The Tube Wall of Cambrian Anabaritids
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The tube wall of Cambrian anabaritids ARTEM KOUCHINSKY and STEFAN BENGTSON Kouchinsky, A. and Bengtson, S. 2002. The tube wall of Cambrian anabaritids. Acta Palaeontologica Polonica 47 (3): 431–444. Celestite/barite−replaced and phosphate−replicated tubes of Early Cambrian anabaritids from the northern part of the Si− berian Platform (Anabar Shield) give new evidence on the wall−structure of these enigmatic fossils. The walls consist of fibres, interpreted as reflecting an original aragonitic fabric. Bundles of fibres are arranged in growth lamellae, and the latter form an angle of at least 45° with the inner tube wall. Where the outer tube surface projects into annular flanges, the lamellae have a chevron−like section due to the backwards deflection of the outer parts. Anabaritids are usually referred to the Cnidaria or left without systematic assignment, but earlier suggestions included affinity to the serpulid polychaetes. The chevron structure resembles that previously exclusively known from serpulids, but the presence of internal tooth−like structures in anabaritid tubes, perhaps compromising up−and−down movement through the tubes, continue to make a di− rect assignment to the Serpulida questionable. Key words: Anabaritida, biomineralization, wall structure, aragonite, celestite, barite, Cambrian, Siberia. Artem Kouchinsky [[email protected]], Department of Earth Sciences, Historical Geology andPalaeontol − ogy, Norbyvägen 22, SE−752 36, Uppsala, Sweden; present adress: Department of Earth & Space Sciences, University of California L.A., Los Angeles, CA 90024, USA; Stefan Bengtson [[email protected]], Department of Palaeozoology, Swedish Museum of Natural History, Box 50007, SE−104 05, Stockholm, Sweden. Introduction trolled mineralization. Whereas biologically induced miner− alization is probably as old as life itself, the ability to control Anabaritids were among the first animal groups to acquire a biominerals for tissue construction did not become wide− mineralized skeleton. Their triradially symmetrical, calcareous spread until the late Neoproterozoic and Cambrian (e.g., tubes are characteristic components of microfossil assemblages Lowenstam and Weiner 1989; Bengtson 1994). The Cam− in Early Cambrian beds. They first appeared in the Manykayan brian explosion of multicellular life was coincident with a (= Nemakit−Daldynian), i.e., pre−Tommotian as recognized on burgeoning of skeletons. These were of various construction the Siberian Platform, and survived until the late Atdabanian or and composition; in all probability they arose independently early Botomian, in the late Early Cambrian (Rozanov et al. in a number of different lineages. In view of the sophisticated 1969; Val’kov and Sysoev 1970; Val’kov 1982; Conway Mor− biochemical controls necessary to nucleate and shape ris and Chen 1989; Bengtson et al. 1990; Towe et al. 1992). biominerals, however, there may well be underlying homolo− Their distribution (summarized by Conway Morris and Chen gous mechanisms behind the evolution of mineralized skele− 1989; and Missarzhevskij 1989) was worldwide. tons (e.g., Lowenstam and Margulis 1980; Kirschvink and Hagadorn 2000). Comparing the constructional and mineral− The systematic affinity of anabaritids is controversial. Except for a few early suggestions that they represent tubi− ogical features of the earliest skeletons in the various lin− colous polychaetes (Voronova and Missarzhevskij 1969; eages may reveal such homologies and, in addition, point to Glaessner 1976), they have either been interpreted as cnidari− unrecognized homologies of skeletal tissues themselves. ans (Missarzhevskij 1974; Val’kov 1982; Fedonkin 1986, As pointed out by Missarzhevsky (1974: 185), the pres− 1987; Kouchinsky et al. 1999) or as being of uncertain sys− ence of inwards directed spines in some anabaritids shows tematic affinity (Val’kov and Sysoev 1970; Matthews and that at least those soft parts adjacent to the interior wall were Missarzhevsky 1975; Abaimova 1978; Conway Morris and firmly attached to the inner surface of the tube, rather than be− Chen 1989; Missarzhevskij 1989; Bengtson et al. 1990; ing able to slide up and down. Abaimova (1978) described Towe et al. 1992). The uncertainty is mainly due to lack of extensive external keels in several taxa, sometimes wider data on anabaritid anatomy, including the structure and com− than the diameter of the tube. She argued (p. 82) that these position of their tube wall. could only have been formed from the outside, and that there− Information on the construction of the skeleton may help fore the anabaritid tube was to be considered an internal, sup− to elucidate anabaritid biology and relationships. A wider porting skeleton. Conway Morris and Bengtson (in Bengtson significance of such data on the early animal skeletons is that et al. 1990: 197) proposed, however, that the secreting tissue they yield insight into the evolution of biologically con− only covered the leading edges of the flanges. Acta Palaeontol. Pol. 47 (3): 431–444, 2002 http://www.paleo.pan.pl/acta/acta47/app47−431.pdf 432 ACTA PALAEONTOLOGICA POLONICA 47 (3), 2002 The original wall of anabaritids has been interpreted to be 108 E Laptev Sea aragonitic (Conway Morris and Chen 1989): a fibrous struc− 100 km ture is sometimes visible on internal moulds, and when the shell is preserved in calcium carbonate it seems invariably recrystallized, with fine structures obliterated. What we know 72 N Kotuj about the wall structure so far derives from phosphatized sam− ples (Conway Morris and Chen 1989; Bengtson et al. 1990). M423 Diagenetic calcium phosphate has a great potential for pre− M419 96−5 Anabar serving delicate structures; however, with regard to skeletal Kotujkan elements it tends to preserve surfaces better than internal structures (Lucas and Prévôt 1991). Thus, satisfactory repli− Bol’shaya Kuonamka Malaya cation of shell surfaces can be achieved, either in positive or 108 E Kuonamka 120 E negative relief, during phosphatization (e.g., Runnegar 1985; Section M423 Section 96−5 Bengtson et al. 1990), but there are few examples of the Section M419 three−dimensional fine structures of aragonitic fossil shells 20m being preserved in phosphate. Conway Morris and Chen K2/26 10m (1989) documented a fibrous inner layer with distinct K2/25 spherulitic structure in phosphatized anabaritid tubes from 96−5/0 Emyaksin Fm China; the structure of the wall external to the spherulitic Medvezhyn Fm layer was considerably less clear, but was interpreted to con− sist of fine concentric laminae with a transverse fibrous fine bedded mainly structure. Phosphatized Australian anabaritids described by K1a/47 limestones sandstones Manykaj Fm Conway Morris and Bengtson (in Bengtson et al. 1990) re− thrombolite sequence Nemakit−Daldyn boundstone boundary vealed only obscure traces of possible fibrous structure. We have investigated new material of anabaritids from Fig. 1. Location maps and generalized stratigraphic columns showing sam− the northern Siberian Platform. In addition to phosphatized pling localities and levels for the described material. specimens, there are tubes preserved in celestite (strontium sulphate, SrSO ) admixed with barite (barium sulphate, 4 guished by its lighter colour in the lower part of the outcrop. BaSO ), which through partial replication of the original 4 Numerous dissolution surfaces occur throughout the unit, and structure give the first direct evidence on the nature of the diabase dykes, about 1 m thick and tens of meters apart, cut the tube wall throughout its thickness. These new data serve to entire section. Visible zones of hot contact with the carbonates elucidate the relationship between the mineralized tubes and extend up to several decimetres. Section M419 (Rozanov et al. the soft tissues secreting them. 1969), situated ca. 2 km upstream of the mouth on the right bank of the Kotujkan, represents the Nemakit−Daldyn Forma− tion; Sample K1a/47 was collected (in 1992, by AK) from the Material and methods lower part of unit 11 (Rozanov et al. 1969) and is a wackestone from the vicinity of an algal build−up. Section 96−5 (A−50 of The material was collected during two expeditions to the Val’kov 1975) is by the Bol’shaya Kuonamka River, at the Anabar Uplift, Siberian Platform (Fig. 1): to the Kotuj River in mouth of a small creek that feeds from the left into the 1992 (participants AK and Pyotr Yu. Petrov) and to the Malaya Ulakhan−Tjulen brook about 1 km from its mouth. The sample and Bol’shaya Kuonamka Rivers in 1996 (participants AK, 96−5/0 from the base of the Emyaksin Formation is a coarse SB, Anatolij V.Valkov, Vladimir V.Missarzhevsky and Shane sandstone with calcareous cement. Pelechaty). Section M423 (Rozanov et al. 1969) is situated at The fossils represent tubes of anabaritids replaced by ce− the mouth of the Kugda brook on the right bank of the Kotuj lestite and barite, and phosphatic internal moulds of tubes River. From a 25–30 m thick succession of richly fossiliferous with replicated microstructures. They were extracted from normal marine carbonates of the Medvezhin Formation, 40 the samples with dilute acetic acid. Pictures were taken with samples were collected (by AK) in 1992. The anabaritids pre− scanning electron microscopes (SEMs; Philips XL−30 and served in celestite are from samples K2/25 and K2/26 from the Hitachi S4300). Thin sections were coated with carbon and uppermost part of a 1.2 m thick wackestone bank distin− investigated with energy−dispersive spectrometers (EDS) at− Fig. 2. Jacutiochrea tristicha (Missarzhevsky), specimen SMNH X3409 (same as in Figs. 5 and 3), sample K2/25. A. Fractured section through a flange and ý wall. B. Septum−like protrusion of the tube wall surrounded by apatitic internal mould (note flattened surface of apatite layer). C. Tube with transverse flanges (arrows correspond to close−ups shown in A, B, D–J). D. Bundle arrangement of fibres in the wall. E. Close−up of D (arrowed) with minute fibres at boundary between adjacent bundles. F. Close−up of G, with different inclination of fibres in adjacent lamellae. G. Transition of wall into flange, and bilamellar construction of the wall at the flange.