Abstract:Antiarch placoderm fishes were an abundant component of the Middle Paleozoic vertebrate assemblages. Despite a large number of known taxa and specimens, the morphology and function of the skeletal elements of their jaws is inadequately known. Because of this, questions regarding their feeding modes and their roles in the trophic webs remain open. We present a skeletomuscular model of the antiarch jaw apparatus with an attempt to reconstruct its potential biomechanical function. The position of the upper jaw su… Show more
“…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.…”
Section: Discussionmentioning
confidence: 99%
“…Dashed leader line indicates uncertain attribution.Figure 4Cheek 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].…”
Section: Discussionmentioning
confidence: 99%
“…This agrees with the general morphological similarities in overall structure between MPC-Fh200/10.4, Radotina , Jagorina and arthrodires (figures 3 and 4 ). The role of the submarginal plate in biting, ‘cleaver-shaped’ palatoquadrate, and small adductor fossa of Bothriolepis represent a clear anatomical outlier (figures 3 and 4 ) and has been attributed recently to ‘scraping’ feeding [ 42 ].…”
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.
“…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.…”
Section: Discussionmentioning
confidence: 99%
“…Dashed leader line indicates uncertain attribution.Figure 4Cheek 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].…”
Section: Discussionmentioning
confidence: 99%
“…This agrees with the general morphological similarities in overall structure between MPC-Fh200/10.4, Radotina , Jagorina and arthrodires (figures 3 and 4 ). The role of the submarginal plate in biting, ‘cleaver-shaped’ palatoquadrate, and small adductor fossa of Bothriolepis represent a clear anatomical outlier (figures 3 and 4 ) and has been attributed recently to ‘scraping’ feeding [ 42 ].…”
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.
“…However, it is clear the tissue covering the sides of the gnathals in arthrodires could not have been either keratin or dentine. The texture of the bone has abundant resorption pits and generally does not resemble the texture in other placoderms proposed to have keratinous beaks [91]. Similarly, it is unlikely these regions were covered by fused sheets of dentine, as if this were the case, flattened sheets of mineralized tissue of unknown homology would be common finds alongside arthrodire fossils.…”
A new species of Dunkleosteus, D. tuderensis sp. nov., is named based on an infragnathal from the Famennian of the Tver Region, Russia. CT scanning of the holotype revealed two high-density bony constituents comparable in position and interrelations to components described for coccosteomorph arthrodires, supported by the presence of at least two clusters of large vascular canals marking separate arterial supplies. Coccosteomorph and dunkleosteid pachyosteomorphs exhibit similar growth patterns including labio-basal depositions of vascularized bone in the infragnathals and basally in the supragnathals. In contrast to coccosteomorphs, dunkleosteid reinforcement of the occlusal margins occurred via the formation of dense osteonal bone, in parallel with resorption forming extensive lingual fossae. Active bone remodeling proceeded without a complete reworking of the primary osteonal bone structure and the original arrangement of vascular canals. Due to inconsistent anatomical terminology in gnathal elements of dunkleosteid arthrodires, a revised terminology is suggested and new terms are introduced.
“…U-shaped) (e.g. [26,35,[49][50][51]). In Alienacanthus, the occlusion of the upper with the lower jaws continues anteriorly past the rostral plate and the ASGs do not meet.…”
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.
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