By means of the analysis of 28 modern individuals of Pterodoras granulosus, a silurid of great importance in the archaeological assemblages of the Amazonas and Paraná basins, allometric equations based on the size of the spines have been derived, allowing the reconstruction of size profile and weight of the individuals recovered in fossil assemblages. These equations have been obtained from four different measures, three of them related to different sections of the spine articulation with the cleithrum, an area that usually has an excellent preservation in the archaeological and palaeontological records. This study allows observation of remarkable individual variability of the captured specimens regarding its total weight, mainly due to the weight variability of viscera, and also probably with other metabolic situations, episodic intakes and/or reproductive aspects, among others. Hence, allometric equations determine more accurately length than weight, resulting in proxy data for the reconstruction of different size profiles of archaeological assemblages. Copyright © 2010 John Wiley & Sons, Ltd.