Rhineceps is an extinct genus of temnospondyl amphibian in the family Rhinesuchidae. Rhineceps was found in Northern Malawi (formerly Nyasaland) in Southern Africa known only from its type species R. nyasaensis. Rhineceps was a late Permian (256-258 Mya) semi-aquatic carnivore that lived in streams, rivers, lakes or lagoons. Rhineceps is an early divergent Stereopondyl within the family Rhinesuchidae, which only existed in the late Permian (Lopingian) and failed to survive the Permian-Triassic extinction unlike other stereospondyl families.
History
Rhineceps was originally discovered in the “Upper Bone Beds” of the Chiweta Beds, North Waller Area in Northern Malawi. The fossil was first described and named by Haughton in 1926, whereby he describes the fossil of a left mandibular ramus of R. nyasaensis as within the genus Rhinesuchus due to its similarity with the fossils of Rhinesuchus whaitsi. Watson later determined in a letter to Parrington that the original fossil was actually the anterior end of a right ramus and not a left ramus. Watson in 1962 described fossils newly found from the same locality collected by Parrington as fossils of R. nyasaensis and determined they were from the same individual. Watson also placed R. nyasaensis as part of a new genus Rhineceps that is distinct from other Rhinesuchus species.
Classification
Rhineceps belongs to the family Rhinesuchidae, which is within the clade Stereospondyli (a Temnospondyl clade). Until recently, both morphology, historical placement and phylogenetic analysis has placed Rhineceps as a sister group to Rhinesuchus within Rhinesuchinae, an early diverging group of Rhinesuchidae.
In 2017, Marsicano et al. created a new phylogeny that places Rhineceps as a sister group to Uranocentrodon rather than within Rhinesuchinae (which is no longer a monophyletic group).
This discrepancy in clearly describing phylogenetic relationships is likely due to the adaptive radiation of stereospondylomorphs that took place during the late Permian and early Triassic. Terrestrial temnospondyls were devastated during this period, caused by selective pressures for semi-aquatic or fully aquatic temnospondyls to acquire many new niches in a relatively short time period.
Description and paleobiology
Rhineceps was a medium-sized, semi-aquatic, carnivorous, basal stereospondyl of the Lopingian of Malawi. They lived in fresh water bodies, including rivers lakes and lagoons, where they had diets mainly of fish. Stereospondyls were historically compared with contemporary crocodilians and giant salamanders due to their similar position as freshwater predators. However, they likely existed in a niche that is unique and independent from crocodilians and giant salamanders. These niche differences are inferred from cranial stress and structural patterns along with labyrinthodont dentition (labyrinthodonty refers to infolding of enamel and dentine within a tooth). The morphology of Rhineceps thus likely restricts its ability to hunt in specific ways that crocodilians and giant salamanders such as sweeping sideways bites. Rhineceps also used its unique morphology in service of catching prey, including its strong powerful bite and its palatal tooth row to grasp and hold fish in its jaw. Many temnospondyls including Rhineceps likely shared certain physiological characteristics with modern amphibians that cannot be directly observed in the fossil record. Rhineceps likely possessed heightened phenotypic plasticity among juvenile individuals like seen in modern amphibians. This plasticity allowed physiological changes during an organism's lifetime to alter its morphology during development to specific and different environmental factors experienced. Plasticity was a means for stereospondyls to adaptively radiate to many different species and niches. Rhineceps like other temnospondyls were ecological indicator species like modern amphibians, and thus likely were sensitive to chemical changes, UV radiation and other mutagens within its ecosystem.