Extreme lower jaw elongation in a placoderm reflects high disparity and modularity in early vertebrate evolution.

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.

Thermal strategies and energetics in two sympatric colubrid snakes with contrasted exposure.

The thermoregulatory strategy of reptiles should be optimal if ecological costs (predation risk and time devoted to thermoregulation) are minimized while physiological benefits (performance efficiency and energy gain) are maximized. However, depending on the exact shape of the cost and benefit curves, different thermoregulatory optima may exist, even between sympatric species. We studied thermoregulation in two coexisting colubrid snakes, the European whipsnake (Hierophis viridiflavus, Lacépède 1789) and the Aesculapian snake (Zamenis longissimus, Laurenti 1768) that diverge markedly in their exposure, but otherwise share major ecological and morphological traits. The exposed species (H. viridiflavus) selected higher body temperatures (approximately 30 degrees C) than the secretive species (Z. longissimus, approximately 25 degrees C) both in a laboratory thermal gradient and in the field. Moreover, this difference in body temperature was maintained under thermophilic physiological states such as digestion and molting. Physiological and locomotory performances were optimized at higher temperatures in H. viridiflavus compared to Z. longissimus, as predicted by the thermal coadaptation hypothesis. Metabolic and energetic measurements indicated that energy requirements are at least twice higher in H. viridiflavus than in Z. longissimus. The contrasted sets of coadapted traits between H. viridiflavus and Z. longissimus appear to be adaptive correlates of their exposure strategies.