''Fossiomanus'' is an extinct genus of

tritylodontid Tritylodontidae ("three-knob teeth", named after the shape of their cheek teeth) is an extinct family of small to medium-sized, highly specialized mammal-like cynodonts, with several mammalian traits including erect limbs, endothermy, and some d ...

mammaliamorph Mammaliamorpha is a clade of cynodonts. It contains the clades Tritylodontidae and Mammaliaformes, as well as a few genera that do not belong to either of these groups. The family Tritheledontidae has also been placed in Mammaliamorpha by some ...

s from the

Early Cretaceous The Early Cretaceous (geochronology, geochronological name) or the Lower Cretaceous (chronostratigraphy, chronostratigraphic name) is the earlier or lower of the two major divisions of the Cretaceous. It is usually considered to stretch from 143.1 ...

of China. It includes one species, ''F. sinensis'', which is known from a single nearly complete skeleton from the

Aptian The Aptian is an age (geology), age in the geologic timescale or a stage (stratigraphy), stage in the stratigraphic column. It is a subdivision of the Early Cretaceous, Early or Lower Cretaceous epoch (geology), Epoch or series (stratigraphy), S ...

Jiufotang Formation The Jiufotang Formation () is an Early Cretaceous geological formation in Chaoyang, Liaoning which has yielded fossils of feathered dinosaurs, primitive birds, pterosaurs, and other organisms (see Jehol Biota). It is a member of the Jehol group. T ...

. Features of its limbs and vertebrae indicate that ''Fossiomanus'' was adapted towards a

fossorial A fossorial animal () is one that is adapted to digging and which lives primarily (but not solely) underground. Examples of fossorial vertebrates are Mole (animal), moles, badgers, naked mole-rats, meerkats, armadillos, wombats, and mole salamand ...

lifestyle. ''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.

Discovery and naming

''Fossiomanus sinensis'' was named in 2021 by Fangyuan Mao, Chi Zhang, Cunyu Liu, and Jin Meng, on the basis of the

holotype A holotype (Latin: ''holotypus'') is a single physical example (or illustration) of an organism used when the species (or lower-ranked taxon) was formally described. It is either the single such physical example (or illustration) or one of s ...

specimen, JZMP-2107500093, a nearly complete skeleton with a damaged skull from the

Liaoning ) , image_skyline = , image_alt = , image_caption = Clockwise: Mukden Palace in Shenyang, Xinghai Square in Dalian, Dalian coast, Yalu River at Dandong , image_map = Liaoning in China (+all claims hatched).svg , ...

. The genus name is derived from Latin ''fossio'' "digging" and ''manus'' "hand" and the species name means "from China".

Description

''Fossiomanus'' was long, excluding the -long tail, with a generally stocky build. Its pointed snout and short tail gave it an overall

fusiform Fusiform (from Latin ''fusus'' ‘spindle’) means having a spindle (textiles), spindle-like shape that is wide in the middle and tapers at both ends. It is similar to the lemon (geometry), lemon-shape, but often implies a focal broadening of a ...

body plan. It had an elongate torso, with 26

thoracic vertebrae In vertebrates, thoracic vertebrae compose the middle segment of the vertebral column, between the cervical vertebrae and the lumbar vertebrae. In humans, there are twelve thoracic vertebra (anatomy), vertebrae of intermediate size between the ce ...

and 5

lumbar vertebrae The lumbar vertebrae are located between the thoracic vertebrae and pelvis. They form the lower part of the back in humans, and the tail end of the back in quadrupeds. In humans, there are five lumbar vertebrae. The term is used to describe t ...

. The front foot was broad and robust, with large claws.

Paleobiology

Tritylodontids such as ''Fossiomanus'' were herbivores. ''Fossiomanus'' was a

taxon, with its powerful forelimbs being used for digging. The long, stocky, fusiform body plan is characteristic of many burrowing mammals.

Evolution

''Fossiomanus'' belongs to

Tritylodontidae Tritylodontidae ("three-knob teeth", named after the shape of their cheek teeth) is an extinct family of small to medium-sized, highly specialized mammal-like cynodonts, with several mammalian traits including erect limbs, endothermy, and some d ...

, a clade of

cynodonts Cynodontia () is a clade of eutheriodont therapsids that first appeared in the Late Permian (approximately 260 mya), and extensively diversified after the Permian–Triassic extinction event. Mammals are cynodonts, as are their extinct ances ...

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. ''Fossiomanus'' is probably the geologically youngest known tritylodontid; its holotype specimen was found just below a

tuff Tuff is a type of rock made of volcanic ash ejected from a vent during a volcanic eruption. Following ejection and deposition, the ash is lithified into a solid rock. Rock that contains greater than 75% ash is considered tuff, while rock co ...

layer that has been determined to be 118.9±0.8 million years old. It appears to be slightly more recent than ''

Montirictus ''Montirictus'' is an extinct genus of tritylodonts known from the Early Cretaceous Kuwajima Formation of Japan. It was among the latest surviving tritylodontids, and is closely related to the earlier ''Xenocretosuchus'' from mainland Asia, and ...

'', 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. Another late-surviving tritylodontid is ''

Xenocretosuchus ''Xenocretosuchus'' is an extinct genus of tritylodont therapsids from the Aptian (Early Cretaceous) Ilek Formation of Siberia, in the Russian Federation. The type species, ''X. sibiricus'', is known only from dental elements, as is ''X. kolossov ...

'', found in the

Ilek Formation The Ilek Formation is a Lower Cretaceous geologic Formation (geology), formation in Western Siberia. Many different fossils have been recovered from the formation. It overlies the Late Jurassic Tyazhin Formation and underlies the Albian Kiya Form ...

of Siberia, the age of which is poorly constrained but estimated to be

Barremian The Barremian is an age in the geologic timescale (or a chronostratigraphic stage) between 125.77 Ma (million years ago) and 121.4 ± 1.0 Ma (Historically, this stage was placed at 129.4 million to approximately 125 million years ago) It is a ...

–

. 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 Oct-4 (octamer-binding transcription factor 4), also known as POU5F1 ( POU domain, class 5, transcription factor 1), is a protein that in humans is encoded by the ''POU5F1'' gene. Oct-4 is a homeodomain transcription factor of the POU family ...

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 Hyraxes (), also called dassies, are small, stout, thickset, herbivorous mammals in the family Procaviidae within the order Hyracoidea. Hyraxes are well-furred, rotund animals with short tails. Modern hyraxes are typically between in length a ...

equal it.

References

{{Taxonbar, from=Q106412798 Tritylodontidae Aptian life Early Cretaceous synapsids of Asia Cretaceous China Fossils of China Jiufotang fauna Fossil taxa described in 2021 Taxa named by Jin Meng