Xenastrapotherium sp.

Goillot, Cyrielle, Antoine, Pierre-Olivier, Tejada, Julia, Pujos, François & Gismondi, Rodolfo Salas, 2011, Middle Miocene Uruguaytheriinae (Mammalia, Astrapotheria) from Peruvian Amazonia and a review of the astrapotheriid fossil record in northern South America, Geodiversitas 33 (2), pp. 331-345 : 335-336

Xenastrapotherium sp. ( Fig. 2 )

? Xenastrapotherium sp. – Antoine et al. 2007: 21.

Granastrapotherium snorki (partim) – Antoine et al. 2007: 21.

Xenastrapotherium sp. – Negri et al. 2010: 247.

MATERIAL EXAMINED. — Edentulous symphysis with canine alveoli ( MUSM 1468) and left i1 ( MUSM 1466), IN-DTC 20 locality; left P3 without ectoloph ( MUSM 1467), IN-DTC 37 locality.

LOCALITY AND DISTRIBUTION. — Río Inuya, c. 2 km downstream the confluence with Río Mapuya ( IN-DTC 20) and c. 500 m upstream the confluence with Río Mapuya ( IN-DTC 37), Atalaya Department, Ucayali, Peru ( Fig. 1 ).

FORMATION AND AGE. — Ipururo Formation, c. 500 m above the base of the formation, late middle M iocene, c. 13 Ma ( Antoine et al. 2007; Espurt et al. 2010).

DESCRIPTION

MUSM 1468 is a much damaged fragment of an edentulous mandibular symphysis ( Fig. 2 A-F). In dorsal view, the symphysis is sagittally elongate (preserved APD = 117 mm), with a shallow sagittal gutter between canine alveoli; only the lingual part of them is preserved, on each side of the symphysis; the preserved part of the concerned alv eolus has a circular transverse section; the canines were hypsodont to hypselodont; they w ere converging frontward in the posterior half of the symphysis, parallel, then diverging in the anterior half; there is neither obligate alveoli for front teeth (incisors) nor for cheek teeth in the preserved part; the posterior border of the symphysis is r egularly concave; the intermandibular space was very wide and reached c. 50 mm. In rostral view, the ventral border of the symphysis is slightly convex; the dorsal border, corresponding to the shallow sagittal gutter described before, is regularly concave between the canine alveoli; three shallow circular pits are observed on the rostral tip of the symphysis; two ar e located on the left side while a single one is pr eserved on the right side; they ar e not perfectly symmetrical with respect to the median axis, and as such, cast a doubt on their meaning; nevertheless, the most probable hypothesis is that they could r epresent vestigial incisor alveoli. In lateral view, both ventral and dorsal borders are parallel and sub-horizontal; canine alveoli are also horizontal in their central part and slightly oriented up ward in their r ostral and caudal parts. In ventral view, four mental foramina open forming a small irr egular semicircle in the rostral part of the symphysis; two larger foramina, more remote than the latter, are visible close to the caudal border of the symphysis; this caudal border is large and regularly concave.

MUSM 1467 is a worn and broken biradiculate tooth (max preserved MDL = 21.6 mm; max preserved LLL = 22.1 mm; est LLL ≈ 25 mm). The labial and lingual r oots are totally fused distally and separate by a deep longitudinal groove on the mesiolingual side ( Fig. 2I, J ). Except where worn or broken, the crown is surrounded by finely wrinkled enamel. The ectoloph is badly broken and enamel is completely lacking on it. I n occlusal view, the crown is triangular, wider distally than mesially, and with a slightly conv ex lingual edge ( Fig. 2G, H ); the single lingual cusp (?pr otocone) is conical and anteriorly connected to the ectoloph b y a thin and low crest interpreted as a protoloph; a short, low and oblique cingulum is visible on the mesiolingual side of the tooth; the preserved part of the ectoloph is slightly higher than the pr otocone; distally, the ectoloph and the protocone are connected by a thick and worn crest, thus forming a metaloph at late stages of wear; the mesiolingual pocket forms a circular pit; the weak distal cingulum is low and smooth, thicker labially, and defining a low distal pocket. MUSM 1466 is a worn incisiform tooth, with a low crown and a long and single root (c. 40 mm long; Fig. 2 K-M). Except where worn, the crown is entirely surrounded by finely wrinkled enamel. In occlusal view, the crown is sub-oval (MDL = 17.4 mm; LLL = 12.4 mm), with two convex and elongate opposite sides (i.e. lingual/labial), slightly converging on the right side; there is a low horizontal cingulid, located just abo ve the neck on both sides, but no vertical groove. The root is flattened transversely and elongate sagittally.

COMPARISON MUSM 1468 was first mentioned by Antoine et al. (2007) as documenting Granastrapotherium snorki , but further observation revealed the putative presence of three incisor alveoli, which precludes its referral to G. snorki (devoid of incisors; Johnson & Madden 1997). At the same time, similar alv eoli occur in early ontogenetic stages of X. kraglievichi from La Venta (UCMP 45069). Orientation of the canines (nearly horizontal) is similar to that observed in X. kraglievichi (UCMP 45069), and at a lesser degree to G. snorki (UCMP 40017), in which they are perfectly horizontal. On the other hand, their orientation is pretty distinct from what is observed in the holotypes of Parastrapotherium martiale Ameghino, 1901 (MACN A52-604) and of Astrapotherium giganteum Ameghino, 1898 (MACN A3274-3278), as well as in the mandible MACN 3207 (A. magnum Burmeister, 1879), i.e. upraised. The triangular occlusal outline and distal widening of MUSM 1467 do not match the P4s referred to G. snorki (rounded) and to X. kraglievichi (quadrangular), while such featur es are observable on the P3 UCMP 38847 fr om La Venta, belonging to the hypodigm of X. kraglievichi , as illustrated by Johnson & Madden (1997: figs 22.1; 22.3). Besides, the estimated dimensions of MUSM 1467 (c. 22 × 25 mm) are similar to those of UCMP 38847 (MDL = 20.5 mm; LLL = 23.4 mm; J ohnson & Madden 1997: table 22.1). Granastrapotherium does not retain P3 and P4 is much larger (range = 26.7- 38.4 mm; mean = 33.2 mm; Johnson & Madden 1997: table 22.4).

The presence of enamel all around the crown of MUSM 1466 impedes r eferring it to a to xodont notoungulate, the only associated astrapothere-sized taxon. When compared to astrapothere teeth, this unicuspid, single-rooted, and brachydont tooth can be identifi ed as a lower incisor. The crown is low, which points either to an i1 or i3, rather than to an i2 (high-crowned). Furthermore, its bilateral symmetry implies it was located axially on the symphysis. As a consequence, it is tentatively interpreted as an i1, which tends to be confirmed by its strong sagittal development and by the strong wear of the crown. Moreover, Astrapotheriidae generally display deep median longitudinal gr ooves on both labial and lingual sides of the incisors (i1-i3), and their absence is only observed in i1s of a few astrapotheriid taxa. MUSM 1466 diff ers from those referred to Astrapotherium (Santacrucian SALMA, Argentina: MACN 3207; Colhuehuapian SALMA, Argentina: A52-513 and A52-525; Kramarz & Bond 2010), Astrapothericulus Ameghino, 1901 (Santacrucian, Argentina: MACN A52-405; Kramarz 2009) and Parastrapotherium (Deseadan SALMA, Argentina: MACN A52-506), but is similar to i1s of the early astrapotheriid Astraponotus (Mustersan/ Deseadan SALMA, Argentina; MLP 52-XI-4-151- 160). Granastrapotherium is devoid of any incisor ( Johnson & Madden 1997). The incisors referred to Xenastrapotherium from the Honda Group, Colombia, have shallow median grooves restricted to the distal half of the cr own, which disappear before late stages of w ear (direct observation by C.G., UCMP, Bogotá). Given the stratigraphical context of IN-DTC 20, MUSM 1466 cannot be referred to Astraponotus ; its morphology would be consistent with that of Xenastrapotherium . Following Johnson & Madden (1997), two species are referred to Xenastrapotherium in the middle Miocene of northern South America, but X. amazonense Paula Couto, 1976 ( Brazil) might be a junior synonym of X.kraglievichi Cabrera,1929 ( Colombia) . However, given the present sample and pending a taxonomic revision of the genus, we refer to the corresponding remains as Xenastrapotherium sp.

Figures

FIG. 2. — Xenastrapotherium sp.from the late middle Miocene Fitzcarrald local fauna,Peruvian Amazonia:A-F, edentulous symphysis with canine alveoli (MUSM 1468), IN-DTC 20 locality;A, dorsal view; B, interpretative reconstruction with canines, same view; C, rostro-dorsal view;D, interpretative reconstruction with alveoli of anterior teeth,same view;E, right lateral view;F, interpretative reconstruction with canines, same view;G-J, left P3 without ectoloph (MUSM 1467), IN-DTC 37 locality;G, occlusal view; H, interpretative sketch of the occlusal view, with main observed features; I, mesiolingual view; J, distal view; K-M, left i1 (MUSM 1466), IN-DTC 20 locality; K, occlusal view; L, labial view;M, lingual view.Leftward and upward arrows show mesial and labial sides,respectively. Scale bars:A-F,50 mm; G-M, 20 mm.

FIG. 1. — Location map of the late middle Miocene astrapotheriid-yielding vertebrate localities in the Río Inuya and Río Mapuya area (“Fitzcarrald local fauna”, Ipururo Formation), Ucayali Department, eastern Peru.