569,

990

566







November

990

TRIBE-AGE CLASSES

N

DIDELPHOIDS

567

judged

o

follow

the former

f

appropriatelyged

individuals howed

dP3

and

M4

present imultaneously,

or

if

P3

was

n the

process

f

erupting

when

M4

was

unctional.

Conversely,ruption

f P3 before

emergence

of

M4,

or

presence

f

fully

grown

P3

while

M4

was

erupting

would ndicate he latter

equence.

RESULTS

As

no

young

material

was

available,

I can

only

assume that classes

1

and

2

for

D.

marsupialis

also

are

applicable

to M. incana.

The

youngest

specimens

in

the

series of

M.

incana

from Alem

Paraiba

corresponded

well with

Tyndale-Biscoe

and Mackenzie s

(1976)

class

3,

with

dP3,

M1,

and M2 all

present.

The

following

stage

is, however,

distinct:

whereas

in

Didelphis

class

4,

dP3

has

been shed and its

permanent

replacement

is

erupting,

and

M3

is

fully

grown

or

nearly

so,

in

M.

incana dP3

is

retained,

M1-M3

are

functional,

and

M4

may

be

erupting.

In the

larger

species

of

didelphids,

M4

erupts

only

after P3

is

fully grown

or

nearly

so.

Thus,

in

Didelphis

class

5,

P3

is functional but

unworn,

and

M4

is

erupting

or

nearly

functional. In

contrast,

for

class 5

in

M.

incana

all

molars

are

functional,

with

either dP3

persisting

or

P3

just

starting

to

erupt. In class 6 for Didelphis all permanent cheek teeth are functional, but there is little wear

on M3-M4.

Because

M4

was

in

use

during

stage

5 for M.

incana,

in

individuals of class

6 it

already

shows some

wear,

and P3

is

half to

fully

erupted.

Class

7

is

used for those

specimens

with

P3

fully

erupted

and

M4

considerably

worn,

thereby resembling

the

corresponding

class

for

Didelphis.

I

noted

in

all

didelphids

that each

lower molar

always appeared

before

its

upper

counterpart;

this

is

understandable,

as the

former shears

against

the

posterior

border of

the

preceding

upper

tooth.

However,

the

upper

deciduous

premolars

are shed and their

permanent

successors

erupt

at

about the

same time

as,

or even

before,

the

lower ones.

Accordingly,

in

both

Didelphis

and

M.

incana,

m4

often

is

erupting

or

is

in

place

before

dp3

has

been

shed

(class 3).

DISCUSSION

Differences

in

the

sequence

of

tooth

eruption

between

Didelphis

and Marmosa

incana must

relate

to their life

histories. A

juvenile

Didelphis

becomes

independent

of its

mother

at

age

class

3

(Motta,

1988),

when it

possesses

three

functional

molariform

teeth

(dP3,

M1-2)

in

both

upper

quadrants.

With

the

eruption

of

M3,

dP3 is

shed,

and

only

three

molariform

teeth remain

to

provide

the total

crushing

surface.

This surface is

slightly

larger

than

before

because

M3

is

longer

and

wider

than

dP3.

P3

is

not

molariform,

but consists

almost

entirely

of

a

large,

conical

cusp

more

suited to

gripping

and

perforating

than

to

grinding

or

crushing. Only

in

age

class

6,

when

M4

has

erupted fully

and come

into

use,

does

the

upper

molariform

series

in

Didelphis

consist

of four teeth.

In

Marmosa

incana,

the

juvenile

does not

lose dP3 when

M3

appears,

but reaches

age

class

4

with four

functional molariform

teeth on each

side

of

the

upper

jaw.

As

dP3 is

replaced

only

when M4

is

full

size,

the animal

is

always equipped

with

this four-tooth

crushing

surface from

age

class

4

onward.

Little is known of the

average

size of

prey

taken

by

didelphids

of various sizes

and

ages.

If

there

is

a

correlation between

prey

size and

length

of

toothrow,

then the

distinct dental

replace-

ment

patterns

of

Didelphis

and

M.

incana

might

imply

differences between the two taxa

in

the

shift from

juvenile

to adult

feeding

habits.

Young

M. incana would become

potential

competitors

of their parents generation sooner than would be the case with Didelphis.

Three

specimens

of Marmosa incana

of

appropriate

age

from two other localities

in

Brazil

(Teres6polis

in Rio de

Janeiro

state,

and Lamario in

Bahia)

corroborated the

tooth-eruption

sequence

found

in

the series from

Alem Paraiba. Evidence for the same

sequence

also was found

in

the

22

species

of

Caluromys,

Glironia,

Marmosa

(sensu

lato-Reig

et

al.,

1985),

and

Mono-

delphis

listed later. Numbers

in

parentheses

ndicate

the number of

specimens

on

which

allocation

to the M. incana

group

was based

(those

in which either

P3

or

M4 was

erupting,

or dP3 and

M4

were

present concurrently):

Caluromys

derbianus

(1,

BMNH),

C. lanatus

(12,

BMNH),

C.

philander

(4,

MN; 1, BMNH),

Glironia

venusta

(1,

BMNH),

Marmosa

alstoni

(1,

BMNH),

M.

canescens

(1,

BMNH),

M.

cinerea

(4,

BMNH),

M.

elegans

(Chile;

3,

BMNH),

M.

fuscata

(1,







568

JOURNAL

OF MAMMALOGY

Vol.

71,

No.

4

BMNH),

M.

germana

(3, BMNH),

M.

impavida

(3,

BMNH),

M.

marica

(1, BMNH),

M.

mexicana

(6,

BMNH),

M.

microtarsus

(1, BMNH),

M.

murina

(3, BMNH),

M.

noctivaga

(5,

BMNH),

M.

quichua

(2, BMNH),

M.

rubra

(1,

BMNH),

Monodelphis

adusta

(2, BMNH),

M.

americana

(1,

MN),

M. dimidiata

(1, BMNH),

M.

scalops (1, BMNH).

One of the two

young

Monodelphis

adusta

examined conformed to the

Marmosa incana

pattern,

whereas the

simultaneous

eruption

of P3

and

M4 in

the other

specimen placed

it in an

intermediate

position.

The

same

occurred

in

two

Marmosa

mexicana,

and

one M.

murina.

These

species

are

retained

in

the M.

incana

group

because

M4

erupted

before P3

in

at

least half

of

each

sample.

In

Caluromys,

M4

is

considerably

reduced in

size

and

apparently may

be

suppressed

altogether.

In two skullsof C. lanatus

(BMNH

54.6.3.1 and

2.7.28.4),

the M4

alveolus

was

absent,

although

the alveolar

ridge

extended

beyond

the

point

at

which

the

tooth would

be

expected

to

develop.

The

first

specimen

also lacked

m4. This is

evidently

not

a

case

of the

Didelphis pattern

of tooth

eruption,

rather one of

suppression

of the last molar.

A few species did not to conform clearly to either pattern of tooth eruption and were placed

in an

intermediate

group:

Marmosa

elegans

(E

of

Andes;

13,

BMNH),

M.

pusilla

(1,

BMNH),

M. robinsoni

(4, BMNH),

Metachirus

nudicaudatus

(16,

MN; 10,

BMNH),

Monodelphis

brevi-

caudata

(1, BMNH),

M. domestica

(2, BMNH).

In these

species,

either

P3

and M4

developed

simultaneously

in

all

or

nearly

all

available

specimens,

or

both

the

Didelphis

and the Marmosa

incana

sequences

were

represented.

In

the

single young specimen

of

Monodelphis

brevicaudata

(BMNH 52.1224),

the

tip

of P3 showed

prematurely

labial to

a

well-rooted

dP3,

and

M4

was

still

rudimentary.

In the

only

available

specimen

of Marmosa

pusilla,

P3 and M4

developed

concurrently.

The

same was

true

of nine of the 13

specimens

identified

as Marmosa

elegans

from east of

the

Andes;

in

the

remaining

four,

M4 was

more

advanced than

P3,

as

expected

in

the M. incana sequence.

Evidence

for the

sequence

of

tooth

eruption provided

by

specimens

of Marmosa

robinsoni,

Monodelphis

domestica,

and Metachirus

nudicaudatus was

ambiguous.

In M.

robinsoni,

two

individuals fitted into the M.

incana

sequence,

one into

the

Didelphis

sequence,

and one

was

intermediate.

In

M.

domestica,

there

was

one

specimen

representing

each

sequence.

The

case

for

M.

nudicaudatus is more

complex:

all 16

specimens

of suitable

age

from

southeastern

Brazil

followed

the

Didelphis pattern,

as

did

two

from

Mato Grosso

and one

from

Ecuador;

two

from

Peru and two from Ecuador were

intermediate;

and

one each from

Mato

Grosso

and

Bolivia

followed

the

M. incana

pattern.

One

Bolivian

specimen

(BMNH 1.6.7.71)

was

aberrant,

retaining

left dP3

in

place

of left

P3,

when

all

other

teeth were

fully

grown.

As

this

genus

requires

revision,

it is

possible

that more than one

species

was

represented.

As

expected,

Didelphis

albiventris

(16,

BMNH),

D. auritus

(8,

MN;

2,

BMNH),

D.

marsupialis

(15,

BMNH),

and D.

virginiana

(1,

BMNH)

followed

the

pattern

of tooth

replacement

described

by Tyndale-Biscoe

and

Mackenzie

(1976),

as did Philander

opossum

(16,

MN;

23,

BMNH)

and

Chironectes

minimus

(2,

BMNH).

Two skulls of

Didelphis

albiventris

(BMNH

84.2.8.30 and

20.2.7.40)

were

slightly

aberrant

in

that

the left

dP3 was retained

in

each

during development

of

P3

and

appearance

of

M4.

Three

specimens

of

Lutreolina

crassicaudata

(5,

BMNH)

conformed

to the

Didelphis pattern,

whereas

the

other two

were

intermediate,

with P3

and M4 at

a

similar

stage

of

development.

The

sequence

of

tooth

eruption

in

the

microbiotheriid

Dromiciops

australis

(2,

BMNH)

also

apparently

follows

that of

Didelphis.

Thus,

the

Didelphis

pattern

of

tooth

eruption

appears

to

be ancestral

within

the

Didelphoidea

because it

is

present

in

both

Didelphidae

and

Microbiotheriidae.

If

retarded

replacement

of

dP3

by

P3

has arisen

only

once in

the

evolutionary

history

of

didelphids,

it

lends

support

to a

monophyletic

group

including

Monodelphis,

Marmosa,

Caluromys,

and

Glironia,

and

argues

against

erection of

subfamily

Caluromyinae

containing Caluromys,

Caluromysiops,

and

Glironia,

whereas

Marmosa

(sensu

lato)

and

Monodelphis

are

retained

in

subfamily

Didelphinae,

tribe

Marmosini

(Reig,

1981;

Reig

et

al.,

1985).

Instead,

Caluromys

may

be a

highly

derived form

that

has

arisen from

a

Marmosa-like

stock.







November

990

TRIBE-AGE CLASSES

N

DIDELPHOIDS

569

ACKNOWLEDGMENTS

Financial

support

during

this

study

came

from

the

Coordenaqio

de

Aperfeigoamento

de Pessoal

de

Ensino

Superior

(1985-1987)

and

the

Conselho

Nacional de Desenvolvimento Cientifico e

Tecnol6gico

(1988-1989),

Brazil. I also benefited from grantsawarded by the latter and by the Financiadora de Estudose Projetosto

the

Departamento

de

Ecologia,

Universidade

Federal

do Rio de

Janeiro,

to which I was attached

(1985-

1987).

I

am

grateful

for

access

to

collections at the Museu

Nacional,

Rio

de

Janeiro,

and

the

Natural

History

Museum,

London. I am indebted to Dr. R.

Cerqueira

for his

encouragement

and advice.

He,

Professor R.

J.

Berry,

and two

anonymous

reviewers

commented on

previous

drafts

of

this

manuscript.

LITERATURE CITED

ARCHER,

M. 1978. The nature of the

molar-pre-

molar

boundary

in

marsupials

and a

reinterpre-

tation of

the

homology

of

marsupial

cheekteeth.

Memoirs of the

Queensland

Museum,

18:157-164.

CERQUEIRA,

R.

1980. A study of Neotropical Di-

delphis

(Mammalia,

Polyprotodontia, Didelphi-

dae).

Ph.D.

dissert.,

University

of

London,

Lon-

don,

430

pp.

-

1984.

Reproduction

in

Didelphis

albiven-

tris in

northeastern Brazil

(Polyprotodontia,

Di-

delphidae).

Mammalia,

48:95-104.

GARDNER,

.

L. 1973.

The

systematics

of

the

genus

Didelphis (Marsupialia,

Didelphidae)

in

North and

Middle America.

Special

Publications,

The Mu-

seum,

Texas

Tech

University,

4:1-81.

MOTTA,

M. F. D.

1988.

Estudo

do

desenvolvimento

extra-uterino de Didelphis aurita Wied, 1826, em

cativeiro-Investigagio

de

critbrios

para

estima-

tiva de

idade.

M.S.

thesis,

Universidade

Federal

do Rio de

Janeiro/Museu

Nacional,

Rio de

Janeiro,

Brazil,

105

pp.

REIG,

O.

A.

1981.

Teoria de la

origen

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