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Ecology of American palaeozoic sponges Nelson, Samuel James

Abstract

Evidence concerning the ecology of American Palaeozoic sponges, excluding hexactinellids and incertae sedis, is discussed and evaluated. In addition, detailed summaries of nearly all the papers describing American Palaeozoic sponges are included. This thesis, for the most part is a compilation of available information, and is intended as such to be a guide for further investigations (especially field studies) of sponge ecology. The environment in which the Palaeozoic sponges lived was very important in deciding whether they would, be preserved as fossils or be destroyed. Muddy waters seem to have been the best for preserving sponges, while only the more solidly built forms such as lithistids and calcisponges are usually preserved on firm bottoms in clear waters. Sponges, because of their physiological make-up, are subject to rapid decay and hence usually leave little trace of their former presence in the sediments. This is especially true of the loosely built manactinellids and tetractinellids, but less so of the more solidly constructed lithistids and calcisponges. It can be seen, therefore, that, at times the ancient seas probably contained large sponge faunas, but that little record of their presence was left behind. The ecology of the sponges, in this thesis, is dealt with by epochs, an exception being that of the Carboniferous period. In the Cambrian, sponges are present in the Waucobian and Albertan series. Besides the hexactinellids, pleosponges and incertae sedis, the monactinellids are the only sponges found. These are nearly all found in shaly sediments. A muddy water environment is suggested, but it is possible that the muds might have been the only sediments capable of preserving the monactinellids. The Ordovician contains the largest sponge fauna, in number of genera and species, of any Palaeozoic system. They are found in all stages. Tetractinellids, lithistids, and calcisponges, as well as monactinellids are present but little information was found on their ecology. A muddy water environment is suggested for the monactinellids and a clear water one for the lithistids and calcisponges. The seas, at all times, appear to have been shallow. In the Cincinnatian epoch the sponges,in the area around Ohio and Kentucky,are believed to have lived in clear, limestone depositing waters and to have been periodically killed by influxes of mud. The Silurian sponges are found only in the Niagaran series. The fauna consists chiefly of lithistids and seems to have lived in relatively deep, quiet,muddy waters.. It is suggested that the salinity of the succeeding Cayugan seas may have contributed to their extinction. After Silurian times, lithistids were never very numerous in the Palaeozoic. Although rich in the remains of hexactinellids, the Devonian system contains very few genera and species of the other sponge groups. The Ulsterian series contains only Hindia fibrosa. This sponge seems to have preferred muddy waters. The Senecan series contains numerous markings attributed to the work of boring sponges. Little is known of the ecology of Carboniferous sponges. Hence the Mississippian and Pennsylvanian systems are discussed together. The Permian system contains a rather large calcisponge and lithistid fauna in which the calcisponges dominate. The Permian sponges are usually found associated with reef deposits. In Leonardian times, calcisponges usually lived on the fore-reef and the lithistids in the near-fore-reef, open sea facies. In Guadalupian times, the calcisponges lived on both the reef and near-fore-reef facies. Lithistids were not numerous, but are found mostly on the fore-reef. The Guadalupian sponges made important contributions to reef building.

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