The Systematic Relationships of the Monotremata: The Ontogenetic Evidence <B>The Systematic Relationships of the Monotremata: The Ontogenetic Evidence</B><P> <EM><B></B></EM><B>Ulrich Zeller</B><BR> Museum f,r Naturkunde, Institut f,r Systematische Zoologie, Invalidenstrasse 43, 10115 Berlin, Germany<B></B><P> Modern systematics must be based on a holistic approach to the organism. This includes not only molecular genetics and its links to morphogenesis, but also morphogenesis by itself has to be considered. In this context the systematic relations of monotremes are reevaluated. The Monotremata and the other Recent Mammalia (Theria) as well as all other groups of Mesozoic mammals share a number of synapomorphies in comparison with their synapsid ancestors that characterize them as a monophyletic group (e.g. squamoso-dentary jaw joint as secondary jaw joint; caudal part of the lower jaw functionally separated from the jaw apparatus transmitting air-born sound; ethmoideal region with extended peripheral olfactory organ, recessus ethmoturbinalis and lamina cribrosa; integument rich in glands including lacteal gland; reproduction characterized by the combination of parental care, lactation, yolk reduction and fetomaternal metabolism (placentation). Furthermore, each of the three Recent mammalian groups (Monotremata, Marsupialia, Placentalia) shows its own mosaic of retained primitive features (plesiomorphies) and derived features (apomorphies).<BR> The Monotremata have retained a series of plesiomorphic characters of the last common ancestor of Recent mammals which were lost in the Theria (e.g. oviparity, cloaca, shoulder girdle, egg tooth, septomaxillary, ectopterygoid, temporal canal, pila antotica, cartilago scleralis). On the other hand, Theria have incorporated plesiomorphic features from their common ancestor which were lost in Monotremata (e.g. pila metoptica, lacrmimal, parasphenoid). Monotremata and Marsuplialia share plesiomorphic mammalian characters, such as meroblastic cleavage, tertiary egg membrancs, precocial offspring, short development within fetal membranes and extensive lactation - Monotremata are autapomorphic with regard to the formation of the secondary side wall of the braincase (lamina obturans). In addition, Marsupialia display autapomorphic characters, e.g. reduction of alantois and mainly choriovitelline placentation, the loss of pila postoptica and the inflection of the angular process of the dentary. The Eutheria are autapomorphic in having achieved a feto-maternal immuno-tolerance as a prerequisite of a long intrauterine development and intensive placentation. Marsupialia and Placentalia are synapomorphic, for example, in the formation of the secondary side wall of the braincase by the alisphenoid. The individual autapomorphies of Monotremata and Theria suggest an early dichotomy of the phyletic lines leading to the Monotremata and the Theria respectively. There is no developmental synapomorphy of Monotremata and Marsupialia suggesting the taxon Marsupionta. Instead, the sister group relationship of monotremes and therians is well established.<P>