A bone fragment of a probable aquatic vertebrate in the Serra Alta Formation (upper permian of Paraná Basin)

A BONE FRAGMENT OF A PROBABLE AQUATIC VERTEBRATE IN THE SERRA ALTA FORMATION (UPPER PERMIAN OF PARANÁ BASIN)

JORGE HACHIRO ¹, PAULO C.F. GIANNINI ¹ AND RENATA N. KINJO ²

¹Departamento de Geologia Sedimentar e Ambiental - IGc / USP.

²Programa de Graduação em Geologia do Instituto de Geociências da USP - IGc / USP.

Presented by ANTONIO C. ROCHA-CAMPOS

We have found a robust bone fragment (4cm long; 2.5cm in average diameter) in a suspension deposit of the Serra Alta Formation, Upper Permian of the Paraná Basin, along the Castelo Branco highway near Cesário Lange (SP). The bone expands towards one end with longitudinal protuberances and has fine superficial striae. The fossil-bearing sediments comprise submetric and metric beds of dark-gray siltstone with centimetric intercalations of dirty carbonate exhibiting bioturbation, coprolites and fossil remains (hardgrounds).

This skeletal fragment is interpreted as probably part of a posterior (bones: tibia; fibula; femur) or anterior (bones: radius; ulna; humerus) limb of an aquatic reptile or amphibian or, possibly, as a portion of the fin of a crossopterygian fish.

On the basis of thin sections of transverse cuts of the bone, it was possible to observe the original phosphatic nature (with low birefringence, concentric lamellae, negative elongation, and wavy extinction) and structure of the submilimetric and milimetric pore-canals filled by microcrystalline silica, as well as subordinate calcite cement.

This bone fragment was preserved due to the relative anoxic depositional conditions of the Serra Alta Sea, the low hydrodynamic energy of sedimentation, and the presence of alkaline fluids during early diagenesis which filled bone voids, mainly its pore-canal system with CaCO₃.

The fragment is massive but slightly broken at both ends. Its massive nature contrasts with the much smaller and more fragile bones of mesosaurids of the subjacent Irati Subgroup (the most stagnant palaeoenvironment of Paraná Basin). Its body may have reached around 1.5m, in length, making it perhaps the largest aquatic organism of its time. If it was carnivorous, it would have been a considerable threat to the other animals of the Serra Alta Sea. — (December 14, 2001) .

Publication Dates

• Publication in this collection
  09 Oct 2002
• Date of issue
  Sept 2002