Hecatoceras
Encyclopedia
Hectoceras is a genus in the nautiloid cephalopod order Discosorida
from the Upper Ordovician of Australia (Tasmania), known from a few isolated siphuncle
specimens.
The siphuncle specimens, which go with two described species, consist of gradually enlarging, expanded segments in a slender series, suggesting similarly slender shells, which themselves are unknown. The interiors of the siphuncles are filled with nested calcareous deposits that form endocones, conformable to the interior shape, leaving a narrow irregular central tube. Connecting with are apparently somewhat thick. Anomalously the bullettes, where the connecting rings attach to the previous septal openings are unswollen.
The slight endogastric curvature, as interpreted by Teichert and Glenister, suggests inclusion in the Cyrtogomphoceratidae
, or ancestral Reudemannoceratidae
; however endosiphuncular desposits are otherwise lacking in both these families. The organic deposits with in the siphuncle suggests it is more likely Hectoceras is a member of the Westonoceratidae
to which it has been assigned.
Discosorida
Discosorida is a unique order of cephalopods that lived from the beginning of the Middle Ordovician, through the Silurian, and into the Devonian. Discosorids are unique in the structure and formation of the siphuncle, the tube that runs through and connects the chambers in cephalopods, which unlike...
from the Upper Ordovician of Australia (Tasmania), known from a few isolated 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...
specimens.
The siphuncle specimens, which go with two described species, consist of gradually enlarging, expanded segments in a slender series, suggesting similarly slender shells, which themselves are unknown. The interiors of the siphuncles are filled with nested calcareous deposits that form endocones, conformable to the interior shape, leaving a narrow irregular central tube. Connecting with are apparently somewhat thick. Anomalously the bullettes, where the connecting rings attach to the previous septal openings are unswollen.
The slight endogastric curvature, as interpreted by Teichert and Glenister, suggests inclusion in the Cyrtogomphoceratidae
Cyrtogomphoceratidae
The Cyrtogomphoceratidae is a family in the cephalopod order Discosorida that comprises genera commonly with compressed, endogastrically curved shells. Siphuncles lie close to the venral side, segments are broadly inflated, connecting rings thick and apically expanded thick bullettes. Chambers are...
, or ancestral Reudemannoceratidae
Reudemannoceratidae
The Reudemannoceratidae are the ancestral and most primitive of the Discosorida, an order of cephalopods from the early Paleozoic. The Reudemannoceratidae produced generally medium sized endogastric and almost straight shells with the siphuncle slightly ventral from the center.-Derivation:The...
; however endosiphuncular desposits are otherwise lacking in both these families. The organic deposits with in the siphuncle suggests it is more likely Hectoceras is a member of the Westonoceratidae
Westonoceratidae
Westonoceratidae are exogastric, mostly compressed, Discosorida of moderate size from the Middle Ordovician to the Lower Silurian.The siphuncle is typically close to the convexly curved outer margin of the phragmocone -the chambered part of the shell- taken to be ventral but may be more central in...
to which it has been assigned.
References
- Flower, R.H.and Curt Teichert 1957. The Cephalopod Order Discosorida. University of Kansas Paleontological Contributions, Mollusca, Article 6. July 1957
- Teichert, C. 1964. Nautiloidea -Discosorida. Treatise on Invertebrate Paleontology Part K. Geological Soc of America and Univ. Kansas Press.