Icochilus rotundatus 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 : 208-209

Icochilus rotundatus Ameghino, 1889

Syntypes. MACN-A 371 ( Fig. 3 N ), incomplete left maxilla with P3–M3 ; MACN-A 372 ( Fig. 3 O ), left mandibular fragment with p3–m1 .

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

Original description ( Ameghino 1889: 473–474). “ La talla de esta especie es apenas un poco menor que la del I. extensus , y con muelas que reúnen en parte los caractéres de las tres precedentes. Los premolares superiores tienen las dos aristas perpendiculares dé la parte anterior de la cara externa, apenas un poco desiguales, la segunda algo mas desarrollada que la primera, pero sin tomar la forma de columna saliente que presenta en los premolares del I. extensus . Además, detrás de la segunda

arista existe la depresión ó surco perpendicular medio, que falta en los mismos dientes de la especie mencionada. Los verdaderos molares presentan las dos aristas anteriores casi confundidas, con la ranura que las separa poco profunda ó rudimentaria, pero sin que se levanten tanto encima de la superficie de la cara externa como en el I. excavatus . Detrás de esta arista, viene la depresión perpendicular mediana, bastante ancha, pero poco profunda. La última muela superior es apenas un poco mas pequeña que la penúltima y casi del mismo ancho. La série dentaria es ligeramente arqueada. […] De la mandíbula inferior conozco un fragmento de la rama horizontal del lado izquierdo, con los dos últimos premolares y el primer verdadero molar. Cada muela consta de dos prismas elípticos casi iguales, apenas un poco mas angostos sobre el lado interno, con su diámetro mayor en sentido transversal. El p. 3 tiene 0 m 0 0 35 de diámetro ántero-posterior y 0 m 0 0 25 de diámetro transverso. El p. 4 y el m. 1 tienen cada uno 4 milímetros de diámetro ántero-posterior y 3 milímetros de diámetro transverso. El agujero mentoniano posterior es de tamaño mucho mayor que en las otras especies, pero está igualmente colocado debajo de la parte anterior del p. 4. La rama horizontal debajo del p. 4 tiene 17 milímetros de alto ”.

English translation. This species is slightly smaller in size than Ic. extensus , and presents cheek teeth that partially combine the characters of the preceding three species. Upper premolars have the two antero-external perpendicular edges slightly dissimilar; the second is somewhat more developed than the first, but without taking the form of a protruding column as in Ic. extensus . In addition, behind the second edge there is a median and perpendicular depression or sulcus, which is absent in the same teeth of the already mentioned species. True upper molars present the two anterior edges nearly confused in a single column, with the groove that separates them being shallow or rudimentary, but without rising above the surface of the external face as much as in Ic. excavatus . Behind this edge, there is the median perpendicular depression fairly broad but shallow. The last upper molar is just a little smaller than the second molar and almost of the same width. The dental series is slightly arched. [...] I know from the mandible a fragment of the left horizontal ramus, with the last two premolars and the first true molar. Each cheek tooth consists of two almost equal and elliptical prisms, just a little narrower on the inner side, with its greater diameter in transverse direction. The p3 has 0.0035 m of antero-posterior diameter and 0.0025 m of transverse diameter. The p4 and m1 have 4 millimetres of antero-posterior diameter and 3 millimetres of transverse diameter. The posterior mental foramen is much larger in size than in the other species, but it is also placed below the anterior part of p4. The horizontal ramus below p4 is 17 millimetres high.

Comments. Ameghino (1889: 473–474) erected and described this species based on a maxilla and a mandible, both incomplete ( Ameghino 1889, plate 15: figs. 15–16). Later, Ameghino (1894a: 15) extended the original description based on a skull that he did not illustrate.

According to Ameghino’s catalogue and Mones (1986), the incomplete skull MACN-A 3588 is the type specimen. MACN-A 3588 corresponds to the skull described by Ameghino (1894a), because it lacks the upper canine and exhibits a long diastema between I3 and dP1. However, it does not match any of the illustrations provided by Ameghino (1889, plate 15: figs. 15–16) and does not exhibit the diagnostic characteristics stated in the original description. Consequently, MACN-A 3588 is neither the holotype nor part of the type series of the species.

On the other hand, MACN-A 371, left maxillary fragment with P3–M3, and MACN-A 372, left mandibular fragment with p3–m1, match the original illustrations ( Ameghino 1889: plate 15, figs. 15–16), which are herein reproduced in Figures. 3 P–Q. It is worth mentioning that it cannot be guaranteed that both specimens belong to the same individual because they do not articulate to each other and there is no mention of this in Ameghino’s catalogue—as he used to do when this was the case—. Therefore, MACN-A 370 and MACN-A 371 are considered here as the syntypes of Ic. rotundatus .

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.