''Triconodon'' ("three-coned tooth") is a genus of extinct

mammal A mammal () is a vertebrate animal of the Class (biology), 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 ...

from the

Early Cretaceous The Early Cretaceous (geochronology, geochronological name) or the Lower Cretaceous (chronostratigraphy, chronostratigraphic name) is the earlier or lower of the two major divisions of the Cretaceous. It is usually considered to stretch from 143.1 ...

England England is a Countries of the United Kingdom, country that is part of the United Kingdom. It is located on the island of Great Britain, of which it covers about 62%, and List of islands of England, more than 100 smaller adjacent islands. It ...

and

France France, officially the French Republic, is a country located primarily in Western Europe. Overseas France, Its overseas regions and territories include French Guiana in South America, Saint Pierre and Miquelon in the Atlantic Ocean#North Atlan ...

with two known species: ''T. mordax'' and ''T. averianovi''. First described in 1859 by

Richard Owen Sir Richard Owen (20 July 1804 – 18 December 1892) was an English biologist, comparative anatomy, comparative anatomist and paleontology, palaeontologist. Owen is generally considered to have been an outstanding naturalist with a remarkabl ...

,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 (''genus typica'') 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-bearin ...

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, tear ...

. 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 Paraphyly is a taxonomic term describing a grouping that consists of the grouping's last common ancestor and some but not all of its descendant lineages. The grouping is said to be paraphyletic ''with respect to'' the excluded subgroups. In co ...

polyphyletic A polyphyletic group is an assemblage that includes organisms with mixed evolutionary origin but does not include their most recent common ancestor. The term is often applied to groups that share similar features known as Homoplasy, homoplasies ...

assemblage respectively, but several lineages of "triconodont" mammals do form a natural,

monophyletic In biological cladistics for the classification of organisms, monophyly is the condition of a taxonomic grouping being a clade – that is, a grouping of organisms which meets these criteria: # the grouping contains its own most recent co ...

group, known as

Eutriconodonta Eutriconodonta is an order (biology), order of early mammals. Eutriconodonts existed in Asia (including Insular India, pre-contact India), Africa, Europe, North America, North and South America during the Jurassic and the Cretaceous periods. 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 (geology), Pennsylvanian sub-period of the late Carboniferous period. By the mid-Triassic, there were many synaps ...

by originating the understanding of the "triconodont" grade and eutriconodont

clade In biology, a clade (), also known as a Monophyly, monophyletic group or natural group, is a group of organisms that is composed of a common ancestor and all of its descendants. Clades are the fundamental unit of cladistics, a modern approach t ...

. Further discoveries on its skeletal anatomy also offer further insights on the palaeobiology of

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 ...

mammals.

Discovery

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

mandible In jawed vertebrates, the mandible (from the Latin ''mandibula'', 'for chewing'), lower jaw, or jawbone is a bone that makes up the lowerand typically more mobilecomponent of the mouth (the upper jaw being known as the maxilla). The jawbone i ...

found in the

Purbeck Group The Purbeck Group is an Upper Jurassic to Lower Cretaceous lithostratigraphy, lithostratigraphic group (stratigraphy), group (a sequence of rock strata) in south-east England. The name is derived from the district known as the Isle of Purbeck 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: * Great Britain, a large island comprising the countries of England, Scotland and Wales * The United Kingdom of Great Britain and Northern Ireland, a sovereign state in Europe comprising Great Britain and the north-eas ...

. These deposits date to the earliest

Cretaceous The Cretaceous ( ) is a geological period that lasted from about 143.1 to 66 mya (unit), million years ago (Mya). It is the third and final period of the Mesozoic Era (geology), Era, as well as the longest. At around 77.1 million years, it is the ...

, 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 143.1 ±0.6 Ma and 137.05 ± 0.2 (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'', or closer to the group containing the rest of the clade, rendering '' Trioracodon'' 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 Plants are the eukaryotes that form the Kingdom (biology), kingdom Plantae; they ar ...

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

, thanks to a broad ontogenetic range presented by the specimens. Through several juvenile specimens we can document the replacement of its lower fourth

premolar The premolars, also called premolar Tooth (human), teeth, or bicuspids, are transitional teeth located between the Canine tooth, canine and Molar (tooth), molar teeth. In humans, there are two premolars per dental terminology#Quadrant, quadrant in ...

, 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 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 mammals of the order Monotremata. They are the only group of living mammals that lay eggs, rather than bearing live young. The extant monotreme species are the platypus and the four species of echidnas. Monotremes are typified ...

, multituberculates and

theria Theria ( or ; ) is a scientific classification, subclass of mammals amongst the Theriiformes. Theria includes the eutherians (including the Placentalia, placental mammals) and the metatherians (including the marsupials) but excludes the egg-lay ...

ns. The

cerebrum The cerebrum (: cerebra), 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 olfac ...

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 Triconodontidae 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