Phylogenetic Relationships Between the Orders Artiodactyla and Cetacea: A Combined Assessment of Morphological and Molecular Evidence

Abstract

A character analysis of selected conservative morphological traits from extant and fossil artiodactyls and cetaceans was combined with a similar analysis of conservative nucleotide positions from the complete mitochondrial cytochrome b sequences of available extant artiodactyls, cetaceans, sirenians, perissodactyls, and other mammals. This combined analysis focuses on the evidence that supports conflicting hypotheses of artiodactyl monophyly, including the affinities of hippopotamids and the monophyly or paraphyly of odontocete cetaceans. Highly conserved morphological traits of the astragalus and deciduous dentition provide strong corroboration of artiodactyl monophyly, including extant and fossil hippopotamids. In contrast, cytochrome b gene sequences are incapable of confirming this monophyly, due to excessive homoplasy of nucleotide and amino acid traits within extant Eutheria. In like manner, highly conserved and uniquely derived morphological features of the skull and auditory regions provide robust corroboration of Odontoceti monophyly, including extant and fossil physeteroids. Several nucleotide similarities do exist between physeteroids and mysticetes; however, most are either silent third-position transversions or occur also in two or more odontocete families. We suggest that increased taxon sampling, combined with functional considerations of amino acids and their secondary structure in protein-coding genes, are essential requirements for the phylogenetic interpretations of molecules at higher taxonomic levels, especially when they conflict with well-supported hypotheses of mammalian phylogeny, corroborated by uniquely derived morphological traits from extant and fossil taxa.