Conodont anatomy, chordate phylogeny and vertebrate classification

Interpretations of conodont anatomy and affinity continue to generate controversy. Fossilized soft-tissue evidence indicates that conodonts possessed eyes, extrinsic eye muscles, a notochord, myomeres, a differentiated tail with fin radiais, possible otic capsules and possible branchial structures. Indirect evidence suggests a differentiated brain and cartilaginous head skeleton. The multi-component phosphatic tissue complexes of the conodont feeding apparatus cannot be compared to the amorphous apatite of extant agnathan otoliths. By limiting cladistic analysis to a restricted selection of these characters the hypothesis that conodonts are a sister group of the clade comprising extant hagfish, lampreys and gnathostomes can be supported. However, exhaustive analysis of a more complete character-set strongly supports the hypothesis that conodonts are more derived than hagfish. From a taxonomic perspective, these two hypotheses have no effect on how conodonts should be classified. Whether they are a stem group (the former hypothesis) or part of the crown group (the latter), conodonts are clearly part of the total group Vertebrata (=Craniata).     

Fossil sister group of craniates: predicted and found

This study investigates whether the recently described Cambrian fossil Haikouella (and the very similar Yunnanozoon) throws light on the longstanding problem of the origin of craniates. In the first rigorous cladistic analysis of the relations of this animal, we took 40 anatomical characters from Haikouella and other taxa (hemichordates, tunicates, cephalochordates, conodont craniates and other craniates, plus protostomes as the outgroup) and subjected these characters to parsimony analysis. The characters included several previously unrecognized traits of Haikouella, such as upper lips resembling those of larval lampreys, the thick nature of the branchial bars, a mandibular branchial artery but no mandibular branchial bar, muscle fibers defining the myomeres, a dark fibrous sheath that defines the notochord, conclusive evidence for paired eyes, and a large hindbrain and diencephalon in the same positions as in the craniate brain. The cladistic analysis produced this tree: (protostomes, hemichordates (tunicates, (cephalochordates, (Haikouella, (conodonts + other craniates))))), with the "Haikouella + craniate" clade supported by bootstrap values that ranged from 81-96%, depending on how the analysis was structured. Thus, Haikouella is concluded to be the sister group of the craniates. Alternate hypotheses that unite Haikouella with hemichordates or cephalochordates, or consider it a basal deuterostome, received little or no support. Although it is the sister group of craniates, Haikouella is skull-less and lacks an ear, but it does have neural-crest derivatives in its branchial bars. Its craniate characters occur mostly in the head and pharynx; its widely spaced, robust branchial bars indicate it ventilated with branchiomeric muscles, not cilia. Despite its craniate mode of ventilation, Haikouella was not a predator but a suspension feeder, as shown by its cephalochordate-like endostyle, and tentacles forming a screen across the mouth. Haikouella was compared to pre-craniates predicted by recent models of craniate evolution and was found to fit these predictions closely. Specifically, it fits Northcutt and Gans' prediction that the change from ciliary to muscular ventilation preceded the change from suspension feeding to predatory feeding; it fits Butler's claim that vision was the first craniate sense to start elaborating; it is consistent with the ideas of Donoghue and others about the ancestor of conodont craniates; and, most strikingly, it resembles Mallatt's prediction of the external appearance of the ancestral craniate head. By contrast, Haikouella does not fit the widespread belief that ancestral craniates resembled hagfishes, because it has no special hagfish characters. Overall, Haikouella agrees so closely with recent predictions about pre-craniates that we conclude that the difficult problem of craniate origins is nearly solved.     

Conodont affinity of the enigmatic Carboniferous chordate Conopiscius

Conopiscius shares V-shaped myomeres with the co-occurring conodont Clydagnathus but instead of a complex oral apparatus it has only a single pair of conical elements, and structures resembling scales are associated with its myomeres. Moreover, the coarsely crystalline crown tissue typical for conodonts has not been identified in the Conopiscius elements, which show only a finely lamellar skeletal tissue. The gap between conodonts and Conopiscius may be filled by isolated elements of similar morphology and structure occurring in the Late Devonian. They reveal a very thin external layer developed mostly at the tooth tip and resembling conodont crown tissue. The pulp cavity is partially filled with layered or spherulitic phosphatic tissue of the kind known also in conodonts (basal filling tissue) and early vertebrates (lamellin). Conodont elements of similar morphology and representing uni-membrate oral apparatuses have not been previously reported from the Devonian or Carboniferous but occur near the Cambrian–Ordovician transition ( Proconodontus ) and in the Late Permian ( Caenodontus ). It is proposed that Conopiscius represents a mostly cryptic conodont lineage extending from the Early Ordovician to the Permian, instead of being directly related to the agnathans.     

Testing hypotheses of element loss and instability in the apparatus composition of complex conodonts: articulated skeletons of Hindeodus

Knowledge of the conodont skeleton, in terms of the morphology of the elements and the positions they occupy, provides the foundation for understanding of homology, taxonomy and evolutionary relationships in conodonts. This knowledge also underpins analyses of conodont functional morphology and feeding. Direct evidence of skeletal anatomy and apparatus architecture comes from natural assemblages: fossils that preserve together the articulated remains of the conodont apparatus, either collapsed onto a bedding plane or as clusters of elements in which juxtaposed and overlapping elements have been fused together by diagenetic minerals. Here we describe six clusters of the biostratigraphically important conodont Hindeodus parvus from the Lower Triassic Shangsi section, Sichuan Province, South China. Five of these clusters represent the partial remains of articulated skeletons, providing direct evidence of the number and arrangement of elements in the apparatus. Combined with data from previously published natural assemblages this provides a test of the hypothesis that Triassic conodonts had a reduced dentition. Hindeodus parvus possessed a complete raptorial array of two M and nine S elements (unpaired S₀; symmetrically paired S₁, S₂, S₃, S₄); the paired P₁ locations were occupied by carminiscaphate elements, but the apparatus lacked P₂ elements. This is consistent with broader evidence for a particularly high degree of integration and constraint operating on the S–M array of morphologically complex conodonts, leading to conserved architecture of the array over a period of more than 250 million years. The loss of elements from the P domain implies a change in food processing ability and, given the predominance of data from P elements in conodont taxonomy and biostratigraphy, the hypothesis of element loss from the P domain has significant implications for the broader understanding of conodont diversity and evolutionary patterns.     

Conodont ecology: pelagic versus benthic

A principal point of disagreement in conodont ecology between the pelagic depth stratification model and the benthic model of lateral segregation is the mode of life of the conodont animal. A study of the literature on certain Recent benthic and planktic organisms that may serve as ecologic analogues (chaetognaths, isopods, foraminifers) indicates that distributional patterns alone are not definitive of a pelagic versus a benthic habit. Therefore, we doubt that it is possible to determine the major ecologic mode of conodonts solely from distributional data. Because there is not consistent congruence between lithofacies and conodont biofacies, it is unlikely that conodonts were infaunal or sessile epifaunal benthos. The probable alternatives are nektobenthic or neritic pelagic animals and these may have been subject to such similar controlling hydrographic factors that it is now extremely difficult to distinguish between them, especially in an extinct group of uncertain zoologic affinity. Apparently, the only significant line of evidence for pelagic mode is the Occurrence of some conodonts in black shales that are devoid of unquestioned benthic fossils and presumably formed under anoxic bottom conditions.     

Conodont element function

There are close similarities between conodont elements and teeth both in general shape and in that they possess pointed tips and have expanded bases with more porous tissue. A number of examples of conodont elements which parallel specific kinds of tooth shapes and organizations are added to earlier known similarities. Both in teeth and in conodont elements the surface structures include cutting edges, striations, and barbs. The change in strength of the conodont denticles caused by the evolution of white matter is also shown to agree with a tooth function. On the other hand, the elements grew throughout the life of the animal by lamellae added to the surface. The solution of this paradox is found in the elements alternating between a growth phase and a functional phase. During growth the oral surface of the elements was enveloped in folds of secreting soft tissue. Structures henceforth termed burrs were formed at the contacts between the folds. Parts of the burrs evolved into cutting edges. Three different bite types occurred among the conodont elements. Many (all?) conodonts were predators which used their elements to seize and to process the food mechanically. The shape of the conodont elements cannot be used for conclusions regarding the affinities of the conodonts. Similarly, an identification of a fossil as belonging to the conodonts must be supported by other evidence than just shape.     

辽河断陷盆地西部晚寒武世至早奥陶世牙形石生物地层

辽河断陷盆地西部凹陷古潜山顶部地层的时代 ,自 70年代末期以来 ,一直被认为属于中、上元古代。本文通过在总面积为 60 0 km2 的西部凹陷中段系统、详尽的牙形石生物地层研究 ,并结合疑源类、小壳化石以及岩石地层学资料 ,确认研究区内古潜山顶部地层的时代主要为晚寒武世至早奥陶世。中、上元古代地层在研究区内虽然存在 ,但分布极其局限。从而 ,彻底改变了2 0年以来对该地区古潜山地层划分和对比上的传统认识。     

Microstructural variation in conodont enamel is a functional adaptation

Recognition that conodonts were the earliest vertebrate group to experiment with skeletal biomineralization provides a window in which to study the origin and early evolution of this developmental system. It has been contended that the conodont skeleton comprised a classic suite of vertebrate hard tissues, while others suggest that conodont hard tissues represent divergent specializations within the early diversification of vertebrate hard tissues, supporting a view that the hard tissues of conodonts, particularly enamel, exhibit a range of microstructural variation beyond that seen in vertebrates. New evidence reveals that, although variable, conodont enamel microstructure is consistent between homologous portions of homologous dentitions. Although there is a correlation between morphology and microstructure, this belies a stronger correlation between the commonality of microstructure and dental function. The enamel of conodonts evolved in response to changes in dental function and differentiation of the microstructural layer into a number of enamel types and can be linked to dental occlusion, heterodonty, a permanent dentition, enamel thickness and, probably above all, the small size of the dental elements.     

Conodont affinity and chordate phylogeny

Current information on the conodonts Clydagnathus windsorensis (Globensky) and Promissum pulchrum Kovács‐ Endrödy, together with the latest interpretations of conodont hard tissues, are reviewed and it is concluded that sufficient evidence exists to justify interpretation of the conodonts on a chordate model. A new phylogenetic analysis is undertaken, consisting of 17 chordate taxa and 103 morphological, physiological and biochemical characters; conodonts are included as a primary taxon. Various experiments with character coding, taxon deletion and the use of constraint trees are carried out. We conclude that conodonts are cladistically more derived than either hagfishes or lampreys because they possess a mineralised dermal skeleton and that they are the most plesiomorphic member of the total group Gnathostomata. We discuss the evolution of the nervous and sensory systems and the skeleton in the context of our optimal phylogenetic tree. There appears to be no simple evolution of free to canal‐enclosed neuromasts; organised neuromasts within canals appear to have arisen at least three times from free neuromasts or neuromasts arranged within grooves. The mineralised vertebrate skeleton first appeared as odontodes of dentine or dentine plus enamel in the paraconodont/euconodont feeding apparatus. Bone appeared later, co‐ordinate with the development of a dermal skeleton, and it appears to have been primitively acellular. Atubular dentine is more primitive than tubular dentine. However, the subsequent distribution of the different types of dentine (e.g. mesodentine, orthodentine), suggests that these tissue types are homoplastic. The topology of relationships and known stratigraphic ranges of taxa in our phylogeny predict the existence of myxinoids and petromyzontids in the Cambrian.     

Overgrowths of large authigenic apatite crystals on the surface of conodonts from Cantabrian limestones (Spain)

Conodonts from the middle to upper Paleozoic limestones of the Cantabrian zone commonly show apatite overgrowths. A large crystal microtexture observed under the SEM corresponds to local rims of euhedral to subhedral apatite crystals, which were preceded by the neoformation of smaller crystals. Four types of this microtexture (blocky, columnar, fan, and denticular) are described on different areas of the oral surface of conodonts, whereas dissolution features may be present in the basal cavity area. The distribution of these types of microtexture in different areas of conodont morphology suggests a general trend to neocrystallization, where crystal size increases towards the top of the conodont ornamentation and a chemical gradient controls the crystalline growth. This arrangement is widely related to the surface morphology and to the general conodont histology. The large crystal microtexture grows during early diagenesis from near surface to moderate burial and is linked to the known secondary apatite cement present in natural fused clusters of conodonts. The features described here are also compared to microtextures developed on conodonts during low- to medium-grade metamorphic conditions, where phosphate in solution is available.     

Zur Biologie, Taxonomie und Chronologie der Conodonten

The actual state of knowledge concerning the conodont animal is critically reviewed. All soft tissue documented by the specimens of the conodont animal were interpreted by the British collectors as Craniota (Vertebrata) features. Opinions on the precise affinities, however, remain controverse: The alleged chorda could have been a gut, bilateral symmetry, myomery, apatitic biomineralisation occur also in Invertebrata, caudal fins do in Chaetognatha. Even the lobes in the capital area interpreted as sensory organs could have served as aiding organs for food intake. Therefore, the originally sensational findings are reduced to the core issue that the conodont animal had an eel-like body and at the blunt end of it, behind peculiar lobes, the mineralized conodont elements were arranged. As a consequence, a relationship to the Protochordata cannot be excluded. A systematic position of the conodont animal as possible Vertebrata is likely only if organized below the Ostracodermata which is particularly so because conodonts occur so early in the Cambrian. In the second part of the article conodonts are discussed as guide fossils. The comprehensive, empirically grown taxonomy of the single elements is the prerequisite for recognizing outstanding, high resolution stratigraphy. In comparison, a multielement taxonomy remains as long problematical, as it is based on assumed, mostly statistically reconstructed rather than on natural multielement apparatuses. Taxonomical and nomenclatorial operations built on assumed apparatus affinity can bear heavily on nomenclature and stratigraphy of platform elements. It can be shown at the example of the Devonian that the platform elements have provided particularly in the pelagic facies a Standard Conodont Zonation that covers time almost without any gaps. The basis for this is that platform single element species and their phylogenetic development are extra-well known. In a special paragraph, history and applicability of the Standard Zonation is discussed in details. Different conodont biofacies are developed between the pelagic realm where the Standard Zonation originated, and the coastal areas so that conodonts can indicate temporal and bathymetric relationships of the mother rocks. In addition alternative conodont zonations were introduced mainly for the shallow water. Because of their conspicuous stratigraphic significance in the Devonian, conodonts were employed by International Committees to redefine Series and Stage Boundaries. For the boundary selection species were utilized from phylognetic lineages as only from tight evolution their point of origination could be well identified and recognized in worldwide correlation. The GSSPs (Global Stratotype Section and Point) within the Devonian are discussed in detail. In recent times, attempts are made at calibrating the duration of conodont zones on the basis of their well known rate of evolution and high temporal resolution. These analyses have to be continued in order to demonstrate the absolute time relationship of condont zones in more and continuous detail. Currently the phyletically controlled Standard Conodont Zonation is the authoritative stratigraphic method in the Devonian. It has provided together with the GSSPs precise and valuable fix points for a Global Time Scale (GTS). In combination with additional stratigraphie methods the Standard Conodont Zonation may provide further refinements in the future.     

The serial transformation hypothesis of vertebrate origins: comment on "The new head hypothesis revisited"

In "The New Head Hypothesis Revisited," R.G. Northcutt (2005. J Exp Zool (Mol Dev Evol) 304B:274-297) evaluates the original postulates of this hypothesis (Northcutt and Gans, 1983. Quart Rev Biol 58:1-28). One of these postulates is that the brain-particularly the forebrain-evolved at essentially the same time as many neural crest and neurogenic placode derivatives-including sensory ganglia, dermal skeleton and sensory capsules of the head, and branchial arches. Northcutt's subsequent paper in 1996 concluded with the idea that transitional forms might not have occurred at the origin of vertebrates. Butler proposed a "Serial Transformation" hypothesis in 2000, which disputed the latter idea in that paired eyes and an enlarged brain (but lacking telencephalon) were envisioned to have been gained before elaboration of most neural crest and neurogenic placodal derivatives. In 2003, J. Mallatt and J.-Y. Chen analyzed fossils of the Cambrian animal Haikouella, which strongly support its affinity to craniates and aspects of several hypotheses, including Butler's transformational model, because although branchial bars are present, most other neural crest and placodal derivatives are absent, while paired eyes and an enlarged brain (but probably without telencephalon) are present. A more complete picture of vertebrate origins can be realized when the various hypotheses are constructively reconciled.     

A cladistic analysis of the anomalocystitid mitrates

A cladistic analysis of the anomalocystitid mitrates is presented. A neutral terminology for the anomalocystitid skeleton is proposed, independent of the zoological interpretation of the fossils as echinoderms or as craniates. The anomalocystitid monophyly is supported by parsimony, although the instability of several basal taxa makes it difficult to ascertain the sequence in which characters were acquired or modified in the transition from the mi-trocystitids to the anomalocystitids. The genus Barrandeocarpus falls outside the anomalocystitids as traditionally defined in the literature, and is either polyphyletic or monophyletic. Diamphidiocystis drepanon is either a basal anomalocystitid or the sister taxon to the group ( Enoploura popei + Allanicytidiidae). Ateleocystites guttenbergensis is placed either at the base of the anomalocystitids or as the sister taxon to a group including mainly boreal forms, the only non-boreal members being the South African Bokkeveldia oosthuizeni and the Australian Victoriacystis wilkinsi.     

The conodont controversies

The discovery of fossilized conodont soft tissues has led to suggestions that these enigmatic animals were among the earliest vertebrates and that they were macrophagous, using their oropharyngeal skeletal apparatus to capture and process prey. These conclusions have proved controversial. There is now a consensus that conodonts belong within the chordates, but their position within the clade is hotly debated. Resolution of these questions has major implications for our understanding of the origin of the vertebrates and the selective pressures that led to the development of the vertebrate skeleton.     

Vertebrate cranial evolution: Contributions and conflict from the fossil record

In modern vertebrates, the craniofacial skeleton is complex, comprising cartilage and bone of the neurocranium, dermatocranium and splanchnocranium (and their derivatives), housing a range of sensory structures such as eyes, nasal and vestibulo-acoustic capsules, with the splanchnocranium including branchial arches, used in respiration and feeding. It is well understood that the skeleton derives from neural crest and mesoderm, while the sensory elements derive from ectodermal thickenings known as placodes. Recent research demonstrates that neural crest and placodes have an evolutionary history outside of vertebrates, while the vertebrate fossil record allows the sequence of the evolution of these various features to be understood. Stem-group vertebrates such as Metaspriggina walcotti (Burgess Shale, Middle Cambrian) possess eyes, paired nasal capsules and well-developed branchial arches, the latter derived from cranial neural crest in extant vertebrates, indicating that placodes and neural crest evolved over 500 million years ago. Since that time the vertebrate craniofacial skeleton has evolved, including different types of bone, of potential neural crest or mesodermal origin. One problematic part of the craniofacial skeleton concerns the evolution of the nasal organs, with evidence for both paired and unpaired nasal sacs being the primitive state for vertebrates.     

Allometry in Anisian (Middle Triassic) segminiplanate conodonts and its implications for conodont taxonomy

Conodonts are a clade of chordates and are valuable indicator fossils for biostratigraphy. The segminiplanate (neogondolelliform) conodonts represent a major morphological group ranging from upper Carboniferous to Upper Triassic marine sediments. However, the morphological similarity of segminiplanate P₁ elements generates problems for taxonomy, especially in the Permian and Triassic clades. This paper represents the first study of morphological variation in Triassic segminiplanate conodonts using a geometric morphometric approach. The laminar microstructures observed in conodont cross‐sections indicate that, within our analysed specimens, smaller conodonts with fewer laminae are generally from an earlier ontogenetic stage while larger conodonts with more laminae are from a later stage of ontogeny. Using linear regressions between relative warp scores from both upper and lateral views and conodont length, we demonstrate strongly allometric growth patterns for the species Paragondolella bifurcata Budurov & Stefanov. Our results indicate that the species‐group taxon Pg. praeszaboi bystrickyi (Kovacs et al.) is an early growth stage of Pg. bifurcata and thus synonymous. We suggest that the allometry of conodonts should be considered seriously, especially when there are numerous transitional morphologies between large‐ and small‐sized conodonts. Reconstructing the ontogenetic series and using larger‐sized conodonts within the numerous transitional morphologies in the population of a rock sample for the definition of new species are suggested for future studies.     

The affinities of conodonts—new evidence from the Carboniferous of Edinburgh, Scotland

Three new specimens which preserve the soft parts of conodonts are described from the Lower Carboniferous of Granton, Edinburgh. The animal was apparently laterally flattened in life and the somites were V-shaped. The nature of the preserved axial lines is equivocal; some may represent the walls of the gut. The elements of one of the new specimens show that it does not belong to Clydagnathus , to which the other soft-bodied specimen from Granton was tentatively assigned. The possibility of a relationship between the euconodonts and the Chaetognatha is discounted. Nor do the conodonts constitute a phylum, but are a separate group of primitive jawless craniates.     

湘西北晚二叠世-早三叠世早期牙形石古生态

通过对牙形石器官的对称性、相分布和地球化学特征的研究,探讨湘西北晚二叠世至三叠纪初牙形动物的生态分布.按器官所包括不同对称型分子的比例,牙形动物器官的对称性可分为4级,器官的对称性越高,动物运动能力越强.湘西北晚二叠世一三叠纪初地层可分为6种沉积相,即盆地相、盆缘-台坡相、开阔台地相、边缘浅滩相、局限台地相和泥坪相.具不同对称级器官的属种在6种沉积相中的差异分布,反映了牙形动物的3种生态类型:自由游泳型、浮游型和底栖型.牙形石地球化学组分的变化与当时环境演变相吻合.随水深增加,化石中的P含量升高,而Ca、s、sr的含量降低;Al、Fe和Ba在近岸浅水环境的牙形石中富集;而Mg的富集指示高盐环境.基于上述研究,划分了4个牙形石相,建立了一个组合横向替代的牙形动物生态分布模式.     

内蒙古达茂旗巴特敖包地区包尔汉图剖面牙形刺生物地层

内蒙古达茂旗(达尔罕茂明安联合旗)巴特敖包地区志留纪、泥盆纪碳酸岩相地层发育,本区珊瑚、腕足类、层孔虫等底栖大化石的研究工作已有一定基础。但志留纪、泥盆纪地层在时代确定和对比上还存在很多问题,必须用主导化石门类牙形刺加以解决。研究表明：包尔汉图剖面的顶部属泥盆系无疑,应归入阿鲁共组,而不是西别河组;巴特敖包地区的海侵,始于罗德洛统卢德福德阶(Ludfordian)早期。本剖面没有发现文洛克世和罗德洛世早期的海相沉积。本文描写了一个志留纪牙形刺新种：Ozarkodina uncrispa sp．nov．。     

Decay of Branchiostoma: implications for soft-tissue preservation in conodonts and other primitive chordates

Decay experiments on the cephalochordate Branchiostoma lanceolatum ('amphioxus') demonstrate that the most decay resistant structures are the notochord sheath and the cartilaginous rods which support the gill bars. However, even more labile soft parts, such as the muscles and skin may survive for at least 124 days under totally anoxic conditions. As the chevron-shaped muscles of the myomeres shrink and collapse, those on opposite sides of the trunk maybe displaced, resulting in pronounced offsetting. Only 1.42% of the initial dry weight of Branchiostoma is resistant to alkali and acid hydrolysis, compared to 46% in the polychaete Nereis virens. Branchiostoma is only likely to be fossilized as a result of decay inhibition and replication by early diagenetic minerals. The results of these experiments cast light on the interpretation of a number of primitive fossil chordates. There is no reason to infer extracellular decay-resistant cuticle in the Burgess Shale Pikaia. The axial lies preserved in the conodont animal specimens from the Carboniferous of Edinburgh, Scotland, represent the notochord. The displacement of the elements to one side of the head reflects the true position of the apparatus - the surrounding tissue has been lost through decay. The chevron-shaped structures in the Carboniferous chordate Conopiscius are the muscles of the myomeres, not external scales. The lines delineating the segments in the Silurian Jamoytius most likely represent the myosepta. There is some doubt about the nature of the only specimen interpreted as a fossil cephalochordate, Palaeobranchiostoma hamatotergum from the Permian of South Africa. □ Taphonomy, decay, softparts, Cephalochordata , Branchiostoma, lancelet, Chordata , Pikaia, conodont , Conopiscius, Jamoytius, Palaeobranchiostoma.