Probainognathus, Romer, 1970

Bento Soares, Marina, Abdala, Fernando & Bertoni-Machado, Cristina, 2011, A sectorial toothed cynodont (Therapsida) from the Triassic Santa Cruz do Sul fauna, Santa Maria Formation, Southern Brazil, Geodiversitas 33 (2), pp. 265-278 : 270-273

cf. Probainognathus sp. (Figs 2; 3)

DESCRIPTION

UFRGS-PV-1121T is represented by a fragment of the right mandibular ramus of approximately 7 mm in length (Fig. 2). The horizontal portion is remarkably low and presents one ellipsoid empty alveolus, two alveoli preserving the base of the crowns and the fourth alveolus with a partial postcanine. The coronoid process is rising immediately behind the last postcanine, whereas the ventral margin of the horizontal ramus is straight ( Fig. 5A ). In medial view the meckelian canal is placed near the ventral margin of the dentary, at approximately one quarter of the height of the bone (Fig. 2B). Most of the meckelian canal is horizontal, except for the posterior portion, at the level of the preserved postcanine, which is directed postero-dorsally (Fig. 2B).

The preserved postcanine is 1.8 mm in anteroposterior length ( Table 2 ) and shows a large main cusp, which is aligned with the anterior and posterior accessory cusps ( Fig. 3 ). The top of the anterior accessory cusp is broken, yet, this cusp seems to be smaller than the posterior accessory one. An additional posterior accessory cusp is somewhat displaced towards the lingual side of the postcanine, but clearly visible in the labial margin ( Fig. 3A ). An also broken anterior lingual cusp is positioned at the base of the anterior accessory cusp, and probably is totally hidden from the labial view of the sectorial margin. There are two bulbous cingular cusps ventrally to the second posterior accessory cusp and the anterior lingual cusp respectively, and a posterior cingular crest ( Fig. 3B ). The root of the penultimate postcanine is single, as evidenced by the removal of bone in the lingual side of the mandibular ramus, with no evidence of a central furrow (Fig. 2).

DISCUSSION

The tiny fragment of lower jaw with tooth described is more likely that of a juvenile individual. The tooth is composed of an aligned series of cusps oriented dorsally, with the most anterior and posterior cusps displaced lingually and with a lingual cingulum formed by bulbous cusps and a posterior crest. A comparison between the crown of UFRGS-PV 1121T and those of other South American sectorial toothed cynodonts follows. We restrict this comparison to the posterior lower postcanines, wherever possible ( Fig. 4 ).

Chiniquodontid cynodonts are recorded in the Santa Cruz do Sul fauna ( Abdala et al. 2001), in the Brazilian Dinodontosaurus Assemblage Zone ( Huene 1936; Teixeira 1982) and also in the Chañares and Ischigualasto formations from Argentina ( Abdala & Giannini 2002). The postcanines of this taxon show a strongly curved main cusp ( Fig. 4A ) and are therefore quite different from the new specimen from Santa Cruz do Sul.

Protheriodon estudianti , represented by a tiny, poorly preserved specimen, was recently described from the Dinodontosaurus AZ of the Santa Maria Formation ( Bonaparte et al. 2006). Its posterior lower postcanines, visible only labially on the right side, show tiny accessory cusps in relation to the main one, being the overall morphology of the crown different from that of UFRGS-PV1121T. The small cynodonts Charruodon tetracuspidatus and Therioherpeton cargnini , from the Brazilian Hyperodapedon AZ are represented only by their holotypes. The posterior upper postcanine of T. cargnini ( Fig. 4B ) and the anterior lower one of C. tetracuspidatus ( Fig. 4C ), the only tooth crown known in each specimen, lack cingulum ( Bonaparte & Barberena 1975, 2001; Abdala & Ribeiro 2000; Oliveira 2006).

Lower postcanines of the tritheledontids Riograndia guaibensis ( Fig. 4D ), Chaliminia musteloides ( Fig. 4E ) and Irajatherium hernandezi ( Fig. 4F ) show a decreasing height of the aligned cusps posteriorly (Bonaparte et al. 2001; Martinelli et al.

2005; Martinelli & Rougier 2007) besides the lack of cingular cusps, being therefore different from the Santa Cruz do Sul specimen. Differences between UFRGS-PV1121T and the Late Triassic brasilodontids ( Fig. 4H ) are also clear because the lower postcanines in this group present a tiny anterior accessory cusp very low in the crown in relation to the main cusp ( Bonaparte et al. 2003, 2005). The posterior accessory cusp is also more developed and high in the specimen from Santa Cruz do Sul.

Sectorial toothed cynodonts more similar to the new record of Santa Cruz do Sul are Prozostrodon brasiliensis from the Santa Maria Formation, Brazil, Probainognathus jenseni from the Middle Triassic Chañares Formation in Argentina and cf. Probainognathus sp. from the Carnian Ischigualasto Formation also in Argentina.

Prozostrodon brasiliensis posterior lower postcanine sectorial margin is also tetracuspidated in labial view but, different from UFRGS-PV1121T, the posteriormost marginal cusp is not displaced lingually and there is a well-developed lingual cingulum formed by a series of clearly differentiated isolated cusps ( Barberena et al. 1987: fig. 4G). In addition, the postcanine roots have a furrow, especially well developed in the most posterior postcanines ( Bonaparte & Barberena 2001).

Several adult specimens of Probainognathus jenseni show a worn out postcanine series with the teeth showing chisel-like edges ( Romer 1970). Postcanines of juveniles of this species present a main cusp with an aligned anterior and two posterior accessory cusps, whereas an additional anterior accessory cusp is displaced lingually ( Fig. 4I ). The lingual cingulum in the lower postcanines of juveniles is mostly formed by cingular crests originating from the anterior and posterior cusps ( Abdala 1996). In some cases it is possible to recognize the presence of a few tiny isolated cuspules contributing to the cingulum. There is no evidence of a furrow in the postcanine roots. Several characters observed in the only preserved postcanine crown of UFRGS-PV1121T, including the lingual location on the crown of the anterior lingual cusp and the second posterior accessory cusp, and the presence of cingular cusps and a lingual cingular crest, are similar to those of P. jenseni juveniles. UFRGS-PV 1121T is remarkably smaller than the juvenile specimens of P. jenseni currently known (see Table 2 for comparative measurements) and presents some important differences that should be mentioned. The posterior lower postcanines of P. jenseni show a comparatively taller and anteroposteriorly shorter crown, and the horizontal ramus of the lower jaw is comparatively taller than that of the specimen of Santa Cruz do Sul.

A tiny (approximately 40 mm of skull length) juvenile specimen from the Ischigualasto Formation was also described as cf. Probainognathus sp. ( Bonaparte & Crompton 1994). The lower postcanines are not visible due to jaw occlusion, whereas the posterior uppers show four aligned cusps in labial view, the second being the largest. The morphology of these upper postcanines is similar to the labial morphology of the lower postcanine of UFRGS-PV1121T, both in the number and degree of development of the marginal cusps as well as in the general proportion between the height and anteroposterior length of the crown. Unfortunately it is not possible to observe the lingual side of the postcanines from the Ischigualasto Formation specimen, hampering a complete knowledge of its tooth morphology.

The postcanine preserved in the new specimen from Santa Cruz do Sul is therefore very similar, in labial view, to the posterior upper postcanines of cf. Probainognathus sp. from the Ischigualasto Formation. There is also a general similarity between the lower postcanine of UFRGS-

A

PV1121T with those of juvenile P. jenseni from the Chañares Formation. Taking into account the fragmentary nature of UFRGS-PV1121T, the differences mentioned above to juveniles of P. jenseni and the overall similarity to the specimen from the Ischigualasto Formation, we provisionally identify this specimen from Santa Cruz do Sul as cf. Probainognathus . Additional material is necessary to provide a more precise taxonomic identification of this taxon in Santa Cruz do Sul as well as in Ischigualasto.

Figures

FIG. 5. — Chrono- and biostratigraphy of southern Brazilian Triassic units: A, Geologic Time Scale 2004 (from Gradstein & Ogg 2004); B, time scale after Muttoni et al. (2004 and reference cited therein), except for the Anisian lower age, after Lehrmann et al. (2006). Dashed line is inferred extension of the base of the Dinodontosaurus Assemblage Zone suggested by the record of the traversodontid Luangwa and the Mariante rhynchosaur (see text).

FIG. 3. — UFRGS-PV1121T detail of the last lower postcanine in buccal (A) and lingual (B) views. Abbreviations: aac, anterior accessory cusp; alc, anterior lingual cusp; cc, cingular cusp; ccr, cingular crest; mc, main cusp; pac, posterior accessory cusp. Scale bar: 1 mm.

FIG. 4. — Lower postcanines of sectorial toothed cynodonts: A-C, in lateral (above) and occlusal (below) views; D-I, in lingual view; A, Chiniquodon; B, Therioherpeton; C, Charruodon; D, Riograndia; E, Chaliminia; F, Irajatherium; G, Prozostrodon; H, Brasilitherium; I, Probainognathus (juvenile); A, B, C, from Abdala & Ribeiro (2000); D, from Soares (2004); E, from Martinelli & Rougier (2007); F, from Martinelli et al. (2005); G, from Bonaparte & Barberena (2001); H, from Bonaparte et al. (2003); I, from Abdala (1996). Scale bars: A-C, 10 mm; D-I, 1 mm.