The Evolution of Order Diprotodontia1

Cura, Maris Hazel A.

dela Cruz, Haiezel L.

de la Rosa, Marianne Michelle Q.

Mora, Jonah Feliza B.

Panerio, Rafael Julian G.

Biology 140 (Evolutionary Biology) – F

Sept. 14, 2011

_______________________

1 A term paper submitted in partial fulfillment of the requirements in Biology 140 under Dr.

William Sm. Gruezo

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The Evolution of Marsupials in Australia

Outline

I. Introduction

II. Discussion

A. Paleontological History

B. Systematics and Phylogeny of Order Diprotodontia

C. Morphology and Diversification of Order Diprotodontia

D. Speciation of Order Diprotodontia

E. Prediction regarding the fate of Order Diprotodontia and Marsupials in

General

III. Summary and Conclusion

IV. Literature Cited

TERM PAPER (Group 1): Order Diprotodontia | 3

I. Introduction

Marsupials belong to Class Mammalia under Subclass Theria. They possess the general

characteristics of mammals such as warm-bloodedness, presence of mammary glands, presence

of insulating hair or fur, viviparity and dentition for mechanical digestion.

The main distinction of marsupials from other mammals is that they bear live, immature

offspring in their marsupium. The young instinctively finds its way into their mother’s nipple

and remain attached there until it becomes fully developed.

During the late Cretaceous period, the first marsupials appeared in North America. They

slowly dispersed southwards towards Antarctica until they reached present-day Australia where

they now thrive.

There are ten extant orders of

marsupials namely Order

Didelphimorphia, the cat-sized

opossums of South America with scaled

and hairy tails that are arboreal in nature

and are insectivorous or omnivorous in

nature; Order Paucituberculata, the

shrew-like rat opossums which live in

the Andes forests of South America

wherein females of the species lack a

pouch – an odd characteristic in

marsupials; Order Microbiotheria, with

only one extant species - Dromiciops

gliroides; Order Notoryctemorphia, the

blind, sand-digging marsupials which

have pouches opening backwards, which

Figure 1. The extant marsupial orders

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live underground and feed on insects; Order Dasyuromorphia, the most carnivorous marsupials

with hairy tails, ranging in size from the size of a mouse to dog; Order Peramelemorphia, the

terrestrial and insectivorous or omnivorous bandicoots and bilbies which are native to New

Guinea and Australia, having pouches which contain the nipples; Order Caenolestidea, nocturnal

rat-sized creatures which may be insectivorous or carnivorous; Order Polyprotodontia, the

known scavengers or omnivores; Order Peramelina, the herbivorous, root-feeding, hopping rat-

sized creatures that live underground; and Order Diprotodontia, the herbivorous endemics of

Australia, which will be the focus of our report.

II. Discussion

A. Paleontological History

The evolutionary history of marsupials is one of the most interesting topics for

scientists; that continuous researches are still held to shed light into their history until today.

One of the products of these endless excavations for answers about their history was the

fossil found in China, specifically in the rocks of the Yixian Formation in Liaoning Province,

by a team of scientists including Zhe-Xi Luo and John Wible, last 2003; and this fossil is

called the Sinodelphys. The Sinodelphys which is about the size of a chipmunk were believed

to have climbed trees. It dates from 125 million years ago (Early Cretaceous Period) and it is

believed to be an early ancestor of the marsupials, since it share many features with the

modern marsupials. This discovery also suggests that marsupials may have originated in

Asia, dispersed in North America, and later migrated to present-day South America. But this

migration eventually took the marsupials to Antartica and Australia, which were believed to

have been united on the ancient landmass, Gondwanaland (MS Encarta, 2006).

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Figure 2. Fossils found in China, the Sinodelphys.

In 1982, scientists identified the only fossil of a land-dwelling mammal in Antartica

as a marsupial and it lived for about 40 million years ago. This fossil then confirmed, the

migration of marsupials from South America, where other fossils of marsupials dating 65

million years ago was found, to Antartica and then to Australia, their final destination, where

marsupial fossils dating 25 million years ago were found (MS Encarta, 2006).

But the Alphadon is the earliest marsupial ever recorded. Its remains were found in

North America about 80 million years ago, during the Cretaceous period.

Figure 3. Tooth remains of the earliest marsupial ever recorded, Alphadon.

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The earliest known fossil of the order is from the Late Oligocene. Hypsiprymnodon

bartholomaii from the Early Miocene is the earliest identified species.

Another fossil of the marsupials, specifically of the Diprotodonts is the Diprotodon. It

is a genus of an extinct, giant marsupial, which is closely related to kangaroos. The fossils of

this marsupial, is found in Australia among Pleistocene deposits. Like the kangaroo, they

were herbivorous, but unlike the kangaroo, they used all four legs for walking. The skull of

the Diprotodon was heavy and less than 1 m (about 3 ft) long; the entire animal was about

the size of a rhinoceros. Diprotodon australis is the only known species of this genus. A

related genus, Nototherium (the bones of which are often found with those of the

Diprotodon), was also a giant marsupial. (MS Encarta, 2006)

Figure 4. Diprotodon Fossil.

B. Systematics and Phylogeny of Order Diprotodontia

The Order Diprotodontia is the largest order of marsupials. This order represents 11

families and over 110 species, which includes kangaroos, wallabies, possums, koalas, gliders

and wombats. Order Diprotodontia is an order which is very diverse in terms of size and

habitat. But this order also contains a family of giant diprotodonts that are now extinct

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Figure 5. Phylogenetic Tree of Order Diprotodontia

The organisms under the Order Diprotodontia share two basic characteristics. These two

characteristic includes the possession of enlarged lower incisor teeth that project forward and an

integument that binds together the second and third digits of their hind feet.

The diversity of species within Diprotodontia is greatly demonstrated by the size range of its

largest and smallest species. And so, along their sizes, their diet is also diversified. Most of the

order is herbivorous but some species prey on invertebrates and small vertebrates. Another point

of diversity within the order is the methods of locomotion. Since some species glide through the

tree tops while the others leap along the grasslands.

C. Morphology and Diversification of Order Diprotodontia

Diversification

In the southern super-continent Gondwanaland, the first mammals with the distinctive

mode of reproduction exhibited by marsupials arose. After the super-continent broke up into

separate continents, marsupial ancestors were separated from placentals and became isolated

in the South American and Australian continents.

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|Plio-

| Pleistocene | ----- North/South America rejoin

|-------------- |

| Miocene | ----- Antarctica/Australia split

|-------------- |

| | ----- Marsupials extinct Asia/North Africa/Europe

| Oligocene | ----- Marsupials in Australia/Asia

|-------------- |

| | ----- Marsupials extinct North America

| |

| | ----- Marsupials reach Antarctica

| Eocene | ----- Radiation of Placentals in North America

| | ----- South/North America split,

|-------------- | Marsupials in Europe

| Paleocene |

|-------------- |

| | ----- South American Marsupials

| Late |

| Cretaceous |

| | ----- First Marsupials (North America)

| |

Figure 6. A portion of The Geologic Time Scale, describing the dispersion of marsupials as the continents drift

Diversity in the Diet

Diprotodontians are often thought of as herbivorous grazers but others have diversified

into numerous ecological niches. Others have shifted to eating fungi (bettongs and potoroos of

Family Potoroidae), nectar (honey possums of Family Tarsipedidae), insects (striped possums of

Family Petauridae), fruits (musky rat-kangaroos of Family Hypsiprymnodontidae) and tree

leaves (phalanger possums of Family Phalangeridae). Other marsupials even changed their diet

to meat. An example of this is Thylacoleonidae, an extinct marsupial family in Australia which is

also known as the large carnivorous marsupial lions.

Diversity in Reproduction

Diprotodonts and all other marsupials are distinguished by their reproduction that is very

different from other mammals. They have a very short gestation period, usually about 28 to 35

days. By this time, the offspring is still short and completely blind but it makes its way to the

mother’s pouch. When the offspring has reached its destination, it attaches to one of the mother’s

teats until the 70th

day. After four to five months, the joey’s head comes out of the pouch. The

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joey still uses the mother’s pouch for warmth and nutrition until the joey is 8 months old. At

some later time, the mother will prevent its offspring to enter the pouch by holding it shut.

Though diprotodonts generally undergo same life cycle and unique reproduction process,

some families deviate from other diprotodonts in the position of marsupium, presence of

placentae, and the number of young per litter.

Koalas are unusual among marsupials in that they form, in a short time, a placenta during

the gestation of their embryos. Members of the families Peramelidae, Peroryctinae,

and Vombatidae are the only other marsupials with placentae.

Members of the family Phalangeridae have a well-developed marsupium that opens

anteriorly. They usually give birth to a single young per litter.

Like most marsupials, the pouches of the members of Burramyidae are well-developed

and open anteriorly. Newly fertilized eggs go through a period of embryonic diapause, a

common trait among diprotodonts.

Macropodids are distinguished by their reproductive cycle which is characterized by a

period of embryonic diapause, during which the blastocyst suspends implantation and

development. At times, females of most species may be supporting young of 3 litters: one in the

uterus, one residing full-time in the pouch and attached to a nipple, and the third living out of the

pouch but returning to nurse.

The pouch of wombats is well developed, but it is oriented so that it opens to the rear,

rather than forward as is more usual in marsupials. The embryo forms an allantoic placenta, as is

true of at least some peramelids and koalas but not other marsupials.

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Diversity in Morphology

Order Diprotodontia, from its name, has a unifying characteristic that involves their teeth.

They all possess a large pair of incisors on the lower jaw and lack canine teeth but have an empty

space where the canine teeth should be. Another defining characteristic of order Diprotodontia is

syndactyly- the second and third digits of their feet are fused but not the claws.

The potoroids are a family of diprotodont marsupials believed to be closely allied with

the kangaroos and wallabies (Macropodidae) and sometimes grouped as a subfamily within that

family. The Potoroidae includes 9 species placed in 5 genera. They are found in Australia.

Like macropodids, these small and secretive animals are diprotodonts and syndactylous.

Also like macropodids, they have enlarged hind feet and powerful hind limbs. At high speeds

they are adept hoppers. At slower speeds, their movement is more rabbit-like; they land with

their weight on their forelimbs as well as hind, then transfer weight to the hind limbs for the next

hop. The forelimbs are smaller than the hind limbs, but the disparity in size is not as great as in

kangaroos and wallabies. As in the case of macropodid hind feet, the fourth toe is the longest and

strongest. It sits in a line with main limb elements and transmits thrust of hopping. It is not as

well developed, however, as the fourth toe in macropodids. The tail is semiprehensile.

The dental formula of potoroids is 3/1, 1-0/0 2/2, 4/4 = 32-34. The second and third

upper incisors are small and placed lateral to and behind the first incisor, not lateral as in

macropodids. In other respects, the skull is similar to that of kangaroos. Canines are present and

well-developed. Themolars are stationary, that is, they don't show pattern of forward

movement with aging that is seen in macropodids. Young potoroids and macropodids have two

upper and lower premolars that are replaced in adulthood by a single, large, blade-like premolar

in both jaws. Members of this family are omnivores and herbivores, feeding mainly on

underground fungi and tubers also taking some seeds and insects.

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Diversity in Habitat

Diprotodonts can be found in Australia, New Zealand, New Guinea, and nearby islands.

They inhabit a wide variety of terrestrial habitats- from grasslands to forests and mountains.

Marsupials have evolved to suit the place where they live. An example of which are the extra

flaps of skin found in diprotodont gliders. These gliders live in heavily forested areas and move

from tree to tree using the flaps on their skin which act as sails. This adaptation allowed them to

use less energy since jumping or climbing up trees would require more. Another good example

of this habitat adaptation is displayed by the possums. Possums have prehensile tails which they

use to easily hang on trees.

Specifically, Kangaroos and Wallabies are exclusively found in Australia, New Guinea

and Tasmania; Possums are found in Australia, New Guinea, and Sulawesi; the Wombats on the

other hand are found in Australia and Tasmania while Koalas are only found in Australia (MS

Encarta).

It is quite noticeable that most Diprotodonts are found in Australia, it is even concluded

by researches that they are restricted to Australasia, which must have been caused by the course

of their ancestor’s evolution and isolation from the other organisms.

D. Speciation of Order Diprotodontia

Marsupials have evolved into a great variety of creatures to suit or adapt to a wide

variety of environments. Some inhibits arboreal habitats, while others live underground.

Through the process of convergent evolution, the adaptation of marsupials has paralleled the

adaptation of placental mammals to same environments. These two groups of mammals have

developed remarkably similar features. For example, placental moles that live underground

has the same curved claws which are used for digging with the marsupial moles.

Unfortunately, as the rule of nature goes, one group could be disadvantaged of these

similarities. And most of the time, the marsupials are the ones being selected against. Like

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how the competition for resources such as food, water and habitat arose when placental

animals were introduced to areas where only marsupials dwell. And so, usually, the

marsupials are induced. Also in some instances, the introduced placental mammals prey

extensively on native marsupials, this putting them into the risk of extinction. Like how

foxes, dogs and domestic cats threatened some kangaroo species in Australia.

But there is one distinguished exception and this is the Virginia opossum in North

America, which is flourishing despite the constant competition posed by the placental

mammals living there as well.

Order Diprotodontia

Pseudocheiridae (ringtails, gliders) 14 spp

Vombatidae (wombats) 3 spp

Acrobatidae (feathertail gliders) 2 spp

Petauridae (wrist-winged gliders, Leadbeater's possum) 10 spp

Macropodidae (kangaroos, wallabies, etc) 45 spp

Potoroidae (potoroos, rat kangaroos) 9 spp

Phascolarctidae (koalas) 1 sp

Phalangeridae (possums, cuscuses) 10 spp

Tarsipedidae (honey possum) 1 spp

Burramyidae (pygmy possums) 8 spp

E. Prediction Regarding the fate of Order Diprotodontia and Marsupials in General

In a few years, if humans will continue to protect marsupial diversity, they will not

become extinct and will continue to flourish in the continent. However, since placental

mammals are now starting to be established in Australia, competition would possibly occur.

And from the previous competitions between the two groups, the marsupials are usually

displaced. Since some placental mammals tend to prey on the marsupials. Like how

kangaroos were threatened by the foxes, dogs and domestic cats. But it is also possible that

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from the past years of isolation, the marsupials might have developed characteristics that

could help them win the competition against the placental mammals that are just introduced

to their million-year long territory. Another possibility is coexistence of the two groups that

is slowly exhibited in North America, as marsupials are still thriving despite the presence of

placental mammals.

In 200 million years, another super continent will be formed because of the

continental drift. Landmasses will connect enabling the dispersal of terrestrial animals like

marsupials. Competition with placental mammals that they may encounter may result to

extinction of one or both animal groups. Marsupials could either be selected against or

selected for. Since Australia is not anymore in isolation, the Hardy-Weinberg Equilibrium

will be disrupted due to migration. Or, if their nutrient and habitat requirement will suffice,

they could coexist.

Another possibility that can be considered is the possibility of evolution. This fusing

of the landmasses could drive the Marsupials, which were for a long time isolated from both

other organisms and other types of environment, to develop a characteristic that could help

them adapt. And this possible fusion of landmasses could possibly just not affect the

marsupials; of course the other organisms must also be affected. And if the marsupials could

adapt to this change and evolve suitable characteristics that would make them better

competitors; then they could win over the placental mammals and possibly other organisms.

III. Summary and Conclusion

Marsupials belong to one of the large groups of mammals. They are distinguished

through their marsupium and unique reproduction. They are divided into ten orders, one of

which is Order Diprotodontia which is composed of the kangaroo, possums, gliders, koalas,

wallabies and wombats. Diprotodonts are grouped as such because of their unifying features:

their unique dentition, lacking canine teeth and having well-developed incisors, and syndactyly.

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However, within this order, they still further diversified into seven families with variations in

diet, habitat, morphology, and reproduction.

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IV. LITERATURE CITED

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‘‘herbivores’’ (Diprotodontia: Koala, wombats, kangaroos and possums). ScienceDirect

(Molecular Phylogenetics and Evolution 46).

Cifelli, R.L. 1993. Early Cretaceous mammal from North America and the evolution of

marsupial dental characters. Oklahoma Museum of Natural History and Department of

Zoology. University of Oklahoma. Norman, OK 73019.

Zihlman, A.L. 2001. The Human Evolution Coloring Book. 2nd

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1995. Anzccart Facts Sheet. The University of Adelaide, Australia. Australia. Vol8 No4.

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and Trade. Commonwealth of Australia, Geoscience Australia, ACRES.

Nedin, C. (1994, June 28). The Natural History of Marsupials. The TalkOrigins Archive.

Exploring the Creation/Evolution Controversy. Retrieved August 30, 2011 from

http://TheNaturalHistoryofMarsupials.htm

Beck, R.M.D. Godthelp, H. Weisbecker, V. Archer, M. Hand, SJ. 2008. Australia's Oldest

Marsupial Fossils and their Biogeographical Implications. School of Biological, Earth

and Environmental Sciences. University of New South Wales, Sydney. New South

Wales, Australia.

2006. Marsupials. Microsoft Encarta. Microsoft Incorporation.