Eutherian Mammals (or Placental Mammals) represent the largest Mammal group alive today. Molecular clock analysis suggests that thy diverged from the Marsupials during the Jurassic, which is supported by the oldest known Eutherian Mammal fossil, Juramaia sinensis, coming from the Late Jurassic Tiaojishan Formation of Liaoning Province in China. However, the fossil record of early Eutherians is sparse, with only Juramaia sinensis known from the Jurassic, and a only a few examples, such as Eomaia, Acristatherium, Ambolestes, Sasayamamylos, Sinodelphys, and Prokennalestes coming from the Early Cretaceous. This makes it hard to make solid predictions about the relationships between these early Eutherian species, with the discovery of new species often leading to the reassessment of the importance of features used to define the group (for example Sinodelphys was considered the earliest known Marsupial until 2018, when a re-evaluation of the group following the discovery of Ambolestes zhoui lead to it being classified as a Eutherian.
In a paper published in the Philosophical Transactions of the Royal Society Series B: Biological Sciences on 7 February 2022, Hai-Bing Wang of the Key Laboratory of Vertebrate Evolution at the Institute ofVertebrate Paleontology and Paleoanthropology, the Centre for Excellence in Life and Paleoenvironment of the Chinese Academy of Sciences, and the Key Laboratory of Palaeobiology and Stratigraphy at the Nanjing Institute of Geology and Palaeontology, Simone Hoffmann of the Department of Anatomy at the New York Institute of Technology, Dian-Can Wang of the Department of Oral and Maxillofacial Surgery at Peking University, and Yuan-Qing Wang, also of the Key Laboratory of Vertebrate Evolution at the Institute of Vertebrate Paleontology and Paleoanthropology, and the Centre for Excellence in Life and Paleoenvironment of the Chinese Academy of Sciences, and of the College of Earth and Planetary Sciences at the University of Chinese Academy of Sciences, describe a new species of Eutherian Mammal from the Early Cretaceous Jehol Biota of Liaoning Province, China.
The new species is described from a single specimen collected from the Jiufotang Formation at the Sihedang Site in Lingyuan City, and has an estimated age of 120 million years. It is named Cokotherium jiufotangensis, where 'Cokotherium' is intended to honour the late Chuan-Kui Li for his contributions to our understanding of the evolution of early Mammals, and 'jiufotangensis' means 'from Jiufotang'.
The specimen is a partial skeleton with a complete skull, forelimbs and part of the trunk and hindlimbs, which is preserved on a single slab of material. The dorsal and right lateral portions of the skull are obscured by the rock-matrix, but could be observed by computerised tomographic imaging.
Cokotherium jiufotangensis has an ossified Meckelian cartilage, something seen in modern Eutherians, but not previous Early Cretaceous examples such as Eomaia, Prokennalestes, Hovurlestes or Ambolestes. This had led to speculation that early members of the group retained a cartilaginous Meckelian sulcus into adult life, as was the case in the contemporary Eutriconodontan and Zhangheotheriid Mammals.
Furthermore, this cartilage is reduced in size, which likely indicates that the middle ear bones have become detached from it, a key development of the ear in modern Mammals, which is not seen in the Eutriconodontans. The ear bones are separated from the Mecklian cartilage in some Zhangheotheriids, although only by a small gap, and this may also be the case in Cokotherium jiufotangensis.
Wang et al. were also able to reconstruct the inner ear of Cokotherium jiufotangensis in three dimensions using computerised tomographic scanning, the first time this has been done for an Early Cretaceous Eutherian Mammal (although the morphology of part of the inner ear has been described in Prokennalestes, a Eutherian from the Early Cretaceous of Inner Mongolia).
The cochlear canal of Cokotherium jiufotangensis comprises a single coil (i.e. 360°). This is similar to the state in most later Cretaceous Eutherian Mammals, with greater coiling seen in most modern Mammals as well as some Cretaceous Zhelestids. Many other features of the ear, including a secondary crus commune, the base of a secondary osseous lamina, the primary osseous lamina and a bony cribriform plate, are similar to those in both later Cretaceous and modern Eutherian Mammals, confirming these arose early in the history of the group.
The earliest Eutherian Mammals generally have a larger number of teeth than modern members of the group, and consequently these teeth are closely packed together. Cokotherium jiufotangensis has a reduced number of both incisors and molars, and consequently a less densely packed dentition, a trait otherwise recorded in Eutherian Mammals from the Late Cretaceous onwards. It still has four premolars on each side of both the upper and lower jaws, but the hindmost premolars are starting to show signs of molarisation, another trait previously known only from later Eutherian species.
Despite these apparently derived traits, a phylogenetic analysis carried out by Wang et al. suggests that Cokotherium jiufotangensis is one of the most basal Eutherians known, and possibly outside the Theria (Eutherians plus Marsupials) altogether. However, it is possible that this placement is an artefact of the small number of early Eutherians known, and our subsequent poor understanding of the relationships between these early Mammals.
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