Caecilians are enigmatic limbless amphibians that, with a few exceptions, all have an at least partly burrowing lifestyle. Although it has been suggested that caecilian evolution resulted in sturdy and compact skulls as an adaptation to their head-first burrowing habits, no relationship between skull shape and burrowing performance has been demonstrated to date. However, the unique dual jaw-closing mechanism and the osteological variability of their temporal region suggest a potential relationship between skull shape and feeding mechanics. Here, we explored the relationships between skull shape, head musculature and in vivo bite forces. Although there is a correlation between bite force and external head shape, no relationship between bite force and skull shape could be detected. Whereas our data suggest that muscles are the principal drivers of variation in bite force, the shape of the skull is constrained by factors other than demands for bite force generation. However, a strong covariation between the cranium and mandible exists. Moreover, both cranium and mandible shape covary with jaw muscle architecture. Caecilians show a gradient between species with a long retroarticular process associated with a large and pennate-fibered m. interhyoideus posterior and species with a short process but long and parallel-fibered jaw adductors. Our results demonstrate the complexity of the relationship between form and function of this jaw system. Further studies that focus on factors such as gape distance or jaw velocity will be needed in order to fully understand the evolution of feeding mechanics in caecilians.
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RBINS Staff Publications 2021
This study is concerned with the Late Holocene floodplain history of the Karkheh River in Lower Khuzestan, and in particular with the role of human action upon its channel shifts. The research was conducted in a multidisciplinary way, in which resources and approaches from different research fields were combined: (1) geomorphological mapping based on the interpretation of Landsat and CORONA satellite imagery, (2) analyses of geological sequences, including the identification of sedimentary facies and radiocarbon dating of organic material, (3) an archaeological field survey of ancient settlements, and (4) consultation of historical documents, mainly Arabic texts from the 9th–14th century and European travel literature from the 16th-early 20th century. Three main channel belts of the Karkheh were identified (labelled Kh1, Kh2 and Kh3), corresponding to successive stages in the evolution of the floodplain. Two river shifts are documented in the datasets, both taking place within the last 2000 years. The first avulsion regards a shift from channel belt Kh1, once a tributary of the Karun, to the straight river bed of Kh2, taking place at least after 1240–1310 cal BP/710–640 AD. The second avulsion, from Kh2 to Kh3, is clearly documented in historical sources and happened in a single night event in the year 1837/113 cal BP. Reactivation of the Kh2 river bed and its irrigation canals can be attributed to the recent construction of an artificial canal bypassing the second avulsion point. Both river shifts were strongly influenced by human interference, whereby an artificial irrigation canal took over the entire river flow from the main channel belt. Most likely, a combination of human-induced factors, such as weakening of the river levees, high sedimentation rates and disadvantageous channel gradients, led to a situation prone to avulsion.
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