Microstructural variation in conodont enamel is a functional adaptation

Abstract

Recognition that conodonts were the earliest vertebrate group to experiment with skeletal biomineralization provides a window in which to study the origin and early evolution of this developmental system. It has been contended that the conodont skeleton comprised a classic suite of vertebrate hard tissues, while others suggest that conodont hard tissues represent divergent specializations within the early diversi¢cation of vertebrate hard tissues, supporting a view that the hard tissues of conodonts, particularly enamel, exhibit a range of microstructural variation beyond that seen in vertebrates. New evidence reveals that, although variable, conodont enamel microstructure is consistent between homologous portions of homologous dentitions. Although there is a correlation between morphology and microstructure, this belies a stronger correlation between the commonality of microstructure and dental function. The enamel of conodonts evolved in response to changes in dental function and di¡erentiation of the microstructural layer into a number of enamel types and can be linked to dental occlusion, heterodonty, a permanent dentition, enamel thickness and, probably above all, the small size of the dental elements.

Testing microstructural adaptation in the earliest dental tools

“…An isotropic model simulated a cusp composed of white matter. For comparison, we used an orthotropic model to simulate a cusp constructed of lamellar tissue where apatite crystallites are orientated orthogonal to the surface, a common microstructure for this tissue [4]. To model these properties, the orthotropic model was partitioned into 16 longitudinal segments (figure 1), and the stiffest axis in each segment orientated approximately perpendicular to the model surface, forming a radiating configuration around the long axis of the model.…”

“…Even greater microstructural variation is present in conodont elements, the earliest vertebrate dental structures [2]. The microstructure of conodont lamellar apatite, a functional analogue and putative biological homologue of enamel, from which element crowns are predominantly constructed [3] was also adapted to specific occlusal kinematics [4]. However, in many conodont taxa, the dental tools responsible for food acquisition and fracture (cusps and denticles) are composed partially or entirely from an additional tissue known as 'white matter' [5] (figure 1a,b).…”

The homology and phylogeny of chondrichthyan tooth enameloid

“…Enameloid matrix contains collagen of ectomesenchymal origin and odontoblast cell processes, in addition to ameloblastic cell secretions (primarily enamelin, though amelogenin and other enamel proteins are also present [Herold et al, 1980;Satchell et al, 2002]), and is secreted beneath the basal lamina of the inner dental epithelium (Shellis andMiles, 1974, 1976;Shellis, 1975;Kemp, 1985;Smith, 1995). There is evidence that the shift from enamel to enameloid has occurred on several occasions during vertebrate evolution (Donoghue et al, 2000;Donoghue, 2001).…”

Histology of the heterostracan dermal skeleton: Insight into the origin of the vertebrate mineralised skeleton

“…In accordance with this interpretation, he regarded the capping layer of heterostracans as enamel (Moss, 1968a). However, true enamel is genetically and developmentally distinct from 'enameloid' present in living teleosts or chondricthyans (Poole, 1967;Sasagawa, 2002;Kawasaki et al, 2004;Kawasaki and Weiss, 2006;Kawasaki et al, 2007) and is restricted to osteichthyans (Donoghue, 2001).…”