The median lingual process of frogs : a bizarre character of Old World ranoids discovered in South American dendrobatids / Taran Grant, Elaine C. Humphrey, and Charles W. Myers. American Museum novitates ; no. 3212

Unexpected discovery of a median protuberance on the tongue of several Colostethus spp. (Dendrobatidae) provides first knowledge of an unusual anatomical structure in New World frogs. The general term median lingual process is suggested for all similarly positioned structures, previously known only in certain Asian and African frogs of the superfamily Ranoidea. References to the process are mostly buried in taxonomic descriptions under the term "median papilla," although it is many times larger than the tongue's fungiform papillae (which carry taste receptors) and the still smaller filiform papillae. Scanning electron microscopy shows a cell-surface ultrastructure of microridges (and microknobs) as is characteristic of certain other epithelia. The function of the median process is unknown but conceivably involved in the sophisticated anuran chemosensory system. From a survey of museum specimens and literature, the median process is shown to occur in one or more species of at least 16 genera of three nominal families and several subfamilies of Old World ranoids. Considerable variation in the median lingual process was found and is preliminarily classified under several morphologies. Type A: Retractile, upright cone-shaped processes occur with a-type retraction in Discodeles, Indirana, Mantidactylus, Rhacophorus, and with 3-type retraction in Platymantis and possibly Staurois. Type B: Retractile, upright rugose processes with a-type retraction characterize Petropedetes only. Type C: Elongate, longitudinally reclining processes characterize several genera of the Petropedetinae-Arthroleptides with aL-type retraction, Dimorphognathus, Ericabatrachus, and Phrynodon, retraction unknown, Nothophryne, possibly nonretractile, and Phrynobatrachus, apparently both retractile and nonretractile. Type D: Retractile and nonretractile bumps or other exceptionally short structures occur in various arthroleptines, petropedetines, and ranines--probably a reflection of parallel reduction in both type-A and type-C processes. Overall homology of the several types of lingual process is tenuous and suggested only by the widespread existence of a retractile mechanism in the Old World ranoids and the apparent loss of such a mechanism in some but not all African petropedetines and in South American Colostethus. In dendrobatid Colostethus, the median lingual process is seemingly nonretractile and, except for a reduced (type D) process in one species, conforms generally to the common petropedetine type (type C), resembling most closely the tapered, pointed process of the East African Nothophryne. The median lingual process therefore looks to be homologous at least in the Dendrobatidae and African Petropedetinae. However, scanning electron micrographs show differences in dorsal tongue-surface morphology that have yet to be put in systematic context: Colostethus has a lingual covering of filiform papillae (as in Rana etc.), whereas at least some petropedetines have the mucosal epithelium folded and ridgelike (as in Bufo and Bombina). But Colostethus and petropedetines similarly have a collar of cilia encircling the taste disc (as in Rana etc. but lacking in Bufo and Bombina). Although current molecular evidence (Ruvinsky and Maxson, 1996) places the Dendrobatidae within South American Bufonoidea (= Hyloidea), the lingual process suggests that dendrobatids are 17