Structure and Affinities of Conodonts
The ultrastructure of 24 form-species of Ordovician conodonts has been examined with the scanning electron microscope. The conodonts represent two provincial and three subprovincial faunas and include hyaline forms, neurodonts (a subgroup of the hyalines), and cancellate forms (conodonts with white matter).
The robust neurodont elements are constructed of cone-in-cone lamellae separated by distinct interlamellar spaces. There is limited fusion between adjacent lamellae and between the long, needle-like crystallites within each lamella. Crystallites become granular in form near the base in some neurodonts. A sheet-like septum bisects the elements longitudinally, but does not pass through the central growth canal. The latter is surrounded by fused lamellae producing a strengthened wall. Minute spheres occur along crystallites in some neurodonts. In the non-neurodont hyaline conodonts, greater fusion occurs between lamellae and between crystallites, and white matter may develop along the central growth canal; no septum or spheres have been noted within this group. Increased fusion is considered to have produced stronger elements capable of acquiring lateral compression, costae, and keels. Cancellate conodonts develop white matter primarily in their cusps and denticles. White matter is finely crystalline with no lamellar structure, but with abundant circular and linear voids. There is no pattern to the occurrence of the holes, but most linear voids are oriented transversely. White matter is formed by secondary transformation from lamellar material; this change is reflected in a transitional zone of incipient white matter, where a reorientation of the hard tissue is evident. Minute spheres occur in white matter; the central growth canal is destroyed and does not penetrate beyond the transition zone. White matter is believed to provide extra strength to the element allowing marked lateral compression and sharp margins.
Although separated on structural criteria, these three conodont groups appear to occupy distinct and evolving ecosystems during the Ordovician. Three theories are proposed which, individually or in combination, may indicate the functional advantages conveyed by the development of white matter: (1) that the factors of element weight and phosphate availability were important, (2) that greater strength to cope with variable stresses was achieved, and (3) that white matter was induced by a disruption in vascular supply, possibly resulting from a partial eruption through the secreting tissues.