640
Journal of Paleontology 91(4):633–642
Table 1. Drilling site comparison on tests of Echinocyamus
stellatus.An ‘X’ in ‘site selectivity’ indicates the exact binomial test (exact p°) is significant, based on a significance level of 0.05.
Sample Phosphatized
Site Oral
Aboral Petal area Ambital area Ambulacral fields Nonphosphatized Oral
Interambulacral fields Anterior Posterior
Aboral Petal area Ambital area Ambulacral fields
Interambulacral fields Anterior Posterior
Total drill holes 64
45 34 56 71 46 28 63
Number of drill holes Relative Observed in each area
53 11 34 11 29 5
34 22 55 16 28 18 21 7
38 25
have resulted in a taphonomic filter against the preservation of larger drill holes on smaller tests. By contrast, the more autochthonous, rapidly buried, nonphosphatized material does not show such a trend even though they feature drill holes of the same length range as the phosphatized material. The absence of size selectivity was also reported for drilled fibulariid samples from the Miocene of Poland (Złotnik and Ceranka, 2005), Oligocene of New Zealand (Meadows et al., 2015), Recent Red Sea (Nebelsick and Kowalewski, 1999), and Recent Mediterra- nean Sea (Grun et al., 2014).
Drilling frequencies.—The drilling frequencies of phosphatized and nonphosphatized samples are clearly different. This mirrors the results from Recent studies in the Red Sea where frequencies differed highly among facies from around 60.6% to 80.2% for Echinocyamus crispus (Mazzetti, 1893) and 22.0% to 83.3% for Fibularia ovulum (Lamarck, 1816) (Nebelsick and Kowalewski, 1999). Grun et al. (2014) also reported highly variable drill hole frequencies in samples taken around a single island in the Mediterranean Sea ranging from absent to 21.7%. It is difficult to ascertain the reason for the different rates of dril- lings in the Recent material (Grun et al., 2014), and even more complicated for the fossil material examined in the present study. The higher frequencies shown by phosphatized (20.5%) over the nonphosphatized (8.1%) samples may be due to a number of ecological parameters both biotic (population densities of both predators and prey) and abiotic (such as sedimentation rates) in their contemporary environment. The frequencies of drilling predation (8.1% and 20.5%) in
Echinocyamus stellatus from the Maltese Globigerina Lime- stone are similar to those reported from the Miocene of Poland. Złotnik and Ceranka (2005) show drilling frequencies of 3.8%, 10.9%, and 15.2% for Echinocyamus linearis (Capeder, 1906), Echinocyamus pusillus (Müller 1776), and Echinocyamus pseudopusillus (Cotteau, 1895), respectively. These results are similar to the Oligocene of New Zealand, where Meadows et al. (2015) reported a drilling frequency for Fibularia sp. of 7.0%. Nebelsick and Kowalewski (1999), however, revealed clearly higher frequencies from the Recent Red Sea (60.6% to 80.2% for Echinocyamus crispus and 22.0% to 83.3% for Fibularia ovulum).
49.6 47.9 52.1 65.6 34.4 54.9 45.1 50.4 49.6 47.9 52.1 65.6 34.4 54.9 45.1
0.828 0.172 0.756 0.244 0.853 0.147 0.607 0.393 0.775 0.225 0.609 0.391 0.750 0.250 0.603 0.397
area (%) probability probability p° 50.4
Test
0.504 0.496 0.479 0.521 0.656 0.344 0.549 0.451 0.504 0.496 0.479 0.521 0.656 0.344 0.549 0.451
Exact
Site selectivity evaluation Selective Nonselective
<0.001 X <0.001 X 0.009 X 0.230
<0.001 X 0.053 0.200 0.231
X
X X X
A possible taphonomic filter against large drill holes in
small tests may result in an underrepresentation of drilling frequencies. Such a filter should, however, have a rather small effect on the overall drilling frequency since the test length distribution of drilled specimens roughly follows the normal distribution of nondrilled specimens (Fig. 4).
Site selectivity.—The selectivity for the aboral test side in the Maltese Echinocyamus stellatus has also been reported for several Echinocyamus species from both fossil and Recent environments (Nebelsick and Kowalewski, 1999; Złotnik and Ceranka, 2005; Grun et al., 2014) as well as for Fibularia (Nebelsick and Kowalewski, 1999; Meadows et al., 2015). The drilling preference to the aboral side, especially to the petalo- dium, may reflect an optimization strategy of the predator reducing the amount of energy (e.g., for producing less acid or by removing less stereom due to pore occurrences) and thus may reduce the time for drilling, leading to potentially less dis- turbance and more drillings over time. The reduction in drilling time for optimization is reasonable since: (1) the predator attacks the buried prey from the aboral side and thus reduces handling time; (2) the aboral side of the test is thinner than the oral side and thus less energy is needed to drill into the test (Złotnik and Ceranka, 2005); (3) the presence of ambulacral pores on the aboral side of the test means that less material needs to be removed; (4) targeted internal organs are located under- neath the petalodium; and (5) in Echinocyamus, internal sup- ports are present beyond the petal area. The preference for the petal area seems to be characteristic
in Echinocyamus prey (Nebelsick and Kowalewski, 1999; Złotnik and Ceranka, 2005; Grun et al., 2014). The fact that there is no significant preference for drilling into the petal and the ambital areas of the nonphosphatized individuals may be due to the generally smaller size of these echinoids. Smaller prey size with constant drill hole diameters results in less specific site selectivity as opposed to the phosphatized examples. There is also evidence for selectivity by predators of the larger phosphatized individuals for the ambulacral areas, which contain relatively large ambulacral pores, rather than the interambulacral areas. Again, the smaller, nonphosphatized individuals show no such selectivity.
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