Genus Protohoplites  Spath, 1923

Suborder Ammonitina – Superfamily Hoplitaceae – Family Hoplitidae – Subfamily Sonneratiinae

Description. According to Casey (1965), this genus is moderatly evolute, with strong ornamentation and compressed or depressed whorls. After an initial Sonneratia stage with a rounded venter, the adolescent develop a ventral depression with a row of round-topped tubercles on each side of it. These tubercles are positioned more or less at right angles to the siphuncle and each one is generally in line with its fellow on the opposite side of the venter. The high, abrupt umbilical wall gradually rounds into the flank and is surmounted by prominent, conical, or radially elongated tubercles, from which simple or bifurcated ribs extend towards the ventrolateral tubercles. In extreme forms the sculpture is reduced to tubercles only. The venter rounds again in adults. The whorl section is subhexagonal, with concave sides and venter and are widest at the umbilical tubercles. The suture line has a first lateral lobe that is quite narrow and more or less symmetrically trifid. It contracts strongly towards the umbilicus.

Subgenera. The subgenus Protohoplites sensu stricto, type Ammonites archiacianus d'Orbigny, 1841, has ventrolateral tubercles that are conical or slightly pinched towards the siphon, bordering a smooth ventral groove. The subgenus Hemisonneratia Casey, 1952 (type Ammonites puzosianus d'Orbigny, 1841) has ribs that cross the venter but with a sag. Following collections in the Perchois quarries (Aube), Amédro et al. (2000) showed that Hemisonneratia is more closely related to Sonneratia than to Protohoplites sensu stricto. This is why Matrion in Colleté (2010) and Amédro & Matrion (2014) elevate the subgenus Hemisonneratia to the rank of a genus.

Remarks. Low ribs sometimes cross the venter in Protohoplites sensu stricto, see the P. (P.) michelinianus of d'Orbigny reproduced in Casey (1965, p. 484, test-fig. 183). On the other hand, ventral attenuation of ribs in Hemisonneratia can be subtle or late. The shape of the ventral tubercles then allows for differentiation: conical or slightly elongated in Protohoplites, distinctly pinched and elongated in Hemisonneratia. In Protohoplites and, to a lesser extent, in Tetrahoplites, we have often observed a few pairs of very divergent ribs (40° or more), with a distinctly rursiradiate posterior branch. This characteristic is never seen in Sonneratia and Pseudosonneratia.

Distribution. Protohoplites is an uncommon genus, known from Western Europe and Transcaspia in the puzosianus zone. It does not appear to have penetrated the Tethyan province. Saveliev (1973, 1992) described highly compressed forms of the subgenus Hemisonneratia in Kazakhstan, such as the one in the photograph on the right, which have not been reported in the Anglo-Paris Basin.