''Bocaconodon'' (meaning "

La Boca La Boca (; "the Mouth", probably of the Matanza River) is a neighborhood (''barrio'') of Buenos Aires, the capital of Argentina. It retains a strong Italian flavour, many of its early settlers having originated in the city of Genoa. Geography ...

conical

tooth 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 ...

") is an early

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 ...

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 n ...

that lived during the

Pliensbachian The Pliensbachian is an age of the geologic timescale and stage in the stratigraphic column. It is part of the Early or Lower Jurassic Epoch or Series and spans the time between 190.8 ± 1.5 Ma and 182.7 ± 1.5 Ma (million years ago). The ...

(

Early Jurassic The Early Jurassic Epoch (in chronostratigraphy corresponding to the Lower Jurassic Series) is the earliest of three epochs of the Jurassic Period. The Early Jurassic starts immediately after the Triassic-Jurassic extinction event, 201.3 Ma ...

) of

Mexico Mexico ( Spanish: México), officially the United Mexican States, is a country in the southern portion of North America. It is bordered to the north by the United States; to the south and west by the Pacific Ocean; to the southeast by Guate ...

. The

type Type may refer to: Science and technology Computing * Typing, producing text via a keyboard, typewriter, etc. * Data type, collection of values used for computations. * File type * TYPE (DOS command), a command to display contents of a file. * Ty ...

and only species, ''Bocaconodon tamaulipensis'' (referring to

Tamaulipas Tamaulipas (), officially the Free and Sovereign State of Tamaulipas ( es, Estado Libre y Soberano de Tamaulipas), is a state in the northeast region of Mexico; one of the 31 states which, along with Mexico City, comprise the 32 Federal Entiti ...

, where 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 seve ...

was found), was named and described in 2008. It is known from a single specimen, a partial right

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 ...

preserving two nearly complete molar teeth and the rear portion of a third molar. The specimen was found at the Huizachal Canyon locality, "a Pliensbachian floodplain

siltstone Siltstone, also known as aleurolite, is a clastic sedimentary rock that is composed mostly of silt. It is a form of mudrock with a low clay mineral content, which can be distinguished from shale by its lack of fissility.Blatt ''et al.'' 1980, ...

in the La Boca Formation".

Description

The only known specimen of ''Bocaconodon'' (IGM 6617) is a partial right

, which preserves most of the last two molar teeth and the rear portion of a third molar. The preserved part of the dentary was quite thin from top to bottom, with the thinnest part being beneath the last molar. Behind the tooth row there was a facet that may have housed the coronoid bone. At the rear end of the dentary, there was a depression known as the

postdentary trough The postdentary trough is a skeletal feature seen in Mesozoic mammals. It is found on the inside of the lower jaw (dentary), at the back behind the molar teeth. It is the hollow in which the postdentary bones and Meckel's cartilage sit. These bone ...

, where the postdentary bones (homologous to some of 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 ...

bones of modern mammals) would have been attached. Since the postdentary bones were attached to the dentary, ''Bocaconodon'' may have retained the primitive

quadrate Quadrate may refer to: * Quadrate bone * Quadrate (heraldry) * Quadrate lobe of liver * Quadrate tubercle The quadrate tubercle is a small tubercle found upon the upper part of the femur. It serves as a point of insertion of the quadratus femori ...

articular The articular bone is part of the lower jaw of most vertebrates, including most jawed fish, amphibians, birds and various kinds of reptiles, as well as ancestral mammals. Anatomy In most vertebrates, the articular bone is connected to two oth ...

jaw joint found in most non-mammalian

synapsid Synapsids + (, 'arch') > () "having a fused arch"; synonymous with ''theropsids'' (Greek, "beast-face") are one of the two major groups of animals that evolved from basal amniotes, the other being the sauropsids, the group that includes rep ...

s, in addition to the newer dentary-

squamosal The squamosal is a skull bone found in most reptiles, amphibians, and birds. In fishes, it is also called the pterotic bone. In most tetrapods, the squamosal and quadratojugal bones form the cheek series of the skull. The bone forms an ancestral co ...

joint found in mammals and other

s. The molars had a "triconodont" shape, with three main cusps aligned in a straight line. In both of the well-preserved teeth, the middle cusp (cusp a) was the largest, the cusp behind it (cusp c) was smaller, and the front cusp (cusp b) was smaller still. Cusp a had an asymmetrical, backwards-curved shape, while cusps b and c were more symmetrical. Cusp b pointed upwards while cusp c was more backwards-pointing. Behind cusp c there was another, smaller cusp (cusp d). The molars bore a narrow ridge on the lingual (inner) side known as the cingulum. The lingual cingulum differed from those of most morganucodonts in lacking any distinct cusps.

Classification

The phylogenetic position of ''Bocaconodon'' within mammaliaforms is shown in the cladogram below:

References

Prehistoric mammaliaforms Prehistoric cynodont genera Pliensbachian life Jurassic synapsids Jurassic Mexico Fossils of Mexico Fossil taxa described in 2008 Taxa named by James Hopson Taxa named by James M. Clark {{Paleo-therapsid-stub