The early history of the Conodonta

ECOS III The slender, spine-shaped, apatitic protoconodonts appear in the fossil record near the Precambrian-Cambrian boundary and persist through the Cambrian . Recent work (Szaniawski 1 982, J. Paleont. 56) suggests that protoconodont elements were homologous to the grasping spines of modern chaetognaths . Paraconodonts are similar to protoconodonts in their mode of growth by basal accretion . However, paraconodonts were more deeply invested in the secreting epithelium, and there are no known transitional forms between the two types. Other Cam brian conodont-like fossils have been investigated to examine the possibility that they may be homologous with paraconodonts. Some of them, such as the funnel-shapcd cones of Fomitehella, have an internal structure which precludes homology with paraconodonts . Others, such as the cone-shaped sclerites of Lapworthella, might have taken on the morphological and structural characteristics ofparaconodonts ifthey became adapted to a tooth or claw function, but there is no direct evidence to support such an interpretation . Morphological, histological and stratigraphi­ cal data indicate that euconodonts evolved from paraconodonts during the late Cambrian by acquiring a crown, a dense apatitic tissue secret ed over the outer surface of the paraconodont cusp. If the paraconodont and protoconodont animals are closely related, as has been suggested earlier, the paraconodont and euconodont animals may represent a branch of the chaetognaths that had developed pharyngeal denticulation . This concept of conodont origin and early evolution fits well with the recently discovered euconodont animal in the Scottish Carboniferous (Briggs, Clarkson & Aldridge 1 983, Lethaia 16) . O Conodonta, Chaetognatha" protoeonodonts, Rhombocorniculum, Fomitchella, Lapworthella, mierostrueture, funetional morphology, evolution, Cambrian.