Earliest fossils of giant-sized bony-toothed birds (Aves: Pelagornithidae) from the Eocene of Seymour Island, Antarctica

Peter A Kloess,Ashley W Poust,Thomas A Stidham

doi:10.1038/s41598-020-75248-6

Abstract

While pelagornithid or ‘bony-toothed’ bird fossils representing multiple species are known from Antarctica, a new dentary fragment of a pelagornithid bird from the middle Eocene Submeseta Formation on Seymour Island, Antarctica represents a species with a body size on par with the largest known species in the clade. Measurements from the partial ‘toothed’ dentary point to a giant body size for the species, although the spacing among the pseudoteeth differs from that published for other pelagornithids. The discrepancy might suggest that previous techniques are not adequate for examination of incomplete material or that another factor such as phylogeny might impact size estimates and comparisons. Combined with a revised stratigraphic position in the early Eocene La Meseta Formation on Seymour Island for the largest pelagornithid tarsometatarsus known, these Antarctic fossils demonstrate the early evolution of giant body size in the clade (by ~ 50 Ma), and they likely represent not only the largest flying birds of the Eocene but also some of the largest volant birds that ever lived (with an estimated 5–6 m wingspan). Furthermore, the distribution of giant-sized pelagornithid fossils across more than 10 million years of Antarctic geological deposits points to a prolonged survival of giant-sized pelagornithids within the southern seas, and their success as a pelagic predatory component of marine and coastal ecosystems alongside early penguins.

Highlights

While pelagornithid or ‘bony-toothed’ bird fossils representing multiple species are known from Antarctica, a new dentary fragment of a pelagornithid bird from the middle Eocene Submeseta Formation on Seymour Island, Antarctica represents a species with a body size on par with the largest known species in the clade
The published specimens from these formations include two partial maxillae, one fragment of a humerus, one dentary fragment, and one distal tarsometatarsus. We add to this assemblage by describing a > 12 cm long pelagornithid dentary fragment, University of California Museum of Paleontology (UCMP) 323792 (Fig. 2)
The reassessed stratigraphic placement of this specimen to the La Meseta Formation, where it joins other La Meseta specimens identified as “large” size-types, indicates that Seymour Island was inhabited by two different size classes of pelagornithids during the early Eocene

Summary

C17 C16 C15 C13

A review of the original locality information indicates that rather than the Submeseta Formation, the tarsometatarsus was recovered from the same stratigraphic horizon as UCMP locality RV8200 (formerly RV-8200) within Telm 5 (the La Meseta Formation). The Cucullaea I Allomember is dated between 51.6 and 49.1 Ma, and Telm 5 is dated between 51.3 and 46.2 Ma13,18 These dates yield an estimate of 51.3–49.1 Ma for the age of fossils from UCMP locality RV8702 (Fig. 4). The reassessed stratigraphic placement of this specimen to the La Meseta Formation (this study), where it joins other La Meseta specimens identified as “large” size-types (estimated 3.5–4.5 m wingspan sensu Cenizo et al.9), indicates that Seymour Island was inhabited by two different size classes of pelagornithids during the early Eocene

Materials and methods

Discussion