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Brasilodon
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Brasilodon
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 12 centimetres (4.7 in). 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.
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 odon, 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.
Brasilodon was a small, derived cynodont, with an estimated total length of around 12 centimetres (4.7 in). 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.
Different specimens of Brasilodon had widely varying skull lengths, ranging from 20 to 55 millimetres (0.79 to 2.17 in), 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.
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Brasilodon
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 12 centimetres (4.7 in). 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.
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 odon, 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.
Brasilodon was a small, derived cynodont, with an estimated total length of around 12 centimetres (4.7 in). 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.
Different specimens of Brasilodon had widely varying skull lengths, ranging from 20 to 55 millimetres (0.79 to 2.17 in), 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.
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