Bonacynodon

Bonacynodon; Temporal range: Ladinian–Carnian PreꞒ Ꞓ O S D C P T J K Pg N
	Skull of the holotype
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Chordata
Clade:	Synapsida
Clade:	Therapsida
Clade:	Cynodontia
Family:	† Probainognathidae
Genus:	† Bonacynodon ; Martinelli et al., 2016
Species:	†B. schultzi
Binomial name
	†Bonacynodon schultzi; Martinelli et al., 2016

Last updated March 06, 2024

Bonacynodon is an extinct genus of cynodonts that lived in what is now southern Brazil during the Triassic period (Ladinian–Carnian ages). The genus is monotypic, containing only the type species Bonacynodon schultzi. B. schultzi is known from two specimens, consisting of two partial skulls and some badly preserved parts of the postcranium. Both specimens were recovered from the Pinheiros-Chiniquá Sequence, part of the Santa Maria Supersequence of the Paraná Basin. This sequence preserves a faunal association known as the Dinodontosaurus Assemblage Zone, which contains numerous other species of cynodonts, dicynodonts and reptiles. Bonacynodon was a small, likely insectivorous cynodont, whose length has been estimated at around 30 centimetres (12 in). It can be distinguished from other cynodonts by its large, serrated (saw-like) canine teeth. Together with the genus Probainognathus of Argentina, it made up the family Probainognathidae, one of the earliest-diverging lineages of the clade Probainognathia. It was a fairly close relative of mammals, the only group of cynodonts alive today.

Discovery and naming

Bonacynodon schultzi is known from two specimens, the holotype MCT-1716-R and the referred specimen (paratype) MCT-1717-R. They were both discovered in the 1940s by the Brazilian palaeontologist Llewellyn Ivor Price in two separate rocky outcrops (sangas) in the Pinheiros region, around 12 kilometres (7.5 mi) south of the town of Candelária, Rio Grande do Sul.^[1] The rocks belong to the Pinheiros-Chiniquá Sequence of the Santa Maria Supersequence, which has been dated to the Ladinian to early Carnian ages.^[2]

The holotype was found in an outcrop known as the Sanga do Janguta, and was preserved together with a specimen of the dicynodont Dinodontosaurus . It consists primarily of a partial skull, including parts of the skull roof, lower jaw, upper and lower dentition, basicranium and palate. The skull roof was heavily damaged during preparation. The holotype also includes some very fragmentary postcranial elements, which have not been described in detail. The referred specimen was found in a different outcrop, the Sanga do Forno. Like the holotype, it preserves parts of the cranium, lower jaw and upper and lower dentition. Its shape has been heavily distorted during fossilisation, but it preserves some parts of the skull that are missing in the holotype. Based on the lack of wear facets on the teeth, it has been suggested that both specimens represent subadult individuals.^[1]

The specimens were first given a proper description in 2016, when they were given a new generic and specific name by Agustín G. Martinelli and colleagues. The first part of the generic name Bonacynodon is derived from the surname of José Bonaparte, an Argentine palaeontologist who specialised in the Mesozoic vertebrates of South America. The second part, cynodon, is partly derived from the Ancient Greek word κύων (kuōn), meaning "dog", and is a reference to it being a cynodont. The specific epithet schultzi honours Cesar L. Schultz, a Brazilian palaeontologist and professor at the Federal University of Rio Grande do Sul.^[1]

Description

Bonacynodon was a small cynodont, with an estimated total length of around 30 centimetres (12 in).^[3]

Skull

The skull of Bonacynodon was about 6–7 centimetres (2.4–2.8 in) long. The temporal region (area behind the eye sockets) was wide, and somewhat longer than the snout. The sagittal crest (a narrow ridge running across the top of the skull) was relatively low. The secondary palate (roof of the mouth) consisted of two parts, a larger section formed by the maxillae (upper jaw bones), and a smaller one formed by the palatine bones. As in the closely related Probainognathus , the palate ended in front of the last postcanine; this is somewhat shorter than what is seen in the more basal (early-diverging) chiniquodontids and the more derived (late-diverging) prozostrodonts.^[1]

The upper jaw consisted of two bones, the premaxilla in the front, and the maxilla in the back. Above the canines and front postcanines were multiple small holes called infraorbital foramina; at least three foramina are seen in the holotype, while the referred specimen preserves two or three. Based on other basal probainognathians, Bonacynodon likely had additional foramina which have not been preserved in the fossils.^[1] The tooth-bearing part of the upper jaw had a sigmoid (S-shaped) curve, sloping downwards near the postcanines before curving upwards again near the orbit. The lower jaw consisted primarily of a single bone, the dentary. The dentary body was somewhat tall, and the symphysis (the connection between the two halves of the dentary) was apparently unfused. The front part of the dentary bore at least three mental foramina on each side. The back part possessed a long and backwards-pointing projection known as the coronoid process. A large basin called the masseteric fossa stretched from near the last postcanine to the tip of the coronoid process. Like in other probainognathians, the postdentary bones (a set of bones at the back of the lower jaw) were highly reduced compared to the condition in more basal cynodonts, forming a small, rod-like structure.^[1]

Dentition

Bonacynodon possessed three types of teeth: incisors, canines and postcanines. It appears to have had four pairs of incisors in its upper jaw; these were slender, with a round cross section. The first three incisors were roughly equal in size, while the fourth one was slightly smaller. The incisors were widely spaced, with a particularly large gap (diastema) between the third and fourth ones. There was a similarly large gap between the last incisor and the canine. The upper canines were large, and flattened from side to side. The canines had strongly serrated (saw-like) back edges, which is an autapomorphy (unique derived feature) of the taxon. The lower incisors are not known from either specimen. Of the lower canines, only a partial left root from the holotype is known. The shape and size of this root was however similar to that of the upper canines.^[1]

Behind each upper canine were six postcanine teeth, which were widely spaced and did not contact each other. The crowns were compressed from side to side, and possessed multiple straight, unserrated cusps (pointed projections of the teeth) which were arranged in a line. The first three upper postcanines bore three cusps, with the one in the middle (cusp A) being the largest, and the ones in front and back (cusps B and C respectively) being smaller. The fourth and fifth upper postcanines additionally possessed a small fourth cusp (cusp D) behind cusp C, and there may also have been a fifth (accessory) cusp.^[1] The sixth upper postcanine was apparently less developed than the fourth and fifth ones, and it bore at least three cusps. The lower postcanines are incompletely known, but they appear to have been similar to the upper ones. The cingula were poorly developed and bore no cusps. Unlike in the prozostrodonts, there was no abrupt change in morphology between the front and back postcanines, and the roots were not constricted.^[1]

Classification

When describing Bonacynodon, Martinelli and colleagues performed a phylogenetic analysis to find out its relationships to other cynodonts. It was recovered as the sister taxon of Probainognathus, a similar probainognathian from the Chañares Formation of Argentina. Together with Probainognathus, Bonacynodon was placed in the family Probainognathidae. The probainognathids were found to be closely related to Prozostrodontia, a clade (group formed by all descendants of a common ancestor) which includes mammals, the only extant (living) cynodonts, as well as several other groups. Subsequent analyses have upheld the sister relationship of Bonacynodon and Probainognathus.^[4]^[5]^[6]^[7]

Below is a cladogram following the analysis of Martinelli et al. (2016):^[1]

Palaeobiology

Early-diverging probainognathians like the ecteniniids and Chiniquodon had postcanine teeth with strongly recurved (and in the case of the ecteniniids, even serrated) cusps, which would have been well-suited for a carnivorous diet. The straight, unserrated postcanine cusps of probainognathids like Bonacynodon were more similar to those of basal prozostrodonts; this is thought to be an adaptation towards insectivory.^[1] As the holotype was preserved together with the remains of a dicynodont, Schwanke & Kellner (2009) hypothesised that the animal might have been an opportunistic scavenger.^[1]^[8] However, in 2016 Martinelli and colleagues suggested that it could instead have eaten insect larvae and other invertebrates that fed on the decomposing dicynodont.^[1]

Palaeoenvironment

Bonacynodon belongs to the Dinodontosaurus Assemblage Zone, the lowermost of the four biostratigraphic units of the Santa Maria Supersequence of the Paraná Basin. The Dinodontosaurus AZ corresponds to the Pinheiros-Chiniquá Sequence, one of the three stratigraphic sequences of the Supersequence. Together with the overlying Santa Cruz Sequence (containing the Santacruzodon AZ) and the lower part of the Candelária Sequence (containing the Hyperodapedon AZ), it comprises the traditional Santa Maria Formation.^[1] The Pinheiros-Chiniquá and Santa Cruz Sequences appear to have been deposited during a dry period, in which the landscape was dominated by loessic plains.^[9]^[2]

Cynodonts make up a large portion of the fauna of the Dinodontosaurus Assemblage Zone. In addition to Bonacynodon, these include the probainognathians Aleodon , Candelariodon , Chiniquodon and Protheriodon , and the traversodontids Luangwa , Massetognathus , Protuberum , Scalenodon and Traversodon . Other vertebrates from this Assemblage Zone include the dicynodonts Dinodontosaurus and Stahleckeria , the parareptile Candelaria , the rhynchosaur Brasinorhynchus , the enigmatic archosauriform Barberenasuchus , several species of pseudosuchians (the group that contains modern-day crocodilians and their extinct relatives), and the aphanosaur (primitive stem-bird) Spondylosoma .^[2]

Related Research Articles

Probainognathidae is an extinct family of insectivorous cynodonts which lived in what is now South America during the Middle to Late Triassic. The family was established by Alfred Romer in 1973 and includes two genera, Probainognathus from the Chañares Formation of Argentina and Bonacynodon from the Dinodontosaurus Assemblage Zone of Brazil. Probainognathids were closely related to the clade Prozostrodontia, which includes mammals and their close relatives.

Probainognathus meaning “progressive jaw” is an extinct genus of cynodonts that lived around 235 to 221.5 million years ago, during the Late Triassic in what is now Argentina. Together with the genus Bonacynodon from Brazil, Probainognathus forms the family Probainognathidae. Probainognathus was a relatively small, carnivorous or insectivorous cynodont. Like all cynodonts, it was a relative of mammals, and it possessed several mammal-like features. Like some other cynodonts, Probainognathus had a double jaw joint, which not only included the quadrate and articular bones like in more basal synapsids, but also the squamosal and surangular bones. A joint between the dentary and squamosal bones, as seen in modern mammals, was however absent in Probainognathus.

Thrinaxodon is an extinct genus of cynodonts, including the species T. liorhinus which lived in what are now South Africa and Antarctica during the Early Triassic. Thrinaxodon lived just after the Permian–Triassic mass extinction event, its survival during the extinction may have been due to its burrowing habits.

Galesaurus is an extinct genus of carnivorous cynodont therapsid that lived between the Induan and the Olenekian stages of the Early Triassic in what is now South Africa. It was incorrectly classified as a dinosaur by Sir Richard Owen in 1859.

Massetognathus is an extinct genus of plant-eating traversodontid cynodonts. They lived during the Triassic Period about 235 million years ago, and are known from the Chañares Formation in Argentina and the Santa Maria Formation in Brazil.

Bauria is an extinct genus of the suborder Therocephalia that existed during the Early and Middle Triassic period, around 246-251 million years ago. It belonged to the family Bauriidae. Bauria was probably a herbivore or omnivore. It lived in South Africa, specifically in the Burgersdorp Formation in South Africa.

<i>Prozostrodon</i> Extinct genus of cynodonts

Prozostrodon is an extinct genus of probainognathian cynodonts that was closely related to mammals. The remains were found in Brazil and are dated to the Carnian age of the Late Triassic. The holotype has an estimated skull length of 6.7 centimetres (2.6 in), indicating that the whole animal may have been the size of a cat. The teeth were typical of advanced cynodonts, and the animal was probably a carnivore hunting reptiles and other small prey.

Gobiconodon is an extinct genus of carnivorous mammals belonging to the family Gobiconodontidae. Undisputed records of Gobiconodon are restricted to the Early Cretaceous of Asia and North America, but isolated teeth attributed to the genus have also been described from formations in England and Morocco dating as far back as the Middle Jurassic. Species of Gobiconodon varied considerably in size, with G. ostromi, one of the larger species, being around the size of a modern Virginia opossum. Like other gobiconodontids, it possessed several speciations towards carnivory, such as shearing molariform teeth, large canine-like incisors and powerful jaw and forelimb musculature, indicating that it probably fed on vertebrate prey. Unusually among predatory mammals and other eutriconodonts, the lower canines were vestigial, with the first lower incisor pair having become massive and canine-like. Like the larger Repenomamus there might be some evidence of scavenging.

<i>Brasilodon</i> Extinct genus of mammaliamorphs

Brasilodon 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.

Progalesaurus is an extinct genus of galesaurid cynodont from the early Triassic. Progalesaurus is known from a single fossil of the species Progalesaurus lootsbergensis, found in the Lystrosaurus Assemblage Zone of the Balfour Formation. Close relatives of Progalesaurus, other galesaurids, include Galesaurus and Cynosaurus. Galesaurids appeared just before the Permian-Triassic extinction event, and disappeared from the fossil record in the Middle-Triassic.

Platycraniellus is an extinct genus of carnivorous cynodonts from the Early Triassic. It is known from the Lystrosaurus Assemblage Zone of the Normandien Formation in South Africa. P. elegans is the only species in this genus based on the holotype specimen from the Ditsong National Museum of Natural History in Pretoria, South Africa. Due to limited fossil records for study, Platycraniellus has only been briefly described a handful of times.

Arctotraversodon is an extinct genus of traversodontid cynodonts from the Late Triassic of Canada. Fossils first described from the Wolfville Formation in Nova Scotia in 1984 represented the first known traversodontid from North America. The type and only species is A. plemmyridon and is represented by teeth and several dentary bones.

Candelariodon is an extinct genus of carnivorous probainognathian cynodonts from the Middle to Late Triassic Santa Maria Formation of the Paraná Basin in Rio Grande do Sul state, Brazil. Candelariodon is known from a partial mandible having some complete teeth. It was first named by Téo Veiga de Oliveira, Cesar Leandro Schultz, Marina Bento Soares and Carlos Nunes Rodrigues in 2011 and the type species is Candelariodon barberenai.

Alemoatherium is an extinct genus of prozostrodontian cynodont which lived in the Late Triassic of Brazil. It contains a single species, A. huebneri, named in 2017 by Agustín Martinelli and colleagues. The genus is based on UFSM 11579b, a left lower jaw (dentary) found in the Alemoa Member of the Santa Maria Formation, preserving the late Carnian-age Hyperodapedon Assemblage Zone. Alemoatherium was among the smallest species of cynodonts found in the rich synapsid fauna of the Santa Maria Formation. Its blade-like four-cusped postcanine teeth show many similarities with those of dromatheriids, an obscure group of early prozostrodontians.

Aleodon is an extinct genus of cynodonts that lived from the Middle to the Late Triassic. Relatively few analyses have been conducted to identify the phylogenetic placement of Aleodon, however those that have place Aleodon as a sister taxon to Chiniquodon. Two species of Aleodon are recognized: A. brachyramphus which was discovered in Tanzania, and A. cromptoni which was discovered most recently in Brazil.

Abdalodon is an extinct genus of late Permian cynodonts, known by its only species A. diastematicus.Abdalodon together with the genus Charassognathus, form the clade Charassognathidae. This clade represents the earliest known cynodonts, and is the first known radiation of Permian cynodonts.

Etjoia is an extinct genus of traversodontid cynodonts that lived during the Middle Triassic or Late Triassic period in southern Africa. This medium-sized omnivorous cynognathian provides important information on the dental evolution of early diverging gomphodonts and traversodontids.

Pseudotherium is an extinct genus of prozostrodontian cynodonts from the Late Triassic of Argentina. It contains one species, P. argentinus, which was first described in 2019 from remains found in the La Peña Member of the Ischigualasto Formation in the Ischigualasto-Villa Unión Basin.

Vetusodon is an extinct genus of cynodonts belonging to the clade Epicynodontia. It contains one species, Vetusodon elikhulu, which is known from four specimens found in the Late Permian Daptocephalus Assemblage Zone of South Africa. With a skull length of about 18 centimetres (7.1 in), Vetusodon is the largest known cynodont from the Permian. Through convergent evolution, it possessed several unusual features reminiscent of the contemporary therocephalian Moschorhinus, including broad, robust jaws, large incisors and canines, and small, single-cusped postcanine teeth.

Santacruzgnathus is an extinct genus of small cynodonts from the Late Triassic (Carnian) Santacruzodon Assemblage Zone of Brazil. It contains one species, S. abdalai. Santacruzgnathus is known from a single partial lower jaw with four postcanine teeth, only one of which is well-preserved. Some features of the specimen, including the slender shape of the jaw and the incipiently double-rooted teeth, indicate that the animal was an early member of Prozostrodontia, a group that includes mammals and their close relatives.

References

1 2 3 4 5 6 7 8 9 10 11 12 13 14 Martinelli, A. G.; Soares, M. B.; Schwanke, C. (2016). "Two New Cynodonts (Therapsida) from the Middle-Early Late Triassic of Brazil and Comments on South American Probainognathians". PLOS ONE. 11 (10): e0162945. doi: 10.1371/journal.pone.0162945 . ISSN 1932-6203. PMC 5051967 . PMID 27706191.
1 2 3 Schultz, C. L.; Martinelli, A. G.; Soares, M. B.; Pinheiro, F. L.; Kerber, L.; Horn, B. L. D.; Pretto, F. A.; Müller, R. T.; Melo, T. P. (2020). "Triassic faunal successions of the Paraná Basin, southern Brazil". Journal of South American Earth Sciences. 104: 102846. doi:10.1016/j.jsames.2020.102846. S2CID 225015757.
↑ Geggel, L. (2016). "Meet the Ancient Reptile that Gave Rise to Mammals". Scientific American.
↑ Guignard, M. L.; Martinelli, A. G.; Soares, M. B. (2019). "The postcranial anatomy of Brasilodon quadrangularis and the acquisition of mammaliaform traits among non-mammaliaform cynodonts". PLOS ONE. 14 (5): e0216672. doi: 10.1371/journal.pone.0216672 . PMC 6510408 . PMID 31075140.
↑ Martinelli, A. G.; Eltink, E.; Da-Rosa, Á. A. S.; Langer, M. C. (2017). "A new cynodont from the Santa Maria formation, south Brazil, improves Late Triassic probainognathian diversity". Papers in Palaeontology. 3 (3): 401–423. doi:10.1002/spp2.1081. S2CID 134049061.
↑ Martinelli, A.; Soares, M. B.; de Oliveira, T. V.; Rodrigues, P. G.; Schultz, C. L. (2017). "The Triassic eucynodont Candelariodon barberenai revisited and the early diversity of stem prozostrodontians". Acta Palaeontologica Polonica. 62. doi: 10.4202/app.00344.2017 . S2CID 54820157.
↑ Pacheco, C. P.; Martinelli, A. G.; Pavanatto, A. E. B.; Soares, M. B.; Dias-da-Silva, S. (2018). "Prozostrodon brasiliensis, a probainognathian cynodont from the Late Triassic of Brazil: second record and improvements on its dental anatomy". Historical Biology. 30 (4): 475–485. doi:10.1080/08912963.2017.1292423. S2CID 90730154.
↑ Schwanke, C.; Kellner, A. W. (2009). "Interações ecológicas no Triássico". In Stock Da-Rosa, Á. A. (ed.). Vertebrados Fósseis de Santa Maria e região (in Portuguese). Santa Maria: Gráfica Editora Pallotti. pp. 279–301.
↑ Horn, B. L. D.; Goldberg, K.; Schultz, C. L. (2018). "A loess deposit in the Late Triassic of southern Gondwana, and its significance to global paleoclimate". Journal of South American Earth Sciences. 81: 189–203. doi:10.1016/j.jsames.2017.11.017.

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[Martinelli2016-1] 1 2 3 4 5 6 7 8 9 10 11 12 13 14 Martinelli, A. G.; Soares, M. B.; Schwanke, C. (2016). "Two New Cynodonts (Therapsida) from the Middle-Early Late Triassic of Brazil and Comments on South American Probainognathians". PLOS ONE. 11 (10): e0162945. doi: 10.1371/journal.pone.0162945 . ISSN 1932-6203. PMC 5051967 . PMID 27706191.

[Schultz2020-2] 1 2 3 Schultz, C. L.; Martinelli, A. G.; Soares, M. B.; Pinheiro, F. L.; Kerber, L.; Horn, B. L. D.; Pretto, F. A.; Müller, R. T.; Melo, T. P. (2020). "Triassic faunal successions of the Paraná Basin, southern Brazil". Journal of South American Earth Sciences. 104: 102846. doi:10.1016/j.jsames.2020.102846. S2CID 225015757.

[SciAm-3] Geggel, L. (2016). "Meet the Ancient Reptile that Gave Rise to Mammals". Scientific American.

[Guignard2019-4] Guignard, M. L.; Martinelli, A. G.; Soares, M. B. (2019). "The postcranial anatomy of Brasilodon quadrangularis and the acquisition of mammaliaform traits among non-mammaliaform cynodonts". PLOS ONE. 14 (5): e0216672. doi: 10.1371/journal.pone.0216672 . PMC 6510408 . PMID 31075140.

[Martinelli2017a-5] Martinelli, A. G.; Eltink, E.; Da-Rosa, Á. A. S.; Langer, M. C. (2017). "A new cynodont from the Santa Maria formation, south Brazil, improves Late Triassic probainognathian diversity". Papers in Palaeontology. 3 (3): 401–423. doi:10.1002/spp2.1081. S2CID 134049061.

[Martinelli2017b-6] Martinelli, A.; Soares, M. B.; de Oliveira, T. V.; Rodrigues, P. G.; Schultz, C. L. (2017). "The Triassic eucynodont Candelariodon barberenai revisited and the early diversity of stem prozostrodontians". Acta Palaeontologica Polonica. 62. doi: 10.4202/app.00344.2017 . S2CID 54820157.

[Pacheco2018-7] Pacheco, C. P.; Martinelli, A. G.; Pavanatto, A. E. B.; Soares, M. B.; Dias-da-Silva, S. (2018). "Prozostrodon brasiliensis, a probainognathian cynodont from the Late Triassic of Brazil: second record and improvements on its dental anatomy". Historical Biology. 30 (4): 475–485. doi:10.1080/08912963.2017.1292423. S2CID 90730154.

[Schwanke2009-8] Schwanke, C.; Kellner, A. W. (2009). "Interações ecológicas no Triássico". In Stock Da-Rosa, Á. A. (ed.). Vertebrados Fósseis de Santa Maria e região (in Portuguese). Santa Maria: Gráfica Editora Pallotti. pp. 279–301.

[Horn2018-9] Horn, B. L. D.; Goldberg, K.; Schultz, C. L. (2018). "A loess deposit in the Late Triassic of southern Gondwana, and its significance to global paleoclimate". Journal of South American Earth Sciences. 81: 189–203. doi:10.1016/j.jsames.2017.11.017.

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