THE ORIGIN OF A MAMMALIAN ORDINAL CHARACTER

By studying the evolutionary trends of organisms paleontologists have become aware of the peculiar circumstances surrounding the origin of higher categories. Most orders, classes and phyla of invertebrates and vertebrates appear abruptly in the geological record with one or more character complexes well differentiated but without any indication of transitional stages. Several theories have been postulated to explain this phenomenon but there have been few detailed discussions of particular examples. This paper represents an. attempt to analyze the evolutionary trend culminating in the appearance of an order of mammals, the even-toed ungulates or Artiodactyla. Of the twenty-six currently recognized orders of eutherian mammals, about fifteen probably had a Paleocene origin. The identification of the earliest memllbers of an order is dependent upon the possibility of identifying one or more definitive ordinal character complexes, which are usually.present in an otherwise still relatively primitive stock. The determination of the ancestral group from which a mammalian order such as the Artiodactyla originated is, by necessity, largely based on inference. Usually but a few heritage characters are available that indicate definite affinity and thus tend to bridge, however incompletely, the structural, if not the chronological hiatus. The first known members of the Artiodactyla are possibly unique among the mammals in that they can be identified positively only on the basis of a single character complex, in the foot, which practically reached its definitive form befnre other skeletal characters were in more than an incipient stage of phylogenetic development. The intermediate steps in the evolution of this complex, which represents the highly specialized form of the tarsus, are Iunknown. The primitive generalized (or relatively unspecialized) type from which it evolved c4n be recognized, however, and the time interval during which the transformation took place call be estimated with reasonable accuracy. It is possible, therefore, to consider the miagnitude of the morphological change during this period as well as the associated functional alternations. In discussing these problems here it will also be of interest to consider possible factors that favored an initial emphasis on the evolution of this particular character complex. A more detailed study of the origin and function of the artiodactyl tarsus has been published elsewhere (Schaeffer, 1947), and only such details of morphology and functiotn as are necessary for the understanding of th-e problem will be repeated. The erection of the Cohort Ferungulata (Simpson, 1945) to include the various orders of extinct and Recent carnivores and ungulates (fig. 1 ) is based on the fact that these greatly diversified groups had a common late Cretaceous origin. The paleontological data suiggest that the most primiitive ferungulates, the arctocyonid creodonts, reached their definitive status before the beginning of the Paleocene. Likewise the earliest ungulates, the condylarths, were differentiated by the early Paleocene. Until recently, the ancestral stock of the Artiodactyla was essentially unrecognized although the condylarths as a group were considered to be structurally antecedent. Simpson (1937) has pointed