Crown Group Mammals

Last week we introduced mammal evolution and discussed the earliest mammals and how they came to be. This week, we'll talk about crown group mammals, including some modern groups and when they evolved. In order to do that, we must first explain what is meant by "crown group". Crown group mammals include extant (still living today) mammals and their relatives, sharing a last common ancestor. 

Looking at the base of the cladogram (think of it as a kind of family tree) of crown mammals, we have monotremes. Monotremes are the most basal mammals alive today, and they include just five animals: one species of platypus, and four species of echidna. Very few monotreme fossils have been found, and the earliest are all from the Cretaceous of Australia. The earliest, Teinolophus, was already a fully evolved platypus, suggesting monotremes evolved earlier, possibly in the Jurassic. Monotremes are considered to be basal mammals because of many features, including the fact that they still lay eggs, unlike other mammals. 

The very strange duck-billed platypus. Image by Maksim

Moving up the "tree" we have a group of mammals called multituberculates. This fully extinct group is characterised by small rodent-like animals that have very distinct molars with tubercles. They lived from the Late Jurassic to the Oligocene (153-35 million years ago), and their position within Mammalia is often debated. Some scientists believe they are in this position, between monotremes and Theria (marsupials and placental mammals), while others don't agree that they are crown group mammals at all, and think they are more primitive than monotremes.

After a few minor extinct groups, we have the therians. This includes the two mammal groups we typically think of: the marsupials and placental mammals. Marsupials are the living representatives of Metatheria, which also include some fossil taxa. The first metatherian, Sinodelphys, lived during the Late Cretaceous of China, about 125 million years ago. Modern groups like opossums evolved during the Late Cretaceous, while diprotodonts (kangaroos, koalas, wallabies, etc.) first appeared in the fossil record during the Oligocene, about 25 million years ago. This is likely review for many people, but marsupials mainly differ from placental mammals in way of their reproduction. Marsupials do not have a placenta, and have very short gestational periods, causing them to "give birth" to very young, under developed offspring. The offspring are then kept in the mother's pouch, where they continue to develop for several months. 

Artists impression of Diprotodon, an extinct marsupial. Image by Nobumichi Tamura

Finally, this brings us to the Eutheria, and specifically, placental mammals. Fossils like Juramaia (160 million years ago) and Eomaia (125 million years ago) have been attributed to Eutheria, while other studies have suggested the earliest true eutherian is Maelestes that lived 91 million years ago. Relationships of modern placental mammals are messy, mainly due to the fact that studies based on physical characters do not agree with those based on molecular features and DNA. Some extant groups of interest include the Xenarthra (anteaters, tree sloths and armadillos) which evolved during the Late Cretaceous, Insectivora (including hedgehogs, moles, and are not to be confused with rodents) also in the Late Cretaceous, while rodents (mice, rats, squirrels, and porcupines) didn't evolve until the Paleocene (approximately 61 million years ago). Bats didn't appear until the Eocene (52 million years ago), with their evolution being fairly unclear as no transitional fossils have yet been found. The major groups of modern mammals all had started to evolve by the Miocene, 20 million years ago, with aardvarks being the last group to evolve. 

Fossil of Icaronycteris, the earliest known fossil of a bat. Image by Andrew Savedra

Skeleton of a modern bat, which is not significantly different from the early fossil bat above. Image by Mnolf. 

And I think that's about it for today on mammals. There are so many more groups of mammals, we could be talking about them for days! So I think this is it for now. Keep watching and maybe we'll touch on mammals a bit more soon!