Marginal dentition and multiple dermal jawbones as the ancestral condition of jawed vertebrates

Abstract

<jats:p>The dentitions of extant fishes and land vertebrates vary in both pattern and type of tooth replacement. It has been argued that the common ancestral condition likely resembles the nonmarginal, radially arranged tooth files of arthrodires, an early group of armoured fishes. We used synchrotron microtomography to describe the fossil dentitions of so-called acanthothoracids, the most phylogenetically basal jawed vertebrates with teeth, belonging to the genera <jats:italic>Radotina</jats:italic>, <jats:italic>Kosoraspis</jats:italic>, and <jats:italic>Tlamaspis</jats:italic> (from the Early Devonian of the Czech Republic). Their dentitions differ fundamentally from those of arthrodires; they are marginal, carried by a cheekbone or a series of short dermal bones along the jaw edges, and teeth are added lingually as is the case in many chondrichthyans (cartilaginous fishes) and osteichthyans (bony fishes and tetrapods). We propose these characteristics as ancestral for all jawed vertebrates.</jats:p>

Dental ontogeny in the most primitive bony fishLophosteusreveals the developmental relationship between teeth and dermal odontodes

“…This suggests an irregular expansion of the dermal epithelium into the region previously occupied exclusively by the oral epithelium. A similar invasion of the tooth field by dermal ornament is observed in Andreolepis (Chen et al, 2016) and an unnamed acanthothoracid from the Canadian Arctic, specimen CPW.9 (Vaškaninová et al, 2020).…”

“…Kosoraspis has a much simpler ontogenetic history without resorption-replacement cycles, essentially corresponding only to the first-generation odontodes of Andreolepis. Kosoraspis show a unidirectional change from small ornament odontodes at the labial margin of the jawbone, through gradually larger and progressively more tooth-like ornament odontodes, to teeth (Vaškaninová et al 2020). It indicates the ossification center locates at the labial margin.…”

“…Comparative context: the dentitions of Andreolepis and 'acanthothoracids'. The marginal dentitions of the stem osteichthyan Andreolepis (Late Silurian, Gotland, Sweden) and the 'acanthothoracid' stem gnathostomes Kosoraspis (Early Devonian, Prague Basin, Czech Republic) are of particular interest as comparators for Lophosteus because they show a similar combination of transverse alternate tooth files with lingual tooth addition, carried on marginal bones that also bear dermal ornament odontodes on their deep facial laminae (Chen et al, 2016;Vaškaninová et al, 2020). Lophosteus aligns with Andreolepis and differs from stem gnathostomes in showing resorptive tooth shedding, a unique osteichthyan characteristic (Chen et al, 2016).…”

“…The same technique has revealed the most primitive gnathostome dentitions (Vaškaninová et al, 2020) and the earliest osteichthyan-style tooth replacement (Chen et al, 2016). The phylogenetically most basal vertebrates known to possess teeth are the Early Devonian armoured fish known as 'acanthothoracids', including Radotina, Kosorapis and Tlamaspis (Figure 1).…”

“…The phylogenetically most basal vertebrates known to possess teeth are the Early Devonian armoured fish known as 'acanthothoracids', including Radotina, Kosorapis and Tlamaspis (Figure 1). All their dentitions suggest that being added lingually but not shed and solely carried by marginal dermal bones are the ancestral conditions of teeth (Vaškaninová et al, 2020). Chondrichthyans and osteichthyans both inherit the lingual tooth addition, but evolve tooth shedding independently; while marginal jawbone ornamented by dermal odontodes is only kept by osteichthyans.…”