<p>Online Resource 1. Mesotheriid forelimb material used for the present study.</p><p>Mesotheriinae (monophyletic; see Billet et al. 2008)</p><p>Plesiotypotherium achirense Villarroel, 1974a: MNHN ACH 34, fragment of left scapula ventral border and fragmented clavicle, distal right humerus, distal left humerus, distal and proximal fragments of right ulna, proximal and distal fragments of right radius, right Mc II-V, phalanges; MNHN ACH 41, fragmented neck of right scapula; </p><p>MNHN-Bol-V 3443, fragmented neck of right scapula; MNHN-Bol-V 11690, fragmented neck of right scapula; MNHN-Bol-V 12659, scapula caudally and cranially broken; MNHN-Bol-V 8507, scapula dorsally broken; MNHN-Bol-V 12617, left scapula laterally broken and right complete scapula; MNHN-Bol-V 12687, right humerus, right ulna, right radius, right elbow sesamoid, right Mc II-III, right lunar, right scaphoid, right pyramidal, right trapezoid, right trapezium, and right magnum; MNHN-</p><p>Bol-V 12768, left humerus; MNHN-Bol-V 9880, distal fragment of left humerus; </p><p>MNHN ACH 46 distal fragment of right humerus, proximal fragment of ulna; MNHN-</p><p>Bol-V 11697, proximal fragment of left radius and distal fragment of right humerus; </p><p>MNHN-Bol-V 12733, proximal fragment of left radius, left lunar, and left scaphoid; </p><p>MNHN-Bol-V 3742, distal fragment of left radius; MNHN-Bol-V 3743, proximal fragment of left right humerus, distal fragment of right humerus, distal fragment of left humerus, right magnum, and proximal fragment of right McV; MNHN-Bol-V 12760, left radius; MNHN-Bol-V 12738, proximal fragment of radius; MNHN-Bol-V 12636, left scaphoid; MNHN-Bol-V 11684,left scaphoid; MNHN-Bol-V 12748, right lunar; </p><p>MNHN-Bol-V 12693, right lunar; MNHN-Bol-V 3336, left lunar, left pyramidal, left trapezoid, left magnum, and right Mc IV; MNHN-Bol-V 11688, right lunar; MNHN-</p><p>Bol-V 12734, right pyramidal and right unciform; MNHN-Bol-V 12744, left pyramidal and proximal fragment of left Mc IV; MNHN-Bol-V 12757, fragmented left pyramidal and right trapezium; MNHN-Bol-V 3740, right magnum and proximal fragment of Mc </p><p>IV; MNHN-Bol-V 12664, right magnum; MNHN-Bol-V 12990, right magnum; </p><p>MNHN-Bol-V 3432, right unciform; MNHN-Bol-V 3432, left unciform; MNHN-Bol-V</p><p>3444, proximal fragment of the right Mc I and proximal fragment of Mc IV; MNHN-</p><p>Bol-V 12984, right Mc V; MNHN-Bol-V 11694, left Mc II; MNHN-Bol-V 11678, proximal fragment of right Mc III; MNHN-Bol-V 3752, proximal fragment of left Mc </p><p>III; MNHN-Bol-V 3440, proximal fragment of right Mc IV; MNHN-Bol-V 3747, proximal fragment of left Mc V.</p><p>Plesiotypotherium casirense Cerdeño et al. 2012: MNHN-Bol-V 3724, left humerus, elbow sesamoid, left and right ulna, right radius, right and left undiforms, right and scaphoid, left lunar, right and left pyramidals, right magnum, left and right trapezium, Mc I-V, several phalanges.</p><p>Pseudotypotherium sp. (Ameghino 1887): MACN PV 7967, right humerus; </p><p>MACN 7962, left radius; MACN PV 7972, right ulna.</p><p>Caraguatypotherium munozi Flynn et al., 2005: SGO PV 22500, fragmented left humerus, left elbow sesamoid, left radius, left ulna, and left Mc II-IV.</p><p>Mesotherium cristatum Serres, 1867: MNHN PAM 2, clavicle, right scapula, right humerus, right ulna, right radius, left scaphoid, left trapezoid, left trapezium, left magnum, left unciform, left pisiform, Mc I-IV, and several phalanges; MACN PV 6871, right humerus; MACN PV 6875, right ulna; MACN PV 6884, right radius, MACN PV </p><p>6879, left scapula.</p><p>Trachytheriinae</p><p>Trachytherus alloxus Billet et al., 2008. The forelimb was studied through photographs of the forelimb UF 91933 (right and left humerus, left ulna, right radius, left Mc I-V, and several phalanges), left humerus MNHN SAL 7, left ulna MNHN </p><p>1738, right ulna MNHN 1740, and left radius MNHN 1743</p>