Romundina and the evolutionary origin of teeth

Abstract

Theories on the origin of vertebrate teeth have long focused on chondrichthyans as reflecting a primitive condition-but this is better informed by the extinct placoderms, which constitute a sister clade or grade to the living gnathostomes. Here, we show that 'supragnathal' toothplates from the acanthothoracid placoderm Romundina stellina comprise multi-cuspid teeth, each composed of an enameloid cap and core of dentine. These were added sequentially, approximately circumferentially, about a pioneer tooth. Teeth are bound to a bony plate that grew with the addition of marginal teeth. Homologous toothplates in arthrodire placoderms exhibit a more ordered arrangement of teeth that lack enameloid, but their organization into a gnathal, bound by layers of cellular bone associated with the addition of each successional tooth, is the same. The presence of enameloid in the teeth of Romundina suggests that it has been lost in other placoderms. Its covariation in the teeth and dermal skeleton of placoderms suggests a lack of independence early in the evolution of jawed vertebrates. It also appears that the dentition-manifest as discrete gnathal ossifications-was developmentally discrete from the jaws during this formative episode of vertebrate evolution.

Keywords: dental development; evolution; jawed vertebrates; modularity; placoderm.

Figure 1.

Acanthothoracid placoderm (same specimen as in [4]) and surface renderings (gold) of Romundina stellina and Compagopiscis croucheri. Upper dental plates (anterior supragnathals, ASG) in occlusal view (a). Right ASG of R. stellina (NRM-PZ P.15956) based on SRXTM data (b–e). (b) Distal, (c) proximal, (d) occlusal and (e) dorsal views. Left posterior supragnathal (PSG) of C. croucheri (NHMUK PV P.50943), based on MicroCT data (f–h). (f) Occlusal, (g) dorsal and (h) distal views. Scale bar represents 1.68 mm in (a), 178 µm in (b–e) and 206 µm in (f–h).

Figure 2.

Segmentation and virtual sections of SRXTM characterizations of a Romundina stellina supragnathal (NRM-PZ P.15956), dermal scale (NRM-PZ P.15952) and Compagopiscis croucheri supragnathal (NHMUK PV P.57629). (a–d) Right ASG of R. stellina. (a) Segmented sclerochronology of the dental plate following lines of arrested growth. Colour scheme (from gold to purple) represents the sequence of tooth addition. (b) Transverse and (c) longitudinal virtual sections showing addition of teeth and basal layer. (d) Detail of (c) showing enameloid/semidentine border and Sharpey's fibres. (e) Detailed virtual section of the right PSG of C. croucheri. (f) Virtual section and (g) dorsal view of the dermal scale of R. stellina. Scale bar represents 180 µm in (a), 97 µm in (b), 86 µm in (c), 50 µm in (d), 157 µm in (e), 96 µm in (f) and 224 µm in (g).