Paleofloristic comparison of the Ayuquila and Otlaltepec basins, Middle Jurassic, Oaxaca, Mexico

Journal of South American Earth Sciences 93 (2019) 1–13

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Paleofloristic comparison of the Ayuquila and Otlaltepec basins, Middle Jurassic, Oaxaca, Mexico

T

Maria Patricia Velasco-de León∗, Erika L. Ortiz-Martínez, Diego E. Lozano-Carmona, Miguel A. Flores-Barragan Facultad de Estudios Superiores Zaragoza UNAM, Mexico

A R T I C LE I N FO

A B S T R A C T

Keywords: Foliar area Paleoflora Paleobasins Palaeoenvironment Gymnosperms

The evolution of the Jurassic sedimentary basins is a topic that has recently begun to be studied in Mexico. The aspects investigated to date involve the formation, size, origin and weathering of the sediments through the deposit, amongst others. However, so far, they have not been related to the plant communities that were established during the phases of basinal evolution. In this paper we compare the paleofloristic listings and vegetation structure of two communities that developed in the Ayuquila and Otlaltepec basins, with the characteristics of the deposit environments reported for each basin that were deposited during the BajocianBathonian. For the Ayuquila basin, alluvial fans are reported combined with flood plains and isolated lakes within a braided river system. While for the Otlaltepec basin the presence of braided river is inferred. The moisture available for each plant community was estimated by the use of seven foliar classes. The vegetation structure was obtained through the identification of leaves, reproductive structures and fossilized wood at the species level; these data allowed the quantification of plant diversity of the communities, which in the Ayuquila basin amounts to 14 genera and 22 species, while in the Otlaltepec basin nine genera and 22 species were identified, which formed plant communities with three well-defined strata. The herbaceous stratum in both basins was made up of Pteridophytes. In the Ayuquila basin they are represented by the genera Equisetum and Cladophlebis, while in the Otlaltepec basin only by Filicales. In the Ayuquila basin, Cladophlebis sp and Otozamites hespera are the most abundant species, while in the Otlaltepec basin it was Zamites lucerensis. In both basins the foliar area exhibited by the floristic elements mostly correspond to microphyll I or smaller leaves, which correspond to Brachyphyllum and to different genera of Bennettitales. Of these latter plants only Otozamites hespera and Zamites lucerensis are considered generalists as they are present in the four localities that comprise the study area. Finally, the lithological characteristics of the watersheds of Ayuquila and Otlaltepec indicate the presence of ephemeral or permanent bodies of water that vary in volume according to the season of the year these provided the necessary humidity for the establishment of ferns and members of Pelourdea, organisms considered ripary or seasonal. While in the dry season the members of Coniferales (Brachyphyllum), Bennettitales and perhaps Caytoniales (Sagenopteris) were favored.

1. Introduction The reorganization of the tectonic plates during the Jurassic controlled the fragmentation of the Pangea continent, in addition to the opening of the Atlantic Ocean and the Gulf of Mexico. As a result, sedimentary basins extensional to transtensional of tectonic origin were formed. The sediments that fill these basins have traditionally been related to the composition of the source rock. In addition, recent works link the origin of these basins with the tectonic conditions that influenced their morphology, size and sedimentation rate (Martini et al.,

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2016). In Mexico sedimentological works have recently been carried out that have documented the origin and evolution of two Jurassic sedimentary basins (Ayuquila and Otlaltepec; (Fig. 1), at the borders of the states of Puebla and Oaxaca (Moran-Zenteno et al., 1993; RamírezCalderón, 2015; Verde-Ramírez, 2015; Martini et al., 2016; ZepedaMartínez, 2017). These studies focused on various basinal aspects, including composition of the sediment or rock and how they related to the protolith, climate, relief, drainage and deposit environment that characterized each of the basins. Earlier paleobotanic studies described the Jurassic flora and its distribution in the Mixteco Terrane, includes the

Corresponding author. E-mail address: [email protected] (M.P. Velasco-de León).

https://doi.org/10.1016/j.jsames.2019.04.008 Received 23 January 2019; Received in revised form 1 April 2019; Accepted 16 April 2019 Available online 23 April 2019 0895-9811/ © 2019 Elsevier Ltd. All rights reserved.

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Fig. 1. Regional map of southern Mexico, northern section of the Mixteco Terrane, showing the location of the Otlaltepec and Ayuquila (yellow boxes) basins and metamorphic and plutonic complexes, as well as sedimentary successions from the Permian to the Cenozoic surrounding the study area. Map based on MoránZenteno et al. (1993); Geologic-Minera Map of Oaxaca E14-9, 1: 250,000 scale, SGM (2000); Campos-Madrigal et al. (2013); Martini et al. (2016). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

(Fm formal unit and informal fm) an approximately 600 m thick sequence consisting of conglomerate, sandstone and limolite of continental origin, beige to reddish colour, covered by Oxfordian sediments of Chimeco fm. According this report the unit lacked fossils. Because of its relationship with the Chimeco fm and its regional stratigraphic relations, they proposed a Middle Jurassic age for these deposits. SilvaPineda (1978) considers that due to the similarity of the flora these layers can be considered as an extension of the Consuelo Group, as Pearson (1976) had already commented. In Campos-Madrigal et al. (2013) formally describe the Ayuquila Fm that, together with the La Mora fm, constitute part of the Ayuquila basin (AB) located on the borders of the states of Puebla and Oaxaca between the towns of Huajuapan de León and Petlalcingo (Figs. 1 and 2). The base of this basin is made up of rocks from the Acatlán complex that are overlaid by a sedimentary sequence from continental to marine. The rocks of the La Mora, Ayuquila and Tecomazúchil formations represent an age range of 210 to 164 Ma and correspond to the continental deposits of AB. Interpretations of the sediments that form the stratigraphic unit of La Mora suggest an extensive alluvial plain that

states of Guerrero, Puebla and Oaxaca, southeast of the national territory (Campa y Coney, 1983; Delevoryas, 1991; Silva-Pineda et al., 2011; Velasco de León et al., 2012). However, so far, they have had no correlation with the evolution of the basins nor has there been a paleofloristic comparison made between them. According to the above in a stratigraphic sequence with plant fossils, changes in temperature and precipitation can be inferred plants can be used as a proxy indicator for climatic conditions (Wolfe and Uemura, 1999; Yang et al., 2015). Objective of this work is to compare paleofloristic elements and their foliar area, with the environmental factors proposed for each basin. In order to infer the conditions under which plant communities were established in the study areas. 2. Background 2.1. Ayuquila basin The first studies in the Ayuquila basin were carried out by PérezIbargüengoitia et al. (1965) who describe as Tecomazúchil Formation 2

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Fig. 2. Geological map of the Ayuquila basin, shows the location of the localities of study: Ayuquila (CFZ-Ay) and Chilixtlahuaca (CFZ-Ch) in the central portion of the basin. Details are observed such as the discordance that separates the Ayuquila and Tecomazúchil formations, and the tectonic boundaries of the basin, in addition to the stratigraphic and undifferentiated units from the Paleozoic to the Cenozoic. Map taken and modified from Campos-Madrigal et al. (2013).

impressions of gymnosperms, Pteridophytes and icnites of dinosaurs. The latter have not been formally reported. The proposed depositional environment is a braided river system (Campos-Madrigal et al., 2013). The reports of plant fossils are from Delevoryas (1991), who proposed new species of female and male cones of Bennettitales (Weltrichia ayuquilana Delevoryas and W. microdigitata Delevoryas) with material collected near the Peña de Ayuquila. More recently Silva-Pineda et al. (2011), collected material that was assigned to a new species, Weltrichia mixtequensis Silva-Pineda et al.

developed in an arid climate, icnofossils and paleosols are common in this unit (Silva-Romo et al., 2015). An angular unconformity separates La Mora fm from the Ayuquila Fm. The latter from the Bajocian-Bathonian age (170-166 Ma) is characterized by the presence of massive and thick monomyctic or oligomictic conglomerate with fragments of sedimentary rock. In the middle part it includes gravel and pebbles contained in a matrix of medium-to fine-grain sandstone and some conglomerates typical of a fluvial succession. In addition to pelecypods and freshwater gastropods of the Unio ogamigoensis Kobayashi and Zuzuki and Pila (turbinicola?) nipponica Kobayashy and Susuki (Mendoza, 2002) species in the lacustrine facies of the formation, as well as impressions of plant fossils. The upper part of the Ayuquila Fm has rocks of a dike-stratum of granodioritic composition. The total thickness of the Ayuquila Fm was estimated at 2000 m and is unconformably covered by the Tecomazúchil Fm deposited during the Callovian (166-163 Ma). This unit was described as a succession of monomyctic conglomerate of metamorphic rock toward the base, interspersed with sublitharenite and subarkose, which to the top changes to limolite. It has abundant

2.2. Otlaltepec Basin The Otlaltepec Basin (OB) is located between the states of Oaxaca and Puebla, in the north-central sector of the Mixteco Terrane, near the towns of Santo Domingo Tianguistengo, Santa Cruz Nuevo and Santo Tomas Otlaltepec (Figs. 1 and 3). It is bounded by lateral faults that show a complex geological history (Moran-Zenteno et al., 1993; VerdeRamírez, 2015; Martini et al., 2016) and with a base that is formed by the rocks of the Acatlán Complex and the Totoltepec Pluton that are 3

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Fig. 3. Geological map of the Otlaltepec basin, shows the ubication of the localities of study: Otlaltepec 1 (CFZ-Ot1) and Otlaltepec 2 (CFZ-Ot2). The outcrops of the hollow stone and Otlaltepec formations are observed, as well as the stratigraphic and undifferentiated units from the Paleozoic to the Cenozoic that emerge in the area. Map taken and modified from Cruz-Cruz (2012) and Martini et al. (2016).

Couper, Manumia Pocock and Clavatispora Boonmee et Hyde, which confirm the Middle Jurassic age. On the other hand, Ramírez-Calderón (2015) reported an enrichment in quartz and faneritic polycrystalline fragments with evidence of recrystallization and crystal-plastic formation. These sediments are formed as a result of a tectonic event associated with Oaxaca fault activity. Martini et al. (2016) in their sedimentological, geochronological and chemical study research the evolution of the OB. measured two stratigraphic columns, one in Arroyo Magdalena and another in Xiotillo with a maximum thickness of 130 m. In the analysis performed on the provenance of the sandstones, of quartz-feldspar to quartzite composition, they conclude that this formation was dominated by a rhythmic deposit in alternating conglomerate and sandstone, based on traction currents (lithofacies Gt, St, Sp, Sr, and Fl; Table 3) and sediments developed during a period without deposits in a subaerial environment that sometimes favored pedogenesis (Farrell, 1987; Miall, 2000; Reading, 2009). The analysis also indicates that the base of this unit came mainly from the Totoltepec Pluton, with minor contributions toward the top of the Oaxaca and Acatlán complexes, of Cozahuico Granite and volcanic rocks located during the Pangea rupture (Martini et al., 2016). Martini et al. (2017), pointed to the first order compositional difference between the Piedra Hueca and Otlaltepec formations, defined mainly by the quartz enrichment of the second unit, as a result of sedimentation occurring in a humid climate during the BajocianBathonian (Middle Jurassic). In 2018 Villanueva-Amadoz et al. Increased the paleopalynological list of the unit to 86 fossil-taxa, where more than half were gymnosperm pollen. In addition, they report for the first time the Ischyosporites Balme and Klukisporites Couper genera, as well as the Aratrisporites minimus Schulz and Leptolepidites bossus (Couper) Schulz species and spores of algae and acritarchs.

overlaid by sediments of continental, transitional and marine origin of four informal formations: Tianguistengo, Piedra Hueca, Otlaltepec and Magdalena (Verde-Ramirez, 2015). The oldest is the Triassic Tianguistengo fm proposed by Ramos-Leal (1989). It was proposed that the formation developed in a flood plain environment, which preserved a scarce fossil record of logs and root impressions (Ramos-Leal, 1989). Recently Verde-Ramírez (2015) stated that this succession was deposited during the lower Triassic-Jurassic in an alluvial environment. The Jurassic formations Piedra Hueca and Otlaltepec contain fossil impressions of leaves and logs, although in the case of the Piedra Hueca fm the fossils are scarce and poorly preserved (Ramírez-Calderón, 2015). The sequence ends with the Magdalena rock fm overlying in angular unconformity to the Otlaltepec fm, which were deposited in a transitional environment (Martini et al., 2016). Due to the fossil content of these formations, only Otlaltepec fm (Middle Jurassic, 167 ± 4 Ma) was considered. Ortega-Guerrero (1989) described the Otlaltepec unit as a succession of sandstone alternating with slate and shale, with a thickness of 1500 m. The sediments were deposited in a fluvial or alluvial environment where the presence of the genus Otozamites Braun was recorded. Subsequently Cruz-Cruz (2012), differed from Ortega-Guerrero and proposed that the Otlaltepec fm has a thickness of 826 m and the deposit occurred in a braided type river environment. It is divided into the lower and upper members. The first has thin layers of coal where the fossil specimens of Bennettitales (leaves, cones and wood) have a better degree of preservation. The upper member is characterized by a cyclical deposit of sandstone and conglomerate that lacks coal. Recently Gerwert et al. (2015), carried out a paleopalynological study into the lower member in which they observed two associations of palynomorphs. At the base the pollen grains of gymnosperms of the Inaperturopollenites Potonie-Venitz-Spheripollenites Couper Group are predominant. Stratigraphically above this association they reported a predominance of verrucous trilateral spores of the genera; Leptolepidites 4

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Fig. 4. Stratigraphic column of the Ayuquila locality (CFZ-Ay) measured east of San José Ayuquila (Fig. 2) indicating stratigraphic levels with fossil plants with black arrows at meters 1, 3, 5–6.

3. Methodology

2002). Subsequently, for each leaf of gymnosperm, the foliar area was calculated and assigned to one of the size classes proposed by OrtizMartínez et al. (2013), and the results are shown in Table 2. Finally, the paleoecological data of the genera and species identified and the percentage of representativity of each class of foliar area were compared with the lithological and environmental characteristics described for each basin (Cruz-Cruz, 2012; Campos-Madrigal et al., 2013; Martini et al., 2016).

Four field trips were made to each of the localities comprising the study area (AB: localities Ayuquila and Chilixtlahuaca; OB: localities Otlaltepec 1 and Otlaltepec 2; (Figs. 2 and 3 respectively). The extraction of the fossil material was carried out with stratigraphic control at each sampled site. Therefore, a stratigraphic column was constructed using a Jacob's staff in each paleofloristic locality (Figs. 4, 5, 7 and 8). The material obtained was deposited in the Collection of Paleontology of the Zaragoza Faculty of Advanced Studies of the National Autonomous University of Mexico (UNAM) with the acronyms, CFZ-Ot1, CFZOt2 (Otlaltepec fm), CFZ-Ay, and CFZ-Ch (Ayuquila Fm). For the identification of leaves, leaflets and reproductive organs of Bennettitales, Caytoniales, Coniferales and ferns, literature from local and regional studies was used (Wieland, 1914; Harris, 1964; Silva-Pineda, 1984; Watson and Sincok, 1992; Pott, 2014, etc.). With the information obtained the paleofloristic listings of each formation was produced (Table 1). In addition, the paleopalynological information of Gerwert et al. (2015) was incorporated. And Villanueva-Amadoz et al. (2018), which correlates with the locality Otlaltepec 1. On the other hand, for the Ayuquila Fm, information on the species of pelecypods and freshwater gastropods of the Ayuquila Fm was incorporated (Mendoza,

4. Results 4.1. Ayuquila basin The paleofloristic richness of the Ayuquila Fm was recorded in two localities (Ayuquila and Chilixtlahuaca) of the central portion of the basin (Fig. 2). In each locality a stratigraphic 20 m section was measured in which the collected material was located (Figs. 4 and 5). The flora is composed by Equisetales, Filicales, Caytoniales, Bennettitales, Coniferales and Incertae sedis elements. In total there are 168 specimens of plants and invertebrates. These fossils were collected in shale, limolite and medium to fine-grained sandstone. (Table 1, Figs. 3–5). In the Ayuquila locality, 90 hand specimens were collected 5

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Fig. 5. Stratigraphic column of the Chilixtlahuaca locality (CFZ-Ch) measured southeast of Chilixtlahuaca (Fig. 2) indicating stratigraphic levels with fossil plants with black arrows at meters 4, 5, 10 and 12. It also shows the location of the clam fossils in the 18–19 m corresponding to the paleolake of Chilixtlahuaca.

order Bennettitales is represented by the species Otozamites hespera Wieland, Zamites lucerensis Wieland, Z. oaxacensis Wieland and Z. tribulosus Wieland, considered rare in this locality. Although the abundance of leaves of these species is low, male and female cones have also been collected of the species Weltrichia mixtequensis and Williamsonia oaxacensis Wieland respectively. Finally, the paleofloristic list is completed by Mexiglossa varia Person and Delevoryas et Pelourdea sp. The foliar area exhibited by the different genera and species of gymnosperms present in this locality include microphyll II and III classes. A second collection locality in the Ayuquila Fm was made at the town of Chilixtlahuaca, which is located below the major angular discordance that marks the upper limit of the Ayuquila Fm with the Tecomazúchil Fm (Campos-Madrigal et al., 2013) and stratigraphically above the Ayuquila locality. In the town of Chilixtlahuaca, 78 impressions of plant fossils were collected from the summit, at meters 4–5, 10 and 12 of the measured stratigraphic section, (Fig. 5). The decrease in the abundance of ferns is noticeable compared to the Ayuquila locality. The floristic diversity of this locality includes eight genera and

containing plant fossils located in the middle-basal part (between meters 1 and 7) of the stratigraphic column (Fig. 4). Among the fossils collected 13 genera and 15 species of plant fossils were identified that are preserved as impressions in limolite. The most abundant taxon with 45% of incidence is Cladophlebis sp. (Table 1; Fig. 6a). It species is associated with fronds of ferns, considered ripary elements. Leaves of the order Caytoniales of the genus Sagenopteris Presl (Fig. 6c) is represented in the fossil association of the Ayuquila Fm by four species (S. colpodes Harris, S. nilssoniana (Brongniart) Ward, S. pualensis Barbacka et al. and S. sp) showing foliar sheets in microphyll III and notophyll classes. Other taxon recorded in the conifer genus is Brachyphyllum Brongniart (B. sp1) (Table 1; Fig. 6b). The rhomboidal-shaped squamiform leaves of species B. sp1 are small (with a total length of 0.1–0.25 cm) and arranged in a spiral, with monocyclic type stomata, and papillae. These specimens, when compared to the characteristics of 22 species from other parts of the world, do not correspond to any of them; ÁngelesFavila (2009) suggest that it may be a new species (Ángeles-Favila, 2009). The leaf area of B. sp1 corresponds to the nanophyll II class. The 6

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Otlaltepec Basin (Fig. 3). In these localities stratigraphic 20 m and 22 m sections were measured, respectively (Figs. 7 and 8). The diversity is composed of Equisetales, Filicales, Bennettitales and Coniferales. It comprises a total of 209 specimens of plant fossils, collected from fineto medium-grain sandstone. In meter 7 of the stratigraphic section of the Otlaltepec 1 locality (Ot1) a total of 32 impressions and autochthonous carbon compressions of plant fossils were collected; preserved in limolite and fine-grain sandstone (Fig. 7). Four genera and five species were identified among them, of which 88.83% correspond to Bennettitales leaves of the species, Zamites lucerensis, Z. sp1 and Z. tribulosus (Fig. 9b, d). This last species is the most abundant in this locality, while the ferns and the Otozamites hespera and Z. sp1 are poorly represented (Table 1). Weltrichia sp (Fig. 9f) is the only taxon of reproductive organ present at the sampled site. The palaeofloristic diversity of this study area is increased with the paleopalynological data obtained by Gerwet et al. (2015) who registered small, monoporated pollen grains from the Inaperturopollenites-Spheripollenites group reported by Kedves (1996) with affinity to gymnosperms (coniferous) in Toarcian localities in Europe. Ascending stratigraphically, the pollinic diversity increases, to 86 fossiltaxa, where more than half were grains of gymnosperm pollen. Villanueva-Amadoz et al. (2018) reported the genera Ischyosporites and Klukisporites as well as the Aratrisporites minimus and Leoptolepidites bossus species. Furhermore, the leaf area measured for the leaflets of the order of the Bennettitales corresponds to the nanophyll II, microphyll I and microphyll III classes. The second collection area within Otlaltepec fm (Ot2) is located at the top of the measured stratigraphic section, between meters 17 to 20 (Fig. 8), where 177 specimens of plant fossils were collected. These are better preserved, more abundant and represent a greater richness than in the Ot1 locality, with eight genera and 18 species (Table 1). This highlights the abundance and diversity of the leaflets of the Zamites Brongniart, represented by seven species (Table 1), of which Z. lucerensis (Fig. 9a) is the most abundant (36.2%). Brachyphyllum (Fig. 9e), although scarce, has a diversity composed of three possible new species. The foliar area of most gymnosperms collected in this locality corresponds to the microphyll I class.

Fig. 6. Ferns, Caytoniales, Coniferales, Bennettitales and invertebrates of the Ayuquila formation. a. cone Williamsonia oaxacensis; b. Brachyphyllum sp1, the arrows indicate the position of the branches and leaves; c. Sagenopteris colpodes, note the anastomotic venation indicated by the arrows; d. Otozamites hespera; e. invertebrates of the Unio ogamoensis (white arrow) and Pila niponica (black Arrow) species. Scale = 1 cm.

5. Discussion 5.1. Floristic comparison

14 species (Table 1). The best represented order is the Bennettitales with 79.5% (Table 1). The most abundant species with 59% is Otozamites hespera (Fig. 6d), whose leaf area corresponds to the microphyll I, II and III classes, while the rest of Bennettitales leaf species are rare. The identified female reproductive structures are represented by the species Williamsonia netzahuacoyotlii Wieland and W. oaxacensis, which are common in the locality; followed abundance by branches of Brachyphyllum sp1 follow (10.25%), with foliar areas of nanophyll II class (Table 2). The presence of the genus Equisetum Linnaeus, three other genera of gymnosperms (Agathoxylon Hartig, Anomozamites Schimper and Ptilophyllum Morris) and of reproductive structures with characteristics related to the Caytonanthus Harris and Wielandiella Nathorst genera, increases the paleofloristic diversity of this formation. Finally, at the top of the sequence between meters 18 and 19 of the stratigraphic section coquines of mollusks of the species Unio ogamoensis and Pila niponica were collected (Figs. 5 and 6e), indicating the presence of the paleolake of Chilixtlahuaca (Mendoza, 2002), this lake is considered small and ephemeral by Campos-Madrigal et al. (2013).

The floristic lists obtained for the Ayuquila and Otlaltepec formations show that they share eight genera and nine species (Table 1). In both basins gymnosperms are the most abundant group of plants. Although in Ayuquila (AB) the Filicales and Caytoniales are better represented. The species shared among the four localities are Otozamites hespera and Zamites lucerensis, which correspond to the most abundant taxa in the Ayuquila and Otlaltepec basins respectively. While in Ayuquila the greatest diversity is made up of species of the Sagenopteris and Zamites genus (Table 1). Overall, 12 exclusive species were identified (Table 1), which we infer was due to particular conditions such as soil types, exposure to environmental agents, topography etc., which favored these groups. 5.2. Plant physiognomy Table 1 shows that the AB (mean basal part of the Ayuquila locality; Fig. 4), registers an abundance of almost 50% of the species Cladophlebis sp (Fig. 6A), abundance that the Filicales do not have again in the rest of the formation. Cladophlebis sp formed the herbaceous stratum of the plant community that was established in this locality during the Middle Jurassic. Ferns are traditionally considered ripary or seasonal organisms (Montañez, 2006), another example are members of the Pelourdea Seward genus (Ash, 1987). Therefore, they are identified as good indicators of humidity, linked to the presence of bodies of water or by

4.2. Otlaltepec Basin The paleofloristic diversity of Otlaltepec fm was recorded in two localities (Otlaltepec 1 and Otlaltepec 2) in the southern portion of the 7

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Fig. 7. Stratigraphic column of the town Otlaltepec 1 (Ot1) measured northwest of Santo Domingo Tianguistengo (Fig. 3) indicating the stratigraphic level with plant fossils with a black arrow at 7 m.

generally extensive during the Triassic–Cretaceous (Taylor and Taylor, 1993). The arboreal stratum in the AB was made up of trees of the Agathoxylon and Brachyphyllum genera. In contrast to what is reported for the OB, where the Agathoxylon genus is not present, and the arboreal stratum is formed only by the Brachyphyllum genus and conifers represented by the Inaperturopollenites-Spheripollenites group (Gerwet et al., 2015). It is worth mentioning that Brachyphyllum has been reported by different authors for high-temperature environments (Francis, 1983; Vakhrameev, 1991; Yang et al., 2006). In both basins, Bennettitales were identified that have generally been reported as trees and shrubs, with trunks that vary from small and rounded to tall and thin, with deciduous leaves, often coriaceous, with little exposed foliar area, thickened cell walls, thick cuticles and trichomes (Taylor and Taylor, 1993). Common characteristics in plants that inhabit regions with high temperatures and low water availability, which were

annual rainfall (Zamora-Martínez et al., 2004). Meanwhile the herbaceous stratum of the OB was formed by a low abundance of ferns (macrofossils), although Villanueva-Amadoz et al. (2018) reportes abundant palynomorphs of the Ischyosporites and Klukisporites genera, as well as the Aratrisporites minimus and Leoptolepidites bossus species. The vertical structure of the plant community established in the AB according to the Aramburu et al. (2006) was made up of two more strata: shrubby and arboreal that included the permanent members of the community. One of the components of the shrubby stratum was the genus Sagenopteris (Table 1; Fig. 6c). In addition, in the locality of Chilixtlahuaca, reproductive structures related to the Caytonanthus genus were identified. McElwin et al. (2007) report the presence of the Sagenopteris genus in thermophilic environments in localities from Late Triassic to Early Jurassic while Petersen and Lindströmm (2012) have found Caytoniales pollen in humid places, where parent plants may have occupied well-drained areas. The distribution of the group was

8

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Fig. 8. Stratigraphic column of the Otlaltepec 2 locality (Ot2) measured southwest of Santo Domingo Tianguistengo (Fig. 3) indicating the stratigraphic level with plant fossils with a black arrow between meters 17–20.

That is, the plant communities established during the BajocianBathonian in the study area lived in seasonal humid conditions (summer-wet sensu Rees et al., 2000), which favored the establishment of Bennettitales and Coniferales with small leaves. The above is reinforced by ecological studies carried out with current plants that propose that organisms with microphyll I class foliar areas inhabit dry or sunny environments, as leaf size is one of the adaptive characters that plants use to control evapotranspiration on the surface of the sheet (Givnish, 1979, 1984; Hickey, 1999). For AB the presence of riparian plants, gastropod and bivalve suggests the existence of intermittent water bodies (Mendoza, 2002). While for OB Martini et al. (2016) proposed a fluvial environment of braided rivers. Although the dominance of small leaves of gymnosperms and paleosols formed under subaerial conditions (Martini et al., 2016) indicate that the water body may have had changes in its volume or may have been seasonal. Besides, the studies of Rees et al. (2000) proposed that

supposed to reduce the risk of water loss by evapotranspiration (Taylor and Taylor, 1993; Saiki and Yoshida, 1999; Rees et al., 2000). Analysis of the foliar area of the gymnosperm leaves reported by both basins indicates that during the Middle Jurassic in the region plant communities of leaves were established, mainly microphylls I or minor, whose percentages vary from 50% to 85.7% (Table 2). These sizes are represented mainly by the Brachyphyllum, Anomozamites, Ptilophyllum, Otozamites and Zamites genera. The analysis indicates that the leaves with the largest exposed foliar area correspond to the nothopyll class to which the Sagenopteris, Mexiglossa and Pelourdea genera belong. In this case the first two genera have been considered deciduous plants (Delevoryas and Person, 1975; Barbacka, 2006). Therefore the existence of plants exhibiting seasonally leaf shedding habits (Bennettitales) and the presence of riparian plants such as Pelourdea and Filicales (Ash, 1987) in both basins, allow the proposal of the existence of seasonal humid conditions. 9

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Table 1 Shows the percentage of each of the taxa identified in the study area. The exclusive species of each basin are highlighted.

Table 2 Size classes of the leaves of fossil gymnosperms, reported for the Mixteco Terrane and percentages obtained in each locality for each of the classes of foliar area. Class

Nanophyll I Nanophyll II Microphyll I Microphyll II Microphyll III Notophyll Mesophyll I

Size (cm2)

< 0.01142 0.01142 to 0.08240 0.08241 to 1.3660 1.3661 to 3.4835 3.4836 to 9.8551 9.8552 to 57.7427 57.7428 to 304.90

Ayuquila Basin

Otlaltepec Basin

Loc. Ayuquila

Loc. Chilixtlahuaca

Loc. Otlaltepec 1

Loc. Otlaltepec 2

37.1 2.85 11.4 17.1 28.57 2.8 0

0 10.52 68.42 10.52 6.57 3.94 0

0 0 85.7 14.2 0 0 0

19.1 1.47 75.73 1.47 0.73 1.47 0

supporting the validity of this analysis.

Cycadophytes and microphyllous conifers were abundant in plant communities established in low latitudes, which coincides with the percentages of the foliar classes reported for the study area. The geographical position of the Ayuquila and Otlaltepec basins (3.3° N) during the Bajocian-Bathonian also supports the existence of a summer-wet climate zone or biome with typical microphyllous elements for waterdeficient seasons (Rees et al., 2000). Finally, the presence of reproductive structures of the Williamsonia Carruthers and Weltrichia Braun genera in both formations makes it possible to infer the reproductive maturity of the plant established in both basins, further

6. Lithological analysis The sedimentological works for Ayuquila Fm supports paleoecological inferences made from the analysis of the fossil flora present within the basin. Campos-Madrigal et al. (2013) propose a subtropical climate to warm and relatively dry in the AB from the presence of abundant red beds and the absence of carbon mantles. 10

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Table 3 Symbology used in the text and Figs. 4, 5, 7 and 8.

Gmm Gt St Sp Sr Sh Ss Sm Fl Fr

Description

Basin

Matrix-supported, poorly sorted conglomerate characterized by the lack of clast framework. Beds of this lithofacies have sharp but non-erosional relationship with underlying deposits. Scoop-shaped bodies of trough-cross-bedded conglomerate typically cut into each other both laterally and vertically. Fine- to very coarse-grained sandstone displaying trough cross-bedding. Fine-to coarse-grained sandstone displaying planar cross-bedding. Very fine- to coarse-grained sandstone displaying ripple cross-lamination. Very fine- to coarse-grained sandstone with horizontal lamination. Sand, very fine to coarse, may be pebbly with broad, shallow scours. Sand, fine to coarse, massive, or faint lamination. Horizontally interlaminated mudstone, siltstone, and very fine-grained sandstone. Mud, silt, massive, roots, bioturbation

Ayuquila; Otlaltepec Otlaltepec Ayuquila; Otlaltepec Ayuquila; Otlaltepec Otlaltepec Ayuquila; Otlaltepec Ayuquila; Otlaltepec Ayuquila; Otlaltepec Ayuquila; Otlaltepec Ayuquila

Fig. 9. Coniferales and Bennettitales of the Otlaltepec formation. a. Zamites lucerensis; b-d. Z. tribulosus, d. approach to the central portion of the leaf of this specimen; c. Z. cf. lucerensis leaf and "flower" of the Williamsonia tlazolteotl Wieland species (white arrow); e. Brachyphyllum sp1; f. Weltrichia sp. abaxial part of the cone. Scale = 1 cm in e and d; 2 cm in a-c and f.

coincide with the work of Cruz-Cruz (2012) in the Gt, Sr, St and Fl facies; the first three related to low sinuosity channels, braided drains and undulations. In addition to FL facies (horizontally spaced sludges and clays). These sediments were deposited intermittently under conditions of high water regimes and their descent, which facilitated the formation of paleosols (Fig. 5).

For OB the observed lithological characteristics also support paleocological inferences. Cruz-Cruz (2012) indicates a high-energy fluvial environment with periods of maximum rainfall, which originated as flood plains during the filling of the basin, which are observed mainly in the upper limb of the stratigraphic column. The work carried out by Martini et al. (2016) and Martini et al. (2017) for the basin 11

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7. Conclusions

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The analysis of the floristic lists obtained for the geological formations deposited during the Bajocian-Bathonian in the limits of the states of Puebla and Oaxaca and their comparison with the data published so far for the region allow us to conclude the following. The floristic diversity is greater in the AB, especially for the herbaceous stratum where the records of spores and pollen increase the presence of short life cycle groups and those more linked to the presence of water bodies. Otozamites hespera, and Zamites lucerensis species are proposed to be generalists, distributed in both basins and probably more tolerant to dry seasonal conditions. In the AB there were conditions that allowed the development of the arboreal, shrubby and herbaceous strata. While in the OB, a shrubby community with few tree elements was established, favored by the establishment of ephemeral or permanent water bodies. The latter had a greater influence on the structure of the plant community that was established in the region during the Bajocian-Bathonian. The temperatures that characterized this region determined the establishment of a vegetation with microphyllous leaves, with some elements of larger foliar areas. While the variation of the volume of water led to the presence of riparian elements of the herbaceous stratum. Acknowledgements The authors thank the work team of the Paleontology Collection of the School of Faculty of Studies Zaragoza (Elizabeth Ortega Chávez and Pedro C. Martínez) for the support in field work and material preparation. This work was financed through project PAPIIT-IN 106010-3 “Distribución de Gimnospermas en el Jurásico de la Región Norte del Terreno Mixteco y Reconstrucción Paleoecológica”. References Angeles-Fávila, R., 2009. El género Brachyphyllum, en el Mesozoico de México. Bachelor Thesis. Universidad Nacional Autónoma de México, México. Aramburu, M.P., Escribano, R., Aguiló, M., 2006. Guía para la elaboración de estudios del medio físico. Ministerio de Medio Ambiente, Secretaría General Técnica, Madrid, pp. 383–429. Ash, S.R., 1987. The upper triassic red bed flora of the Colorado plateau, western United States. J. Ariz. Nev. Acad. Sci. 22, 95–105. Barbacka, M., Palfy, J., Smith, P.L., 2006. Hettangian (early jurassic) plant fossils from puale bay (peninsular terrane, Alaska). Rev. Palaeobot. Palynol. 142, 33–46. Campa, M.F., Coney, P.J., 1983. Tectonostratigraphic terranes and mineral resource distribution in Mexico. Can. J. Earth Sci. 20, 1040–1051. Campos-Madrigal, E., Centeno, G.C., Mendoza, R.C., y Silva, R.G., 2013. Sedimentología, reconstrucción paleoambiental y significado tectónico de las sucesiones clásticas del Jurásico Medio en el área de Texcalapa, Puebla-Huajuapan de León, Oaxaca: revisión de las formaciones Ayuquila y Tecomazúchil. Revista Geológica Mexicana 30 (1), 24–50. Cruz-Cruz, M.A., 2012. Análisis estratigráfico de la secuencia jurásica de la región de Santo Domingo Tianguistengo, Oaxaca Santa Cruz Nuevo, Puebla. Bachelor Thesis. Universidad Nacional Autónoma de México, México. Delevoryas, T., Person, C.P., 1975. Mexiglossa varia gen. et sp. nov., a new genus of glossopteroid leaves from the Jurassic of Oaxaca. México. Palaeontobrap. Abt. B. 154, 114–120. Delevoryas, T., 1991. Investigations of north American cycadeoids: Weltrichia and Williamsonia from the jurassic of Oaxaca, Mexico. Am. J. Bot. 78, 177–182. Farrell, K.M., 1987. Sedimentology and facies architecture of overbank deposits of the Mississippi River, False River region, Louisiana. In: In: Ethridge, F.G. (Ed.), Recent Developments in Fluvial Sedimentology: Society of Economic Paleontologists and Mineralogists Special Publication, vol 39. pp. 111–120. Francis, J.E., 1983. The dominant conifer of the jurassic purbeck formation, england. Palaeontology 26, 277–294. Gerwert, N.M., Villanueva, A.U., Martini, M., Ramírez, C.M., 2015. Asociaciones palinológicas mesozoicas en Santo Domingo de Tianguistengo. XIV Congreso Nacional de Paleontología. Coahuila, México. Givnish, T.J., 1979. On the adaptative significance of the leaf fron. In: Solbring, O.T.S., Jain, G. B. Johnson, Raven, P.H. (Eds.), Tropics in Plant Population Biology. Columbia University. Press, New York. Givnish, T.J., 1984. Leaf and canopy adaptations in tropical forest. In: Medina, E., Mooney, H.A., Vazquez-Yañez, C. (Eds.), Phisiological Ecology of Plants of the Wet Tropics. The Hague, pp. 51–84.

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