Cladistic Analysis of Primitive Equids, with Notes on Other Perissodactyls

Abstract

MacFadden, B. J. (Department of Vertebrate Paleontology, The American Museum of Natural History, New York, New York, 10024) 1976. Cladistic analysis of primitive equids, with notes on other perissodactyls. Syst. Zool. 25:1-14.-The following hypotheses are developed by cladistic methodology, using only synapomorphic characters to assess phylogenetic relationships. The sister group of perissodactyls is formed by phenacodont condylarths, as evidenced in both groups by reduction (or absence) of metapodials and reduction (or absence) of the paraconid-paralophid complex on the lower cheek teeth. The taxon Perissodactyla is monophyletic, as defined by the presence of a saddle-shaped, or concave, navicular facet on the astragalus. Within perissodactyls, Hyracotherium and other equids form a monophyletic group, distinct from all non-equid perissodactyls, as evidenced by confluence of the foramen ovale and middle lacerate foramen in the ventral basicranium, and by the optic foramen that has migrated close to, or has become confluent with, a group of posteroventral foramina: anterior lacerate, rotundum, and anterior opening of the alisphenoid canal. The Palaeotheriidae are included in the Equidae at a lesser taxonomic rank. Within equids, Orohippus has apomorphic dental characters (greater degree of molarization of the premolars) relative to Hyracotherium. Orohippus and Haplohippus are morphologically similar. The premolars of Epihippus have a degree of molarization greater than those of Orohippus and Haplohippus. The subfamily Hyracotheriinae is a paraphyletic taxon and should be abandoned. [Hyracotheriinae; Hyracotherium; Equidae; Perissodactyla; cladism.] Cladism is a logical framework for reconstructing phylogeny, and among systematists it is gaining acceptance as a valid method for relating taxa. The primary purpose of this paper is to apply cladistic analysis to perissodactyls, not to discuss it in theoretical terms. The theory of cladism has been extensively discussed elsewhere: in the definitive work by Hennig (1950, in German; 1966, English translation) and in many more concise articles (e.g., Hennig, 1965; Brundin, 1968; Kavanaugh, 1972). Many terms used in the present paper are adopted from those discussions. The methods of determining that a character is plesiomorphic (primitive), symplesiomorphic (shared primitive), apomorphic (derived), or synapomorphic (shared derived) conform with the discussion of Schaeffer, Hecht, and Eldredge (1972). The purpose of this paper is to apply cladistic analysis to perissodactyls, particularly to equids. After equids are set apart as a monophyletic group, distinct from other perissodactyls on the basis of apomorphic characters, then several primitive equids will be discussed. Various taxa discussed in this paper need taxonomic revision, e.g., phenacodontid condylarths. Also, some taxa are not satisfactory (monophyletic) cladistic categories. Occasionally, these problems will be mentioned when germane to the discussion. It is not within the scope of this paper to discuss problems of all perissodactyl taxa. Unless otherwise indicated, the nomenclature used represents the present and imperfect state of the art. Names of institutions are abbreviated as follows: AMNH, The American Museum of Natural History, New York; BMNH, British Museum (Natural History), London; CMNH, Field Museum of Natural History, Chicago; YPM, Peabody Museum of Natural History, New Haven. SISTER GROUP OF PERISSODACTYLS A basic procedure of cladistic analysis is to search for the sister group. What is the sister group of perissodactyls? Numerous