A comparative study on auditory and hyoid bones of Jurassic euharamiyidans and contrasting evidence for mammalian middle ear evolution

Fossils document evolutionary changes of jaw joint to mammalian middle ear

The dual jaw joint of Morganucodon1,2 consists of the dentary-squamosal joint laterally and the articular-quadrate one medially. The articular-quadrate joint and its associated post-dentary bones constitute the precursor of the mammalian middle ear. Fossils documenting the transition from such a precursor to the mammalian middle ear are poor, resulting in inconsistent interpretations of this hallmark apparatus in the earliest stage of mammaliaform evolution1-5. Here we report mandibular middle ears from two Jurassic mammaliaforms: a new morganucodontan-like species and a pseudotribosphenic shuotheriid species6. The morganucodontan-like species shows many previously unknown post-dentary bone morphologies1,2 and exhibits features that suggest a loss of load-bearing function in its articular-quadrate joint. The middle ear of the shuotheriid approaches the mammalian condition in that it has features that are suitable for an exclusively auditory function, although the post-dentary bones are still attached to the dentary. With size reduction of the jaw-joint bones, the quadrate shifts medially at different degrees in relation to the articular in the two mammaliaforms. These changes provide evidence of a gradual loss of load-bearing function in the articular-quadrate jaw joint-a prerequisite for the detachment of the post-dentary bones from the dentary7-12 and the eventual breakdown of the Meckel's cartilage13-15 during the evolution of mammaliaforms.

Geometric parameters sensitivity evaluation based on projection trajectories for X-ray cone-beam computed laminography

BACKGROUND

X-ray cone-beam computed laminography (CL) is widely used for large flat objects that computed tomography (CT) cannot investigate. The rotation angle of axis tilt makes geometric correction of CL system more complicated and has more uncertain factors. Therefore, it is necessary to evaluate sensitivity of the geometric parameters of CL system in advance.

OBJECTIVE

This study aims to objectively and comprehensively evaluate sensitivity of CL geometric parameters based on the projection trajectory.

METHODS

This study proposes the Minimum Deviation Unit (MDU) to evaluate sensitivity of CL geometric parameters. First, the projection trajectory formulas are derived according to the spatial relationship of CL system geometric parameters. Next, the MDU of the geometric parameters is obtained based on the projection trajectories and used as the evaluation index to measure the sensitivity of parameters. Then, the influence of the rotation angle of the axis tilt and magnification on the MDU of the parameters is analyzed.

RESULTS

At low magnification, three susceptible parameters (η, u0, v0) with MDU less than 1 (° or mm) must be calibrated accurately to avoid geometric artifacts. The sensitivity of CL parameters increases as the magnification increases, and all parameters become highly sensitive when the magnification power is greater than 10.

CONCLUSION

The results of this study have important guiding significance for the subsequent further parameter calibration.

Jurassic mammaliaform petrosals from Western Siberia (Russia) and implications for early mammalian inner-ear anatomy

Five partially preserved mammaliaform petrosals recovered from Middle Jurassic sediments of the Berezovsk coal mine (Krasnoyarsk Krai, Russia) show similarities to other early mammaliaforms like the morganucodontan Morganucodon and the docodontan Haldanodon in having an inflated promontorium and a curved and apically inflated cochlear canal, but they are distinct from dryolestoid and derived mammalian petrosals by the weak coiling of the cochlear duct and the presence of a perilymphatic foramen with an open perilymphatic sulcus. The two larger and robust specimens exhibit striking similarities to docodontan petrosals. Inside the bone an intricate circumpromontorial venous plexus was discovered, as recently described for the docodontan Borealestes, confirming that this structure is consistently present in basal non-mammalian mammaliaforms. The three smaller and slender petrosals probably belong to haramiyidans and are unique in showing a septum-like structure medially along the cochlear nerve entrance. The protruding perforated bony bar, which is preserved in two of the three, is interpreted here to be a remnant of a bony septum with multiple foramina for cochlear nerve fibres, representing an autapomorphic feature of Haramiyida. This newly described passageway for nerve fibres shows that the formation of the osteological structure surrounding the nervous pathways of the cochlea is more plastic among the non-mammalian mammaliaforms than previously thought.

Exploring ancestral phenotypes and evolutionary development of the mammalian middle ear based on Early Cretaceous Jehol mammals

We report a new Cretaceous multituberculate mammal with 3D auditory bones preserved. Along with other fossil and extant mammals, the unequivocal auditory bones display features potentially representing ancestral phenotypes of the mammalian middle ear. These phenotypes show that the ectotympanic and the malleus-incus complex changed notably during their retreating from the dentary at various evolutionary stages and suggest convergent evolution of some features to extant mammals. In contrast, the incudomalleolar joint was conservative in having a braced hinge configuration, which narrows the morphological gap between the quadroarticular jaw joint of non-mammalian cynodonts and the incudomalleolar articulations of extant mammals. The saddle-shaped and abutting malleus-incus complexes in therians and monotremes, respectively, could have evolved from the braced hinge joint independently. The evolutionary changes recorded in the Mesozoic mammals are largely consistent with the middle ear morphogenesis during the ontogeny of extant mammals, supporting the relation between evolution and development.

A monotreme-like auditory apparatus in a Middle Jurassic haramiyidan

Among extant vertebrates, mammals are distinguished by having a chain of three auditory ossicles (the malleus, incus and stapes) that transduce sound waves and promote an increased range of audible-especially high-frequencies¹. By contrast, the homologous bones in early fossil mammals and relatives also functioned in chewing through their bony attachments to the lower jaw². Recent discoveries of well-preserved Mesozoic mammals have provided glimpses into the transition from the dual (masticatory and auditory) to the single auditory function for the ossicles, which is now widely accepted to have occurred at least three times in mammal evolution^3-6. Here we report a skull and postcranium that we refer to the haramiyidan Vilevolodon diplomylos (dating to the Middle Jurassic epoch (160 million years ago)) and that shows excellent preservation of the malleus, incus and ectotympanic (which supports the tympanic membrane). After comparing this fossil with other Mesozoic and extant mammals, we propose that the overlapping incudomallear articulation found in this and other Mesozoic fossils, in extant monotremes and in early ontogeny in extant marsupials and placentals is a morphology that evolved in several groups of mammals in the transition from the dual to the single function for the ossicles.

Tip dating supports novel resolutions of controversial relationships among early mammals

The estimation of the timing of major divergences in early mammal evolution is challenging owing to conflicting interpretations of key fossil taxa. One contentious group is Haramiyida, the earliest members of which are from the Late Triassic. Many phylogenetic analyses have placed haramiyidans in a clade with multituberculates within crown Mammalia, thus extending the minimum divergence date for the crown group deep into the Triassic. A second taxon of interest is the eutherian Juramaia from the Middle-Late Jurassic Yanliao Biota, which is morphologically very similar to eutherians from the Early Cretaceous Jehol Biota and implies a very early origin for therian mammals. Here, we apply Bayesian tip-dated phylogenetic methods to investigate these issues. Tip dating firmly rejects a monophyletic Allotheria (multituberculates and haramiyidans), which are split into three separate clades, a result not found in any previous analysis. Most notably, the Late Triassic Haramiyavia and Thomasia are separate from the Middle Jurassic euharamiyidans. We also test whether the Middle-Late Jurassic age of Juramaia is 'expected' given its known morphology by assigning an age prior without hard bounds. Strikingly, this analysis supports an Early Cretaceous age for Juramaia, but similar analyses on 12 other mammaliaforms from the Yanliao Biota return the correct, Jurassic age. Our results show that analyses incorporating stratigraphic data can produce results very different from other methods. Early mammal evolution may have involved multiple instances of convergent morphological evolution (e.g. in the dentition), and tip dating may be a method uniquely suitable to recognizing this owing to the incorporation of stratigraphic data. Our results also confirm that Juramaia is anomalous in exhibiting a much more derived morphology than expected given its age, which in turn implies very high rates of evolution at the base of therian mammals.