A well-preserved 'placoderm' (stem-group Gnathostomata) upper jaw from the Early Devonian of Mongolia clarifies jaw evolution

Abstract

The origin of jaws and teeth remains contentious in vertebrate evolution. 'Placoderms' (Silurian-Devonian armoured jawed fishes) are central to debates on the origins of these anatomical structures. 'Acanthothoracids' are generally considered the most primitive 'placoderms'. However, they are so far known mainly from disarticulated skeletal elements that are typically incomplete. The structure of the jaws-particularly the jaw hinge-is poorly known, leaving open questions about their jaw function and comparison with other placoderms and modern gnathostomes. Here we describe a near-complete 'acanthothoracid' upper jaw, allowing us to reconstruct the likely orientation and angle of the bite and compare its morphology with that of other known 'placoderm' groups. We clarify that the bite position is located on the upper jaw cartilage rather than on the dermal cheek and thus show that there is a highly conserved bite morphology among most groups of 'placoderms', regardless of their overall cranial geometry. Incorporation of the dermal skeleton appears to provide a sound biomechanical basis for jaw origins. It appears that 'acanthothoracid' dentitions were fundamentally similar in location to that of arthrodire 'placoderms', rather than resembling bony fishes. Irrespective of current phylogenetic uncertainty, the new data here resolve the likely general condition for 'placoderms' as a whole, and as such, ancestral morphology of known jawed vertebrates.

Keywords: Devonian; Mongolia; acanthothoracid; evolution; jaws; placoderm.

Figure 1.

MPC-Fh200/10.4 upper jaw complex in virtual three-dimensional rendering from synchrotron tomography. (a) Lateral view; (b) lateral view with in-fill of lateral line canals (red); (c) lateral view with suborbital plate rendered semi-transparent to reveal internal cavities and canals; (d) mesial view; (e) ventral (labial) view; (f) dorsal view; (g) posterior view. Blue-grey indicates suborbital (dermal) plate; mauve indicates perichondral sheath of palatoquadrate. Exact boundaries between these units are approximate. Dashed leader line indicates uncertain attribution.

Figure 2.

(a) Lateral line canal pattern on the cheek of ‘acanthothoracid’ placoderms showing the position of the canals relative to the palatoquadrate (dotted line). (b) Drawing based on data from [8]; (c) interpretive drawing of specimen from [32].

Figure 3.

Upper jaws of placoderm fishes in internal view. (a) Jagorina (based on Museum für Naturkunde, Berlin, specimen MB.f.510.5); (b) Buchanosteid based on data published in [39]; (c) Radotina based on surface models from [8]; (d) MPC-Fh200/10.4 (original); (e) Bothriolepis (after [40]). Arrow indicates anterior. Horizontally hatched areas indicate articulation facet. Dashed label lines indicate uncertainty concerning facet as part of quadrate or possible hyoid attachment. Dashed leader line indicates uncertain attribution.

Figure 4.

Cheek morphology and bite pattern in placoderm fishes with suborbital plates. (a–d) Placoderms displaying pronounced overbite with lower jaw contact on internal surface of upper jaw; (e) highly ventralized jaw mechanics of antiarchs for inferred ‘scraping’ feeding [42]. Illustration in (a) is original composited from [39,43,44]. Illustrations in (b) and (c) adapted from [45]; (e) adapted from [42,46]. Red lines indicate paths of the sensory line canals.