The oldest Megalonychidae (Xenarthra: Tardigrada); phylogenetic relationships and an emended diagnosis of the family

CARLINI A.A. Y SCILLATO-YANÉ G.J.

NEUES JAHRBUCH FUR GEOLOGIE UND PALAONTOLOGIE-ABHANDLUNGEN

E SCHWEIZERBARTSCHE VERLAGS

Lugar: Stuttgart; Año: 2004 vol. 233 p. 423 - 443

0077-7749

THE OLDEST MEGALONYCHIDAE (XENARTHRA: TARDIGRADA); PHYLOGENETIC RELATIONSHIPS AND AN EMENDED DIAGNOSIS OF THE FAMILY ALFREDO A. CARLINI AND GUSTAVO J. SCILLATO-YANÉ Dto. Co. Paleontología Vertebrados, Museo de La Plata, Paseo del Bosque s/n, 1900 La Plata, Argentina. CONICET. e-mail: acarlini@museo.fcnym.unlp.edu.ar ABSTRACT The origins and sister groups of most basal Xenarthran lineages are either controversial or still unknown. The absence of most major groups of Xenarthra in South American mammal-bearing sediments of Late Cretaceous and Early Paleocene age is the greatest enigma in the study of this Superorder. The aim of this paper is to describe a new genus and species of Megalonychidae, Deseadognathus riggsi, from the type locality of the Deseadan Land- Mammal Age, Oligocene of Patagonia,Argentina, the oldest megalonychid thus far known. This new gen. et sp. is caracterized by a robust caniniform with subtriangular cross section emplaced in an oblique and lateral position outside the axis of the molariform series, subtrapezoidal first molariform (mf1) and subrectangular mf2 cross-sections. In addition, this new taxon is similar in size to Eucholoeops fronto but smaller than E. externus and E. curtus, and much smaller than E. ingens, and is larger than living species of Bradypus and Choloepus. Using 17 characters of the mandible (osteological and dental), a phylogenetic analysis of 14 genera of megatherioids yielded two equally parsimonious, and highly congruent trees of 40 steps (CI of 0.514 and a RI of 0.684). The polarity of several characters is supported by ontogeny, i.e. progressively increasing divergence of dental series, increase in size of caniniforms, it sub-circular to sub-triangular section, and shortening of the rostrum. The results of our phylogenetic analysis confirm the monophyletic status of the Megalonychidae with an enlarged membership, and we provide an emended diagnosis for this family level group. Considering that there is almost no pre-Deseadan record of these Xenarthrans, Deseadan tardigrades had attained remarkable diversity. Available evidence indicates a South American origin for the Megalonychidae, followed by later radiations at different times to the West Indies and North America.