''Megaconus'' 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
allotheria
Allotheria (meaning "other beasts", from the Greek , '–other and , '–wild animal) is an extinct branch of successful Mesozoic mammals. The most important characteristic was the presence of lower molariform teeth equipped with two longitudin ...
n mammal from the
Middle Jurassic 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
Inner Mongolia
Inner Mongolia, officially the Inner Mongolia Autonomous Region, is an autonomous region of the People's Republic of China. Its border includes most of the length of China's border with the country of Mongolia. Inner Mongolia also accounts for a ...
, China. The
type and only
species
In biology, a species is the basic unit of classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species is often defined as the largest group of organisms in which any two individuals of the appropriate s ...
, ''Megaconus mammaliaformis'' was first described in the journal ''
Nature
Nature, in the broadest sense, is the physical world or universe. "Nature" can refer to the phenomena of the physical world, and also to life in general. The study of nature is a large, if not the only, part of science. Although humans are ...
'' in 2013. ''Megaconus'' is thought to have been a
herbivore that lived on the ground, having a similar posture to modern-day
armadillos and
rock hyrax
The rock hyrax (; ''Procavia capensis''), also called dassie, Cape hyrax, rock rabbit, and (in the King James Bible) coney, is a medium-sized terrestrial mammal native to Africa and the Middle East. Commonly referred to in South Africa as the da ...
es. ''Megaconus'' was in its initial description found to be member of a group called
Haramiyida. A
phylogenetic
In biology, phylogenetics (; from Greek φυλή/ φῦλον [] "tribe, clan, race", and wikt:γενετικός, γενετικός [] "origin, source, birth") is the study of the evolutionary history and relationships among or within groups o ...
analysis published along its description suggested that haramiyidans originated before the appearance of
true mammals, but in contrast, the later description of the haramiyidan ''
Arboroharamiya'' in the same issue of ''Nature'' indicated that haramyidans were true mammals.
If haramiyidans are not mammals, ''Megaconus'' would be one of the most
basal ("primitive") mammaliaforms to possess fur, and an indicator that fur evolved in the ancestors of mammals and not the mammals themselves. However, later studies cast doubt on the euharamiyidan intrepretation, instead finding it to be a basal allotherian mammal.
Description
''Megaconus'' is one of the few early mammaliaforms known from a complete skeleton. The skeleton includes both the jaw bones and the teeth, which are the most informative features because they allow for comparisons with other mammaliaforms known only from dental features. ''Megaconus'' has a
dentition similar to those of
rodent
Rodents (from Latin , 'to gnaw') are mammals of the order Rodentia (), which are characterized by a single pair of continuously growing incisors in each of the upper and lower jaws. About 40% of all mammal species are rodents. They are n ...
s, with large
incisor
Incisors (from Latin ''incidere'', "to cut") are the front teeth present in most mammals. They are located in the premaxilla above and on the mandible below. Humans have a total of eight (two on each side, top and bottom). Opossums have 18, wher ...
s at the front of the jaws and broad
molars in the back. One distinguishing feature of ''Megaconus'' is a pair of enlarged
premolar teeth in the lower jaw. The teeth of ''Megaconus'' have many
cusp
A cusp is the most pointed end of a curve. It often refers to cusp (anatomy), a pointed structure on a tooth.
Cusp or CUSP may also refer to:
Mathematics
* Cusp (singularity), a singular point of a curve
* Cusp catastrophe, a branch of bifurc ...
s, allowing them to interlock tightly when the jaws are closed. If ''Megaconus'' is a non-mammalian mammaliaform, it is one of the most basal mammaliaforms to possess such complex teeth.
[
The ]middle ear
The middle ear is the portion of the ear medial to the eardrum, and distal to the oval window of the cochlea (of the inner ear).
The mammalian middle ear contains three ossicles, which transfer the vibrations of the eardrum into waves in the ...
of ''Megaconus'' is more primitive than that of modern mammals. The three bones that make up the middle ear in modern mammals — the malleus, incus, and stapes — originated from the lower jaw in the ancestors of mammals. In ''Megaconus'', these bones are still associated with the lower jaw, placed within a groove behind the dentary bone
In anatomy, the mandible, lower jaw or jawbone is the largest, strongest and lowest bone in the human facial skeleton. It forms the lower jaw and holds the lower teeth in place. The mandible sits beneath the maxilla. It is the only movable bone ...
of the lower jaw.[
''Megaconus'' is estimated to have weighed about . It probably had an outwardly similar appearance to ]multituberculate
Multituberculata (commonly known as multituberculates, named for the multiple tubercles of their teeth) is an extinct order of rodent-like mammals with a fossil record spanning over 130 million years. They first appeared in the Middle Jurassic, a ...
s, a major group of 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 ...
mammals. However, its body is longer than those of multituberculates and most other Mesozoic mammaliaforms, having more back vertebrae
The spinal column, a defining synapomorphy shared by nearly all vertebrates, Hagfish are believed to have secondarily lost their spinal column is a moderately flexible series of vertebrae (singular vertebra), each constituting a characteristi ...
(24) than other early mammals. The transition between the thoracic
The thorax or chest is a part of the anatomy of humans, mammals, and other tetrapod animals located between the neck and the abdomen. In insects, crustaceans, and the extinct trilobites, the thorax is one of the three main divisions of the crea ...
and lumbar vertebrae
The lumbar vertebrae are, in human anatomy, the five vertebrae between the rib cage and the pelvis. They are the largest segments of the vertebral column and are characterized by the absence of the foramen transversarium within the transverse p ...
is more gradual in ''Megaconus'' than it is in multituberculates, and the boundary between the anterior and posterior epaxial muscles (the muscles that cover the front part and the back part of the back, respectively) is positioned farther back.[
''Megaconus'' is inferred to have been ]ambulatory
The ambulatory ( la, ambulatorium, ‘walking place’) is the covered passage around a cloister or the processional way around the east end of a cathedral or large church and behind the high altar. The first ambulatory was in France in the 11th ...
, meaning that it walked on the ground. Its claws are short, meaning that they were not suitable for digging, and only slightly curved, meaning that they were not suitable for climbing. The lack of an elongated calcaneus or "heel" on the ankle means that it could not have run fast. However, ''Megaconus'' differs from most ambulatory mammals in having a fused tibia
The tibia (; ), also known as the shinbone or shankbone, is the larger, stronger, and anterior (frontal) of the two bones in the leg below the knee in vertebrates (the other being the fibula, behind and to the outside of the tibia); it connects ...
and fibula
The fibula or calf bone is a leg bone on the lateral side of the tibia, to which it is connected above and below. It is the smaller of the two bones and, in proportion to its length, the most slender of all the long bones. Its upper extremity i ...
in its lower leg, a feature that is normally seen in jumping or bipedal
Bipedalism is a form of terrestrial locomotion where an organism moves by means of its two rear limbs or legs. An animal or machine that usually moves in a bipedal manner is known as a biped , meaning 'two feet' (from Latin ''bis'' 'double' ...
mammals. Among the only living ambulatory mammals to have a fused tibia and fibula are the armadillo and the aardvark
The aardvark ( ; ''Orycteropus afer'') is a medium-sized, burrowing, nocturnal mammal native to Africa. It is the only living species of the order Tubulidentata, although other prehistoric species and genera of Tubulidentata are known. Unlik ...
.[
Remains of fur are preserved on parts of the skeleton. The fur consists of dark ]guard hair
Guard hair or overhair is the outer layer of hair of most mammals, which overlay the fur. Guard hairs are long and coarse and protect the rest of the pelage (fur) from abrasion and frequently from moisture. They are visible on the surface of the fu ...
s and a lighter layer of underfur
Fur is a thick growth of hair that covers the skin of mammals. It consists of a combination of oily guard hair on top and thick underfur beneath. The guard hair keeps moisture from reaching the skin; the underfur acts as an insulating blanket ...
. Fur seems to be absent on the underside of the abdomen
The abdomen (colloquially called the belly, tummy, midriff, tucky or stomach) is the part of the body between the thorax (chest) and pelvis, in humans and in other vertebrates. The abdomen is the front part of the abdominal segment of the to ...
. A keratinous
Keratin () is one of a family of structural fibrous proteins also known as ''scleroproteins''. Alpha-keratin (α-keratin) is a type of keratin found in vertebrates. It is the key structural material making up scales, hair, nails, feathers, ho ...
spur on the rear leg may have been used to deliver poison, like the spur of the living platypus
The platypus (''Ornithorhynchus anatinus''), sometimes referred to as the duck-billed platypus, is a semiaquatic, egg-laying mammal endemic to eastern Australia, including Tasmania. The platypus is the sole living representative or mono ...
.[
]
Discovery
The 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 ...
skeleton of ''Megaconus'' was discovered in the Daohugou Beds
The Haifanggou Formation (), also known as the Jiulongshan Formation (), is a fossil-bearing rock deposit located near Daohugou () village of Ningcheng County, in Inner Mongolia, northeastern China.
The formation consists of coarse conglomerat ...
of the Tiaojishan Formation in Inner Mongolia, China. The layer in which ''Megaconus'' was found is about 165 million years old ( Ma). In addition to ''Megaconus'', several other mammaliaforms are known from the Daohugou Beds: the non-mammalian mammaliaform '' Castorocauda'', the basal mammals ''Volaticotherium
''Volaticotherium antiquum'' (meaning "ancient gliding beast") is an extinct, gliding, insectivorous mammal that lived in Asia during the Jurassic period, around 164 mya. It is the only member of the genus ''Volaticotherium''.
The discovery of ...
'' and ''Pseudotribos
''Pseudotribos'' ("false chewing") is an extinct genus of mammal that lived in Northern China during the Middle Jurassic some , possibly more closely related to monotremes than to theria (placental and marsupial mammals), although other studies i ...
'', and the placental
Placental mammals (infraclass Placentalia ) are one of the three extant subdivisions of the class Mammalia, the other two being Monotremata and Marsupialia. Placentalia contains the vast majority of extant mammals, which are partly distinguishe ...
mammal ''Juramaia
''Juramaia'' is an extinct genus of very basal eutherian mammal known from the Late Jurassic ( Oxfordian stage) deposits of western Liaoning, China. It is a small shrew-like mammal with a body length of approximately 70–100 mm, making it ...
''.
The genus name ''Megaconus'' means "large cusp", coming from the 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 ...
''mega'' ("large") and the Greek
Greek may refer to:
Greece
Anything of, from, or related to Greece, a country in Southern Europe:
*Greeks, an ethnic group.
*Greek language, a branch of the Indo-European language family.
**Proto-Greek language, the assumed last common ancestor ...
''conus'' ("cusp"), in reference to the pair of large premolars in its lower jaw. The species name ''mammaliaformis'' references its ancestral mammalian features.[
]
Relationships
''Megaconus'' is part of the clade Haramiyida, which is otherwise known almost exclusively from teeth. The relationships of haramiyidans to other early mammals is contested.
One idea is that haramiyidans are close relatives of Multituberculata, the most diverse group of Mesozoic mammals, and that both are part of the larger clade Allotheria
Allotheria (meaning "other beasts", from the Greek , '–other and , '–wild animal) is an extinct branch of successful Mesozoic mammals. The most important characteristic was the presence of lower molariform teeth equipped with two longitudin ...
. According to this evolutionary hypothesis (phylogeny
A phylogenetic tree (also phylogeny or evolutionary tree Felsenstein J. (2004). ''Inferring Phylogenies'' Sinauer Associates: Sunderland, MA.) is a branching diagram or a tree showing the evolutionary relationships among various biological spe ...
), haramiyidans fall within the crown group Mammalia — the clade originating from the most recent common ancestor
In biology and genetic genealogy, the most recent common ancestor (MRCA), also known as the last common ancestor (LCA) or concestor, of a set of organisms is the most recent individual from which all the organisms of the set are descended. The ...
of living mammals. Within the crown group Mammalia, Haramiyida would be a basal offshoot of Boreosphenida
Tribosphenida is a group (infralegion) of mammals that includes the ancestor of ''Hypomylos'', Aegialodontia and Theria (the last common ancestor of marsupials and placentals plus all of its descendants). Its current definition is more or less sy ...
, the clade including marsupial
Marsupials are any members of the mammalian infraclass Marsupialia. All extant marsupials are endemic to Australasia, Wallacea and the Americas. A distinctive characteristic common to most of these species is that the young are carried in a ...
s, eutherians (placental mammals), and all mammals more closely related to them than to monotremes (which are part of the clade Australosphenida
The Australosphenida are a clade of mammals, containing mammals with tribosphenic molars, known from the Jurassic to Mid-Cretaceous of Gondwana. They are thought to have acquired their tribosphenic molars independently from those of Tribosphenid ...
).
A second hypothesis holds that haramiyidans originated before the appearance of crown group Mammalia as more basal members of the larger clade Mammaliaformes and that multituberculates are deeply nested within the crown group, splitting up Allotheria. This phylogeny was supported by the phylogenetic analysis that included ''Megaconus''. Features of ''Megaconus'' that link it with basal mammaliaforms include a groove in the lower jaw holding the middle ear bones (in multituberculates and other true mammals, these bones are completely separated from the lower jaw) and a calcaneum or ankle bone that lacks an enlarged heel. Below is a cladogram
A cladogram (from Greek ''clados'' "branch" and ''gramma'' "character") is a diagram used in cladistics to show relations among organisms. A cladogram is not, however, an evolutionary tree because it does not show how ancestors are related to ...
modified from the analysis:[
A 2022 study found it to be a basal allotherian instead, due to lacking apomorphies characteristic of euharamiyidans.] Furthermore, a 2020 study found it to fall outside Mammaliaformes entirely, as the sister taxon 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, bearing several mammalian traits like erect limbs, endothermy and detail ...
.
References
{{Taxonbar, from=Q15220921
Haramiyida
Prehistoric cynodont genera
Callovian life
Middle Jurassic synapsids of Asia
Jurassic China
Fossils of China
Paleontology in Inner Mongolia
Fossil taxa described in 2013
Taxa named by Chang-Fu Zhou
Taxa named by Shaoyuan Wu
Taxa named by Thomas Martin (paleontologist)
Taxa named by Zhe-Xi Luo