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1. Introduction
Mollusks are cosmopolitan invertebrates. Bivalves and gastropods live mainly in marine and freshwater environments (Donovan and Hensley, 2003; Checa et al., 2009; Smitha and Mustak, 2017). In Mexico, the fossil record of bivalves and gastropods date from the Paleozoic (Quiroz-Barroso and Perrilliat, 1997), and the Mesozoic (Stanley et al., 1994). In particular, the fossil record of these mollusks is more frequent in Cretaceous deposits from Baja California, Nuevo León, Chihuahua, Coahuila, San Luis Potosí, Zacatecas, Querétaro, Jalisco, Michoacán, Guerrero, Puebla, and Chiapas (e.g. Vega and Perrilliat, 1990; Perrilliat et al., 2008; Vega et al., 2019). Despite the presence of Campanian continental mollusks in Coahuila, these are scarce in the Olmos Formation. The only formal report is that of the gastropod Tympanotonus cretaceus from the uppermost Olmos Formation (Perrilliat et al., 2008). T. fuscatus is a brackish gastropod, living nowdays in mangroves and lagoons along the West African coast (Dockery, 1993; Bandel and Kowalke, 1999; Reid et al., 2008). The Olmos Formation includes different sub-environments such as swamps, flood plains, interlocking rivers, and meandering rivers, derived from the epicontinental Cretaceous sea present in North America (Estrada-Ruiz et al., 2013). In addition, the formation has a great diversity of life forms, mainly composed of fossil plants similar to those found in tropical and paratropical rainforests (Estrada-Ruiz, 2009; Estrada-Ruiz et al., 2007, 2010, 2011; Centeno-González et al., 2019, 2021). Other organisms preserved in fine-grained sandstones of the Olmos Formation include fragments of Theropoda, Tyrannosauridae, Ankylosauria, Ceratopsidae, cf. Chasmosaurus sp., Hadrosauridae, tracks of turtles, crocodiles and small birds (Ojeda-Rivera et al., 1968; Silva-Bárcenas, 1969; Meyer et al., 2005; Torres-Rodríguez et al., 2010; Porras-Múzquiz and Lehman, 2011; Ramírez-Velasco et al., 2014; López-Conde et al., 2021).
We studied the freshwater mollusks from a coal-mine of the Olmos Formation in Coahuila, NE Mexico. Our principal aim was to contribute to the knowledge of the molluscan shells in the Upper Cretaceous of northern Mexico. Some plants were found associated to the mollusks here described.
2. Geological setting
The Sabinas Basin includes mainly Upper Cretaceous strata subdivided in several lithostratigraphic units, among which the Olmos Formation includes of coal deposits extending to the north of Piedras Negras and the south of Monclova, between parallels 26° and 28° North and meridians 100° to 102° West (Weber, 1972) (Figure 1). The Campanian age of this formation is based on presence of ammonites (Flores-Espinoza, 1989), along with the presence of the lower Maastrichtian bivalves Exogyra costata and Pycnodonte mutabilis, collected from the base of the overlying Escondido Formation in Piedras Negras, Coahuila. The age was corroborated by means of U-Pb zircon methods (Campanian 76.1 ± 1.2 Ma) (González-Partida et al., 2022).
Figure 1 Geographic location of Coahuila, Mexico, and sites where fossils were collected. A. Mexico, Coahuila is remarked in black. B. Coahuila, star = Tajo La Lulú, where most of the mollusks were collected.
The Olmos Formation has a composed thickness of approximately 540 meters (Flores-Espinoza, 1989). It is represented by coal and a minor proportion of gray shale, carbonaceous shale, siltstone, and fine- medium-grained sandstone with parallel lamination and cross-stratification (Corona-Esquivel et al., 2006; Estrada-Ruiz, 2009). Initially, it was divided into five units (Robeck et al., 1956). Later, based on cores and stratigraphic and sedimentological studies from different quarries, the Olmos Formation was classified in two systems (Flores-Espinoza, 1989). The first and lower one is a delta plain, while the second and upper one consists of a river plain with river facies and flood plains (Flores-Espinoza, 1989). Later, Estrada-Ruiz (2009) described four depositional sub-environments: 1) Lithofacies A rich in coal, suggesting that they correspond to marshy areas with restricted circulation. 2) Lithofacies B composed of shales and sandstones, which might represent floodplain environments and/or lagoons with open circulation. 3) Lithofacies C from fluvial environment, probably with intertwined rivers, as suggested by the geometry of the sand bars and canal fillings. 4) Lithofacies D with sandstones with cross-stratification, interpreted as channel infills and lateral bars deposited in a meandering river (Flores-Espinoza, 1989; Estrada-Ruiz, 2009), where dinosaurs and Tympanotonus cretaceus were reported.
3. Material and methods
A total of 44 samples with bivalves and gastropods were collected in sediments of the Olmos Formation in an open pit or coal mine known as Tajo La Lulú, with coordinates 27º 55’ 36.6 “N and 101º 11’ 30.2” W (Figure 1). All the samples come from a layer overlying the coal mantle, from where leaf and fruit impressions have also been reported (Estrada Ruiz, 2009). The material is deposited in the Paleontological Collection of the Instituto Politécnico Nacional, Mexico City, under the acronym IPN-PAL. The fossils were cleaned with Dremel 290-01 electric hammer. For taxonomic recognition, the fossils were described according to their morphology. Later, we made a morphological comparison with similar fossil gastropods and bivalves (Lucas et al., 1995; Perrilliat et al., 2008; Taparila and Roberts, 2013). For a reliable identification, we used only complete specimens preserving ornamentation and other morphological features. The paleoenvironmental interpretation was based on the identified mollusks and comparison with close living representatives.
4. Results
Systematic Palaeontology
Class Gastropoda Cuvier, 1797
Order Caenogastropoda Cox, 1959
Superfamily Ampullaroidea Gray, 1842
Family Viviparidae Gray, 1847
Subfamily Viviparinae Gray, 1847
Genus Viviparus Montfort, 1810
Viviparus sp.
Figure 2 Samples of different freshwater species from the Olmos Formation. A. Viviparus sp. (IPN-PAL 207). B-D. Mesolanistes sp. (IPNPAL 208, IPN-PAL 209, IPN-PAL 210). E. Pseudolimea sp. (IPN-PAL 211). F. Proparreysia sp. (IPN-PAL 212). G, H. Unio sp. (IPN-PAL 213, IPNPAL 214). I. Unionelloides? sp. (IPN-PAL 215). J-R. Plesielliptio sp. (IPN-PAL 216 - IPN-PAL 224.). S. Ostrea sp. (IPN-PAL 225).
Description. Small, pyramidal shell, only body and second whorls preserved; shell smooth, suture between first and second whorl weakly marked.
Material. One specimen, IPN-PAL 207.
Measurements. Length = 26.2 mm, width = 22.3.
Observations. Viviparus was very abundant in Cretaceous and Paleocene deposits of North America, specially in the Parras Basin, with specimens preserved in situ in red layers (delta plain) of the Paleocene Las Encinas Formation (Perrilliat et al., 2008).
Order Basommatophora Schmidt, 1855
Superfamily Lymnaeoidea Rafinesque, 1815
Family Physidae Fitzinger, 1833
Genus Mesolanistes Yen, 1945
Mesolanistes sp.
Description. Medium to large, dextral shell, involute, with four convex whorls, the outer one being the most prominent; acute anterior, posterior almost flat; shell surface with longitudinal lines.
Material. Three specimens, IPN-PAL 208, IPNPAL 209 and IPN-PAL 210.
Measurements. IPN-PAL 208, length = 29.4 mm, width = 32.4 mm, height = 13.3 mm (Figure 2B); IPN-PAL 209, length = 59.8 mm, width = 31.7 mm, height = 18.6 mm (Figure 3B); IPN-PAL 210, width = 18.3 mm, length = 15.0 mm (Figure 2D).
Figure 3 Some specimens of the flora recorded in the Olmos Formation; A. Palm leaf; B. Fern; C. Conifer leaf; D. Angiosperm leaf and aquatic fern Salvinia sp.; E. Aquatic fern Marsilea mascogos Estrada-Ruiz et al. 2018; F, G. Two fruits, a complete capsule and other inclomplete fruit of angiosperm.
Observations. Mesolanistes was reported in deposits of the Upper Cretaceous of Sonora and other lithostratigraphic units in North America. In the Cerro del Pueblo Formation (late Campanian) it is very abundant, both in river deposits (green layers) and in marshes and lakes (Lucas et al., 1995; Perrilliat et al., 2008).
Class Bivalvia Linnaeus, 1758
Order Pterioida Newell, 1965
Superfamily Limoidea Rafinesque, 1815
Family Limidae Rafinesque, 1815
Genus Pseudolimea Arkell, 1933
Pseudolimea sp.
Description. Triangular shell, ornamented by strong and scaly radial ribs quite numerous.
Material. A left valve, IPN-PAL 211.
Measurements. Length = 24.6 mm, width = 18.0 mm.
Observations. Given the poor preservation of the specimen, it is likely that it has been transported from the coastal area.
Order Unionoida Gray, 1854
Suborder Unionidina Gray, 1854
Superfamily Unionoidea Rafinesque, 1820
Family Unionidae Rafinesque, 1820
Genus Proparreysia Wanner, 1921
Proparreysia sp.
Description. Medium, elongated, convex semitriangular shell; concentric growth lines, very fine, little marked; umbo weakly inclined.
Material. Left valve, IPN-PAL 212.
Measurements. Length = 39.0 mm, width = 41.6 mm, height = 15.2 mm.
Observations. The genus is represented by several species in the Kaiparowits Formation (Campanian) of Utah (Taparila and Roberts, 2013).
Genus Unio Philipsson, 1788
Unio sp.
Description. Elongated, large, suboval shell, moderately compressed laterally, weakly marked umbons and located in the anterior third of the shell, which has fine growth lines: strongly marked hinge, with teeth and pits evident in a specimen that preserves the left leaflet. Ligament marks preserved posterior to hinge.
Material. A left valve (IPN-PAL 213) and an articulated specimen (IPN-PAL 214).
Measurements. IPN-PAL 213, length = 30.7 mm, width = 50.0 mm (Figure 2G); IPN-PAL 214, length= 40.0 mm, width = 56.9 mm (Figure 2H).
Observations. Unio is common in Mesozoic and Cenozoic freshwater deposits around the world. Although there is similarity with the specimens reported by Lucas et al. (1995) for the Cabullona de Sonora Group, the specimens of the Olmos Formation present a smoother and less compressed shell in ventro-dorsal view.
Genus Unionelloides Gu, 1962
(sensu Fang et al., 2009)
Unionelloides? sp.
Description. Globose shell, surface covered by wide concentric growth lines; small umbons, inclined almost 180 degrees with respect to the dorsal portion; anterior region almost straight, posterior rounded; ventral margin rounded to triangular; small hinge.
Material. A left valve, IPN-PAL 215.
Measurements. Length = 39.3 mm, height = 43.3 mm, width = 18.4 mm.
Observations. Although the valve shows a certain degree of deformation, the characteristics seem to coincide with the genus.
Genus Plesielliptio Russell, 1934
Plesielliptio sp.
Description. Medium, slightly elongated, subtriangular shell; umbons located a third of the longitudinal distance from the anterior margin; ornamentation of fine concentric lines; hinge made up of pits and teeth inclined to transversal.
Material. Eight right valves and one left. IPNPAL 215, IPN-PAL 216, IPN-PAL 217, IPN-PAL 218, IPN-PAL 219, IPN-PAL 220, IPN-PAL 221, IPN-PAL 222 (left valve) and IPN-PAL 223.
Measurements. IPN-PAL 215, length = 21.6 mm, width = 27.9 mm (Figure 2J); IPN-PAL 216, length = 25.5 mm, width = 40.1 mm (Figure 2K); IPN-PAL 217, length = 24.8 mm, width = 31.5 mm (Figure 2L); IPN-PAL 218, Length = 20.6 mm, width = 26.5 mm (Figure 2M); IPN-PAL 219, length = 7.6 mm, width = 8.8 mm (Figure 2N); IPN-PAL 220, length = 20.2 mm, width = 25.2 mm (Figure 2O); IPN-PAL 221, length = 21.7 mm, width = 27.5 mm (Figure 2P); IPN-PAL 222 (left leaflet), length = 22.2 mm, width = 27.5 mm (Figure 2Q); IPN-PAL 223, length = 10.7 mm, width = 11.9 mm (Figure 2R).
Observations. The specimens of the Olmos Formation are similar to Plesielliptio sonoraensis Kues, in Lucas et al., 1995, but the preservation does not allow a certain specific allocation.
Suborder Ostreina Férrusac, 1822
Superfamily Ostreoidea Rafinesque, 1815
Family Osteidae Rafinesque, 1815
Subfamily Ostreinae Rafinesque, 1815
Genus Ostrea Linnaeus, 1758
Ostrea sp.
Description. Small, flattened right valve, with some concentric growth lines.
Material. A right valve, IPN-PAL 225.
Measurements. Length = 9.5 mm, width = 6.2 mm.
Observations. Ostrea sensu stricto is a common bivalve marine environments, so it is probable that the specimen has been transported to the freshwater deposits of the Olmos Formation.
5. Discussion
We found that almost all the specimens identified in the Tajo La Lulú from the Olmos Formation were related to freshwater organisms, such as Viviparus, Mesolanistes, Unio, Unionelloides?, Proparreysia, and Plesielliptio. Nevertheless, Pseudolimea and Ostrea have been recorded inhabiting a range of environments, from shallow marine sediments such as estuaries to the deep sea (Mikkelsen and Bieler, 2008). The amount of these brackish specimens was scarce compared to Unionidae. In addition the poor preservation of Pseudolimea and Ostrea, suggest that they may had been transported from the coastal or mangrove areas to the freshwater deposits of the Olmos Formation.
Unionidae has been collected in different Cretaceous assemblages of North America (Stanton, 1917; Russell, 1934; Tozer, 1956; Taylor, 1975; Hartman, 1976, 1999). Unionoids are abundant and diverse in fluvial, lacustrine, and estuarine environments. These organisms only survive under running and oxygenated water (Scholz and Hartman, 2007; Roberts et al., 2008). In the case of Unio, their species generally prefer a muddy environment with a bottom free of vegetation (Teng-Chien, 1950). This feature may indicate a minimum transport due to the usually fragile shells of Unionidae (Kotzian and Simöes, 2006). This portion of the Olmos Formation represents freshwater systems allowing the unionid community establishment.
For the gastropods we found Viviparus, a relatively abundant genus in the Cretaceous and Paleocene deposits of North America (Perrilliat et al., 2008). Both Recent and fossil species of Viviparus inhabit numerous freshwater low-energy environments, founding in part buried in the mud or silt of lakes, ponds, or slower portions of streams where there is some vegetation and muddy substrate (Pace, 1973; Van Damme, 1984; Glöer and Meier-Brook, 1998). Regarding Mesolanistes, it has been reported from river, marshes, ponds and lake deposits adjacent to the marine environment of the Upper Cretaceous of Sonora, Coahuila, and in other lithostratigraphic units from North America (e.g., Yen, 1945; Lucas et al., 1995; Perrilliat et al., 2008). In future studies, this assemblage from the Olmos Formation could be compared with the landscape to that described in the Cerro del Pueblo Formation (Upper Campanian), from where Mesolanistes and Viviparus were also recorded (Lucas et al., 1995; Perrilliat et al., 2008). The Cerro del Pueblo Formation (upper Campanian) in the Parras Basin (south of the Olmos Formation outcrops) contains more diverse paleoenvironments that changed in time affected by high-frequency changes in relative sea level and coastal storm events (Eberth et al., 2004). The geographic location of the tajo La Lulú is further south of the rest of the fossiliferous outcrops in the Sabinas-Múzquiz Basin, from where different specimens of plants have been recorded (e.g. Estrada-Ruiz et al., 2018; Centeno-González et al., 2021, 2019, Figure 3). In most localities, no samples of mollusks have been found, except by a few and fragmentary specimens. These northern localities contain a high concentration of leaves and fruits related to paratropical or tropical environments, such as palms, conifers, and paratropical angiosperms (e.g., Weber, 1972, 1973, 1975, 1978; Serlin et al., 1980; Cevallos-Ferriz, 1992; Estrada-Ruiz et al., 2007, 2010, 2011; Sainz-Resendiz et al., 2015; Centeno-González et al., 2019, 2021). In addition, aquatic plants have been recorded, including ferns such as Salvinia sp. and Marsilea mascogos, linking them to stagnant freshwater bodies (Estrada-Ruiz et al., 2018). In the Tajo La Lulú, we collected fragmented and poorly preserved samples of leaves, organic matter, and a leaf that resembled a conifer (Figure 3). This suggests that this area corresponded to a transitional zone, where freshwater bodies and estuaries or marshes coexisted.
6. Conclusions
The record of freshwater bivalves and gastropods in the Olmos Formation help understand the paleoenvironments of this region during the Late Cretaceous. The identification of the two freshwater gastropods Viviparus sp. and Mesolanistes sp., allow us to assume the presence of freshwater low-energy environments, such as swamps, ponds, and streams. This is supported by the relative abundance of Unionidae (Proparreysia sp., Unio sp., Unionelloides? sp., and Plesielliptio sp.), a freshwater family that was present in fluvial, lacustrine, and estuarine environments of Cretaceous assemblages of North America. Presence of some specimens with articulated valves indicate minimum transport of this community. Additionally, we recorded Limidae (Pseudolimea sp.) and Ostreidae (Ostrea sp.), both related to marine or marshes o estuarine environments. Nevertheless, the record of these specimens was scarce and with poor preservation of the valves, indicate a possible transport derived by a storm event from the coastal or mangrove area to the freshwater deposits of the Olmos Formation. Other features are related to the record of plants present in different localities of the Olmos Formation, including the plants recollected along with the mollusk assemblage.
