The Adequacy of Morphology for Reconstructing the Early History of Placental Mammals

Abstract

Mammalian anatomists and paleontologists working primarily with osteological data have long been intrigued with the problem of eutherian diversification, including both interordinal relationships and the timing of the placental mammal radiation (McKenna, 1975; Novacek, 1992). Cladistic analyses of morphological characters for placental mammal orders have suggested a variety of superordinal hypotheses (Table 1). Molecular trees based on single gene segments were often in conflict with each other and with morphology, but larger nuclear gene data sets that include longer and/or multiple gene segments have converged on a well-supported superordinal tree topology that divides placental orders into four major groups: Afrotheria, Xenarthra, Laurasiatheria, and Euarchontoglires (Madsen et al., 2001; Murphy et al., 2001a, 2001b; Scally et al., 2001; Amrine-Madsen et al., 2003). Analyses of independent molecular and genomic data sets, specifically whole mitochondrial genomes and rare genomic changes (RGCs), are congruent with this four-clade classification (Hudelot et al., 2003; Waddell and Shelley, 2003; Murphy et al., 2004; Reyes et al., 2004; Springer et al., 2004, 2005; Gibson et al., 2005; Kriegs et al., 2006; Nishihara et al., 2006; Kjer and Honeycutt, 2007). For example, Nishihara et al. (2006) found six, nine, and nine L1 retroposon insertions supporting the monophyly of Afrotheria, Euarchontoglires, and Laurasiatheria, respectively.