Digging through the past: the evolutionary history of burrowing and underground feeding in rhinophrynid anurans

The Rhinophrynidae, endemic to North America, are a highly specialised group of fossorial anurans that, despite a long geologic history, has low diversity. Today, rhinophrynids are represented by one taxon, Rhinophrynus dorsalis , which occurs in extreme southern Texas, USA, and southwards through Mexico and into Central America, in dry tropical to subtropical forests along coastal lowlands. The skeleton of R. dorsalis is highly specialised for burrowing and subterranean feeding, with individuals spending most of their time underground feeding on ants and termites and emerging only after periods of heavy rain to breed. In the fossil record, rhinophrynids with skeletons specialised for burrowing first appear in the late Paleocene (Tiffanian) of Wyoming, USA, and are subsequently known from the Western Interior of North America until the late Eocene (Chadronian), with the exception of one late Pleistocene occurrence in Mexico. From the late Paleocene (Tiffanian) until the present, rhinophrynids became more specialised for burrowing. The tibiale and fibulare became relatively shorter and stouter, the distal condyle of the femur became expanded and divided into two condyles, and the distal prehallux bone and distal phalanx of the first metatarsal were modified as spades by at least the middle Eocene (Uintan). Adaptations for feeding underground might have occurred as early as the middle Eocene (Bridgerian), as suggested by the elongate neural arch of Eorhinophrynus septentrionalis . The middle Eocene (Uintan) Chelomophrynus bayi shares several specialisations with Rhinophrynus dorsalis for feeding underground, so it is presumed that it too was a subterranean feeder. These specialisations include a somewhat protracted snout, lack of teeth on the jaws and vomers, forward shift of the jaw suspensorium and most likely the shoulder girdle, and possibly the structure of the hyoid apparatus. As the climate became more seasonal and subfreezing temperatures developed after the Eocene–Oligocene transition, rhinophrynids apparently never developed the ability to hibernate to avoid freezing and instead shifted their range south.