Neocnus toupiti, R D E Macphee & Jennifer L White & Charles A Woods, 2000

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 : 15-20

Neocnus toupiti , new species

HOLOTYPE: This species is founded on a partial associated skeleton collected at the type locality on 3 June 1984 by D. Cordier and party. In addition to the skull ( UF 156892), chosen for illustration here (fig. 9), the Jérémie #5 holotypic skeleton includes the following separately numbered elements (fig. 11): right humerus, right and left radii and ulnae (all numbered UF 170073); right and left femora ( UF 76235 , 76236 ); right and left tibia, and right calcaneus (all numbered UF 170281); and left scapula ( UF 170072).

TYPE LOCALITY AND AGE: Trouing Jérémie #5, Plain Formon, Département du Sud, Hai­ ti; late Quaternary (see appendix 1 for additional details).

SYNONYMS: None.

REFERRED SPECIMENS: Hypodigm also includes a referred skull (UF 156894) from Trou Wòch Dadier, Étang de Miragôane; mandible (UF 171296; fig. 10) from Trou Ni­ colas, Morne la Visite; scapula (UF 170005) from Trujin Bridge, Morne la Visite; astragalus (UF 170286) from Trouing Jérémie #5; calcaneus (UF 170443) from Trouing Jérémie #3.

ETYMOLOGY: Haitian Creole adjective meaning ‘‘tiny,’’ pronounced approximately [too­pitee]; reference is to the very small body size of this species. Recommended common name: Least neocnus.

DISTRIBUTION: Haiti and Dominican Republic.

DIAGNOSIS OF NEW SPECIES: With respect to known elements, agrees with features that define the genus. Differs from other Neocnus species in having very small, extremely gracile skeleton; upper caniniform with very deep lingual groove; symphyseal spout long, narrow, and untapered (partly broken on holotype); femur lacking third trochanter; fem­ oral shaft cylindrical with reduced anterior prong; femoral head tiny; distal tibial articular surface very narrow and divided only at anterior edge; astragalus tiny with relatively long neck; ectal and sustentacular facets very close together; calcaneal tuberosity triangular and L­shaped (rather than J­shaped); ectal facet distinctly humped; most anterior aspect of glenoid fossa of scapula pointed; supracondylar ridge reduced; pronator quadratus flange gentle and reduced; ulnar shaft extremely laterally compressed; sigmoid notch of ulna unsegmented and shallow; proximal fibular facet of tibia round, reduced, and posteriorly oriented .

DISCUSSION: Neocnus toupiti is the smallest known Antillean sloth, being significantly smaller in linear dimensions and long­bone cortical cross­sectional thickness than either of the extant tree sloths, Bradypus and Choloepus (3–4 kg body mass; Silva and Downing, 1995). Neocnus comes , not otherwise discussed here, is larger and differs in a number of cranial and postcranial features; its inclusion within Neocnus is discussed elsewhere (see fn. 6; White and MacPhee, in press).

Species of Hispaniolan Neocnus can be distinguished from each other and from Cuban Neocnus by unique discrete traits as well as morphometric analyses (White, 1993a; White and MacPhee, in prep.). Whereas some discrete features are found in more than one Neocnus species , the sum total of all differentiae for each species serves to distinguish them from one another. Table 1 presents a sample of morphometric measurements that distinguish the two new species of Neocnus diagnosed here. The chosen measurements are all taken on elements that are also distinguished by discrete traits, to demonstrate that size is not the primary criterion for taxonomic assignment. Coefficients of variation (table 2) computed for some representative measurements also support the occurrence of two separate species; in most cases, coefficients of variation for each species alone fall within the normal range of mammalian variation, whereas coefficients of variation for the combined samples are higher than one would expect from a single species (Simpson et al., 1960).

Further evidence for the distinctiveness of Neocnus species comes from articulating elements (fig. 11). For example, N. toupiti humeral heads are all too small for N. dousman glenoid fossae, and all N. toupiti glen­ oid fossae are too small for N. dousman humeral heads. The collection also revealed some immature bones of larger Neocnus species that are the same size as, or larger than, adult examples of smaller Neocnus species. For example, UF 76292 is an immature femur that lacks epiphyses yet exhibits a third trochanter. On size alone it evidently belonged to either N. dousman or N. comes , yet it is the same size as fully adult femora of N. toupiti .