Genus Neosilesites Breistroffer, 1951

Suborder Ammonitina – Superfamily Desmocerataceae – Family Silesitidae

Position. According to Wright (1996), the family Silesitidae Hyatt, 1900, probably derives from the Barremian genus Barremites. It comprises evolute ammonites with an oval or compressed whorl cross-section, a slowly increasing whorl height, smooth young whorls but with constrictions, and later whorls with fine, spaced or dense ribs that cross venter. The suture line has a generally concave appearance, with the sequence of auxiliary elements tending to curve forward towards the umbilical suture and thus to extend beyond the first lateral saddle.

The family comprises four genera. Silesites Uhlig, 1883 (type Ammonites seranonis d'Orbigny, 1841) has regularly spaced constrictions and radial ribs that project strongly forward near the venter, crossing it in a chevron pattern. It may have ribs bifurcating near venter and intercalary ribs. The ribs of Neosilesites Breistroffer, 1951 (type Silesites seranonis var. balearensis Fallot, 1920) are slightly convex on the flanks and consistently bifurcated near venter. Neoastieria Egoian, 1969 (type N. reliqua by original designation) has the same bifurcated ribs, but with tubercles at the points of bifurcation. Finally, Miyakoceras Obata, 1967 (type M. tanohatense by original designation), has very marked constrictions, followed by a rib stronger than the others.

Description. Neosilesites has flat or convex flanks, a broadly rounded venter, and close, sharp primary ribs that bifurcate at the top of the flanks to form secondary ribs that cross the venter. The constrictions are weaker than in Silesites and correspond to wider intercostal spaces than in other species. This genus was separated from Silesites by Breistroffer to distinguish forms with ribs bifurcating near venter.

Age and distribution. Neosilesites is a Tethyan genus found from the Late Aptian to the Early Albian in the Balearic Islands, Austria, Tunisia, Egypt (Sinai), and Madagascar. Silesites, that can be found in France, is limited to the Barremian. Neoastieria is known only from the Late Aptian of Caucasus, and Miyakoceras from the Late Aptian of Japan.

Species. Five species of Neosilesites are known: N. nepos H. Douvillé, 1917 (initially classified among the Silesites); N. ambatolafiensis Collignon, 1963; N. ampakabensis Collignon, 1963; N. madagascariensis Collignon, 1963; and N. maximus Collignon, 1963. The last four are illustrated in fascicle X (Albian) of Collignon's Atlas of Characteristic Fossils of Madagascar, but in our opinion, N. ampakabensis and N. madagascariensis fall within the range of variation of N. nepos. These ammonites are generally very small: 2 to 3 cm in diameter, except for N. maximus which can reach 6 cm, hence its name.