NOTA PALEONTOLÓGICA

New evidence for the age of the Jurassic Flora from Cañadón del Zaino, Sierra de Taquetrén, Chubut

Ignacio Escapa¹,², Rubén Cúneo¹,² and Gerardo Cladera²

¹Consejo Nacional de Investigaciones Científicas y Técnicas.
²Museo Paleontológico Egidio Feruglio, Av. Fontana 140, 9100 Trelew, Argentina.

Introduction

Jurassic taphofloras from the Chubut Province have been only sporadically studied in the past. These studies were based on a limited number of specimens from a few localities (Cazaubón, 1947; Frenguelli, 1949; Bonetti, 1964; Herbst and Anzótegui, 1968; Cortes and Baldoni, 1984). These initial studies were later reviewed by Stipanicic and Bonetti (1970), Nullo and Proserpio (1975) and Baldoni (1980; 1981; 1990), who included revisions of the plant taxa and their chronostratigraphical settings.
Of the different localities bearing Jurassic floras, the Cañadón del Zaino site (Sierra de Taquetrén, Chubut Province) (figure 1), which was originally discovered by Prof. R. Casamiquela in the early ‘60s, has yielded the most diverse assemblage consisting of several plant groups. Fossil plants collected there were studied by Bonetti (1964) who described materials belonging to 14 species now deposited at the Paleobotanical Collection of the Museo Argentino de Ciencias Naturales "Bernardino Rivadavia" (BA Pb). A second collection was later studied by Herbst and Anzótegui (1968), who added two new taxa to those initially described by Bonetti (op. cit.). These specimens are stored at the Paleobotanical Collection of the Facultad de Ciencias Exactas, Físicas y Naturales de la Universidad del Nordeste (PB-CTES).

Figure 1. Location map of fossil plant locality of Cañadón del Zaino, Sierra de Taquetrén, Chubut Province / mapa de ubicación de la locadidad plantífera en el Cañadón del Zaino, Sierra de Taquetrén, provincia de Chubut. Modified from Lizuain and Silva Nieto (2005) and Aragón et al. (2000) / modificado de Lizuain y Silva Nieto (2005) y Aragón et al. (2000).

The plant-bearing sediments at the Cañadón del Zaino locality in the Sierra de Taquetrén were assigned to the local Taquetrén Formation by Nullo and Proserpio (1975), who suggested a Late Jurassic age based on its paleobotanical content. Later, Lizuain and Silva Nieto (1996) regarded these deposits as volcaniclastic facies of the more regional Lonco Trapial Formation, which has been considered Middle-Late Jurassic in age (Nullo, 1983; Pankhurst et al., 1998) based on radiometric dating. We agree here with Lizuain and Silva Nieto (2005) in that the former Taquetrén Fm. corresponds to volcaniclastic facies of the Lonco Trapial Fm. In this context, the plant-bearing deposits here analyzed stratigraphically lay between the lower Las Leonera Formation of Early Jurassic age (Nakayama, 1972) and the upper and slightly younger classic Lonco Trapial Fm. lavic facies.
The age of the Cañadón del Zaino flora, as suggested by Bonetti (1964) and Herbst and Anzótegui (1968), was mainly based on close comparison with the Middle Jurassic flora from Hope Bay in Antarctica (Halle, 1913; Gee, 1989), which has been widely used, together with the Botany Bay flora also from Antartica, as geochronological markers for other Jurassic floras in Gondwana. Stipanicic and Bonetti (1970) agreed on the similarities between the Jurassic floras from Chubut Province and Antarctica but suggested a relatively longer time span (Middle to Late Jurassic or Callovian-Oxfordian) for the Chubut floras. However, recent collections from Antarctica reviewed by Rees (1993), Cantrill (2000) and Rees and Cleal (2004), indicate an early Jurassic age for the Hope Bay Flora, which was the age suggested earlier by Orlando (1971). On these grounds, the Cañadón del Zaino flora at the Sierra de Taquetrén might also be considered as Early Jurassic based on the taxonomic comparisons with the Hope Bay Flora made by Bonetti (1964). This hypothesis is supported here based upon new collections from the classical Cañadón del Zaino locality (Escapa et al., 2007), which incorporate an even greater number of plant species shared with the Antarctic Hope Bay Flora. The new specimens include all the species previously described for the locality (Bonetti, 1964; Herbst and Anzótegui, 1968), as well as some new taxa (Escapa and Cúneo, in prep.)

Comparisons between the Hope Bay and Cañadón del Zaino (Sierra Taquetrén) Floras

Based on the presence of six shared morphospecies, Bonetti (1964) suggested a close similarity between the Hope Bay and Cañadón del Zaino (Sierra Taquetrén) floras. These species are: Cladophlebis denticulata (Brongniart) Fontaine emend. Harris; Cladophlebis cf. antartica Halle; Sphenopteris nordenskjöldii Halle emend. Rees and Cleal; Elatocladus conferta (Oldham and Morris) Halle, Pagiophyllum feistmanteli Halle emend. Townrow and Archangelskya furcata (Halle) Herbst emend. Rees and Cleal (see also Stipanic and Bonetti, 1970; Nullo and Proserpio, 1975). However, the real value of these morphospecies for taxonomic comparisons is highly questionable since, as it has been discussed by Halle (1913), several morphospecies do not necessarily reflect natural systematic entities. For example, the morphogenus Cladophlebis Brongniart has been associated with the family Osmundaceae (Phipps et al., 1998) as well as with the families Schizaeaceae and Cyatheaceae (Halle, 1913). Sphenopteris is another example in this regard. Addition examples include the conifers Pagiophyllum and Elatocladus, which include species referred to more than one genus and even family (e.g. some forms of Pagiophyllum are often related to the families Araucariaceae and Cheirolepidiaceae). Thus, considering that a comparative floristic analysis of two floras for geochronological correlation purposes is basically a comparative study of biotic similarity, the use of morphospecies based on potentially homoplasic characters (i.e., resemblance not originated from common ancestry) could easily mislead the interpretations. Therefore, although the information originating from comparisons of morphotaxa cannot be completely discarded (especially if similarities involve a large number of morphotaxa), the shared presence of a few morphospecies should not be considered as decisive evidence for postulating the similarities of two floras.
Despite the previous considerations, it is important to emphasize that primary similarities found by Bonetti (1964) between the Hope Bay and Cañadón del Zaino (Sierra Taquetrén) Floras is remarkable. This similarity is further enhanced by the presence of taxa with systematic affinities to the genera Sagenopteris (Caytoniaceae) and Goeppertella (Dipteridaceae) (figures 2.1, 4).

Figure 2. 1, MPEF-Pb 1758. Goeppertella sp., frond fragment showing the classical reticulate venation pattern also recognized in most of the dipteridaceous ferns / Goeppertella sp., fragmentos de fronde mostrando el clásico patrón de venación reticulada típico de Dipteridaceae. 2, MPEF-Pb 2074. Sagenopteris nilssoniana, compound leaf with four leaflets / Sagenopteris nilssoniana, hoja compuesta mostrando cuatro folíolos. 3, MPEF- Pb 2094. Goeppertella sp., bipinnate frond bearing pinnae and rachial pinnules / Goeppertella sp., fronde bipinnada portando pinnas y elementos subsidiarios. 4, MPEF-Pb 2713. Sagenopteris nilssoniana, detail of leaflets morphology and venation / Sagenopteris nilssoniana, detalle de la morfología y venación de un folíolo. 5,6, MPEF-Pb 2147a and 2146a. Clathropteris obovata, oishi laminar fragments / Clathropteris obovata, fragmentos de la fronde. Bar scale 1 cm / escala 1 cm.

The occurrence of the genus Sagenopteris in the Cañadón del Zaino flora was first noted by Bonetti (1964) with the species S. nilssoniana (Brongniart) Ward (=S. rhoifolia Herbst and Anzótegui). New specimens recently collected from the same locality (figures 2.2, 4), as well as from a new site that correlates stratigraphically with the original site, confirms the definitive presence of this taxon in Chubut, under-scoring its close resemblance with the Early Jurassic Antarctic floras. The chronostratigraphic distribution of the genus Sagenopteris in the Northern Hemisphere is mostly from Lower-Middle Jurassic units (Cantrill, 2000). On the other hand, the dipteridaceous genus Goeppertella Oishi and Yamasita spans from the Late Triassic through the Early Jurassic in the Northern Hemisphere, and has never been found in younger floras (Oishi and Yamasita, 1936; Harris, 1946 and references therein). In the Southern Hemisphere, Goeppertella has been reported from the Lower Jurassic of Argentina and Antarctica (Herbst, 1964; 1966; 1975; 1991; Arrondo and Petriella, 1982; Rees, 1993; Rees and Cleal, 2004). However, it must be taken into account that Goeppertella can be and has been easily confused with the genus Dictyophyllum, another dipteridaceous fern, especially when only fragmentary pieces of lamina occur, due to the reticulate venation pattern present in both genera. In this regard, the specimens collected from the Cañadón del Zaino locality are very well preserved and most of the characters that define the genus such as bipinnate fronds and the presence of subsidiary pinnular elements on the rachis, can be observed (figures 2.1, 2.3).
The coeval occurrence of Goeppertella and Sagenopteris in the Cañadón del Zaino (Sierra Taquetrén) Flora is highly relevant for comparisons with the Early Jurassic Hope Bay flora especially in regards to age determination. Similarly, the pteridosperm Archangelskya furcata is considered an indicator for the Lower Jurassic. Furthermore, the great dominance and diversity of the Cycadophyta is another trait of the Cañadón del Zaino Flora (Bonetti, 1964; Herbst and Anzótegui, 1968) that probably characterizes Early Jurassic floras of Argentina, since this group shows a marked decrease in abundance and diversity in most of the Middle-Late Jurassic floras (see Frenguelli, 1949; Baldoni, 1990). For instance, in the entire Cañadón Asfalto Formation plant record (Middle-Late Jurassic) from the Chubut Province, not one Cycadophyte has been recorded so far (Frenguelli, 1949; Cortes and Baldoni, 1984; Escapa et al., 2007), and a sparse and sporadic record occurs at other localities.
The cooccurrence of Goeppertella-Sagenopteris has also been recorded in other Early Jurassic floras in Antarctica (Botany Bay Flora; see Rees and Cleal, 2004), Chubut (Cerro Meschio Flora, Herbst, 1964) and Neuquén (Piedra Pintada flora, Herbst, 1966), while the genus Goeppertella has been exclusively mentioned from a number of Early Jurassic floras in Patagonia (Cazaubón, 1947; Arrondo y Petriella, 1980, 1982 and references therein).
The flora from Piedra Pintada in Neuquén Province closely resembles that of Cañadón del Zaino in Chubut Province. In addition to all taxa mentioned above, the presence of Clathropteris obovata Oishi (figures 2.5-6), Archangelskya furcata and other morphotaxa are common for the two floras. It is also noteworthy that the age of the Piedra Pintada flora has been established as Early Jurassic (Pliensbachian) based on the presence of associated invertebrate faunas (see Damborenea et al., 1975; Riccardi et al., 2000).

Conclusions

The fossil flora from Cañadón del Zaino (Sierra de Taquetrén), originally thought to be Middle-Late Jurassic, is actually Early Jurassic in age. This is based on the presence of natural taxa (e.g., Goeppertertella, Sagenopteris, Clathropteris, Archangelskya) that are good chronostratigraphic markers, as they have been recorded from other localities with additional paleontological age control. In this regard, it is important to emphasize that none of these taxa are reliable age indicators by themselves. Moreover, some of them could show a time span that extends into the early Middle Jurassic. However, their common occurrence represents strong evidence in support of an Early Jurassic age for those floras where they occur. Therefore, the original Middle-Late Jurassic age for the Cañadón del Zaino flora should be discarded based on the new evidence and comparisons presented here.
The Cañadón del Zaino Flora shares a large number of natural plant taxa with coeval floras from both Patagonia and Antarctica. In particular, the Piedra Pintada flora in Neuquén Province and the Hope Bay flora in Antarctica are both highly comparable, and represent along with the Cañadón del Zaino Flora in Chubut, the most highly diverse and best-preserved Early Jurassic floras in the Southern Hemisphere.
Finally, from a geological viewpoint, it is clear that the local stratigraphy and further delimitation of formational units should be revised in the light of the new age suggested for the strata bearing the Cañadón del Zaino flora in the Sierra of Taquetrén.

Acknowledgments

We thank the reviewers A. Lizuain, D. Silva Nieto, C. Gee and E. Aragón who provided critical suggestions that enhanced the quality of this contribution. M. Caffa, P. Puerta, L. Canesa, M. Cárdenas and R. Gonzáles were most helpful during the field-work. This contribution was partially funded by grant BID 1728/OC-AR - PICT 1516 "Estudio macro y micro florístico del Jurásico Medio y Superior de Chubut: Sistemática, Bioestratigrafía y Paleoecología".

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Recibido: 27 de septiembre de 2007.
Aceptado: 30 de julio de 2008.