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Memoir 2—Studies of the Actinoceratida

Part 1: The Ordiovician deveopment of the Actinoceratida, with notes on Actinoceratida morphology and Ordovician stratigraphy
Part 2: Macroloxeras, a Devoinian homeomorph of the Actinoceratida

By R. H. Flower, 1957, 101 pp., 5 figs., 13 plates, 1 index.

Descriptions of new Ordovician species of Actinoceratida are combined with a review of the evolution and faunal development of the order in the Ordovician. New morphological information includes spatial relationships of cameral and siphonal deposits in complete shells of several species, and a revised concept of the canal system. Occurrences of antinoceroids in the Ordovician of North America, eastern Asia, and northern Europe are summarized. A revised concept of evolution of the group is presented. Reason is given for regarding Bathmoceras as the ancestor of the order; it gave rise to Polydesmia having thick primitive rings and dendritic radial canals. Reticulate canals characterize Whiterock actinoceroids, of which the genera Wutinoceras, Cyrtonybyoceras, and Adamsoceras are characteristic. Ormoceras, developed from Adamsoceras, continues beyond the close of the Ordovician, giving rise to Deiroceras, of which Troedssonoceras is a synonym. Actinoceras yields itself to division into species groups of faunal and stratigraphic value. Leurorthoceras is a synonym of this genus, and Saffordoceras and Troostoceras are doubtfully distinct. Kochoceras is derived from Actinoceras. Internal differences indicate that Gonioceras and Lambeoceras are homeorphic but not closely related.

Stratigraphic notes include a general discussion of the Ordovician, with special reference to the recent revisions, recognition of a pre-Chazyan interval, the Chazy-Black River hiatus, and the correlation of the Red River beds with the Cobourg and Eden. A revision of the Montoya Group is presented. The importance of austral-boreal oscillations is emphasized, the actinoceroids being predominantly boreal in faunal affinities, although in Cobourg time Deiroceras and Orthonybyoceras invaded the austral realm. The systematic portion includes discussion of 15 genera, one of which, Adamsoceras, is new. Thirty-one species are described and illustrated, all but two of which are new; these are listed in the table of contents.

This work combined the descriptions of new species of actinoceroids with a review of the order as developed in the Ordovician of North America. A primary concern has been the tracing of stocks with special reference to their faunal and stratigraphic significance. This has involved some contributions to the morphology and phylogeny of the group.

The new genus Macroloxoceras is described from two species; the genotype M. magnum, from the Dyer Dolomite of Colorado, and M. minor, from the Percha Shale of New Mexico, both very late Devonian age. Together with Bergoceras, Paraloxoceras, and Pseudactinoceras, this genus comprises the new subfamily Macroloxoceratinae of the Pseudorthoceratidae. Although in the entire subfamily there is a strong resemblance to Rayonnoceras, reexaminations of the evidence suggests this relationship is homeomorphic, though a remarkable example of contemporaneous convergence. Pseudocyrtoceras, sp. Schindewolf is assigned to the Pseudorthoceratinae. In spite of its resemblance to the Macroloxoceratinae, Rayonnoceras retains actinoceroid features favoring its derivation from Ormoceras.

The remarkable new genus Macroloxoceras, on which the present study is based, first came to the attention of the writer through two specimens collected near Glenwood Springs, Colorado, by Dr. William H. MacQuown. Both specimens came from loose boulders at the foot of an escarpment in which both the Devonian Dyer Dolomite member of the Challee Limestone and the Mississippian Leadville Formations were exposed. The specimens not having been found in situ, there was, of course, some uncertainty as to their Devonian or Mississippian age.

The specimens themselves, here described as Macroloxoceras magnum, provided no clear evidence of age. Although they seemed to be closely related to forms previously known only from the Mississippian of Belgium, they were obviously not close enough to them to be considered congeneric. Consequently this resemblance was far from being conclusive evidence of a Mississippian age. An origin in the Devonian was suggested by the lithology of the specimens. Also, one of them was found at the foot of the escarpment at a point at which the Mississippian had been eroded some 300 yards back from the edge. It was not until some 10 years later that a second species of the genus was found by the writer in the Percha Shale of New Mexico. This occurrence greatly strengthens the inferred Devonian origin of the Colorado form.

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