Feeding in the Devonian antiarch placoderm fishes: a study based upon morphofunctional analysis of jaws

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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 suborbital bones within the plane of the ventral side of the fish armor is suggested to represent the natural “mouth closed” position. During mouth opening, the suborbitals rotated rostrally with simultaneous depression and inward rotation of the infragnathals. The ball-and-socket jaw articulation might ensure this combined movement. Recently described lower jaw elements of Livnolepis zadonica (Obrucheva, 1983) and Bothriolepis sp. from the Upper Devonian (lower Famennian) of Central Russia demonstrating very deep and porous blades of the oral division of the infragnathals queried the structure of these bones in other antiarchs. Observed porosity reflects intense vascularization to supply blood to a connective tissue underlying a supposed keratinous sheath, which protected and strengthened the jaws, as well as made possible scraping tough food objects, such as thallus algae, from the substrate.

Having evolved during the Silurian in the Pan-Cathaysian zoogeographical province, antiarchs migrated to Gondwana during the Emsian and later to Euramerica during the Eifelian. Supposedly, antiarchs became the first macrophytophagous vertebrates occupying the trophic level of primary consumers during the late Silurian–Early Devonian. This event diversified the only previously existing predator–prey interrelationships between filter-feeding agnathans and predatory gnathostomes.

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Placoderms (Armored Fish): Dominant Vertebrates of the Devonian Period

Gavin C. Young (2010)

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Loss of teeth and enamel in tetrapods: fossil record, genetic data and morphological adaptations.

Tiphaine Davit-Béal, Abigail Tucker, Jean-Yves Sire (2009)

Since their recruitment in the oral cavity, approximately 450 million years ago, teeth have been subjected to strong selective constraints due to the crucial role that they play in species survival. It is therefore quite surprising that the ability to develop functional teeth has subsequently been lost several times, independently, in various lineages. In this review, we concentrate our attention on tetrapods, the only vertebrate lineage in which several clades lack functional teeth from birth to adulthood. Indeed, in other lineages, teeth can be absent in adults but be functionally present in larvae and juveniles, can be absent in the oral cavity but exist in the pharyngeal region, or can develop on the upper jaw but be absent on the lower jaw. Here, we analyse the current data on toothless (edentate) tetrapod taxa, including information available on enamel-less species. Firstly, we provide an analysis of the dispersed and fragmentary morphological data published on the various living taxa concerned (and their extinct relatives) with the aim of tracing the origin of tooth or enamel loss, i.e. toads in Lissamphibia, turtles and birds in Sauropsida, and baleen whales, pangolins, anteaters, sloths, armadillos and aardvark in Mammalia. Secondly, we present current hypotheses on the genetic basis of tooth loss in the chicken and thirdly, we try to answer the question of how these taxa have survived tooth loss given the crucial importance of this tool. The loss of teeth (or only enamel) in all of these taxa was not lethal because it was always preceded in evolution by the pre-adaptation of a secondary tool (beak, baleens, elongated adhesive tongues or hypselodonty) useful for improving efficiency in food uptake. The positive selection of such secondary tools would have led to relaxed functional constraints on teeth and would have later compensated for the loss of teeth. These hypotheses raise numerous questions that will hopefully be answered in the near future.

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From snout to beak: the loss of teeth in birds.

Antoine Louchart, Laurent Viriot (2011)

All living birds are toothless, constituting by far the most diverse toothless vertebrate clade, and are striking examples of evolutionary success following tooth loss. In recent years, an unprecedented number of Mesozoic birds have been described, illustrating the evolution of dentition reductions. Simultaneously, major advances in experimental embryology have yielded new results concerning avian edentulism. Reviewing these lines of evidence, we propose hypotheses for its causes, with a prominent role for the horny beak during development. A horny beak and a muscular gizzard functionally 'replaced' dentition for food acquisition and processing, respectively. Together with edentulism itself, these features and others contributed to the later success of birds, as a result of their high performance or additional functionality working in concert in these complex organisms. Copyright © 2011 Elsevier Ltd. All rights reserved.