First record of Tripneustes gahardensis (Seunes, 1896) in the Oran region (northwestern Algeria)

INTRODUCTION

The genus Tripneustes Agassiz, 1841 comprises several species of sea urchins with distinctive ecological niches and morphological adaptations. T. gahardensis (Seunes, 1896) provides valuable insights into the Miocene marine environments of the Mediterranean province. Previous studies have established its presence in various localities from southwestern Europe (France, Spain) but overlooked Algeria until the recent discovery of specimens in Oran. This paper focuses on filling this palaeogeographical gap by analysing the morphology and ecology of T. gahardensis (Seunes, 1896) in relation to its coeval marine environments.

The discovery of T. gahardensis in the Upper Miocene deposits around Oran provides new insights into the biogeographical and ecological heritage of this region. The taxonomic and morpho-functional analyses evidenced the significant adaptability of this species, encouraging further exploration of other localities from Algeria and enhancing a deeper understanding of its ecological role in ancient marine ecosystems.

The present study aims to present a taxonomic and palaeoecological investigation of T. gahardensis through biometric measurements and morpho-functional analysis. By comparing these newly collected specimens with previously documented specimens from France and Spain, we purported to search for their morphological affinities and ecological contexts. Through this comprehensive analysis, we intend to increase knowledge of the biogeography and evolutionary history of T. gahardensis within the broader framework of Miocene echinoids.

MATERIAL AND METHODS

The specimens at hand in the present study come from a quarry section located northwest of the Boutlelis village in the western Oran province. The newly collected specimens are housed at the Basin geodynamics and sediment balance Laboratory, University of Oran, under registration label: TR-G1 (Half test), TR-G2 (Fragment of inter- and ambulacral zones) and TR-G3 (Fragment of the peribuccal zone).

These are fragmentary material collected from algal limestone, consisting of half-test whose oral face displays a few spines with some partial well-preserved ambulacral areas; another specimen only showing the ambulacral and interambulacral areas and the last specimen documenting the ambulacral area of the peristomial region with the perioral notches. Systematics follows Kroh (2020). The specimen description combines both biometric and morphological analyses (Fig. 1). The dimensions were taken using a Vernier caliper (150x0.05 mm); the measures are expressed in millimetres (mm). The biometric data are reported in Table 1 and compared with those of Bajo & Borghi (2009). The standard nomenclature for test plating used herein follows Lovén’s method (Lovén, 1874). The drawings are made using Corel Draw Suite X6 software.

Fig. 1 Anatomical diagrams of a sea urchin test from top and side views, labeling diameter, height, and ambulacral zones.

GEOLOGICAL SETTING

The study area (Fig. 2) is located northwest of the Boutlelis village in the western Oran province, about 20 km of Oran city, Algeria (Global Positioning System coordinates: N 35°37’7.05” / W 0°52’35.79”). This area is characterised by its coastal location and diverse topography, featuring a series of prominent reliefs to the south of the Andalusian plain. The mountain ranges in this region are oriented along a NE/SW direction, Djebel Murdjajo being the most noticeable peak, rising to about 430 m. Other significant elevations include Jebel Djorf Lalia, reaching approximately 250 m and overlooking the M’sila forest, and Jebel Bougoug. The mountain ranges present steep slopes on their northern side, while gently sloping towards the extensive Sebkha (salt flat) area on the southern side. This topography reflects the complex geological history of the area, primarily associated with the pre-Miocene terrain. The landscape is also characterised by several Wadis (seasonal rivers), the most prominent of which include: Oued Bougoug, Oued Hammadi and Oued Madagh.

Fig. 2 A geological map series of Algeria showing a detailed survey area near Oran, complete with a color-coded stratigraphic legend.

GEOLOGICAL STRATIGRAPHY

In 1903, Gentil attributed a Pontian age, equivalent to the Messinian, to the sediments exposed on the southern slope of the M’sila forest (Fig. 2). Rouchy (1979, 1980, 1982) highlighted the presence of fringing coral reefs in the region and confirmed the Messinian age of the series. Rouchy et al. (1982), Saint Martin (1984) and Rouchy et al. (1986) specified the arrangement of the facies pattern within the context of the Messinian “crisis”. The work of Sadran (1952, 1958) addressed the characteristics of the Miocene volcanic activity on the Oran coast. Bellon et al. (1984) provided new chrono- and biostratigraphic data concerning this issue confirming the Messinian cycle. A micro-palaeontological study based on planktonic foraminifers from the Brachiopods Yellowish Marly Sandstone, interbedded within algal limestone, evidenced the presence of Globorotalia miocenica mediterranea “Catalano & Sprovieri, 1969” assigning a Messinian age to these deposits [identification performed by Dr Mahfoud Hadj Chrif, Chlef University, personal communication].

According to Saint Martin et al. (1995), the Upper Miocene stratigraphy in this region is divided into five distinct units, each representing different depositional environments and geological processes:

- First Unit: This unit indicates a transgressive phase characterised by bio-detrital limestones, primarily comprising the foraminiferal genus Heterostegina and the echinoids genus Clypeaster. This level can be observed in locations such as Wadi Sly near Chlef, Ain Oumata, the western sector of the Tessala village, the southern-western slope of Djebel Murdjajo, and at Cape Figalo.

- Second Unit: This unit includes deposits of Algal limestone or Marl-diatomite, reflecting a transitional environment exhibiting fluctuating depositional conditions.

- Third Unit: Representing a reef complex, indicative of the vibrant marine ecosystems that had thrived during the Miocene.

- Fourth Unit: Characterised by Whitish micritic limestones or Oolithic limestones, this unit provides evidence of varied sedimentary processes, likely influenced by changes in sea level and marine chemistry.

- Fifth Unit: This final unit is composed of Oolithic or Stromatolithic limestones, suggesting an environment that had fostered the formation of carbonate structures through both biological and physical processes.

The sequence (Fig. 4) exposed in the quarry working face (Fig. 3), displays mostly marine deposits, essentially composed of Whitish marls and Algal limestones with echinoid fauna, thus corresponding to the second unit according to the classification of Saint Martin (1995).

Fig. 3 Five field photos A to E showing a layered sedimentary rock outcrop with labels, rock hammer scales, and fossil-rich layers.

Fig. 4 A Messinian stratigraphic log with outcrop photos showing chalky marl, algal limestone, and calcarenites alongside a fossil and lithology legend.

The section starts with a thick bed (18 m) of Chalky whitish marl. This marly bed is overlain by algal limestones (6 m) interbedded by a Yellowish marly sandstone brachiopod bed [Terebratula sinuosa (Brocchi, 1814)] (Gentil, 1903; Ruggiero et al., 2008) (0.4 m). This bed also presents bryozoans [Calpensia nobilis (Esper, 1796)] (Gentil, 1903; Haddadi-Hamdane, 1996), incrusted on the brachiopods, alongside a fauna of bivalves [Aequipecten opercularis (Linnaeus, 1758); Nodipecten nodosus (Linnaeus, 1758) (Cosel & Gofas, 2019); Gigantopecten latissimus (Brocchi, 1814) (Ben Moussa, (1994)] and echinoids [Tripneustes gahardensis; Trachypatagus oranensis Pomel, 1887; Cidaris saheliensis (Pomel, 1887); and Echinolampas hayesianus Desor in Agassiz & Desor, 1947] all dispersed in the algal limestones.

The lithological succession terminates with a 0.60 m-thick, calcarenites bed. Noticeably, the transition from the Algal limestones to the calcarenites occurs across a ravinement erosion surface, an unconformity formed by erosional processes linked to sea-level fluctuations.

SYSTEMATIC PALAEONTOLOGY

Phylum Echinodermata Klein, 1734

Class Echinoidea Schumacher, 1817

Subclass Euechinoidea Bronn, 1860

Infraclass Carinacea Kroh & Smith, 2010

Crown group Calycina Kroh & Smith, 2010

Echinacea Claus, 1876

Order Camarodonta Jackson, 1912

Infraorder Echinidea Kroh & Smith, 2010

Section Odontophora Kroh, 2020

Family Toxopneustidae Troschel, 1872

Genus Tripneustes Agassiz, 1841

Type-species: Echinus ventricosus Lamarck, 1816, by original designation (Agassiz, 1841: 7).

Tripneustes gahardensis (Seunes, 1896)

1884 Hipponoë sp. Bazin, p. 36 pl. I, figs. 22-25.

1896 Hipponoë parkinsoni var. gahardensis Seunes, pp. 84-86, pl. I, Fig. I.

1906 Tripneustes gahardensis (Seunes), Lambert, pp. 75-77, Fig. I, pl. 5, figs. 12-13.

2009 Tripneustes gahardensis (Seunes), Bajo et al., pl. I, Fig. 1a-c, pl. II, 1c, 2a-c. al.

Plate. I Fossil sea urchin specimen 1 A, 1 D with magnified views 1 B, 1 C showing clusters of fossilized spines, complete with 10 millimeters and zoom scales.

Plate. II: Close-up views 1 A, 2 B, and a full fragment 3) of a fossil echinoid test, showing ambulacral pores and primary tubercles with 10-millimeter scales.

Diagnostic features

Test: Large size (D) ranging from 151 to 155 mm, with an average diameter of 153 mm (based on 2 measurements deduced from the curvature of the ambital area). It displays a circular outline with a sub-conoidal appearance in lateral view. The oral face is slightly concave and displays a slight depression around the peristome (at the edge of the opening). The average thickness of the plates at the ambital zone is 2.6 mm.

Ambulacra: Composed of three rows of isopores (separated by granules). The adradial and perradial rows are straight, while the middle row displays a zig-zag pattern (Fig. 5). The rows are separated by non-perforated, and non-crenulated secondary tubercles with extended platforms, decreasing in size towards the peristome. The interporiferous zone consists of two rows of well-developed, non-perforated, and non-crenulated primary tubercles along the perradial zone and the median suture. These two rows of primary tubercles are flanked by two rows of less-developed secondary tubercles, one at each side, also decreasing in size towards the peristome, with size comparable to those of the primary tubercles in the ambital zone. The poriferous zone tightens towards the peristome, lined with miliary granules of irregular size along the perradial suture.

Fig. 5 An anatomical diagram of half ambulacral plates, detailing tubercles, pores, sutures, and a 10-millimeter scale bar.

Interambulacra: Comprises rows of non-perforated, non-crenulated, vertical and horizontal primary tubercles. In the ambital region, up to six tubercles are present on each plate, reducing to two tubercles in the vicinity of the apical disc and peristome (Fig. 6). Similar trends are observed for secondary tubercles that decrease in size towards the peristome.

Fig. 6 An anatomical diagram of sea urchin interambulacral plates, detailing tubercles, areole, sutures, and a 10-millimeter scale bar.

Very dense miliary granules of irregular-size are found around scrobicles (small depressions or pits) of both primary and secondary tubercles. Buccal notches are deeply indented and bordered on the perradial side by a decreasing row of vertical tubercles near the peristome (Fig. 7; Plate II, 3).

Fig. 7 An anatomical diagram of sea urchin plates, highlighting ambulacral pores, primary tubercles, buccal notches, and a 10-millimeter scale bar.

Primary spines: These are long spines attached to the oral face of the test, exhibiting longitudinal streaks, with long and slender shaft. The base of the spine is about as wide as the basal part of the shaft, only the ring being wider (Plate I; 1A, 1B, 1C).

Based on the morphological characteristics mentioned in the description, the Algerian Tripneustes specimen corresponds to T. gahardensis for several reasons, especially its much larger diameter, reaching 150 mm, whereas T. planus measures 98 mm (Philippe, 1998). The primary and secondary tubercles of T. gahardensis are similar in size in the ambital zone, which is more uniform than that of T. planus. T. gahardensis has a much higher number of tubercles in the ambital zone while retaining the same size. In T. planus, the interambulacral zones are depressed on the aboral side (Philippe, 1998), a characteristic absent in T. gahardensis. The interambulacral plates of T. planus are proportionally higher: in the ambital zone, they reach an average H/L ratio of 0.31 and 0.53 towards the apex, compared to 0.21 and 0.32 in T. gahardensis (Bajo & Borghi, 2009). In Algerian specimens, these ratios are 0.21 at the ambitus and 0.37 towards the oral zone. Finally, T. planus has a much lower profile than T. gahardensis.

T. ventricosus (Lamarck, 1816) is the species most closely morphologically related to T. gahardensis in terms of test dimensions, but differs in the primary tubercles of the interambulacra, which reach areas closer to the apical apparatus in T. gahardensis. In T. ventricosus, the density of small secondary and miliary tubercles is higher than in T. gahardensis. The high density of miliary tubercles sometimes results in the absence of bare areas in some specimens, particularly in the buccal region of the echinoid (Mortensen, 1943). Bare areas in the interambulacra on the aboral surface of T. ventricosus are very rare (Mortensen, 1943), whereas they are always present in T. gahardensis. The ambulacral areas near the peristome are wider than in the interambulacral areas in T. ventricosus (Mortensen, 1943), whereas in the study specimens of T. gahardensis, the ambulacral areas are always slightly narrower than the interambulacral areas.

Kroh’s (2005) work on T. cf. ventricosus from the Langhian of Austria evidenced several morphological similarities with T. gahardensis, particularly in terms of tubercle distribution, nearly bare areas and tuberculation density. However, Kroh emphasised that T. cf. ventricosus would have had a tall, almost conical shell, a feature that does not align with the morphology of T. gahardensis. Taking these observations into account, alongside the overall morphological characteristics of the specimen, its attribution to T. gahardensis is further supported and refined.

DISCUSSION

The history of classification and nomenclature of echinoid species within the genus Tripneustes reflects a complex interplay of scholarly contributions. To summarize and clarify the key points from the provided text:

Bazin (1884) initially illustrated the ambulacral and interambulacral plates of a Tripneustes (Hiponae) but did not attribute it to a specific species. Subsequently Seunes, (1896) was the first to establish the taxon Tripneustes gahardensis, under the original name Hiponae parkinsoni var. gahardensis. Lambert (1906) transferred the species gahardensis to the genus Tripneustes, because he considered that the genus Hipponae, which was overtaken by that of Hipponoa Audouin, was a nomen nudum and that it was only published in 1855 while Tripneustes is fourteen years older, by Lambert (1910) considered Tripneustes parkinsoni to be closely related to T. gahardensis by the homogeneity of its tubercles, the extreme width of its ambulacra and the development of its secondary tubercles at the ambitus. He separated T. gahardensis from T. planus despite attempts by Cotteau (1877) to unify them, citing differences such T. planus’s flatter lower face, non-inflated interambulacra, narrower peristome, the equal size of primary tubercles, and its finer granules arranged around the tubercles. Philippe (1998) expressed doubt regarding the similarities between T. planus and T. gahardensis but refrained from making a definitive decision due to the scarcity of palaeontological material available for study. Bajo & Borghi (2009) noted that the Seunes collection is currently held at the University of Rennes (France), but the holotype of T. gahardensis is missing, the “neotype” informally cited by Lambert (1906), which belonged to the Almera collection and is partially preserved at the Natural Science Museum in Barcelona, is also considered missing. These factors would require the establishment of a new neotype, in accordance with the provisions of Article 75, paragraph 3.6 of the International Code of Zoological Nomenclature (ICZN).

Based on measurements and comparison, it appears that the species T. gahardensis can indeed exceed sizes previously recorded in former studies. The references to maximum sizes from Lambert (1906) and Bajo & Borghi (2009), suggest that there has been a consistent upper limit noted for this species, with the largest verified specimen being 98 mm. However, our estimates from the half test fragments at hand indicate that T. gahardensis may reach larger sizes, with 152 mm and 155 mm respectively.

The respective ratios (H/D, Amb/D, Iamb/D, P/D) also document similar values between T. gahardensis from the Oran region and T. gahardensis previously documented by Bajo & Borghi (2009) from Spain (Table. 1). The consistency of all these ratios with the data provided by Bajo & Borghi (2009) (Table. 1) provides strong evidence for the increasing size of T. gahardensis beyond the formerly known size records.

ECOLOGICAL INTERPRETATIONS

The genus Tripneustes appear to be a generalist in habitat and food in tropical and subtropical, shallow water habitats subject to disturbance (Lawrence & Agatsuma, 2013).

T. ventricosus is distributed across shallow waters from Bermuda and southern Florida throughout the West Indies down to Brazil, including Trinidad and Ascension Islands, as well as along the west coast of Africa from the Gulf of Guinea to Walfish Bay (Mortensen, 1943). In Brazil, its range extends to Rio de Janeiro (Tommasi, 1972) and it has been recorded in Panama (Lessios, 1985) and Quintana Roo (Caso, 1974). Serafy (1979) reported its presence in both the southwestern and southeastern Gulf of Mexico. T. ventricosus and T. variegatus inhabit various environments, often associated to habitats differing in the proportion of turtle grass (Thalassia testudinum) versus coral rubble or shell. T. ventricosus is most common in very shallow waters, although individuals have been found at depths up to 30 m, with T. gratilla reaching depths of 75 m (Mortensen, 1943). They both occur in habitats such as seagrass beds, algal beds, sand with rubble, rocks and coral reef flats. Ogden & Lobel (1978) observed T. ventricosus occupying algal beds with high hydrodynamics in the Caribbean, while Aseltine (1982) reported its presence within seagrass beds and limestone shelves in the Bahamas, particularly where water flow was strong. Lilly (1975) noted T. ventricosus was most common at the seaward edge of grass flats in Barbados, whereas Moore et al. (1963) found it mainly confined to the outer edges of seagrass beds in Miami. However, Aseltine (1982) documented its occurrence within seagrass beds at the Bahamas. The distribution of tripneustids is limited to tropical and subtropical regions by temperature constraints, with T. ventricosus found in southern Florida at temperatures between 20 and 31 °C (McPherson, 1965). Regarding diet, Lewis (1958) reported that the guts of T. ventricosus in Barbados almost always contained algae, sometimes mixed with sand. Maharavo et al. (1994) suggested that the diet of T. gratilla is influenced by food availability. Tertschnig (1989) observed that T. ventricosus fed even when completely covered by fragments of algae, seagrass or small pieces of coral, though less so when moving, indicating behavioural adaptations related to feeding in different cover conditions.

In our study area, T. gahardensis was found in Messinian deposits within algal limestones, (Spanish and French T. gahardensis occur respectively in clayey silt, and molassic sediments). It was associated with echinoids, bivalves and brachiopods, which are indicative of an infralittoral environment. As with its modern relatives, particularly T. ventricosus, T. gahardensis favoured algal habitats that provided shelter and food. These environments are influenced by temperature and hydrodynamics. As with its current congener, T. ventricosus, which inhabits tropical and subtropical regions, it is highly probable that T. gahardensis was also affected by similar climatic conditions. Its large size reflects a rapid growth.

The morphological adaptation of T. gahardensis, like its current congeners, reflects its evolutionary adjustments to thrive in relatively turbulent environments, characterised by higher temperatures and firm substrates, often enhanced by the presence of seagrass or algae. As a result, T. gahardensis, is commonly found in subtropical climate, typically inhabiting coastal to infralittoral zones.

CONCLUSION

T. gahardensis is a rare echinoid species found within the Miocene outcrops of the Oran region. Its first occurrence in the Miocene deposits of northwestern Algeria upgrades former knowledge about this species and enhances our understanding of its palaeogeographical distribution. This discovery also provides valuable insights into the palaeoecological and palaeo-environmental conditions of the Mediterranean Basins, shedding light on the evolutionary history of marine ecosystems in the region.