Parvamussium pizpuezense Schneider, Crampton & Lukeneder, 2013
SCHNEIDER, S., J. S. CRAMPTON & A. LUKENEDER. 2013. Propeamussiidae, Inoceramidae, and other Bivalvia from the Lower Cretaceous Puez Formation (Valanginian–Cenomanian; Dolomites, South Tyrol, Italy). Cretaceous Research, 46: 216-231, figs. 1-12. [p. 221, figs. 7A-7F, 8A-8G]
2013 Parvamussium pizpuezense Schneider, Crampton & Lukeneder, 2013
S. Schneider, J. S. Crampton &
A. Lukeneder, 2013, figures 7, 8. |
«1888 Pecten Agassizi Pict. et Lor.: Uhlig, p. 101.
1888 Pecten (Amussium) Agassizi Pict. & Lor.: Haug, p. 258. 1928 Pecten Agassizi Pict. u. Lor.: Reithofer, p. 304. 2006 Propeamussium sp.: Lukeneder and Aspmair, pl. 8, figs 6, 9. Diagnosis. Medium-sized (L < 30 mm) Parvamussium with up to 19 internal costae. Shell inaequilateral, length exceeding height. Left valve ornamented with two alternating sets of radial costellae of different strength, crossed by less prominent commarginal lirae, resulting in regular reticulate pattern.
Material. The holotype is an articulated specimen with slightly detached and displaced valves (Fig. 7; NHMW 2013/0292/0026). Paratype 1, a left valve, with internal mould and shell preserved (Fig. 8A–C; internal mould; NHMW2005z0245/0004 (A–B); shell; NHMW 2005z0245/0027 (C)). Paratype 2, a small fragmentary semi-translucent left valve (Fig. 8D–E; NHMW 2013/0292/0027). Paratype 3, a right valve, with internal mould and shell preserved (Fig. 8F–G; internal mould and shell; NHMW 2013/0292/0028ae b). In addition, six left valves and two right valves have been examined (NHMW 2013/0292/0029e0036).
Measurements. Holotype: L = 26 mm; H = 23 mm; Paratype 1: L = 26 mm; H = 25 mm; Paratype 2 [fragment]: L = 12 mm; H = 17.5 mm; Paratype 3 [fragment]: L = 16 mm; H = 19 mm.
Type locality. Piz de Puez, Dolomites, South Tyrol, Italy.
Type stratum. Puez Limestone Member, Puez Formation; Barremian; the holotype is from the Upper Barremian of horizon P1 174 (see Fig. 2).
Derivation of name. Referring to the name of the type locality, Piz de Puez.
Description. Medium-sized Parvamussium (L < 30 mm). Shell fragile, semi-translucent, slightly longer than high, markedly inaequilateral, with extended allometric growth in posterior-ventral direction (Figs. 7A–B and 8A–C). Dorsal margin straight. Apical angle c. 120°. Auricles relatively small, more or less equal sized, with straight margins, meeting at almost right angles. Set of c. 25 well-developed resilial teeth visible on inside of posterior auricle in one specimen (Fig. 7F). Prodissoconch unknown.
Shell interior ornamented with up to 19 distinctly antimarginal internal costae; 8 costae present at the earliest visible growth stage; additional costae becoming gradually inserted in interspaces with growth (Fig. 8A–C). Internal costae extending down to c. 75–80% of shell height. Internal costae often detached from shell and transposed onto internal mould (Fig. 8A–C, F–G). External radial costellae and commarginal lirae of left valve visible as incisions also on inside of shell in central part of disc (Fig. 8C). Numerous minute, slightly undulating radial incisions along inner margin of left valve (Fig. 8C). Inside of marginal apron additionally strengthened with two or three low commarginal lirae (Fig. 7B, E). Outside of left valve ornamented with up to more than 120 radial costellae and more than 100 commarginal lirae. Radial costellae at each growth stage consisting of two alternating sets; primary costellae twice as high and wide as secondary costellae. With growth, secondary costellae become more prominent, approaching primary costellae in height and strength. At this stage, tertiary costellae become inserted in interspaces between primary and secondary costellae (Fig. 7B–D). Insertion of secondary and tertiary costellae gradual; not coincident in all parts of shell. In late growth stages, all radial costellae attain similar height and strength (Fig. 7B). Commarginal lirae equal to or less prominent than secondary radial costellae. Radial and commarginal interspaces almost equally wide (approximately twice as wide as costellae or lirae), causing very regular reticulate pattern. Internal costae and pallial margin visible from outside of left valve due to semi-translucent shell; often dark reddish-brown in colour (Figs. 7A and 8A). External surface of right valve and nature of byssal notch, if any, unknown. Ornament of regular commarginal lirae on exterior of right valve visible clearly also from inside in the form of narrow commarginal incisions (Fig. 8F–G). Remarks. Compared to most extant and fossil species of Parvamussium, P. pizpuezense n. sp. is of medium size, as are most Mesozoic representatives of the genus. Parvamussium agassizi (Pictet and Loriol, 1858) (Fig. 9A–C; Muséum d’histoire naturelle de la Ville de Genève; BVIII-44-30933) is probably the most closely related congener from the Hauterivian to Barremian of the Montagne des Voirons (Haute-Savoie, Rhône-Alpes, southeast France). Thus, it corresponds well, both with regard to geological age and palaeogeography, with specimens from Piz de Puez. Both species are similar in shape and number of internal antimarginal costae. However, P. agassizi differs significantly from P. pizpuezense n. sp. in the ornamentation of the left valve. The specimen figured by Pictet and Loriol (1858) on plate 9, figure 2, which is herein designated lectotype (see Fig. 9B), is much smaller than the holotype of P. pizpuezense n. sp. It obviously is an internal mould with remnants of shell, providing, however, a relatively good impression of external morphology. Parvamussium agassizi is ornamented with approximately 50 radial costellae, which occur in two alternating sets of different strength, just as in P. pizpuezense n. sp., and even correspond in number to the appropriate growth stage in the latter. However, in P. agassizi the costellae are much broader than in P. pizpuezense n. sp.,with narrow interspaces and sharp crests. Most significantly, however, the left valve of P. agassizi lacks any commarginal ornamentation.
Two species from the Berriasian of the Crimea Peninsula (Ukraine; Arkadiev and Bogdanova, 2012) that presumably belong to Parvamussium were first described by Retowski (1893). Pecten (Amusium) pawlovi Retowski, 1893 clearly differs from Parvamussium pizpuezense n. sp. in its significantly higher number of internal costae, >22 according to Retowski (1893) and 33–34 according to Arkadiev and Bogdanova (2012). Pecten (Amusium) sokolowi Retowski, 1893 is slightly smaller than Parvamussium pizpuezense n.sp. and has almost straight radial internal costae, whereas they are distinctly antimarginal in the new species. Moreover, Pecten (Amusium) sokolowi has only up to 70 external radial costellae and much weaker commarginal lirae than P. pizpuezense n. sp.
As far as we are aware, no additional species of Parvamussium have been described from Tethyan Cretaceous rocks in Europe. The Turonian to Maastrichtian boreal species Parvamussium inversum (Nilsson, 1827) reaches only up to 8mmin length and height, and is thus distinctly smaller than P. pizpuezense n. sp. Moreover, it lacks radial ornamentation on the left valve (Griepenkerl, 1889; Dhondt, 1971). Johnson (1984) revised the Jurassic Propeamussiidae from Europe and adjacent regions and recognised three species, which he assigned to the genus Propeamussium; however, that author did not discuss the relationships between Propeamussium, Parvamussium and allied genera. From his descriptions, however, it becomes clear that the Jurassic species all retain at least a weak byssal notch in adulthood and thus belong to Parvamussium according to the taxonomic concept applied herein. This observation is supported by Waller (2006), although he regarded the Early to Middle Jurassic representatives of the Propeamussiidae as intermediate between the Triassic genus Filamussium Waller, 2006 and the Late Jurassic to Recent Parvamussium, based on differences in the ordering of calcite prisms. Since Waller (2006) did not establish a separate genus for Early to Middle Jurassic taxa, we herein continue to treat the respective species as representatives of Parvamussium. Parvamussium pizpuezense n. sp. differs from the three Jurassic species in being longer than high, in its inaequilateral shell and in its left valve that is more densely ornamented with more than 100 radial costellae and commarginal lirae. Additionally, the Late Pliensbachian to Early Bajocian (see Johnson, 1984) Parvamussium laeviradiatum (Waagen, 1868) clearly differs from all other congeners in its dorsally extended, horn-like auricles in the right valve. This species was placed in a separate genus, Varientolium, by Andreeva (1966). Parvamussium laeviradiatum and the Late Pliensbachian to Bathonian P. pumilum (Lamarck, 1819) also differ from P. pizpuezense n. sp. in having no more than 13 near-straight radial internal costae; no additional internal costae are intercalated. The Late Jurassic (Late Oxfordian to Early Tithonian) Parvamussium nonarium (Quenstedt, 1857) has slightly antimarginal internal costae and intercalary costae, but differs from P. pizpuezense n. sp. in being much smaller, and in having much coarser reticulate ornamentation on the left valve. An additional species not mentioned by Johnson (1984), Pecten spendiarovi Abel, 1897, from the Kimmeridgian to Tithonian Klentnice Beds of northeast Austria (Schneider et al., 2013), probably also belongs to Parvamussium. It is much more slender than P. pizpuezense n. sp. and has radial external costellae on both valves, according to the drawings of Abel (1897); the holotype of P. spendiarovi could not be traced in Vienna, and may be lost. Several Mesozoic species of Parvamussium from the circum- Pacific area (California, Japan, Indonesia, Thailand) were discussed in detail by Sundberg (1989) and Saengsrichan et al. (2011). These all are distinctly smaller than P. pizpuezense n. sp. and differ in the number and arrangement of internal costae and external disc ornamentation. They are thus not explicitly considered herein.» SIMON SCHNEIDER, JAMES S. CRAMPTON & ALEXANDER LUKENEDER, 2013
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