The foot of Archicebus – an early primate

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.

Figure 1. Archicebus is close the ancestry of tarsiers and monkeys. It retained a lemur-like foot,

Figure 1. Archicebus is close the ancestry of tarsiers and monkeys. It retained a lemur-like foot, but with distinctly tarsier-like proportions starting to show. Here in a published illustration pedal digit 1 is not illustrated as more robust and the metatarsals are inaccurately similar in length. The size was about that of the smallest living lemur, the pygmy mouse lemur. Moderate size eye sockets are distinct from the giant eye sockets of living nocturnal tarsiers.

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).

Figure 2. Click to enlarge. The two pedes of Archicebus, bottom sides flipped to match top sides. Toes colorized for reconstruction (Fig. 3).

Figure 2. Click to enlarge. The two pedes of Archicebus, bottom sides flipped to match top sides. Toes colorized for reconstruction (Fig. 3). The talus/astragalus is missing from both pedes.  Note the massive proximal articulation of the big toe.

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).

Figure 3. The reconstructed foot of Archicebus alongside that of the basal lemur, Notharctus. Note the gathering of metatarsals 2-4, as in tarsiers.

Figure 3. The reconstructed foot of Archicebus alongside that of the basal lemur, Notharctus. Note the gathering of metatarsals 2-4, as in tarsiers. The missing astragalus/talus sits on top of the calcaneum, a trait first appearing on cynodonts like Probelesodon. I suppose this is an example of modular evolution: first the toes, then the ankle.

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.

Figure 4. Archicebus pes compared to a living tarsier  pes. Note the elongated proximal tarsals in the tarsier. Archicebus has the elongate digit 3 retained by tarsiers.

Figure 4. Archicebus pes compared to a living tarsier pes. Note the elongated proximal tarsals and shorter metatarsals in the tarsier. Archicebus has the elongate digit 4 and short digit 2 retained by tarsiers. Both of these reconstructions are flattened, which is not the way tarsiers hold their toes (Fig. 5). The lengthening of the ankle makes tarsiers excellent leapers.

PIL continuity
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.

Figure 6. The tarsier foot and PILs are shown in action at right angles to the tree cylinder and parallel to the long axis. The use of pads appears to change the way the foot operates, without the strong PILs a grasping or walking foot has.

Figure 5. The tarsier foot and PILs are shown in action at right angles to the tree cylinder and parallel to the long axis. The use of pads appears to change the way the foot operates, without the strong PILs a grasping or walking foot has. The fingers and toes don’t lie flat, but strongly flex at the interphalangeal joints. This messes with PILs that are applied to flat reconstructions (Fig. 3, 4)

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.”

References
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

wiki/Archicebus

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