Module 5

Question 1

Define the differences between marsupial and eutherian reproduction.

Answer 1

Eutherians diverge from marsupials based on the presence of holoblastic cleavage (the complete division of an egg during early embryonic development, where the entire zygote is cleaved into separate blastomeres), placentation, viviparity (development of the embryo inside the body of the mother), and testicular descent.

While marsupials have this “conveyor belt of offspring”, eutherians have a more segmented reproductive history.

The placenta provides this close interface between the material and fetal blood cells so they are no longer relying much on the nutrients of the yolk, as seen with marsupials. However, because eutherian placental implantation in the uterine lining is much more invasive, highly specialized tissues are growth to prevent the mother’s body from rejecting the fetus (foreign DNA, male DNA).

This results in a greatly decreased lactation period in eutherians compared to marsupials. This is important because majority of energy expenditure in marsupials occurs during lactation.

Question 2

Evaluate the ecological constraints on marsupial diversity, including physical limitations arising from short gestations, such as limb development.

Answer 2

Eutherians do not need the strongly developed girdles for crawling to the pouch, as seen with marsupials. This offered huge benefits for eutherian locomotion and the evolution of more sophisticated girdle structure.

Question 3

Describe the causes and consequences of the K-T extinction event (the end of the Cretaceous period).

Answer 3

Eutherians didn’t diversify until after the Cretaceous period (66 mya).

“The K-T extinction event (now referred to as the Cretaceous-Paleogene or K-Pg extinction) occurred approximately 66 million years ago and was caused by a combination of catastrophic events. The primary cause was a massive asteroid impact near present-day Mexico, creating the Chicxulub Crater. This impact generated intense heat, shockwaves, and global wildfires, while ejecting dust and debris into the atmosphere, blocking sunlight and causing a “nuclear winter.” Additionally, massive volcanic eruptions in the Deccan Traps of India released enormous amounts of gases like carbon dioxide and sulfur dioxide, contributing to severe climate changes and acid rain. These factors, combined with long-term shifts in temperature and sea levels, left ecosystems vulnerable even before the asteroid’s impact.

The consequences of this extinction were profound. About 75% of all species were wiped out, including non-avian dinosaurs, marine reptiles, and many plants and marine organisms. Ecosystems collapsed as photosynthesis declined due to the lack of sunlight, disrupting food chains from the bottom up. However, this extinction also marked a turning point in Earth’s history, as mammals diversified and became the dominant terrestrial animals, eventually leading to the evolution of modern species, including humans.” (chat)