There’s one clade of placental mammals
that few are interested in. That may be because they are a confusing lot, difficult to determine internal and external affinities.
They are all extinct today.
These are the mesonychids
(Fig 1) and they are largely known from skulls. Some were likely carnivores. Others were likely dog-like to hippo-like herbivores with hooves.
Basal taxa were small. Derived taxa were huge.
Figure 1. The LRT clade of mesonychids, including Mesonyx and Andrewsarchus.
Where does this clade nest?
That depends on what day it is. Today it is the phenacodontids. These taxa finally nest together. That seems stable. Where they nest together changes when rescoring distantly related taxa. That’s the current challenge.
Since these are all terrestrial taxa,
they probably evolved after the asteroid impact, giving placentals the second of their two radiations. The first radiation of tiny taxa occurred during the Middle Jurassic and produced only arboreal taxa (according to the present data, depending largely on size , currently lacking post-crania).
Seeking a little assistance from Wikipedia – Mesonychidae
they report, “Mesonychidae (meaning “middle claws”) is an extinct family of small to large-sized omnivorous-carnivorous mammals. They were endemic to North America and Eurasia during the Early Paleocene to the Early Oligocene, and were the earliest group of large carnivorous mammals in Asia. Once considered a sister-taxon to artiodactyls, recent evidence now suggests no close connection to any living mammal.”
See what I mean? 1) “Carnivorous” and 2) “basal to artiodactyls” and 3) no close connection. That’s why work continues on the placental subset of the LRT.
“Mesonychid taxonomy has long been disputed and they have captured popular imagination as “wolves on hooves”, animals that combine features of both ungulates and carnivores.”
Mesonychidae is a difficult clade to understand. For everyone.
Let’s not forget that hippos have huge canines and hooves.
PS
For those interested in elephant ancestors, that recent post was updated yesterday. Now hooved, herbivorous Coryphodon is the ancestor with saberteeth.
References
https://en.wikipedia.org/wiki/Mesonychidae
The Pterosaur Heresies 