The age structure of an irregular echinoid population, Heteraster oblongus (Brongniart, 1821), has been evaluated. The study was conducted by random sampling in a bed belonging to the inferior Aptian from Mas de Borràs (Morella, Maestrat basin, Iberian Chain), Western Mediterranean, that was interpreted to be a palaeobiocoenosis. It has been considered that length intervals, represent the growth stages. We concluded that the age structure was typical of a species in optimum conditions, but it is interpreted that the earliest growth stages are underrepresented due to biostratinomy processes. The species is infaunal, developing the life cycle buried in a soft substratum, which facilitates fossilization after death. But we point to the hypotheses the younger specimens are very superficially buried and therefore more subject to bioturbation and wave action. That would prevent younger echinoids to get permanently buried in a ratio similar to adults. The fossilizing probability would be directly proportional to the depth at which echinoids are buried in life, until reaching a certain threshold of depth where the probability will tend to be the highest and would remain without variations.