''Santacruzgnathus'' is an extinct genus of small

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 from 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 ...

(Carnian) ''Santacruzodon'' Assemblage Zone of

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 ...

. 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 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 ...

, a group that includes

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 and their close relatives.

Discovery and naming

The holotype and only known specimen of ''Santacruzgnathus'' (UFRGS-PV-1121-T) consists of a partial right dentary bone with a well-preserved final postcanine tooth and fragments of three other postcanines. The specimen was discovered at the Schoenstatt site near the town of Santa Cruz do Sul within the state of

Rio Grande do Sul Rio Grande do Sul (, ; ; "Great River of the South") is a Federative units of Brazil, state in the South Region, Brazil, southern region of Brazil. It is the Federative units of Brazil#List, fifth-most populous state and the List of Brazilian s ...

. The outcrop in which the specimen was found belongs to the Santa Cruz Sequence of the Santa Maria Supersequence, which has been dated to the early Carnian age of the

. The specimen was first described in a 2011 paper by Soares ''et al.'' They interpreted it as a juvenile, and provisionally assigned it to cf. '' Probainognathus'' due to similarities with ''Probainognathus'' juveniles from the Chañares Formation of Argentina, as well as a supposed juvenile ''Probainognathus'' specimen from the Argentine Ischigualasto Formation. The specimen was redescribed in a 2016 paper by Martinelli ''et al.'' They identified it as a new species of cynodont unrelated to ''Probainognathus'', which was given the name ''Santacruzgnathus abdalai''. The generic name derives from Santa Cruz do Sul where the holotype was found, and the

Neo-Latin Neo-LatinSidwell, Keith ''Classical Latin-Medieval Latin-Neo Latin'' in ; others, throughout. (also known as New Latin and Modern Latin) is the style of written Latin used in original literary, scholarly, and scientific works, first in Italy d ...

word meaning a lower jaw. The specific epithet ''abdalai'' references the Argentine palaeontologist Fernando Abdala. The authors reinterpreted the holotype as a likely subadult or adult; the last postcanine is fully erupted in the specimen, which is usually not the case in juvenile cynodonts.

Description

While only a small portion of the dentary is preserved, the bone appears to have been quite slender, as was the case with many early

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. On the lingual (inner) side near the lower edge of the bone, a thin meckelian groove can be seen. Behind the tooth row, most of the coronoid process has broken off with only the base remaining. On the lingual side of the coronoid process, part of the coronoid bone has been preserved. Of the four teeth preserved in the specimen, the first two are represented only by fragmentary roots, while the third also preserves part of the base of the crown. Unlike the others, the fourth tooth is nearly complete. The crown bears a large central cusp (cusp a), a small front cusp (cusp b), a rear cusp (cusp c) somewhat larger than cusp b, and an even smaller cusp d behind cusp c. Cusp a is symmetrical and not recurved. The tip of cusp b is broken off. Cusp b is shifted somewhat towards the labial (outer) side of the tooth, and is separated from cusp a by a deep groove. On the lingual side of the tooth, there is a discontinuous cingulum divided into mesiolingual (front) and distolingual (back) portions. The mesiolingual cingulum has a crenulated edge but no distinct cusps. The root is not fully divided, but has a prominent constriction on the labial side, as seen in most early prozostrodontians.

Classification

Based on features of its jaw and teeth, Martinelli ''et al.'' (2016) interpreted ''Santacruzgnathus'' as a member of Prozostrodontia, a group of probainognathian cynodonts that includes

s and close relatives of them. The genus was however not included in any

phylogenetic In biology, phylogenetics () is the study of the evolutionary history of life using observable characteristics of organisms (or genes), which is known as phylogenetic inference. It infers the relationship among organisms based on empirical dat ...

analysis in that paper. In 2017, the prozostrodontian ''

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, ...

'' was described by Martinelli ''et al.'' Three phylogenetic analyses were performed, two of which included ''Santacruzgnathus''. The resulting

cladogram A cladogram (from Greek language, Greek ''clados'' "branch" and ''gramma'' "character") is a diagram used in cladistics to show relations among organisms. A cladogram is not, however, an Phylogenetic tree, evolutionary tree because it does not s ...

s placed ''Santacruzgnathus'' within a large polytomy at the base of Prozostrodontia. The cladogram from the third analysis is shown below:

References

{{Taxonbar, from=Q37578523 Prozostrodontia Carnian genera Late Triassic synapsids of South America Triassic Brazil Fossils of Brazil Santa Maria Formation Fossil taxa described in 2016