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A Review and Classification of Fossil Didemnid Ascidian Spicules

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Journal of Micropalueontolo~,~,15: 135- 149. 0262-821 X/96 $07.00 0 1996 British Micropalaeontological Society. A review and classification of fossil didemnid ascidian spicules OSMAN VAROLI & SIMON D. HOUGHTON' 'Varol Research, 12 Tan-y-bryn Road, Rhos-on-Sea, North Wales, LL28 4AE, UK. 'FibreCount UK Ltd, Suite 22, Dinnington Business Centre. Outgang Lane, Dinnington, Sheffield S31 7QY, UK. ABSTRACT - 'This study discusses and classifies iossil didemnid ascidian spicules. Threc new fossil genera and nine new fossil species are described based on spicule morphology. The genera are Bonrtia gen. nov.. Rrgardia gem nov. and Monniofia gen. nov. The species are Bonetia arum sp. nov., B. hrrvis sp. nov., R. qitusitrirnc'afa sp. nov.. B. trirncatri sp. nov.. Rigairdia mulrirudiata sp. nov., R. prurcisu sp. nov., Micrascidites partciruilinfits sp. nov., Monnrofia acirforrnis sp. nov.. and M. fascicrtlata sp. nov. Recognizing these distinctive fossil didemnid spicules in tine-grained sediments should provide useful pnlaeoenvironmental information and may stimulate interest in their biostratigraphy. .I. Microprilnronfol. 15(2): 135-149, October. 1996 INTRODUCTION The class Ascidiacea, also known as sea-squirts, are Ascidians, often called sea squirts, are sessile, filter feeding sessile, mostly colonial tunicates and are common marine tunicates (subphylum Urochordata) which are important invertebrates worldwide. The sack-like zooic ranges in size members of marine benthonic communities throughout shelf from 1 to 10cm. Most (c. 95%) ascidians form colonies in seas. Living ascidians have attracted a widespread interest the shallow water of the continental shelf where they are from biologists because of their evolutionary position as attached to rocks and shells; or they are occasionally fixed in close relatives of the vertebrate (Plough, 1978). mud and sand by filaments or stalks. Colonial organization Although most ascidians are soft-bodied, some species varies within the class, but although the colony itself may secrete distinctive aragonitic spicules. Didemnid spicules are grow to a considerable size, usually the individuals forming often found in slides prepared for calcareous nannofossil it are very small (see Plough, 1978, plate VII). examination. These stellate or spherical-shaped spicules Ascidians are filter feeders and extract plankton from range in size between 10 and 70 pm and, more rarely up to water which passes through the pharynx. The water currents 125 pm. Although living didemnid ascidians are well known are drawn in through branchial slits by ciliary action. Some from the continental shelves throughout the world, fossil deep-sea ascidians obtain their food from the surrounding didemnid spicules are very rarely reported by palaeon- sediments. Other deep-sea ascidians feed on minute animals tologists and, therefore, at present are of little use as (nematodes and epibenthic crustaceans) which are caught palaeoenvironmental or biostratigraphic markers. with lobes situated around the buccal siphon. Ascidians are This study summarizes the present knowledge of ascidians hermaphroditic, each individual having male and female in relation to didemnid ascidians and fossil ascidian spicules. gonads. When the eggs are externally fertilized they hatch All type material has been deposited in the Department of into free-swimming larvae similar in appearance to tadpoles. Palaeontology, Natural History Museum. However. as the larvae mature, they usually attach themselves to some suitable object and lose their tail. SCOPE OF STUDY DIDEMNID ASCIDIANS This study is based on observations by the authors during All didemnid ascidians are sessile and colonial, they are investigations of nannofossil assemblages from worldwide common and have a global distribution ranging from the localities, ranging from Asia, the Middle East, North Africa, Arctic to the Antarctic (Plough, 1978). Most are found in Europe and the Gulf of Mexico, over a period of fifteen shallow water (0-50 m) attached to rocks. shells and other years. Additionally, the published records of ascidian spicule hard surfaces. They are rarely found in deep water, distribution have been compiled. exceptions include 1,rptoclinide.s faroensis (1500 m) and Dirlernnitrn alhiurn polare (1430 m). Didemnid ascidians are BIOLOGY AND ECOLOGY OF ASCIDIANS depth-sensitive and different species occupy well-defined Forms belonging to the subphylum Tunicata have a body areas on the sea bottom, conditioned by ocean currents and (zooid) covered by a complex tunic (from which the name water temperatures. Ascidians are usually very vulnerable to tunicate is derived) containing a substance chemically almost prolonged freshwater influences. Heavy cyclonic rainfall identical with cellulose. The tunicates consist of three regularly kills large colonies of ascidians close to the classes: the scssile Ascidiacea and the free-floating Australian coastline (P. Mather, pers. comm. in Heckel, Thalaliacea and Larvacea. Over 1300 species of Tunicata 1973). Dead ascidian bodies may float on the surface of the have been descriibed; the great majority of which belong to sea, where wind, tides and current drift may play a part in the Ascidiacea (Barnes, 1980). their distribution. 135 Varol & Houghton DIDEMNID ASCIDIAN SPICULES also favours spicule secretion, whereas attachment to a Ascidians are soft-bodied animals and, with the exception of flexible object which allows even slight bending or spicules in didemnid and polycitorid ascidians, they are movement of the test, frequently results in secretion of rarely found as fossils. Monniot (1970) described Cystodyfes smaller types of spicules with shorter and less well-formed (Polycitorida) from Pliocene deposits of Brittany, France. rays or points. Didemnid spicules secreted in polar and Didemnid ascidians secrete fibrous spicules which, according subpolar species are frequently only sparsely distributed in to Matthews (1966), are composed of aragonite with high the surface layer of the test and are burr-like with levels of strontium (6.5%). In this study, several specimens poorly-developed spicule rays (Van Name, 1945; Kott, recovered in sediments from the Red Sea (DSDP Leg 23, 1969). Fossil didemnid ascidian spicules of varying shape Site 229A) were analysed using energy dispersive X-rays have been recorded in sediments of Jurassic to Quaternary (EDAX) to identify the strontium level. However, only age (Boekschoten, 1981). Living didemnid ascidians are traces of strontium (<1%)were observed in the spicules. classified primarily on the characteristics of the zooid; so it is The origin of the spicules remains unknown. Loewig & difficult to assign individual spicules to living species. Until Koelliker (1846), Hardman (1886), Woodland (1907) and now, therefore, Tertiary didemnid spicules which are Prenant (1925) suggested that the spicules were developed spherical in shape have been placed in Micvuscidites and independently of the zooid, whereas Michelsen (1919), PCrks those which are disc-shaped are assigned to Neanthozoites (1947) and Van Name (1952) indicated that the spicules (produced by Polycitoridae). were products of the zooid and originatcd in the lateral If a direct relationship can be established between fossil organs. In living ascidians, the spicules are surrounded by a spicules and living species, the information obtained from double-layered membrane (Lafargue & Kniprath, 1978). living forms may be applied to fossil spicules, assuming they Spicule formation is therefore not simply the result of have not changed their habitat with time, and provide useful physico-chemical processes. palaeoenvironmental information. Spicule characteristics have not been used by biologists The occurrence of recent didemnid-spicule-rich sediments for generic level taxonomy of living ascidians. Classification seems to be restricted to tropical and subtropical is based instead on a range of soft body elements (see carbonate-rich sediments. Heckel (1973) found high Elredge, 1966). However, at specific level, spicules and their concentrations of didemnid spicules (>lo% of the characteristics have been used by many authors as the nannoplankton fractions) occurred around the main primary diagnostic feature, in conjunction with characteris- carbonate reef areas of the Great Barrier Reef. Heckel tics of the zooids. concluded that the spatial distribution of the spicules in the Presence or absence of spicules, their diameter, ray sediments was controlled by selective preservation of the count, arrangement, distribution and density of rays are aragonitic spicule rays. Freshwater discharge into the basin usually considered as primary diagnostic criteria, in severely reduced the preservation potential of the spicules in conjunction with the characteristics of the zooid. However, the sediments. Berrill (1950) and Elredge (1966) treated these criteria as of When studying fossil occurrences of didemnic spicules, secondary importance or disregarded them as given species special attention must be taken to distinguish in siiu can be spicular or aspicular depending on environmental occurrences. The durability of didemnid spicules to the controls. processes of erosion, transport and deposition in warm Variations in spicule presence in ascidians could, however, waters is shown by their occurrence in turbidite deposits be the result of confusing more than one species whose adjacent to carbonate-rich shelf environments (Beall & definitions are only based on the characteristics of the zooid. Fischer,
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