Parvihebetoceras wahli, 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-6003-FCE6-FB22E459FEE9 |
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treatment provided by |
Felipe |
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scientific name |
Parvihebetoceras wahli |
| status |
sp. nov. |
Parvihebetoceras wahli sp. nov.
Figs. 4C View Fig , 7C View Fig , 9 View Fig .
Derivation of the name: In honour to the Estonian geologist Arwed von Wahl who collected some of the specimens mentioned here during his work in Porkuni in the 1920s ( Wahl 1923).
Holotype: TUG 1227/13.
Type locality: Porkuni, north Estonia.
Type horizon: Siuge Member, Porkuni Regional Stage,Late Ordovician.
Material.— Nine specimens, displaying juvenile, adolescent and mature growth stages in collection of TUG .
Diagnosis.—Same as for genus, by monotypy.
Description.— Holotype TUG 1227/13 ( Fig. 7C View Fig ) is most complete fragment of collection representing complete body chamber and three chambers of phragmocone, including septum of truncation. Holotype preserves fine surface ornamentation with simple transverse growth lines with distance of 0.1 mm. Body chamber length is 12 mm, slightly bulged at most adoral one−third of fragment.
Specimen TUG 1227/41 ( Fig. 9 View Fig ) is 5.55 mm long fragment of conch with three chambers. Apical septum, which is septum of truncation, is strongly concave, 0.19 mm thick, with anvil−shaped septal necks and septal perforation that is 0.18 mm in diameter. Siphuncular tube is thin, tubular. Diaphragm occurs at 0.6 mm adoral of septum of truncation, there, siphuncle diameter is 0.28 mm. Siphuncular tube apical of diaphragm is inflated. Adoral two septa are 0.03 mm thick, respectively and each septal perforation has diameter of 0.26 mm. Septal necks are thickened, suborthochoanitic. Outer shell is fractured at position of septum of truncation.
Specimen TUG 1227/12 is fragment of conch with four chambers, displaying position of septum of truncation, one adapical and one adoral septum. Length of fragment is approximately six millimetres. Cross section is circular with diameter 3.5 mm. Distance between succeeding adoral septum and septum of truncation is 1.4 mm. Distance between septum of truncation and preceding adapical septum is 1.36 mm, adoral and adapical septum have shallow curvature, septum of truncation with deep curvature. Thickness of septum of truncation is 0.12 mm, of adoral and adapical septum is 0.08 mm, shell thickness is 0.3 mm. Septal necks and remains of siphuncular tube are preserved at adoral septum. Position of siphuncle is subcentral. At cross section diameter 3.4 mm septal perforation is one millimetre from conch margin. Diameter of septal perforation is 0.3 mm (about 0.1 of cross section diameter). Remains of siphuncular tube are slightly expanded. Dorsal side of septal necks are suborthochoanitic−cyrtochoanitic with length of 0.1 mm. Sepal necks at ventral side are strongly recursive, cyrtochoanitic with length of 0.12 mm.
Specimen TUG 1227/11 is fragment of truncated conch displaying three adoral chambers. Length of fragment is 5.4 mm. Cross section is circular with four millimetres in diameter. Distance of truncated septum from succeeding adoral septum is 1.5 mm and from proceeding adoral septum is two millimetres. Truncated septum is 0.08 mm thick, cap−shaped with deep curvature (1.4 mm deep). Adoral septa are 0.04 mm thick with shallow curvature (0.3 mm deep). Shell thickness is 0.14 mm mural part of septa inclusive. Septum of truncation occurs at cross section diameter of five millimetres. Distance to succeeding youngest septum is 1.7 mm from the preceding oldest septum of fragment is one millimetre.
Additional five specimens display only two or a single septum with cross section diameter of 2–3 mm. Conch smooth with circular cross section. Specimen TUG 1227/10 ( Fig. 4C View Fig ) is fragment of apex with two millimetres length, 1.7 mm maximum diameter. Fragment is slightly cyrtocone. Shell of conch is smooth and very thin (> 0.01 mm). Three chambers are preserved with distance between septa from apical to adoral chamber: 0.88 mm (initial chamber), 0.54 mm (second chamber), 0.42 mm (third chamber). Cross section diameter at first septum is 1.4 mm, at second septum 1.5 mm. Septa have very shallow curvature. Siphuncle is empty, subcentral with ventral side toward concave side of curvature of shell. Siphuncle is slightly expanded. Septal necks are asymmetrical with ventral side suborthochoanitic and dorsal side achoanitic. Caecum is 0.7 mm long, 0.4 mm thick, with distance of 0.12 mm from outer shell of tip. Apex is bullet shaped, slightly bent, with oblique surface of 0.7 mm and approximately 45 ° toward growth axis near ventral side of tip.
Remarks.—It is not known if the truncation of the shell in Parvihebetoceras is a single event in the ontogeny or if there are multiple truncations. However, the presence of three fragments with a septum of truncation that shows nearly identical cross section diameters at around four millimetres indicates a single event when reaching maturity.
The attribution of the apex fragment TUG 1227/10 ( Fig. 4C View Fig ) to the species under consideration is somewhat questionable. However, it is justified by the characteristic asymmetric shape of the septal necks and the characteristic very shallow curvature of the septa that occur in the apical fragment as well as in later growth stages. Moreover, the only known apex of Ascocerida described by Lindström (1890) in Parascoceras Miller, 1932 , resembles the fragment of TUG 1227/10 with respect to general and siphuncular shape. The apex of Parascoceras is slightly cyrtocone and bullet−shaped, the siphuncle is subcentral, and septal necks are suborthochoanitic. This is similar to specimen TUG 1227/10. Furthermore, specimen TUG 1227/10 could not be attributed to any other cephalopod fragment of Porkuni. It is therefore concluded that it represents an apex of Parvihebetorthoceras wahli sp. nov.
The presence of a cicatrix is assumed by a very characteristic oblique surface at the tip of the apex. Such a surface is also present in Pseudorthoceras Girty, 1911 ( Kröger and Mapes 2004) and the living Nautilus . This surface displays the same dimension of the lentoid structure of the cicatrix (initial shell of the conch) in Nautilus (compare Tanabe and Uchiyama 1997). The presence of the cicatrix in Ascocerida gives evidence for a close relationship between Ascocerida and Pseudorthocerida
Stratigraphic and geographic range.—Chert of the Siuge Member, Porkuni Regional Stage, Late Ordovician, Estonia.
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