Finite element,occlusal, microwear and microstructural analyses indicate that conodont microstructure is adapted to dental function

Conodonts constitute the earliest evidence of skeletal biomineralization in the vertebrate evolutionary lineage, manifest as a feeding apparatus of tooth‐like elements comprised of enamel‐ and dentine‐like tissues that evolved in parallel with these canonical tissues in other total‐group gnathostomes. As such, this remarkable example of evolutionary parallelism affords a natural experiment in which to explore the constraints on vertebrate skeletal evolution. Using finite element analysis, informed by occlusal and microwear analyses, we tested the hypothesis that coincidence of complex dental function and microstructural differentiation in the enamel‐like tissues of conodonts and other vertebrates is a consequence of functional adaptation. Our results show topological co‐variation in the patterns of stress distribution and crystallite orientation. In regions of high stress, such as the apex of the basal cavity and inner parts of the platform, the crown tissue comprises interwoven prisms, discontinuities between which would have acted to decussate cracks, preventing propagation. These results inform a general occlusal model for platform conodont elements and demonstrate that the complex microstructure of conodont crown tissue is an adaptation to the dental functions that the elements performed.     

Pseudooneotodusi a histological study of an Ordovician to Devonian vertebrate lineage

Detailed histological investigations have shed new light on the nature of Pseudooneotodus Drygant, 1974 (?Arenig/ljanvirn - Emsian). The genus has generally been interpreted as a conodont and is represented by squat phosphatic cones. These conodont dements show a differentiation into a lamellar cap, indistinguishable from vertebrate enamel, which is underlain by a spherulitic basal tissue with several characters indicative of dentine. The presence of these two issues in the elements of a conodont argues persuasively for the. vertebrate classification of the cladc, and illustrates that at least some conodonts have a hard tissue complex which is histologically indistinguishable from those of other primitive vertebrates. These observations have potentially important implications for conodont classification and the stratigraphic first appearances of vertebrate hard tissues.     

Testing microstructural adaptation in the earliest dental tools

Conodont elements are the earliest vertebrate dental structures. The dental tools on elements responsible for food fracture—cusps and denticles—are usually composed of lamellar crown tissue (a putative enamel homologue) and the enigmatic tissue known as ‘white matter’. White matter is unique to conodonts and has been hypothesized to be a functional adaptation for the use of elements as teeth. We test this quantitatively using finite-element analysis. Our results indicate that white matter allowed cusps and denticles to withstand greater tensile stresses than do cusps comprised solely of lamellar crown tissue. Microstructural variation is demonstrably associated with dietary and loading differences in teeth, so secondary loss of white matter through conodont phylogeny may reflect changes in diet and element occlusal kinematics. The presence, development and distribution of white matter could thus provide constraints on function in the first vertebrate dental structures.     

Skeletal ontogeny and feeding mechanisms in conodonts

The growth and function of the conodont skeletal apparatus have important implications for early vertebrate relationships and the evolution of vertebrate hard tissues, yet they are poorly understood. Analysis of element length, platform linear dimensions, and platform area in discrete Pa elements of Carboniferous Idiognathodus and Gnathodus bilineatus reveals that the platform increased in size at a rate significantly above that required to maintain geometric similarity. Measurements of P, M and S elements in bedding-plane assemblages of Idiognathodus and G. bilineatus indicate that relative to Pa element length, Pb and S element growth was isometric, whereas M elements grew with negative allometry. There is no evidence to support loss or resorption of S and M elements in later growth stages, or to indicate periodic shedding and replacement of elements. These results are important for understanding apparatus and element Function. The positive allometry of the Pa element platform supports interpretations of a mashing or grinding tooth-like Function for platformed Pa elements. If conodonts were active suspension-feeders, the increasing food requirements of a growing conodont would require the filter array formed by the S and M elements to have grown at a rate significantly above isometry. The lack of positive allometry of S and M elements indicates that conodonts were not suspension-feeders and supports hypotheses that conodonts fed with a raptorial apparatus and teeth. □ Conodonts, vertebrates, skeletal apparatus, ontogeny, allometry, function, suspension-feeding, teeth.     

Growth and patterning in the conodont skeleton

Recent advances in our understanding of conodont palaeobiology and functional morphology have rendered established hypotheses of element growth untenable. In order to address this problem, hard tissue histology is reviewed paying particular attention to the relationships during growth of the component hard tissues comprising conodont elements, and ignoring a priori assumptions of the homologies of these tissues. Conodont element growth is considered further in terms of the pattern of formation, of which four distinct types are described, all possibly derived from a primitive condition after heterochronic changes in the timing of various developmental stages. It is hoped that this may provide further means of unravelling conodont phylogeny. The manner in which the tissues grew is considered homologous with other vertebrate hard tissues, and the elements appear to have grown in a way similar to the growing scales and growing dentition of other vertebrates.     

The sharpest tools in the box? Quantitative analysis of conodont element functional morphology

Conodonts have been considered the earliest skeletonizing vertebrates and their mineralized feeding apparatus interpreted as having performed a tooth function. However, the absence of jaws in conodonts and the small size of their oropharyngeal musculature limits the force available for fracturing food items, presenting a challenge to this interpretation. We address this issue quantitatively using engineering approaches previously applied to mammalian dentitions. We show that the morphology of conodont food-processing elements was adapted to overcome size limitations through developing dental tools of unparalleled sharpness that maximize applied pressure. Combined with observations of wear, we also show how this morphology was employed, demonstrating how Wurmiella excavata used rotational kinematics similar to other conodonts, suggesting that this occlusal style is typical for the clade. Our work places conodont elements within a broader dental framework, providing a phylogenetically independent system for examining convergence and scaling in dental tools.     

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.     

BASAL TISSUE STRUCTURE IN THE EARLIEST EUCONODONTS: TESTING HYPOTHESES OF DEVELOPMENTAL PLASTICITY IN EUCONODONT PHYLOGENY

Abstract:  The hypothesis that conodonts are vertebrates rests solely on evidence of soft tissue anatomy. This has been corroborated by microstructural, topological and developmental evidence of homology between conodont and vertebrate hard tissues. However, these conclusions have been reached on the basis of evidence from highly derived euconodont taxa and the degree to which they are representative of plesiomorphic euconodonts remains an open question. Furthermore, the range of variation in tissue types comprising the euconodont basal body has been used to establish a hypothesis of developmental plasticity early in the phylogeny of the clade, and a model of diminishing potentiality in the evolution of development systems. The microstructural fabrics of the basal tissues of the earliest euconodonts (presumed to be the most plesiomorphic) are examined to test these two hypotheses. It is found that the range of microstructural variation observed hitherto was already apparent among plesiomorphic euconodonts. Thus, established histological data are representative of the most plesiomorphic euconodonts. However, although there is evidence of a range in microstructural fabrics, these are compatible with the dentine tissue system alone, and the degree of variation is compatible with that seen in clades of comparable diversity.     

Conchodontus,Mitrellataxis and Fungulodus: Conodonts,fish or both?

Conchodontus, Mitrellataxis and Fungulodus are phosphatic microfossils from the Late Devonian of China and North America, alternatively interpreted as conodont elements or fish scales. The histology and microornament of these sclerites have been studied in an attempt to resolve their affinity, and to determine characters for distinguishing between conodont elements and the ichthyoliths of other lower vertebrates. The histology of all three genera is directly comparable to conodont elements, dispelling the notion that conodonts are histologically indistinguishable from the teeth and scales of other vertebrates. Microornament is found not to be useful in discriminating between high-level taxonomic groupings. White matter and thickness of prismless enamel are suggested as apomorphies of the Conodonta.     

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.     

Evidence of parallel evolution in the dental elements of Sweetognathus conodonts

The repeated emergence of similar morphologies in the dental elements of Permian Sweetognathus conodonts has been a hypothesized example of parallel evolution. To test if morphological parallelisms occur between isolated Sweetognathus lineages, this study uses two-dimensional-based geometric morphometrics combined with a revised and expanded phylogeny of Permian Sweetognathus conodonts to quantify dental element trait distributions and compare the phenotypic trajectories between lineages. A hierarchical clustering method was used to identify recurrent species pairs based on principal component scores describing their morphological variation, with the further incorporation of widely used ecological metrics such as limiting similarity and morphological overlap. Our research implies that a major contributor to conodont diversity in Palaeozoic marine trophic networks is the emergence of recurrent parallel morphologies via disruptive and directional selection. This study illustrates the mechanisms through which conodonts achieved their status as hyper-diverse predators and scavengers, contributing substantially to the complexity of Palaeozoic marine communities.     

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.     

Early evolution of vertebrate skeletal tissues and cellular interactions, and the canalization of skeletal development

The stratigraphically earliest and the most primitive examples of vertebrate skeletal mineralization belong to lineages that are entirely extinct. Therefore, palaeontology offers a singular opportunity to address the patterns and mechanisms of evolution in the vertebrate mineralized skeleton. We test the two leading hypotheses for the emergence of the four skeletal tissue types (bone, dentine, enamel, cartilage) that define the present state of skeletal tissue diversity in vertebrates. Although primitive vertebrate skeletons demonstrate a broad range of tissues that are difficult to classify, the first hypothesis maintains that the four skeletal tissue types emerged early in vertebrate phylogeny and that the full spectrum of vertebrate skeletal tissue diversity is explained by the traditional classification system. The opposing hypothesis suggests that the early evolution of the mineralized vertebrate skeleton was a time of plasticity and that the four tissue types did not emerge until later. On the basis of a considerable, and expanding, palaeontological dataset, we track the stratigraphic and phylogenetic histories of vertebrate skeletal tissues. With a cladistic perspective, we present findings that differ substantially from long-standing models of tissue evolution. Despite a greater diversity of skeletal tissues early in vertebrate phylogeny, our synthesis finds that bone, dentine, enamel and cartilage do appear to account for the full extent of this variation and do appear to be fundamentally distinct from their first inceptions, although why a higher diversity of tissue structural grades exists within these types early in vertebrate phylogeny is a question that remains to be addressed. Citing recent evidence that presents a correlation between duplication events in secretory calcium-binding phosphoproteins (SCPPs) and the structural complexity of mineralized tissues, we suggest that the high diversity of skeletal tissues early in vertebrate phylogeny may result from a low diversity of SCPPs and a corresponding lack of constraints on the mineralization of these tissues.     

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.     

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.     

Effect of microstructure upon elastic behaviour of human tooth enamel

Tooth enamel is the stiffest tissue in the human body with a well-organized microstructure. Developmental diseases, such as enamel hypomineralisation, have been reported to cause marked reduction in the elastic modulus of enamel and consequently impair dental function. We produce evidence, using site-specific transmission electron microscopy (TEM), of difference in microstructure between sound and hypomineralised enamel. Built upon that, we develop a mechanical model to explore the relationship of the elastic modulus of the mineral–protein composite structure of enamel with the thickness of protein layers and the direction of mechanical loading. We conclude that when subject to complex mechanical loading conditions, sound enamel exhibits consistently high stiffness, which is essential for dental function. A marked decrease in stiffness of hypomineralised enamel is caused primarily by an increase in the thickness of protein layers between apatite crystals and to a lesser extent by an increase in the effective crystal orientation angle.     

Frasnian conodonts from the eastern Russian Platform

The Frasnian conodonts of the eastern Russian Platform from the Timan in the north to the Orenburg Region in the south are characterized in detail both paleontologically and stratigraphically. This paper is a continuation of a previously published monograph (Ovnatanova and Kononova, 2001) on the Frasnian conodonts of the shallow-water strata of the central Russian Platform, where polygnathids prevail. The conodont assemblages from the shallow-water and basinal sections of the Volga-Ural province and Southern Timan are analyzed. Problems and difficulties associated with the correlation of the shallow-water and deep-water Frasnian sections of the eastern Russian Platform and their correlation with the existing zonal conodont scales are discussed. The correlation between the deep-water Mendym deposits and the shallow-water Rechitza and Voronezh strata of the Volga-Ural province with the Vetlasyan and Sirachoi deposits of the Southern Timan is substantiated. The stratotype of the Semiluki Regional Stage of the Central Devonian Field contains equivalents of the lowermost beds of the Domanik Formation (unit 1), based entirely on polygnathids. Phylogenetic reconstructions for Palmatolepis and Polygnathus are suggested based on the ontogenetic series for some species of these two genera and the presence of transitional forms between some of the species. Based on these phylogenetic reconstructions, conodont zonal scales for the shallow-water and deep-water sections of the Frasnian Stage of the Russian Platform are proposed and their correlation with the existing conodont zonal scales is also adduced. Some aspects of biofacies control are considered based on the distribution of conodonts in the sections studied. In Systematic Paleontology, 91 conodont species of the genera Ancyrodella, Mesotaxis, Palmatolepis, and Polygnathus are described, including the new species Palmatolepis menneri, P. kaledai, P. acutangularis, and Polygnathus reitlingerae. DEDICATION To the blessed memory of the outstanding geologist Vladimir Vladimirovich Menner     

The tick tock of odontogenesis

Although a big deal of dental research is being focused to the understanding of early stages of tooth development, a huge gap exist on our knowledge on how the dental hard tissues are formed and how this process is controlled daily in order to produce very complex and diverse tooth shapes adapted for specific functions. Emerging evidence suggests that clock genes, a family of genes that controls circadian functions within our bodies, regulate also dental mineralized tissues formation. Enamel formation, for example, is subjected to rhythmical molecular signals that occur on short (24 h) periods and control the secretion and maturation of the enamel matrix. Accordingly, gene expression and ameloblast functions are also tightly modulated in regular daily intervals. This review summarizes the current knowledge on the circadian controls of dental mineralized tissues development with a special emphasis on amelogenesis.     

牙形石Histiodella Hass 四个种的讨论及地层对比意义

作者描述了产自塔里木盆地的HistiodellaHass的四个种,并探讨了它们与中国北方和北美中大陆暖水型牙形石及中国南方和北大西洋冷水型牙形石的对比关系。认为Histiodellasinuosa带对比北美中大陆牙形石动物群3的下部、中国南方Amorphognathusvariabilis带下部和中国北方Aurilobodusleptosomatus-Loxodusdissectus带下部。Histiodellaholodentata带下部相当于中国北方Aurilobodusleptosomatus-Loxodusdissectus带中部或中国南方的Amorphognathusvariabilis带至Eoplacognathussuecicus带的下部。Histiodellakristinae带对比北大西洋区的Eoplacognathussuecicus带中上部和中国北方的Tangshanodustangshanensis带。Histiodellabellburnensis带对比北大西洋区的Eoplacognathussuecicus带的上部到Eoplacognathusfoliaceus带下部。HistiodellaHass与北大西洋区和中国北方区典型牙形石的共同出现可以作为一个桥梁,建立起北美中大陆、北大西洋、塔里木盆地及华北地区牙形石的对比关系。     

内蒙古大兴安岭乌奴耳地区泥盆纪的两个牙形刺动物群

在大兴安岭乌奴耳地区发现两个牙形刺动物群。一个采自北矿组,牙形刺Caudicriodus angulatus cf.cauda指示北矿组的时代为Eifelian早期。另一个样品采自霍博山组(大民山组)上部的角砾岩层,产有牙形刺的混合动物群。Palmatolepis hassi,Pal.ljashenkovae,Pal.kireevae,Pal.subrecta,Polygnathus decorosus,Icriodus ex-pansus,I.symmetricus等牙形刺主要来自Frasnian晚期晚rhenana带。而Ancyrodella binodosa,Ancyrodella nodosa,Schimitognathus cf.hermanni则主要来自Gevitian期晚期和Frasnian期早期;Ancyrognathus ubiquitus是重要的事件种,见于linguiformis带至早triangularis带(F/F界线层);这个角砾岩层最后形成的时代可能是早triangularis带,含有多层位的再沉积的牙形刺。文中共描述了牙形刺11属22种,包括2个未定新种和1个未定种。