Algeria has several exposed Cainozoic terrestrial deposits producing a range of fossils, but these have received relatively little attention, either for their faunal content or their stratigraphic significance. In order to correlate such deposits to one another it is useful to study index fossils (fossils with known chronological ranges and environmental preferences) obtained from these sites. For marine environments this is relatively easy, as a wide range of invertebrate groups produce easily preserved calcareous shells, but for terrestrial deposits this is much more difficult to a less diverse invertebrate fauna. One group that can be useful in terrestrial deposits are Snails, as these produce calcareous shells, are often easy to identify to species level, have a relatively high species turnover (i.e. individual species have a limited chronological age) and well understood environmental preferences.
In a paper published in the journal Annales de Paléontologie on 22 November 2018, Rodrigo Salvador of the Museum of New Zealand Te Papa Tongarewa, Mohammed Adaci of the Department Earth and Universe Sciences at Abou-Bekr Belkaid University, and Madani Benyoucef of the Faculty of Natural and Life Sciences at Mustapha Stambouli-Mascara University, describe the Land Snail fauna of the Plio-Pleistocene deposits exposed on the sea cliffs overhanging Beni Saf at Playa Port on the right bank of the Oued Sidi Boucif in the Orania Region of northwest Algeria.
These deposits form part of the Tafna Basin sequence, which begins in the Early Miocene, when the entire Orania Region was covered by the sea. The cliffs at Playa Port consist of four exposed units, the lower three of which are thought to be Pliocene and the highest, and most extensive, Pleistocene in origin.
The unit at the base of the cliffs (Unit A) is a conglomerate made up of mixed schist, quartzite, sandstone and limestone pebbles in a brown sandstone matrix, with bivalve fragments, about 24 m thick. Towards the top of this unit the conglomerate pebbles become restricted to channels, alternating with sandstone levels producing the Sea Urchin Clypeaster sp.. Above this is a second conglomerate unit (Unit B), about 5 m thick, containing angular clasts of Palaeozoic schist and quartzite and Messinian reef limestone, again in a brown sandstone matrix, which again becomes dominated by sandstone towards the top, with the conglomerate material reduced to channels and the sandstone containing more shelly material. The third unit (Unit C) is about 32 m thick and comprises a yellowish bedded sandstone with a variety of sedimentary structures, including wave-ripple cross-stratification, hummocky cross-stratification, mud-drape, and planar and herringbone cross-stratification, with a centimetre thick conglomerate layer, comprising rounded pebbles in a yellow sandstone, at the top.
The uppermost unit (Unit D) is about 35 m in thickness, and is divided into three subunits. The lowest of these is red, haematite rich sandstone, with palaeosol (fossil soil) horizons and pedotubes (root traces), as well as one Snail-producing level, identified as Gastropod level 1. Above this, the second unit comprises a yellowish fine-grained sandstone with mega-ripple (dune) structures, which yields no fossil material. The third, uppermost, subunit has a massive structure and a homogeneous grey appearance, with irregular stratification surfaces, with a thin calcrete layer at its upper surface. This subunit contains two further Snail producing layers, identified as Gastropod Level 2, close to its bottom, and Gastropod Level 3, close to its top.
These deposits form part of the Tafna Basin sequence, which begins in the Early Miocene, when the entire Orania Region was covered by the sea. The cliffs at Playa Port consist of four exposed units, the lower three of which are thought to be Pliocene and the highest, and most extensive, Pleistocene in origin.
The unit at the base of the cliffs (Unit A) is a conglomerate made up of mixed schist, quartzite, sandstone and limestone pebbles in a brown sandstone matrix, with bivalve fragments, about 24 m thick. Towards the top of this unit the conglomerate pebbles become restricted to channels, alternating with sandstone levels producing the Sea Urchin Clypeaster sp.. Above this is a second conglomerate unit (Unit B), about 5 m thick, containing angular clasts of Palaeozoic schist and quartzite and Messinian reef limestone, again in a brown sandstone matrix, which again becomes dominated by sandstone towards the top, with the conglomerate material reduced to channels and the sandstone containing more shelly material. The third unit (Unit C) is about 32 m thick and comprises a yellowish bedded sandstone with a variety of sedimentary structures, including wave-ripple cross-stratification, hummocky cross-stratification, mud-drape, and planar and herringbone cross-stratification, with a centimetre thick conglomerate layer, comprising rounded pebbles in a yellow sandstone, at the top.
The uppermost unit (Unit D) is about 35 m in thickness, and is divided into three subunits. The lowest of these is red, haematite rich sandstone, with palaeosol (fossil soil) horizons and pedotubes (root traces), as well as one Snail-producing level, identified as Gastropod level 1. Above this, the second unit comprises a yellowish fine-grained sandstone with mega-ripple (dune) structures, which yields no fossil material. The third, uppermost, subunit has a massive structure and a homogeneous grey appearance, with irregular stratification surfaces, with a thin calcrete layer at its upper surface. This subunit contains two further Snail producing layers, identified as Gastropod Level 2, close to its bottom, and Gastropod Level 3, close to its top.
(A) Panoramic view of the Beni Saf maritime cliff. (B) Lithostratigraphic log of the Plio-Quaternary Playa Port outcrop. (C) Closer view of Unit D showing the positions of the three Gastropod-bearing levels. Salvador et al. (2018).
The second Snail recorded is placed in the genus Leonia, but not assigned to a specific species as it cannot be confidently assigned to an extant species in the genus, though it resembles the modern Leonia mammillaris, which is found in Algeria, Morocco, and mainland. A single specimen of Leonia sp. is reported, from Gastropod Level 1 at Beni Saf. This is 22.7 mm high 10.9 mm wide and has about 6 whorls. The specimen is tall and narrow, and preserved as an internal mould with little actual shell remaining, though part of the second whorl is present and shows a pattern of five strongly marked spiral cords.
The first Snail recorded is Tudorella sulcata, a species still found in Algeria today, favouring rocky habitats with open vegetation, is also known from Pliocene deposits in Oran and Cap de Garde, and which is present in Gastropod Level 1 at Beni Saf. Two specimens of this species were found, measuring 18.8 and 20.5 mm in height, 14.0 and 14.5 mm in width, and having four and four-and-a-half whorls. These Snails have conical shells with strongly convex whorls that increase in size rapidly, and have deep sutures. The Beni Saf material comprises two internal moulds.
Tudorella sulcata moulds from Beni Saf, Algeria. Salvador et al. (2018).
The second Snail recorded is placed in the genus Leonia, but not assigned to a specific species as it cannot be confidently assigned to an extant species in the genus, though it resembles the modern Leonia mammillaris, which is found in Algeria, Morocco, and mainland. A single specimen of Leonia sp. is reported, from Gastropod Level 1 at Beni Saf. This is 22.7 mm high 10.9 mm wide and has about 6 whorls. The specimen is tall and narrow, and preserved as an internal mould with little actual shell remaining, though part of the second whorl is present and shows a pattern of five strongly marked spiral cords.
Leonia sp. from Beni Saf, Algeria. Salvador et al. (2018).
The next species reported is Rumina decollata, which is more abundant at Beni Saf, with five specimens reported from Gastropod Levels 1 and 2. These have an average height of 28.0 mm, an average width of 11.1 mm and five to five-and-a-half whorls. The whorls of these specimens are notably tall, with the body (final) whorl making up about half of the shell height, and have a broader shell and wider aperture than other members of the same genus. This species has a fairly long fossil record, being known from Miocene deposits in Algeria, and possibly Spain, as well as unconfirmed reports of the species from Oligocene deposits in Algeria. Today it is found around the Mediterranean Basin, including the north coast of Africa, and has been introduced to a number of other parts of the world. Rumina decollata can be found in a wide variety of environments, but appears to favour drier habitats, where it is typically more numerous.
Rumina decollata, adult, from Beni Saf, Algeria. Salvador et al. (2018).
Salvador et al. also record a second species of Rumina, Rumina atlantica, from Beni Saf, based upon six specimens from Gastropod Level 1. These average 32.4 mm in height and 12.1 mm in width, with about six-and-a-half whorls, so that the shells are broader than those of Rumina decollata, with more whorls, and these whorls are each shorter. This species has only previously been described from the Pliocene of Oran in Algeria.
Rumina atlantica, adult, from Beni Saf, Algeria. Salvador et al. (2018).
Eremina sp. from Beni Saf, Algeria. Salvador et al. (2018).
The fifth species of Snail recorded is Trochoidea trochoides, of which four specimens were obtained from Gastropod Level 1. These are 5.0-5.5 mm high, 5.1-5.4 mm wide and have about five whorls, and a pyramidal shape. Trochoidea trochoides is today found in dry areas around the western Mediterranean, and has been found in Quaternary deposits from Aïn-el Bey in Algeria.
Trochoidea trochoides from Beni Saf, Algeria. Salvador et al. (2018).
The sixth type of Snail recorded from Beni Saf is assigned to the genus Xerosecta, but not to a specific species due to poor preservation. Xerosecta sp. was found in Gastropod Levels 1 and 2, with a total of twelve specimens found. These shells are 6.4-7.4 mm high and 10.5-11.5 mm wide, with four-and-a-half to five whorls. The spire of the shells is almost flat, with rounded whorls with a slight keel and a rounded aperture. Species of this genus have previously been discovered from Pliocene deposits in Algeria, and are known in Iberia and Morocco today.
Xerosecta sp. from Beni Saf, Algeria. Salvador et al. (2018).
The seventh Snail from Beni Saf is assigned to the genus Cernuella, but again not to a specific species. Seventeen specimens of this Snail are reported from Gastropod Levels 1 and 2. They are about 6.5 mm high and 10.5-11.0 mm wide, with about 4.5 whorls. They have rounded whorls with a step-like spire, a deep suture and an elongate aperture. This genus has been recorded from the Pliocene of Oran, and is today found in dry calcareous habitats.
Cernuella sp. from Beni Saf, Algeria. Salvador et al. (2018).
The eighth Snail from Beni Saf is assigned to the genus Sphincterochila, but again not to a specific species, although the described specimens are notably smaller than all living members of the genus except one, Sphincterochila debeauxi, which is found in Algeria and Morocco. Two specimens of this Snail are recorded, both from Gastropod Level 2. They are 5.5 mm high and 7.5 mm wide, with four-and-a-half whorls, and are conical with an irregular whorl profile and a rounded aperture. This genus is recorded in deposits from Algeria from the Oligocene onwards.
Sphincterochila sp. from Beni Saf, Algeria. Salvador et al. (2018).
The ninth Snail recorded is assigned to the species Massylaea massylaea, a species found in Algeria today, but not in the northwest of the country (i.e. the area around Beni Saf); fossil specimens have previously been recorded from the Quaternary of Constantine, in the northeast of the country. A total of thirteen specimens are reported from all three Gastropod Levels at Beni Saf. These are 18.5-22.0 mm high and 28.0-33.5 mm wide, with four whorls, with a broad conical spire, a broad body whorl and a quadrangular aperture.
Massylaea massylaea from Beni Saf, Algeria. Salvador et al. (2018).
Salvador et al. also record a second species of Massylaea, Massylaea vermiculata, based upon forty specimens from Gastropod Levels 1 and 2. These shells are 20.0-23.0 mm high, 29.5-31.5 mm wide and have four to four-and-a-half whorls. Thes have more compact shells than Massylaea massylaea, with higher and more rounded body whorls and shorter spires. This species is found around much of the Mediterranean today, though not in western Algeria or Morocco, however, fossils have been found in Pliocene deposits in Oran.
Massylaea vermiculata from Beni Saf, Algeria. Salvador et al. (2018).
The eleventh and twelth types of Snail recorded from Beni Saf are placed in the genus Otala, since these can be told apart but not confidently placed in any known species or assigned to new ones, they are simply referred to as Otala sp. 1 and Otala sp. 2, with Otala sp. 1 being found in Gastropod Levels 1 and 2, while Otala sp. 2 is only found in Gastropod Level 2. Salvador et al. report 35 specimens of Otala sp. 1, ranging from 19.0-21.0 mm in height and 27.5-31.0 mm in width, with four to four-and-a-half whorls. These have large, flattened shells with rapidly expanding whorls. Otala sp. 2 is described as being notably smaller with a more globose shell, though no dimensions are given. Species of Otala are found today in Algeria, Morocco, and Spain, favouring dry, rocky habitats, and the genus has been found in Pliocene deposits at Oran and Late Tertiary deposits near Constantine.
Otala sp. 1, (G)-(I), and Otala sp. 2, (J)-(L), from Beni Saf, Algeria. Salvador et al. (2018).
The final Snails from Beni Saf are placed in the genus Eremina, but again not assigned to species level. Nineteen specimens of this Snail are recorded from Gastropod Levels 1 and 3. They are 21.0-30.0 mm high and 32.5-40.0 mm wide with four to four-and-three-quarter whorls, and have depressed spires, rapidly expanding whorls and a large aperture. This genus is widespread in North Africa today, living in dry rocky areas and in some cases on desert sand.
Salvador et al. note that almost all of the Snails recorded at Beni Saf are restricted to dry areas in distribution, but that the deposits in which they are found at Beni Saf are clearly aquatic in nature. Since the Snails are restricted to three horizons within these beds, they suggest that these beds represent rare flood evens which swept the Snails from their natural habitats.
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