Vilevolodon: the atavistic reappearance of post-dentary bones

Preface
I’ve been wondering about the traditional nesting of Multituberculata and kin outside of the Mammalia for years. All have a dentary jaw joint, but some have post-dentary bones. given the opportunity multituberculates nest with rodents and plesiadapiformes in the large reptile tree (LRT, 1047 taxa).  No other pre-mammals resemble them. Traditionally Haramiyava (Fig. 1) has been considered a pre-mammal link to Haramiyida + Multituberculata. In the LRT Haramiyava nests with the mammaliaforms Brasilodon, Sinoconodon and Therioherpeton – far from any other taxa considered Haramiyida + Multituberculata currently and provisionally nesting deep within the Mammalia.

Figure 1. Haramiyavia reconstructed and restored. Missing parts are ghosted. The fourth maxillary tooth appears to be a small canine. The post-dentary bones are imagined from Vilevolodon (figure 4).

Vilevolodon diplomylos
(Luo et al. 2017; Jurassic, 160 mya; BMNH2942A, B; Figs. 2-4) was originally considered a stem mammal (= mammaliaform), a eleutherodontid in the clade Haramiyida AND it had clearly defined gliding membranes (Fig. 2). By contrast the LRT nests Vilevolodon with the Late Jurassic para-rodent Shenshou and the extant rodents, Rattus and Mus, not far from members of the Multituberculata.

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

Figure 2. Vilevolodon in situ, plate, counterplate, original drawing, DGS color, and restored manus and pes. Note the gliding membrane (patagium) and fur.

But there’s a big problem
Vilevolodon doesn’t have tiny ear bones, like mammals do. It has post-dentary bones, like pre-mammals do (Figs. 3, 4).

Figure 2. Vilevolodont skull in situ, without color, DGS color tracing, that tracing reconstructed and a CT scan form Luo et al. 2017.

Figure 3. Vilevolodont skull in situ, without color, DGS color tracing, that tracing reconstructed and a CT scan form Luo et al. 2017.

The ear problem in Jurassic rodents
Luo et al. report, “a mandibular middle ear with a unique character combination previously unknown in mammaliaforms.” Pre-mammals have post-dentary bones (articular, angular, surangular). Therian mammals shrink and migrate those bones to the base of the skull where they become middle ear bones with new names (malleus, incus, ectotympanic). The stapes remains the stapes in all tetrapods. So what is happening with Vilevolodon and its sisters? Why don’t the pre-mammal post-dentary bones define it as a pre-mammal? After all, that’s the current paradigm.

Figure 3. There is no doubt that Vilevolodon has a pre-mammal type of posterior jaw bones. Otherwise they nest with rodents and plesiadapiformes. This appears to be a mammal with an atavism, a reversal. These elements simply stopped developing as in other mammals.

Figure 3. There is no doubt that Vilevolodon has pre-mammal type post-dentary bones. There is also no doubt that the dentary formed the main jaw joint with the squamosal. How does one reconcile both sets of traits? In the LRT Vilevolodon nests with rodents. This appears to be a mammal with an atavism, a reversal. These elements simply stopped developing as in other mammals.

Mammals are defined by
the evolution and migration of their posterior jaw bones into middle ear bones with a jaw joint switch from quadrate/articular to dentary/squamosal. Multituberculates and haramiyids appear to bend or break that rule because they have cynodont-like posterior jaw bones, not tiny middle ear bones, and yet otherwise they nest with rodents and plesiadapiformes. This is one reason why you don’t want to pull a Larry Martin with post-dentary bones. You want to nest a taxon based on a long list of traits, not just one, two or a dozen.

The massive jaw joint
Mammals, such as Vilevolodon, with atavistic post-dentary bones also have a massive jaw joint with a long articulating surface on the dentary contacting the squamosal. All mammals have such a jaw joint. Pre-mammals don’t. While Vilevolodon has a large dentary/squamosal jaw joint, the post-dentary articular, still contacts the quadrate. It’s clearly not the main jaw joint.

Filan 1991
traced the development of post-dentary bones in embryonic Monodelphis specimens. She reported, “Neonates of Monodelphis possess neither mammalian (dentarysquamosal) nor reptilian (quadrate-articular) jaw articulations, nor does the contact between the incus and crista parotica offer a joint surface. Elasticity in Meckel’s cartilage allows minimal deflection of the lower jaw.” After all, those neonates are just sucking milk, not biting, and the embryos don’t even do that. Does that make neonates like this not mammals? No. The evidence indicates that in multituberculates and haramiyds the embryological transformation of posterior jaw bones stopped before development transformed them into middle ear bones. This is an atavism, a phylogenetic reversal. The timing of development changed. In the case of Vilevolodon, the middle ear bones stop evolving during embryological development and the post-dentary bones they would have evolved from continue to appear in adults. What was a rare mutation probably spread throughout an isolated population. Perhaps this had something to do with the increase in size of the dentary jaw joint.

Haramiyavia and the Haramiyida clade
Seems at this point that only Haramiyavia is a haramiyid, unless Brasilodon is one as well. Members traditionally assigned to the clade Eleutherodontidae also nest in various locations in the LRT, not all in one clade.

Meng et al. 2017 report,
“Stem mammaliaforms are morphologically disparate and ecologically diverse in their own right, and they developed versatile locomotor modes that include arboreal, semiaquatic, and subterranean specializations, which are all distinct from generalized mammaliaforms.” Unfortunately, the LRT nests a long list of mammaliaforms at various nodes within the Mammalia. They are not from a single diverse clade.

Contra Meng et al. 2017
the LRT reduces the niches and body shapes of stem mammals down to a few small, generalized taxa like Sinoconodon and Megazostrodon. Derived taxa nest at derived nodes.

The LRT nests rodents close to Plesidapiformes,
including the extant aye-aye, Daubentonia as first reported here. So it comes as no surprise when Luo et al. report, “Eleutherodontids show a marked similarity to the primate Daubentonia in the ventrally bent rostrum and deep mandible, and both features are interpreted to be reinforcement for incisor gnawing.” That’s the case only with Vilevolodon this time. Others may be by convergence.

Molars
The jaw joint of the rodent allows for rostral-caudal and dorsal-ventral motion of the jaws. Luo et al. report, in Villevolodon it is not possible for the mandible to move posteriorly or horizontally, but their images show a continuous anteroposterior trough/furrow in the three molars, though not to the extent seen in sister taxon Shenshou. Molars with a long and continuous trough for rostral-caudal grinding appear by convergence in several reptile/mammal clades.

Incisor replacement
Luo et al. report, “Incisor replacement is prolonged until well after molars are fully erupted, a timing pattern unique to most other mammaliaforms. In rodents incisors never stop growing. The growth pattern in Vilevolodon may be the first step toward that. Not sure why Luo et al. are missing all these strong rodent clues.

Gliding?
Meng et al. 2017 note: “They [Vilevolodon and kin] are the most primitive known gliders in mammal evolution, evolving approximately 100 million years before the earliest known therian gliders.” Earlier, with the appearance of the stem pangolin, Zhangheotherium at the start of the Cretaceous, the ghost lineage for primates, flying lemurs and bats was also set to that time or earlier. Before the advent of flying birds, but after the advent of predatory theropods, many mammals had evidently taken to the trees. And one way to get from tree to tree without descending to the dangerous turf is to jump, glide and fly. I predict we’ll find the big-handed ancestors of bats in Jurassic and Cretaceous strata someday. They are already volant shortly after the K-T extinction event.

Hearing in Vilevolodon
With the reappearance of post-dentary bones in taxa like Vilevolodon, the auditory acuity that was more highly developed in its ancestors must have suffered a setback. By the evidence provided, the massive jaw joint must have been more important for its survival.

Figure 8. Multituberculate Kryobaatar mandible in lateral and medial views. Here post-dentary bones are absent. The malleus (quadrate) and ectotympanic are on the skull.

Figure 4. Multituberculate Kryobaatar mandible in lateral and medial views. Here post-dentary bones are absent here. The malleus (quadrate) and ectotympanic are on the skull.

Getting back to the purported patagium of Maiopatagium
which we looked at yesterday. It is not apparent and the authors do not describe it. Rather, Meng et al. 2017 sidestep this by reporting, “Furthermore, we report a second eleutherodont specimen (BMNH2942) preserved with a halo of carbonized fur and patagial membranes, similar to those of Maiopatagium.” The patagial taxon remains unnamed in the Maiopatagium paper (Meng et al. 2017), but is named in a second paper appearing on the same day. It is today’s subject, Vilevolodon (Fig. 1)

References
Filan SL 1991. Development of the middle ear region in Monodelphis domestica (Marsupialia, Didelphidae): marsupial solutions to an early birth. Journal of Zoology 225(4): 577–588 DOI: 10.1111/j.1469-7998.1991.tb04326.x
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
Jenkins FA, Jr, Gatesy SM, Shubin NH and Amaral WW 1997. Haramiyids and Triassic mammalian evolution. Nature 385(6618):715–718.
Luo Z-X, Gatesy SM, Jenkins FA, Jr, Amaralc WW and Shubin NH 2015. Mandibular and dental characteristics of Late Triassic mammaliaform Haramiyavia and their ramifications for basal mammal evolution. PNAS 112 (51) E7101–E7109.

wiki/Haramiyavia
wiki/Vilevolodon
wiki/Maiopatagium

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