Grun et al.—Drilling Predation on Miocene Echinocyamus from Malta N


rho p


= = <


134 0.93 0.001


639


drill hole length (mm)


Figure 8. Echinocyamus stellatus from the Miocene of Malta. Spearman’s correlation between drill hole length and drill hole width. N = number of involved individuals; rho = Spearman’s correlation coefficient; p = p-value of the statistical test.


Figure 10. Echinocyamus stellatus from the Miocene of Malta. Spearman’s correlation between test length and drill hole length among phosphatized and nonphosphatized individuals. N = number of involved individuals; rho = Spearman’s correlation coefficient; p = p-value of the statistical test.


to the drill holes in the Maltese echinoids, which are perpendi- cular to the test surface. In addition, attachment scars on the test surface attributed to parasitic eulimids are missing. Echinocyamus stellatus from middle Miocene sediments of


Figure 9. Echinocyamus stellatus from the Miocene of Malta. Drilling frequencies compared between phosphatized and nonphosphatized individuals. N = number of involved individuals.


(Bromley, 1981) (see Wisshak et al., 2015 for a discussion of purported synonymy between Oichnus and Sedilichnus). Cassids are known drilling predators of echinoids (e.g., Hughes and Hughes, 1971, 1981) and are interpreted as the cause of drill holes in both Recent (Nebelsick and Kowalewski, 1999; Grun et al., 2014) and fossil (Złotnik and Ceranka, 2005) fibulariids. According to previous morphological descriptions of the drill holes (Hughes and Hughes, 1971, 1981; Nebelsick and Kowa- lewski, 1999; Grun et al., 2014; Meadows et al., 2015), with respect to shape and size, cassid gastropods (Tonnacea) are the most likely drillers of the Maltese Echinocyamus. It is highly unlikely that eulimids are the producers of drill holes in the studied Maltese samples as Lützen and Nielsen (1975) reported much smaller drill hole diameters for eulimid drillings. SEM micrographs of eulimid drill holes published by Warén and Crossland (1991) are obliquely cylindrical or conical as opposed


Malta do not show allometric growth during ontogeny. These results are similar to those found in Recent Echinocyamus samples from the Red Sea (Nebelsick and Kowalewski, 1999) and the Mediterranean Sea (Grun et al., 2014), as well as in fossil examples from Poland (Złotnik and Ceranka, 2005). The studied echinoids from the two preservation modes show clear differences in average test length, suggesting that they indeed belong to separate samples (Fig. 5). There may be several reasons for such separation, including primary differences in the test size or taphonomic bias due to transport mechanisms during winnowing. By contrast, drill hole length does not vary significantly


between the samples. The drill hole diameter can be correlated to the size of the radula and thus to the size of the gastropod predators (Hughes and Hughes, 1971, 1981). The similar drill hole lengths in both echinoid samples suggest similar predators of similar size range. The drill holes in Echinocyamus stellatus are between 0.25 to 4.00mm in length, reflecting the lower end of drill hole diameters reported for Recent cassid gastropods by Hughes and Hughes (1981) on regular adult sea urchins. This is


similar to the size range observed for drill holes in modern fibulariids (Nebelsick and Kowalewski, 1999). The variation from small to larger drill holes in Echinocyamus stellatus and the comparison to drill hole lengths produced by adult cassids species suggest that Echinocyamus stellatus served as a food source for both juvenile and adult cassid gastropods. There is a trend of increasing drill hole length with


increasing test length in the phosphatized sample. It cannot be rejected that predators selected the prey by test size, but it is likely that relatively large drill holes weaken the test integrity more than smaller drill holes. Bioturbation, transport, and extended exposure on the seafloor during phosphogenesis may


drill hole width (mm)


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