CARBONIFEROUS ACTINOCERATOID NAUTILOIDEA (CEPHALOPODA)-A NEW PERSPECTIVE

Journal of Paleontology, Jul 2007 by Kr�ger, Bj�rn, Mapes, Royal

ABSTRACT-

New material from the Late Mississippian Fayetteville and Caney Formations of the south-central USA supports the taxonomic revision of the Mississippian actinoceroid cephalopods. New representatives of the previously poorly known Rayonnoceras solidiforme, Campyloceras striatulum, and Campyloceras imoense are described. Carbactinoceras procerum new species and Elmoceras graffhami new genus and species are described, and the genus Campyloceras is emended. Character evaluation suggests the shape of the endosiphuncular deposits in the Actinoceratida and Pseudactinocerida are homoplastic because similar patterns of endosiphuncular deposits occur in distantly related nautiloid clades. Apex shape, however, is shown to have previously unrecognized potential for taxon discrimination. Cladistic analysis of 13 actinoceroid and pseudorthocerid nautiloids supports the splitting of the Orthocerida and the creation of a sister group to the Actinoceratida which is identical with the largely neglected Pseudorthocerida.

INTRODUCTION

THE EVOLUTIONARY history of ectocochleate cephalopods, especially of cephalopods with straight shells, is one of repeated appearance of homoplastic similarity. This is the main cause for many of the ongoing difficulties in the systematic approaches that are typically applied in orthocerids, actinocerids, and bactritids. When comparing the definitions of such geographically widespread Paleozoic cephalopod orders as the Actinocerida (Teichert, 1933), Orthocerida (Kuhn, 1940), or Bactritida (Shimansky, 1954), insufficient knowledge of real diagnostic characters (autapomorphies) is apparent from the overlapping borders. The order to which many species should be assigned is not yet clear, and consequently there have been strong differences in opinion as to the scope and membership of these orders (for Actinocerida: see Teichert, 1964a; Dzik, 1984; for Orthocerida: see Balashov and Zhuravleva, 1962; Teichert, 1964b; Sweet, 1964; Barskov, 1968; Engeser, 1996; for Bactritida: see Shimansky, 1962; Erben, 1964; Dzik, 1984; House, 1993).

Actinoceroid cephalopods are straight cephalopods with a large siphuncle containing endosiphuncular deposits. These deposits are ring-like or collar-like (annulosiphuncular), concentrated around the curved septal necks (cyrtochoanitic), and display a special morphology consisting of a central and multiple radial canals. Actinoceroid cephalopods are generally long orthocones (including the largest known ectocochleate cephalopods), with some specimens reaching lengths of several meters; they are the giants among Carboniferous marine invertebrates (e.g., Manger et al., 1999).

During the Carboniferous these cephalopods reached their highest diversity since the Late Ordovician. Although a quantitative measure of this diversity peak is lacking at this time, a simple review of the major monographs is revealing (compare Koninck, 1880; Gordon, 1965; Liang and Wang, 1991; Histon, 1998). The following investigation adds new descriptions of Carboniferous actinocerids and gives a new systematic interpretation. These are made possible by the recent discovery of the apical portions of several actinoceroid taxa. The new information that is provided by these features impacts the overall understanding of these animals.

NOTES ON NAUTILOID CLASSIFICATION

In order to discuss the classification of the Carboniferous actinoceroids, a brief review of the long history of the higher-level taxonomic classification of the Nautiloidea is necessary.

Hyatt (1900) defined the nautiloid orders on the basis of their characteristic septal necks. One of these orders was the Cyrtochoanites, which was defined by cyrtochonitic septal necks. He divided the Cyrtochoanites into three suborders on the basis of the shape of the endosiphuncular deposits. One of these was the Annulosiphonata, which included all forms with annular siphuncular deposits. Hyatt's Annulosiphonata contained gyrocone or coiled forms (Uranoceratidae Hyatt, 1900), the orthoconic actinoceratids (Actinoceratidae Saemann, 1852), and the orthoconic loxoceratids (Loxoceratidae Hyatt, 1900). The most distinctive group in Hyatt's Annulosiphonata was the family Actinoceratidae, which was not carefully studied until Foerste and Teichert (1930) and Teichert (1933). Teichert (1933) elevated this group to ordinal level. The group that was most difficult to define was Hyatt's Loxoceratidae. An important step towards classifying these nautiloids was made by Flower and Caster (1935), who defined the Pseudorthoceratidae. When describing his intention to erect the group, Flower (1939, p. 78) wrote: "The family Pseudorthoceratidae Flower and Caster was referred to the Annulosiphonata of the Cyrtochoanites as a receptacle to contain forms customarily assigned to the uncertain Loxoceratidae. The family Sactoceratidae was previously proposed by Troedsson [Troedsson 1926] for the same purpose."

It is important to note that Flower's Pseudorthoceratidae soon became, by definition, part of the Orthocerida which was erected by Kuhn (1940). At this point, the Anglo-American classification scheme of Hyatt (1900) which placed the Pseudorthoceratidae within the Cyrtochoanites was superceded by the German classification, in which Hyatt's orders had never been adopted (see Zittel, 1923). Kuhn's Orthocerida rapidly gained acceptance; thus the origin and relationships of the pseudorthocerids were sought within the orthocerids (Sweet, 1964; Balashov and Zhuravleva, 1962; Zhuravleva, 1978; Dzik, 1984; Flower, 1988; Teichert, 1988), and the older concept of a close relationship to the Actinocerida was largely forgotten. This is remarkable, because the question of whether forms like Sactorthoceras Flower, 1946 and Rayonnoceras Croneis, 1926 are actinocerids or pseudorthocerids has been answered very differently by different authors (see Dzik, 1984; Histon, 1998; Doguzhaeva et al., 1999). Thus, the midtwentieth century marks the shift from a widespread formal acceptance of the importance of general shell shape (longicone, orthocone) over septal neck shape in the ordinal level classification of nautiloids.