LOWER PERMIAN PERRINITID AMMONOID FAUNAS FROM THAILAND

Journal of Paleontology, Mar 2004 by Zhou, Zuren, Liengjarern, Malai

Sixty years ago perrinitids were known to be widespread and abundant in America, but were thought to occur elsewhere only in Timor and the Pamirs (Miller and Furnish, 1940). Tharalson (1984) documented the basic distribution of perrinitids in the Americas-from the northwestern Yukon through the western United States to Columbia/Venezuela in the southeast-as a belt along the geosyncline and its extension along the west edge of the Midcontinent (Fig. 5).

As for the Eastern Hemisphere, there were only four localities (areas), namely Crimea, Pamirs, Afghanistan, and Timor (Tharalson, 1984). However, with increasing discoveries of new fossil localities, the distribution area of perrinitids has been enlarged, not confined to such a few localities as previously thought. In recent years, many perrinitid localities have been reported from China and Thailand. Properrinites was discovered from Liuzhai, Landan County in Guangxi, South China (Zhou, 1987). The genera Metaperrinites and Perrinites have much wider distributions in China and Thailand. The former is known from Akqi in Xinjiang (Lai and Xiao, 1987), Lingyun in Guangxi, and Ziyun, and Cehong in Guizhou in China; and from Ban Na Pang, Loei, in northcentral Thailand. The latter occurs in Qiemo in Xinjiang, Tianjun, and Maduo in Qinghai in China; Khao Nong Hoi in Amphoe Muak Lek, Changwat Nakhon Ratchasima; and the Saraburi quarry, Amphoe Phra Phutthabat, Changwat Saraburi in southcentral Thailand (Fig. 5).

The original Asia-Europe perrinitid localities, Crimea, Afghanistan, Pamirs, and Timor, plus the new localities noted above in China and Thailand also indicated a beltlike distribution along the Tethys geosynclines and adjacent shelves. An important discovery was the appearance of Metaperrinites involutus (Liang, 1982) in Shuangyang, Jilin, northeast China (Liang, 1982), which provided evidence of the possible seaway for the communication between the perrinitid ammonoid faunas of the Tethys and West America geosynclines and their adjacent shelves.

The distribution of the Permian perrinitids could be summed up as follows. 1) Perrinitids lived in the active linear belt-geosynclines, the "open-sea" (Zhou, 1986). 2) They are found only in the Tethys, west Americas geosynclines and their adjacent shelves, and are not found from Franklinian, Andean, Italian, and Tasmanian geosynclines and their adjacent shelves. 3) The extension of the perrinitid distribution in the western Midcontinent of America was due to post-mortem draft. For example, the conchs of Perrinites at Salt Croton Creek, Stonewall County, Texas, are preserved exclusively in the gypsum-bearing beds, phragmocones of almost the same diameter only, but without body chamber. This indicates the allochthonous origin of these conchs, with rigid sorting. 4) The highly specialized inner constriction of the conch, i.e., the extremely digitate suture and crowded alignment of septa, led to the extinction of perrinitids by the end of Roadian stage. There was no way to make any improvement in their inner construction when the environment in which they lived had undergone even very small change. 5) The communication of the perrinitid belts between the East and West Hemispheres presumably was accomplished through the Beishan-Nei Mongolian and Far East Primoria geosynclines.