''Triconodon'' ("three coned tooth") is a genus of extinct mammal from the Early Cretaceous of

England England is a country that is part of the United Kingdom. It shares land borders with Wales to its west and Scotland to its north. The Irish Sea lies northwest and the Celtic Sea to the southwest. It is separated from continental Europe b ...

and

France France (), officially the French Republic ( ), is a country primarily located in Western Europe. It also comprises of overseas regions and territories in the Americas and the Atlantic, Pacific and Indian Oceans. Its metropolitan area ...

with two known species: ''T. mordax'' and ''T. averianovi''. First described in 1859 by Richard Owen,R. Owen. 1859. Palaeontology. '' Encyclopedia Britannica, 8th ed.'' 17:91-176 . Wagner/P. Wagner it is the

type genus In biological taxonomy, the type genus is the genus which defines a biological family and the root of the family name. Zoological nomenclature According to the International Code of Zoological Nomenclature, "The name-bearing type of a nominal ...

for the order Triconodonta, a group of mammals characterised by their three-cusped (triconodont) molar

teeth A tooth ( : teeth) is a hard, calcified structure found in the jaws (or mouths) of many vertebrates and used to break down food. Some animals, particularly carnivores and omnivores, also use teeth to help with capturing or wounding prey, t ...

. Since then, this "simplistic" type of dentition has been understood to be either ancestral for mammals or else to have evolved multiple times, rendering "triconodonts" a paraphyletic or

polyphyletic A polyphyletic group is an assemblage of organisms or other evolving elements that is of mixed evolutionary origin. The term is often applied to groups that share similar features known as homoplasies, which are explained as a result of conver ...

assemblage respectively, but several lineages of "triconodont" mammals do form a natural, monophyletic group, known as

Eutriconodonta Eutriconodonta is an order of early mammals. Eutriconodonts existed in Asia, Africa, Europe, North and South America during the Jurassic and the Cretaceous periods. The order was named by Kermack ''et al.'' in 1973 as a replacement name for the ...

, of which ''Triconodon'' is indeed part of. ''Triconodon'', therefore, is significant in the understanding of the

evolution of mammals The evolution of mammals has passed through many stages since the first appearance of their synapsid ancestors in the Pennsylvanian sub-period of the late Carboniferous period. By the mid-Triassic, there were many synapsid species that looked l ...

by originating the understanding of the "triconodont"

grade Grade most commonly refers to: * Grade (education), a measurement of a student's performance * Grade, the number of the year a student has reached in a given educational stage * Grade (slope), the steepness of a slope Grade or grading may also ref ...

and eutriconodont clade. Further discoveries on its skeletal anatomy also offer further insights on the palaeobiology 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.

Discovery

The type specimen of ''Triconodon'' is BMNH 47764, a single

mandible 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 ...

found in the

Purbeck Group The Purbeck Group is an Upper Jurassic to Lower Cretaceous lithostratigraphic group (a sequence of rock strata) in south-east England. The name is derived from the district known as the Isle of Purbeck in Dorset where the strata are exposed in ...

, pertaining to the type species (''T. mordax''). Since then, several other specimens have been found in this region, mostly represented by skulls and jaws, making it the most common mammal fossils in this area of

Britain Britain most often refers to: * The United Kingdom, a sovereign state in Europe comprising the island of Great Britain, the north-eastern part of the island of Ireland and many smaller islands * Great Britain, the largest island in the United King ...

. These deposits date to the earliest

Cretaceous The Cretaceous ( ) is a geological period that lasted from about 145 to 66 million years ago (Mya). It is the third and final period of the Mesozoic Era, as well as the longest. At around 79 million years, it is the longest geological period of ...

, to the

Berriasian In the geological timescale, the Berriasian is an age/stage of the Early/Lower Cretaceous. It is the oldest subdivision in the entire Cretaceous. It has been taken to span the time between 145.0 ± 4.0 Ma and 139.8 ± 3.0 Ma (million years ago ...

at around 145-140 million years of age. The second species, ''T. averianovi'', was named in 2020 based on fossils found in the

-aged

Lulworth Formation The Lulworth Formation is a geologic formation in England. It dates from the late Tithonian to the mid Berriasian. It is a subunit of the Purbeck Group. In Dorset, it consists of three members, which are in ascending order, the Mupe Member, the ...

. A single specimen has also been found in the Champblanc Quarry in

, dating to roughly the same age. It is unclear if it belongs to the same species as the British form, though given the close temporal and geographical proximity it seems likely.

Classification

''Triconodon'' is known from two species, represented only by ''T. averianovi'' and ''T. mordax'' (though see above). Besides being the type genus and species for Eutriconodonta as seen above, it is also the type genus and species for

Triconodontidae Triconodontidae is an extinct family of small, carnivorous mammals belonging to the order Eutriconodonta, endemic to what would become Asia, Europe, North America and probably also Africa and South America during the Jurassic through Cretaceous p ...

, erected in 1887 by Charles Marsh. Within this group it is usually recovered in a basal position, sometimes as sister taxa to ''

Trioracodon ''Trioracodon'' is an extinct genus of Late Jurassic to Early Cretaceous eutriconodont mammal found in North America and the British Isles. It was named in 1928 ''T. bisulcus'' is known from the Morrison Formation, where it is present in strat ...

'', or closer to the group containing the rest of the clade, rendering ''

'' in the basalmost position.

Biology

Like most eutriconodonts, ''Triconodon'' was probably a

carnivore A carnivore , or meat-eater (Latin, ''caro'', genitive ''carnis'', meaning meat or "flesh" and ''vorare'' meaning "to devour"), is an animal or plant whose food and energy requirements derive from animal tissues (mainly muscle, fat and other s ...

, its triconodont teeth being well adapted for shearing, and possessing other speciations such as long

canines Canine may refer to: Zoology and anatomy * a dog-like Canid animal in the subfamily Caninae ** ''Canis'', a genus including dogs, wolves, coyotes, and jackals ** Dog, the domestic dog * Canine tooth, in mammalian oral anatomy People with the surn ...

and powerful jaw musculature. It was about as large as a modern cat, suggesting that it hunted vertebrate prey such as other mammals or small dinosaurs. A study detailing Mesozoic mammal diets ranks it among carnivorous taxa.

Tooth replacement

''Triconodon'' is one of the few Mesozoic mammals with direct evidence of

tooth eruption Tooth eruption is a process in tooth development in which the teeth enter the mouth and become visible. It is currently believed that the periodontal ligament plays an important role in tooth eruption. The first human teeth to appear, the deciduo ...

, thanks to a broad ontogenetic range presented by the specimens. Through several juvenile specimens we can document the replacement of its lower fourth premolar, erupting and coming into use when at least three out of its four molars were already fully erupted.G. G. Simpson. 1928. A Catalogue of the Mesozoic Mammalia in the Geological Department of the British Museum 1-215

Brain

One of the earliest fossil brain

endocast An endocast is the internal cast of a hollow object, often referring to the cranial vault in the study of brain development in humans and other organisms. Endocasts can be artificially made for examining the properties of a hollow, inaccessible sp ...

studies has been performed for ''Triconodon''. The olfactory lobe is large, with a teardrop-shaped outline, suggesting a well developed sense of smell. The cerebral hemisphere is long, oval and flat, lacking the inflated appearance present in

monotremes Monotremes () are prototherian mammals of the order Monotremata. They are one of the three groups of living mammals, along with placentals (Eutheria), and marsupials ( Metatheria). Monotremes are typified by structural differences in their br ...

multituberculates 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 ...

and

theria Theria (; Greek: , wild beast) is a subclass of mammals amongst the Theriiformes. Theria includes the eutherians (including the placental mammals) and the metatherians (including the marsupials) but excludes the egg-laying monotremes. ...

ns. The

cerebrum The cerebrum, telencephalon or endbrain is the largest part of the brain containing the cerebral cortex (of the two cerebral hemispheres), as well as several subcortical structures, including the hippocampus, basal ganglia, and olfactory bulb ...

is neither expanded anteriorly to overlap the posterior part of the olfactory lobe, nor is it hemispherical. It is similar to that of multituberculates in that it has a large, roughly triangular bulge, now thought to be the superior cistern. The midbrain was apparently exposed to the dorsal side of the brain as with many other non-

n mammals. What this indicates about the animal's intelligence is currently unclear, though its overall brain proportions are somewhat smaller than those of more derived mammals like multituberculates and therians.Harry Jerison, Evolution of The Brain and Intelligence, 02/12/2012

References

{{Taxonbar, from=Q778777 Eutriconodonts Berriasian life Early Cretaceous mammals of Europe Cretaceous England Fossils of England Cretaceous France Fossils of France Fossil taxa described in 1859 Taxa named by Richard Owen Prehistoric mammal genera