Stratigraphy and age

The Ixtaltepec Formation is divided into eight informal intervals (API-1 to API-8), each characterized by its fossil association (Quiroz-Barroso and Perrilliat, Reference Quiroz-Barroso and Perrilliat1997, Reference Quiroz-Barroso and Perrilliat1998). This stratigraphical distribution of the biota has allowed identification of the approximate depositional ages of the informal intervals, with brachiopods being the more useful proxy. This is the case for the rhynchonellid and spire-bearing brachiopods herein described, which are present in the distinct levels of the formation. Thus, we observed in the interval API-1 the presence of Anthracospirifer occiduus; in API-2 Allorhynchus scientiana n. sp., A. occiduus, and the indeterminate martiniid; and in API-3 Leiorhynchoidea sp. and Alispirifer tamaulipensis. Next, in API-5, we found A. occiduus, Anthracospirifer cf. A. “opimus”, Alispirifer transversus, and Spiriferellina campestris. In API-6, we found A. occiduus, Anthracospirifer newberryi, and S. campestris. In API-7, we found Leiorhynchoidea perrilliatae n. sp., Composita ovata, Huestedia rotunda, Crurithyris expansa, Anthracospirifer oaxacaensis n. sp., Anthracospirifer sp., and S. campestris, whereas in level API-8 we found only A. oaxacaensis n. sp.

Of these taxa, A. occiduus represents one of the most relevant, particularly given its currently recognized stratigraphic range. When Pantoja-Alor (Reference Pantoja-Alor1970) described the Ixtaltepec Formation, he assigned an age of Middle–Upper Pennsylvanian employing the occurrence of this species throughout the unit. At that time, A. occiduus was known as a Moscovian index fossil (e.g., Dunbar and Condra, Reference Dunbar and Condra1932; Hoare, Reference Hoare1961; B.O. Lane, Reference Lane1962; N.G. Lane, Reference Lane1963, Reference Lane1964; Sturgeon and Hoare, Reference Sturgeon and Hoare1968), making the age unquestionable. Later, given the relative age inferred by this brachiopod, along with the presence of some Pennsylvanian bivalves (Quiroz-Barroso and Perrilliat, Reference Quiroz-Barroso and Perrilliat1997, Reference Quiroz-Barroso and Perrilliat1998), the Ixtaltepec Formation rocks were assigned to Bashkirian–Moscovian (Lower–Middle Pennsylvanian), maintaining these stratigraphical stages for many years, until the brachiopods were carefully studied. Although A. occiduus has been widely recorded in the United States, in Mexico it has only been reported twice: (1) in the Bashkirian (Lower Pennsylvanian) of the La Joya Formation of Sonora (Jiménez-López et al., Reference Jiménez-López, Sour-Tovar, Buitrón-Sánchez and Palafox-Reyes2018), a Mississippian–Pennsylvanian unit (Navas-Parejo et al., Reference Navas-Parejo, Palafox, Villanueva, Buitrón-Sánchez and Valencia-Moreno2017); and (2) in most fossiliferous levels of the Ixtaltepec Formation, with lower and upper Carboniferous strata. Although most records of A. occiduus belong to the Bashkirian–Moscovian, its potential occurrence in Serpukhovian (Upper Mississippian) rocks of North America (e.g., Anthracospirifer cf. A. occiduus in Butts, Reference Butts2007) has raised controversy about whether this taxon is really a Lower–Middle Pennsylvanian index fossil.

Thus, the presence of brachiopods such as Orbiculoidea caneyana (Serpukhovian), Semicostella sp. (Serpukhovian), Productus concinnus (Visean–Bashkirian), Keokukia sp. (Visean), Inflatia inflata (Serpukhovian), Echinoconchella elegans (Serpukhovian–Bashkirian), Stegacanthia bowsheri (Serpukhovian), Marginovatia minor (Serpukhovian), Ovatia muralis (Serpukhovian), Undaria manxensis? (Serpukhovian), Sinuatella sp. (Visean–Serpukhovian), and Alispirifer tamaulipensis (Tournaisian–Serpukhovian) in the intervals API-1 to API-3 has allowed assignment of lower strata of the formation to the Serpukhovian (=Chesterian) (Upper Mississippian), despite A. occiduus occurring in levels API-1 and API-2. As for interval API-4, there are only ichnofossils and a few remains of plants; hence its age is still uncertain, although it is considered as the Mississippian-Pennsylvanian transition (Hernández-Ocaña and Quiroz-Barroso, Reference Hernández-Ocaña and Quiroz-Barroso2018). Interval API-5 can be correlated with the Bashkirian (=Morrowan) by the presence of Orbiculoidea capuliformis (Bashkirian–Moscovian), Anthracospirifer cf. A. “opimus” (Bashkirian–Moscovian), Neospirifer dunbari (Bashkirian–Gzhelian), and especially Echinoconchella elegans (Serpukhovian–Bashkirian) and Spiriferellina campestris (Bashkirian), as well as Alispirifer transversus, which is a typical species from the Lanipustula Zone of the late Serpukhovian–Moscovian (Upper Mississippian–Middle ennsylvanian) (Taboada and Shi, Reference Taboada and Shi2011; Cisterna and Sterren, Reference Cisterna and Sterren2016). The next interval (API-6) was related to the upper Bashkirian (=upper Morrowan–lower Atokan) because of the occurrence of Spiriferellina campestris (Bashkirian), but especially by the record of Anthracospirifer newberryi, which is an index fossil from the upper Bashkirian of the United States (Sutherland and Harlow, Reference Sutherland and Harlow1973; Gordon, Reference Gordon and McKee1982). Finally, the intervals API-7 and API-8 have been dated as Moscovian (=upper Atokan–Desmoinesian) mainly by the presence of Orbiculoidea missouriensis (Pennsylvanian–middle Permian), Reticulatia huecoensis (Pennsylvanian–lower Permian), Buxtonia websteri (Moscovian), Echinaria knighti (Moscovian), Linoproductus prattenianus (Moscovian–Gzhelian), Hustedia rotunda (Moscovian), and Crurithyris expansa (Moscovian–Gzhelian).

Paleogeographical significance

As the supercontinent Rodinia fragmented during the Proterozoic, the microcontinent Oaxaquia (currently, part of Oaxaca, Hidalgo, and Tamaulipas territories) separated from the Grenvillean belt, migrating and joining Gondwana during the Cambrian–Ordovician (Ortega-Gutiérrez et al., Reference Ortega-Gutiérrez, Ruíz and Centeno-García1995). During the Devonian, Oaxaquia detached from Gondwana and moved until it collided with Euramerica, completing the union of both continental masses at the beginning of the Carboniferous (Ortega-Gutiérrez et al., Reference Ortega-Gutiérrez, Ruíz and Centeno-García1995; Centeno-García, Reference Centeno-García2005). During the Mississippian, Euramerica began to move towards Gondwana, and the Rheic Ocean was reduced to a narrow sea between the western edge of Gondwana and the southwestern edge of Euramerican. Additionally, the Paleotethys remained surrounded to the west by Euramerica and Gondwana, while to the east it continued to be restricted by the smaller islands of China (McNamara, Reference McNamara and Burnie2009). During the Late Mississippian (Serpukhovian), the Rheic Ocean was still open as a narrow passage between continental masses, allowing interchange of marine currents from the east to the west side. At the end of the Serpukhovian and during the Bashkirian, Euramerica and Gondwana were completely merged, interrupting flow of the Rheic Ocean, which caused an alteration of ocean currents and the dispersal pattern of marine fauna (Groves and Yue, Reference Groves and Yue2009; Qiao and Shen, Reference Qiao and Shen2013). During the Early Pennsylvanian (late Bashkirian–Moscovian), as a result of the Rheic Ocean closure, the circum-equatorial current from east to west was redirected to the north and south of the supercontinent that was forming, along the west coast of the Paleotethys. This event forced displacement of warm equatorial waters towards high and cold latitudes (Smith and Read, Reference Smith and Read2000; Qiao and Shen, Reference Qiao and Shen2013).

In this paleogeographic context, deposition of the Carboniferous units found at Santiago Ixtaltepec occurred south of Oaxaquia, which was located in southwestern Euramerica at paleolatitudes close to Ecuador. During the Carboniferous, the Nochixtlán area was subjected to different geological processes, mainly resulting from the merger of Euramerica and Gondwana on their way towards the formation of Pangea. This process triggered numerous environmental variations observed throughout the Carboniferous succession, with environments related to reef, tidal plain (within the intertidal zone), shallow subtidal, peri-reef, and offshore observed within the continental platform (Torres-Martínez, Reference Torres-Martínez2014; Torres-Martínez and Sour-Tovar, Reference Torres-Martínez and Sour-Tovar2016b; Hernández-Ocaña and Quiroz-Barroso, Reference Hernández-Ocaña and Quiroz-Barroso2018). Such environmental modifications affected the distribution of the marine invertebrate associations from the region, especially influencing those composed of brachiopods.

Because of this, variations of taxonomic associations through the Carboniferous succession provide significant information about the paleogeographic events that occurred at the end of the Mississippian and during the Early Pennsylvanian. Thus, we see that numerous taxa located in the intervals API-1 to API-3 displayed a cosmopolitan distribution at both the specific and generic level (Table 4), with stratigraphical distributions mainly confined to the Serpukhovian (Late Mississippian). This dispersal can be correlated with the presence of the circum-equatorial current that flowed continuously through the Panthalassa Ocean, Paleotethys, and the Rheic Ocean, allowing colonization of several brachiopod species in very disjointed geographical areas. According to Waterhouse (Reference Waterhouse1973), a determinant factor for this distribution is that brachiopods are very sensitive to extreme temperature variations and inhabit areas geographically separated with similar latitudinal ranges (Qiao and Shen, Reference Qiao and Shen2013). This proposal coincides with the pattern of distribution of the Mississippian Oaxacan brachiopods, which mostly display a migration pathway within tropical paleolatitudes, even if the regions were very distant from each other (e.g., Productus concinnus, Marginovatia minor, Inflatia inflata, Echinoconchella elegans, Undaria manxensis?, and Stegacanthia bowsheri) (Fig. 7).

Figure 7.

Reference map of the Serpukhovian, showing the geographical distribution of the Late Mississippian brachiopods from the Ixtaltepec Formation. Numbers indicate oceanic corridors: 1) Franklinian; 2) Rheic ocean; 3) Austropanthalassic-Rheic. The map includes a close-up of the current territory of the United States, displaying the location of brachiopods related to the Oaxacan unit.

Table 4.

Previous records of species reported from Serpukhovian (Upper Mississippian) units of the Ixtaltepec Formation.

Inflatia inflata and S. bowsheri have been recorded in Mexico and Australia, which were extremely separated areas during the Serpukhovian. To explain the occurrence of the same taxon in both regions, Taboada (Reference Taboada2010) noted that the location of the Austropanthalassic-Rheic corridor could have favored the dispersion of many species along the south Polar circle. However, there are no reports of similar species to Serpukhovian Oaxacan taxa in the south Polar circle, suggesting that Mexican brachiopods did not use such a connection during the Late Mississippian. If this is true, the migration pathway of I. inflata and S. bowsheri could have been related to the equatorial current of the Tropical circles instead of the austral corridor.

The interval API-4 contains only ichnofossils and fossil plants associated with interference ripples and flaser-like stratification. This suggests a shallow stage during the Mississippian-Pennsylvanian transition (Hernández-Ocaña and Quiroz-Barroso, Reference Hernández-Ocaña and Quiroz-Barroso2018), possibly coinciding with closure interval of the Rheic Ocean.

In the case of the brachiopod fauna from the Bashkirian (Lower Pennsylvanian) of the Ixtaltepec Formation (intervals API-5 and API-6), we observed a significant taxonomic provincialism in the west side of the continent (Table 5). Although the fauna shows low diversity, it is evident that the cosmopolitan nature of most species diminishes (Fig. 8). Nonetheless, the presence of Alispirifer transversus in Oaxaca, the distribution of which (Australia, Colombia, Argentina, and Mexico) certainly coincides with the Austropanthalassic-Rheic corridor, favors the exchange of cool- to cold-water tolerant brachiopods between southwestern and eastern Gondwana at the beginning of the Pennsylvanian. The presence of A. transversus in Santiago Ixtaltepec corroborates that this species not only reached localities within tropical latitudes of southwestern Gondwana (e.g., Colombia), but also equatorial zones (e.g., Oaxaca, Mexico). Regarding Echinoconchella elegans, it is not possible to corroborate if the taxon could have migrated between Mexico and Spain during the Bashkirian due to the lack of a fossil record. Because the species shows a similar stratigraphic range (Serpukhovian–Bashkirian) in both countries (Winkler Prins, Reference Winkler Prins and Renema2007; Martínez-Chacón and Winkler Prins, Reference Martínez-Chacón and Winkler Prins2009; Torres-Martínez and Sour-Tovar, Reference Torres-Martínez and Sour-Tovar2012), it can be suggested that E. elegans only endured in both paleoequatorial regions until the Early Pennsylvanian.

Figure 8.

Reference map of the Bashkirian with the geographical distribution of the Early Pennsylvanian species from the Ixtaltepec Formation. Numbers indicate oceanic corridors: 1) Franklinian; 2) Equatorial; 3) Austropanthalassic-Rheic. The map includes a close-up of the current territory of the United States, displaying the location of brachiopods related to the Oaxacan unit.

Table 5.

Previous reports of the Bashkirian (Lower Pennsylvanian) species of the Ixtaltepec Formation.

For the Ixtaltepec Formation Moscovian rocks, we found a greater number of exclusively North American taxa whose species were previously recorded in Missouri, Illinois, Nebraska, Wyoming, Arkansas, Montana, Kansas, Ohio, New Mexico, Utah, Oklahoma, Colorado, Texas, Idaho, Nevada, and Iowa in the United States (Table 6). This suggests that stronger taxonomic provincialism (Mexico-USA) occurred in the Middle Pennsylvanian, indicating a possible direct marine connection between the shallow waters of Oaxaca and the Mid-Continent epicontinental sea of the United States (Missouri, Iowa, Nebraska, Kansas, Arkansas, Oklahoma, Texas, New Mexico) (Sour-Tovar, Reference Sour-Tovar1994; Quiroz-Barroso and Perrilliat, Reference Quiroz-Barroso and Perrilliat1997; Torres-Martínez et al., Reference Torres-Martínez, Sour-Tovar and Pérez-Huerta2008, Reference Torres-Martínez and Sour-Tovar2018; Torres-Martínez and Sour-Tovar, Reference Torres-Martínez and Sour-Tovar2012, Reference Torres-Martínez and Sour-Tovar2016a, Reference Torres-Martínez and Sour-Tovarb). There also may have been a link with the Great Basin (Nevada, Utah), the Illinois Basin (Illinois, Indiana), the Appalachian Basin (Ohio), and the Alliance Basin (Wyoming, Idaho, Montana) seas (see Algeo and Heckel, Reference Algeo and Heckel2008, p. 207, fig. 2). This coincides with the results of Porras-López (Reference Porras-López2017), highlighting that the main affinity between taxa from Mexico and the United States occurred until the Pennsylvanian and not the Mississippian, as had been noted previously (Fig. 9).

Figure 9.

Reference map of the Moscovian, showing the geographical distribution of the Middle Pennsylvanian brachiopods from the Ixtaltepec Formation. Numbers indicate oceanic corridors: 1) Franklinian; 2) Equatorial. The map includes a close-up of the current territory of the United States, displaying the location of brachiopods related to the Oaxacan unit.

Table 6.

Previous records of the species found in the Moscovian (Middle Pennsylvanian) rocks of the Ixtaltepec Formation.

Nonetheless, Karavankina fasciata shows a wider distribution within Pennsylvanian equatorial warm waters, occurring in China, Canada, and Mexico (Torres-Martínez and Sour-Tovar, Reference Torres-Martínez and Sour-Tovar2012). As happens with A. occiduus, K. fasciata could have gone from east to west through the Franklinian corridor (Davydov and Cózar, Reference Davydov and Cózar2019), coinciding with migration pathways of other Moscovian taxa from the Great Basin of the United States (Pérez-Huerta, Reference Pérez-Huerta2007). This is an example of how not all Middle Pennsylvanian taxa were exclusively from a determined region, given that many species had wider distribution patterns. Such “provincialism” was decreasing through the Pennsylvanian, returning again to a cosmopolitan distribution of diverse brachiopods during the early–middle Permian (Shen et al., Reference Shen, Xie, Zhang and Shi2009, Reference Shen, Zhang, Shi, Li, Xie, Mu and Fan2013; Tazawa et al., Reference Tazawa, Okumura, Miyake and Mizuhara2016; Torres-Martínez et al., Reference Torres-Martínez, Sour-Tovar and Barragán2016, Reference Torres-Martínez, Heredia-Jiménez, Quiroz-Barroso, Navas-Parejo, Sour-Tovar and Quiroz-Barragán2019). It is necessary to consider that the paleogeographic discussion is based on taxa to a specific level whereby there could be a margin of error, considering that such taxa may need revision or reassignment.