Giganhinga is a genus of giant darter that lived during the Late Miocene to Early Pleistocene in what is now Uruguay and Argentina. The largest species of anhinga known to science, estimates suggest it may have weighed around and was likely flightless. Its weight likely helped it dive for prey and the anatomy of the pelvis indicates that it was a good and maneuverable swimmer. Only a single species is currently recognized, G. kiyuensis.
History and naming
Giganhinga was described by Rinderknecht and Noriega in the year 2002 based on specimen MNHN 1632, an incomplete pelvis. Due to the fossils immense size and weight, it was at first thought to belong to a terror bird before it was identified as an anhinga. The exact stratigraphic layer of Giganhinga is unknown, as there are three outcrops present in the region where the fossil has been found. These three outcrops belong to the Camacho Formation, the San José Formation and the Libertad Formation. However, based on sand and gravel attached to the fossil Rinderknecht and Noriega rule out both the Camacho and Libertad Formations, concluding that the material could only stem from the Pliocene to Pleistocene sediments of the San José Formation. Another indication of this is the colour of the fossil. Although most of the surface has taken on a darker coloration, areas filled with sand managed to preserve its original colour, which is pinkish and thus matches those of other fossils from the San José Formation. A second specimen, the distal end of an enormous femur, was found in the Argentinian Ituzaingó Formation and tentatively assigned to cf. Giganhinga based on its massive size.
The name Giganhinga is a combination of the Greek "gigas" (γίγας) meaning giant and the name Anhinga, which is the genus including all modern darter species (and itself derives from the Tupi language and means "devil bird" or "snake bird"). The species name "kiyuensis" is based on the beach of Kiyü, a balneario in southern Uruguay on the Rio de La Plata.
Description
The holotype pelvis of Giganhina is somewhat deformed and heavily eroded, having lost most of its original texture and its original colour and having undergone remineralisation. The pelvis is notably more robust than those of any extant or extinct darter species and has strong attachment points for musculature while lacking pneumatization. In addition to being incredibly robust, the fossil is also of large size, approximately 500% larger than the known material of the second-largest anhinga species, Macranhinga paranensis. The cranial section of the synsacrum is composed of strongly united vertebrae which are taller than they are wide and laterally compressed, said compression growing more pronounced further up the vertebral column. However this compression is relatively less developed than in cormorants and other anhingas. Like in other anhingas, the first vertebra of the synsacrum is opisthosacral. The ventral margin of the synsacrum remains parallel to the axis of the body while opposing the first two vertebrae, only to project more dorsally with the onset of the third vertebra, creating an angle in the bone similar to what is observed to other South American fossil anhingas. At the level of the sacral vertebrae there is a notable fossa which is deeper towards its caudal end similar to Anhinga but not as developed as in either Meganhinga or Macranhinga. The type material shows six preacetabular foramina on each side of the fossil, growing smaller in diameter the further back they are located and with a somewhat larger gap between the last two foramina. The socket that received the femur is closed and notably enlarged and the space between these cavities (of which only one is preserved) is located further ventrally than in the relatives of Giganhinga. The neural spine before the sockets is higher than in any other pelecaniform and gradually decreases in height as it approaches the acetabulum, bifurcating over the socket and ending in two robust postrochanteric processes. This process too is higher than in Anhinga and even Meganhinga and terminates further back than in either of these two taxa, bearing a closer resemblance to Macranhinga. The antitrocanter has a large surface and is oriented in a more perpendicular fashion relative to the synsacrum.