Traditionally the extant mouse lemur is the smallest primate.
This omnivore is native to Madagascar. Microcebus (Fig 1) is the single genus for several genetically split species. The smallest of these, M myoxinus (Fig 1) is much smaller than the pen-tailed tree shrew, Ptilocercus lowii (Fig 1). All species of Microcebus are likewise much smaller than Notharctus (Fig 1), the most primitive tested primate in the large reptile tree (LRT, 2061 taxa, subset Fig 2). Microcebus first entered the LRT in 2020 basal to Primates.
After taxon addition and score correction in the LRT
Microcebus now nests basal to the Ptilocercus + colugo + pandgolin + bat clade (= Volitantia), rather than anywhere in or before the Primates clade. Despite nesting one node apart from Primates, (Fig 2), forcing Microcebus back to Primates adds seven steps with current scoring and taxa.
According to Janecka et al. 2007
“In order to resolve the ancestral relationships among primates and their closest relatives, we searched multispecies genome alignments for phylogenetically informative rare genomic changes within the superordinal group Euarchonta, which includes the orders Primates, Dermoptera (colugos), and Scandentia (treeshrews).”
The LRT adds shrews and Glires (gnawing taxa) to this list with Notharctus, a primitive adapid (= extinct lemurs living outside of Madagascar) at the base of all Euarchonta.
“We also constructed phylogenetic trees from 14 kilobases of nuclear genes for representatives from most major primate lineages, both extant colugos, and multiple treeshrews, including the pentail treeshrew, Ptilocercus lowii, the only living member of the family Ptilocercidae.”
Excessive splitting here.
The LRT relates every tested taxon to every other. None stand alone.
“A relaxed molecular clock analysis including Ptilocercus suggests that treeshrews arose approximately 63 million years ago. Our data show that colugos are the closest living relatives of primates and indicate that their divergence occurred in the Cretaceous.”
According to the LRT, Notharctus, Microcebus and Ptilocercus (Fig 1) must be
much older: Early to Middle Jurassic in origin because derived members of their clade, the Multituberculates, appear in the Middle to Late Jurassic.
According to the LRT Euarchonta should be redefined to include Glires and Multituberculata.
Did someone mention phylogenetic miniaturization?
If not, I’ll bring it up. Since Microcebus is a tiny Notharctus and it nests at the genesis of a new clade it qualifies as a phylogenetically miniaturized taxon, something we’ve seen over and over. Note the neotony in Microcebus: shorter rostrum, larger orbits, smaller overall as an adult, compared to Notharctus (Fig 1). Take a moment to do this: scroll up and down to compare the two and you’ll get an appreciation for the way evolution really works, sometimes with phylogenetic miniaturization = precocious sexual maturity.
The mouse lemur is not the first former primate to leave the clade.
Long time readers might remember the aye-aye, Daubentonia left the primates back in 2016. Likewise, the classic plesiadapiform, Plesiadapis, is a Daubentonia relative and closer to Carpolestes and multituberculates (Fig 2) than to any primate.
This latest bit of heresy was brought to you by taxon inclusion.
Whenever taxa are tested together that have never been tested together, sometimes they switch clades. Sometimes the unexpected happens. We’ve seen this time and again.
References
Janečka, JE et al. (seven co-authors) 2007. Molecular and Genomic Data Identify the Closest Living Relative of Primates. Science. 318 (5851): 792–794. Bibcode:2007Sci…318..792J. doi:10.1126/science.1147555.
The Pterosaur Heresies 