NEW EVIDENCE FOR PELECANIFORM (AVES: PELECANIFORMES) DIVERSITY DURING THE EOCENE OF WYOMING

The Fossil Butte Member of the Green River Formation boasts some of the most well-preserved specimens of Eocene avians. Among these birds are two genera of Pelecaniformes: a new pelecaniform, the sister-taxon to Fregata (frigatebirds), and Limnofregata, the sister-taxon to a clade composed of Sula (boobies) and Morus (gannets). The new pelecaniform genus differs from Limnofregata azygosternon by the presence of 1. a deeply hooked rostrum that curves below the tip of the mandible, 2. an orbital fenestra with dorsal margin deeply incised into the frontal in conjuction with a drastic drop in the caudal edge of the orbit as it approaches the processus postorbitalis such that the margin is straightened, 3. a broader jugal, 4. a flat-edged ventral termination of the facies articularis caudalis in the cervical vertebrae, 5. the gradual closure pattern of the bases from which the processes costalis projects in the cervical vertebrae such that fusion of these bases occurs in association with keel formation, 6. a medially-depressed facies ventralis in medial and caudal cervicals, 7. a faint, ventrally-raised ridge in the caudal region of the synsacrum, 8. a pointed dorsal termination of the crista cnemialis cranialis of the tibiotarsus, 9. a tibiotarsus lacking a prominent internal ligamental prominence on its distal end, 10. a shallower sulcus extensorius that does not extend distally to the level of the foramen vasculare distale in the tarsometatarsus, and 11. a nonangular distal termination of the trochlea metatarsi. Some of the morphological features that distinguish the two pelecaniform genera may represent adaptations to feeding behaviors, which serve to separate the ecological niches filled by these taxa as in modern assemblages of multiple pelecaniform species.