Hypotarsus Morphology of the Ralloidea Supports a Clade Comprising Sarothrura and Mentocrex to the Exclusion of Canirallus

Molecular data suggest that the traditional Rallidae (rails) are polyphyletic and that some species actually belong to a clade that is the sister taxon of the Heliornithidae (sungrebes). This clade includes the African taxon Sarothrura and the Madagascan Mentocrex and has been termed Sarothruridae. It was noted that White-spotted Flufftail Sarothrura pulchra and Sungrebe Heliornis fulica share a distinctive morphology of the hypotarsus, which guides the tendons for the flexor muscles of the toes. Owing to the scarcity of skeletons in osteological collections, however, other species of the Sarothruridae and Heliornithidae have not yet been studied. Here, the hypotarsus of extant and fossil members of the Ralloidea is examined. It is shown that the characteristic hypotarsus morphology is also present in Buff-spotted Flufftail Sarothrura elegans (Sarothruridae), Madagascan Wood Rail Mentocrex kioloides (Sarothruridae), and African Finfoot Podica senegalensis (Heliornithidae). Grey-throated Rail Canirallus oculeus, by contrast, which was traditionally considered closely related to Sarothrura and Mentocrex, exhibits the hypotarsus morphology found in the Rallidae. To foster future communication, it is here proposed to use the taxon name Heliornithes for the clade including Heliornithidae, Sarothrura, and Mentocrex. A previously unknown derived hypotarsus morphology is described, which characterizes the rallid taxon Porphyrio (swamphens) and may be functionally correlated with the fact that the species of this taxon manipulate food items with their feet.