Model-based analysis of postcranial osteology of marsupials from the Palaeocene of Itaboraí (Brazil) and the phylogenetics and biogeography of Metatheria

Frederick S. SZALAY & Eric J. SARGIS

en Geodiversitas 23 (2) - Pages 139-302

Published on 29 June 2001

Over 130 specimens of metatherian bones (predominantly long bones, one scapula, and several innominates and unguals) from the Palaeocene Itaboraí fissures of Brazil are described and analyzed from a functional-adaptive perspective, in order to 1) understand the adaptations of species they represent; 2) obtain taxonomic properties of the species taxa they represent; and 3) estimate their phylogeny within the Metatheria. To provide context for the analysis, the descriptions are preceded by a survey of postcranial variation in the extant Didelphis virginiana Kerr, 1792, together with an examination of a number of heritage and habitus properties in several extant species designated as models. The study of extant models served as the basis for explanations of locomotion-related form-function features that predict a range of positional behaviors ranging from arboreal to terrestrial in the fossils. The purpose of such adaptation-oriented character analysis is to differentiate causally-related adaptive aspects of a species in its realtime (the entire life history of the individual organisms) from features retained from its ancestry, i.e. those that were contingent in the past of its lineage. Such level-specific primitive attributes, while fully adaptive, are less directly related to the more recently acquired adaptive strategies of the species taxa analyzed, the derived attributes. This procedure allows the ordering and polarization of lineage- and taxon-specific properties. As a cornerstone of our methodology, prior to the analyses of didelphid variation, the model species, and the fossils, we examined 1) the general theoretical issues that relate to adaptations in fossils; 2) some of the competing views of what functional biology is supposed to be in relation to phylogenetic analysis; and 3) the connections between a broad concept of functional biology and phylogenetic analysis. Model construction for character state evaluations, using extant marsupials, was a basic part of our methodology. The medium-sized models employed were the didelphids Caluromys derbianus (Watrhouse, 1841) and Metachirus nudicaudatus (Demarest, 1817) and the small models were the microbiotheriid Dromiciops gliroides Thomas, 1894 and the caenolestid Rhyncholestes raphanurus Osgood, 1924. In each group the former species are arboreal and the latter terrestrial. The attributes of the models were explained in the light of locomotor and myological attributes, joint mechanics, as well as substrate preference. The various attribute-related hypotheses were tested by comparisons with the extant Didelphis virginiana and Monodelphis domestica (Wagner, 1842), as well as behaviorally less well-known species of Marmosa Gray, 1821, Micoureus Lesson, 1842, Chironectes Illiger, 1811, and Lutreolina Thomas, 1910. This permitted us to ascertain and extend, and thus generalize in a metatherian context: 1) the close causal association of many of these attributes with similar or divergent adaptations in other taxa in addition to the models; 2) their consolidation into robust character complexes; and 3) an understanding of which are primitive or advanced didelphid, didelphimorph, or didelphidan and/or sudameridelphian characters. In addition to the living species, the nearly complete skeletal remains of early Palaeocene Tiupampa Pucadelphys Marshall & Muizon, 1988 and Mayulestes Muizon, 1994 provided complete skeletal comparisons and therefore increased confidence in the evaluation of isolated fossil bony element, although fossils are not proper tests of the models, as there is no direct information about their habits. Following description and analysis of the extant and fossil specimens, a variety of phylogenetic proposals based mostly on genetic and molecular evidence have been examined in light of the morphological evidence. Together with an examination and critique of the recent contributions of Kirsch et al. (1997) and Springer et al. (1998) on metatherian phylogeny and biogeography, some aspects of the nature of morphological and fossil evidence are highlighted, and marsupial suprafamilial relationships outlined. Biogeographic implications of the phylogeny presented are also detailed, particularly the Gondwanan distribution of lineages and taxa and their various possible interpretations.

Keywords :

Metatheria, Palaeocene, Itaboraí, Brazil, functional analysis, adaptations of fossil species, extant models, Marsupiala, phylogeny, biogeography

Download full article in PDF format