Absolute [Hg]Fossil and [Hg]Rock, as well as [Hg]Sample values, are presented in Table 1, while [Hg]Sample averages are summarized in Figure 2. The lower the trophic level of Romualdo taxa (represented by smaller species with villiform dentition)[18], presented the lower (< 1.00) [Hg]sample, respectively: R. buccalis (0.27); T. araripis (0.87) and V. comptoni (0.92). The position of V. comptoni in the Romualdo food web is still dubious. It was inferred as a predatory species that fed on small fish, based on stomach content analysis of some specimens[20–22] –although there are no published images detailing this record. Similar studies[23], on the other hand, found no small fish fragments while performing stomach content analysis, and linked V. comptoni’s reduced dentition and well-developed gill apparatus to morphologies observed in recent filter-feeding taxa, such as paddlefishes (genus Polyodon).
The observed average [Hg]sample in V. comptoni is higher than those observed in the lower trophic levels represented by R. buccalis and T. araripis, indicating piscivore feeding habits based in consumption of the latter taxa – and possibly even smaller ones, such as Santanichthys diasii[24]. However, it is important to note that these ratios show large variability per specimen (0.45–2.01), compared to other fish species currently analysed – a possible evidence of foraging variation that could either be tied to geographical[26] and/or ontogenetic[27] factors. Thus, this broad range of [Hg]sample values in V. comptoni might be caused either by occupying different ecological niches through life, or simply presenting less Hg accumulation at a different ontogenic stage at time of death.
With the increase in trophic level (transition from villiform to molariform dentition, and from the latter to conical teeth, accompanied by increase in overall size of species), [Hg]sample becomes higher, indicating shifts to higher levels of the trophic web. Molariform, mid-tying level species interpreted as durophagous taxa, which fed on larger, opportunistic invertebrates[18], presented [Hg]sample of ~1.00, as in Batoidea indet. (1.02) and N. penalvai (1.12). Values of [Hg]Sample around 1.00 were also found in individuals of Cl. gardneri, along with others up to 1.31, which is expected for upper ties of the trophic web (Fig. 3), occupied by the largest fish taxa, with conical dentition, of the Araripe. The highest [Hg]sample (1.52–21.08) are observed in specimens of Ca. cylindricus; these might have acted as apex predators in the aquatic environments of the Romualdo, being able to feed on whatever fit into their mouths – including delving into cannibalism[28].