Description. Type Lytoceras argonautarum Anderson, 1902. According to Kennedy & Klinger (1978), these are evolute ammonites with tangent or barely overlapping whorls, a subrectangular or more or less depressed trapezoidal whorl section, and which grow very rapidly in height and thickness. The umbilical wall is very tall and subvertical. The test bears fine, closely spaced ribs or lirae, radial to proverse, straight or slightly convex, which cross venter in a straight line. The inner whorls may have narrow constrictions. The first lateral lobe L1 is special: it is deeper than L and L2 and deeply incised to form two bifid sublobes. Rare genus found from the Middle Aptian (Gargasian) to the Middle Albian (benettianus zone): southeastern France, Bulgaria, Caucasus, Mozambique, South Africa, Madagascar and California.
Species. Kennedy & Klinger (1978) describe the four known species: A. argonautarum Anderson, 1902; A. belliseptatum (Anthula, 1899); A. depereti (Kilian, 1892) (Upper Aptian); and A. besairiei Collignon, 1949 (Late Aptian - Middle Albian, photographs on the left). They consider the first two to be junior synonyms of A. depereti, but maintain A. besairiei. However, Förster (1975) synonymizes the latter with A. belliseptatum.
Remarks. We follow the Treatise (Wright, 1996) which places the genus in the subfamily Lytoceratinae alongside Ammonoceratites Bodwich, 1822, while Anderson and Kennedy & Klinger (1978) divide the genus Ammonoceratites into two subgenera: Ammonoceratites sensu stricto and Argonauticeras. The former appears identical in profile, but its whorl section is rounded, its constrictions are infrequent, and its fine ribs are crenulated (their rear side is finely wavy).
Confusions. Labeling mistakes are frequent on the web, as some genera in the same family (Lytoceratidae) are similar in profile, but their whorl section is round to oval. E.g., Eogaudryceras numidum (see its entry) also has whorls that grow very rapidly, but they are more overlapping, while Protetragonites aeolus (d'Orbigny, 1850) has whorls that grow less rapidly, a wider umbilicus, and more widely spaced constrictions. The family also includes genera with a rectangular section. Eotetragonites umbilicostriatus has a wider umbilicus, numerous proverse and convex grooves, and whorls increasing less rapidly (see its entry). Tetragonites heterosulcatus (Anthula, 1899) displays ribs with a concave sinus on venter; see Kennedy & Klinger (1977). In fact, Argonauticeras is distinguished by its whorls which are barely covered, with a depressed rectangular section and growing very quickly, by the presence of very fine ribs or lirae, not crenulate and without a notable ventral sinus, and by constrictions only in the young.