Protoplotus beauforti Lambrecht, 1931 is possibly among the earliest representatives of the Fregatidae/Suloidea clade. The species is based on a nearly complete skeleton from lacustrine sediments of Sumatra. The age of these deposits is debated, but they probably date from the Eocene or even the Paleocene (van Tets et al. 1989; Stidham et al. 2005).
P. beauforti is a small species, of a size similar to that of the extant Little Pied cormorant, Phalacrocorax ("Microcarbo") melanoleucos. The tarsometatarsus is very short, and the trochlea for the second toe protrudes distally beyond those for the other toes. As in the stem group frigatebird Limnofregata (Sect. 7.1.2), the narial openings of P. beauforti are long and slit-like, whereas they are greatly reduced in extant Fregatidae and Suloidea. Protoplotus differs from Limnofregata in, for example, the shape of the mandible and the proportionally longer legs.
P beauforti was originally described as a representative of the Anhingidae (Lambrecht 1931b). Van Tets et al. (1989) classified the species into a new taxon, Protoplotidae, and detailed that its anhingid affinities have not been well established. P. beauforti differs in many osteological features from crown group Anhingidae, for example, the mandibular rami are tightly joined in their rostral half (as in the stem group tropicbird Prophaethon; Sect. 7.11), the cervical vertebrae are proportionally shorter, and the limb elements have different proportions, with ulna and tarsometatarsus being relatively longer. Although the species more closely resembles the Anhingidae and Phalacrocoracidae in skeletal features than the Fregatidae and Sulidae, a closer relationship to the former two taxa has not yet been established with derived characters.
The holotype specimen of P. beauforti is remarkable because it preserves a dense accumulation of large gastroliths, which led Lambrecht (1931b) to hypothesize that the species was granivorous. By contrast, van Tets et al. (1989) assumed that it foraged under water and fed on small fish and invertebrates. These authors further noted that the gastroliths may have served as ballast to aid diving, whereas Zhou et al. (2004) argued that Protoplotus could have employed seasonal diet switching from fish to plant material.
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