A large Middle Devonian eubrachythoracid \u2018placoderm\u2019 (Arthrodira) jaw from northern Gondwana

Melina Jobbins,Thodoris Argyriou,Christian Klug,Martin Rücklin

doi:10.1186/s13358-020-00212-w

Abstract

For the understanding of the evolution of jawed vertebrates and jaws and teeth, ‘placoderms’ are crucial as they exhibit an impressive morphological disparity associated with the early stages of this process. The Devonian of Morocco is famous for its rich occurrences of arthrodire ‘placoderms’. While Late Devonian strata are rich in arthrodire remains, they are less common in older strata. Here, we describe a large tooth-bearing jaw element of Leptodontichthys ziregensis gen. et sp. nov., an eubrachythoracid arthrodire from the Middle Devonian of Morocco. This species is based on a large posterior superognathal with a strong dentition. The jawbone displays features considered synapomorphies of Late Devonian eubrachythoracid arthrodires, with one posterior and one lateral row of conical teeth oriented postero-lingually. μCT-images reveal internal structures including pulp cavities and dentinous tissues. The posterior orientation of the teeth and the traces of a putative occlusal contact on the lingual side of the bone imply that these teeth were hardly used for feeding. Similar to Compagopiscis and Plourdosteus, functional teeth were possibly present during an earlier developmental stage and have been worn entirely. The morphological features of the jaw element suggest a close relationship with plourdosteids. Its size implies that the animal was rather large.

Highlights

For the understanding of the evolution of jawed vertebrates and jaws and teeth, ‘placoderms’ are crucial as they exhibit an impressive morphological disparity associated with the early stages of this process
The partial preservation of the posterior superognathal (PSG) of Actinolepis does not allow the evaluation of the presence of tooth rows, it has been previously suggested that it bears numerous teeth, possibly in rows, as in the Anterior superognathal (ASG) (Mark-Kurik 1985)
An isolated bone like the PSG of Leptodontichthys implies that the Conclusion We describe a large posterior superognathal from the Middle Devonian Drotops layer of the eastern AntiAtlas

Summary

Introduction

For the understanding of the evolution of jawed vertebrates and jaws and teeth, ‘placoderms’ are crucial as they exhibit an impressive morphological disparity associated with the early stages of this process. Despite the lack of some processes such as tooth resorption, ‘placoderms’ exhibit structures homologous to crown gnathostome teeth, which include a dentinous tissue, a pulp cavity and a successional development out of bite (Donoghue and Rücklin 2014; Johanson and Smith 2005; Rücklin et al 2012, 2014; Smith and Johanson 2003a, b; Vaškaninová et al 2020) These dental structures grow through successive addition and form rows, as in the arthrodire Compagopiscis (Donoghue and Rücklin 2014; Rücklin et al 2012) and inferred based on gross morphology for the arthrodire Plourdosteus Issues regarding the evolutionary history and putative function of these dental structures remain open

Methods

Discussion

Conclusion