Huazhcmg Pan Xianggen

Gastropods (snails and slugs) are the most abundant, most diverse, and perhaps the best known of extant mollusks. They have approximately 37,500 living species, and represent some 80 percent of the living mollusks, only less than insects in number among all invertebrates. More than half of these snails and slugs are marine animals, and they are the only molluscan class to have spread both into freshwater and on land. Gastropods include three subclasses: Prosobranchia, Opisthobranchia, and Pulmonata. Both the prosobranchs and the opisthobranchs are mostly marine inhabitants with high diversity and wide distribution. Generally, many marine prosobranchs have well-developed and colorful shells with beautiful sculptures. A great number of them are found on rocky, sandy, and muddy bottoms in the shallow sea areas. The Pulmonata are the only molluscan subclass that lead a successful life on land, with some returning to a secondarily freshwater dwelling. Some gastropods burrow into the soft bottoms, while others swim or float at the surface, often forming a conspicuous part of the plankton, but the overwhelming majority of snails are crawling on the bottoms. In the inland freshwaters, gastropods are also common, and they are represented by some forms of the Prosobranchia as well as the Basommatophora from the Pulmonata, e.g., viviparids, bithynids, planorbids, lymnaeids, etc., commonly found in lakes or rivers. Their fossils often occurred in non-marine deposits from the Mesozoic to Cenozoic. Some forms of Stylommatophora may be seen in terrestrial habitats from deserts to tropical rainforests and montane altitudes of nearly 5,000 m above the sea level. The variations and special adaptations of these snails reflect a wide range of habitats. The majority of gastropods are carnivores, herbivores or occasionally, scavengers. Identification of fossil gastropod taxa mainly depends on protoconch characters, shape of shell, aperture and ornamentation.

Fossil gastropods are very common in the Jehol Biota, and widely o

ms 30 Shell of Probaicalia gerassinwvi (3.34 mm long, 1.3 mm wide), a micromelaniid snail in apertural and ventral views, from Jianshangou locality (Yixian Formation) in Beipiao, Liaoning. (Left)

■n 31 Shell ofPseudarimayushugouensis (1.81 mm long. 0.80 mm wide), a cyclophorid snail in apertural view, from Pijiagou locality (Jiufotang Formation) in Yixian, Liaoning. (Right)

Yixian FormationYixian Formation

33 Shell of Gyran I us sp. (0.75 mm long, 2.25 mm wide), a planorbid snail in apertural. apical and basal views, from Siheiiin locality (Yixian Formation) in Beipiao, Liaoning.

distributed in eastern Asia (including Transbaikalian area of Russia; southeastern Mongolia; Zhejiang, Anhui provinces, southern China and Hebei, Shandong, Henan and some other provinces of northern China). They are mainly found in the Dabeigou, Yixian and Jiufotang Formations of northern Hebei and western Liaoning, and equivalent horizons of other areas. The fauna is largely represented by Probaicalia (Fig. 30), Pseudarinia (Fig. 31), opercula of Reesidella, viviparids of the Prosobranchia, Ptychostylus (Fig. 32) and Zaptychius of Ellobiidae, Gyraulus (Fig. 33) of Planorbidae and Galba sphaira of Lymnaeidae (Fig. 34). Sometimes the faunas are relatively low in diversity or even absent in some horizons. Once they were found they were always found in a large number of individuals. The gastropod fauna, scattered in the tuffaceous siltstone of Bed 9 below bird-bearing rocks of the lower part of the Yixian Formation near Sihetun village, western Liaoning, has a low diversity and a high density, e.g., one specimen (227 cm2 in area) with 117 individuals. The fauna contains Amplovalvata sp., Probaicalia vitimensis, Probaicalia gerassimovi, Ptychostylus philippii, Ptychostylus harpaeformis and Gyraulus sp., but it is dominated overwhelmingly by Ptychostylus, constituting about 66.5% of the total number of specimens in the gastropod fauna. Pseudarinia yushugouensis, Gyraulus loryi and Galba from the Jiufotang Formation of the Pijiagou section in Yixian, western Liaoning are persevered in purple siltstone.

It is noteworthy that these gastropod faunas include mostly very tiny forms, in general less than 5 mm long.

Based on its stratigraphic distribution and fossil characteristics, the gastropod fauna of the Jehol Biota is closely related to those of the Middle Purbeck Bed of Dorset, southern England, the Serpulit Formation of northwestern Germany, the Lower Cretaceous of southeastern Mongolia and Russian Transbaikalia. This indicates that the gastropod fauna bears obviously characteristics of those of the Early Cretaceous.

Gastropods have wide range of habitats, but they are sensitive to slight changes in environment, such as stability of the bottom, type of sediments, salinity, food supplies, water depth, temperature, oxygen content, and turbidity. Thus, there are different gastropod assemblages in different settings. Many scholars have examined the environments of modern snails, so the habitats of fossil gastropods can be extrapolated from the comparisons with their living counterparts. Different assemblages of fossil gastropods are found in several deposits that are interpreted to represent different kinds of habitat areas.

Analogous to their modern counterparts, population of the fossil gastropods Ptychostylus increased individual in number but decreased body in size usually indicates a brackish water living environment. Ptychostylus mainly occurred in the Serpulit Formation of North-West Germany associated with Hydrobia and neomiodontids (bivalve), showing the brackish water environment; Ptychostylus from Inter-marine Beds or Upper Building Stone near marine Cinder Beds, and Corbula Beds of Middle Purbeck Beds in Durlston Bay, England is associated with turtles, fish and bivalve Neomiodon of brackish water affinities. There are a few gypsum beds in the lower part of the Yixian Formation near Sihetun village, western Liaoning. All the factors indicate that Ptychostylus-dominated assemblage from the lower part of the Yixian Formation near Sihetun Village, w. Liaoning might be of brackish water setting, but Gyraulus loryi- and Galba-dommaxtd assemblage from the Jiufotang Formation may represent a drifting assemblage frequently occurred along the shore of a river or lake.

(All photographs in this chapter were taken by Liu-Ping Yuan/ NIGP)

mti34 Shell of Calba sphaira (1.20 mm long, 0,74 mm wide). a lymnaeid snail in apertural, ventral and apical views, From Pijiagou locality (Jiufotang Formation) in Yixian, Liaoning.

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