The Chondrocranium of the Mexican Burrowing Toad, Rhinophrynus dorsalis

Abstract

-The chondrocranium of Rhinophrynus dorsalis is described and illustrated. Autapomorphies of R. dorsalis are the presence of the symplectic cartilage and a cartilaginous process on the ascending process of the palatoquadrate. Synapomorphies of Rhinophrynus shared with the Pipidae are: (1) early formation of the lower jaw and (2) presence of a suprarostral plate. Alternative developmental pathways for the formation of the suprarostral plate are considered. Histological data indicates that the suprarostral plate forms from: (1) the early fusion of the cornua trabeculae with the suprarostral cartilages, (2) an anterior growth of the ethmoid plate, and (3) subsequent fusion of the cornua trabeculae and the anterior process of the ethmoid plate. The larval crista parotica of R. dorsalis resembles the one described for microhylid taxa. Taylor (1942) described pipid-like anuran larvae from a sample of tadpoles collected in Guerrero, Mexico. However, these larvae remained unidentified until the tadpole of Rhinophrynus dorsalis was described by Orton (1943). Orton's description included a few chondrocranial characteristics, particularly focusing on the jaw apparatus. She noted the early formation of the lower jaw and suggested that this early development may be correlated with a macrophagous diet. Subsequently, Sokol (1977, 1981) made references to the chondrocranial anatomy for Rhinophrynus dorsalis larvae while evaluating the use of larval characteristics in systematics. The sequence of cranial ossification for Rhinophrynus larvae was described by Trueb, 1985. Description of adult osteology has been previously described (Trueb, 1973; Trueb and Cannatella, 1982). Hay et al. (1995) placed Rhinophrynus as the sister group to Pipidae based on 12S and 16S rRNA sequence data. Previous morphological studies suggested the following synapomorphies for the Pipoidea: absence of mentomeckelian bones, lack of parasphenoid alae, presence of a single frontoparietal, greatly enlarged otic capsules, and larvae with paired spiracles and lacking comified beaks and denticles (Ford and Cannatella, 1993). Of the approximately 28 species of the Pipoidea, the chondrocranium has been described thoroughly for Xenopus laevis (Trueb and Hanken, 1992) and partially for Rhinophrynus dorsalis, Pipa carvalhoi, and Xenopus tropicalis (Sokol, 1975). Herein we provide a complete description and illustration of the chondrocranium of Rhinophrynus dorsalis. 1 Corresponding Author. MATERIALS AND METHODS Thirty tadpoles of the Mexican Burrowing Toad, Rhinophrynus dorsalis, representing Neiuwkoop and Faber (1956) stages 48-62 were cleared and double-stained for bone and cartilage using standard procedures. Drawings were obtained with the help of a Wild M3C stereomicroscope with camera lucida attachment. Collection localities of specimens used in this study are: Guanacaste, Costa Rica (USNM 515945515958); and Aguacate Lagoon, Cayo District, Belize (USNM 515959-515974). Collection numbers, measurements, and stages of material examined are given in Table 1. Nieuwkoop and Faber's (1956) staging table for Xenopus was used to stage Rhinophrynus tadpoles. The similarity of external morphology of Xenopus and Rhinophrynus during development makes this table preferable to the Gosner's (1960) staging table. Chondrocranial terminology follows De Jongh (1968), Haas (1995), and Trueb and Hanken (1992).