Vilevolodon: the atavistic reappearance of post-dentary bones

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.

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.

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)

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)
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.


Vintana and the vain search for the clades Allotheria and Gondwanatheria

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.

Earlier we looked at Vintana (Fig. 1, Krause et al. 2014a, b). To Krause et al. Vintana represented the first specimen in the clades Allotheria and Gondwanatheria to be known from more than teeth and minimal skull material.

To Krause et al. 
Allotheria included Multituberculata and nested between the clade Eutriconodonta (including Repenomamus and Jeholodens) and the clade Trechnotheria (including the spalacotheres Maotherium and Akidolestes) and Cronopio, Henkelotherium, Juramaia, Eomaia, Eutheria and Metatheria.

Taxon exclusion issues
The large reptile tree (LRT, 1005 taxa) did not recover the above clades or relationships. Alotheria does not appear in the LRT.

  1. Multituberculata, Henkelotherium and Maotherium nest within Glires (rats and rabbits and kin) in the LRT.
  2. Repenomamus and Jeholodens nest within the pre-mammalian trityllodontid cynodonts in the LRT.
  3. Akidolestes nests within basal Mammalia, close to Ornithorhynchus in the LRT.
  4. Cronopio and Juramaia nest within basal Mammalia between Megazostrodon and Didelphis in the LRT.
  5. Eomaia nests at the base of the Metatheria in the LRT.
  6. Vintana nests with Interatherium among the derived Metatheria (marsupials), with wombats, like Vombatus and Toxodon in the LRT.

Despite a paper in Nature
and a memoir of 222 pages in the Journal of Vertebrate Paleontology; despite CT scans and firsthand examination with electron microscopes; despite being examined and described by many of the biggest name and heavy hitters in paleontology… Krause et al. never understood that Vintana was just a derived wombat, evidently due to taxon exclusion problems.

Figure 3. Interatherium does not nest with notoungulates or other purported interotheres. Rather cat-sized Interatherium nests with wombats, between Vombatus and the giant Toxodon.

Figure 2. Interatherium does not nest with notoungulates or other purported interotheres. Rather cat-sized Interatherium nests with wombats,with Vintana,  between Vombatus and the giant Toxodon

The large reptile tree now includes
1005 taxa, all candidates for sisterhood with every added taxon. Despite the large gamut of 74 taxa employed by Krause et al. they did not include the best candidates for Vintana sisterhood. Perhaps the fault lies in the reliance of prior studies and paradigms. Perhaps the fault lies in the over reliance by Krause et al. and other mammal workers, on dental traits. Perhaps the fault lies in the absence of pertinent sisters to the above-named taxa, including Interatheriium for Vintana.

In any case
Vintana does not stand alone as the only taxon in its clade represented by skull material. Based on its sisterhood with Interatherium, we have  pretty good idea what its mandibles and post-crania looked like. Yes, Vintana is weird. But Interatherium is also weird in the same way, just not as weird.

The LRT has dismantled and invalidated
several other clades, too, Ornithodira and Parareptilia among them.

Krause DW, Hoffmann S, Wible JR, Kirk EC, and several other authors 2014a. First cranial remains of a gondwanatherian mammal reveal remarkable mosaicism. Nature. online. doi:10.1038/nature13922. ISSN 1476-4687.
Krause DW et al. 2014b. Vintana sertichi (Mammalia, Gondwanatheria) from the Late Cretaceous of Madagascar. Journal of Vertebrate Paleontology Memoir 14. 222pp.

pterosaur heresies – Vintana

What is Darwinius?

Franzen et al. 2009
reported on a well-preserved small primate from 50mya named Darwinius.

From the Franzen et al. 2009 conclusion:
“Darwinius masillae represents the most complete fossil primate ever found, including both skeleton, soft body outline and contents of the digestive tract. Study of all these features allows a fairly complete reconstruction of life history, locomotion, and diet. Any future study of Eocene-Oligocene primates should benefit from information preserved in the Darwinius holotype. Of particular importance to phylogenetic studies, the absence of a toilet claw and a toothcomb demonstrates that Darwinius masillae is not simply a fossil lemur, but part of a larger group of primates, Adapoidea, representative of the early haplorhine diversification.”

In a published comment Beard 2009 wrote:
“Unbridled hoopla attended the unveiling of a 47-million-year-old fossil primate skeleton at the American Museum of Natural History in New York on 19 May. Found by private collectors in 1983 in Messel, Germany, the press immediately hailed the specimen as a “missing link” and even the “eighth wonder of the world.”

“Overall proportions and anatomy resemble that of a lemur, and the same is true for other adapiform primates. A new genus and species of adapiform primate, Darwinius masillae (Franzen et al., 2009; Eocene, 50 mya ). The adapids are a branch of the primate tree that leads to modern lemurs. Ida would have to have anthropoid-like features that evolved after anthropoids split away from lemurs and other early primates. Here, alas, Ida fails miserably.” The reasons for that “fail” were not listed in the Beard note.

Taxon exclusion?
The large reptile tree, (LRT, 896 taxa), currently tests only a few primates. At this stage, Darwinius does indeed nest at the base of higher primates (simians), alongside Tarsius, the extant tarsier, but there are many dozen primate taxa that have not been included in the LRT.

Figure 1. Darwinius overall plus an X-ray showing the transition from milk teeth to adult teeth in this juvenile specimen.

Figure 1. Darwinius overall plus an X-ray showing the transition from milk teeth to adult teeth in this juvenile specimen.

In the LRT,
nesting only a few primates at present, the adapid prosimian, Notharctus, is basal to higher primates including humans (genus Homo). Tarsius, the tarsier, nests between Notharctus and Proconsul, a basal anthropoid (ape). Darwinius nests with Tarsius, but lacks the many specialized autapomorphies that characterize extant tarsiers like:

  1. oversized eyes
  2. distally fused tibiafibula
  3. elongated pedal digits 4 and 5.
  4. hyperelongated astragalus and calcaneum
  5. cervicals insert further beneath the skull
Figure 2. Tarsius, the extant tarsier. Note the several autapomorphies displayed here vs. the many plesiamorphies in Darwinius.

Figure 2. Tarsius, the extant tarsier. Note the several autapomorphies displayed here vs. the many plesiamorphies in Darwinius.

Wikipedia reports
“Most experts hold that the higher primates (simians) evolved from Tarsiidae, branching off the Strepsirrhini before the appearance of the Adapiformes.” If true, Darwinius is close to the lineage of humans. “A smaller group agrees with Franzen et al. that the higher primates descend from Adapiformes (Adapoidea). The view of paleontologist Tim White is that Darwinius is unlikely to end the argument.” 

NBC news reports,
here that “Ida is as far removed from the monkey-ape-human ancestry as a primate could be, says Erik Seiffert of Stony Brook University in New York. The new analysis says Darwinius does not belong in the same primate category as monkeys, apes and humans. Instead, the analysis concluded, it falls into the other major grouping, which includes lemurs.”

Nature reports on the “media frenzy”
here in a paper entitled: A hyped-up fossil find highlights the potential dangers of publicity machines.  To be fair, the authors’ claims at the press conference were appropriately measured. Nonetheless, the researchers were fully involved in the documentaries and the media campaign, which associate them with a drastic misrepresentation of their research.”

“Another damaging aspect of the events was the unavailability of the paper ahead of the press conference and initial media coverage. This prevented scientists other than those in the team from assessing the work and thereby ensuring that journalists could give a balanced account of the research.

“There is no reason to think that PLoS ONE’s editors and reviewers did less than their duty to the paper. Nonetheless, the clock was ticking at the time of submission.”

“In principle, there is no reason why science should not be accompanied by highly proactive publicity machines. But in practice, such arrangements introduce conflicting incentives that can all too easily undermine the process of the assessment and communication of science.”

The primate experts can hash this out.
At present, with so few primates tested, Darwinius is still a candidate to be at the transition from prosimian to simian in the LRT, as it presently nests… until additional taxa knock it out.

Added within minutes of posting
I ran across this reference:
Gingerich PD et al. 2010. Darwinius masillae is a Haplorhine — Reply to  Williams et al. (2010). Journal of Human Evolution. 59(5)574-576 where they report, “Williams et al. (2010) imply that ‘total evidence’ means study of hundreds of characters in a great many taxa. However, total evidence is about combining data before analysis and not about the size of the resulting matrix. “We agree with Seiffert et al., 2009 and Williams et al., 2010, and others that there is a strepsirrhine–haplorhine dichotomy in primate evolution. We employ the same cladistic methods. We accept that total evidence drawn from many sources is advantageous. Why then do we reach such a different conclusion about the systematic position of Darwinius? Given that our methods are the same, then our contrasting results can only be explained by differences in the number and balance of taxa chosen for study, the character matrix used to analyze higher-level primate phylogeny, the outgroup chosen to root a phylogenetic network, or some combination of these.”
More details on their arguments are found here.

Beard C 2009. Why Ida is fossil is not the missing link. Comment, NewScientist.
Online here.
Franzen JL, Gingerich PD, Habersetzer J, Hurum JH, Von Koenigswald W and Smith BH 2009. Complete primate skeleton from the Middle Eocene of Messel in Germany: morphology and paleobiology PLoS ONE. 4 (5): e5723.

wiki/Darwinius article that touches on Darwinius