Crioceratites (Balearites) theodomirensis: This

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 : 15-16

Crioceratites (Balearites) theodomirensis: This species

is interpreted as the first descendant of C. (B.) balearis

in this heterochronocline, because its ornamentation

is closest to that of C. (B.) balearis . It shows three

ornamental ontogenetic stages.

1. The first stage is characterized by a fine dense uniform ribbing with lateral tubercles and small ventrolateral tubercles on every rib; this stage ends at a diameter of 7 mm.

2. The second stage is called the balearis stage and is characterized by fine dense uniform ribbing without lateral tubercles, but with ventrolateral tubercles on every rib; the appearance of umbilical bullae, which mark the future main ribs, begins at a diameter of 13 mm. This fine-ribbed stage ends at a diameter of c. 30 mm and is much shorter than in the ancestor through the process of acceleration.

3. The third stage is an ‘innovation’ characterized by a gradual coarsening and a distinct differentiation of the ribbing.

4. Thick ribs lined by constrictions begin to appear at a diameter of 36-40 mm. This is the ‘adult variation’ which characterizes all species of Crioceratites (Balearites) .

Ventrolateral tubercles are present on every rib up to

the aperture. This extension of the rows of ventrolateral

tubercles up to the aperture can be interpreted as the

result of the process of ‘progressive deviation’ in the ontogeny of the descendant. Progressive deviation has been defined as the process in which “a character, which makes its appearance in the young stage of an ancestral animal, may in the ontogeny of the descendant appear in the young and adult stage, producing a substitution of a new adult condition for the old condition” (De Beer, 1930; Gould, 1977). No heterochrony is involved.

The whorl height equals the umbilical width at a diameter of 50 mm. There is no heterochrony in the shape of the conch in relation to the aperamorphic ancestor. The size of a full-grown specimen is not known; the largest diameter found in the material of the author is 60-65 mm.

Pseudothurmannia (Kakabadziella) mortilleti: This species is the second descendant of this heterochronocline and considered to belong to another genus than the first descendant, because it lost the crioceratitic initial ornamental stage with lateral tubercles (this loss is interpreted as the product of persistent acceleration of the first ornamental stage of its ancestor until it has disappeared) and it lost the typical crioceratitic adult ornamental variation with constrictions lined by thick ribs. Besides, this species has fine uniform ventrolateral clavi on every rib instead of rounded tubercles, it displays a strong sexual dimorphism, the macroconch reaches a large size, and the young specimen has strongly overlapping whorls and a narrow umbilicus. Nevertheless, the ornamentation of the inner whorls is similar to that of C. (B.) theodomirensis , and that is the reason why the author is convinced that the change-over from Crioceratites to Pseudothurmannia is made from C. (B.) theodomirensis to P. (K.) mortilleti . In the macroconch of P. (K.) mortilleti four ontogenetic stages were defined. 1. A fine, uniformly ribbed balearis stage, which ends

at a diameter of 20 mm; umbilical bullae appear at a

diameter of 15 mm, which is not significantly different

from their appearance in C. (B.) theodomirensis ;

later in this stage, the umbilical bullae become more

prominent, the ribs slightly more pronounced and

differentiated into 3-5 intermediate ribs to one main

rib; this stage is equivalent to the later part of stage

two of C. (B.) theodomirensis and ends at a diameter

of c. 33 mm, which is not significantly different from

that diameter in C. (B.) theodomirensis . 2. The second stage is characterized by slightly wider

spaced ribs with mainly two intermediate ribs to one

main rib and ends at a diameter of c. 40 mm, which is

not significantly different from the diameter at the end

of the equivalent stage three in C. (B.) theodomirensis . 3. In the third stage the number of intermediate ribs

increases to mainly 3-4 to one main rib. 4. The adult stage starts at a diameter of c. 130 mm

with the addition of distantly spaced, extra thick ribs

separated by 5-7 intermediate ribs per interval. Small

uniform ventrolateral clavi occur on every rib in all

ontogenetic stages. Up to a diameter of 40 mm P. (K.) mortilleti is quite similar to C. (B.) theodomirensis except for the shape of the conch, which is more involute in P. (K.) mortilleti . In C. (B.) theodomirensis the adult ornamentation starts at a diameter of 40 mm, in P. (K.) mortilleti at a diameter of c. 130 mm. The largest macroconch of P. (K.) mortilleti reaches a diameter of 188 mm, and has two ornamental ontogenetic stages more than C. (B.) theodomirensis of which the largest specimen known has a diameter of 65 mm.

As for the ontogeny of the shape of the conch: the point where the whorl height equals the umbilical width is at a diameter of c. 100 mm, which is at a smaller size than the start of the adult ornamentation. In C. (B.) theodomirensis this point is reached at a diameter of 50 mm, which is a larger diameter than the start of the adult ornamentation. This means that in P. (K.) mortilleti this point has reached in an earlier ontogenetic stage than in C. (B.) theodomirensis . This means that this point accelerated in the ontogeny of P. (K.) mortilleti , not retarded as it seems at first sight. It means furthermore that the ornamental ontogenetic stages 1-3 also accelerated with respect to those in the ontogeny of C. (B.) theodomirensis . In view of the additional two ornamental ontogenetic stages in P. (K.) mortilleti , the type of heterochrony between the ontogenies of C. (B.) theodomirensis and P. (K.) mortilleti could be interpreted as hypermorphosis.