New rhynchonellid and spire-bearing brachiopods from the Carboniferous of Mexico. Paleogeographical significance of the Oaxacan brachiopod fauna through the Serpukhovian–Moscovian

Abstract This work describes a rhynchonellid and spire-bearing brachiopod fauna from the Ixtaltepec Formation of Oaxaca, Mexico. Leiorhynchoidea perrilliatae, Allorhynchus scientiana, and Anthracospirifer oaxacaensis are new species. The specific determination, along with information of previously described taxa, allowed the establishment of precise relative ages of the different fossiliferous intervals (API-1 to API-8) of the formation. The occurrence of Serpukhovian taxa in API-1 to API-3 allowed assignment of the strata to the Upper Mississippian. The presence of Bashkirian species allowed the assignment of the rocks of API-5 and API-6 to the Lower Pennsylvanian. Likewise, Middle Pennsylvanian brachiopods in API-7 and API-8 enabled correlation of the strata with the Moscovian stage. This study shows that the Ixtaltepec Formation is represented by a succession of well-delimited Serpukhovian, Bashkirian, and Moscovian rocks. Regarding paleogeography, the brachiopod fauna displays clear taxonomic variations that concur with global geological changes that occurred between the Serpukhovian to Moscovian. In the Serpukhovian intervals, we recorded numerous cosmopolitan taxa of tropical waters, coinciding with the migration pathway of the Rheic Ocean. For the Bashkirian, we observed a North American provincialism; however, because of the presence of Australian and South American species, it is proposed that the Austropanthalassic-Rheic corridor had a close connection with Oaxaca. The main provincialism was observed in the Moscovian association because most of those taxa have been reported from different localities in the United States. This study supports that the main resemblance between Oaxacan and North American faunas continued until the Pennsylvanian and not the Mississippian, as was previously proposed. UUID: http://zoobank.org/181e49cf-a08a-4e99-8200-65f4a90dafcd


Introduction
The Carboniferous sedimentary rocks of Mexico, which are globally recognized for their wide exposure and fossil content, crop out in different localities throughout the country. Four specific regions stand out: Chicomuselo, Chiapas (Torres-Martínez et al., 2016); central Sonora (Navas-Parejo, 2018); La Peregrina, Tamaulipas (Sour-Tovar et al., 2005); and Santiago Ixtaltepec, Oaxaca (Sour-Tovar, 1994). Each of these regions is characterized by wide geographical extension, successional layers, and high diversity in marine invertebrates (the last does not apply to the Santa Rosa Formation from Chiapas). Brachiopods are the most abundant and diverse phylum, but sponges, rugose corals, bivalves, gastropods, ammonoids, ostracodes, trilobites, bryozoans, and crinoids are also present. Other important taxonomic groups, such as benthic foraminifera and conodonts, are present as well (Navas-Parejo, 2018).
faunas have allowed correlation of the Oaxacan units with those recorded in other geographical regions, not only of North America but also globally as key components in the stratigraphical and paleogeographical works of the region.
Even though the Ixtaltepec Formation has been studied previously, there are still several poorly known taxa whose finds have allowed new and refined stratigraphical, paleoenvironmental, and paleogeographical interpretations. Therefore, this work aims to describe the rhynchonellid and spire-bearing brachiopods from the Ixtaltepec Formation, contributing to the discussion on the stratigraphical and paleogeographical significance of the Oaxacan brachiopod fauna from the Serpukhovian, Bashkirian, and Moscovian.

Geological setting
The Santiago Ixtaltepec region is in the Oaxaquian Geological Province ( Fig. 1.2), and the stratigraphy of Paleozoic outcrops in this area is shown in Figure 2. The basement comprises metamorphic and metasedimentary rocks from the Oaxacan Complex, represented by Proterozoic gneiss and slate (Fries et al., 1962;Solari et al., 2003). Overlying the Precambrian basement is the Tiñú Formation (Robison and Pantoja-Alor, 1968), which is divided into two members: the inferior calcareous with upper Cambrian invertebrates; and the superior, mainly composed of Lower Ordovician shale with graptolites. The hiatus that divides both units is considered the Cambrian-Ordovician boundary (Sour-Tovar and Buitrón, 1987).
Above the Tiñú Formation, two Carboniferous formations crop out (Pantoja-Alor, 1970). The former is the Santiago Formation (Lower-Middle Mississippian) with a unit that currently is considered to be informal that is 164 m thick, followed by the Ixtaltepec Formation, which is ∼560 m thick (Upper Mississippian-Middle Pennsylvanian). The base of the Carboniferous sequence (Santiago Formation) is composed of shallow marine facies of Tournaisian-Visean age (=Osagean) (Quiroz-Barroso et al., 2000, Navarro-Santillán et al., 2002, followed by Visean strata (=Meramecian) deposited in offshore environments (Sour-Tovar and Quiroz-Barroso 1991;Castillo-Espinosa et al., 2010). The stratigraphical transition to the Ixtaltepec Formation from the Serpukhovian-Moscovian (=Chesterian-Desmoinesian) is still unclear. Nonetheless, the occurrence of carbonate terrigenous facies representing shallow environments and reef patches in the lower strata of the Ixtaltepec Formation imply a hiatus at the base of the unit. The rest of the Ixtaltepec Formation is made up of alternating external marine and shallow-water environments that were subjected to tide changes (Torres-Martínez, 2014;Torres-Martínez and Sour-Tovar, 2016a, b;Hernández-Ocaña and Quiroz-Barroso, 2018). Consequently, is possible to observe numerous changes in paleoenvironments (reef, peri-reef, lagoon, and offshore environments) throughout the stratigraphic unit. In some strata, it is possible that sea level reached the continental shore, as suggested by plant remains and supratidal ichnofossils (Hernández-Ocaña and Quiroz-Barroso, 2018).
The Paleozoic succession ends with the Yododeñe Formation, which rests above the Ixtaltepec Formation and is composed of Permian-Triassic? conglomerate (Torres-Martínez and Sour-Tovar, 2012). In addition, calcareous rocks from the Lower Cretaceous are exposed in the region (Sánchez-Beristain et al., 2019).

Ixtaltepec Formation fauna
The Ixtaltepec Formation is the unit with the most marine invertebrate diversity in the upper Paleozoic deposits of the Santiago Ixtaltepec area. Different groups have been described previously, including rugose corals (Peña-Salinas, 2014), bivalves Perrilliat, 1997, 1998), trilobites of the species Griffithides ixtaltepecensis Morón-Ríos and Perrilliat, 1988,  The continuous thick black lines on each species indicate the fossiliferous units of the Ixtaltepec Formation where rhynchonellid and spire-bearing brachiopods were found. Thin black lines are guidelines connecting species names with occurrences. The thin dashed line between the two stratigraphic occurrences of Anthracospirifer occiduus indicates the absence of that species in API-3 and API-4, even though it is present in API-1 through API-3, API-5, and API-6.

Materials and methods
The material consists of 85 type specimens belonging to 15 species. Brachiopods are preserved as internal and external molds of both valves, and some samples are permineralized. In many cases, the internal regions of ventral valves are preserved as composite molds where it is possible to see some of the external morphology. All taxa were recollected in different fossiliferous intervals of the Ixtaltepec Formation, particularly from the API-1 to API-3 and API-5 to API-8. The most-representative samples were photographed and illustrated. Supraspecific morphological features were studied employing the Treatise on Invertebrate Paleontology, specifically those chapters of the orders Rhynchonellida (Savage et al., 2002), Athyridida (Alvarez and Rong, 2002), Spiriferida , and Spiriferinida . Likewise, we took into consideration the information recorded online in Fossilworks (http://fossilworks.org) and the Paleobiology Database (https://paleobiodb.org).
Diagnosis.-Medium to large shell, subcircular, with the greatest width at total mid-length; commissure broadly uniplicate; ventral valve slightly rounded in lateral profile, Torres-Martínez and Sour-Tovar-New Carboniferous brachiopods from Oaxaca, Mexico 93 beak protrudes 4-5 mm beyond the hinge, with ∼85°apical angle; shallow sulcus, originated posterior to total mid-length, with five costae, beginning ∼7 mm anterior to beak; concentric lamellae more obvious on the anterior region; dorsal fold originated at mid-length, with 4-5 costae; dorsal median septum moderately long, extended slightly posterior to the mid-length; septalium short; crural bases thin, short, close to each other; dental sockets long and deep.
Description.-Medium to large shell, subcircular, with the greatest width at total mid-length; posterior margins gently convex; commissure broadly uniplicate. Ventral valve slightly rounded in lateral profile, with the greatest convexity in the umbonal region; short and acute beak protrudes 4-5 mm beyond the hinge, with ∼85°apical angle; delthyrium open apically; flanks laterally flattened; broad and shallow sulcus, originated posterior to total mid-length; ornamentation consists of five costae on sulcus, beginning ∼7 mm anterior to beak; tongue low, serrate; concentric lamellae more obvious on the anterior region. Interior with ventral muscle field in a triangular shape, anteriorly elongate. Dorsal valve slightly convex with the greatest convexity in the umbonal region; umbonal region swollen and smooth; lateral flanks of the valve are gently convex; fold convex and elevated, originated at mid-length; ornamented by 4-5 costae and conspicuous lamellae on the entire valve. Interior with median septum moderately long, extended about posterior to the mid-length of the valve, becoming narrow towards the anterior region; septalium short; dorsal muscle field narrow; crural bases thin, short, close to each other; dental sockets long and deep. Measurements shown in Table 1.
Etymology.-Named in honor of María del Carmen Perrilliat, a distinguished Mexican paleontologist dedicated to invertebrate paleontology.
Remarks.-Leiorhynchoidea carbonífera (Girty, 1911) from the Arco Hills Formation of the Visean-Serpukhovian of Idaho, USA (Butts, 2007) is different from the species described in its smaller size, weaker costae on flanks, and in its umbonal apical angle of ∼90°. Leiorhynchoidea rockymontana (Marcou, 1858) from the Tiawah Limestone of the Moscovian of Missouri, USA (Hoare, 1961) differs from L. perrilliatae n. sp. in its smaller subtriangular external shape, and the presence of 2-3 costae on the sulcus, as well as 3-4 costae on the fold. Leiorhynchoidea cloudi (Cooper in Cooper et al., 1953) from the upper levels of the Monos Formation (Capitanian) of Sonora, Mexico (Cooper et al., 1953;Lara-Peña et al., 2021) is characterized by its two broad costae on the sulcus and three on the fold, 4-5 weak costae on flanks, and crural bases enveloped by thickening of the hinge plate. Leiorhynchoidea schucherti from the Las Delicias Formation of the Wordian-Capitanian of Coahuila, Mexico (Cloud, 1944) is different from the new species by its subtriangular shape, ventral sulcus originating slightly anterior to the mid-length, the occurrence of 2-6 rounded costae on the sulcus, 3-7 costae on the fold, and the septum extended beyond total mid-length. The Santiago Ixtaltepec specimens also were compared with Leiorhynchoidea carbonífera from the Heath Formation of the Mississippian of Montana, USA, described by Easton (1962), whose assignation was questioned by Butts (2007). This taxon is distinguished from the Oaxacan species by its more numerous costae on sulcus and fold, apical angle of the umbo 130-140°, and the presence of a septum that reaches two-thirds of the total length.
Description.-Medium-size valve, subtriangular in outline, greatest width at mid-length, measuring 25 mm in width; fold low, originated to the mid-length; ornamentation consists of  three broad and rounded costae; numerous and narrow concentric lamellae cover the entire valve.

Materials.-Internal and external mold of dorsal valve (FCMP 1479).
Remarks.-The material is different from Leiorhynchoidea perrilliatae n. sp. of the level API-7 by the subtriangular outline, arrangement of lamellae, and the fewer number of rounded costae on the dorsal fold. Our specimen is dissimilar to other taxa previously described; however, the lack of material and poor preservation prevents us from a specific assignment.
Diagnosis.-Subpentagonal outline, with the greatest width anterior to mid-length, anterior commissure uniplicate, and denticulate; ventral valve convex, mainly at the anterior region; beak curved dorsally with 43-56°angle; delthyrium open, triangular, deltidial plates narrow; sulcus initiating about one-third anterior to beak, forming a low tongue at the anterior region; flanks slightly convex; ornamented by simple and subangular costae, with four costae on the sulcus and eight costae on each flank of the valve; with two postero-cardinal costae not very noticeable; interior with ventral muscle field anteriorly elongate; dorsal valve with fold broad, ornamented by five costae, with eight costae on each flank of the valve; interior with posterior adductors elongate and narrow.
Description.-Small, biconvex, and subpentagonal shell, with greatest width anterior to mid-length, anterior commissure uniplicate, and denticulate. Ventral valve convex, mainly at the anterior region; beak straight, short, slightly curved dorsally with 43-56°angle; delthyrium open, triangular, deltidial plates narrow; shallow sulcus, originating about one-third anterior to beak, narrow in the beginning, becoming slightly broad towards the anterior margin where it forms a low tongue; flanks faintly convex; ornamentation consists of complete, simple, and subangular costae; four costae ornament the sulcus, eight costae are on each flank of the valve; the two adjacent costae to postero-cardinal margins are very thin and not very noticeable; interior with ventral muscle field anteriorly elongate. Dorsal valve strongly convex at the anterior region; fold broad, corresponding to ventral sulcus; fold ornamented by five costae, with eight costae on each flank of the valve; the two last postero-cardinal costae are thinner than the rest. Interior with posterior adductors elongate and narrow; dorsal median septum absent. Measurements shown in Table 2.
Etymology.-Named for the Faculty of Sciences, UNAM, trainer institution of numerous Mexican paleontologists.
Paratypes.-An internal mold of articulated specimen in lateral view (FCMP 1481), three internal molds of ventral valve (FCMP 1482(FCMP -1484, and six internal molds of dorsal valve, showing the posterior region of the ventral valve (FCMP 1482(FCMP , 1485(FCMP -1489, and an external mold of dorsal valve (FCMP 1490). In addition to this material, >90 specimens represent both valves in the collection.
Remarks.-Allorhynchus heteropsis (Winchell, 1865) from the Tournaisian of Burlington, Iowa (Weller, 1914) differs from A. scientiana n. sp. by its smaller size, different angle of the beak, sulcus originating to the middle of the total length, and fewer costae on each flank of both valves. Allorhynchus macra (Hall, 1858b) from the Visean of Salem Limestone, Indiana (Weller, 1914) is different from the new species by its smaller size, less angular beak, angular costae, and concentric striae. Allorhynchus acutiplicatum Weller, 1914, from the Serpukhovian of the Carterville Formation of Missouri (Weller, 1914) is dissimilar from A. scientiana n. sp. by its smaller size, less angular beak, sulcus initiating on the corpus, and a greater number of not very noticeable postero-cardinal costae.
Description.-Biconvex shell, outline subovate to subcircular, with the greatest width at mid-length; shells up to 25.6 mm in length and 24.5 mm in width; ventral valve with greatest convexity in the posterior region, shallow sulcus initiating near umbonal region; shell ornamented by sublamellar growth lines and fine radial striae; interior with narrow diductor scars; dorsal valve with a low fold that, along with the sulcus, forms a deflection in the commissure, and a dorsal interior with a moderately long myophragm; adductor scars extended and narrow; triangular inner hinge plate.
Description.-Small, biconvex, and subovate shells, with commissure rectimarginate; shells up to 12.8 mm in length and 10.8 mm in width; ventral valve with greatest convexity in the posterior region, weak sulcus; both valves ornamented with 22-25 simple and rounded costae, with depressions of the same width as the costae, central costae slightly greater; dorsal valve subcircular, and dorsal fold absent.
Description.-Large-sized shell for the genus, ventribiconvex shape, and subovate outline; with greatest width at mid-length, shells up to 11.2 mm in length and 12.4 mm in width; ventral valve gibbous in the umbonal region, beak strongly curved; narrow and shallow sulcus; cardinal extremities rounded; dorsal valve with a weak sulcus; ornamentation of both valves is composed of fine growth lines.
Remarks.-The morphological traits allowed us to relate the Oaxacan specimens with Crurithyris expansa from the Moscovian of Nebraska (Dunbar and Condra, 1932, p. 348, 349). This species is clearly distinguished from others of the genus by its greater size, transverse shape, smaller umbo, and beak strongly curved (Dunbar and Condra, 1932). The Ixtaltepec Formation specimens display a slightly greater size than those described in the Pennsylvanian of Nebraska. In addition to this material, >20 specimens represent both valves in the collection.

Superfamily Martinioidea Waagen, 1883
Family Martiniidae Waagen, 1883 Martiniid gen. and sp. Indeterminate Remarks.-The morphological features allowed us to relate these specimens with taxa belonging to the Family Martiniidae ; however, the preservation did not allow us to make a reliable generic assignment.
Anthracospirifer occiduus (Sadlick, 1960) Figure 5.1-5.8  Occurrence.-Intervals API-1, API-2, API-5, and API-6, Ixtaltepec Formation, Arroyo las Pulgas. Serpukhovian-Bashkirian (Upper Mississippian-Lower Pennsylvanian). Description.-Small-or medium-sized biconvex shell, subrectangular in outline, moderately transverse, with the greatest width at the hinge-line; large shells up to 28.3 mm in length and 43.2 mm in width; cardinal extremities with 85-90°angle; commissure uniplicate; ventral valve with beak short, dorsally curved; delthyrium subtriangular; interarea slightly concave, apsacline, ∼4 mm in the largest specimen, with parallel striae; shallow sulcus, initiating at the beak, with linguliform shape at the commissure; sulcus ornamented by one simple central costa, followed by two costae on each side originating from the delimitating costae of the sulcus, which in turn are bifurcated once to the outside; 11 simple and rounded costae on each lateral flank, with fine concentric lirae on the entire shell; dorsal valve with fold originating in the umbonal region, displaying four costae derived from two costae bifurcated once, as well as 10 costae on each lateral flank, those nearest to the fold are bifurcated but the rest are simple.
Remarks.-Although the species had already been recorded in the initial works of the Ixtaltepec Formation, this is the first formal study where the taxon is described and corroborated. In addition to this material, >40 specimens represent both valves in the collection.
Diagnosis.-Medium-sized and subpentagonal shell, more transverse in juvenile specimens, with the greatest width at the hinge-line; cardinal extremities with ∼70-75°angle; ventral interarea slightly denticulate at margin, faintly concave, apsacline, with parallel and slightly diagonal striae; sulcus moderately deep, with a costellation resembling A. occiduus; 7-8 subrounded costae on each lateral flank, the two costae nearest the sulcus are bifurcated once; fine concentric and successive lirae cover the entire shell, and some juvenile specimens display anterior concentric lamellae; sulcus linguliform initiating at the beak, curved dorsally; delthyrium subtriangular; well-developed ventral adminicula; dorsal interarea narrow; fold with two bifurcate costae; lateral flanks with seven costae, the two costae nearest the fold are derived from a bifurcation.
Description.-Medium-sized and subpentagonal shell, more transverse in juvenile specimens, with greatest width at the hinge-line; cardinal extremities with ∼70-75°angle; commissure uniplicate; ventral valve convex, more gibbous in the posterior region; interarea slightly denticulate at margin, faintly concave, apsacline, 4 mm in height in the largest specimen, with parallel and slightly diagonal striae; sulcus moderately deep, with a costellation resembling A. occiduus; each lateral flank displays 7-8 subrounded costae, the four costae nearest the sulcus originate from two bifurcated costae, the others are simple; interspaces are slightly narrower than the costae; fine concentric and successive lirae cover the entire shell, although some juvenile specimens display anterior concentric lamellae; sulcus linguliform initiating at the beak, which is short and curved dorsally; delthyrium subtriangular; dental adminicula well developed but moderate; slightly divergent dental flanges; diductor scars narrow. Dorsal valve convex; interarea narrow; fold beginning in the umbonal region, with four costae originating from two bifurcate costae; lateral flanks with seven costae, the two costae nearest the fold are derived from a bifurcation, the rest are simple. Measurements shown in Table 3.
Remarks.-Anthracospirifer opimus from the Moscovian of the Cherokee Shale of Nebraska (Dunbar and Condra, 1932) is different from the new species by its subtriangular outline, strongly convex shell, rounded cardinal extremities, shallow sulcus, and arrangement of costellation. Anthracospirifer rockymontanus from the Moscovian of the Tiawah Formation, Seville Limestone, and Burgner Formation of Missouri (Hoare, 1961) is dissimilar to A. oaxacaensis n. sp. in its smaller size, greatest width near the hinge-line, arched beak, rounded cardinal extremities, and more numerous costae on the fold. Anthracospirifer birdspringensis from the Bashkirian of the Bird Spring Formation of Nevada (Lane, 1963) is different from A. oaxacaensis n. sp. by its more transverse outline, a greater number and distinct arrangement of costae, and shallower sulcus. The specimens of A. oaxacaensis n. sp. resemble Anthracospirifer occiduus from Santiago Ixtaltepec, Oaxaca; however, the new species differs in its subpentagonal shape, more acute cardinal extremities, deeper sulcus, striae from the interarea slightly diagonal, and different costellation arrangement on the lateral flanks. Description.-Medium-sized and strongly convex valve, subtriangular in outline, with the greatest width anterior to hinge-line; measuring ∼20.5 mm in length and 26.6 in width; beak and umbo strongly arched, cardinal extremities rounded; interarea apsacline; shallow sulcus originating at the beak, with a simple central costa followed on each side by one costa derived from the bifurcation of the costae that delimit the sulcus, as well as nine simple, rounded, and broad costae on each lateral flank, with very narrow interspaces.
Description.-Small-or medium-sized, biconvex and very transverse shells, with cardinal extremities alate; commissure uniplicate; ventral valve convex, 11.9 mm in length and 26.4 mm in width; interarea low and orthocline, with striae; sulcus shallow and obscure, originating 2 mm from beak and becoming inconspicuous anteriorly, ornamented by a central simple costa followed on each margin of the sulcus by one bifurcated costa; 13 low costae ornament each lateral flank, broader anteriorly; the intercostal grooves are subangular; fine, and transverse lirae on the entire shell; interior with dental adminicula short and diductor scars narrow; dorsal valve more convex than the opposite valve; fold low, well delimited by a pair bifurcated costae to the center; 10 similar costae on the opposite valve on each lateral flank; microornamentation capillate, with slightly lamellose growth lines.
Remarks.-Anthracospirifer newberryi is distinguishable from the other Oaxacan species of the genus by the shape of sulcus and fold, the orthocline ventral interarea, and arrangement and number of costae.
Description.-Small, convex, and subpentagonal valve, ∼19.5 mm in length and 19 mm in width; interarea apsacline; beak minute and strongly curved, cardinal extremities rounded; shallow sulcus initiating at the beak; sulcus with a simple central costa, followed on each side by one costa derived from the bifurcation of the delimiting costae of the sulcus, which are broader, with six simple and rounded costae on each lateral flank, becoming slender towards the anterior region; intercostal grooves strongly narrow.
Description.-Small and biconvex shell, extremely transverse with with a length-width proportion of ∼1:3; cardinal extremities alate and acute; commissure uniplicate; shells up to 11.8 mm in length and 44.8 mm in width; ventral valve with interarea moderately high, ∼2 mm, denticulate, and apsacline; delthyrium with ∼ 55°angle, and an apical callus; small and gently curved beak; shallow, smooth, and well-delimited sulcus, originating at the beak; shell ornamented by rounded plications, with slightly angular interspaces, displaying 8-9 plications on each lateral flank, slightly indistinct on the cardinal extremities; small growth-lamellae abundant; interior with short and divergent dental adminicula; muscle field rhomboidal; dorsal valve with fold gently high; and ornamentation similar to the opposite valve.
Description.-Medium-sized, biconvex, transverse, and strongly punctate shell; cardinal extremities slightly extended; shells up to 14.9 mm in length and 28.4 mm in width; commissure uniplicate; ventral valve with interarea high, apsacline; sulcus angular, well delimited by a pair of rounded plications, gently broad; 5-6 thinner plications on lateral flanks; microornamentation of imbricate, thin, and closely spaced lamellae; interior with median septum moderately high, measuring one-third of the total length; dental adminicula short; dorsal valve with fold gently high, composed of a median plication distinctly higher at the anterior margin than the lateral ones; six plications on lateral flanks, and interarea orthocline.
Remarks.-The specimens display the traits mentioned by Sutherland and Harlow (1973, p. 87) for the species, based on the lectotype of S. campestris described by White (1877). This species was also recorded by Beus and Lane (1969) from the Moscovian of the Ely Limestone of Nevada; however, their identification was based on Girty's (1903) material, which is a different taxon than Spiriferina campestris (Sutherland and Harlow, 1973). The samples of "S. campestris" from Nevada (Beus and Lane, 1969, p. 997, 998, pl. 119, figs. 4, 8.) differ from our material in their more transverse shape and bigger size, as well as the arrangement and greater number of plications on both valves. In addition to this material, >20 specimens represent both valves in the collection.

Discussion
Stratigraphy and age.-The Ixtaltepec Formation is divided into eight informal intervals (API-1 to API-8), each characterized by its fossil association (Quiroz- Perrilliat, 1997, 1998). 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 (1970) 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, 1932;Hoare, 1961;B.O. Lane, 1962;N.G. Lane, 1963N.G. Lane, , 1964Sturgeon and Hoare, 1968), making the age unquestionable. Later, given the relative age inferred by this brachiopod, along with the presence of some Pennsylvanian bivalves Perrilliat, 1997, 1998), 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., 2018), a Mississippian-Pennsylvanian unit (Navas-Parejo et al., 2017); 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, 2007) has raised controversy about whether this taxon is really a Lower-Middle Pennsylvanian index fossil.
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., 1995). 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 Journal of Paleontology 97(1):90-111 (Ortega-Gutiérrez et al., 1995;Centeno-García, 2005). 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, 2009). 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, 2009;Qiao and Shen, 2013). 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, 2000;Qiao and Shen, 2013).
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, 2014;Torres-Martínez and Sour-Tovar, 2016b;Hernández-Ocaña and Quiroz-Barroso, 2018). 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 (1973), 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, 2013). 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).
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 (2010) 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 flaserlike stratification. This suggests a shallow stage during the Mississippian-Pennsylvanian transition (Hernández-Ocaña and Quiroz-Barroso, 2018), 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  (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, 1994;Quiroz-Barroso and Perrilliat, 1997;Torres-Martínez et al., 2008, 2018Torres-Martínez andSour-Tovar, 2012, 2016a, b). 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  Journal of Paleontology 97(1):90-111 (Wyoming, Idaho, Montana) seas (see Algeo and Heckel, 2008, p. 207, fig. 2). This coincides with the results of Porras-López (2017), 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). Nonetheless, Karavankina fasciata shows a wider distribution within Pennsylvanian equatorial warm waters, occurring in China, Canada, and Mexico (Torres-Martínez and Sour-Tovar, 2012). As happens with A. occiduus, K. fasciata could have gone from east to west through the Franklinian corridor (Davydov and Cózar, 2019), coinciding with migration pathways of other Moscovian taxa from the Great Basin of the United States (Pérez-Huerta, 2007). 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., 2009Tazawa et al., 2016;Torres-Martínez et al., 2016. It is necessary to consider that the paleogeographic Table 6. Previous records of the species found in the Moscovian (Middle Pennsylvanian) rocks of the Ixtaltepec Formation.

Species
North America South America Europe Africa Asia Oceania Orbiculoidea missouriensis X Neochonetes (N.) granulifer X Reticulatia huecoensis X Desmoinesia muricatina X Buxtonia websteri X Echinaria knighti X Karavankina fasciata X X X Linoproductus platyumbonus X Linoproductus prattenianus X Marginovatia pumila X Marginovatia aureocollis X Hustedia rotunda X Composita ovata X Crurithyris expansa X Torres-Martínez and Sour-Tovar-New Carboniferous brachiopods from Oaxaca, Mexico 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.
From the Serpukhovian to the Moscovian, the Nochixtlán region was subjected to diverse paleogeographical changes related to closure of the Rheic Ocean and the subsequent formation of Pangea. This can be shown with the changes of the brachiopod fauna throughout the Ixtaltepec Formation. In Serpukhovian strata, we find the usual cosmopolitan taxa of warm waters, which could have migrated through the paleoequatorial pathway of the Rheic Ocean. For the Bashkirian, and with closure of the Rheic Ocean, we observe the beginning of a certain taxonomic provincialism. However, the presence of species from Australia and South America suggests that the Austropanthalassic-Rheic corridor might have influenced the Nochixtlán area. Finally, major provincialism was observed in the Moscovian intervals, highlighting that almost all brachiopods previously were recorded in different Middle Pennsylvanian localities of the United States.
Contrary to previous proposals, our study suggests that the main taxonomic affinity-between the brachiopods from Mexico and the United States-continued until the Pennsylvanian and not from the Mississippian.