What is the sister group of marsupials?
- Eutheria (placentals)
- Marsupials and Eutheria = Theria (clade) → sister taxa to monotremes → under mammalia
Unique skeletal features of marsupials
- Epipubic bones
- (can also be found in monotrmes)
- Thought to be support and protection of the pouch
- Recently locomotion is thought to be an adaptation of the bone
- Not present in thylacia
How do marsupial skulls and jaws differ from other groups ?
Skull
- Large face - small brain case
- Rest is similar to primitive mammal
Jaw
- Rear part in turned inward instead of outward
How are marsupial teeth different from other groups?
- More teeth than placental mammals
- 3 premolars and 4 molars in both upper and lower jaw
- 4 or more incisors - polyprotodont
- 2 lower incisors - diprotodont - Exception to the rule (possum etc)
What is the main different between marsupials and other groups?
- The reproductive system
Presence of a pouch (50% of species)
Features of the female reproductive system
- Female : completely doubled
- Fertilisation will occur via one of the
- Vaginas
- At birth both vaginas fuse and create
- Pseudo birth canal
- In kangaroos birth canal remains open
Features of the male reproductive system
- Mostly similar to placentals
- Penis behind the scrotum
- Some species have a bifurcate (forked) penis (in this case the scrotum is behind penis)
How long is the gestation time of marsupials?
- Short gestation time (8-43 days)
- Extremely altricial young - 1% of adults body mass
- Average length of gestation = length of menstrual cycle
- Some marsupials have a simple form of placenta - gestation period is still very short
how does marsupial body temperature differ from placentals?
2.5 degrees lower
What three groups of marsupials are found in the Americas?
Classification
- Didelphimorphia (opossum)
- Microbiotheria (only one extant species: monetalmente)
- Paucituberculata (7 species of shrew opossum)
What are the 4 groups of marsupials found in Australia and Australasia?
Dasyuromorphia (carnivorous species)
Diprotodontia (largest order - wombats, koalas, kangaroos etc) 155 species
Notoryctemorphia (marsupial moles)
Peramelemorphia (bandicoots.. )
What is the distribution of Marsupials?
- 3 groups are found in the Americas
- 4 groups are found in Australia/Australasia
What is essential when looking at the classifcation of marsupials?
The ankle bone pattern
- Continuous lower ankle pattern (derived feature)
- Separate ankle structure (primitive feature)
These features of the ankle bone match to their distribution with one exception: The monetalmente
What is the primitive ankle bone condition?
- Seperate ankle structure
- Seen in the American groups (except monetalmente)
What is the derived ankle bone feature?
- Continuous ankle structure
- Seen in the Australasian groups & the monetalmente
What are the names given to the American and Australasian groups of marsupials?
- Ameridelphia
- Australidelphia
How did marsupials get to Australia?
- Widest accepted theory
- They dispersed from the Americas via Antarctica mainland when everything was connected during the late cretaceous or paleocene
What there a single, multiple or continuous disperal of marsupials?
Single dispersal event from SA to Australia (through Antarctica)
Main characteristics of marsupial reproduction
- Pouch/ Marsupium in 50% of species
- Paired vaginae transport sperm
- Birth is via a pseudovaginal canal
- Two uteri
- Simple placenta in some cases
How is pouch positioned?
- Pouch is either upward or backward facing depending on how animals walks/stands
- Kangaroos are upward facing because they stand up
How can fertilisation be altered?
- Fertilisation can be put on hold if mother has a young already
- Can pause embryo development until mother is more available to care
Newborn marsupial features
- Lack cranial skeletal segments
- No eye pigments or eyelids
- Separation of heart ventricles incomplete
- Strong forelimb locomotion to enable climbing to pouch
- Developed olfactory - sense where pouch is
- Mouth and nose/tongue? Well developed to latch onto nipple
Benefits of such a short gestation period?
- Much less of an energetic cost
- No need to protect the embryo from the maternal immune system
- Less risks for the female (extended pregnancy, large embryo….) - risk of predation (can expel embryo and run away)
- Flexible reproduction
- Non seasonal breeding in most marsupials
Risks of such a short gestation period?
- Tiny newborn is very underdeveloped - at risk
- Constrains evolution of marsupial forelimb (due to strong initial development of forelimb)
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Intro and origins of verts54
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The Agnatha and evolution of the jaw31
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Fishes: Chondrichthyes52
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Practice questions7
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Osteichyes (Bony fish)45
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EVOLUTION OF THE TETRAPODS18
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Amphibians34
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Amniotes and the conquest of the land reptiles68
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DINOSAURS23
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Origin on birds22
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Origin of Flight24
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MAMMALS38
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Monotremes6
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Marsupials25
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PRIMATES28
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Primate vision18
