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Mammals are vertebrates, and the main distinctive feature separating them from other animals is the presence of mammary glands. Other differentiating characteristics among mammals include integument, skeleton, and internal anatomical features. In modern times, this class of animals exists in three clades marsupials, monotremes, and placental mammals. Marsupials are currently found in Australia, and they include kangaroos, bandicoot, koala, and other various species. The majority of these animals have a pouch where their young ones reside after birth for nurturing. Marsupials, like any other mammals, have a long evolution history dating back to over 90 million years ago. This paper discusses marsupials to present a phylogeny of their in-group and out-group (sister) taxa, key features that unite the clade, and ecological roles played by some of these features. It also describes the evolutionary history of marsupials with respect to geological time scale and factors to the success or failure of this clade in evolutionary time.
Phylogeny
On the other hand, in the Australian orders, the largest, Diprotodontia, has 39 genera and 143 species followed by Dasyuromorphia with 23 genera and 71 species, then Peramelemorphia with 8 genera and 21 species, and finally Notoryctemorphia with 1 genus and 2 species (Meredith et al., 2007). Both the American and Australian orders form the sister taxa of marsupials because they are closely related, and they share many common physical and anatomical features.
Key Features
Marsupials have distinctive features or synapomorphies that unite the clade morphologically, behaviorally, or physiologically. From a general outlook, marsupials share the common mammalian characteristics, including mammary glands, true hair, and 3 middle ear bones. However, some features, such as the pouch with multiple nipples, differentiate these animals from their out-group taxa. Morphologically, they have small and tight skulls with enlarged and extended cheekbone to the rear side of the animal. The lower jaw has an angular extension bending toward the center.
They also have a hard palate that differentiates them from their evolutionary ancestors, Eutherians, which have foramina with many openings. Additionally, they have between 40 and 50 teeth, with some created permanently, that is, they do not grow. Some, such as kangaroos, have muscular and enlarged hind legs and reduced forelimbs. The majority of the extant species have a pouch, which is a permanent bag opening at the front. However, in some species, the pouch grows during gestation in the form of skin folds where the young ones are hidden. The pouch is only present in females, but some males, such as water opossum, have a bag-like feature, specifically for holding their genitalia when running or swimming.
Physiologically, the reproductive system of marsupials is different in many ways from that of placental mammals. For instance, marsupials have a cloaca, which is directly linked to a urinogenital pocket where waste materials are briefly stored before being expelled. Females have double vaginas and uteri whereby during gestation, a canal forms between them to make the median vagina for birthing. In other words, each vagina leads to a different uterus, but they all emerge externally via the same opening. The median vagina or birth canal is solely used for giving birth, and in most cases, it is impermanent.
Marsupials are born during the early stages of development, whereby they crawl until they enter their mothers pouch and attach to a nipple where they stay for several weeks. Ultimately, after enough nurturing, the young ones can leave the pouch intermittently and come back for protection and warmth. Similarly, males have double phallus located at the front of the scrotum. The bifurcated penis is designed to fit into the two female vaginas (Sharman & Pilton, 1964). The phallus is only used for mating, thus these mammals have a different urinary tract. Additionally, some species have very short gestation periods of about 12 days, which is a distinguishing feature from other mammals.
Ecological Roles of Key Features
Each distinguishing key feature of marsupials has specific roles to play in the animals ecology. The pouch is specifically meant to nurture and protect the young ones, also called joeys. As mentioned earlier, joeys are born prematurely at very early stages of development after which they find their way into their mothers pouch, where they are attached to a nipple for suckling. Inside the pouch, the joeys are taken care of until they are strong enough to step into their surrounding environment. Therefore, the pouch is an ecological adaptation to ensure that joeys are protected. In its absence, the probability of the immature young ones dying after birth would be high almost guaranteed. In the process, these mammals would be extinct by now, because the chances of survival for joeys would be minimal.
Kangaroos are specifically adapted to grassland environment, as they have strong hind legs and reduced forelimbs for jumping, which is their characteristic locomotive style supported by a thick and muscular tail for balance. The honey possum, because it is nocturnal, has three spectrally cone photoreceptors (Karlen & Krubitzer, 2007). Additionally, their paws are designed to fit their natural habitats. For example, some have spade-shaped paws for digging, while others like the striped possum have elongated digit for catching insects. In general, marsupials inhabit different environments, such as aquatic, arboreal, terrestrial, and burrowing. Additionally, some are solitary, while others are socialized feeding on various food materials as carnivores, herbivores, and omnivores. Karlen and Krubitzer (2007) argue that marsupials have evolved an array of morphological, behavioral, and cortical specializations that are strikingly similar to those observed in placental mammals occupying similar habitats, which indicate that there are constraints imposed on evolving nervous systems that result in recurrent solutions to similar environmental challenges (p. 122). As such, these animals use their distinct features to adapt and survive in their environments.
Evolutionary History
The available fossil records show that marsupials evolved from Therapsida, one of the reptilian orders, which developed complex characteristics believed to be part of the Mammalia class, in around 230 million years ago during the Triassic period (Clemens, 1968). In dental morphology, these animals had various features ranging from the multituberculates, whose complex opposing molars equipped them for existence on a herbivorous diet, to the symmetrodonts and triconodonts, probably carnivorous or insectivorous mammals, with dentitions admirably modified for cutting and slicing their food (Clemens, 1968, p. 2). The early marsupial fossils were discovered in Texas and Spain, specifically the Multituberculata, Triconodonta, and Symmetrodonta orders. The earliest known mammals from which marsupials evolved were members of the infraclass Pantotheria found in Wales (Clemens, 1968). Two orders, Symmetrodonta and Eupantotheria, evolved from this infraclass. The Welsh Pantotheria had strong lower jaws for the chopping of food materials. The Symmetrodonta continued with the evolution process during the Early Cretaceous period with remarked changes to adapt to the environment, especially after the emergence of angiosperms. During the mid-Jurassic period, marsupials are thought to have split with placental mammals from metatherians.
About 100 million years ago, metatherians spread from the Gondwana continent (currently China and Australia) toward the west into modern-day North America. However, from these early fossil records, it is difficult to differentiate marsupials from other mammals because most distinctive characteristics are physiological. The first definite marsupial fossil is close to 65 million years old, and it belongs to the Peradectes minor (OLeary et al., 2013). However, marsupials in Laurasia became extinct for various reasons, including intense competition from placental mammals (Sánchez-Villagra, 2012). In South America, Didelphimorphia, specifically opossums, evolved, but they could not grow into large animals because they were outcompeted by the native placental ungulates (Prevosti et al., 2011). With North and South America connected, opossums spread through Central America to reach Canada.
Around 50 million years ago, marsupials entered Australia after it was split from current-day China. Marsupials seen in Australia today are progenitors of these early variants, which invaded different parts of the continent. They continued to evolve into carnivores, omnivores, and herbivores, to adapt to the changing environmental conditions. Therefore, based on the available fossil and DNA data, Nilsson et al. (2010) point that the evolution of marsupials started with Didelphimorphia and evolved to Paucituberculata before becoming Microbiotheria and the process ended with the modern-day Australian marsupials. These mammals survived due to their ability to adapt to the changing environment, especially through dentition modifications for feeding purposes. Those that could not change with the evolving environment became extinct.
Conclusion
Marsupials are part of the three orders of mammals, and they are currently found mainly in Australia and some species in North America. Based on their phylogeny, marsupials outgroups are Eutherian, while ingroups the over 330 extant species the three American and four Australian orders. These mammals have peculiar features, with the pouch being the most outstanding, and it is mainly used to nurture and protect joeys, which are born prematurely. Without the pouch, marsupials would be extinct because the young ones cannot survive the harsh weather conditions. The evolutionary history of marsupials is long starting from over 230 million ago, but the first definite fossils related to this order are 65 million years old, and they belong to Peradectes minor. The current marsupials in Australia entered the continent 50 million years ago and evolved into various species as they adapted to the changing environment.
References
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Clemens, W. (1968). Origin and early evolution of marsupials. Evolution, 22(1), 1-18.
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Gallus, S., Janke, A., Kumar, V., & Nilsson, M. A. (2015). Disentangling the relationship of the Australian marsupial orders using retrotransposon and evolutionary network analyses. Genome Biology and Evolution, 7(4), 985-992.
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Karlen, S. J., & Krubitzer, L. (2007). The functional and anatomical organization of marsupial neocortex: Evidence for parallel evolution across mammals. Progress in Neurobiology, 82(3), 122-141.
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Meredith, R. W., Westerman, M., Case, J. A., & Springer, M. S. (2007). A phylogeny and timescale for marsupial evolution based on sequences for five nuclear genes. Journal of Mammalian Evolution, 15(1), 1-36.
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