Vilevolodon jaw, pectoral and ankle issues

Vilevolodon diplomylos (Luo et al. 2017; Jurassic, 160 mya; Figs. 1–5) was originally considered a relative of the Jurassic porcupine, Maiopatagium and then the pre-mammal, Haramiyavia.

Figure 1. Vilevolodon in situ, plate, counterplate, original drawing, DGS color, and restored manus and pes. Note the gliding membrane (patagium) and fur.

Figure 1. Vilevolodon in situ, plate, counterplate, original drawing, DGS color, and restored manus and pes. Note the gliding membrane (patagium) and fur.

Here flying squirrel mimic Vilevolodon
nests with the Jurassic squirrel-like multituberculate, Shenshou. According to Luo et al., tiny middle ear bones fail to develop here (Fig. 2). Rather these bones remain as post-dentary jaw bones. True, but in the scheme of things, balanced against a larger suite of synapomorphies, this is an atavism (= reversal). Jaw and ankle trait misinterpretations (see below) caused Meng et al. 2017 to consider Vilevolodon a pre-mammal. Rather it is a derived mammal with an odd throw-back ear region for reasons currently unknown.

Figure 1. Basal mammals and Vilevolodon as figured by Meng et al. Note in the other taxa the two jaw joints are nearly coincident. Not so in Vilevolodon.

Figure 2. Basal mammal (Morganucodon), pre-mammal (Yanoconodon) and Vilevolodon (rodent) as figured by Meng et al. Note in the other taxa the two jaw joints are nearly coincident and side-by-side. Not so in Vilevolodon where substantial distance separates the two putative jaw joints.

Where is the jaw joint?
Most basal mammals (and many pre-mammals) have a large dentary bone that includes a 1. large coronoid process, 2. a large articular process and 3. a large retro process posteroventrally.  Vilevolodon (Fig. 1) lacks #2, but originally the huge retro process was considered the articular process. Tiny posterior jaw bones were purported to arise from a medio-ventral trough (Figs 2, 3). In all other mammals of this grade (like Morganucodon and Yanoconodon, Fig. 2) the two articular surfaces are next to each other. That’s not the case in the Vilevolodon where the two purported articulations are far apart.

No basal mammals or pre-mammals
have the same tooth count with giant incisors seen in Vilevolodon (Fig. 2. In the large reptile tree (LRT, 1269) Vilevolodon has the opportunity to nest anywhere in the Tetrapoda, and it nests within Rodentia. In many rodents the jaws are free to move on a sliding jaw joint supported by a complex sling of jaw muscles. That seems to be the case here, as well.

Many rodents with sliding jaws
have flat-topped molars and pre-molars for grinding seeds and other plant materials. These taxa have a sliding jaw joint. That’s not the case with Vilevolodon, where the teeth occlude precisely, any sort of jaw joint is absent and the articular surface angles the other way.

Figure 4. Paramys is an extinct rodent with a sliding jaw joint and flattened premolars and molars. Compare to Vilevolodon.

Figure 4. Paramys is an extinct rodent with a sliding jaw joint and flattened premolars and molars. Compare to Vilevolodon.

Paramys
(Fig. 4) is similar to modern rats and mice in having flattened teeth and sliding jaw joint. Similar to, yet distinct from those of Vilevolodon where the molars must occlude. Everything else in the mandible has to be engineered so that becomes the end point (Fig. 5).

Figure 5. Vilevolodon in CT scan from Luo et al. There is no jaw glenoid, Lacking a jaw joint the mandible was held in place by large, interweaving jaw muscles that slightly rotate the mortar and pestle molars within one another during grinding of the food. Those ‘ear’ bones are indeed former posterior jaw bones making an atavistic reappearance in this highly derived taxon.

Then there is the ankle issue.
Meng et al. interpreted a primitive ankle joint (astragalus and calcaneum side-by-side) for Vilevolodon. Reexamination indicates a fairly typical rodent ankle here (Fig. 6; astragalus on top of the calcaneum) simply lacking a large tuber for the calcaneum. We saw a similar error in Maiopatagium earlier here.

Figure 6. Original and DGS tracing of the Vilevolodon left pes, here flipped for consistency of presentation. Note the astragalus sits atop the calcaneum, as in other therian mammals and rodents.

Figure 6. Original and DGS tracing of the Vilevolodon left pes, here flipped for consistency of presentation. Note the astragalus sits atop the calcaneum, as in other therian mammals and rodents.

 

Finally
I present a possible solution to the procoracoid/coracoid issue, almost following the reconstruction of Meng et al. with modification. In Vilevolodon the procoracoid and coracoid are loose (Fig. 7) and reconstructed like the same bones in a juvenile platypus, many nodes distant.  No other related taxa back to prototheres have a procoracoid and coracoid. These few traits appear to be atavisms or reversals because they do not tip the balance enough to nest Vilevolodon in or around prototheres. On that note, no prototheres have a suite of rodent-like traits to match those of rodents.

Figure 7. The pectoral girdle of BMNH 3258, fossil lacking a skull and any post lumbar data. Here a tentative reconstruction is presented that locates the procoracoid and coracoid anterior to the humerus with concave coracoid curves matching convex humerus curves. 

Figure 7. The pectoral girdle of BMNH 3258, fossil lacking a skull and any post lumbar data. Here a tentative reconstruction is presented that locates the procoracoid and coracoid anterior to the humerus with concave coracoid curves matching convex humerus curves.

Reversals like this are real problems for paleontologists.
Without the rest of the skeleton to tip the scores toward rodents, the presence of a procoracoid and coracoid would have been protothere indicators, which is what traditional paleontologists have thought for decades. Don’t pull a Larry Martin! One, two or a dozen traits do not nest taxa. Only a suite of 200+ traits can be trusted to correctly nest taxa, overcoming the phylogenetic problems of convergence, reversal and ‘eyeballing’ phylogeny based on a short list of cherry-picked traits.

Whenever Maiopatagium and Vilevolodon
are someday tested with squirrels and porcupines by other workers,  then we’ll see if those two nest basal to Theria or within Rodentia. At present, taxon exclusion, the use of suprageneric taxa and some misinterpretations appear to be problems with prior studies.

We’re all learning as we go.
I better understand things today than yesterday, then sharing those new thoughts with you.

Added six hours later:
The extant aye-aye (genus: Daubentonia), the closest living relative of Vilevolodon, seems to provide insight into the derived/atavistic rodent pectoral girdle and ankle described above. The precise drawing of Daubentonia (Fig. 8) seems to show a procoracoid and coracoid medial to the humerus, as shown above. It also shows an astragalus side-by-side with the calcaneum, except that the tibia and fibula are rotated medially (= dorsal pes rotated laterally), so the astragalus, relative to the tibia, still sits atop the calcaneum. Such solutions must be acknowledged when they appear in nature— or is this just artistic liberty?

Figure 7. Highlights of the aye-aye (Daubentonia) skeleton focusing on the small bones medial to the humerus (procoracoid + coracoid) and the lateral rotation of the ankle and pes where the astragalus still sits on top of the calcaneum, as the dorsal surface of the pes is now lateral.

Figure 7. Highlights of the aye-aye (Daubentonia) skeleton focusing on the small bones medial to the humerus (procoracoid + coracoid) and the lateral rotation of the ankle and pes where the astragalus still sits on top of the calcaneum, as the dorsal surface of the pes is now lateral.

References
Luo Z-X, Meng Q-J, Grossnickle DM, Neander AI, Zhang Y-G and Ji Q 2017. New evidence for mammaliaform ear evolution and feeding adaptation in a Jurassic ecosystem. doi:101.1038/nature 23483\
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

wiki/Shenshou
wiki/Vilevolodon

2 thoughts on “Vilevolodon jaw, pectoral and ankle issues

  1. You are assuming your identification of “the Jurassic squirrel” is correct, but is your reptile family tree the appropriate place to try and fit it in? Wouldn’t a tree of synapsids with a single sauropsid to root the tree make more sense? This also applies to Vilevolodon , a Jurassic mammal that is classified very differently, as a basal mammaliaform (not even a member of crown group Mammalia) in your references, and also in Wikipedia.

    The lower jaw of Vilevolodon reminds me very much of that of the multituberculate Ptilodus but I suspect this is just parallelism. Did the two nest close together in your tree?

    • To answer your queries, please click the link to the LRT. You’ll see ancestors extending back to Devonian tetrapods for all included taxa, which minimizes problems with taxon exclusion that hamper smaller studies.

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