''Docofossor'' is an extinct

mammaliaform Mammaliaformes ("mammalian forms") is a clade that contains the crown group mammals and their closest extinct relatives; the group radiated from earlier probainognathian cynodonts. It is defined as the clade originating from the most recent comm ...

(a docodont) from 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 ...

. Its remains have been recovered in China from 160 million years old rocks. It appears to have been the earliest-known subterranean mammaliaform, with adaptations remarkably similar to the modern

Chrysochloridae Golden moles are small insectivorous burrowing mammals endemic to Sub-Saharan Africa. They comprise the family Chrysochloridae and as such they are taxonomically distinct from the true moles, family Talpidae, and other mole-like families, all ...

, the golden moles.

Discovery

The fossil of ''Docofossor brachydactylus'',

holotype A holotype is a single physical example (or illustration) of an organism, known to have been used when the species (or lower-ranked taxon) was formally described. It is either the single such physical example (or illustration) or one of sever ...

BMNH 131735, along with that of '' Agilodocodon scansorius'', was originally found by farmers near Nanshimen in the province of

Hebei Hebei or , (; alternately Hopeh) is a northern province of China. Hebei is China's sixth most populous province, with over 75 million people. Shijiazhuang is the capital city. The province is 96% Han Chinese, 3% Manchu, 0.8% Hui, and 0 ...

in a layer of the Chinese

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 ...

( Oxfordian) and acquired by the Beijing Museum of Natural History. The holotype consists of a compressed skeleton with skull and lower jaws, preserved on a plate and counterplate, along with soft-tissue remnants. The shoulder girdle area and the tail have been damaged. The

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 specime ...

''Docofossor brachydactylus'' was named and described by Zhe-Xi Luo, Meng Qingjin, Ji Qiang, Liu Di, Zhang Yuguang, and April I. Neander in the journal ''

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

'' in 2015. The generic name refers to the membership of the Docodonta and a burrowing lifestyle, ''fossor'' meaning "digger" in

Latin Latin (, or , ) is a classical language belonging to the 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 the power of the ...

. The specific name is derived from Greek βραχύς, "short", and δάκτυλος, "finger", referring to the reduction of the finger phalanges.

Skeleton

''Docofossor'' was at least nine centimetres long, exempting the tail, and weighed at least nine grams, perhaps sixteen. It had a skeletal structure and body proportions strikingly similar to the modern day African

golden mole Golden moles are small insectivorous burrowing mammals endemic to Sub-Saharan Africa. They comprise the family Chrysochloridae and as such they are taxonomically distinct from the true moles, family Talpidae, and other mole-like families, all ...

. It had shovel-like

finger A finger is a limb of the body and a type of digit, an organ of manipulation and sensation found in the hands of most of the Tetrapods, so also with humans and other primates. Most land vertebrates have five fingers ( Pentadactyly). Chambers ...

s for digging, short and wide upper

molars The molars or molar teeth are large, flat teeth at the back of the mouth. They are more developed in mammals. They are used primarily to grind food during chewing. The name ''molar'' derives from Latin, ''molaris dens'', meaning "millstone to ...

typical of mammals that forage underground, and a sprawling posture indicative of subterranean movement. The sprawling is proven by a short hindlimb of just twenty-three millimetres, a massive

olecranon The olecranon (, ), is a large, thick, curved bony eminence of the ulna, a long bone in the forearm that projects behind the elbow. It forms the most pointed portion of the elbow and is opposite to the cubital fossa or elbow pit. The olecranon ...

as an adaptation for digging and a projecting parafibula forcing the knee joint into a bent position. Its snout point was blunt and slightly overhanging.

Characteristics

''Docofossor'' shows two unique derived traits or

autapomorphies In phylogenetics, an autapomorphy is a distinctive feature, known as a derived trait, that is unique to a given taxon. That is, it is found only in one taxon, but not found in any others or outgroup taxa, not even those most closely related to t ...

. The upper

have a grooved (prestylar) shelf at the front. The fourth molar has a single root. ''Docofossor'' had a reduced number of phalanges in its fingers. The phalangeal formula was 2-2-2-2-2 instead of the ancestral 2-3-3-3-3. This led to shortened but wide digits. Furthermore, the claw-bearing sections were enlarged and the upper phalanges shortened. African golden moles possess almost exactly the same adaptation, which provides an evolutionary advantage for digging mammals. This characteristic is due to the fusion of the bone joints between the upper and middle phalanges during embryonic development – a process influenced by the

gene In biology, the word gene (from , ; "... Wilhelm Johannsen coined the word gene to describe the Mendelian units of heredity..." meaning ''generation'' or ''birth'' or ''gender'') can have several different meanings. The Mendelian gene is a b ...

s BMP and GDF-5. Because of the many anatomical similarities, the researchers hypothesize that this genetic mechanism may have played a comparable role in early mammaliaform evolution, as in the case of ''Docofossor''. The spines and ribs of ''Docofossor'' also show evidence for the influence of genes seen in modern mammals, since they feature a gradual thoracic to lumbular vertebrae transition. These shifting patterns of thoracic-lumbular transition have been seen in modern mammals and are known to be regulated by the genes Hox 9-10 and Myf 5-6. That these ancient mammaliaforms had similar developmental patterns is evidence that these gene networks could have functioned in a similar way long before true mammals evolved.

Phylogeny

''Docofossor'' was placed in the

Docodontidae Docodonta is an order of extinct mammaliaforms that lived during the Mesozoic, from the Middle Jurassic to Early Cretaceous. They are distinguished from other early mammaliaforms by their relatively complex molar teeth, from which the order ge ...

together with ''

Docodon ''Docodon'' (meaning 'beam tooth') was a mammaliaform from the Late Jurassic of western North America. It was the first docodontan cynodont to be named. Description ''Docodon'' was the first docodontan cynodont found and named, and later gave ...

'' and ''

Haldanodon ''Haldanodon'' is an extinct docodont mammaliaform which lived in the Upper Jurassic (Kimmeridgian, about 145 million years ago). Its fossil remains have been found in Portugal, in the well-known fossil locality of Guimarota, which is in the Al ...

''. Docodontidae are basal Mammaliaformes outside of the crown group

Mammalia 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 o ...

Paleobiology

Early mammaliaforms were once thought to have limited ecological opportunities to diversify during the

dinosaur Dinosaurs are a diverse group of reptiles of the clade Dinosauria. They first appeared during the Triassic period, between 243 and 233.23 million years ago (mya), although the exact origin and timing of the evolution of dinosaurs is t ...

-dominated

Mesozoic The Mesozoic Era ( ), also called the Age of Reptiles, the Age of Conifers, and colloquially as the Age of the Dinosaurs is the second-to-last era of Earth's geological history, lasting from about , comprising the Triassic, Jurassic and Cretace ...

era. However, ''Docofossor'' and numerous other fossils – including '' Castorocauda'', a (related) swimming, fish-eating mammaliaform – provide strong evidence that forms ancestral to the true mammals adapted to wide-ranging environments despite competition from dinosaurs.

References

{{Taxonbar, from=Q19258785 Docodonts Prehistoric cynodont genera Middle Jurassic synapsids of Asia Jurassic China Fossils of China Paleontology in Hebei Fossil taxa described in 2015 Taxa named by Zhe-Xi Luo Taxa named by Qing-Jin Meng Taxa named by Qiang Ji (paleontologist) Taxa named by Di Liu Taxa named by Yu-Guang Zhang Taxa named by April I. Neander