Early Cretaceous Jeholbaatar kielanae: middle ear origin or reversal?

FIgure 1. Tiny Jeholbaatar in situ, full scale.

FIgure 1. Tiny Jeholbaatar in situ, full scale.

Wang, Meng  and Wang 2019 introduce us 
to a tiny new multituberculate, Jeholbaatar kielanae (Figs. 1, 2), in which the much tinier and displaced middle ear bones are found in articulation (Figs. 3, 4) along with a displaced surangular!

Figure 2. Jeholbaatar with certain bones colorized using DGS methods.

Figure 2. Jeholbaatar with certain bones colorized using DGS methods.

Figure 5. Jeholobaatar images from Wang, Meng and Weng 2019. Rat ear bones photo from Li, Gao, Ding and Salvi 2015. Correction label added here. The rat middle ear, no surprise, is phylogenetically similar to that of the multituberculates, Jeholbaatar and Arboroharamiya.

Figure 3. Jeholobaatar images from Wang, Meng and Weng 2019. Rat ear bones photo from Li, Gao, Ding and Salvi 2015. Correction label added here. The rat middle ear, no surprise, is phylogenetically similar to that of the multituberculates, Jeholbaatar and Arboroharamiya.

Figure 4. The tiny displaced middle ear bones of Jeholbaatar colorized

Figure 4. The tiny displaced middle ear bones of Jeholbaatar colorized

Wang, Meng and Wang present two cladograms
(Fig. 3) of multituberculate relationships. The LRT agrees with the ‘a’ oversimplified cladogram, the one that does not split Arboroharamiya from Jeholobaatar. 

In similar fashion,
the large reptile tree (LRT, 1612+ taxa) nests multituberculates as derived rodents, despite what appears to be, in this phylogenetic context, a more primitive middle ear morphology than found in extant rats (Rattus, Fig. 4), mice (Mus) and their closest living relatives, the aye-aye (Daubentonia). In the LRT Jeholbaatar nests at the base of the Villevolodon + Shenshou clade.

Tradition continues in Wang, Meng and Wang
as they omit rodents from multituberculate studies while force fitting multis into a dissimilar nesting with basal mammals, including prototheres, the egg-laying mammals.

It is worthwhile comparing the skulls
of the rodent, Rattus (Fig. 6), and the multituberculate, Kryptobaatar (Fig. 5). This is much more than convergence. That is why they nest close to one another in the LRT, but not as sisters.

Figure 1. Animation of the mandible of the multituberculate Kryptobaatar showing the sliding of the jaw joint producing separate biting and grinding actions, just like rodents, their closest relatives in the LRT.

Figure 5. Animation of the mandible of the multituberculate Kryptobaatar showing the sliding of the jaw joint producing separate biting and grinding actions, just like rodents, their closest relatives in the LRT. In multis the middle ear bones remain attached to the posterior dentary and ride along with jaws as they slide distinct from most placental taxa. Note the lack of a middle ear (tympanic) bulla. Compare to figure 1.

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

Figure 6 The brown rat (Rattus norvegicus) skull has a mandible glenoid separate from the retroarticular process. Whenever the jaw slides back and forth, these two processes slide above and below the middle ear (tympanic) bulla without interfering with the ear canal. Compare to figure 2.

From the Wang, Meng and Wang 2019 abstract:
“The evolution of the mammalian middle ear is thought to provide an example of ‘recapitulation’—the theory that the present embryological development of a species reflects its evolutionary history.”

This has been documented in embryo dissections.

“Accumulating data from both developmental biology and palaeontology have suggested that the transformation of post-dentary jaw elements into cranial ear bones occurred several times in mammals.”

In the LRT this happened once in crown mammals, and once again in Repenomamus, a pre-mammal, Cretaceous mammal-mimic

“In addition, well-preserved fossils have revealed transitional stages in the evolution of the mammalian middle ear. But questions remain concerning middle-ear evolution, such as how and why the post-dentary unit became completely detached from the dentary bone in different clades of mammaliaforms.”

The LRT does not recognize the clade Mammaliaformes, defined as, “the clade originating from the most recent common ancestor of Morganucodonta and the crown group mammals.” Morganucodon is a basal metatherian in the LRT.

“Here we report a definitive mammalian middle ear preserved in an eobaatarid multituberculate mammal, with complete post-dentary elements that are well-preserved and detached from the dentary bones. The specimen reveals the transformation of the surangular jaw bone from an independent element into part of the malleus of the middle ear, and the presence of a restricted contact between the columelliform stapes and the flat incus.”

Congratulations to the preparator. The middle ear bones are microscopic.

“We propose that the malleus–incus joint is dichotomic [= classification based upon two opposites] in mammaliaforms, with the two bones connecting in either an abutting or an interlocking arrangement, reflecting the evolutionary divergence of the dentary–squamosal joint. In our phylogenetic analysis, acquisition of the definitive mammalian middle ear in allotherians such as this specimen was independent of that in monotremes and therians.”

The LRT does not recover the clade Allotheria. Rather multituberculates nest within Glires, within Rodentia close to Rattus (Fig. 4), Carpolestes and the aye-aye, Daubentonia.

“Our findings suggest that the co-evolution of the primary and secondary jaw joints in allotherians was an evolutionary adaptation allowing feeding with unique palinal (longitudinal and backwards) chewing. Thus, the evolution of the allotherian auditory apparatus was probably triggered by the functional requirements of the feeding apparatus.”

Unfortunately,
the authors do not compare their find to rodents. Note the similarity of the middle ear bones in Rattus (Fig. 3) to those of multituberculates. So, once again: taxon exclusion spoils a perfectly grand discovery and description.

Figure 4. Evolution of the tetrapod mandible and ear bones leading to humans.

Figure 7. Evolution of the tetrapod mandible and ear bones leading to humans.

A little backstory:
As mentioned by Wang, Meng and Wang, a mammal embryo develops ontogenetically it more or less recapitulates its entire phylogenetic development, from one cell, to a ball of cells, to an embryo with gills, to an embryo with several primordial jaw bones. Three of these detach from the dentary in placentals, migrate posteriorly and become middle ear bones.

Multis are different.
Multis appear to have large, attached, more primitive middle ear bones, like those found in egg-laying pre-mammals. The question few appear to have asked is: why would this happen?

Multis are renown
for losing (or never developing in the tradition model) their cylinder and socket jaw joint to develop a sliding jaw joint, harnessed by large muscles. A sliding jaw joint is also present in rodents.

Here’s a thought:
Perhaps nature found it more important for the jaw joint to slide posteriorly in multis, over the spot where tiny ear bones are found in rodents, so the middle ear bones remained in a state of arrested development, more or less attached to the posterior dentary, moving along with the sliding jaw, never attaching themselves to the base of the braincase, as in other placental mammals. This can happen by a simple matter of stopping the development of primitive large ear bones to tiny ear bones. Evidently this reversal was a successful gambit, as multis survived from the Jurassic deep into the Tertiary before finally going extinct for reasons unknown.

For the general public (popular press),
the following online article describes the specimen and its authors.

“Researchers from the Institute of Vertebrate Paleontology and Paleoanthropology (IVPP) of the Chinese Academy of Sciences and the American Museum of Natural History (AMNH) have reported a new species of multituberculate – a type of extinct Mesozoic “rodent” – with well-preserved middle ear bones from the Cretaceous Jehol Biota of China. The findings were published in Nature on November 27.”

Note the use of quotation marks around the word “rodent” indicate their understanding that Jeholbaatar was not a rodent, but looked like one.

“The new mammal, Jeholbaatar kielanae, has a middle ear that is distinct from those of its relatives. WANG Yuanqing and WANG Haibing from IVPP, along with MENG Jin from AMNH, proposed that the evolution of its auditory apparatus might have been driven by specialization for feeding.”

According to the LRT, it is not the evolution of an auditory apparatus, but the reversal to to a more primitive state, because sisters all have tiny middle ear bones.

“Fossil evidence shows that postdentary bones were either embedded in the postdentary trough on the medial side of the dentary or connected to the dentary via an ossified Meckel’s cartilage in early mammals, prior to their migration into the cranium as seen in extant mammals.”

See figure 7 for an illustration of this trough and migration in several taxa.

“Detachment of the mammalian middle ear bones from the dentary occurred independently at least three times. But how and why this process took place in different clades of mammals remains unclear.”

It remains unclear in the Wang, Meng and Wang paper due to taxon exclusion, leading to an invalid tree topology. They why was likely due to the nocturnal and arboreal displacement of surviving mammals, requiring better hearing abilities, during the age of dinosaurs, which were their chief predators during the day.

“The Jeholbaatar kielanae specimen was discovered in the Jiufotang Formation in China’s Liaoning Province. It displays the first well-preserved middle-ear bones in multituberculates, providing solid evidence of the morphology and articulation of these bony elements, which are fully detached from the dentary.”

Fully detached and displaced, between the teeth.

“It reveals a unique configuration with more complete components than those previously reported in multituberculates. The new fossil reveals a transitional stage in the evolution of the surangular – a “reptilian” jawbone.”

Remember, ontogeny recapitulates phylogeny, and Jeholbaatar is a tiny specimen (Fig. 1), a precocial and phylogenetically miniaturized taxon, retaining juvenile traits, including a middle ear in an arrested state of development.

“In light of current evidence, scientists argue that the primary (malleus-incus) and secondary (squamosal-dentary) jaw joints co-evolved in allotherians, allowing a distinct palinal (anteroposterior) jaw movement while chewing.” 

In the LRT anteroposterior jaw movement was well established in more primitive taxa (Figs, 3, 4). The LRT does not recognize the traditional clade, ‘Allotheria’.

“Selective pressure to detach the middle ear bones could have been stronger in order to increase feeding efficiency, suggesting that evolution of the middle ear was probably triggered by functional constraints on the feeding apparatus in allotherians.”

Actually, just the opposite, according to the LRT.  Primitive placentals already had, for millions of generations, tiny middle ear bones. In multis alone neotony + phylogenetic miniaturization led to the arrested development of the middle ear bones, which moved along with the dentary during palinal (anteroposterior) jaw movement. I suggest workers add more taxa to their phylogenetic analyses. Test rodents and their relatives along with multituberculates and see what pops out.


References
Li P, Gao K, Ding D and Salvi R 2015. Characteristic anatomical structures of rat temporal bone. ScienceDirect Journal of Otology 10:118–124.
Wang H, Meng J and Wang Y-Q 2019. Cretaceous fossil reveals a new pattern in mammalian middle ear evolution. Nature  online

http://english.cas.cn/newsroom/research_news/life/201911/t20191127_226412.shtml

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