''Bonacynodon'' is an extinct

genus Genus ( plural genera ) is a taxonomic rank used in the biological classification of living and fossil organisms as well as viruses. In the hierarchy of biological classification, genus comes above species and below family. In binomial nom ...

of cynodonts that lived in what is now southern Brazil during the

Triassic The Triassic ( ) is a geologic period and system (stratigraphy), system which spans 50.6 million years from the end of the Permian Period 251.902 million years ago (Year#Abbreviations yr and ya, Mya), to the beginning of the Jurassic Period 201.36 ...

period (

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 ...

–

Carnian The Carnian (less commonly, Karnian) is the lowermost stage (stratigraphy), stage of the Upper Triassic series (stratigraphy), Series (or earliest age (geology), age of the Late Triassic Epoch (reference date), Epoch). It lasted from 237 to 227 m ...

ages). 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

type species In zoological nomenclature, a type species (''species typica'') is the species name with which the name of a genus or subgenus is considered to be permanently taxonomically associated, i.e., the species that contains the biological type specime ...

''Bonacynodon schultzi''. ''B. schultzi'' is known from two specimens, consisting of two partial skulls and some badly preserved parts of the

postcranium Postcrania (postcranium, adjective: postcranial) in zoology and vertebrate paleontology is all or part of the skeleton apart from the skull. Frequently, fossil remains, e.g. of dinosaurs or other extinct tetrapods, consist of partial or isolated s ...

. Both specimens were recovered from the Pinheiros-Chiniquá Sequence, part of the

Santa Maria Supersequence The Santa Maria Formation is a sedimentary rock formation found in Rio Grande do Sul, Brazil. It is primarily Carnian in age (Late Triassic), and is notable for its fossils of cynodonts, "rauisuchian" pseudosuchians, and early dinosaurs and other ...

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 ...

. This sequence preserves a faunal association known as the ''Dinodontosaurus'' Assemblage Zone, which contains numerous other species of cynodonts,

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 ...

s and reptiles. ''Bonacynodon'' was a small, likely insectivorous cynodont, whose length has been estimated at around . It can be distinguished from other cynodonts by its large,

serrated 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 ...

(saw-like)

canine teeth In mammalian oral anatomy, the canine teeth, also called cuspids, dog teeth, or (in the context of the upper jaw) fangs, eye teeth, vampire teeth, or vampire fangs, are the relatively long, pointed teeth. They can appear more flattened howeve ...

. Together with the genus ''

Probainognathus ''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 South America. ''Probainognathus'' is a member of the family Probainogna ...

'' of Argentina, it made up the family

Probainognathidae 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 ...

, 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 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 ...

MCT-1716-R and the referred specimen (

paratype In zoology and botany, a paratype is a specimen of an organism that helps define what the scientific name of a species and other taxon actually represents, but it is not the holotype (and in botany is also neither an isotype nor a syntype). O ...

) MCT-1717-R. They were both discovered in the 1940s by the Brazilian palaeontologist

Llewellyn Ivor Price Llewellyn Ivor Price (October 9, 1905 – June 9, 1980) was one of the first Brazilian paleontologists. His work contributed not only to the development of Brazilian but also to global paleontology. He collected '' Staurikosaurus'', the first ...

in two separate rocky outcrops (''sangas'') in the Pinheiros region, around south of the town of

Candelária, Rio Grande do Sul Candelária is a municipality in the state of Rio Grande do Sul, Brazil. Paleontology The Museum Aristides Carlos Rodrigues, in this city, has fossils. See also *List of municipalities in Rio Grande do Sul This is a list of the municipali ...

. The rocks belong to the Pinheiros-Chiniquá Sequence of the

, which has been dated to the

to early

ages. The holotype was found in an outcrop known as the ''Sanga do Janguta'', and was preserved together with a specimen of the

Dinodontosaurus ''Dinodontosaurus'' (meaning "terrible-toothed lizard") is a genus of dicynodont therapsid. It was medium to large dicynodont of the Triassic (with skull up to long) and had a beak corneum. It lived in the Middle Triassic but disappeared in the ...

''. It consists primarily of a partial skull, including parts of the skull roof, lower jaw, upper and lower dentition, basicranium and

palate The palate () is the roof of the mouth in humans and other mammals. It separates the oral cavity from the nasal cavity. A similar structure is found in crocodilians, but in most other tetrapods, the oral and nasal cavities are not truly separ ...

. The skull roof was heavily damaged during preparation. The holotype also includes some very fragmentary

postcranial Postcrania (postcranium, adjective: postcranial) in zoology and vertebrate paleontology is all or part of the skeleton apart from the skull. Frequently, fossil remains, e.g. of dinosaurs or other extinct tetrapods, consist of partial or isolated s ...

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 A juvenile is an individual organism that has not yet reached its adult form, sexual maturity or size. Juveniles can look very different from the adult form, particularly in colour, and may not fill the same niche as the adult form. In many o ...

individuals. 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 José Fernando Bonaparte (14 June 1928 – 18 February 2020) was an Argentine paleontologist who discovered a plethora of South American dinosaurs and mentored a new generation of Argentine paleontologists . One of the best-known Argentine paleo ...

, an Argentine palaeontologist who specialised in the Mesozoic vertebrates of South America. The second part, ''cynodon'', is partly derived from the

Ancient Greek Ancient Greek includes the forms of the Greek language used in ancient Greece and the ancient world from around 1500 BC to 300 BC. It is often roughly divided into the following periods: Mycenaean Greek (), Dark Ages (), the Archaic p ...

word (), 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 The Federal University of Rio Grande do Sul ( pt, Universidade Federal do Rio Grande do Sul, UFRGS) is a Brazilian public federal research university based in Porto Alegre, Rio Grande do Sul. UFRGS is among the largest and highest-rated univers ...

Description

''Bonacynodon'' was a small cynodont, with an estimated total length of around .

Skull

The skull of ''Bonacynodon'' was about long. The temporal region (area behind the

eye sockets 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 ...

) 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 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 ...

(roof of the mouth) consisted of two parts, a larger section formed by the

maxilla The maxilla (plural: ''maxillae'' ) in vertebrates is the upper fixed (not fixed in Neopterygii) bone of the jaw formed from the fusion of two maxillary bones. In humans, the upper jaw includes the hard palate in the front of the mouth. T ...

e (upper jaw bones), and a smaller one formed by the palatine bones. As in the closely related ''

'', 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. The upper jaw consisted of two bones, the

premaxilla The premaxilla (or praemaxilla) is one of a pair of small cranial bones at the very tip of the upper jaw of many animals, usually, but not always, bearing teeth. In humans, they are fused with the maxilla. The "premaxilla" of therian mammal has ...

in the front, and the maxilla in the back. Above the

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 front

postcanine Cheek teeth or post-canines comprise the molar and premolar teeth in mammals. Cheek teeth are multicuspidate (having many folds or tubercles). Mammals have multicuspidate molars (three in placentals, four in marsupials, in each jaw quadrant) and ...

s were multiple small holes called

infraorbital foramina In human anatomy, the infraorbital foramen is one of two small holes in the skull's upper jawbone ( maxillary bone), located below the eye socket and to the left and right of the nose. Both holes are used for blood vessels and nerves. In anatomic ...

; 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. The tooth-bearing part of the upper jaw had a

sigmoid Sigmoid means resembling the lower-case Greek letter sigma (uppercase Σ, lowercase σ, lowercase in word-final position ς) or the Latin letter S. Specific uses include: * Sigmoid function, a mathematical function * Sigmoid colon, part of the l ...

(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 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 ...

. The dentary body was somewhat tall, and 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 connection between the two halves of the dentary) was apparently unfused. The front part of the dentary bore at least three

mental foramina The mental foramen is one of two foramina (openings) located on the anterior surface of the mandible. It is part of the mandibular canal. It transmits the terminal branches of the inferior alveolar nerve and the mental vessels. Structure The ...

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 Masseteric is an adjective meaning "of or pertaining to the Masseter muscle", such as: * Masseteric artery The masseteric artery is small and passes laterally through the mandibular notch to the deep surface of the masseter muscle, which it suppl ...

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.

Dentition

''Bonacynodon'' possessed three types of teeth:

incisors 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 ...

, 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 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 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

(saw-like) back edges, which is an

autapomorphy In phylogenetics, an autapomorphy is a distinctive feature, known as a derived trait, that is unique to a given taxon. That is, it is found only in one taxon, but not found in any others or outgroup taxa, not even those most closely related to ...

(unique derived feature) of the taxon. The lower incisors are not known from either specimen. Of the lower canines, only a partial left

root In vascular plants, the roots are the organs of a plant that are modified to provide anchorage for the plant and take in water and nutrients into the plant body, which allows plants to grow taller and faster. They are most often below the su ...

from the holotype is known. The shape and size of this root was however similar to that of the upper canines. Behind each upper canine were six postcanine teeth, which were widely spaced and did not contact each other. The

crowns 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 ...

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. 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 Morphology, from the Greek and meaning "study of shape", may refer to: Disciplines * Morphology (archaeology), study of the shapes or forms of artifacts * Morphology (astronomy), study of the shape of astronomical objects such as nebulae, galaxies ...

between the front and back postcanines, and the roots were not constricted.

Classification

When describing ''Bonacynodon'', Martinelli and colleagues performed a

phylogenetic analysis 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 ...

to find out its relationships to other cynodonts. It was recovered as the sister taxon of ''Probainognathus'', a similar probainognathian from the

Chañares Formation The Chañares Formation is a Carnian-age geologic formation of the Ischigualasto-Villa Unión Basin, located in La Rioja Province, Argentina. It is characterized by drab-colored fine-grained volcaniclastic claystones, siltstones, and sandstone ...

of Argentina. Together with ''Probainognathus'', ''Bonacynodon'' was placed in the family

. 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''. Fig 7

Below is a

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 ...

following the analysis of Martinelli ''et al.'' (2016):

Palaeobiology

Early-diverging probainognathians like the

ecteniniid Ecteniniidae is an extinct family (biology), family of probainognathian cynodonts from the Triassic of South America. They are notable for their large size, as well as for being among the first synapsids with specializations towards cursoriality. ...

s and ''

Chiniquodon ''Chiniquodon'' is an extinct genus of carnivorous cynodonts, which lived during the Late Triassic (Carnian) in South America (Argentina and Brazil) and Africa (Namibia and Madagascar). ''Chiniquodon'' was closely related to the genus '' Aleodo ...

'' 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 A robber fly eating a hoverfly An insectivore is a carnivorous animal or plant that eats insects. An alternative term is entomophage, which can also refer to the human practice of eating insects. The first vertebrate insectivores wer ...

. 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. However, in 2016 Martinelli and colleagues suggested that it could instead have eaten insect

larva A larva (; plural larvae ) is a distinct juvenile form many animals undergo before metamorphosis into adults. Animals with indirect development such as insects, amphibians, or cnidarians typically have a larval phase of their life cycle. ...

e and other

invertebrate Invertebrates are a paraphyletic group of animals that neither possess nor develop a vertebral column (commonly known as a ''backbone'' or ''spine''), derived from the notochord. This is a grouping including all animals apart from the chordate ...

s that fed on the

decomposing Decomposition or rot is the process by which dead organic substances are broken down into simpler organic or inorganic matter such as carbon dioxide, water, simple sugars and mineral salts. The process is a part of the nutrient cycle and is e ...

dicynodont.

Palaeoenvironment

''Bonacynodon'' belongs to the ''Dinodontosaurus'' Assemblage Zone, the lowermost of the four biostratigraphic units of the Santa Maria Supersequence of the

. 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 The Santa Maria Formation is a sedimentary rock formation found in Rio Grande do Sul, Brazil. It is primarily Carnian in age (Late Triassic), and is notable for its fossils of cynodonts, "rauisuchian" pseudosuchians, and early dinosaurs and other ...

. 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. Cynodonts make up a large portion of the fauna of the ''Dinodontosaurus'' Assemblage Zone. In addition to ''Bonacynodon'', these include the probainognathians ''

Aleodon ''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 siste ...

'', ''

Candelariodon ''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 mandibl ...

'', ''Chiniquodon'' and ''

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 ...

'', and 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 ...

s '' Luangwa'', '' Massetognathus'', '' Protuberum'', '' Scalenodon'' and '' Traversodon''. Other vertebrates from this Assemblage Zone include the dicynodonts ''Dinodontosaurus'' and ''

Stahleckeria ''Stahleckeria'' is an extinct genus of Middle Triassic (Ladinian) dicynodonts.parareptile Parareptilia ("at the side of reptiles") is a subclass or clade of basal sauropsids (reptiles), typically considered the sister taxon to Eureptilia (the group that likely contains all living reptiles and birds). Parareptiles first arose near th ...

'' Candelaria'', the

rhynchosaur Rhynchosaurs are a group of extinct herbivorous Triassic archosauromorph reptiles, belonging to the order Rhynchosauria. Members of the group are distinguished by their triangular skulls and elongated, beak like premaxillary bones. Rhynchosaurs ...

Brasinorhynchus ''Brasinorhynchus'' is an extinct genus of derived stenaulorhynchine known from the late Middle Triassic (Ladinian stage) Santa Maria Formation of Rio Grande do Sul, Brazil.archosauriform '' Barberenasuchus'', several species of

pseudosuchia Pseudosuchia is one of two major divisions of Archosauria, including living crocodilians and all archosaurs more closely related to crocodilians than to birds. Pseudosuchians are also informally known as "crocodilian-line archosaurs". Prior to ...

ns (the group that contains modern-day

crocodilia Crocodilia (or Crocodylia, both ) is an order of mostly large, predatory, semiaquatic reptiles, known as crocodilians. They first appeared 95 million years ago in the Late Cretaceous period ( Cenomanian stage) and are the closest livi ...

ns and their extinct relatives), and the aphanosaur (primitive stem-bird) ''

Spondylosoma ''Spondylosoma'' (meaning "vertebra body") is a genus of avemetatarsalian archosaur belonging to the clade Aphanosauria from the late Ladinian-age Middle Triassic Lower Santa Maria Formation in Paleorrota Geopark, Brazil. History Friedrich v ...

''.

References

{{Taxonbar, from=Q37577828 Prehistoric probainognathians Prehistoric cynodont genera Ladinian genera Carnian genera Middle Triassic synapsids of South America Late Triassic synapsids of South America Triassic Brazil Fossils of Brazil Santa Maria Formation Fossil taxa described in 2016