Despite the long history of embryological studies of squamates, many groups of this huge clade have received only limited attention. One such understudied group is the anguimorphs, a clade comprising morphologically and ecologically very diverse lizards. We describe several stages of embryonic development of Anguis fragilis, a limbless, viviparous anguimorph. Interestingly, in several clutches we observe high morphological variation in characters traditionally important in classifying embryos into developmental stages. The causes of this variation remain unknown but environmental factors do not seem to be very important. Additionally, we describe the state of ossification in several perinatal specimens of A. fragilis. The cranial skeleton is relatively poorly ossified around the time of birth, with all of the bones constituting the braincase unfused. On the other hand, the vertebral column is well ossified, with the neurocentral sutures closed and the neural arches fused in all postatlantal vertebrae. Such an advanced state of ossification may be related to the greater importance of the vertebral column in locomotion in limbless species than in ones with fully-developed limbs. Numerous factors seem to affect the state of ossification at the time of hatching or birth in squamates, including phylogenetic position, mode of reproduction and, potentially, limblessness. However, data from a greater number of species are needed to reach firmer conclusions about the relative importance of these variables in certain clades.
The blue-spotted phenotype in a slow worm can be considered as an alternative colour morph or a secondary sexual characteristic. This phenotype is known to entail an elevated predation risk; thus, its continuous presence in a population must be balanced by additional and positive fitness
consequences. In this study, we show that blue-spotted males are characterised by a greater snout-vent length (SVL) than typical specimens. Importantly, the SVL of blue-spotted males reaches the magnitude of the average female size. This indicates that the presence of blue spots may involve
a correlated positive effect on growth and body size. The greater body size of the blue-spotted males could enhance their survival and mating success, and thus facilitate the continued presence of a high fraction of this morph within the population. In addition, we found that the blue-spotted
phenotype is more common in the eastern than the western slow worm, and the proposed fitness consequences of the blue-spotted phenotype might enhance its tendency to spread in the eastern Anguis lineage.
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