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A detailed analysis of 813 tests of the irregular echinoid Fibularia sp. collected from multiple depositional horizons at a single Oligocene locality on South Island, New Zealand revealed 57 specimens with drill holes, representing a 7% drilling frequency (57 drilled individuals). Among those drilled, 38% (22 individuals) had multiple drill holes with up to four holes present. Most holes were complete, circular to sub-circular in outline, averaging 0.5 mm in diameter, and belong to the ichnospecies Sedilichnus simplex. The distribution of holes on the test was non-random: 1) the aboral surface was drilled preferentially, and 2) the central region of the aboral surface had a significantly higher number of holes (p < 0.005) than expected. No evidence of size selectivity was detected: the relationship between drill hole size and test size of Fibularia was not significant (p > 0.1). The distribution of drill holes as well as their shape and size make it likely that they were produced by a predatory gastropod, such as the cassid Galeodea that is known from the Oligocene of New Zealand. However, the high frequency of multiply drilled Fibularia allows for the possibility of a parasite, such as the eulimid Niso, another New Zealand gastropod coeval with Fibularia. This study adds to the growing body of literature on the drilling of echinoids, shedding light on the ecology of this group.
Predation is frequently suggested to be a key biotic process that can shape ecological communities and drive coevolution. The premise behind these hypotheses is that predators select prey to ensure maximum gain per unit effort; for example, by selecting species that are more abundant or accessible. In this study, we tested for predator selectivity in a tropical molluscan assemblage by quantifying the influence of relative abundance (encounter frequency) on predation frequencies. We collected macromollusks (> 4 mm) from 15 sites in three soft-sediment reef lagoons at One Tree Reef (southern Great Barrier Reef, Australia). Dead mollusks were counted and identified to species level (61 species, n = 8131), and species predation frequencies were calculated as the proportion of shells with drill holes. We found that in this infauna-dominated community, levels of drilling predation were low (7.14% on average), and there was no evidence that predators selected prey based on encounter frequency. This result was consistent across prey species and lagoons. Thus, drilling predators did not specialize on more accessible prey species and were not a major cause of mortality in this modern macromollusk assemblage. Since drilling gastropods are size selective, lack of selectivity in our samples only applies to the prey size range considered. Detailed studies of prey morphological traits, as well as accounting for predator non-consumptive effects could shed light on the preferences and relevance of drilling gastropods in this soft-sediment carbonate reef assemblage.
Pocilloporid corals dominated shallow-water environments in the Caribbean during much of the Cenozoic; however, the regional diversity of this family declined over the last 15 My, culminating with the extinction of its final member, Pocillopora palmata, during the latest Pleistocene. Here we present a new record of P. palmata from Dry Tortugas National Park in the Florida Keys and infer its likely age. Although most existing records of P. palmata are from the sub-aerial reef deposits of MIS5e (~ 125 ka), the presently submerged reef in the Dry Tortugas was too deep (> 18 m) during this period to support significant reef growth. In contrast, the maximum water depth during MIS5a (~ 82 ka) was only ~ 5.6 m, which would have been ideal for P. palmata. Diagenetic alteration prevented direct dating of the samples; however, the similarity between the depths of the Pleistocene bedrock in the Dry Tortugas and other reefs in the Florida Keys, which have been previously dated to MIS5a, support the conclusion that P. palmata likely grew in the Dry Tortugas during this period. Our study provides important new information on the history of P. palmata, but it also highlights the vital need for more comprehensive studies of the Quaternary history of Caribbean reef development. With modern reef degradation already driving yet another restructuring of Caribbean coral assemblages, insights from past extinctions may prove critical in determining the prognosis of Caribbean reefs in the future.
Oxygen bubbles produced during photosynthesis internally deform filamentous cyanobacterial mats, producing distinctive fenestral patterns. Similar textures preserved in ancient microbialites are useful biosignatures when filaments are no longer preserved, but have typically been observed within stromatolites. This study describes bubble-associated fenestrae within oncoids from the early Cambrian Bayan Gol Formation of Mongolia. Fenestrae appear in mm-scale micritic laminae which contain dense accumulations of large (10 × 300 μm) filamentous Girvanella microfossils. Many laminae are not spherical, often occurring with one flat side opposite a conical peak. Up to six generations of conical geometry are present, with each cone rotated with respect to the previous peak. We hypothesize that the oncoids experienced intermittent disturbances followed by periods of stasis and vertical growth. During resting periods, we hypothesize that flat areas formed the oncoid resting base and peaked areas the top. The presence of bubble laminae within peaks implies formation in part via entrapment of microbially produced gases. Examples of resting oncoids growing into stromatolites are well known, as well as irregularly laminated oncoids with no cones; the Bayan Gol Formation samples are intermediate between typical spherical oncoids and stromatolites. The preservation of cones also provides evidence for relatively rapid mineralization in the Cambrian ocean, as antecedent microbial tufts would likely have collapsed if disturbed before calcification.