Ancyloceratina Wiedmann, 1966

Hoedemaeker, Philip, 2013, Genus Pseudothurmannia Spath, 1923 and related subgenera Crioceratites (Balearites) Sarkar, 1954 and C. (Binelliceras) Sarkar, 1977 (Lower Cretaceous Ammonoidea), Revue de Paléobiologie 32 (1), pp. 1-209 : 38-39

Suborder Ancyloceratina Wiedmann, 1966 Superfamily Ancyloceratoidea Gill, 1871 Family Ancyloceratidae Gill, 1871

Subfamily Crioceratitinae Gill, 1871

7. Genus Pseudothurmannia Spath, 1923

Type species: Pseudothurmannia picteti, Sarkar, 1955 .

Planospiral shells with evolute, advolute or slightly crioconic whorls. The first whorl is uncoiled and leaves an open umbilicus. The whorls egress during ontogeny (excentric umbilicus). The whorl section is compressed with flat to slightly convex parallel flanks, and has a flattened to arched ventral side. The dorsal side of the whorls is flat to deeply concave, and has the same threelayered constitution as the shell on the flanks and venter. In the earliest ontogenetic stages the ribs are simple and fine to moderately fine; lateral tubercles are lacking. With growth they become coarser and differentiate into main ribs and intermediate ribs. The main ribs are prominent only near the umbilicus, where they may have umbilical bullae; on the venter all ribs are characteristically equal or virtually equal in thickness except in the adult stages of a few species of the subgenus Pseudothurmannia (Pseudothurmannia) . There are characteristically only a few, generally short, intermediate ribs between every two main ribs, normally 1-4, rarely five or six. On the innermost whorls the ribs are weakened at mid-venter, on the outer whorls they are not. Small uniform clavi are present on each rib forming two delicate rows on the ventral shoulders, which continue to be present up to the aperture. The suture line consists of four trifid lobes and bifid saddles. The saddle L/U is higher than E/L. For several species sexual dimorphism is established; the macroconchs are markedly larger than the coarse-ribbed microconchs.

Remarks: The main diagnostic characteristics, by which Pseudothurmannia can be distinguished from the other species of laterally tuberculate Crioceratitinae , are their having at least the inner whorls in contact; the absence of lateral tubercles on the innermost whorl; the absence of constrictions alongside thick ribs (except for weak constrictions in the latest growth stage of some large macroconchs); and, last but not least, the presence of small uniform clavi instead of tubercles on each rib forming two rows on the ventrolateral shoulders, which continue up to the aperture. The smallness of these clavi is the reason for their defective preservation.

In Pseudothurmannia the shell of the dorsal side of a whorl does not consist, as in normally coiled ammonites, of merely an inner prismatic layer (Runzelschicht), but has the same three-layered constitution as the shell of the flanks and ventral side; it consists of an outer prismatic layer, a nacreous layer and an inner prismatic layer. The two outer prismatic layers, one of the dorsal side of the outer whorl and one of the ventral side of the inner whorl, are not welded together, but merely in contact, or not ( Fig. 40 C-D); there is no real umbilical seam, but a more or less deep fissure. It is, therefore, not possible to judge whether the whorls are really in contact or not. When the dorsal side is concave, the whorls may exhibit an overlap, but may easily be separated from each other. The significance of the remarks ‘touching each other’, or ‘not touching each other’, or ‘slightly crioconic’ is not really relevant. A three-layered dorsal side is probably a feature of most heteromorphic ammonites. The three-layered dorsal side of the whorls, the perforate umbilicus, and the quadrilobate suture lines betray the heteromorphic nature of Pseudothurmannia and a systematic position within the Crioceratitinae .

As to sexual dimorphs, the macroconchs are markedly larger than the microconchs and may return to crioceratitic features in late ontogenetic stages. In microconchs, the egression of the whorls starts markedly earlier that in the macroconchs of the same species, and exhibit a coarsening and a greater irregularity of the ornamentation.

The genus Pseudothurmannia is here subdivided into three subgenera, namely Pseudothurmannia (Pseudothurmannia) Spath, 1923 , Pseudothurmannia (Parathurmannia) Busnardo, 2003 , and Pseudothurmannia (Kakabadziella) subgen. nov. In the ideas of ammonitologists, the latter subgenus includes the species that, with the exception of Ammonites angulicostatus d’Orbigny, 1841 , and Pseudothurmannia picteti Sarkar, 1955 , always have formed part of the genus Pseudothurmannia , and that lack lateral tubercles in any ontogenetic stage. Subgenus Pseudothurmannia (Pseudothurmannia) consists of species that do not match the former fixed idea of how Pseudothurmannia should look; the presence of lateral tubercles in the late growth stages was not considered characteristic of Pseudothurmannia , but of Crioceratites . Subgenus Pseudothurmannia (Pseudothurmannia) , together with the small subgenus Pseudothurmannia (Parathurmannia) , first has been renamed Sornayites by Wiedmann (1962) and later Prieuriceras by Vermeulen (2004). However, this group of species appears to be the typical group of Pseudothurmannia , because it includes the type species P. (P.) picteti Sarkar. Only the late ontogenetic part of large macroconchs of the species of Pseudothurmannia more or less returns to a Crioceratites -like morphology, the start of which accelerates during phylogeny. This shows that it is embedded within the family Ancyloceratidae .