Pseudothurmannia (Pseudothurmannia), 1923

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 : 22

Pseudothurmannia (Pseudothurmannia) arundicos- tata: This species is the second descendant species in this

peramorphocline and has three ornamental ontogenetic

stages.

1. The first stage has a similar ornamentation as P. (P.) picteti without lateral tubercles up to a diameter of 58 mm.

2. The second stage has a similar ornamentation as P. (P.) simionescui with lateral tubercles up to a diameter of 140 mm. So there is no ‘predisplacement’ of the start of the lateral tubercles as in P. (P.) simionescui .

3. The last stage is characterized by a late ‘ornamental innovation,’ viz. from a diameter of 140 mm begins a crioceratites -like ornamentation consisting of thick trituberculate ribs separated by 3-4 long intermediate ribs; this ornamentation continues up to the aperture. The simionescui -like ornamentation stops earlier than in P. (P.) simionescui , which can be interpreted as heterochrony through acceleration of the ontogeny in relation to P. (P.) simionescui . The innovation of the crioceratites -like ornamentation is presumably a case of hypermorphosis, since the adult size of P. (P.) arundicostata is more than 320 mm. Perhaps P. (P.) arundicostata returns to a crioceratitic way of life. As it is not clear whether P. (P.) simionescui has a crioceratites- like ornamentation, the author cannot assess where this innovation has occurred for the first time, in P. (P.) simionescui or in P. (P.) arundicostata .

There is a clear heterochrony in the shape of the conch. In

P. (P.) arundicostata the diameter at which the umbilical

width equals the whorl height is 84 mm instead of

113 mm in P. (P.) simionescui , the ancestor. This can be

attributed to heterochrony by the process of acceleration.

The early whorls of P. (P.) arundicostata are in touch

with each other, the later whorls are just not in touch; it

is difficult to judge where the whorls stop touching each

other. It seems as if crioconic whorls go together with

crioceratitic ornamentation.

Pseudothurmannia (Pseudothurmannia) perevoluta: This species is the third and last descendant of the cladogenetic peramorphocline. The ribbing of P. (P.) perevoluta can be considered an acceptable variation of the picteti / simionescui -like ribbing, as it consists of long ribs separated by one or two shorter ribs. However, not all the long ribs are main ribs; the main ribs are provided with umbilical and later also lateral tubercles, and have thicker lower parts. The adult part figured by Dimitrova (1967) shows crioceratites -like ornamentation with 4-5 intermediate ribs to one trituberculate main rib.

There is a clear heterochrony in the shape of the conch. The whorls are not touching each other, probably with the exception of the innermost whorls, and the whorl height grows very slowly. The whorl height equals the umbilical width already at a diameter of 23 mm. The early start of the egression and the diameter at which the shell becomes crioconic may be attributed to heterochrony by the process of acceleration. The lateral tubercles already begin at a diameter of 37 mm, and this earlier start of the lateral tuberculation is considered to be due to the process of ‘predisplacement.’ The ornamental innovation set in by P. (P.) arundicostata , is inherited by P. (P.) perevoluta . The largest known specimen of P. (P.) perevoluta is 290 mm, but the species may reach a diameter of more than 300 mm).