Pseudothurmannia (Kakabadziella) ohmi

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 : 18-20

Pseudothurmannia (Kakabadziella) ohmi valbon- nettensis: It was possible to detect significant hetero-

chronies in the ornamental ontogenies at the anagenetic

transition from P. (K.) ohmi ohmi to P. (K.) ohmi val-

bonnettensis. Four ornamental ontogenetic stages can be

distinguished in the latter species.

1. The balearis stage, which ends at a diameter of 13 mm.

2. The mortilleti stage, which ends at a diameter of 55 mm. The stages one and two end later in the ontogeny than in P. (K.) ohmi ohmi . This retardation in the ornamental ontogeny results in a slightly finer and denser ribbing of P. (K.) ohmi valbonnettensis in contrast to P. (K.) ohmi ohmi .

3. Stage three shows an irregular disposition of 1-4 intermediate ribs to one main rib and the presence of umbilical bullae on the main ribs, which are absent in P. (K.) ohmi ohmi ; this stage is equivalent to stage three in P. (K.) ohmi ohmi and ends at a diameter of 98 mm, which is not significantly earlier in the ontogeny than in P. (K.) ohmi ohmi .

4. The adult ornamentation shows extra thick, broad main ribs and prominent umbilical bullae.

There is also a clear heterochrony in the shape of the conch. The whorl height equals the umbilical width at a diameter of 56 mm, which is significantly later in the ontogeny than in P. (K.) ohmi ohmi . This retardation in the ontogeny of the conch shape is correlated with the slightly smaller umbilicus of P. (K.) ohmi valbonnettensis in comparison with P. (K.) ohmi ohmi .A smaller umbilicus and more sigmoid ribs promote swimming velocity.

The retardation in the ontogeny of the conch and ornamentation is indicative of the process of ‘neoteny.’ The evolutionary trend of the anagenetic transition from P. (K.) ohmi ohmi to P. (K.) ohmi valbonnettensis is a paedomorphic one. The differences between P. (K.) ohmi ohmi and P. (K.) ohmi valbonnettensis are too small to warrant the introduction of a new species, but the recognition of this subspecies is a way to identify the Catulloi Zone. The rapid transformation from P. (K.) ohmi ohmi to P. (K.) ohmi valbonnettensis probably occurred during the time span of the same precession of the equinoxes as the transition from P. (K.) mortilleti to P. (K.) catulloi and P. (K.) caravacaensis .

Conclusions: Fig. 4. The evolutionary trend of the anagenetic transition from P. (K.) mortilleti to P. (K.) catulloi is characterized by a paedomorphic tendency, the one from P. (K.) mortilleti to P. (K.) caravacaensis by a peramorphic one. In contrast to speciation by branching (cladogenesis), anagenesis is the tendency for a lineage to achieve greater functional efficiency owing to natural selection, that is, evolutionary improvement. Commonly, a gradual transition (gradualism) is implied, but in the cases described here the sea-level fall preceding major sequence boundary Ha7 caused the phyletic transition to become punctuated. As the morphological separation of the ancestor and descendant within the lineage is total, that is, without intermediate forms, both parts of the lineage can be considered genuine species, chronospecies, instead of subspecies.

The transition from P. (K.) ohmi ohmi to P. (K.) ohmi valbonnettensis is, like in P. (K.) catulloi , characterized by a paedomorphic tendency.

This transition from P. (K.) mortilleti to the chronospecies P. (K.) catulloi or P. (K.) caravacaensis and from P. (K.) ohmi ohmi to P. (K.) ohmi valbonnettensis occurred in a very short time geologically, probably not longer than the time span of one precession of the equinoxes, 20.000 years, because the change took place in a deep water (upper bathyal) environment (implying no hiatus) between two successive beds at the boundary between the Ohmi and Catulloi zones, and because intermediate forms were not found. The change was induced by the major sea-level fall, that is, during the Falling Stage Systems Tract that precedes major sequence boundary Ha7. As a consequence of this deep sea-level fall many biotopes of ammonite species were pushed over the shelf edge and severely telescoped. This reduction of the extension of their biotopes would have enhanced selection pressure and ultimate extinction (Hoedemaeker, 1995a).

4.3. Stepped cladogenetic dissociated peramorphocline within subgenus Pseudothurmannia (Pseudothurmannia) ( Fig. 5)

It should be stressed that species of the subgenera Pseudothurmannia (Pseudothurmannia) and Pseudothurmannia (Parathurmannia) were never found below the base of the Catulloi Zone. All nine species with trituberculate main ribs in the adult are restricted to the Catulloi Zone , which begins just one or two orbital precessions earlier than major sequence boundary Ha7. This could be confirmed along the Río Argos, along the River Veveyse, in the Angles section, in the Tornajo section, in the Vergons section, in the Chamaloc section and in the Valbonnette section. In this basal bed (bed A153) along the Río Argos P. (K.) caravacaensis , P. (K.) catulloi , P. (K.) ohmi valbonnettensis , P. (Pa.) dissiticostata , P. (Pa.) sarasini , and P. (P.) picteti start their ranges. There is a strong radiation of Pseudothurmannia at the base of the Catulloi Zone.

Ancestry of P. (P.) picteti : The first ornamental ontogenetic stage of P. (P.) picteti closely resembles the second ornamental ontogenetic stage of P. (K.) mortilleti and P. (K.) catulloi ; P. (P.) picteti has, like P. (K.) mortilleti , a truncated venter. The last argument may indicate that P. (P.) picteti could be a direct descendant of P. (K.) mortilleti . Moreover P. (P.) picteti has a small umbilicus and strongly overlapping whorls like P. (K.) catulloi and P. (K.) mortilleti . However, the slightly wider spacing of the ribs makes the resemblance with the ribbing of the second and third stages of P. (K.) catulloi so great, that many specimens of this species figured in literature have been identified as P. picteti . Moreover P. (K.) catulloi and P. (P.) picteti start their ranges in the same bed (Río Argos bed A153). All these arguments make it probable that P. (P.) picteti have descended from