Module 4 Flashcards
Subclass Theria Clade Metatheria
Explore the diversity of marsupials, including their evolutionary history and extinct forms.
- Opossum spp.
- Kangaroos and other Macropodidae
- Wombats (teeth: convergent evolution with rodents)
- Koalas
- Marsupial mole
- Borhyaenids were a carnivorous group of otter/wolverine-shaped marsupials (Order Sparassodonta) [Extinct]
- Thylacosmilus sabre-toothed jaguar-sized predator (marsupial equivalent to Smilodon) [Extinct]
- Carnivorous, shrew-like marsupials in of South America
- Water opossum looks a bit similar to a muskrat (possesses webbed feet; Central America)
- The Monitor del Monte “little mountain monkey” is a semi-arboreal South American marsupial which is thought to be more closely related to the marsupials of Australia than those in the Americas (mouse-like).
- Quoll (Qu-ole); the marsupial equivalent to a rat
- Fat-tailed dunnart (also similar to a mouse; small and nocturnal; Australia)
- Tasmanian Devil
- Numbat
- Thylacine (Tasmanian Wolf) [Extinct– for now]
- Bandicoot
- sugar glider (closest to a “flying” marsupial)
- Thylacoleo carnifex (marsupial lion; diprotodont condition; teeth example of convergent evolution with eutherian cats) [Extinct]
Identify key physical and physiological traits that differentiate marsupials from eutherian mammals.
Marsupials, like monotremes, possess the epipubic bone, which is believed to provide pouch support.
Some marsupials possess syndactylous feet (kangaroos, koalas, possums, and wombats; bilbies and bandicoots; extinct Diprotodon sp.), which is where the second and third toes of a foot are fused together by skin.
Some marsupials, called diprotodonts, have unique canine teeth (incisors) on their lower jaw that appear like two buck-teeth that jut out (as opposed to polyprotodonts that have multiple lower incisors). Most marsupials are polyprotodonts. Diprotodont marsupials include kangaroos, wallabies, possums, gliders, wombats, koalas, pygmy possums, honey possums, and feathertail gliders, showcasing diverse adaptations for hopping, climbing, gliding, and herbivorous or omnivorous diets.
Prehensile tails (can hold onto things) are also a common feature in marsupials. Kangaroos do not possess prehensile tails but instead use theirs as an additional appendage for balance and movement.
Some marsupials, like the wombat and the koala, have these pronounce front incisors, displaying convergent evolution with rodents. Note that both of these species are herbivorous (most marsupials are carnivores).
Discuss traits shared between marsupials and Prototherians, highlighting evolutionary connections; and the major share traits of Theria.
“Marsupials and Prototherians (monotremes) share several traits linked to their evolutionary history, such as a cloaca for excretion and reproduction, lower metabolic rates compared to placental mammals, and less complex placental structures (or none, in monotremes). Both groups also lay eggs (in Prototherians) or give birth to highly underdeveloped young (in marsupials), relying heavily on post-birth care, highlighting their intermediate evolutionary position between reptiles and more advanced mammals.
Theria, which includes marsupials and eutherians, is characterized by live birth (viviparity), the presence of a tribosphenic molar (adapted for diverse diets), and more complex auditory structures like three middle ear bones. They also share mammary glands with nipples for lactation and a more advanced brain with a neocortex, supporting higher cognitive functions. These traits emphasize Theria’s evolutionary advancement toward more efficient reproduction and survival strategies.” (chat)
Explain the reproductive biology of marsupials and compare it to that of Prototherians and Eutherians; choriovitelline (or yolk sac) placenta; and instances of independent evolution of a more invasive chorioallantoic placental, closer to the Eutherian placenta.
Adult female marsupials are pretty much always pregnant. Because their young are born so underdeveloped, the females can become pregnant again while still rearing young in the pouch.
“The marsupial placenta is primarily derived from the yolk sac, hence the term “choriovitelline placenta.”
This placenta forms a less invasive connection with the uterine wall than the chorioallantoic placenta of eutherians. It facilitates gas exchange and nutrient transfer between the mother and the developing embryo, but it is less efficient compared to the eutherian placenta.
As a result, marsupial embryos are born at a very early stage of development, often resembling an embryo more than a fully-formed neonate.” (chat)
“The eutherian placenta supports a much longer gestation period, sometimes lasting many months (e.g., humans at ~9 months). In contrast, marsupial development heavily relies on lactation and the pouch rather than extended gestation.” (chat)
The bandicoot is an exception to this, as they have a more invasive, chorioallantoic placenta (closer to the eutherian placenta).
“The chorioallantoic placenta in bandicoots is a unique adaptation among marsupials, as it is more developed and efficient than the typical marsupial choriovitelline placenta. Bandicoots form a temporary chorioallantoic placenta that connects more deeply with the uterine wall, allowing for improved nutrient and gas exchange compared to the standard yolk-sac placenta found in other marsupials. However, it is still less invasive and less efficient than the chorioallantoic placenta of eutherians, which supports prolonged in-utero development. This makes bandicoots an interesting exception, bridging the gap between marsupial and eutherian placental structures.” (chat)
What are the advantages of longer gestation?
“The marsupial strategy minimizes the risks associated with prolonged pregnancy, such as maternal death during a long gestation. However, it also means marsupials are highly dependent on their pouch and lactation for neonatal survival.
Eutherians, on the other hand, have adapted to invest heavily in the prenatal phase, which may reduce the risks of neonatal mortality but increases the energetic and physiological demands on the mother during pregnancy.” (chat)
Evaluate the ecological constraints on marsupial diversity, including physical limitations arising from short gestation, such as limb development.
“Marsupial diversity is shaped by ecological constraints, particularly their reproductive strategy of short gestation and reliance on a pouch for postnatal development. This limits the prenatal development of marsupial neonates, especially limb and organ development, which must be sufficient at birth for crawling to the pouch. Consequently, marsupials have fewer opportunities for ecological niches requiring highly specialized or precocial young, such as fully aquatic environments or prolonged in-utero development. Additionally, marsupials’ slower reproductive rates and high dependency on maternal care make them more vulnerable to environmental changes and predation, further restricting their diversity compared to eutherians. These constraints have shaped their adaptive radiation, favoring generalist and terrestrial niches over highly specialized lifestyles.” (chat)
Describe the current diversity of marsupials and engage in discussions on conservations strategies for the group.
Distribution of marsupials from the Late Crustaceous period to present day has drastically decreased from a broad range over all continents to South America, eastern Central America, and Oceania.
