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''Brasilodon'' ("tooth from Brazil") is an extinct genus of small, mammal-like cynodonts that lived in what is now Brazil during the Norian age of the
Late Triassic The Late Triassic is the third and final epoch of the Triassic Period in the geologic time scale, spanning the time between Ma and Ma (million years ago). It is preceded by the Middle Triassic Epoch and followed by the Early Jurassic Epoch. ...
epoch, about 225.42 million years ago. While no complete skeletons have been found, the length of ''Brasilodon'' has been estimated at around . Its dentition shows that it was most likely an insectivore. The genus is
monotypic In biology, a monotypic taxon is a taxonomic group (taxon) that contains only one immediately subordinate taxon. A monotypic species is one that does not include subspecies or smaller, infraspecific taxa. In the case of genera, the term "unispe ...
, containing only the species ''B. quadrangularis''. ''Brasilodon'' belongs to the family
Brasilodontidae ''Brasilodon'' ("tooth from Brazil") is an extinct genus of small, mammal-like cynodonts that lived in what is now Brazil during the Norian age of the Late Triassic epoch, about 225.42 million years ago. While no complete skeletons have been foun ...
, whose members were some of the closest relatives of mammals, the only cynodonts alive today. Two other brasilodontid genera, ''Brasilitherium'' and ''Minicynodon'', are now considered to be junior synonyms of ''Brasilodon''.


Discovery and naming

The first three specimens referred to ''Brasilodon quadrangularis'' were found at the Linha São Luiz site, a quarry near the town of
Faxinal do Soturno Faxinal do Soturno is a municipality in the state of Rio Grande do Sul, Brazil. See also *List of municipalities in Rio Grande do Sul This is a list of the municipalities in the state of Rio Grande do Sul (RS), located in the South Region of Br ...
in the state of Rio Grande do Sul. The rocks where ''Brasilodon'' was found belong to the upper part of the Candelária Sequence of the Santa Maria Supersequence, corresponding to the traditional
Caturrita Formation The Caturrita Formation is a rock formation found in Rio Grande do Sul, Brazil. Its sediments were deposited in the Paraná Basin. The formation is from the Upper Triassic and forms part of the Santa Maria Supersequence in the upper section of th ...
, which has been dated to the early Norian age of the
Late Triassic The Late Triassic is the third and final epoch of the Triassic Period in the geologic time scale, spanning the time between Ma and Ma (million years ago). It is preceded by the Middle Triassic Epoch and followed by the Early Jurassic Epoch. ...
. 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 ...
(UFRGS-PV-0611-T) consists of a well-preserved skull retaining both the left and right upper postcanine teeth, but missing the lower jaw. The referred specimen UFRGS-PV-0716-T consists of the left front part of the skull, preserving 6 postcanines. The specimen UFRGS-PV-0628-T consists of a partial skull including the lower jaw, with most of the upper and lower dentition preserved. Many other specimens of ''Brasilodon'' have since been discovered, both in Faxinal do Soturno and in Candelária, and comprising both cranial and postcranial material. The genus ''Brasilodon'' was named in a 2003 paper by
José F. Bonaparte José is a predominantly Spanish and Portuguese form of the given name Joseph. While spelled alike, this name is pronounced differently in each language: Spanish ; Portuguese (or ). In French, the name ''José'', pronounced , is an old vernacul ...
, Agustín G. Martinelli, Cesar L. Schultz and Rogerio Rubert. The generic name ''Brasilodon'' is derived from the country of Brazil where it was found, and from the Greek word , meaning "tooth". The intended meaning is "tooth from Brazil". The specific epithet ''quadrangularis'' refers to the rectangular shape of the upper postcanine teeth. In the same 2003 article, the species ''Brasilitherium riograndensis'' was named, based on six specimens. Specimens attributed to ''Brasilitherium'' have been found at the same localities as ''Brasilodon'', and have been distinguished from this taxon largely by their smaller size, different skull proportions, and by the presence of a cusp d in the lower postcanines of ''Brasilitherium'', but not in ''Brasilodon''. A 2005 paper erected the family
Brasilodontidae ''Brasilodon'' ("tooth from Brazil") is an extinct genus of small, mammal-like cynodonts that lived in what is now Brazil during the Norian age of the Late Triassic epoch, about 225.42 million years ago. While no complete skeletons have been foun ...
for the two genera. In 2010 a third brasilodontid species, ''Minicynodon maieri'', was named by Bonaparte ''et al.'', based on a single well-preserved skull from Faxinal do Soturno. This species was differentiated from ''Brasilodon'' by the firm attachment of the bones of the skull roof, and from ''Brasilitherium'' by the lack of a cusp b in the lower postcanines. ''Minicynodon'' was also interpreted as possessing a double jaw joint, unlike the other two species. Later studies have cast doubt on the validity of ''Brasilitherium'' and ''Minicynodon'', proposing instead that they, along with ''Brasilodon'', merely represent individual variation within a single species. In that case, ''Brasilodon'' is the valid taxon, whereas ''Brasilitherium'' and ''Minicynodon'' are invalid junior synonyms.


Description

''Brasilodon'' was a small, derived cynodont, with an estimated total length of around . It exhibited many mammal-like features, including a well-developed
secondary palate The secondary palate is an anatomical structure that divides the nasal cavity from the oral cavity in many vertebrates. In human embryology, it refers to that portion of the hard palate that is formed by the growth of the two palatine shelves medi ...
, symmetrical tooth development, and a more derived ear anatomy than in earlier cynodonts.


Skull

Different specimens of ''Brasilodon'' had widely varying skull lengths, ranging from , owing mainly to differences in age. The skull had a low, elongated shape when seen from the side. The top of the braincase was fairly wide, with a short and low parietal crest. ''Brasilodon'' lacked prefrontal and
postorbital The ''postorbital'' is one of the bones in vertebrate skulls which forms a portion of the dermal skull roof and, sometimes, a ring about the orbit. Generally, it is located behind the postfrontal and posteriorly to the orbital fenestra. In some ...
bones, which were found in more basal probainognathians like ''
Prozostrodon ''Prozostrodon'' is an extinct genus of advanced cynodonts that was closely related to the ancestors of mammals. The remains were found in Brazil and are dated middle to late Triassic. It was originally described as a species of ''Thrinaxodon' ...
''. There was no
postorbital bar The postorbital bar (or postorbital bone) is a bony arched structure that connects the frontal bone of the skull to the zygomatic arch, which runs laterally around the eye socket. It is a trait that only occurs in mammalian taxa, such as most strep ...
behind the
eye socket In anatomy, the orbit is the cavity or socket of the skull in which the eye and its appendages are situated. "Orbit" can refer to the bony socket, or it can also be used to imply the contents. In the adult human, the volume of the orbit is , o ...
. The
zygomatic arch In anatomy, the zygomatic arch, or cheek bone, is a part of the skull formed by the zygomatic process of the temporal bone (a bone extending forward from the side of the skull, over the opening of the ear) and the temporal process of the zygo ...
(cheek bone) was quite low and slender. The lower jaw consisted mainly of 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 ...
, which bore the teeth. The tip of the lower jaw was bent upwards. The
symphysis A symphysis (, pl. symphyses) is a fibrocartilaginous fusion between two bones. It is a type of cartilaginous joint, specifically a secondary cartilaginous joint. # A symphysis is an amphiarthrosis, a slightly movable joint. # A growing togethe ...
, the joint between the two halves of the dentary, was unfused, with a rough surface where ligaments would have been attached. The
meckelian groove The Meckelian groove (or Meckel's groove, Meckelian fossa, or Meckelian foramen, or Meckelian canal) is an opening in the medial (inner) surface of the mandible (lower jaw) which exposes the Meckelian cartilage.quadrate bone The quadrate bone is a skull bone in most tetrapods, including amphibians, sauropsids (reptiles, birds), and early synapsids. In most tetrapods, the quadrate bone connects to the quadratojugal and squamosal bones in the skull, and forms upper pa ...
(a bone of the cranium, homologous to the
incus The ''incus'' (plural incudes) or anvil is a bone in the middle ear. The anvil-shaped small bone is one of three ossicles in the middle ear. The ''incus'' receives vibrations from the ''malleus'', to which it is connected laterally, and transmit ...
of modern mammals) and the
articular bone 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 ...
(one of the postdentary bones, homologous to the
malleus The malleus, or hammer, is a hammer-shaped small bone or ossicle of the middle ear. It connects with the incus, and is attached to the inner surface of the eardrum. The word is Latin for 'hammer' or 'mallet'. It transmits the sound vibrations f ...
). There may also have been a contact between the dentary and the
squamosal bone 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 c ...
, with the articular process of the dentary having a thickened end, apparently a precursor condition to the fully developed dentary condyle of more derived
mammaliamorph Mammaliamorpha is a clade of cynodonts. It contains the clades Tritylodontidae and Mammaliaformes, as well as a few genera that do not belong to either of these groups. The family Tritheledontidae has also been placed in Mammaliamorpha by some ...
s. The quadrate bone had a well-developed stapedial process, similar to that of the basal
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 comm ...
'' Morganucodon''. The front part of the roof of the mouth consisted of a well-developed
secondary palate The secondary palate is an anatomical structure that divides the nasal cavity from the oral cavity in many vertebrates. In human embryology, it refers to that portion of the hard palate that is formed by the growth of the two palatine shelves medi ...
, formed by the maxillae and the palatine bones, which extended about as far back as the last postcanine. The secondary palate had a groove that the lower postcanines would have fit into when the mouth was closed. Behind the secondary palate was the primary palate, formed by the
vomer The vomer (; lat, vomer, lit=ploughshare) is one of the unpaired facial bones of the skull. It is located in the midsagittal line, and articulates with the sphenoid, the ethmoid, the left and right palatine bones, and the left and right max ...
and the
pterygoid bone The pterygoid is a paired bone forming part of the palate of many vertebrates Vertebrates () comprise all animal taxa within the subphylum Vertebrata () (chordates with backbones), including all mammals, birds, reptiles, amphibians, and fis ...
s. There was a pair of gaps between the pterygoids called the interpterygoid vacuities. Well-developed interpterygoid vacuities are known in basal cynodonts like ''
Procynosuchus ''Procynosuchus'' (Greek: "Before dog crocodile") is an extinct genus of cynodonts from the Late Permian. It is considered to be one of the earliest and most basal cynodonts. It was 60 cm (2 ft) long. Remains of ''Procynosuchus'' have ...
'', but the vacuities are generally reduced or absent in more advanced groups, so their presence in ''Brasilodon'' is likely a derived condition. The pterygoids had a transverse contact with the basipterygoid process of the basisphenoid bone. The basicranium was wide when seen from below. The prootic and opisthotic bones were fused into a
petrosal bone The petrous part of the temporal bone is pyramid-shaped and is wedged in at the base of the skull between the sphenoid and occipital bones. Directed medially, forward, and a little upward, it presents a base, an apex, three surfaces, and three an ...
, which had a well-defined promontorium. There were separate
foramina In anatomy and osteology, a foramen (;Entry "foramen"
in
(holes) for the maxillary and
mandibular 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 tooth, teeth in place. The mandible sits beneath the maxilla. It is the only movabl ...
nerves. Next to the
crista interfenestralis The crista interfenestralis is an anatomical feature found in the inner ear of many reptiles. It is a bone ridge that divides the inner ear into two parts, the anterior and the posterior. The anterior part contains the fenestra ovalis and seems ...
there was a large depression called the "stapedial recess", which bore several foramina. The
occipital condyle The occipital condyles are undersurface protuberances of the occipital bone in vertebrates, which function in articulation with the superior facets of the atlas vertebra. The condyles are oval or reniform (kidney-shaped) in shape, and their anteri ...
s were located further back than the lambdoid crest. The cranial endocast, the interior surface of the braincase, is an important source of information about the brains of prehistoric animals, as the brains themselves are very rarely preserved in fossils. A 2013 study by Rodrigues ''et al.'' described the endocast of the specimen UFRGS-PV-1043-T, which was originally assigned to the genus ''Brasilitherium''. The endocast had a length of , which is 46.5% of the total length of this skull. The maximum width of the endocast was . The shape of the endocast indicates that the animal possessed well-developed
olfactory bulb The olfactory bulb (Latin: ''bulbus olfactorius'') is a neural structure of the vertebrate forebrain involved in olfaction, the sense of smell. It sends olfactory information to be further processed in the amygdala, the orbitofrontal cortex ( ...
s, the parts of the brain associated with the sense of smell. On the other hand, the
cerebral hemisphere The vertebrate cerebrum (brain) is formed by two cerebral hemispheres that are separated by a groove, the longitudinal fissure. The brain can thus be described as being divided into left and right cerebral hemispheres. Each of these hemispheres ...
s were smaller than in basal mammaliaforms like ''Morganucodon''. It is unclear if ''Brasilodon'' possessed a neocortex, a part of the brain found in modern mammals. The encephalisation quotient (relative brain size) was interpreted to likely be higher than in more basal cynodonts, such as the
traversodontid Traversodontidae is an extinct family of herbivorous cynodonts. Traversodonts were primarily Gondwanan, with many species known from Africa and South America. Recently, traversodonts have also been found from Europe and eastern North America. Tra ...
'' Massetognathus'', but lower than in basal mammaliaforms and modern mammals. Like other non-mammalian cynodonts, ''Brasilodon'' likely had a lissencephalic (smooth) brain surface.


Dentition

Like most cynodonts, ''Brasilodon'' possessed a heterodont dentition, divided into
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,
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 postcanines. The specimens originally assigned to ''Brasilodon'' do not preserve any of the upper incisors, but some specimens attributed to ''Brasilitherium'' bore 4 pairs of incisors in the upper jaw. There were 3 pairs of lower incisors; the first lower incisor was procumbent (forwards-pointing). In adult specimens, the canines were large and flattened from side to side, but had no
serration Serration is a saw-like appearance or a row of sharp or tooth-like projections. A serrated cutting edge has many small points of contact with the material being cut. By having less contact area than a smooth blade or other edge, the applied p ...
s. In small individuals attributed to ''Brasilitherium'', there were two pairs of functional canines, which were only slightly bigger than the incisors. There were up to 8 pairs of postcanines in both jaws. The postcanines had a rectangular shape when looking towards the
crown A crown is a traditional form of head adornment, or hat, worn by monarchs as a symbol of their power and dignity. A crown is often, by extension, a symbol of the monarch's government or items endorsed by it. The word itself is used, partic ...
. There was little variation in shape in the upper postcanines, while the lower ones exhibited more variation. The upper postcanines were considerably smaller than the lower ones, a condition observed in many other probainognathians. The postcanines were "triconodont-like", with three main cusps arranged in a roughly straight line. There was a large central cusp, termed cusp A in the upper teeth and cusp a in the lower ones, a smaller cusp in front of the central cusp (cusp B/b), and another small cusp behind the central cusp (cusp C/c). In the lower postcanines of some specimens, there was an additional cusp (cusp d) behind cusp c; the absence or presence of this cusp has been used to distinguish ''Brasilodon'' from ''Brasilitherium'', but it is likely that the lack of a cusp d in some specimens is caused by
tooth wear Tooth wear refers to loss of tooth substance by means other than dental caries. Tooth wear is a very common condition that occurs in approximately 97% of the population. This is a normal physiological process occurring throughout life; but with i ...
in older individuals. The postcanines bore varying numbers of accessory cusps as well, sometimes forming a cingulum. In older specimens, there was a large gap (
diastema A diastema (plural diastemata, from Greek διάστημα, space) is a space or gap between two teeth. Many species of mammals have diastemata as a normal feature, most commonly between the incisors and molars. More colloquially, the condition ...
) between the canines and postcanines, which was created by the first postcanine being shed but not replaced. There was a tongue-and-groove system in the middle lower postcanines for the interlocking between neighbouring teeth, with cusp d fitting between cusp b and the accessory cusp e, but this system was apparently not functional due to the distance between the teeth. The roots of the postcanines were not fully divided as they are in modern mammals, but they had a noticeable constriction in the middle, giving them an 8-shaped cross-section. The nutritious canals of the roots were divided too. The postcanine roots were secured to the jaw by a ring of ossified
periodontal ligament The periodontal ligament, commonly abbreviated as the PDL, is a group of specialized connective tissue fibers that essentially attach a tooth to the alveolar bone within which it sits. It inserts into root cementum one side and onto alveolar b ...
.


Vertebrae and ribs

The
vertebra 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 ...
e of ''Brasilodon'' are poorly known. One specimen (UFRGS-PV-1043-T) preserves 4 isolated presacral vertebrae, all of which are badly preserved. The vertebral centra were ''platycoelous'', having a flat front end and a concave back end. Such a shape is also seen in some
tritylodontid 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 details ...
s and basal mammaliaforms, but not in more basal cynodonts, which generally had ''amphicoelous'' vertebrae that were concave on both sides. The
neural canal In the developing chordate (including vertebrates), the neural tube is the embryonic precursor to the central nervous system, which is made up of the brain and spinal cord. The neural groove gradually deepens as the neural fold become elevated, ...
was fairly large, while the lateral walls were thin. The
neural arches 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 characteristic ...
were fused to the centra in this specimen, showing that it was an adult. One of the vertebrae preserves a small and cylindrical diapophysis (transverse process). The zygapophyses (articular processes, four extensions of each vertebra which connect neighbouring vertebrae together) are poorly preserved, but the
postzygapophyses The articular processes or zygapophyses ( Greek ζυγον = " yoke" (because it links two vertebrae) + απο = "away" + φυσις = " process") of a vertebra are projections of the vertebra that serve the purpose of fitting with an adjacent vert ...
apparently were nearly horizontal and located rather high on the vertebrae. The shape of the base of the
neural spine 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 characteristic ...
(a pointed extension on the top surface of the vertebra) indicates that it was elongated anteroposteriorly (in a front-to-back direction). The specimen UFRGS-PV-1043-T also preserves 4 isolated rib fragments. The ribs were flattened anteroposteriorly. Many non-probainognathian cynodonts, including most
cynognathia Cynognathia ("dog jaw") is one of two major clades of cynodonts, the other being Probainognathia. Cynognathians included the large carnivorous genus '' Cynognathus'' and the herbivorous traversodontids. Cynognathians can be identified by several ...
ns, had wide extensions known as ''costal plates'' on their ribs; these plates are absent in most probainognathians, including ''Brasilodon''.


Limbs and limb girdles

The lower part of a right
scapula The scapula (plural scapulae or scapulas), also known as the shoulder blade, is the bone that connects the humerus (upper arm bone) with the clavicle (collar bone). Like their connected bones, the scapulae are paired, with each scapula on eith ...
(shoulder blade) is preserved in the specimen UFRGS-PV-1043-T, with two other associated bone fragments likely representing the middle part of the dorsal (upper) margin, and the anterior (front) margin, respectively. The scapula was a thin bone, with a constricted base as in other epicynodonts. The upper margin was somewhat convex, while the posterodorsal (upper back) angle was pointed. The front and back margins had sideways-projecting flanges, as seen in other non-mammaliaform epicynodonts. There was a well-developed
acromion In human anatomy, the acromion (from Greek: ''akros'', "highest", ''ōmos'', "shoulder", plural: acromia) is a bony process on the scapula (shoulder blade). Together with the coracoid process it extends laterally over the shoulder joint. The ac ...
process at the end of the front margin. The acromion was forwards-pointing as in many other cynodonts. There was a deep
infraspinous fossa The infraspinous fossa (infraspinatus fossa or infraspinatous fossa) of the scapula is much larger than the supraspinatous fossa; toward its vertebral margin a shallow concavity is seen at its upper part; its center presents a prominent convexi ...
on the side of the scapula, but a
supraspinous fossa The supraspinous fossa (supraspinatus fossa, supraspinatous fossa) of the posterior aspect of the scapula (the shoulder blade) is smaller than the infraspinous fossa, concave, smooth, and broader at its vertebral than at its humeral end. Its ...
was seemingly absent. The
glenoid The glenoid fossa of the scapula or the glenoid cavity is a bone part of the shoulder. The word ''glenoid'' is pronounced or (both are common) and is from el, gléne, "socket", reflecting the shoulder joint's ball-and-socket form. It is a sha ...
facet of the scapula (the area where it connected to the humerus) was concave and had a somewhat ovoid shape. The glenoid appears to have been downwards-pointing, unlike the more sideways-facing glenoids of certain early-diverging cynodonts. The
coracoid A coracoid (from Greek κόραξ, ''koraks'', raven) is a paired bone which is part of the shoulder assembly in all vertebrates except therian mammals (marsupials and placentals). In therian mammals (including humans), a coracoid process is prese ...
and procoracoid bones have not been preserved, but there were articular surfaces for these bones on the scapula. The specimen UFRGS-PV-1043-T preserves a complete left humerus (upper arm bone), with a length of . The shaft of the humerus was slender. It was somewhat twisted, with an angulation of 15 degrees between the opposite ends of the bone. A twisted humerus is found in most cynodonts outside
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. ...
(the clade containing modern
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 and
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 ...
s), including in modern monotremes. However, the twisting of the humerus was relatively low in ''Brasilodon'' when compared to that of many other non-therian cynodonts. On the proximal side (the side connecting to the shoulder), the humeral head had a hemispherical shape typical of mammaliaforms, but unlike that of more basal cynodonts, where the humeral head was generally more ovoid. Unlike in most therians, the humeral head faced posterolaterally (backwards and to the side). There was a ridge extending from the humeral head to the ectepicondylar crest on the other end of the bone. Unlike more basal cynodonts, ''Brasilodon'' had a distinct
greater tubercle The greater tubercle of the humerus is the outward part the upper end of that bone, adjacent to the large rounded prominence of the humerus head. It provides attachment points for the supraspinatus, infraspinatus, and teres minor muscles, thr ...
, though it was quite small. The
lesser tubercle The lesser tubercle of the humerus, although smaller, is more prominent than the greater tubercle: it is situated in front, and is directed medially and anteriorly. The projection of the lesser tubercle is anterior from the junction that is found ...
had a thick and bulbous shape. It was larger than the greater tubercle, as in basal mammaliaforms and monotremes, but unlike in
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,
spalacotheriid Spalacotheriidae is a family of extinct mammals belonging to the paraphyletic group ' Symmetrodonta'. They lasted from the Early Cretaceous to the Campanian in North America, Europe, Asia and North Africa. Spalacotheriids are characterised by ha ...
s and therians. The
deltopectoral crest Deltopectoral may refer to; * Clavipectoral triangle, also known as the deltopectoral triangle * Deltopectoral groove * Deltopectoral lymph nodes One or two deltopectoral lymph nodes (or infraclavicular nodes) are found beside the cephalic vein, b ...
was confluent with the greater tubercle, and extended across 48% of the length of the humerus. The crest was quite large and robust, like in other non-mammalian cynodonts, monotremes and fossorial (burrowing) therians such as
moles Moles can refer to: * Moles de Xert, a mountain range in the Baix Maestrat comarca, Valencian Community, Spain * The Moles (Australian band) *The Moles, alter ego of Scottish band Simon Dupree and the Big Sound People *Abraham Moles, French engin ...
. On the other side of the bone, there was another crest known as the teres crest. This crest was found in most
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 ...
cynodonts, but missing in ''
Vincelestes ''Vincelestes'' ("Vince's thief") is an extinct genus of actively mobile mammal, that lived in what would be South America during the Early Cretaceous from 130 to 112 mya, existing for approximately . Description ''Vincelestes neuquenianus'' ...
'' and therians. The teres crest was about as long as the deltopectoral crest, and extended from the lesser tubercle to a large and ovoid protrusion near the middle of the bone. This protrusion may have served as an anchor for the
teres major muscle The teres major muscle is a muscle of the upper limb. It attaches to the scapula and the humerus and is one of the seven scapulohumeral muscles. It is a thick but somewhat flattened muscle. The teres major muscle (from Latin ''teres'', meanin ...
. A similar structure existed in many other Mesozoic cynodonts, but not in multituberculates, spalacotheriids or therians. On the front surface of the humerus, there was a wide
bicipital groove The bicipital groove (intertubercular groove, sulcus intertubercularis) is a deep groove on the humerus that separates the greater tubercle from the lesser tubercle. It allows for the long tendon of the biceps brachii muscle to pass. Structure ...
, bordered by the deltopectoral and teres crests, similar to that of many other Mesozoic cynodonts. The bicipital groove likely served as an attachment point for the
coracobrachialis muscle The coracobrachialis muscle is the smallest of the three muscles that attach to the coracoid process of the scapula. (The other two muscles are pectoralis minor and the short head of the biceps brachii.) It is situated at the upper and medial part ...
. The distal side of the humerus (the side connected to the
radius In classical geometry, a radius ( : radii) of a circle or sphere is any of the line segments from its center to its perimeter, and in more modern usage, it is also their length. The name comes from the latin ''radius'', meaning ray but also the ...
and
ulna The ulna (''pl''. ulnae or ulnas) is a long bone found in the forearm that stretches from the elbow to the smallest finger, and when in anatomical position, is found on the medial side of the forearm. That is, the ulna is on the same side of t ...
of the
forearm The forearm is the region of the upper limb between the elbow and the wrist. The term forearm is used in anatomy to distinguish it from the arm, a word which is most often used to describe the entire appendage of the upper limb, but which in ...
) had a width of 43% the length of the humerus. This was narrower than in many other non-mammalian cynodonts, as well as in monotremes, but wider than in the basal mammaliaform ''Morganucodon'' and the therian ''Didelphis''. The entepicondyle was quite robust, and had a protrusion that pointed towards the proximal end of the bone. The ectepicondyle was more narrow and less protruding than the entepicondyle. The entepicondylar foramen was a large and elongated hole. Conversely, there was no ectepicondylar foramen; this hole was found in most cynodonts more basal than ''Brasilodon'', but missing in tritylodontids and most mammaliaforms. The
ulnar The ulna (''pl''. ulnae or ulnas) is a long bone found in the forearm that stretches from the elbow to the smallest finger, and when in anatomical position, is found on the medial side of the forearm. That is, the ulna is on the same side of t ...
and
radial Radial is a geometric term of location which may refer to: Mathematics and Direction * Vector (geometric), a line * Radius, adjective form of * Radial distance, a directional coordinate in a polar coordinate system * Radial set * A bearing f ...
condyles (the parts connecting to the ulna and radius, respectively) had a bulbous shape, as seen in other non-mammalian cynodonts. The condyles were separated by a narrow groove, and lacked the trochlear (pulley-like) shape seen in many crown-group mammals, like therians or the stem-monotreme ''
Kryoryctes ''Kryoryctes'' is a genus of prehistoric monotreme mammal from the Early Cretaceous (Albian) Eumeralla Formation of Victoria, Australia from the Otway Group of Dinosaur Cove. It is known only from a partial right humerus, estimated at 106 m ...
''. The ulnar condyle was visible both on the front and back sides of the humerus. Its front side was somewhat transversely compressed. The radial condyle was larger than the ulnar condyle, but was not visible at the back side of the bone. The
olecranon fossa The olecranon fossa is a deep triangular depression on the posterior side of the humerus, superior to the trochlea. It provides space for the olecranon of the ulna during extension of the forearm. Structure The olecranon fossa is located on ...
(a depression on the back side of the humerus) was quite shallow, unlike the deeper fossa seen in multituberculates, ''Vincelestes'' and therians. The left radius of UFRGS-PV-1043-T is long. The radius was slender, with a circular cross-section near the proximal end (closer to the elbow). The head of the radius (where it articulated with the humerus) was approximately circular, with a cup-shaped depression in the middle surrounded by a bulbous rim. The head was angled somewhat towards the anteromedial (front right) side. On the posteromedial (back right) side of the head, there was a small articular facet where it would have articulated with the
radial notch The radial notch of the ulna (lesser sigmoid cavity) is a narrow, oblong, articular depression on the lateral side of the coronoid process; it receives the circumferential articular surface of the head of the radius. It is concave from before bac ...
of the ulna. There was no
radial tuberosity Beneath the neck of the radius, on the medial side, is an eminence, the radial tuberosity; its surface is divided into: * a ''posterior, rough portion'', for the insertion of the tendon of the biceps brachii. * an ''anterior, smooth portion'', on w ...
for the attachment of the biceps muscle. The distal end of the bone (closer to the wrist) was shaped like a flattened rectangle. The distal end was somewhat curved posteromedially, which would have allowed the bone to cross over the front of the ulna, as in tritylodontids, ''Morganucodon'' and opossums. The distal articular surface (where it connected to the wrist) also had a rectangular shape. The right ulna of UFRGS-PV-1043-T has a length of . As in most cynodonts, the ulna was narrow mediolaterally (from side to side), and had a sigmoid (s-like) shape. On the sides of the bone, there were two grooves running from one end of the bone to the other, enclosed by thickened edges on the front and back sides. The grooves are thought to have served as attachment points for extensor and flexor muscles. On the proximal end of the bone, there was a well-developed and ossified
olecranon The olecranon (, ), is a large, thick, curved bony eminence of the ulna, a long bone in the forearm that projects behind the elbow. It forms the most pointed portion of the elbow and is opposite to the cubital fossa or elbow pit. The olecranon ...
process. With the exception of ''
Trucidocynodon ''Trucidocynodon'' is an extinct genus of ecteniniid cynodonts from Upper Triassic of Brazil. It contains a single species, ''Trucidocynodon riograndensis''. Fossils of ''Trucidocynodon'' were discovered in Santa Maria Formation outcrops in Pal ...
'' and tritylodontids, no cynodonts more basal than ''Brasilodon'' are known to have had an ossified olecranon, and it is thought to have been
cartilaginous Cartilage is a resilient and smooth type of connective tissue. In tetrapods, it covers and protects the ends of long bones at the joints as articular cartilage, and is a structural component of many body parts including the rib cage, the neck a ...
in those taxa. The olecranon of ''Brasilodon'' made up a bit less than 20% of the length of the ulna, and was directed forwards relative to the rest of the bone. The olecranon was somewhat thicker from side to side than the rest of the bone. The
semilunar notch The trochlear notch (), also known as semilunar notch and greater sigmoid cavity, is a large depression in the upper extremity of the ulna that fits the trochlea of the humerus (the bone directly above the ulna in the arm) as part of the elbo ...
, where the ulna articulated with the ulnar condyle of the humerus, was large and semicircular. The anconeal process was quite small, like in more basal cynodonts and the docodont mammaliaform ''
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 Al ...
'', but unlike in many other mammaliaforms, where the anconeal process forms a prominent crest. The pelvis (hip bone) of UFRGS-PV-1043-T preserves a complete left acetabulum and pubis, and an incomplete left ilium and ischium. The acetabulum is where the
femur The femur (; ), or thigh bone, is the proximal bone of the hindlimb in tetrapod vertebrates. The head of the femur articulates with the acetabulum in the pelvic bone forming the hip joint, while the distal part of the femur articulates wit ...
of the hindlimb articulates with the pelvis. The acetabulum of ''Brasilodon'' was a deep, spherical and sideways-facing depression. The three bones making up the acetabulum (pubis, ilium and ischium) were fused together, with no sutures being visible; this is a feature that ''Brasilodon'' shared with mammaliaforms, while the bones were more distinct in some more basal cynodonts such as
traversodontid Traversodontidae is an extinct family of herbivorous cynodonts. Traversodonts were primarily Gondwanan, with many species known from Africa and South America. Recently, traversodonts have also been found from Europe and eastern North America. Tra ...
s. The bones formed three distinct crests known as ''supracetabular buttresses'' around the acetabulum. The buttresses were separated by gaps of , unlike in modern therians, where there is a fully ossified rim around the acetabulum. The gaps would likely have been filled by fibrocartilage in the living animal. The blade of the ilium is badly preserved, but the shape of the base indicates that the postacebular (backwards-pointing) part of the blade was either reduced or absent in ''Brasilodon'', much like in other
prozostrodontia Prozostrodontia is a clade of cynodonts including mammaliaforms and their closest relatives such as Tritheledontidae and Tritylodontidae. It was erected as a node-based taxon by Liu and Olsen (2010) and defined as the least inclusive clade cont ...
ns. The pubic tuberosity was located below the acetabulum as in most probainognathians, including modern therians. In basal epicynodonts, monotremes and the spalacotheriid ''
Akidolestes ''Akidolestes'' is an extinct genus of mammals from the family Spalacotheriidae, spalacotheriid. Although Akidolestes do not have any modern relatives, they are early mammal, mammals related to Theria, therians (the subclass containing marsupi ...
'', the pubic tuberosity was located in front of the acetabulum instead. The pubis and ischium formed a large and ovoid
obturator foramen The obturator foramen (Latin foramen obturatum) is the large opening created by the ischium and pubis bones of the pelvis through which nerves and blood vessels pass. Structure It is bounded by a thin, uneven margin, to which a strong membran ...
. The right femur (thigh bone) of UFRGS-PV-1043-T is long. The shaft of the femur was mostly straight, but with a prominent forward bend close to the hip joint, as in other non-mammalian cynodonts; in modern mammals, this bend is less well-developed. The proximal part of the shaft (closer to the hip) was mostly square-shaped, but it became more compressed from front to back more distally (towards the knee), while simultaneously becoming wider from side to side. The proximal and distal ends of the bone had the same width. On the proximal end of the bone, the
femoral head The femoral head (femur head or head of the femur) is the highest part of the thigh bone (femur). It is supported by the femoral neck. Structure The head is globular and forms rather more than a hemisphere, is directed upward, medialward, and a l ...
(which articulated with the acetabulum of the hip) had a hemispherical shape. The head was separated from the rest of the bone by a short
femoral neck The femoral neck (femur neck or neck of the femur) is a flattened pyramidal process of bone, connecting the femoral head with the femoral shaft, and forming with the latter a wide angle opening medialward. Structure The neck is flattened from ...
, as in tritylodontids and mammaliaforms; more primitive cynodonts lacked a femoral neck. The head was angled around 60 degrees medially (to the right) compared to the longitudinal axis of the femur, resembling the condition in multituberculates and spalacotheriids. Unlike in mammaliaforms, there was no fovea capitis on the femoral head. Slightly distally to the femoral head, there was a well-developed projection known as the
greater trochanter The greater trochanter of the femur is a large, irregular, quadrilateral eminence and a part of the skeletal system. It is directed lateral and medially and slightly posterior. In the adult it is about 2–4 cm lower than the femoral head.Stan ...
on the left side of the bone. A distinct greater trochanter is also found in tritylodontids and mammaliaforms, but the greater trochanter was confluent with the femoral head in more basal cynodonts. On the right side of the bone, there was a short crest called the
lesser trochanter The lesser trochanter is a conical posteromedial bony projection of the femoral shaft. it serves as the principal insertion site of the iliopsoas muscle. Structure The lesser trochanter is a conical posteromedial projection of the shaft of the fe ...
. The lesser trochanter pointed to the right, and was visible on the front side of the bone, as in many other probainognathians. In more basal cynodonts, it generally was more backwards-pointing, and not visible on the front side. The lesser trochanter did not extend as far towards the proximal end of the bone as in basal mammaliaforms. There was no
third trochanter In human anatomy, the third trochanter is a bony projection occasionally present on the proximal femur near the superior border of the gluteal tuberosity. When present, it is oblong, rounded, or conical in shape and sometimes continuous with the g ...
. On the back side of the femur, between the greater and lesser trochanters, there was a deep and narrow depression called the intertrochanteric fossa. On the distal edge of this fossa, there was an
intertrochanteric crest The intertrochanteric crest is a prominent bony ridge upon the posterior surface of the femur at the junction of the neck and the shaft of the femur. It extends between the greater trochanter superiorly, and the lesser trochanter inferiorly. Ana ...
which connected the two trochanters. This crest is also found in most therians, but was missing in most Mesozoic cynodont groups. There was a relatively shallow, triangular depression located distally to the intertrochanteric crest; this probably served as an attachment point for the hip adductor muscles. On the back side of the distal end of the femur, there were two condyles that would have connected to the
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 ...
, the medial and
lateral Lateral is a geometric term of location which may refer to: Healthcare *Lateral (anatomy), an anatomical direction * Lateral cricoarytenoid muscle * Lateral release (surgery), a surgical procedure on the side of a kneecap Phonetics *Lateral co ...
condyles. The medial condyle was compressed from side to side. The
popliteal fossa The popliteal fossa (also referred to as hough, .html" ;"title="/sup>">/sup> or kneepit in analogy to the cubital fossa) is a shallow depression located at the back of the knee joint. The bones of the popliteal fossa are the femur and the tibia ...
had a triangular shape and was located close to the condyles. UFRGS-PV-1043-T preserves a nearly complete left tibia (shin bone). It was a slender bone, with the preserved parts having a length of . The shaft was mostly straight, but with a slight medial (rightwards) curve in the proximal part (towards the knee); more basal cynodonts generally had a stronger medial curve of the tibia. The shaft became progressively more flattened towards the distal end (towards the ankle). On the proximal part, there were two articular facets, the medial and
lateral Lateral is a geometric term of location which may refer to: Healthcare *Lateral (anatomy), an anatomical direction * Lateral cricoarytenoid muscle * Lateral release (surgery), a surgical procedure on the side of a kneecap Phonetics *Lateral co ...
facets, which articulated with the condyles of the femur. The lateral facet was wider than the medial one. The distal end of the tibia has not been preserved. UFRGS-PV-1043-T preserves two left
tarsal bone In the human body, the tarsus is a cluster of seven articulating bones in each foot situated between the lower end of the tibia and the fibula of the lower leg and the metatarsus. It is made up of the midfoot (cuboid, medial, intermediate, and la ...
s, the
calcaneum In humans and many other primates, the calcaneus (; from the Latin ''calcaneus'' or ''calcaneum'', meaning heel) or heel bone is a bone of the tarsus of the foot which constitutes the heel. In some other animals, it is the point of the hock. ...
and
astragalus ''Astragalus'' is a large genus of over 3,000 species of herbs and small shrubs, belonging to the legume family Fabaceae and the subfamily Faboideae. It is the largest genus of plants in terms of described species. The genus is native to tempe ...
. The calcaneum (heel bone) of ''Brasilodon'' had a low and broad shape, unlike the narrow calcaneum found in multituberculates, the " symmetrodont" ''
Zhangheotherium ''Zhangheotherium'' is a genus of symmetrodont, an extinct order of mammals. Previously known from only the tall pointed crowned teeth, ''Zhangheotherium'', described from Liaoning Province, China, fossils in 1997, is the first symmetrodont known ...
'' and therians. It had a convex upper surface and a concave lower surface. There was a well-developed tuber calcis on the back end of the bone; this feature is found in most eucynodonts, but is missing in more basal taxa like ''
Thrinaxodon ''Thrinaxodon'' is an extinct genus of cynodonts, most commonly regarded by its species ''T. liorhinus'' which lived in what are now South Africa and Antarctica during the Early Triassic. ''Thrinaxodon'' lived just after the Permian–Triassic m ...
''. There was a
peroneal shelf Peroneal may refer to: * Relating to the lateral compartment of leg * Peroneal artery * Peroneal vein * Peroneus muscles * Peroneal nerve See also *Perineal (disambiguation) *Peritoneal The peritoneum is the serous membrane forming the ...
on the lateral (left) side of the calcaneum. On the medial (right) side, there was a projection known as the sustentaculum tali, which would have connected to the underside of the astragalus (ankle bone). In basal cynodonts like ''Thrinaxodon'', no sustentaculum tali has been found, suggesting that it was cartilaginous, if it even existed in those taxa. The left astragalus was smaller than the calcaneum. It had a hemispherical dorsomedial (top right) surface and a flat lateroplantar (bottom left) surface. The front end of the astragalus had a small head. The neck that separated the head from the rest of the bone was shorter than in modern therians. UFRGS-PV-1043-T also preserves two incomplete
metatarsal The metatarsal bones, or metatarsus, are a group of five long bones in the foot, located between the tarsal bones of the hind- and mid-foot and the phalanges of the toes. Lacking individual names, the metatarsal bones are numbered from the me ...
s, comprising a proximal and a distal end, which would have connected to the tarsals and to the
phalange The phalanges (singular: ''phalanx'' ) are digital bones in the hands and feet of most vertebrates. In primates, the thumbs and big toes have two phalanges while the other digits have three phalanges. The phalanges are classed as long bones. ...
s, respectively. The proximal end bore a circular depression, while distal end had two symmetrical condyles. A possible metapod (metatarsal or
metacarpal In human anatomy, the metacarpal bones or metacarpus form the intermediate part of the skeletal hand located between the phalanges of the fingers and the carpal bones of the wrist, which forms the connection to the forearm. The metacarpal bones ar ...
) fragment is also known from the specimen UFRGS-PV-0765-T, which had a broad proximal part and a more narrow shaft. Both of these specimens also preserve some
phalange The phalanges (singular: ''phalanx'' ) are digital bones in the hands and feet of most vertebrates. In primates, the thumbs and big toes have two phalanges while the other digits have three phalanges. The phalanges are classed as long bones. ...
s (finger bones), with UFRGS-PV-0765-T preserving a nearly complete middle phalange. This phalange was rather short, with a concave proximal end and two small condyles on the distal end.


Classification

The genus ''Brasilodon'' belongs to Brasilodontidae, a family of advanced probainognathian cynodonts. Along with ''Brasilodon'', two contemporary genera (''Brasilitherium'' and ''Minicynodon'') have been assigned to the family, both of which are likely to be synonyms of ''Brasilodon''. ''
Protheriodon ''Protheriodon'' is an extinct genus of probainognathian cynodonts which existed in the Santa Maria Formation of the Paraná Basin in southeastern Brazil during the middle Triassic period. It contains the species ''Protheriodon estudianti''. It ...
'', a genus from the
Middle Triassic In the geologic timescale, the Middle Triassic is the second of three epochs of the Triassic period or the middle of three series in which the Triassic system is divided in chronostratigraphy. The Middle Triassic spans the time between Ma and ...
(
Ladinian The Ladinian is a stage and age in the Middle Triassic series or epoch. It spans the time between Ma and ~237 Ma (million years ago). The Ladinian was preceded by the Anisian and succeeded by the Carnian (part of the Upper or Late Triassic ...
) of Brazil, and '' Panchetocynodon'', a poorly known cynodont from the
Early Triassic The Early Triassic is the first of three epochs of the Triassic Period of the geologic timescale. It spans the time between Ma and Ma (million years ago). Rocks from this epoch are collectively known as the Lower Triassic Series, which is a un ...
(
Induan The Induan is the first age of the Early Triassic epoch in the geologic timescale, or the lowest stage of the Lower Triassic series in chronostratigraphy. It spans the time between 251.902 Ma and Ma (million years ago). The Induan is sometime ...
) of India, were also placed in Brasilodontidae in a 2013 paper by José Bonaparte. However, ''Protheriodon'' has more recently been found to be an early-diverging probainognathian unrelated to brasilodontids, while ''Panchetocynodon'' is otherwise treated as an ''
incertae sedis ' () or ''problematica'' is a term used for a taxonomic group where its broader relationships are unknown or undefined. Alternatively, such groups are frequently referred to as "enigmatic taxa". In the system of open nomenclature, uncertain ...
'' taxon, whose relationships to other taxa are unclear, due to its incompleteness. Features of the skull and dentition of ''Brasilodon'' indicate that it was a derived cynodont closely related to mammals, the only extant cynodonts.
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 ...
analyses have commonly recovered it as a member of the clade Mammaliamorpha, lying closer to mammals than the tritylodontids, but outside Mammaliaformes, a more exclusive clade containing mammals and their closest relatives, such as
morganucodont Morganucodonta ("Glamorgan teeth") is an extinct order of basal Mammaliaformes, a group including crown-group mammals (Mammalia) and their close relatives. Their remains have been found in Southern Africa, Western Europe, North America, India an ...
s, docodonts and haramiyidans. The
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 ...
below is adapted from a 2019 analysis by Wallace ''et al.'':


Palaeobiology


Locomotion

Features of the postcranium indicate that ''Brasilodon'' was a generalised animal capable of diverse modes of
locomotion Locomotion means the act or ability of something to transport or move itself from place to place. Locomotion may refer to: Motion * Motion (physics) * Robot locomotion, of man-made devices By environment * Aquatic locomotion * Flight * Locomo ...
, including digging and climbing. The large tuberosity near the deltopectoral crest of the humerus implies that the teres major muscle (an important retractor muscle of the forelimb) was quite large, as seen in modern fossorial
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. The digging adaptations of ''Brasilodon'' were however less pronounced than in modern fossorial mammals, as well as many other non-mammalian cynodonts. Evidence for scansorial (climbing) abilities includes the hemispherical humeral and femoral heads, which would have allowed a wide range of rotation of the shoulder and hip joints, the well-developed ectepicondylar crest, the hemispherical capitulum and nearly circular radial head, which would have conferred high mobility to the elbow, and the small anconeal process of the ulna. Several features of the humerus, including the twisted shaft, indicate that ''Brasilodon'' had sprawling or semi-sprawling forelimbs. On the other hand, features of the pelvis and hindlimbs, such as the poorly developed postacetabular portion of the iliac blade, indicate a more erect (
parasagittal The sagittal plane (; also known as the longitudinal plane) is an anatomical plane that divides the body into right and left sections. It is perpendicular to the transverse and coronal planes. The plane may be in the center of the body and divid ...
) posture of the hindlimbs.


Tooth replacement

According to one theory, outlined in detail in a 2010 paper by Martinelli ''et al.'', ''Brasilodon'' had a
polyphyodont A polyphyodont is any animal whose teeth are continually replaced. In contrast, diphyodonts are characterized by having only two successive sets of teeth. Polyphyodonts include most toothed fishes, many reptiles such as crocodiles and geckos, ...
tooth replacement, where the teeth were replaced more than once throughout the animal's lifetime, though it is unclear how many successive postcanine replacements there were in ''Brasilodon''. The postcanine replacement ceased long before the death of an individual, allowing extremely strong wear to develop in the teeth in older individuals. In some cases, the individual's teeth wore down over time to less than half the height of the crown still remaining. The size of the postcanine diastema indicates that the front postcanines may have been lost faster than in the primitive cynodont ''Thrinaxodon''. According to Martinelli ''et al.'' (2010), the postcanines were replaced in an alternating manner. The postcanine replacement would have occurred from the back to the front, like in several other non-mammaliaform probainognathians, and would thus have differed from that of early mammaliaforms, where the postcanines were replaced from the front towards the back. This may have been due to the unlimited skull growth of early probainognathians. Tooth replacement variation can perhaps also be attributed to diet, with the herbivorous/omnivorous traversodontids having widened postcanines and a sequential tooth replacement, and carnivorous and insectivorous non-mammaliaform probainognathians like ''Brasilodon'' having an alternating tooth replacement instead. A 2022 study by Cabreira ''et al.'' suggests that ''Brasilodon'' actually was diphyodont, replacing its teeth only once, like most modern mammals. According to this theory, the postcanines in ''Brasilodon'' consisted of a set of deciduous teeth, which were replaced, and a set of
permanent teeth Permanent teeth or adult teeth are the second set of teeth formed in diphyodont mammals. In humans and old world simians, there are thirty-two permanent teeth, consisting of six maxillary and six mandibular molars, four maxillary and four mand ...
divided into premolars and
molars The molars or molar teeth are large, flat teeth at the back of the mouth. They are more developed in mammals. They are used primarily to grind food during chewing. The name ''molar'' derives from Latin, ''molaris dens'', meaning "millstone to ...
. The study suggests that the deciduous postcanines in ''Brasilodon'' erupted sequentially in a back-to-front direction, while the molars were added in the opposite direction, from the front to the back.


Palaeoecology

Specimens of ''Brasilodon'' have been found in two localities. The holotype was found at the Linha São Luiz locality, within the municipality of Faxinal do Soturno. Other specimens have been found at the Sesmaria do Pinhal locality in the municipality of Candelária; both of these locations lie within the Brazilian part of the
Paraná Basin The Paraná Basin ( pt, Bacia do Paraná, es, Cuenca del Paraná) is a large cratonic sedimentary basin situated in the central-eastern part of South America. About 75% of its areal distribution occurs in Brazil, from Mato Grosso to Rio Grande d ...
. The rocks where ''Brasilodon'' was found belong to the upper part of the Candelária Sequence, which corresponds to a biostratigraphic unit known as the ''Riograndia'' Assemblage Zone. The ''Riograndia'' AZ has been dated to the early Norian age of the Late Triassic epoch, around 225.42 million years ago. Cynodonts are one of the most common and taxonomically diverse elements in this Assemblage Zone, and are represented by many well-preserved specimens. In addition to ''Brasilodon'', cynodonts are represented by the
tritheledontid Tritheledontidae, the tritheledontids or ictidosaurs, is an extinct family of small to medium-sized (about 10 to 20 cm long) cynodonts. They were highly mammal-like, specialized cynodonts, although they still retained a few reptile-like ana ...
s '' Irajatherium'' and ''
Riograndia ''Riograndia'' is an extinct genus of tritheledontid cynodonts from the Late Triassic of South America. The type and only species is ''Riograndia guaibensis''. Remains have been found in the Caturrita Formation of the geopark of Paleorrota. I ...
'', the basal mammaliamorph '' Botucaraitherium'', and indeterminate
traversodontid Traversodontidae is an extinct family of herbivorous cynodonts. Traversodonts were primarily Gondwanan, with many species known from Africa and South America. Recently, traversodonts have also been found from Europe and eastern North America. Tra ...
s. Other animals include the
dicynodont Dicynodontia is an extinct clade of anomodonts, an extinct type of non-mammalian therapsid. Dicynodonts were herbivorous animals with a pair of tusks, hence their name, which means 'two dog tooth'. Members of the group possessed a horny, typic ...
'' Jachaleria'', the
procolophonid Procolophonidae is an extinct family of small, lizard-like parareptiles known from the Late Permian to Late Triassic that were distributed across Pangaea, having been reported from Europe, North America, China, South Africa, South America, Antarc ...
''
Soturnia ''Soturnia'' is an extinct genus of procolophonid parareptile. It is known from rocks of the Late Triassic-age Caturrita Formation of the municipality of Faxinal do Soturno in the geopark of Paleorrota, Brazil. ''Soturnia'' was named in 2003 by ...
'', the
lepidosauromorph Lepidosauromorpha (in PhyloCode known as ''Pan-Lepidosauria'') is a group of reptiles comprising all diapsids closer to lizards than to archosaurs (which include crocodiles and birds). The only living sub-group is the Lepidosauria, which cont ...
s ''
Cargninia ''Cargninia'' is an extinct genus of basal lepidosauromorph from the Late Triassic of Brazil. The type and only known species is ''Cargninia enigmatica''. It is known from the holotype UFRGS PV 1027 T, a partial left dentary (l ...
'', ''
Clevosaurus ''Clevosaurus'' (meaning "Gloucester lizard") is an extinct genus of rhynchocephalian reptile from the Triassic, Late Triassic and the Jurassic, Early Jurassic Period (geology), periods. Species of ''Clevosaurus'' were widespread across Pangaea, ...
'' and '' Lanceirosphenodon'', and the
avemetatarsalia Avemetatarsalia (meaning "bird metatarsals") is a clade of diapsid reptiles containing all archosaurs more closely related to birds than to crocodilians. The two most successful groups of avemetatarsalians were the dinosaurs and pterosaurs. ...
ns '' Faxinalipterus'', ''
Guaibasaurus ''Guaibasaurus'' is an extinct genus of basal saurischian dinosaur known from the Late Triassic Caturrita Formation of Rio Grande do Sul, southern Brazil. Most analyses recover it as a sauropodomorph, although there are some suggestions that it ...
'', ''
Macrocollum ''Macrocollum'' is a genus of unaysaurid sauropodomorph dinosaur that lived during the Late Triassic period (early Norian) in what is now Brazil. It is one of the oldest dinosaurs known. Discovery ''Macrocollum'' was discovered in 2012 in Rio G ...
'', '' Maehary'', '' Sacisaurus'' and ''
Unaysaurus ''Unaysaurus'' is a genus of unaysauridae, unaysaurid sauropodomorpha, sauropodomorph herbivore dinosaur. Discovered in southern Brazil, in the geopark of Paleorrota, in 1998, and announced in a press conference on Thursday, December 3, 2004, it ...
''. Indeterminate remains of
temnospondyl Temnospondyli (from Greek τέμνειν, ''temnein'' 'to cut' and σπόνδυλος, ''spondylos'' 'vertebra') is a diverse order of small to giant tetrapods—often considered primitive amphibians—that flourished worldwide during the Carbo ...
s,
phytosaur Phytosaurs (Φυτόσαυροι in greek) are an extinct group of large, mostly semiaquatic Late Triassic archosauriform reptiles. Phytosaurs belong to the order Phytosauria. Phytosauria and Phytosauridae are often considered to be equivalent g ...
s and other groups have also been found. The location where these fossils were found is a fluvial system, characterized by large quantities of fine sandstone forming sandy beds, resulting from sedimentation in the basin during peak flow events.


References

{{Taxonbar, from=Q1064317 Prehistoric prozostrodonts Prehistoric cynodont genera Norian genera Late Triassic synapsids of South America Triassic Brazil Fossils of Brazil Paraná Basin Transitional fossils Fossil taxa described in 2003 Taxa named by José Bonaparte