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Mellopegma georginensis

Helcionelloida - Helcionellida - Stenothecidae

Taxonomy
Mellopegma georginensis was named by Runnegar and Jell (1976). It is not extant. Its type locality is QML128. 6 km SSW of Thorntonia Homestead, which is in a Floran carbonate limestone in the Gowers Formation of Australia.

It was corrected as Mellopegma georginense by Vendrasco et al. (2011).

Synonymy list
YearName and author
1976Mellopegma georginensis Runnegar and Jell
2002Mellopegma georginensis Landing et al. p. 298 figs. f. 8.1-8.2
2010Mellopegma georginensis Vendrasco et al. pp. 101 – 106 figs. txt. f. 3A; pl. 1
2011Mellopegma georginense Vendrasco et al. p. 11A9 figs. 11A9 f. 5-6, 7.3-7.4
2023Mellopegma georginensis Wagner p. 5427

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RankNameAuthor
kingdomAnimalia()
Bilateria
EubilateriaAx 1987
Protostomia
Spiralia
superphylumLophotrochozoa
phylumMollusca
RankNameAuthor
classHelcionelloidaPeel 1991
orderHelcionellida
superfamilyYochelcionelloidea
familyStenothecidae
genusMellopegma
speciesgeorginensis

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 georginensis Runnegar and Jell 1976
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Diagnosis
ReferenceDiagnosis
M. J. Vendrasco et al. 2010Polygons are less common than angular imprints on internal moulds of M. georginensis, as would be expected if they reflect prismatic microstructure of the outer shell layer, but they are well preserved in some specimens (Pl. 1, figs 4–5). The polygons are consistently concave in these specimens, with the boundaries between polygons raised. These often ropy polygon walls have been interpreted as the result of phosphate that filled in the spaces between crystal prisms following organic framework degradation (Kouchinsky 1999), but several lines of evidence suggest they represent a direct phosphate replacement of the interprism conchiolin walls.
M. J. Vendrasco et al. 2011Laterally compressed, elongate shell with prominent comarginal ridges that continue around the anterior and posterior margins. Supra-apical dorsal margin gently to strongly convex. Sub-apical margin strongly concave, terminating in a flat to downward sloped shelf. Internal moulds covered with regularly spaced pores and ropy comarginal ridges.

Shell elongate, laterally compressed, 2-3 times longer than tall (character 10, state 1); 5-7 times longer than wide. Aperture narrow at mid-point (character 20, state 1); widened at either end; upturned at both sub-apical and supra-apical ends. Apex located close to most distal point of sub-apical margin (~20% of shell length); short part of dorsum sharply concave; long part of dorsum mildly convex. Inner shell texture of coarse comarginal rugae; often with fine, sinuous striations. Periostracum with curved ridges that extend from the apex to the aperture. Tubercles (granules) common on internal moulds and in some specimens occur over entire surface. Innermost shell layer consists of highly organized calcitic semi-nacre; outer shell layer prismatic (character 11, state 1), with thick-walled organic (conchiolin) matrix. Prismatic shell layer expressed over much of surface of internal mould, except at and near apex. Juveniles slightly less elongate than adults.