Early Permian conifer remains from Central Mexico and reevaluation of Paleozoic conifer morphotaxa

Restos de coníferas del Pérmico temprano de México central y reevaluación de los morfotaxones de coníferas del Paleozoico

Genaro R. Hernández-Castillo^1,*, Silvia A. Silva-Pineda², Sergio R.S. Cevallos²

¹ Paleoconifer Consulting Ltd., Edmonton, AB, T6X 0X7, Canada. ^*paleoconifer@gmail.com

Manuscript received: February 22, 2013.
Corrected manuscript received: April, 10, 2013.
Manuscript accepted: September 7, 2013.

Abstract

This paper describes the first Permian walchian conifer from Mexico based on several specimens preserved as compressions/impressions. The material comes from the Tuzancoa Formation in the state of Hidalgo, Central Mexico. It is described as Calnalia hidalguensis gen. nov. et sp. nov. a new conifer morphotaxon based on isolated lateral branches with leaves. This new conifer has some of the smallest branches and leaves found among previously described walchians. Calnalia hidalguensis has two types of branches (ovoid and deltoid) with simple and helically arranged leaves. Leaves are narrowly triangular to linear in front view, slightly concave and spreading (penultimate shoots) to slightly concave to slightly S-shaped (ultimate shoots), in side view. Leaves range from 3.2 mm long by 0.5 mm wide on penultimate shoots, to 1.3 - 2.9 mm long by 0.4 - 0.9 mm wide on ultimate shoots, with angles of leaf insertion ranging from 16º to 39º. The new species is most similar to Emporia lockardii Mapes and Rothwell, Walchia goeppertiana Florin, Walchia mucronata Florin, and Otovicia hypnoides Kerp, Poort, Swinkels & Verwer. However, none of these have the same ranges of morphological variation found in the leaves of Mexican species. The new taxon expands the diversity of walchian conifers at the end of the Paleozoic in America, and marks the beginning of new research of Permian floras in Mexico. The presence of C. hidalguensis and other gymnosperms, to be described, reinforces the presence of an arc along the Oaxaquia block that connected Mexico to the North American craton during the late Paleozoic. These new data also allow us to test hypotheses about the origin and evolution of Paleozoic floras in Euramerica.

Keywords: conifer, Mexico, morphotaxa, Permian, Walchia.

Este artículo describe la primera conífera pérmica "walchiana" con base en varias compresiones e impresiones fósiles de la Formación Tuzancoa en el Estado de Hidalgo, Centro de México. Se describe como Calnalia hidalguensis gen. nov. et sp. nov., un nuevo morfotaxón de coníferas basados en ramas laterales y hojas. Esta nueva conífera tiene unas de las ramas más pequeñas descritas hasta ahora para los miembros "walchianos" en el registro fósil, y cuenta con dos tipos de ramas (ovoide y deltoide), las cuales portan hojas simples con filotaxia helicoidal. Las hojas son ligeramente triangulares a lineares; lateralmente son ligeramente cóncavas a abiertas (penúltimo orden), y ligeramente cóncavas a ligeramente en forma de "S" (último orden). El tamaño de las hojas de penúltimo orden es de 3.2 mm de largo por 0.5 mm de ancho y en las hojas de último orden de 1.3 - 2.9 mm de largo por 0.4 - 0.9 mm de ancho, con ángulos de inserción de las hojas de 16º a 39º. La nueva especie posee mayor similitud con Emporia lockardii Mapes and Rothwell, Walchia goeppertiana Florin, Walchia mucronata Florin y Otovicia hypnoides Kerp, Poort, Swinkels & Verwer. Sin embargo, ninguna de estas especies tiene los mismos rangos de variación morfológica que las hojas de la nueva especie Mexicana presentan. Este nuevo taxón aumenta la diversidad de estas coníferas conocida a finales del Paleozoico en el continente Americano. A su vez, la descripción de esta nueva especie abre la investigación paleobotánica durante el Pérmico en México, y su presencia, así como la de otras especies de plantas aún por describir, refuerzan la teoría de la existencia de un arco a lo largo del bloque Oaxaquilla que conectaba a México con el Cratón de Norteamérica a finales del Paleozoico. Estos datos nos ayudan a proponer hipótesis sobre el origen y evolución de las floras Paleozoicas de Euramérica.

Palabras clave: conífera, México, morfotaxa, Pérmico, Walchia.

1. Introduction

The earliest fossil conifer remains of Mexico have been described from early Permian sediments of the San Mateo area between the states of Hidalgo and Veracruz (Carrillo-Bravo, 1965; Silva-Pineda, 1987). This fossil material consisted of few isolated and highly fragmentary ultimate shoots of a Walchia-like plant. Additional conifer remains have been collected from Paso Hondo, Tuzancoa, and Patlanoaya Formations from Chiapas, Hidalgo, Veracruz and Puebla respectively (Carrillo-Bravo, 1965; Hernández-García, 1973; Moreno-Cano and Patiño-Ruiz, 1981; Silva-Pineda, 1987; Silva-Pineda et al., 2000; Vachard et al., 2000; Silva-Pineda, 2003). These conifers are few in number and preserved as impressions and compressions with poor cuticles. The early Permian age (Wolfcampian-Leonardian) determinations for Mexican localities are based on key brachiopods, pelecipeds and fusulinids species (Carrillo-Bravo, 1965; Hernández-García, 1973; Moreno-Cano and Patiño-Ruiz, 1981; Vachard et al., 2000).

New material has been collected from Tuzancoa Fm. near Calnali river, Hidalgo (Figure 1). These fossils allow us to describe a new conifer species from the early Permian of Mexico. The material is characterized by isolated and fragmentary lateral branches with simple leaves that resemble many other walchian conifers around the world (Florin, 1938-45; Rothwell, 1982; Clement-Westerhof, 1987; Meyen, 1997; Rothwell et al., 1997; Kerp and Clement-Westerhof, 1991). Traditionally, fragmentary conifer remains were described as species belonging to form-genera (Florin, 1938-45; Mapes and Rothwell, 1991; Visscher et al., 1986), now known as morphotaxa (Greuter in McNeill et al., 2006). Paleozoic conifer morphotaxa lack familial affinities due to their incomplete nature and broad ranges of morphological and cuticular variation (Hernandez-Castillo et al., 2001; Hernandez-Castillo, 2005; Rothwell et al., 2005). Thus, prompting the use of different approaches and classifications to understand Paleozoic walchian conifers.

Three classification systems have been proposed for Paleozoic conifers (Mapes and Rothwell, 1991). The first classification was proposed by Florin (1927, 1938-45) and it was later improved and modified by Clement-Westerhof and colleagues (Clement-Westerhof, 1984, 1987; Visscher et al., 1986; Kerp and Clement-Westerhof, 1991). Although both classification systems are and/or have been used widely, they do not recognize nor integrate overlapping ranges of morphological and cuticular variation found among most walchians species (Hernandez-Castillo et al., 2001). A third classification does employ these broad ranges of variation for species delimitation (Mapes and Rothwell, 1991), therefore providing robust organcorrelations that allow reliable and complete conifer reconstructions (Hernandez-Castillo et al., 2001; Rothwell and Mapes, 2001; Hernandez-Castillo, 2005; Rothwell et al., 2005).

Although, this last method has proven to be most effective in reconstructing conifers as species of extinct plants, no new methods have been recently proposed for dealing with poorly preserved conifer morphotaxa. Therefore, the new Mexican material, from Calnali, offers an opportunity to reevaluate and update current approaches for describing and naming Paleozoic conifer morphotaxa. Furthermore, the discovery of walchian conifers, in central Mexico, may serve to test hypothesis on the origin and accretion of Mexican terrenes and blocks at the end of the Paleozoic, as previously discussed by other authors (Ortega-Gutierrez et al., 1995; Sánchez-Zavala et al., 1999; Vachard et al., 2000; Centeno-García, 2005).

The fossil material was collected 0.5 km from Calnali, Hidalgo, near Calnali River (Figure 1). This locality was originally described as part of the Guacamaya Formation (Carrillo-Bravo, 1965). However, Ochoa-Camarillo (1996) divides the Guacamaya Formation into the Tuzancoa and Otlamalacatla Formations. Recent work has provided evidence that Calnali Permian strata correspond to the Tuzancoa Formation (Rosales-Lagarde et al., 1997, 2002; Sour-Tovar et al., 2005). The Tuzancoa Formation is exposed in the northeast and central-east portions of Hidalgo state, Mexico (20º 55'- 20º 35' N and 98º 40'- 98º 30 W, Figure 1). This formation is part of the Sierra Madre Oriental and belongs to the Huayacocotla Anticlinorium (Rosales-Lagarde et al., 1997). Tuzancoa Formation is a siliclastic sequence that switches to lava flows and andesitic and basaltic breccia, intercalating with sandstones, shales and lutites of volcanic origin (Figure 2). This formation is found between Precambrian metamorphic rocks (Huiznopala Gneiss) and discordant Mesozoic units (Sour-Tovar et al., 2005; Figure 2). The fossil material comes from a turbiditic section characterized by mid-fine grain vulcanoclastic sandstones alternating with siltstones that correspond to the upper most section described at Tuzancoa's Fm. type section (section 5 of Rosales-Lagarde, 2002).

The Wolfcampian to Leonardian age on this material is constrained by at least two invertebrate localities that are located above and below the fossil plant bearing strata (Figure 2; Carrillo-Bravo, 1965; Moreno-Cano and Patiño-Ruiz, 1981; Rosales-Lagarde, 2002; Sour-Tovar et al., 2005). This fossil conifer material is associated to fragmentary remains of Pecopteris, Neuropteris, Odontopteris, cf. Gangamopteris and Taeniopteris (Silva-Pineda, 1987), and poorly preserved gigaptopterid-like? leaf remains.

The specimens are preserved as compressions/impressions in fine to medium grain sandstones and fine-grained shales associated to large grain sandstones, limestones and tobes (Figure 2; Carrillo-Bravo, 1965; Ochoa-Camarillo, 1996; Silva-Pineda et al., 2000). Specimens were prepared using needles to remove sediments on top of branches and leaves. Light microscopy was conducted using Zeiss Stemi DV4 microscope. Images were captured with a Nikon CoolPix 4300 (Nikon Corp., Japan) and an AxioCam MRc Rev. 2/3.3V (Zeiss Inc., Germany) digital cameras. Images were processed using Adobe Photoshop CS3. Measurements of digitized specimens were taken using Image J ver. 1.36 b (Abramoff et al., 2004) according to Hernandez-Castillo (2005). All specimens are stored in the Colección del Laboratorio de Paleobotanica, Instituto de Geología, Mexico City, Mexico with collection numbers IGM-PB 1254- IGM-PB 1284.

3. Systematics

Class – Coniferopsida
Order – Voltziales
Family – Unknown
Genus – Calnalia Hernandez-Castillo, Silva-Pineda et Cevallos Ferriz

Calnalia hidalguensis diagnosis – Lateral branches plagiotropic, ovoid to deltoid, 4.0 to 15 cm long and 4.5 to 9.4 cm wide; shoots of penultimate order up to 15 cm long, 0.4 cm wide; shoots of ultimate order up to 4.8 cm long, 0.21 cm wide; leaves on penultimate shoots helical, simple, narrowly triangular to linear (front view), slightly concave and spreading (side view), 2.6-3.2 mm long, 0.3-0.5 mm wide; leaves on ultimate shoots helical, simple, narrowly triangular to linear (front view), slightly concave to slightly S-shaped (side view), 1.3-2.9 mm long, 0.4-0.9 mm wide, angles of insertion 16º-39º (Figures 2-11).

Holotype: Penultimate shoot bearing ultimate shoots with leaves. Specimen IGM-PB 1254, Figures 2, 6.

Paratypes: Branches showing leaf morphology on penultimate and ultimate shoots, IGM-PB 1256, IGM-PB 1258 (Figures 3, 4). Leaves of penultimate shoots IGM-PB 1263, IGM-PB 1264 and IGM-PB 1254 (Figure 5, 6, 7). Leaves on ultimate shoots IGM-PB 1263 (Figures 8, 11), IGM-PB 1274 (Figure 9), IGM-PB 1279 (Figure 10).

Collecting locality: Calnali River (20º 55'- 20º 35' N and 98º 40'- 98º 30 W), Municipio de Calnali, Hidalgo, Mexico. Figure 1.

Stratigraphic occurrence and age: Huayacocotla Anticlinorium, Tuzcanoa Formation, early Permian (Wolfcampian-Leonardian/Sakmarian-Artinskian), Figure 2.

Etymology: The specific epithet hidalguensis refers to Hidalgo State in Central Mexico.

4. Description

The new species is represented by small lateral plagiotropic branches with a penultimate shoot bearing several ultimate shoots (Figures 2-5) (3, 4). Both branching orders are covered by helically arranged simple leaves (Figures 6-11). Branches range in shape from ovoid to slightly deltoid (Figures 2-5). Branches are small (up to 15 cm long), but most of them are often broken. Penultimate shoots are up to 15 cm and up to 4 mm wide. Ultimate order shoots may be straight or slightly recurved depending on branch type and size (Figures 2-5)(3, 4). Some branches display straight ultimate shoots with insertion angles ranging from 45º to 90º (Figures 2-4), while other ones have ultimate shoots that are recurved giving the branch an ovate shape (Figures 3, 5).

Shoots of ultimate order are found attached to penultimate shoots in lateral branches, or isolated and often fragmentary (Figures 2-4; Figures 9, 10). Simple and helically arranged leaves cover these shoots. Leaves range from 1.3 to 2.9 mm in length and 0.4 to 0.9 mm in width. They are narrowly triangular to linear in front view, and slightly S-shaped to slightly concave in side view (Figures 8-11). They can be adpressed with angles of insertion ranging from 16º to 24º (Figures 8-11)to slightly adpressed with angles ranging from 25º to 39º (Figures 9, 10). Although leaves on ultimate shoots may be slightly to fully adpressed, their angles of insertion at the base and apex of the ultimate shoot are rather similar. Leaf shape (in side view) and angles of insertion depend on the branch type, position within the branch, and overall preservation (Figures 8-11). No heterophylly has been observed between the leaves on penultimate and ultimate shoots so far.

5. Disscusion

5.1. Calnalia hidalguensis and its relationship to Paleozoic walchian conifers

Calnalia hidalguensis is described as a new morphotaxon for vegetative lateral plagiotropic branches of Paleozoic conifers, with simple, helically arranged leaves and no cuticles preserved. Lateral branches of C. hidalguensis conform to the traditional walchian conifer morphology (Florin, 1938-45). Walchian conifers are common gymnosperms from Upper Paleozoic sediments of Euramerica (Florin, 1938-45; Rothwell, 1982; Clement-Westerhof, 1984). These conifers typically display lateral plagiotropic branches composed of a penultimate shoot that bears several ultimate shoots similar to those of C. hidalguensis. This new taxon is similar to commonly known Euramerican Paleozoic conifers previously described by Florin (1938-45), Winston (1984), Kerp et al. (1990), Mapes and Rothwell (1991), Hernandez-Castillo et al.(2001), and Hernandez-Castillo (2005).

Calnalia hidalguensis have simple leaves on penultimate shoots that contrast with taxa bearing forked penultimate leaves (Table 2). The presence or absence of simple or forked leaves on penultimate shoots is used to distinguish taxa and heterophylly in conifers (Hernandez-Castillo et al., 2001). This is important because some of these conifers display different types of heterophylly, suggesting different tree architectural types, similar to those found in living conifers (Rothwell and Mapes, 2001; Hernandez-Castillo et al., 2003; Hernandez-Castillo, 2005). Plagiotropic branches of C. hidalguensis display penultimate order shoots bearing leaves that are most similar to those of Lebachia piniformis var. solmsii Florin (Table 2). Unfortunately, not many penultimate order leaves have been found, which prevents us from more comprehensive comparisons to the very few walchian species with well-preserved penultimate leaves.

Shoots, of ultimate order, bear some of the smallest leaves found among walchian conifers of Europe and North America (Table 3). Leaves range from 1.3 to 3.2 mm long and fall within the ranges of variation of species such as Emporia lockardii (Mapes & Rothwell) Mapes & Rothwell, two new Emporia species, Emporia cryptica and Emporia royalii, from the Hamilton Quarry (Hernandez-Castillo et al., 2009b, 2009c), Lebachia goeppertiana Florin, Lebachia mucronata Florin, and Otovicia hypnoides Kerp, Poort Swinkels & Werver (Table 3). In contrast, other euramerican conifers, such as Lebachia americana Florin, Lebachia angustifolia Florin, Lebachia garnettensis (Florin) Winston, Thucydia mahoningensis Hernandez-Castillo, Rothwell and Mapes, and Utrechtia floriniformis Mapes and Rothwell, display larger leaves than those of Calnalia (Table 3). Angles of insertion of leaves, in most euramerican taxa, either overlap or are greater than those of Calnalia (Table 3). Therefore, no other previously described walchian conifer morphotaxa have leaves with same ranges of variation as those recorded for Calnalia hidalguensis.

5.2. Conifer morphotaxa

Most Paleozoic conifer morphotaxa lack familial affinities due to broad and/or overlapping ranges of morphological and cuticular variation of leaves and leaf-like structures such as, pollen cone microsporophylls, ovulate cone bracts and sterile scales (Hernandez-Castillo et al., 2001; 2009b). Species identification, including familial affinities among these conifers, are controversial due to these overlapping ranges of variation and the absence of organic connections (Hernandez-Castillo et al., 2001; Rothwell et al., 2005; Hernandez-Castillo, 2005). The lack of unequivocal sets of diagnostic characters, used to characterize and distinguish walchian conifer species, has prompted different approaches to solving Paleozoic conifer systematics. Hence, each approach creates different classifications with alternative morphotaxa (Table 1; Visscher et al., 1986; Mapes and Rothwell, 1991).

Traditional approaches allow workers to describe conifer remains based on conventional morphological and/or cuticular characters (Florin, 1927, 1938-45; Clement-Westerhof, 1984, 1987, 1988; Visscher et al., 1986; Kerp et al., 1990). However, these approaches fail to acknowledge and deal with known broad ranges of variation within and among walchian conifer species (Hernandez-Castillo et al., 2001; Hernandez-Castillo, 2005). A third approach proposed by Mapes and collaborators allows us to accurately correlate different conifer organs in order to reconstruct new conifer plants and survey known ranges of variation (Rothwell and Mapes, 2001; Hernandez-Castillo et al., 2001; Hernandez-Castillo et al., 2003). Nevertheless, this approach requires of dozens or hundreds of conifer remains.

Due to the small amount of fossil conifer specimens and poor preservation (no cuticles), we decided to use a combination of techniques from previous approaches, where few specimens are described using conventional morphological characters (i.e., Florin and Clement-Westerhof) and their ranges of variation (Hernandez-Castillo, 2005), by surveying fourteen morphological characters of branches and leaves and comparing them to previously described species (Hernandez-Castillo and Silva-Pineda, 2009). This will permit employing a combination of approaches and preparing our material for future comparisons with the best-known and reconstructed walchian conifers from Euramerica.

Calnalia hidalguensis lateral branches and leaves are some of the smallest ever found among walchian conifers (Table 1). Comparisons to previous taxa suggest than both shape and size of Calnalia branches relate to apical and medium-size branches of young or juvenile walchian and araucarian trees (Hernandez-Castillo et al., 2003, 2008, 2009). This assumption is supported by the lack of a) fully developed oblong-shaped lateral branches (basal and/or mature branches); b) narrowly oblong-shaped branches (most apical branches), and c) reproductive structures, as seen in most previously described morphotaxa and newly reconstructed Paleozoic conifers (Florin, 1938-45; Clement-Westerhof, 1994; Mapes and Rothwell,1991; Hernandez-Castillo et al., 2003, 2008, 2009a, 2009c; Looy, 2007).

Calnalia is here represented by a small sample of young branches and leaves with unique ranges of morphological variation that do not correspond to any known walchian conifer, and as such deserves to be placed in its own genus and species. The use of previously assigned morphotaxa names is not recommended for newly discovered species with distinct ranges of morphological variation if 1) the taxon is undoubtedly correlated to unknown and/or overlapping ranges of morphological variation (e.g., Walchia); and 2) if the taxon is in conflict with current classifications (Visscher, et al., 1986; Mapes and Rothwell, 1991; Hernandez-Castillo et al., 2001). This approach prevents us from using ambiguous morphotaxa names that would lead to more confusing walchian conifer species concepts.

5.3. Permian outcrops and conifers of Mexico.

Walchia hidalguensis is the first species of conifer fossils to be described from the Permian of Mexico and its general features resemble most closely Euramerican Lebachioid conifers. In addition, Walchia expands the present distribution for walchian conifers in North America. Typical Lebachioid conifers (Rothwell et al., 2005) have been described and reconstructed from the Upper Carboniferous to the Late Permian of Europe and North America. However, additional records have been found in North Africa and the Middle East (El Wartiti et al., 1986; Broutin et al., 1986, 1998; Berthelin et al., 2003), and new sites in Texas (USA) and Blanzy (France) (Bercovici and Broutin, 2007; Looy, 2007), therefore expanding the geographic distribution of Lebachioid walchians at the end of the Paleozoic (Florin, 1938-45; Rothwell, 1982; Mapes and Rothwell, 1984; Rothwell et al., 1997).

Walchian fossils have been collected from sediments that currently are located in central Mexico, but this region was once part of the southern most region of the Sierra Madre terrane (Rosales-Lagarde, 2002; Centeno-García, 2005). The Sierra Madre terrane was part of the Oaxaquia block along with Coahuila terrane. It is believed that the Oaxaquia block was part of Gondwana until it collided with North America at the Carboniferous-Permian boundary (Pindell, 1985; Centeno-García, 2004, 2005). Different models are now trying to explain this accretion process and posterior formation of the Mexican paleogeographic regions (Pindell, 1985; Ortega-Gutierrez et al., 1995; Dickinson and Lawton, 2001). One of the models with the strongest evidence (Pindell's model) relies on strong faunal associations during the Mississippian among the Coahuila and Sierra Madre terranes, and the mid-continent province in North America (Stewart et al., 1999; Navarro-Santillán et al., 2002; Centeno-García, 2005), suggesting that these terranes were part of the North American Craton at the end of the Paleozoic. The presence of Calnalia and other taxa yet to be described in the Sierra Madre terrane reinforces this model. Preliminary surveys of at least six localities in the Tuzancoa Formation suggest similarities with Permian floras of Texas (DiMichele et al., 2000, 2001). Furthermore, walchian conifer remains found in Texas resemble typical Lebachioid conifers and Walchian Voltziales (DiMichele et al., 2000, 2001; Looy, 2007, 2007a). Some of these Texan conifers resemble branches/leaves of Calnalia, while others resemble typical branches/leaves and cones of Upper Permian Majonicaceae conifers (Looy, 2007). These facts add more evidence of the presence of an arc found along the Coahuila and Sierra Madre terranes (Oaxaquia block) that connected Texas to these Mexican terranes by the early Permian (Centeno-García, 2005).

Acknowledgement

We thank Hector Ochoa-Camarillo (1996), Blanca Buitrón and Carlos Castañeda-Posadas for their help in collecting some of the material, and Laura Calvillo-Canadell, Sergio R. S. Cevallos-Ferriz and Gar W. Rothwell for their helpful comments of this manuscript. This research was supported in part by DGAPA and CONACyT (Postdoctoral Grants 051006 and 050213) and CONACyT research grant 054521 to GRHC.

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