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Introduction

  • The middle ear of modern mammals has evolved significantly from that of their synapsid predecessors.

  • It is detached from the mandible and has a soft-tissue eardrum, allowing for enhanced hearing capabilities encompassing a wide range of frequencies.

  • This study focuses on the fossil evidence and finite element analysis (FEA) of the middle ear function in the synapsid Thrinaxodon to understand early mammalian auditory evolution.

Significance

  • The research highlights the evolutionary importance of the middle ear in synapsids, particularly pertaining to how these structures evolved from jaw bones.

  • The findings suggest that Thrinaxodon, an ancient cynodont, possessed the capability for tympanic hearing similar to modern mammals, indicating that significant auditory advancements occurred early in mammal evolution.

Author Contributions

  • Research was designed by A.T.W., C.F.R., and Z.-X.L.

  • Research performed by A.T.W. and Z.-X.L.

  • New reagents and analytic tools contributed by C.C.G.S., C.F.R., and Z.-X.L.

  • Data analyzed by A.T.W.

  • Manuscript written by A.T.W., C.C.G.S., C.F.R., and Z.-X.L.

Evolution of the Middle Ear

  • Evolutionary evidence shows that mammalian middle ear bones (malleus, incus, ectotympanic) originated from the postdentary bones of Paleozoic therapsids and Mesozoic cynodonts via detachment from the mandible, changes in shape, and size reduction.

  • The ectotympanic serves as an attachment point for the tympanic membrane, which evolved from a precursor membrane in earlier synapsids.

  • The shifts in middle ear morphology resulted in enhanced auditory function, moving from dual functions in chewing and hearing to a more specialized auditory apparatus.

Functional Significance of the Mandibular Ear

  • Thrinaxodon functioned with both hearing and feeding roles attributed to its middle ear structures.

  • The articular bone (malleus homolog) and the quadrate bone (incus homolog) acted as load-bearing jaw joints while also operating within the auditory system.

  • Hypotheses suggest that postdentary bones could transmit sound through bone conduction, a method still present in many tetrapods today.

  • There is limited understanding of how bone conduction versus airborne sound affects the hearing sensitivity and frequency range in cynodonts.

Hypotheses and Objectives

  • This study hypothesizes that the tympanic membrane played a vital role in Thrinaxodon’s auditory capabilities, potentially relying on sound reception through airborne conduction more than through bone conduction.

  • The research aims to perform a finite element analysis comparing the efficacy of these sound transmission routes in Thrinaxodon, particularly focusing on their respective auditory performance across various frequencies.

Materials and Methods

Fossil Specimens and CT Scanning

  • The study is based on anatomical models derived from micro-computed tomography (µCT) scans of two Thrinaxodon specimens: UCMP V40466 and BP/1/7199.

  • Digital reconstructions of the mandible and middle ear bones were performed to analyze hearing function.

Finite Element Modeling

  • Finite element modeling (FEM) techniques were used to calculate auditory response in Thrinaxodon.

  • The model included various parts of the middle ear, and the harmonic response was examined under various sound pressure scenarios.

Results

Analysis of Auditory Parameters

  • Key auditory parameters like lower frequency limits, best frequency, and hearing thresholds were estimated.

  • Estimated hearing range for Thrinaxodon reached from 10 Hz to 2,650 Hz with the highest sensitivity around 1,000 Hz.

  • The tympanic membrane was confirmed as the primary sound receiver yielding favorable responses for hearing sensitivity compared to the other components powered by bone conduction.

Comparative Analysis of Sound Conducting Routes

  • Various sound conduction routes were analyzed, revealing that the tympanum alone yielded sufficient pressure at the stapedial footplate to enable hearing across the frequency range.

  • Bone conduction from the reflected lamina was less effective, struggling to meet hearing thresholds alone.

Discussion

  • The findings reinforce the notion that the soft-tissue tympanum significantly enhances Hearing capabilities, offering superior sensitivity to airborne sound compared to bone conduction alternatives.

  • Analysis indicates that Thrinaxodon provides evidence of an early transition to tympanic hearing dating back much earlier in synapsid evolution than previously understood, showcasing a functional tympanum by leveraging direct airborne sound reception.

  • The evolutionary trajectory of the tympanum illustrates varying functional demands throughout synapsid-mammal evolution, ultimately reflecting adaptations favoring auditory sensitivity in diverse environments.

Conclusion

  • The research provides compelling evidence for the evolutionary significance of the tympanic middle ear in early mammals.

  • It establishes Thrinaxodon as a pivotal link in understanding the morphological and functional advancements accompanying the evolution of mammalian hearing.

Acknowledgments

  • Gratitude is extended to contributors from the University of California Museum of Paleontology and all individuals involved in facilitating this research, alongside funding from the University of Chicago and NSF.

References

  • A list of references cited throughout the study is provided to support the methodology and discussions presented in the research article.