Brasilodon is an extinct genus of cynodonts that lived during the Norian age of the Late Triassic Period, about 228 to 208.5 million years ago. The average length of Brasilodon is approximately 12 cm and weighed about 20 grams. The average size of this cynodont species is about the size of extant mice. Brasilodontids are most likely insectivores due to their teeth. The two species in this genus are B. tetragonus and B. quadrangularis, two very small cynodonts belonging to the family Brasilodontidae.
Description
Brasilodon is a genus of small derived cynodont that exhibits many mammalian features including a secondary palate, symmetrical tooth development, and more developed sense of hearing than other cynodonts. The study of Brasilodons shows that the expansion of advanced cynodonts with mammalian features occurred in several species that still retained many primitive features.
Skull
Skull size in Brasilidontids varies from individual to individual about 35-40%, and is about 20% larger than Brasilitherium, the other genus in the family Brasilodontidae. Brasilodon species lack a prefrontal bone, postorbital bone, and postorbital bar, which is seen in most other Probainognathians. Some derived features shown in Brasilodon include: reduced postdentary bones, nasal-frontal bone contact on the lateral sides, reduced canines close to the size of the last incisor, and the vascular features of their "stapedial recess". The well developed stapedial process, as well as the presence of several foramina in Brasilodon indicates that the middle ear of Brasilodontids was another feature showing that it is a more derived Probainognathian. The interpterygoid vacuity seen in Brasilodontids is considered to be possibly derived or a reversed character of the primary palate found in mammals, which extends from the choana to the interpterygoid vacuities. The presence of a secondary palate shows another derived feature found in Brasilodon, and the posterior end of the secondary osseous palate extends posteriorly to the last postcanine.
Dentition
Brasilodon quadrangularis is characterized by quadrangular upper and lower postcanines, which are secured to the jaw by ossified periodontal ligament in the root. Each tooth row contains 6-8 upper postcanines and 8-9 lower postcanines; there is little morphological variation to be seen in the upper tooth rows, while the lower tooth rows exhibit more variable morphology. The upper postcanines are considerably smaller than lower postcanines, a condition observed in many Probainognathians. Brasilodon can be differentiated from Brasilitherium by its more derived lower postcanines. This consists of a prominent central cusp, an anterior accessory cusp, and a posterior accessory cusp, resulting in a symmetrical distribution. The postcanines seen in Brasilitherium, on the other hand, has morganucondontid-like plan in lower postcanines. Due to the size of the diastema between canines and postcanines in the largest Brasilodontids, this shows that anterior postcanines were most likely lost faster than it was in Thrinaxodon. Although it is not known how many successional postcanine replacements there are in Brasilodon, there is more than one replacement, and it is done the same way as many cynodonts, in which postcanines are replaced in an alternate manner. Postcanine replacement in Brasilodon was slow enough to allow extremely strong wear to develop in the teeth. This replacement also slowed over the individual's lifespan and ceased long before the death of an individual. In some cases, the individual's tooth wore down over time to less than half the height of the crown still remaining. Brasilodontids postcanine replacement occurs the same way as many other advanced Probainognathians, and differs from most mammaliforms in the direction of tooth replacement. This is most likely due to their unlimited skull growth. Tooth replacement variation can perhaps also be attributed to cynodont diet. Herbivorous/omnivorous traversodontids species present widened postcanines and tooth replacement is sequential. Carnivores and Insectivores non-mammaliaform probainognathians like Brasilodon present alternating tooth replacement instead. Brasilodon can be seen as more derived than Morganucodon, a very close mammal relative, due to its upper and lower symmetrical dentition pattern.
Classification and species
The genus Brasilodonbelongs to the family Brasilodontidae along with Brasilitherium, Minicynodon, Panchetocynodon, and Protheriodon. Because of the skull and dentition traits seen in Brasilodon, this genus appears to be more derived than any previously known cynodont, and therefore more closely related to mammals than either tritheledontids or tritylodontids. Since Brasilodontidae exhibits more derived features and many strong differences than other cynodonts, it has been considered that this family is connected with the origin of mammals, although it is younger than the oldest mammals. Despite presenting apomorphies in the skull and dentition, Brasilidontids' postcanines also share many synapomorphies with primitive mammaliaformes. This includes a tongue, a system in middle postcanines for mesiodistal interlocking between teeth, similar size and distribution of tooth cusps, a close pattern of wear facets in postcanines, and a root with an eight-shaped cross-section. This shows that mammalian tooth replacement evolved after many craniodental features in cynodont evolution. Using C.T. scans of Brasilodon quadrangularis and Didelphis, an extant mammal with primitive cranial morphology, the nasal cavity of Brasilodon'' and other cynodonts has been shown to be very close to mammalian forms. This indicates that some cynodonts had high respiratory rates and therefore endothermic metabolic status as seen in eucynodonts. Therefore, all of the adaptations needed for endothermy could already be present in Triassic cynodonts.
Discovery
Brasilodon was found in the Late Triassic outcrops of Rio Grande do Sul, Brazil from the Paraná Basin in the Paleorrota geopark, along with another closely related mammalian ancestor Brasilitherium. The taxa was named by J. Bonaparte, A. Martinelli, C. Schultz, and R. Rubert in 2003. Non-mammliaform cynodonts are one of the most common and taxonomically diverse elements in this location and therefore are well represented by many well-preserved specimens and represented by genera like Riograndia, Irajatherium, and Brasilodontids including Brasilodon quadrangularis. 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. From there, a partial skull of a Brasilodon quadrangularis was first discovered, displaying 6 worn quadrangular postcanines. The overall length of this individual skull found is about 26 mm.
