''Prozostrodon'' is an extinct genus of

probainognathia Probainognathia is one of the two major subgroups of the clade Eucynodontia, the other being Cynognathia. The earliest forms were carnivorous and insectivorous, though some groups eventually also evolved herbivorous diets. The earliest and most b ...

cynodont Cynodontia () is a clade of eutheriodont therapsids that first appeared in the Late Permian (approximately 260 Megaannum, mya), and extensively diversified after the Permian–Triassic extinction event. Mammals are cynodonts, as are their extin ...

s that was closely related to

mammal A mammal () is a vertebrate animal of the Class (biology), class Mammalia (). Mammals are characterised by the presence of milk-producing mammary glands for feeding their young, a broad neocortex region of the brain, fur or hair, and three ...

s. The remains were found in

Brazil Brazil, officially the Federative Republic of Brazil, is the largest country in South America. It is the world's List of countries and dependencies by area, fifth-largest country by area and the List of countries and dependencies by population ...

and are dated to the

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 227.3 ...

age of the

Late Triassic The Late Triassic is the third and final epoch (geology), epoch of the Triassic geologic time scale, Period in the geologic time scale, spanning the time between annum, Ma and Ma (million years ago). It is preceded by the Middle Triassic Epoch a ...

. The

holotype A holotype (Latin: ''holotypus'') is a single physical example (or illustration) of an organism used when the species (or lower-ranked taxon) was formally described. It is either the single such physical example (or illustration) or one of s ...

has an estimated skull length of , 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.

Discovery and naming

''Prozostrodon brasiliensis'' was originally described as a species of ''

Thrinaxodon ''Thrinaxodon'' is an extinct genus of cynodonts which lived in what are now South Africa and Antarctica during the Late Permian - Early Triassic. ''Thrinaxodon'' lived just before, during, and right after the Permian–Triassic mass extinction ...

'' in a 1987 paper by Mário C. Barberena, José F. Bonaparte and A. M. Sá Teixeira. The

(UFRGS-PV-0248-T) includes a well-preserved skull preserving the front half of the cranium, a mostly complete lower jaw and all of the teeth, but missing most of the

braincase In human anatomy, the neurocranium, also known as the braincase, brainpan, brain-pan, or brainbox, is the upper and back part of the skull, which forms a protective case around the brain. In the human skull, the neurocranium includes the calv ...

sagittal crest A sagittal crest is a ridge of bone running lengthwise along the midline of the top of the skull (at the sagittal suture) of many mammalian and reptilian skulls, among others. The presence of this ridge of bone indicates that there are excepti ...

and

zygomatic arches In anatomy, the zygomatic arch (colloquially known as the cheek bone), is a part of the skull formed by the zygomatic process of the temporal bone (a bone extending forward from the side of the skull, over the opening of the ear) and the temporal ...

. It also preserves multiple postcranial elements, including parts of the

vertebral column The spinal column, also known as the vertebral column, spine or backbone, is the core part of the axial skeleton in vertebrates. The vertebral column is the defining and eponymous characteristic of the vertebrate. The spinal column is a segmente ...

rib In vertebrate anatomy, ribs () are the long curved bones which form the rib cage, part of the axial skeleton. In most tetrapods, ribs surround the thoracic cavity, enabling the lungs to expand and thus facilitate breathing by expanding the ...

interclavicle An interclavicle is a bone which, in most tetrapods, is located between the clavicles. Therian mammals ( marsupials and placentals) are the only tetrapods which never have an interclavicle, although some members of other groups also lack one. In ...

humeri The humerus (; : humeri) is a long bone in the arm that runs from the shoulder to the elbow. It connects the scapula and the two bones of the lower arm, the radius and ulna, and consists of three sections. The humeral upper extremity consists of ...

, right ilium,

femora The femur (; : femurs or femora ), or thigh bone is the only bone in the thigh — the region of the lower limb between the hip and the knee. In many four-legged animals the femur is the upper bone of the hindleg. The top of the femur fits in ...

and right foot. The specimen was found at the Faixa Nova locality in the municipality of Candelária, Rio Grande do Sul, in layers belonging to the Candelária Sequence of the

Santa Maria Supersequence The Santa Maria Formation is a sedimentary geological formation, 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 ear ...

. It is deposited in the palaeontology museum of the UFRGS. The assignment of the species to ''Thrinaxodon'' was disputed by Battail (1991), who instead suggested that it might be a member of

Chiniquodontidae Chiniquodontidae is an extinct family of basal probainognathian cynodonts that lived in what is now Africa and South America during the Middle and Late Triassic. It is currently thought to include four valid genera: ''Aleodon'', '' Chiniquodon'' ...

, a group of cynodonts closer to mammals than ''Thrinaxodon''. A 2001 redescription by Bonaparte & Barberena indicated that ''T. brasiliensis'' was indeed more closely related to mammals than to the other ''Thrinaxodon'' species; as a result, it was reclassified into the new genus ''Prozostrodon''. The generic name references the well-developed cingulum of the lower postcanines, being derived from the

Greek Greek may refer to: Anything of, from, or related to Greece, a country in Southern Europe: *Greeks, an ethnic group *Greek language, a branch of the Indo-European language family **Proto-Greek language, the assumed last common ancestor of all kno ...

word , meaning a girdle or band, and the

Latin Latin ( or ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally spoken by the Latins (Italic tribe), Latins in Latium (now known as Lazio), the lower Tiber area aroun ...

prefix , meaning "before". A 2018

histological Histology, also known as microscopic anatomy or microanatomy, is the branch of biology that studies the microscopic anatomy of biological tissue (biology), tissues. Histology is the microscopic counterpart to gross anatomy, which looks at large ...

analysis found that the holotype represents a

subadult A juvenile is an individual organism (especially an animal) 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 t ...

, as its bones were still growing when it died. A second specimen (CAPPA/UFSM 0123) was described in 2018 from the Marchezan locality in the municipality of

São João do Polêsine São João do Polêsine is a municipality in the state of Rio Grande do Sul, Brazil. Paleontology In this city there are outcrops with fossils. Sauropodomorph dinosaur '' Buriolestes'' and dinosauromorph '' Ixalerpeton'' were found in the Car ...

. It consists of a right

dentary bone In jawed vertebrates, the mandible (from the Latin ''mandibula'', 'for chewing'), lower jaw, or jawbone is a bone that makes up the lowerand typically more mobilecomponent of the mouth (the upper jaw being known as the maxilla). The jawbone i ...

preserving three

postcanine teeth Cheek teeth or postcanines 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 p ...

. Because of its small size and lack of a

diastema A diastema (: diastemata, from Greek , '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 may be referred to ...

behind the canine, this specimen is thought to be a juvenile. A third specimen (UFRGS-PV-0543-T) was described in 2020 from the Faixa Nova locality, consisting of the front part of the cranium and fragments of the right dentary. This specimen notably preserves a natural

endocast An endocast is the internal cast of a hollow object, often referring to the cranial vault in the study of brain development in humans and other organisms. Endocasts can be artificially made for examining the properties of a hollow, inaccessible ...

of the

nasal cavity The nasal cavity is a large, air-filled space above and behind the nose in the middle of the face. The nasal septum divides the cavity into two cavities, also known as fossae. Each cavity is the continuation of one of the two nostrils. The nas ...

. In 2023, a nearly complete skull (CAPPA/UFSM 0210) was described from the Marchezan locality. Unlike any of the other known specimens, it preserves the back portion of the skull, including the

basicranium The base of skull, also known as the cranial base or the cranial floor, is the most inferior area of the skull. It is composed of the endocranium and the lower parts of the calvaria. Structure Structures found at the base of the skull are for ...

Description

With the holotype having an estimated skull length of , ''Prozostrodon'' was considerably larger than the other prozostrodontians (''

Alemoatherium ''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, ...

'' and ''

Therioherpeton ''Therioherpeton'' is an extinct genus of small, carnivorous cynodonts belonging to the clade Prozostrodontia, which lived in what is now Brazil during the Late Triassic. Its type species is ''Therioherpeton cargnini''. It was named in 1975 by th ...

'') from its locality.

Skull

On the snout tip, 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 mammals h ...

bore a narial process that connected with the

nasal bone The nasal bones are two small oblong bones, varying in size and form in different individuals; they are placed side by side at the middle and upper part of the face and by their junction, form the bridge of the upper one third of the nose. Eac ...

. Further back, the premaxilla had a thin posterodorsal process that was wedged between the septomaxilla and

maxilla In vertebrates, the maxilla (: maxillae ) 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. The two maxil ...

. Above the

eye socket In anatomy, the orbit is the cavity or socket/hole 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 ...

(orbit), ''Prozostrodon'' retained small

prefrontal Prefrontal may refer to: *Prefrontal bone, a skull bone in some tetrapods *Prefrontal cortex, a region of the brain of a mammal *Prefrontal scales The prefrontal scales on snakes and other reptiles are the scales adjacent and anterior to the fr ...

and

postorbital The ''postorbital'' is one of the bones in vertebrate skulls which forms a portion of the dermal skull roof and, sometimes, a ring about the orbit. Generally, it is located behind the postfrontal and posteriorly to the orbital fenestra. In some ve ...

bones, unlike in most other

prozostrodontia Prozostrodontia is a clade of cynodonts including mammaliaforms and their closest relatives such as Tritheledontidae and Tritylodontidae. It was erected as a node-based taxon by Liu and Olsen (2010) and defined as the least inclusive clade contai ...

ns where these bones are completely absent. The

postorbital bar The postorbital bar (or postorbital bone) is a bony arched structure that connects the frontal bone of the skull to the zygomatic arch, which runs laterally around the eye socket. It is a trait that only occurs in mammalian taxa, such as most strep ...

, which had divided the eye socket from the

temporal fenestra Temporal fenestrae are openings in the temporal region of the skull of some amniotes, behind the orbit (eye socket). These openings have historically been used to track the evolution and affinities of reptiles. Temporal fenestrae are commonly (al ...

in earlier cynodonts, was missing in ''Prozostrodon'', as in most other prozostrodontians. Within the eye socket, there was a broad contact between the frontal,

palatine A palatine or palatinus (Latin; : ''palatini''; cf. derivative spellings below) is a high-level official attached to imperial or royal courts in Europe since Roman Empire, Roman times.

and

orbitosphenoid The lesser wings of the sphenoid or orbito-sphenoids are two thin triangular plates, which arise from the upper and anterior parts of the body, and, projecting lateralward, end in sharp points ig. 1 In some animals, they remain as separate bones ...

bones. A small hole called the

sphenopalatine foramen The sphenopalatine foramen is a foramen of the skull that connects the nasal cavity and the pterygopalatine fossa. It gives passage to the sphenopalatine artery, nasopalatine nerve, and the superior nasal nerve (all passing from the pterygopala ...

was present within the eye socket, as in other prozostrodontians. In ''Prozostrodon'' the foramen was completely enclosed by the palatine bone, unlike in some later prozostrodontians like ''

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 foun ...

'' and ''

Morganucodon ''Morganucodon'' ("Glamorgan tooth") is an early mammaliaform genus that lived from the Late Triassic to the Middle Jurassic. It first appeared about 205 million years ago. Unlike many other early mammaliaforms, ''Morganucodon'' is well represent ...

'', where the foramen was bordered by both the orbitosphenoid and palatine. The secondary (hard) palate extended slightly behind the last postcanine tooth. On the lingual (inner) side of the upper postcanines, there was a deep groove where the lower teeth would have fit when the mouth was closed. The lower jaw consisted primarily of the

. The dentary was fairly robust, with an upturned tip. The

symphysis A symphysis (, : 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 together o ...

(joint) between the two halves of the dentary was unfused. More basal

ns generally had a fused symphysis, and an unfused symphysis is one of the main

synapomorphies In phylogenetics, an apomorphy (or derived trait) is a novel character or character state that has evolved from its ancestral form (or plesiomorphy). A synapomorphy is an apomorphy shared by two or more taxa and is therefore hypothesized to ...

(shared derived features) of Prozostrodontia. At the rear end of the dentary, behind the tooth row, a somewhat tall and broad coronoid process extended upwards. Behind the coronoid process, there was a backwards-pointing projection called the

articular process The articular process or zygapophysis ( + apophysis) of a vertebra is a projection of the vertebra that serves the purpose of fitting with an adjacent vertebra. The actual region of contact is called the ''articular facet''.Moore, Keith L. et al. ...

. In modern mammals, the articular process bears a dentary condyle that articulates with the

squamosal bone The squamosal is a skull bone found in most reptiles, amphibians, and birds. In fishes, it is also called the pterotic bone. In most tetrapods, the squamosal and quadratojugal bones form the cheek series of the skull. The bone forms an ancestral ...

of the cranium, but this condyle was absent in ''Prozostrodon''. On the lingual side of the dentary, a

meckelian groove The Meckelian groove (or Meckel's groove, Meckelian fossa, or Meckelian foramen, or Meckelian canal) is an opening in the medial (inner) surface of the mandible (lower jaw) which exposes the Meckelian cartilage.

Teeth

Like most cynodonts, ''Prozostrodon'' had a

heterodont In anatomy, a heterodont (from Greek, meaning 'different teeth') is an animal which possesses more than a single tooth morphology. Human dentition is heterodont and diphyodont as an example. In vertebrates, heterodont pertains to animals wher ...

dentition divided into

incisor 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 ...

s, canines and postcanines (cheek teeth). There were four pairs of incisors in both the upper and lower jaws. The lower incisors were somewhat procumbent (forwards-pointing) with recurved tips, and decreased in size further back in the mouth. Conversely, the upper incisors were backwards-pointing, and increased in size further back. The canines were large, and the lower one had fine

serration 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 pr ...

s on its back edge. As in other cynodonts, the postcanines bore multiple cusps, which in ''Prozostrodon'' were placed in a straight line. They consisted of a large central cusp (cusp A in the upper teeth, cusp a in the lower ones), a smaller front cusp (cusp B or b) in front of the central cusp, and a small rear cusp (cusp C or c) behind the central cusp. Cusp C/c was often followed by an even smaller cusp D/d. Some of the postcanines also bore a shelf-like feature called a cingulum parallel to the main cusps. The

roots A root is the part of a plant, generally underground, that anchors the plant body, and absorbs and stores water and nutrients. Root or roots may also refer to: Art, entertainment, and media * ''The Root'' (magazine), an online magazine focusin ...

of the postcanines were incipiently divided by a groove that ran lengthwise down the root. ''Prozostrodon'' appears to have had up to ten lower postcanines. The lower postcanines can be divided into two main shapes, located in the front and back respectively. The first shape (the "simple" type) had a large cusp a and significantly smaller cusps b and c, and sometimes a small cusp d behind cusp c. In teeth of this type, the cingulum was poorly developed or absent. The second postcanine shape (the "complex" type) had a relatively smaller cusp a and larger cusps b and c. These teeth also had a well-developed cingulum on the lingual side, which could bear up to nine distinct cusps. In the holotype, the first five postcanines (pc1–5) are of the simple type, while the last four (pc7–10) are inferred to be of the complex type. A third, "transitional" type, with similar cusp proportions to the first type but with a cingulum like the second, is found in the sixth postcanine of the holotype. In the juvenile specimen CAPPA/UFSM 0123, which only preserves the fourth, fifth and sixth postcanines, the fourth tooth was of the first, simple type. Unlike the holotype, the fifth and sixth postcanines were both of the second, more complex type, more closely resembling the seventh and eighth postcanines of the holotype. This indicates that juveniles had more complex anterior postcanines than adults, and that these were replaced with simpler teeth during growth, simultaneously with the eruption of additional complex postcanines at the back of the jaws.

Classification

Ever since the genus was named in 2001, ''Prozostrodon'' has generally been found to be a relatively derived probainognathian, diverging after 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. ...

chiniquodontid Chiniquodontidae is an extinct family of basal probainognathian cynodonts that lived in what is now Africa and South America during the Middle and Late Triassic. It is currently thought to include four valid genera: ''Aleodon'', ''Chiniquodon'', ...

s and probainognathids, but before the

tritheledontid Tritheledontidae, the tritheledontids or ictidosaurs, is an extinct family of small to medium-sized (about 10 to 20 cm long) cynodonts. They were highly mammal-like, specialized cynodonts, although they still retained a few reptile-like ana ...

tritylodontid Tritylodontidae ("three-knob teeth", named after the shape of their cheek teeth) is an extinct family of small to medium-sized, highly specialized mammal-like cynodonts, with several mammalian traits including erect limbs, endothermy, and some d ...

s and more derived groups. The

clade In biology, a clade (), also known as a Monophyly, monophyletic group or natural group, is a group of organisms that is composed of a common ancestor and all of its descendants. Clades are the fundamental unit of cladistics, a modern approach t ...

formed by ''Prozostrodon'' and Mammalia was given the name Prozostrodontia in a 2010 paper. A 2023 phylogenetic analysis recovered ''Prozostrodon'' as the sister taxon of ''

Pseudotherium ''Pseudotherium'' ("false beast") 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 th ...

'', a prozostrodontian from the Carnian

Ischigualasto Formation The Ischigualasto Formation is a Late Triassic geological formation in the Ischigualasto-Villa Unión Basin of southwestern La Rioja Province, Argentina, La Rioja Province and northeastern San Juan Province, Argentina, San Juan Province in northw ...

of Argentina. The clade formed by the two genera was named

Prozostrodontidae Prozostrodontia is a clade of cynodonts including mammaliaforms and their closest relatives such as Tritheledontidae and Tritylodontidae. It was erected as a node-based taxon by Liu and Olsen (2010) and defined as the least inclusive clade contai ...

Limb bone osteohistology of Prozostrodon brasiliensis

References

{{Taxonbar, from=Q1653541 Prozostrodontia Carnian genera Late Triassic synapsids of South America Triassic Brazil Fossils of Brazil Paraná Basin Fossil taxa described in 2001 Taxa named by Mário Costa Barberena Taxa named by José Bonaparte