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.     

The apparatus of panderodontid conodonts

A cluster of thirteen elements of Panderodus unicosfatus (Branson & Mehl) from the Llandoverian of Po-dolia, Ukraine, is shown to represent an almost complete apparatus; only one element is lacking. The restored original arrangement of the elements is consistent with the model of the conodont apparatus with seven pairs of elements ordered in two parallel rows, with cusps opposing in every pair. The apparatus of Panderodus is closely similar to that of the Cambrian 'protoconcdonts' and their Recent chaetognath successors. Nevertheless, general trends in the evolution of elements of the Panderodontidae towards increased elongation, smoothness of the cusp curvature, and depth of the basal cavity indicate that the grasping apparatus of Panderodus developed independently of, and convergently to, that of the Chaetog-natha.     

Conodontes de la coupe de Reun Ar C'Hrank en Lanvéoc (Rade de Brest) emsien du massif Armoricain (France) corrélations biostratigraphiques

In the Reun ar C'Hrank section (Rade de Brest), the Conodonts from just above the base of the Beg an Arreun Formation of the Troaon Group in the Armorican Massif indicate the Polygnathus laticostatus Zone, which belongs to the highest Zlichvian and the base of the Dalejan. Species of Icriodus from the same beds indicate a correlation with the lower and middle part of the Hierges Formation, which represents the Upper Emsian in the Ardennes. Conodonts from Le Fret Formation are tentatively assigned an Upper Emsian/Dalejan age.     

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.     

Variation in the outline and distribution of epithelial cell imprints on the surface of polygnathacean conodont elements

The elements of many conodont taxa exhibit a polygonal surface micro-ornamentation. Four main types are recognized (striation, linear texture, regular (idiomorphic) texture and granular texture) and their distribution over the conodont elements of different morphology is considered. The intraspecific (ontogenetic and ecological) and interspecific (phylogenetic) causes of the reticulation texture variations are also considered.     

A critical look at the Treatise family-group classification of Conodonta: an exercise in eclecticism

The family-group classification of conodonts presented in the revised edition of the Treatise on Invertebrate Paleontology, Part W, Supplement 2, Conodonta, published in 1981, is shown to contain errors with regard to the International Code of Zoological Nomenclature. The classification lacks a concise philosophical and methodological foundation and exhibits very poor conceptualization of the taxa it recognizes. The classification is analyzed and discussed in terms of, among Linnkan taxonomists, generally accepted objectives and conventions with regard to the internal structure and phylogenetic message of a hierarchical natural classification. The obvious errors in terms of the ICZN are pointed out and corrected. As a demonstration of an alternative approach, a classification of the superfamilies Prioniodontacea, Hibbardellacea, and Icriodontacea is presented.