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Pelagiella deltoides
Taxonomy
Pelagiella deltoides was named by Runnegar and Jell (1976). Its type locality is QML128. 6 km SSW of Thorntonia Homestead, which is in a Floran carbonate limestone in the Gowers Formation of Australia.
Synonymy list
| Year | Name and author |
|---|---|
| 1976 | Pelagiella deltoides Runnegar and Jell p. 135 figs. f. 7A-I |
| 1986 | Pelagiella deltoides Geyer p. 93 |
| 1998 | Pelagiella deltoides Brock p. 584 figs. f. 5.5-7 |
| 2010 | Pelagiella deltoides Vendrasco et al. pp. 114 – 115 figs. txt. f. 3N-O; pl. 8 f. 1 – 11 |
| 2023 | Pelagiella deltoides Wagner p. 5430 |
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If no rank is listed, the taxon is considered an unranked clade in modern classifications. Ranks may be repeated or presented in the wrong order because authors working on different parts of the classification may disagree about how to rank taxa.
†Pelagiella deltoides Runnegar and Jell 1976
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Diagnosis
| Reference | Diagnosis | |
|---|---|---|
| M. J. Vendrasco et al. 2010 | Most internal moulds of Pelagiella from the Gowers Formation are relatively smooth, although many have faint transverse (i.e. parallel to growth lines) striations (Pl. 8, fig. 10), a pattern also often seen in other contemporaneous gastropod-like forms such as Protowenella (see below). A few specimens, however, preserve partial phosphatized casts of the shell (Pl. 8, figs 2, 5–7), and these indicate that, as in the Early Cambrian species Pelagiella subangulata from the Parara Limestone in Australia (Runnegar 1985), Pelagiella deltoides had a lamellofibrillar shell microstructure, with an innermost layer consisting of fibres arranged parallel to growth lines, and an overlying layer consisting of fibres arranged perpendicular to the growth lines (Pl. 8, figs 6–7). Lamello-fibrillar shell microstructure is a laminar structure where ‘horizontal fibers in successive laminae differ in orientation by irregularly varying angles’ (Carter 1990, p. 611). This type of shell microstructure appears to be common in early molluscs (Feng and Sun 2003).
This microstructure observed in Pelagiella deltoides (and Pelagiella subangulata) is similar to crossed lamellar (Runnegar, 1985) in that different bundles of fibres have different orientations, but it differs from crossed lamellar in that the long axes of the fibres are horizontal, whereas in crossed-lamellar microstructure the fibres dip vertically (Carter 1990). However, it is possible that the ‘fibers’ in P. deltoides represent the imprint of first-order lamellae that are composed of second-order crystals, as the ‘fibers’ bear similarities to striations (that represent first-order lamellae of a crossed-lamellar microstructure) on the inner surface of the shell of some modern molluscs (Pl. 10, figs 10–11). This hypothesis would help explain why these striations often extend for a long distance and appear to follow the curvature of the shell. No clear sign of lower order crystals has yet been seen on the fossils of P. deltoides, but specimens of the similar genus Aldanella from the Early Cambrian of Siberia show probable lower order crystallites within striations that likely represent second- order lamellae (MV, pers. obs.). It is possible that a similar state exists in P. deltoides but requires finer preservation to detect. If these ‘fibers’ represent first-order lamellae, they are so narrow as to be better described as crossed acicular. |