Bivalvia are a class of bivalved moilusks, also known as Pelecypoda and Lameliibranchiata in the literature. The bivalves are aquaric animals with a shell consisting of two matching lateral valves united by a dorsal horny ligament. The valves can be drawn together by a pair of adductor muscles (reduced to one in some) against the opening counter-force of the ligament. Today the bivalves are among the most common benthic invertebrates, especially on marine shelves, although there are many species in fresh-water or brackish-water environments and even in abyssal habitats. The laterally compressed shape characteristic of the great majority of bivalves renders them well adapted for burrowing in sandy or muddy substrates. Some bivalves are borers in solid or soft rocks, whereas the most highly specialized boring forms are wood-borers ("ship-worms"). Among the benthic epifauna, bivalves form an important element. Some rely on the weight or the shape of their shell to maintain their position on the seafloor; others attach themselves to stationary objects by means of their byssus or by cementation of one of the valves. Some highly mobile bivalves, for example the pectinids, can "hop" near the sea bottom for short distances. No living bivalves are pelagic, except for larval stages and very few species with strongly reduced shells. However, some extinct thin-shelled bivalves are considered to have possibly lived pelagically.
The first occurrences of Bivalvia are known from the Lower Cambrian at a few localities in the United States, southern Australia, and China. During the Early Paleozoic periods, bivalves were not abundant. Since the Devonian, bivalves became very frequent and diversified. The class reached the acme of its development in the Mesozoic and Cenozoic. In most instances, Mesozoic bivalves are useful for stratigraphical correlation.
The Jehol Fauna arose after an important biotic extinction event, which took place between the latest Middle Jurassic and early Late Jurassic in China, accompanied by the strong tectonic movements during the Oxfordian and early Kimmeridgian (Late Jurassic). Owing to this extinction event, the Eolamprotula-Psilunio fauna and the Psettclocardinia fauna, flourished in the Middle Jurassic of the Chinese continent, are missing from the Jehol Fauna. The bivalves of the Jehol Fauna manifested themselves with a new feature
135 Multiple species of Arguniella (15—25 mm long of the individuals), a sibireconchid bivalve in aggregation, from the roadside ofSihetun to Ljbalanggou (lower part of Yixian Formation) in Beipiao, Liaoning. (Photo: Da-jian Li/CAS)
Sb/j Internal moulds of Mengyinaia. a unionid bivalve, from Ningjiagou locality (Mengyin Formation) in Mengyin. Shandong, a, Mengyinaia mengyinensis (65 mm long); b. Mengyinaia tugrigensis (90 mm long). (Photo; Zhou-qing Chen/ NtGP)
jt , Shell of Nakamuranaia chingshanensis (5t mm long), a nippononaiid bivalve, from Jiazhangzi locality (Jiufotang Formation) injianchang. Liaoning. (Photo: Zhou-qing Chen/ N1GP)
ïv. Internal moulds of Sphaeriutn, a small-sized pisidiid bivalve, from Xiwa locality (Xiwa Formation) in Mengyin. Shandong, a, Sphaerium jeliolense (3 mm king): I), Sphaerium pujiangense (7 mm long). (Photo: Zhou-qing Chen/ NIGP)
greatly differing from the Middle Jurassic ones. Among them, three freshwater assemblages are recognizable on the basis of bivalve samples respectively from the formations yielding the indices of rhe Jehol Fauna. The three-assemblages are as follows.
1. Argunitlla assemblage. The assemblage includes two species of a single genus, i.e., Argunitlla lingyuanensis and A. yamhanvnsis, found from the Dabeigou Formation in eastern Hebei province, showing a very low diversity of the bivalves at the beginning or the early phase of the faunal development of the Jehol Fauna. The assemblage is comparable with that in the Upper Jurassic Mayakskaya Formation in the upper reaches of the Argun River, easrern Siberia, Russia.
2. Arguniella-Sphaerium assemblage. This bivalve assemblage belongs to the main phase of the Jehol Fauna, found from the lower Yixian Formation in western Liaoning and northern Hebei provinces. Particularly rich bivalves of the assemblage occur just below the bird [Confuctusornisybcd at the type locality of this primitive fossil bird (Fig. >5). Although it is very abundant in number of individuals, the assemblage possesses only four species in two genera, Argutiielht lingyuaricnsis, A. yanshanemis, Sphaerium jeholense and S. iinderssoni, indicating emerging but still low diversity compared with some-other groups of invertebrates (e.g., ostracods and insects) and some vertebrates (e.g., birds) in the same phase. Therefore, the bivalve radiation in the Jehol Fauna was delayed, probably owing to the slow evolutionary rates of these bivalve mollusks, which were protected by their solid shells and most probably had adapted to the adverse environments at that time.
3. Metigyinaiti-Nakamuranditt-Spbaerium assemblage. It is from the fossiliferous horizons of the late phase of the Jehol Fauna, the Jiufotang Formation in western Liaoning and its coevals. The assemblage is more abundant and diverse in bivalves than the above two, consisting of more than 30 species designated to five genera including five species to AlengyinaM (Fig. 36), three to Nakamuranaia (Fig. 37), I 2~ 1 3 to Sphaurmm (Fig. 38), six to
Arguniella and five to Weicbangella (Fig. 39), representing a distinct faunal radiation. This assemblage was succeeded by rhe Nippononaia itnensts-N. cf. tetoriensis-Tetoria cf. yokoyamai assemblage found from the Shahai Formation in Liaoning, which overlays the Jiufotang Formation and can be correlated with the beds of the upper Itoshiro Subgroup of the Teton Group in Japan. The latter beds of Japan have been recently dated, based on ammonites and marine bivalves, as Llauterivian (liarly Cretaceous). Therefore, the present assemblage is regarded to be around Valanginian in age.
Pei-ji Chen onchostracans, or clam shrimps, are small, bivaived branchiopod crustaceans (Fig. 40). They have a short laterally compressed body enclosed between two lateral valves that constitute the chkinous carapace with an interior membranous lining. The head is located anteriorly and bears sessile, paired compound eyes with well-developed ocelli. Posteriorly, the telson is distinguished by two flattened, upwardly curved processes or claws. The antennules are small and simple, but the antennae are modified into powerful biramous swimming organs. During mating, males use them to aid in seizing the female carapace. Trunk legs (swimming legs) vary from 10 to 32 pairs.
These animals occur in non-marine fades sporadically, but often abundantly from the Devonian to the present day. Many paleontologists still refer to them informally using the pre-occupied generic name "Esthetic", originally used for a fly.
The clam shrimps flourished twice in the Late Paleozoic of Europe and in the Mesozoic of Asia with about 200 genera in 27 families, bur rapidly-declined in the Cenozoic with only 14 genera in 5 families still living today.
The living conchostracans reside in small and temporary inland ponds, flood-plain pools, rice fields, roadside ditches, and almost any shallow depressions filled with water. They have also been reported from spring water, along margins of large lake and on coastal salt flats. They appear in the Yangtze River drainage area during spring or in North China, Northwest China, Northeast China, and Inner Mongolia during summer and early autumn. The most favorable temperature for the majority of living forms is
There arc two kinds of conchosrracan eggs: the thin-shell eggs can hatch after several days of ovulation by females; the thick-shell dormant eggs could withstand long desiccation or frozen of the pools, so as subsequently to become dispersed by wind, water or birds and hatch even a number of years later. This may explain the worldwide distribution of fossil clam shrimps, which has great potential in biost rat ¡graphical research, especially for the non-marine Jurassic and Cretaceous in eastern Asia.
»40 Eocyzicus mongolianus, a recent conchostracan arthropod from Zhenglanqi, Inner Mongolia. (Photo: Ji-liang Mao/NICP)
i41 Carapace valves of Nestoria pissovi (14 mm long), a loxomegaglyptid conchostracan with several brawny growth lines ornamented by large reticulations in between, from Dadianzi locality (Dabeigou Formation) in Luanping, Hebei. (Photo: Da-jian Li/CAS)
The taxonomy of living conchostracans is based on the structures of soft body, which is seldom preserved as fossils. Thus, we have to pay more attention to the variation of carapace and the minute ornamentation of growth bands of valve in fossil clam shrimps. For example, the euesrheriids and loxomegaglyptids bear reticulate sculpture and the orthestheriids radial lirae, whereas the eosestheriids have the mixed sculpture of reticulation and radial lirae. From the Jehol Biota, three conchostracan assemblages can be clearly identified chronologically as follows.
Nestoria-Keratestheria assemblage: Besides its namesake forms, the assemblage includes Sen test heria, Abrestheria, Ambondla, and ]ibeilimnadi<j. Generally, they have a large carapace with several brawny growth lines sometimes bearing spines or nodes. Growth bands are broad and flatted with big reticulate sculpture (Fig. 4 1). These fossils were found from the Dabeigou Formation of northern Hebei Province, the Baoshi, Murei, or Baiyingaolao rr
Formations of Inner Mongolia of China and the Argun and Ust'kal Forma tions of eastern Transbaikalia.
un 42 Carapace valves of Eosestheria aff. middendnrjii (¡3—14 mm long), an eosestheriid conchostracan with mixed sculpture of reticulation and radial lirae on growth bands (enlarged detail), from Zaocishan locality (Jingangshan Bed of the upper Yixian Formation) in Yixian, Liaoning, (Photo: Da-jian Li/ CAS, Mao-fang Pang/' NIGP)
Ma 43 Carapace valves of Eosestheria lingyuanensis (11 mm long), an eosestheriid conchostracan, from Sihetun locality (Jianshangou Red of' the lower Yixian Formation) in Ueipiao. Liaoning. (Photo: Da-jian Li/ CAS)
Eosestheria-Diestberia assemblage: These two genera were absolutely predominant in quantity among tins assemblage when they lived in the Paleo-Amur River valley including Inner Mongolia, Northeast and North China, Mongolia and Transbaikalia. Their fossils are collected from the Yixian Formation of western Liaoning Province, NE China and the relevant horizons in other regions. The well-known forms of" this assemblage are Eosestheria aft. middendorfii (Fig. 42), l:. itngyuanensis (Fig. 43), E. oval a (Fig. 44), E. jingangihanemis, Dkstberia yixianetuis (Fig. 45) and D.jeholensis. In addition, some other species of Eosestheriupsis has been recognized recently from the Jehol Fauna, but in the past were found only from the Upper Jurassic deposits such as the Penglaizhen and Tuodian Formations of southwestern China.
Eosestheria has a relative large carapace with moderate growth lines but without spines or nodes. Growth bands are ornamented with irregular
m»45 Carapace valve of Diestheriayixianensis (21 mm long), a diestheriid conchostracan, from Zhoujiatun locality (Bakangpu Bed of the middle Yixian Formation) in Yixian. Liaoning. (Photo:Da-jian Li'CAS)
medium reticulations transferring gradually into the radial lirae near ventral and posterior margins of the valve. The ornamentation of Diestheria is similar to that of Eosestheria, with additional overlapped big reticulations near the posteroventral margin of the valve. The impressions of these overlapped reticulations are large and flattened tubercles in the mould fossils,
E os est he ria- Yd n j /1 j theria assemblage: It includes Allestberia, Yumenestheria, Diestheria, in addition to its namesake forms. During this period, Loses theria was bearing numerous growth lines and the corresponding growth bands changed into narrow ornamentation with relatively small reticulate sculpture only near the dorsal side but fine and crosvded radial lirae in the rest parts of the valve. The genus Yanjiestheria was evolved from Ensestheria. It has smaller reticulate sculpture near the dorsal side and finer and more crowded radial lirae near ventral and posterior parts of the valve. The conchostracan fossils of this assemblage were collected from the Jiufotang Formation in lacustrine fades of western Liaoning or similar rocks of other regions of China, Mongolia, Transbaikalia, Korea peninsula, and southwestern Japan.
■«44 Carapace valve of Eosestheria ovala (19 mm long), an eosestherijd conchostracan with mixed sculpture of reticulation and radial lirae on the chitinous growth bands and corresponding small and polygonal platforms and radial grooves in the externa! mould, from Sihetun locality (Jianshangou Bed of the tower Yixian Formation) in Beipiao, Liaoning. (Photo: Da-jian l_i/ CAS)
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