Supporting information for: Flight Performance of the Largest Volant Bird Daniel T. Ksepka National Evolutionary Synthesis Center, 2024 W. Main Street, Durham, NC 27705 USA. Department of Marine, Earth and Atmospheric Sciences, North Carolina State University, Raleigh, NC 27695 USA. Contents: Supporting text: Additional details of geological context and species diagnosis. Figure S1. Additional fossil birds represented in the Chandler Bridge avifauna. Table S1. Alternate estimates of primary feather length in P. sandersi. Table S2. Estimated Gliding Flight Parameters for P. sandersi. Figure S2. Individual results of analyses 1-12 in Flight 1.25 showing glide polars. Figure S3. Individual results of analyses 13-24 in Flight 1.25 showing glide polars Figure S4. Individual results of analyses 1-12 in Flight 1.25 showing lift:drag versus speed. Figure S5. Individual results of analyses 13-24 in Flight 1.25 showing lift:drag versus speed. Table S3. Estimated Power Parameters for P. sandersi. Figure S6. Individual results of power curve analyses 1-12 in Flight 1.25. Figure S7. Individual results of power curve analyses 13-24 in Flight 1.25. Supporting References. Locality Data ChM PV4768 was collected during construction at the Charleston Airport, at a central channel approximately 1.2km from Dorchester Road (SC route 642) by Albert Sanders and colleagues. All elements were collected in tight association from a single block excavated from Bed 2 of the Chandler Bridge Formation. The Chandler Bridge Formation is a thin (up to 5m) unit composed of fine-grained, quartz-phosphate sand with silty, calcareous clays occurring locally near the base of the unit (1). Although original efforts to obtain microfossils were unfruitful due to leaching of this permeable unit (2), calcareous nannofossils referable to NP25 have subsequently been discovered (3). These provide the basis for an age of ~25-27Ma (4), in close agreement with an estimate of 27- 28Ma based on the cetacean fauna (5), and placing the unit in the lower Chattian (Late Oligocene). The Chandler Bridge Formation can be subdivided into three units: Bed 1, Bed 2, and Bed 3 (1). Bed 2, which yielded ChM PV4768, has also yielded numerous whales and marine turtles (5-6). Bed 2 appears to have been deposited in an open shelf or open bay environment below the wave base (6-7). Several additional pelagornithid elements have been recovered from Bed 2 of the Chandler Bridge Formation including a coracoid and two partial humeri (Fig. S1). These elements are substantially smaller than those of the holotype individual of P. sandersi. Because of poor preservation, it is not possible to determine whether they represent an immature individual of the same species, or an adult of a smaller pelagornithid taxon. Seven other avian species are represented in the Chandler Bridge Formation, all of which belong to marine groups (Fig. S1). Four species of Sulidae and three species of Procellariiformes are represented, and will be fully described elsewhere. A total of 28 elements can be assigned to Sulidae (boobies and gannets) based on features including presence of multiple pneumatic foramina on the processus acrocoracoideus of the coracoid, (also present in the otherwise dissimilar Fregatidae and Pelecanidae), a mediolaterally narrow facies articularis sternalis of the coracoid, an extremely deep fossa olecrani of the humerus, and presence of a pneumatic foramen near the dorsal border of the impressio m. brachialis of the ulna (also present in Fregatidae and Pelecanidae) (see 8). Six specimens can be assigned to the small albatross taxon Plotornis (9) based on the combination of a shallow, poorly defined fossa m. brachialis, moderately projected processus supracondylaris dorsalis (in contrast to the elongate processus in extant Diomedeidae), and elongate tuberculum supracondylare ventrale which extends proximally beyond the level of the processus supracondylaris dorsalis. Finally, two humeri represent Procellariidae (petrels) based on the absence of a pneumatic foramen in the fossa tricipitalis, deep, pit-shaped fossa m. brachialis, and well-developed processus supracondylaris dorsalis. Figure S1: Additional avian remains from the Chandler Bridge Formation. Pelagornis sp.: (a) right humerus (ChM uncataloged) and (b) left coracoid (ChM uncataloged). Sulidae: (c) left coracoid (ChM PV2851), (d) distal end of right humerus (ChM PV2846), and (e) right tarsometatarsus (ChM PV4737). Plotornis sp.: (f) distal end of right humerus (ChM PV4740). Procellariidae: (g) right humerus (ChM PV 3407). Abbreviations: fmb - fossa m. brachialis, fo - fossa olecrani, pf - pneumatic foramina, psd - processus supracondylaris dorsalis. Additional Comparisons to other species of Pelagornithidae Comparisons follow recent taxonomy proposals (10-11) to synonymize Palaeochenoides, Pseudodontornis, Tympanonesiotes, and Osteodontornis with Pelagornis. The suggestion that Odontopteryx and Macrodontopteryx be synonymized with Dasornis (12) is also adopted. Comparisons below are intended to support the diagnosis of a new species. A full osteological description will be published elsewhere. Three species of Pelagornithidae were identified from South Carolina prior to the discovery of P. sandersi. Unfortunately, the stratigraphic horizon remains uncertain for all of these specimens. They were originally assigned to the Late Miocene Hawthorne Formation, but this unit is not exposed in South Carolina. It was previously determined that the fossils are more likely from Oligocene deposits of either the Cooper Formation or Chandler Bridge Formation (13). The Cooper Formation has since been elevated in status to the Cooper Group and it is evident that the specific unit referred in past work (13) is the Ashley Formation (formerly the Ashley Member). Where both units are in contact, a burrowed unconformity separates the Chandler Bridge Formation from the underlying Ashley Formation (3). The Ashley Formation has yielded microfossils referable to nannoplankton zones NP24 and NP25 (3) and so is close in age to the Rupelian/Chattian boundary. Pelagornithid fossils collected from the Ashley Formation are thus most likely ~2Ma older than those from the Chandler Bridge Formation (5). Pelagornis (=Palaeochenoides) mioceanus was named based on the distal end of a femur (14), which limits comparisons. P. sandersi differs from P. mioceanus in having a shallow sulcus on the trochlea fibularis, which is considered a derived feature within Pelagornithidae (10). P. mioceanus instead exhibits a deep, well-defined sulcus. Based on the overall proportions of the femur, P. sandersi was also approximately 15% larger than P. mioceanus. Hopson (15) assigned a fragment of a mandible including several pseudoteeth to Pelagornis (=Pseudodontornis) longirostris. The holotype of this species is a partial skull of unknown age and locality (16) that appears to have been destroyed in World War II (13). Comparisons to P. longirostris must rely on figures and descriptions of the holotype, which indicate the caudal portion of the mandible was substantially deeper than in P. sandersi. Although the presence of only one small pseudo-tooth between the largest pseudo-teeth was considered characteristic for P. longirostris (10) the smallest teeth were likely lost due to wear, and thus this pattern may not to be useful for diagnosis (15,17). While the shape of the mandible differentiates P. sandersi from P. longirostris, the fragmentary nature of Hopson's South Carolina specimen does not permit a confident species referral and it should be considered Pelagornithidae indet. Pelagornis (=Tympanonesiotes) wetmorei was named based on a fragmentary tarsometatarsus (15). Its exact age is even less certain than the other two specimens discussed here due to reworking, though it is likely Oligocene in age (13). This specimen indicates an individual less than 2/3rds the size of P. sandersi, which suggests conspecificity is unlikely. Meaningful comparisons are otherwise precluded by the fragmentary state of the material. In comparison to other species of Pelagornis from outside the Atlantic coast of North America, P sandersi can be distinguished from the Miocene Pelagornis miocaenus of Europe (known only from a humerus; note that this is a separate species from Pelagornis mioceanus) and the Pliocene Pelagornis (=Osteodontornis) stirtoni of New Zealand by a >50% size difference, and additionally differs from P. stirtoni in markedly more robust femur. In P. stirtoni, the mandible is described as having an intraramal suture (18), whereas the caudal and rostral portions were connected only by a thin splint of bone in P. sandersi (see 19). Aside from size (>15% larger), P. sandersi differs from Pelagornis mauretanicus from the Pliocene of Morocco in the different pattern of pseudotooth emplacement in the caudal part of the mandibular tooth row (the only directly comparable section). Previous anatomical studies (20) classified pseudoteeth into four easily distinguished types: rank 1 (very large), rank 2 (medium), rank 3 (small), and rank 4 (very thin and spine-like). The caudalmost rank 1 pseudotooth is separated from the next rank 1 pseudotooth by a sequence of a rank 3 pseudotooth, a rank 2 pseudotooth and a rank 3 pseudotooth in P. sandersi. In P. mauretanicus, the pattern in this portion of the jaw is instead rank 2, rank 4, rank 3, rank 4, rank 1. Although it is uncertain how much intraspecific variation exists in pseudotooth pattern, the large amount of temporal (~20Ma) and geographical disparity