Polygnathus semicostatus Branson & Mehl, 1934 from the P. trachytera Zone at Kowala, K. crepida Zone at Kadzielnia, and the C. quadrantinodosa Zone at Łagów in the Holy Cross Mountains.

Diagnoza

Tongue-like dorsal end of the platform in P₁ element much narrower than the main part, ornamented with continuous transverse ribs.

Porównanie

The P₁ elements of the species differ from superficially similar elements of the Eifelian P. linguiformis in the mode of arching of the platform and in having a shorter blade. The geologically oldest specimens showing platform ornamentation typical for this species occur in samples Ka-3 and Ka-10 at Kadzielnia. They are associated with the dominant morph, in which tubercles near the dorsal end of the platform tend to be arranged into densely distributed transverse ribs in her P. szulczewskii. The type specimen of that species seems to be a dextral element of the early P. semicostatus, as suggested by its co-occurrence in the same sample (Minkowice 1 borehole depth 2385,4-2391,4; cf.) with a sinistral element of P. semicostatus. Such initially profound apparatus asymmetry tends to disappear in the evolution of P. semicostatus. This is connected with a wider appearance of the ventral part of the platform, which no longer develops vertical margins. The same refers to P. planirostratus Dreesen & Dusar, 1974. Polygnathus limbatus Matyja, 1993 from the lower marginifera zone of the Chojnice borehole in western Pomerania seems to be a juvenile of P. semicostatus.

There is an interesting aspect of changes in the pattern of variability in the evolution of P. semicostatus. In the preceding P. praecursor the asymmetry in the P₁ element pair is stable, that is the sinistral element is narrower and with no apparent elevation of the anterior margin of the platform. In early populations of P. semicostatus rare dextral elements with ‘sinistral’ morphology (and opposite) appear. In late populations such an inversion in asymmetry is very common. In effect both dextral and sinistral elements became dimorphic, with some intermediate forms, too. This loss of strict control of asymmetry is coeval with the general increase in population variability. Apparently the developmental mechanisms were losing their rigidity.

P. semicostatus is probably the most variable of all the Famennian polygnathids. This refers also to the organization of its basal cavity. In samples Ko-15, ŁSł73-2 and Ł-27 specimens with a depression in the base (the character used to define Neopolygnathus) occur, otherwise they do not differ from the associated typical elements of the species. Moreover, specimens with a virtually smooth narrow part of the platform occur, similar to Polygnathus sp. A of Matyja and Żbikowska (1974) from the crepida Zone of the borehole Minkowice 1 in the Lublin area or Polygnathus padovanii of Perri and Spalletta (1990) from coeval strata of the Carnic Alps. Combination of these two characters in specimens from Łagów (sample Ł-27) results in a morphology virtually indistinguishable from Neopolygnathus communis. Such specimens have been classified as Polygnathus depressus by Metzger (1989).

As indicated by the distribution of imprints of ameloblasts, the dorsal part of the platform was more intensely secreted than the ventral one.

Autekologia

Występowanie geograficzne

Jabłonna, Kadzielnia, Wietrznia, Kowala, Miedzianka, and Ostrówka.
The species occurs also in the Dębnik area

Zasięg czasowy

The late K. crepida to L. styriacus Zones.  

Materiały muzealne

ZPAL: 1,896 specimens.

Literatura

Dzik, J. 2006. The Famennian "Golden Age" of conodonts and ammonoids in the Polish part of the Variscan sea. Palaeontologia Polonica 63, 1-359.