Introduction
The fossil record of ibises is largely incomplete and very patchy. In this sense, the early record of ibises remains poorly documented, being represented by an isolated tibiotarsus from the Eocene of Myanmar (Stidham et al. Reference Stidham, Tsubamoto, Egi, Nishioka and Takai2016), and the enigmatic genus Rynchaeites from the Middle Eocene of Germany and Denmark (Mayr & Bertelli Reference Mayr and Bertelli2011).
In South America and Antarctica, the oldest records correspond to the Lower Miocene Protibis cnemialis Ameghino 1891 (Tonni, Reference Tonni1980), and an indeterminate Late Eocene ibis-like bill from Antarctica (Jadwiszczak et al. Reference Jadwiszczak, Gazdzicki and Tatur2008). The latter specimen was originally described by Jadwiszczak et al. (Reference Jadwiszczak, Gazdzicki and Tatur2008) who tentatively ascribed the bill to Threskiornithidae. If correctly identified, the specimen would constitute the oldest record of the clade for the Southern Hemisphere.
The Antarctic specimen is represented by two fragments of a possible bill belonging to a long-beaked bird (Jadwiszczak et al. Reference Jadwiszczak, Gazdzicki and Tatur2008). The fossil was collected at the Seymour Island, at the upper part of the La Meseta Formation, representing a nearshore or shallow marine environment, associated with penguin and gadiform fish remains. However, a review of the published information sheds doubts on the affinities of the specimen, and the aim of the present note is to challenge the conclusions of Jadwiszczak et al.
Discussion
The specimen was described in detail by Jadwiszczak et al. (Reference Jadwiszczak, Gazdzicki and Tatur2008). As those authors pointed out, it shows important differences to available beaks of Threskiornithidae and most long-beaked birds, including the notably slender and sharply pointed tip, an acute and keeled "dorsal margin", and the total absence of openings for sensory nerves. The latter is a key feature of Threskiornithidae, being represented by a dense honeycomb texture at the end of the beak (Cunningham et al. Reference Cunningham, Corfield, Iwaniuk, Castro, Alley, Birkhead and Parsons2013).
Further, in contrast to threskiornithids, it lacks the schizorhinal nares that are anteriorly extended by means of a deep longitudinal groove that reaches the tip of the beak (Ferreira & Donatelli Reference Ferreira and Donatelli2005). Finally, the element shows important longitudinal ridges absent in any bird beak.
These features are highly suggestive that the specimen does not represent a threskiornithid, or even a bird, rostrum. On the contrary, the specimen shows remarkable similarities with dorsal spine of chondrichthyan fishes. Specifically, chimaeroid fishes show numerous spaces within the spine (large vascular canals), show numerous longitudinal ridges along the spine blade, and a highly compressed and acute tip (e.g. Patterson Reference Patterson1965).
Results
Assuming correct identification, the spine could belong to any of the reported chimaeroid fishes known from the La Meseta Formation (e.g. Ischyodus, Chimaera, see Kriwet & Gazdzicki Reference Kriwet and Gazdzicki2003).
Conclusion
This re-examination of the work by Jadwiszczak et al. (Reference Jadwiszczak, Gazdzicki and Tatur2008), describing a possible ibis-like bill from the Eocene of Antarctica, results in the rebuttal of its threskiornithid, and even avian affinities. In fact, the specimen appears to belong to the dorsal spine of a chimaeroid chondrychthian. Whatever its identification among vertebrates, the specimen shows features that are not congruent with a threskiornithid beak. In this way, unambiguous fossil threskiornithids are excluded from the Eocene of Antarctica.
Acknowledgements
We thank G. Lio, J. D'Angelo, M. Motta, M. Aranciaga, F. Brissón Egli, G. Lo Coco and M. Cerroni for their comments and discussion on fossils from Antarctica and other southern landmasses. We also thank F. Novas for his constant guidance. We are sincerely grateful to Cliff Atkins, Sharon Cooke and an anonymous reviewer for their valuable comments on the manuscript.
