Chimaera monstrosa, Linnaeus, 1758

Smith, Moya, Manzanares, Esther, Underwood, Charlie, Healy, Chris, Clark, Brett & Johanson, Zerina, 2020, Holocephalan (Chondrichthyes) dental plates with hypermineralized dentine as a substitute for missing teeth through developmental plasticity, Journal of Fish Biology 97 (1), pp. 16-27 : 18

3.1 | Chimaera monstrosa and Chimaera sp.

In Chimaera , the close fit of the anterior and posterior upper dental plates is apparent (Figures 1c,d, 2a and 3a,c, a.dpl, p.dpl, 2b, white arrows; Supporting Information File S1), such that when the density of whitlockin is segmented out (Figures 1d, arrows, 2c and 3c), the highly mineralized rods and ovoids of the anterior plate are in close alignment with those of the posterior, presumed to result from tight morphological constraints. One feature on the lingual surface of the anterior plate in all growth stages is the iterative, sequentially developed ridges of the outer dentine layer. In the youngest individual examined (14 cm long; Figure 1c, rdg), three to four widely spaced ridges are present, with small oval dentine units at the midline (Figure 1c, asterisk) of each dental plate. Notably these are not hypermineralized tissues as are the rods found in these plates (Figure 1d, ap.rd, pp.rd) but just elevations of the outer dentine layer. Also at this growth stage, the hypermineralized rods are shorter and do not appear to extend through the dental plate.

At the next growth stage (Figure 2), there has been substantial wear on both anterior and posterior dental plates, as indicated by the thickness of the sclerotic dentine (sensu Smith et al., 2019; Figure 2b, g,h, sod), forming in response to wear. The hypermineralized rods of the anterior plate are longer and extend through more of the anterior plate, while on the posterior dental plate more rods are present when compared to the 14 cm individual (Figure 2c). The number of ridges on the lingual surface of the plate has increased as well, to seven, with the most oral ridge showing wear (Figure 2a,b). These are absent from the comparable surface on the posterior dental plate (Figure 2a). The posterior end of these ridges is expanded and bulbous, particularly the most aboral ridges (Figure 2b,d,f, asterisk), the histology of the lower jaw plates of Harriotta showing that this outer dentine layer does become even more mineralized ( Smith et al., 2019). Just below these ridges, the aboral margin of the plate is composed of an open network of trabecular dentine (Figure 2f,h, tb). In section, the ridges surround trabecular dentine (Figure 2g,h), with co-incidence of early whitlockin formation of the rod and the ridge itself (Figure 2g, arrows). Sclerotic dentine fills the ridges in the same manner as it fills the spaces within the trabecular dentine closer to the oral surface (Figure 2g,h). Additionally, the ridges themselves appear to be set off from the rest of the plate, with a shallow furrow developing posteriorly (Figure 2f, arrow 1) and an elongate oral-aboral ridge anteriorly (Figure 2f, arrow 2).

The next two growth stages are represented by a subadult C. monstrosa and a juvenile Chimaera sp. (Figure 3), and in the upper dental plate of the younger of these individuals (Figure 3a–e) the hypermineralized dentine forms a series of ovoids in both the anterior and posterior dental plates (Figure 3b, ov). Along with this is a small rod located posteriorly and parasymphysially (Figure 3b). The posterior dental plate includes multiple series of ovoids, along with a small number of rods, at the anterior end. In addition, there are two elongate rods, lingual and medial to the ovoids (Figure 3a,b). There are six or seven ridges on the lingual surface of the anterior plate, although the bulbous ends are not as apparent (Figure 3c,e), and a shallow furrow and elongate ridge mark the boundaries of this region within the plate (Figure 3e, arrows 1 and 2). In section (Figure 3d), there again appears to be some spatial relationship between the series of rods and the lingual ridges, including those with developing rods that are less mineralized (d in Figure 3d; see also Figure 8). In the older individual (Figure 3f–i), the number of lingual ridges stays more or less constant. The anterior dental plate includes both a series of ovoids and a series of rods, but in the posterior dental plate, the rods are modified to produce the tritoral pads, with spaces for the vasculature appearing aborally within the whitlockin tissue (Figure 3g).