Trocholitidae
Encyclopedia
The Trocholitidae are Tarphycerida with whorls in close contact as with the Tarphyceratidae
, but in which the siphuncle
, similar in structure, becomes dorsal. The Trocholitidae are derived from the Tarphyceratidae, perhaps from different tarphyceratids.
Members of the Trocholitidae vary in whorl section and siphuncle position. Most are tightly coiled with the dorsum impressed to some degree. As exceptions Arkoceras and Wichitoceras have their whorls touching but without a dorsal impression. Discoceras
and Curtoceras
have somewhat subquadrate whorl sections and are moderately impressed. In Graftonoceras and Jasperoceras the venter is rounded and continues smoothly through the flanks to a broad dorsum which in each is moderately impressed. The siphuncle in Graftonoceras is on the dorsal margin, in Jasperoceras it is between the center and dorsum. Whorls in Trocholites have a wide cross section and low profile, in Wichitoceras they are laterally compressed resulting in a high profile. Litoceras has a broad cross section and a deeply impressed dorsum. As with Discoceras and Curtoceras the siphuncle is between the center and dorsum.
Arkoceras, Trocholitoceras, and Wichitoceras are limited to the upper Lower Ordovician, along with the enigmatic Beekmanoceras
which may or may not belong. Curtoceras, Litoceras, and Hardmanoceras begin in the Lower Ordovician and continue into the middle of the Period, Hardmanoceras questionably. Jasperoceras is known only from the Middle Ordovician; Trocholites and Discoceras from the Middle and Upper. Graftonoceras comes from the Upper Silurian
. Hardmanoceras which is strongly ribbed may be the ancestor of the Ophidioceratidae of the Upper Silurian
Part L (Furnish and Glenister 1964) are found in three principal realms, North America, Australia, and Northern Europe (Balto-Scandia).
Genera known only from North America
Genera known from North America and Europe ---
Genera known from North America and Australia. ---
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 the...
, but in which the siphuncle
Siphuncle
The siphuncle is a strand of tissue passing longitudinally through the shell of a cephalopod mollusk. Only cephalopods with chambered shells have siphuncles, such as the extinct ammonites and belemnites, and the living nautiluses, cuttlefish, and Spirula...
, similar in structure, becomes dorsal. The Trocholitidae are derived from the Tarphyceratidae, perhaps from different tarphyceratids.
Members of the Trocholitidae vary in whorl section and siphuncle position. Most are tightly coiled with the dorsum impressed to some degree. As exceptions Arkoceras and Wichitoceras have their whorls touching but without a dorsal impression. Discoceras
Discoceras
Discoceras, a member of the Trocholitidae in the Tarphycerida, not to be confused with Discosorus.Discoceras is characterized by closely coiled, gradually expanding shells with a subquadrate cross section, that may be ribbed or smooth. The sides are broadly rounded; the venter wide and slightly...
and Curtoceras
Curtoceras
Curtoceras is a genus in the tarphycerid family Trocholitidae found widespread in the late Early and Middle Ordovician of North America and northern Europe. Curtoceras has a shell that is gradually expanded, with half the fully mature body chamber divergent from the preceding volution. Whorl...
have somewhat subquadrate whorl sections and are moderately impressed. In Graftonoceras and Jasperoceras the venter is rounded and continues smoothly through the flanks to a broad dorsum which in each is moderately impressed. The siphuncle in Graftonoceras is on the dorsal margin, in Jasperoceras it is between the center and dorsum. Whorls in Trocholites have a wide cross section and low profile, in Wichitoceras they are laterally compressed resulting in a high profile. Litoceras has a broad cross section and a deeply impressed dorsum. As with Discoceras and Curtoceras the siphuncle is between the center and dorsum.
Arkoceras, Trocholitoceras, and Wichitoceras are limited to the upper Lower Ordovician, along with the enigmatic Beekmanoceras
Beekmanoceras
Beekmanoceras is a small cephalopod from the Middle Canadian of New York with a gyroconic shell in which the siphuncle is on the inner or concave side of the whorl. Furnish and Glenister placed Beekmanoceras in the Trocholitidae ,interpreting the curvature to be exogastastric and the siphuncle to...
which may or may not belong. Curtoceras, Litoceras, and Hardmanoceras begin in the Lower Ordovician and continue into the middle of the Period, Hardmanoceras questionably. Jasperoceras is known only from the Middle Ordovician; Trocholites and Discoceras from the Middle and Upper. Graftonoceras comes from the Upper Silurian
Silurian
The Silurian is a geologic period and system that extends from the end of the Ordovician Period, about 443.7 ± 1.5 Mya , to the beginning of the Devonian Period, about 416.0 ± 2.8 Mya . As with other geologic periods, the rock beds that define the period's start and end are well identified, but the...
. Hardmanoceras which is strongly ribbed may be the ancestor of the Ophidioceratidae of the Upper Silurian
Trocholitid Genera
Genera assigned to the Trocholitidae in the Treatise on Invertebrate PaleontologyTreatise on Invertebrate Paleontology
The Treatise on Invertebrate Paleontology published by the Geological Society of America and the University of Kansas Press, is a definitive multi-authored work of some 50 volumes, written by more than 300 paleontologists, and covering every phylum, class, order, family, and genus of fossil and...
Part L (Furnish and Glenister 1964) are found in three principal realms, North America, Australia, and Northern Europe (Balto-Scandia).
Genera known only from North America
- BeekmanocerasBeekmanocerasBeekmanoceras is a small cephalopod from the Middle Canadian of New York with a gyroconic shell in which the siphuncle is on the inner or concave side of the whorl. Furnish and Glenister placed Beekmanoceras in the Trocholitidae ,interpreting the curvature to be exogastastric and the siphuncle to...
(affin?). Tiny,fragmentary cyrtocones from the uL Ordovician of New York state, presumed exogastric with a dorsal siphuncle. Flower (1964) thought they were endogastric ellesmeroceratidsEllesmeroceratidaeThe Ellesmeroceratidae constitute a family within the cephalopod order Ellesmerocerida. They lived from the Upper Cambrian to the Lower Ordovician... - Jasperoceras: subglobular, rapidly expanded, weakly ribbed; whorls uniformly rounded across flanks and venter, dorsum well impreseed. Siphuncle proportionally large, subdorsal. Known only from the Middle Ordovician of Arkansas
- LitocerasLitocerasLitoceras is a trocholitid genus that has been found the Lower and early Middle Ordovician of Newfoundland. Whorls in Litoceras have a broadly rounded cross section with the width greater than the height and somewhat resembles the tarphyceratid Pionoceras from the same time, except for the...
: fairly large, tightly coiled, with broadly rounded whorls and a deeply impressed dorsum, found in the upper Lower and lower Middle Ordovician of Newfoundland (Can). Whorl section similar to that of Pionoceras (TarphyceratidaeTarphyceratidaeThe 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 the...
) except for the position of the siphuncle. - Wichitoceras : small, laterally compressed, with a narrowly rounded venter and slightly impressed dorsum, widespread in the uL Ordovician of N Am (U.S.A.). Cross section similar to Shumardoceras (EstonioceratidaeEstonioceratidaeThe Estonioceratidae is a family of loosely coiled tarphycerids in which the inner side of the whorls, which forms the dorsum, is rounded or flat with no impression; and in which the siphuncle, composed of thick tubular segments, is located ventrally...
).
Genera known from North America and Europe ---
- CurtocerasCurtocerasCurtoceras is a genus in the tarphycerid family Trocholitidae found widespread in the late Early and Middle Ordovician of North America and northern Europe. Curtoceras has a shell that is gradually expanded, with half the fully mature body chamber divergent from the preceding volution. Whorl...
: gradually expanded with a divergent mature body chamber; whorls moderately impressed, subquadrate in section; siphuncle subdorsal after the first volution. (L-M Ord.) - DiscocerasDiscocerasDiscoceras, a member of the Trocholitidae in the Tarphycerida, not to be confused with Discosorus.Discoceras is characterized by closely coiled, gradually expanding shells with a subquadrate cross section, that may be ribbed or smooth. The sides are broadly rounded; the venter wide and slightly...
: gradually expanded,smooth or ribbed,dorsum slightly to moderately impressed. Siphuncle starts off central, becomes subdorsal in the first half volution.(M-U Ord) Has also been found in China. - TrocholitesTrocholitesTrocholites is a tarphycerid genus in the fa mily Trocholitidae from the Middle and Upper Ordovician with a gradually expanding, weakly ribbed shell; whorls in contact, dorsum slightly impressed; cross section depressed, venter and sides rounded; siphuncle close to but not at the dorsal margin.The...
: gradually expanded, weakly ribbed, whorl section broad and smoothly rounded, dorsum with broad shallow impression; suphuncle submarginal. (M-U Ord)
Genera known from North America and Australia. ---
- Arkoceras: somewhat rapidly expanded; whorls in contact but without dorsal impression, cross section subcircular (resembles Picnoceras (EstonioceratidaeEstonioceratidaeThe Estonioceratidae is a family of loosely coiled tarphycerids in which the inner side of the whorls, which forms the dorsum, is rounded or flat with no impression; and in which the siphuncle, composed of thick tubular segments, is located ventrally...
);sihuncle small,subdorsal.(L Ord) - Trocholitoceras: graduallly expanded,weakly ribbed,like Trocholites except for being deeply impressed and the siphuncle being close to the dorsum in all but the innermost volution (L Ord)
References
- Furnish, W.M. and Glenister, Brian F 1964; Nautiloidea -Tarphycerida, in The Treatise on Invertebrate Paleontology, Part K, Nautiloidea; Geological Society of America and University of Kansas press.