Distribution Of Seymouriamorpha

No Late Carboniferous seymouriamorphs are recognized, but members of two families, Discosauriscidae and Seymouriidae, were widely distributed during the Early Permian, although by the lower Upper Permian their record is reduced to two occurrences of the former from China (Zhang et al., 1984) and Russia (Ivakhnenko, 1981) and, one of the latter from Oklahoma (Olson, 1980). The taxa and their ages and localities, as well as the bibliographic sources of this data, used to plot the Early Permian distributions of the Discosauriscidae and Seymouriidae in Figure 2 are as follows:

• Discosauriscidae

Ariekanerpeton (Tadghikistan), Discosauriscus (Czech Republic, France, and Germany), Letoverpeton (Czech Republic and Germany), and Utegenia (Kazakhstan); Ivakhnenko (1981), Kuznetsov and Ivakhnenko (1981), and Werneburg (1989, 1990).

• Seymouriidae

Seymouria (Germany, New Mexico, Oklahoma, Prince Edward Island, Texas, and Utah); Langston (1963), Vaughn (1966), Olson (1979a, 1980), Berman et a/.(1987), and Berman and Martens, (1993).

Although the discosauriscids listed above are assigned an Early Permian age, this is not certain for some of their occurrences. For example, Utegenia is from beds that range in age from the end of the Carboniferous to the beginning of the Permian, and no precise age is known for Ariekanerpeton. These Asian discosauriscids have been given an Early Permian age apparently on the basis on correlations with European forms. In addition, as already noted above in the discussion of the Carboniferous-Permian Boundary, many of the European discosauriscid localities may actually be latest Carboniferous rather than Early Permian. Regarding the age of this group, it is also impotant to note that Discosauriscus has been reported from the lower Upper Permian in the southern cis-Uralian forelands of Russia (Ivakhnenko, 1981) and Urumqia from China is Upper Permian (Zhang et al., 1984). With the exception of one suspicious description, Seymouria is restricted to the Early Permian. On the basis of a single partial skeleton that includes only a small portion of the skull, Olson (1980) described a new species of Seymouria from the lower Upper Permian of Oklahoma. Generic identification was based almost entirely on the shape of the presacral neural arches, which it could conceivably share with more distantly related groups, particularly the diadectomorphs. In addition, there are aspects of the appendicular skeleton which are unlike those of any known seymouriid.

Despite the described age and taxomonic resolution difficulties, there exists an interesting stratigraphic replacement in central Europe of the discosauriscids {Discosauriscus and Letoverpeton) by Seymouria. The smaller and morphologically more primitive, aquatic discosauriscids (Laurin, 1995), preserved typically in lacustrine grey sediments and black shales (as are the Russian and Asian forms), have been known for well over a century and reported, often in large numbers, from numerous localities in the Lower and possibly the basalmost level of the Upper Rotliegend (Werneburg, 1989, 1990; Ivakhnenko, 1990). On the other hand, until very recently Seymouria was restricted to the Lower and possibly the lowermost Upper Permian of North America (Olson, 1980; Berman et al., 1987). From the well-known Bromacker locality in central Germany two specimens of Seymouria were described from the Lower Permian Tambach Formation, which lies near the base of the Upper Rotliegend (Berman and Martens, 1993). Not only does the Tambach Formation occupy a stratigraphic level distinctly above those yielding the

European discosauriscids, but it also consists of typical fluvial red-bed sediments like those in which the Permian Seymouria specimens of North America are preserved. The replacement of the basically upper Lower Rotliegend discosauriscids by the later, basal Upper Rotliegend Seymouria in Europe may reflect either an evolutionary event or response to an ecological succession, or both.

The absence of discosauriscids in North America seems unexpected in view of that region's widespread occurrences of extensive, Late Pennsylvanian-Early Permian deposits similar to those of the Lower Rotliegend of central and western Europe. One explanation can be offered here to explain this distribution: the origin of the seymouriamorphs took place in Europe with the appearance first of the strictly aquatic, Lower Rotliegend discosauriscids, which subsequently gave rise to the semiterrestrial Early Permian seymouriids. With the exception of the few Asian and Russian discosauriscids, specimens of this family are preserved in sediments that accumulated during the the Late Carboniferous and Early Permian in elongate, fault-bounded, northeast-trending, intramontane basins distributed in a broad band across central and western Europe from western Poland and the Czech Republic southwest through central Germany and south-central France. The basins formed on the northern margin of the Variscan mountains of the Baltic region and apparently represented a physical environmental setting unique from those of other regions in Euramerica in which fossil-bearing sediments of the same age accumulated. This has prompted the suggestion that the Late Paleozoic intramontane basins of Europe may have been a site of endemism for several families of amphibians and the stem amniote discosauriscids as well (Milner, 1993). On the basis of this premise, it can be further speculated that with the gradual replacement of the coal swamps by the terrestrial, fluvial red-bed environments in the intramontane basins of the Baltic region of Europe the discosauriscids gave rise to and were replaced by Seymouria, the earliest occurring and morphologically most primitive member of the Seymouriidae.

Figure 2. Early Permian distributions of the seymouriamorph families Discosauriscidae (D) and Seymouriidae (S). Localities, latitidue and longitude may be found in Appendix 1.

This event, coupled with the widespread development of red-bed environments, may have allowed the semiterrestrial Seymouria the opportunity to expand its range to North America.

If the discosauriscids originated in the Baltic region of Europe, then, as indicated by recent paleogeographic reconstructions (Scotese and Langford, 1995; Ziegler, 1996), they could not have reached the Kazakhstan continent and the cis-Uralian forelands of Russia until at least the Early Permian. Although Kazakhstan and Euramerica were widely separated during the Late Carboniferous, even after their Early Permian union a persistant shallow sea extended across the eastern margin of Euramerica to intervene between the terrestrial areas of Euramerica to the west and the Uralian mountains and their forelands and Kazakhstan to the East. However, a narrow corridor between these two regions was available for faunal interchange. During the Permian a broadening isthmus of emergent land surrounding a string of high mountains that was an eastward extension the Variscan Orogeny, spanned the shallow sea to join the Baltic region of Euramerica with the southern margin of the Kazakhstan. The hypothesis that discosauriscids were endemic to the Baltic region of Euramerica and expanded their range to Kazakhstan and Russia during the Permian becomes more plausible if, as discussed earlier, the Lower Rotliegend deposits of Europe containing the discosauriscids are considered to be at least in part latest Carboniferous.

Of important consideration to this discussion is an explanation for why Seymouria remained undetected in Europe until recently (Berman and Martens, 1993), despite a long history of intensive prospecting of the productive deposits of the Rotliegend. The same question can also be asked about the recent discoveries at the Bromacker locality of the diadectomorph Diadectes, discussed below, and a trematopid temnospondyl amphibian (Sumida et al., 1994, 1996). Both are terrestrially adapted forms known almost exclusively as common elements in the Early Permian red-bed assemblages of North America (a few Late Pennsylvanian, North American trematopids are known). Martens (1988, 1989) and Berman and Martens (1993) have suggested that the rare occurrences of representatives of North American Early Permian terrestrial tetrapods in the Rotliegend deposits of Europe are due to a bias in exploration that has greatly ignored the red-bed sediments where such forms are most likely to be discovered. Poor exposures of the Rotliegend red-bed deposits and the long-standing, widely accepted misconception that they represent an inhospitable dry climate in which preservation of vertebrate skeletal remains would have been unlikely has fostered a history of little interest in their exploration. This has had the expected results of there being only a paucity of vertebrates collected from the red-beds of the Rotliegend, because most investigators have concentrated on the lacustrine gray sediments and black shales that are noted for their highly productive sites yielding mainly obligatory aquatic amphibians and stem amniote discosauriscids. It seems very apparent that the explanation for the similarity between the widely separated Early Permian assemblages of the Bromacker locality and those of the United States is that similar environments are being sampled, and that the fluvial red-bed sediments are the most likely source of terrestrial tetrapods in the Rotliegend.

Was this article helpful?

0 0

Post a comment