See discussions, stats, and author profiles for this publication at: https://www.researchgate.net/publication/282505841 Premaxillae of the Extinct Megalonychid Sloths Acratocnus, Neocnus, and Megalonyx, and their Phylogenetic Implication.... Article in Journal of Mammalian Evolution · September 2015 DOI: 10.1007/s10914-015-9308-7 CITATIONS READS 2 169 4 authors, including: Lauren Lyon H. Gregory McDonald East Tennessee State University BLM - The Bureau of Land Management Salt … 8 PUBLICATIONS 2 CITATIONS 127 PUBLICATIONS 2,512 CITATIONS SEE PROFILE SEE PROFILE Timothy J. Gaudin University of Tennessee at Chattanooga 61 PUBLICATIONS 1,618 CITATIONS SEE PROFILE Some of the authors of this publication are also working on these related projects: Digital endocasts of extinct and extant sloths View project Palaeobiology and functional anatomy of fossil Carnivora View project All content following this page was uploaded by Timothy J. Gaudin on 27 November 2015. The user has requested enhancement of the downloaded file. Premaxillae of the Extinct Megalonychid Sloths Acratocnus, Neocnus, and Megalonyx, and their Phylogenetic Implications (Mammalia, Xenarthra) Lauren M. Lyon, Chelsea Powell, H. Gregory McDonald & Timothy J. Gaudin Journal of Mammalian Evolution ISSN 1064-7554 J Mammal Evol DOI 10.1007/s10914-015-9308-7 1 23 Your article is protected by copyright and all rights are held exclusively by Springer Science +Business Media New York. This e-offprint is for personal use only and shall not be self- archived in electronic repositories. If you wish to self-archive your article, please use the accepted manuscript version for posting on your own website. You may further deposit the accepted manuscript version in any repository, provided it is only made publicly available 12 months after official publication or later and provided acknowledgement is given to the original source of publication and a link is inserted to the published article on Springer's website. The link must be accompanied by the following text: "The final publication is available at link.springer.com”. 1 23 Author's personal copy J Mammal Evol DOI 10.1007/s10914-015-9308-7 ORIGINAL PAPER Premaxillae of the Extinct Megalonychid Sloths Acratocnus, Neocnus,andMegalonyx, and their Phylogenetic Implications (Mammalia, Xenarthra) Lauren M. Lyon1,2 & Chelsea Powell 1 & H. Gregory McDonald3 & Timothy J. Gaudin1 # Springer Science+Business Media New York 2015 Abstract In most folivorans, the premaxilla is loosely at- maxilla in two species of Megalonyx from North America, the tached to the maxilla, so that it is often missing in otherwise Blancan North American Land Mammal Age (NALMA) very well-preserved fossil skulls. Despite its infrequent pres- M. leptostomus and Rancholabrean NALMA M. jeffersonii. ervation in sloths, the premaxilla has been shown to have These species show a progressive reorientation of the premax- phylogenetically significant variation among the taxa that do illa within Megalonyx from a primitive horizontal element preserve the element. In the family Megalonychidae, the to a nearly vertical element, and some significant changes in premaxilla is known only in the early taxon Eucholoeops the anatomy of the incisive foramen. Morphological evidence (Santacrucian South American Land Mammal Age suggests that a broadened, plate-like premaxilla constitutes a [SALMA]), the extant two-toed sloth Choloepus,andthe synapomorphy for the entire clade Megalonychidae. North American Neogene taxon Megalonyx, the last described Furthermore, although Eucholoeops retains a short anterior only in an unpublished Master’s thesis. We report here process of the premaxilla like that of megatherioid sloths, this the discovery of the premaxilla in two genera of extinct process is lacking in the other megalonychids, suggesting that megalonychids, Neocnus and Acratocnus. These small bodied, the loss of this process may unite late Miocene to Recent semiarboreal megalonychid sloths are endemic to the islands megalonychids. of the Greater Antilles. Though the presence of sloths in the Caribbean dates at least to the late Oligocene, the best known Keywords Megalonychidae . Premaxilla . Neocnus . taxa derive from Pleistocene to Holocene cave deposits in Acratocnus . Megalonyx . Folivora Puerto Rico, Hispaniola, and Cuba. We also describe the pre- Introduction * Timothy J. Gaudin [email protected] The premaxilla is normally the anteriormost bone of the ven- Lauren M. Lyon tral rostrum in the mammalian skull. Among placentals it has [email protected] an extensive sutural attachment to the maxilla posteriorly, a H. Gregory McDonald dorsal attachment to the nasal and occasionally the frontal [email protected] bone, and often a small posterior contact with the vomer (Novacek 1993). It is typically tripartite, consisting of a body, a palatal process, and a dorsal process. The body, which is 1 Department of Biological & Environmental Sciences, University of Tennessee at Chattanooga, 615 McCallie Avenue, horizontal and lateral to the incisive foramen, forms the an- Chattanooga, TN 37403-2598, USA terolateral margin of the hard palate and houses the incisor 2 Department of Geosciences and Don Sunquist Center of Excellence teeth in most mammals, although these may be secondarily in Paleontology, East Tennessee State University, 325 Treasure Lane, lost as, for example, in some artiodactyls, sirenians, and in Johnson City, TN 37614, USA most xenarthrans (Ungar 2010). The palatal process is also 3 Museum Management Program, National Park Service, 1201 horizontal but lies medial to the incisive foramen. The dorsal Oakridge Drive, Fort Collins, CO 80525, USA process extends along the lateral margin of the external nares Author's personal copy J Mammal Evol (Evans and Christiansen 1979). In Folivora (=Tardigrada; investigate the systematic implications of premaxillary morphol- Fariña and Vizcaíno 2003), the incisors normally housed in ogy within Megalonychidae, and between Megalonychidae the body of the premaxilla have been lost, and the dorsal and other sloths. process is greatly reduced or absent (Engelmann 1985; Gaudin 2004). Thus, only the body and the palatal process remain, described herein as the lateral and medial rami of Material and Methods the premaxilla, respectively. This follows terminology long in use among sloth specialists, who refer to these portions of The present study was occasioned by the discovery of two iso- the bone as the medial and lateral Barms^ or Bbranches^ (see, lated premaxillary bones in museum collection drawers. Both e.g., Scott 1903–4; Stock 1925) of the premaxilla, formalized represent the first instance in which the bone has been recovered as the medial and lateral rami by De Iuliis (1994). There is for the relevant taxa, despite the fact that both taxa have been often an additional anterior process in sloths, defined as a thoroughly described in the literature, and are known from nu- distinct projection extending forward from the junction of merous well-preserved skulls (e.g., see Anthony 1926;Paula the lateral and medial rami (following De Iuliis 1994;De Couto 1967;MacPheeetal.2000; Gaudin 2011). The first pre- Iuliis et al. 2011). The xenarthran premaxilla typically forms maxilla was found among the extensive collection of sloth re- most, if not all, of the margin of the incisive foramen, which mains from Holocene cave deposits in Haiti housed at the Florida serves as the opening into the oral cavity for the nasopalatine Museum of Natural History, and derives from the small bodied duct (Wible and Gaudin 2004). megalonychid sloth, Neocnus comes (UF 248501, right side In most folivoran species, the premaxilla is not sutured to the only; Fig. 2). The second was found at the American Museum maxilla. This means that the premaxilla is rarely found articu- of Natural History, and pertains to the mid-sized late Pleistocene/ lated with fossil skulls (indeed, the premaxilla is frequently Holocene megalonychid sloth from Puerto Rico, Acratocnus missing in museum specimens of the extant three-toed sloth odontrigonus (AMNH 17716, left side only; Fig. 3). Figures 2 Bradypus), and its morphology is poorly known. There are and 3 show the bones in dorsal and ventral view, and include exceptions, e.g., the Mio-Pliocene nothrothere, Thalassocnus reconstructions showing the positions the bones would occupy (McDonald and Muizon 2002). In the megathere in the intact skull. One of us (HGM) had also noted the preser- Megatherium and the scelidothere, Scelidotherium, the lateral vation of the premaxilla in specimens of the late Pliocene ramus is often fused to the skull and the premaxilla is (Blancan NALMA) North American species Megalonyx commonly preserved, though curiously this is not the case in leptostomus (UF 216900, right side only; Fig. 4) from the their respective close Neotropical relatives, Eremotherium and Florida Museum of Natural History, and the late Pleistocene Catonyx. Despite its infrequent preservation, De Iuliis (1994) (Rancholabrean NALMA) North American species Megalonyx has shown that the bone exhibits significant variation among jeffersonii from the Florida Museum of Natural History living and fossil sloths that can be systematically informative. (UF 213841), the Idaho Museum of Natural History (IMNH In the present study, descriptions of the morphology of the 23034), and the Denver Museum of Nature and Science premaxilla are provided in six species of megalonychid sloths: (DMNS EVP. 65000). Descriptions of M. jeffersonii are based Choloepus