Variability in pulmonary reduction and asymmetry in a serpentiform lizard: The sheltopusik, Pseudopus apodus (Pallas, 1775)

Besides snakes, numerous lineages of squamates gave rise to limb-reduced and elongated (serpentiform) species, indicating the evolutionary success of this modification of the plesiomorphic lizard Bauplan. Concerted with a serpentiform habitus are several morphological adaptations, many of which also concern the structure and arrangement of the viscera, such as frequently a pronounced pulmonary asymmetry in which one lung is reduced or even absent. The European glass lizard or sheltopusik, Pseudopus apodus, is the largest species of the exclusively serpentiform Anguinae. Driven by pre-existing conflicting statements on pulmonary asymmetry, we examined the lungs of 14 sheltopusiks and compared the condition to 11 slow worms (Anguis fragilis). We consistently found the left lung pronouncedly shorter for the slow worm, but indeed a highly variable pulmonary asymmetry between left and right sides in the sheltopusik. This is the first verified case of such variability in pulmonary reduction for any serpentiform squamate and raises several questions about the underlying developmental program for this otherwise taxon-specifically conservative trait.