Contribution to the head anatomy of the basal frog Barbourula busuangensis and the evolution of the anura

Barbourula busuangensis Taylor and Noble, 1924 is a poorly known basal aquatic frog from the Philippines. Here we describe some features of the head anatomy of B. busuangensis, with a focus on jaw and eye musculature and the cranial skeleton, based on 3D reconstructions from serial histological sections of a metamorphosed but not fully grown (subadult) female and two fully grown adults, one of which was investigated by synchrotron x-ray imaging. Examination of two different post-metamorphic stages allowed tentative assessment of the development of some characters from the subadult to the adult condition, e.g., shape and size of the parahyoid or arrangement of some jaw adductors. The subadult specimen of B. busuangensis also possesses some unique cranial characters, including a salamander-like arrangement of the jaw adductors, which is obscured in fully grown adults, and a well-developed basal (= basicranial) articulation resembling that of temnospondyls. The facial nerve of B. busuangensis does not fuse with the trigeminal ganglion. Head anatomy confirms that Barbourula is a basal anuran, and comparison with other basal taxa suggests that B. busuangensis most closely resembles the hypo-ossified taxa Alytes and Bombina, and Discoglossus with respect to degree of ossification.