Megatherium gallardoi C. Ameghino & Kraglievich, 1921

Brandoni, Diego, Soibelzon, Esteban & Scarano, Alejo, 2008, On Megatherium gallardoi (Mammalia, Xenarthra, Megatheriidae) and the Megatheriinae from the Ensenadan (lower to middle Pleistocene) of the Pampean region, Argentina, Geodiversitas 30 (4), pp. 793-804 : 797-800

publication ID	https://doi.org/ 10.5281/zenodo.4651208
persistent identifier	https://treatment.plazi.org/id/240F87CC-FFA4-FFBB-FEFB-FB8ADFAEFA5D
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scientific name	Megatherium gallardoi C. Ameghino & Kraglievich, 1921
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HOLOTYPE. — MACN Pv 5002, partial skull (left zygomatic arch and molariforms missing), complete mandible lacking molariforms 1 and 4, right humerus, left ulna and left femur ( Figs 2 View FIG ; 3 View FIG ; Tables 1; 2).

TYPE LOCALITY. — Excavations for water filters made as part of the building work of the public water supply system (34°33’23.53’’S, 58°25’46.66’’W, Buenos Aires, Argentina, Ameghino & Kraglievich 1921) ( Fig. 1 View FIG ).

TYPE LEVEL. — Ensenadan Stage (lower to middle Pleistocene).

DESCRIPTION

Skull ( Fig. 2 View FIG ; Table 1)

As mentioned above, the skull and mandible of Megatherium gallardoi have been thoroughly described by Ameghino & Kraglievich (1921). Among the features of the specimen MACN Pv 5002,

Ameghino & Kraglievich (1921) mentioned that the skull roof profile does not show an abrupt transition between the frontal and nasal regions ( Fig. 2A View FIG ), as observed in several specimens of M. americanum ; that the temporal fossa is located entirely below the lambdo-nasal plane, whereas in most specimens of M. americanum the dorsal portion of the temporal fossa is located above this plane; and, that the postorbital processes of the frontal bone are reduced ( Fig. 2A View FIG ; Ameghino & Kraglievich 1921: fig. 2).

In addition to the characters described by Ameghino & Kraglievich (1921), the premaxillae in Megatherium gallardoi are weakly united to the maxillae ( Fig. 2B View FIG ), without strong fusion between these bones that occurs in most specimens of M. americanum and M. altiplanicum . In M. celendinense (see Pujos 2006: fig. 3.6), M. tarijense and M. sundti the condition is similar to that of M. gallardoi . The premaxillae of M. gallardoi are not fused to each other as in the abovementioned

species ( Fig. 2B View FIG ).

Brandoni D. et al.

TABLE 2. — Measurements of Megatherium gallardoi C. Ameghino & Kraglievich, 1921 (MACN Pv 5002) and specimens referred to

M. americanum Cuvier, 1796 . All measurements are in mm, except Humerus Robustness Index (HRI). Abbreviation: p, preserved.

Mandible ( Fig. 2C, D View FIG ; Table 1)

The mandibles of Megatherium gallardoi and M. americanum appear similar in size and general morphology. However, they differ in the shape of the ventral bulge and in the ventral margins of the mandibular body, both anteriorly and posteriorly to the ventral bulge, as well as in the angle of inclination of the spout.

In this clade, the degree of hypsodonty may be evaluated through comparisons of molariform crown height, or by the height of the horizontal mandibular ramus relative to tooth row length (maximum height of mandibular ramus/tooth row length × 100, see De Iuliis 1996; Bargo et al. 2006a; among others).According to this method, the values of Hypsodonty Index (HI) ( Zetti 1964; De Iuliis 1996; Saint-André & De Iuliis 2001; Bargo 2001; Bargo et al. 2006a) obtained for MACN Pv 5002 are similar to those observed in several species of tertiary megatheriines (e.g., Pyramiodontherium Rovereto, 1914 , Megatheriops C. Ameghino & Kraglevich, 1921 , see Brandoni 2006). The value of HI is 87 in MACN Pv 5002, 101 in Megatherium altiplanicum (see Bargo et al. 2006a), 92-112 in M. americanum (see Saint-André & De Iuliis 2001), and 107 in Anisodontherium halmyronomum (Cabrera, 1928) (see Brandoni & De Iuliis 2007). However, as discussed by De Iuliis et al. (2004) and Brandoni & De Iuliis (2007), the HI value of A. halmyronomum

is not directly comparable with others due to the particular shape of the molariforms in this species (i.e. molariforms mesiodistally compressed and thus rectangular rather than square in cross section). The molariforms of MACN Pv 5002 are nearly rectangular in section ( Fig. 2D View FIG ; Table 1), though not as rectangular as in A. halmyronomum or Megathericulus patagonicus .

Humerus ( Fig. 3A View FIG )

The humerus of Megatherium gallardoi is mostly similar to that of other megatheriines, as well as those of Megalonychidae and Prepotheriinae, in that it is relatively elongated and gracile. An entepicondylar foramen is absent as in other megatheriines. According to the general condition of sloths, the posterior surface is flattened and bears a shallow olecranon fossa, and the distal third of the diaphysis is markedly expanded mediolaterally and compressed anteroposteriorly. The humerus of MACN Pv 5002(795 mm long) is longer than that of most specimens of M. americanum (Table 2; De Iuliis 1996: appendix 4C), and similar in length to several humeri of Eremotherium laurillardi (Lund, 1842) (730-878 mm; De Iuliis 1996: appendix 4C); whereas the humerus of M. celendinense (see Pujos 2006: fig. 4.F, G) is relatively shorter and more massive.

As in other Pleistocene megatheriines ( Megatherium americanum , M. sundti , Eremotherium

laurillardi), in MACN Pv 5002, the deltopectoral

Megatheriinae (Mammalia) from the Ensenadan of Argentina

crest is reduced to an elongated triangular ridge; this structure is even more reduced in M. celendinense and M. tarijense (see Pujos 2006). Among pre-Pleistocene Megatheriinae such as Pyramiodontherium scillatoyanei De Iuliis, Ré & Vizcaíno, 2004 (see De Iuliis et al. 2004: fig. 5.A, B), Megathericulus patagonicus and Megatheriops rectidens (Rovereto, 1914) (see De Iuliis 2003: figs 3 and 4.A respectively), the deltopectoral crest is strongly raised distally and deflected laterally (see De Iuliis 2003

for a discussion of the muscular and functional implications related to size and form of the deltopectoral crest in megatheriines). The presence of a well-developed deltopectoral crest is considered to be plesiomorphic ( De Iuliis 1996; Brandoni 2006; Pujos 2006).

As in other Megatherium species and in E. laurillardi , the humerus of M. gallardoi is markedly robust; whereas it is relatively slender in the majority of the pre-Pleistocene species (except Eremotherium eomigrans De Iuliis & Cartelle, 1999 ). An

index to evaluate humeral robustness, the Humerus

Brandoni D. et al.

RobustnessIndex (HRI), is calculated as: humeral distal transversal width/humeral total length × 100 ( Brandoni 2006). In those megatheriines with a robust humerus ( M. americanum , E. laurillardi ), HRI values are over 40; whereas in others with a slender humerus (e.g., Pyramiodontherium , Megatheriops ) HRI is under 40. The value of HRI is 46 for MACN Pv 5002, 42 for M. tarijense , 45-50 for M. americanum , and 42-46.5 for E. laurillardi (see Brandoni 2006: table 3).

Ulna ( Fig. 3B View FIG )

The ulna of MACN Pv 5002 (694 mm long) is longer than that of other Megatherium species (Table 2) except MACN Pv 10148, in which the ulna is almost the same length (see De Iuliis 1996). However, the general morphology of this bone resembles that of M. americanum . As in the latter, the diaphysis is mediolaterally compressed and tapered distally. In Urumaquia robusta and FMNH P14511, the ulna is very gracile and proportionally much longer than in any other Megatheriinae (e.g., Megatheriops , Megatherium and Eremotherium Spillmann, 1948 ) (see Carlini et al. 2008).

Femur ( Fig. 3C, D View FIG )

The femur of MACN Pv 5002 is morphologically similar to that of M. americanum , but larger than many specimens of the latter species (Table 2; De Iuliis 1996: appendix 4F). It is 764 mm in total lateral length and 315 mm in minimum transverse diameter at the middle of the diaphysis, and is expanded transversally at both its proximal and distal portions (455 and 458 mm respectively).

The femoral head and neck are prominent, as in E. laurillardi and M. americanum (see De Iuliis & Saint-André 1997: figs 4.1 and 4.2 respectively); whereas in M. altiplanicum (Saint-André & De Iullis 2001: fig. 7.A) the neck is more constricted, but not as much as in E. sefvei (De Iuliis & Saint- André 1997: fig. 5.1, 2). As in Eremotherium and other species of Megatherium , the greater trochanter is markedly deflected backward from the plane that passes through the distal condyles and head, so that the femur shows vertical torsion. In MACN Pv 5002, vertical torsion of the diaphysis is appro-

ximately 30°; whereas it ranges between 31 and 57° in M. americanum and between 10 and 41° in Eremotherium laurillardi ( De Iuliis 1996; De Iuliis et al. 2004). Its lateral margin is sigmoidal in lateral view, a feature also present in M. americanum (De Iuliis & Saint-André 1997: fig 6.3) and M. altiplanicum (Saint-André & De Iuliis 2001: fig. 7.B).

The lateral and medial femoral margins are notably concave ( Fig. 3C View FIG ). This feature occurs in M. americanum (De Iuliis & Saint-André 1997: fig. 4.2), M. altiplanicum (Saint-André & De Iuliis 2001: fig. 7.A), M. sundti ( De Iuliis 2006: fig. 2.C) and in the species of Pyramiodontherium (see Carlini et al. 2002; De Iuliis et al. 2004; Brandoni 2006); whereas in M. medinae ( Casamiquela & Sepulveda 1974: pl. VI), M. urbinai ( Pujos & Salas 2004: fig. 2.I, J), Eremotherium laurillardi (De Iuliis & Saint-André 1997: fig. 4.1) and E. sefvei (De Iuliis & Saint-André 1997: fig. 5.1, 2), the proximal and distal portions of the femur are less expanded transversally and the margins are nearly rectilinear and parallel.

As in most megatheriines, the patellar trochlea of M. gallardoi is separated from the internal condyle and continuous with the external condyle, forming a single articular surface ( Fig. 3D View FIG ). Whereas the patellar trochlea contacts both condyles in Megathericulus patagonicus F. Ameghino, 1904 (MLP 92-XI-15-2, see De Iuliis et al. 2008) and Megathericulus primaevus Cabrera, 1939 (MLP 39-VI-24-1). As in Megatherium americanum and M. altiplanicum (Saint-André & De Iuliis 2001: fig. 7.C), the patellar trochlea of the femur is extremely reduced as an anteromedial extension of the lateral condyle ( Fig. 3D View FIG ). Whereas in Megatherium sundti , M. tarijense , M. medinae , and M. urbinai the patellar trochlea is well developed; and is also well developed in E. laurillardi and E. sefvei (De Iuliis & Saint-André 1997: figs 1a and 2 respectively).