Yesterday we looked at a paper that compared
the squirrel, Sciurus, to the squirrel-like aye-aye, Daubentonia (Fig. 1) using µCT scans. The authors considered this a case of convergence and nested Daubentonia (from Madagascar) with lemurs (from Madagascar) based on gene studies.
As a follow-up
I’ll show the ancestry of Daubentonia based on the large reptile tree (LRT, 1370 taxa). The short list includes Ignacius and Plesiadapis, two other taxa also previously considered basal primates, but nest in the LRT with rodents. In the LRT Ignacius is sister to the Mus/Sciurus (= mouse/squirrel) clade.
Daubentonia has a postorbital bar,
convergent with that found in primates. Don’t be guilty of ‘Pulling a Larry Martin’. One character can appear on unrelated clades. Use an unbiased suite of traits and let the taxa nest wherever they want to. Compared to Plesiadapis, Daubentonia appears to be neotonous with a shorter rostrum and mandible, along with a smaller zygomatic arch and large braincase. The mandible has a similar morphology to multituberculates.
Basal taxa in the LRT
are what they are supposed to be: generalized forms ready to evolve into wild and exotic types. In that regard, wild and exotic Daubentonia (Fig. 4) is a poor candidate as a basal primate. In the LRT it nests as a highly derived wild and exotic rodent with no known descendants.
Ignacius frugivorus (formerly Phenacolemur; Matthew and Granger 1921) was originally based on upper jaw with teeth. It was originally and is here considered a plesiadapiform, close to Plesiadapis.
Plesiadapis tricuspidens (Gervais 1877) Paleocene ~55 mya. The Plesiadapiformes were widely thought to be the earliest representatives of the primate order, but here they nest wihthin Glires. Derived from a sister to Ignacius, Plesiadapis phylogenetic preceded the living aye-aye, Daubentonia. This clade nests between traditional rodents and multituberculates. Distinct from Ignacius, the skull of Plesiadapis had a deeper shorter rostrum and a higher orbit, but a smaller braincase. The jugal was more robust. The ear was raised. The mandible was more robust with deeper surfaces for muscle attachement and a more robust angular process and a longer coronoid process. The cervicals were shorter. The dorsals, ribs and lumbars were more robust along with the caudals. Chevrons developed at a likely sitting point. The limbs and girdles were more robust. The radius was anteriorly boewed and the ulna developed a large olecranon process (elbow). The unguals were large and deep. The feet were larger than the hands. The joints were nearly all transversely aligned indicating a simple extension/flexion motion for the fingers and toes.
Daubentonia madagascariensis (Gmelin 1788, Sciurus madagascariensis; Geoffrey Saint-Hilaire 1795; 40 cm snout-to-vent length) is the extant aye-aye. Originally considered a squirrel, then traditionally an odd sort of lemur-like primate with rodent-like teeth, here Daubentonia returns to Glires to nest with Plesiadapis, which has also been wrongly considered a basal primate. This nocturnal arboreal mammal has a long slender digits, particularly manual digit 3, which is used to probe for insects below tree bark. Note the hallux-like pedal digit 1. Like primates, a postorbital bar appears in this taxon, but the eyeballs are no more rotated or stereoscopic than ancestors.
An expert on mammals
(name omitted) replied to a recent query with the following note on Daubentonia: “At the moment, your phylogenetic results in many ways resemble 19th century studies in which superficial similarities were interpreted as evidence of close relationship – for example, it’s striking that you find that Daubentonia is not a primate: this was debated during the early part of the 19th century, before researchers collectively reached a consensus that it is a primate, and this is overwhelmingly supported by molecular data too.”
I’m still looking for the phenomic cladogram
that includes the above taxa and others in the LRT. To my knowledge there is none as genomics has taken paleontologists to fantasyland. And, it’s not just superficial similarities… it’s a suite of 231 traits that overwhelms the few convergent traits with primates. 21st century phylogenetics should be accepted over 19th century debates. Let’s hope science and unbiased experimentation will someday triumph over tradition and ego.
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Bloch JI and Boyer DM 2006. Grasping primate origins. Science 298:1606-1610.
Gervais P 1877. Enumeration de quelques ossements d’animaux vertebres recueillis aux environ de Reims par M. Lemoine. Journal de Zoologie (Paris) 6:74–79.
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Matthew WD and Grange W 1921. New genera of Paleocene mammals. American Museum Novitates 13:1-7
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