Our mystery skeleton is a mammal, or part of the Mammalia class
There are three main groups, or subclasses, of mammals: pouched mammals (like kangaroos), egg-laying mammals (like the platypus), and placental mammals, whose young develop within the female. Humans and dolphins are two examples of placental mammals. Our mystery skeleton is a placental mammal, a subclass that is divided into 18 orders that include everything from bats to sea lions.

Mammals are so diverse that there are many ways to differentiate one order from another—from their teeth to their toes! One way to rule out a few of the orders is simply by looking at the size of our mystery skeleton. Animals of some orders of mammals are relatively small, while animals of other orders can be very large. Use the ruler to measure our mystery skeleton’s length. Which category does it fall into?
  Never over 3 feet long (or tall) Can be over 3 feet long (or tall)
    Order (example): Chiroptera (bat)
Dermoptera (flying lemur)
Endentata (anteater)
Hyracordea (hyrax)
Insectivora (mole)
Lagomorpha (rabbit)
Macroscelia (elephant shrew)
Pholidata (pangolin)
Rodentia (squirrel)
Zenarthra (sloth)
  Order (example):
Artiodactyla (giraffe)
Carnivora (dog)
Cetacea (whale)
Perissodactyla (rhinoceros)
Pinnipedia (seal)
Primates (monkey)
Proboscidea (elephant)
Sirenia (manatee)
Tubulidentata (aardvark)
Kingdom: Animalia  
Phylum: Chordata  
Class: Mammalia  
Order: ???  
Family: ???  

NOTE: Animals can be classified even further into genus and species.

Fossil image courtesy of Smithsonian's National Museum of Natural History.
Picture: Steve Fugikawa