Eriditidae Chen
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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-7F36-600F-FCE6-FCFBE393FD03 |
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treatment provided by |
Felipe |
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scientific name |
Eriditidae Chen |
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Family Eriditidae Chen in Chen et al., 1981
Gen. et sp. indet.
Figs. 4A View Fig , 5C View Fig , 6D View Fig , 7E View Fig , 8 View Fig .
Material.— Two specimens displaying internal characters of nearly adult growth stages ( TUG 101 /66a, TUG 1227 /13), one fragment displaying adolescent growth stages , two specimens with preserved apex ( TUG 1227 /44, 47) from Chert of the Siuge Member , Porkuni Regional Stage , Late Ordovician from Porkuni, north Estonia .
Description.—Specimen TUG 101/66a has shell length of 49 mm, minimum cross section diameter seven millimetres, maximum diameter of 13 mm, and angle of expansion of seven degrees. Outer shell is poorly preserved, but apparently smooth ( Fig. 7E View Fig ). Cross section is circular. Chamber height is approximately one−third of shell diameter. Conch margin is slightly concave in lateral view. Twelve septa occur over total length of fragment. Septal distance is approximately one−fourth of cross section diameter. Septa are shallowly concave and slightly oblique, sloping in adoral direction at pro−siphuncular side. Siphuncle is slightly subcentral. Septal perforation is approximately 0.1 of conch cross section. Septal necks are achoanitic to very short loxochoanitic ( Fig. 4A View Fig ). Siphuncular segments are expanded within chambers, oval in longitudinal section with maximum diameter approximately 0.26 of conch cross section. Siphuncular segments are more expanded within chambers at side that is closer to conch margin. Episeptal and mural deposits occur in all chambers.
E. Pomerantsoceras tibia sp. nov., TUG 1227/20, the apparent spherical shape of the apex is an artefact that resulted from the oblique section of a part of the specimen, the partially preserved thin siphuncular tube, fakes a prochoanitic septal neck at the second septum, but the septal necks are always achoanitic. F. Graciloceratidae gen. et sp. indet. B., TUG 1227/25, apical part (F 1), enlarged part of same specimen (F 2), note the suborthochoanitic septal necks. G. Graciloceratidae gen. et sp. indet. A, TUG 1227/34. H. Orthoceratidae gen. et sp. indet. B, TUG 1227/2 detail of most apical part of fragment (H 1), overview of apical part of the fragment (H 2). I. Piersaloceras gageli Teichert, 1930 . TUG 1227/32, section represents not a median section, but a section along the growth axis, sectioned roughly from flank to flank of the shell. Scale bars 1 mm.
Apex TUG 1227/147 ( Fig. 6D View Fig ) has oblique, flat, saucer−shaped tip with 0.9 mm diameter. Surface of apex is smooth with only very fine growth lines that form faint sinus parallel to apex tip. Sinus occurs at anti−siphuncular side, thus growth lines aperture of embryonic shell slightly bendforward at opposite side of siphuncle. Faint shell constriction occurs at distance of about one millimetre from tip. Surface of shell is ornamented with faint, angular, kinked longitudinal ridges but otherwise smooth.
Apex TUG 1227/44 ( Fig. 8 View Fig ) has first septum 0.7 mm from tip, second at one millimetre from tip. Less than two septa occur at distance similar to cross section. Septal curvature is very shallow. Septal necks are very short loxochoaniticachoanitic. Siphuncle position is eccentric. Septal perforation is 0.1 of conch cross section. Thin episeptal deposits occur at prosiphuncular side.
Remarks.—The specimens described above are assigned to the same species because of the similarity of internal characters. In all the specimens, the septal necks are achoanitic or nearly so, the siphuncle is subcentral, and the septa have a very shallow concavity. In fragments displaying later growth stages, remnants of the connecting rings are preserved. The siphuncular segment of these specimens is very conspicuous, having a more expanded side in direction toward the prosiphuncular conch margin. The septal spacing and the expansion rate of apical fragments are larger than that of specimens representing later growth stages. However, in many known orthoceridans the apicalmost conch is slender with wider septal distance in later growth stages.
The surface of the fragment representing later growth stages is smooth. In the contrary, the apical fragments have shallow longitudinal structures starting a few millimetres from the tip. Supposedly, these faint structures do not occur in more adolescent growth stages or are not visible because of the comparatively poor preservation of larger specimens.
Very similar apices from the earliest Silurian of Siberia have been described by Miagkova (1967: table 3, fig. 1) as Kionoceras muyagkirum Miagkova, 1967 . These apices also show a blunt cap−shaped tip and kinked longitudinal lirae appear at some distance from the tip. Miagkova (1967: 11) described in the diagnosis of K. muyagkirum the septal necks as very short orthochoanitic but provides a figure (Miagkova 1967: fig 2.) that shows suborthochoanitic septal necks in later growth stages, leaving an ambiguous impression for the reader. Her assignment of the species to Kionoceras Hyatt, 1884 emphasizes the aspect of her figure 2, because Orthoceras doricum Barrande, 1868 , the type species of Kionoceras , displays septal necks that are at the tenuous border between orthocoanitic and suborthochoanitic. However, species with very short to achoanitic septal necks occur in the same horizon that are very similar to K. muyagkirum with respect to the general conch shape and ornamention, such as Malgaoceras Miagkova, 1967 , and Mongoceras Miagkova, 1967 . The latter two display very short septal necks and expanded siphuncular segments and represent Eriditidae in the sense of Chen et al. (1981). Malgaoceras , Mongoceras , and K. muyagkirum represent a group of Early Silurian orthocones that are very similar to each other. They are ornamented with faint longitudinal and transverse elements or smooth; the siphuncle of these orthocones is sometimes slightly expanded, sometimes nearly tubular and the septal necks are generally very short. They are grouped in Eridites Zhuravleva, 1961 or Kionoceras Hyatt, 1884 , respectively (e.g., Kionoceras styliforme Chen and Liu in Chen et al., 1981, and Eriditus crassus Chen in Chen et al., 1981). This entire group is in need of a revision. The assignation of the specimens described above within the Eriditidae emphasizes the close relationship with the kionoceratidans of Miagkova (1967) and the Chinese eriditidans.
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