Pseudothurmannia (Pseudothurmannia) crioceroides, Torcapel, 1884

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 : 22-23

Pseudothurmannia (Pseudothurmannia) crioceroides :

This species does not fit in the cladogenetic peramorpho-

cline of Pseudothurmannia (Pseudothurmannia) . P. (P.)

crioceroides has two ornamental ontogenetic stages.

1. A picteti -like stage with commonly 2-3 intermediate ribs to one main rib, including a very short stage with 1-2 intermediate ribs to one main rib, ends at a diameter of 80-90 mm.

2. A second stage characterized by a crioceratites -like ornamentation with commonly 3-5 intermediate ribs between every two thick trituberculate main ribs. This late ontogenetic innovation possibly indicates a return to a crioceratitic way of life during the adult stage. This late innovation continues up to the aperture at a diameter of more than 365 mm. The interpretation given for this deviating ontogenetic development is that P. (P.) crioceroides forms a side branch of the peramorphocline, which descended directly from P. (P.) picteti . The strongly overlapping whorls and the small umbilicus in the young ontogenetic stage are arguments in favour of this derivation.

With respect to the shape of the conch, the whorls start

egressing much earlier than in P. (P.) picteti , and the

umbilical width equals the whorl height at a diameter

of 74 mm. This can be ascribed to heterochrony by the

process of acceleration. In a variety of P. (P.) crioceroides

[which might belong to P. (P.) monasteriensis ] the

trituberculation begins earlier, a case of predisplacement.

Pseudothurmannia (Pseudothurmannia) monasteriensis: This species looks like P. (P.) crioceroides and may be a variety of the latter, but it has a different shape (rounded whorls), thicker sturdier ribs and an earlier start of lateral tubercles. The latter start at a diameter of c. 50 mm, whereas in the typical P. (P.) crioceroides they start at a diameter of 80-90 mm, with the exception of the variety that has an early start of the lateral tubercles. The ontogeny of this variety and of P. (P.) monasteriensis would show heterochrony with respect to the ontogeny of P. (P.) crioceroides through the process of ‘predisplacement.’

The shape of the conch is quite similar to the conch of P. (P.) crioceroides with the exception of the rounded whorl section, but whether this is due to the process of acceleration or retardation in the ontogeny is not clear.

Conclusion: Fig. 5. It can be concluded that the peramorphosis in the shape of the shells in the successive steps of the cladogenetic Pseudothurmannia (Pseudothurmannia) peramorphocline is produced by the acceleration of the start of the egression of the whorls to ever younger ontogenetic stages. U = Wh at a diameter of 150 mm, 113 mm, 84 mm, and 37 mm in P. (P.) picteti , P. (P.) simionescui , P. (P.) arundicostata and P. (P.) perevoluta respectively. This means that also the umbilicus becomes wider at the same diameter. Also P. (P.) crioceroides and P. (P.) monasteriensis yielded to the same trend with D = 74 mm and D = 50 mm respectively, though they form a side branch of the main lineage. The overall trend is to become slightly crioconic. This trend is also recognizable in the late ontogenetic return to a crioceratites -like ornamentation consisting of thick trituberculate main ribs with many long intermediate ribs. Perhaps this is connected with a return to a crioceratitic way of life. The author also observed predisplacement of the beginning of lateral tuberculation, which moved in the successive ontogenies from D = 75 mm in P. (P.) picteti via D = 55-58 mm in P. (P.) simionescui and P. (P.) arundicostata to D = 37 mm in P. (P.) perevoluta . This is not a case of acceleration, because all the species of the subgenus Pseudothurmannia (Pseudothurmannia) have comparable sizes of more than 300 mm.

The ontogeny of the aperamorphic ancestral species, P. (P.) picteti , shows a neotenic heterochrony with respect to the ontogeny of P. (K.) catulloi , its ancestor, which in its turn shows a neotenic heterochrony with respect to P. (K.) mortilleti ( Fig. 4). So, starting from P. (K.) mortilleti the first appearing species of Pseudothurmannia , and following the phylogenetic line to P. (P.) perevoluta one has to do with a stepped mosaïc heterochronocline with an early paedomorphic part and a later peramorphic part. The polyphyletic origin of the non-tuberculate species and the tuberculated species of Pseudothurmannia (Busnardo, 2003, p. 62, table 6) is untenable; the latter group originated from the former.

4.4. Evolutionary trends in subgenus Pseudothurmannia (Parathurmannia) ( Fig. 6)

Ancestry of P. (Pa.) Sarasini : The ornamentation of the inner whorls of Pseudothurmannia (Parathurmannia) sarasini does not resemble the ornamentation of P. (K.) catulloi or P. (P) picteti in any sense. However, P. (Pa.) sarasini has a small umbilicus, and the only remaining small-umbilicated ancestors are P. (K.) mortilleti and P. (K.) caravacaensis . The ribbing of the inner whorls of P. (Pa.) sarasini is fine and dense, but regularly spaced umbilical bullae are already present at a diameter of 8 mm. This means that the ornamental ontogenetic stage with fine uniform balearis -like ribbing is absent, for in P. (K.) caravacaensis and P. (K.) mortilleti umbilical bullae appear at the end of the balearis stage. Young P. (Pa.) sarasini have regularly, but widely spaced main ribs separated by 4-5, sometimes even six intermediate ribs. This feature is characteristic for all species of Pseudothurmannia (Parathurmannia) . Wide spacing of main ribs in potentially ancestral species occurs only in P. (K.) caravacaensis in which irregularly spaced main ribs are commonly separated by 1-2, but also by 3-4, and even by 6-7 intermediate ribs; this wide separation of the main ribs is not rare. The presence of more than four intermediate ribs between two main ribs in young specimens is an exclusive characteristic of the subgenus Parathurmannia . This implies that P. (K.) caravacaensis is likely the ancestor of P. (Pa.) sarasini . The latter species must have split off at much the same time as the origination of P. (K.) caravacaensis from P. (K.) mortilleti , that is, at the base of the Catulloi Zone when the sea level was rapidly falling to a very low level and the decreasing biotope caused severe selection pressure.