Functional-Adaptive Analysis of the Postcranial
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FUNCTIONAL-ADAPTIVE ANALYSIS OF THE POSTCRANIAL SKELETON OF A LAVENTAN BORHYAENOID, LYCOPSIS LONGIROSTRIS (MARSUPIALIA, MAMMALIA) Author(s): CHRISTINE ARGOT Source: Journal of Vertebrate Paleontology, 24(3):689-708. 2004. Published By: The Society of Vertebrate Paleontology DOI: http://dx.doi.org/10.1671/0272-4634(2004)024[0689:FAOTPS]2.0.CO;2 URL: http://www.bioone.org/doi/full/10.1671/0272-4634%282004%29024%5B0689%3AFAOTPS %5D2.0.CO%3B2 BioOne (www.bioone.org) is a nonprofit, online aggregation of core research in the biological, ecological, and environmental sciences. BioOne provides a sustainable online platform for over 170 journals and books published by nonprofit societies, associations, museums, institutions, and presses. Your use of this PDF, the BioOne Web site, and all posted and associated content indicates your acceptance of BioOne’s Terms of Use, available at www.bioone.org/page/terms_of_use. 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Journal of Vertebrate Paleontology 24(3):689–708, September 2004 © 2004 by the Society of Vertebrate Paleontology FUNCTIONAL-ADAPTIVE ANALYSIS OF THE POSTCRANIAL SKELETON OF A LAVENTAN BORHYAENOID, LYCOPSIS LONGIROSTRIS (MARSUPIALIA, MAMMALIA) CHRISTINE ARGOT Laboratoire de Pale´ontologie, UMR 5143 du CNRS, Muse´um national d’Histoire naturelle, Paris, France ABSTRACT—Lycopsis longirostris, a middle Miocene Colombian borhyaenoid, is compared functionally with early Miocene borhyaenoids from Patagonia and the Tasmanian thylacine Thylacinus cynocephalus, the largest Recent mar- supial carnivore. The postcranium of Lycopsis shows a mosaic of features. Although several features characterizing the long bones are consistent with a primarily terrestrial mode of life (e.g., a straight ulna and tibia, a semi-digitigrade forefoot), other features like the development of mm. spinati, pectoralis and biceps and the pseudo-opposable pollex indicate that the forelimb had manipulative and grasping abilities. On the hindlimb, the poorly stabilized lower ankle joint, the short metatarsals, and the well-developed hallux also preclude fast running. When compared to three early Miocene borhyaenoids, Prothylacinus patagonicus, Cladosictis patagonica, and Borhyaena tuberata, which form a mor- phological gradient from the most arboreally-adapted taxon to an incipient cursorial one, Lycopsis is placed between Cladosictis and Borhyaena. Lycopsis was more likely an ambush hunter than a pounce-pursuit predator, stalking small- to medium-sized prey. INTRODUCTION sis longirostris, comparisons are made with Thylacinus cyno- cephalus, the largest marsupial carnivore in historic times (Jones Only a few highly carnivorous borhyaenoids, ranging from the and Stoddart, 1998), and similar in size to Lycopsis. The skeletal early Paleocene to the late Miocene of South America, are comparisons between both species will take advantage of eco- known by postcranial remains that are complete enough to infer logical and behavioral data that have been compiled about T. potential locomotor activities. Therefore, the analysis of the lo- cynocephalus (Smith, 1982; Johnson and Wroe, 2003). comotor features characterizing the middle Miocene Lycopsis longirostris from Colombia is expected to represent a significant MATERIALS AND METHODS part of the comprehensive review of the evolution of the loco- motion within Borhyaenoidea. The specimen UCMP 38061 is represented by the right half of The fossils described from the Honda Group section in the the skull, the right mandibular ramus, and most of the articulated upper Magdalena River valley (Villavieja Formation, Depart- postcranial skeleton. Originally, the animal was found lying on ment of Huila, Colombia) constitute the La Venta Fauna. This its right side in a partially flexed position (Marshall, 1977). The fauna is of particular interest as it represents the most complete postcranial elements of Lycopsis longirostris are partially record of Tertiary mammals from tropical South America, as crushed, and the articular facets and muscular attachments are well as the youngest endemic fauna known from the northern relatively poorly preserved. However, most of the long bones part of the continent before the arrival of North American im- have been removed from matrix for examination, which allows a migrants (Patterson and Pascual, 1968). Madden et al. (1997) functional comparative analysis. Following the methodology proposed a correspondence between the Laventan Stage, a chro- found relevant for the study of Mayulestes ferox, the oldest nostratigraphic unit well known for the abundance of fossil mam- borhyaenoid known (early Paleocene, Bolivia), it is assumed in mals occuring in the Honda Group, and a geochronologic sub- the description of the postcranial morphology of Lycopsis that division called the Laventan Age, representing the time interval the major muscle scars observed on the skeleton, and therefore between 13.5 and 11.8 Ma. The La Venta Fauna contains a di- the corresponding muscles, are homologous in borhyaenoids and verse assemblage of endemic ungulates and xenarthrans, and the living marsupials (Argot, 2001, 2002, 2003a). The myological in- earliest representatives of several modern South American sub- ferences concerning Lycopsis will occasionally refer to data I families of armadillos, anteaters, bats, opossums, and New World obtained from dissections performed on various genera of living monkeys (Flynn and Wyss, 1998). Paleoenvironmental data sug- Guyanese didelphids. The scarce myological data used in the gest a mosaic of biotopes typical of modern lowland meandering discussion about Thylacinus cynocephalus refer to data from the stream systems (Kay and Madden, 1997). nineteenth century compiled by Smith (1982). The postcranial skeleton of the species Lycopsis longirostris is Thylacinus cynocephalus is the largest carnivorous marsupial known by a single specimen (the holotype UCMP 38061) that for which any firsthand accounts of biology are available. Three has never been described in detail, especially from a functional thylacinid species existed in the Australian late Oligocene, and point of view. The only comments that exist are from Marshall ten species are known from Miocene sites, whereas a single one (1977), who found no major differences between the skeleton of is recognized in Plio-Pleistocene deposits (Wroe and Musser, L. longirostris and those of Prothylacinus patagonicus, Borhy- 2001; Wroe, 2003). The dental remains suggest a considerable aena tuberata, and Cladosictis patagonica. However, it is clear variation in size and diet between these species (the largest ones that the three early Miocene (Santacrucian) borhyaenoids, Pro- being also the most carnivorous: Wroe, 2001; Wroe and Musser, thylacinus, Borhyaena, and Cladosictis, exhibit distinct adaptive 2001). Most of these species are known only from dentitions, features (Argot, 2003b, c), and it is of interest to compare the rarely by cranial elements, and therefore, the potential diversi- younger Lycopsis with these taxa. fication of the locomotion within the family is unknown. Little is In order to assess the functional implications of the morpho- known about the ecology and behavior of the Recent taxon, data logical features characterizing the postcranial skeleton of Lycop- consisting mainly of nineteenth century observations from farm- 689 690 JOURNAL OF VERTEBRATE PALEONTOLOGY, VOL. 24, NO. 3, 2004 ers and other bush workers, and observations made on captive gentina; MNHN, Muse´um national d’Histoire naturelle, Paris, animals (Smith, 1982). However, Jones and Stoddart (1998) have France; PU, Peabody Museum of Yale University, New Haven, recently reconstructed the likely hunting and killing methods of USA; UCMP, University of California, Museum of Paleontol- the thylacine based on canine tooth strength and femur/ ogy, Berkeley, USA. metatarsal length ratio, and conclude that this marsupial was a pounce-pursuit predator of fairly open habitats, which killed me- DESCRIPTION dium-sized prey (1–5 kg), quite small relative to its body size (mean weight: 25 kg). However, a selection of small prey species Axial Skeleton may have resulted from human disturbance as the two largest potential Tasmanian prey species of the thylacine, the emu and Cervical Vertebrae—The dorsal arch of the atlas of Lycopsis eastern grey kangaroo, were being decimated or eliminating by is robust and anteroposteriorly wide. The anterior edge is less the nineteenth century, while the thylacine was restricted to mar- convex than in Borhyaena (Fig. 1A). Moreover, in contrast to ginal habitats (Johnson and Wroe, 2003). The thylacine was char- Borhyaena, the atlas of Lycopsis does not have deep grooves on acterized by a dog-like external aspect: large size, deep chest, the lateral extremities of the anterior edge for the passage of the enlarged brain, long snout, and especially a digitigrade stance first cervical nerve and the vertebral artery. In Lycopsis, the and straight forelimb with non-retractile claws suggesting a simi- anterior sulcus is closed in an intervertebral (or atlantal) fora- lar adaptation for running