Abstract
In 2006, a partial avian femur (South Dakota School of Mines and Technology (SDSM) 78247) from the Upper Cretaceous (Maastrichtian) Sandwich Bluff Member of the López de Bertodano Formation of Sandwich Bluff on Vega Island of the northern Antarctic Peninsula was briefly reported as that of a cariamiform—a clade that includes extant and volant South American species and many extinct flightless and cursorial species. Although other authors have since rejected this taxonomic assignment, SDSM 78247 had never been the subject of a detailed description, hindering a definitive assessment of its affinities. Here we provide the first comprehensive description, illustration, and comparative study of this specimen. Comparison of characters that may be assessed in this femur with those of avian taxa scored in published character matrices refutes the inclusion of SDSM 78247 within Cariamiformes, instead supporting its assignment to a new, as-yet unnamed large-bodied species within the genus Vegavis, and therefore its referral to a clade of semiaquatic anseriforms. Important character states diagnostic of Vegavis + Polarornis include strong craniocaudal bowing of the femoral shaft, the presence of a distinct fossa just proximal to the fibular trochlea, and the broad and flat shape of the patellar sulcus. Referral to Vegavis is based on the presence of a distinctive proximocaudal fossa and distolateral scar. This genus was previously known only from Vegavis iaai, a smaller-bodied taxon from the same locality and stratigraphic unit. Our reassignment of SDSM 78247 to Vegavis sp. removes the record of cariamiform landbirds from the Antarctic Cretaceous.
Highlights
In 2005, an expedition co-directed by one of us
The stratigraphic position of the specimen places it in the lower part of the uppermost Cretaceous (Maastrichtian) Sandwich Bluff Member of the López de Bertodano Formation, at a level some 12 m upsection from the concretionary horizon that yielded the holotype and referred skeletons of the anseriform bird Vegavis iaai (Clarke et al, 2005, 2016)
Of the four femoral character states proposed by Agnolín et al (2017) as diagnostic for Vegaviidae that are scorable in SDSM 78247, the new femur is consistent with three: (1) craniocaudal bowing of the shaft; (2) the presence of a distinct fossa just proximal to the fibular trochlea; and (3) a broad and flat shape of the patellar sulcus
Summary
In 2005, an expedition co-directed by one of us (J.A. Case) recovered a partial avian left femur as part of a field investigation of Upper Cretaceous sedimentary units in the James Ross Basin of the northern Antarctic Peninsula (Fig. 1). Gastrocnemialis lateralis tubercle forms a rugose scar on the lateral face proximocranial to the fibular trochlea; (211) the medial condyle comprises approximately half of the total width across the condyles; (213) the patellar sulcus is broad and flat in cranial view; (214) the presence of a notch for the m. Gastrocnemialis lateralis as a round scar near the fibular trochlea; (209) the absence of a distinct depression immediately proximal to the caudal articular surface of the fibular trochlea; (211) the medial condyle contributing more than half of the maximum mediolateral width across the condyles; (215) a deep popliteal fossa; (218) the lateral edge of the distal end of the shaft in caudal view is smoothly curving and continuous with the condyle; and (220) the fibular trochlea is caudally directed, merging smoothly into the shaft. In the new femur, the patellar sulcus is shallow and broad
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