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Southern Brazil A new therioherpetid cynodont from the Santa Maria Formation (middle Late Triassic), southern Brazil Fernando ABDALA Museu de Ciências e Tecnologia, Pontifícia Universidade Católica de Rio Grande do Sul, Avenida Ipiranga 6681, 90619-900 Porto Alegre, RS (Brazil) [email protected] Ana Maria RIBEIRO Museu de Ciências Naturais, Fundação Zoobotânica do Rio Grande do Sul, Rua Dr Salvador França, 1427, 90690-000 Porto Alegre, RS (Brazil) [email protected] Abdala F. & Ribeiro A. M. 2000. — A new therioherpetid cynodont from the Santa Maria Formation (middle Late Triassic), southern Brazil. Geodiversitas 22 (4) : 589-596. ABSTRACT A new cynodont with sectorial postcanines, from the Santa Maria Formation of southern Brazil, is described. Charruodon tetracuspidatus n. gen. n. sp., KEY WORDS represented by a fragmentary lower jaw, shows a postcanine morphology close Cynodontia, to that of Therioherpeton cargnini, thereby justifying a tentative inclusion of Therioherpetidae, the new species within the family Therioherpetidae. Comparisons among middle Late Triassic, Brazil, cynodonts with sectorial teeth are suggestive of a sequence of morphological sectorial tooth. changes in the sectorial postcanines of advanced cynodonts. RÉSUMÉ Un nouveau cynodonte thérioherpétidé de la Formation Santa Maria (Trias supé- rieur) du Sud du Brésil. Un nouveau cynodonte aux dents postcanines sectoriales, provenant de la Formation Santa Maria au Sud du Brésil, est décrit. La morphologie des dents postcanines de Charruodon tetracuspidatus n. gen. n. sp., représentée par un MOTS CLÉS fragment de mandibule, est proche de celle observée chez Therioherpeton car- Cynodontia, gnini, ce qui par conséquent l’inclut dans la famille des Therioherpetidae. Des Therioherpetidae, comparaisons avec d’autres cynodontes ayant une dentition sectoriale vien- Trias supérieur, Brésil, nent suggérer une série de modifications morphologiques qui affectent les dent sectoriale. postcanines sectoriales chez les cynodontes évolués. GEODIVERSITAS • 2000 • 22 (4) © Publications Scientifiques du Muséum national d’Histoire naturelle, Paris. www.mnhn.fr/publication/ 589 Abdala F. & Ribeiro A. M. INTRODUCTION crown morphology resembling that seen in Therioherpeton. The purpose of this paper is to The Santa Maria Formation in southern Brazil is provide a description of this new cynodont mate- well-known for its abundant Triassic continental rial and compare it with that of other sectorial- fauna (Barberena et al. 1985; Schultz 1995). toothed cynodonts. Ranging from ?Ladinian to Norian, this form- ation preserves a heterogeneous sample of the ABBREVIATIONS vertebrates of the epoch (see Barberena et al. MCP PV Paleontological collection of the Museu de 1985 for a list of taxa). Ciências e Tecnologia of the Pontifícia Important components of the Santa Maria fauna Universidade Católica de Rio Grande do are non-mammalian cynodonts, a paraphyletic Sul; PVL Paleontologia de Vertebrados Lillo collec- assemblage that includes the closest relatives of tion, Universidad Nacional de Tucumán; the early mammals (i.e. Mammaliaformes in The cusp terminology follows that used by Hopson & Rowe 1988). The transition to mammals is one Crompton (1969: 23) to describe the molars of of the best documented evolutionary events in Morganucodon (= Eozostrodon). the fossil record, allowing detailed examination of the successive changes leading to the origin of a major group (Kemp 1982; Hopson & SYSTEMATIC PALEONTOLOGY Barghusen 1986; Sidor & Hopson 1998). Non-mammalian cynodonts are represented in Order THERAPSIDA Broom, 1905 Brazil by the numerically dominant traversodont- Suborder CYNODONTIA Owen, 1861 ids, characterized by transversely broad postca- Family THERIOHERPETIDAE nines (Barberena 1974), and by the medium- Bonaparte & Barberena, 1975 to-large-sized chiniquodontids, distinguished by sectorial postcanine teeth (Romer 1969; Teixeira Genus Charruodon n. gen. 1982; Abdala 1996). In addition, there are smal- TYPE SPECIES BY ORIGINAL DESIGNATION.— ler forms such as Thrinaxodon brasiliensis Charruodon tetracuspidatus n. sp. Barberena, Bonaparte & Teixeira, 1987, which ETYMOLOGY. — In honor to the Charruas, a native features cingular cusps along the base of the South American tribe that lived in southern Brazil, crown of postcanine teeth, and Therioherpeton and from the Greek odon which means tooth. cargnini Bonaparte & Barberena, 1975, which DIAGNOSIS. — As for the species, by monotypy. has fully sectorial postcanines. The latter species, represented only by the holotype, comes from Santa Maria city and was proposed as a probable Charruodon tetracuspidatus n. sp. mammal ancestor by Bonaparte & Barberena (Figs 2; 3; Tables 1; 2) (1975). HOLOTYPE. — Fragment of the left dentary horizontal In the paleontological collection of the Museu de ramus, with alveoli for three incisors, roots of the Ciências e Tecnologia, Pontifícia Universidade canine and three postcanines, and a fifth tooth with a Católica de Rio Grande do Sul, a fragment of the well-preserved crown (Figs 2; 3) (MCP-3934 PV). left lower jaw of a small cynodont was found bear- ETYMOLOGY.— tetracuspidatus, in reference to the ing a sectorial tooth. The specimen comes from four-cusped crown morphology. the same locality (Fig. 1) in which the traverso- LOCALITY. — 29°42’07.44’’S, 52°51’43.95’’W, dontid cynodont Exaeretodon Cabrera, 1943 and approximately 8 km West of the city of Candelaria in the State of Rio Grande do Sul, southern Brazil. the basal archosaur Proterochampsa nodosa Barberena, 1982 were discovered. HORIZON. — Santa Maria Formation. The presence of Exaeretodon and Proterochampsa here is strongly Close inspection of the jaw fragment revealed the suggestive of a Carnian age for this stratum of the for- presence of a replacement tooth with a sectorial mation (Rogers et al. 1993). 590 GEODIVERSITAS • 2000 • 22 (4) A new cynodont from the Brazilian middle Late Triassic Rio Pardo Rio Jacui VENÂNCIO SANTA AIRES MARIA CANDELÁRIA BR 287 ARGENTINA N SANTA CRUZ DO SUL RIO GRANDE DO SUL 30° CACHOEIRA BR BRAZIL DO SUL 471 URUGUAY BR 290 PANTANO GRANDE 120 km 25 km 54° 53° FIG. 1. — Map showing the location of the fossil outcrop. DIAGNOSIS. — Lower jaw with a high horizontal pear relatively small compared with the anterior ramus when compared with the postcanine size, height of the jaw. Originally, the region preserved approximately 0.31 with respect to the anteroposterior length of the crown of the fifth postcanine; four well- extended as far posteriorly as the fifth alveolus, developed cusps on the crown of the postcanine; sec- which contained a replacing tooth that preserved a ond cusp “a” is the largest, followed closely in size by complete crown. This tooth was isolated and pre- the third (“c”); the three posterior cusps (“a”, “c” and pared in order to examine its crown morphology “d”) are directed posteriorly; a sharp constriction dis- tinguishes the crown from the root; roots undivided. (Fig. 3). Four aligned cusps are present and there is no indication of a cingulum. The main cusp “a” is slightly larger than the first posterior accessory DESCRIPTION cusp “c” from which it is separated by a notch. The dentary fragment extends from an open and Both of these cusps are slightly recurved posterior- high symphysis to the level of the third postcanine ly. The second posterior accessory cusp “d” is (Fig. 2A, B). There are three alveoli for the inci- the lowest. It is posteriorly directed and well- sors. The root of the canine is larger than those of separated from cusp “c” by a notch. The anterior the incisors and postcanines, but has undergone accessory cusp “b” is relatively small and closely some transverse compression during fossilization. united with the main cusp by a high crest. In oc- The mental foramen is located at the level of the clusal view, the crown is bulbous, with convex la- second postcanine, and closer to the symphysis are bial and lingual sides, the latter being particulary some nutritive foramina (Fig. 2B). Three postca- rounded. The root is undivided and distinguished nines are represented only by their roots. All ap- from the crown by a prominent constriction. GEODIVERSITAS • 2000 • 22 (4) 591 Abdala F. & Ribeiro A. M. A B FIG. 2. — Stereo pair and line drawing of the mandibular ramus of Charruodon tetracuspidatus n. sp. (MCP-3934 PV); A, labial view; B, lingual view. Scale bar: 10 mm. TABLE 1. — Charruodon tetracuspidatus n. sp., holotype. DISCUSSION AND CONCLUSION Measurements of the specimen MCP-3934 PV (mm). Several new Late Triassic European cynodonts Total length of specimen 23.5 Height at the level of the first postcanine 15.2 have been described over the past two decades Anteroposterior length of the root of the canine 5.0 mainly on the basis of isolated teeth (Hahn et al. Anteroposterior length of the root of postcanine 1 2.8 1984, 1988, 1994; Sigogneau-Russell & Hahn Anteroposterior length of the root of postcanine 2 3.0 1994; Godefroit 1997; Godefroit & Battail Anteroposterior length of the root of postcanine 3 3.0 1997). As outcome of these findings, different systematic proposals for cynodont taxonomy have emerged to incorporate the new evidence. TABLE 2. — Charruodon tetracuspidatus n. sp., holotype. Measurements of the fifth postcanine (mm). Battail (1991) recognized Dromatheriidae, Chiniquodontidae and Therioherpetidae, among Crown length 4.8 others, as valid cynodont families, all character- Crown height 3.0 ized by the presence of sectorial postcanines bea- Crown width 1.8 ring a faint cingulum or lacking one
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