«To cite this version: Aihua Yuan. Latest Permian Deep-Water Ostracod (Crustacea) Fauna from South China. Pa- leontology. Universit´ Pierre et Marie ...»
The eggs hatch in the expanded posterior brood space of the adult female carapace or are laid directly into sea. The larva hatched from egg is called nauplius (1st instar, A-8) which is already enclosed by a bivalve carapace and has 3 pairs of appendages. Ostracod body grows until the nauplius carapace can not encase the body anymore. The nauplius will molt the old carapace and secret a new larger one.
Then the larva gets into the 2nd instar (A-7). Before getting adult (A), the larva gives 8 instars as described above in Podocopida and 4 to 7 instars in Mydocopida (Fig. 2-1-2-A). At every instar transition, the old carapace is rejected and a new larger one is formed and calcified. Along with carapace molting and secreting, ostracod body grows larger and larger and inner organs and appendages get formed and matured.
Ostracod shell has not the growth lines and is double layered. This shell structure gradually forms during the ontogeny. The double layers are respectively outer lamella and inner lamella composed of calcite and chitinous cuticle (Fig. 2-1-2-B). The soft body parts are just housed in the body cavity between the outer and inner lamellae. During the ontogeny, the epidermis invaginated from the join of cephalic and thoracic segments until envelope the whole body within the double layers. The outer lamella calcifies completely, while the inner lamella calcifies partially, with the rest remaining chitinous.
The calcified inner lamella is called duplicature which appears only when the ostracod becomes adult.
the latter one. Males sometimes inflated posterior to accommodate the relatively large copulatory appendages. But in most taxa, female is larger and more inflated due to the brood care.
2.1.3 Shell morphology In fossil record, the shell is generally the only preserved part. Consequently, shell morphology is the most useful tool (the only tool in most instances) to identify fossil ostracods. In this part, the shell morphological features involved in this thesis will be described. Some other features of hingement, adductor muscle scar and marginal zone are also very important taxonomic characters but not preserved in our specimens, whereas, the related descriptions will be omitted. This part is mainly referred to Moore (1961), Pokorný (1978), Crasquin (1984), Hao & Mao (1993) and Holmes & Chivas (2002). In all figures of this part, the arrow parallel to the length indicates the front of the carapace.
As described above, ostracod carapace is formed of two valves articulated dorsally. The two valves are respectively named as right valve (RV) and left valve (LV) (Figs 2-1-3-A, 2-1-3-C). The border bearing the hinge line is dorsal border (DB) / dorsum. The borders which are not concerned by the hinge are called free margins. Free margins are divided into anterior border (AB), posterior border (PB), and ventral border (VB). Accordingly, the lateral surface of the shell is divided into different portions (Fig. 2-1-3-B) which provide great convenience for ostracod describing.
Fig. 2-1-3-A: measurements of a Bairdia carapace.
Generally, LV and RV are not simply mirror images of each other. They are sub-equal or unequal in size so that the larger valve overlaps the smaller one along part or all of its margins (Figs 2-1-3-C, 2-1-3-F (b), (f)).
Ostracod carapaces vary greatly in shape. The shape/outline of the shell is a rather important taxonomic character especially in fossil ostracods. In lateral view, the shell may be ovate, rectangular, triangular, rhombic, pentagonal, fusiform, bean-shaped or kidney-shaped. For some typical shapes the certain words are adopted as “bairdian shape” is used to indicate the rhombic outline with anterior and 24 Yuan Aihua: Latest Permian Deep-Water Ostracod (Crustacea) Fauna from South China 2008/5 posterior beaks upwarded (Figs 2-1-3-A, 2-1-3-B (a), 2-1-3-C (a)). Preplete, amplete and postplete are also referred to the outline based on the position of the maximum height of the shell (Fig. 2-1-3-D). If the maximum height at the mid-length, it is considered as amplete, if anterior to the mid-length as preplete, if posterior to the mid-length as postplete. In dorsal or ventral view the carapace usually has symmetrically or asymmetrically biconvex lensoid shape. DB may be straight or convex/arched/bowed.
When ostracod has a straight DB, cardinal angle (CA), the angle between hinge line and AB/PB, would be introduced to be an important classification index. There are anterior cardinal angle (ACA) and posterior cardinal angle (PCA) (Fig. 2-1-3-E). If DB is convex, two other “border” concepts would be defined as anterior dorsal border (ADB) and posterior dorsal border (PDB) (Fig. 2-1-3-B (a)). VB is straight, convex or slightly concave. There are also anterior ventral border (AVB) and posterior ventral border (PVB) accordingly (Fig. 2-1-3-B (a)). AB and PB appear usually rounded (broadly or acutely) or pointed. The variation of curvature radius is used to qualitatively describe the curving extent of AB and PB.
Fig. 2-1-3-B: morphological features of the lateral surface ((a) Bairdia sp.4 sensu Yuan & Crasquin-Soleau from Dongpan section, right lateral view of valve; (b) Kellettinidae indet. from Liuqiao section, in right lateral view of valve).
Nomenclature referred to Scott (1961). MA: mid-anterior; MP: mid-posterior; M: median; MD: mid-dorsal; AD:
anterodorsal; PD: posterodorsal; MV: mid-ventral; AV: anteroventral; PV: posteroventral; AM: anteromedian; PM:
posteromedian; DM: dorsomedian; VM: ventromedian.
Fig. 2-1-3-F: sketch showing common ornamentation for ostracod shell. (modified after Scott, 1961) The ostracod shell is smooth or ornamented (Fig. 2-1-3-F). Ornamentation may sometimes vary within a species due to the different environments, but the pattern of ornamentation is usually used as a specific character. In the view of whether or not reflected on the internal surface of the shell, the ornamentation can be divided into primary ornamentation (valvular vaulting) and secondary ornamentation (sculpture). In another view, if the ornamentation depresses below the surface, it is called negative ornamentation, by contrarily, if elevates, called positive ornamentation. In this thesis, both the two division systems are adopted for better describe the ornamentation (Tab.2-1-3-A).
26 Yuan Aihua: Latest Permian Deep-Water Ostracod (Crustacea) Fauna from South China 2008/5
Fig. 2-1-3-G: some examples of ornamentations. All specimens from this work ((a) Aurikirkbya cf. ultima (Kozur, 1985), right lateral view of valve; (b) Permokegelites cf. beichuanensis Becker & Wang, 1992 sensu Yuan & Crasquin-Soleau 2007, right lateral view of valve; (c) left lateral view of carapace; (d) right lateral view of carapace; (e) Paraberounella ?cf. laterospina Kozur, 1991 sensu Yuan et al. 2007, left lateral view of valve; (f) Spinomicrocheilinella anterocompressa Yuan & Crasquin-Soleau, 2007, right lateral view of carapace; (g) Petasobairdia bicornuta Chen, 1982, left lateral view of valve; (h) Macronotella ? sp.1 sensu Yuan et al. 2007, left lateral view of carapace; (i) ?Libumella athabascensis Green, 1963 sensu Yuan et al. 2007, right lateral view of valve; (j) Denticupachydomella spinosa n.gen.n.sp., left lateral view of valve; (k) Hollinella sp., right lateral view of valve; (l) Monoceratina sp.1 sensu Yuan et al.
2007, right lateral view of valve. Scale bas is 100µm.
27 2008/5 PhD dissertation of University of Pierre Marie Curie & China University of Geosciences (Wuhan) Primary ornamentation (Figs 2-1-3-G (a)-(d)) is the elevation or depression reflected the inner anatomic structures. The lobe (bilobate / trilobite / quadrilobate valve, L1, L2, L3, L4) is positive ornamentation. And the sulcus (unisulcate / bisulcate / trisulcate valve, S1 usually found in very ancient species, S2 in center of which kirkbyan pit/spot is situated, S3) is the negative ornamentation (Fig.
Secondary ornamentation (Figs 2-1-3-G (e)-(l)) is the sculpture in the external surface and has no corresponding trace on the internal surface, which includes the positive ornamentation of ridge, carina, rim (marginal ridge), stria, frill (ornamental border on a garment or certain gathered or pleated at one edge), flange (raised outside edge), spine, tubercle (hollow), node (solid), knob, granule, pustule, papillae and the negative ornamention as puncate, pit and so on. For the ornamentation of reticulate or mesh, the muri is of course the positive ornamentation, whereas, the foveolation between muris belongs to the negative ornamentation.
Remarks: some comparisons between the similar features:
(1) tubercle, node & knob: tuberclenodeknob;
(2) ridge, rim, carina and velate structure: ridge is a general nomenclature for all elongate or linear elevation of lateral surface. It is called rim (=marginal ridge/rim) if it is the linear elevation along free margins. When it is parallel to VB, it is equal to velate structure. When it is strong elevated (in Podocopida) or parallel to velate structure on its dorsal side, it is named as carina.
1962; Pokorný, 1978; LSP, 1981; Hao & Mao, 1993) Orientation and measurement are the premilinary work for the identification. For the living ostracods, depending on the features of the appendages and the other soft parts, the orientation is much easier recurring to the ontogeny and phylogeny. But it is not so easy for fossil ostracods without the soft parts preserved. As follows I summarize the common criteria adopted in peers and my own research.
The two tables (Tab. 2-1-4-A, Tab. 2-1-4-B) present respectively the criteria for distinguishing anterior with posterior and dorsal with ventral. It should be noted that every criterion is not independent. In the practical identification, all available criteria that can be got from the material should be considered. In addition, there are some special but useful criteria for certain taxa, e.g. Palaeocopida has the greatest height anteriorly; in some bairdiids, the venter appears as a flat plane.
28 Yuan Aihua: Latest Permian Deep-Water Ostracod (Crustacea) Fauna from South China 2008/5
After orienting, it is the step to measure the shell. A shell is characterized by length (L), height (H) and width (W) / thickness (T) (commonly written as maximum length / height / width in systematic descriptions) (Fig. 2-1-3-A).
I will describe the measurements here in a little detail since I feel important and not easy to carry out in practical work. As mentioned above, the outline of ostracod shell is various and the hingement is usually not preserved in fossil record. Furthermore, different literatures present varied measurement principles which would confuse the following study.
The reason, resulting in different measurement principles, is the definition of the three dimensions.
No doubt the three dimensions are perpendicularly interrelated and W is the maximum distance between the lateral extremities of the carapace which is perpendicular to the sagittal plane. But for L and H, both have possibly two kinds of measurements. Take L for example.
29 2008/5 PhD dissertation of University of Pierre Marie Curie & China University of Geosciences (Wuhan) Fig. 2-1-4-A: measurements for ostracod with different outlines. The red dashed indicates the hinge line.
In the classical literature Treatise (Moore, 1961), L was respectively defined for the straight-backed and curve-backed ostracods. For the former, it is no problem. L is the maximum dimension in direction parallel to hinge line (Fig.2-1-4-A (a)). For the latter, L was defined as the maximum dimension parallel to the longitudinal axis of shell. In fact, for the curve-backed shell with straight hinge line as in Figs 2-1-4-A (c)-(e), if the shell axis is drawn, we can find out that the axis is parallel to the hinge line. In this way, the measurements for convex-backed (straight hinge line) ostracods have the same principle as the straight-backed ostracods. Then the division of measurement should be based on hinge line (whether or not straight) instead of “back”. And if the measuring is conducted in this way, L would be Ls sensu Hinz-Schallreuter & Schallreuter (1999, p.16). However, in practical work, ostracodologists commonly measure Lb (Hinz-Schallreuter & Schallreuter, 1999, p.16), the “maximum distance of end points of the carapaces parallel to the basal line” (Pokorný, 1978) (Figs 2-1-4-A (b)-(e)). I calculated the relationship between the two measurements (Fig. 2-1-4-B). Lh and Lb
can be transformed by the equation:
Lh=Lb cos (angle h&b) + (Ha-Hp) sin (angle h&b) There are two special situations, if hinge line is horizontal, Lh and Lb will be equal; if maximum extremities of AB and PB are situated at the same height, Lh will be smaller than Lb. The most common status is Ha≠Hp, angle h&b≠0, when the relationship of Lh and Lb can not be evaluated generally. But it seems that they would be equal only in some instances. Commonly, different data are arisen by these two measurements.
30 Yuan Aihua: Latest Permian Deep-Water Ostracod (Crustacea) Fauna from South China 2008/5
As many other ostracodologists, I also measured the Lb in my thesis. In my opinion, the basal line can be considered as the balanced surface for ostracod shell and also indicates the advancing direction of the ostracods. It would be more practical to consider L as the maximum measurement parallel to the advancing direction, especially for fossil ostracods without hinge line preserved. Consequently, in my thesis, L is unified as the maximum dimension between maximum anterior and posterior curvatures parallel to the basal line. This principle is applicable to all ostracods (for straight-backed ostracods, the hinge line is parallel to the basal line). Accordingly, H is defined as the maximum dimension between DB and VB perpendicular to L.
(1) If there are lobes, spines or other kinds of projections beyond the shell, the L, H and W should be marked “with” or “without” including the projection. Both of the dimension data will be acceptable.
(2) In addition, H/L is a taxonomic feature at specific level. Thus it is calculated commonly and listed with the L, H and W in systematic description.