A NEW FOSSIL ALGA FROM THE SILURIAN OF MICHIGAN

The record of Paleozoic algal diversity is strongly biased toward calcareous and stromatolitic types due to the general paucity of well preserved compression fossils of other taxa. A new genus, Thalassocystis (type species T. striata) is described on the basis of compression fossils in middle Silurian dolomite from the northern peninsula of Michigan. The plant is characterized by a short branched thallus, each branch terminated by an enlarged bladder-like structure. The plants were deposited in a tropical, shallow water marine environment but whether they were benthic, adventive to the pelagic zone, or completely pelagic cannot be determined from the evidence available. RECOGNITION AND CLASSIFICATION of Paleozoic non-calcareous algae is particularly difficult despite the rather voluminous literature on fossils of this type. Impressions and compressions resembling thalli of modern brown algae (Phaeophyta) are relatively common in Paleozoic marine deposits. A number of genera of doubtful biological significance have been erected despite the fact that most specimens of this type appear to consist of an amorphous carbon film with little or no structural detail. As early as 1893, James attempted to define the scope of the form genus Fucoides, to which a large number of these problematic forms had been assigned. Fucoides, Sphenophycus, and Buthotrephis are all form genera which bear a superficial resemblance to modern fucoid thalli. Fenton's attempts to verify the biological status of much of the material assigned to these genera (1943, 1946) were severely limited by a lack of preservation of distinctive structures in the type specimens. Fucoid-like fossils can actually be produced by a variety of biological entities and it is possible that a significant amount of impression material of this character may actually represent animal tracks or burrows. Only rarely has compression material beensufficiently well-preserved to state definitively that the source organisms were actually algae (Fry and Banks, 1955). Despite the presumed diversity of algae in marine and freshwater habitats in the Paleozoic, the fossil record for many groups is incomplete. The best representation is found in the stromatolitic blue-green and calcareous green and red algae whose presence is revealed in a variety of car'Received for publication 8 January 1976. 2 Present address: Biological Science Department, California Polytechnic State University, San Luis Obispo, California 93407. bonate reefs and banks of presumed algal origin. The paucity of well-preserved compression material results in a strong bias toward calcareous and stromatolitic types in our perception of Paleozoic algal diversity. The material to be described is in remarkable contrast to the bulk of known compression material in that the major aspects of the external form of the plant thallus are well preserved. The collection site for the fossil material is located approximately two miles east of the town of Manistique in Schoolcraft County, Michigan. The site is on the north shore of Lake Michigan and the fossils occur in light to dark gray dolomites outcropping in the form of gently dipping beds cut into shallow ledges by wave action. The rocks outcropping in this area are part of the Manistique Group (Ehlers and Kesling, 1957) in the Niagran Series and are essentially mid-Silurian in age. Marine invertebrates and palynomorphs in the exposed rocks at the collection site confirm the Silurian age determination. SYSTEMATIC DESCRIPTION-Thalassocystis Taggart and Parker gen. n.-Marine alga; organism consisting of a branched thallus, each branch terminating in an inflated bladder; bladders elliptical to obovate. Generitype-Thalassocystis striata Taggart and