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

Goodbye Scrotifera. Goodbye Euarchontaglires. Goodbye Scandentia. etc. etc.

Earlier the large reptile tree
found that several former clades, like Parareptilia, PterodactyloideaCetacea, Testudinata (Chelonia) Notoungulata, Pseudosuchia, Ornithodira and Pinnipedia were not monophyletic… and that list keeps growing.

The large reptile tree (LRT, 1044 taxa) does not replicate the following mammalian clades:

  1. Scandentia – tree shrews: yes, closely related, but at the bases of different clades.
  2. Euarchontaglires – rodents, rabbits, tree shrews, flying lemurs and primates,  (Fig. 1)
  3. Euarchonta – tree shrews, flying lemurs, primates and plesiadapiformes.
  4. Glires – rodents, rabbits
  5. Scrotifera – Eulipotyphla (see below), bats, pangolins, Carnivora, Euungulata (including whales)
  6. Eulipotyphla – hedgehogs, shrews, solenodons, moles (moles are Carnivora))
  7. Euungulata – perissodactyls, artiodactyls (including whales)
  8. Tenrecidae – tenrecs, some are closer to shrews, others closer to odontocetes
  9. Macroscelidea – elephant shrews, some are closer to tenrecs
  10. Primates – Plesiadapiformes and extant primates, including Daubentonia (the aye-aye. No giant anterior dentary teeth in valid primates.
  11. there are a few more I’m overlooking. I’ll add them as they come to me.
Figure 1. Glires and Euarchonta are two clades within the Mammalia in the LRT.

Figure 1. Glires and Euarchonta are two clades within the Mammalia in the LRT.

Let’s focus on Plesiadapiformes
Bloch et al. 2007 found plesiadapiforms (Plesiadapis, Carpolestes and kin) more closely related to primates than to any other group. They did not test against rodents and multituberculates. The LRT does not replicate these results, but finds plesiadapiforms more closely related to multituberculates and rodents when included.

According to Bloch & Boyer 2002
“Plesiadapiforms share some traits with living primates, including long fingers well designed for grasping, and other features of the skeleton that are related to arboreality.” That’s fine, but there are other taxa in the tree topology with long fingers, too.

Paromomyidae
Krause 1991 reports, “Paromomyids …have long been regarded by most workers as members of the Plesiadapiformes.” Again, the LRT does not support this, but nests Paromomyids, like Ignacius (Fig. 2), with rodents, like Mus and Paramys. Paromomyids have squared off and flat molars, but Paromomys does not.

Figure 2. The skull of Ignacius nests with other rodents, not plesiadapiformes.

Figure 2. The skull of Ignacius nests with other rodents, not plesiadapiformes. Ironically it is closer to the squirrel-like Paramys than to Paromomys.

Beard 1990 thought paromomyids,
as plesiadapiforms, where close to colugos or “flying lemurs”. The LRT (Fig. 1) does not support this relationship. Rather paromomyids, like Ignacius, were squirrel-like, able to scamper both in the trees and on the ground. Ignacius graybullianus (USNM 421608, Fig. 1) is a new taxon that nests as a basal rodent in the LRT.

Figure 3. Ignacius clarkforkensis known parts.

Figure 3. Ignacius clarkforkensis known parts.

Remmber, no primates 
have giant anterior dentary teeth. The aye-aye, Daubentonia, has such teeth, but the LRT finds it nests with Plesiadapis and multituberculates and rodents, not primates. Yes, plesiadapiformes and Ignacius had long limbs, big brains and binocular vision, but by convergence with primates.

References
Beard KC 1990. 
Gliding Behavior and palaeoecology of the alleged primate family Paromomyidae (Mammalia, Dermoptera). Nature 345, 340-341.
Bloch J, Silcox MT, et al. 2007.
New Paleocene skeletons and the relationship of plesiadapiforms to crown-clade primates.  Proceedings of the National Academy of Science 104, 1159-1164.
Kay RF, Thewissen JG and Yoder, AD 1992. Cranial anatomy of Ignacius graybullianus and the affinities of the Plesiadapiformes. American Journal of Physical Anthropology. 89 (4): 477–498. doi:10.1002/ajpa.1330890409.
Krause DW 1984. Mammal Evolution in the Paleocene: Beginning of an Era. In: Gingerich, P. D. & Badgley, C. E. (eds.): Mammals: notes for a short course. Univ. of Tennessee, Department of Geological Sciences.
Krause DW 1991. Were paromomyids gliders? Maybe, maybe not. Journal of human evolution 21:177-188.

Multituberculates and rodents: cousins? or not?

The big question is: what are they?
The LRT nests multituberculates with rodents, but currently that’s a minority view of one.

Kielan-Jaworowska Z and Hurum 2001 wrote:
“Traditionally palaeontologists believed that multituberculates might have originated from cynodonts independently from all other mammals, or diverged from other mammals at a very early stage of mammalian evolution.” Unfortunately these authors do not say which taxa attract multis to the the pre-eutherian grades and clades.

Simpson (1945, p. 168) stated:
“The multituberculate structure was so radically distinctive throughout their history that it seems hardly possible that they are related to other mammals except by a common origin at, or even before, the class as such”

Hahn et al. (1989) and Miao (1993)
reported that multituberculates might be a sister taxon of all other mammals. On the other hand, Kielan-Jaworowska et al. 1986; Miao 1988; Wible 1991; Rougier et al. 1992; Wible and Hopson 1993, 1995; Hurum 1994, 1998a, b) demonstrated the homogeneity of the internal structure of the skull and vascular system of all mammals, including multituberculates.

Hurum et al. 1996; Rougier et al. 1996a report
Multituberculate ear ossicles display the same pattern as those of all other mammals.

The notion
that multituberculates might form a sister taxon of all other mammals is related to the idea that they are close relatives to the Haramiyidae, a family represented until recently only by isolated teeth, with numerous cusps arranged in longitudinal rows, known from the Late Triassic and Early Jurassic mostly in Europe. Key to these thoughts are the idea that most fossil material comes from teeth.

Jenkins et al. (1997)
described from the Upper Triassic of Greenland Haramiyavia clemmenseni, assigned to the Haramiyidae, represented by dentaries and partial maxillae with teeth and fragments of the postcranial skeleton. Haramiyavia has been interpreted as having orthal jaw movement (standard up-down rotation on a glenoid axis). On this basis Jenkins et al. excluded the Haramiyida from the Allotheria, which have propalinal (fore-and-aft) movement of the dentary and backward (palinal) power stroke. In turn Butler (2000) revised all known allotherians and argued that dental resemblance supports the hypothesis that the Multituberculata originated from the Haramiyida.

Kielan-Jaworowska Z and Hurum 2001 wrote:
“Finally, the most recent analyses of mammalian relationships, including analysis of the skeleton of a symmetrodont Zhangheotherium (Hu et al. 1997; here recovered as a pangolin ancestor), and the skeleton of the eutriconodont Jeholodens (Ji et al. 1999; here recovered as a tritylodontid), did not support multituberculate-therian sister-group relationship. In both of these papers the Multituberculata were placed between Monotremata (Ornithorhynchus) and Symmetrodonta (Zhangheotherium), being a sister taxon of all the Holotheria” (last common ancestor of Kuehneotherium and Theria). In other words, close to monotremes.

Kielan-Jaworowska Z and Hurum 2001 wrote about the multi brain:
“The multituberculate brain, designated cryptomesencephalic (characterised by an expanded vermis, no cerebellar hemispheres, and lack of the dorsal midbrain exposure) is very different from that in Theria, which originally had eumesencephalic brains (characterised by a wide cerebellum with extensive cerebellar hemispheres and large dorsal midbrain exposure).”

This appears to assume only one direction for brain development, with no evolutionary backsliding. Unfortunately Kielan-Jaworowska and Hurum employed a hypothetical ancestor for their multituberculate cladogram.

We’ve already seen teeth in whales reverse from the typical W and Y molar cusp patterns, to linear molar cusps to simple pegs.

Figure 1. Rodent and multituberculate right pedes dorsal view. Note the derived pes of Kryptobaatar based on the primitive pedes of Shenshou and Paramys. Multis have a reduced astragalus (orange) for a looser ankle joint for an arboreal niche.

Figure 1. Rodent and multituberculate right pedes dorsal view. Note the derived pes of Kryptobaatar based on the primitive pedes of Shenshou and Paramys. Multis have a reduced astragalus (orange) for a looser ankle joint for an arboreal niche.

Kielan-Jaworowska Z and Hurum 2001 wrote about the multituberculate foot:
“Another character neglected until recently in phylogenetic analyses of early mammals involves the foot structure. In the multituberculate foot the middle metatarsal (M3) is abducted from the longitudinal axis of the tuber calcanei, while the calcaneus contacts distally the 5th metatarsal (Kielan-Jaworowska and Gambaryan 1994). This type of foot appeared at that time to be unique among mammals, but Ji et al. (1999) described a similar type of foot in the eutriconodont Jeholodens. It follows that there are two groups of characters related to brain and foot structure, which ally multituberculates with eutriconodonts.”

Figure 2. Squirrel pes.

Figure 2. Squirrel pes, not similar to a multi ankle yet still able to clamber and roared on tree trunks.

The trouble is
a large gamut analysis of mammalian relationships does not find a better nesting for those highly-derived, but primitive-brained, rodent-like mutituberculates than with rodents. They have similar teeth, similar extremities, similar skulls. And that twisted heel-bone (calcaneum) is a derived trait. So why are multis supposed to nest with mammals earlier than placentals?

If anyone can produce a pre-therian that attracts multis, please bring it to my attention. So far, I have failed to find out, and so multis continue to nest with rodents and plesiadapids.

References
Kielan-Jaworowska Z and Hurum JH 2001. Phylogeny and Systematics of multituberculate mammals. Paleontology 44, 389–429.

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.

 

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

Insectivores and Rodents (including a Multituberculate) added to the LRT

Today several Insectivores (clade within the Mammalia)
and other taxa are added to the large reptile tree (now 744 taxa). And, no surprise, the little fur-balls nest together (subset Fig. 1). Talpa is a mole. Scutisorex is a shrew. Solenodon is, well, a solenodon!

Figure 1. the mammal subset of the large reptile tree, still fully resolved. Here insectivores nest with rodents and rabbits. Plesiadapis is often traditionally nested with primates in analyses performed by others, but does not do so here.

Figure 1. The mammal subset of the large reptile tree, still fully resolved. Here insectivores nest with rodents and rabbits. Plesiadapis is often traditionally nested with primates in analyses performed by others, but does not do so here. If some of these taxa are unfamiliar, you can Google them to retrieve their more familiar names. It’s a pretty simple tree. And, doggone it, it makes sense, too!

As you can see,
Tenrecs are no longer insectivores. Insectivores are sisters to Solenodon and that clade is a sister to the rodents + Rugosodon (Jurassic, Yuan et al. 2013), a multiuberculate mammal. Rugosodon is the oldest multituberculate so far described, so it should be one of the least derived. They have flexible ankles, according to Wikipedia.

About multituberculates
Wikipedia reports, “Multituberculates are extinct rodent-like mammals usually placed outside either of the two main groups of living mammals—Theria, including placentals and marsupials, and Monotremata[9]—but closer to Theria than to monotremes. (references on that page). Sorry to report this, but after testing in the large reptile tree, multituberculates appear to be rodent-like because they are rodent sisters. If you have any suggestions for taxa that might attract Rugosodon away from rodents, toward the base of the Mammalia, please pass them on.

A few carnivores were added.
Phoca is a seal. Procyon is a raccoon (and a bright star).

If you want to check my work
I’ll have images and pages set up at ReptileEvolution.com after the upcoming weekend. Otherwise, nearly all the images I used for data were Googled. So you can find the same data online.

References
Yuan CX, Ji Q, Meng QJ, Tabrum AR, Luo ZX 2013. Earliest evolution of multituberculate mammals revealed by a new Jurassic fossil”. Science 341 (6147): 779–783. doi:10.1126/science.1237970.