Meizhen Cao Ycinxia

Ostracods are tiny crustaceans of the phyium Arthropoda. They are important microfossils and useful in geological prospecting. The most conspicuous feature of these animals is the bearing of a bivalved carapace in which the soft body is enclosed (Fig. 46). The two valves are not symmetrical, and are constituted by calcareous layer and chitinous coating. The left valve is normally somewhat larger and overlapping the right one. The exterior surface of valves may be ornamented or smooth. The ostracod carapace is generally between 0.5 and 1.5 mm in length, with a few either less than 0.5 mm or more than 1.5 mm, and very few reaching 70 mm such as Paramoelkritia from che Devonian carbonate rocks of Guangxi Province, China.

Ostracods have a long history and a worldwide distribution. They first appeared in the Cambrian of 500 million years ago and are still flourishing today. They have a high salinity and temperature tolerance and can live in seawacer, freshwater, and brackish water, but they are especially common in shallow-sea and lakes. They even live in hot or sulfur springs. Most ostracods are crawling benthos, but some are plankton, and still others swim or burrow into mud. Ostracods are gonochoristic and can reproduce ail year round. In suitable environmental conditions, they reproduce bisexually, whereas in the unsuitable environments they are parthenogenetic. Ostracods are oviparous animals. Their eggs, round or oval in shape, can withstand desiccation and cold, spreading for long distance and hatching in a suitable condition. The ontogeny of ostracods is discontinuous. As is in other arthropods, after the larva hatched out from the egg, with the growth of the soft body the hard carapace shed when the new carapaces are formed. There are approximately eight molts during their ontogeny. And the shape and ornament of the carapace change with the growth. The chitin layers of the carapace are very thin and rarely preserved and the soft body is prone to decay. Therefore, only the calcareous layers are usually preserved as fossils.

The fossil ostracods of the Jehol Biota from the northern Hebei and western Liaoning are very abundant and all of freshwater types. Fossil ostracods usually occur in great abundance on the rock surface of the Dabeigou, Yixian, and Jiufotang Formations, like a "sesame bread" (Fig. 47). Up to now, more than 130 species belonging to 25 genera of fossil ostracods have been found in the Jehol Biota, of which 5 species in 5 genera were collected from the Dabeigou Formation, over 80 species of 11 genera from the Yixian Formation, and more than 80 species of 19 genera from the Jiufotang Formation.

146 Cyprinotus sp., a recent ostracod arthropod in lateral view, Irom Wuhan. Hubei,

147 Fossil ostracods, on the rock surface like a "sesame bread" ( x 4). from Senjitu locality (Yixian Formation) in Fengning, Hebei, (Photo: Hai-chun Zhang/ NICPi

The lossil ostracods from the Dabeigou Formation are so tar only known from Dabeigou, Jingshang and Zhangjiegou areas, Luanping County, Hebei Province. They arc- dominated by the large-sized (about 2 mm in length) and smooth-surfaced Luatipingella (Fig, 48) and Torinina (Fig. 19), associated with small-sized (less than I mm) Rhinocypris with small spines and tubercles on ilie surface (Fig, 50) as well as the- smooth-surfaced Danvimila . Torinina first appeared in the Late Jurassic Turga Formation in the Argun River valley of East Transbaikalia. In the past, some ostracod specialists identified Torinina from the Dabeigou Formation as Eoparacypris. Because both of the Dabeigou and Turga Formations share the Torinina, they may be closely

dip48 Left valve of Luanpingella postacuta ( x 27), external lateral view, from Jingshang locality (Dabeigou Formation) in Luanping, Hebei.

riip50 Carapace of Rliinocypris jurassica ( x 80), external lateral view, from Sihetun locality (tower part of Yixian Formation) in Beipiao, Liaoning.

Mfe49 Right internal mould of Torinina tersa ( x 30). external lateral view, from Jingshang locality (Dabeigou Formation) in Luanping, Hebei.

ttp 51 Carapace of Cypridea (Cypndea) sihetunensis ( x 50), external lateral view, from Sihetun locality (lower pari of Yixian Formation) in Beipiao, Liaoning.

M* 52 Carapace of Cypridea (Cypridea) dabeigouensis ( x 50), external lateral view, from Dabeigou locality (lower part of Yixian Formation) in Luanping. Hebei,

» 53 Carapace of Cypridea (Ulwellia) beipianensis ( x 50), external lateral view, from Libalanggou locality (lower part of Yixian Formation) in Beipiao, Liaoning.

riip54 Carapace of Timiriasevia jianshangouensis (x «0), external lateral view, from Jianshangou locality (lower part of Yixian Formation) in Beipiao, Liaoning.

155 Carapace of Darwinula legumineHa | x 70), external lateral view, from Sihetun locality (lower part of Yixian Formation) in Beipiao. Liaoning,

• 56 Carapace of Cypridea (Cypridea) jingangshancnsis ( * 45). external lateral view, from Zaocishan locality (upper part of Yixian Formation) in Yixian, Liaoning.

157 Carapace of Cypridea [Cypridea) zaocishanensis ( x 45). external lateral view, from Zaocishan locality (upper part of Yixian Formation) in Yixian. Liaoning.

Carapace of Yumenia casta ( x 40), external lateral view, from Pijiagou locality (upper part of Jiufotang Formation) in Yixian. Liaoning,

■¡59 Carapace of Yumenia jianchangensis ( * 40), external lateral view, from Pijiagou locality (upper part of Jiufotang Formation) in Yixian. Liaoning,

■*60 Carapace of Yixianella marginulata ( * 40|. external lateral view, from Pijiagou locality (upper part of Jiufotang Formation) in Yixian, Liaoning.

correlated. Their age is Late Jurassic.

Ostracods from the Yixian Formation are very rich in quantity and more diverse than those from the Dabeigou Formation. Cypridea is the predominant form. These ostracods may be grouped into two assemblages. The lower assemblage, in addition to Cypridea (Figs. 5 I — 53), includes abundant large-sized and smooth-surfaced forms such as Yambanina. Djungarica and small-sized forms Trmiriasevia (Fig. 54), Darwinula (Fig. 55), etc.; The upper assemblage consists chiefly of species of genus Cypridea in which the subgenus Cypridea (Cypridea) (Figs. 56, 57), with left valve larger than the righr, occupied a dominant position. The ostracods from the lower part of the Yixian Formation are similar to the two ostracod assemblages in the lower part of the Purbeck Group of England and may be correlated with the ostracod assemblage from the Bulum Member of the Kuti Formation in the Argun River valley of the eastern Transbaikalia. Judging from the above-

mentioned facts, the lower part of the Yixian Formation should be larc Tithonian or late Tithonian to Berriasian in age.

Ostracods from the Jiufotang Formation are extremely abundant and highly diverse. They may be grouped also into two assemblages. The lower assemblage is comparatively similar to that from the upper part of the Yixian Formation predominant with forms of Cypridea (Cypridea). The fossil ostracods from the upper parr of the Jiufotang Formation are abundantly represented with rapidly evolved and widely distributed forms such as Yi/menia (Figs. 58, 59), ¡Jmnocypridea and a lot of Cheilocypridea, Yixianella (Fig. 60) and Cypridea (Uhvellia) with the right valve larger than the left one. This assemblage is characteristically identical to the Early Cretaceous Valanginian-Barremian ostracods in and outside China.

(Except where indicated, all photos in this chapter were taken by Yong-qiang Mao/ NIG Pi

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