An avian femur from the Late Cretaceous of Vega Island, Antarctic Peninsula: removing the record of cursorial landbirds from the Mesozoic of Antarctica

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.

Keywords: Anseriformes; Antarctica; Bird; Cariamiformes; Cretaceous; Femur; López de Bertodano Formation; Maastrichtian; Vega Island; Vegavis.

Figure 1. Map of Vega Island, Antarctica, modified from Roberts et al. (2014).

Star indicates approximate location of Sandwich Bluff, the site that yielded the avian femur (SDSM 78247) described herein.

Figure 2. Photographs of left femur of Vegavis sp. (SDSM 78247).

Proximal fragment in (A) cranial, (B) caudal, (C) lateral, (D) medial, and (E) proximal views; distal fragment in (F) cranial, (G) caudal, (H) lateral, (I) medial, and (J) distal views. Arrows in E and J indicate cranial (top of image) and medial (left side of image) directions.

Figure 3. Line drawings of left femur of Vegavis sp. (SDSM 78247).

(A) Cranial, (B) caudal, (C) lateral, (D) medial, (E) proximal, and (F) distal views. Arrows in E and F indicate cranial (top of image) and medial directions. Hatching indicates worn or broken areas. Abbreviations: af, antitrochanteric face; ds, distolateral scar; fc, fibular crest; ft, fibular trochlea; ftc, fovea for insertion of tendon of m. tibialis cranialis; imi, insertion for m. iliotrochantericus; lc, lateral condyle; mc, medial condyle; msc, medial supracondylar crest; pf, proximocaudal fossa; pop, popliteal fossa; ps, patellar sulcus; tfc, tibiofibular crest; tmg, tubercle for insertion of m. gastrocnemialis. Scale bar = 1 cm.

Figure 4. Comparison of left femora.

(A, E, I, M, Q) Vegavis sp. (SDSM 78247), (B, F, J, N, R) Vegavis iaai (cast of MACN-PV 19.148, reversed), (C, G, K, O, S) Polarornis gregorii (TTU P 9265), and (D, H, L, P, T) Cariama cristata (TMM M-10446), in cranial (A–D), lateral (E–H), caudal (I–L), medial (M–P), and distal (Q–T) views. Abbreviations as in Fig. 3 except: h, femoral head; is, intercondylar sulcus; le, lateral epicondyle; me, medial epicondyle; t, femoral trochanter. Scale bar = 1 cm.