Subphylum Vertebrata Cuvier, 1812

Infraphylum Gnathostomata Zittel, 1879

Class Chondrichthyes Huxley, 1880

Subclass Elasmobranchii Bonaparte, 1838

Cohort Euselachii Hay, 1902

Order †Hybodontiformes Maisey, 1975

Superfamily †Hybodontoidea Owen, 1846

Family †Hybodontidae Agassiz, 1843

Genus †Planohybodus Rees & Underwood, 2008

The fossil fishes catalogued into the Type Collection of CNP-UNAM are from 32 paleontological localities belonging to four undefined geological units (Fig. 1; Suppl. material 1), seven marine formations, and five freshwater formations. The oldest strata found is from Oxfordian “Caliza con Cidaris” geological unit, Oaxaca, while the youngest fish fossil beds are from the Pleistocene of Laguna de Media Luna (San Luis Potosí), the Tarango Formation, near Tula (Hidalgo), and Pie de Vaca, near Tepexi de Rodríguez (Puebla) (Fig. 2). This range represents approximately the last 150 million years of Earth’s history. Below are the main features of these Mexican lithostratigraphic units.

This geological unit was first known as the Gloria Formation and based on an outcrop exposed few kilometers away from Santa Rosalía town, Baja California Sur (Wilson 1948). Given that this last name was pre-occupied for a unit of Jurassic rocks from Coahuila (Imlay 1936), Applegate and Espinosa-Arrubarrena (1981) suggested the name of Cañada Gloria Formation for the rocks of Baja California Sur; however, the name of this geological unit changed to Tirabuzón Formation (Carreño 1981), based on Wilson (1948) and on nomenclatural incongruences present in the previous suggestions. The stratotype of the Tirabuzón Formation is inside the Bole Basin, at Santa Rosalía, Baja California Sur (Wilson 1948; Carreño 1981) and is a discontinuous basal conglomerate covered with potentially fossiliferous marine sandstone sediments that are overlaid with conglomerate strata containing a gradual lateral transition to littoral, deltaic, and nonmarine facies (Carreño 1981; Ortlieb and Colleta 1984; Carreño and Smith 2007). The abundant fossil content in this formation includes ichnofossils, foraminifers, mollusks, echinoderms and cetaceans (Wilson 1948; Applegate 1978; Carreño 1981; Ortlieb and Colleta 1984; Quiroz-Barroso and Perrilliat 1989; Barnes 1998; Carreño and Smith 2007; Shroat-Lewis 2007). The age of the unit is Late Miocene-Early Pliocene based on planktonic foraminifers (Carreño 1981), nannofossils (Ortlieb and Colleta 1984), and the shark fauna (Applegate 1978).

Pantoja-Alor and Carrillo-Bravo (1966) first described this unit which is also located in Baja California Sur. Its stratotype is near the intersection of the Coyote and La Trinidad Streams, in the western margin of the Coyote Stream; however, these outcrops are from both San José del Cabo and Los Barriles Basins (Pantoja-Alor and Carrillo-Bravo 1966; Martínez-Gutiérrez and Sethi 1997; Schwennicke et al. 2017). The unit mainly has a fine-grained sandstone, siltstone, and mudstone but is also composed of gray-greenish, laminated, fine to medium marine sandstone, shale and siltstone, and some diatomite laminate toward the center of the San José del Cabo Basin (Martínez-Gutiérrez and Sethi 1997; Schwennicke et al. 2017). The age of the Trinidad formation range from the middle Miocene to upper Pliocene and its depositional environment has three distinct types of strata: 1) a basal nearshore-lagoon deposit; 2) a deep marine environment; 3) a high-energy, shallow marine waters, which they relate to inner shelf shoals and bars (Martínez-Gutiérrez and Sethi 1997; McCloy 1984; Carreño 1992; Fierstine et al. 2001; Schwennicke et al. 2017). The fossil record of the unit comprises nanoplanktons (Schwennicke et al. 2017), foraminifers and diatomite (McCloy 1984; Carreño 1992), mollusks (Martínez-Gutiérrez and Sethi 1997) fishes (Fierstine et al. 2001), and trace fossils (Schwennicke et al. 2017).

Located in Coahuila state, this formation was described at the beginning 20^th century (Imlay 1936); however, its formal description as Cerro del Pueblo Formation was later (Murray et al. 1962). Belonging to the Difunta Group (Eberth et al. 2004), its stratotype is northeast to Saltillo City, and its outcrops are in several localities in the Parras Basin (Murray et al. 1962). The lithology of the unit is mainly composed of mudstone and sandstone but also of lesser amounts of limonite, conglomerates, and limestone (McBride et al. 1974; Kirkland et al. 2000). The unit contains a vast fossil record, comprising plants (fruiting structures, palm fronds, conifer cones), rudists, bivalves, gastropods, cephalopods, elasmobranchs, bony fishes, dinosaurs, crocodiles, turtles (Kirkland et al. 2000), and insects (Cifuentes-Ruiz et al. 2006). There are seven distinct facies, and sediments were laid down in a low coastal plain and shallow marine conditions which were strongly influenced by frequent changes in the relative sea-level or coastal physical processes (Eberth et al. 2004). Hence, the paleoenvironment is a cyclic alternation of marine, estuarine, and freshwater environments (Cifuentes-Ruiz et al. 2006). The age of the formation ranges from uppermost late Campanian to Maastrichtian (Kirkland et al. 2000).

This formation is in the Valley of Mexico (which includes the states of Ciudad de México, Estado de México, and Hidalgo) and was first proposed based on sediments exposed about 4 km southwest of Mixcoac, Mexico City (Bryan 1948; Ferrusquía-Villafranca et al. 2017). This geological unit is composed of sandstone and poorly cemented conglomerate, poorly cemented sandstone and interleaved clay, layers of clay, some layers of basalt interspersed with detrital units, lightly compacted conglomerate lenses, insulated limestone lenses, thin lenses of volcanic ash and tuff, and caliche nodules tuff, tuff-breccia, fluvial volcanic gravel, and thin pumice layers (Bryan 1948; Cervantes-Medel and Armienta 2004; Ferrusquía-Villafranca et al. 2017). The fossil record includes ostracods and diatoms (Ferrusquía-Villafranca et al. 2017), bony fishes (Alvarado-Ortega et al. 2006), and mammals of the genera Equus Linnaeus, 1758, Cuvieronius Osborn, 1923, Mammuthus Brookes, 1828, Sylvilagus Gray, 1867, Canis Linnaeus, 1758, and Bison Smith, 1827 (Castillo-Cerón et al. 1996). The depositional environment is a series of fluvial/lacustrine conditions with sandstone crossbedding which indicates deltaic conditions (Segerstrom 1962; Alvarado-Ortega et al. 2006; Ferrusquía-Villafranca et al. 2017). Based on the paleofauna, geochronology, and fault systems, the Tarango Formation is currently considered to present a Pliocene-Quaternary age (Castillo-Cerón et al. 1996; Suter et al. 2001).

The El Doctor Formation outcrops from the eastern portion of Queretaro state to the western edge of Hidalgo (Wilson et al. 1955). Its type locality is in the north flank, near El Doctor village in the Sierra Gorda, Queretaro, northeastern Peña de Bernal (Wilson et al. 1955; Segerstrom 1961; Aguirre-Díaz et al. 2013). This unit has a large limestone bank of varied textures with some chert lenses, dolomite interbeds, and shale partings (Segerstrom 1961). Based on lithology, there are four subunits: 1) La Negra, deposited in the deep of the neritic zone; 2) San Joaquín, with the same sediments of La Negra but deposited at a depth and under storm wave action; 3) Cerro Ladrón, a calcareous bank formed in shallow waters; and 4) Socavón, with clastic sediments deposited not far from the input origin (Wilson et al. 1955). The fossil record found in the El Doctor Formation includes rudists (Wilson et al. 1955), miliolids, corals, oysters, gastropods, ammonites, echinoids (Segerstrom 1962), planktic and benthic foraminifers, radiolarians (Bravo-Cuevas et al. 2009), crustaceans (Feldmann et al. 2007), and fishes (Carranza-Castañeda and Applegate 1994; Bravo-Cuevas et al. 2009). Its deposits originate on a platform shelf followed by a transitional marine system, including open sea to deep shelf margin, with alternation of neritic and open oceanic waters and, occasionally, influx of near-shore waters, probably during storms (Wilson et al. 1955; Carrillo-Martínez 1981; Bravo-Cuevas et al. 2009). The age of this formation is Albian-Cenomanian (Wilson et al. 1955; Bravo-Cuevas et al. 2009).

The stratotype is a few kilometers north of Tepexi de Rodriguez town, Puebla state, eastern Mexico (Pantoja-Alor 1992). There is a subdivision of the Tlayúa Formation: 1) the Lower Member, formed by micritic limestones (mudstone to wackestone) with silica concretions and chert lenses; 2) the Middle Member, formed by red lithographic laminar limestone (mudstone) and chert lenses; and 3) the Upper Member, with layers of dolomite and dolomitic limestone (Pantoja-Alor 1992). The fossil record is preserved in the Middle Member and, by its abundance and exceptional quality of preservation, the Tlayúa Formation is currently considered the first fossil-Lagerstätte site found in Mexico (Alvarado-Ortega et al. 2007). The paleontological record are both marine and terrestrial fauna and flora, including rudists (Alencáster 1973), foraminifera, sponges, gorgonians, gastropods, ammonoids (Cantú-Chapa 1987), belemnoids (Seibertz and Buitrón 1987), bivalves, arthropods (Feldmann et al. 1998), asteroids (Buitrón-Sánchez et al. 2015), holothurians (Applegate et al. 1996), ophiuroids, fishes (Applegate 1987; Alvarado-Ortega 2004), lizards, crocodiles, turtles (Reynoso 1997, 2000), pterosaurs (Cabral-Perdomo and Applegate 1994), algae, and gymnosperms (Espinosa-Arrubarrena et al. 1996). The paleoenvironment is a double-enclosed shallow lagoon behind a barrier reef with stagnant, anaerobic, and hypersaline conditions and bounded by semi-arid land, on the other side, by a barrier bordering a deeper, well-oxygenated lagoon (Applegate 1987; Pantoja-Alor 1992; Espinosa-Arrubarrena and Applegate 1996). Nevertheless, some influences of an open sea have been proposed (Kashiyama et al. 2004). The age of Tlayúa quarry strata ranges between the Aptian and Albian stages in the Early Cretaceous between 125 and 100 Ma (Cantú-Chapa 1987; Seibertz and Buitrón 1987; Kashiyama et al. 2004).

The outcrops of the Pie de Vaca Formation are in the southern portion of Puebla, a few kilometers northeast of Tepexi de Rodríguez town very close to the Tlayúa Formation (Pantoja-Alor 1992). The lithology of the formation consists of continental deposits of fluvial-lacustrine and alluvial environments formed by conglomerates, gravel, silt, clay, marl, limestone, travertine, and volcanic rocks (Pantoja-Alor 1992). These are followed by micritic sandstone with siliciclastic bands and intraclasts of flint, limestone, and volcanic rocks (Cabral-Perdomo et al. 2018). The fossil records of the unit are ichnites belonging to birds, camelids, felids, proboscideans, small artiodactyls (Cabral-Perdomo 1995, 2013), and bony fishes (González-Rodríguez et al. 2013; Guzmán 2015). Fungus, leaves, leaflets, wood, and fruits of angiosperms are also well conserved in this unit, as well as ostracods and stromatolites (Beraldi-Campesi et al. 2006). The paleoenvironmental condition is a tropical paleolake which evolved from a basin with alluvial conditions to shallow, alkaline, and evaporitic lacustrine circumstances, indicating the gradual desertification of the environment (Beraldi-Campesi et al. 2006). Although the palynological record indicates an Eocene-Oligocene age for this formation (Martínez-Hernández and Ramírez-Arriaga 1999), the paleobotanical and ichnological record with associated mammals indicates with more robustness an Oligocene-Pleistocene age (Cabral-Perdomo 1995, 2013; Cabral-Perdomo et al. 2018; Ramírez and Cevallos-Ferriz 2002).

The Sabinal Formation is in northeastern Oaxaca, with its outcrops (Yosobé and La Lobera) in the southern portion of Tlaxiaco Basin (López-Ticha 1985; Meneses-Rocha et al. 1994; Alvarado-Ortega et al. 2014). The lithology of the formation consists of a sequence of mudstone and wackestone clay, marl, and dark gray to black bituminous shale strata with abundant calcareous concretions arranged in thin laminar layers and showing abundant light oil impregnations (López-Ticha 1985; Felix 1891; Meneses-Rocha et al. 1994; Alvarado-Ortega et al. 2014). The fossil contents of the unit are microfossils (ostracods), plants, invertebrates (mostly ammonites), marine reptiles, and bony fishes (Alvarado-Ortega et al. 2014; Barrientos-Lara et al. 2015). The paleoenvironment is as a transitional environment under the marine influence (Alvarado-Ortega et al. 2014). A Kimmeridgian-Tithonian age is attributed to the Sabinal Formation based on the ammonite assemblage (Alvarado-Ortega et al. 2014).

This Jurassic geological unit was informally named because it carries numerous remains of urchins belonging to the genus Cidaris Leske, 1778. Main outcrops of this unit are present between Tlaxiaco and Mixtepec in Oaxaca State (Buitrón 1970). This includes the outcrops in “La Titana” hills, near Tlaxiaco, firstly reported by Felix (1891) (also see Alvarado-Ortega et al. 2014). The numerous marine invertebrates recovered in the gray marls and limestones interbedded with shales present in this site suggest that its age could extend from the Late Callovian to the Early Kimmeridgian (Buitrón 1970). Besides the first fossil fishes from the Lobera site reported by Alvarado-Ortega et al. (2014), where “Caliza con Cidaris” sediments outcrops; the fossils already documented in this geological unit include three bivalves, echinoids, calcareous sponges, gastropods, annelids, crinoids, a brachiopod (Buitrón 1970; Felix 1891).

Located to the northwest of Tuxtla Gutiérrez in Chiapas state (Licari 1960; Allison 1967; Ferrusquía-Villafranca 1996), the San Juan Formation have light-brown shales and yellowish-brown fine-grained calcarenite, composed of conglomerates, sandstone, siltstone, limestone, marl, and coquina (Ferrusquía-Villafranca 1996; Perrilliat et al. 2003). The fossil record in the unit contains foraminifers (Ferrusquía-Villafranca 1996; Perrilliat et al. 2003), calcareous algae, wood, bivalves, corals, annelids, gastropods, nautiloids, bivalves, echinoderms (Perrilliat et al. 2003), crustaceans, bony fishes (Vega et al. 2001), and sharks (Ferrusquía-Villafranca et al. 1999). The paleoenvironments are episodes of shallow, marine waters with high organic productivity and low terrigenous influence, combined with well-oxygenated shallow waters influenced by continental sedimentation and, probably, with marsh conditions from a deltaic lagoon system (Ferrusquía-Villafranca 1996; Perrilliat et al. 2003). Based on the foraminifer fossil record, the age assigned to the San Juan Formation is middle Eocene (Ferrusquía-Villafranca 1996; Perrilliat et al. 2003).

Also located in Chiapas, the Ixtapa Formation is 28 km east from Tuxtla Gutiérrez City and its outcrops mainly at the east side of the Soyaló-Ixtapa highway (State Road 195) next to the bridge that crosses the Río Hondo 1 km north of Ixtapa Municipality (Langenheim and Frost 1963; Ferrusquía-Villafranca 1996). This unit has a sequence of pyroclastic materials interbedded with calcitic pebbly gravels and tuffs, which become more frequent towards the base of the unit forming part of interbedded layers of conglomerates, sandstones, and clays where crystalline and calcareous conglomerates are sporadically present (Langenheim and Frost 1963; Martínez-Hernández 1992). The fossil record in Ixtapa Formation is diverse and includes charophytes, foraminifers, mollusks (Daily and Durham 1966), palynomorph assemblages (e.g. dinoflagellates and mangrove pollen), proboscides, horses, rhinoceros (Langenheim and Frost 1963; Daily and Durham 1966), freshwater turtles (Ferrusquía-Villafranca 1996), and only one record of a bony fish (Cantalice and Alvarado-Ortega in press). The Ixtapa Formation was formed under a low-energy fluvial-lacustrine conditions over the Middle Miocene continental sandstones of the Coyolar Formation and is below the Pliocene-Pleistocene volcanic deposits of the Punta de Llano Formation (Ferrusquía-Villafranca 1996; Martínez-Amador et al. 2004; Hernández-Villalva et al. 2013). The paleoenvironment is a continental lacustrine or a brackish transitional environment near the coast (Daily and Durham 1966; Martínez-Hernández 1992). The age assigned to this Formation ranges from the Middle to Late Miocene (Ferrusquía-Villafranca 1996).

The Tenejapa-Lacadón geological unity was first mentioned by Islas-Tenorio et al. (2005). This layer represents the union of two contemporary and laterally continuous formations: Tenejapa, first described from outcrops of San Cristóbal de las Casas City (Quezada-Muñetón 1987), and Lacandón, primary known from Petén, Guatemala (Vinson 1962; Fourcade et al. 1999). Because it is not possible to determine the boundaries between the two Formations and the proper identity of each unit is poorly understood, here we interpret both formations as a single element, the Tenejapa-Lacandón geological unity (see Alvarado-Ortega et al. 2015). This layer outcrops in diverse portions of Chiapas state (Islas-Tenorio et al. 2004, 2005); however, the fossils housed in the CNP are from the División del Norte and Belisario Dominguez quarries. The first one is approximately 2 km southeast of the archeological site of Palenque City, while the last is 9.5 km from Palenque City. Both strata are limestone marls deposited in laminated and parallel strata, which show yellowish-creamy colors with some dark-grey silicified bands and nodules (Alvarado-Ortega et al. 2015; Cantalice et al. 2018a). The fossil specimens are many poor-preserved impressions of plant remains and a singular paleoicthyofauna (Alvarado-Ortega et al. 2015; Cantalice and Alvarado-Ortega 2016, 2017; Cantalice et al. 2018a). The number of well-preserved specimens (mostly on massive mortality) makes this geological unity a fossil-Lagerstätte site. The paleoenvironment of Tenejapa-Lacandón geological unity is a marine platform with influences of external conditions to the west, originating the Tenejapa Formation, and to the east, shallow waters influenced by primarily internal conditions composes the Lacandón Formation (Quezada-Muñetón 1987; Alvarado-Ortega et al. 2015). Studies based on stable strontium isotopes indicate a Paleocene (Danian) age for this geological unity (Alvarado-Ortega et al. 2015).