6-477: Campbelloceras is a tarphyceratid nautiloid known from the Lower Ordovician , Upper Canadian Epoch of North America, where it is widespread. Campbelloceras was named by Ulrich and Foerste in 1936. The shell of Campbelloceras has a circular whorl section, only slightly impressed, and a siphuncle that is close to the venter in all growth stages. Campbelloceras differs from Tarphyceras in that
12-659: The Estonioceratidae . Tarphyceratids vary in form and siphuncle position. The siphuncle may be subcentral as in Centrotarphyceras ; intermediary as in Tarphyceras , or subventral as in Campbelloceras . Cross sections vary. Tarphyceras and Campbelloceras are rounded. Centrotarphyceras is subquadrate with broadly rounded flanks and a slightly rounded venter. Eurystomites is subquadrate and slightly compressed, with
18-758: The Lower Ordovician (M-U Canad); gave rise to the Trocholitidae and provided the sources for the more advanced barrandeocerids Genera in the Tarphyeratidae as named in the Treatise on Invertebrate Paleontology Part K (Furnish and Glenister 1964) are restricted to the middle and upper parts of the Lower Ordovician in North America. For purposes of discussion only, they are divided into those where dorsum, which
24-526: The maximum width closer to the dorsum. Pionoceras is subrounded and slightly depressed with the maximum width closer to the venter. The dorsal impression in Tarphyceras and Eurystomites is broad and deep and in Pionoceras and Centrotarphyceras , broad and shallow. In Campbelloceras and Cycloplectoceras the impression is slight, running along the dorsal median. The Tarphyceratidae are known mainly from
30-468: The outer whorl of Estonioceras are similar, except for Campbelloceras being slightly impressed. Both have proportionally large siphuncles, near the venter. Tarphyceratidae The Tarphyceratidae are tightly coiled, evolute Tarphycerida with ventral siphuncles . The dorsum is characteristically impressed where the whorl presses against the venter of the previous. The Tarphyceratidae are derived from Bassleroceras or possibly from some member of
36-518: The rate of expansion is greater, the siphuncle is proportionally larger, and an impression is shallower. Campbelloceras may have given rise to the barrandeocerid Plectoceras ( Plectoceratidae ) through simplification of the connecting rings, and to Tarphyceras through tighter coiling, development of a deeper impression, and reduction in the size of the siphuncle. Campbelloceras may be conceivably derived from Estonioceras ( Estonioceratidae ). The cross section of Campbelloceras and that of
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