Palaeontologia Electronica palaeo-electronica.org Dimorphism in Quaternary Scelidotheriinae (Mammalia, Xenarthra, Phyllophaga) Ángel R. Miño-Boilini and Alfredo E. Zurita ABSTRACT The contributions concerning possible cases of sexual dimorphisms in fossil and living sloths are scarce. Until now, studies in fossil ground sloth sexual dimorphism have been limited to the subfamilies Megatheriinae (Eremotherium) and Mylodontinae (Paramylodon) from the Pliocene and Pleistocene of South America and North Amer- ica. Scelidotheriinae constitutes an endemic lineage of ground sloths from South American, with a biochron age ranging the lapse “Friasian”-Lujanian SALMAs (middle Miocene-early Holocene). An integral phylogenetic and taxonomic revision of the Qua- ternary Scelidotheriinae shows that it is possible to recognize three genera and six species: Scelidotherium Owen (Scelidotherium leptocephalum and S. bravardi), Val- gipes Gervais (Valgipes bucklandi), and Catonyx Ameghino (Catonyx cuvieri, C. tari- jensis, and C. chiliensis). One of the most noticeable aspects in some specimens analyzed (n= 47) was the presence of two morphtypes in each species at the level of the dorsal crests of the skull (parasagittal crests and sagittal crest) and at the level of the distal-most region of the mandible (only in C. tarijensis). In all but two species (S. leptocephalum and S. bravardi) the two types involve the absence and presence of a sagittal crest. We suggest that specimens with sagittal crest are males, and specimens lacking sagittal crest are females. This represents the third reported ground sloth clade with evidence of sexual dimorphism of the skull and mandible. Ángel R. Miño-Boilini. Centro de Ecología Aplicada del Litoral (CECOAL-CONICET) y Facultad de Ciencias Exactas y Naturales y Agrimensura, Universidad Nacional del Nordeste. Ruta 5, km 2,5 (CP 3400, CC 128) Corrientes, Argentina [email protected] Alfredo E. Zurita. Centro de Ecología Aplicada del Litoral (CECOAL-CONICET) y Facultad de Ciencias Exactas y Naturales y Agrimensura, Universidad Nacional del Nordeste. Ruta 5, km 2,5 (CP 3400, CC 128) Corrientes, Argentina [email protected] Keywords: Ground sloths; Mylodontidae; South America; variability; skull; mandible PE Article Number: 18.1.12A Copyright: Society for Vertebrate Paleontology March 2015 Submission: 30 September 2013. Acceptance: 26 February 2015 Miño-Boilini, Ángel R. and Zurita, Alfredo E. 2015. Dimorphism in Quaternary Scelidotheriinae (Mammalia, Xenarthra, Phyllophaga). Palaeontologia Electronica 18.1.12A: 1-16. palaeo-electronica.org/content/2015/1102-dimorphism-in-scelidotheriinae MIÑO-BOILINI & ZURITA: DIMORPHISM IN SCELIDOTHERIINAE INTRODUCTION six species are to be considered valid (Miño-Boilini, 2012; Corona et al., 2013). Sexual dimorphism (i.e., differences in form The first studies on sexual dimorphism of between males and females belonging to the same some fossil ground sloths (e.g., Megatheriinae: species) is a widespread attribute in extant mam- Eremotherium laurillardi and Mylodontinae: Param- mals (Ralls, 1977; Smith and Fisher, 2013), mainly ylodon harlani) were carried out by Cartelle and represented by differences in body size (Isaac, Bohórquez (1982) and McDonald (2006). These 2005), but also including coloration, horns, and authors were able to recognize two morph types on antlers (McDonald, 2006). In most species, males the basis of crania, and concluded that they repre- are larger than females. However, the existence of sent males and females. taxa in which females are larger has also been Among the valid taxa of Scelidotheriinae, one reported (e.g., Ralls, 1977; Lara-Ruiz and Chi- of their most interesting characteristics are the arello, 2005). morphological variations observable at the level of Until now, most of the contributions on sexual the dorsal crests (parasagittal crests and sagittal dimorphism in extant mammals were limited to crest of the skull) and of the most distal region of northern hemisphere taxa, whereas studies in trop- the mandible. The goal of this contribution is to ical regions and the southern hemisphere were describe the two morph types present at skull level scarce (Isaac, 2005). Sexual dimorphism has in each species. The possibility that this morphol- recently been postulated for several extinct taxa, ogy represents sexual dimorphism is discussed. especially dinosaurs and mammals (Barden and Maidment, 2011; Smith and Fisher, 2013). This MATERIAL AND METHODS involves differences in body size and osteological characters (McDonald, 2006). McDonald (2006) We studied skulls and mandibles of speci- and Czerwonogara and Fariña (2013) have mens of Quaternary Scelidotheriinae deposited in recently suggested that the recognition of sexual the collections of various institutions in Argentina dimorphism in extinct taxa could allow for the infer- and abroad (see institutional abbreviations). The ence of some aspects of the ethological and social following genera are recognized for the Quaternary structure of populations under study. Sexual dimor- period: Scelidotherium Owen, 1839, Valgipes Ger- phism also has taxonomic implications, because vais, 1873, and Catonyx Ameghino, 1891 (see several species or subspecies may be actually Pujos, 2000; McDonald and Perea, 2002; Cartelle based on males and females (see Kurtén, 1969; et al., 2009; Miño-Boilini, 2012; Corona et al., Frailey, 1986; McDonald, 2006). 2013). Among mammals, the orders Artiodactyla, Most of the skulls and mandibles of the speci- Carnivora, and Primates constitute the paradig- mens correspond to adult individuals, are in good matic groups concerning studies on sexual dimor- state of preservation and fairly complete. A total of phism (including morphological and ethological 47 specimens were studied (n = 10 Scelidotherium differences), and thus have been used as analo- leptocephalum; n = 6 S. bravardi; n = 3 Valgipes gous models for other groups of extinct mammals. bucklandi; n = 19 Catonyx tarijensis; n = 4 C. chil- However, as mentioned by McDonald (2006), one iensis; n = 5 C. cuvieri). The measures were taken key question is whether these groups can be used with a digital caliper, with a range of error of 0.5 to interpret possible sexual dimorphism in other mm (Figure 1). The sagittal crest is defined as an clades such as ground sloths. evident dorsal hump on the sagittal line of the skull The order Xenarthra is a particular group of formed by the junction of sagittal crests and with a placental mammals, characteristic of the Neotropi- maximum transverse diameter of 25 mm (Figures cal region, and restricted to the American continent 2-10). (Rose and Gaudin, 2010). This group includes Differences in skull measurements between three large clades: Cingulata (armadillos), Vermi- the two morphotypes (Scelidotherium leptocepha- lingua (anteaters), and Tardigrada [=Phyllophaga lum and Catonyx tarijensis) were assessed by Kru- or Folivora (ground sloths)]. Vermilingua and Tardi- skal-Wallis test. This analysis was not performed in grada constitute the clade Pilosa (Delsuc et al., the remaining species because of the low number 2002; Gaudin and McDonald, 2008). Scelidotheri- of available specimens. Differences were consid- inae (middle Miocene-early Holocene) constitute ered to be statistically significant at p <0.05. Statis- one of the most diverse clades of Tardigrada, tical analyses were performed using Infostat restricted to South America. According to the last Software (Di Rienzo et al., 2012). taxonomic revision, three Quaternary genera and 2 PALAEO-ELECTRONICA.ORG FIGURE 1. Measurements used for comparisons. Institutional abbreviations. BM(NH)M: British cago, USA; MACN: Sección Paleontología Verteb- Natural History Museum, London, England; CTES- rados, Museo Argentino de Ciencias Naturales PZ: Paleozoología Corrientes, Facultad de Cien- “Bernardino Rivadavia”, Buenos Aires, Argentina; cias Exactas y Naturales y Agrimensura, Universi- MCL: Museu de Ciências Naturais da Pontifícia dad Nacional del Nordeste, Corrientes, Argentina; Universidade Católica de Minas Gerais, Belo Hori- FMNH P: Field Museum of Natural History, Chi- zonte, Brazil; MD: Museo Municipal “Carlos Dar- 3 MIÑO-BOILINI & ZURITA: DIMORPHISM IN SCELIDOTHERIINAE FIGURE 2. Scelidotherium leptocephalum. Skulls in dorsal view. 1, MMP 1155-M; 2, MACN 9625. Scale bar equals 10 mm. win”, Punta Alta, Buenos Aires, Argentina; MFCA: Museum Universitat Copenhagen, Copenhagen, Museo Universitario “Florentino y Carlos Amegh- Denmark. ino”, Universidad Nacional de Rosario, Santa Fe, Anatomical abbreviations. Mf/mf: upper/lower Argentina; MLP: División Paleontología Vertebra- molariform tooth. dos, Facultad de Ciencias Naturales y Museo, Uni- Measurement abbreviations. WB: width bicondy- versidad Nacional de La Plata, Buenos Aires, lar; LMC: length maxillar-condylar; LDS: length of Argentina; MHN-UNSL-GEO V: Museo de Historia dental series; MWP: minimum width postorbital; Natural Universidad Nacional de San Luis, OMf5L: distance between the occipital condyles Geología Vertebrados, San Luis, Argentina; MMP: and the posterior edge of molariform 5; WS: width Museo Municipal de Ciencias Naturales de Mar del of snout; HS: height of snout. Plata, “Lorenzo Scaglia”, Buenos Aires, Argentina; MNPA: Museo Nacional Paleontológico-Arque- SYSTEMATIC PALEONTOLOGY ológico, Tarija (ex MUT: Museo Universitario de (Tables 1 and 2) Tarija), Bolivia; MNHN: Muséum national d’Histoire Superorder XENARTHRA Cope, 1889 naturelle, Paris, France; NRM-M: Swedish Order TARDIGRADA