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

The hypothesis that conodonts are vertebrates rests solely on evidence of soft tissue anatomy. This has been corroborated by microstructural, topological and develop- mental evidence of homology between conodont and verteb- rate hard tissues. However, these conclusions have been reached on the basis of evidence from highly derived eucono- dont taxa and the degree to which they are representative of plesiomorphic euconodonts remains an open question. Fur- thermore, the range of variation in tissue types comprising the euconodont basal body has been used to establish a hypo- thesis of developmental plasticity early in the phylogeny of the clade, and a model of diminishing potentiality in the evo- lution 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 hypo- theses. It is found that the range of microstructural variation observed hitherto was already apparent among plesiomorphic euconodonts. Thus, established histological data are represen- tative of the most plesiomorphic euconodonts. However, although there is evidence of a range in microstructural fab- rics, these are compatible with the dentine tissue system alone, and the degree of variation is compatible with that seen in clades of comparable diversity.