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 found, the length of Brasilodon has been estimated at . Its dentition shows that it was most likely an insectivore. The genus is monotypic, containing only the species B. quadrangularis. Brasilodon belongs to the family Brasilodontidae, 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 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, which has been dated to the early Norian age of the Late Triassic. The holotype (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, 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 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, 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 bones, which were found in more basal probainognathians like Prozostrodon. There was no postorbital bar behind the eye socket. The zygomatic arch (cheek bone) was quite low and slender.
The lower jaw consisted mainly of the dentary bone, which bore the teeth. The tip of the lower jaw was bent upwards. The symphysis, the joint between the two halves of the dentary, was unfused, with a rough surface where ligaments would have been attached. The meckelian groove was located near the lower edge of the dentary. The postdentary bones, a set of bones in the lower jaw located behind the dentary, were highly reduced compared to the condition in more primitive cynodonts. Like in most non-mammalian cynodonts, the jaw joint in Brasilodon involved the quadrate bone (a bone of the cranium, homologous to the incus of modern mammals) and the articular bone (one of the postdentary bones, homologous to the malleus). There may also have been a contact between the dentary and the squamosal bone, with the articular process of the dentary having a thickened end, apparently a precursor condition to the fully developed dentary condyle of more derived mammaliamorphs. The quadrate bone had a well-developed stapedial process, similar to that of the basal mammaliaform Morganucodon.
The front part of the roof of the mouth consisted of a well-developed secondary palate, 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 and the pterygoid bones. There was a pair of gaps between the pterygoids called the interpterygoid vacuities. Well-developed interpterygoid vacuities are known in basal cynodonts like Procynosuchus, 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, which had a well-defined promontorium. There were separate foramina (holes) for the maxillary and mandibular nerves. Next to the crista interfenestralis there was a large depression called the "stapedial recess", which bore several foramina. The occipital condyles 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 bulbs, the parts of the brain associated with the sense of smell. On the other hand, the cerebral hemispheres 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 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 incisors, canines 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 serrations. 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. 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 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) 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.
Vertebrae and ribs
The vertebrae 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 tritylodontids and basal mammaliaforms, but not in more basal cynodonts, which generally had amphicoelous vertebrae that were concave on both sides. The neural canal was fairly large, while the lateral walls were thin. The neural arches 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 apparently were nearly horizontal and located rather high on the vertebrae. The shape of the base of the neural spine (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 cynognathians, had wide extensions known as costal plates on their ribs; these plates are absent in most probainognathians, including Brasilodon.
Limbs and limb girdles