We haven’t looked at mammals or synapsids for awhile.
If you want to check the large reptile tree, they’re still reptiles — just hairier. Todays let’s look at Archicebus (Early Eocene, 55 mya, Ni et al. 2013), the oldest primate known from a skeleton. Notharctus is 5 million years younger, but more primitive, just as living lemurs are more primitive than modern apes and humans are. Ipso facto, the discovery of Archicebus pushes the origin of lemurs back even further. The younger lemuroid, Smilodectes, had a similar short-snout skull.
My interest in feet
and PILs (parallel interphalangeal lines) goes back a long way (Peters 2000a, 2010, 2011). Pterosaur PILs are instructive, helping flatten or elevate plantigrade and digitigrade pedes. Cats put an interesting twist on PILs due to their retractable claws. Primates do too, because they are adapted to cylindrical substrates (branches), not flat (the ground).
Nature reports, “By analyzing almost 1,200 morphological aspects of the fossil and comparing them to those of 156 other extant and extinct mammals, the team put the ancient primate near the base of the tarsier family tree.” I haven’t repeated that experiment, buy it looks to me that pedal characters alone would tell the tale Figs. 3,4).
Nature reports, “The mammal sports an odd blend of features, with its skull, teeth and limb bones having proportions resembling those of tarsiers, but its heel and foot bones more like anthropoids.” Actually the Archicebus foot is also an “odd blend.” The ankle is short, like that of most other primates (Fig. 3, not just anthropoids). But digit 2 is short and digit 4 is long, like those of tarsiers (Fig. 4). Really it comes down to just these two traits for an accurate nesting of Archicebus. Perhaps an accurate reconstruction would have helped. I took my data (Fig. 1) from online photos of Ni et al. 2014, but I have not seen the paper.
The foot of Archicebus appears to lose the continuity of many PILs (Fig. 4) when laid flat. But that’s not the way tarsiers hold their toes in vivo (Fig. 5). Similarly in the human hand the PILs become more continuous in use, like when you grasp a golf club, hold a baseball bat or make a fist. And, of course, the opposable thumb does not work as part of the lateral four digit sets. It goes its own way.
The elongation of the proximal ankle elements in tarsiers enables them to leap tremendous distances. Archicebus did not have that ability. I suppose Archicebus is an example of modular evolution: first the toes, then the ankle, but, of course, it’s never as simple as that.
Archicebus is well-deserving of its celebrity.
According to Nature, “Because A. achilles sits near the base of the tarsier family tree, scientists say it probably resembles the yet-to-be-discovered creatures that lie at the base of most primate groups — including the anthropoid lineage that ultimately gave rise to humans. “If you retrace primate evolution to its beginning, [A. achilles] is what our ancestors most likely looked like,” says Luo.”
Ni X, Gebo DL, Dagosto M, Meng J, Tafforeau P, Flynn JJ, Beard KC 2013. The oldest known primate skeleton and early haplorhine evolution. Nature 498 (7452):60–64.
Peters D 2000a. Description and Interpretation of Interphalangeal Lines in Tetrapods. Ichnos, 7: 11-41.
Peters D 2010. In defence of parallel interphalangeal lines. Historical Biology iFirst article, 2010, 1–6 DOI: 10.1080/08912961003663500
Peters D 2011. A Catalog of Pterosaur Pedes for Trackmaker Identification. Ichnos 18(2):114-141. http://dx.doi.org/10.1080/10420940.2011.573605