Yesterday
we looked at similarities between the skull of the extinct basal primate, Notharctus, and the extinct basal condylarth, Phenacodus.
Today:
a closer look at the manus and pes of these two taxa, with parallel interphalangeal lines (PILs) added (Fig 1). All patterns are similar between the two given an arboreal vs terrestrial lifestyle at a much larger size requiring more robust elements and shorter digits.
Figure 1. The manus and pes of Notharctus and Phenacodus compared. Both are quadrupeds at least part of the time. Notharctus is arboreal. Phenacodus is developing hooves for terrestrial locomotion in a digitigrade fashion. Even so, The similarities are commonplace heere.
In Phenacodus
(Fgi 1) what look like small pre-hooves actually evolve into robust hooves in descendant taxa. That doesn’t mean they would have been hooves in Phenacodus. Instead the digits may appear as fingers and toes, as in primates including humans, or some transitional morphology we no longer see.
Even the wrist (carpus) and ankle (tarsus) bones appear similar (Fig 1).
References
This topic may not have been covered yet in academic papers. The descent from the trees to the ground seems not to have been covered except to describe the origin of bipedal locomotion in humans.
The Pterosaur Heresies 