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16. Chordate Affinities of the Conodonts

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16. Chordate Affinities of the Conodonts 16. CHORDATE AFFINITIES OF THE CONODONTS Jerzy Dzik Until recently virtually nothing was known about ence of soft tissue above the crown tissue. Several the morphology of conodont animals, and zool­ solutions have been proposed. It is suggested that ogic affinities of the group are still regarded as ob­ conodont elements (1) formed a skeleton of the scure. The present contribution evaluates the hy­ ciliary apparatus (Lindstrom 1973, 1974; Conway pothesis that agnathan vertebrates are the closest Morris 1976, 1980; Nicoll 1977), (2) grew within relatives of conodonts. The microstructure of ele­ epithelial pockets and were protruded while func­ ment of conodont apparatuses, their organization tional (Bengtson 1976; Carls 1977), (3) were cov­ into skeletal structures, and the inferred morphol­ ered by a hard horny tissue secreted above the ep­ ogy of the soft body parts are compared to the ithelium (Priddle lq74), or (4) originated in a anatomy of early vertebrates. Other problematic manner analogous to vertebrate teeth and were re­ phosphatic dermal sclerites are also reviewed. placed in ontogeny (Carls 1977; Jeppsson 1980). A complete phosphatic element of the cono­ None of these interpretations is convincing. dont apparatus consists of two distinct parts. The Bengtson’s pocket hypothesis can hardly explain oral part (crown of Nicoll 1977) is developed by healed fractures of conodont cusps (see Muller centrifugal external accretion. It is rather compact and Nogami 1971) and brushlike appearance of in structure, and its surface is smooth or regularly platform elements not uncommon in the Devo­ ornamented. The basal part (basal filling), which nian (Muller 1981). Carls’ concept of ontogenetic is developed by inner accretion, varies greatly in replacement implies the occurrence of smaller, re­ microstructure and morphology. The mode of se­ generating elements in at least some natural as­ cretion of the crown tissue is here considered di­ semblages. However, nothing like this has ever agnostic for the group; fossil sclerites secreted dif­ been reported. The high rate of evolution and re­ ferently are not considered to be conodont peated development of platform elements, with elements (see Bengtson 1976, 1983a, for opposing their molarlike appearance, are evidence against view). the filtratory apparatus concept (Dzik 1976; Jeppsson 1979a). Neither were scleritized organic caps covering conodont elements recognized in CROWN TISSUE articulated apparatuses, nor were phosphatic nu­ The crown of conodont elements is composed of clei identified in teeth of Paleozoic hagfishes, to relatively large, elongated crystallites of apatite. support Priddle’s model of the conodont element. They are parallel to each other, and normal to the element surface (Fryxellodontus, Cordylodus) or, most commonly, parallel to the cusp or denticle BASAL FILLING axis. Frequent disconformities in distribution of Unlike the crown, apatite crystallites in the basal lamellae marking subsequent stages in secretion filling are roughly isometric and usually do not of the crown clearly indicate that the surface of at show any preferred orientation (Pietzner et al. least platform elements was periodically worn 1968; Lindstrom and Ziegler 1981). The bound­ out (Muller and Nogami 1971; Muller 1981). ary between the crown and basal filling tissues is Whether this was caused by mechanical abrasion always sharp and distinct (Fig. 1). or chemical resorption, cannot be determined. Most conodont elements lack any remnants of The shape of apatite crystallites and the pattern of the basal filling. Sometimes, however, a black, their distribution persisted from the Late Cam­ carbonized spongy tissue penetrated by meander­ brian Teridontus nakamurai (Nogami 1967) (see ing canals can be found inside the basal cavity. It Landing et al. 1980) to the extinction of cono­ may represent shrunken remnants of unmineral­ donts (Pietzner et al. 1968; Lindstrom and Ziegler ized organic matrix of the basal filling tissue (Fig. 1971; Barnes et al. 1973; Muller 1981; Bitter and 1B). In some other cases the basal filling tissue is Merrill 1983). The internal surface of the crown, well mineralized, but its morphology suggests that visible in the basal cavity, always shows distinct partial shrinkage occurred during the early dia­ growth lines (Pietzner et al. 1968; Lindstrom and genesis (Lindstrom and Ziegler 1971, 1981). Irreg­ Ziegler 1981). ular canals and tubuli penetrate this kind of tissue A basic problem is to explain the function of (Barnes et al. 1973). When the tissue underwent the conodont elements by allowing for the pres­ more advanced mineralization while alive, the Fig-1- Microstructure of the basal filling in elements of the conodont Semiacontiodus cornuformis (Sergeeva 1963) from the Llanvimian of the Baltic area (glacial erratic boulders). A. Basal view, openings of dentine tubuli (dt) visible, sample E-079. X300. B. Basal view of a black, unmineralized basal fillilng, sample E-113. X500. C. Medial thin section, dentine tubuli (dt) in longitudinal section, sample E-079. X200. D. Etched oblique sec­ tion through conspicuous callus of the basal filling, sample E-079. X300. 241 242 PROBLEMATIC FOSSIL TAXA tubuli appear to be confined to the axial parts of clusters illustrated by Briggs et al. (1983) indicate the element and they open basally (Fig. 1A, C). that, despite different arrangement of processes, Most commonly, the basal filling tissue shows la­ the cusps of all the elements point in the same di­ mellar organization. Except for growth lines, no rection. Because the sp element, at one end of the other internal structures can be recognized apparatus, usually is the most robust one (fre­ (Muller and Nogami 1971; Lindstrom and Ziegler quently with a platform), while the ne element, at 1981). In the histogeny of some primitive Early the opposite end, usually bears a sharp, swordlike Ordovician conodonts, such a laminated basal cusp, they were inferred to represent the posterior filling tissue followed the spongy tissue (Fig. 1D). and anterior ends of the apparatus, respectively Most post-Ordovician conodonts had compact (Dzik 1976). A functional gradient along the ap­ basal filling tissue only. This variation in struc­ paratus, from the grasping function of the ne ele­ ture of the basal filling tissue seems to represent ment to the masticatory function of the sp ele­ consecutive stages in its mineralization. ment, was proposed. This has been corroborated Generally, the basal filling tissue contributes by the discovery of soft body parts in the cono­ only a small portion of the total mass. There is dont animal (Briggs et al. 1983). The pattern of only a single group of Ordovician conodonts with conodont apparatus thus is well established (Fig. the basal filling tissue dominant. Archaeognathus 2), although there still are some doubts regarding primus Cullison 1938 from the Dutchtown For­ the position of symmetric (tr) elements inside or mation (Late Llanvirnian?) of Missouri is the best outside the apparatus. In Mashkova’s (1972) ap­ known representative of this group (Cullison paratus, the tr element (identified by Jeppsson 1938), which is widespread and abundant in 1979b) has the same orientation as the others, and North America (Mosher and Bodenstein 1969) even though its counterpart is not preserved, and Siberia (Moskalenko 1972, 1976; Barskov et there is no evidence for its medial positon as pro­ al. 1982). The crown tissue is restricted to the posed by Jeppsson (1971, 1979b). Jeppsson cited working edge of the element, and it forms only a the commonness of numerical underrepresenta­ thin crown, frequently split into coniform denti­ tion of tr elements to support his reconstruction. cles. The basal filling reveals a complex internal Such underrepresentation, however, may be structure with wide, branching canals (Barskov et caused by hydrodynamic factors alone, for tr ele­ al. 1982). Although these so-called coleodontids ments usually are the most ramified and gracile in still await detailed taxonomic description and re­ the apparatus. construction, their conodont nature seems to be Apparatuses of ramiform conodont elements well established. can be easily homologized in the Early Ordovi­ cian (Dzik 1983) through Late Triassic (Sweet 1981). They have 7 pairs of elements each, except CONODONT APPARATUSES for some Late Triassic apparatuses with possibly The original organization of apparatuses pre­ 10 pairs of elements each [5 pairs of hindeodelli- served on rock bedding plane (natural assem­ form elements; Ramovs (1978), Dzik and Tram­ blages) generally is obscured by sediment mer (1980)] and Devonian Icriodus apparatuses, compression during lithification. This has which bear hundreds of coniform elements (Ni­ prompted many contradictory reconstructions coll 1982) and which perhaps represent disinte­ (Jeppsson 1971; Collinson et al. 1972; Lindstrom grated denticles of originally ramiform elements. 1973, 1974; Nicoll 1977; Hitchings and Ramsay The closest functional analog to apparatuses of 1978). A fortunate exception in the mode of pres­ coniform conodont elements is the grasping ap­ ervation is represented by the natural assemblage paratus of the Chaetognatha (Fig. 3A; Szaniawski of Ozarkodina steinhornensis (Ziegler 1956) de­ 1982), while apparatuses of ramiform elements scribed by Mashkova (1972) from Early Devo­ have their closest analog in the buccal apparatus nian limestones. Its elements collapsed obliquely of hagfish (Fig. 3B; Priddle 1974). The buccal ap­ from the
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