Growth and feeding ecology of coniform conodonts

Conodonts were the first vertebrates to develop mineralized dental tools, known as elements. Recent research suggests that conodonts were macrophagous predators and/or scavengers but we do not know how this feeding habit emerged in the earliest coniform conodonts, since most studies focus on the derived, 'complex' conodonts. Previous modelling of element position and mechanical properties indicate they were capable of food processing. A direct test would be provided through evidence of in vivo element crown tissue damage or through in vivo incorporated chemical proxies for a shift in their trophic position during ontogeny. Here we focus on coniform elements from two conodont taxa, the phylogenetically primitive Proconodontus muelleri Miller, 1969 from the late Cambrian and the more derived Panderodus equicostatus Rhodes, 1954 from the Silurian. Proposing that this extremely small sample is, however, representative for these taxa, we aim to describe in detail the growth of an element from each of these taxa in order to the test the following hypotheses: (1) Panderodus and Proconodontus processed hard food, which led to damage of their elements consistent with prey capture function; and (2) both genera shifted towards higher trophic levels during ontogeny. We employed backscatter electron (BSE) imaging, energy-dispersive X-ray spectroscopy (EDX) and synchrotron radiation X-ray tomographic microscopy (SRXTM) to identify growth increments, wear and damage surfaces, and the Sr/Ca ratio in bioapatite as a proxy for the trophic position. Using these data, we can identify whether they exhibit determinate or indeterminate growth and whether both species followed linear or allometric growth dynamics. Growth increments (27 in Pa. equicostatus and 58 in Pr. muelleri) were formed in bundles of 4-7 increments in Pa. equicostatus and 7-9 in Pr. muelleri. We interpret the bundles as analogous to Retzius periodicity in vertebrate teeth. Based on applied optimal resource allocation models, internal periodicity might explain indeterminate growth in both species. They also allow us to interpret the almost linear growth of both individuals as an indicator that there was no size-dependent increase in mortality in the ecosystems where they lived e.g., as would be the case in the presence of larger predators. Our findings show that periodic growth was present in early conodonts and preceded tissue repair in response to wear and damage. We found no microwear and the Sr/Ca ratio, and therefore the trophic position, did not change substantially during the lifetimes of either individual. Trophic ecology of coniform conodonts differed from the predatory and/or scavenger lifestyle documented for "complex" conodonts. We propose that conodonts adapted their life histories to top-down controlled ecosystems during the Nekton Revolution.

Morphometric variance, evolutionary constraints and their change through time in Late Devonian Palmatolepis conodonts

Phenotypic variation is the raw material of evolution. Standing variation can facilitate response to selection along "lines of least evolutionary resistance", but selection itself might alter the structure of the variance. Shape was quantified using 2D geometric morphometrics in Palmatolepis conodonts through the Late Devonian period. Patterns of variance were characterized along the record by the variance-covariance matrix (P-matrix) and its first axis (Pmax). The Late Frasnian was marked by environmental oscillations culminating with the Frasnian/Famennian mass extinction. A shape response was associated with these fluctuations, together with a deflection of the Pmax and the P-matrix. Thereafter, along the Famennian, Palmatolepis mean shape shifted from broad elements with a large platform to slender elements devoid of platform. This shift in shape was associated with a reorientation of Pmax and the P-matrix, due to profound changes in the functioning of the elements selecting for new types of variants. Both cases provide empirical evidences that moving adaptive optimum can reorient phenotypic variation, boosting response to environmental changes. On such time scales, the question seems thus not to be whether the P-matrix is stable, but how it is varying in response to changes in selection regimes and shifts in adaptive optimum. This article is protected by copyright. All rights reserved.

The Materials of Mastication: Material Science of the Humble Tooth

Dental functional morphology, as a field, represents a confluence of materials science and biology. Modern methods in materials testing have been influential in driving the understanding of dental tissues and tooth functionality. Here we present a review of dental enamel, the outermost tissue of teeth. Enamel is the hardest biological tissue and exhibits remarkable resilience even when faced with a variety of mechanical threats. In the light of recent work, we progress the argument that the risk of mechanical degradation across multiple scales exhibits a strong and continued selection pressure on structural organization of enamel. The hierarchical nature of enamel structure presents a range of scale-dependent toughening mechanisms and provides a means by which natural selection can drive the specialization of this tissue from nanoscale reorganization to whole tooth morphology. There has been much learnt about the biomechanics of enamel recently, yet our understanding of the taxonomic diversity of this tissue is still lacking and may form an interesting avenue for future research.

A new icriodontid conodont cluster with specific mesowear supports an alternative apparatus motion model for Icriodontidae

Increasing numbers of conodont discoveries with soft tissue preservation, natural assemblages and fused clusters of the hard tissue have strengthened the hypothesis regarding the function and mechanism of the conodont feeding apparatus. Exceptional fossil preservation serves as a solid basis for modern reconstructions of the conodont apparatus illustrating the complex interplay of the single apparatus elements. Reliable published models concern the ozarkodinid apparatus of Pennsylvanian and Early Triassic conodonts. Recognition of microwear and mammal-like occlusion, especially of platform elements belonging to individuals of the genus Idiognathodus, allows rotational closure to be interpreted as the crushing mechanism of ozarkodinid platform (P1) elements. Here we describe a new icriodontid conodont cluster of Caudicriodus woschmidti that consists of one pair of icriodontan (I) and 10 pairs of coniform (C1-5) elements, with I elements being preserved in interlocking position. The special kind of element arrangement within the fused cluster provides new insights into icriodontid apparatus reconstruction and notation of elements. However, orientation of coniform elements is limited to a certain degree by possible preservational bias. Four possible apparatus models are introduced and discussed. Recognition of specific wear on denticle tips of one of the icriodontan elements forms the basis for an alternative hypothesis of apparatus motion. Analysis of tip wear suggests a horizontal, slightly elliptical motion of opposed, antagonistically operating I elements. This is supported by similar tip wear from much better preserved, but isolated, elements of Middle Devonian icriodontids. More detailed interpretation of the masticatory movement will allow enhanced understanding of anatomical specifications, diet and palaeobiology of different euconodont groups.

Conodonts and the Paleoclimatological and Paleoecological Applications of Phosphate Δ18O Measurements

Oxygen isotopic analysis of the phosphate in bioapatite has become a standard paleoclimatological tool with results documented in a rapidly expanding literature. Phosphate-based measurements are particularly important for samples where carbonates preservation is suspect (as is the case for many Paleozoic sites). Important analytical and observational advances that have fueled the expansion of phosphate-based studies include: 1) Oxygen isotopic ratios of biogenic apatite can be measured on small enough samples (≥ ~300 μg), quickly enough, cheaply enough, and accurately enough to permit meaningful high resolution paleoclimatic studies of trends through time, along spatial transects, and/or among taxa, 2) biogenic apatite is precipitated in approximate equilibrium with ambient waters and thus records the interplay of temperature and the isotopic composition of the water in which a sample grew, 3) tooth enamel and conodont crown material are quite resistant to diagenetic alteration and are preferred targets for both paleotemperature and paleoecological studies, 4) Paleozoic conodont δ18O records seem to provide robust paleotemperature information on time scales ranging from thousands of years to 100's of millions of years, and generation of increasingly refined paleotemperature records from this diagenetically resistant phase is likely to continue to be a useful field of study, 5) paleoenvironmental variations in δ18O values of seawater have been documented (e.g., differences between glacial and interglacial oceans), but whether and by how much the δ18O value of the hydrosphere may have increased since the Cambrian remains unresolved, and 6) differences in δ18O values among conodont taxa are increasingly well documented and, coupled with the potential to study growth series using ion microprobe techniques, are providing novel perspectives on and important tests of conodont paleoecology.

Constraining the Deep Origin of Parasitic Flatworms and Host-Interactions with Fossil Evidence

Novel fossil discoveries have contributed to our understanding of the evolutionary appearance of parasitism in flatworms. Furthermore, genetic analyses with greater coverage have shifted our views on the coevolution of parasitic flatworms and their hosts. The putative record of parasitic flatworms is consistent with extant host associations and so can be used to put constraints on the evolutionary origin of the parasites themselves. The future lies in new molecular clock analyses combined with additional discoveries of exceptionally preserved flatworms associated with hosts and coprolites. Besides direct evidence, the host fossil record and biogeography have the potential to constrain their evolutionary history, albeit with caution needed to avoid circularity, and a need for calibrations to be implemented in the most conservative way. This might result in imprecise, but accurate divergence estimates for the evolution of parasitic flatworms.

The characters of Palaeozoic jawed vertebrates

Newly discovered fossils from the Silurian and Devonian periods are beginning to challenge embedded perceptions about the origin and early diversification of jawed vertebrates (gnathostomes). Nevertheless, an explicit cladistic framework for the relationships of these fossils relative to the principal crown lineages of the jawed vertebrates (osteichthyans: bony fishes and tetrapods; chondrichthyans: sharks, batoids, and chimaeras) remains elusive. We critically review the systematics and character distributions of early gnathostomes and provide a clearly stated hierarchy of synapomorphies covering the jaw-bearing stem gnathostomes and osteichthyan and chondrichthyan stem groups. We show that character lists, designed to support the monophyly of putative groups, tend to overstate their strength and lack cladistic corroboration. By contrast, synapomorphic hierarchies are more open to refutation and must explicitly confront conflicting evidence. Our proposed synapomorphy scheme is used to evaluate the status of the problematic fossil groups Acanthodii and Placodermi, and suggest profitable avenues for future research. We interpret placoderms as a paraphyletic array of stem-group gnathostomes, and suggest what we regard as two equally plausible placements of acanthodians: exclusively on the chondrichthyan stem, or distributed on both the chondrichthyan and osteichthyan stems.

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.

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.

Synchrotron-aided reconstruction of the conodont feeding apparatus and implications for the mouth of the first vertebrates

The origin of jaws remains largely an enigma that is best addressed by studying fossil and living jawless vertebrates. Conodonts were eel-shaped jawless animals, whose vertebrate affinity is still disputed. The geometrical analysis of exceptional three-dimensionally preserved clusters of oro-pharyngeal elements of the Early Triassic Novispathodus, imaged using propagation phase-contrast X-ray synchrotron microtomography, suggests the presence of a pulley-shaped lingual cartilage similar to that of extant cyclostomes within the feeding apparatus of euconodonts ("true" conodonts). This would lend strong support to their interpretation as vertebrates and demonstrates that the presence of such cartilage is a plesiomorphic condition of crown vertebrates.

Origin and early evolution of vertebrate skeletonization

Data from living and extinct faunas of primitive vertebrates imply very different scenarios for the origin and evolution of the dermal and oral skeletal developmental system. A direct reading of the evolutionary relationships of living primitive vertebrates implies that the dermal scales, teeth, and jaws arose synchronously with a cohort of other characters that could be considered unique to jawed vertebrates: the dermoskeleton is primitively composed of numerous scales, each derived from an individual dental papilla; teeth are primitively patterned such that they are replaced in a classical conveyor-belt system. The paleontological record provides a unique but complementary perspective in that: 1) the organisms in which the skeletal system evolved are extinct and we have no recourse but to fossils if we aim to address this problem; 2) extinct organisms can be classified among, and in the same way as, living relatives; 3) a holistic approach to the incorporation of all data provides a more complete perspective on early vertebrate evolution. This combined approach is of no greater significance than in dealing with the origin of the skeleton and, combined with recent discoveries and new phylogenetic analyses, we have been able to test and reject existing hypotheses for the origin of the skeleton and erect a new model in their place.

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.