THE ZOOLOGICAL AFFINITIES OF THE CONODONTS
Authors: RHODES F. H. T. , PHILLIPS R. 
Published: January 2008

SUMMARY

Conodonts are minute, tooth‐like fossils, which exhibit considerable variation in form. Two main types of structure are recognizable, laminated and fibrous. Lamellar conodonts occur in sedimentary rocks from Ordovician to Triassic, while fibrous conodonts appear to be confined to the Ordovician.
Conodonts have been classified as isolated specimens, upon which a binomial system of classification has been erected. This classification does not, however, represent a true zoological classification, since recent work has shown that a number of ‘form‐genera’ appear to have been present in an individual conodont‐bearing animal. This has formed the basis for a zoological classification, which now exists alongside the earlier ‘form‐classification’.
The fibrous conodonts are frequently attached to basal ‘bone‐like’ material. They are apparently confined to the Ordovician, and this suggests that they may represent a distinct group from the lamellar conodonts.
The main variations in form in the lamellar conodonts are described.
The basal cavity of the lamellar conodonts is variable in form. Keels are developed on the aboral surfaces of a number of types, and a few lamellar conodonts are attached to bone‐like material.
Their microstructure indicates that conodonts were formed by accretion around a basal cavity. The presence of radial canals, extending from the basal cavity to the surface of the units, has been detected.
Conodonts are composed of calcium phosphate which has the structure of the apatite series. Analysis shows their composition to be essentially similar to that of the ‘bone‐like’ material to which they are sometimes attached, ridged, bone‐like, fragmentary plates, found associated with the conodonts and a typical Devonian fish‐plate fragment. There appears to be no difference in composition between the fibrous and the lamellar conodonts.
Specimens are recorded in which broken parts of conodonts appear to have been regenerated. The significance of this is discussed.
Palaeontological studies indicate that the conodont‐bearing animals were adapted to a wide variety of shallow‐water, marine environments.
Natural conodont assemblages appear to indicate that conodonts are paired, generally in an antero‐posteriorly elongated arrangement. A single assemblage may contain 14–22 component conodonts, representing 3–5 different ‘form‐genera’.
Conodonts are extinct, having existed from Ordovician to Triassic times. Their geological history is discussed and compared with the histories of other animal groups.
The denticulated jaw Archeognathus may not necessarily represent a group of fibrous conodonts. It is, however, similar to these, and its form tends to support the theory that fibrous conodonts represent a distinct group of animals from those which bore the lamellar conodonts.
The structures on a specimen of Coelacanthus lepturus described by Demanet do not appear to represent conodonts.
The arrangement, form, number, chemical composition and faunal associations of conodonts do not appear to favour the theories of their crustacean or molluscan origin.
The suggestion of the skeletal function of conodonts does not appear to be favoured by their general form, their assemblage occurrences or the form of their basal cavity.
The evidence for the suggestion of the annelid affinities of the conodonts is discussed. The significance of the chemical composition of the conodonts is considered.
The chemical composition, size and general form of conodonts, and the arrangement of conodont assemblages, appear to contradict the theory that they functioned as copulatory structures in worms.
The reasons for regarding the conodonts as being parts of fish are discussed. The two most important are the chemical composition and the basal attachment of conodonts, but neither of these appears to offer conclusive evidence of the origin of conodonts from fishes. It is suggested that, if the vertebrate origin of conodonts is accepted, they may represent some group of vertebrates, other than fishes, now extinct, and apart from the conodonts, entirely unknown.
It is suggested that the general lack of wear, form, size and assemblage arrangement of conodonts tend to support a theory of their annelid affinities. The main problem appears to be whether the internal secretion of calcium phosphate must be regarded as an indication of vertebrate, rather than invertebrate, origin. The answer to this problem largely determines whether the conodonts are considered as representing vertebrates or worm‐like creatures. It is suggested that the present state of knowledge does not justify a final conclusion as to the affinities of the conodonts, although they appear to represent an extinct group of either worm‐like creatures or primitive vertebrates.

It is a pleasure to acknowledge the assistance given by Mr Adrian P. Rhodes in preparing Figs. 1 and 3, Mr Roy Philips for his section on the composition of conodonts, Prof. Harold Scott, of the University of Illinois, for a number of helpful discussions on this subject, Prof. James Cullison, of Florida State University, for supplying information on Archeognathus, Prof. J. B. Cragg and Dr John Phillipson for reading the manuscript, Dr George Kohnstam for translating a paper, Mr G. O'Neill and Mrs J. Harker for assistance in the preparation of the illustrations, and the secretarial staff of the Geology Department of the Durham Colleges for their assistance in the preparation of the typescript.