Fossil Fish Teeth in Phosphatic Series of Jebel Dyr (Algerian-Tunisian Border Area)

The Thanetian phosphatic series of Jebel Dyr syncline, northeastern Algeria, are analyzed for their paleontological content. Materials were concentrated by sample washing and outcrops surface collecting of friable phosphorites facies, yielding a multitude of phosphatised fish teeth rich in species of variable morphology, representing 28 species of Elasmobranchii. The fish fauna indicates tropical to temperate paleo-environmental conditions. Most of the species represent benthic/nektonic forms of coastal areas with neritic and midwater depths of the continental slope.


Introduction
The Thanetian phosphatic series of the Algerian-Tunisian border region and their paleontological contents are still insufficiently known, despite the contributions of [1] [2] [3].
The fish fauna characterizing these rocks includes a multitude of species represented by teeth and some isolated vertebrae. We have studied tens of samples collected from phosphatic outcrops of the southeastern flank of Jebel Dyr ( Figure 1). For the first time, we characterise and establish a general systematic overview of the many fish (elasmobranchs). Furthermore, the assemblages are analysed in order to reconstruct the paleoenvironment during this upper Paleocene period individualized by phosphatogenesis in the area.

Methods and Materials Used
Jebel Dyr sector research was carried out during 2015-2016. The sampling of Thanetian phosphatic layers (total rock) was systematically conducted on outcrops of three sections crossing the southeastern slope of Djebel dyr. The hard layers, with dolomite cement, make the collection of teeth impossible by existing means, therefore, were not sampled. Concurrently, direct investigations for teeth were carried out on the field. However, the most important collecting of teeth materiel was completed after washing the sediment in the Laboratory of Geosciences at the Tebessa University in Algeria. Clean residues were obtained from pre-dried sediment samples washed over a standard set of nested sieves (the last sieve used of a fine mesh is 0.1 mm) after disaggregation in tap water mixed with a wetting agent (detergent). Some samples need subsequent treatment with 5% -10% hydrogen peroxide (H 2 O 2 ), neutralised with a few drops of ammonia. Fossils were picked from dried residues under a binocular microscope.
Photographs and measurements were taken, where appropriate, to aid in identification. Specimens were subsequently identified to genus level.
Unfortunately many Elasmobranchii teeth are incomplete or broken (existing of just crown or part of the root). These teeth are not resistant to postmortems mechanical damage. So, many of these cannot be identified or determinable to the family level only.

Results and Discussion
Fieldwork through three parallel sections (Figure 1(b)), carried out on the outcrops of Djebel Dyr formation (Figure 2(a)), which shows sub-tabular layers, as well as the laboratory tasks, allowed us to combine and finalize a stratigraphic log for this site (Figure 2(b)). This lithostratigraphic column established shows a formation revealing 08 layers of phosphates of Thanetian age, relatively friable of decimetric order, alternating with layers of limestone and marl-limestone and two layers of hard phosphates supported in Ypresian flint limestone to at the top.
The Thanetian fish assemblages of sampled phosphate of Jebel Dyr formation include many remains cartilaginous fish. Twenty-eight species (Elasmobranchii) are identified (Table 1), (Figures 3-6), twenty-three of which are sharks (Euselachii), however only five are rays (Batoidea). All together, they represent eighteen families of eight orders (Table 1). Systematic specification of these fossil groups has not been published previously for the Paleogene basin of the Algerian-Tunisian border area.
The Lamniformes with 9 species and ~25% of the total number of collected spe-       Fish with this dentition prefer a benthic lifestyle.   • Grinding type in Dasyatdae. Teeth are high crowned and of polygonal shape, closely intertwined and forming a real dental plaque with an almost flat surface. This type is found in fishes preferring benthic habits where they can feed on prey with hard resistant envelopes. • Clutching-Grinding type of Heterodontidae. Anterior teeth cusps generally provided with lateral denticles are of clutching type, while the lateral teeth, with massive spreading crown more or less cambered are of grinding type.
The genera with dentition type prefer benthic habits and live mainly on hard bottoms.
In summary, a relationship between dentition type and living environment, can observed as following: • The Tearing type is specifically confined to the Lamniformes, confined to coastal and epi-bathyal areas.  Tearing type; large dagger-shaped teeth, long and tapering, often with two small, sharp denticles next to the main crown; small and strong tapering in Odontaspis. The crown is smooth, the root is short with spaced lobes in Brachycarcharias, or long and close together to Carcharias.

MITSUKURINIDAE (Goblin sharks)
Tearing type; small unsymmetrical teeth, thrown towards the commissure with a blunted apex; wide and low denticles; root large and relatively flat with a deep groove (Striatolamia). Uncommon

OTODONTIDAE (Megatoothed sharks)
Cutting type; Teeth large robust with unsserrated triangular crown, smooth cutting edges and roughly triangular side cusps (Otodus). Cretolamna teeth have a compressed root which has a rectangular looking shape. The main cusp is broad and there are two stocky triangular shaped cusplets.

LAMNIDAE (Mackerel sharks)
Cutting type; Teeth slack cusplets and have thick, but mesio-distally compressed, roots with nearly vertical lateral margins and a straight cusp. The lingual protuberance bears an elliptical foramen (Isurolamna). Uncommon

CARCHARHINIDAE (Requim sharks)
Cutting-Clutching type; teeth are generally small, inclined to a sharp notch between the posterior dentil and the rest of the crown (Adounia). In general, the crown is blade-like and distally directed. The mesial cutting-edge continues on to the shoulder and bears weak serrations and can be strongly serrate with up to four cusplet-like serrations (Physogaleus).

JAEKELOTODONTIDAE
Tearing type, Mennerotodus teeth have a fine and high main cusp and lateral cuspids that are not well developed.
They are distinguished by the presence of fine crenulations between the crown and the lateral denticles. Rare

TRIAKIDAE (hound sharks)
Clutching-crushing type: Small ring of teeth flat main cusp highly developed and curved. The denticles decreasing from top to bottom edge (Galeorhinus). The Mustelus teeth are characterised by a crushing type dentition. They are slightly asymmetrical and transversely elongated, with a reduced, distally directed cusp. The crown is as high as the root. Crushing type; small triangular tooth, its root behind rejects the crown angles. The base of the root has a rhombic shape (Squatina). Rare

HETERODONTIDAE (bullhead sharks)
Clutching-grinding type: flat teeth and stretched, without sharp cusps. The labial face of the crown extends over the splayed root lobes and the apron may bear ornamentation (Heterodontus). Uncommon

GINGLYMOSTOMAT1D AE (Nurse sharks)
Clutching type: teeth generally small, the number of denticles can multiply gradually and laterally. Clutching type, Teeth fairly compressed laterally to high cusp, pointed, inclined inwards. The outer face of the crown is quite convex. The inner medial protuberance is well developed, long and rather slender at its end. The root is tall, squat, flat basal face in profile, but concave anteriorly in labial view (Squtiscyllium).

MYLIOBATIDAE (Eagle rays)
Crushing type; Online teeth, dentition plates or real millstones. Palace mills high crown of polygonal contour. The indented portion is the root of the tooth, the thin crown, is a smooth portion and hard (Myliobatis). Rare

DASYATIDAE (Whiptail stingrays)
Grinder type: tiny teeth about 2 mm, ornate. The forms found are loops. The root is higher in profile (Dasyatis). Uncommon

MOBULIDAE (Mantarays, Devilrays)
Cutting-Clutching type; Teeth Archaeomanta are easily recognized by their peg-like design and bulbous root. Unlike the living manta ray, the crown rises directly from the root, bears a median ridge and is fully covered with enameloid.

RAJEDAE (skates fish)
Crushing type, teeth usually small shaped like inverted trumpets. The anterior and lateral have a cusp more or less elongated (Raja). Rare

PRISTIDAE (Sawfish)
Rostral "teeth" (referred herein as spines). The tooth is longer than broad, the crown is globular & rounded, has a transverse crest. In general, elongated rostral spines with sharp or smoothly rounded posterior edges tend to be channeled ones (Pristis). • The crushing, grinding, grinding/clutching and clutching types are essentially restricted to benthic and nektic forms of the neritic zone. Table 3 specifies climatic, environmental and bathymetric preference of the species represented in the phosphate series of Jebel Dyr, allow deductions and Table 3. Environmental preferences of the thanetian ichthyological assemblage recovered from Jebel Dyr phosphatic layers (Algerian-Tunisian border area). of sharks and rays demonstrates shows their biostratigraphic potential.

Rare
Finally, given the geological data and the absence of the purely pelagic and bathyal forms, it can be assumed that this Thanetian assemblage occupied a relatively narrow gulf. The last was between mainland and open sea, agreeing with the paleogeographical interpretation of [33]. It indicating that Tebessa (Dyr) area was immersed between the Algerian promontory and the island of Kasserine ( Figure 7).

Conclusions
1) The Jebel Dyr phosphatic serie yielded a wealth of fish remains with many  predators, represented by 28 species of Elasmobranchii (Euselachii and Batoidea).
2) The fossil forms of the Elasmobranchii recognized in these phosphate layers confirm the Thanetian age for this formation of Jebel Dyr.
3) Dentition types mainly characterize benthic and nectic forms of the neritic littoral.
4) The fish fauna indicates a marine, coastal, shallow water environment with both rocky and sandy bottom, and temperature to subtropical climatic conditions. We conclude that Jebel Dyr fish fauna preferred inhabit the neritic zonal environments, which gave the highly opportunistic predator ability to prey on various animal groups, such as molluscs (bivalves, gastropods and cephalopods), crustaceans, echinoderms, annelids, sipunculids and fish. The wealth of biota on the continental shelf and upper slope contrasts strongly to the pelagic or bathyal zones, where potential food is less abundantly available, more dispersed and less varied. Open Journal of Geology helping in fieldwork. We are grateful for the help received from Hassan A. in his technical assistance in laboratory. The contributions and facilities of Mr. Omar the head of the Department of Earth Sciences and the Universe, Tebessa University are also much appreciated. We especially thank M. Arie W. Janssen, Naturalis Biodiversity Center, Netherlands for his critically reading the manuscript and mainly his linguistic improvement. Finally, we warmly thank M. Adnet S., Montpellier University, France for his improvement to the manuscript and the redetermination of some species.