Megalocnus Leidy, 1868

R D E Macphee, Jennifer L White & Charles A Woods, 2000, New Megalonychid Sloths (Phyllophaga, Xenarthra) from the Quaternary of Hispaniola, American Museum Novitates 3303, pp. 1-32 : 6-7

Megalocnus Leidy, 1868

Classically, Megalocnus is distinguished from all other phyllophagans by its specialized maxillary incisiform teeth (Matthew and Paula Couto, 1959). However, it also exhibits highly distinctive postcranial elements (White, 1993a; see also White and MacPhee, in press), exhibiting a number of hitherto unrecognized or underappreciated skeletal apomorphies. Megalocnus is also the only Antillean sloth taxon other than Parocnus that can be justifiably called megafaunal. ( M. rodens was estimated by Paula Couto [1979] to have weighed ± 270 kg, but it would be useful to check this guess against results using cortical cross­sectional areas [Biknevicius et al., 1993] or other modern methods.) Megalocnus rodens , the first Antillean sloth to be identified as such (Leidy, 1868), is known from a large number of sites on the mainland of Cuba as well as Isla de Pinos (= I. de la Juventud) (Matthew and Paula Couto, 1959). A large number of species— and even subspecies—of Cuban Megalocnus have been named over the years; all are synonymized (as M. rodens ) by White and MacPhee (in press), obviating the need for separate comparisons here.

DIAGNOSIS OF GENUS: Differs from Parocnus (= Mesocnus ) 4 (the only valid genus

4 Taxa conventionally grouped under Mesocnus and Parocnus are considered congeneric by White and MacPhee (in press); Parocnus (Miller, 1929) has priority over Mesocnus (latter not validly named until Matthew’s [1931] publication).

with which it might be confused on the basis of size) in many details of cranial and postcranial construction, as follows: maxillary teeth pseudorodentiform or incisiform rather than caniniform (i.e., broad and anteroposteriorly compressed); femoral and humeral heads more convex; acetabular rim with large gap. In the case of the scapula (see fig. 2A–F), differences include: fossa for teres major is more developed; (2) post­ and prescapular fossae are unequal (prescapular fossa much larger); (3) rostral and caudal borders of scapular spine are divergent (not parallel); (4) second scapular spine is more prominent at inferior angle; (5) anterior scapular border is smoothly curved (border damaged in UF 169977, but shape discernible in other specimens); and (6) glenoid fossa is more concave. Differs from M. rodens from Cuba in that fossa for teres major is much more capacious and expands abruptly beneath secondary scapular spine. These illustrations should be compared to the right scapula of M. rodens (AMNH 49968) depicted by Matthew and Paula Couto (1959: pl. 13). Although much reconstructed, the caudal border of AMNH 49968 is intact and shows only a comparatively small fossa for teres major (cf. Parocnus ).

major much larger, expanded into a blade; pre­ and postscapular (i.e., supra­ and infraspinous) fossae unequal, with prescapular fossa being much the larger; rostral and caudal borders of scapular spine divergent (rath­ er than parallel); second scapular spine more prominent; anterior scapular border smoothly curved (rather than sinusoidal); glenoid fossa strongly anteroposteriorly concave.