From sedimentary reworking to collection bias: dual taphonomic filters in shark fossil remains

Shark fossils can undergo taphonomic processes that mix spatial and temporal information, impacting data interpretation. Sampling selection bias, arising from non-random collection, affects result representativeness. The present study evaluates the effect of sampling bias on shark teeth collected along the southernmost coast of Brazil, where biodetritic gravel deposits extend for hundreds of metres along the coast. To identify possible biases, a controlled sampling protocol was employed, including regular field trips with random collection of all fossils remaining within an 80 × 20 m delimited area. Four collectors with different levels of experience (high, moderate, low, and none) conducted independent, time-controlled sampling sessions of 30 minutes each. In total, 16 field trips were conducted between 2021 and 2024, yielding 138 shark teeth. As expected, the highly experienced collector recovered the greatest number of specimens (53; 38.40%), followed in decreasing order by the moderately experienced (30; 21.73%), low experience (28; 20.28%), and no experience (27; 19.56%) collectors. Notably, the no experience collector recovered several specimens comparable to those of the moderate and low experience collectors. This pattern may reflect a more meticulous and time-intensive search strategy commonly observed in novice individuals, who lacking the ability to identify specimens quickly, compensate through greater attentiveness and thoroughness during collection. Regarding diagenesis, colour variation from reddish-brown to black is associated with iron and manganese oxide impregnation, indicating fossilization under unstable redox conditions typical of shallow coastal marine environments. External factors such as weather and physical conditions can affect collection efficiency, leading to sampling variation among collectors.