Eutherians

Question 1

Difference between Placentalia and Eutheria

Answer 1

Placentalia - all extant placentals and their most recent common ancestor

Eutheria - all extinct mammals that share a more common ancestor with placentals than they do with Methateria

Question 2

Give some common shared features of Eutheria

Answer 2

Jaw and teeth:

• Joint between dentary and squamosal bone
• Formed in embryo
• Support feeding and vocalisation from birth
• Formula 4-3-3-1 with some exceptions

Presence of placenta:

• organ that facilitated nutrient and waste exchange between mother and foetus during pregnancy

Ankle joint:

• enlarged malleolus (bottom of tibia)

Question 3

How diverse is the Eutheria?

Answer 3

1135 extant genera

~5000 species

Afrotherians:

• 6 orders and 90 species
• Examples being elephant and ardvark

Xenarthra:

• 1 group and 31 species
• Giant anteater and three-toed sloth

Euarchontoglires:

• 5 orders and 2000 species
• Ring-tailed lemurs and rabbits

Laurasiatheria:

• 6 orders and 1775 species
• Wild dogs, tigers and bears

Question 4

Give some features of the hypothetical placental ancestor

Answer 4

Tree-climbing

Insect-eating

Weight around 6-245g

Furry

Long tail

Complex brain with a large lobe

Question 5

Give some features of Eutherian reproduction

Answer 5

Viviparous

Oocytes are yolk free

Develop in ovary

Oestrus cycle

Testicles are scrotal or inguinal

Fertilisation in the uterine tubes for most taxa

2-6 days later zygotes arrive in uterine cavity for implantation

Single evolutionary origin

Question 6

What is the placenta?

Answer 6

Transient organ that provides an interface for metabolic exchanges between foetus and mother

Composed of foetal chorion and maternal uterine endoemtrium

Question 7

Give the different types of placenta

Answer 7

Diffuse:

• placenta forms continuous interface with uterine lining
• mother and foetal tissue in direct contact
• common in ungulates

Discoid:

• disc-shaped structure
• interaction of mother and foetal tissue confined to small area
• common in primates

Cotyledonary:

• multiple structures that serve as contact points between foetal and maternal tissue
• common in ruminants

Zonary:

• placenta forms a band that completely (or almost) surrounds the foetus
• common in carnivores

Question 8

Why was there repeated evolution of a less invasive placenta?

Answer 8

Invasive placenta is ancestral but evolved to become less invasive

No link between placental traits and invasivness and growth

Shorter gestation in invasive placenta species

Less invasive placenta = less transmission risk for parasites? - but found lower parasite richness in the envrionment in species with non-invasive placenta

So may be beneficial to pass on maternal antibodies when protozoan infection risk is high so selection pressure driving retention of invasive placenta