A large Middle Devonian eubrachythoracid 'placoderm' (Arthrodira) jaw from northern Gondwana

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.

Keywords: Arthrodira; Dentition; Food web; Givetian; Maïder basin; Palaeoecology.

Fig. 1

Map of the Anti-Atlas of Morocco, showing where the Drotops layer crops out and Jebel Zireg, where the holotype of Leptodontichthys ziregensis gen. et sp. nov. was found

Fig. 2

Posterior superognathal (PSG) of Leptodontichthys ziregensis gen. et sp. nov. (PIMUZ A/I 5022), holotype, Drotops-Limestone, Middle Devonian, Jebel Zireg, Morocco in labial (a) and dorsal view (b). Teeth point caudally. The PSG is highlighted in black in the reconstruction of the eubrachythoracid Coccosteus. Scale bar represents 10 mm

Fig. 3

CT reconstruction of the posterior superognathal of Leptodontichthys ziregensis gen. et sp. nov. (PIMUZ A/I 5022) in labial (a), anterior (b), lingual (c), ventro-lingual (d), posterior (e) and dorsal (f) view. Putative occlusal marks of the inferognathal are indicated by arrows in c and d. Scale is 100 mm. dp, dorsal process; ddp, depression of the dorsal process; lp, lateral process; pp, posterior process

Fig. 4

CT slices of the posterior superognathal of Leptodontichthys ziregensis gen. et sp. nov. (PIMUZ A/I 5022). a Overview in labial view. b–f, both tooth rows showing layers of dentin and the pulp cavities. b Parasagittal view of the posterior row and c of the lateral row. d, e, transverse sections (e: close-up). f Frontal section. g Growth development of Plourdosteus canadensis according to Ørvig (1980). h Posterior row and i lateral row. Both tooth rows of L. ziregensis gen. et sp. nov. were 3D reconstructed to show these layers and some preserved basal canals. Scale bars represent 10 mm (a–d, g), 2 mm (e), 4 mm (f) and 5 mm (h, i). LT, Lateral Tooth; PT, Posterior Tooth

Fig. 5

Line drawings of labial views of eubrachythoracid posterior superognathals. Note the great differences in dimensions. a L. ziregensis. b Plourdosteus canadensis. c Coccosteus cuspidatus. d Harrytoombsia elegans e Mcnamaraspis kaprios. f Eastmanosteus calliaspis. g Torosteus pulchellus. h Compagopiscis croucheri. All scale bars equal 10 mm