Megadolodus molariformis McKenna 1956
Creators: name: Carrillo, Juan D.
name: Suarez, Catalina
name: Benites-Palomino, Aldo Marcelo
name: Vanegas, Andrés
name: Link, Andrés
name: Rincón, Aldo F.
name: Luque, Javier
name: Cooke, Siobhán B.
name: Tallman, Melissa
name: Billet, Guillaume
Publisher: name: Zenodo
Year: 2023
Subjects: Biodiversity
Taxonomy
Animalia
Chordata
Mammalia
Litopterna
Didolodontidae
Megadolodus
Megadolodus molariformis

description: Megadolodus molariformis McKenna, 1956 (Figs 1-4)
HOLOTYPE. — UCMP 39270, partial left mandible bearing the labial portion of the crown of p4, the m1, and the alveoli of p3 and m2 (Fig. 3B).
NEW REFERRED MATERIAL. — VPPLT 974, partial mandible with the symphysis and left body, bearing the alveoli of?i2, right and left?i3, alveolus of right c and p1, roots of left p1, and the left p2-m3 (Fig. 3A, C-E). VPPLT 1588, isolated left P1 and P2, right P3/P4, left P3/P4, and partial left maxilla with M1, M2 and lingual part of M3 (Fig. 2).
TYPE LOCALITY AND HORIZONS. — La Victoria Formation. VPPLT 974 comes from Las Gaviotas 1 (3°18’41”N, 75°12’27”W) in the bed set below the Cerbatana conglomerate (StL 7; Mora-Rojas et al. 2023) and VPPLT 1588 comes from Cerro Pan de Azúcar (3°21’26”N, 75°10’33”W) in the bed set below Cerro Gordo (StL1; Mora-Rojas et al. 2023) (Fig. 1).
DIAGNOSIS. — Same as for the genus due to monotypy.
REMARKS
The upper and lower incisors, canines and most rostral premolars were only partially preserved in known specimens of Megadolodus molariformis. Previously known craniodental material included partial mandibles that preserved dp3-4, p1-p3, fragmentary p4, m1-m3. Fragments of maxilla and isolated upper teeth included the dP3-4, P2-M2 and fragmentary M3 (Table 3; Cifelli & Villarroel 1997). The known postcranial elements include an almost complete vertebral column, ribs, fragments of the humerus, radius and ulna, most of the manus, pelvis and almost all the elements of the hind limbs (Cifelli & Villarroel 1997).
Fragments of the rostral upper and lower dentition have been previously identified, but given that these were isolated and fragmentary elements there has been uncertainty regarding the number and morphology of the upper and lower incisors, canines and most rostral premolars (p1-2 and P1-2) (Cifelli & Villarroel 1997). VPPLT 974 provides new evidence that clarifies for the first time the number of lower incisors and shows that the lower tusks are incisors (Fig. 3A, C), not lower canines as previously hypothesized (Cifelli & Villarroel 1997). The dental formula of Proterotheriidae is characterized by having one upper incisor, which is enlarged and developed into a tusk, and two lower incisors, the most mesial one is small and the most distal one is tusk-like (Scott 1910; Soria 2001). The homology of the upper and the two lower incisors is uncertain since it has not been established which loci were lost (Soria 2001). Scott (1910) hypothesized that the upper tusk corresponded to I2, whereas the mesial-most lower incisor was the i2 and the distal one was the i3, as in toxodontid notoungulates. Although we acknowledge the uncertainty in the homology on the upper and lower incisors, we follow Scott (1910) and tentatively identify the upper incisor as?I2, and the two lower incisors as?i2 and?i3.
DESCRIPTION
VPPLT 1588 and VPPLT 974 have large bunodont dentitions (Table 4) with thick enamel characteristic of Megadolodus (McKenna 1956; Cifelli & Villarroel 1997). In addition, VPPLT 1588 is referred to Megadolodus based on the partly molariform P3/P4 (with a triangular arrangement of the metacone with the protocone, paracone, the presence of paraconule and metaconule, but without a hypocone; Fig. 2C, E), and the presence of a hypocone in the upper molars (Fig. 2D, F) (Cifelli & Villarroel 1997). The referral of VPPLT 974 to Megadolodus is further supported by the robust mandible, the p4 fully molariform, and the absence of paraconid in p3-m3 (Fig. 3A, D) (McKenna 1956; Cifelli & Villarroel 1997).
VPPLT 1588 preserves two isolated teeth, here interpreted as left P1 (Fig. 2A) and P2 (Fig. 2B), because the crowns are not molariform, as is the case in P3 and P4 (Cifelli & Villarroel 1997). The crown of the putative P1 is simple and it has a single cusp with crests extending mesially and distally. It is triangular in labial view, with some wear on the lingual side (Fig. 2A). P2 is similar to P1, but with a more extended lingual edge. The crown of the putative P2 is also triangular in labial view, as it has one main cusp on the labial side of the crown with crests extending mesially and distally from it and an extended lingual edge. The latter is composed of a basin lingual to the main cusp and exhibits a low cusp on its lingual aspect, which resembles an incipiently developed protocone. A faint cingular crest extends from the low lingual cusp along the mesial and distal edges of the tooth (Fig. 2B).
In Megadolodus, the P3 and P4 have a very similar morphology and size, being partly molariform, but without an hypocone (Cifelli & Villarroel 1997). Therefore, we cannot establish if the isolated left and right upper premolars correspond to P3 or P4 (Fig. 2C, E; Table 4). Both have a well-developed protocone, paracone, metacone, and worn metaconule, the paraconule is only visible in the left tooth (Fig. 2C, E), as in other specimens of Megadolodus (Cifelli & Villarroel 1997). The P3/P4 have mesiolingual and labial cingula. The mesiolingual cingulum isolates a small basin and forms a small cuspule on its lingual end. The lingual face of the P3/P4 is rounded, in contrast to lingual face of the molars which is straight (Fig. 2 C-E). There is no mesostyle in the P3/P4, the parastyle is present and the metastyle region of the crown is not well-preserved in both teeth.
The upper molars show a more quadrangular outline in occlusal view than P3/P4 due to the presence of the cingulum-derived hypocone (Cifelli & Villarroel 1997). The crown of M1 is broken in the distolingual portion where the hypocone would be found, and the crown of M3 is broken, missing most of its labial and distal portions (Fig. 2D). We identified the M1 because of its more quadrangular outline in occlusal view and larger size in comparison with the P3 and P4 (Fig. 2D, F). It also shows more advanced occlusal wear than P3 and/ or P4, M2 and M3, as expected for the M1 given that is the first permanent tooth to erupt. All upper molars have a strong mesiolingual cingulum that isolates a small basin (narrower on M1-2), similar to what is seen on P3 and/or P4. M1 and M2 have a small mesostyle in addition to the faint parastyle and metastyle (Fig. 2F). There are two faint labial ridges, one on the paracone and one in the metacone on M1-2 (Fig. 2F). The paraconule and metaconule are well developed. On the M1 and M2 the paraconule is equidistant between the protocone and paracone, as in other specimens of Megadolodus (Cifelli & Villarroel 1997). The molars of VPPLT 1588 have a preprotocrista (connecting the protocone with the paraconule) and a preparaconular crista (connecting the paraconule with the parastyle). There is no postprotocrista that would connect the protocone and metaconule (Fig. 2F). The metaconule is isolated and closer to the metacone than the protocone, as seen in other specimens of Megadolodus (Cifelli & Villarroel 1997). The hypocone is well-developed and isolated. The labial cingula, which can be seen on M1 and M2, are very thin and extending across the whole labial side (Fig. 2F). Although M1 and M3 are only partially preserved, it appears that M2 is the largest molar, especially in the labiolingual dimension (i.e., width; Table 4).
The mandible of Megadolodus (VPPLT 974) is robust with a deep horizontal ramus that appears to reach its maximum depth at the level of p2-3 (Figs 3-4). No foramina are observed. The rostroventral portion of the left ascending ramus is preserved; it is wide and suggests that the masseteric fossa was large (Fig. 3A). The symphysis extends caudally up to the level of p3 (Fig. 3A). Only the alveoli of the of right and left?i2 were preserved (Fig. 3C). The?i3 is tusk-like and extroverted (projecting anterolaterally). It is not ever-growing, and its root closes at the level of the distal edge of p2 (Fig. 4). The lingual face of?i3 is flat, the labial face is convex, giving a nearly oval shape in cross-section. The anterior tip bears an oblique wear-facet marking the occlusion with the upper dentition (Fig. 3C). Only the alveoli of the right canine and of the p1 are preserved (Fig. 3A). There is a short diastema (8.6 mm) between the right?i3 and the alveolus of the canine, and a slightly longer diastema (13.5 mm) between the alveoli of the right canine and the p1 (Fig. 3A). The alveolus of the canine indicates it was single rooted, and the p1 has two roots, as shown by the preserved part of the left p1 and alveolus of the right p1. The p2 has one main cuspid which is located approximately at the mid-length of the tooth. It is connected distally to a crest directed downwards and towards a small distal basin. This basin (incipient talonid) is bordered distally by a small cuspulid. A crest also connects the mesial edge of the crown with the main cuspid, and there is no mesial bulge, as described in other specimens of Megadolodus (Cifelli & Villarroel 1997). The p2 has two roots, one mesial and one distal, the latter divides in two smaller roots at the apical end, one labial and one lingual (Fig. 4).
The p3-4 are molariform and lack a hypoconulid (Fig. 3A, E), as in other specimens of Megadolodus (Cifelli & Villarroel 1997). There is an increase in size from the p3 to m1 (Table 4). The p3 has three roots, a large distal one, and two mesial. It lacks a paraconid and the paracristid forms a small bulge at the anterolingual border of tooth. The protoconid, metaconid, hypoconid and entoconid are similar in size. The p4 has four roots, two mesial and two distal.
The p4 and the lower molars of VPPLT 974 show an advanced state of wear, and no crest are visible. The trigonid and talonid of the molars are comparable in length and width (Cifelli & Villarroel 1997). The paraconid is absent in all the lower molars.The protoconid, metaconid, and hypoconid have similar size, while the entoconid and hypoconulid are smaller. Due to wear, the hypoconulid is only distinct in the m3, where it is located distal to the other talonid cusps (Fig. 3D). The hypoconulid is slightly closer to the hypoconid, whereas in other Megadolodus specimens it is closer to the entoconid (Cifelli & Villarroel 1997). Unlike upper molars (VPPLT 1588) where M2 is bigger than M1 and M3, the lower molars (VPPLT 974) have a similar size (Table 4). The lower molars have four roots, two mesial and two distal (Fig. 4).
description: Published as part of Carrillo, Juan D., Suarez, Catalina, Benites-Palomino, Aldo Marcelo, Vanegas, Andrés, Link, Andrés, Rincón, Aldo F., Luque, Javier, Cooke, Siobhán B., Tallman, Melissa & Billet, Guillaume, 2023, New remains of Neotropical bunodont litopterns and the systematics of Megadolodinae (Mammalia: Litopterna), pp. 409-447 in Geodiversitas 45 (15) on pages 416-421, DOI: 10.5252/geodiversitas2023v45a15, http://zenodo.org/record/8319166