SVPCA TALKS A Devonian tetrapod-like fish from China reveals parallelism in stem tetrapod evolution

The fossils assigned to the tetrapod stem group document the evolution of terrestrial vertebrates from lobe-finned fishes. During the past 15 years the phylogenetic structure of this stem group has remained remarkably stable, even when accommodating major new discoveries such as the elpistostegid Tiktaalik. Here we present a large tetrapodomorph fish from the Late Devonian of China that disrupts this stability. This new taxon, provisionally named Hongyu, comes from the Famennian Zhongning Formation of Qingtongxia, Ningxia, north-central China. It is the first Devonian sarcopterygian fish to be described from the North China Block, a craton complex that at this time formed a small continent located near the equator. Hongyu is represented by a single specimen comprising a partial skull, gill skeleton, shoulder girdle and anterior vertebral column. It combines characteristics of rhizodont fishes with derived elpistostegid-like and tetrapod-like characters such as a plate-like scapulocoracoid, a cleithrum without a ventral lamina, and a hyomandibula that ends distally at the opercular facet. A ventrally flattened lower jaw with an extremely large and robust retroarticular process suggests a benthic "fish trap" lifestyle similar to that proposed for the aberrant Triassic temnospondyl Gerrothorax. Phylogenetic analyses incorporating Hongyu suggest two very different topologies for the tetrapod stem group: either rhizodonts clade with Hongyu and elpistostegids + tetrapods, or at the cost of one extra step rhizodonts + Hongyu are widely separated from elpistostegids + tetrapods. It appears that evolutionary parallelism, ecological diversity and biogeographical provinciality in the tetrapod stem group may all have been underestimated.