Earliest chitinozoans discovered in the Cambrian Duyun fauna of China Cen Shen1, Richard J. Aldridge2, Mark Williams2, Thijs R.A. Vandenbroucke3, and Xi-guang Zhang1* 1
Key Laboratory for Paleobiology, Yunnan University, Kunming, Yunnan 650091, China Department of Geology, University of Leicester, Leicester LE1 7RH, UK 3 Géosystèmes, UMR 8217 du CNRS, Université Lille 1, 59655 Villeneuve d’Ascq, France 2
ABSTRACT Chitinozoans are an enigmatic group of organic-walled marine microfossils with an abundant Lower Ordovician to Upper Devonian fossil record. They achieved maximum species diversity during the Middle Ordovician–Silurian, and their history during the group’s acme is well documented. Nevertheless, information about their origin and early evolution is sparse. Here we report three phosphatized flask-shaped vesicles recovered from the Cambrian Stage 5 (~510 m.y. old) Duyun fauna of southern China as the earliest known chitinozoans, Eisenackitina? sp., extending the record of the microfossil group back by at least 20 m.y. Their exceptional occurrence within an Orsten-type Lagerstätte might imply a benthic mode of life. This discovery is significant in that it supplies critical data for evaluation of the basic morphology of Cambrian chitinozoans and assessment of their early evolution.
near Balang village in Duyun, Guizhou, China (for locality details, see Zhang et al., 2011, their figure 1), were digested in acetic acid of low concentration (5%), and the insoluble residue was taken out of the acid solution every day. All phosphatized specimens were picked from the residue under a stereomicroscope (at 20× magnification), and then images were taken (secondary electron: 10–20 kV) using scanning electron microscopy (model FEI Quanta 200). SYSTEMATIC DESCRIPTION Despite being based solely on morphological characters, the biological significance of which is largely unknown, a Linnaean classification is adopted for chitinozoans. Here we follow the most recent generic and suprageneric classification of Paris et al. (1999). Chitinozoa Family Desmochitindae Eisenack, 1931, emend. Paris, 1981 Subfamily Eisenackitininae Paris, 1981 Genus Eisenackitina Jansonius, 1964, restrict. Paris, 1981 Eisenackitina? sp. (Figs. 1A–1I) Repository Key Laboratory for Paleobiology, Yunnan University. Specimen YKLP 12001 (Figs. 1A–1C) is a nearly complete, three-dimensionally preserved vesicle; YKLP 12002 has a broken aperture but a funnel-like copula (Figs. 1D–1F); YKLP 12003 is an internal mold of a chamber (Figs. 1G–1I). Occurrence Cambrian Stage 5 (Middle Cambrian), Gaotai Formation, Balang section in Duyun, Guizhou Province, southern China. The specimens co-occur with numerous lingulid brachiopod valves, bradoriid arthropod carapaces, rare scalidophoran embryos (Zhang et al., 2011), various small shelly fossils, and exoskeletons of the eodiscoid trilobite Pagetides qianensis (Zhang and Clarkson, 2012), which, as a key index fossil, confirms the chronostratigraphic age for the fossil assemblage. Description Vesicle flask-shaped and claviform, with an ovoid chamber, 520–670 µm long and 230–325 µm wide, exhibiting a length/width ratio of ~2:1, and tapering gradually from the basal region toward the aperture. Body chamber is subspherical to ovoid, displaying a convex and rounded base. No flexure, neck indistinct, but ~1/3–1/2 of the total length of the vesicle, gently tapering forward and ending with a flaring collarette with a sinuate rim around the aperture. Surface of test wall slightly spiny to glabrous with many small, simple spines (length 3–8 µm) that are mostly concentrated on the neck. The spiny ornamentation fades away on the chamber. Well-developed copula on the base with some tiny holes (perforations) scattered around the base (Figs. 1C, 1F, and 1I). The fossils are phosphatized (Fig. DR2 in the Data Repository), as is to be expected with Orstenstyle preservation (Maas et al., 2006). Remarks Individual chitinozoans are organic-walled, urn-, flask-, tube-, or bottle-shaped vesicles that are distinctly polar (they have an aperture on one side and the other side is closed) and are radially symmetrical around their longitudinal axis. They have a chamber and a neck (largely missing in desmochitinids), are limited to between 50 µm and 2 mm in size (most
INTRODUCTION Since their first discovery ~80 yr ago (Eisenack, 1931), chitinozoans have become an important fossil group; they have a rich record in Paleozoic rocks of Ordovician to Devonian age (Paris et al., 1999; Grahn and Paris, 2011). Their temporal and spatial occurrences are used for biostratigraphy (Webby et al., 2004) and for the assessment of paleoenvironments and paleoclimates (Vandenbroucke et al., 2010). The organic-walled vesicles of chitinozoans are usually preserved as isolated individuals, but may also be found in chains or in aggregate masses, sometimes associated with organic envelopes (cocoons) (Kozlowski, 1963; Gabbott et al., 1998). Theories on the biological affinity of chitinozoans have long attracted attention. The current consensus is that these microfossils are the egg cases of small, entirely soft-bodied marine metazoans (Paris and Nõlvak, 1999); this is supported by the discovery of three-dimensional coiled catenary structures, interpreted as immature eggs, within the decayed body of a parent organism (Paris and Nõlvak, 1999). Some tear- and flask-shaped microfossils from the late Precambrian Chuar Group of the Grand Canyon (Arizona, United States) were initially regarded to be the oldest chitinozoans (Bloeser et al., 1977). Subsequently, on the basis of evidence from increasingly documented Precambrian material, these essentially vase-shaped microfossils were suggested to be Protozoa (Schopf, 1992), most closely related to the testate amoebae (Porter and Knoll, 2000; Porter et al., 2003). To date, the origin, early evolution, and systematic position of chitinozoans have remained unclear. Thus, the discovery of chitinozoans from Cambrian Stage 5, reported here, offers fresh insight into the nature and early history of these organisms. MATERIAL AND METHODS Approximately 600 kg of limestone nodules, collected from the Gaotai Formation at the outcrop (see Fig. DR1 in the GSA Data Repository1)
*E-mail:
[email protected]. 1 GSA Data Repository item 2013047, Figure DR1 (stratigraphy of the Balang outcrop) and Figure DR2 (SEM-EDX analysis of the chemical composition of Cambrian fossils from the Balang outcrop), is available online at www .geosociety.org/pubs/ft2013.htm, or on request from
[email protected] or Documents Secretary, GSA, P.O. Box 9140, Boulder, CO 80301, USA. GEOLOGY, February 2013; v. 41; no. 2; p. 191–194; Data Repository item 2013047
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doi:10.1130/G33763.1
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Published online 28 November 2012
© 2012 Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or
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Figure 1. Cambrian Stage 5 chitinozoan Eisenackitina? sp. from Gaotai Formation in Duyun, Guizhou, China. A–C: Three-dimensionally preserved, vase-shaped vesicle (specimen YKLP 12001; Key Laboratory for Paleobiology, Yunnan University). A: Lateral view; basal spine or process may be artifact. B: Detail of aperture with flaring collarette. C: Detail of copula with perforations (arrowed) around base. D–F: Vase-shaped vesicle with broken aperture (specimen YKLP 12002). D: Lateral view. E: Posterior view showing hollow copula blocked by residue near boundary between copula and vesicle bottom. F: Reverse view of D showing perforations (arrowed) around base of copula. G–I: Internal mold showing broken aperture and copula with laterally expanding edge (specimen YKLP 12003). G: Lateral view showing solid internal of vesicle. H: Posterior view showing blocked vesicle. I: Detail of perforations? (arrows) around edge of copula.
commonly between 100 µm and 1000 µm) and their aperture can be sealed with a plug (Paris et al., 1999). A wide variety of chamber morphologies and surface ornamentation exists. They often exhibit structures that reflect their original chained nature (e.g., a copula). The Duyun specimens conform to most of these characteristics, although the plug has not been preserved (it is often missing in younger specimens) and the phosphatization process obscures the original test composition. In the Duyun specimens, the copula is hollow but contains some residue near the vesicle bottom (Figs. 1E and 1H). Therefore, we cannot prove the presence of a membrane between the copula and chamber, but evidence of noncommunication between the inside of the vesicle and the outside at the aboral pole, an essential trait in chitinozoans, is provided by differential preservation of the chamber (filled and/or internal mold) and copula (hollow) in Figures 1G and 1H. More specifically, the Duyun chitinozoans display some diagnostic characters of the genus Eisenackitina within the family Desmochitinidae and the order Operculatifera, especially the spiny ornamentation and the overall vesicle shape. This assignment is tentative, as the vesicle shape is a little elongate (indistinct neck). An apparent basal “spine” is not typical of this genus but is likely an artifact (YKLP 12001; Fig. 1A). The
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material also resembles the usually smooth-walled genera Linochitina and Urnochitina, which are other desmochitinids with a short copula. Chitinozoan classification at the order level is based on the nature of the vesicle seal, either a prosome for the order Prosomatifera or an operculum for the order Operculatifera. This plug is a structure that is not preserved in the Duyun specimens (Fig. 1B), so although the overall shape suggests that the specimens should be within the Operculatifera, they could also be within the Prosomatifera. Some conochitinids and belonechitinids (Order Prosomatifera, Family Conochitinidae) have a very well developed mucron, similar to the copula structure seen here, and could therefore be a potential genus for these specimens within this order (the basal structure of the Cambrian specimens is too small to be an Eremochitina copula). The exact systematic position of the Duyun chitinozoans within the Chitinozoa thus remains somewhat circumspect, with the available morphological evidence suggesting they are in between Eisenackitina and Belonechitina, and this is symptomatic of most form-taxonomic classifications. It is interesting that some of the characters (copula, urn and/or claviform shape, spines) recognized are not generally considered to be basal (Paris et al., 1999), as they are not known from the earliest Ordovician species, but from stratigraphically younger taxa.
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until their later colonization of pelagic niches (Grahn and Paris, 2011) and preservation in organic-rich pelagic mudstones. Alternatively, the early chitinozoans may already have adopted a floating mode of life, with the small number of chitinozoans preserved in the Duyun fauna representing chance material drifting to the seabed from the overlying water column; this hypothesis, however, does not explain why such pelagic microfossils are not more widely recovered from Cambrian strata. In this context, our hypothesis of a benthic antecedent for planktonic chitinozoans (Fig. 2) would be consistent with many other organism groups that have colonized the water column, including ostracods, graptolites, trilobites, decapods, and chaetognaths (Rigby and Milsom, 2000). Previously known chitinozoans have an organic wall, which is not made out of chitin, but consists of a kerogen network dominated by aromatic units and with few aliphatics (Jacob et al., 2007). The three-dimensionally phosphatized Duyun chitinozoans show little sign of carbon (Fig. DR2), but display the typical preservation associated with Orstenstyle deposits (Maas et al., 2006). In Orsten-style preservation the tissues of small organisms are rapidly impregnated or coated with phosphate. This process occurs early and is responsible for the exceptional preservation of many Cambrian soft-bodied faunas. The chitinozoans, although well known to be unaffected by many other diagenetic and geochemical processes, were presumably phosphatized in the same manner as all other small organisms in the Duyun fauna, which had various original compositions (see the Data Repository). Younger, Ordovician–Silurian chitinozoans are not usually recovered as phosphatized vesicles, but by that time, the typically Cambrian Orsten-style preservation had disappeared. Evolutionary changes in chitinozoan tests have been reported to include size decrease, shape change toward the development of a distinct neck with a spherical chamber, and acquisition of surface ornament (spines, carinae, processes) (Paris et al., 1999; Armstrong and Brasier, 2005). Paris (1981) predicted that early chitinozoans were large; this is consistent with the Cambrian material described here, and with some Early Ordovician representatives. Moreover, the Cambrian chitinozoans had some characteristic ornament and structures associated with chitinozoans of the Ordovician. Thus, these early representatives of Chitinozoa, as with the initial radiation of many metazoan clades during the Cambrian explosion, appear to show rapid innovation of some advanced traits (Lee
OTHER FLASK-LIKE FOSSILS These Eisenackitina? specimens also superficially resemble some tintinnid loricae, which are flask shaped with a spherical chamber, a caudal appendage, and an outward-turned flaring collar around the aperture (Colom, 1948), but the average size of typical tintinnids,
