Short answers, pulled from the story.
The defining feature of the order Diprotodontia is a pair of large, forward-slanting incisors on the lower jaw. This dental arrangement, combined with a short jaw and the absence of lower canines, creates a specialized tool for cropping vegetation that has remained largely unchanged for millions of years.
The earliest known fossils of the order Diprotodontia date back to the late Oligocene, roughly 23 to 28 million years ago. The gaps in the fossil record suggest their origins lie even further in the past, with rapid divergence occurring well before the fossil record begins.
The extinction of the large species in the order Diprotodontia coincided with the arrival of humans approximately 50,000 years ago. The use of fire by early humans likely caused widespread habitat changes that these large animals could not adapt to, leading to their rapid decline.
The Potoroidae family, which includes bettongs, potoroos, and rat-kangaroos, is almost unique among vertebrates in being largely fungivorous. These creatures feed primarily on fungi rather than plants or meat, showcasing the order's ability to exploit niche ecological roles.
Syndactyly is the fusion of the second and third digits of the foot up to the base of the claws. This condition is generally posited as an adaptation to assist in climbing, allowing the animal to grip branches with greater precision.