«To cite this version: Aihua Yuan. Latest Permian Deep-Water Ostracod (Crustacea) Fauna from South China. Pa- leontology. Universit´ Pierre et Marie ...»
Hou & Chen, 1962; LSP, 1981; Hao & Mao, 1993; Holmes & Chivas, 2002; Liebau, 2005; ∗) Since 1961, many researchers have been contributing to perfect the classification (the milestones of ostradological development in early times summarized here in Tab. 2-1-5-A). It seems as if
impossible to form a stable framework. Liebau (2005) proposed the preliminary revised classification of ostracoda. But the common result has not been made officially. In my work, I still followed mainly the Treatise classification (Moore, 1961) (Tab. 2-1-5-B) but distil the later achievements by other ostracodologists for some new taxa. At the same time, integrated with the study on living ostracoda, I agree with getting to the unified routine in taxa, e.g. “oidea” is adopted as the uniform postfix for Superfamily here (Holmes & Chivas, 2002, p.8).
Tab. 2-1-5-A Milestones of ostradological development in the early times (before 1961) Ostracoda was first discovered by Danish naturalist Müller who gave the oldest generic names Cypris and 1776 Cythere∗.
“Ostracoda” was named by French scientist Latreille (derived from the Greek “ostrakon” which means “shell”).
1806 Ostracoda has formally become a taxonomic nomenclature (order name).
1821 Based on the appendages, Sars proposed four suborders (Myodocopa, Podocopa, Platycopa and Cladocopa) mostly of which are still adopted now for recent ostracods. Until then ostracoda has had its 1866 rudiment of classification.
Norwegian scientist Henningsmoen proposed Palaeocopa for straight dorsal ostracods.
1953 Czechish scientist Pokorný combined the Paleozoic and post-Paleozoic ostracod study and established the 1958 Subclass Ostracoda.
Treatise adopted the classification by Sylvester-Bradley and gave the foundation of today’s classification.
1961 Tab. 2-1-5-B Classification adopted in this work (mainly modified after Moore, 1961) Phylum Arthropoda Siebold & Stannius, 1845 Subphylum Crustacea Pennant, 1777 Class Ostracoda Latreille,1806 (O-Rec.) Order Palaeocopida Henningsmoen, 1953 (O-?P) Suborder Beyrichicopina Scott, 1961 Suborder Kloedenellocopina Scott, 1961 Order Podocopida Mueller, 1894 (O-Rec.) Suborder Podocopina Sars, 1866 Suborder Metacopina Sylvester-Bradley, 1961 Suborder Platycopina Sars, 1866 Order Myodocopida Sars, 1866 (O-Rec.) Suborder Myodocopina Sars, 1866 Suborder Cladocopina Sars, 1866 There are two main reasons which influence the establishment of a new classification: (1) continuous designation of new taxa; (2) inflation or devaluation of known taxa. However, the root ∗ www.ucl.ac.uk/GeolSci/micropal/ostracod.html 32 Yuan Aihua: Latest Permian Deep-Water Ostracod (Crustacea) Fauna from South China 2008/5 resulting from the disunity on taxonomy is the classification criteria. As mentioned above, the living ostracods are classified by variations in their appendages and other soft parts. Although exceptionally well preserved fossil ostracods with the soft parts intact have been found, these are very rare and therefore the morphological features of the shell have become vital in fossil ostracod classification (Holmes & Chivas, 2002, p.19). Furthermore, due to the ontogeny (instars and dimorphism), ostracods may have the two adult morphtypes and different stages of juveniles which may misunderstanding the single species in fossils. In a word, many indeterminate factors arise and result in the difficulties in the fossil identification.
The common features adopted in high level classification are listed in Tab. 2-1-4-A, Tab. 2-1-4-B and Tab. 2-1-5-C. For some taxa, there are easily recognizable features, e.g. Palaeocopida usually have typical ornamentation (lobe and sulcus) (Figs 2-1-3-G (a)-(c)), Kirkbyidae has well developed kirkbyan pit (Fig.2-1-3-G (a), (b)), Bairdia has typical rhombic lateral outline (bairdian shape) (Figs 2-1-3-A, 2-1-3-B(a), 2-1-3-C (a)), Microcheilinella has very inflated carapace and strong overlap (Figs.2-1-3-C (c), 2-1-3-G (f)). At the same time, there are some practical features such as the location of the maximum height and length and the position of the maximum curvature of AB and PB. In a word, general and certain objective features should be integratively considered in the practical identification.
33 2008/5 PhD dissertation of University of Pierre Marie Curie & China University of Geosciences (Wuhan)
2.2.1 Sampling In this thesis, I focus on the important geological boundary interval and planed to discuss paleoenvironmental variation and extinction process. Thus I carried out the sampling bed by bed even sub-bed by sub-bed to get continuous and precise results along each section.
When sampling, I chose the fresh rocks without weathering and recrystallizing. The weight for each sample is about 400-500g (Lethiers, 1979). Each sample should be well enclosed in a sample bag and labelled by corresponding number to its horizon.
The extracting method depends on the microfossil-bearing rock types and the type of microfossils themselves. The extracting method may be chemical or physical. In my work, the ostracod-bearing samples are siliceous rocks, muddy rocks or calcareous rocks. Chemical methods were mainly applied.
As follows, the processing method for calcareous rocks will be thoroughly described at first. The description for the same steps such as sample crushing and rinsing will be simplified or left out in the two other methods.
Calcareous rocks & Calcareous ostracods Generally, if the fossils and the matrixes have different compositions or different endurance to some reagents, the chemical method can be applied. Here, in the calcareous rocks, the ostracod shells are also composed of calcium carbonate. The adopted methodology, known as “hot-acetolysis”, was perfected and published by Lethiers and Crasquin-Soleau (1988), derived from Bourdon (1957, 1962).
The dehydrated samples and application of pure acetic acid effectively avoid the corrosion to the fossils during the reaction. It is based on abundant experiments over 20 years on thousands of samples collected from the Devonian-Triassic strata in France, Belgium, Canada, China, Arabia and Turkey by Prof. Lethiers and Dr. Crasquin-Soleau. Compared with the methodology of Bourdon (1962), it is easier and more efficient. The most importantly is that the fossils will be released and retrieved without corrosion (Crasquin-Soleau & Kershaw, 2005; Crasquin-Soleau et al., 2005).
Peng & Wang (2002) introduced the heating-acid-digestion method for sample preparation of ostracods from carbonate rocks. Generally speaking, there is not far difference from our method. It was also derived from Bourdon (1957, 1962). But they use the desiccant CuSO4 to absorb the water 34 Yuan Aihua: Latest Permian Deep-Water Ostracod (Crustacea) Fauna from South China 2008/5 produced during the disaggregation. According to Coen (1985), CuSO4 has definite protection to the shells. And the condenser was adopted to maintain the concentration and the pressure of the container.
Equipments: heating-store, sand-bath in a ventilated cabinet, a battery of three sieves (2mm mesh,
0.5mm mesh and 0.1mm mesh), hammer, glass bottles with cover (acid and heat proof), several rockered petrie dishes (porcelain or Teflon), large container for acid recycling, aluminium foil, funnel, filter paper, rubber gloves, respirator.
Chemicals: pure acetic acid (purity99.5%), blue methylene powder.
Step I: sample crushing In order to increase the surface area and further accelerate the reaction, the sample should be crushed into fragments (several cubic centimetres, like the size of hazelnut) with hammer. Generally speaking, 400-500g sample is enough. It is suggested using paper to limit the shard loss. After crushing, the sample placed into a glass bottle labelled with the sample number (the bottle should be large and heatproof so as to be stable in the drier and provide enough space for the later reaction) (Fig. 2-2-A).
Step II: dehydration To avoid destroying the fossils during the reaction, the sample should not contain any water. For this reason, after crushing, the bottle is placed in the heating-store or a heated sand-bath. The temperature should be controlled at lower than 100ºC (to avoid the boiling of the sample) (Fig. 2-2-B).
It usually takes 2 or 3 days for most calcareous rocks.
Step III: acidization & disaggregation / acetolysis The aim of acidization is to separate the fossils from the enclosing matrixes. Take the sample out of the heating-store or the heated sand-bath. If the temperature is too high to hold the pot in the hand, the operator should leave it cooling for a moment to avoid the breakage of the glass. Then carefully pour the pure acetic acid into the bottle making sure the acid has covered all of sample. When pouring the acid, the operator should observe the sample to see if there are bubbles and to hear if there is a tittering sound. If effervescence occurs, the pouring should be stopped and make the sample re-dried. The possible reasons are that the drying is insufficient or/and the purity of the acetic acid does not reach the requirement. Then cover the bottle but do not tightly and completely close so as to free the volatilizing of the acid vapours. To prolong the life duration of the bottle, the use of aluminium foil is suggested.
Place the bottle on the sand-bath at a temperature of 60ºC -80ºC (lower than the boiling point of acetic acid which is 118ºC) (Fig. 2-2-C). There are a series of complex reactions. The reaction time also varies from 1 day to several weeks (months). Generally, the coarser the crystallinity, the quicker the reaction proceeds. For the health and safety, the operator should wear the rubber gloves and conduct this step under the ventilated cabinet.
Step IV: acid recycling and residua rinsing From the beginning of acidization, the operator should examine the sample everyday to see if the reaction is enough or more acid should be added. When the solution is turbid and the rock has turned into very small fragments (without complete disaggregation) and there is enough muddy deposit at the 35 2008/5 PhD dissertation of University of Pierre Marie Curie & China University of Geosciences (Wuhan) Fig. 2-2 shows the complete procedures of hot-acetolysis. A: sample crushing; B: sample dehydration in heater; C: sample acidization; D: acid recycling and residua rinsing with a battery of three sieves; E: residua dehydration; F: hand picking and sorting under a binocular stereoscope; G: stubs prepared for scanning.
36 Yuan Aihua: Latest Permian Deep-Water Ostracod (Crustacea) Fauna from South China 2008/5 bottom of the pot, the sample can be washed. Generally, if this kind of phenomenon has not occurred after three weeks, the sample should be washed and this step repeated. Before rinsing, the excess acid is poured off and filtered, using a funnel and filter paper, into a special container for recycling (Fig. 2-2-D).
Althougu after the reaction, the acid has become red or yellow in colour, it can be reused with the same efficiency. The sample is then poured onto a battery of three sieves (2mm mesh for undisaggregated sediment, 0.5mm mesh for adults and large individuals and 0.1mm mesh for larvae and small specimens) (Fig. 2-2-D). To avoid reaction between the water and the sample, the rinsing should be conducted quickly. The part remaining on the 2mm sieve can be kneaded slightly and gently before being re-dried for the second acidization (make a signal of have been washed once). Successively rinse the residua remaining on the 0.5mm sieve and 0.1 mm sieve, until the escaping water from the sieve is clean. Sieve and place the two residua in different dishes with labels and place them in the heating-store (Fig. 2-2-E). When the residua are dry, they can be transferred to the little bags or bottles with labels.
The dishes should be washed carefully and completely for the after use.
Before rinsing the next sample, clean the sieves in order to inhibit contamination of other samples.
Usually, I immerse the sieves into the aqueous solution of methylene blue for a few minutes. This kind of solution can permeate into the rock and is very effective to dye the possibly mixed specimen (except the pyritized fossils) in the mesh so as to inhibit the contamination (Beckmann, 1959). During the operation, wearing gloves and the respirator even blinkers are needed to avoid the strong acid vapour.
Then the second acetolysis is conducted. Generally the reaction is more quickly in the second operation.
And it has been found that more specimens are obtained from the second operation. If it is necessary, i.e.
more specimens are necessary for an important species, a third operation is needed.
Siliceous rocks & calcareous/silicified/pyritized ostracods The ostracods studied here were obtained from the hydrofluoric acid (HF) technique, which is a special method for extracting the radiolarians from the cherts (Pessagno and Newport, 1972). Some detail procedures are improved in our own practical operation depending on the laboratory conditions and nature of rocks. In fact, the use of hydrofluoric acid in micropaleontology is not new, Wetzel (1921) recommended it as a preparatory technique to extract calcareous microfossils from non-calcareous rocks. The process, which transforms the opaque calcium carbonate into translucent calcium fluoride, was termed as “fluoridization” by Upshaw et al. (1957). The pseudomorphs of the fossils are believed to be a molecule-by-molecule replacement and reveal the calcareous skeletal and fine sculptures very well (Schallreuter, 1982).
The same as the calcareous rocks, the siliceous samples should be crushed into fragments (several cubic centimetres, like the size of walnut) so as to increase the surface area and further accelerate the reaction. After crushed, each sample was put into a meshy bag (made from nylon window screening or something like that. It provides convenience of changing the acid) and then place in a plastic beaker (beaker 1) labelled with the sample number. At the same time, for every sample, another plastic beaker (beaker 2) should be prepared and labelled for containing the residua.