A neontological interpretation of conodont elements based on agnathan cyclostome tooth structire function, and development

Krejsa, R. J., Bringas, P. Jr. & Slavkin, H. C. 1990 10 15: A neontological interpretation of conodont elements based on agnathan cyclostome tooth structure, function, and development. Lethaia , Vol. 23, pp. 359–378. Oslo. ISSN 0024–1164.
Speculation about a conodont-cyclostome connection has led us to search for and establish a biological basis for various characteristic structures in conodont elements. Measurements of juvenile hagfish palatal and lingual teeth overlap those of representative conodont elements, demonstrating a size correspondence of conodonts with teeth of living vertebrates. When hagfish tooth histology is compared with internal and surface topography (SEM) of hagfish, keratinous teeth and mineralized conodont elements, microspaces and tubules similar to those found in hagfish functional tooth coverings and replacement elements are also found within the white matter' of conodont elements. It is provisionally suggested that the primary organic matrix of conodont elements could be keratin and/or keratin-related molecules, and that individual conodont elements could represent shed tooth coverings. The basal bodies' found in certain conodont elements could be replacement elements. These interpretations are contrary to several paradigms of orthodox conodontology. ▭ Agnatha, conodonts, cyclostomes, hagfish, keratin, paleo-biology, shedding, teeth .     

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.     

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.     

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.     

Convergent dental adaptations in the serrations of hypercarnivorous synapsids and dinosaurs

Theropod dinosaurs are well known for having a ziphodont dentition: serrated, blade-shaped teeth that they used for cutting through prey. Serrations along the carinae of theropod teeth are composed of true denticles, a complex arrangement of dentine, enamel, and interdental folds. This structure would have supported individual denticles and dissipated the stresses associated with feeding. These particular serrations were previously thought to be unique to theropod dinosaurs and some other archosaurs. Here, we identify the same denticles and interdental folds forming the cutting edges in the teeth of a Permian gorgonopsian synapsid, extending the temporal and phylogenetic distribution of this dental morphology. This remarkable instance of convergence not only represents the earliest record of this adaptation to hypercarnivory but also demonstrates that the first iteration of this feature appeared in non-mammalian synapsids. Comparisons of tooth serrations in gorgonopsians with those of earlier synapsids and hypercarnivorous mammals reveal some gorgonopsians acquired a complex tissue arrangement that differed from other synapsids.     

Eine Conodontengruppe von Prooneotodus tenuis (MÜller, 1959) in natürlichem Zusammenhang aus dem Oberen Kambrium von schweden

A natural conodont assemblage,Prooneotodus tenuis (MÜller, 1959) was discovered in shales in Zone I of the Upper Cambrian of Hunneberg, Väastergötland, Sweden. It is composed of 12 similar single cone elements, which form 6 pairs of different size. The largest is more than twice the size of the smallest. This variation in size together with the occurrence of between 8-12 elements in conspecific assemblages recorded by Miller & Rushton (1973) makes it likely that during growth the animal added new elements to the apparatus. Should this be applicable to all conodonts, this observation would be of some relevance in the statistical reconstruction of conodont apparatusses.     

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.     

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.     

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.     

Soft anatomy and the affinities of conodonts

Recent claims that conodonts are members of the Craniata or Vertebrata are based in part upon soft tissue features that have been preserved in a small number of specimens. These features include what appear to be radials in the caudal fin and paired structures that have been identified as eye remnants. The evidence for radials is limited, but credible. However, the anatomy of extant cyclostomes suggests that the paired structures are more reasonably interpreted as otic capsules than the remnants of sclerotic eye capsules. Moreover, even if these structures are the remnants of eyes, conodonts might equally well be a sister group to the craniates as a member of that group. Aside from these paired structures, conodont fossils exhibit no features that are suggestive of a cartilaginous skeleton. Given that cyclostome fossils sometimes show evidence of the cartilages of the head, the apparent absence of a similar skeleton in conodont animals calls into question the claim that they are craniates. The simple single chevron shape of conodont myomeres also suggests that they lie outside of the Craniata. All living craniates have double-chevron myomeres as adults, whereas simple myomeres of the conodont type are found in the non-craniate cephalochordates. Thus the available soft tissue evidence suggests that conodonts are best regarded as the sister group of the craniates.     

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.     

Cutting the first ‘teeth’: a new approach to functional analysis of conodont elements

The morphological disparity of conodont elements rivals the dentition of all other vertebrates, yet relatively little is known about their functional diversity. Nevertheless, conodonts are an invaluable resource for testing the generality of functional principles derived from vertebrate teeth, and for exploring convergence in a range of food-processing structures. In a few derived conodont taxa, occlusal patterns have been used to derive functional models. However, conodont elements commonly and primitively exhibit comparatively simple coniform morphologies, functional analysis of which has not progressed much beyond speculation based on analogy. We have generated high-resolution tomographic data for each morphotype of the coniform conodont Panderodus acostatus. Using virtual cross sections, it has been possible to characterize changes in physical properties associated with individual element morphology. Subtle changes in cross-sectional profile have profound implications for the functional performance of individual elements and the apparatus as a whole. This study has implications beyond the ecology of a single conodont taxon. It provides a basis for reinterpreting coniform conodont taxonomy (which is based heavily on cross-sectional profiles), in terms of functional performance and ecology, shedding new light on the conodont fossil record. This technique can also be applied to more derived conodont morphologies, as well as analogous dentitions in other vertebrates and invertebrates.     

Biology meets engineering: The structural mechanics of fossil and extant shark teeth

The majority of studies on the evolution and function of feeding in sharks have focused primarily on the movement of cranial components and muscle function, with little integration of tooth properties or function. As teeth are subjected to sometimes extreme loads during feeding, they undergo stress, strain, and potential failure. As attributes related to structural strength such as material properties and overall shape may be subjected to natural selection, both prey processing ability and structural parameters must be considered to understand the evolution of shark teeth. In this study, finite element analysis was used to visualize stress distributions of fossil and extant shark teeth during puncture, unidirectional draw (cutting), and holding. Under the loading and boundary conditions here, which are consistent with bite forces of large sharks, shark teeth are structurally strong. Teeth loaded in puncture have localized stress concentrations at the cusp apex that diminish rapidly away from the apex. When loaded in draw and holding, the majority of the teeth show stress concentrations consistent with well designed cantilever beams. Notches result in stress concentration during draw and may serve as a weak point; however they are functionally important for cutting prey during lateral head shaking behavior. As shark teeth are replaced regularly, it is proposed that the frequency of tooth replacement in sharks is driven by tooth wear, not tooth failure. As the tooth tip and cutting edges are worn, the surface areas of these features increase, decreasing the amount of stress produced by the tooth. While this wear will not affect the general structural strength of the tooth, tooth replacement may also serve to keep ahead of damage caused by fatigue that may lead to eventual tooth failure. J. Morphol., 2011. © 2010 Wiley‐Liss, Inc.     

The conodont animal

A unique specimen of a small, elongate, soft-bodied animal from the Lower Carboniferous of the Edinburgh district, Scotland, is described. The head expands anteriorly into two lobate structures flanking a central lumen; behind this lies a conodont apparatus, apparently in situ, consisting of an aligned set of ramiform elements followed by a pair of ozarkodiniform elements and one of platform elements. From the morphology of the platform elements the animal has been identified as Clydagnathus? cf. cavusformis. Repeated structures which may represent segments are evident in the posterior part of the trunk, which bears a posterior and a caudal fin, each supported by rays. The animal shows similarities to both chordates and chactognaths, but the evidence supports its assignment to a separate phylum, the Conodonta. The function of the conodonts remains equivocal, but it seems more likely that they served as teeth than as internal supports.     

Patterns of bilateral asymmetry and allometry in Late Devonian Polygnathus conodonts

Conodont animals were early jawless vertebrates equipped with a feeding apparatus composed of several tooth‐like elements. The P1 elements, at the rear of the apparatus, were characterized by a robust shape and rapid morphological evolution. Occlusion occurred between paired right and left P1 elements, occasioning some bilateral asymmetry, which, together with allometric growth, may partially obliterate the temporal differences. The present study aims to disentangle these different components of morphological variation in Late Devonian Polygnathus P1 conodont elements. An extensive 2D geometric morphometric analysis of the platform shape was performed through the Famennian record of two outcrops. This analysis was completed by a 3D study on a subset of conodont elements. The 2D and 3D morphometric quantifications provided highly congruent results, showing that the 2D shape constitutes a good approximation of the element geometry. The 3D analysis delivered further insights into the relationship between the geometry of the elements and the constraints related to occlusion. The 2D analysis allowed a quantitative assessment of the variation among species and through time. Allometry and bilateral asymmetry were differently expressed depending on the species considered, suggesting that constraints imposed on pairing by the morphology of the elements varied even among related species. The within‐species variation was so important that it largely obliterated temporal trends; a relationship of Polygnathus shape and conodont biofacies variations through the Famennian nevertheless suggested an evolution driven by ecological interactions between conodont genera.     

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.     

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.     

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.     

The mechanics of cutting and the form of shark teeth (Chondrichthyes,Elasmobranchii)

Summary A rich engineering literature exists that is applicable to many aspects of vertebrate jaw mechanics and has been referred to in many studies in this sector. But mechanical engineering technology has provided few theoretical bases that are directly helpful in the study of predator teeth. Hence, analyses of puncturing and slicing functions of these teeth have lacked a firm physical technology as a background. Predator teeth have evolved to pierce and cut animal tissues that are usually compliant in that they readily undergo relatively large deformations under applied stress before they actually yield. The bulk of engineering theory is directed toward such noncompliant materials as wood and metal, the design of tools that cut them, and the mechanics involved in this. The purpose of the present paper is to scan the mechanical implications of different tooth designs, pose hypotheses that relate to primary considerations of the physics of cutting compliant substrates, and offer a preliminary approach that is intended as a useful guide to further studies on sharks and on other vertebrate groups. Thus, in this paper I have attempted to formulate some tentative and preliminary generalizations concerning the mechanics of cutting compliant materials. Then comes a survey of the teeth of a particular group of predators, three families of sharks, in terms of these preliminary formulations. The approach views the shark teeth in isolation from the complex cranial mechanism (presently under study) that functionally integrates with the teeth. Therefore, adaptive conclusions are minimal, because the evolutionary significance of tooth form cannot properly be assessed outside of an integrated study. However, certain correlations do exist between structural tooth characteristics and mechanics. Slender, smooth-edged (or nearly so) teeth can readily pierce prey, but are of less use in slicing it. Such teeth are typical of the lower jaw dentition in many sharks and, in a few species, they are present in both upper and lower jaws. Usually these slender teeth display a reversed curvature at their tips, so that although most of the tooth's crown is curved inward toward the mouth cavity, the tip is turned outward. This outward turning of the tip can enhance the probability of initial prey penetration, without much compromising the prey-retaining properties of the inward curvature of the greater, more proximal portion of the tooth. Many sharks possess upper teeth with serrations along the edges. The serrations vary from one species to another in coarseness and in distribution along tooth edges. Serrated teeth can make greater use of the available biting forces, and they have a greater cutting effect than do smooth-edged teeth. These latter depend upon friction which, because the coefficient friction is always less than 1.0 (often very much less), can make use of only a fraction of the total bite force. However, smooth tooth blades can pierce prey with less resistance and are less prone to binding (becoming immobilized) in the prey tissue. In many shark species serrations are concentrated along the proximal portions of the tooth crown, where the bases of adjacent teeth are in near contact along the jaw margin. In these regions food can be pressed during feeding, resulting in a binding of the teeth in the prey. Release of the binding must be accomplished by cutting the jammed food, to permit clearance of the prey material so it can slip past the tooth rows. The more prominent serrations in such regions may act to puncture and slice the jammed tissue. It is noted that commercial saws are typically designed in various ways to promote clearance between adjacent saw teeth. The pitch or rake of the teeth of sharks is discussed, as is the overall form of the tooth rows along the jaw margins. The relationship between the distribution of teeth along the jaw margins and surface irregularities of the prey surfaces is also considered.     

Late Devonian dimpled phosphatic microspherules associated with conodonts are possible otoliths of short‐lived and opportunistic organisms

Dimpled phosphatic microspherules, contradictorily associated with conodonts, are widely distributed in strata ranging in age from the Cambrian to Carboniferous. These microspherules have attracted much attention from palaeobiologists and were suggested to be ‘conodont pearls’, ‘conodont otoliths’ or ‘fish otoliths’ due to their similar chemical composition and co‐occurrence with conodonts or fish teeth. However, these hypotheses are still highly controversial. Here, we report ‘checks’, ‘rhythmic growth patterns’ and ‘sub‐diurnal increments’ from growth annuli of the Late Devonian phosphatic microspherules from South China, on the basis of quantitative microstructure analysis. The annulus width of phosphatic microspherules becomes narrower with increasing radius. These microstructural characteristics of growth annuli are most typical indicators of modern animal otoliths. In addition, a maximum value of about 90 annuli is encountered from all the specimens. We propose that these microspherules are essentially phosphatic otoliths, which might have been secreted by a specific kind of marine organisms with very short lifespans (less than 90 days). Furthermore, the sudden enrichment of phosphatic microspherules in the late Frasnian may represent a biological response of short‐lived animals to ecological crisis such as ocean eutrophication.