Lagostomus (Lagostomopsis) incisus ( Ameghino, 1888 )

Rasia, Luciano Luis & Candela, Adriana Magdalena, 2013, Systematic and biostratigraphic significance of a chinchillid rodent from the Pliocene of eastern Argentina, Acta Palaeontologica Polonica 58 (2), pp. 241-254 : 244-249

Lagostomus (Lagostomopsis) incisus ( Ameghino, 1888)

Figs. 3–5, 6 .

1888 Lagostomus incisus sp. nov.; Ameghino 1888: 9.

1888 Lagostomus spicatus sp. nov.; Ameghino 1888: 10 (nov. sin.).

1888 Lagostomus angustidens sp. nov.; Moreno 1888: 15.

1888 Lagostomus intermedius sp. nov.; Moreno 1888: 15–16 (nov. sin.) 1889 Lagostomus incisus Ameghino, 1888 ; Ameghino 1889: 182, pl. 9: 22.

1889 Lagostomus angustidens Moreno, 1888 ; Ameghino 1889: 182.

1989 Lagostomus spicatus Ameghino, 1888 ; Ameghino 1889: 184, pl. 9: 9, 15 (nov. sin.).

1889 Lagostomus intermedius Moreno, 1888 ; Ameghino 1889: 184 (nov. sin.).

Holotype: The holotypes of Lagostomus (Lagostomopsis) incisus and “ L. (L.) spicatus ” were originally deposited at the MLP ( Ameghino 1889), but both are currently lost. We therefore propose MACN−A 1112 ( Fig.3A) as the neotype, because it was referred by Ameghino (1889: pl. 9: 22) to L. (L.) incisus , and was recovered from the same locality as the holotype.

Type locality: Farola Monte Hermoso, Buenos Aires Province, Argentina.

Type horizon: Monte Hermoso Formation, Montehermosan–Lower Chapadmalalan, Early Pliocene.

Referred material.— MACN−A 1112 , anterior portion of skull with both incisors, complete tooth rows, and right fragment of basicranium (referred to L. [L.] incisus in Ameghino 1889: pl. 9: 22; proposed neotype) ; MACN−A 1654 , left mandible with p4–m3, and rostral fragment with right upper incisor (referred to “ L. [L.] spicatus ” in Ameghino 1889: pl. 9: 9, 15) ; MACN−Pv 7388 , ventral portion of skull, with both incisors and complete tooth rows ; MLP 46 −V−13−72, right mandible fragment with incisor and p4–m2, isolated right M2, and right tibia ; MLP 48 −XII−16−194, palatal fragment with right P4–M1, M3, and left M2–3 ; MLP 86 −VI−20−13, anterior portion of rostrum with both incisors ; MLP 88 − VI−1−2, skull fragment with right P4–M3 and left P4–M2, left mandible fragment with p4–m3, two isolated upper incisors, one caudal vertebra, right humerus, fragments of pelvic girdle, and left femur ; MLP 91 −IV−5−258, almost complete skull with complete tooth rows, right radius, shaft fragment of right ulna, right femur, portion of left femur, right tibia, left calcaneus, right astragalus, right second, third and fourth metatarsals, and right second, third and fourth proximal phalanges ; MLP 63 −VI−10−59, incomplete skull with complete

RASIA AND CANDELA—A CHINCHILLID RODENT FROM THE PLIOCENE OF ARGENTINA 245

10 mm

tooth rows; MLP 91−III−1−18, almost complete skull complete tooth rows; MLP 91−III−1−36, palate with complete tooth rows, portion of rostrum, atlas, axis, cervical vertebra, and right and left portions of pelvic girdle; MLP 94−II−1−136, palatal fragment with complete tooth rows; MLP 91−III−1−9, right mandible with incisor and p4–m3; MLP 94−II−1−146, mandibles with incisors and complete tooth rows, right mandible with p4–m3, right maxilla with P4–M3, two isolated upper incisors, right and left humerus, right portion of pelvic girdle, right and left femur, right and left tibia, distal portion of right fibula, right and left astragalus, left calcaneus, right navicular, right ectocuneiform, right second and third metatarsals, and right second and third proximal phalanges; material represents at least two individuals; MLP 91−III−1−88, mandibles with complete cheek tooth rows, fragment of rostrum with incisors, left humerus, and several phalanges; MLP 91−IV−5−214, left mandible fragment with incisor and p4–m2, and isolated left upper incisor.

Emended diagnosis.—Lagostomine 20% smaller than Lagostomus maximus . Maxillae much more expanded transversely, palatines much more reduced in ventral view, and upper cheek teeth more obliquely implanted in the maxillae than in L. (L.) pretrichodactylus , L. (L.) insolitus , L. (L.) angulatus , L. (L.) loberiaensis , L. (L.) euplasius , L. (L.) compressidens , L. (L.) indefinitus , L. cavifrons , and L. maximus . Posterior palatine apophyses of the premaxillae very reduced and not at the same dorsoventral level as the diastema, clearly differing from L. (L.) pretrichodactylus , L. (L.) insolitus , L. (L.) loberiaensis , L. (L.) euplasius , L. (L.) compressidens , L. cavifrons , and L. maximus . Lower cheek teeth much more compressed anteroposteriorly and more obliquely implanted in the mandible than in L. (L.) pallidens , L. (L.) antiquus , L. (L.) insolitus , L. (L.) euplasius , L. (L.) definitus , L. (L.) compressidens , L. debilis , L. heterogenidens , L. minimus , and L. maximus . Humerus with canalis supracondyloideus (entepicondyloideus) not entirely closed.

Comparative description

Skull: The skulls of adult specimens ( MACN−A 1112, MACN−Pv 7388, MLP 91−III−1−18; Figs. 3A, B, 4A ) of Lagostomus (Lagostomopsis) incisus are about 20% smaller than those of L. maximus , ranging in length from 73.85 mm ( MLP 91−IV−5−258) to 97.92 mm ( MLP 91−III−1−18), with an average length of 85.88 mm. The nasals are shorter than the premaxillae ( Fig. 4A 2), as in L. (L.) pretrichodactylus , L. (L.)

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insolitus , L. (L.) loberiaensis , and L. (L.) euplasius . In most of the studied specimens of L. maximus the nasals are longer than the premaxillae, as in L. (L.) compressidens and L. cavifrons . The posterior processes of the premaxillae extend beyond the anterior edge of the dorsal zygomatic root, as is also the case in L. (L.) pretrichodactylus , L. (L.) insolitus , L. (L.) loberiaensis , and L. (L.) euplasius , but not in L. (L.) compressidens , L. cavifrons , and L. maximus . The frontals are depressed along the midline ( Figs. 4A 2, 5A 2), being as long as the nasals in young individuals and slightly shorter than the nasals in adult ones. In L. (L.) pretrichodactylus , L. (L.) insolitus , L. (L.) loberiaensis , and L. (L.) euplasius the frontals are slightly longer than the nasals, whereas in L. (L.) compressidens , L. cavifrons , and L. maximus they are shorter. The parietals are more vaulted than in L. maximus . As in L. maximus , the sagittal and temporal crests are weakly developed in young individuals ( Fig. 5A 2), but well developed in adults ones ( Fig. 4A 2).

The posterior palatine apophyses of the premaxillae (located between the interpremaxillary and incisive foramina) are small and not at the same dorsoventral level as the diastema ( Figs. 3A, 4A 1, B 1 , 5A 1 ), clearly differing from L. (L.) pretrichodactylus , L. (L.) insolitus , L. (L.) loberiaensis , L. (L.) euplasius , L. (L.) compressidens , L. cavifrons , and L. maximus , in which the posterior palatine apophyses are strong and frequently project to the level of the diastema. The interpremaxillary foramen is always present, unlike in L. maximus , in which it can be closed in adults. The incisive foramen is wide and long relative to the length of the diastema. The maxillae are more expanded in palatal view and the palatines are much more reduced ( Figs. 3A, B, 4A 1, B 1 , C, D, 5A 1, B 1 ) than in the living species. In young individuals, the palatines are fused along the midline and form a posterior process ( Figs. 4B 1 , C, D, 5A 1 ), as in L. maximus . In adults, the maxillae cover the palatines ventrally, thus forming the “cleft palate” described by Ameghino (1888) (see Discussion). The posterior maxillary foramina are small. The tympanic bullae are rounded and inflated in all individuals, as opposed to the more elongate shape observed in adult L. maximus .

Other skull features, such as foramina of the orbital region and the morphology of the basisphenoid and occipital regions, are indistinguishable from those of L. maximus .

Upper dentition: The upper incisors range in transverse diameter from 2.53 mm to 5.46 mm ( Table 1). The upper cheek teeth are more obliquely implanted in the maxillae ( Figs. 3A, B, 4A 1, B 1 , C, D, 5A 1, B 1 ) than in L. (L.) pretrichodactylus , L. (L.) insolitus , L. (L.) angulatus , L. (L.) loberiaensis , L. (L.) euplasius , L. (L.) indefinitus , L. (L.) compressidens , L. cavifrons , and L. maximus , with the angle of the tooth laminae with respect to the sagittal plane ranging from 39 ° –45 ° in L. (L.) incisus , as opposed to 46 ° –62 ° in all of the other species. In P4, the anterior lamina is slightly narrower transversely than the posterior one, which widens labially, and the enamel is thinner on the labial side of the tooth. As in L. maximus , there is no enamel on the labial side of the upper molars. The third lamina of M3 is large and resembles that of L. maximus in shape.

Lower dentition: The transverse diameter of the lower incisors ranges from 3.42 mm to 5.26 mm (see Table 1). The lower cheek teeth are more compressed anteroposteriorly (see Table 1) and more obliquely implanted in the mandible ( Figs. 3C 1, E 1 , 4E 1, F 1 , 5B 1, C 1 , 6A 1, B 1 ) than in L. (L.) pallidens , L. (L.) antiquus , L. (L.) insolitus , L. (L.) euplasius , L. (L.) compressidens , L. (L.) definitus , L. debilis , L. heterogenidens , L. minimus , and L. maximus . The only other species possessing highly anteroposteriorly compressed lower cheek teeth is L. (L.) laminosus ( Ameghino 1891; Vucetich 1984), but the relatively scarcity of the material assigned to this taxon currently prevents more detailed comparisons. The transverse axis of p4 is almost parallel to the sagittal plane, and the anterior lamina of this tooth is anteroposteriorly longer than the posterior one. While the anteroposterior length of the lower molars decreases from front to back, their transverse diameter increases, with m1 being much narrower than m2 and m3.

Mandibles: The mandibles are less divergent than in L. maximus . In other preserved features, such as the fossa for the insertion of the masseter medialis pars posterior, the masseteric notch, the morphology of the coronoid process, or the relative length of the diastema, L. (L.) incisus does not differ from L. maximus .

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Postcranial material: The canalis supracondyloideus or entepicondyloideus ( Figs. 4F 3, 6D ) is not entirely closed, as opposed to L. (L.) loberiaensis and L. (L.) euplasius , in which the canal is fully closed, as well as the extant L. maximus and the other living chinchillids, Chinchilla and Lagidium , in which the canal is absent.

Remarks.— Lagostomus (Lagostomopsis) incisus was originally described by Ameghino (1888) as a species slightly smaller than Lagostomus maximus , and characterized by the presence of a “cleft palate”, as well as more slender and shorter premaxillae, and more compressed and more obliquely implanted upper cheek teeth than in the living species. “ L. (L.) spicatus ” was distinguished by Ameghino (1888, 1889) by being just half the size of L. (L.) incisus , the presence of strongly striated incisors, more dorsally directed coronoid processes, and more compressed and more obliquely implanted lower cheek teeth than in L. maximus . Note that the differences between L. (L.) incisus and “ L. (L.) spicatus ” observed by Ameghino (1888, 1889) correspond mainly to differences in the size of the skull and the width of the incisors, as well as the enamel striation pattern of these teeth. According to Ameghino (1888, 1889), L. (L.) incisus has incisors with a transverse width exceeding 5 mm, while in “ L. (L.) spicatus ” the transverse width of the incisors ranges from 3.5–4 mm. The diagnostic value of these features in light of the ontogenetic variability and sexual dimorphism observed in the living L. maximus is discussed below.

Geographic and stratigraphic range.— MACN−A 1112, MACN−A 1654, MACN−PV 7388, MLP 46−V−13−72, MLP 48−XII−16−194, and MLP 86−VI−20−13 come from Farola Monte Hermoso ( Fig. 1 ), with the first three having no precise stratigraphic provenance within the Monte Hermoso Formation (Montehermosan–Lower Chapadmalalan, Early Pliocene), while the last three come from the upper section of the Monte Hermoso Formation (Lower Chapadmalalan, late Early Pliocene, Cione and Tonni 1995b; see Fig. 2); MLP 88−VI−1−2 was recovered from Las Vertientes (near Mar del Plata; Fig. 1 ), from the Chapadmalal Formation (Upper Chapadmalalan, early Late Pliocene; Fig. 2); MLP 91−IV−5−214 and MLP 91−IV−5−258 come from Fortín−88 ( Fig. 1 ), from the lower levels of the Chapadmalal Formation (Upper Chapadmalalan, early Late Pliocene); MLP 63−VI−10−59 was recovered from Quequén Salado River ( Fig. 1 ), from the Irene “Formation” (Chapadmalalan, late Early–early Late Pliocene); MLP 91−III−1−9, MLP 91−III−1−18, MLP 91−III− 1−36, MLP 91−III−1−88, MLP 94−II−1−136, and MLP 94− II−1−146 come from Cascada Grande ( Fig. 1 ), from the middle levels of the Irene “Formation” (Chapadmalalan, late Early–early Late Pliocene; Fig. 2).

Figures

Fig. 6. The chinchillid rodent Lagostomus (Lagostomopsis) incisus (Ameghino, 1888) from the Irene “Formation”, Buenos Aires Province, Argentina (late Early–early Late Pliocene); MLP 94−II−1−146. A. Mandibles with incisors and complete tooth rows in occlusal (A1) and labial (A2) views. B. Right mandible with p4–m3 in occlusal (B1) and labial (B2) views. C. Right maxilla with P4–M3 in ventral view. D. Right humerus in anterior view.

Fig. 4. The chinchillid rodent Lagostomus (Lagostomopsis) incisus (Ameghino, 1888) from the Irene “Formation”, Buenos Aires Province, Argentina (late Early–early Late Pliocene). A. MLP 91−III−1−18, skull with complete tooth rows in ventral (A1), dorsal (A2), and lateral (A3) views. B. MLP 63−VI−10−59, partial skull with complete tooth rows in ventral (B1) and dorsal (B2) views. C. MLP 91−III−1−36, palate with complete tooth rows. D. MLP 94−II−1−136, palate with complete tooth rows. E. MLP 91−III−1−9, right mandible with p4–m3 in occlusal (E1) and labial (E2) views. F. MLP 91−III−1−88, mandibles with complete tooth rows in occlusal (F1) and labial (F2) views, and left humerus (F3) in anterior view.

Fig. 5. The chinchillid rodent Lagostomus (Lagostomopsis) incisus (Ameghino, 1888) from the Chapadmalal Formation, Buenos Aires Province, Argentina (early Late Pliocene). A. MLP 91−IV−5−258, skull in ventral (A1), dorsal (A2) and lateral (A3) views. B. MLP 88−VI−1−2, palate with right P4–M3 and left P4–M2 in ventral view (B1), and left mandible with p4–m3 in occlusal (B2) and labial (B3) views. C. MLP 91−IV−5−214, left mandible with p4–m2 in occlusal (C1) and labial (C2) views.

Fig. 1. Map of Buenos Aires Province, Argentina (A), showing the localities mentioned in the text (B). Black circles indicate fossil localities, while open circles indicate towns and cities.