The aye-aye and squirrel revisited in µCT scans

Morris, Cobbb and Cox 2018
used µCT scans to compare the traditional ‘unusual primate’ from Madagascar, Daubentonia (the aye-aye, Fig. 1) to the grey squirrel, Sciurus. In the large reptile tree (LRT) these two both nest within the larger clade Glires and the smaller clade, Rodentia, not close to Primates. Daubentonia could have nested with any one of several included primates, but did not do so, although other putative primates with rodent-like teeth, Ignacius and Plesiadapis, also nests within Rodentia in the Daubentonia clade.

Figure 1. From Morris, Cobb and Cox 2018, µCT images of Daubentonia and Sciurus.

Figure 1. From Morris, Cobb and Cox 2018, µCT images of Daubentonia and Sciurus.

The Morris, Cobb and Cox study labeled the obvious similariities
of the aye-aye and squirrel “Convergence—the independent evolution of similar phenotypes in distantly related clades”.

Unfortunately,
the Morris, Cobb and Cox cladogram, compiled from three earlier genomic studies, did not include any fossil taxa. In their study, Daubentonia nests with other taxa from the island of Madagascar, adding evidence to the curious notion that placental mammals fall into genomic clades determined by land masses, like Afrotheria.

According to Sterling and McCreless 2007 (citations deleted), “Owen’s definitive study of aye-aye anatomy (Owen, 1866) finally quelled the debate over the species’ taxonomic position, focusing attention away from the animal’s rodentlike anterior teeth and towards its primatelike characteristics, such as a postorbital bar, stereoscopic vision, and an opposable hallux. Although its placement within the primates is still being debated, Daubentonia is considered a member of the family Indridae; as a sister taxon to the other Malagasy primates; and as the most basal branch of the strepsirrhines.”

Owen 1866 had no idea
of the fossil taxa that now surround Daubentonia in the LRT, attracting it away from primates and toward fossil rodents, none of which have a postorbital bar, but all of which have an opposable hallux and rodent-like dentition. In the LRT a postorbital bar is retained in taxa basal to Glires (Ptilocercus and Tupaia), but lost in all derived taxa, except Daubentonia. A postorbital process appears in the Late Cretaceous multituberculate, Catopsbaatar. A pseudo-postorbital bar created by the anteriorly displaced squamosal appears in the clade that includes Pectinator, Chinchilla and Allactaga.

Figure 2. Lemur catta skull in 3 views.

Figure 2. Lemur catta skull in 3 views. Compare this skull to that of Daubentonia in figure 1.

A little backstory
Gmelin 1788 and Cuvier 1797 assigned Daubentonia to the Rodentia, under the genus and species: Sciurus madagascariensis. Geoffroy 1795 coined the present genus name, in honor of his professor L-J-M Daubenton. Shaw 1800 called Daubentonia a “long-fingered lemur” perhaps because it lived in the land of lemurs, Madagascar. Wikipedia provides no references for phylogenetic studies that include Daubentonia and fossil taxa nesting as sisters and near-sisters in the LRT. So… taxon exclusion once again appears to have unnecessarily created an enigma.

Oxnard 1981 reported,
“The differences that have been found are large enough that it can be confidently asserted that in its postcranial skeleton, Daubentonia is more different from the primates as a whole than is any other primate genus. paralleling the enormous differences of Daubentonia from other primates in its dentition, skull and cheiridia, that we may prefer to keep open minds about its taxonomic placement.”

No other primates
have the ever-growing rodent-like incisors that Daubentonia has. But all rodents do.

There is one family of rodents native to Madagascar,
the Nesomyinae. Genera include:

  1. Brachytarsomys
  2. Brachyuromys
  3. Eliurus
  4. Gymnuromys
  5. Hypogeomys
  6. Macrotarsomys
  7. Monticolomys
  8. Nesomys
  9. Voalavo

None of these taxa
are currently included in the LRT. None appear to be more closely related to Daubentonia that to Mus and Rattus.

Sciurus was recently added to the LRT
and, not surprisingly, it nested with Ratufa, the giant squirrel, within the clade Rodentia.

Lemur catta was also recently added to the LRT
just to be fair, because Morris, Cobb and Cox 2018 nested Daubentonia with Lemur catta. Not surprisingly Lemur nested at the base of the Notharctus clade, derived from the IVPP V 5235 specimen of Hapalodectes.

The worldwide dispersion of post-Cretaceous basal primates
actually signals a radiation preceding the splitting of Madagascar and South America from Africa in the Early Cretaceous (rather than lemurs rafting to Madagascar and New World monkey to South America), as discussed earlier here. Madagascar provided a refuge for lemurs and lemur-like rodents, like Daubentonia.

References
Cuvier G 1797. Tableau e´lementaire de l’histoire naturelle des animaux. Paris, France: Baudouin.
Gmelin JF 1788. Caroli a Linné systema naturae per regna tria naturae, secundum classes, ordines, genera, species, cum characteribus, differentiis, synonymis, locis. Tomus I. Editio decima tertia, aucta, reformata. – pp. [1-12], 1-500. Lipsiae. (Beer).
Groves CP 2005. Order Primates. pp. 111–184 In Wilson, D. E.; Reeder, D. M. Mammal Species of the World: A Taxonomic and Geographic Reference (3rd ed.). Johns Hopkins University Press.
Morris PJR, Cobb SNF and Cox PG 2018. Convergent evolution in the Euarchontoglires. Biology Letters 14: 20180366. http://dx.doi.org/10.1098/rsbl.2018.0366
Owen R 1866. On the aye-aye (Chiromys, Cuvier: Chiromys madagascariensis, Desm.; Sciurus madagascariensis, Gmelin, Sonnerat; Lemur psilodactylus, Schreber, Shaw). Transactions of the Zoological Society of London 5:33–101.
Oxnard CE 1981. The uniqueness of Daubentonia. American Journal of Physical Anthropology 54(1):1–21.
Sterling EJ and McCreless 2007. Adaptations in the Aye-Aye: A review. Chapter 8 (pp. 159–184) in Lemurs, Ecology and Adaptation. Gould L and Sauther ML Eds. Part of the 47 volume Developments in Primatology: Progress and Prospect book series. Springer Nature Switzerland.

PR on Morris, Cobb and Cox 2018
wiki/Aye-aye

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