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

An avian vocal organ from the Mesozoic.

The Vegavis iaai specimen showing the location of the syrinx. (Adapted from Clarke et al., 2016)

The Vegavis iaai specimen showing the location of the syrinx. (Adapted from Clarke et al., 2016)

Birds originated from a theropod lineage more than 150 million years ago. Their evolutionary history is one of the most enduring and fascinating debates in paleontology. In recent years, several discovered fossils of theropods and early birds have filled the morphological, functional, and temporal gaps along the line to modern birds. The earliest diversification of extant birds (Neornithes) occurred during the Cretaceous period and after the mass extinction event at the Cretaceous-Paleogene (K-Pg) boundary, the Neoaves, the most diverse avian clade, suffered a rapid global expansion and radiation. Today, with more than 10500 living species, birds are the most species-rich class of tetrapod vertebrates.

In the mid-nineteenth century, T. H. Huxley recognized that birds were most closely related to dinosaurs. He also named the unique vocal organ in birds as the syrinx. Located at the base of a bird’s trachea, the syrinx consists of specialised cartilaginous structures, connective tissue masses, membranes and muscles. The oldest known remains of a syrinx was found within the fossilised, partial skeleton of a bird, known as Vegavis iaai, from the Late Cretaceous (66 mya) of Antarctica.

Vegavis iaai by Gabriel Lio. / Photo: CONICET

Vegavis iaai by Gabriel Lio. / Photo: CONICET

The Vegavis iaai holotype specimen 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). It belonged to the clade Anseriformes, a group that includes ducks, geese and swans. Vegavis exhibits the fusion of cartilage rings and asymmetry between the left and right sides of the syrinx, that are useful for making comparisons with structural data from the present-day birds. Fused rings in Vegavis form a well-mineralized pessulus, a derived neognath bird feature, proposed to anchor enlarged vocal folds or labia. Although mineralized structures of the syrinx in Vegavis and many parts of extant Anatidae show asymmetry, Presbyornis, Chauna and Galliformes lack this feature. The absence of known tracheobronchial remains in all other Mesozoic dinosaurs may be indicative that a complex syrinx was a late arising feature in the evolution of birds, well after the origin of flight and respiratory innovations.

 

References:

Julia A. Clarke, Sankar Chatterjee, Zhiheng Li, Tobias Riede, Federico Agnolin, Franz Goller, Marcelo P. Isasi, Daniel R. Martinioni, Francisco J. Mussel and Fernando E. Novas. Fossil evidence of the avian vocal organ from the Mesozoic. Nature, 2016 DOI: 10.1038/nature19852

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.

Larsen, O. N.; Franz Goller (2002). “Direct observation of syringeal muscle function in songbirds and a parrot”. The Journal of Experimental Biology. 205 (Pt 1): 25–35.

Xing Xu, Zhonghe Zhou, Robert Dudley, Susan Mackem, Cheng-Ming Chuong, Gregory M. Erickson, David J. Varricchio, An integrative approach to understanding bird origins, Science, Vol. 346 no. 6215, DOI: 10.1126/science.1253293.