Orthoceratidae M’Coy, 1844
|
publication ID |
https://doi.org/ 10.5281/zenodo.13741383 |
|
persistent identifier |
https://treatment.plazi.org/id/039187AC-7F30-600D-FF20-FA78E343FCCC |
|
treatment provided by |
Felipe |
|
scientific name |
Orthoceratidae M’Coy, 1844 |
| status |
|
Family Orthoceratidae M’Coy, 1844 View in CoL
Gen. et sp. indet. A
Figs. 4B View Fig , 5A View Fig .
Material.— Four specimens displaying apex characters and juvenile growth stages from Chert of the Siuge Member ; Porkuni Regional Stage, Late Ordovician, Estonia .
Description.—Specimen TUG 1227/9 is fragment with three chambers. Minimum cross section diameter is 1.2 mm, length 2.4 mm, chamber length 0.8 mm. Cross section is slightly compressed. Shell is smooth, no growth lines are visible. Septa have shallow curvature. Siphuncle is subcentral.
Specimen TUG 1227/1 is a fragment of most apical nine millimetres of the conch ( Fig. 4B View Fig ). Diameter of conch is at adoral end 1.4 mm, at position of first chamber 1.3 mm. Maximum diameter of fragment is 1.45 mm at distance 1.4 mm from tip, forming slightly inflated interval of first six chambers. Extreme tip of apex is not known. Distance between first and second chamber is 0.4 mm, significantly shorter than subsequent chambers (see Table 1 View Table1 ). Between sixth and seventh septum (at distance of 3.8 mm from apex) abrupt increase of thickness of outer shell of conch (0.03–0.05 mm) occurs. Siphuncle is slightly subcentral, diameter approximately 0.1 of conch cross section. Septal necks are suborthochoanitic, with length of 0.08 mm at sixth septum.
Remarks.—The species under consideration resembles Michelinoceras sp. A of Ristedt (1968) but differs in having an apex diameter that is more then two times larger, a more centrally positioned siphuncle, and more narrowly spaced septa. The species under consideration differs from Michelinoceras elongatum ( Yü, 1930) in having an eccentric siphuncle, a shallower septal curvature and short septal necks. The diagnostic characters of Michelinoceras are a tubular siphuncle with long orthoconic septal necks and an apex with inconspicuous spherical initial chamber, which differs considerably from the species under consideration. Because characters of more mature specimens are not available a generic or species determination is impossible.
Gen. et sp. indet. B
Figs. 4H View Fig , 5C View Fig .
Material.—Three fragments of apical parts of the phragmocone (TUG 1227/2, 3, 4) from Chert of the Siuge Member, Porkuni Regional Stage, Late Ordovician, Estonia.
Description.—Specimen TUG 1227/2, is a fragment of most apical part of shell with length of 12 mm, maximum cross section diameter 1.6 mm, and 18 chambers ( Fig. 4H View Fig ). Conch is slightly bent. Subspherical initial chamber has length of 0.66 mm and cross section diameter of one millimetre. Slight constriction occurs at 0.5 mm from apex. Caecum elongate, starting at 0.14 mm from apex, with siphuncular diameter 0.16 mm. Length of second chamber is significantly shorter (0.5 mm) than initial and subsequent chambers (for length of subsequent chambers see Table 1 View Table1 ). Septal spacing in younger chambers is narrower than in older chambers. Septal necks are suborthochoanitic.
Specimen TUG 1227/4 is a complete embryonic specimen, representing single−chambered phragmocone and body chamber. Length of entire shell is 2.4 mm, maximum diameter 1.2 mm, length of single chamber of phragmocone is 0.45 mm, diameter at septum is 1 mm. Surface of shell is smooth.
Specimen TUG 1227/3 is a fragment that is broken off close to apex with minimum diameter 1.35 mm, maximum diameter 1.6 mm, length 5.2 mm, and angle of expansion is less than three degrees. Thickness of siphuncle is 0.2 mm at diameter 1.52 mm. Siphuncle is very slightly expanded within chambers. Septal necks are suborthochoanitic. Septal curvature shallow. Shell surface is smooth.
Remarks.—These fragments resemble Pleurorthoceras Flower, 1962 because they display suborthochoanitic septal necks and their very slightly expanded siphuncle is constricted at the septal foramina. However, the fragments differ from known species of Pleurorthoceras in having a slightly cyrtoconic growth axis, a higher angle of expansion and a shallow septal concavity. These differences may represent specifics of the early growth stages that are not known in the type of the genus. Because all Porkuni fragments are from minute juvenile shells a specific taxonomic attribution is impossible.
The specimens may be conspecific with one of the species of a group of poorly known smooth longicone orthochoanites of the Middle and Late Ordovician of Baltoscandia that have been mentioned and listed in Balashov (1953) as nomina nuda such as Michelinoceras balticum nom. nud., M. kukersiense nom. nud., and a species falsely referred to as Orthoceras dimitatum Münster, 1840 (= Hemicosmorthoceras dimidiatum Sowerby in Murchinson, 1839, a characteristic Devonian species) by Eichwald (1860) and Balashov (1953). Smooth orthochoanitic longicones represent also a common element of the cephalopod fauna of the Haljala Regional Stage, Middle Ordovician of north Estonia. The entire group of these species needs a revision.
No known copyright restrictions apply. See Agosti, D., Egloff, W., 2009. Taxonomic information exchange and copyright: the Plazi approach. BMC Research Notes 2009, 2:53 for further explanation.