Urogenital Morphology of Dipnoans, With Comparisons to Other Fishes and to Amphibians

The morphology of the urogenital system of extant dipnoans is compared among the three genera, and to that of other fishes and amphibians. Analysis is based on dissections, sectioned material, and the literature. Urogenital system morphology provides no support for the hypothesis of a sister-group relationship between dipnoans and amphibians, for virtually all shared characters are primitive, and most characters shared with other fishes are also primitive. Urogenital morphology is useful at the familial level of analysis, however, and synapomorphies support the inclusion of Lepidosiren and Protopterus in the family Lepidosirenidae separate from Neoceratodus of the family Ceratodontidae. Analysis of relationships of the Dipnoi to other fishes and to tetrapods have largely been based on the morphology of hard tissues-the skeleton, tooth plates, and scales. Several studies of other systems are available in the literature, but most are descriptive and few are comparative, at least in considering all three genera of living lungfish. Soft-tissue systems have had limited evaluation in terms of assessing evolution and relationships. Several current hypotheses of the relationships of the Dipnoi exist. These include the proposal by Romer (’66) that dipnoans are the sister group to actinistians and rhipidistians; that of Miles (‘77) classifying dipnoans as the sister group of rhipidistians only; and that of Rosen et al. (’81) in which dipnoans are the sister group of tetrapods, with actinistians the outgroup and rhipidistians (porolepiforms) in an unresolved cladistic position. Within the Recent Dipnoi, Neoceratodus (one species: forsteri) is placed in the family Ceratodontidae; Lepidosiren (one species: paradoxa) and Protopterus (four species: amphibius, aethiopicus, annectens, and dolloi) are placed in the Lepidosirenidae. Soft-tissue morphology, because of the extreme paucity of extinct forms, can offer virtually nothing to our understanding of the relationships of dipnoans to rhipidistians, and little to the association of dipnoans with actinistians (Latimeria not necessarily being “representative”). However, soft-tissue morphology can contribute to information about dipnoan-tetrapod relationships (Rosen et al., ’81, utilized muscle characters, for example) and to generic and familial relationships. This survey and review of urogenital morphology of the three genera of extant dipnoans and of the literature is undertaken with the following objectives: 1) to provide information from a direct comparison of representatives of all three extant genera; 2) to see if the urogenital system offers support for any of the current hypotheses of dipnoan relationships; and 3) to determine whether or not the urogenital system offers additional characters for the familial allocation of the genera. Urogenital morphology and physiology have received little consideration in these contexts (see, for example, Thomson, ’69; Rosen et al. ’81). In this paper I review the morphology of the components of the urogenital system in dipnoans, adding new information where possible, and I compare the morphology of these components among dipnoan species as well as with the urogenital structures of other fishes and amphibians. For each component I discuss the implications of the morphology for assessment of relationships and suggest directions for further research. MATERIALS AND METHODS Urogenital organs from five specimens of Lepidosiren paradoxa (220-300 mm standard length) from Laguna Oca, Rio Paraguay, Formosa, Argentina, six of Protopterus aethiopicus (230-320 mm standard length) from Kusa Beach, Lake Victoria, near Kisumu, Kenya,