Orthomyctera rigens ( Ameghino, 1888 )

Jaén, María Carolina Madozzo & Pérez, María Encarnación, 2024, Redescription of a small Caviidae (Rodentia: Hystricognathi) from the Neogene of northwestern Argentina and its systematic implications, Comptes Rendus Palevol 23 (20), pp. 269-292 : 273-275

Orthomyctera rigens ( Ameghino, 1888)

( Figs 2; 3; Tables 1 ; 2 )

Cavia rigens Ameghino, 1888: 12 .

Orthomyctera Ameghino, 1889: 372 .

TYPE MATERIAL. — Lectotype. Argentina. MACN-A 1661 , incomplete skull, with left P4-M3 and right P4-M2. TYPE LOCALITY. — Argentina, Buenos Aires Province, 60 km east from Bahía Blanca locality, Monte Hermoso locality, ( Ameghino 1889) ( Fig. 1A).

GEOGRAPHICAL AND STRATIGRAPHIC PROVENANCE. — Argentina, Buenos Aires, Monte Hermoso locality, Monte Hermoso Formation, Early Pliocene ( Deschamps et al. 2012; Tomassini et al. 2013; Prevosti et al. 2021).

DESCRIPTION AND COMPARISON

Cranium

The skull and only known specimen of Orthomyctera rigens is poorly preserved ( Fig. 2 A-D), missing most of the rostrum, the zygomatic process, and the braincase. The anteroposterior length of the skull is incomplete because it is broken, but larger than in Microcavia australis , and it seems smaller than of Dolicavia Ameghino, 1916 ( Table 1 ; Appendices 4; 5).

Premaxilla. The premaxilla ( Table 1 ) is poorly preserved in the ventral view ( Fig. 2D), the incisive foramina seem triangular-shaped, as in M. australis and D. minuscula .

Maxilla. The lateral ridge of the infraorbital groove (rMx; Fig. 2B) is similar than Neocavia lozanoi and some specimens of Dolicavia (e.g. MMP 536-S), while dorsoventrally is lower than in M. australis and other specimens of Dolicavia (e.g. MACN-Pv 10617). In lateral view, the posterior border of the diastema is vertical, and the suture between both maxillae forms a small bulge ( Fig. 2A), as in M. australis and in D. minuscula . A small portion of the maxillary zygomatic process is preserved on the left side at the level of P4-M1. In ventral view, the notch for the tendon of the medial part of the masseter muscle in the origin of the zygomatic arch is an elliptical concavity with well-defined edges ( Fig. 2D) as D. minuscula . The palate is terraced (maxillopalatal portion of the palate more dorsal than the lateral maxillary portion; Fig. 2D). This terracing is similar to the condition of “ Orthomyctera ” andina and N. lozanoi and is shallower than in Dolicavia and M. australis .

Frontal. In dorsal view, the anterior portion of the frontal are flat ( Fig. 2B, C), at the level of the orbital region. Other characteristics (e.g. ledge or the dorsal hole or orbital constriction) described by Ameghino (1889) cannot be confirmed due to poor preservation of the remains.

Parietal. The parietal is displaced from its original position and broken in the posterior portion. The conserved portion is convex as in M. australis , D. minuscula , and N. lozanoi ; the left temporal fossa seems as shallow as in other caviines (e.g. M. australis ) the right portion is not preserved.

Palatine. The palatine bone occupies 25% of the anteroposterior length (APL) of the palate ( Fig. 2D). The medial suture between both maxillary counterparts and between maxilla and palatine are not completely fused, indicating that it could be a still young individual. The anterior border of the mesopterygoid fossa is U-shaped, extending up to the anterior lobe of M3, as in other Caviinae (e.g. Dolicavia , G. musteloides , M. australis ). However, it differs from Cavia , which presents a palatal process; it also differs from Dolichotinae in which the mesopterygoid fossa extends up to M2 and has a V-shape (see Ameghino 1889).

Cheek teeth

In the publication of 1889, Ameghino wrote: “ todas las muelas del lado derecho y las tres primeras del izquierdo ”, that means “all the molars on the right side and the first three on the left”. However, this is a simple confusion that needs to be clarified, since the specimen MACN-A 1661 preserves the left P4-M3 and right P4-M2 ( Fig. 3A).

The cheek teeth are euhypsodont and double heart-shaped, with a constriction in the lingual apex of the lobes, and a transverse dentine crest in the middle of the occlusal surface of each lobe ( Fig. 3A), as in other Caviinae and Dolichotinae . The hypoflexus is funnel-shaped with cement; enamel is interrupted at the labial wall, except along the external secondary flexus (HSE). The HSE of molariforms of O. rigens is similar to Dolicavia and shallower than in other cavies (e.g. Cavia , Neocavia ) and, differs from the flexus of Dolichotis and “ O. ” andina in that these taxa have the flexus as a labial furrow opposite to the hypoflexus ( Madozzo Jaén et al. 2021; but see Ameghino 1888, 1889; see below).

The APL of M1 is the shortest of the cheek teeth, while the APL of P4 is shorter than that of M2, and M2 is shorter than that of M3 ( Table 2 ). It differs from M. australis in which the APL of M1 is similar to that of M2. The M3 has a developed posterior projection of the posterior lobe, shorter than in Neocavia , Dolicavia , and “ O. ” andina ; and differs from M. australis , in which the posterior projection is an incipient lobe.

REMARKS

The type species of Orthomyctera

Orthomyctera rigens was designated the type species of the genre Orthomyctera by Kraglievich (1930: 71). Posteriorly, Pascual (1966) considered Orthomyctera lata as the type species of the genre, without any kind of justification. According to the nomenclature code (International Code of Zoological Nomenclature: art. 69.1) if no type species was designated in the original publication, the first designation is valid.

The type material of Orthomyctera rigens

Ameghino (1888) established Cavia rigens based on characters of the upper and lower teeth without specifying on which specimens the determination was made. Posteriorly, Ameghino (1889) erected the genus Orthomyctera , which includes the species Cavia rigens , among others ( Orthomyctera vaga , O. lata , and Dolichotis lacunosa Ameghino, 1888 ). In that work he figured a skull ( MACN-A 1661 see Lam. XI, 6-7), and a jaw ( MACN-A 1662) as Orthomyctera rigens . Subsequently, both specimens have been considered the type material of this species (e.g. Croft et al. 2011). From the morphological analysis here, it is inferred that the skull and the mandible do not correspond to the same individual and even correspond to different taxonomic entities because of the follow morphological differences:1) lower m1 of MACNA 1662 smaller than the upper molars of MACN-A 1661 (see Table 2 ) lower m1 of MACN-A 1662 with an opposite furrow to the hypoflexid, contrasting with that condition of the upper molars of MACN-A 1661 in which the HSE is present. Since Ameghino (1889) focused the description of this species on the skull MACN-A 1661, we designated this specimen as the lectotype of O. rigens .

Referred material to Orthomyctera rigens

Appendices 6-8 provides a revision of the fossil specimens previously referred to as O. rigens in the literature. However, here is necessary to clarify that the mention of O. rigens in Pardiñas et al. (2017) (that was taken in turn of Frenguelli (1928)), from the Quequén Salado River, “Irenense” Fauna (Buenos Aires Province), could not be corroborated because the authors did not provide details of the material or the corresponding collection number.