Flying squirrels and aye-ayes: convergent with multituberculates

For those in a hurry, a two-part summary:
1. By convergence, basal multituberculates in the Jurassic (Figs. 1, 4), had a distinct  flying squirrel (Glaucomys, Figs. 2, 3)-like patagial (= gliding membrane) morphology.
2. Also by convergence, multituberculates in the Jurassic had a short post-dentary skull length with a sliding jaw joint and a nearly absent angular process as seen in the extant aye-aye (Daubentonia, Figs. 5, 6).

Today’s blogpost had its genesis
when I finally noticed several basal multituberculates that preserved soft tissue had flying-squirrel-like patagia preserved in the sediment (Fig. 1)… and squirrels nested more or less close to the origin of multituberculates. So, I added a flying squirrel, Glaucomys (Fig. 1) to the large reptile tree (LRT, 1810+ taxa) to see what would happen.

It should come as no surprise
that Glaucomys nested with the extant red squirrel Sciurus, NOT any closer to multituberculates. Thus, the ability to glide in the manner of a flying squirrel turned out to be by convergence in basal multituberculates of the Jurassic.

Based on the phylogenetic position
of squirrels and other rodents as sisters to multituberculates, either flying squirrels were also gliding from tree-to-tree during the Mesozoic, or they took their time and only appeared after the Mesozoic. That is the current paradigm based on present evidence.

End of part 1. Scroll down for part 2.

Part 2.
By convergence, the aye-aye, Daubentonia
(Fig. 5) has a multituberculate-like mandible lacking an angular process along with a large circumference, sliding jaw joint and reduced post-dentary skull.

By convergence, Carpolestes
has an enlarged posterior lower premolar, as in multituberculates. So, lots of convergence surrounds the multituberculates.

The aye-aye is a traditional basal primate,
based on gene studies (Dene et al1980; Rurnpler et al 1988; Del Pero et al 1995; Porter et al 1995).

By contrast
the large reptile tree (LRT, 1810+ taxa; subset Fig. 4) nests the aye-aye (Daubentonia) with rodents, plesiadapiformes, carpolestids and multituberculates. We’ve seen how genomic studies produce false positives. Add Daubentonia to that list of flubs. Note that both lemurs and aye-ayes are both from Madagascar, lending more evidence to the hypothesis that geography and geology (e.g. Afrotheria, Laurasiatheria) affect genomics to a greater degree than professionally realized over deep time.

Like rodents:
The aye-aye does not have mammary glands on the chest, as in primates, but along the groin, as in non-primates. The aye-aye has a large diastema between the incisors and molars, as in plesiadapiformes and rodents, distinct from primates.

Like primates:
The aye-aye has a postorbital bar, stereoscopic vision and an opposable hallux. Owen 1863 considered such traits ‘must be ordained’ in arguments for God and against Darwin’s then novel hypothesis of natural selection and evolution.

Like rodents,
Perry et al. 2014 report: “the single pair of incisors consists of continuously growing, elongate, open-rooted chisels, both upper and lower incisors.”

Based on the LRT
mutltuberculates are netonous rodents, growing to adulthood without ontogenetically incorporating post-dentary bones into the tympanic and periotic (inner ear enclosing bones), as we learned earlier here.

By convergence
the aye-aye (Daubentonia. Fig. 6) likewise reduces the tympanic and periotic along with the angular process of the dentary, producing a sliding joint that would have interfered with the ear bones if allowed to develop as in most placentals.

Carter 2009 notes
(while mistakenly assuming a lemur affinity for Daubentonia), “The overall dimensions of the D. madagascariensis auditory ossicles are large and they have a unique morphology.” Carter also reports on the elongate manubrium of the malleus (the former articular). This is in accord with similar structures in the neotonous (not primitive!) multituberculate auditory bone chain you can see here.

What does the angular process of the plancental dentary do?
According to Meng et al. 2003, a huge angular process was present in Rhombomylus, an extinct gerbil. Meng et al. mapped insertions for the deep masseter and superficial masseter externally. Then they mapped insertions for the medial pterygoid and superficial masseter internally. The Rhomboylus glenoid has a small diameter and rotates. It does not slide.

Meng et al. write: “As the major muscle to move the mandible forward, the superficial masseter must be long enough so that it can work to bring the jaw forward at least the minimum working distance. In general, the action line of the anterior deep masseter is nearly perpendicular to the moment arm of the mandible, while the posterior one has an acute angle to the moment arm and, therefore, less mechanical advantage. the deep masseter must have been sizable and supplies the main force for mastication as in rodents.”

The point of which is: multituberculates and the aye-aye reduce and eliminate the angular process. So we can imagine the muscles listed by Meng et al. either migrate or are lost in multituberculates and the aye-aye.

We can’t leave Jurassic flying squirrels
without a quick review of Maiopatagium (Early Jurassic, Fig. 7, Meng et al. 2017), which was hailed ever since as a gliding mammal or mammaliaform.

Contra Meng et al. 2017
phylogenetic analysis nested Maiopatagium with the extant porcupine (Coendou), not with gliding multituberculates, like Vilevolodon. Maiopatagium has long straight hairs and lacks any trace of a patagium. Those long straight hairs are the precursors to porcupine quills according to the LRT.

Phyogenetic analysis puts rodents and all their precursors
(Tupaia, Henkelotherium, Nasua) squarely and clearly in the earlier part of the Early Jurassic, though not yet recovered in fossils.

The myth about the patagium surrounding Maiopatagium
seems to have had its genesis in the fact that Vilevolodon was described at the same time,  by the same authors, in the same publication. Vilevolodon (Fig. 1) has a no-doubt, flying sqirrel-like patagium. Maiopatagium (Fig. 7) was described with a misidentified patagium and a misidentified bat-like calcar. No patagium is present, but long straight hairs are. As noted above, these are precursors to porcupine quills. Getting taxa into a proper phylogenetic context is the key to understanding soft tissue and taxonomy.

References
Carter Y 2009. Monkey Hear: A morphometric analysis of the primate auditory ossicles. Master of Arts thesis, The U of Manitoba.
Del Pero M et al (4 co-authors) 1995. Phylogenetic relationships among Malagasy lemuls as revealed by mitochrondrial DNA sequence analysis. Primates 36: 43I-440.
Dene H, Goodman M and Prychodlco V 1980. Immunodiffusion systematics of the primates. Mamalia 44:27-31.
Luo Z-X, (6-co-authors) 2017. New evidence for mammaliaform ear evolution and feeding adaptation in a Jurassic ecosystem. Nature. in press (7667): 326–329. doi:10.1038/nature23483
Meng et al. 2003. The osteology of Rhombomylus (Mammalia, Glires): Implications for phylogeny and evolution of Glires. Bulletin of the American Museum of Natural History 275: 1–247.
Meng Q-J, Grossnickle DM, Liu D, Zhang Y-G, Neander AI, Ji Q and Luo Z-X 2017.
New gliding mammaliaforms from the Jurassic. Nature (advance online publication)
doi:10.1038/nature23476
Owen R 1863. On the characters of the aye-aye as a test of the Lamarckian and Darwmian hypothesis of the transmutation and origin of the species. Rep Br Assoc Adv Sci 1863: 114-116.
Perry JM et al. (4 co-authors) 2014. Anatomy and adaptations of the chewing muscles in Daubentonia (Lermuriformes). The Anatomical Record 297:308–316.
Porter CA et al (5 co-authors) 1995. Evidence on primate phylogeny from e-globin gene sequences and flanking regions. Journal of Molecular Evolution 40: 30-55.
Rurnpler Y et al (4 co-authors) 1988. Chromosomal evolution of the Malagasy lemurs. Folio Primatologica 50 124-129.
Sterling EJ 1994. Taxonomy and distribution of Daubentonia madagascariensis: a historical perspective. Folio Primatologica 62: 8-I3.

wiki/Maiopatagium
wiki/Coendou
wiki/Multituberculata

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