Abstract
The numerous cushion-shaped tooth-bearing plates attributed to the stem group osteichthyan Lophosteus superbus, which are argued here to represent an early form of the osteichthyan inner dental arcade, display a previously unknown and presumably primitive mode of tooth shedding by basal hard tissue resorption. They carry regularly spaced, recumbent, gently recurved teeth arranged in transverse tooth files that diverge towards the lingual margin of the cushion. Three-dimensional reconstruction from propagation phase-contrast synchrotron microtomography (PPC-SRµCT) reveals remnants of the first-generation teeth embedded in the basal plate, a feature never previously observed in any taxon. These teeth were shed by semi-basal resorption with the periphery of their bases retained as dentine rings. The rings are highly overlapped, which evidences tooth shedding prior to adding the next first-generation tooth at the growing edge of the plate. The first generation of teeth is thus diachronous. Successor teeth at the same sites underwent cyclical replacing and shedding through basal resorption, producing stacks of buried resorption surfaces separated by bone of attachment. The number and spatial arrangement of resorption surfaces elucidates that basal resorption of replacement teeth had taken place at the older tooth sites before the addition of the youngest first-generation teeth at the lingual margin. Thus, the replacement tooth buds cannot have been generated by a single permanent dental lamina at the lingual edge of the tooth cushion, but must have arisen either from successional dental laminae associated with the individual predecessor teeth, or directly from the dental epithelium of these teeth. The virtual histological dissection of these Late Silurian microfossils broadens our understanding of the development of the gnathostome dental systems and the acquisition of the osteichthyan-type of tooth replacement.
Highlights
Lophosteus superbus Pander from the Late Silurian (Pridoli) of Ohesaare cliff, Saaremaa, Estonia, was described in 1856 and was among the first Silurian vertebrates to be studied [1]
The virtual histological dissection of these Late Silurian microfossils broadens our understanding of the development of the gnathostome dental systems and the acquisition of the osteichthyan-type of tooth replacement
The cushion-shaped plates of Lophosteus and Andreolepis, referred to below as ‘tooth cushions’, are certainly internal to the margins of the jaws and could in principle, represent either the inner dental arcade, that is to say the coronoid-dermopalatine series, or still more internal elements such as branchial dental plates, or both. We return to this question in the Discussion, where we argue that they are more likely to belong to the inner dental arcade
Summary
Lophosteus superbus Pander from the Late Silurian (Pridoli) of Ohesaare cliff, Saaremaa, Estonia, was described in 1856 and was among the first Silurian vertebrates to be studied [1] It is represented by a rich but disarticulated material of scales, spines, dermal plates and tooth-bearing elements [2,3,4]. In 2010, Friedman & Brazeau [5] established a synapomorphy scheme for the osteichthyan stem, crown and total group, and placed several Silurian–Devonian osteichthyans in the osteichthyan stem, but most of these are ‘scale taxa’ represented by very fragmentary material Their scheme proved controversial; Schultze [7] revised the scale characters of these taxa and argued that Lophosteus, interpreted by Friedman and Brazeau as a possible (but not definite) stem osteichthyan, is the only known member of the osteichthyan stem group
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