Initial radiation of jaws demonstrated stability despite faunal and environmental change

Abstract

More than 99 per cent of the roughly 58,000 living vertebrate species have jaws. This major clade, whose members are collectively known as gnathostomes ('jawed mouths'), made its earliest definitive appearance in the Silurian period, 444-416 million years (Myr) ago, with both the origin of the modern (crown-group) radiation and the presumptive invasion of land occurring by the end of the Devonian period (359 Myr ago). These events coincided with a major faunal shift that remains apparent today: the transition from Silurian ecosystems dominated by jawless fishes (agnathans) to younger assemblages composed almost exclusively of gnathostomes. This pattern has inspired several qualitative descriptions of the trophic radiation and ecological ascendance of the earliest jawed vertebrates. Here we present a quantitative analysis of functional variation in early gnathostome mandibular elements, placing constraints on our understanding of evolutionary patterns during this critical interval. We document an initial increase in functional disparity in the Silurian that stabilized by the first stage of the Devonian, before the occurrence of an Emsian (∼400 Myr ago) oxygenation event implicated in the trophic radiation of vertebrates. Subsequent taxonomic diversification during the Devonian did not result in increased functional variation; instead, new taxa revisited and elaborated on established mandibular designs. Devonian functional space is dominated by lobe-finned fishes and 'placoderms'; high disparity within the latter implies considerable trophic innovation among jaw-bearing stem gnathostomes. By contrast, the major groups of living vertebrates--ray-finned fishes and tetrapods--show surprisingly conservative mandibular morphologies with little indication of functional diversification or innovation. Devonian gnathostomes reached a point where they ceased to accrue further mandibular functional disparity before becoming taxonomic dominants relative to 'ostracoderm'-grade jawless fishes, providing a new perspective on classic adaptive hypotheses concerning this fundamental shift in vertebrate biodiversity.

The shapes of bird beaks are highly controlled by nondietary factors

“…Further research is therefore warranted to investigate whether biomechanical function is similarly integrated (41) and how the shape of the upper beak is affected by the skull musculature as it develops. Finally, the phenomena of "many-to-one" and "one-to-many" mapping between form and function mean that similarities in shape do not necessarily imply similarity in function (42)(43)(44), further justifying the need for biomechanical analyses of avian skulls. Evolutionary history plays a significant role in dictating skull shape.…”

“…The 62 landmarks and semilandmarks were collated for each specimen, and the semilandmarks were slid to minimize bending energy in the Geomorph package for R (43). The slid configurations for all birds were then imported to MorphoJ (44) and subjected to a Procrustes superimposition.…”

Evolution of long-toothed fishes and the changing nature of fish–benthos interactions on coral reefs

“…As top predators in marine systems for the last 450 million years (My), fishes have played a key role in these interactions 4,5 . Changes in jaw and tooth structure of vertebrates have been key indicators of changing feeding modes through time [6][7][8] . However, no comprehensive overview of tooth structure in fish assemblages has been undertaken.…”

Musculoskeletal modelling under an evolutionary perspective: deciphering the role of single muscle regions in closely related insects

“…As the dicondylous insect mandible is a third-order lever just like in vertebrates [47,48], the MA is the ratio between the inner lever arm and the outer lever arm. The MA indicates the relative force transmissions of the muscle force to the food item, which is correlated with diet in, for example, vertebrates and thus is a frequently used proxy to assess the biomechanical disparity of the mandibular muscle insertions and joints among taxa [49,50].…”