Icochilus excavatus Ameghino, 1889

Fernández, Mercedes, Fernicola, Juan Carlos & Cerdeño, Esperanza, 2019, On the type materials of the genera Interatherium Ameghino, 1887 and Icochilus Ameghino, 1889 (Interatheriidae, Notoungulata, Mammalia) from early Miocene of the Santa Cruz Province, Argentina, Zootaxa 4543 (2), pp. 195-220 : 205-206

Icochilus excavatus Ameghino, 1889

Syntypes. MACN-A 9681 ( Fig. 3 A ), incomplete right maxilla with alveoli of C and dP1, and series P2–M3 ; MACN-A 177 ( Figs. 3 B–C), mandibular symphysis with alveoli of both i1 and left series i2 –p2; MACN-A 178 ( Figs. 3 D–E), left mandibular fragment with c–m1; and MACN-A 179 ( Fig. 3 F ), right partial maxilla with two molariforms (probably M1–2) . MACN-A 180 (maxillary or mandibular fragment according to the catalogue) and an unnumbered isolated m3 (originally illustrated) are currently missing .

Age and provenance of the type material. Santacrucian SALMA, early Miocene , Santa Cruz Province ( Argentina).

Original description ( Ameghino 1889: 472–473). “ Esta especie era de un tercio mas pequeña que la precedente [ Icochilus extensus ], de la que se distingue por la conformación de la cara externa de los premolares y molares superiores, que es bastante diferente. Los premolares 2, 3 y 4, presentan las dos aristas perpendiculares de la parte anterior de la cara externa, mas estrechas, y de tamaño igual, sin que la segunda tome la forma de una columna saliente como en la especie anterior; además la parte posterior externa de cada uno de estos dientes en vez de ser plana como en los del I. extensus es excavada por un surco perpendicular que viene inmediatamente detrás de la segunda arista, como sucede con los verdaderos molares de la especie mencionada.

Los verdaderos molares superiores del I. excavatus tienen las dos aristas perpendiculares de la parte anterior de la cara externa, casi confundidas en una sola, que toma el aspecto de una columna ancha en la que apenas se vé un pequeño vestigio de la ranura que en los premolares y en los molares de la otra especie la dividen en dos aristas. Detrás de esta columna, sobre la parte media de la cara externa, viene el surco perpendicular, mas estrecho y mas profundo que en los de la otra especie. Las muelas forman una série dentaria mas arqueada, pero mas corta, pues las siete muelas superiores juntas solo ocupan un espacio longitudinal en línea recta de 22 milímetros. La mandíbula inferior se distingue por un tamaño un poco menor, por la parte sinfisaria mas comprimida transversalmente, por el diastema entre el canino y el premolar todavía bastante mas corto, y por los dos prismas que componen cada muela que son mas elípticos y de consiguiente menos prismáticos y menos oblicuos que en la otra especie ”.

English translation. This species was a third smaller in size than the previous one [ Icochilus extensus ], and it is distinguished from it by some characteristics of the external face of upper premolars and molars. P2–4 present the two antero-external perpendicular edges narrower and equal in size, the second edge that does not acquire the form of a projecting column as in the previous species; in addition, the postero-external region of each of these teeth, instead of being flat as in those of Ic. extensus , is excavated by a perpendicular groove that appears immediately behind the second edge, as it happens with the true molars of the previous species. M1–3 of Ic. excavatus have the two perpendicular edges of the anterior part of the external face almost confused in a single one, which takes on the appearance of a broad column in which barely a small vestige of a groove is visible, in the premolars and molars of the other species this is a groove that divides the column into two edges. A perpendicular depression appears behind this column, on the middle part of the external face of the tooth, and it is narrower and deeper than in the other species. The cheek tooth series is arranged in a more arched position, but it is shorter because the space occupied, in straight line, by the seven upper molars is only 22 millimetres. The mandible is distinguished by a quite smaller size, by its symphyseal region being more transversally compressed, by the diastema between the canine and the premolar that is still quite shorter, and by the two prisms that constitute each tooth which are more elliptical and, consequently, less prismatic and less oblique than in the other species.

Comments. Ameghino (1889: 472–473, plate 15: figs. 10–13) described and illustrated this taxon based on different characteristics of the upper and lower cheek teeth.

According to Ameghino’s catalogue and Mones (1986), MACN-A 177 to MACN-A 180 are the type material of the species. The mandibular symphysis MACN-A 177 and the fragmented mandible MACN-A 178 match the original figures 12 and 13, respectively ( Ameghino 1889: plate 15), which are herein reproduced ( Figs. 3 G–K). Fortunately, we could locate the right maxilla MACN-A 9681, which matches figure 11 of the Atlas ( Ameghino 1889) herein reproduced in Figure 3 G . On the other hand, even though the fragmented maxilla MACN-A 179 ( Fig. 6 F ) and missing MACN-A 180, a mandibular or maxillary fragment according to the inventory, were not originally illustrated, they cannot be discarded as syntypes because both are part of the same lot as MACN-A 177 and MACN-A 178. Unfortunately, it was not possible to find an m3 that matches the figure 10 of the Atlas ( Ameghino 1889: plate 15), but it is also considered as part of the type series of the species. Consequently, MACN- A 177 to MACN-A 180 and MACN-A 9681 are the syntypes of Ic. excavatus .

Figures

FIGURE 3. Icochilus excavatus, syntype MACN-A 9681, right maxilla with alveoli of C and dP1 and complete P2–M3, in occlusal view (A); syntype MACN-A 177, broken symphysis, in labial (B) and lingual (C) views; syntype MACN-A 178, fragmented mandible with c–m1, in labial (D) and occlusal (E) views; syntype MACN-A 179, fragmented right maxilla with two molariforms, in occlusal view (F); reproductions of figures 11 (G), 12 (H), 12a (I), 13 (J) and 13a (K) from Ameghino (1889, plate 15). Icochilus undulatus, holotype MACN-A 373, left maxilla with P3–M3, in occlusal view (L); reproduction of figure 14 (M) from Ameghino (1889, plate 15). Icochilus rotundatus, holotype MACN-A 371, incomplete left maxilla with P3–M3, in occlusal view (N); holotype MACN-A 372, left mandibular fragment with p3–m1, in occlusal view (O); reproductions of figures 15 (P) and 16 (Q) from Ameghino (1889, plate 15). Scale bar = 10 mm.

FIGURE 6. Icochilus multidentatus, lectotype MACN-A 3637, right maxilla with alveolus of extra tooth and C and dP1–M3, in occlusal view (A); paralectotype MACN-A 3638, right mandibular fragment with dp1–4 and m1–3, in oclusal (B) and lingual (C) views. Icochilus hegetotheroides, holotype MACN-A 9855, incomplete skull, anterior view of the premaxilla with labial view of both I1 (D) and right lateral view (E); holotype MACN-A 9856, incomplete mandible, in occlusal (F) and left lateral (G) views. Icochilus ulter, holotype MACN-A 11601, right mandibular fragment with p3–m3, in oclusal (H) and labial (I) views. Scale bar = 10 mm.