Amorphognathus tvaerensis Bergstrom, 1962 from the Mójcza limestone.

Apparatus of Amorphognathus tvaerensis Bergstrom, 1962.

Diagnoza

The sp elements do not form a mirror-image pair, the sinistral one being more highly arched and bears an additional antero-internal lobe; branches of the posterior process of similar length, the external one being somewhat longer. The elements ne with a low cusp that usually does not differ in size from neighbouring denticles .

Synonims:

Early form:
Amorphognathus inaequalis RHODES; DZIK, 1976
Late form :
 Amorphognathus tvaerensis n. sp.; BERGSTROM, 1962;  Amorphognathus tvaerensis BERGSTROM; BERGSTROM, 1971; Amorphognathus ordovicica BRANSON et MEHL; BEDNARCZYK, 1971; Ambalodus frognoeyensis HAMAR; BEDNARCZYK,1971; Ambalodus triangularis BRANSON et MEHL; BEDNARCZYK, 1971;
 Amorphognathus tvaerenensis BERGSTROM; DZIK, 1976; Amorphognathus tvaerensis BERGSTROM; NOWLAN, 1981; Amorphognathus tvaerensis BERGSTROM; BURRETI et al., 1983; Amorphognathus tvaerensis BERGSTROM; BERGSTROM and ORCHARD, 1985

Porównanie

BERGSTROM (197 1) proposed the first complete reconstruction of the apparatus and homologized elements with the prioniodontids. He used the presence of the antero-intern al lobe in the left sp element as the diagnostic feature. The lobe is usually armed with a single denticle in the earliest populations of the lineage in Mójcza. Gradually it becomes larger and larger and armed with a row of up to four denticles . It branches relatively late in ontogeny but in the course of evolution the branching developed earlier and earlier in histogeny as indicated by a migration of the point of branching towards the cusp. There is thus an acceleration of the ontogeny which also concerns the bifurcation of the posterior process. Its external lobe becomes more and more elongated in the course of evolution.

The right sp element is flat , with a wide platform and lacks any additional lobes on its inner process. However, in some rare specimens a denticle corresponding to that in the left element is present. Single elements of this morphology were found in samples MA-76, 86 , and 88; the sample MA- 82 contains two such elements associated with four undenticulated ones. Even if the denticle is present the left elements remain flat and clearly different from their right counterparts.

The denticulation in the ne elements is extremely variable, usually with a group of 3-5 radially arranged denticles; the posterior process remains undenti culated or bears very small denticles. In early populations the external process is prominently denticulated . In the course of evolution this denticulation gradually disappears and the process itself becomes smaller and smaller.

Autekologia

Występowanie geograficzne

Zasięg czasowy

The lineage immigrated to the Holy Cross Mountains area already in the Llandeilo.
The early populations in Mójcza (1.0 m below the bentonite - the base of the P. anserinus Zone) are thus at least partially coeval with the Welsh  populations of Rhodesognathus naequalis, possibly being conspecific with them. Although in R. inaequalis the lobes of the posterior process in the sp elements are variably developed (see BERGSTROM et al. 1985) the external lobe is usuall y smalle r than the remaining one, which is a state more primitive than in any of the Mójcza populations. It seems probable that both the Polish-Baltic lineage of A. tvaerensis and the Welsh lineage of Rhodesognathus developed from R. inaequalis, the former by further development of the external lobe, the latter by a secondary reduction of the platform. As there is at least a possibility of an allopatricaly divergent evolution of these lineages I prefer to pro visionally attribute the early Mójcza populations to A.tvaerensis , as its early form , instead of assigning them to R. inaequalis. Being intermediate between typical populations of the se species they still do not deserve separation into a new subspecies because the overlap of the population variabilities seems too large. To solve the problem more data on the evolution of Rhodesognathus in its type area is required.
Both the lower boundary of the typical (late) form of A. tvaerensis and its transition into the succeeding A. superbus is very unclear due to a great population variability of all the diagnostic characters. The first sp elements without accessory lobes on their inner lobes appear 2.0 m above the bentonite, the last with the lobe occur 2.5 m above the bentonite (see DZIK 1989). This makes zonal boundaries based on these species rather fuzzy unless they coincide with immigrations in some areas , which was evidently the case with the lower boundary of the A. tvaerensis Zone in the Baltic area. As evidenced by the much earlier appearance of the line age in the Holy Cross Mountains there is a possibility that a boundary so defined may prove to be significantly heterochronous.

Materiały muzealne

Literatura

Dzik, J. 1994a Conodonts of the Mójcza Limestone. In: J. Dzik, E. Olempska, & A. Pisera. Ordovician carbonate platform of the Holy Cross Mountains. Palaeontologia Polonica 53, 43-128.