 |
| Dianoconodon youngi Mao, C. Zhang, Ren, T. Wang, G. Wang, F. Zhang, Rich, Vickers-Rich & Meng, 2024 Reconstruction by Chuang Zhao |
Abstract
The dual jaw joint of Morganucodon 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 evolution. Here we report mandibular middle ears from two Jurassic mammaliaforms: a new morganucodontan-like species and a pseudotribosphenic shuotheriid species. The morganucodontan-like species shows many previously unknown post-dentary bone morphologies 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 dentary and the eventual breakdown of the Meckel’s cartilage during the evolution of mammaliaforms.
 |
| Reconstruction of the newly described species Dianoconodon youngi. illustration by Chuang Zhao |
Mammaliaformes Rowe, 1988
Dianoconodon youngi gen. et. sp. nov.
Holotype. A complete skull with associated dentitions and partialpostcranial skeleton (IVPP V4257, Institute of Vertebrate Paleontologyand Paleoanthropology, Chinese Academy of Sciences, Beijing) (Figs. 1and 2; Methods and Extended Data Figs. 1–6).
Etymology. Dian, the short form (Chinese phonetic Pinyin) of Yunnan Province, where the Lufeng fossil site is located; konos, Greek ‘cone’ or ‘cusp’; odon, Greek ‘tooth’. The specific name honours C. C. Young, who initiated the study of mammaliamorphs in Lufeng.
 |
| Mandibular middle ears and transformation of the articular-quadrate joints in mammaliaforms. |
Fangyuan Mao, Chi Zhang, Jicheng Ren, Tao Wang, Guofu Wang, Fakui Zhang, Thomas Rich, Patricia Vickers-Rich and Jin Meng. 2024. Fossils document Evolutionary Changes of jaw joint to mammalian middle ear. Nature. DOI: doi.org/10.1038/s41586-024-07235-0 [03 April 2024]
