A new giant darter (Pelecaniformes:Anhingidae) from the upper Miocene of Argentina

ARETA, JUAN IGNACIO; NORIEGA, JORGE IGNACIO; AGNOLIN, FEDERICO

Quillan, Francia

Simposio; VI Meeting of the Society of Avian Paleontology and Evolution; 2004

Society of Avian Paleontology and Evolution-CNRS

Darters, also known as anhingas or snakebirds, are fairly large fresh waterbirds presently distributed in tropical regions on all continents. Until recently, fossil darters were mainly known from North America, Europe, and Africa. In the last decade, descriptions of various paleospecies in three different genera (Meganhinga, Macranhinga, and Giganhinga) revealed a radiation of large-bodied darters in the Tertiary of South America. Two of the largest species, Macranhinga paranensis and M. ranzii, came from upper Miocene and upper Miocene-lower Pliocene localities of Argentina and Brazil, respectively. A third one, Giganhinga kiyuensis, was recorded in upper Pliocene-lower Pleistocene geologic unit in Uruguay. In this contribution, we report the finding of the largest specimen of Anhingidae known to date. This specimen comes from the basal part of the Ituzaingó Formation, informally known as the Mesopotamian, which outcrops discontinuosly along the eastern cliffs of the Paraná River in Entre Ríos Province at Argentina. These beds are biochronologically assigned to the Late Miocene (Huayquerian Land-Mammal Age). The material, housed in the collections of the Museo Argentino de Ciencias Naturales Bernardino Rivadavia of Buenos Aires (MACN 12179), consists on a well preserved distal fragment of shaft of a femur with its distal end. It clearly separates from the Phalacrocoracidae by having the distal portion of the shaft straighter; fibular condyle less protruded laterally; posterior intercondylar fossa not as deep; crista supracondylaris medialis more developed and sharper. MACN 12179 can be distinguished from the genus Anhinga and resembles Macranhinga by presenting some characters with an intermediate condition between those observed in cormorants (Phalacrocoracidae) and those typical of living darters: shaft more robust; distal end wider and more produced posteriorly; origin of the external head (Pars lateralis) of M. gastrocnemius located more proximally, with the presence of the tuberculum M. gastrocnemialis lateralis correspondingly greater and located more proximal. The specimen cannot be directly compared with Giganhinga kiyuensis because the latter is known only by a pelvis with its fused synsacrum. However, the possible fit of this femur with the pelvis of G. kiyuensis can be inferred by analyzing the proportions between both elements: if the head of the former is supposed to measure approximately half the length of its preserved distal end, then it would be congruent with the size of the acetabulum of G. kiyuensis. The most striking feature of the femur herein described is its large size, much greater than in all other known fossil or extant anhingas. A quantitative approach in estimating the body mass of this bird by using Campbell & Marcus?s scaling model, throws a result of 17.7 kg which is 30% larger than that of G. kiyuensis (reestimated on 12.8 kg) and 300 % larger than in Macranhinga paranensis (5.4 kg). Due to the lack of well defined diagnostic features at a generic level in the fragment of femur herein described we avoid to make a more accurate assignment.