Marsupials Research Paper
Adaptive Radiation, Convergence and the Marsupials
Abstract
Metatheria (Marsupalia) originated upon Laurasia in the Cretaceous with the oldest fossilised remains having been dated to 125 Ma. The first period of metatherian diversification occurred in the late Cretaceous. The K-T mass extinction created an ecological void in which mammalian radiation occurred in the Palaeocene. With South America isolated from North America throughout the Tertiary, there was the second great period of metatherian diversification and there were numerous examples of placental and marsupial convergence. In the Eocene, one line of metatheria radiated across the Antarctic to Australasia and from that lineage the modern Australasian marsupials are descended. There …show more content…
are numerous examples of convergence within this marsupial population, which are now becoming evident with advances in DNA analysis. In the Pliocene the Great American Interchange occurred when the Isthmus of Panama was created and many South American marsupial fauna became extinct.
Introduction
To examine the relationship between marsupials and placentals throughout the tertiary one must first understand the concepts of adaptive radiation and convergent evolution.
In 1953, G.G. Simpson defined adaptive radiation as the ‘more or less simultaneous divergence of numerous lines from much the same adaptive type into different, also diverging adaptive zones.’ Adaptive radiation is the process by which a single common ancestor differentiates into myriad species, which have morphological and ecological adaptations to aid their survival in a wide range of environments. These adaptations arise through both speciation and phenotypic adaptation (Schluter, 2000).
Convergence is the process by which two or more non-monophyletic organisms evolve analogous traits due to occupying similar ecological niches. Simon Conway Morris has argued that a set number of potential ecological niches exist rather than evolution and natural selection leading to the creation of new ecological niches. From this one could infer that there are also a finite number of morphological adaptations that promote the successful functionality of an organism in available ecological niches (Ruse, 2008).
Conway Morris cites the shear number of examples of convergence as proof that evolution is constrained by natural selection along certain evolutionary pathways, which lead to the successful occupation of ecological niches. Geographical isolation can also lead to the duplication of certain traits through both convergence and parallel evolution (Springer et. al, 1997).
The Cretaceous-Tertiary mass extinction event left an ecological void. Many modern eutheria and metatheria can trace their origins back to the late Cretaceous but it was not until the early Cenozoic that the true mammalian adaptive radiation occurred. In modern times there are approximately four thousand three hundred extant species of placental mammals (Wilson & Reeder, 2005). This number is factors of ten larger than the number of species of marsupials. The marsupial biodiversity in this smaller number of species is therefore astounding. This biodiversity, both morphological and ecological, can be attributed to the geographical isolation of the Gondwanan continents from the more northern continents throughout the majority of the Cenozoic.
The marsupial radiation throughout the previously Gondwanan continents has been a result of at least two periods of adaptive radiation, followed by periods of progressive occupation and the convergence of not only placentals and marsupials but also within the marsupial population itself (Springer et al., 1997).
It is tempting to view the placental sister group of marsupials as an alternative evolutionary pathway of Mammalia but this comparison is of limited use. Metatheria did not evolve to fill as an extensive an array of ecological niches as the placental mammals did throughout the Tertiary and therefore any comparison of this nature is very limited in scope.
Metatherian Origination and Radiation
Marsupialia, Placentalia and Monotremata are the extant lineages of the Mammalian class. Modern marsupials are part of the infraclass metatheria. This infraclass includes all existing marsupials and the all known extinct sister taxa. Metatherians and eutherians are part of the sub-group Boreosphenida. The oldest known metatherian fossil was found in China in the Yixian Formation. The Sinodelphys szalayi has been dated as being one hundred and twenty-five million years, placing the fossil in the early Cretaceous. Without a known common ancestor for the metatheria and eutheria, this fossil provides us with best information for comparisons with chronologically analogous eutherian fossils to be based upon.
Geographically, it is interesting that the oldest metatheria has been discovered in China as this indicates that the metatherians have radiated globally over the course of at least sixty-five million years, with marsupials arriving in Australia approximately fifty million years ago in the Eocene (Marshall et al., 1990). Metatherian fossils have been found in Utah, dating back to one hundred and one million years ago (Cifelli, 1993). No South American metatheria have been found dating back to any earlier than this as of yet. In the late Cretaceous, the metatheria appear to have been exclusively in Laurasia. This would indicate that the metatherian origination and initial phase of diversification occurred exclusively in Laurasia. Furthermore, the South American fauna seem to have been endemic from those of North America for much of the Cretaceous.
Time Period Event
3 Ma Pliocene Great American Interchange
Creation of the Isthmus of Panama
15 Ma Miocene Antarctica covered by ice sheet
23 Ma Miocene Antarctica and Australia Separate
Oligocene Marsupials extinct in Afr., Eur., and Asia
50 Ma Eocene Marsupials radiate to Australasia
50 Ma Eocene North American Marsupials Extinct
65.5 Ma Palaeocene Start of Mammalian Radiation
65.5 Ma Cretaceous K-T Mass Extinction
125 Ma Cretaceous Oldest metatherian Sinodelphys szalayi alive
Cretaceous Oldest Common Ancestor of Metatheria and Eutheria
195 Ma Jurassic Origination of Mammalia
At end of the Cretaceous, metatheria could be found living in both Laurasia and upon the South American craton. Approximately sixty-five million years ago, North America became separated from South America. The mammals living on the two continents then underwent significantly different evolutionary pathways during their thirty five million year period of isolation from one another Tyndale-Briscoe. Whilst placentals came to dominate North America, filling most ecological niches, in South America metatheria filled the ecological niches of rodents, small insectivores, and shared the niche of large predator with early tertiary birds. The South American placentals, however, evolved to become large herbivores and none evolved into a large predatory niche unlike their North American brethren.
The Cretaceous-Tertiary mass extinction event created an environment in which the mammalian taxa could adapt and flourish. It is during this period of mammalian adaptive radiation in the Palaeocene that the second wave of metatherian diversification is supposed to have occurred. Research by Mark S. Springer, John A.W. Kirsch, and Judd A. Case (1997) on molecular divergence times of extant marsupial species has supported this theory of two metatherian periods of diversification. When combined with the palaeontological data it confirms that the first wave of diversification happened in the late Cretaceous exclusively in Laurasia. Furthermore, they state that all extant marsupials have arisen from a Gondwanan common ancestor, which would have diversified in the late Cretaceous or early Palaeocene.
About forty million years ago in the mid-Eocene, South America’s geographical isolation ended when a landmass formed between the southern tip of South America and the Antarctic landmass. The nature of the connecting landmass is somewhat disputed but is believed to have an island arc. At this point the South American metatheria radiated onto the southern continent and from there, onto the adjoining Australasian continent.
It appears that only one metatherian lineage successfully radiated to Australasia and is the common ancestor for all marsupial fauna on the continent. Currently, the Tingamarra fauna is the oldest metatherian that has been identified in the Australasian fossil record (Godthelp et al., 1992).
Convergent Evolution in the Cenozoic Sabre-toothed Predators
One clear and distinctive example of convergence is that of the saber-toothed predator organisms of the Cenozoic. The evolution of the distinctive canines of both taxa must be one of the most striking mammalian examples of evolutionary convergence to be found within the Tertiary. Mammalian sabre-toothed tigers of North America (from the Eocene onwards) and the marsupial sub-families of sabre-toothed predators of South America (from the Miocene onwards) appeared throughout the Cenozoic. This is evidence that there was obviously an ecological niche for ‘feline’ predators with large canines used for the stabbing and shearing of their prey. It has previously been theorised that the allosaurids had occupied this ecological niche throughout the upper Jurassic and lower Cretaceous (Bakker, 1998).
Fig 1.1 A – Borhyaenid Thylacosmilus, B – Smilodon (see references).
These borhyaenids were the top mammalian predators of South America for around twenty million years. Their legs were shorter than placental contemporaries and they are believed to have hunted in a manner similar to that of wolverines and badgers (Tyndale-Briscoe, 2005). One of the most recent borhyaenid sabretooths was the Thylacosmilus, which is thought to have become extinct around three to two and a half million years ago. The Thylacosmilus lived exclusively in South America and is thought to have become extinct when the Isthmus of Panama formed three million years ago and an event called the Great American Interchange began.
During the Great American Interchange, placentals migrated from North America along the newly formed landmass connecting the two continents. South American marsupials and placentals moved in the opposite direction. It is thought that the North American fauna out-competed their South American contemporaries and led to the extinction of some of the South American fauna.
Thylacosmilus may have fallen victim to this competition perhaps with the placental Smildodon, which were apex predators of North America, entering the same ecological niche in South America as that of the Thylacosmilus. It is interesting to note, however, that no obvious successor emerged to fill the ecological niche vacated by the Thylacosmilus and this has led to some debate upon whether or not the influx of North American fauna was in fact the true cause of its extinction (Tyndale-Briscoe, 2005). Another major effect of the American Interchange was the reintroduction of marsupial fauna to the North American continent. Native North American marsupial fauna became extinct in the late Eocene and the marsupials that inhabit areas of North America today are actually descended from South American ancestors.
Marsupial Morphological Overview
When looking at modern marsupial morphology the most readily apparent feature may be that of the pouch in the females. The first explorers of South America and Australia in the sixteenth century applied the Greek prefixes of ‘thyla’ and ‘pera’, translated to ‘pouch’ and ‘pocket’, attached to the known species of placental which they resembled best to describe the species.
The pouch is just part of the distinctive female reproductive system which is the criterion upon which early zoologist, Henri de Blainville, based his analysis of the marsupial anatomy in 1816. The female marsupial also has two vaginae, two cervix and two oviducts, which obviously distinguish marsupials females from placental females anatomically (Tyndale-Briscoe, 2005). This distinction also leads to differing lengths and modes of pregnancy and the rearing of young. Marsupials tend to have very short pregnancies compared to placentals, with the much smaller young undergoing further maturation in the pouch.
Fig 1.2 A comparison of the standard metabolic rate of terrestrial vertebrates (Tyndale-Briscoe, 2005).
Another major difference between marsupials and placentals is physiological.
Placentals have an average body temperature of 38oC whereas marsupials have an average temperature of 35.5oC. This equates to a slower metabolic rate. Advantages of this slower metabolic rate include lower food and water requirements when compared to a placental of a comparable body mass. This adaptation of the entire marsupial infra-class may be indicative of the conditions in South America and Australasia during which time the marsupials were evolving and is evidence of the constraint of marsupial evolution by the overlying ecological niche (in this case on a continental scale) in which they …show more content…
lived.
A slower metabolic rate would indicate the marsupials have adapted to survive a harsher climate than the average placental. It does, however, confer several disadvantages upon the marsupials in comparison to placentals. Placentals have higher rates of growth and reproduction, faster nerve conduction and smoother muscle contraction (Tyndale-Briscoe, 2005).
Phylogeny and Morphology
Incidents of convergence can often be attributed to external morphological features that can disguise the true phylogenetic relationships between taxa (Springer et al., 1997). When attempting to disentangle the web of convergence and synapomorphic and homoplastic similarities within an infra-class, such as the Marsupalia, it can often be contentious to label a superficial morphological similarity as convergence without taking into account the phylogenetic data now available due to enhancements in the study of DNA. When evaluating modern taxa, the zoologist has the luxury of using both phylogenetic and morphological data.
Fig 1.3 C: Examples of didactyl hindfeet of marsupials. D: Examples of the syndactyl hindfeet of Vombatus ursinus (iii) and Hypsipyrmnodon moschatus (iv). (Springer et al., 1997).
The relationship between the Australasian Thylacinidae family and the South American Borhyaenid family can lead to vastly conflicting conclusions. Both taxa demonstrate similar morphological and ecological traits throughout the Cenozoic. The Borhyaenid family of metatherians became extinct around the end of the Miocene and the Thylacine was declared extinct in 1982 (despite no Thylacine sightings being officially recorded since 1936 when the last Thylacine in captivity died). The oldest borhyaenid fossils date back to the early Palaeocene and the Thylacinidae is believed to have originated in the late Oligocene. If the Borhyaenids and the Thylacinidae were both descended from a common ancestor, then this would imply a closer relationship between South American and Australasian taxa throughout the Cenozoic than is currently thought to have existed. On the other hand, if the similarities in morphology and ecology are in fact convergence then the current theories of metatherian radiation and diversification are upheld and further questions are raised upon the subject of convergence versus relationships based upon shared traits within the modern marsupial community. The lack of phylogenetic data makes the relationship between the two hard to ascertain. Other examples of this quandary of convergence versus inheritance can be observed in the relationship between the American didelphid opossums and Australian dasyurids, and the South American caluromyid opossums and the Australian folivorous ringtails (Springer et. al, 1997). Within just the Australasian taxa, there are three genera of opossums that have independently developed gliding adaptations (Acrobates, Petauroides, and Petaurus).
Traditionally, syndactyl peramelinans and syndactyl diprotodontians have been thought to be very closely related based upon this anatomical similarity. Bandicoots, of the order Peramelamorphia, have always based been placed uncertainly upon cladograms as their relationship to other marsupials has been poorly understood. Phylogenetic analysis and studies based upon single-copy DNA/DNA hybridisation and DNA sequencing techniques have been used to assess the degree of relationship between extant genera and species of marsupials and placentals. In the marsupial studies, the relationships between most genera were largely confirmed but it has also been decided that the Peramelidae family are actually the terminus of their own branch of the cladogram, which has branched off from the other diprotodontian species before their subsequent diversification (See Fig 1.4)(Springer et. al, 1997). Syndactyly would appear to not be synapomorphous in the Australian peramelinans and diprotodontians but is either a trait inherited from a common ancestor, which has been subsequently lost within some taxa or is an example of convergence within the marsupial population.
Fig. 1.4 ‘DNA hybridisation tree of relationships among twenty-one of diprotodontian marsupials, rooted with two peramelid bandicoots.’ (Springer et al., 1997).
The main point of these inclusion of this examples is to illustrate the way in which modern DNA analysis can aid in identification of convergences within populations. As an extension of this line of inquiry, it may also be the case that some palaeontological relationships based solely upon morphological synapomorphies in the marsupial fossil record may not be true relationships but may in fact be convergence. In the case of the Peramelidae, they have never been placed comfortably upon any cladogram and there has always been a degree of doubt as to its position within the Marsupial infra-class.
Conclusions
The history of the marsupial infra-class has had three defining periods of adaptive radiation and progressive occupation. The first period of diversification being the in late Cretaceous, and restricted to Laurasia prior to the Cretaceous-Tertiary mass extinction event. In the ecological void left by this extinction event, the second period of diversification occurred once the metatherian population had radiated to South America. In the Eocene, the metatheria radiated across Antarctica to Australasia and thereupon began the final period of diversification into many of the orders whose descendants are most identifiable today.
The evolution of the marsupial infra-class throughout the Cenozoic has included many instances of convergence with placentals, mainly due to the geographical isolation of the populations. Marsupial evolution, however, has also had many occurrences of convergence within the infra-class itself which now, with techniques for the analysis of DNA, can be examined and ultimately be used to ascertain the correct evolutionary history.
It is important to unravel the numerous examples of convergence within the extant marsupial population because as long as there is ambiguity as to whether synapomorphies have arisen between Australasian and South American marsupials due to convergence or through inheritance from a common ancestor the original radiation of the Australasian marsupials cannot be entirely discerned.
References
Bakker, R.T., 1998, Brontosaurus Killers: Late Jurassic allosaurids as sabre-tooth cat analogues, Gaia, vol. 15, Pg. 145-158.
Cifelli, R.L., 1993, Early Cretaceous mammals from North America and the evolution of marsupial dental characters, Proceedings of the National Academy of Sciences USA, 90 Pg. 9413-9416.
Conway-Morris, S., 2003, Life’s Solution: Inevitable Humans in a Lonely Universe, Cambridge University Press, Pg. 126-194.
Fig 1.1: From: http://www.catbg.net/divi/pictures/Paleo/Thynacosmilus/Th_Smilodon.JPG Accessed 21/11/11.
Godthelp.
H., Archer, M., Cifelli, R., Hand, S.J., & Gilkeson, C.F., 1992, Earliest known Australian Tertiary mammal fauna, Nature 256, Pg. 514-516.
Marshall, L.G., Webb, S.D., Sepkoski, J.J., Jr., and Raup, D.M., 1982, Mammalian evolution and the great American interchange, Science 215: Pg. 1351-1357.
Marshall, L.G., Case, J.A., and Woodburne, M.O., 1990, Phylogenetic relationships of the families of marsupials, Current Mammology, vol. 2, H.H. Genoways, NY: Plenum Press.
Schluter, D., 2000, The Ecology of Adaptive Radiation, Oxford University Press, Pg. 10-21.
Simpson, G.G., 1953, The major features of evolution, New York, NY:Columbia University Press.
Springer, M.S., Kirsch, J.A.W., & J.A. Case, 1997, The Chronicle of Marsupial Evolution, Molecular Evolution and Adaptive Radiation, Cambridge University Press, Pg. 129-161.
Tyndale-Briscoe, C.H., 2005, Life of Marsupials, Csiro Publishing, Pg. 1-36, & 103-138.
Wilson, D.E., & Reeder, A.M., 1993, Mammal species of the world: a taxonomic and geographic reference (2nd ed.), Washington, D.C., Smithsonian Institution Press.
Wilson, D.E., & Reeder, A.M., 2005, Mammal Species of the World (3rd ed.), Preface and Introductory Material, Baltimore: John Hopkins University
Press.
Abstract
Metatheria (Marsupalia) originated upon Laurasia in the Cretaceous with the oldest fossilised remains having been dated to 125 Ma. The first period of metatherian diversification occurred in the late Cretaceous. The K-T mass extinction created an ecological void in which mammalian radiation occurred in the Palaeocene. With South America isolated from North America throughout the Tertiary, there was the second great period of metatherian diversification and there were numerous examples of placental and marsupial convergence. In the Eocene, one line of metatheria radiated across the Antarctic to Australasia and from that lineage the modern Australasian marsupials are descended. There …show more content…
are numerous examples of convergence within this marsupial population, which are now becoming evident with advances in DNA analysis. In the Pliocene the Great American Interchange occurred when the Isthmus of Panama was created and many South American marsupial fauna became extinct.
Introduction
To examine the relationship between marsupials and placentals throughout the tertiary one must first understand the concepts of adaptive radiation and convergent evolution.
In 1953, G.G. Simpson defined adaptive radiation as the ‘more or less simultaneous divergence of numerous lines from much the same adaptive type into different, also diverging adaptive zones.’ Adaptive radiation is the process by which a single common ancestor differentiates into myriad species, which have morphological and ecological adaptations to aid their survival in a wide range of environments. These adaptations arise through both speciation and phenotypic adaptation (Schluter, 2000).
Convergence is the process by which two or more non-monophyletic organisms evolve analogous traits due to occupying similar ecological niches. Simon Conway Morris has argued that a set number of potential ecological niches exist rather than evolution and natural selection leading to the creation of new ecological niches. From this one could infer that there are also a finite number of morphological adaptations that promote the successful functionality of an organism in available ecological niches (Ruse, 2008).
Conway Morris cites the shear number of examples of convergence as proof that evolution is constrained by natural selection along certain evolutionary pathways, which lead to the successful occupation of ecological niches. Geographical isolation can also lead to the duplication of certain traits through both convergence and parallel evolution (Springer et. al, 1997).
The Cretaceous-Tertiary mass extinction event left an ecological void. Many modern eutheria and metatheria can trace their origins back to the late Cretaceous but it was not until the early Cenozoic that the true mammalian adaptive radiation occurred. In modern times there are approximately four thousand three hundred extant species of placental mammals (Wilson & Reeder, 2005). This number is factors of ten larger than the number of species of marsupials. The marsupial biodiversity in this smaller number of species is therefore astounding. This biodiversity, both morphological and ecological, can be attributed to the geographical isolation of the Gondwanan continents from the more northern continents throughout the majority of the Cenozoic.
The marsupial radiation throughout the previously Gondwanan continents has been a result of at least two periods of adaptive radiation, followed by periods of progressive occupation and the convergence of not only placentals and marsupials but also within the marsupial population itself (Springer et al., 1997).
It is tempting to view the placental sister group of marsupials as an alternative evolutionary pathway of Mammalia but this comparison is of limited use. Metatheria did not evolve to fill as an extensive an array of ecological niches as the placental mammals did throughout the Tertiary and therefore any comparison of this nature is very limited in scope.
Metatherian Origination and Radiation
Marsupialia, Placentalia and Monotremata are the extant lineages of the Mammalian class. Modern marsupials are part of the infraclass metatheria. This infraclass includes all existing marsupials and the all known extinct sister taxa. Metatherians and eutherians are part of the sub-group Boreosphenida. The oldest known metatherian fossil was found in China in the Yixian Formation. The Sinodelphys szalayi has been dated as being one hundred and twenty-five million years, placing the fossil in the early Cretaceous. Without a known common ancestor for the metatheria and eutheria, this fossil provides us with best information for comparisons with chronologically analogous eutherian fossils to be based upon.
Geographically, it is interesting that the oldest metatheria has been discovered in China as this indicates that the metatherians have radiated globally over the course of at least sixty-five million years, with marsupials arriving in Australia approximately fifty million years ago in the Eocene (Marshall et al., 1990). Metatherian fossils have been found in Utah, dating back to one hundred and one million years ago (Cifelli, 1993). No South American metatheria have been found dating back to any earlier than this as of yet. In the late Cretaceous, the metatheria appear to have been exclusively in Laurasia. This would indicate that the metatherian origination and initial phase of diversification occurred exclusively in Laurasia. Furthermore, the South American fauna seem to have been endemic from those of North America for much of the Cretaceous.
Time Period Event
3 Ma Pliocene Great American Interchange
Creation of the Isthmus of Panama
15 Ma Miocene Antarctica covered by ice sheet
23 Ma Miocene Antarctica and Australia Separate
Oligocene Marsupials extinct in Afr., Eur., and Asia
50 Ma Eocene Marsupials radiate to Australasia
50 Ma Eocene North American Marsupials Extinct
65.5 Ma Palaeocene Start of Mammalian Radiation
65.5 Ma Cretaceous K-T Mass Extinction
125 Ma Cretaceous Oldest metatherian Sinodelphys szalayi alive
Cretaceous Oldest Common Ancestor of Metatheria and Eutheria
195 Ma Jurassic Origination of Mammalia
At end of the Cretaceous, metatheria could be found living in both Laurasia and upon the South American craton. Approximately sixty-five million years ago, North America became separated from South America. The mammals living on the two continents then underwent significantly different evolutionary pathways during their thirty five million year period of isolation from one another Tyndale-Briscoe. Whilst placentals came to dominate North America, filling most ecological niches, in South America metatheria filled the ecological niches of rodents, small insectivores, and shared the niche of large predator with early tertiary birds. The South American placentals, however, evolved to become large herbivores and none evolved into a large predatory niche unlike their North American brethren.
The Cretaceous-Tertiary mass extinction event created an environment in which the mammalian taxa could adapt and flourish. It is during this period of mammalian adaptive radiation in the Palaeocene that the second wave of metatherian diversification is supposed to have occurred. Research by Mark S. Springer, John A.W. Kirsch, and Judd A. Case (1997) on molecular divergence times of extant marsupial species has supported this theory of two metatherian periods of diversification. When combined with the palaeontological data it confirms that the first wave of diversification happened in the late Cretaceous exclusively in Laurasia. Furthermore, they state that all extant marsupials have arisen from a Gondwanan common ancestor, which would have diversified in the late Cretaceous or early Palaeocene.
About forty million years ago in the mid-Eocene, South America’s geographical isolation ended when a landmass formed between the southern tip of South America and the Antarctic landmass. The nature of the connecting landmass is somewhat disputed but is believed to have an island arc. At this point the South American metatheria radiated onto the southern continent and from there, onto the adjoining Australasian continent.
It appears that only one metatherian lineage successfully radiated to Australasia and is the common ancestor for all marsupial fauna on the continent. Currently, the Tingamarra fauna is the oldest metatherian that has been identified in the Australasian fossil record (Godthelp et al., 1992).
Convergent Evolution in the Cenozoic Sabre-toothed Predators
One clear and distinctive example of convergence is that of the saber-toothed predator organisms of the Cenozoic. The evolution of the distinctive canines of both taxa must be one of the most striking mammalian examples of evolutionary convergence to be found within the Tertiary. Mammalian sabre-toothed tigers of North America (from the Eocene onwards) and the marsupial sub-families of sabre-toothed predators of South America (from the Miocene onwards) appeared throughout the Cenozoic. This is evidence that there was obviously an ecological niche for ‘feline’ predators with large canines used for the stabbing and shearing of their prey. It has previously been theorised that the allosaurids had occupied this ecological niche throughout the upper Jurassic and lower Cretaceous (Bakker, 1998).
Fig 1.1 A – Borhyaenid Thylacosmilus, B – Smilodon (see references).
These borhyaenids were the top mammalian predators of South America for around twenty million years. Their legs were shorter than placental contemporaries and they are believed to have hunted in a manner similar to that of wolverines and badgers (Tyndale-Briscoe, 2005). One of the most recent borhyaenid sabretooths was the Thylacosmilus, which is thought to have become extinct around three to two and a half million years ago. The Thylacosmilus lived exclusively in South America and is thought to have become extinct when the Isthmus of Panama formed three million years ago and an event called the Great American Interchange began.
During the Great American Interchange, placentals migrated from North America along the newly formed landmass connecting the two continents. South American marsupials and placentals moved in the opposite direction. It is thought that the North American fauna out-competed their South American contemporaries and led to the extinction of some of the South American fauna.
Thylacosmilus may have fallen victim to this competition perhaps with the placental Smildodon, which were apex predators of North America, entering the same ecological niche in South America as that of the Thylacosmilus. It is interesting to note, however, that no obvious successor emerged to fill the ecological niche vacated by the Thylacosmilus and this has led to some debate upon whether or not the influx of North American fauna was in fact the true cause of its extinction (Tyndale-Briscoe, 2005). Another major effect of the American Interchange was the reintroduction of marsupial fauna to the North American continent. Native North American marsupial fauna became extinct in the late Eocene and the marsupials that inhabit areas of North America today are actually descended from South American ancestors.
Marsupial Morphological Overview
When looking at modern marsupial morphology the most readily apparent feature may be that of the pouch in the females. The first explorers of South America and Australia in the sixteenth century applied the Greek prefixes of ‘thyla’ and ‘pera’, translated to ‘pouch’ and ‘pocket’, attached to the known species of placental which they resembled best to describe the species.
The pouch is just part of the distinctive female reproductive system which is the criterion upon which early zoologist, Henri de Blainville, based his analysis of the marsupial anatomy in 1816. The female marsupial also has two vaginae, two cervix and two oviducts, which obviously distinguish marsupials females from placental females anatomically (Tyndale-Briscoe, 2005). This distinction also leads to differing lengths and modes of pregnancy and the rearing of young. Marsupials tend to have very short pregnancies compared to placentals, with the much smaller young undergoing further maturation in the pouch.
Fig 1.2 A comparison of the standard metabolic rate of terrestrial vertebrates (Tyndale-Briscoe, 2005).
Another major difference between marsupials and placentals is physiological.
Placentals have an average body temperature of 38oC whereas marsupials have an average temperature of 35.5oC. This equates to a slower metabolic rate. Advantages of this slower metabolic rate include lower food and water requirements when compared to a placental of a comparable body mass. This adaptation of the entire marsupial infra-class may be indicative of the conditions in South America and Australasia during which time the marsupials were evolving and is evidence of the constraint of marsupial evolution by the overlying ecological niche (in this case on a continental scale) in which they …show more content…
lived.
A slower metabolic rate would indicate the marsupials have adapted to survive a harsher climate than the average placental. It does, however, confer several disadvantages upon the marsupials in comparison to placentals. Placentals have higher rates of growth and reproduction, faster nerve conduction and smoother muscle contraction (Tyndale-Briscoe, 2005).
Phylogeny and Morphology
Incidents of convergence can often be attributed to external morphological features that can disguise the true phylogenetic relationships between taxa (Springer et al., 1997). When attempting to disentangle the web of convergence and synapomorphic and homoplastic similarities within an infra-class, such as the Marsupalia, it can often be contentious to label a superficial morphological similarity as convergence without taking into account the phylogenetic data now available due to enhancements in the study of DNA. When evaluating modern taxa, the zoologist has the luxury of using both phylogenetic and morphological data.
Fig 1.3 C: Examples of didactyl hindfeet of marsupials. D: Examples of the syndactyl hindfeet of Vombatus ursinus (iii) and Hypsipyrmnodon moschatus (iv). (Springer et al., 1997).
The relationship between the Australasian Thylacinidae family and the South American Borhyaenid family can lead to vastly conflicting conclusions. Both taxa demonstrate similar morphological and ecological traits throughout the Cenozoic. The Borhyaenid family of metatherians became extinct around the end of the Miocene and the Thylacine was declared extinct in 1982 (despite no Thylacine sightings being officially recorded since 1936 when the last Thylacine in captivity died). The oldest borhyaenid fossils date back to the early Palaeocene and the Thylacinidae is believed to have originated in the late Oligocene. If the Borhyaenids and the Thylacinidae were both descended from a common ancestor, then this would imply a closer relationship between South American and Australasian taxa throughout the Cenozoic than is currently thought to have existed. On the other hand, if the similarities in morphology and ecology are in fact convergence then the current theories of metatherian radiation and diversification are upheld and further questions are raised upon the subject of convergence versus relationships based upon shared traits within the modern marsupial community. The lack of phylogenetic data makes the relationship between the two hard to ascertain. Other examples of this quandary of convergence versus inheritance can be observed in the relationship between the American didelphid opossums and Australian dasyurids, and the South American caluromyid opossums and the Australian folivorous ringtails (Springer et. al, 1997). Within just the Australasian taxa, there are three genera of opossums that have independently developed gliding adaptations (Acrobates, Petauroides, and Petaurus).
Traditionally, syndactyl peramelinans and syndactyl diprotodontians have been thought to be very closely related based upon this anatomical similarity. Bandicoots, of the order Peramelamorphia, have always based been placed uncertainly upon cladograms as their relationship to other marsupials has been poorly understood. Phylogenetic analysis and studies based upon single-copy DNA/DNA hybridisation and DNA sequencing techniques have been used to assess the degree of relationship between extant genera and species of marsupials and placentals. In the marsupial studies, the relationships between most genera were largely confirmed but it has also been decided that the Peramelidae family are actually the terminus of their own branch of the cladogram, which has branched off from the other diprotodontian species before their subsequent diversification (See Fig 1.4)(Springer et. al, 1997). Syndactyly would appear to not be synapomorphous in the Australian peramelinans and diprotodontians but is either a trait inherited from a common ancestor, which has been subsequently lost within some taxa or is an example of convergence within the marsupial population.
Fig. 1.4 ‘DNA hybridisation tree of relationships among twenty-one of diprotodontian marsupials, rooted with two peramelid bandicoots.’ (Springer et al., 1997).
The main point of these inclusion of this examples is to illustrate the way in which modern DNA analysis can aid in identification of convergences within populations. As an extension of this line of inquiry, it may also be the case that some palaeontological relationships based solely upon morphological synapomorphies in the marsupial fossil record may not be true relationships but may in fact be convergence. In the case of the Peramelidae, they have never been placed comfortably upon any cladogram and there has always been a degree of doubt as to its position within the Marsupial infra-class.
Conclusions
The history of the marsupial infra-class has had three defining periods of adaptive radiation and progressive occupation. The first period of diversification being the in late Cretaceous, and restricted to Laurasia prior to the Cretaceous-Tertiary mass extinction event. In the ecological void left by this extinction event, the second period of diversification occurred once the metatherian population had radiated to South America. In the Eocene, the metatheria radiated across Antarctica to Australasia and thereupon began the final period of diversification into many of the orders whose descendants are most identifiable today.
The evolution of the marsupial infra-class throughout the Cenozoic has included many instances of convergence with placentals, mainly due to the geographical isolation of the populations. Marsupial evolution, however, has also had many occurrences of convergence within the infra-class itself which now, with techniques for the analysis of DNA, can be examined and ultimately be used to ascertain the correct evolutionary history.
It is important to unravel the numerous examples of convergence within the extant marsupial population because as long as there is ambiguity as to whether synapomorphies have arisen between Australasian and South American marsupials due to convergence or through inheritance from a common ancestor the original radiation of the Australasian marsupials cannot be entirely discerned.
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