The spermatozoon of the basal frog Leiopelma hochstetteri Fitzinger, 1861 is examined and compared with previously investigated lissamphibian spermatozoa. L. hochstetteri spermatozoa share with those of urodeles and Ascaphus the plesiomorphic (primitive) features of an elongate conical acrosome, and an elongate nucleus, with distinct nuclear shoulders, which tapers to a point (as the nuclear rostrum) in the subacrosomal space. The spermatozoon of L. hochstetteri, alone among the Anura, resembles those of urodeles in having a juxta-axonemal fibre at doublet 8. L. hochstetteri shares with Ascaphus truei (homoplastically) the reduction of the undulating membrane to a short, thick paraxonemal rod. L. hochstetteri spermatozoa have the plesiomorphic condition, also seen in dipnoans, of a cytoplasmic collar, containing mitochondria, around the axoneme. Although the sperm of L. hochstetteri lacks the endonuclear perforatorium seen in Ascaphus, Bombinidae, urodeles and caecilians, it has what may be remnants of an endonuclear perforatorium within the nuclear rostrum. No synapomorphic spermatozoal characters are present to support the placement of Leiopelma and Ascaphus in the same family. Spermatologically, Leiopelma is the plesiomorph sister taxon of all other anurans and is thus confirmed as the most basal (primitive) anuran. A further finding is that the Pelodytidae, sometimes considered basal archaeobatrachians, share the synapomorphy of a conical perforatorium divided into fibres with the Bufonoidea.
Amphibia, Anura, Leiopelmatidae, spermatozoa, ultrastructure, phylogeny.