Requieniidae Kutassy,1934

MASSE, JEAN-PIERRE, FENERCI-MASSE, MUKERREM & BUCUR, IOAN I., 2024, New Species Of The Lower Cretaceous Genus Matheronia Munier-Chalmas (Bivalve Hippuritida) In Romania, Acta Palaeontologica Romaniae 20 (1), pp. 3-15 : 6-7

Family Requieniidae Kutassy,1934

? Subfamily Matheroniinae Scott et al., 2010

Genus Matheronia Munier-Chalmas, 1873

Type species Caprotina virginiae Gras, 1852

Generic characters of the genus Matheronia and comments on the subfamily Matheroninae

The shell is attached by the LV, mostly matheroniform sensu Masse (2002), i.e. spirogyrate. RV uncoiled with growth lines concentric to the apex. On antero-posterior transverse sections the RV pm is a crest, am a bulge. LV myophores being on inner shell wall. The striking difference with the other Requieniinae, e.g. Requienia , is the existence of a RV posterior myophoral plate, lacking in Matheronia , as established by Douvillé (1915).

The foregoing shows that among the Requieniidae the generic identification of Matheronia requires the presence of both valves, especially the RV which bears the diagnostic myophoral characters; an antero-posterior section showing the posterior myophoral crest and the anterior myophoral bulge is necessary and sufficient for this purpose.

Emended diagnosis

Shell attached by the LV, mostly matheroniform, i.e. spirogyrate, with a short beak. RV uncoiled with growth lines concentric to the apex, The RV pm is a crest, am a bulge. LV myophores being on inner shell wall. Radial bands usually absent or inconspicuous.

Several additional characters

The presence of a well-defined LV anterior tooth socket was considered by Paquier (1903) as a diagnostic character of M atheronia, a point of view shared by Mongin (1971), but not retained by Douvillé (1915) and most of the subsequent workers.

The existence of an additional tooth (PII) was noticed by Douvillé (1915) in the LV cardinal area of Matheronia virginiae , but this feature, recorded in some other rudists, is considered to have a negligeable taxonomic value.

The dorso-ventral length of the RV anterior and posterior myophores is distinct: ma being longer than mp ( Douvillé, 1887; Paquier, 1903).

Radial bands (pro siphonal bands of Douvillé, 1915) appear to be lacking in most of Matheronia species, whereas they are present in Requienia species ( Douvillé, 1915, 1918; Mongin & Trouvé, 1953; Mongin, 1971). Nevertheless, radial bands are somewhat crudely outlined in Matheronia baksanensis and M. aptiensis , but not frankly sketched in Requienia migliorinii , even if the generic name of this taxon cannot be ascertained ( Masse, 2002).

There is consensus among palaeontologist to recognize the short beak of LV and the absence of coiling of RV, two correlated characters, as typical attributes of Matheronia . But this trait does not match the morphology of the Valanginian Matheronia ? taurica Yanin (1985), an atypical form with a LV having a corkscrew shape, a RV with a raised dorsal margin and a myophoral organisation close to that of Hypelasma .

By contrast the large size and thick shell of Matheronia invoked by Paquier (1903), Douvillé (1935) and Dechaseaux et al. (1969) as an attribute of the genus, cannot be retained, e.g. Matheronia aptiensis has a modest size and a limited shell thickness, some of our new species described herein belong to this category.

The definition of the subfamily Matheroniinae (in Skelton, 2013; after Scott et al., 2010) states that “the LV myophores are as expanded plates on the valve wall”: a character which applies to Lovetchenia Masse but not to Matheronia , and the “RV pm extends from the cardinal platform” (see also Dechaseaux et al., 1969) a character lacking for instance in Matheronia virginiae . As noticed earlier ( Masse, 2002) the shell morphology of Matheronia is not always “matheroniform”, for instance it is “requieniform” in Matheronia ? taurica and somewhat “toucasiform” in Matheronia baksanensis . The foregoing shows that the definition of the subfamily Matheroniinae has to be reappraised.

In the following we compare the Romanian material with the classical Urgonian species of SE France. Matheronia aptiensis , M. munieri and M. virginiae , here illustrated to document some of their generic attributes ( Fig. 3 ).

Comparisons are based on the recent revision of the corresponding species collections stored in the Urgonia Museum of Orgon. The age of this reference fauna is Barremian ( Frau et al., 2018; Masse et al., 2020).

By contrast with the Urgonian species from SE France represented by well-preserved whole shells, the Romanian forms, but one, are represented by variously oriented sections; we have selected the rather rare, bivalve, antero-posterior sections exhibiting the RV myophores, but such material makes uncertain the recognition of the radial bands. Hence the vast majority of sections represent left valves, which cannot be safely assigned either to Requienia or Matheronia and are just identified as “requieniids”, excluding Toucasia with a myophoral crest or ridge on LV.

Specific attributes of Matheronia combine the following traits: size (Dap and Dvd), transverse cross sectional shell outline, presence or absence of a ventral carina, development of the LV shell lamellar surface, usually more important on the anterior side, and shape of the RV. The use of Dap as a size parameter, is a standard for requieniids, but Dvd does not fit the common use of this parameter. The usual dimensional approach for requieniids deals with the W/L ratio (e.g. Masse et al., 1998) in which W is the width, equivalent to Dap, L being the distance between the dorsal and ventral shell margin, i.e. Ddv sensu stricto, acquired in the plane of the RV. In the following Ddv does not conform this definition but corresponds to the distance between the ventral margin of the LV and the plane of the RV, hence the smeasurements of Dap and Dvd are tranversally orthogonal. This method was introduced to acknowledge a quantitative approach based on transverse sections rather than well-preserved entire specimens.

Figures

Fig. 3. Illustration of classical Matheronia species from the Barremian type locality of Orgon (SE France). Matheronia munieri Paquier. a. posterior view of a bivalve specimen showing the LV lamellar habit and flat RV (sample DG 110). b. internal view showing the myocardinal apparatus of the RV (sample DG 113); c. same specimen showing the uncoiled RV. d. Matheronia aptiensis Matheron, transverse antero-posterior section showing the myophores (Urgonia Museum collections). Specimens a to c, reference DG (Urgonia Museum), have been cleaned and prepared by Dominique Gesbert. Scale bar 10 mm.