''Rugosodon'' is an extinct

genus Genus ( plural genera ) is a taxonomic rank used in the biological classification of living and fossil organisms as well as viruses. In the hierarchy of biological classification, genus comes above species and below family. In binomial nom ...

of multituberculate (rodent-like)

mammal Mammals () are a group of vertebrate animals constituting the class Mammalia (), characterized by the presence of mammary glands which in females produce milk for feeding (nursing) their young, a neocortex (a region of the brain), fur ...

s from eastern

China China, officially the People's Republic of China (PRC), is a country in East Asia. It is the world's List of countries and dependencies by population, most populous country, with a Population of China, population exceeding 1.4 billion, slig ...

that lived 160 million years ago during the

Jurassic period The Jurassic ( ) is a geologic period and stratigraphic system that spanned from the end of the Triassic Period million years ago (Mya) to the beginning of the Cretaceous Period, approximately Mya. The Jurassic constitutes the middle period of ...

. The discovery of its

type species In zoological nomenclature, a type species (''species typica'') is the species name with which the name of a genus or subgenus is considered to be permanently taxonomically associated, i.e., the species that contains the biological type specim ...

and currently only known species ''Rugosodon eurasiaticus'' was reported in the 16 August 2013 issue of ''

Science Science is a systematic endeavor that builds and organizes knowledge in the form of testable explanations and predictions about the universe. Science may be as old as the human species, and some of the earliest archeological evidence ...

''.

Description

''Rugosodon'' is represented by a nearly complete fossilized skeleton, including a

skull The skull is a bone protective cavity for the brain. The skull is composed of four types of bone i.e., cranial bones, facial bones, ear ossicles and hyoid bone. However two parts are more prominent: the cranium and the mandible. In humans, th ...

, that bears a strong resemblance to a small rat or a

chipmunk Chipmunks are small, striped rodents of the family Sciuridae. Chipmunks are found in North America, with the exception of the Siberian chipmunk which is found primarily in Asia. Taxonomy and systematics Chipmunks may be classified either as ...

. The mammal is estimated to have weighed between 65 and 80 g, about that of an average chipmunk. The generic name ''Rugosodon'' (

Latin Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally a dialect spoken in the lower Tiber area (then known as Latium) around present-day Rome, but through ...

for "wrinkly tooth") refers to the rugosity, or wrinkliness, of the distinctively shaped teeth. Its teeth indicate that the animal was an

omnivore An omnivore () is an animal that has the ability to eat and survive on both plant and animal matter. Obtaining energy and nutrients from plant and animal matter, omnivores digest carbohydrates, protein, fat, and fiber, and metabolize the nu ...

, well-adapted to gnawing both plants and animals, including

fruits In botany, a fruit is the seed-bearing structure in flowering plants that is formed from the ovary after flowering. Fruits are the means by which flowering plants (also known as angiosperms) disseminate their seeds. Edible fruits in partic ...

and

seeds A seed is an embryonic plant enclosed in a protective outer covering, along with a food reserve. The formation of the seed is a part of the process of reproduction in seed plants, the spermatophytes, including the gymnosperm and angiosperm ...

worms Worms may refer to: *Worm, an invertebrate animal with a tube-like body and no limbs Places *Worms, Germany Worms () is a city in Rhineland-Palatinate, Germany, situated on the Upper Rhine about south-southwest of Frankfurt am Main. It had ...

insects Insects (from Latin ') are pancrustacean hexapod invertebrates of the class Insecta. They are the largest group within the arthropod phylum. Insects have a chitinous exoskeleton, a three-part body (head, thorax and abdomen), three pairs of j ...

and small

vertebrates Vertebrates () comprise all animal taxa within the subphylum Vertebrata () (chordates with backbones), including all mammals, birds, reptiles, amphibians, and fish. Vertebrates represent the overwhelming majority of the phylum Chordata, with ...

. Its

ankle joint The ankle, or the talocrural region, or the jumping bone (informal) is the area where the foot and the leg meet. The ankle includes three joints: the ankle joint proper or talocrural joint, the subtalar joint, and the inferior tibiofibular joi ...

s were highly mobile at rotation. This means that the ankle is remarkably flexible, allowing the foot to hyper-extend downward — like a

ballerina A ballet dancer ( it, ballerina fem.; ''ballerino'' masc.) is a person who practices the art of classical ballet. Both females and males can practice ballet; however, dancers have a strict hierarchy and strict gender roles. They rely on ye ...

standing on pointed toes — and to rotate through a wide range of motion. This feature, along with highly mobile digits, defines the multituberculates and is not seen in other mammalian lineages of the era. ''Rugosodon'' also had a highly flexible

spine Spine or spinal may refer to: Science Biology * Vertebral column, also known as the backbone * Dendritic spine, a small membranous protrusion from a neuron's dendrite * Thorns, spines, and prickles, needle-like structures in plants * Spine (zoolo ...

, which would have allowed it to twist both left to right and front to back. Due to the proportions of its hand bones, it is thought to have been terrestrial rather than arboreal. Its diet was likely

omnivorous An omnivore () is an animal that has the ability to eat and survive on both plant and animal matter. Obtaining energy and nutrients from plant and animal matter, omnivores digest carbohydrates, protein, fat, and fiber, and metabolize the nut ...

Discovery and taxonomic significance

In 2009, a local fossil hunter unearthed an unusual fossil in the

Tiaojishan Formation The Tiaojishan Formation is a geological formation in Hebei and Liaoning, People's Republic of China, dating to the middle-late Jurassic period (Bathonian- Oxfordian stages). It is known for its exceptionally preserved fossils, including those of ...

of China's

Liaoning Liaoning () is a coastal province in Northeast China that is the smallest, southernmost, and most populous province in the region. With its capital at Shenyang, it is located on the northern shore of the Yellow Sea, and is the northernmo ...

province, dating to 160 million years ago. He turned the fossil over to the

Chinese Academy of Geological Sciences The Chinese Academy of Geological Sciences (CAGS; ) is an institution that engages in geoscience research in the People's Republic of China. The academy was established in 1956 and reorganized in 1999. Administratively it is under the PRC Ministr ...

, where it was eventually identified as a nearly complete skeleton and given the name ''Rugosodon eurasiaticus''. The fossil was discovered on what was the shore of an ancient lake. It was preserved in two shale slabs and measures about 17 cm (6.5 inches) long from head to rump. The site of the discovery consists of lake sediments with embedded volcanic layers, which also contained fossils of

feathered dinosaur A feathered dinosaur is any species of dinosaur possessing feathers. While this includes all species of birds, there is a hypothesis that many, if not all non-avian dinosaur species also possessed feathers in some shape or form. It has been s ...

Anchiornis ''Anchiornis'' is a genus of small, four-winged paravian dinosaurs, with only one known species, the type species ''Anchiornis huxleyi'', named for its similarity to modern birds. The Latin name ''Anchiornis'' derives from a Greek word meaning " ...

'' and the

pterosaur Pterosaurs (; from Greek ''pteron'' and ''sauros'', meaning "wing lizard") is an extinct clade of flying reptiles in the order, Pterosauria. They existed during most of the Mesozoic: from the Late Triassic to the end of the Cretaceous (228 ...

Darwinopterus ''Darwinopterus'' (meaning "Darwin's wing") is a genus of pterosaur, discovered in China and named after biologist Charles Darwin. Between 30 and 40 fossil specimens have been identified, all collected from the Tiaojishan Formation, which dates t ...

''. The dental features of ''Rugosodon'' most resemble those of multituberculates of the Late Jurassic of

Western Europe Western Europe is the western region of Europe. The region's countries and territories vary depending on context. The concept of "the West" appeared in Europe in juxtaposition to "the East" and originally applied to the ancient Mediterranean ...

, suggesting that Europe and Asia had extensive mammal

fauna Fauna is all of the animal life present in a particular region or time. The corresponding term for plants is '' flora'', and for fungi, it is '' funga''. Flora, fauna, funga and other forms of life are collectively referred to as '' biota''. ...

l interchanges (hence the specific name, ''eurasia'') during the Jurassic. Prior to the discovery of ''Rugosodon'', scientists knew that multituberculates living 66 million years ago had highly flexible ankles. However, older species were mostly known from small fragments, and it was not proven that the trait was ancestral. Additionally, it was unknown what sort of diet was primitive in the lineage. The presence of the characteristic flexible ankles in ''Rugosodon'' demonstrates that the trait is ancestral and provides a strong clue that the trait was a major factor in the lineage's evolutionary success. The animal's diet provides a bridge between very early mammals, which were mostly

insectivore A robber fly eating a hoverfly An insectivore is a carnivorous animal or plant that eats insects. An alternative term is entomophage, which can also refer to the human practice of eating insects. The first vertebrate insectivores were ...

s, and later multituberculates, which were mostly

herbivore A herbivore is an animal anatomically and physiologically adapted to eating plant material, for example foliage or marine algae, for the main component of its diet. As a result of their plant diet, herbivorous animals typically have mouthpar ...

s. In the initial description, ''Rugosodon'' was attributed to

Paulchoffatiidae Paulchoffatiidae is a family of extinct mammals that lived predominantly during the Upper Jurassic period, though a couple of genera are known from the Early Cretaceous. Fossils have been reported from Europe (Portugal, Spain, Germany and Engla ...

, a group of multituberculates otherwise known from Western Europe. However, later studies suggested that it lacked key morphological features of the family, and was instead placed as a member of the more inclusive Paulchoffatiid-line outside of any defined family, or possibly even more basally than that.

References

External links

Supplemental Material to Yuan ''et al.'' 2013 (169 p.)Paleobiology DatabaseNews with photograph at World Fossil Society
{{Taxonbar, from=Q14752840 Fossil taxa described in 2013 Fossils of China Late Jurassic mammals Prehistoric monotypic mammal genera Multituberculates Prehistoric mammal genera