COMPARATIVE MORPHOLOGY AND EVOLUTION OF THE LUNGLESS CAECILIAN ATRETOCHOANA-EISELTI (TAYLOR) (AMPHIBIA, GYMNOPHIONA, TYPHLONECTIDAE)

Atretochoana eiselti is a radically divergent aquatic caecilian until recently known from only a single specimen from South America. In addi tion to its status as the largest lungless tetrapod known. and the onl y known lungless caecilian, this species has a suite of highly unusual morphological features that sets it apart from all other tetrapods, i ncluding sealed choanae (internal nostrils), complete loss of pulmonar y arteries and veins, novel cranial architecture, and a novel stapedia l muscle. The external, buccal, skeletal, muscular and cardiovascular anatomies of Atretochoana eiselti are described and compared to these features in other caecilians, particularly representatives of all typh lonectid genera which are its closest relatives. The comparative morph ological data are used as a basis for interpretations of the ecology a nd evolution of Atretochoana eiselti. It is argued that lunglessness a nd the transition to cutaneous gas exchange is correlated with life in cold, montane, fast-flowing streams. Here, high oxygen concentrations and reduced metabolic rate serve to relax the physiological limitatio ns on body size imposed by a reliance upon cutaneous gas-exchange, and lungs can produce disadvantageous buoyancy. Cranial evolution has inc reased the gape of Atretochoana eiselti relative to other caecilians, and seems likely to be associated with a shift in prey size and possib ly type. Several modifications of the cranium appear to be associated with enhanced cranial kinesis in which a distinct cheek unit is highly mobile. The novel stapedial musculature is interpreted as contributin g to this cranial kinesis. Respiratory and cranial evolution are argue d to be correlated, with the ventilatory function of the buccopharynge al pump constraining the evolution of the skull. The evolution of lung lessness removed this constraint facilitating repatterning of the skul l. (C) 1997 The Linnean Society of London.