Microtragulus bolivianus Hoffstetter and Villarroel, 1974

Babot, M. Judith & García-López, Daniel A., 2016, Redescription of the argyrolagid Microtragulus bolivianus (Metatheria, Polydolopimorphia, Bonapartheriiformes) based on new remains from Northwestern Argentina, Palaeontologia Electronica (Ottawa, Ont.) 51 (9), pp. 1-23 : 4-15

Microtragulus bolivianus Hoffstetter and Villarroel, 1974

Figure 2 , Figure 3 , Figure 4 , Figure 5 , Figure 6 ,

Figure 7

Holotype. MNHN-BOL-V-011707 (= GB-0001), right mandibular fragment with i1 and m3-m4 (GB refers to the currently nonexistent GEOBOL [Servicio Geológico Boliviano] collection).

Referred Material. JUY-P- 0065, fragment of right maxilla with M3-M4; JUY-P 50, edentulous fragment of right maxilla preserving the zygomatic process; JUY-P- 0066, right mandibular body with i1, alveoli of i2 and p3, and complete m1-4; JUY-P-0067, right mandibular body with complete i1 and i2, alveolus of p3, and complete m1-4; JUY-P-0068, anterior fragment of right mandibular body with i1 and alveoli of i2 and p3; JUY-P- 0069, almost complete right humerus except for the proximal third; JUY-P- 0070, almost complete left humerus except for the proximal third; JUY-P-0071, distal half of left humerus; JUY-P- 0072, distal end of right humerus; JUY-P 59, proximal half of right ulna; JUY-P 60, proximal half of left ulna; JUYP 61, proximal fragment of radius; JUY-P 52, right proximal epiphysis and right distal end of tibiofibula; JUY-P- 0073, complete left calcaneus; JUY-P-0074, body of left calcaneus; JUY-P- 0075, right astragalus; JUY-P 55, complete right metatarsal III; JUY-P 56, lot containing proximal and distal portions of right metatarsal IV (not associated); JUY-P 57, distal portion of left metatarsal IV; JUY-P 58, lot containing three ungueal phalanges.

Range and Occurrence. The holotype comes from Viscachani, 94 km SE of La Paz, Bolivia; Umala Formation (Pliocene levels dated between 5.3–2.9 Ma [ Marshall et al., 1992; MacFadden et al., 1994]). The materials studied herein come from San Roque (26º 14’ 32’’ S, 65º 21’ 55’’ W; 2940 m), 4.4 km SSW of Humahuaca town, Humahuaca Department, Jujuy Province, Argentina; Uquía Formation (late Pliocene-early Pleistocene; Marplatan SALMA, probably Vorohuan subage; Ortiz et al., 2012).

Emended Diagnosis. Microtragulus bolivianus differs from Proargyrolagus by the presence of euhypsodont teeth, simplified occlusal structure, and reduced dental formula (which is also a difference with Hondalagus and Anargyrolagus ). Additionally, M. bolivianus differs from the species of Hondalagus , Anargyrolagus , and Argyrolagus by the following combined features: M3 subcircular in outline and with a single labial cusp (metacone); M4 proportionally smaller than M3 and without differentiated cuspidal relief; lower molars with talonid proportionally shorter than the trigonid, reduced or absent entoflexid; and proportionally smaller m4. Within Microtragulus , M. bolivianus differs from M. reigi by the absence of an entoflexid in m2, and a shallower entoflexid in m3; and from M. catamarcensis by a proportionally larger m4 and the presence of a well-marked entoflexid in m3.

Description

Maxilla. This bone is represented by two poorly preserved fragments characterized by the presence of several sulci, crests, and foramina ( Figure 3.1-3 ). A deep sulcus is present in the medial side above M4; this sulcus becomes progressively shallower towards the anterior side, and its trajectory matches with the infraorbital canal for the passage of the maxillary nerve (a derivate of the maxillary branch of the trigeminal nerve) and accompanying vessels. This condition is inferred from the observation of the specimen GHUNLPam 8549 (holotype of Argyrolagus rusconii ), which preserves the infraorbital foramen connected to a medial sulcus of the maxilla. Other specimen referred to Microtragulus sp. (PVL 6594; García-López and Babot, 2015), shows similar structures. Given this arrangement, we infer that a proper infraorbital canal is absent, being replaced by a sulcus (corresponding to the medial wall of the canal) connected anteriorly to a foramen piercing the maxilla anterior to the orbit. This modified condition is probably

PALAEO- ELECTRONICA.ORG related to the great development of the palatal vacuities typical of this family.

The specimen preserving the zygomatic process (JUY-P 50) probably corresponds to a juvenile individual, judging by the posterior position of the process (above the alveolus of the last present molariform, which is very large). This would indicate that M4, which is the smallest molar, was not yet fully erupted at the time of death of the individual.

Mandible. The mandibular body is short and high and strongly convex ventrally. The point of maximum height is located at the level of m2 ( Figure 3.4-7 ); in the labial side, it measures 5.65 mm in JUY-P-0067 and 4.94 mm in JUY-P-0066.

In lateral view the body exhibits the mental foramina. In JUY-P-0066 there are three foramina, which are progressively smaller toward the posterior side; the first one is located at the level of p3. The second one is placed at the level of the mesial border of m1, and the third opening is located at the level of the mesial border of m2 ( Figure 3.5 ). Only two foramina are visible in JUY-P-0067, although the surface where the third foramen should be is actually broken ( Figure 3.7 ). These openings present the same location than the first and second apertures in JUY-P-0066. Nevertheless, the second foramen is proportionally smaller in this case. The specimen JUY-P-0068 ( Figure 3.9 ) shows a different arrangement: the two preserved foramina open together inside a larger aperture (feature also present in Proargyrolagus bolivianus ; Wolf, 1984). Other apertures are several tiny vascular openings scattered all over the lateral surface of the mandibular body.

The lateral side of the mandibular body also preserves a blunt but conspicuous coronoid crest. This structure projects anteroventrally towards the level of the anterior half of m4.

In medial view, the mandible shows an unfused symphysis, the borders of which are not well defined. However, the surface is easily recognized by the presence of several shallow depressions and low tubercles. It extends to the level of the anterior portion of m2. Both specimens JUY-P-0066 and JUY-P-0067 bear several tiny nutritious foramina and fenestrations towards the posteroventral portion of the body ( Figure 3.4 , 6 ). The angular process is partially preserved; its anterior end is located at the level of the talonid of m4. The visible portion of this process indicates that it was strongly inflected forming a well-developed concavity facing upwards, as in other argyrolagids ( Rusconi, 1933; Simpson, 1970a; Sánchez-Villagra et al., 2000). Two foramina are visible above the process: the dorsal one is the proper mandibular foramen and the aperture located immediately below is interpreted here as an accessory opening of this canal. These secondary openings are also present in the holotype of Microtragulus bolivianus as well as in other argyrolagids. The maxillary canal (see Discussion) is not preserved in the specimens recovered from Uquía Formation but is clearly present in the holotype of M. bolivianus ( Hoffstetter and Villarroel, 1974) .

Dentition. The dentition is euhypsodont and deeply implanted in the maxilla and dentary. The teeth are prismatic, with central dentine surrounded by a layer of enamel. The teeth are also partially surrounded by a thin layer of cementum that thickens in the area of the sulci. All the recovered dental pieces (except the only i2 of the sample studied herein) show a central fossa/fossid. This structure is particularly conspicuous in i1, where the layer of enamel surrounding the inner cavity is clearly present ( Figure 4.1 ). This feature, described for Hondalagus ( Villarroel and Marshall, 1988) , is also present in Argyrolagus ( Simpson, 1970b) and Microtragulus ( García-López and Babot, 2015) , and probably in Anargyrolagus MPEF-PV 5291.

Upper dentition. M3 is sub-quadrangular ( Figures 2.1 , 3.3 ) and larger than M4 (L= 1.51 mm and W= 1.41 mm). The intraalveolar portion of the crown is slightly curved, vertically implanted, and divergent with respect to the intraalveolar portion of M4. The layer of enamel is continuous, although it is thinner in the mesiolabial area. The labial side is clearly higher than the lingual one, which is about the same level of the alveolar border. In the labial edge there is a poorly developed mesiolabial lobe and a distal elevation that we interpret here as the metacone (metacone + StD after Goin and Abello, 2013). The lingual side is rounded and bears the protocone and the metaconule, which is very reduced and located slightly labially in relation to the protocone. The central area of the occlusal surface bears a mesiodistally elongated and slightly oblique fossa.

The M4 is simplified in relation to M3; it is small (L= 0.87 mm and W=0.71) and in occlusal view the crown is roughly triangular ( Figures 2.1 , 3.3 ). The labial edge is convex and lacks flexi and cusps. Lingually, there are no proper cusps but the presence of a protocone can be inferred, following the arrangement in the precedent molar. The metaconule is absent. The enamel distribution is irregular; it is absent in the mesiolabial side of the tooth and very thin in the distolingual part. As in other Argyrolagidae , the occlusal surface of M4 is mesioventrally directed, contrasting with the distoventrally orientation of the preceding molar. A subcircular fossa occupies a central position in the occlusal surface.

Lower dentition. The i1 is kidney-shaped (the labial border is convex and the lingual one is concave), labiolingually compressed, procumbent, and deeply implanted (the intraalveolar section extends up to the level of m1) ( Figures 3.4-9, 4.1 ; Table 2). The tooth shows a continuous layer of enamel. In lateral view, the occlusal surface is concave; the mesial side is acute and high, and the distal one is blunt and low. It shows a small and oval fossid near the medial area of the lingual wall.

The second incisor is located immediately behind i1 ( Figure 3.6-7 ). It is smaller and oval in outline. Although hypsodont, this tooth is not deeply implanted since the length of its alveolus is ventrally limited by the alveolus of i1. In lateral view, the crown is mesially orientated. In contrast to i1, i2 is surrounded by a thick layer of cementum. The occlusal surface presents a well-developed wear facet facing labially.

The third premolar is not preserved in any of the specimens of Microtragulus bolivianus here studied. Its alveolus is separated from i2 by a short diastema, whose mesiodistal length is about the same length of the alveolus of i2. The alveolar size and shape suggest that this tooth was markedly smaller than the first molar (as in other argyrolagids) and not deeply implanted ( Figure 4.2-3 ).

All the lower molars are euhypsodont. The enamel distribution is mostly continuous around the tooth, with small interruptions in the mesial and distal edges of m1-3 (and a small portion of the mesial edge of m4), and the ectoflexid of all molars. Additionally, the layer of enamel of the mesial side of m2 and m3 can be absent or be very thin. The cementum has an irregular arrangement. In most cases, it is thicker in the deepest portion of the ectoflexid and in the labial wall of the talonid. The central fossid is mesiodistally elongated ( Figures 2 , 4.2-3 ).

The first molar shows a distinct ectoflexid, which separates the trigonid from the talonid. The entoflexid is absent in m1, as in Microtragulus catamarcensis . This trait is present but vestigial in M. reigi ( Simpson, 1970a) and clearly demarcated in all the other argyrolagids. The metaconid is the dominant cusp; it is a high crest-like structure mesially displaced; slightly posterior, the lingual wall of the tooth exhibits a tiny bulge that could be an accessory cusp (entostylid?; see Hershkovitz, 1971). The ectostylid is lower and presents a well-developed column in the labial wall of the tooth. Mesially, a groove separates this structure from the protoconid which points mesiolabially. The paraconid is absent. The talonid, two thirds shorter than the trigonid, is labiolingually extended with its main axis transversal to the tooth row. It has a distinctive tongue-like hypoconid and a well-developed entoconid. The hypoconulid is a vestigial structure barely distinctive in the distolingual corner of the talonid and adjacent to the entoconid.

The second molar differs from the first one in its larger size, a shallower mesial groove in the trigonid (absent in JUY-P-0066), and a wider ectoflexid. As in m1, the entoflexid is absent. The hypoconid, entoconid, and hypoconulid have the same development and arrangement than in m1.

The third molar is shorter than the preceding teeth but longer than m4. The trigonid is very similar in outline to that of m2, although both in JUY-P-0066 and in JUY-P-0067 the groove mesial to the ectostylid is absent. The talonid is well differentiated in this tooth, since a visible but shallow entoflexid lingually separates these structures. The ectoflexid is very similar to that of m2. The entoconid is more conspicuous and the hypoconulid is absent. The layer of enamel appears to be reduced in the mesial end of the trigonid in JUY-P-0066 and in the mesial and distolingual edge in JUY-P-0067.

The fourth molar is very different in shape and size from the preceding molars. It is the smallest molar, both in length and width, and the talonid is labiolingually reduced. As in m3, the trigonid is separated from the talonid by the ecto and entoflexids. The hypoconid is small and low, and the entoconid is a higher crest-like structure. The distal border of the tooth bears a shallow flexid that separates the hypoconid from the distally extended hypoconulid. This arrangement is evident in JUY- P-0067 but is masked in JUY-P-0066 given the poor development of the hypoconulid.

Postcranial Skeleton. Among argyrolagids, the postcranium is known from partially preserved remains of Microtragulus argentinus (cuboid, navicular, ectocuneiform, metatarsals III and IV, and caudal vertebrae), M. reigi (humerus, femur, calcaneus, and tibiofibula), and Argyrolagus scagliai (partially preserved vertebrae, scapula, humerus, radius, ulna, pelvic girdle, femur, tibiofibula, tarsals, metatarsals, and phalanges). In addition, an isolated calcaneus (not directly associated to any Anargyrolagus specimen, but coming from the same level and locality; A. Carlini, personal commun., 2016) was recovered at Gaiman, Chubut Province and described by Szalay (1994). Up to now, the anatomical descriptions came from the work of Simpson (1970a), which was focused on A. scagliai , M. reigi , and M. argentinus , and Szalay (1994) who emphasized on the morphology of the Gaiman calcaneus and the crurotarsal joint and tarsal anatomy of Argyrolagus . Here we present a detailed description of isolated pieces of the postcranium of Microtragulus bolivianus , which includes partially preserved humeri, ulnae, radii, tibiofibulae, and complete astragalus, calcaneus, metatarsals, and distal phalanges. According to our observations, there are no significant differences in the morphology of the skeletal pieces among these species. However, in the following section we include several comparisons in order to highlight inter and intraspecific variations.

Humerus. Four partially preserved humeri (two left and two right) were recovered from the Uquía fossil association ( Figure 5.1 -16). These elements vary in size and robustness: JUY-P-0070 ( Figure 5.5-8 ) is larger and stouter than the other three humeri, a feature that could be related to sexual or ontogenetic variation. In anterior view the diaphysis is straight. The proximal half of the shaft appears to be twisted in relation to the distal portion although the absence of the head prevents to evaluate clearly this feature. This half bears the deltoid crest. In contrast to the general elongated shape of this structure in many mammals, in argyrolagids (e.g., Microtragulus bolivianus , Argyrolagus ) the place of attachment of the M. deltoideus and M. pectoralis major is a short, raised plate-like and ovoid area facing anterolaterally. Laterally, the distal half of the humerus bears a well-developed and hook-shaped ectepicondylar crest, which faces anterolaterally and ends proximally in an acute tip. Even if this crest is not fully preserved, it appears to have a different arrangement in relation to Argyrolagus where it does not flare laterally ( Simpson, 1970a). Medially, the distal portion of the diaphysis exhibits a large entepicondylar foramen in which the medial aperture is hidden in anterior view. The same morphology has been described by Simpson (1970a, p. 27) in Argyrolagus scagliai . The distal epiphysis exhibits a rounded capitulum in continuity with an ectepicondyle not expanded laterally. The trochlea is bounded medially by a sharp edge separating it from the entepicondyle. This structure is not medially expanded. The intercondylar notch is deep. Posteriorly, the supratrochlear foramen opens in the supratrochlear fossa which widens medially. Several nutritious foramina pierce the humerus. The medial wall of the entepicondylar foramen bears one (JUY-P-0069), two (JUY-0072), or three (JUY-P-0070, 0071) apertures. Other openings are visible in the proximal edge of the ectepicondylar crest, adjacent to the diaphysis, where the specimens JUY-P-0069 and JUY-P-0070 bear one foramen. Finally, foramina are also visible above the posterior side of the supratrochlear foramen (JUY-P-0070, 0071, and 0072 show one aperture and JUY-P-0069 shows two apertures).

Ulna. Two incomplete ulnae were preserved ( Figure 5.17 -22). The distal end is lost in both elements. The olecranon is short and stout; the anteroposterior length is greater than the proximodistal length. The anconeal process faces laterally and is not part of the anterior border of the olecranon which is well-projected anteriorly. The trochlear notch is deep. The coronoid process is prominent and its articular surface is proximally directed. Distally, the shaft exhibits a concave and triangular area related to the insertion of the M. biceps brachii. The radial notch is concave, facing more laterally, instead of anteriorly. It is well limited posteriorly by an acuminate process. The body is incomplete; nonetheless, it is clear that it narrowed distally. The most conspicuous feature of the shaft is the thinness of the bone and the extremely concave fossa for the M. abductor pollicis longus. This fossa is extended along almost the entire shaft and is limited anteriorly by a sharp crest which is laterally bent in anterior view. In medial view, the proximal third of the ulna exhibits a shallow fossa for the insertion of the M. flexor digitorum profundus.

Radius. The proximal end of a radius is preserved ( Figure 5.23 ). The incompleteness of this element prevents us from determining if it is a right or a left bone. The head is round and the fovea capitis is slightly concave. The neck is conspicuous; it shows a well-developed radial (bicipital) tuberosity, as in Argyrolagus ( Simpson, 1970a) . The body has a deep posterior fossa probably related with the origin of the M. abductor pollicis longus. Laterally, this fossa is bounded by a sharp pronator crest where the M. pronator teres would be inserted.

Tibiofibula. This complex is represented by a right proximal epiphysis and a fragment of a right distal third. The proximal epiphysis is triangular in proximal view. This area exhibits the medial condyle which is subtriangular and slightly convex; the lateral one is kidney-shaped, concave, and longer anteroposteriorly. The popliteal notch is conspicuous and the intercondylar eminence exhibits both the lateral and medial tuberosities (being the lateral remarkably smaller than the medial one). The extensor sulcus is absent. Anteriorly, the material preserves part of the tibial tuberosity that is a small and smooth surface.

The distal portion of the tibia and the fibula is completely fused ( Figure 5.24 -25); there is no evidence of sutures distinguishing neither this pair of bones nor the diaphysis from the distal epiphysis. In anterior view this epiphysis exhibits a marked fossa, proximodistally elongated. In posterior view, the shaft has a blunt and oblique longitudinal crest. The tibia bears in its medial face a very shallow notch that indicates the position of the passage of the tendons of the muscles flexing the foot. In turn, the extensor muscles in the lateral side of the fibula pass through a deep groove ( Szalay, 1994).

The joint between the tibiofibula and the tarsus includes the articulation with the astragalus (medial) and with the calcaneus (lateral). As Szalay (1994) described for Argyrolagus , the crurotarsal joint is highly restricted in Microtragulus ; both the fibular and tibial sides contact closely with the astragalus. In addition, the fibular portion joints with a large area in the calcaneus. The astragalotibial facet is formed by two well-differentiated medial and lateral facets. In the same way the medial astragalotibial facet exhibits a horizontal and vertical surface. The horizontal one is concave and anteroposteriorly elongated and the vertical is convex and located in the lateral wall of the medial malleolus. The lateral astragalotibial facet is very convex. On the fibula, the astragalofibular facet is a small surface located in the medial side of the lateral malleolus. This facet is continuous with the lateral astragalotibial facet; therefore, both facets are hard to identify. The calcaneofibular facet is concave, oval, and anteroposteriorly elongated. Its main axis is shorter than the medial astragalotibial facet. Both the morphology of the crurotarsal joint, as well as that of the tarsals is essentially the same in Argyrolagus and Microtragulus .

Astragalus. The astragalar body (corpus tali) is remarkably large; it contrasts with a small head, which occupies one third of the total length of the dorsal face ( Figure 6.1-5 ). In dorsal view the body exhibits the large astragalotibial lateral facet (ATil). It is well extended, both distally and proximally, and is somewhat pulley-like, being the medial half steeper than the lateral one. The astragalar fibular facet is small and the astragalar foramen is absent. The head exhibits the dorsal portion of the astragalonavicular contact, which is proximolaterally-distomedially extended. Medially, this facet shows the astragalar distal tuber. The proximal extension of the lateral tibial contact, the ectal (calcaneoastragalar) facet, and the sustentacular and navicular facets are visible in plantar view. The ectal facet is oval and strongly concave. The main axis is roughly orientated proximomedially-distolaterally. The sustentacular facet is ribbonlike and contacts with the homonymous facet in the calcaneus; it comprises the sustentacular facet properly and the superior sustentacular facet. The first one is formed by two areas, one occupies the central exposition of the plantar face and the other is rod-like and oblique. Both are tangential and its contact is located near the proximomedial end of the rod-like area. The central area of the sustentacular facet is oval, slightly wider distally, and is almost parallel to the ectal facet. The rod-like portion of the sustentacular facet is well exposed in plantar view. It is remarkably convex, and occupies the full width of the astragalar head. A deep and narrow astragalar sulcus (interarticular sulcus) separates this facet from the calcaneoastragalar articulation. The superior sustentacular facet is represented as a deep sulcus between the plantar extension of the astragalotibial lateral facet and the sustentacular facets. The astragalar distal tuber is small and is restricted to the proximomedial portion of the plantar aspect. The astragalonavicular facet is restricted to the distolateral extremity of the head.

In medial view the dominant structure is the semicircular astragalotibial medial facet, clearly defined by a blunt dorsal edge. On the proximal and plantar corner there is a small astragalar medial plantar tuberosity located adjacent to the superior sustentacular facet. Distally this aspect exhibits the medial extension of the contact with the navicular and the astragalar distal tuber. In lateral view the astragalus exhibits the astragalofibular facet which is small, triangular, and faces laterally. The distinctive feature of the distal view is the astragalonavicular joint, which covers almost the entire surface of the head; there is no articulation between the astragalus and the cuboid.

Calcaneus. The tuber is a cylindrical lateromedially compressed structure which widens toward its posterior end ( Figure 6.6 -10). This end exhibits two smooth surfaces separated by a low step; one is mainly posterior and the other is more plantarly directed. In lateroplantar view the tuber shows an oval scar which was probably related to with the attachment of the M. abductor digiti quinti. A blunt crest runs anteroposteriorly in plantar view and ends in a small anterior plantar tubercle. The peroneal process is located in the laterodistal corner of the plantar aspect. This process is rounded and well developed. Adjacent to the peroneal process, and distal to it, there is a small groove for the M. peroneus longus.

The body of the calcaneus is wide and much shorter than the tuber. In dorsal view the body shows the proximal process bearing the calcaneoastragalar (or ectal) facet and the calcaneofibular facet. Both facets are separated by a shallow groove in Argyrolagus and Microtragulus , differing from the Gaiman calcaneus in which they are close together (see Szalay, 1994, figure 7.28 A, C). The calcaneoastragalar facet faces dorsally and dorsomedially and is smaller and more dorsally projected than the calcaneofibular facet (particularly visible in medial and distal views). Both in Argyrolagus as in Microtragulus , the calcaneoastragalar facet is smaller than that of the Miocene Gaiman calcaneus ( Szalay, 1994). The sustentacular facet comprises the sustentacular facet properly and the superior sustentacular articulation. As in the astragalus, the sustentacular is divided in two portions: one is proximal, oval, and distally oriented and the other is more distally located, smaller, and faces dorsomedially. The superior sustentacular facet is a small articular surface restricted to the dorsal border of the proximal part of the sustentacular facet. In distal view, this facet is smaller in Argyrolagus than in the Gaiman calcaneus ( Szalay, 1994), a condition also present in Microtragulus .

The contact with the cuboid is formed by two discontinuous facets separated by a step. The proximal facet (CaCup) is smaller and somewhat triangular; the distal one (CaCud) is dorsoplantarly elongated and oval.

Metatarsals. One complete right Mt III and three fragments of the Mt IV were recovered ( Figure 7.1- 2 ). The Mt III measures 29.87 mm, slightly larger than Microtragulus argentinus (27 mm; Ameghino, 1904) and smaller than Argyrolagus scagliai (35.6 mm; Simpson, 1970a, p. 68). The Mt III exhibits a slightly concave T-shaped facet for the articulation with the ectocuneiform. Laterally a marked notch lodges a medial process for the Mt IV; medially, there is a distinctive facet which is interpreted as the contact with a vestigial Mt II, as proposed for Argyrolagus ( Simpson, 1970a, figure 15E). The proximal end of the Mt IV exhibits a triangular convex surface which articulates with the cuboid. Laterally this bone presents a proximally projected process that probably covered partially the lateral side of the cuboid, as in Microtragulus argentinus MACN 12925 and Argyrolagus scagliai MMP 785- S ( Simpson, 1970a). The only peculiarity of the diaphysis of the metatarsals is the flattened area of contact between Mt III and IV. In this last bone, the distal half of the diaphysis is thinner than in Mt III. The distal end of the metatarsals is simple. In the dorsal aspect of the head the articular surface is condylar; in plantar view this structure shows a notorious longitudinal crest.

Phalanges. Three ungual phalanges were recovered ( Figure 7.3-5 ). They are long, slightly curved, and lateromedially compressed. The flexor tubercle is long, constricted in the middle portion, and is pierced by the ungual foramen. The articular facet is strongly concave; the dorsal border is more posteriorly projected than the ventral one.

Comparison

When compared with Oligocene and Miocene argyrolagids ( Proargyrolagus , Hondalagus , and Anargyrolagus ), Microtragulus bolivianus shows clear differences involving several features. Regarding Proargyrolagus , the main dental differences include the degree of hypsodonty, the morphology of the occlusal surface, and the dental formula. In Proargyrolagus the molars are hypsodont but rooted (i.e., high crown, not evergrowing; Wolf, 1984; protohypsodont in Goin and Abello, 2013) while in M. bolivianus the molars are euhypsodont (i.e., high crown and rootless; Hoffstetter and Villarroel, 1974). The occlusal surface of the upper molars in Proargyrolagus exhibits a complex outline including several distinctive cups and structures (e.g., mesiolabial lobe, paracone + StB, metacone + StD, protocone, and metaconule). In M. bolivianus this pattern is simplified with most of the occlusal features largely masked. Another distinctive feature of M. bolivianus regarding Proargyrolagus is related to the crown implantation in M3- 4. In Proargyrolagus the crowns of both molars present an almost parallel implantation, while in M. bolivianus the intraalveolar part of the crowns is strongly divergent. This divergence determines that the occlusal surface of M3 faces ventrally and that of M4 faces mesioventrally. These differences in implantation are also visible in the lower molars. Additionally, M. bolivianus also lacks several dental pieces compared to Proargyrolagus (dental formula 2.0.1.4 for M. bolivianus and 3.1.2.4 or 4.0.2.4 for P. bolivianus ; see Sánchez-Villagra and Kay [1997] for different interpretations on the lower dental formula in the Oligocene species). Finally, the euhypsodont nature of the lower molars in M. bolivianus also determines that the height of the mandibular body is greater in this species.

The upper teeth of the Miocene genus Hondalagus differ mainly in size (smaller in relation to Microtragulus bolivianus ), outline (more transverse in Hondalagus ), and the distinctive layer of cementum ( Sánchez-Villagra et al., 2000), which is much thinner in M. bolivianus . Hondalagus has an extra anterior lower tooth ( Sánchez-Villagra et al., 2000), a well-developed entoflexid in m1 and m2 (absent in M. bolivianus ), and proportionally larger talonids (particularly in m4).

The dental formula is the main difference between Anargyrolagus and Microtragulus bolivianus , since the former presents a canine and two more premolars than Microtragulus . In Anargyrolagus M3 is more quadrangular, with a well-developed metaconule, and M4 shows a marked occlusal relief in contrast with M. bolivianus , which presents an almost flat occlusal surface. The first lower incisors show a clear fossettid near the medial area of the lingual wall in M. bolivianus ,

PALAEO- ELECTRONICA.ORG absent in Anargyrolagus . This genus also contrasts with M. bolivianus by the higher mandibular body, the well-developed entoflexid and proportionally larger talonid in all the molars, and a proportionally larger m4.

Regarding the genus Argyrolagus , which includes four species ( A. scagliai , A. parodii , A. palmeri , and A. rusconii ), the upper dentition will be compared only with A. scagliai and A. rusconii since A. parodii and A. palmeri are represented only by lower teeth. These two species show M3 longer than wide, with the paracone present and well separated from the metacone, the occlusal outline with more distinctive corners and welldefined angles (especially in the protocone area), and the mesial border much wider than the distal one; all these features represent clear differences regarding M. bolivianus . Additionally, the labial wall in both species is straight in M3, contrasting with the convex wall of M. bolivianus . Regarding M4, in A. scagliai and A. rusconii this molar is proportionally larger and both the paracone and metacone are distinctive. Finally, the lower molars of all the species of Argyrolagus are characterized by a visible entoflexid (in m1-m4) and a larger talonid in all molars, particularly in m4.

The new material of Microtragulus bolivianus described herein can be compared more precisely with the specimens known for M. reigi and M. catamarcensis . In M. reigi , M3 is longer than wide, while in M. bolivianus the ratio between the length and the width is approximately one. Moreover, M 3 in M. reigi has a small paracone and sharp corners (i.e., a more angled outline). Additionally, the flexus between the mesiolabial lobe and the metacone, present in M. bolivianus , is absent in M. reigi ; the mesial border of the tooth is wider than the distal one; and the layer of cementum is thicker in M. reigi . The relative size and shape of M4 is similar between these species, but the presence of differentiated paracone and metacone in M. reigi is not observed in M. bolivianus . In the lower dentition, the morphology of the incisors is similar in both species. The molars differ in the presence of a small but distinctive entoflexid in m 2-3 in M. reigi , and a more reduced talonid in its m4 which also lacks the distal flexid distinguishable in M. bolivianus .

Microtragulus catamarcensis is smaller than M. bolivianus , and the diastema between i2 and p3 is relatively shorter. Additionally, the talonid of m3 lacks the entoflexid, clearly present in M. bolivianus . The talonid in m4 has a simpler morphology in M. catamarcensis : the cusps are indistinguishable and the distal flexid present in M. bolivianus is absent.

Figures

FIGURE 2. Line drawings (occlusal view) of Microtragulus bolivianus showing the anatomical nomenclature used in the text. 1. Right M3-4. 2. Right m1-4 (image reflected). Abbreviations: cf, central fossa; cfd, central fossid; ectf, ectoflexid; ectd, ectostylid; ent, entoconid; entd, entostylid?; entf, entoflexid; gr, groove; hyp, hypoconid; hypd, hypoconulid; mesl, mesiolabial lobe; met, metacone; metd, metaconid; metl, metaconule; prt, protocone; prtd, protoconid.

FIGURE 3. Microtragulus bolivianus. 1-3. JUY-P-0065, fragment of right maxilla with M3-M4 in: 1, lateral; 2, medial; and 3, occlusal views. 4-5. JUY-P-0066, right mandibular body with i1, alveoli of i2 and p3, and complete m1-4 in: 4, medial; and 5, lateral views. 6-7. JUY-P-0067, right mandibular body with complete i1 and i2, alveolus of p3, and complete m1-4 in: 6, medial; and 7, lateral views. 8-9. JUY-P-0068, anterior fragment of right mandibular body with i1 and alveoli of i2 and p3 in: 8, medial; and 9, lateral views. Scale bar equals 5 mm.

FIGURE 4. Microtragulus bolivianus, right lower dentition in occlusal view. 1. JUY-P-0066, detail of i1. 2. JUY-P-0066, m1-4. 3. JUY-P-0067, m1-4. Scale bar equals 5 mm.

FIGURE 5. Microtragulus bolivianus, fore and hindlimb bones. 1-4. JUY-P-0069, right humerus almost complete except the proximal third in: 1, anterior; 2, posterior; 3, medial; and 4, lateral views. 5-8. JUY-P-0070, left humerus almost complete except the proximal third in: 5, anterior; 6, posterior; 7, medial; and 8, lateral views. 9-12. JUY-P- 0071, distal half of left humerus in: 9, anterior; 10, posterior; 11, medial; and 12, lateral views. 13-16. JUY-P-0072, distal end of right humerus in 13, anterior; 14, posterior; 15, medial; and 16, lateral views. 17-19. JUY-P 59, proximal half of right ulna in: 17, anterior; 18, lateral; and 19, medial views. 20-22. JUY-P 60, proximal half of left ulna in: 20, anterior; 21, lateral; and 22, medial views. 23. JUY-P 61, proximal fragment of radius. 24-25. JUY-P 52, distal end of right tibiofibula in: 24, anterior; and 25, posterior views. Scale bar equals 10 mm.

FIGURE 6. Microtragulus bolivianus, astragalus and calcaneus. 1-5. JUY-P-0075, complete right astragalus in: 1, dorsal; 2, plantar; 3, medial; 4, lateral; and 5, distal views. 6-10. JUY-P-0073, complete left calcaneus in: 6, dorsal; 7, plantar; 8, medial; 9, lateral; and 10, distal views. Scale bar equals 5 mm.

FIGURE 7. Microtragulus bolivianus, metatarsal III and IV and phalanges. JUY-P 55, complete right metatarsal III and JUY-P 56, distal portion of right metatarsal IV (not associated) in: 1, anterior; and 2, posterior views. Scale bar equals 10 mm. 3-5. JUY-P 58, ungual phalanges in lateral view. Scale bar equals 5 mm.