Journal of Paleontology, 91(4), 2017, p. 633–642 Copyright © 2017, The Paleontological Society 0022-3360/17/0088-0906 doi: 10.1017/jpa.2016.123


Comparative drilling predation on time-averaged phosphatized and nonphosphatized assemblages of the minute clypeasteroid echinoid Echinocyamus stellatus from Miocene offshore sediments (Globigerina Limestone Formation, Malta)


Tobias B. Grun,1 Andreas Kroh,2 and James H. Nebelsick1


1Department of Geosciences, University of Tübingen, Hölderlinstraße 12, 72074 Tübingen, Germany ⟨tobias.grun@uni-tuebingen.de⟩, ⟨nebelsick@uni-tuebingen.de⟩ 2Department of Geology and Palaeontology, Natural History Museum Vienna, Burgring 7, 1010 Wien, Austria ⟨andreas.kroh@nhm-wien.ac.at⟩


Abstract.—Fossilized tests of 1,053 Echinocyamus stellatus (Capeder, 1906) from the Miocene Globigerina Limestone Formation exposed on the northern coast of Gozo (Maltese Islands) were analyzed for predation traces. Specimens mixed by time-averaging processes can be clearly separated into two distinct samples according to their preservation as phosphatized or nonphosphatized individuals. Overall, 11.1% of the tests reveal holes that are referred to the ichnospecies Oichnus simplex (Bromley, 1981). Because of the hole morphology and diameter, the holes are interpreted as predatory drill holes, most likely produced by cassid gastropods. Redeposited phosphatized echinoids derived from an earlier period of reduced sedimentation rates show drilling frequencies of 20.5%. Younger, autochthonous, nonphosphatized echinoids show drilling frequencies of 8.1%. In both samples, predators predominantly targeted the aboral side of the echinoid test, particularly on the petalodium.


Introduction


The geological deep-time record has revealed that drilling predation can be used as a tool to recognize ecological interactions in both fossil and extant communities (e.g., Kelley, 1989; Kowalewski et al., 1998;Harper, 2003). Drilling frequencies have increased over time (e.g., Huntley and Kowalewski, 2007) and thus represent a useful proxy for a better understanding of the development of ecological interactions through time (e.g., Bengtson and Zhao, 1992; Conway Morris and Bengtson, 1994; Kowalewski et al., 1998; Kowalewski et al., 2005). Early exam- ples of predatory drill holes in echinoderms have been reported from Late Ordovician stylophorans and other Paleozoic echino- derms (Deline, 2008 and references therein). There is a rich record of predation on echinoids that has generally been attributed to a number of different organisms (e.g., Nebelsick, 1999; Kowalewski and Nebelsick, 2003). Maximum rates of drilling predation on echinoids increase in intensity from the Early Cretaceous to the Recent (Kowalewski and Nebelsick, 2003). The comparison of drilling predation patterns on the fossil


and Recent clypeasteroid echinoid Echinocyamus, which can be studied in large numbers from different habitats, serves to increase the knowledge of predator–prey relationships among echinoids through time. Data on drilling predation exist for several Recent fibulariids including Echinocyamus (Nebelsick and Kowalewski, 1999; Grun et al., 2014; Grun and Nebelsick, 2015) and Fibularia (Nebelsick and Kowalewski, 1999). In addition, drillings have been examined for fossil counterparts of


Echinocyamus (Ceranka and Złotnik, 2003; Złotnik and Ceranka, 2005) and Fibularia (Meadows et al., 2015). Echinocyamus can be abundant in near-to-offshore habitats varying from tropical to cold-water environments (e.g., Mortensen, 1927, 1948; Ghiold, 1982; Schultz, 2006). The test of these echinoids can be well preserved due to internal supports linking the oral and aboral side and thus strengthening the test (e.g., Seilacher, 1979; Mooi, 1989; Nebelsick, 1999, 2008; Grun et al., 2014). This study aims to investigate differences in drilling


intensity and site selectivity for predation in two time-averaged Miocene Echinocyamus stellatus (Capeder, 1906) (Fig. 1) samples from different paleoenvironmental settings of the Globigerina Limestone Formation in Malta. The tests were analyzed with respect to: (1) test length; (2) drilling frequencies; (3) drill hole length; (4) size selectivity; and (5) site selectivity. The results are compared to previous work on drilling predation on both Recent (Nebelsick and Kowalewski, 1999; Grun et al., 2014) and fossil (Ceranka and Złotnik, 2003; Złotnik and Ceranka, 2005) Echinocyamus.


Drill holes, predators, and echinoids


Drilling predation in marine shells can be used as an ecological signal for the interpretation of predator–prey relationships in both modern and fossil environments and has thus been inves- tigated for a number of Recent and fossil organisms such as foraminifera and ostracods (e.g., Reyment, 1966), polychaetes (e.g., Young, 1969), mollusks (e.g., Kelley, 1988, 2001;


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