Simomylodon uccasamamensis, SAINT-ANDRE ET AL., 2010

SIMOMYLODON UCCASAMAMENSIS SAINT-ANDRÉ ET AL., 2010

( FIGS 2–12 View Figure 2 View Figure 3 View Figure 4 View Figure 5 View Figure 6 View Figure 7 View Figure 8 View Figure 9 View Figure 10 View Figure 11 View Figure 12 ; SUPPORTING INFORMATION, APPENDIX S1)

Glossotheriscum dalenzae Saint-André, 1994: 174– 183 , fig. 18, pl. 13.

Simotherium uccasamamense Saint-André, 1994: 184–228, figs 19–20, pls 14–20.

Glossotheridium chapadmalense Anaya & MacFadden, 1995: 94–98 , figs 3–5, table 1.

Pleurolestodon dalenzae Saint-André et al., 2010: 261–269 , figs 2–4, table 1.

Nomenclatural observations

The species Pleurolestodon dalenzae was erected by Saint-André et al. (2010) only a few pages before S. uccasamamensis . However, the ‘page priority criterion’ has no formal standing in the International Code of Zoological Nomenclature (ICZN), and these species have to be considered equally old (see ICZN, 1999). In these cases, the order of priority is determined by the first reviser [i.e. the first author(s) to consider their synonymy; ICZN, 1999, article 24.2]. As first revisers, we therefore establish that Pleurolestodon dalenzae is a junior synonym of S. uccasamamensis , on the basis of the more abundant and well-determined material of the latter species in comparison with the former ( Saint-André et al., 2010).

Holotype

MNHN-Bol V 11731 View Materials (ex GB 078; Fig. 2 View Figure 2 ), anterior part of cranium without dentition ( Saint-André et al., 2010).

Measurements

See the Supporting Information (Appendix S5).

Revised stratigraphic and geographical occurrence Latest Miocene–earliest Pliocene of Choquecota , early Pliocene of Pomata-Ayte, Casira and Inchasi (Oruro and Potosí Departments, Bolivia) and late Pliocene of Ayo Ayo –Viscachani (La Paz Department, Bolivia) .

Revised diagnosis

Fossil sloth smaller in size than Glossotheridium chapadmalense , Paramylodon garbanii and Pleurolestodon acutidens , and roughly biometrically similar to Glossotheriopsis pascuali ; long zygomatic processes of squamosal; wide braincase in relationship to the total cranial length and wide V-shaped palate; in ventral view, the medial palatal process of the maxilla is more extended mediolaterally than anteroposteriorly and the occipital condyles are well separated from the condyloid foramina, as in the Miocene species Pleurolestodon acutidens ; the foramen magnum shows a detached notch located on its dorsal border, similar to that observed in Pleurolestodon and Mylodon ; long and slender ascending process of the jugal, which strongly resembles that of Glossotheridium chapadmalense ; the diastema between Cf1 and Mf1 is absent or extremely reduced and, in lateral view, Cf1 presents an almost vertical wear facet similar to the condition in Pleurolestodon acutidens ; Mf2 and Mf3 possess a marked lingual sulcus, comparable to Pleurolestodon acutidens and Paramylodon garbanii ; cf1 is bevelled, with well-developed mesial and distal wear surfaces; mf3 is marked by a deep apicobasal sulcus on its lingual side, absent on the labial side and not covered by the ascending ramus of the mandible in lateral view (like Glossotheridium chapadmalense and unlike Pleurolestodon acutidens and Paramylodon garbanii ); the symphyseal spout is anteriorly flat as in Glossotheridium chapadmalense and not rounded as in Pleurolestodon acutidens and Paramylodon garbanii .

Paratype

MNHN-Bol V 3321 ( Fig. 3 View Figure 3 ), maxillary and premaxillary fragments with left Mf1–Mf3 and right Mf2–Mf4 ( Saint-André et al., 2010).

Referred specimens

See the Supporting Information (Appendix S1). Description

Cranium: The cranium of S. uccasamamensis appears elongated, with the cranial roof and cranial base roughly horizontal in lateral view. The nasal region is slightly depressed in relationship to the braincase. In dorsal view, MNHN-Bol V 3348 ( Fig. 4A View Figure 4 ), 3711 ( Fig. 5A View Figure 5 ) and 3726 ( Fig. 6A View Figure 6 ) are more slender than MNHN-Bol V 3717 ( Fig. 7A View Figure 7 ) and 3718 ( Fig. 8A View Figure 8 ) (see measurements in Supporting Information, Appendix S5). The snout is elevated and widened anteriorly and relatively short; the braincase is wide in relationship to the total cranial length ( Figs 5A View Figure 5 , 7A View Figure 7 , 8A View Figure 8 ).

In dorsal view, the nasals are narrow at the level of the antorbital constriction, and gradually broaden anteriorly and posteriorly ( Figs 2A View Figure 2 , 4A View Figure 4 , 5A View Figure 5 , 6A View Figure 6 , 7A View Figure 7 , 8A View Figure 8 ). The nasals are probably wider in MNHN-Bol V 3718 ( Fig. 8A View Figure 8 ) than in the other specimens as a result of diagenetic dorsoventral compression of this skull. A V-shaped, posteriorly pointed nasofrontal suture is present in all specimens examined. The anterior border of the nasals is strongly convex in MNHN-Bol V 3711 and 3726 ( Figs 5A View Figure 5 , 6A View Figure 6 ) and straighter in MNHN-Bol V 11731 View Materials , 3717 and 3718 ( Figs 2A View Figure 2 , 7A View Figure 7 , 8A View Figure 8 ); in MNHN-Bol V 3348 ( Fig. 4A View Figure 4 ), the nasals are strongly convex but with a narrow, anteriorly projecting median process.

The temporal fossa and the temporal lines are not clearly observable in all the specimens, owing to breakage and/or diagenetic deformation ( Figs 6 View Figure 6 , 7 View Figure 7 ). In general, the temporal lines appear straight and roughly parallel to one another, diverging anteriorly as they approach the postorbital processes of the frontal bones. The temporal lines curve laterally and ventrally at their posterior limit, extending parallel and slightly anterior to the nuchal crest ( Figs 4 View Figure 4 , 5 View Figure 5 , 8 View Figure 8 ).

In lateral view, the anterior part of the cranium is dominated by the maxilla. This bone contacts the nasal and the frontal dorsally, the lacrimal in its middle part and the palatine and the alisphenoid posteriorly. The premaxillae are preserved in six specimens. They are V-shaped in MNHN-Bol V 3348, 3711 and 3726 ( Figs 4C View Figure 4 , 5C View Figure 5 , 6C View Figure 6 ), whereas they show a more arched profile (and even flatten anteriorly as they approach their midline junction) in MNHN-Bol V 11731 View Materials , 3717 and 3718 ( Figs 2C View Figure 2 , 7C View Figure 7 , 8C View Figure 8 ). Their medial and lateral rami are nearly equivalent in size, with the lateral ramus slightly longer than the medial one, in MNHNBol V 3348, 3711 and 3726 ( Figs 4C View Figure 4 , 5C View Figure 5 , 6C View Figure 6 ), whereas the lateral ramus is substantially longer than the medial ramus in MNHN-Bol V 11731 View Materials , 3717 and 3718 ( Figs 2C View Figure 2 , 7C View Figure 7 , 8C View Figure 8 ). The palate is rugose and strongly widened anteriorly, especially in MNHN-Bol V, 11731, 3321, 3717 and 3718 ( Figs 2C View Figure 2 , 3B View Figure 3 , 7C View Figure 7 , 8C View Figure 8 ); whereas it is narrower in MNHN-Bol V 3348, 3711 and 3726 ( Figs 4C View Figure 4 , 5C View Figure 5 , 6C View Figure 6 ; see also ‘Morphometric analyses’ subsection below). In lateral view, the palate is concave at the level of Cf1–Mf1 and convex posteriorly at the level of Mf2–Mf4. The anterior palatal foramina (sensu De Iuliis et al., 2011) are always clearly delineated ( Figs 2C View Figure 2 , 3B View Figure 3 , 4C View Figure 4 , 5C View Figure 5 , 6C View Figure 6 , 7C View Figure 7 , 8C View Figure 8 ) and continue into distinct anterior grooves that extend medially to connect with the incisive foramina (not visible in ventral view). Also, enlarged postpalatal foramina (sensu Gaudin, 2011) are observable in all specimens ( Figs 2C View Figure 2 , 3B View Figure 3 , 4C View Figure 4 , 5C View Figure 5 , 6C View Figure 6 , 7C View Figure 7 , 8C View Figure 8 ).

The sphenorbital fissure and the optic and sphenopalatine foramina are visible in MNHN-Bol V 3711, 3717 and 3718 ( Figs 5 View Figure 5 , 7 View Figure 7 , 8 View Figure 8 ). These foramina open into a common depression and are approximately aligned horizontally. The sphenorbital fissure is the posteriormost and largest of the three foramina mentioned above. The optic foramen is adjacent to the sphenorbital fissure and the sphenopalatine foramen is farther anterior, at the anterior margin of the common depression. Posteriorly and ventrally, the foramen ovale is located between the squamosal and the lateral plate of the pterygoid and opens onto the lateral wall of the cranium.

In ventral view, the structures of the middle region of the basicranium are difficult to observe, owing to poor preservation and complete fusion of the sutures. The pterygoids are inflated at their base ( Fig. 5 View Figure 5 ). The descending laminae of the pterygoid are broad and deep, but only preserved fully in MNHN-Bol V 3711 and 3717 ( Figs 5 View Figure 5 , 7 View Figure 7 ).

In lateral view, the lacrimal is more elongated anteroposteriorly than dorsoventrally and is pierced by a rounded lacrimal foramen. The orbital portion of the bone is larger than its facial portion. The jugal is firmly attached to the lacrimal and possesses ascending, descending and middle processes ( Figs 5E View Figure 5 , 6B View Figure 6 , 7B View Figure 7 , 8B View Figure 8 ), as is typical for sloths ( Gaudin, 2004). The ascending process is the longest of the three. It is wide at its base, becoming narrower posteriorly, and ends as a rounded tip located at the level of the anteroposterior midpoint of the zygomatic process of the squamosal. The ascending and middle processes are strongly divergent in MNHN-Bol V 3717 and 3718 ( Figs 7B View Figure 7 , 8B View Figure 8 ) and more convergent in MNHN-Bol V 3711 and 3726 ( Figs 5E View Figure 5 , 6B View Figure 6 ). The ascending process of the jugal is marked by a weak postorbital process near its base ( Figs 5E View Figure 5 , 6B View Figure 6 , 7B View Figure 7 , 8B View Figure 8 ). The middle process of the jugal is triangular in shape and closely approaches the zygomatic process of the squamosal posteriorly. The descending process of the jugal bears a posteriorly or posteromedially extended hook in MHNH-BOL V 3717 and 3718 ( Figs 7 View Figure 7 , 8 View Figure 8 ) that is not present in MNHN-Bol V 3711 and 3726 ( Figs 5 View Figure 5 , 6 View Figure 6 ). The long zygomatic process of the squamosal is almost horizontal in lateral view and anterolaterally directed in dorsal view ( Figs 4–8 View Figure 4 View Figure 5 View Figure 6 View Figure 7 View Figure 8 ).

In lateral view, the occipital is inclined anteriorly in MNHN-Bol V 3711 ( Fig. 5B View Figure 5 ), whereas it appears more vertical in MNHN-Bol V 3718 ( Fig. 8B View Figure 8 ). In posterior view ( Fig. 5D View Figure 5 ), its transverse breadth exceeds its dorsoventral height. It is marked by a slight median external occipital crest that does not continue dorsally into a sagittal crest and culminates ventrally in a well-marked notch on the posterior edge of the foramen magnum ( Fig. 5D View Figure 5 ). The nuchal crests are strongly developed and clearly visible in dorsal, lateral and posterior views ( Figs 5 View Figure 5 , 8 View Figure 8 ). The occipital condyles, in ventral view, are roughly triangular in shape, with a slightly concave medial edge, and are slightly more elongated mediolaterally than anteroposteriorly ( Fig. 8C View Figure 8 ). In lateral view, they appear more prominent in MNHN-Bol V 3711 ( Fig. 5B View Figure 5 ) than in MNHN-Bol V 3718 ( Fig. 8B View Figure 8 ), a difference that is probably related to the inclination of the occiput already discussed.

Upper dentition: The upper tooth rows of S. uccasamamensis , composed of five teeth, are divergent anteriorly. The most mesial tooth is caniniform, whereas the remaining four are molariform ( Figs 4–8 View Figure 4 View Figure 5 View Figure 6 View Figure 7 View Figure 8 ). There is no diastema between Cf1 and Mf1 ( Figs 2–8 View Figure 2 View Figure 3 View Figure 4 View Figure 5 View Figure 6 View Figure 7 View Figure 8 ). Cf1 is roughly semicircular in cross-section in most specimens: MNHN-Bol V 3711 and 3726 ( Figs 5 View Figure 5 , 6 View Figure 6 ), a shape that is also observable in the alveoli of MNHN-Bol V 11731 View Materials ( Fig. 2 View Figure 2 ). The straight side, corresponding ideally to the diameter of this semicircle, faces linguomesially, whereas the arched outline faces labiodistally. Variations of this shape are observable in the almost triangular caniniform of MNHN-Bol V 3717 ( Fig. 7 View Figure 7 ) and the nearly circular Cf1 (or correspondent alveoli) of MNHN-Bol V 3718 and 3321 ( Figs 3 View Figure 3 , 8 View Figure 8 ). In all the specimens where it is preserved, the occlusal surface is almost vertical in lateral view and directed lingually and distally in occlusal view.

Mf1 is ovate in cross-section, elongated along the main axis of the tooth row. This tooth bears a lingual apicobasal sulcus in MNHN-Bol V 3718 ( Fig. 8 View Figure 8 ), absent in MNHN-Bol V 11371 View Materials , 3321, 3711 and 3726 ( Figs 2 View Figure 2 , 3 View Figure 3 , 5 View Figure 5 , 6 View Figure 6 ). In MNHN-Bol V 3717 ( Fig. 7 View Figure 7 ), the lingual sulcus of Mf1 is slightly marked on the left and absent on the right, indicative of the great variability of this character. Mf1 bears a bevelled occlusal surface, with a mesial wear facet that is larger than the distal facet.

Mf2 and Mf3 are bilobate and exhibit a deep lingual apicobasal sulcus ( Figs 3–8 View Figure 3 View Figure 4 View Figure 5 View Figure 6 View Figure 7 View Figure 8 ). In occlusal view, these two teeth are roughly triangular in cross-section, with the orthogonal angle disposed mesiolingually ( Figs 3–8 View Figure 3 View Figure 4 View Figure 5 View Figure 6 View Figure 7 View Figure 8 ). Mf2 and Mf3 vary in their occlusal outlines, but Mf2 is generally longer mesiodistally than transversely, whereas in Mf3 the transverse width is equal to or exceeds the mesiodistal length. The wear facet of Mf2 is more pronounced distally than mesially, whereas in Mf3 it is more pronounced in the central part of the tooth than in its labial and lingual extremities (e.g. MNHN-Bol V 3711 and 3717; Figs 5 View Figure 5 , 7 View Figure 7 ).

Mf4 is T-shaped in occlusal view, with the distal lobe clearly narrower transversely than the mesial one. The last upper tooth presents both lingual and labial longitudinal sulci, the latter more pronounced than the former (e.g. MNHN-Bol V 11731 View Materials , 3321, 3711 and 3717; Figs 2 View Figure 2 , 3 View Figure 3 , 5 View Figure 5 , 7 View Figure 7 ).

Mandible and lower dentition: The lower caniniform, like the upper one, is also generally semicircular in cross-section, with some exceptions represented by the triangular shape of MNHN-Bol V 3296 ( Fig. 9A View Figure 9 ) and the ovate shape in MNHN-Bol V 3711 and 3371 ( Figs 5F View Figure 5 , 10D View Figure 10 ). It appears as the highest tooth of the lower tooth row and is nearly equal in size to mf1 ( Figs 5–7 View Figure 5 View Figure 6 View Figure 7 , 9–12 View Figure 9 View Figure 10 View Figure 11 View Figure 12 ). The cf1 is bevelled, with the mesial wear facet broader than the distal one. It also presents a slight lingual apicobasal sulcus in MNHN-Bol V 3726 ( Fig. 6 View Figure 6 ), 3296 ( Fig. 9 View Figure 9 ) and the juvenile mandibles ( Fig. 12 View Figure 12 ), a feature not observable in the other specimens ( Figs 5 View Figure 5 , 7 View Figure 7 , 10 View Figure 10 , 11 View Figure 11 ).

A lingual apicobasal sulcus is a consistent feature of mf1. This tooth is transversely wider mesially than distally and bears an oblique, distally inclined wear facet ( Figs 5–7 View Figure 5 View Figure 6 View Figure 7 , 9–12 View Figure 9 View Figure 10 View Figure 11 View Figure 12 ). The irregular cross-section of mf2 resembles a parallelogram ( Figs 5–7 View Figure 5 View Figure 6 View Figure 7 , 9–12 View Figure 9 View Figure 10 View Figure 11 View Figure 12 ). The presence of longitudinal sulci on this tooth is variable among the observed specimens. In MNHN-Bol V 3296 ( Fig. 9 View Figure 9 ), sulci are present on all four sides, whereas in MNHN-Bol V 3371 ( Fig. 10D View Figure 10 ) they are almost absent. These two morphologies represent the extremes of the observable variation for mf2.

In occlusal view, mf3 is strongly bilobate, with the mesial lobe wider transversely than the distal lobe ( Figs 5–7 View Figure 5 View Figure 6 View Figure 7 , 9 View Figure 9 , 10 View Figure 10 , 12 View Figure 12 ). The mesial lobe is extended mesiolabially, with an apicobasal sulcus that faces mesiolingually. The distal lobe of mf3 is rounded distally. The two lobes are separated by a thin isthmus accompanied lingually by a deep, broad apicobasal sulcus, absent on the labial side ( Figs 5–7 View Figure 5 View Figure 6 View Figure 7 , 9 View Figure 9 , 10 View Figure 10 , 12 View Figure 12 ).

The mandible of S. uccasamamensis is short and deep, with the ventral border of the horizontal ramus nearly horizontal in lateral view ( Figs 5–7 View Figure 5 View Figure 6 View Figure 7 , 9 View Figure 9 , 10 View Figure 10 , 12 View Figure 12 ). The tooth row is aligned in occlusal view, with the exception of the cf1, which is slightly displaced laterally. The dorsoventral depth of the horizontal ramus of the mandible is constant along the tooth row, becoming narrower towards the symphyseal spout and deepening posteriorly at the base of the ascending ramus. The profile of the symphyseal spout in lateral view is irregular, with a strong convexity flanked by marked dorsal and ventral concavities ( Figs 5–7 View Figure 5 View Figure 6 View Figure 7 , 9–11 View Figure 9 View Figure 10 View Figure 11 ). In occlusal view, the symphysis is wider distally than proximally, with a visible constriction anterior to the caniniforms. The mandibular foramen is located on the medial side of the mandible, well posterior and slightly ventral to the base of mf3. The posteroexternal opening of the mandibular canal (a characteristic feature of sloths among xenarthrans; see Gaudin, 2004; De Iuliis et al., 2011) faces laterally at the level of the posterior edge of the root of mf3, well ventral to the dorsal edge of the horizontal ramus. The mandibular canal emerges anteriorly through the mental foramina situated on the anterolateral surface of the symphyseal spout. The mental foramina are highly variable in size and number (e.g. a single one in MNHN-Bol V 3296 and six in MNHN-Bol V 3298; Figs 9 View Figure 9 , 11A–C View Figure 11 ), even in the two dentaries of a single individual (e.g. MNHN-Bol V 3717, 3371).

On the ascending ramus, the angular, condyloid and coronoid processes are equally divergent ( Fig. 9 View Figure 9 ). The angular process is deeply concave medially, with a strongly convex ventral edge that is clearly demarcated from the ventral edge of the horizontal ramus ( Fig. 9 View Figure 9 ). The transverse width of the mandibular condyle is much greater than its anteroposterior length. It has a hooked aspect in dorsal view, and its lateral portion is oriented horizontally in posterior view, whereas the medial portion is downturned ventrally. The coronoid process is tall, with its posterior edge orthogonal to the main mandibular axis in lateral view ( Figs 6E View Figure 6 , 9B View Figure 9 , 12G View Figure 12 ). It is somewhat hooked posteriorly in MNHN-Bol V 3726 and 3359 ( Figs 6E View Figure 6 , 12G View Figure 12 ), but not in MNHN-Bol V 3296 ( Fig. 9B View Figure 9 ). However, a complete coronoid process belonging to an adult individual is lacking in the present sample and therefore this feature cannot be assessed properly.

No important differences have been detected among the mandibular features of the specimens of S. uccasamamensis . In general, the mandibles MNHN-Bol V 3717, 3358 and 3298 ( Figs 7 View Figure 7 , 10A–C View Figure 10 , 11A–C View Figure 11 ) are more robust than the specimens MNHN-Bol V 3711, 3726, 3296, 3371 and 12518 ( Figs 5 View Figure 5 , 6 View Figure 6 , 9 View Figure 9 , 10D–F View Figure 10 , 11D–F View Figure 11 ). This greater robustness is exemplified by the two specimens MNHN-Bol V 3358 and 3371, illustrated in Fig. 10 View Figure 10 , with the former ( Fig. 10A–C View Figure 10 ) showing a more robust dentition and more anteriorly divergent, thicker and deeper horizontal rami than the latter ( Fig. 10D–F View Figure 10 ). These variations were treated by Anaya & MacFadden (1995) as possible indicators of different ontogenetic stages and/or the existence of sexual dimorphism.

The S. uccasamamensis sample also includes four mandibular corpora of juvenile individuals (i.e. MNHNBol V 3359, 11758, 12001 and MNHN.F. AYO 165; Fig. 12 View Figure 12 ). In juvenile specimens of Simomylodon , cf1 is larger and wider than mf1–mf2, as in Choloepus ( Hautier et al., 2016) View in CoL and Glossotherium tropicorum ( De Iuliis et al., 2017) . The cf1 also shows a semicircular section and a feeble apicobasal sulcus in the lingual side ( Fig. 12A–I View Figure 12 ). The mf1 and mf2 ( Fig. 12J–L View Figure 12 ) are somewhat more rounded and simple but already show the same occlusal pattern described above for adults ( Fig. 12A–I View Figure 12 ). Additionally, mf3 already shows the same peculiar shape as the adult tooth, with a wide mesial and narrow distal lobe and the presence of a single lingual apicobasal sulcus ( Fig. 12A–I View Figure 12 ). Also, the coronoid and condyloid processes appear simpler, without the small incisura on the posteriormost tip of the coronoid process ( Fig. 12G–I View Figure 12 ). In posterior view, the condyle appears to be inclined laterally, rather than medially. Overall, the juvenile specimens of S. uccasamamensis already display the diagnostic features of the adults, appearing largely as smaller scale versions of the adult bone.

Comparison

Cranium and upper dentition: In S. uccasamamensis , the presence of a high braincase, a deep and anteriorly elevated snout, and an approximately horizontal cranial base in lateral view are typical features of Mylodontidae ( Gaudin, 2004) . The width of the braincase relative to total skull length is comparable to Pleurolestodon acutidens (FMNH P14495; Rovereto, 1914) and is greater than Glossotheridium chapadmalense , Glossotherium robustum , Mylodon darwinii or Paramylodon harlani ( Owen, 1842; Kraglievich, 1925, 1934; Stock, 1925; McAfee, 2009; Brandoni et al., 2010).

Other features, above all in the rostral region, allow assignment of Simomylodon to the Mylodontinae . In particular, the snout is relatively short, widened anteriorly in dorsal view and depressed in lateral view ( Figs 2 View Figure 2 , 4–8 View Figure 4 View Figure 5 View Figure 6 View Figure 7 View Figure 8 ).

The wide rostrum of S. uccasamamensis is also accompanied by an anterior enlargement of the nasals. The nasal becomes narrower posteriorly and widens again at the level of the nasofrontal suture ( Figs 2 View Figure 2 , 4–8 View Figure 4 View Figure 5 View Figure 6 View Figure 7 View Figure 8 ). This expansion is a recurrent feature in mylodontids, as is the great enlargement of the external nares ( Gaudin, 2004). The characters observed on the rostrum are closely related to the morphology of the palate. Indeed, S. uccasamamensis shows a V-shaped palate ( Figs 2–8 View Figure 2 View Figure 3 View Figure 4 View Figure 5 View Figure 6 View Figure 7 View Figure 8 ), comparable with all the mylodontines and lestodontines ( Gaudin, 2004) except Mylodon , which has secondarily lost Cf1s, and consequently, reduced anterior palatal width ( Kraglievich, 1934; Brandoni et al., 2010). The medial anterior palatal processes of the maxilla are projected farther anteriorly than the lateral ones ( Figs 2–8 View Figure 2 View Figure 3 View Figure 4 View Figure 5 View Figure 6 View Figure 7 View Figure 8 ). This characteristic is also observed in Glossotheridium , Glossotherium , Paramylodon , Pleurolestodon and Mylodon ( Owen, 1842; Rovereto, 1914; Kraglievich, 1925, 1934; Stock, 1925; McAfee, 2009; Brandoni et al., 2010). The extension of the medial palatal processes of the maxilla is moderate anteroposteriorly, but the processes are broad mediolaterally ( Figs 2–8 View Figure 2 View Figure 3 View Figure 4 View Figure 5 View Figure 6 View Figure 7 View Figure 8 ). In this respect, S. uccasamamensis resembles more Pleurolestodon acutidens (FMNH P14495) than Glossotheridium chapadmalense ( Kraglievich, 1925) .

On the lateral cranial wall, the lacrimal is wide, with its orbital portion larger than its facial portion. The lacrimal is pierced by a small lacrimal foramen ( Figs 4B View Figure 4 , 7B View Figure 7 ), the diminutive size of this opening being a feature of all mylodontines and lestodontines ( Gaudin, 2004).

All members of Mylodontidae are characterized by complex jugals with distinct ascending, descending and middle processes. The middle process is elongated and triangular, and the descending process is hooked posteriorly ( Gaudin, 2004). All these features have been observed in the specimens attributed to Simomylodon ( Figs 5E View Figure 5 , 6B View Figure 6 , 7B View Figure 7 , 8B View Figure 8 ). Moreover, this taxon presents a weak postorbital process of the zygomatic arch, as in Glossotheridium , Paramylodon , Pleurolestodon and the lestodontines ( Kraglievich, 1925; Stock, 1925; Webb, 1989; Gaudin, 2004; McAfee, 2009). This is situated at the base of the ascending process, which is long and slender ( Figs 5E View Figure 5 , 6B View Figure 6 , 7B View Figure 7 , 8B View Figure 8 ), resembling that of Glossotheridium , Glossotherium , Paramylodon and Mylodon ( Kraglievich, 1925, 1934; Stock, 1925; McAfee, 2009; Brandoni et al., 2010). In contrast, Pleurolestodon , Lestodon and Thinobadistes have shorter and more robust ascending processes of the jugal ( Rovereto, 1914; Webb, 1989; Bargo et al., 2006). Finally, the ascending process of S. uccasamamensis is nearly horizontal, as in most mylodontids ( Figs 5E View Figure 5 , 6B View Figure 6 , 7B View Figure 7 , 8B View Figure 8 ; Gaudin, 2004).

The length of the zygomatic process of the squamosal is peculiar in Simomylodon because it is the longest (relative to the total cranial length) ever observed among mylodontids. Its almost horizontal orientation and its broad and flattened tip ( Figs 4–8 View Figure 4 View Figure 5 View Figure 6 View Figure 7 View Figure 8 ), however, are common features of mylodontines and lestodontines ( Gaudin, 2004).

In dorsal view, the frontals and parietals are anteroposteriorly and mediolaterally flattened and the sagittal crest is absent ( Figs 4–8 View Figure 4 View Figure 5 View Figure 6 View Figure 7 View Figure 8 ). This morphology is present in all mylodontines and lestodontines except Lestodon ( Gaudin, 2004) . The presence of a flat temporal fossa, delimited by non-connecting temporal lines, is shared with Pleurolestodon , Glossotherium , Paramylodon and Mylodon ( Owen, 1842; Rovereto, 1914; Stock, 1925; Kraglievich, 1934; McAfee, 2009; Brandoni et al., 2010).

The strongly developed nuchal crest of constant width and aligned with the posterior surface of the occiput (this latter showing a median external occipital crest connecting the nuchal crest to the dorsal edge of the foramen magnum) are features of Simomylodon shared with all mylodontids ( Gaudin, 2004). Simomylodon also exhibits a detached notch on the dorsal border of the foramen magnum, comparable with that observed in Pleurolestodon (FMNH P14495) and Mylodon ( Kraglievich, 1934) . In posterior view, the foramen magnum is limited laterally by roughly triangular occipital condyles. These lie at the level of the dentition in lateral view ( Figs 5B View Figure 5 , 8B View Figure 8 ). Both are common conditions in Mylodontidae ( Gaudin, 2004) . As in all mylodontines and lestodontines, the occipital condyles extend posteriorly to the posteriormost edge of the foramen magnum in ventral view ( Figs 5C View Figure 5 , 8C View Figure 8 ). The condyles are widely separated from one another, in this respect resembling Glossotherium , Pleurolestodon and Thinobadistes ; also, they are mediolaterally elongated in ventral view to an extent that is comparable with Mylodon , Pleurolestodon and the lestodontines Thinobadistes and Lestodon ( Owen, 1842; Rovereto, 1914; Stock, 1925; Kraglievich, 1934; Webb, 1989; McAfee, 2009; Brandoni et al., 2010). In ventral view, the occipital condyles are well separated from the condyloid foramina ( Figs 5C View Figure 5 , 8C View Figure 8 ), as in Pleurolestodon (FMNH P14495).

The upper dentition of S. uccasamamensis is similar to the other representatives of Mylodontidae , with five teeth on each side aligned in two divergent tooth rows ( Figs 2–8 View Figure 2 View Figure 3 View Figure 4 View Figure 5 View Figure 6 View Figure 7 View Figure 8 ; Gaudin, 2004). Exceptions to this pattern are represented by Scelidotheriinae, which have parallel tooth rows ( McDonald, 1987), and Octomylodon and Mylodon , which exhibit anterior tooth loss ( Scillato-Yané, 1977; Brandoni et al., 2010).

The Cf1–Mf1 diastema is absent or extremely reduced ( Figs 5C View Figure 5 , 8C View Figure 8 ), as in most mylodontids. Exceptions to this mylodontid pattern are represented by Mylodon and Octomylodon ( Scillato-Yané, 1977; Brandoni et al., 2010), in which Cf1 is secondarily lost, and Lestodon , which shows a derived and extremely elongated diastema ( Czerwonogora & Fariña, 2013).

In Simomylodon , the most mesial upper tooth is strongly caniniform, as observed in all mylodontines and lestodontines, with the exception of Pseudoprepotherium ( Hirschfeld, 1985) . Cf1 is also the smallest of the upper tooth row ( Figs 4–8 View Figure 4 View Figure 5 View Figure 6 View Figure 7 View Figure 8 ), a feature in which Simomylodon closely resembles Pleurolestodon , Glossotheridium , Glossotherium and Paramylodon ( Owen, 1842; Rovereto, 1914; Kraglievich, 1925; Stock, 1925; Robertson, 1976; McAfee, 2009). In occlusal view, Cf1 of Simomylodon is located at the anterior edge of the maxilla ( Figs 2–8 View Figure 2 View Figure 3 View Figure 4 View Figure 5 View Figure 6 View Figure 7 View Figure 8 ), as in Pleurolestodon , Thinobadistes , the megalonychid sloths and the extant Bradypus ( Gaudin, 2004) View in CoL . The posterior curvature of Cf1 ( Figs 2–8 View Figure 2 View Figure 3 View Figure 4 View Figure 5 View Figure 6 View Figure 7 View Figure 8 ) is present in all mylodontines, whereas the alignment of both Cf1 and cf1 with the remainder of the tooth row is a feature of both mylodontines and scelidotheriines, although absent in Lestodon ( McDonald, 1987; Gaudin, 2004; Czerwonogora & Fariña, 2013). The Cf1 of S. uccasamamensis exhibits almost vertical wear ( Figs 5–7 View Figure 5 View Figure 6 View Figure 7 ), like that found only in Pleurolestodon acutidens (FMNH P14495) among Neogene mylodontines. This feature is absent in Paramylodon garbanii ( Robertson, 1976) , Glossotheriopsis pascuali ( Scillato-Yané, 1976) and Glossotheridium chapadmalense ( Kraglievich, 1925) .

Mf1 is also recurved posteriorly ( Figs 3–7 View Figure 3 View Figure 4 View Figure 5 View Figure 6 View Figure 7 ), as in several Mylodontidae but not in Mylodon , Pseudoprepotherium , Lestodon and Scelidotheriinae, where Mf1 is nearly straight ( Gaudin, 2004). The ovate cross-section and the anteroposterior elongation of Mf1 observed in Simomylodon ( Figs 2–8 View Figure 2 View Figure 3 View Figure 4 View Figure 5 View Figure 6 View Figure 7 View Figure 8 ) is widespread in Mylodontidae , lacking only in Octomylodon , Catonyx and Scelidotherium ( Scillato-Yané, 1977; Gaudin, 2004). Mf2 is longer mesiodistally than transversely, as is typical among mylodontines and lestodontines. Mf2 and Mf3 present some peculiar features highly similar to the condition in Pleurolestodon acutidens and Paramylodon garbanii , such as their marked lingual sulcus and the almost orthogonal mesiolingual corner ( Figs 3–7 View Figure 3 View Figure 4 View Figure 5 View Figure 6 View Figure 7 ). In contrast, a marked lingual apicobasal sulcus is found only on Mf2, with a weak sulcus on Mf3, in Glossotheridium chapadmalense ( Kraglievich, 1925) . Finally, Mf4 is T-shaped ( Figs 3–8 View Figure 3 View Figure 4 View Figure 5 View Figure 6 View Figure 7 View Figure 8 ), a peculiar feature of all members of the family Mylodontidae except Octomylodon , which possesses a bilobate Mf4 ( Scillato-Yané, 1977; Gaudin, 2004).

Mandible and lower dentition: In occlusal view, cf1 is equivalent in size to mf1 ( Figs 5–7 View Figure 5 View Figure 6 View Figure 7 , 9–12 View Figure 9 View Figure 10 View Figure 11 View Figure 12 ), and mf3 is the largest lower tooth, an invariant trait of Mylodontidae also known to occur in Bradypodidae ( Gaudin, 2004) . The cf1 is roughly semicircular in cross-section in most specimens, exceptionally displaying ovate or triangular cross-sections ( Figs 5–7 View Figure 5 View Figure 6 View Figure 7 , 9–12 View Figure 9 View Figure 10 View Figure 11 View Figure 12 ). The latter two conformations are typical among Mylodontinae and Lestodontini , respectively ( Gaudin, 2004). The caniniform of Simomylodon has a bevelled occlusal surface ( Figs 5–7 View Figure 5 View Figure 6 View Figure 7 , 9 View Figure 9 , 10 View Figure 10 , 12 View Figure 12 ), resembling that of Pleurolestodon acutidens ( Rovereto, 1914) and Paramylodon garbanii ( Robertson, 1976) . The cf1 is bevelled also in Glossotheridium chapadmalense , but both wear facets are well developed in Simomylodon and Pleurolestodon , whereas the distal facet is extremely reduced in Glossotheridium chapadmalense ( Kraglievich, 1925) . Whereas cf1 of Paramylodon garbanii projects strongly mesially and labially ( Robertson, 1976), that of S. uccasamamensis and Glossotheridium chapadmalense is implanted vertically ( Kraglievich, 1925). The irregularly lobate mf1 and mf2 ( Figs 5–7 View Figure 5 View Figure 6 View Figure 7 , 9–12 View Figure 9 View Figure 10 View Figure 11 View Figure 12 ) are very similar to those of Pleurolestodon acutidens , Glossotheridium chapadmalense and Paramylodon garbanii , but in Simomylodon they are significantly smaller in size. Moreover, Paramylodon garbanii shows deeper apicobasal sulci on the lingual and distal sides of mf2 ( Robertson, 1976), whereas Glossotheridium chapadmalense has a more elongated and almost straight-walled mf2 ( Kraglievich, 1925). The elongated and bilobate mf3 is a recurrent feature in Mylodontinae and Lestodontini ( Gaudin, 2004). As already noted, mf3 has asymmetrically developed lingual and labial apicobasal sulci in S. uccasamamensis , with the former markedly deeper than the latter ( Figs 5–7 View Figure 5 View Figure 6 View Figure 7 , 9 View Figure 9 , 10 View Figure 10 , 12 View Figure 12 ). Among late Miocene–Pliocene Mylodontinae , the pattern of S. uccasamamensis is very similar to that of Glossotheridium chapadmalense , but differs from that of Pleurolestodon acutidens (FMNH P14495, 14521), in which both labial and lingual apicobasal sulci are well developed, and Paramylodon garbanii (UF 10922), which displays an extra bulge on the labial side of mf3.

The mandible of Simomylodon presents some typical mylodontid features, such as the straight horizontal ventral edge of the horizontal ramus in lateral view, and a condyle located at the same level as the tooth row ( Figs 5–7 View Figure 5 View Figure 6 View Figure 7 , 9 View Figure 9 , 10 View Figure 10 , 12 View Figure 12 ) ( Gaudin, 2004; Saint-André et al., 2010). In general, the mandible of Simomylodon is smaller than that of all the other late Miocene– Pliocene mylodontids (i.e. Pleurolestodon acutidens , Paramylodon garbanii and Glossotheridium chapadmalense ; Rovereto, 1914; Kraglievich, 1925; Robertson, 1976). The articular condyle of Simomylodon is convex and medially hooked in dorsal view, as in many Mylodontidae , in contrast to the condyle of lestodontines, which is extended both laterally and medially ( Gaudin, 2004).

The angular process is the posteriormost process of the mandible ( Fig. 9 View Figure 9 ), another common mylodontid trait. Among Mylodontidae , only Nematherium and Octomylodon show an equally posterior extension of the condyloid process ( Gaudin, 2004). In medial view, the mandible of Simomylodon shows a detached oblique ridge that extends from the anteroventral edge of the angular process towards the root of the last tooth ( Fig. 9D View Figure 9 ). This last condition is shared with Pleurolestodon , Glossotherium , Paramylodon ( Gaudin, 2004) and specimen MACN Pv 8675 of Glossotheridium chapadmalense . In lateral view, the ascending ramus does not cover mf3 ( Figs 6E View Figure 6 , 9B View Figure 9 , 10E View Figure 10 , 12D, G View Figure 12 ), resembling the condition observed in Glossotheridium chapadmalense and Paramylodon garbanii ( Kraglievich, 1925; Robertson, 1976). This partial coverage of mf3 is observed in some other mylodontid genera, such as Octodontotherium , Pseudoprepotherium , Mylodon and Pleurolestodon ( Gaudin, 2004) . The anterior edge of the mandibular spout is broad and flat in occlusal view, as it is in Glossotherium , Glossotheridium and Lestodon ( Owen, 1842; Kraglievich, 1925; McAfee, 2009; Czerwonogora & Fariña, 2013) ( Figs 5–7 View Figure 5 View Figure 6 View Figure 7 , 9 View Figure 9 , 10 View Figure 10 ). Other mylodontine genera, such as Mylodon , Paramylodon and Pleurolestodon , possess anteriorly rounded mandibular spouts ( Stock, 1925; Kraglievich, 1934; McAfee, 2009; Brandoni et al., 2010).

Four mandibles can be ascribed to juvenile individuals of S. uccasamamensis , based on their reduced size and lack of wear on the lower dentition ( Fig. 12 View Figure 12 ). These remains already display the main diagnostic features that have been found in the adults (e.g. a straight ventral margin, the absence of a diastema between cf1 and mf1, and the extreme reduction of the labial apicobasal sulcus on mf3; Fig. 12 View Figure 12 ). All these features are in strong contrast to those observed in the juvenile mandibular fragment described by Oliva & Brandoni (2012) and tentatively attributed to a ‘mylodontid cf. Simomylodon ’ (Huayquerian SALMA, Buenos Aires Province, Argentina). The present data suggest that the specimen described by Oliva & Brandoni (2012) does not belong to Simomylodon , but rather is more compatible with the genus Pleurolestodon , also recognized in the Huayquerian SALMA of Argentina ( Rovereto, 1914). However, given that no juvenile mandibular remains are known for that taxon, we prefer to consider the latter specimen as Mylodontinae indet.

MORPHOMETRIC ANALYSES

The results of PCAs among the Neogene Mylodontinae are depicted in Figures 13 View Figure 13 and 14 View Figure 14 , showing the two distinct modules: cranium and upper dentition ( Fig. 13 View Figure 13 ) vs. mandible and lower dentition ( Fig. 14 View Figure 14 ). We followed this approach in order to overcome the problem of the paucity of the data, thus maximizing the number of specimens that could be included in the analyses (for further details, see Material and Methods and Supporting Information, Appendix S5).

In the cranial dataset ( Fig. 13 View Figure 13 ), principal component (PC) 1 explains 51.80% of the variance and, given that all the variables have positive loadings, it reflects body size. Size is lower on the left side and higher on the right. Principal component 2 ( Fig. 13A View Figure 13 ) explains 17.42% of the variance. Positive values on this axis reflect skulls that have slender palates and thin rostra relative to total skull lengths, whereas negative values represent skulls with relatively wider palates and rostra. Finally, PC3 ( Fig. 13B View Figure 13 ) explains 12.77% of the total variance. Positive values for PC3 are associated with robust dentitions and a long and deep snout relative to total skull lengths, whereas negative values are correlated with a reduced dental series and a short and slender snout (in relationship to total length). The Miocene–Pliocene sloths are well segregated along PC1 ( Fig. 13 View Figure 13 ), with S. uccasamamensis and Glossotheriopsis pascuali as the smallest taxa, and Glossotheridium chapadmalense as the largest taxon in the dataset. Pleurolestodon acutidens and Paramylodon garbanii occupy intermediate positions ( Fig. 13 View Figure 13 ).

On PC2, the extreme morphologies are represented by Glossotheriopsis pascuali in the positive range and Paramylodon garbanii in the negative range ( Fig. 13A View Figure 13 ). However, these morphologies must be treated cautiously, because the result may be affected by the lack of total skull length measurements for both species, given that neither is represented by complete skulls. Simomylodon shows important variation along PC2, whereas Pleurolestodon acutidens and Glossotheridium chapadmalense do not overlap ( Fig. 13A View Figure 13 ). This means that Glossotheridium chapadmalense exhibits a wider palate and rostrum relative to total skull length than is the case for Pleurolestodon acutidens .

On PC3, S. uccasamamensis still shows high variation, including Glossotheriopsis pascuali from southern Argentina in its morphometric range ( Fig. 13B View Figure 13 ). The most extreme morphologies are represented by Pleurolestodon acutidens (the highest values) and Glossotheridium chapadmalense (the lowest values). These two taxa, together with Paramylodon garbanii , partly overlap along PC3 ( Fig. 13B View Figure 13 ).

In Fig. 13 View Figure 13 , the S. uccasamamensis specimen MNHNBol V 3348 ( Fig. 3 View Figure 3 ) is represented by a red triangle. This cranium was previously attributed to the species Pleurolestodon dalenzae by Saint-André et al. (2010). The present dataset shows that this specimen falls far outside the morphometric range of the genus Pleurolestodon , but well within the range of variation for Simomylodon ( Fig. 13 View Figure 13 ).

Likewise, a second PCA ( Fig. 14 View Figure 14 ) was performed on the variables of the mandible and lower dentition, yielding the same general pattern as the cranial analysis. Principal component 1, which explains 57.96% of the total variance, is again a representation of size, and Simomylodon occupies the lowest positions on the left side of the graph ( Fig. 14 View Figure 14 ). Glossotheridium chapadmalense shows the largest mandibular values, whereas Pleurolestodon acutidens and Paramylodon garbanii are recovered in intermediate positions.

Principal component 2 explains 19.42% of the variance. Higher values are correlated with a long dental series and a deep mandibular ramus at the level of the dentition, relative to total mandibular length, whereas lower values correspond to a shorter dental series and less robust mandible.

Finally, PC3 explains 9.45% of the variance and reflects mandibles with a long horizontal ramus and anteroposteriorly narrow ascending ramus (positive values) vs. mandibles displaying a shorter horizontal ramus and a more anteroposteriorly enlarged ascending ramus (negative values).

On both PC1 and PC2 ( Fig. 14A View Figure 14 ), Simomylodon shows the greatest range of variation, probably attributable to the inclusion of juvenile individuals in the dataset ( Fig. 12 View Figure 12 ). These specimens are retrieved in the far bottom-left portions of the graph depicting PC1 vs. PC2 ( Fig. 14A View Figure 14 ), corresponding to the lowest values for both principal components. This means that they are the smallest specimens in the dataset (as expected), but they also possess a dental series that is reduced in length and horizontal rami that are of moderate depth relative to total mandibular length. Glossotheridium chapadmalense shows the highest variation on PC3 ( Fig. 14B View Figure 14 ), an effect that is probably related to the incompleteness of the dataset for this taxon (no complete mandibles are known).

As before, the red triangles indicate the S. uccasamamensis specimens that were formerly assigned to another taxon. These are the specimens MNHN-Bol V 3358 ( Fig. 10A–C View Figure 10 ), 3371 ( Fig. 10D–F View Figure 10 ) and 3359 ( Fig. 12G–I View Figure 12 ) that Anaya & MacFadden (1995) assigned to Glossotheridium chapadmalense . The more extensive data of the present analysis support their inclusion in the genus Simomylodon instead.

PHYLOGENETIC ANALYSIS

The phylogenetic analysis recovered a single most parsimonious tree (tree length: 755 steps, consistency index = 0.662, retention index = 0.927), with a topology compatible to that of the consensus tree from the analysis by Gaudin (2004). In our dataset, Simomylodon is deeply nested within Mylodontinae , more precisely as the sister taxon of the monospecific genus Pleurolestodon ( Fig. 15 View Figure 15 ). The node uniting the latter two taxa is well supported, with bootstrap and jackknife values of 53 and 70, respectively. These values are even greater than those supporting Mylodontinae (Supporting Information, Appendix S7). However, and in accordance with the previous study of Gaudin (2004), other groups are better supported, such as Lestodontini , Mylodontidae , Scelidotheriinae and Folivora (Supporting Information, Appendix S7).

The unambiguous synapomorphies that link Simomylodon and Pleurolestodon include: the Cf1 placed at the edge of the premaxilla ( Gaudin, 2004: character 21, 1→0), the relatively wide braincase ( Gaudin, 2004: character 82, 2→3) and the pronounced separation of the occipital condyles from the hypoglossal foramina ( Gaudin, 2004: character 194, 1→2). The close morphological affinity of Simomylodon and Pleurolestodon , suggested by the present phylogenetic analysis, is probably the cause of the taxonomic misunderstanding of Saint-André et al. (2010), who assigned a gracile specimen of Simomylodon to a new species of Pleurolestodon .

Autapomophies of S. uccasamamensis , as retrieved by the present phylogenetic analyses are as follows: an intermediate shape of the coronoid process ( Gaudin, 2004: character 47, 2→1), a posterodorsal inclination of the mandibular condyloid process in lateral view ( Gaudin, 2004: character 52, 1→0), a short mandibular symphysis ( Gaudin, 2004: character 62, 2→1), a weak buccinator fossa of the maxilla ( Gaudin, 2004: character 106, 0→1), an ascending process of the jugal longer than the descending process ( Gaudin, 2004: character 151, 0→1), a very elongate zygomatic process of the squamosal ( Gaudin, 2004: character 168, 2→3), an enlarged condyloid foramen ( Gaudin, 2004: character 187, 1→2) and a narrow and fairly deep mastoid depression ( Gaudin, 1995: character 34, 1→0).