Anisodontherium sp.

Brandoni, Diego, Powell, Jaime E. & González, Osvaldo E., 2012, Anisodontherium from the Late Miocene of north-western Argentina, Acta Palaeontologica Polonica 57 (2), pp. 241-249 : 243-246

Anisodontherium sp.

Figs. 2 , 3A .

Referred material.— PVL 6425 , a nearly complete mandible lacking both angular processes and the left articular condyle ( Fig. 2A–C ); and digit III of the manus ( Fig. 2D ). Cerro Castillo de Las Brujas (26 ° 38' 07.26”S, 65 ° 50' 53.04”W), Tucumán Province, Argentina, Saladillo Formation (Upper Miocene) ( Fig. 1B ) .

Emended diagnosis (modified from Brandoni and De Iuliis 2007).—Low, elongated skull; palate flattened; lateral walls of rostrum rectilinear and parallel; short tooth row; anterior margin of coronoid process sigmoid; posterior margin of mandibular symphysis located anterior to m1 and rounded; both dentaries well separated; posterolateral opening of mandibular canal located at the base of the coronoid process, nearly on the alveolar plane; anterolateral opening of mandibular canal located anterior to posterior margin of mandibular symphysis; m4 anterior to anterior margin of coronoid process; lower molariforms small and mesiodistally compressed (especially m2 and m3); molariforms with a markedly deep, V−shaped valley between the transverse crests, particularly on m1; hard dentine, especially that of the anterior wall of each molariform, not well developed; anterior transverse crest of molariforms without a well−developed wear facet.

Description.—In lateral view ( Fig. 2A, B ), the ventral margin of the dentary is generally convex, but ascends posterior to the base of the m4 alveolus, thus forming a notch between the level of the alveolus and the angular process. In this PVL 6425 closely resembles Anisodontherium halmyronomum (see Brandoni and De Iuliis 2007: fig. 3A) and Plesiomegatherium hansmeyeri (see Roth 1911: fig. 1), although the ventral bulge is more marked in those species.

While the angular processes of PVL 6425 are incompletely preserved, their broken bases suggest a ventral position, as also seen in Anisodontherium halmyronomum (see Brandoni and De Iuliis 2007: fig. 3A), Megatheriops rectidens , Megathericulus patagonicus Ameghino, 1904 (see De Iuliis et al. 2008: fig. 2D), Eomegatherium andinum Kraglievich, 1930 , Pliomegatherium lelongi (see Brandoni 2006: figs. 3.2, 4), Eremotherium laurillardi ( Lund, 1842) , and Megatherium tarijense Gervais and Ameghino, 1880 (see De Iuliis et al. 2009: fig. 2C); by contrast, the angular processes are more dorsally positioned in some species of Megatherium Cuvier, 1796 , such as M. americanum and M. gallardoi Ameghino and Kraglievich, 1921 (see Brandoni et al. 2008: fig. 2C).

The anterior margin of the coronoid process is sigmoid in lateral view ( Fig. 2A, B ), with the ventral part being concave

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and the dorsal part convex. The posterior margin of the coronoid process is concave. The highest point of the coronoid process is located 135 mm, and the articular condyle 100 mm above the alveolar plane. In Eremotherium Spillmann, 1948 , the articular condyle is relatively low, whereas in Megatherium it occupies a more dorsal position ( De Iuliis and Cartelle 1999).

In PVL 6425, m4 lies anterior to the anterior margin of the coronoid process and is entirely visible in lateral view ( Fig. 2B ), as is the case in Anisodontherium halmyronomum (see Brandoni and De Iuliis 2007: fig. 3A) and probably also Megathericulus patagonicus (see De Iuliis et al. 2008: fig. 2C, D) and Eomegatherium andinum . In other megatheriines, m4 is located more posteriorly with respect to the coronoid process, although there is some variation in its exact position: in Pliomegatherium lelongi , the distal margin of m4 nearly coincides with the plane of the anterior margin of the coronoid process ( Brandoni 2006: figs. 3.1–4); by contrast, in Eremotherium eomigrans De Iuliis and Cartelle, 1999 (see De Iuliis and Cartelle 1999: fig. 4B), Plesiomegatherium hansmeyeri , Megatheriops rectidens , Pyramiodontherium bergi , and Pyramiodontherium brevirostrum (see Carlini et al. 2002: fig. 2), only the mesial half of m4 is visible in lateral view; finally, m4 is located entirely posterior to the anterior margin of the coronoid process and hidden from lateral view in several specimens of Megatherium americanum .

The posterolateral opening of the mandibular canal lies at the base of the coronoid process, almost reaching the alveolar plane ( Fig. 2A, B ). In Anisodontherium halmyronomum , the position of this opening is not observable owing to the poor state of preservation of this part of the dentary. The condition in Megathericulus patagonicus resembles that of PVL 6425, whereas in other megatheriines, such as Pliomegatherium , Megatheriops , Pyramiodontherium , Megatherium , and Eremotherium , the foramen is located dorsally, medial to the base of the coronoid process, and usually opposite m4. As in Pyramiodontherium and Megatheriops , the anterolateral opening of the mandibular canal is situated anterior to the posterior margin of the mandibular symphysis.

In occlusal view ( Fig. 2C ), the posterior margin of the mandibular symphysis is located ventromedial and anterior to the level of m1, as in Anisodontherium halmyronomum (see Brandoni and De Iuliis 2007: fig. 3B), Megathericulus patagonicus (see De Iuliis et al. 2008: fig. 2C), Eomegatherium andinum , and E. nanum (see Kraglievich 1930: fig. 5A). The position of the posterior margin of the mandibular symphysis in other megatheriines varies: in Pyramiodontherium bergi and Eremotherium laurillardi , it reaches approximately to a point halfway along m1. It is even farther posterior, reaching the plane of the alveolar septum between m1 and m2, in Plesiomegatherium hansmeyeri (see Roth 1911: fig. 1), Pyramiodontherium brevirostrum (see Carlini et al. 2002: fig. 2), Megatherium altiplanicum Saint−André and De Iuliis, 2001 (see Saint−André and De Iuliis 2001: fig. 4B), and M. tarijense (see De Iuliis et al. 2009: fig. 2D). Finally, the posterior margin of the mandibular symphysis generally reaches to a point halfway along the m 2 in M. americanum and M. gallardoi (see Brandoni et al. 2008: fig. 2D), although in M. americanum it occasionally also lies at the level of the m1/m2 alveolar septum. In PVL 6425, the posterior margin of the symphysis is rounded and the dentaries are well separated. By contrast, the dentaries approach each other more closely at the level of the symphysis and form a nearly V−shaped outline in dorsal view in Megatherium altiplanicum , M. gallardoi , M. tarijense , and most specimens of M. americanum . In Anisodontherium sp. , the articular condyle is nearly oval in outline, while being subcircular in M. tarijense (see De Iuliis et al. 2009: fig. 2D).

The most notable feature of the dentition of PVL 6425, and especially m2 and m3, is the mesiodistally compressed, rather than isodiametric (length and width being almost equal) shape of the molariforms ( Figs. 2C , 3A ; Table 1). This mesiodistal compression also occurs in Anisodontherium halmyronomum (see Brandoni and De Iuliis 2007: fig. 3B, D), Megathericulus patagonicus and Eomegatherium andinum (note that no teeth have so far been described for the latter two species; however, their form can be inferred from their alveoli). By contrast, in Pyramiodontherium , Megatheridium , Megatheriops , Pliomegatherium , Megatherium , Eremotherium , and Plesiomegatherium hansmeyeri the molariforms are isodiametric, albeit differently shaped. In occlusal view, megatheriine molariforms have two crests of hard dentine separated by a deep valley excavated in the soft dentine ( Fig. 3 ). Unlike Pyramiodontherium and Megatherium ( Fig. 3B and 3C , respectively), Anisodontherium sp. lacks well−developed hard dentine, especially along the anterior wall of the molariforms, as well as wear facets on the anterior transverse crests ( Fig. 3A ).

In PVL 6425, m1 has a nearly trapezoidal outline in occlusal view, whereas m2 and m3 are clearly rectangular, and the margins of m4 (the smallest) are flat anteriorly and convex posteriorly. In Anisodontherium halmyronomum the shape of the molariforms is similar to those of PVL 6425, whereas in Pyramiodontherium bergi the anterior and posterior crests of m1 and m2 are oriented obliquely with respect the anteroposterior axis, giving those teeth a more trapezoidal outline ( Fig. 3B ). By contrast, all of the molariforms of Megatherium and Eremotherium are nearly squared ( Fig. 3C ). In addition, the molariforms, and particularly m1, of Anisodontherium sp. and A. halmyronomum possess a deep V−shaped valley between the transverse crests ( Fig. 2A, B ). While present, this valley is generally shallower in other taxa, such as Pyramiodontherium , Megatheriops , and Megatherium .

In taxa showing mesiodistal compression of the molariforms, the molariform tooth row is relatively shorter than in megatheriines with isodiametric cheek teeth. This has implications for the Hypsodonty Index (HI), calculated as the greatest height of the mandibular ramus/length of the molariform tooth row × 100, which has been used as an indicator of hypsodonty of the molariforms in sloths (see Zetti 1964; Bargo et al. 2006; Brandoni and De Iuliis 2007; Fields 2009). For PVL 6425, the HI is nearly 99 (taking into account an average of the measurements of both dentaries), compared to a value of 103–107 in Anisodontherium halmyronomum . In other Tertiary megatheriines from Argentina, the HI ranges from 75–113 ( Table 2), whereas among Quaternary megatheriines the HI is greatest in Megatherium americanum , in which it varies between 92 and 112 ( Saint−André and De Iuliis 2001). However, as was noted by De Iuliis et al. (2004)

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and Brandoni and De Iuliis (2007), HI values for Anisodontherium and M. americanum are not directly comparable (see below).

Digit III is composed of a proximal phalanx (fusion of phalanges 1+2) and a strong, claw−shaped ungual phalanx ( Fig. 2D ). The proximal surface of the phalanx (1+2) bears a wide facet for Mc III, which is elongated along the dorsolateralventromedial axis. The well−developed and relatively short and robust ungual phalanx is higher than wide, particularly in its distal portion, and oriented obliquely to the sagittal plane. It articulates with the proximal phalanx (1+2) along a deeply depressed surface, which is divided into two oval parts.

Figures

Fig. 2. Megatheriine Anisodontherium sp. (PVL 6425), Cerro Castillo de Las Brujas, Tucumán Province, Argentina. Saladillo Formation (Upper Miocene). A–C. Mandible in left lateral (A), right lateral (B), and occlusal (C) views. D. Digit III of the manus in lateral view.

Fig. 3. Lower molariforms of megatheriines in occlusal view. A. Anisodontherium sp. (PVL 6425), Cerro Castillo de Las Brujas, Tucumán Province, Argentina. Saladillo Formation (Upper Miocene). B. Pyramiodontherium Rovereto, 1914 (MLP 31−XI−12−25), Bajo de Andalhuala, Catamarca Province, Andalhuala Formation (Upper Miocene). C. Megatherium Cuvier, 1796 (CICYTTP−PV−M−2−356), Entre Ríos Province, Tezanos Pinto Formation (Upper Pleistocene). Explanatory drawings (A1–C1), photographs (A2–C2).

Fig. 1. A. Map of Argentina showing the distribution of fossil Megatheriinae during the Late Miocene–Pliocene. 1, Abra Pampa (Jujuy Province); 2, Saladillo Formation (Tucumán Province); 3, Andalhuala and Corral Quemado formations (Catamarca Province); 4, Toro Negro Formation (La Rioja Province); 5, Huayquerías de San Carlos (Mendoza Province); 6, “conglomerado osífero”, Ituzaingó Formation (Entre Ríos Province); 7, Arroyo Chasicó Formation (Buenos Aires Province); 8, Río Negro Formation (Río Negro Province). B. Geologic map of Cerro Castillo de Las Brujas area (Tucumán Province). C. Schematic profile of the sequence indicating the position of the fossiliferous level and the dated tuff.