CYNODONT POSTCRANIAL ANATOMY AND THE "PROTOTHERIAN" LEVEL OF MAMMALIAN ORGANIZATION.

Abstract

The origin of mammals from synapsid reptiles occurred during the Late Triassic. Although evolutionary events both preceding and following that time were important to the ultimate development of living mammals, certain Late Triassic forms possess such an even balance of reptilian and mammalian characters that they are truly transitional between the two classes. It should be emphasized that this transition comprised a biological series of events, and that these are meaningful only when interpreted in biological terms. However, the usual techniques of direct observation and experimentation on living animals are unavailable for these unique biological events. Instead, investigations of mammal origins must utilize indirect means, viz., functional interpretations of transitional forms and comparative studies of primitive living mammals. This paper uses both approaches to evaluate the question of the kind of biological organization that characterized the transitional forms on or near the reptilemammal boundary. Some authors (e.g., Winge, 1941; Kermack, 1963; MacIntyre, 1967) have implied or overtly claimed that Mesozoic mammals were advanced little toward a mammalian level of organization, and that it was only in late Mesozoic or early Tertiary times that taxonomic Mammalia underwent the transition from a basically reptilian to a mammalian level of biological organization. Bakker (1968) claimed that advanced mammal-like reptiles and Jurassic mammals were no more mammalian in posture and locomotion than modern lizards. New evidence bearing on the origin of mammalian characters is here Dresented through (1) a functional interpretation of the postcranial anatomy of certain advanced mammal-like reptiles and (2) through the phylogenetic implications of monotreme biology. Cynodonts, a diverse group of PermoTriassic therapsids (mammal-like reptiles), possess a suite of cranial features indicative of a probable ancestral relationship to mammals. Among these features are ethmoturbinals, double occipital condyles, secondary palate (cf. Brink, 1956), jaw structure (Barghusen, 1968), and dentition (Crompton and Jenkins, 1968). Previously, the origin of mammals was obscured by an apparent hiatus in the paleontological record between the last major therapsid faunas of the early Triassic and the earliest Mesozoic mammalian faunas of the middle Jurassic. Recently, however, the remains of Late Triassic mammals have been recovered in relative abundance (Kermack, 1967a; Parrington, 1967); this new evidence now provides a more substantial basis for deductions on the derivation and initial radiation of Mesozoic mammalian orders. Crompton and Jenkins (1968) concluded that cynodonts are the most likely ancestors of mammals on the basis of similarities in dentition; the same conclusion was reached by Barghusen (1968) on evidence of the jaw apparatus. With a few exceptions (Gregory and Camp, 1918; Romer, 1922; Simpson, 1928, 1929; Brink, 1956; Parrington, 1961), students of early mammalian evolution have generally neglected the postcranial skeleton-primarily because postcranial material of Mesozoic mammals is relatively scarce or, when available, is