Comparative morphology and evolution of the lungless caecilian Atretochoana eiselti (Taylor) (Amphibia: Gymnophiona: Typhlonectidae)

Abstract

Atretochoana eiseltiis a radically divergent aquatic caecilian until recently known from only a single specimen from South America. In addition to its status as the largest lungless tetrapod known, and the only known lungless caecilian, this species has a suite of highly unusual morphological features that sets it apart from all other tetrapods, including sealed choanae (internal nostrils), complete loss of pulmonary arteries and veins, novel cranial architecture, and a novel stapedial muscle. The external, buccal, skeletal, muscular and cardiovascular anatomies ofAtretochoana eiseltiare described and compared to these features in other caecilians, particularly representatives of all typhlonectid genera which are its closest relatives. The comparative morphological data are used as a basis for interpretations of the ecology and evolution ofAtretochoana eiselti. It is argued that lunglessness and 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 limitations on body size imposed by a reliance upon cutaneous gas-exchange, and lungs can produce disadvantageous buoyancy. Cranial evolution has increased the gape ofAtretochoana eiseltirelative to other caecilians, and seems likely to be associated with a shift in prey size and possibly 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 contributing to this cranial kinesis. Respiratory and cranial evolution are argued to be correlated, with the ventilatory function of the buccopharyngeal pump constraining the evolution of the skull. The evolution of lunglessness removed this constraint facilitating repatterning of the skull.