''Docofossor'' is an extinct
mammaliaform
Mammaliaformes ("mammalian forms") is a clade of synapsid tetrapods that includes the crown group mammals and their closest extinct relatives; the group radiated from earlier probainognathian cynodonts during the Late Triassic. It is defined a ...
(a
docodont
Docodonta is an Order (biology), order of extinct Mesozoic Mammaliaformes, mammaliaforms (advanced cynodonts closely related to true Crown group, crown-group mammals). They were among the most common mammaliaforms of their time, persisting from t ...
) from the
Jurassic
The Jurassic ( ) is a Geological period, geologic period and System (stratigraphy), stratigraphic system that spanned from the end of the Triassic Period million years ago (Mya) to the beginning of the Cretaceous Period, approximately 143.1 Mya. ...
period. Its remains have been recovered in China from 160-million-year-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 Endemism, endemic to Sub-Saharan Africa. They comprise the family Chrysochloridae (the only family in the suborder Chrysochloridea) and as such they are Taxonomy (biology), taxonomically dis ...
, the golden moles.
Discovery

The fossil of ''Docofossor brachydactylus'',
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 ...
BMNH 131735, along with that of ''
Agilodocodon scansorius'', was originally found by farmers near
Nanshimen in the province of
Hebei
Hebei is a Provinces of China, province in North China. It is China's List of Chinese administrative divisions by population, sixth-most populous province, with a population of over 75 million people. Shijiazhuang is the capital city. It bor ...
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 (stage), Oxfordian stages). It is known for its Lagerstätte, exceptionally preser ...
(
Oxfordian) and acquired by the
Beijing Museum of Natural History
The National Natural History Museum of China (NNHM; ), previously the Beijing Museum of Natural History, is a municipal public natural history museum in Dongcheng, Beijing, China. It is affiliated with the Beijing Academy of Science and Technolo ...
. 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 International_Code_of_Zoological_Nomenclature, 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 spe ...
''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 discipline that builds and organises knowledge in the form of testable hypotheses and predictions about the universe. Modern science is typically divided into twoor threemajor branches: the natural sciences, which stu ...
'' 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 languages, Italic branch of the Indo-European languages. Latin was originally spoken by the Latins (Italic tribe), Latins in Latium (now known as Lazio), the lower Tiber area aroun ...
. The
specific name Specific name may refer to:
* in Database management systems, a system-assigned name that is unique within a particular database
In taxonomy, either of these two meanings, each with its own set of rules:
* Specific name (botany), the two-part (bino ...
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 (the only family in the suborder Chrysochloridea) and as such they are taxonomically distinct from the true moles, f ...
. It had shovel-like
finger
A finger is a prominent digit (anatomy), digit on the forelimbs of most tetrapod vertebrate animals, especially those with prehensile extremities (i.e. hands) such as humans and other primates. Most tetrapods have five digits (dactyly, pentadact ...
s for digging, short and wide upper
molars
The molars or molar teeth are large, flat tooth, teeth at the back of the mouth. They are more developed in mammal, mammals. They are used primarily to comminution, grind food during mastication, chewing. The name ''molar'' derives from Latin, '' ...
typical of mammals that forage underground, and a sprawling posture indicative of
subterranean
Subterranean(s) or The Subterranean(s) may refer to:
* Subterranea (geography), underground structures, both natural and man-made
Literature
* ''Subterranean'' (novel), a 1998 novel by James Rollins
* ''Subterranean Magazine'', an American fa ...
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 process on the proximal, posterior end of the ulna. It forms the protruding part of the elbow and is opposite to the cubital fossa or elbow pit (trochlear notch). The olecranon serves as a lever ...
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. The upper
molars
The molars or molar teeth are large, flat tooth, teeth at the back of the mouth. They are more developed in mammal, mammals. They are used primarily to comminution, grind food during mastication, chewing. The name ''molar'' derives from Latin, '' ...
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 has two meanings. The Mendelian gene is a basic unit of heredity. The molecular gene is a sequence of nucleotides in DNA that is transcribed to produce a functional RNA. There are two types of molecular genes: protei ...
s BMP and GDF-5. Because of the many
anatomical
Anatomy () is the branch of morphology concerned with the study of the internal structure of organisms and their parts. Anatomy is a branch of natural science that deals with the structural organization of living things. It is an old scien ...
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
rib
In vertebrate anatomy, ribs () are the long curved bones which form the rib cage, part of the axial skeleton. In most tetrapods, ribs surround the thoracic cavity, enabling the lungs to expand and thus facilitate breathing by expanding the ...
s 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 Mesozoic mammaliaforms (advanced cynodonts closely related to true crown-group mammals). They were among the most common mammaliaforms of their time, persisting from the Early Jurassic to the Early Cretaceous acr ...
together with ''
Docodon'' 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 Alco ...
''. Docodontidae are basal
Mammaliaformes
Mammaliaformes ("mammalian forms") is a clade of synapsid tetrapods that includes the crown group mammals and their closest extinct relatives; the group radiated from earlier probainognathian cynodonts during the Late Triassic. It is defined as ...
outside of the
crown group
In phylogenetics, the crown group or crown assemblage is a collection of species composed of the living representatives of the collection, the most recent common ancestor of the collection, and all descendants of the most recent common ancestor ...
Mammalia
A mammal () is a vertebrate animal of the class Mammalia (). Mammals are characterised by the presence of milk-producing mammary glands for feeding their young, a broad neocortex region of the brain, fur or hair, and three middle ear bon ...
.
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 Geological period, period, between 243 and 233.23 million years ago (mya), although the exact origin and timing of the #Evolutio ...
-dominated
Mesozoic
The Mesozoic Era is the Era (geology), era of Earth's Geologic time scale, geological history, lasting from about , comprising the Triassic, Jurassic and Cretaceous Period (geology), Periods. It is characterized by the dominance of archosaurian r ...
era. However, ''Docofossor'' and numerous other fossils—including ''
Castorocauda
''Castorocauda'' is an extinct, semi-aquatic, superficially otter-like genus of docodont mammaliaforms with one species, ''C. lutrasimilis''. It is part of the Yanliao Biota, found in the Daohugou Beds of Inner Mongolia, China dating to the ...
'', 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
Docodonta
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