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Mellopegma simesi
Taxonomy
Anabarella simesi was named by MacKinnon (1985) [= Mellopegma georginensis Runnegar & Jell 1976 f. 8b12-13]. It is a 3D body fossil. Its type locality is Trilobite Rock, Cobb Valley, South Island, which is in a Boomerangian carbonate limestone in the Tasman Formation of New Zealand.
It was recombined as Mellopegma simesi by Vendrasco et al. (2011), Oh et al. (2025).
It was recombined as Mellopegma simesi by Vendrasco et al. (2011), Oh et al. (2025).
Synonymy list
| Year | Name and author |
|---|---|
| 1985 | Anabarella simesi MacKinnon p. 71 figs. f. 3A-J |
| 1998 | Anabarella simesi Brock pp. 578 - 580 figs. f. 4.11-13 |
| 2010 | Anabarella simesi Vendrasco et al. p. 106 figs. txt. f. 3B; pl. 2 |
| 2011 | Mellopegma simesi Vendrasco et al. p. 11A:15 figs. 10-11 |
| 2023 | Anabarella simesi Wagner p. 5497 |
| 2025 | Mellopegma simesi Oh et al. p. 11 |
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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.
†Mellopegma simesi MacKinnon 1985
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Diagnosis
| Reference | Diagnosis | |
|---|---|---|
| M. J. Vendrasco et al. 2010 | The type of laminar microstructure in A. simesi is most similar to calcitic semi-nacre, although it shares some similarities with nacre as well as foliated calcite and aragonite (Table 2). None of these shell microstructures is known from any of the other Cambrian species assigned to Anabarella, although the Early Cambrian Anabarella plana had a stepwise microstructure (Kouchinsky 1999) that has been interpreted as laminar (Rogalla et al. 2003). Shell microstructure is well preserved in many specimens of A. simesi. In some cases, partial or complete edges of tablets are preserved (Pl. 2, figs 6–7, 11), revealing that their shape was that of a parallelogram with unequal to roughly equal sides and with spiral growth (Pl. 2, fig. 7), similar to rhombs of calcitic semi-nacre (Text-fig.4P). The distribution of interfacial angles is similar to that in Mellopegma georginensis – there are two clusters of interfacial angles, one group with a mean of 75.1 degree (n = 25; standard deviation = 5.1), and other group with greater dispersion with a mean of 113.9 degree (n = 28; standard deviation = 12.3) similar to the expected angles (78 degree and 102 degree) of the rhombic version of calcitic semi-nacre. As with Mellopegma georginensis, the microstructure lacks any sign of elongate blades or laths or the regular-toothed pattern of laminae that characterize foliated calcite. However, the 75 degree acute angle is more suggestive of calcite than aragonite (see discussion of Mellopegma georginensis above). Commonly, the imprints of shell microstructure in A. simesi look like relatively broad, shallow stairs, formed by layers of laterally merged tablets that dip slightly in the direction of the supra-apical end of the shell (Pl. 2, figs 9–10, 12). This characteristic of A. simesi is similar to the ‘foliated aragonite’ of modern monoplacophorans (Checa et al. 2009), but again no elongate laths – that also characterize foliated aragonite – are known from A. simesi. Overall, as with M. georginensis, the laminar microstructure shares the most similarities with the rhombic type of calcitic semi-nacre (Table 2).
In addition, somewhat regularly spaced, often faint granules or tubercles occur over much of the internal mould surface (Pl. 2, fig. 5). These granules achieve significant height in some specimens and so very likely represent the filling of pores that opened on the inner surface of the shell. | |
| M. J. Vendrasco et al. 2011 | Laterally compressed, elongate shell with less than half a coil. Margin below apex strongly concave; dorsal margin weakly to strongly convex. Faint internal ridges on some specimens. Raised sub-apical margin, forming a shelf ranging from nearly horizontal to angling downward at 45º. Supra-apical margin tends to be wider than sub-apical margin (Figure 10.17, 10.19-20; cladistic character 21, state 1; Appendix 1). Granules apparent in some internal moulds, paralleling growth lines (Figure 10.17, Figure 11.1). Prominent protoconch (Figure 10.12, 10.17) bulbous. Pegma or pegma-like structure developed to varying degrees (Figure 10, Figure 11.2-3; character 1, state 1). |
Measurements
No measurements are available
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| Source: f = family, c = class | |||||
| Reference: Hendy et al. 2009 | |||||
Age range: base of the Wuliuan to the top of the Boomerangian or 506.50000 to 499.00000 Ma
Collections (4 total)
| Time interval | Ma | Country or state | Original ID and collection number |
|---|---|---|---|
| Wuliuan | Greenland (Peary Land) | Mellopegma simesi (275942) | |
| Floran | Australia (Queensland) | Anabarella simesi (93409) | |
| Floran - Undillan | Australia (New South Wales) | Anabarella simesi (9394) | |
| Boomerangian | New Zealand (Nelson) | Anabarella simesi (type locality: 7897) |