Vintana: a weird wombat, not an ‘allothere’

Figure 1. Vintana as originally illustrated. I added colors to certain bones. Note the high angle of the ventral maxilla and the deep premaxilla. Lateral view reduced to scale with other views.

Figure 1. Vintana as originally illustrated. I added colors to certain bones. Note the high angle of the ventral maxilla and the deep premaxilla. Lateral view reduced to scale with other views.


Vintana sertichi
(Krause et al. 2014; Late Cretaceous, UA 9972; 12.4 cm skull length; Figs. 1,2) is a Madagascar mammal originally considered a member of the Allotheria and Gondwanatheria, two clades that do not yet appear in the large reptile tree (LRT). Krause et al. compared by analogy and function the odd and very derived skull of Vintana with that of Myocastor the coypu, nutria or river rat Fig. 2).

Figure 2. Vintana and Myocastor from Krause et al. 2014. They invented the dentary and anterior incisor, ignored the posterior incisor alveolus. I added Vombatus and Zalambdalestes for comparison and all color overlays including a new dentary based on Vombatus.

Figure 2. Vintana and Myocastor from Krause et al. 2014. They invented the dentary and anterior incisor, ignored the posterior incisor alveolus. I added Vombatus and Zalambdalestes for comparison and all color overlays including a new dentary based on Vombatus. Note the premaxilla of Zalambdalestes leaned down when corrected See figure 3.

In contrast to Krause et al. the LRT nests Vintana
between the wombats, Vombatus and Zalambdalestes (Fig. 2). This may be yet another case of taxon exclusion: finding a ‘by default’ nesting in a clade, the ‘Allotheria’, whose members, like multituberculates, keep leaving for extant clades.

Krause et al reported,
“The new taxon is the largest known mammaliaform from the Mesozoic of Gondwana. Its craniofacial anatomy reveals that it was herbivorous, large-eyed and agile, with well-developed high-frequency hearing and a keen sense of smell. The cranium exhibits a mosaic of primitive and derived features, the disparity of which is extreme and probably reflective of a long evolutionary history in geographic isolation.”

Krause et al. labeled a septomaxilla
posteroventral to the naris. If so that is an autapomorphy. Perhaps more likely that ‘bone’ is the rest of the premaxilla and the ‘suture’ is a crack. Then it’s not an autapomorphy. The long premaxillary tooth root passes through that area. Based on comparisons to Vombatus and Zalambdalestes, the septomaxilla is reduced to a tiny triangle posterodorsal to the naris in Vintana.

Four molars per quadrant were present
as in other marsupials and a large diastema separated them from the chisel-like incisors, as in Vombatus. However only three molars were operational. Only Zalambdalestes has a similar deeply convex maxilla, but it is lined with more premolars and only three molars. The occlusal surfaces are sharply inclined in Vintana.

Klinorhynchy
This is a new term, at least for me, as of today. It means “downwardly bent face.” That applies to VintanaVombatus and to a less extent, Zalambdalestes.

Mosaic traits
Krause et al. report, “an array of primitive features reminiscent of the most basal mammaliaforms, or even non-mammaliaform cynodonts, coupled with highly derived features unknown in any otherMesozoic mammaliaform.” My comments below  follow their comments:

  1. “The snout of Vintana exhibits a number of features that are unique among mammaliaforms including the retention of a septomaxilla with both a large posterodorsal facial process and an intranarial process.” That same area houses tooth roots in Vombatus and internally is taken over by neuromuscular grooves in Vintana. In therapsids, the external expression of the septomaxilla IS the lacrimal with the dorsal maxilla laminated over it. In any case, here with the reduction of the maxilla combined with lacrimal – premaxilla contact combined with a fine network of cracks and sutures, I would lean toward not creating an autapomorphy here and just calling the present suture a crack.
  2. “The lacrimal bone is enormous and extends anteriorly to contact both the septomaxilla and premaxilla.” Enormous yes, but likewise large and in contact with the premaxilla in Vombatus where it is apparent that the lacrimal / septomaxilla is overlapped by a dorsal extension of the premaxilla. 
  3. “The lacrimal and palatine bones of Vintana contribute significantly to the nasal cavity” No internal nasal data for Vombatus, but it would be interesting to see if the well-developed set of turbinals found in Vintana matches.
  4. “No Mesozoic mammaliaform has a jugal as enlarged, and with as massive a ventral flange, as that of Vintana.” Lacking only a descending process, Vombatus also has a massive jugal.
  5. “The palate of Vintana is very narrow, has a rugose texture, and lacks vacuities or any sizable foramina (other than the incisive foramina anteriorly). The palatines are fused in the midline and extend forward to contact thepremaxillae, thus excluding the maxillae from contacting one another in the midline.” Based on DGS tracings, I don’t see the palatines excluding contact between the maxillaries (Fig. 1) nor do the palatines extend to the premaxilla. The vomers were overlooked. The premaxillae are narrower than reconstructed. Vombatus has a similar, but still odd triangular palatine though much wider.
  6. Krause et al. list several braincase traits that I cannot comment on for lack of data and knowledge
  7. Likewise with several basicranium traits for channels, sinuses and lappets that I will leave to the wombat experts
  8. “In the occipital region, Vintana is the only Mesozoic mammaliaform for which both a postparietal and paired tabulars, together comprising the interparietal, have been identified as discrete elements.” Could not find data for Vombatus. Send it if you have it.
  9. Encephalization quotient for Vintana: (0.28-0.56) Vombatus: (0.64), brain volume: 24 percent.
  10. “The olfactory bulbs were very large, occupying over 14% of endocranial volume.” Vombatus: 6 percent.
  11. “The cochlear canal is only slightly curved and short, resembling non mammalian cynodonts.” This link provides X-rays and CT scans of Vombatus, but I cannot determine the shape of its cochlear canal from images there.
  12. “The direction of the power stroke of the chewing cycle was distal as in haramiyidans, multituberculates and other gondwanatherians.”
    Without a dentary sporting a narrow articular surface that claim cannot be substantiated for Vintana. In Vombatus and nearly all other tetrapods the chewing stroke is dorsal. See the reconstruction in figure 2.
  13. “Vintana appears to be unique among these clades in possessing a significant buccal component to the power stroke.” No data for Vombatus, but look at those super thick jugals!
  14. Vintana had significantly higher bite forces than the similarly sized extant rodent Myocastor.” I don’t see much room for a large coronoid process between the last molar and the jaw glenoid, except if one creates a lingual underlap of the posterior dentary molar. Small and large hypothetical coronoids are shown in Fig. 2). In Vombatus the coronoid does not extend above the lateral temporal arch, if this is a guide. The Vombatus dentary also includes a deep masseteric fossa for the insertion of a large superficial masseter, as envisioned for Vintana.

Wombats are unique among marsupials
in that all teeth are open-rooted and continuously growing. The crowns in Vintana were described as ‘extremely worn, essentially flat’ with ‘multiple short roots.’

Madagascar,
“together with the Indian subcontinent, separated from Africa approximately 165 mya and became fully isolated from Antarctica and Australia approximately 115–112 Myr ago, with Madagascar and the Indian subcontinent separating from each other about 88 Myr ago.” Vintana lived 72-66 mya and so enjoyed isolation for tens of millions of years.

About Zalambdalestes
Wible et al. 2004 reports, “Included among the primitive cranial features of Zalambdalestes are: the last upper incisor in the maxilla; nasals expanded posteriorly to contact the lacrimals; double lacrimal foramina; pterygoids meeting on the midline; jugal extending posteriorly to the glenoid process; and a relatively posterior position of the glenoid fossa, opposite the anterior half of the promontorium of the petrosal, almost entirely on the zygoma and not the braincase proper. This evidence contraverts the few craniodental apomorphies that have been cited as allying Zalambdalestes with certain placentals, such as lagomorphs and Glires.” That’s because Zalambdalestes specimens are wombats with marsupial traits. This team was unable to nest Zalambdalestes into a large clade but did headline their paper with the term ‘eutherian.’ Did they preconceive this taxon based on the number of molars (3)?

FIgure 3. Two Zalambdalestes skulls along with Wible et al. drawing and select bones colorized. Here the septomaxilla / lacrimal is orange.

FIgure 3. Two Zalambdalestes skulls along with Wible et al. drawing and select bones colorized. Here the septomaxilla / lacrimal is orange. Note how the many differences one can find between the two specimens of Zalambdalestes and the drawing representing the taxon. They did not reset the anterior rostrum to align with the posterior portion, which affects scoring (Fig. 3).

Note
how the many differences one can find between the two specimens of Zalambdalestes (Fig. 3) and the drawing representing the taxon presented by Wible et al. They did not lower the anterior rostrum to align with the posterior portion in the referred PSS-MAE130 specimen, which affects scoring. I did so (Fig. 3).

References
Krause DW, Hoffmann S, Wible JR, Kirk EC, and several other authors 2014. First cranial remains of a gondwanatherian mammal reveal remarkable mosaicism. Nature. online. doi:10.1038/nature13922. ISSN 1476-4687.
Wible JR, Novacek MJ and Rougier GW 2004. New data on the skull and dentition in the Mongolian Late Cretaceous eutherian mammal Zalambdalestes. Bulletin of the American Museum of Natural History 281:144pp.

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