Deciduous dentition and dental eruption sequence in Interatheriinae (Notoungulata, Interatheriidae): implications in the systematics of the group

Abstract

Studies focused on deciduous dentition, ontogenetic series, and tooth eruption and replacement patterns in fossil mammals have lately increased due to the recognized taxonomic and phylogenetic weight of these aspects. A study of the deciduous and permanent dentition of Interatherium and Protypotherium (Interatheriinae) is presented, based mainly on unpublished materials. Deciduous cheek teeth are brachydont and placed covering the apex of the respective permanent tooth; in addition, some morphological and metrical differences are observed along the crown height. Five dental ontogenetic stages are distinguished among the juvenile specimens on the basis of the degree of wear, the replacement of the deciduous premolars, and the eruption of the molars. The crown height and the wear degree of different Interatheriinae taxa show: (1) eruption pattern of molars in an anterior–posterior direction (M/m1 to M/m3); (2) pattern of replacement of deciduous premolars and eruption of permanent premolars in a posterior–anterior direction (dP/dp4 to dP/dp2 and P/p4 to P/p2); and (3) eruption of M/m3 before the replacement of dP/dp4. Results allow evaluating the diagnostic dental characteristics used to describe some interatheriines, as well as reinterpreting some taxonomic assumptions: the holotype of Protypotherium diversidens Ameghino, 1891 is recognized as a juvenile of another species of the genus, and the species is not validated, considering it as Protypotherium sp.; the holotype of Eudiastatus lingulatus Ameghino, 1891 falls in the variability of Protypotherium, becoming P. lingulatus new combination, tentatively maintaining the species and implying the synonymy between Eudiastatus and Protypotherium; and the holotype of Eopachyrucos ranchoverdensis Reguero, Ubilla, and Perea, 2003 is reinterpreted as bearing deciduous premolars.

Systematic Revision of Interatherium and Icochilus (Interatheriidae, Notoungulata) from the Santa Cruz Formation (Early To Middle Miocene), Santa Cruz Province, Argentina

“…Interatheriines are represented in the SCF by the genera Protypotherium (= Eudiastatus; Fernández et al, 2021b), Patriarchus, Interatherium, and Icochilus, which historically have contained more than 40 described species. Recent work (Fernández et al, 2019b(Fernández et al, , 2021aFernández, 2020) and results presented herein indicate this figure was a remarkable overestimation of the species-level taxonomic diversity of the Santacrucian interatheriines.…”

“…For example, out of the almost 30 historically described species belonging to Patriarchus, Icochilus, and Interatherium in the SCF, we now recognize just four species, distributed among Patriarchus, Neocochilus, and Interatherium (=Icochilus); this is mainly due to synonymy (Fernández et al, 2019b(Fernández et al, , 2021aFernández 2020;this paper). (See Fernández, 2020 for a similar revisionist pattern with respect to Protypotherium).…”

“…-Icochilus senilis was differentiated by the lack of grooves in P2, which was described as being sub-elliptical in section (Ameghino, 1894: 15), but grooves diminish with wear and the holotype of this species shows a very advanced stage of wear and vestiges of such grooves on its both P2s (see Fernández et al, 2021a). The marked reduction of the ectoflexid and the entoflexid identified by Ameghino (1894: 15) in the lower cheek teeth, particularly in p2, cannot be considered as diagnostic also due to their ontogenetic variation.…”

“…We use a modified version of the data matrix of Fernández et al (2021b) because it is most complete in terms of the taxon sampling of Interatheriidae at the genus level. In turn, that matrix (Fernández et al, 2021b) was a modified version of those presented by García-López and Babot (2015), Hitz et al (2006), andFernández et al (2019b). The outgroup includes the basal notoungulate Simpsonotus Pascual et al, 1978 (Henricosborniidae), Pleurostylodon Ameghino, 1897 (Isotemnidae), and the basal typothere Colbertia Paula-Couto, 1952 (Oldfieldthomasiidae).…”

“…Early to Middle Miocene; known from, probably, the 'Pinturan Age' and the Santacrucian and Colloncuran SALMAs of Argentina.DescriptionSinclair (1909) provided an exhaustive description of Interatherium, particularly with respect to the skull and mandible. The most relevant dental characteristics of (d)I/i1-(d)C/c and (d)P/p2-4 were described byFernández et al (2021a). In order to reduce the information presented here and avoid repetitions in the respective commented species, we only describe the main tooth features and the variations that justify the proposed synonyms (see below), as well as the characteristics of M/m1-3.The upper and lower dentitions ofInteratherium have cementum on the labial and lingual faces of each tooth, but markedly reduced in thickness in the mesial and distal imbrication areas.…”