Eucholoeops ingens Ameghino, 1887

Iuliis, Gerardo De, Pujos, François, Toledo, Nestor, Bargo, Susana & Vizcaíno, Sergio F., 2014, Eucholoeops Ameghino, 1887 (Xenarthra, Tardigrada, Megalonychidae) from the Santa Cruz Formation, Argentine Patagonia: implications for the systematics of Santacrucian sloths, Geodiversitas 36 (2), pp. 209-255 : 220-239

Eucholoeops ingens Ameghino, 1887

Eucholoeops ingens Ameghino, 1887: 21 .

Eucholoeops latirostris Ameghino, 1891: 322 .

Eucholoeops externus Ameghino, 1891: 322 .

Eucholoeops curtus Ameghino, 1894: 410 (not p. 154 as incorrectly indicated in Scott 1904: 276).

NEOTYPE. — MPM-PV 3401 , designated here ( Figs 2 ; 6A, B ; 8 A-C; 9A, B).

LOCALITY AND HORIZON. — Puesto Estancia La Costa , Santa Cruz Province, Argentina ; Estancia La Costa Member , Santa Cruz Formation (late Early Miocene, Santacrucian SALMA). The specimen is also figured in Bargo et al. (2009: fig. 2; 2012: fig. 13.3) .

DIAGNOSIS. — Caniniform teeth very large; maxilla does not extend beyond alveolar wall of C1 laterally or ventrally and forms a pillar-like sheath surrounding C1; premaxillar margins of maxillae form wide V-shaped notch, with small anterior flange; preorbital width of rostrum considerably greater than width at postorbital constriction; differing from other Eucholoeops species, in which caniniforms are smaller, maxillae extend slightly anterior to caniniform alveolus, premaxillar margins of maxillae forming narrower V-shaped notch, with more pronounced anterior flange, and preorbital rostrum width slightly greater than that of postorbital constriction; dentary with marked concavity anterior to c1 alveolus; Mcs II-V subequal in length.

REFERRED MATERIAL. — FMNHP13125, FMNHP13139, MACN-A 6413 (type of E. curtus ), MACN-A 6414, MACN-A 4639 (type of E. latirostris ), MACN-A 4640, 4641 (type of E. externus ), MACN-A 4642, MPM-PV 3451, MPM-PV 3452, and MPM-PV 15046.

DESCRIPTION AND COMPARISON

Skull, mandible, and teeth

See Figures 2-7 and Tables 1 , 2 .

Skull ( Figs 2-5 ; 7 A-H)

The dorsal profile of the skull of Eucholoeops ingens is gently and regularly convex, with a very slight depression at its middle and another posteriorly, as opposed to the generally declining profile anterior to the middle of the cranium in Hapalops . The general form of the skull in Eucholoeops has long been conceptualized as having an enormously robust muzzle with a prominent depression between the facial and cranial parts of the skull, based on the illustration of E. externus by Scott (1904: pl. 56, fig. 1). This specimen is assigned here to E. ingens , and Scott’s figure is considered inaccurate (see Discussion).The skull is usually regularly convex in some megalonychids (e.g., Choloepus , Acratocnus , and Neocnus : see Taboada et al. 2007) though more strongly so. The difference is due mainly to the more robust rostrum of E. ingens compared with these taxa. In other megalonychids such as Neocnus , Parocnus Miller, 1929 and, to a lesser degree, Megalocnus Leidy, 1868 and Acratocnus the rostrum is slender and elongated so that the profile over the orbital and facial regions (see Matthew & Paula Couto 1959; Paula Couto 1967; Taboada et al. 2007) is nearly rectilinear. In still others such as Megalonyx , the rostrum is very deep and the profile is doubly convex, over the cranium and the rostrum. In Ahytherium the rostrum is considerably shortened and the profile is shallowly concave at the nasofrontal contact.

The maxillae end abruptly anterior to C1, mainly following the curvature of this tooth, so that the nasal overhangs the maxilla. A similar condition exists in Acratocnus , Choloepus , and Megalonyx , but these are easily distinguished from Eucholoeops in that the rostrum tapers notably anteriorly in lateral view in Acratocnus and Choloepus and the teeth are not triangular in section in Megalonyx . The lacrimal foramen appears relatively lower owing to the deeper height of the muzzle in E. ingens compared to Hapalops . In E. ingens a pronounced and triangular depression lies on the ventral surface of the palate posterior to C1 and narrows into a trough extending along length of the diastema to M1. The presence of the depression and trough seem unrelated to accommodating c1. The depression and trough are ridged laterally and medially and do not extend onto the lateral surface of the maxilla. In Hapalops there is only a much shallower trough in this position and it is not consistently present (for example, it is not evident on the left side of FMNH P13141). Naples (1982) and Gaudin (2004) noted the presence of a postcanine fossa in the maxilla of Choloepus that receives the tip of c1. This differs from that of E. ingens in being deeper and restricted to the region immediately posterior to C1, rather than continuing as a trough to M1. Also, the fossa in Choloepus extends onto the lateral surface of the maxilla and is thus observable in lateral view, in contrast to Eucholoeops , in which it is restricted to the palatal surface. Gaudin (2004) signaled a postcanine fossa as characteristic of Megalonychidae , but the fossa is not present in a Megalonyx wheatleyi specimen (ROM 55503a, b, cast of skull and mandible) we examined and is apparently absent in the Megalonyx specimen figured by Leidy (1855), in which the caniniforms occlude differently; this is also true of the Acratocnus specimen illustrated by Anthony (1926), in which the teeth occlude similarly.

The premaxillae of Eucholoeops ingens , loosely connected to the maxilla and each other (as is usual in sloths), are approximately triangular, although their anterior tip is missing in MPM-PV 3401 ( Fig. 2D, E ). It is likely that the overall form of the premaxillae would have resembled the Y-shaped structure noted for Eucholoeops fronto and Hapalops by Scott (1903, 1904). Gaudin (2004) also scored Eucholoeops as having a Y-shaped premaxilla, and the preserved portions of MPM-PV 3401 do not necessarily contradict this author’s assessment. The premaxilla of Hapalops , known from several species (e.g., H. longiceps, YPM-VPPU 15523, and H. ruetimeyeri, AMNH 9250), differs mainly in being more gracile, bearing a longer anterior process, and a wider opening between the lateral and medial rami. The lateral ramus of the premaxilla of E. ingens is considerably more robust though notably shorter than the medial ramus. The slender medial ramus tapers posteriorly, whereas the lateral ramus widens posteriorly into a blunt process that contacts the anteromedial margin of the maxilla, as occurs also in the Miocene nothrotheriid Mionothropus De Iuliis, Gaudin & Vicars, 2011 from Peru and Hapalops ( De Iuliis et al. 2011) . In these last two genera, however, the medial ramus is about as long as the lateral ramus, although in Hapalops the posterior end of the medial ramus appears to have been at least partially reconstructed for H. longiceps (YPM-VPPU 15523), so that illustrations in Scott (1903: pl. 31) are somewhat misleading in this regard.

The anterior margins of the maxillae of Eucholoeops ingens , with which the premaxillae articulate, describe a widely open V, with nearly rectilinear edges. A small and delicate triangular flange, barely detectable in some specimens and variable in size between left and right sides, is present in some specimens. A similar morphology occurs in a specimen of Neocnus illustrated by Gaudin (2011: fig. 21C). Typically in sloths, the anterior margin of the maxilla bears lateral and medial palatal processes that extend anteriorly in contacting the premaxillae. The medial process is triangular and fits between the lateral and medial rami of the premaxilla. In considering the extent of the lateral and medial palatal processes of the maxilla, Gaudin (2004: ch. 111) scored Eucholoeops as having a shorter medial than lateral margin. While technically and probably phylogenetically accurate, this does not capture the form in E. ingens : a lateral process is nonexistent (the maxilla ends with the wall of the alveolus of C1, as noted above), and the medial is barely, if at all, present. Nonetheless, the latter does fit between the lateral and medial premaxillary rami and so is homologous with the medial palatal flange of the maxilla.

The lateral rostral wall between C1 and M1 is strongly concave ventrally, forming a deep antorbital or buccinator fossa in Eucholoeops ingens . Dorsally, however, the rostral wall bulges laterally, forming a “roof” over the fossa, so that the preorbital constriction (measured dorsally) is wider than the postorbital constriction. This resembles the condition in Acratocnus , but not Hapalops , in which the bulge is much less prominent, so that the preorbital constriction is narrower than the postorbital. In other megalonychids the preorbital constriction is narrower in Neocnus and Parocnus , about equal to the postorbital constriction in Megalocnus ( Taboada et al. 2007) , and wider in Megalonyx ( Leidy 1855) and Ahytherium , in which the rostrum is also extremely shortened ( Cartelle et al. 2008).

The lacrimal of Eucholoeops ingens lies dorsal to the more anterior molariforms and extends anterior to the mesial margin of M 1 in E. ingens , Mionothropus , and Choloepus but not Hapalops ; in Acratocnus the lacrimal lies almost entirely anterior to M1. The position of the lacrimal foramen, naturally, reflects this position, lying above the mesial margin of M 1 in E. ingens , Choloepus , Neocnus , Parocnus , and Mionothropus , well anterior to M 1 in Acratocnus , Megalonyx , and Ahytherium , and approximately between M1 and M 2 in Hapalops . The relative size of the foramen is similar in all taxa except Choloepus , in which it is smaller. The posterior surface of the lacrimal is deeply concave in E. ingens .

The postorbital process of the frontal lies dorsal to M 3 in Eucholoeops ingens , approximately as in Mionothropus and Choloepus . A small foramen for the frontal diploic vein lies just anterior to the process (see De Iuliis et al. 2011). The process lies farther posteriorly in Hapalops , approximately dorsal to M4, whereas in Acratocnus it lies above M1 and M2. The process of E. ingens is slightly better developed than in Hapalops , but much less prominent than in Choloepus and Acratocnus . The region between the postorbital process and the lacrimal is more elongated in Hapalops .

The temporal lines of Eucholoeops ingens (MPM- PV 3401, Fig. 2C ) curve posteromedially, meeting just anterior to the frontoparietal suture to form a small sagittal crest that extends posteriorly to the occiput. The crest is similar in MPM-PV 3403 ( E. cf. fronto ), but not as raised in MPM-PV 3451 ( Fig. 3C ). There is variation in the form of the temporal lines and sagittal crest in Hapalops . Some species, such as H. longiceps ( Scott 1903: fig. 17a) and H. vulpiceps ( Scott 1904: pl. 44:1), follow the pattern just described for E. ingens . However, in other species, such as H. brachycephalus ( Scott 1904: pl. 36:2) and H. ruetimeyeri ( Scott 1903: fig. 17b; 1904: pl. 38:4), the temporal lines similarly converge, but do not meet to form a sagittal crest; they pass instead on either side of the midsagittal plane over the frontals, diverge as they extend onto the parietals and then converge again over the posterior part of the parietals. Hapalops elongatus ( Scott 1904: pl. 38: 1, 2, 5) exhibits both patterns. Cartelle & Bohórquez (1982; see also De Iuliis 1996) observed similar intraspecific variation in the megatheriid Eremotherium laurillardi ( Lund, 1842) and suggested that the states reflected sexual dimorphism. In Acratocnus the temporal lines converge more markedly, meeting well in advance of the frontoparietal suture to form a prominent sagittal crest. In Mionothropus the lines are not prominent and do not approach each other closely. In contrast, those of Choloepus are prominent and converge gradually; they may remain relatively far apart, though some specimens possess a short, distinct sagittal crest on the parietals.

The nasals of Eucholoeops ingens are broad posteriorly, narrow approximately at midlength and then widened again. This pattern seems typical for the other megalonychid sloths considered here (though not Megalocnus , in which the nasals are narrowed posteriorly) and Hapalops and Mionothropus (but not Nothrotheriops Hoffstetter, 1954 , in which they are nearly of the same width throughout their length, and Nothrotherium Lydekker, 1889 , in which they are slightly wider posteriorly)(see Cartelle& Fonseca 1983; De Iuliis et al. 2011; Scott 1903, 1904; Stock 1925; Taboada et al. 2007). Although the nasals are generally broken anteriorly, in specimens with reasonably well-preserved nasals, these bones extend about as far forward as or slightly beyond the maxillae (e.g., Neocnus , Parocnus , Megalocnus ; see Taboada et al. 2007: 40, 41) and their anterior margins may be nearly straight and transversely oriented ( Neocnus , Megalocnus ), slightly convex anteriorly ( Parocnus ), or concave anteriorly ( Acratocnus ) (see Taboada et al. 2007: 40, 41; Anthony 1926: pl. 40). In Hapalops , Mionothropus , Nothrotheriops , and Nothrotherium the anterior margin bears a V-shaped notch between lateral and medial laminae (see Cartelle & Fonseca 1983, De Iuliis et al. 2011, Scott 1904, Stock 1925). This condition differs from that of E. ingens , in which the nasals are well preserved in MPM-PV 3451 ( Fig. 3 ): the nasals extend well beyond and overhang the maxillae ( Fig. 3A, B ) and the medial half of the anterior margin is nearly straight and transversely oriented and the lateral half extends posterolaterally, so that overall the margin is, in dorsal view,convex ( Fig.3D ). The strongly developed nuchal crests of Eucholoeops ingens overhang the occiput, resembling Acratocnus , although the crests are not as well developed in the latter. In contrast, the occiput of Hapalops , Mionothropus , and Choloepus is widely exposed in dorsal view. The supraoccipital of Hapalops bears a well-developed median crest, more so than that of E. ingens . The occipital condyles of the latter are relatively larger and project more prominently than in Hapalops , Mionothropus , Acratocnus , and Choloepus . The basioccipital, and thus the condyles, lie at approximately the same level as the alveolar margins (i.e. the palate) in E. ingens , as in Hapalops and Choloepus , but lie more dorsally in Mionothropus and Acratocnus . The occipital condyles extend considerably ventral to the external auditory meatus in E. ingens , but slightly less so in Hapalops and Mionothropus and are directly posterior to the meatus in Choloepus and Acratocnus . A more detailed analysis of the basicranium and auditory region is in progress.

The jugal of Eucholoeops ingens is loosely attached to the lacrimal and maxilla and does not contact the zygomatic process of the squamosal (i.e. the zygomatic arch is incomplete), as occurs in most sloths (see Gaudin 2004), but not all (e.g., the Pleistocene megatheriid Eremotherium Spillmann, 1948 , the mylodontid Mylodon Owen, 1839 , and the megalonychids Megalonyx and Ahytherium ; Gaudin 2004; Cartelle et al. 2008). The base of the jugal of Eucholoeops ingens has an orbital wing that prevents a maxillolacrimal contact in the orbital region, in contrast to the condition noted in Gaudin (2004: ch. 109; the jugal is missing in the Eucholoeops specimens examined by this author, although its facet is preserved). Eucholoeops ingens thus resembles the condition that De Iuliis et al. (2011) described for most nothrotheriids. The contact, however, is present in most megalonychids ( Gaudin 2004). The jugal forms the ventral rim of the orbit, which lies in about the same position typical for other mammals. This position occurs in sloths such as Acratocnus and Choloepus , as opposed to being displaced ventrally near the level of the alveolar margin, as occurs in some megalonychids such as Megalonyx ( Gaudin 2004) , Ahytherium ( Cartelle et al. 2008) , and other sloths, such as Hapalops ( Scott 1904) , the nothrotheriids Nothrotherium , Nothrotheriops , and the megatheriid Eremotherium (see Gaudin 2004).

The posterior part of the jugal bears three processes, as occurs in most sloths ( Gaudin 2004). The ascending process of Eucholoeops ingens , resembling that of Hapalops and Mionothropus , is elongated and slender and extends posterodorsally above the zygomatic process of the squamosal, but is longer and tapers more gradually than in these taxa. A weak postorbital process is present along the anterior surface of its base. The middle process projects posteriorly toward the zygomatic process of the squamosal. It is prominent and approximately triangular, resembling that of Hapalops and Nothrotheriops , but more robust and pointed at its posterior extremity, rather than blunt. The robust descending process is shorter than the ascending process, but less so than in Hapalops . It is wide at its base and narrows ventrally, but not as strongly as occurs in Hapalops . Although the ventral extremity is incomplete, it is sufficiently preserved to indicate that the posterior margin is concave, thus imparting a hook-like appearance to the end of the process; it is not as curved as in Nothrotheriops , resembling more that of Mionothropus .

The auditory region of Eucholoeops is complex and will be described in a subsequent study.

Mandible ( Figs 6 ; 7I, J )

The mandible of Eucholoeops ingens generally resembles that of Hapalops (and many other sloths, see below) in having a prominent spout, a deep horizontal ramus, and prominent processes. A main difference between E. ingens and Hapalops is that the dentary is more robust in the former. In Hapalops the horizontal ramus appears relatively gracile and elongated, with the spout extending farther anteriorly and the body being less deep. In both, as in many but not all sloths (see below), the ventral margin of the horizontal ramus is convex and nearly reaches or projects below the level of the ventral margin of the angular process. This ventral bulge lies ventral to the molariform tooth row. More anteriorly the margin of the dentary has a small ventral protuber- ance approximately coincident with the posterior end of the mandibular symphysis. This feature may be related to the presence of a caniniform tooth (although the base of this tooth is posterior to the protuberance), as the protuberance is not present in sloths such as in the nothrotheriid Nothrotheriops , and the megatheriids Megatherium Cuvier, 1796 and Eremotherium , in which the caniniform is either lost or molariform and part of the molariform tooth row (i.e. a pronounced diastema is absent). It is present in the nothrotheriid Mionothropus , in which small caniniforms are retained.

The ventral profile of the horizontal ramus of Eucholoeops ingens strongly resembles that of Acratocnus , although the spout is somewhat shorter and the angular process is positioned notably more dorsally in the latter. The profile varies among the remaining megalonychids, but is usually convex, as in Neocnus , Parocnus , Megalocnus , Megalonyx , and Ahytherium . A slight ventral protuberance is also present (though very slight in Ahytherium ), except in Megalocnus ; the caniniform, however, is strongly procumbent in the latter. In Choloepus , on the other hand, the ventral profile is nearly rectilinear and the angular process projects slightly below it. This condition resembles the typical condition described for mylodontid sloths, such as the mylodontine Paramylodon Brown, 1903 and the scelidotheriine Scelidotherium Owen, 1840 (see Stock 1925; Mc- Donald 1987).

The spout of Eucholoeops ingens is shorter than in Hapalops , thus conforming to the condition in most megalonychids when the spout is present (it is strongly reduced in Megalonyx and Megalocnus ). The spout extends slightly anterodorsally. Its ventral margin is gently concave in lateral view. A prominent trough-like depression lies on the dorsolateral margin of the dentary just anterior to c1. The depression faces dorsolaterally and receives C1. The depression is not present in Hapalops , although in lateral view the dorsal margin of the spout is gently concave just anterior to c1. Gaudin (2004) noted the presence of a depression on the lateral surface of the dentary just posterior to c 1 in Hapalops and E. ingens . This depression is prominent and easily

noted in some Hapalops specimens, such as H. longiceps (YPM-VPPU 15523) but appears absent in others, such as H. elongatus (YPM-VPPU 15597). In E. ingens MPM-PV 3401 ( Fig. 6A, B ) the depression is present but shallower than in H. longiceps YPM-VPPU 15523, whereas it is not notable in E. ingens FMNH P 13125 ( Fig. 6E, F ), although this region is not well preserved in this specimen.

The coronoid process of Eucholoeops ingens and Hapalops are about equally high, but that of Hapalops appears relatively shorter because the condyle is more dorsally positioned in the latter. In E. ingens the condyle lies just dorsal to the level of the molariforms, and so resembles the condition of Choloepus among megalonychids (and mylodontids such as Paramylodon and Scelidotherium ). In other megalonychids, such as Acratocnus , Ahytherium , Megalocnus , Megalonyx , Neocnus , and Parocnus , the condyle is more dorsal (especially Megalocnus ), and the height of the coronoid varies. For example, it is well above the condyle in Choloepus , Ahytherium , and Megalonyx , at about the same level as the condyle in Acratocnus , Neocnus , and Parocnus (in these the condyle is somewhat shorter than in E. ingens and Hapalops ), and lower than the condyle in Megalocnus (in which the coronoid is nonetheless tall). Except for a slightly more dorsal position of the condyle in Hapalops , the form and relative positions of the condyle and coronoid process in E. ingens resemble more those in Hapalops than in other megalonychid sloths.

The angular process in Eucholoeops ingens is prominent, as in Hapalops . Its ventral margin, in both taxa, reaches to about the same level as the ventral bulge of the dentary, as is also true in Neocnus . It lies slightly dorsal to the ventral margin in Acratocnus , and much more so in Parocnus and Megalocnus (see Scott 1904; Anthony 1926; Matthew & Paula Couto 1959; Taboada et al. 2007). The margin between the condyle and angular process outlines a fairly deep and smooth semicircular notch, as also occurs usually in Hapalops , as well as Megalonyx and Ahytherium among megalonychids; it is much shallower in Choloepus and in the remaining megalonychids and more angular in the latter group, except Megalocnus (see Taboada et al. 2007). It is also somewhat angular in H. elongatus (YPM-VPPU 15597). In overall relative positions, form, and sizes of the angular, condyloid, and angular processes, E. ingens most resembles Hapalops . A single mental foramen is present on either side in MPM-PV 3401 ( Fig. 6B ). The foramen lies in a depression of the dentary and opens anterior to the deep notch before the c1 alveolus.

Position of the posterolateral opening. Descriptions of the position of the posterolateral opening of the mandibular canal are somewhat ambiguous. In sloths it is always near or on the base of the anterior margin of the ascending ramus. It has been described, except for megatheriines (see below), as either opening anterolaterally or laterally from the base of the ascending ramus or from the lateral surface of the horizontal ramus. However, the opening is always associated with the base of the anterior margin of the ascending ramus, but depending on the opening’s dorsoventral position and relation to the molariform teeth, it may appear as opening from the ascending ramus or the horizontal ramus. For such reasons, it is useful to also consider the position of the opening with respect to these other features. In Eucholoeops ingens the opening lies entirely ventral to the alveolar margin and below the distal half of m3, and thus appears on the lateral surface of the horizontal ramus. This is the typical condition in other megalonychids, although there is variation relative to the molariform teeth and dorsoventral position. For example, it is, exceptionally, more ventral in Neocnus and lies below m2. In some megalonychids, such as Megalonyx and Parocnus , its position is approximately as in E. ingens . In Choloepus the opening is below m3 but its posterior portion may intersect the alveolar border. In Acratocnus and Megalocnus it lies below m2. In Ahytherium the position of the opening varies, and may lie just medial or lateral to the base of the anterior margin of the coronoid process, and from the distal margin of m3 to the mesial half of m3. In MCL 21834, the holotype of Ahytherium aureum , the more posterior position occurs in the R dentary and the more anterior position in the L dentary. The condition in Mionothropus and Nothrotheriops is similar, with the opening lying ventral to the alveolar margin below m3, and as well in Scelidotherium and Paramylodon , with the difference being that it lies below m4 (which is equivalent to m 3 in the above discussed taxa, as all four lower teeth are molariform). In contrast, it lies more dorsally in Hapalops (e.g., YPM-VP- PU 15523, 15597; AMNH 9222), lying about level with the alveolar margin and posterior to m3, and thus appears on the anterior margin of the base of the ascending ramus. Scott (1903) indicated that its position varies intraspecifically, at least for H. elongatus , as is apparent in two specimens that Scott (1904: pl. 40: figs 2a, 4; respectively AMNH 15155 and 15531) assigned to this species: in AMNH 15531 it is entirely ventral to the alveolar margin; in AMNH 15155 it intersects the margin. In megatheriines it lies medial to the base of the ascending ramus.Taxonomic importance has also been given to the presence of two openings in this region. However, this is individual variation, as is indicated by the presence of one and two openings in the same individual (e.g., MPM-PV 4251, Hyperleptus sp., and MPM-PV 3458, Hapalops sp.). Scott (1903) reported such variation in a specimen of H. longiceps , and considered it an abnormality.

It might be expected that, to some degree at least, the position of the opening would be correlated to the position of the anterior margin of the ascending ramus, but a survey of sloths suggests that any possible correlation is not straightforward. In E. ingens , as in Choloepus , Hapalops , Mionothropus , and Nothrotheriops , the anterior margin of the coronoid process is posterior to m3 and leaves this tooth entirely exposed in lateral view. Megalonyx and Ahytherium are similar except that m3 is partially covered. In other megalonychids (e.g., Acratocnus , Megalocnus , Neocnus , and Parocnus ), however, the anterior margin of the coronoid lies farther anteriorly, with m3 being usually entirely (and in some taxa m2 partially) covered in lateral view.

Dentition

Bargo et al. (2009) noted the main features of the dentition of Eucholoeops ingens and provided detailed descriptions of the occlusal surfaces for their analysis of mastication in this sloth. Here, comparative descriptions among taxa are provided, as well as an assessment of intraspecific variation in E. ingens . The dentition includes an upper and lower caniniform and four upper and three lower molariforms. The caniniform is considerably enlarged, projecting beyond the level of the molariforms. C1 and c1 are approximately triangular in section and with a nearly vertical wear facet, that of C1 on the distal and of c1 on the mesial surface of the tooth. The apex of each tooth is sharply pointed. C1 projects more strongly past the upper molariforms than does c1 past the lower molariforms. The diastema is pronounced, particularly in the upper tooth row. The form and size of the caniniform teeth are sufficient to distinguish E. ingens from any other Santacrucian sloth. Enlarged caniniforms are characteristic of megalonychids, though they are not always sharply pointed. Eucholoeops ingens strongly resembles Choloepus and Acratocnus and some species of Neocnus (see MacPhee et al. 2000) in this respect. Those of Ahytherium are also sharply pointed, but they are not as large as in E. ingens , Choloepus , and Acratocnus . The c1 of E. ingens projects about as strongly as in Acratocnus (as illustrated by Gaudin 2004: fig. 10). In contrast to the condition of Choloepus , c1 does not project far enough to require accommodation by a fossa posterior to C1. The caniniforms in megalonychids such as Megalonyx , Megalocnus , and Parocnus are clearly distinguished from those of E. ingens in being incisiform (see Leidy 1855; Matthew & Paula Couto 1959; Taboada et al. 2007).

The upper molariforms are generally oval to rectangular in shape, with long axis oriented vestibulolingually. This is a common pattern in megalonychids (but see below), nothrotheriids, and planopsines, and occurs in some less derived megatheriines (e.g., Megathericulus Ameghino, 1904 and Anisodontherium Brandoni & De Iuliis, 2007 , Middle and Late Miocene, respectively; Pujos et al. 2013).

Eucholoeops ingens is notable in that its molariforms, particularly M2 and M3, tend to be transversely expanded, and in some specimens these teeth appear mesiodistally compressed. M2 and M3 are the largest molariforms, although there is variation, including individual, as to which is largest ( Fig. 4B ). M4 is the smallest upper tooth. It may be transversely oval (MPM-PV 3451; Fig. 3C ) or nearly trapezoidal (MPM-PV 3401; Fig. 2B ), but its section varies, even within individuals. For example, its distal surface may be convex or slightly indented, producing either an oval or somewhat reniform section. Despite the overall general resemblance of the molariforms among megalonychids, there are some notable differences, as is evident from figures and descriptions in, for example, Anthony (1926), Cartelle et al. (2008), Matthew & Paula Couto (1959), and Taboada et al. (2007). Among these differences, we may note that some molariforms tend to be somewhat triangular to trapezoidal rather than oval in Ahytherium , Megalonyx , Neocnus and, to a lesser extent, Parocnus . In some megalonychids, the long axis of some molariforms tends to be oriented obliquely to the long axis of the tooth row, as occurs in M1 to M 3 in Acratocnus , M 3 in Ahytherium , M1-M 2 in Parocnus , and M 1 in Megalocnus . Lastly, several molariforms may be reniform, with a distal apicobasal sulcus, in Acratocnus , Megalocnus , and Parocnus .

The sample of mandibles securely associated with skull material is considerably smaller than skull remains, so that the lower teeth are less well represented than the uppers. Indeed, only two individuals, MPM-PV 3401 ( Fig. 6A, B ) and FMNH P13125 ( Fig. 6 C-F), possess reasonably well-preserved skulls and associated mandibular remains (and that for FMNH P13125 is not particularly complete). The m1 and m2 are of similar size and shape, resembling the upper molariforms in being transversely oval in section. The section of m3, though still oval, is nearly circular (MPM-PV 3401, Fig. 6A ) or squared (FMNH P13125, Fig. 6C, D ), and its long axis is oriented linguodistally to vestibulomesially rather than nearly transverse. As for the upper molariforms, differences may be noted in the lower molariforms of other megalonychids. For example, m1 is more nearly trapezoidal to oval in Neocnus and Ahytherium (based on alveolar morphology), reniform (with mesial apicobasal sulcus) in Parocnus , and triangular in Megalocnus , as is m 2 in the latter.

As noted already in this section, there is variation in size, shape, and section of any particular tooth. In addition to these general dental characteristic, there is also variation in the occlusal surface and features of the teeth. This is easily apparent in MPM-PV 3401, in which features such as size, shape, and height of cusps and crests differ between left and right side teeth ( Fig. 6A, B ; see also Bargo et al. 2009: fig. 2G). These differences do not appear to be related to preservation.

Postcranial Skeleton

See Figures 8-11 .

Humerus ( Fig. 8 A-C, G-I)

The humerus of Eucholoeops ingens is known from MPM-PV 3401, FMNH P13125 (measurements for these are given at the end of this section), and MACN-A 6414 (see Discussion). The humerus has a proximal, subcylindrical proximal portion that widens markedly into a flattened distal portion, which, as noted by De Iuliis et al. (2011), occurs in nearly all sloths except mylodontids.The head is well exposed between the tubercles in anterior view as in Acratocnus antillensis Matthew, 1931 ( Taboada et al. 2007), Ahytherium , Neocnus , Hapalops elongatus , Mionothropus , and Nothrotherium ; it is less evident in anterior view in Acratocnus odontrigonus Anthony, 1916 ( Anthony 1926) , Megalonyx , Parocnus , and Nothrotheriops , whereas it is more exposed in Megalocnus (see Anthony 1926, Cartelle et al. 2008, Matthew & Paula Couto 1959, and Taboada et al. 2007). The tubercles are widely separated in E. ingens , as in the sloths mentioned above, with the lesser tubercle larger than the greater tubercle, as in Hapalops ( Scott 1904: pl. 41). The tubercles are subequal in the megalonychids Acratocnus , Ahytherium , Megalocnus , Neocnus , and the nothrotheriids Mionothropus , Nothrotheriops ( Stock 1925: pl.8; McDonald 1985), and Nothrotherium ( Reinhardt1878; GDI pers.obs.).The greater tubercle projects farther proximally than the lesser tubercle in E. ingens , as occurs in Ahytherium , Megalonyx , Parocnus , and Nothrotherium , and as is usual in sloths neither tubercle projects more proxi-

bg he

mally than the head (one exception being Parocnus ). In Acratocnus antillensis , Mionothropus and Hapalops , the lesser tubercle projects slightly farther proximally than the greater tubercle, whereas in A. odontrigonus , Megalocnus , Neocnus ,and Nothrotheriops , the tubercles project to approximately the same level.The bicipital groove is broad and shallow in all taxa.

The deltopectoral shelf, as noted by De Iuliis (2003) and De Iuliis et al. (2011), is a raised, flattened, and distally tapered structure on the central third of the anterior diaphyseal surface of many sloths. The shelf is defined laterally by the deltoid ridge and medially by the pectoral ridge. The latter begins just distal to the medial margin of the greater tubercle.The deltoid ridge begins laterally on the diaphyseal surface, just distal to the greater tubercle.The ridges become more pronounced distally, converging to form a strongly raised triangular eminence.The deltopectoral shelf of Eucholoeops ingens (MPM-PV 3401; Fig. 8 A-C) is a well-developed and prominently raised structure, as described for Hapalops ( De Iuliis et al. 2011) , in which the ridges are strongly raised into crests and flared.The position of the shelf in E. ingens occupies just more than the lateral half of the diaphysis, approximately as in Hapalops elongatus (FMNH P13133 and YPM- VPPU 15160) and H. ruetimeyeri (AMNH 9250; Scott 1904: pls 41, figs 4 and 42, fig. 4, respectively, illustrated the YPM-VPPU and AMNH specimens) whereas in H. longiceps (YPM-VPPU 15523; Scott 1903: pl. 32, fig. 2) the shelf nearly covers the entire anterior diaphyseal surface. Among megalonychids, E. ingens resembles, in the form and extent of the shelf, Neocnus and Megalocnus (though in the latter the shelf is wider distally), whereas Acratocnus resembles more the condition in H. longiceps . Parocnus resembles E. ingens , but the shelf ends farther proximally. In Ahytherium the shelf is not as raised, owing to weaker ridges that do not project beyond the margins of the diaphysis, and in Megalonyx the shelf is less raised as well, owing to a weaker pectoral ridge.

The supinator ridge extends proximally almost vertically from the ectepicondyle, and forms a distinct angle to continue proximomedially. This resembles the condition in Acratocnus , Ahytherium , Megalonyx , and Neocnus , but the vertical portion is shorter in E. ingens and Ahytherium . It is notably elongated in Acratocnus and Neocnus and in Acratocnus major (which is a synonym of A. odontrigonus according to White & MacPhee 2001) it is especially prominent, forming a robust hook-like proximal extension ( Anthony 1926: pl. 49, fig. 2a). The ridge is somewhat less prominent and more sloped proximomedially in Megalocnus and Parocnus and a distinct angle is not present. The entepicondyle of E. ingens is drawn proximomedially into a robust process, resembling that of Megalonyx and Hapalops in forming a small proximal protuberance, as illustrated by Scott (1903: pl. 32, fig. 2); but caution is necessary, as in many cases the humeri illustrated by Scott, and indeed other skeletal elements, have at least some reconstruction, whereas his illustrations disguise this fact and give the false impression of being based on complete specimens.

Measurements of the humerus

MPM-PV 3401. Greatest length: 201 mm; anteroposterior/transverse diameter of humeral head: 30/ 29 mm; maximal width of deltopectoral shelf: 27 mm; diaphyseal thickness at deltopectoral shelf: 27 mm; distal width: 76 mm.

FMNH P13125. Greatest length: 191 mm.

Radius ( Fig. 6 D-F)

The radius of Eucholoeops ingens is known from MPM-PV 3451 ( Fig. 6 D-F) and FMNH P13125 (measurements given at the end of this section). The head is strongly inclined distomedially, approximately as in Acratocnus odontrigonus ( Anthony 1926: pl. 48, fig. 5), Hapalops longiceps ( Scott 1903: pl. 32, fig. 3), and H. ruetimeyeri ( Scott 1904: 42, fig. 6). The head is less steeply inclined in other megalonychids (e.g., Ahytherium , Megalocnus , Megalonyx , and Parocnus ). The neck is short and slightly constricted. The bicipital tuberosity is prominent; its position is approximately as in Neocnus and Acratocnus antillensis , whereas it is farther distal in A. odontrigonus (see Anthony 1926; Taboada et al. 2007). The tuberosity projects mainly posteriorly. Distal to the tuberosity the diaphysis extends gently distomedially to just past its midlength, and then continues more nearly distally. This produces a prominent bend in the diaphysis, so that the distal end is offset medially with respect to the proximal end (see De Iuliis et al. 2011), as occurs in many sloths (e.g., some megalonychids and megatheriines), although in other sloths the bend is very slight – e.g., in the megalonychids Megalonyx and Parocnus the proximal and distal ends are essentially aligned longitudinally. The position of the bend also varies. For example, it occurs in about the same position as in E. ingens in Ahytherium , Megalonyx , and Hapalops , but much farther proximally in Acratocnus and Megalocnus , and farther distally in Neocnus . The medial diaphyseal surface bears an angle at this position, which is associated with the prominent medial muscular scar, probably for the insertion of the m. pronator teres. In Eucholoeops ingens , as in Ahytherium , Megalonyx , and Neocnus , this angle is fairly gradual or rounded, but it is much more abrupt in Megalocnus and Hapalops . Apparently, this character varies in Acratocnus odontrigonus (compare Anthony 1926: fig. 72 and pl. 48, fig. 5a). Distally from this angle the lateral and medial margins extend nearly parallel to each other to the distal articular surface, as is typical among sloths. The distal articular surface is extensive and concave for articulation with the proximal carpals, and faces more distally whereas in other Santacrucian sloths, it faces more distolaterally. The diaphysis of E. ingens is also curved, bowing anteriorly. The two specimens differ in degree of curvature, with that in MPM-PV 3451 ( Fig. 6 D-F) being less prominent. However, it is not clear that this represents intraspecific variation, as the diaphysis of FMNH P13125 is more heavily damaged.

Measurements of the radius

MPM-PV 3401. Greatest length: 164 mm, anteroposterior/transverse diameter of radial head: 20/ 23 mm; minimal width at neck: 19 mm; width at pronator teres process: 25 mm; distal width: 37 mm.

FMNH P13125. Greatest length: 164 mm.

Manus ( Fig. 9 )

The manus of Eucholoeops ingens is incompletely known. The carpals are known mainly from

Mc III Mc IV

FMNH P13125 ( Fig. 9C, D ), with only the R scaphoid, trapezium, trapezoid, magnum, and unciform, and L lunar (this specimen may not belong to the same individual; a hand-written noted accompanying the specimen in the FMNH collections has the last digit of the specimen number, following “P1312”, blotted out and illegible; the note suggests it may belong to P13133, assigned to Hapalops elongatus ) being reasonably complete.These elements, however, have been incorporated into plaster reconstructions of the R and L manus and are unavailable for detailed study. All the metacarpals are known. MPM- PV 3401 preserves Mcs I-III, and FMNH P13125 preserves MC III-V. Phalanges are only preserved in MPM-PV 3401.

The lunar of Eucholoeops ingens resembles that typical for sloths, with a strongly convex proximal surface for articulation with the radius. Medially the lunar articulates with the scaphoid, which has a broad though similarly convex proximal articular surface. The scaphoid articulates distally with the trapezium and trapezoid and laterally with the magnum. The magnum articulates proximally with the lunar, medially with the scaphoid, as noted, and trapezoid, distally with Mc III, and laterally with the unciform. The magnum is wider distally than proximally, as occur in Megalonyx and nothrotheriids, whereas in Hapalops it is wider proximally (see De Iuliis et al. 2011). The unciform is deeper laterally than medially, articulating distolaterally with Mc V, distally with Mc IV, and medially with the magnum. It may also have articulated with MC III, as in Hapalops ( Scott 1903: pl. 33, fig.2) and Mionothropus ( De Iuliis et al. 2011: fig. 11), but the proximal end of Mc III is apparently incomplete in FMNH P13125 ( Fig. 9C, D ) and not well preserved in MPM-PV 3401 ( Fig. 9A, B ).

Mc I (MPM-PV 3401) is the smallest metacarpal, being only about half the length of the others. It articulates with the trapezium proximally, via a saddle-shaped facet, and Mc II laterally. The remaining metacarpals of Eucholoeops ingens are approximately equal in length to each other, with Mc III (as usual among sloths) being the most robust, and more robust in MPM-PV 3401 than in FMNH P13125 ( Fig. 9A, B, D ). Mcs II and IV are about equally robust, followed by Mc V. The pattern of metacarpal lengths in Eucholoeops is atypical among sloths, in which Mcs II and III are usually about equal in length, but roughly half the length of Mcs IV and V, as in Megalonyx , Hapalops , Mionothropus , Nothrotheriops , and Nothrotherium (see De Iuliis et al. 2011). In this regard, E. ingens resembles more the condition in Megalocnus , in which Mcs II-IV are subequal in length, with Mc V being slightly longer. Mcs I-IV have a keeled distal articular surface, whereas Mc V is blunt distally. The proximal and distal phalanges of digit 1, the middle phalanx of digit 2, and all three phalanges of digit 3 are known for E. ingens . The distal phalanges of digits I and III are unguals, the latter being much the larger. The morphology of the phalangeal elements does not differ from that reported for other fossil sloths.

Femur ( Fig. 10 A-C)

The femur of Eucholoeops ingens is known only from FMNH P13125 ( Fig. 10 A-C; greatest length = 212 mm; width at third trochanter approximately 46 mm). It is damaged, missing part of the head and parts of the diaphysis, but is sufficiently well preserved to provide a general description and some measurements. The femur is wide and flattened anteroposteriorly, as occurs generally in ground sloths. It is relatively elongated and gracile, with its proximal half slightly wider than and at a distinct angle to the distal half (not considering trochanters). In these regards it closely resembles the form in Acratocnus odontrigonus AMNH 17363a ( Matthew & Paula Couto 1959: pl. 41, fig. 2), although the femur assigned by Taboada et al. (2007: 51) to A. antillensis is nearly rectilinear, as is the femur, AMNH 17363b, assigned by Anthony (1926: pl. 51, fig. 3a, b) to A. odontrigonus . It would appear that either there is considerable variation in Acratocnus in the form of the femur or this genus is in need of revision. In any event, the femur of E. ingens is more strongly “bent” than in AMNH 17363a, and the form and relationships among the head and greater trochanter differ from those of that specimen.A bent femur also occurs in Megalocnus but the diaphysis is markedly wider and the greater and third trochanters are much more prominent in the latter. In other megalonychids, such as Ahytherium , Megalonyx , and Neocnus (and Acratocnus AMNH 17363b), the diaphysis is much more nearly rectilinear. That of Megalonyx differs from the others in being particularly wide and of nearly uniform width. That of Ahytherium , not as wide but also of nearly uniform width, is distinguishable by the great width across the epicondyles. The greater trochanter of Eucholoeops ingens , though missing its proximal tip, is approximately as in Megalonyx and Parocnus in relative size and degree of proximal projection, being well distal to the head. The trochanter extends farther proximally, nearly reaching the level of the head, in Megalocnus , in which it is particularly massive (note that the images of Megalocnus and Parocnus are apparently reversed in Taboada et al. 2007: 54). Variation is apparent in this character in Acratocnus odontrigonus (see Anthony 1926: pl. 51, figs 2, 3a) and Neocnus gliriformis (see Taboada et al. 2007: fig. 2.4), with the greater trochanter being distal to and nearly at the same level as the head. In Ahytherium the greater trochanter is distally displaced.The lesser trochanter is not preserved in Eucholoeops ingens ( Fig. 10A, B ). The third trochanter is prominent and is positioned at about midlength, approximately as in other megalonychids (except Parocnus , in which it appears to be absent). The prominent appearance of this trochanter in E. ingens compared to other megalonychids may be partly due to the marked bend of the diaphysis.

The distal articular surface of the femur bears three articular surfaces. The patellar trochlea, for articulation with the patella, is transversely broad, as in other megalonychids.Although the region between the patellar trochlea and the medial articular condyle has been partly reconstructed in FMNH P13125, the two surfaces appear to have been contiguous, connected by a smooth and narrow isthmus. The trochlea and lateral articular condyle are not contiguous, as is clear from Fig. 10C . The patellar trochlea is contiguous with the medial and lateral articular surfaces for the tibia in some megalonychids, such as Acratocnus , Megalocnus , and Parocnus . In Megalonyx the medial and lateral surfaces butt against the patellar trochlea, whereas in Neocnus , the lateral is separated from the medial surface, which butts against the trochlea. They are not contiguous in Ahytherium . The femur of Hapalops is relatively less elongated than that of Eucholoeops ingens , but like the latter is slightly wider proximally than distally.Its diaphysis differs in being nearly rectilinear and the greater trochanter projects more proximally, nearly reaching the level of the head, as described above for some megalonychids. The third trochanter appears more prominent in Hapalops and lies slightly more proximally than in E. ingens , and the distal articular facets are contiguous.

Tibia and fibula ( Fig. 10D, E ).

The R tibia (greatest length, measured along anterior surface = 156 mm; minimal diaphyseal width, measured just past midshaft = 18 mm) and fibula (greatest length = 146 mm; maximal diaphyseal width = 12 mm) are known only from FMNH P13125. The tibia is elongated and slender, resembling more that of Acratocnus and Neocnus rather than Megalocnus , Megalonyx , and Parocnus , whereas the fibula is relatively stout compared to those of other Santacrucian sloth such as Hapalops (e.g., Scott 1903:pl. 33, fig.3; 1904: pl. 41, fig. 6; pl. 42, fig. 8), the mylodontid Analcimorphus Ameghino, 1891 (YPM-VPPU15561; Scott 1904: pl. 49, fig. 4), and the megatheriid Prepotherium Ameghino, 1891 (YPM-VPPU 15568; Scott 1904: pl. 61, fig. 2; note that Scott labeled the figured specimen as YPM-VPPU 15368 but this is probably an error as the specimen is recorded as 15568on the catalogue card and YPM-VPPU online database, whereas the database describes 15368 as consisting of proterotheriid litoptern remains). The fibular diaphysis is of nearly constant diameter in Eucholoeops ingens , whereas it tends to be wider proximally ( Hapalops , Analcimorphus ) or distally( Prepotherium ). In this regard among megalonychids, it resembles more that of Megalocnus than Acratocnus , Parocnus , and Neocnus (see Anthony 1926; Matthew & Paula Couto 1959; Taboada et al. 2007).

Pes ( Fig. 11 )

Only a few elements of the pes of Eucholoeops ingens are known, all from FMNH P13125. These include Mt II-V ( Fig. 11 A-I), the cuboid ( Fig. 11 K-M), the ectocuneiform, and navicular ( Fig. 11 N-P). In addition, there are several other preserved bones that may be pes elements, but they cannot be articulated with the elements just listed or compared with elements of other ground sloths, given the dearth of published descriptions and illustrations of ground sloth pedal elements, and so we are unable to identify them unambiguously.The elements,being generally similar to those described and illustrated by Stock(1925) for Hapalops and Nothrotheriops , are described only briefly below.

The cuboid is a blocky element with three well-defined articular surfaces ( Fig. 11 K-M).The proximal surface bears two nearly flat facets that meet at an acute angle: a medial facet articulating with the astragalus and a lateral facet articulating with the calcaneum.Distally, the cuboid bears a transversely elongated, smooth and slightly saddle-shaped facet for Mts IV and V. The navicular is an ovate, bowl-like element ( Fig. 11 N- P). Its proximal surface bears a deep, concave facet, with a central prominence, for articulation with the astragalar head. Its distal surface bears articular facets for the ecto-, meso- and entocuneiform.

Mt II. Based on the preserved portions (the plantar projection is missing) of Mt II in Eucholoeops ingens , this element strongly resembles that of Hapalops in shape and proportions ( Fig. 11 A-C). In lateral view the proximal end of Mt II in Hapalops is dorsoplantarly expanded, compared to its shaft, with much of this expansion due to its plantarly projecting portion, whereas the dorsal portion projects little beyond the shaft. In Nothrotheriops the proximal end is also expanded but dorsal and plantar projections are about equal.

Mt III of E. ingens resembles that of Hapalops , but is relatively longer and with a slightly better developed shaft ( Fig. 11D, E ). This condition contrasts with that in Megalonyx ( McDonald 1977) and, particularly, Nothrotheriops ( Stock 1925) , in which Mt III is strongly compressed proximodistally so that it effectively has no shaft. The proximal surface bears the dorsoplantarly elongated and concave facet for the ectocuneiform, resembling more that of Nothrotheriops than that of Hapalops .

Mt IV. The proximal surface of Mt IV is dorsoplantarly elongated, more like that of Hapalops though not as wide plantarly, in contrast to the more quadrate shape in Nothrotheriops ( Fig. 11D, G, H ). It also resembles that of Hapalops in lacking a facet dorsomedially. A similar facet was described in Nothrotheriops as articulating with the ectocuneiform ( Stock 1925).

Mt V. Although Mt V is incomplete proximally, a flange was apparently present proximolaterally ( Fig. 11D, I, J ). The flange is particularly large in Megalonyx and Nothrotheriops , but less so in Hapalops . Based on the preserved portions in FMNH P13125, the flange in E. ingens may not have been as large as in Hapalops . As in Hapalops and Nothrotheriops , the shaft narrows considerably before widening again distally. The distal end resembles that of Hapalops but is more expanded. The distal articular surface bears a convex and dorsoplantarly elongated facet, similar to the elliptical convexity described by Scott (1903) for Hapalops longiceps , which contrasts with the keeled articular surfaces of Mts II-IV. The form of the surface in Mt V suggests the presence of at least one phalanx for digit 5, possibly nodular as described by Stock (1925) for Nothrotheriops . This author noted that Scott illustrated a complete digit 5 for Hapalops longiceps , but this occurs in the figure of the mounted reconstruction ( Scott 1903: pl.30). Scott (1903:203; 1904) did not illustrate any digit 5 phalanges in the plates of individual skeletal elements and noted only that all “the phalanges of the fifth digit have been lost,” implying that he lacked direct evidence for the reconstruction of a complete digit 5.

In terms of length, Mt IV is the longest of the series ( Fig. 11C, D ), exceeding slightly the length of Mt V, as occurs also in Megalonyx ( McDonald 1977) , Nothrotheriops , Paramylodon ( Stock 1925) , and Hapalops ( Scott 1903, 1904). Mt V of Megalocnus is longer than Mt IV, which is slightly longer than Mt III ( Matthew & Paula Couto 1959). In Eucholoeops ingens Mt III is slightly shorter than Mt V, and Mt II is slightly shorter than Mt III. In H. longiceps Mts II and III are subequal (length given as 3.8 cm by Scott 1903: 197 and as reflected in his pl. 33, fig. 4). The condition in H. elongatus is unclear: Scott’s illustrations (1904: pl. 41, figs 1, 2) indicate that Mt III is decidedly shorter than Mt II, but his reported measurements (1904: 231) indicate only a marginal difference of between 1 and 2 mm. In Megalonyx Mts II and III are subequal in length ( McDonald 1977). The condition in Nothrotheriops is distinctly different than in E. ingens and H. longiceps , as Mt III is notably shorter than Mt II ( Stock 1925). In Megalocnus Mt II is longer than Mts III and IV, and nearly as long as Mt V ( Matthew & Paula Couto [1959]). In Paramylodon Mt III is notably longer than Mt II ( Stock 1925).

Figures

FIG. 2. — Eucholoeops ingens Ameghino, 1887, specimen MPM-PV 3401 (neotype): A-C, skull in left lateral (A), ventral (B), and dorsal (C) views; D, E, premaxillae in ventral (D) and lateral (E) (anterior towards left) views. Scale bar: 5 cm.

FIG. 6. — Dentaries of Eucholoeops ingens Ameghino, 1887, MPM-PV 3401: A, B, neotype; C-F, FMNH P13125; A, C, D, occlusal views; B, E, F, lateral views. Abbreviations: anp, angular process; c1, lower caniniform tooth; condp, condyloid process; corp, coro- noid process; emef, external mental foramen; hrm, horizontal ramus; ifmc, internal foramen of the mandibular canal; m1-m3, lower molariform teeth; pc1d, pre-c1 depression; peomc, posterior external opening of the mandibular canal; spo, spout. Scale bar: 5 cm.

FIG. 8. — Forelimb long bone elements of Eucholoeops ingens Ameghino,1887:A-C, right humerus (MPM-PV 3401,neotype) in anterior (A), medial (B) and posterior (C) views (proximal towards top); D-F, left radius (MPM-PV 3451) in anterior (D), lateral (E) and posterior (F) views (proximal towards top); G-I, right humerus (FMNH P13125) in anterior (G), medial (H), and posterior (I) views (proximal towards top).Abbreviations:bg, bicipital groove;bt, bicipital tuberosity;ca, capitulum;das, distal articular surface;dps, del- topectoral shelf; enc, entepicondylar crest; enf, entepicondylar foramen; ecp, ectepicondyle (lateral epicondyle); enp, entepicondyle (medial epicondyle); gt, greater tubercle; he, head; lt, lesser tubercle; ne, neck; supr, supinator ridge; tr, trochlea. Scale bar: 5 cm.

FIG. 3.— Skull of Eucholoeops ingens Ameghino,1887 MPM-PV 3451 in left lateral (A, B), ventral (C), and dorsal (D) views (anterior towards left).Abbreviations:bf, buccinator fossa;bo, basioccipital;C1, caniniform tooth;eam, external auditory meatus;foo, foramen ovale;fl, flange of the premaxillar margin of the maxilla;fr, frontal;frpas, frontoparietal suture;la, lacrimal; laf, lacrimal foramen; M1-M4, upper molariform teeth;max, maxilla;maxfrs, maxillofrontal suture;mcr, median crest; na, nasal; nafrs, nasofrontal suture; nucr, nuchal crest;oc, occipital condyle;pa, parietal;pt, pterygoid;sacr, sagittal crest;so, supraoccipital;sq, squamosal;sqpas, squamosoparietal suture.Scale bar:5 cm.

FIG. 4. — Skulls of Eucholoeops ingens Ameghino, 1887, specimens: A, C, E, MPM-PV 3452; B, D, F, MPM-PV 15046. A, B, ventral views; C, D, left lateral views; E, F, dorsal views (anterior towards left). Abbreviation: fl, flange of the premaxillar margin of the maxilla. Scale bar: 5 cm.

FIG. 5. — Skulls of Eucholoeops ingens Ameghino, 1887, specimens: A, C, E, FMNH P13125; B, D, F, FMNH P13139. A, B, left lateral views; C, D, ventral views; E, F, dorsal views (anterior towards left). Scale bar: 5 cm.

FIG. 7. — Type specimens referred to Eucholoeops ingens Ameghino, 1887: A-C, MACN-A 4639, type of E. latirostris, anterior part of skull in lateral (A), dorsal (B) and ventral (C) views (A reversed from original; anterior towards left);D-F, MACN-A 6413,type of E. curtus, in lateral (D), dorsal (E) and ventral (F) views (anterior towards left); G-J, MACN-A 4640, type of E. externus, skull remains: G-H, ante- rior towards left; I, J, R dentary (anterior towards right) in dorsal (G), ventral (H), lateral (I) and occlusal (J) views. Scale bar: 5 cm.

FIG. 9.— Manus elements of Eucholoeops ingens Ameghino,1887: A, B, L manus elements of MPM-PV 3401 (neotype) in dorsal and medial views respectively (proximal towards top); C, D, partial R and L manus respectively of FMNH P13125 (proximal towards top). Abbreviations:lun, lunar;mag, magnum;Mc I-Mc V, metacarpals 1 to 5; P1-P3, first-second-third phalanx, followed by Roman numeral indicating digit number; sc, scaphoid; tm, trapezium; td, trapezoid; unc, unciform. Scale bar: 5 cm.

FIG. 10. — Hind limb long bones of Eucholoeops ingens Ameghino, 1887, FMNH P13125: A-C, R femur in anterior, posterior, and distal views respectively (proximal towards top in A, B; anterior towards top in C); D, E, R tibia and fibula in anterior and posterior views respectively (proximal towards top). Abbreviations: gt, greater trochanter; he, head; lac, lateral articular condyle; mac, medial articular condyle; pt, patellar trochlea; tt, third trochanter. Scale bar: 5 cm.

FIG. 11. — Pes elements of Eucholoeops ingens Ameghino, 1887, FMNH P13125: A-C, R Mt II in proximal, medial, and dorsal views respectively;D, R Mts III-V in dorsal view; E, F, R Mt III in proximal and lateral views respectively;G, H, R Mt IV in proximal and lateral views respectively; I, J, R Mt V in proximal and lateral views respectively; K-M, L cuboid in distal, proximal, and dorsal views respec- tively; N-P, R navicular in proximal, distal, and dorsal views respectively. Scale bar: 2 cm.