New findings in a 400 million-year-old Devonian placoderm shed light on jaw structure and function in basal gnathostomes

Arthodire placoderms have been proposed as the sister group of Chinese ‘maxillate’ placoderms plus all the more crownward gnathostomes. These basal groups provide key information for understanding the early evolution of jaws. Here, we test previous assumptions about placoderm jaw structure and function by using high-resolution computed tomography, digital dissection, and enlarged 3D printouts on a unique articulated 400 million-year-old buchanosteid arthrodire. The upper jaw has a double ethmoid and a palatobasal connection, but no otic connection; the dermal bone attachment for the quadrate is different to other placoderms. A separately ossified cartilage behind the mandibular joint is comparable to the interhyal of osteichthyans. Two articular facets on the braincase associated with the hyomandibular nerve foramen supported a possible epihyal element and a separate opercular cartilage. Reassembling and manipulating 3D printouts demonstrates the limits of jaw kenetics. The new evidence indicates unrecognized similarities in jaw structure between arthrodires and osteichthyans, and will help to clarify the sequence of character acquisition in the evolution of basal gnathostome groups. New details on the hyoid arch will help to reformulate characters that are key in the heated debate of placoderm monophyly or paraphyly.

fossil fish assemblage is the most diverse fossil fish fauna known from the Devonian, 48 and also the oldest coral reef fish assemblage from the fossil record 1 .

Institutional abbreviations 51
Institutional abbreviations for registered specimens are: ANU, College of Science, 52 The Australian National University, Canberra, ACT; WAM, Western Australian 53 Museum, Perth, WA.

55
Taxonomy 56 ANU V244 was previously referred to as a 'new buchanosteid' 6 , a member of a 57 complex of small to medium-sized arthrodires from the Burrinjuck fossil fish 58 locality 7 . The genus Buchanosteus was erected 8 for a skull specimen from Early 59 Devonian limestones of Buchan, Victoria, first named as 'Coccosteus osseus' 9 . New has a different shape to the isolated bone that was referred to Parabuchanosteus 10 , and 79 the postmarginal plate forming the lateral corner of the skull is a small triangular 80 bone, in this respect resembling Buchanosteus rather than Parabuchanosteus 7 .

81
However, the ridges on the lateral trunk-armour plates are absent in the trunk-armour 82 specimen referred to Buchanosteus confertituberculatus 7 . For these reasons we 83 consider that ANU V244 is probably a new taxon within the family Buchanosteidae, 84 and it is referred to in the text as a 'buchanosteid'.

85
The genus Buchanosteus has been scored as a basal arthrodire in data matrices 86 supporting various gnathostome phylogenies, but the described morphology 4 derives 87 from a range of incomplete specimens including some earlier described as 88 Parabuchanosteus 10 , on the assumption there was only one genus (Buchanosteus) in 89 one family 4 . However, with at least three genera in two families now recognised from 90 SE Australia 7 , and many new specimens indicating greater diversity, the morphology 91 used in phylogenetic analyses is considered a composite of closely related forms, so 92 we have referred to the taxon as 'Buchanosteus'.    The clarification of the mandibular joint position (Supplementary Fig. 3a, b) is  The Meckel's cartilage was noted above to be deepest towards the anterior end, 240 and using 3D printouts to place it in occlusion with the palatoquadrate shows this 241 corresponds to the anterior extent of the adductor fossa ( Supplementary Fig. 2e). In  Fig. 6a). A full interpretation will depend on 256 segmentation of internal braincase structures, not yet carried out, but we note here that 257 the grooves indicate a more complex pattern of arteries than recently represented for 258 'Buchanosteus' as a primitive gnathostome 18 .

259
The left groove for the lateral dorsal aorta is best preserved, with a similar 260 position to the previous description for 'Buchanosteus' 4 ( Supplementary Fig. 6a). with the arterial groove system, it can be assumed that the blood supply to the floor of 319 the orbital cavity passed up through this foramen. 320 The same system is connected to the 'transverse groove' on the palatoquadrate 321 attachment surface for the posterior supragnathal (Fig. 3), so presumably also carried 322 arterial blood to this region. We note that there seems no corresponding arterial  (Fig. 1c) did not fill the anterior groove of the palatoquadrate, 328 leaving an 'anterior notch' (Supplementary Fig. 3c). In articulation, this notch sat 329 opposite a 'lateral notch' (Supplementary Fig. 3d), formed between the anterior 330 supragnathal and a groove inside the ectethmoid process of the braincase (Fig. 6).