Extreme lower jaw elongation in a placoderm reflects high disparity and modularity in early vertebrate evolution

Abstract

Jaws are a key vertebrate feature that arose early in our evolution. Placoderms are among the first jawed vertebrates; their fossils yield essential knowledge about the early diversification of gnathostome feeding strategies, diets and modularity. Modularity can be expressed through disproportional lengths of lower and upper jaws as in swordfish or halfbeaks. Alienacanthus malkowskii is an arthrodire from the Famennian of Morocco and Poland, whose most remarkable feature is its lower jaw, which is twice as long as the skull. This is the oldest record of such extreme jaw elongation and modularity in vertebrates. The gnathal plates of Alienacanthus possess sharp, posteriorly recurved teeth that continue anterior of the occlusion in the inferognathals. The dentition suggests a catching and trapping live prey function, and the jaw occlusion is unique among placoderms. This armoured 'fish' expands the morphological and ecological diversity during one of the first radiations of jawed vertebrates with a combination of features so far unrecorded for arthrodires.

Keywords: disparity; gnathostome; jaw elongation; modularity; placoderm.

Figure 1.

Map of the Late Devonian (Famennian) showing the Alienacanthus localities.

Figure 2.

Alienacanthus malkowskii, skull, PIMUZ A/I 5239. In right (a,b), left (c,d) and dorsal (e,f) view; inferognathals, PIMUZ A/I 5238, in lingual (g), lateral (h) and dorsal (i) view. Each bone is differentiated by a separate colour. Black arrow points to lingual depression. Scale bars correspond to 100 mm.

Figure 3.

Histology of Alienacanthus. Section of an ankylosed tooth from MCD200 (a,b) and inferognathal from 1809.II.24 (c–f), overview (c), detail of the area where both ossifications meet (d), dense outer layer (e,f). Tiny bone cell lacunae and multiple cracks are respectively visible in (e) and (f). Scale bars correspond to 1 mm (a,b,d), 2 mm (c), 200 microns (e), and 250 microns (f).

Figure 4.

Eubrachythoracid phylogeny including Alienacanthus and simplified morphological diversity of skulls and jaws in different groups. Based on the computed 50% majority-rule consensus tree. Outlines of Dicksonosteus, Rolfosteus, Coccosteus, Dunkleosteus, and Melanosteus are modified from [,–35], and [36]. Comparison to other known taxa with an elongated lower jaw through time. Reconstructions not to scale and lower jaws are separated from the skull to show their shapes.

Figure 5.

Live reconstructions of Alienacanthus. Based on the body morphology of extinct and modern fishes with elongated jaws (elongated, fusiform, bodies). Artwork by Beat Scheffold (Zürich) and Christian Klug.

Figure 6.

Pharyngeal gill arches of placoderms. Reconstructed gill arches of placoderms (in green box) and lobe-finned fish Glyptolepis from [80]. Alienacanthus only has the first arch preserved and thus the basihyal is subjective and the other arches unknown. Colours: pink, basihyal; blue, first arch; green, basibranchials; dark blue, ceratobranchials. Image not to scale.