Tunicate spicules and their syntaxial overgrowths: examples from the Pleistocene Ironshore Formation, Grand Cayman, British West Indies

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Pleistocene limestones of the Ironshore Formation, deposited approximately 125 000 years ago, contain stellate tunicate spicules that are morphologically similar to those of the modern Trididemnum strigosum Kott, 1980 from Australia. The aragonite spicules, up to 0.1 mm in diameter, have a 1–4 μm diameter nucleus formed of anhedral crystals (<1 μm long), surrounded by radially disposed composite crystals that have a pseudohexagonal cross section. The spines, which have a hexagonal cross section, are formed of subcrystals.During diagenesis the tunicate spicules were modified by (i) the development of concentric laminae of aragonite needles (up to 2.2 μm long and 0.2 μm wide), which are tangential to the outer surface of the spicule but randomly orientated in the tangential plane, and (or) by (ii) syntaxial aragonite overgrowth of the crystals in the spicule. If the spicules were surrounded by densely packed mud, no overgrowths occurred. Conversely, significant overgrowth occurred if the spicules were surrounded by loosely packed mud or faced into an open cavity. Where completely encased by syntaxial overgrowths, the original spicule is difficult to recognize. Interruptions in the growth of the syntaxial overgrowths produced concentric growth rings, which are marked by a physical discontinuity or a thin zone of concentrically arranged aragonite crystals. Some spicules are encased by two or more zones of aragonite overgrowths with well-developed growth lines. Available evidence suggests that the syntaxial overgrowths on the tunicate spicules probably developed soon after deposition of the sediments while they were still in a marine setting.The recognition of tunicate spicules in ancient limestones is important because they provide evidence of soft-bodied animals that are rarely documented from ancient faunas despite their abundance in modern oceans. Although superficially similar to inorganically precipitated aragonite spherulites and Microcodium, the tunicate spicules can be recognized by careful consideration of their morphological attributes.