Middle Jurassic record of the limnic ostracode genus Rosacythere (Limnocytheridae, Timiriaseviinae): implications on the origin and evolution of the Kovalevskiella group

Two species of the limnic ostracode genus Rosacythere Colin (Limnocytheridae, Timiriaseviinae) have been identified in the uppermost Bajocian (Middle Jurassic) of the southern part of the Paris Basin, France. This record is the earliest for this genus and for the ‘Kovalevskiella group’ of Colin & Danielopol (1978 Colin & Danielopol (1980). This study also confirms that most of the Timiriaseviinae morphological groups really started to diversify during the Middle Jurassic.


INTRODUCTION
The limnic ostracod genus Rosucyrhere, defined by Colin (in Colin & Danielopol, 1980), belongs to the family Limnocytheridae, subfamily Timiriaseviinae. It is one of the genera forming the 'Kovalev.skiella group' defined by Colin & Danidopol (1978, 1980. According to the authors, the main diagnostic characters o f this lineage are: small-sized carapace, about 0.5 mm (or less): c-)mamentation in 'rosettes' or micropustulose, 'raspberry-typc' (Colin,199 I); inverse hingement with positive elements on the left valve: right valve < left valve: no, or oine to two vertical sulci: marked sexual dimorphism, females having a developed brood pouch.
The 'raspberry-type' of ornamentation is in fact derived from the 'rosettes'-type by the development of a small hemispherical pustule at the intersection of the alveolae muri. These types of ornamentation which are always present in the genus tiovalevskiella are sometimes absent in some species o f Rosac,ythere (Theriosynoecum? sp. 4 and 7'heriosynoecurn? sp. 2 Andreu, 1978 from the uppermost Albian-Cenomanian of northern Spain) and Frunihocythere (Frunihocyther~z cf. riiniiensis ferreri Colin in Babinot, 1980 from thts Maastrichtian of SE France) which are totally smooth.
Until recently, the earliest known representatives of this lineage which comprises the genera Rosacythere Colin. 1 980, Frumbocythere Colin, 1980, Kovalevskiella Klein, 1963, and probably Ahrotocythere Zhao, 1987 were not older than Early Cretaceous as indicated by the presence of Timiriascwia' cardiiformis Rosenfeld & Raab, 1984 in the 'Neocomian' of Israel. Zhao (1987) considered the genus Rhrorocyi'here as a 'Tertiary derivative' of Rosucythere, which differs from other genera of the Kovalrv.skiella lineage by its normal hinge with positive elements on the right valve (left valve > right valve) and its less regular type o f ornamentation.
The genus Kosucythere was considered to be restricted to the Late Aptian to Cenomanian interval ( Fig. 1) of southern Europe, being known especially from SW France, Spain. Portugal ,and Hungary (Colin, 1974: Colin & Danielopol, 1980: Andreu, 1978, 1983Cabral, 1995;Zalanyi, 1959) (Fig. 2). The genus Framhocythere, originally thought to be restricted to the Late Maastrichtian to Early Eocene interval from southern Europe (southern Belgium, France and Spain), India and China, has recently been identified in the Albian of Zaire (Colin, 1993). The oldest known species of Kovalevskiella dates from the Oligocene of Germany (Carbonnel & Ritzkowski, 1969), and in the present day this genus is confined to the hypogean and interstitial realms in south-central Europe (Colin & Danielopol, 1980: Carbonel er al., 1986. The genus Ahrotocythere has until now been only found in the Miocene (or possibly Oligocene) of SW China (Zhao, 1987).
Although non-marine sediments were widespread during the Late Jurassic and the Early Cretaceous ('Purbecko-Wealden' facies) in many parts of the world, no representative of this group, other than the 'Neocomian' Israeli record of 'Timiriasevia' cardiiformis (Rosenfeld & Raab, 1984), has been reported from this period, whereas other Timiriaseviinae such as Timiriasevia and Theriosynoecum ( = Bisiilcocypris Pinto &Sanguinetti, 1958 andDryelha Sohn, 1982, auct.) were common.

THE GENUS ROSACYTHERE IN THE MIDDLE JURASSIC
Although it is currently accepted that the first Timiriaseviinae probably appeared during the Triassic (Colin & Danielopol, 1980), they really started to develop and to diversify only during the Middle Jurassic and especially during the Bathonian as indicated by the presence of numerous species of the genera Timirimevia Mandelstam, 1947, and Theriosynoecum Branson, 1936(Whatley, 1990. Until now, representatives of the Kovalevskiella Group were not known from this period, although few species showed strong affinities with it.
In the Late Bathonian of SW France, Rohr (1976) described a small species (L = 0.45-0.53 mm) of Timiraseviinae, 'Bisulcocvpris' pusilla (also reported and illustrated as Theriosynoecum sp. by Malz et a / . (1985) in the Early Bathonian of Sardinia and as Metacypris sp. 2 by Mette (1995) in the Callovian of southern Tunisia) showing strong morphological affinities with the Kovalevskiella Group, but from which it differs mainly by its weak reticulate ornamentation.    A re-examination of uppermost Bajocian material (Mourier, unpublished) from the southern part of the Paris Basin (Vienne valley, about 35 km SE of Poitiers), previously studied and illustrated by Oertli (in Bernard et ul., 1957 andOertli, 1963), and DCpEche (1984) allowed us to attribute to the genus Rosacythere two forms originally tentatively assigned to the genus Gomphocythere Sars, 1924. Pinto & Sanguinetti (1962 recognized that these two species belonged to a new indeterminate genus and DCpEche (1984) attributed one of this species to the genus Kovaleu.skiella and the other to the genus Rosacythere. The two species encountered possess all the diagnostic features of the genus Rosacythrre: size, ornamentation, inverse hingement, sexual dimorphism. In the type-locality (La Tour-au-Cognum, near the town of Civaux, Vienne valley : IGN map 1/25000 Chauvigny 8 ) they have been found in a thin shaly interval rich in organic matter, 2 m below a limestone level having yielded the uppermost Bajocian ammonite Parkinsonia aff. suhtilis Arkell. In the studied horizon Rosacythere species are associated with Cypridea? postelonguta Oertli, Tiriiiriaseuiu sp. (=Gomphocythere nov. sp. 1 Oertli), Erpetocypris'? sp. and charophyte gyrogonites.

COMMENTS ON THE ORIGIN AND EVOLUTION OF THE KOVALESKZELLA GROUP
This study shows that the oldest known representatives of the Kovuleuskiella lineage already possessed most of the diagnostic characters of the group: size, ornamentation, sexual dimorphism, inverse hinge.

Ostracocle genus Rosacythere
A close observation shows that the Kovalevskiella ornamentation, i.e. 'raspberry-type', which was generally developed by the end of the Cretaceous (in the genus Frarnbocythere) and later in the Cenozoic (in the genus Kovalev.skiella), was derived from the 'rosettes'-type by the developrnent of a small hemispherical pustule at the intersection of the alveolae muri. It is therefore suggested that the 'roset1.es'-type is a more primitive character.
It is also suggested that the absence of sulcus is a primitive character. Bisulcate forms make their first appearance in the Albian with the genus Frarnbocythere. Monosulcate forms, appearing in 1.he Oligocene with the genus Kovalevskiella would therefore be more evolved. The fact that one of the youngest known species of Frarnbocythere (F. valrrorii Tambareau) from the Early Eocene of southern France (Tambareau el' al., 1991) shows a tendency toward a single sulcus would support this hypothesis.  Colin, 1980 Type species (by original designation): Theriosynoecurn grekofi Colin, 1974. Fig. 3. 1. Rosncythere sp. 2 (Ocrtli, 1957). left valve. lateral view, X200. 2. Rosnryrhere sp. 3 (Oertli, 1957 Diagnosis (translated from Colin & Danielopol, 1980). Timiriaseviinae with small-sized carapace (about 0.5 mm), ornamented by triangular alveolae disposed in 'rosettes'. Hinge lophodont, inverse, with terminal positive elements on the left valve. Presence of a brood pouch in females.

TAXONOMIC NOTES
This diagnosis is emended to include species having a micropustulose, 'raspberry'-type of ornamentation, and the smooth species of Rosacythere found by Andreu (1978) in the Late Albian of northern Spain.
Rosacythere sp. 2 (Oertli, 1957) ( Fig. 3, 1 Oertli, pl. 27-2. 1984Koualevskiellu 'nov. sp. 2' (Oertli): DCpeche: 210, pl. 5 , 1985 Remarks. This species differs from all other described species of Rosacythere by its 'raspberry'-type of ornamentation similar to the genera Koualevskiellu and Frumbocythere. Such an ornamentation in the genus Rosacythere has been observed in a new species from the Late Aptian of Portugal (Cabral, 1995 and Cabral & Colin in prep.). On adult specimens, pustules are often papillate. Juvenile specimens show a spine at the postero-dorsal and postero-ventral angles (Oertli in Bernard et al., 1957, pl. 13 (Oertli, 1957) ( Fig. 3, 2) 1957 Gornphocythere ? nov. sp. Remarks. This species possess the characteristic 'rosette' ornamentation of Rosacythere, and. differs from other known species of this genus essentially by the presence of three well developed conical spines on the posterior half of the ventral margin. It possesses a conical spine on the dorsal margin anterior to the posterodorsal angle. That is characteristic of the type-species Rosacythere grekofi (Colin, 1974)  pl. 5, fig. 4.

CONCLUSIONS
The identification of two species of the limnic ostracod genus Rosacythere (Limnocytheridae, Timiriaseviinae) in the uppermost Bajocian of France is the earliest record of representatives of the 'Kovalevskiella group' of Colin & Danielopol (1 980) which will have two periods of maximum development during the Late Maastrichtian-Early Eocene with the genus Frurnbocythere and Oligocene-Recent with the genus Kovulsvekiella. This discovery also confirms that most of the Timiriaseviinae morphological groups (Tirniriaseuia-Metacypris, Theriosynoecurn, Kovalev.skiella) which appeared during the Triassic. really started their diversification during the Middle Jurassic.