Pseudothurmannia (Parathurmannia) dissiticostata, Hoedemaeker, 2013

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 : 23-24

Pseudothurmannia (Parathurmannia) dissiticostata :

In this species only three ontogenetic stages could be

established:

1. The first stage is similar to the first one of P. (Pa.) sarasini , and consists of flexuous main ribs with 4-6 intermediate ribs between every two main ribs. This stage ends at a much larger diameter than in P. (Pa.) sarasini , viz. at a diameter of c. 70 mm.

2. The second stage consists of 3-4 intermediate ribs to one main rib. Lateral tubercles appear on the main ribs and continue to be present at least up to a diameter of 115 mm. Thus also the end of the second stage is clearly retarded in relation to the second stage in P. (Pa.) sarasini .

3. The author collected a whorl fragment belonging to a specimen at least 150 mm in diameter with 2-4 flexuous intermediate ribs to one main rib; this stage is equivalent to stage three of P. (Pa.) sarasini . However, it ends at a much larger diameter and is retarded in relation to P. (Pa.) sarasini . As all the stages of P. (Pa.) dissiticostata are longer than the equivalent stages of P. (Pa.) sarasini , one may have to do with ‘proportioned gigantism,’ which is caused by more rapid growth with a constant rate of somatic (= ornamental) development.

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

The inner whorls are overlapping and have a small

umbilicus, and the diameter at which the umbilicus is equal to the whorl height is c. 70 mm, which is smaller than in P. (Pa.) sarasini . This diameter has shifted from 100 mm to 70 mm, which is indicative of the process of acceleration. The author is not sure whether it is possible that one species can show two opposite evolutionary trends.