ABOUT THE SKETCH ON COVE R- A simple orthocone with gas in the camerae would have an anterior center of gravity (G1) and an apical center of buoyancy (B1). Such a situation would cause the shell to occupy a vertical position, forcing the head into the mud on the sea floor—making the animal very unhappy and uncomfortable, as can be plainly seen from its expression. Weighting the apex of the shell with cameral deposits or siphonal deposits, or both (only cameral deposits are shown here), would move the center of gravity backward, from 01 to G2, and move the center of buoyancy forward, from B1 to B2, permitting the animal to swim and move in a horizontal position—making it very happy and comfortable, as can be seen from the ecstatic expression. Further growth of the shell requires further growth of the deposits, moving the center of gravity forward to midlength (from G2 to G3) and moving the center of buoyancy forward also from B2 to B3. Such an adjustment requires careful physiological skill. If the animal now has a rather smug expression, who can blame it? As indicated elsewhere (Flower, 1955, fig. 1, p. 247), balance is attained completely by siphonal filling in the Endoceratida, by a combination of siphonal and cameral deposits in the Actinoceratida, and mainly by cameral deposits in the Michelinoceratida. However, Hook and Flower (1976) subsequently found that the Michelinoceratidae and Troedssonellidae probably spring from different parts of the Baltoceratidae, in which there are both cameral and siphonal deposits (rods, linings, and annuli in varying combinations) in a siphuncle close to the venter. In all instances, concentration of deposits on the ventral side of the shell assured stability, with the venter down and the dorsum up. This illustration was first published in Geological Society of America Memoir 67, Treatise on Marine Ecology and Paleoecology. New Mexico Bureau of Mines & Mineral Resources A DIVISION OF NEW MEXICO INSTITUTE OF MINING AND TECHNOLOGY Late Canadian (Zones J, K) Cephalopod Faunas from Southwestern United States by Stephen C. Hook and Rousseau H. Flower SOCORRO 1977 Preface This study, for which the senior author is primarily responsible, explores in detail the internal morphology and taxonomic relationships of the Baltoceratidae and Protocycloceratidae of the Ellesmeroceratida (Flower, 1964a), and the Michelinoceratidae and Troedssonellidae, the first of the Michelinoceratida, in the latest Canadian of the southwestern United States. The work is a doctoral thesis by the senior author, under the direction of the associate author, who has found the detailed and painstaking work most gratifying. The material was disappointingly fragmentary, and much was so extensively replaced or otherwise altered that it presented most perplexing dilemmas. The morphological facts are, however, beyond dispute. Where one approaches possible transitions among major groups, the problem emerges, as it has here, as to which of the several major divisions some of these new forms should be assigned. Where major divisions are distinct, it is either because of spontaneous generation, sudden and drastic morphological mutations (both unlikely), or it is because we lack knowledge of transitional forms. It is intriguing to reflect that where taxonomic compartments seem perfect, it is not because of our knowledge but because of our lack of knowledge. In this work new and surprising patterns of internal morphology have been found in the small orthoconic cephalopods of the latest Canadian, in contempo- raneous faunas from New Mexico, west Texas, and western Utah. The various new forms often present a basis for conflicting interpretations. While alternate hypotheses of relationships can be derived from this material, this vexation is surely offset by the importance of recording the unexpected morphological variation found in this material. In 1935 a joint paper by Flower and Caster on Conewango cephalopods was published; in this my position might well have been that of junior author. Kenneth Caster gave much support to this work that I cannot repay, but here I have had an opportunity to pass it on. The present study has made fullest use of the materials available, and has brought forth unexpected morphological surprises, based upon the only known latest Canadian cephalopod faunas preserved in limestones and, thus, suitable for study by thin sections. I have added some passages, mainly noting more of what we do not know, rather than what we do know. I can only end by quoting from the introduction to the Conewango paper, "The principal author of this paper is such a student whose zeal and enthusiasm have been exceptional and whose sustained interest has been most gratifying." Rousseau H. Flower Senior Paleontologist Socorro, New Mexico New Mexico Bureau of Mines Dec. 2, 1975 and Mineral Resources 9 Abstract The Wahwah Limestone of western Utah and the rod-bearing Baltoceratidae is further documented both Florida Mountains Formation of west Texas and south- stratigraphically and morphologically through the western New Mexico have yielded the only large, well- discovery of the new genus Veneficoceras, which is preserved cephalopod faunas of latest Canadian age morphologically more similar to the rod-bearing Bal- (latest Early Ordovician) in North America. These toceratidae than to the Troedssonellidae. The first faunas are completely Canadian in aspect and are North American representative of the genus Manchur- dominated by the small orthocones belonging to the oceras, previously known only from Manchuria and Ellesmeroceratida and the Michelinoceratida ( = Or- Australia and questionably from Siberia and Argentina, thoceratida). Surprisingly, the Tarphyceratida and was discovered in the Florida Mountains Formation of Endoceratida, the dominant cephalopods of the Middle- west Texas. Late Canadian, are minor faunal constituents in these New criteria based on thin- and opaque-section analyses faunas. are established for recognition of ventral lobes in suture Analyses of more than 100 vertical (sagittal) thin patterns. sections of these orthocones revealed a morphological Twenty-five new taxonomic names are proposed, diversity previously unrecognized this early in the including 6 new genera and 19 new species. The new evolutionary history of the Cephalopoda. Complex genera are Amsleroceras, Rangeroceras, Veneficoceras, patterns and combinations of cameral and siphonal Bakeroceras, Wardoceras, and Enigmoceras. The new deposits were observed in members of the Baltocerati- species are Amsleroceras gracile, Rangeroceras hintzei, dae, Protocycloceratidae, Michelinoceratidae, and Rhabdiferoceras planiseptatum, Veneficoceras susanae, Troedssonellidae. Three types of siphonal deposits Protocycloceras laswelli, P. rhabdiferum, Catoraphiceras (rods, annuli, and linings) and three types of cameral ibexense, C. pearsonae, C. sinuatum, C. staceyae, Kym- deposits (episeptal, hyposeptal, and mural) were ob- inoceras kottlowskii, Rudolfoceras keadyi, R. russelli, served in members of these families. The complexity of Bakeroceras wahwahense, Manchuroceras lemonei, these patterns suggests a fairly long evolutionary Michelinoceras floridaense, M. melleni, Wardoceras history preceding the latest Canadian. orygoforme, and Enigmoceras diaboli. The derivation of the Troedssonellidae from the Introduction PROBLEM AND PREVIOUS WORK pods of the Chazyan than to those of the Canadian (Flower, 1968a, 1971, 1976a, b, c). Although the publication of the nautiloid volume of The use of the term Canadian goes much farther the Treatise on Invertebrate Paleontology (Teichert and back than this. Bypassing various vicissitudes of usage, others, 1964) made a complete synthesis of cephalopod Ulrich (1911) proposed using both Ozarkian and evolution possible (Teichert, 1967), the fact remains Canadian as systems, between the Cambrian and that very little detailed information is available on the Ordovician. Flower, the late Dr. Josiah Bridge, and evolution, morphology, and taxonomy of the cephalo- others agreed that the Lower and Middle Ozarkian were pods of the Late Canadian (late Early Ordovician). The equivalent to the Trempealeauian (Upper Cambrian), Late Canadian is a critical time in the evolutionary but that a Canadian succession was recognizable with history of the Cephalopoda because the close of the the emendation that Ulrich's Lower Canadian and Canadian is marked by the extinction of 7 cephalopod Upper Ozarkian are equivalents. families (Flower, 1964a, 1971, 1976c), whereas the The origin of cameral deposits in the chambers of Chazyan (Middle Ordovician) is marked by the appear- orthoconic cephalopods has long been a subject of ance of 6 new cephalopod orders and 17 new families controversy (for latest summary, Fischer and Teichert, (Flower, 1964a); additionally, the Late Canadian marks 1969). However, their organic nature is now considered the earliest known geological occurrence of cameral an established fact (Flower, 1964b; Gregorie and Tei- deposits in orthoconic cephalopods (Fenton and Fenton, chert, 1965; Lowenstam, 1963; Fischer and Teichert, 1958, p. 180; Flower, 1962, 1964a, b). 1969). Cameral deposits, which are calcareous deposits The contrast between the cephalopod faunas of the secreted against the original walls of the camerae during Canadian and Chazyan is so great that Flower (1957a, the life of the animal, are interpreted as adaptive devices p. 17-18) proposed that the Canadian be recognized as a used to counteract the buoyant effect of gas in the