New fossils from the Jurassic period have been found by an international team of paleontologists led by the American Museum of Natural History and the Chinese Academy of Sciences. These discoveries offer new insights into the early history of mammals. The discoveries, which were reported in two consecutive papers published in the journal Nature, may alter the way that researchers reconstruct the earliest branches of the mammalian tree of life.
The shuotheriid family of mice-sized mammals—whose molars differ from those of any other living mammal—is the subject of the first study. Researchers observed that the molars of two recently uncovered and well-preserved shuotheriid skeletal remains from Inner Mongolia, which are thought to have lived between 168 and 164 million years ago, resembled the molars of another extinct mammal group known as the docodontans. Furthermore, they concluded that the two specimens are part of a recently discovered genus and species, which they have called Feredocodon chowi.
Fig 1 Primary tooth patterns of mammaliaforms in the phylogenetic frame (From the published paper)
The fossil skulls of two new species, Dianoconodon youngi and Feredocodon chowi, which lived between 201 and 184 million years ago, served as the basis for the second study, which was also headed by Meng and Mao. The middle ear, which provides modern mammals with the best hearing on Earth, was the structure that the researchers examined. Three bones, known as auditory ossicles, are specific to mammals and are found in the middle ear of modern mammals. This region sits just inside the eardrum and converts air vibrations into ripples in the fluids of the inner ear.
The new fossils, which start with an ancestral mammal having a double jaw joint, offer compelling fossil evidence of this shift in progress. Examinations of the more aged fossil (Dianoconodon youngi) reveal that the reptile joint, one of the two, was beginning to lose in its capacity to withstand the stresses produced by chewing. Feredocodon chowi, the younger individual, already has a mammal middle ear that was developed and suited solely for hearing.
Fig2 Mandibular middle ears and transformation of the articular–quadrate joints in mammaliaforms (From the published paper)
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CITATIONS
Mao, F., Li, Z., Wang, Z. et al. Jurassic shuotheriids show earliest dental diversification of mammaliaforms. Nature 628, 569–575 (2024). https://doi.org/10.1038/s41586-024-07258-7
Mao, F., Zhang, C., Ren, J. et al. Fossils document evolutionary changes of jaw joint to mammalian middle ear. Nature 628, 576–581 (2024). https://doi.org/10.1038/s41586-024-07235-0
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