Tritylodontids are close relatives of mammals with specialized teeth adapted for herbivory. Despite their diversification during the Jurassic, the fossil record of this clade suggests they declined significantly in the Cretaceous when they are mainly represented by fragmentary dental remains. The exception is the Early Cretaceous taxon Fossiomanus sinensis Mao et al., 2021. Here we describe a new mandible of this species from the same locality as the holotype specimen. The new specimen provides more complete information on mandible shape and tooth morphology, filling a knowledge gap for this iconic Cretaceous tritylodontid, given that cranial morphology in the holotype of F. sinensis remains insufficiently studied. Additionally, the fossil record of F. sinensis represents the youngest known tritylodontid (~119 Ma, Aptian) and the latest non-mammaliaform cynodont, shedding light on the evolutionary history of early mammalian relatives.

Haibing Wang [wanghaibing@ivpp.a.c.cn, ORCID: https://orcid.org/0000-0001-6811-7262], Zifan Zhu [zhuzifan24@mails.ucas.ac.cn; ORCID: https://orcid.org/0009-0008-6938-4774], and Yuanqing Wang [wangyuanqing@ivpp.ac.cn, ORCID: https://orcid.org/0000-0002-7467-6586], Key Laboratory of Vertebrate Evolution and Human Origins of Chinese Academy of Sciences, Institute of Vertebrate Paleontology and Paleoanthropology, Beijing, 100044, China. College of Earth and Planetary Sciences, University of Chinese Academy of Sciences, Beijing, 100049, China. Wenjie Zheng [zhengwj@zmnh.com, ORCID: https://orcid.org/0000-0002-3610-806X] and Junfang Xie [xiejf@zmnh.com; ORCID: https://orcid. org/0009-0001-7629-9972], Zhejiang Museum of Natural History, Hangzhou, Zhejiang Province, 310014, China. Zhiqiang Yu [zhiqiangyu@mail.iggcas.ac.cn; ORCID: https://orcid.org/0000-0001-5486-8372], Key Laboratory of Planetary Science and Frontier Technology, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China. Lun Hai [hailun@ivpp.ac.cn; ORCID: https://orcid.org/0009-0007-7610-4323], Key Laboratory of Vertebrate Evolution and Human Origins of Chinese Academy of Sciences, Institute of Vertebrate Paleontology and Paleoanthropology, Beijing, 100044, China. School of Earth Sciences and Resources, China University of Geosciences, Beijing, 100083, China.