Parvihebetoceras, Kröger, 2007
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publication ID |
https://doi.org/ 10.5281/zenodo.13741383 |
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persistent identifier |
https://treatment.plazi.org/id/039187AC-7F3A-6001-FF20-FA60E4E0FB16 |
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treatment provided by |
Felipe |
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scientific name |
Parvihebetoceras |
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Genus Parvihebetoceras nov.
Derivation of the name: From Latin parvus, small, referring to the small adult size of the genus.
Type species: Parvihebetoceras wahli sp. nov., by monotypy.
Diagnosis.— Ascocerida with orthoconic adult shell with slightly depressed or circular cross section. Apical angle of conch is low. Truncated adult shell is nearly tubular. Parvihebetoceras differs from all other ascoceridans in having small, truncated adult shell, displaying few closely spaced, shallow concave septa with straight, directly transverse sutures. Conch surface is smooth or ornamented with faint growth lines. Sutures adapical of septum of truncation are straight, shallowly concave, and closely spaced. Chamber height is less than one−third of cross section diameter. Septum of truncation occurs at cross section diameter of about four millimetres. Adult cross section diameter is approximately seven millimetres. Siphuncular diameter is approximately one−tenth of conch diameter. Siphuncle is positioned between centre of conch and conch margin. Siphuncular diaphragm occurs slightly adorally of the septum of truncation. Septal necks are asymmetric, more elongated at ventral side, achoanitic and suborthochoanitic in juvenile growth stages, suborthochoanitic in adult growth stages. Apex is bullet−shaped, slightly cyrtoconic, cicatrix present, with diameter at first septum approximately 1.4 mm. No endosiphuncular deposits are known.
Remarks.—The genus in consideration differs from the enigmatic Hebetoceras Flower, 1941 , with which it is comparable because of its orthoconic truncated conch, in having a smaller adult size and a smooth shell. Hebetoceras mirandum Flower, 1941 , the type species of Hebetoceras is poorly known making a more precise comparison difficult. The genus of consideration represents without doubt an ascoceratid. Evidence for this comes from the shape of the septal necks and the type of truncation. The Middle–Late Silurian Sphooceras Flower, 1962 differs in having a septum of truncation that is covered by a conspicuous callus and does not show the typical septal diaphragm. It is assumed that the truncated specimens represent adult individuals because the nearly completely preserved body chamber reveals a terminal constriction, which is visible as convex outline of the lateral view ( Fig. 7B View Fig ).
Stratigraphic and geographic range.—Siuge Member, Porkuni Regional Stage, Late Ordovician from Porkuni, north Estonia.
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