Fossiomanus

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Fossiomanus is an extinct genus of tritylodontid mammaliamorphs from the Early Cretaceous of China. It includes one species, F. sinensis, which is known from a single nearly complete skeleton from the Aptian Jiufotang Formation. Features of its limbs and vertebrae indicate that Fossiomanus was adapted towards a fossorial lifestyle.^[1]

Fossiomanus lived roughly 120 million years ago, making it potentially the geologically youngest known tritylodontid, which would also make it the last known non-mammalian synapsid.

Fossiomanus sinensis was named in 2021 by Fangyuan Mao, Chi Zhang, Cunyu Liu, and Jin Meng, on the basis of the holotype specimen, JZMP-2107500093, a nearly complete skeleton with a damaged skull from the Jiufotang Formation of Liaoning. The genus name is derived from Latin fossio "digging" and manus "hand" and the species name means "from China".^[1]

Fossiomanus was 316 millimetres (12.4 in) long, excluding the 65 millimetres (2.6 in)-long tail, with a generally stocky build. Its pointed snout and short tail gave it an overall fusiform body plan. It had an elongate torso, with 26 thoracic vertebrae and 5 lumbar vertebrae. The front foot was broad and robust, with large claws.^[1]

Tritylodontids such as Fossiomanus were herbivores. Fossiomanus was a fossorial taxon, with its powerful forelimbs being used for digging. The long, stocky, fusiform body plan is characteristic of many burrowing mammals.^[1]

Fossiomanus belongs to Tritylodontidae, a clade of cynodonts that were the last surviving lineage of non-mammalian synapsids. Tritylodontids were widely distributed during the Early Jurassic, but had become restricted to Asia by the Late Jurassic.^[2] Fossiomanus is probably the geologically youngest known tritylodontid;^[1] its holotype specimen was found just below a tuff layer that has been determined to be 118.9±0.8 million years old.^[3] It appears to be slightly more recent than Montirictus,^[1] another late-surviving tritylodontid, which was found in strata constrained to be between 121.2±1.1 and 130.7±0.8 million years old.^[4] Another late-surviving tritylodontid is Xenocretosuchus, found in the Ilek Formation of Siberia, the age of which is poorly constrained but estimated to be Barremian–Aptian.^[5]

Earlier tritylodontids, such as Kayentatherium, did not have the elongate body plan that characterized Fossiomanus. The evolution of an elongate body in Fossiomanus may have been the result of a change in the GDF11 or OCT4 genes, which regulate the development of the transition from the trunk to the tail. With a total of 38 presacral vertebrae, Fossiomanus may have been at the upper limit of the number of presacral vertebrae possible in mammaliamorphs; no known terrestrial mammal exceeds this number, although hyraxes equal it.^[1]