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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Rodent, rabbit, tree shrew and multituberculate skulls compared

In an effort to understand 
a clade that was giving me trouble on the LRT, I put together the following set of skulls (Fig. 1, click here to enlarge) from the redefined clade Glires (still rodents and rabbits, but also their closest kin).

Figure 1. Click to enlarge. See text for explanation.

Figure 1. Click to enlarge. See text for explanation.

In the above illustration.
Select members of the Glires (rodents, rabbits and relatives, all derived, ultimately from the basal placental, Monodelphis, not to scale. Numbers refer to columns:

  1. Tupaia is a tree shrew. Macroscelides is an elephant shrew. Chrysochloris is a golden mole.
  2. Scutisorex is a hero shrew. Apatemys is a an arboreal apatemyid. Trogosus is a terrestrial apatemyid or tillodont. 
  3. Solenodon is a solendontid. Zalambdalestes is another solenodontid. 
  4. Nambaroo is a primitive rabbit and a kangaroo mimic. Brachyerix is an extinct hedgehog. Gomphos is an extinct rabbit. Orytolagus is an extant rabbit.
  5. Carpolestes is an arboreal plesiadapiform. Plesiadapis is another arboreal plasiadapiform. Taeniolabis is a terrestrial plesiadapiform often considered a multituberculate. 
  6. Shenshou is a pre-rodent. Paramys is an extinct rodent. Rattus is an extant rodent. Ignacius has not been tested but usually nestes with plesiadapiform. 
  7. Kryptobaatar is a multituberculate. Ptilodus is a multituberculate. Rugosodon is a multituberculate. Megconus is another multituberculate often considered a mammaliaform.

Still have not found evidence
that multituberculates nested in a clade more primitive than placentals. You’ll note that Zalambdalestes now nests with Solenodon despite the epipubes (found occasionally in other placentals, too). Glires nests between Carnivora and Ptilocercia. See yesterday’s post on basal placentals for basalmost taxa.

 

A Jurassic ancestor to both rodents and multituberculates

I did not know of
the early mammal, Shenshou (Bi et al. 2014; Oxfordian, earliest Late Jurassic; Fig. 1), when producing an earlier blog you can read here that pushed back the origin of mammals to the Triassic. Little did I know, that hypothesis originated two years ago by the Bi et al. team describing their find (see below). On close examination, however, there were problems with that description that have bearing on their phylogenetic placement of Shenshou.

Figure 1. The Jurassic mammal Shenshou, which nests within Allotheria (Haramiyida + Mutituberculata) within the Mammalia, as I proposed based on the LRT without knowledge of this paper.

Figure 1. The Jurassic mammal Shenshou, which nests within Allotheria (Haramiyida + Mutituberculata) within the Mammalia, as I proposed based on the LRT without knowledge of this paper.

Figure 3. Mammal tree according to Bi et al. 2014. Taxa duplicated in the large reptile tree are in yellow. Some taxa here do not look like their sisters. Other sisters do not nest close to one another.

Figure 2. Mammal tree according to Bi et al. 2014. Taxa duplicated in the large reptile tree are in yellow. Some taxa here do not look like their sisters. Other sisters do not nest close to one another.

From the Bi et al. abstract
“The phylogeny of Allotheria, including Multituberculata and Haramiyida, remains unsolved and has generated contentious views on the origin and earliest evolution of mammals. Here we report three new species of a new clade, Euharamiyida, based on six well-preserved fossils from the Jurassic period of China. These fossils reveal many craniodental and postcranial features of euharamiyidans and clarify several ambiguous structures that are currently the topic of debate. Our phylogenetic analyses recognize Euharamiyida as the sister group of Multituberculata, and place Allotheria within the Mammalia. The phylogeny suggests that allotherian mammals evolved from a Late Triassic (approximately 208 million years ago) Haramiyavia-like ancestor and diversified into euharamiyidans and multituberculates with a cosmopolitan distribution, implying homologous acquisition of many craniodental and postcranial features in the two groups. Our findings also favour a Late Triassic origin of mammals in Laurasia and two independent detachment events of the middle ear bones during mammalian evolution.”

This blog post was prompted by a comment made by reader M. Mortimer, who responded to my challenge, “Where among the pre-eutherians is there a better match for multituberculates”? by writing “Where they go in the consensus (e.g. “sister” to Shenshou in your terminology- Bi et al., 2014), based on more characters and taxa than you use.”

First of all
the challenge was to find a mismatched sister and a better match WITHIN the taxon list of the large reptile tree. Bringing in an outside taxon, does not answer the challenge. That Shenshou was already considered a member of the clade under question establishes that M. Mortimer clearly did not understand or chose to ignore the parameters of the challenge. Be that as it may…

The Bi et al. tree
nests Allotheria (haramiyids and triconodonts) traditionally between monotremes and spalacotheres + eutherians + megatherians. Bi et al. used 113 taxa and 495 characters, and several magnitudes more dental traits than used here. I found several mistakes made in the description of Shenshou, so we’ll start with those.

Skull
I was able to trace some elements of the skull using Photoshop (Fig. 2). Shenshou has the basic buck-toothed rodent appearance with a narrow lateral temporal bar (zygomatic arch) and  broad skull (as determined by the triangular displaced post parietal. I was not able to recover the same posterior dentary shape that Bi et al. produced (Fig. 7). The one traced here is more traditional.

Figure 2. Shenshou skull traced in colors.

Figure 3. Shenshou skull traced in colors. The lateral temporal bar is gracile. The vomers are visible. There is a nice ectotympanic hemicylinder there.

Figure 3. Scapula and vertebrae of Shenshou. One centrum was misidentified as an unusually shaped coracoid by Bi et al.

Figure 4. Scapula and vertebrae of Shenshou. One centrum was misidentified as an unusually shaped coracoid by Bi et al.

Scapula and coracoid issues
Bii et al. reported a scapula with a spine along the anterior ridge (as in pre-mammal tritylodontids like Oligokyphus and Kayentatherium). Unfortunately I place the ridge at mid scapula. They also report the presence of a small coracoid (Fig. 7), but here that is interpreted as a series of half-buried vertebral centra (Fig. 3). If present, these traits would tend to push Shenshou toward more primitive taxa, but here they only produced autapomorphies in an otherwise firm nesting.

Manus and pes
re-reconstructions. The manus was pretty easy to reconstruct (Fig. 4. As it was preserved almost intact. Not so the pes (Fig. 5), which was scattered about and the proximal portions of two metatarsals were lost during excavation. In any case, the long fingers and toes indicate that Shenshou was arboreal, as Bi et al. originally described it.

Figure 4. Manus of Shenshou in situ and reconstructed. Compare this the original reconstruction shown in figure 6.

Figure 5. Manus of Shenshou in situ and reconstructed. Compare this the original reconstruction shown in figure 7.

The fibula
should be separated from the tibia (Fig. 7) as it is in the fossil (Fig. 1). The anterior caudals have shallow, but long centra that are not shown in the Bi et al. reconstruction (Fig. 7). The femur was drawn too long with regard to the spinal column and the pelvis was drawn to short. The ectotympanic ring was ignored.

Figure 5. Shenshou pes reconstrution. The short calcaneum also indicates an arboreal taxon.

Figure 6. Shenshou pes reconstrution. The short calcaneum also indicates an arboreal taxon.

In the large reptile tree
(subset Fig. 8) Shenshou nests within the Eutheria, basal to rodents and multituberculates + haramiyids and not far from the branch that produced the other arborealist, Plesiadapis, an ancestral arboreal rabbit with similar long fingers and toes. A comparison of Shenshou with Rattus, the rat (Fig. 6), shows the two taxa have much in common despite the minor mistakes made by Bi et al. in their attempt at fulfilling an erroneous paradigm that Shenshou should nest far from arboreal eutherian mammals.

Figure 6. Shenshou original art by or traced from Bi et al. 2014, compared to Rattus, the rat.

Figure 7 Shenshou original art by or traced from Bi et al. 2014, compared to Rattus, the rat. Note the extremely deep posterior dentary here is not a good reflection of the actual fossil (Fig. 3). Note the original interpretation of a coracoid.  Even that looks like a vertebral centrum. Note, the large reconstruction is a chimaera.

 

Figure 8. Shenshou nests basal to Rattus and the former Allotheria (= Multituberculata + Haramiyida).

Figure 8. Shenshou nests basal to Rattus and the former Allotheria (= Multituberculata + Haramiyida).

If you can show that
Allotheria belongs outside of the Theria, please bring that data to my attention. So far, based on traits from all over the body and not so much the teeth, those rodent-like taxa are nesting with rodents.

Thanks, Mickey, for bringing this paper to my attention. Sorry it didn’t hold up as evidence for your negative POV. Next time, please bring up two taxa within the LRT.

References
Bi S, Wang Y-Q, Guan J, Sheng X and Meng J 2014.
Three new Jurassic euharamiidan species reinforce early divergence of mammals. Nature 514:579-584. online here.

Haramiyidans and Multituberculates are rodents, not pre-mammals.

Wikipedia reports
Haramiyidans have been known since the 1840s, but only from fossilized teeth and a single partial lower jaw. However, several features of the teeth have shown for many years that haramiyidans are among the most basal of mammaliaforms. Megaconus (Middle Jurassic, Zhou et al. 2013, Fig. 1) is a member.”

Wikipedia also reports
Multituberculata is is an extinct taxon of rodent-like mammals. At least 200 species are known, ranging from mouse-sized to beaver-sized. Multituberculates are usually placed outside either of the two main groups of living mammals—Theria, including placentals and marsupials, and Monotremata—but closer to Theria than to monotremes. The oldest known species in the group is Rugosodon from the Jurassic.” 

Figure 1. Megaconus in situ. Original tracing and DGS color tracing which appears to show that both hind limbs and the vernal pelvis have been displaced posteriorly -- unless their is a counter plate that preserves skeletal parts that don't appear to be present here.

Figure 1. Megaconus in situ. Original tracing and DGS color tracing which appears to show that both hind limbs and the vernal pelvis have been displaced posteriorly — unless their is a counter plate that preserves skeletal parts that don’t appear to be present here.

Recently added taxa
to the LRT (749 taxa) include the purported haramiyid allothere mammaliaform. Megaconus mammaliaformis (Zhou et al., 2013) the mutituberculates Rugosodon (Yuan et al. 2013) and Kryptobaatar (Kielan-Jaworowska  1970). Unfortunately most of the other known haramiyid allothere mammaliaformes are known from too few traits to test in the LRT. So far as I know, only Kryptobataar (Fig. 8), Rugosodon (Fig. 9) and Megaconus (Fig. 1) are known from complete skeletons. There may be others, but these three are enough to test the nesting. In the LRT they nest together with Rattus (the rat. Fig. 3).

Figure 1. Mammals include rodents. Haramiyadens and multituberculates nest with rodents.

Figure 2. Mammals include rodents. Haramiyidans and multituberculates nest with rodents. Click to enlarge.

By contrast…
Zhou et al. 2013 report: “Here we describe a new fossil from the Middle Jurassic that has a mandibular middle ear, a gradational transition of thoracolumbar vertebrae and primitive ankle features, but highly derived molars with a high crown and multiple roots that are partially fused. The upper molars have longitudinal cusp rows that occlude alternately with those of the lower molars.” The first three traits put Megaconus among the pre-mammal cynodonts. The last three traits are specialiizations. The broader traits employed by the LRT put Megaconus in the rodent clade. Rattus (the rat) and Oryctlagus (the rabbit) were included taxa in both studies.

So now we have a phylogenetic problem.
Do Megaconus and Rugosodon nest more primitively than monotremes? According to Zhou et al. they do. The Zhou team employed more taxa, more traits and more dental traits — by far.

Figure 5. Megaconus soul with original outline tracing. Note they missed lots of detail, but marked the tiny angular (green triangle). In DGS tracing I don't see what Zhou et al. saw.

Figure 3. Megaconus soul with original outline tracing. Note they missed lots of detail, but marked the tiny angular (green triangle). In DGS tracing I don’t see what Zhou et al. saw.

Unfortunately,
Megaconus otherwise looks so much like a rodent that it has been given the nickname, ‘the Jurassic squirrel.’ The LRT tests only the relatively easy traits to see, not dental details. In the LRT, shifting Megaconus and Rugosodon to Juramaia adds 37 steps. That’s a big hump to get over. I do not know how many additional steps would be added by shifting Megaconus to Rattus in the Zhou et al. study.

Figure 3. Megaconus mandible showing cynodont-like posterior mandible bones, not tiny mammal-like ear bones.

Figure 4. Megaconus mandible showing cynodont-like posterior mandible bones, not tiny mammal-like ear bones. Unfortunately this key trait cannot be confirmed with present photo resolution. Mammals and reptiles call the same bones different names in some cases and some of these are labeled here.

If the Zhou et al. team is correct
then we have a problem. If the Zhou et al. team is not correct, they have a problem. They have identified an angular/ectotympanic where there is none. Rugosodon and Kryptobataar likewise do not have pre-mammal-type posterior jaw bones prior to their evolution into tiny ear bones.

Figure 3. Skull of Rattus, the rat. Note the similarities to Megaconus. Not identical but similar.

Figure 5. Brown Rat (Rattus norvegicus) skull showing how lower incisors are used to scrape away and sharpen upper incisors The ear bones are located inside the circular ectotympanic posterior to the mandible and below the skull.

How can we reconcile this problem? 

  1. If Megaconus indeed nests with Rattus, then the ankle, posterior jaw and other traits may represent reversals to a more primitive state. 
  2. If Megaconus is indeed primitive, then it anticipates and converges on a long list of traits with Rattus under the LRT, Given that living monotremes have a long list of special traits, it is not unreasonable to accept that Megaconus diid likewise. The only caveat to that hypothesis is that monotreme special traits are not shared with or converge with those of other mammals.
  3. If Megaconus parts have been misidentified, then (no exceptions) all other traits indicate it is a rodent sister.
Figure 4. Haramiyava dentary showing what a more typical stem mammal dentary and teeth look like. Earlier studies linked this clade to multituberculates, but this dentary was cause to reject that association.

Figure 6. Haramiyava dentary showing what a more typical stem mammal dentary and teeth look like. Earlier studies linked this clade to multituberculates, but this dentary was cause to reject that association. Just the appearance of that poster medial groove is enough to indicate the presence of tiny jaw bones that had not transformed into ear bones. From Luo et al. 2015.

Stem mammals have lots of teeth
(Fig. 6) and Megaconus does not have lots of teeth. It has rodent-like teeth and everything else is rodent-like, too. And check out that overbite!

Figure 7. Eomaia skull traced and reconstructed. Eomaia nests between marsupials and placentals. Note the unspecialized skull and dentition. Megaconus has a very specialized dentition.

Figure 7. Eomaia skull traced and reconstructed. Eomaia nests between marsupials and placentals. Note the unspecialized skull and dentition. Megaconus has a very specialized dentition.

The skull of
Kryptobataar, (Fig. 6) another purported multituberculate, likewise shows no trace of tiny post-dentary bones, either here or in a Digimorph scan.

Figure 8. The skull of the multituberculate Kryptobataar, which now nests as a rodent in the LRT.

Figure 8. The skull of the multituberculate Kryptobataar, which now nests as a rodent in the LRT. B&W image copyright Digimorph.org and used with permission. 

The skull of
of Rugosodon (Fig. 9) likewise shows no trace of long, gracile post dentary bones, either here or originally.

Figure 9. The skull of Rugosodon. There are no tiny post dentary bones present here according to the original authors or my own tracings.

Figure 9. The skull of Rugosodon. There are no tiny post dentary bones present here according to the original authors or my own tracings.

References
Kielan-Jaworowska Z 1970. New Upper Cretaceous multituberculate genera from Bayn Dzak, Gobi Desert. In: Kielan-Jaworowska (ed.), Results of the Polish-Mongolian Palaeontological Expeditions, pt. II. Palaeontologica Polonica 21, p.35-49.
Luo Z-X, Gatesy SM, Jenkins FA Jr., Amarai WW and Shubin NH 2015. Mandibular and dental characteristics of Late Triassic mammaliaform Haramiyavia and their ramifications for basal mammal evolution. PNAS 112(41) E71010-E7109. doi: 10.1073/pnas.1519387112
Wible Jr, Rougier GW 2000. Cranial anatomy of Kryptobaatar dashzevegi (Mammalia, Multituberculata), and its bearing on the evolution of mammalian characters. Bulletin of the American Museum of Natural History 247: 1–120. doi:10.1206/0003-0090(2000)2472.0.
Yuan CX, Ji Q, Meng QJ, Tabrum AR and Luo ZX 2013. Earliest evolution of multituberculate mammals revealed by a new Jurassic fossil.. Science 341 (6147): 779–783. doi:10.1126/science.1237970.
Zhou CF, Wu S, Martin T, Luo ZX 2013. A Jurassic mammaliaform and the earliest mammalorian evolutionary adaptations. Nature 500 (7461): 163. doi:10.1038/nature12429.

wiki/Rugosodon
wiki/Megaconus

Rats! – (or where Mickey Mouse diverged from Walt Disney)

We don’t talk about mammals much, but as reptiles they (we) do qualify as subjects to be covered by ReptileEvolution.com.

A new online study by Wu et al. (2012) finds evidence for a post-Cretaceous origin for rodents. Rodents (everything from porcupines and guinea pigs to squirrels and mice) are related to rabbits (lagomorphs) which are related to primates (including readers of this blog and lemurs) which all were derived from arboreal carnivores like Vulpavus.

The Wu et al 2012 study on rodents and their post-Cretaceous appearance.

The Wu et al 2012 study on rodents and their post-Cretaceous appearance.

How are they all related? 
Near (but not at) the base of the primates is an interesting set of taxa known as tree shrews. Essentially they are micro lemurs with shifted teeth.

Tupaia, the large tree shrew,

Figure 1. Tupaia, the large tree shrew, a living taxon close to the base of rabbits and rodents with origins in the Paleocene, just following the Cretaceous. Click to learn more.

The most common one, Tupaia (Raffles 1821) was found to be basal to the equally arboreal Plesiadapis (Fig. 3) and by extension to the terrestrial rabbits, and by further extension to rodents (keeping on topic), like the porcupine. It’s worthwhile to see the porcupine skull and how close it resembles that of Plesiadapis.

Plesiadapis

Figure 3. Plesiadapis, formerly considered a basal primate, is here considered a basal arboreal lagomorph (rabbit ancestor).

The other arboreal tree shrew, Ptilocercus, was found to be basal to bats and colugos (flying lemurs), all three with relatives extending back to the Paleocene (post-Cretaceous).

Ptilocercus, pen-tailed tree shrew

Figure 2. Ptilocercus, pen-tailed tree shrew, a living relative to the ancestor of bats and colugos.

I don’t know much about rodents, but given what I do know about the initial appearance of their outgroups, the Wu et al. (2012) study makes perfect sense in the present context!

References
Wu S, Wu W, Zhang Z, Ye J, Ni X, Sun J, Edwards SV, Meng J and Organ CL 2012. Molecular and Paleontological Evidence for a Post-Cretaceous Origin of Rodents. PLoS ONE 7(10): e46445. doi:10.1371/journal.pone.0046445
http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0046445