Brasilodon

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Brasilodon
Temporal range:
Ma
Reconstructed skull in lateral view
Scientific classification Edit this classification
Domain: Eukaryota
Kingdom: Animalia
Phylum: Chordata
Clade: Synapsida
Clade: Therapsida
Clade: Cynodontia
Clade: Mammaliamorpha
Family: Brasilodontidae
Bonaparte et al., 2005
Genus: Brasilodon
Bonaparte et al., 2003
Species:
B. quadrangularis
Binomial name
Brasilodon quadrangularis
Bonaparte et al., 2003
Synonyms
  • Brasilitherium Bonaparte et al., 2003
  • Minicynodon Bonaparte et al., 2010

Brasilodon ("tooth from Brazil") is an extinct genus of small, mammal-like

junior synonyms
of Brasilodon.

Discovery and naming

Map of part of Rio Grande do Sul, showing the two localities (Linha São Luiz and Sesmaria do Pinhal) where Brasilodon has been found

The first three specimens referred to Brasilodon quadrangularis were found at the Linha São Luiz site, a quarry near the town of

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.[1] Many other specimens of Brasilodon have since been discovered, both in Faxinal do Soturno and in Candelária, and comprising both cranial and postcranial material.[1][2]

The genus Brasilodon was named in a 2003 paper by

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.

Description

Brasilodon was a small, derived cynodont, with an estimated total length of around 12 centimetres (4.7 in).[7] 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.[4]

Skull

Skull of UFRGS-PV-0929-T, originally attributed to Brasilitherium
Skull of UFRGS-PV-1030-T, originally attributed to Minicynodon

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.

eye socket. The zygomatic arch (cheek bone) was quite low and slender.[2][4]

The lower jaw consisted mainly of the

The front part of the roof of the mouth consisted of a well-developed

occipital condyles were located further back than the lambdoid crest.[4]

The

traversodontid Massetognathus, but lower than in basal mammaliaforms and modern mammals. Like other non-mammalian cynodonts, Brasilodon likely had a lissencephalic (smooth) brain surface.[8]

Dentition

Comparison of the upper postcanines of UFRGS-PV-0611-T (L) and the lower postcanines of UFRGS-PV-0603-T, traditionally attributed to Brasilitherium (M)

Like most cynodonts, Brasilodon possessed a heterodont dentition, divided into incisors, canines and postcanines. The specimens originally assigned to Brasilodon do not preserve any of the upper incisors, but some specimens attributed to Brasilitherium bore 4 pairs of incisors in the upper jaw.[9] There were 3 pairs of lower incisors; the first lower incisor was procumbent (forwards-pointing).[2] In adult specimens, the canines were large and flattened from side to side, but had no serrations.[4] In small individuals attributed to Brasilitherium, there were two pairs of functional canines, which were only slightly bigger than the incisors.[9]

There were up to 8 pairs of postcanines in both jaws.

periodontal ligament.[11]

Vertebrae and ribs

Vertebra and scapula

The

neural spine (a pointed extension on the top surface of the vertebra) indicates that it was elongated anteroposteriorly (in a front-to-back direction).[2]

The specimen UFRGS-PV-1043-T also preserves 4 isolated rib fragments. The ribs were flattened anteroposteriorly. Many non-probainognathian cynodonts, including most cynognathians, had wide extensions known as costal plates on their ribs; these plates are absent in most probainognathians, including Brasilodon.[2]

Limbs and limb girdles

The lower part of a right

ovoid shape. The glenoid appears to have been downwards-pointing, unlike the more sideways-facing glenoids of certain early-diverging cynodonts. The coracoid and procoracoid bones have not been preserved, but there were articular surfaces for these bones on the scapula.[2]

Photos and drawings of the left humerus

The specimen UFRGS-PV-1043-T preserves a complete left humerus (upper arm bone), with a length of 15.6 millimetres (0.61 in). The shaft of the humerus was slender. It was somewhat twisted, with an angulation of 15 degrees between the opposite ends of the bone. A twisted humerus is found in most cynodonts outside

trochlear (pulley-like) shape seen in many crown-group mammals, like therians or the stem-monotreme Kryoryctes.[12] The ulnar condyle was visible both on the front and back sides of the humerus. Its front side was somewhat transversely compressed. The radial condyle was larger than the ulnar condyle, but was not visible at the back side of the bone. The olecranon fossa (a depression on the back side of the humerus) was quite shallow, unlike the deeper fossa seen in multituberculates, Vincelestes and therians.[2]

Photos and drawings of the left radius and right ulna

The left radius of UFRGS-PV-1043-T is 14 millimetres (0.55 in) long. The radius was slender, with a circular cross-section near the proximal end (closer to the elbow). The head of the radius (where it articulated with the humerus) was approximately circular, with a cup-shaped depression in the middle surrounded by a bulbous rim. The head was angled somewhat towards the anteromedial (front right) side. On the posteromedial (back right) side of the head, there was a small articular facet where it would have articulated with the radial notch of the ulna. There was no radial tuberosity for the attachment of the biceps muscle. The distal end of the bone (closer to the wrist) was shaped like a flattened rectangle. The distal end was somewhat curved posteromedially, which would have allowed the bone to cross over the front of the ulna, as in tritylodontids, Morganucodon and opossums. The distal articular surface (where it connected to the wrist) also had a rectangular shape.[2]

The right ulna of UFRGS-PV-1043-T has a length of 16.5 millimetres (0.65 in). As in most cynodonts, the ulna was narrow mediolaterally (from side to side), and had a sigmoid (s-like) shape. On the sides of the bone, there were two grooves running from one end of the bone to the other, enclosed by thickened edges on the front and back sides. The grooves are thought to have served as attachment points for extensor and flexor muscles. On the proximal end of the bone, there was a well-developed and ossified

docodont mammaliaform Haldanodon, but unlike in many other mammaliaforms, where the anconeal process forms a prominent crest.[2]

Photo and drawing of the pelvis

The

traversodontids. The bones formed three distinct crests known as supracetabular buttresses around the acetabulum. The buttresses were separated by gaps of 2–3 millimetres (0.079–0.118 in), unlike in modern therians, where there is a fully ossified rim around the acetabulum. The gaps would likely have been filled by fibrocartilage in the living animal. The blade of the ilium is badly preserved, but the shape of the base indicates that the postacebular (backwards-pointing) part of the blade was either reduced or absent in Brasilodon, much like in other prozostrodontians. The pubic tuberosity was located below the acetabulum as in most probainognathians, including modern therians. In basal epicynodonts, monotremes and the spalacotheriid Akidolestes, the pubic tuberosity was located in front of the acetabulum instead. The pubis and ischium formed a large and ovoid obturator foramen.[2]

Photos and drawings of the right femur

The right femur (thigh bone) of UFRGS-PV-1043-T is 15.9 millimetres (0.63 in) long. The shaft of the femur was mostly straight, but with a prominent forward bend close to the hip joint, as in other non-mammalian cynodonts; in modern mammals, this bend is less well-developed. The proximal part of the shaft (closer to the hip) was mostly square-shaped, but it became more compressed from front to back more distally (towards the knee), while simultaneously becoming wider from side to side. The proximal and distal ends of the bone had the same width. On the proximal end of the bone, the

fovea capitis on the femoral head. Slightly distally to the femoral head, there was a well-developed projection known as the greater trochanter on the left side of the bone. A distinct greater trochanter is also found in tritylodontids and mammaliaforms, but the greater trochanter was confluent with the femoral head in more basal cynodonts. On the right side of the bone, there was a short crest called the lesser trochanter. The lesser trochanter pointed to the right, and was visible on the front side of the bone, as in many other probainognathians. In more basal cynodonts, it generally was more backwards-pointing, and not visible on the front side. The lesser trochanter did not extend as far towards the proximal end of the bone as in basal mammaliaforms. There was no third trochanter. On the back side of the femur, between the greater and lesser trochanters, there was a deep and narrow depression called the intertrochanteric fossa. On the distal edge of this fossa, there was an intertrochanteric crest which connected the two trochanters. This crest is also found in most therians, but was missing in most Mesozoic cynodont groups. There was a relatively shallow, triangular depression located distally to the intertrochanteric crest; this probably served as an attachment point for the hip adductor muscles. On the back side of the distal end of the femur, there were two condyles that would have connected to the tibia, the medial and lateral condyles. The medial condyle was compressed from side to side. The popliteal fossa had a triangular shape and was located close to the condyles.[2]

Photo and drawing of the tibia

UFRGS-PV-1043-T preserves a nearly complete left tibia (shin bone). It was a slender bone, with the preserved parts having a length of 16.1 millimetres (0.63 in). The shaft was mostly straight, but with a slight medial (rightwards) curve in the proximal part (towards the knee); more basal cynodonts generally had a stronger medial curve of the tibia. The shaft became progressively more flattened towards the distal end (towards the ankle). On the proximal part, there were two articular facets, the medial and lateral facets, which articulated with the condyles of the femur. The lateral facet was wider than the medial one. The distal end of the tibia has not been preserved.[2]

UFRGS-PV-1043-T preserves two left

sustentaculum tali, which would have connected to the underside of the astragalus (ankle bone). In basal cynodonts like Thrinaxodon, no sustentaculum tali has been found, suggesting that it was cartilaginous, if it even existed in those taxa. The left astragalus was smaller than the calcaneum. It had a hemispherical dorsomedial (top right) surface and a flat lateroplantar (bottom left) surface. The front end of the astragalus had a small head. The neck that separated the head from the rest of the bone was shorter than in modern therians.[2]

UFRGS-PV-1043-T also preserves two incomplete

phalanges (finger bones), with UFRGS-PV-0765-T preserving a nearly complete middle phalange. This phalange was rather short, with a concave proximal end and two small condyles on the distal end.[2]

Classification

The genus Brasilodon belongs to Brasilodontidae, a family of advanced probainognathian cynodonts. Along with Brasilodon, two contemporary genera (Brasilitherium and Minicynodon) have been assigned to the family, both of which are likely to be synonyms of Brasilodon.[2][6] Protheriodon, a genus from the Middle Triassic (Ladinian) of Brazil, and Panchetocynodon, a poorly known cynodont from the Early Triassic (Induan) of India, were also placed in Brasilodontidae in a 2013 paper by José Bonaparte.[13] However, Protheriodon has more recently been found to be an early-diverging probainognathian unrelated to brasilodontids,[14] while Panchetocynodon is otherwise treated as an incertae sedis taxon, whose relationships to other taxa are unclear, due to its incompleteness.[9][15]

Features of the skull and dentition of Brasilodon indicate that it was a derived cynodont closely related to mammals, the only extant cynodonts.

morganucodonts, docodonts and haramiyidans. The cladogram below is adapted from a 2019 analysis by Wallace et al.:[16]

phalange
Probainognathia

Palaeobiology

Restoration of Brasilodon (left) and Riograndia (right)

Locomotion

Features of the postcranium indicate that Brasilodon was a generalised animal capable of diverse modes of

parasagittal) posture of the hindlimbs.[2]

Tooth replacement

According to one theory, outlined in detail in a 2010 paper by Martinelli et al., Brasilodon had a polyphyodont tooth replacement, where the teeth were replaced more than once throughout the animal's lifetime, though it is unclear how many successive postcanine replacements there were in Brasilodon.[11] The postcanine replacement ceased long before the death of an individual, allowing extremely strong wear to develop in the teeth in older individuals. In some cases, the individual's teeth wore down over time to less than half the height of the crown still remaining.[4] The size of the postcanine diastema indicates that the front postcanines may have been lost faster than in the primitive cynodont Thrinaxodon.[11][10]

According to Martinelli et al. (2010), the postcanines were replaced in an alternating manner. The postcanine replacement would have occurred from the back to the front, like in several other non-mammaliaform probainognathians, and would thus have differed from that of early mammaliaforms, where the postcanines were replaced from the front towards the back. This may have been due to the unlimited skull growth of early probainognathians.[11] Tooth replacement variation can perhaps also be attributed to diet, with the herbivorous/omnivorous traversodontids having widened postcanines and a sequential tooth replacement,[17] and carnivorous and insectivorous non-mammaliaform probainognathians like Brasilodon having an alternating tooth replacement instead.[11]

A 2022 study by Cabreira et al. suggests that Brasilodon actually was diphyodont, replacing its teeth only once, like most modern mammals. According to this theory, the postcanines in Brasilodon consisted of a set of deciduous teeth, which were replaced, and a set of permanent teeth divided into premolars and molars. The study suggests that the deciduous postcanines in Brasilodon erupted sequentially in a back-to-front direction, while the molars were added in the opposite direction, from the front to the back.[7]

Palaeoecology

stratigraphic
position of Brasilodon and other probainognathians within the Santa Maria Supersequence

Specimens of Brasilodon have been found in two localities. The holotype was found at the Linha São Luiz locality, within the municipality of Faxinal do Soturno. Other specimens have been found at the Sesmaria do Pinhal locality in the municipality of Candelária; both of these locations lie within the Brazilian part of the

temnospondyls, phytosaurs and other groups have also been found.[18]

The location where these fossils were found is a fluvial system, characterized by large quantities of fine sandstone forming sandy beds, resulting from sedimentation in the basin during peak flow events.[1][19]

References

  1. ^ a b c d e f g h i j Bonaparte, J. F.; Martinelli, A. G.; Schultz, C. L.; Rubert, R. (2003). "The sister group of mammals: small cynodonts from the Late Triassic of southern Brazil". Revista Brasileira de Paleontologia. 5: 5–27.
  2. ^
    PMC 6510408
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  3. ^ .
  4. ^ a b c d e f g h i j k Bonaparte, J. F.; Martinelli, A. G.; Schultz, C. L. (2005). "New information on Brasilodon and Brasilitherium (Cynodontia, Probainognathia) from the Late Triassic of southern Brazil". Revista Brasileira de Paleontologia. 8 (1): 25–46.
  5. ^ .
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  9. ^ a b c Martinelli, A. G. (2017). "Contribuição ao conhecimento dos Cinodontes Probainognátios (Therapsida, Cynodontia, Probainognathia) do Triássico da América do Sul e seu impacto na origem dos Mammaliaformes" (in Portuguese).
  10. ^ .
  11. ^ a b c d e f Martinelli, A. G.; Bonaparte, J. F. (2010). "Postcanine replacement in Brasilodon and Brasilitherium (Cynodontia, Probainognathia) and its bearing in cynodont evolution". Paleontología y dinosaurios desde América Latina: 179–186.
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