Cifelliodon is an extinct genus of mammaliaforms from the Lower Cretaceous of North America. In the describing paper, it was considered one of the latest surviving haramiyids yet known, belonging to the family Hahnodontidae. Its discovery led to the proposal to remove hahnodontids from the larger well-known group, the multituberculates. However, later papers have considered it to be a basal allotherian outside of Haramiyida.
The sole species in the genus, Cifelliodon wahkarmoosuch, was found in the geological rock unit called the Yellow Cat Member, part of the Cedar Mountain Formation in Grand County, Utah. This rock unit dates to between 139–124 million years old. It was found alongside the remains of several dinosaurs — a large iguanodontian, a dromaeosaur, and an ornithopod — and parts of a crocodyliform.[1]
Discovery and naming
Cifelliodon was described by the American paleontologist Adam Huttenlocker and colleagues in 2018 based on an "exceptionally preserved skull". The skull was recovered from the Yellow Cat Member of the Cedar Mountain Formation. The site is Early Cretaceous in age, between 139 and 124 million years old according to radiometric dating estimates. The type species of the genusCifelliodon was named Cifelliodon wahkarmoosuch. Cifelliodon means 'Cifelli's tooth', in honor of the mammal paleontologist Richard Cifelli, while wahkarmoosuch comes from the Ute language words for 'yellow' (wahkar) and 'cat' (moosuch).[1]
Description
The skull of Cifelliodon was noted for its large size relative to those of other mammaliaforms of the Mesozoic: 70 millimeters (2.8in) in length, corresponding to an estimated total body mass of 0.91–1.27 kilograms (2.0–2.8lb). Most of its upper dental features are missing, but two alveoli (tooth sockets), belonging to the incisors; one canine; and four postcanines, including an unerupted pair of molars, are preserved. The incisor roots are reclined back and converge centrally, indicating procumbency (inclination toward the lips). The alveoli of the postcanines appear to be similar in diameter to that of the canine. The final molar possesses a well-defined central valley that is closed-off from the front by a tall cusp (raised tooth point), as well as a lingual (tongue-facing) ridge that connects to a posterolingual (at the base of the tongue) cusp below, a pattern which the researchers compared with the teeth of the hammer-headed bat (Hypsignathus monstrosus), a fruit-eater. The molar was noted for its extensive similarity, aside from size and proportion, to that of Hahnodon taqueti, a mammaliaform from the Early Cretaceous of Morocco which had been previously identified as a multituberculate. Huttenlocker and colleagues wrote that the features of the molars of Cifelliodon and H. taqueti appeared more haramiyidan than multituberculate.[1]
The type specimen also displays a variety of ancient traits more commonly seen in mammaliamorphs of the Triassic in than those of the Cretaceous. The include the shape of its parietal bones, which are waisted along their lateral sides rather than bulging outward; the presence of a tabular bone; and the extension of the front part of the jugal bone. However, some of the features exhibited are found in crown-groupmammals, including an extensive process (outgrowth of tissue) on the premaxilla that borders the nasal region; reduced premaxilla process between the nostrils; and the limited extent of the rear end of the jugal bone. X-ray-computed tomography of the inner skull revealed traits intermediate between primitive mammal relatives and crown mammals. The size of the cranial vault was described as "modest", with an encephalization quotient (a measure of actual brain mass compared to expected brain mass based on body size) of between 0.25 and 0.30, which the authors wrote was smaller than those of the mammaliaform Hadrocodium and the Cretaceous crown mammals Vincelestes and Pucadelphys. They also wrote that much of the brain volume would have been taken up by olfactory bulbs and the piriform cortex (both associated with the sense of smell). The neocortex would have been small, its features poorly defined, especially compared with those of Cretaceous crown mammals. The authors concluded that the features of the brain support the hypothesis that haramiyidan brain development and behavior were primarily scent-oriented.[1]
Classification
In their description of Cifelliodon, Huttenlocker and colleagues expressed support for the recognition of Haramiyida, the order to which they assigned Cifelliodon, as a monophyletic group closely related to Mammalia, as opposed to belonging to Mammalia itself as a relative to the multituberculates, as some authors had proposed. They defined Haramiyida as all mammaliaforms more closely related to Thomasia and Haramiyavia than to Didelphis and placed Cifelliodon as forming a polytomy with Hahnodon and Vintana, the latter of which is a gondwanathere. Their study supported the placement of Gondwanatheria within Haramiyida and outside of Mammalia. They wrote that the well preserved remains of Cifelliodon supported the relationship of hahnodontids to Haramiyida (placing the former group within the latter).[1]
In 2020, the Canadian paleontologist David W. Krause and colleagues reassessed the relationships of mammaliaform lineages and placed Cifelliodon within Mammalia at the base of Allotheria. According to their work, Allotheria also contains Euharamiyida and a clade formed by Multituberculata and Gondwanatheria. The haramiyidans Haramiyavia and Thomasia, as well as the poorly studied Megaconus, were placed outside of Mammaliaformes altogether.[2] The placement of Cifelliodon within Allotheria was supported by the Chinese paleontologist Fangyuan Mao and colleagues, although they placed it as the sister group of a clade formed by Euharamiyida and Gondwanatheria and also moved Haramiyavia and Thomasia within Allotheria.[3] A September 2023 study by the Chinese paleontologist Zhiqiang Yu and colleagues instead placed Cifelliodon outside of Mammalia and as a close relation of Haramiyavia and Thomasia (all three of which were placed within Mammaliaformes). This study rejected the idea of Haramiyida as a monophyletic group, finding no close relation between the three aforementioned genera and euharamiyidans (with the latter group were recognized as crown mammals).[4]
Paleoecology
The skull of Cifelliodon is have been remarkably large for an Early Cretaceous mammal. The animal would have been approximately the size of a medium hare at a time when most mammals were around shrew- or mouse-sized. Only one known North American contemporary was larger: the carnivorous Gobiconodon ostromi.[5] Huttenlocker and colleagues wrote that the discovery of Cifelliodon supported the idea that haramiyidans, with which they classified the genus, survived until the Early Cretaceous and ranged across the ancients continents of Laurasia and Gondwana. They also described the presence of related dinosaur taxa across the continents as evidence of later intercontinental links than previously thought. They identified the mid-Cretaceous radiation of other mammalian groups, namely multituberculates and tribosphenids, as coinciding with the extinctions of many of their more primitive relatives. They labelled Cifelliodon as evidence that large-bodied mammaliamorphs with diverse ecological roles persisted in North American ecosystems until the mid-Cretaceous.[1]
123456Huttenlocker, Adam K.; Grossnickle, David M.; Kirkland, James I.; Schultz, Julia A.; Luo, Zhe-Xi (June 2018). "Late-surviving stem mammal links the lowermost Cretaceous of North America and Gondwana". Nature. 558 (7708): 108–112. doi:10.1038/s41586-018-0126-y.
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