Yesterday we looked at a new primate family tree (revised in Fig. 2 with one more taxon) that separates the more primitive New World monkeys from the more derived and convergent Old World monkeys. According to traditional paleontology, primates had their genesis in Eocene China.
Recapping yesterday, according to the LRT,
New World monkeys evolved among adapid lemuroids that migrated North and East into North America, then South America.
Old World monkeys evolved from lemurs, then tarsiers, that migrated South and West into India, Asia Minor, Europe and Africa.
Ni et al. 2013
described, “The oldest known primate skeleton and early haplorhine evolution,” Archicebus achilles (Fig. 1, Eocene, China) and nested this taxon as THE basal member of the Tarsiiformes.
The large reptile tree (LRT, 1353 taxa, subset Fig. 2) more or less confirms that nesting, but nests Archicebus between the more basal Aegyptopithecus and the more derived Tarsius + Darwinius. Aegyptopithecus (from Egypt) is 22 million years younger than Archicebus despite its more primitive status.
The tarsier clade is not the most primitive primate clade.
That accolade currently goes to Hapalodectes (IVPP V5235, Fig. 3), which is a mere one million years older (56 mya), but is much more primitive, more basal than lemurs and adapids. It is known from just a skull, so no skeleton has been recovered here. This specimen has been ignored in prior primate studies.
Earlier we looked at the just the foot of Archicebus. We also looked at basal-most placental mammals, like Caluromys, here. Caluromys has a prehensile tail, a primitive trait retained by South American monkeys, but lost in tarsiers, Old World monkeys and most other derived placentals.
Another basal tarsier,
Teilhardina (Fig. 5, worldwide distribution, early Eocene) greatly resembles Archicebus. It has not yet been tested in the LRT.
So how far back do the primates go?
My guess is deep into the Mesozoic, with lemurs bounding through the trees worldwide above the heads of most dinosaurs. More primitive outgroups to primates are also small, arboreal, omnivorous climbers, like the extant Caluromys (Fig. 4).
The rare and few Eocene primate fossils
represent the worldwide radiation after a Jurassic genesis. This seems to be the case given the fossil appearance of the pangolin ancestor, Zhangheotherium in the Earliest Cretaceous and the derived rodent, Megaconus, in the Middle Jurassic. All were arboreal taxa, out of reach of predatory dinosaurs. That these taxa lived in jungles would have reduced their chances to fossilize. Their discovery in Mesozoic strata will someday be big news.
Finally, about that new postorbital bone
that does not fuse to the frontal in Archicebus and Teilhardina. I have not yet sought the embryological evidence that might recapitulate the phylogeny presented here, but I note the human skull (Fig. 8) appears to retain a separate ossification center fused to the frontal that is otherwise identical to the Y-shaped postorbital in Archicebus and Teilhardina. Phylogenetically, the postorbital disappeared millions of years earlier with the pre-mammal, Pachygenelus. The atavistic reappearance of long lost bones occurs whenever previously unused genes ore reawakened. We’ve seen this before with the reappearance of digit zero in dinosaurs like Limusaurus and the extant screamer, Chauna, long after originally appearing in basal tetrapods with more than five fingers, like Acanthostega.
Ni X, Gebo DL, Dagosto M, Meng J, Tafforeau P, Flynn JJ and Beard KC 2013. The oldest known primate skeleton and early haplorhine evolution. Nature 498: doi:10.1038/nature12200