Of the four genera, Osteocephalus is the most generalized in morphology, and, like Phrynohyas, has no specialized habits. Osteocephalus and Argenteohyla are similarly distinguished from Phrynohyas and Trachycephalus on the basis of vocal sac structure. The vocal sacs of Osteocephalus and Argenteohyla are posterior and protrude posterolateral to the angles of the jaws when they are inflated, whereas those of Phrynohyas and Trachycephalus are more lateral and protrude posterior to the angles of the jaws when inflated.
Although Osteocephalus and Argenteohyla have similar vocal sac structure, they are obviously distinct. The monotypic Argenteohyla is a rather specialized, semifossorial frog (Trueb, 1970b), characterized by smooth skin, moderate-sized digital discs, and a large inner metatarsal tubercle. The general architecture of the skull is not unlike that of Osteocephalus; the skulls of both are well roofed, broader than long, and characterized by posterolaterally oriented parasphenoid alae. Argenteohyla bears small, slightly curved prevomerine dentigerous processes in contrast to the large, angular processes of Osteocephalus. The skull of Argenteohyla shows specializations, apparently adaptations to its semifossorial mode of existence, which further distinguish the genus from Osteocephalus. In comparison with Osteocephalus, the cranium of Argenteohyla is slightly depressed anteriorly, the roofing bones extensively casqued, and the palatines robust.
Osteologically, Osteocephalus more closely resembles Phrynohyas than either of the other two genera, but Osteocephalus and Phrynohyas are clearly distinct on the basis of their respective vocal sac structure. Like Osteocephalus, skulls of the members of the genus Phrynohyas are broader than long, have extensive dermal roofing bones, and have posterolaterally oriented parasphenoid alae. In contrast to Osteocephalus, the dentigerous processes of the prevomers are curved, rather than angular in Phrynohyas. Furthermore, the latter genus is singularly distinguished from Osteocephalus, Argenteohyla, and Trachycephalus by having extensively developed parotoid glands that produce a viscous, milky volatile secretion.
Trachycephalus is the most readily identifiable of the four genera under discussion. Members of this genus are large frogs with heavily casqued and co-ossified skulls (Trueb, 1970a). The dermal roofing bones bear ornate and characteristic patterns of sculpturing. The medial ramus of the pterygoid does not articulate with the otic capsule, and the parasphenoid alae are laterally, rather than posterolaterally, oriented. A dermal sphenethmoid is present, and the parasphenoid bears odontoids. The basic structure of the skull has many characters in common with both Osteocephalus and Phrynohyas. The obvious modifications of dermal roofing bones and of palatal and suspensory elements seem to be specializations adapting members of the genus Trachycephalus to their peculiar phragmotic habits. The vocal sac structure of Trachycephalus is like that of Phrynohyas and therefore further distinguishes it from Osteocephalus.
Morphologically, Osteocephalus seems to be sufficiently diverse and generalized so as to represent a modern derivative of an ancestral type which might have given rise to Phrynohyas, Trachycephalus, and Argenteohyla. The specialized vocal sac structure in Phrynohyas and Trachycephalus suggests that these two genera may be rather closely allied and represent a single phyletic line from an ancestral stock similar to Osteocephalus. Argenteohyla is quite distinct from Phrynohyas and Trachycephalus and apparently represents a distinct phyletic line from the ancestral stock.
OCCURRENCE OF OSTEOCEPHALUS IN AMAZONIAN ECUADOR
All of our observations on members of this genus have been made at four localities: 1) Santa Cecilia at an elevation of 340 meters on the Río Aguarico, a tributary of the Río Napo, 2) Lago Agrio, 330 meters, about 14 kilometers east of Santa Cecilia, 3) Puerto Libre, 570 meters, on the Río Aguarico just east of its formation by the confluence of the Río Cofanes and Río Chingua, and 4) south slope of the Cordillera del Dué, above the Río Coca, 1150 meters. Osteocephalus leprieurii was found at all four localities, and buckleyi was found at all but the last; taurinus was found at Santa Cecilia and Lago Agrio, and verrucigerus was found only in the Cordillera del Dué. Our data are based on collections of 113 frogs and three lots of tadpoles, as well as observations on calling sites and young. The observations are summarized by species, as follows:
Osteocephalus buckleyi.—No breeding activity was observed. Males were found only at night in March, June, and July. One was perched on a Heliconia leaf in a swamp at Puerto Libre, and two were on bushes in the forest at Santa Cecilia. A gravid female was found on a recently felled tree at Lago Agrio on the night of 12 May 1969.
Osteocephalus leprieurii.—Males were heard calling sporadically at Puerto Libre in July 1968, and at Santa Cecilia in May 1969. A small chorus was found on the night of 12 May 1969 at Lago Agrio, where the frogs were perched on branches of fallen trees over a temporary pool. The call is a soft rattling chuckle. In late April and May many gravid females and males with well-developed nuptial excrescences were obtained from trees as they were felled at Lago Agrio. The reproductive condition of the frogs indicates that they probably breed in May. One individual called nearly every night from a large tree at Puerto Libre between 4-17 July 1968. The tree was felled on the latter date, but no frog was found. Two nights later apparently the same individual called from a bromeliad at a height of about 10 m on a large bamboo adjacent to the felled tree; the frog was collected when the bamboo was cut down.
Throughout the rainy months that we have worked in Ecuador (April-August) we have found occasional individuals perched on bushes or low trees at night. Large numbers of adults were observed only during a clearing operation which resulted in the felling of many large trees. Thus, it seems likely that leprieurii is a tree-top inhabitant. A partially digested adult male was removed from the stomach of a Hemiphractus proboscideus.