The ostracod genus Paijenborchella and some of its species in the Early Tertiary of Pakistan

Six species of the genus Paijenborchella are described. Five of these species are new, namely Paijenborchella (Eopaijenborchella) angulosa, P. (E.) viriosa, P. (E.) ventricaudata, P. (E.) noar and P. (Paijenborchella) inponticulata. The stratigraphic distribution of these species in the Early Tertiary of Pakistan is given; they range from the Lower Eocene to the Upper Eocene in the Sulaiman Range and one species occurs in the Upper Paleocene of the Sor Range.


INTRODUCTION
The distribution of Early Tertiary rocks in parts of Pakistan is shown in Figure 1. The stratigraphic succession of these rocks in the Rakhi Nala of the southern Sulaiman Range, is that published by Siddiqui (1971, modified after Eames, 1952. These rocks also occur in the northern Sulaiman Range (Fig. 2). For full information on the stratigraphy of the Sulaiman Range and Sor Range and the horizons from which samples were taken, see Siddiqui (1971).
Paijenborchella (Upper Cretaceous-Recent) is a marine genus with an interesting development in its carapace structure. The carapace in this genus is punctate, pitted, reticulate or smooth, rarely spinose (or a combination of these) and always sulcate (with a vertical groove placed slightly anterior to the middle of each valve). The sulcus forms a corresponding raised ridge on the inside of the valve; the adductor muscle scars are located on this. A median longitudinal ridge usually occurs, often forming a bridge over the sulcus. There are also generally one or two ventral longitudinal ridges, which may end in posterior spines or alae.
Paijenborchella is divided into two subgenera: Paijenborchella sensu stricto, with smooth carapace; and Eopaijenborchella in which the surface is ornamented. In this material from the Sulaiman and Sor ranges of Pakistan, there is one example of the smooth subgenus, P. (Paijenborchella) inponticulata. This is the only species which lacks a median longitudinal ridge and is therefore without a bridge over the shallow interrupted sulcus.
The muscle scar field in Cytheridae, found on the inside of the sulcus in sulcate species, is often a weak part of the carapace. This can be seen in Cythere lutea when contraction of adductor muscles at death ruptures the valves. In several Cytherura species the first signs of erosion occur in the muscle scar field.
In the genus Paijenborchella, which has relatively robust carapaces and deep sulci, the reverse side of the sulcus is a raised internal ridge. One to three external longitudinal ridges, one crossing the sulcus and one or two supporting the ventral margin, also serve to oppose any central weakness. The development of a bridge crossing the sulcus is an interesting feature of a family in which ornamentation is prominent and varied. It invites speculation as to the arrangement of the appendages inside these carapaces, and the environmental conditions which elicit such adaptations.
All the specimens described in this paper are deposited in the Natural History Museum, London and the specimen numbers quoted are those identifying them in the fossil ostracod collection of that institution.  (Siddiqui, 1971: 11).
Description. Carapace subtriangular in lateral view. Sexual dimorphism is present, males being longer, but shorter in height, than females. Anterior margin broadly and obliquely rounded, posterior with short and pointed subventral caudal process. Dorsal margin slightly curved in left valve, almost straight in right valve, postero-dorsal margin nearly straight, ventral margin slightly incurved anterior to the middle, but appears to be sinuous because of ventro-lateral expansion. Left valve larger than the right, over-reaching it at the dorsal, antero-dorsal and postero-dorsal margins. Surface ornamentation consists of reticulations (stronger in the posterior part of the carapace), with a vertical sulcus located slightly anterior to the middle and confined to the upper two-thirds of the carapace. The sulcus is traversed by a short median bridge which merges into the shell surface on both sides of the sulcus. Anterior marginal rim narrow. Both anterior and posterior marginal areas compressed. Internal features not seen.

Dimensions.
Remarks. This species shows some resemblance to Paijenborchella (Eopaijenborchella) dattai Bhandari, 1992, but differs from it in having a narrower anterior marginal rim and a shorter median bridge. Paijenborchella indegina Sarma, 1973 (Pl.  Derivation of name. Latin, angulosus, full of corners; with reference to surface ornamentation.

Diagnosis.
A species of Eopaijenborchella in which posterior part of carapace has ridges defining large angular polygons. Median bridge short and connected posteriorly to the corner of a polygon; anterior marginal area divided by a short horizontal median ridge. Valve surface finely punctate.
Description. Carapace subrectangular in lateral view. Sexual dimorphism apparent, male dimorphs being more elongate than females. Anterior margin broadly and obliquely rounded towards the ventral margin, posterior with a short caudal process with a concavity above. Dorsal margin straight, ventral margin straight but appears convex because of ventral ridge. Greatest length passes through midpoint, and greatest height at the anterior cardinal angle. Anterior cardinal angle rounded, posterior cardinal angle obtuse and well marked. Left valve larger than the right, over-reaching it at the dorsal, anteroventral and posteroventral margins. Valve surface finely punctate. The posterior part of the carapace has ridges defining large angular polygons. Median bridge short, connected posteriorly to the corner of a polygon. Sulcus well developed, almost vertical, widest near the dorsal margin, lying slightly anterior to the middle. Ventral ridge prominent and alate. Anterior and posterior marginal areas compressed, anterior with a short horizontal median ridge. Internal details unknown.

Dimensions.
Remarks. This species seems to be identical to Paijenborchella narediensis (Bhandari, 1996;non Guha, 1974) from subsurface rocks of Jaisalmer Basin, Rajasthan, of the Khuiala Formation, Lower Eocene. However, the specimen illustrated by Bhandari (1996) appears to be somewhat eroded. Paijenborchella narediensis Guha, 1974 from the Lower Eocene of Kutch differs from the present species in having a narrower caudal process and a ventral ridge ending posteriorly in a sharp spine. P. (E.) angulosa sp. nov. appears to be restricted to the Lower Eocene and is thus a useful horizon marker in the area. Description. Carapace obovate in lateral outline. Sexual dimorphism strong, presumed males more elongate and not as high as females. Anterior end broadly rounded, posterior with a distinct caudal process almost in the middle, dorsal margin curved, posterodorsal margin slightly concave, ventral margin curved (over-reached by lower ventral ridge in lateral view), anteroventral flange preserved in some specimens. Left valve overreaches right valve at the dorsal and posterodorsal region. A subvertical deep sulcus extends from the dorsal margin to the ventral ridge. It lies subcentrally in females but within anterior two-fifths in male dimorphs. The sulcus is crossed by a welldeveloped median bridge which is short but in some specimens extends for a short distance anteriorly, posteriorly or both. The upper ventral ridge is curved convexly downward in the middle; the ventral part of the sulcus ends against this ridge. In some specimens it terminates posteriorly in a marked spine.  Sohn, 1970 shows some affinity with the male dimorph of the present species but has a reverse overlap and only one, rather than two, ventral ridges. Guha (1968) assigned some specimens from the Middle Eocene of Kutch to Paijenborchella trisulcata Rozieva, 1962; his illustration shows a juvenile which is not P. trisulcata but is probably conspecific with the present species. This becomes obvious when a large population of the present species is examined. The male dimorph of P. (E.) viriosa shows some resemblance to the male dimorph of P. (E.) bhatiai Bhandari, but has a slightly upturned caudal process particularly in the right valve, while that of P. Description. Carapace subovate excluding caudal process. Sexual dimorphism strong, males being more elongate and not as high as females. Anterior margin broadly rounded, posterior with a well-marked subventral and subtriangular caudal process, particularly in the left valve. Dorsal margin almost straight in right valve but slightly curved in left valve, ventral margin almost straight. Greatest height at the anterior third with greatest length passing through midventral region. In dorsal view greatest width is almost in the middle. Left valve larger than the right, over-reaching it in the dorsal, posterodorsal, anterodorsal and anteroventral regions. Shell surface reticulate (except for anterior and posterior marginal areas which are punctate, and the sulcus which is almost smooth). An almost flat tubercle-like elevation is present in both valves in the subanterodorsal region anterior to the sulcus. The sulcus is deep, slightly curved and lies anterior to the middle. It is crossed by a median longitudinal bridge above and culminates at the upper ventral ridge. The lower ventral ridge is curved convexly downward and joins the ventral ridge anteriorly. Anterior and posterior areas (including the caudal process) are compressed. Anterior margin ornamented with 8 or 9 small tubercles. Internal details unknown.

Dimensions.
Remarks. This species differs from all other known species of the subgenus Eopaijenborchella by its subventral and subtriangular caudal process. The present species has so far been found in the Middle and Upper Eocene of the Rhaki Nala and Zao River sections of the Sulaiman Range, Pakistan, and from subsurface samples of Late Eocene from Rajasthan, India (Bhandari, 1991(Bhandari, , 1996. Paijenborchella (Eopaijenborchella) noar sp. nov. (Pl. 2, figs 5-8)

Derivation of name.
Greek, noar, phantom; with reference to the ghost-like remnants of the reticulation.
Diagnosis. Species of the subgenus Eopaijenborchella with mildly reticulate shell surface, fossae finely punctate, median bridge short and crossing over a shallow sulcus, valves with a flat anterodorsal tubercle lying above the sulcus, ventral ridge not well developed, but ending in a posteroventral tubercle.
Description. Sexual dimorphism present, females being wider in proportion than males. Carapace subrhomboidal in lateral outline. Anterior margin broadly and obliquely rounded, posterior with a short caudal process slightly above midline. Dorsal margin almost straight with a flat anterodorsal tubercle. Posterodorsal corner with an obtuse cardinal angle, ventral margin slightly convex. Left valve larger than right, overreaching it at anteroventral, anterodorsal and posterodorsal margins. In dorsal view the greatest width lies at the posterior third, in the region of the posteroventral tubercle. mildly reticulate, with finely punctate fossae. A wide and shallow sulcus lies in the anterior third. It is crossed by a short median bridge; ventrolateral ridge not well developed but culminates in a distinct rounded tubercle. Internal features not seen. Khosla & Pant, 1989, shows some resemblance to the present species, but has an upturned, well-developed, subdorsal caudal process and a wellmarked reticulated shell, absent from the present species. Neomonoceratina monocornuta Khosla & Nagori, 1989, has a shorter subdorsal caudal process and a pointed alar prolongation. The present species occurs only in the Upper Eocene of the Rakhi and Zao River sections. Description. Sexual dimorphism apparent, males being more elongate and not as high as females. Carapace subovate in lateral outline. Anterior margin broadly and obliquely rounded towards the venter, anteroventral margin with a flange (preserved in some specimens), posterior with a well-developed truncate caudal process. Dorsal margin almost straight with a posterodorsal concavity, ventral margin convex downward, but appears to be sinuate because of venterolateral ala. Greatest length passes through midpoint and greatest height at anterior third. Left valve greater than the right, over-reaching it at the dorsal and posterodorsal margins. Surface smooth. A shallow almost vertical sulcus which lacks a median bridge lies at the anterior third; it is shallower in the middle. Ventral ridge ends posteriorly in an ala which is rounded at its extremity. Internal features not seen.

Dimensions.
Remarks. This species somewhat resembles Paijenborchella caudata (Lienenklaus), Triebel, 1949 but lacks a median bridge. The caudal process in the present species is blunter at the posterior end and is situated at midline rather than being subventral. Paijenborchella (Paijenborchella) solitaria Ruggieri (Keij, 1966) also lacks a median bridge, but has a more prominent alar process and a longer caudal process than P. (P.) inponticulata sp. nov. The present species is currently known from the Middle Eocene of the Rakhi Nala section. This may be the oldest species of the subgenus.

STRATIGRAPHIC OCCURRENCE
Paijenborchella (Eopaijenborchella) species A from the Upper Paleocene of the Sor Range section (Fig. 2) is currently the oldest known species of the genus from Pakistan. All five of the newly described species occur in the Rakhi Nala section of the Sulaiman Range but only three of them are found in the Zao River section. Paijenborchella (Eopaijenborchella) angulosa is restricted to the upper part of the Lower Eocene. Paijenborchella (Paijenborchella) inponticulata occurs only in the Middle Eocene and has been found only in the Rakhi Nala section. Paijenborchella (Eopaijenborchella) viriosa and Paijenborchella (Eopaijenborchella) ventricaudata range through the Middle and Upper Eocene. Paijenborchella (Eopaijenborchella) noar occurs only in the uppermost Eocene. Since these Paijenborchella species have short vertical ranges, they make good horizon markers.

CONCLUSIONS
Paijenborchella is a marine unisulcate genus with a world-wide distribution. It ranges from the Upper Cretaceous to Recent. It is divided into two subgenera: Paijenborchella sensu stricto in which the surface is smooth and Eopaijenborchella in which the surface is ornamented. In the material from Pakistan only one species of Paijenborchella sensu stricto and five species of Eopaijenborchella occur. These range from the Upper Paleocene to the Upper Eocene.