DISCUSSION

El Espinal and El Chango quarries are in the area where the Sierra Madre Formation crops out, as previously noted (see Ovalles-Damián and Alvarado-Ortega 2002; Ovalles-Damián et al. 2006; Vega et al. 2006, 2007). Nevertheless, no index fossil has been collected in these localities so far, preventing an accurate biostatigraphical correlation of these localities and the lithological zones recognized by Steele (1986) and Waite (1986), who performed a geological-paleontological study nearby these localities, about 10 km northwest from El Espinal quarry.

Vega et al. (2006, p. 324, 2007) suggested that El Espinal and El Chango strata correspond to "the lowermost lithofacies defined by Steele (1986, figure 4) and Waite (1986) as dolomite and dolomitic breccia (unit 1), located between 650 and 700 m from the base of the Sierra Madre Formation, which lies conformably over the Upper Jurassic–Lower Cretaceous San Ricardo Formation." Nonetheless, Steele (1986) and Waite (1986) described their unit 1 as corresponding with the first 828-895 m of the Sierra Madre Formation sequence and suggested that it was deposited under supratidal or intertidal conditions. Unfortunately, these authors observed but did not sample the sections 400-748 m within this unit 1 and the whole unit 3 (which could be about 384 m thick). In this scenario, it is not possible to perform a proper comparison between El Espinal and El Chango strata with these non-sampled sections of the Sierra Madre Formation. Actually, additional research is required to determine the positions of El Espinal and El Chango strata within the Sierra Madre Formation.

Figure 8 summarizes the biostatigraphical ranges of the fossils from El Espinal and El Chango quarries identified by Vega et al. (2006, 2007) and the fishes described above. Although a detailed biostratigraphic analysis has not been performed to specify the age of the El Espinal and El Chango fossiliferous beds, the similarity of the Chiapas fossil assemblage and other Cenomanian localities from Europe and the Middle East makes it likely that the Chiapas fossils could have accumulated during the Cenomanian and not through the Albian as suggested by Vega et al. (2006, 2007). However, further analyses are necessary to confirm this hypothesis.

The fossil assemblage from El Espinal and El Chango quarries described by Vega et al. (2006, 2007) includes gastropods (cerithiids and probably a Perissoptera Tate 1865); crustaceans [tanaidaceans, isopods (Sphaeromatidae and Cirolanidae), and decapods (Palirinus palaciosi Vega, García-Barrera, Perrilliat, Coutiño, and Mariño-Pérez 2006; and Roemerus robustus Bishop 1983)], insects (an odonate-zigopteran nymph and a hemipteran-?belostomatid), as well as unidentified bivalves, echinoids (remains), ichnites, ostracods, and plant remains. Perissoptera is an Aptian-Campanian gastropod from Europe and North America (Saul 1998). Tanaidaceans have been a defined group since the Carboniferous (Knopf et al. 2006). The cerithiid gastropods are an extant group that includes well-documented paleozoic species (Batten 1985). The spehaeromitid isopods are extant forms with a well-known Triassic record (Brandt et al. 1999); and unmistakable cirolanids appeared first in the Cretaceous, but their record must be older (Wieder and Feldman 1992). Palirinus occurs in sediments of Europe and Lebanon from upper Barremian- middle Eocene, but today this genus has a restricted distribution along the Eastern Atlantic Ocean, Mediterranean Sea, and off South East Africa (Palero and Abelló 2007). Roemerus, which only includes the species R. robustus, was firstly known and named based on specimens from the lower Albian of the Glen Rose Limestone, Texas (Bishop 1983). The range of zygoptera is Permian-recent (Wootton 1981). The oldest belostomatid known is from the Triassic (Fraser et al. 1996).

Vega et al. (2006, p. 324) wrote about El Espinal quarry that "similar fossils have been found at the Tlayúa Formation lithographic limestones of Albian age in Puebla." Unfortunately, this proposition cannot be applied to fishes. Although these authors considered only the macrosemiid and the Paraclupea-like form referred here, which have closely related forms within the Tlayúa fish assemblage, there are hitherto more fishes to consider from Tlayúa, El Espinal, and El Chango quarries. Besides, unlike González-Rodríguez et al. (2002), who identified the macrosemiid from El Espinal as Macrosemius fourneti, a Jurassic (Kimmeridgian) species known from Cerin in France, we determine it as Macrosemiocotzus, a genus we described based on Albian specimens from Tlayúa. Ovalles-Damián (2004) described the Paraclupea-like taxon (considering that the order Ellimmichthyiformes is composed only of Paraclupea, Ellimmichthys, and Diplomystus), Zaragüeta-Bagils (2004) and Alvarado-Ortega et al. (2008) performed phylogenetic analyses of ellimmichthyiforms that suggest a more complex taxonomical composition of the group. Today, Macrosemiocotzus is the only fish common to Tlayúa and Sierra Madre formations.

Tlayua quarry is located in Tepexi de Rodríguez, Puebla, Central Mexico. Often this Albian Konservat-Lagerstätte locality is referred to as the "Mexican Solnhofen" due the extraordinary preservation, abundance, and biodiversity represented in the Tlayúa fossil assemblage (Alvarado-Ortega et al. 2007). Although the better represented fossils in Tlayúa are the fishes (Applegate 1996; Applegate et al. 2006; Alvarado-Ortega et al. 2006b; among others); a large part of taxa still require detailed taxonomic analysis. Based on the actual knowledge on the fossil fishes found in Tlayúa and El Espinal plus El Chango, the different composition of these assemblages is revealed.

Forey et al. (2003) provided a taxonomical list of the Cenomanian fishes of the Tethys Sea domains found in localities of Africa, Europe, and the Middle East. González-Rodríguez and Bravo-Cuevas (2005) provide similar information for the Albian-Cenomanian strata of the Muhi quarry, Hidalgo, Mexico. Recently, Alvarado-Ortega (2005) and Applegate et al. (2006) provided a general review of the Albian fossil fishes found in Tlayúa quarry. The comparison of these fish assemblages shows that Enchodus is present in two Mexican Cenomanian assemblages, El Espinal + El Chango and Muhi, as well as in all the other localities except Tlayúa (Figure 9). Additionally, Triplomystus and Saurorhamphus are common genera in El Espinal + El Chango and localities from the Middle East.

A better taxonomic comparison between fishes from Mexican localities and other localities around the world requires much more detailed anatomical studies of the Mexican forms. Although El Espinal and El Chango quarries were deposited in the western part of the Tethys Sea, in contrast with the Cenomanian localities of Europe and Middle East (i.e., Comen, Namoura), which were deposited in the eastern part of the Tethys Sea, the fish assemblage of the former resembles those from the latter localities (Figure 9). This fact exposes the bias in our early knowledge on the temporal and geographical distribution and the systematic composition of certain fish groups in Mexico.