LETHAIA SEMINAR @ Lefhaia, Vol. 23, pp. 11S114. “$I, 1990 01 15

‘Spicules’ in halysitids: a reply RACHEL WOOD, PAUL COPPER AND JOACHIM REITNER

Ever since the rediscovery of living examples of coralline spon- ges, attention has focused on the possibility of whether various fossil reef-building groups would be considered better as mem- bers of the Porifera rather than the Cnidaria. Representatives of these groups, which include stromatoporoids and chaetetids, resemble living forms in possessing not only a basal calcareous skeleton, but also siliceous spicules. Detection of such spicule remnants is considered therefore as conclusive proof of pori- feran affinity. As search techniques for their identification become more sophisticated, so an increasing number of posi- tively assigned forms can be rescued from the welter of Pro- blematica (e.g. Gray 1980; Kaimierczak 1979). The resultant reclassification has been radical and had considerable impli- cations for our understanding of poriferan evolution. It also establishes firmly the major ecological role of sponges in ancient communities, especially within reefs.

Kaimierczak (1989) has recently proposed that halysitid tabu- lates be added to the growing roster of proposed fossil coralline sponges. The basis for this is his report of pyrite pseudomorphs of ‘spicules’ in a single specimen of the halysitid Quepora ?uggfomerufiformis (Whitfield), from the Ordovician of Baffin Island, Canada. On the basis of these putative ‘spicules’, Kai- mierczak allied the halysitids to the choristids or sublithistids within the Class Demospongiae. We believe, however, that interpretation of these structures as spicules is erroneous. They are interpreted better as pyrite-replaced endolithic borings for the following reasons:

(1) The appearance of the halysitid ‘spicules’ is unlike sponge spicules. They are sinuous and branched, and show no obvious tapering axes. His Figs. 3A, B clearly show a considerable infestation of borings, and we are unable to detect any ordered arrangement of these structures, as would be expected in a spicular framework. Figs. 4B, C, although presenting a more convincing spicule-like morphology and some semblance. of regularity, are also probably borings. The figure given here shows a section through a spiculate Mesozoic stromatoporoid. Note the contrast between the form and arrangement of pyrite pseudomorphs of genuine spicules, which are club-shaped and organized into plumose tracts, in comparison to the pyrite- replaced endolithic borings which tend to ramify perpendicular to the skeletal surface. Such borings are commonplace. in corals, calcified sponges and coralline algae, and for fuller descriptions of endolithic activity the reader is referred to GolubiC et al. (1975) and Schneider (1977).

(2) Pyrite-replaced endolithic borings have been misinterpreted previously as spicules. Twitchell (1929) described ‘spicules’ from the Palaeozoic stromatoporoid Sfromaroporu. TO test whether such an attribution for these structures was sensible, Finks (1986) dissolved the specimen in weak acid. He found them to be insoluble and flexible, and concluded that they were endolithic algae or fungi. In 1984, KaAnierczak noted ‘spicules’ from the favositid tabulate Thamnopora. However, Copper’s (1985)

Fig. 1. Longitudinal section through the spiculate Mesozoic stromatoporoid Dehornella crustam. BM(NH) H5168a. Oxfor- dian, Makhtesh Hagadol, Israel. Compare the form and arrangement of the pyrite pseudomorphs of genuine spicules, which are club-shaped and organized in plumose tracts, to the pyrite-replaced endolithic borings which ramify perpendicular to the skeletal surface. Scale bar = 100 p.

114 Rachel Wood and others LETHAIA 23 (1990)

subsequent description of exquisitely preserved cnidarian polyps in the calicles of Faoosites provided sufficient evidence for their affinity to lie with the anthozoans, rather than any putative poriferan.

(3) In both Stromatopora and Thamnopora, the ‘spicules’ are restricted to the peripheral regions of the specimens, a surficial distribution consistent with endolithic invasion. Kaimierczak does not mention the distribution of the ‘spicules’ in his Quepora specimen, but we suspect they too are limited to the exposed surfaces of the colony.

(4) The organization of the halysitid skeleton seems to be suited for the lodgement of zooids in a colonial arrangement, and shows no expression of a poriferan filtration system. One of the characteristics of many halysitids and favositids, including Quepora ?agglomeratiformis, is a twelve-fold symmetry of sep- tal insertion, as emphasized by Mistiaen (1989). No sponges show the 4, 6, 8 or 12-fold symmetry found in Cnidarians.

We conclude that the structures discussed by Kaimierczak represent extensive post-mortem infestation by endolithic organisms (see especially Fig. 3A), and the finding of these structures within specimens of the associated fauna of brachio- pods and trilobites from Baffin Island would provide definitive proof. We welcome Kaimierczak‘s comments on the intriguing problem of the convergent formation of calcareous skeletons in sponges, and are equally eager to increase our understanding of reef-building forms. However, we feel that the inclusion of favositid and halysitid tabulates within the ranks of Porifera is unfounded on the evidence available.

References Copper, P. 1985: Fossilised polyps in 430-Myr-old Favosites

corals. Nature, London 316, 142-144.

Finks, R. M. 1986: ‘Spicules’ in Thamnopora. Fossil Cnidaria 15(1.2), 22.

GolubiC, S . , Perkins, R. D. & Lukas. R. 1975: Boring micro- organisms and microborings in carbonate substrate. In Frey, R. (ed.): Study of Trace Fossils, 22F259. Springer-Verlag, New York.

Gray, D. I. 1980: Spicule pseudomorphs in a new Palaeozoic chaetetid, and its sclerosponge affinities. Palaeontology 23, 80S820.

Kaimierczak, J . 1979: Sclerosponge nature of chaetetids evi- denced by spiculated Chaetetopsis faorei (Deninger 1906) from the Barremian of Crimea. Neues Jahrbuch fur Geologie und Palaontologie, MonaDheft (1979), 2 , 97-108.

Kaimierczak, J. 1984: Favositid tabulates: evidence for pori- feran affinity. Science 225, 835-837.

Kafmierczak, J. 1989: Halysitid tabulates: sponges in corals’ clothing. Lethaia 22, 195-205.

Mistiaen, B. 1989: Importance de la symmetrie dordre douze chez les Tabulata. Comptes Rendus hebdomadaires des Sian- ces de I‘Acadimie des Sciences, Paris 308, 451-456.

Schneider, J. 1977: Carbonate construction and decomposition by epilithic and endolithic micro-organisms in salt- and fresh- water. In Fliigel, E. (ed.): Fossil Algae, 24S260. Springer- Verlag, Heidelberg.

Twitchell, B. G. 1929: The structure and relationships of the true stromatoporoids. American Midland Naturalist 40, 27C- 306:

Rachel Wood, Department of Earth Sciences, University of Cam- bridge, Downing St., Cambridge CB2 3EQ, UK; Paul Copper, Department of Geology, Laurentian University, Ramsey Lake Road, Sudbury P3E 2C6, Ontario, Canada; Joachim Reitner, Institut fur Palaontologie der Freien Universitat, Schwen- denerstrase 8, 0-1000 Berlin 33, FRG; 9 June 1989.