The youngest egg with avian affinities from the Cretaceous of Patagonia

FERNÁNDEZ, MARIELA S.; SALGADO LEONARDO

Historical Biology

Taylor and Francis

Lugar: London; Año: 2018

1029-2381

We report the youngest fossil egg of avian affinities from the Cretaceous (Allen Formation, Campanian-Maastrichtian) of Patagonia (Salitral Ojo de Agua, Río Negro Province, Argentina). The fossil consists in most of the internal cast of a single egg, preserving some eggshell fragments on one of its poles. Its greater axis is 4.5 cm whereas its lesser axis is 3.06 cm. The eggshell surface is smooth, and its thickness is about 400µm. The eggshell ultrastructure reveals two layers, with an ornithoid basic type and a ratite morphotype. The well-developed mammillary layer is 154µm thick, with petal-shaped mammillae; in turn, the squamatic layer is 251 µm thick. The ratio between the thickness of the continuous layer and that of the mammillary layer is 1:1.6 (CL:ML). Shell microstructure is consistent with Laevisoolithidae, an ootaxon traditionally associated with enantiornithid birds. The shape of fibers of the testaceous membrane (TM) is preserved; these are flattened, with a thickness of 1.1µm and a width of 4.8µm each. The arrangement of the fibers of the TM is random, similar to that observed in extant reptiles. The preservation of the TM suggests extraordinary site conditions. EDAX analysis reported that fibers are enriched in traces of Mg, Al, Si, P, F, and K, elements that are also present in both extant and fossil shell membrane. Cladistic analysis performed reveals avian affinities, which is consistent with the macro, micro and ultrastructural characteristics.