THE ANATOMY OF CONODONTS

Specimens from the Carboniferous Granton shrimp bed of Edinburgh, Scot land, provide the most complete record of conodont anatomy. Ten specim ens are now known, six of which are previously undescribed, and form t he basis of a new description and restoration of the conodont animal. The feeding apparatus is present in eight of the specimens; all but tw o of these can be assigned to Clydagnathus on the basis of the element s. A different genus and species is represented by the other two. The soft tissue morphology of all the specimens is similar. The Granton an imals are elongate, 21-55 mm in preserved length with a short head, a trunk with V-shaped myomeres, and a ray-supported caudal fin. The head is characterized by two lobate structures, which are interpreted as h ollow sclerotic cartilages indicating the position of large eyes. One specimen preserves traces of possible otic (auditory) capsules and bra nchial structures. Ventral and immediately posterior to the eyes lies the feeding apparatus, with the ramiform elements at the anterior end. There is no evidence of tissue surrounding this apparatus, indicating incomplete preservation of ventral soft parts, at least at the anteri or end of the specimens. The trunk of most specimens displays the noto chord as a pair of axial lines which represent its margins. In one spe cimen the area between the two lines is mineralized and displays a tra nsverse fibrous structure. The notochord tapers anteriorly and posteri orly; it extends as far forwards as the ramiform elements of the feedi ng apparatus but does not reach the anterior tip of the head. Two spec imens show a possible trace of the dorsal nerve cord. V-shaped myomere s are particularly well-preserved along the trunk of some of the new s pecimens; their preservation as distinct chevrons is attributed to a l ittle post-mortem shrinkage. Possible traces of original muscle fibres are preserved in the myomeres of one specimen. The tail is present on two specimens, with fin rays representing a caudal fin that may be sy mmetrical or may be slightly more extensive on the ventral margin. The evidence of the soft-part anatomy, together with features of element histology, show that the conodonts are vertebrates. Hypotheses that co nodonts have affinities with nemerteans, molluscs, chaetognaths, or ce phalochordates are refuted. Nor do the conodonts represent a separate phylum. Within the Vertebrata, the conodonts are considered to lie cro wnwards of the myxinoids, forming a primitive sister group of the Hete rostraci+Myopterygii.