Osteohistological analysis of Vegavis iaai

Vegavis iaai by Gabriel Lio. / Photo: CONICET

The earliest diversification of extant birds (Neornithes) occurred during the Cretaceous period. Today, with more than 10500 living species, birds are the most species-rich class of tetrapod vertebrates. Vegavis iaai is the first unquestionable neornithine bird from the Cretaceous and is known by the holotype and specimen MACN-PV 19.748. The holotype specimen MLP 93-I-3-1 (Museo de La Plata, Argentina) from Vega Island, western Antarctica, was discovered in 1992 by a team from the Argentine Antarctic Institute, but was only described as a new species in 2005 (Clarke et al., 2005). Polarornis gregrorii, from the López de Bertodano Formation of Seymour Island, Antarctica, and Vegavis form a monophyletic basal clade of foot-propelled anseriform birds (Agnolín 2016), a group that includes ducks, geese and swans.

Osteohistological analysis of the femur and humerus of V. iaai. shows a highly vascularized fibrolamellar matrix lacking lines of arrested growths, features widespread among modern birds. The femur has some secondary osteons, and shows several porosities, one especially large, posterior to the medullar cavity. The humerus exhibits a predominant fibrolamellar matrix, but in a portion of the anterior and medial sides of the shaft there are a few secondary osteons, some of them connected with Volkman’s canals, and near to these canals, there are a compact coarse cancellous bone (CCCB) with trabeculae. This tissue disposition and morphology suggests that Vegavis had remarkably high growth rates.

Detail of the humerus of Vegavis iaai (MACN-PV 19.748) in polarised light. Scale = 1 mm. (From G. Marsà et al., 2017)

Many studies on avian microanatomy have established a relationship between high bone compactness (i.e., considerable degree of osteosclerosis) and diving behavior. Differences in the degree of osteosclerosis could be tentatively related to variations in diving behaviour. Vegavis was a diver, characterised by a medium level of limb osteosclerosis. Polarornis, with more massive bones, was possibly adapted to deeper and more prolonged diving than Vegavis, as occurs in modern penguins.

The value of Relative Bone Thickness (RBT) in Vegavis is comparable with two genera of extant foot-propelled diving ducks. A high RBT is related with increased stiffening the forelimb, regardless of body mass or depth of diving. Flightless Pan-Alcidae and penguins, have a very rigid, flipper-like wings suggesting that decreased wing flexion and increased cortical thickness of forelimbs are somehow correlated. Based on  the values of RBT present in both Vegavis and Polarornis is possible to infer that these taxa were foot-propelled birds.

References:

Jordi Alexis Garcia Marsà, Federico L. Agnolín & Fernando Novas (2017): Bone microstructure of Vegavis iaai (Aves, Anseriformes) from the Upper Cretaceous of Vega Island, Antarctic Peninsula, Historical Biology, DOI: 10.1080/08912963.2017.1348503

Agnolín FL. 2016. A brief history of South American birds. Contribuciones del MACN 6:157–172

Clarke, J. A., C. P. Tambussi, J. I. Noriega, G. M. Erickson, and R. A. Ketcham. 2005. Definitive fossil evidence for the extant avian radiation in the Cretaceous. Nature 433:305-308. DOI: 10.1038/nature03150

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