Neithea striatocostata robusta Dhondt & Dieni, 1993
DHONDT, A. V. & I. DIENI. 1993. Non-rudistid bivalves from Late Cretaceous rudist limestones of NE Italy (Col di Schiosi and Lago di S. Croce areas). Memorie di Scienze Geologiche, 43: 165-241, pls. 1-20, text-figs. 1-18. [p. 197, pl. 5, figs. 8-18; pl. 6, figs. 2a-4b ]
1993 Neithea (Neithea) striatocostata robusta Dhondt & Dieni, 1993
A. V. Dhondt & I. Dieni, 1993,
plates 5, 6. |
«1892 Neithea quadricostata Sowerby - Futterer, p. 80, pI. 3, fig. 6 [non J. Sowerby, 1814 = N. gibbosa (Pulteney,1813)].
1932 Pecten (Neithea) quadricostatus Sow. - Parona, p. 98 [non J. Sowerby, 1814 = N. gibbosa (Pulteney, 1813 )]. Holotype: Specimen B129 (right valve; PI. V, fig. 8).
Paratypes: Twenty nine right valves (B48, B49, B54-56, B61, B63, B66, B68, B70-74, B77, B78, B94, B163, B 164, P16-18, P40, DGP 26708, five unregistered specimens, IGF colI. Meneguzzo), and three left valves (B83, B84, B96), all from Pinei. Three right valves from "Calloniche" (= Pinei, in this paper) identified by Parona (MS) as "Nelthea quadricostata (Sowerby) (secondo Futterer)" (MGB/Ve 801-803).
Type locallty: Pinei, Lago di S. Croce (Belluno, eastern Venetian Prealps, Italy).
Type horizon: Fadalto Limestone of latest Coniacian-earliest Campanian age.
Derivation of name: From robustus, -a, -um (lat.) = sturdy, because of the unusual thickness of the shell.
Diagnosis: Subspecies of Neithea (Neithea) striatocostata (Goldfuss, 1833) with very thick shell, subdivided on the outside by grooves in sixteen flat topped ribs: one wider major rib alternates with two narrower intercalary ones.
Description: Most specimens from Pinei are missing the outer shell layer and as a result are indistinguishable from the nominal subspecies. However, a few which have a complete shell preserved show a drastically different ornamentation of the right valve, which has an almost smooth look, only subdivided by very narrow grooves which individualise six wide radial very flat topped ribs, intercalated by two narrower ones. On the anterior and posterior areas in the same way numerous (up to eight have been counted) narrow radial grooves delimit riblets. Towards the ventral disc margin the wider radial ribs show on their sides indications of further subdivision.
Specimens with a less complete shell have the typical ornamentation of N. (N.) striatocostata striatocostata: six sharp major radial ribs with two smaller intercalary ribs, all of them subdivided in numerous riblets. The further the shell of N. (N.) striatocostata robusta is peeled off, the more numerous the subdivisions are (e.g. PI. V, fig. 14 and PI. VI, fig. 4a). The auricles are relatively large and bear a few radial riblets on their lower half. The left valves are flat and show the typical pronounced subdivision of the ribs. The specimens are often partially covered by matrix and therefore the value B' has been omitted because it is not possible to measure it accurately.
The variation of L/H in right valves is shown in the graph of Text-fig. 7, which illustrates that the ratio remains virtually constant during the adult ontogeny of this bivalve. Discussion: As stated in the description, the new subspecies Neithea (N.) striatocostata rabusta differs from the nominal subspecies mainly by its very thick shell. Similar very thick shells are found on N. (N?) deshayana (Matheron, 1843) and on N. (N.) zitteli (Pirona, 1884), two species also described from rudist facies, but are not known in Neithea species generally. Therefore this thick shell is explained here as a special adaptation to a high energy environment.
N. (N.) striatocostata robusta subsp. nov. differs from other Neithea taxa by: - fewer intercalary ribs between the major ribs when compared to N. (N.) zitteli (Pirona, 1884), N. (N.) coquandi (Peron, 1877), N. (N.) regularis (Schlotheim, 1813) (2 instead of 3), to N. (N.) quinquecostata (J. Sowerby, 1814) (2 instead of 4), to N. (N.) sexcostata (Woodward, 1833) (2 instead of 5); - less recurved umbo than in N (N) sexcostata, but more recurved than in N (N.) alpina (d'Orbigny, 1847), N. (N.) regularis, N. (N.) quinquecostata; - with complete shell the intercalary ribs are much less marked, with incomplete shell they are subdivided and more marked than in N. (N.) alpina; - much less angular ventral margin than in N. (N.) sexangularis (d'Orbigny, 1847). In his description of the Lago di S. Croce faunas, from the locality "Calloniche" Futterer mentioned and figured a "Neithea quadricostata" (1892, p. 80, pl. 3, fig. 6); this is almost certainly a reference to the taxon erected here. The specimen of "N. quadricostata" from Aurisina briefly discussed by Parona (1932) is at present missing at the Museo di Geologia e Paleontologia dell'Universita of Bologna where it was deposited. Therefore it is impossible to confirm the statement of Parona that his specimen closely resembles Futterer's figure. Distribution: Neithea (N.) striatocostata robusta subsp. nov. is recorded here from the uppermost Coniacian-lowermost Campanian of NE Italy (eastern Venetian Prealps and possibly from the Aurisina area, Trieste).
N. (N.) striatocostata striatocostata (Goldfuss, 1833) has been recorded from Coniacian to uppermost Maastrichtian of Europe, North Africa and the Arabian Peninsula.» ANNIE VALÉRIE DHONDT & IGINIO DIENI, 1993
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