ZOOLOGY Zoology 108 (2005) 307–315 The retroarticular process, streptostyly and the caecilian jaw closing system Adam P. Summers a,Ã , Marvalee H. Wake b a Department of Ecology and Evolutionary Biology, University of California, Irvine, CA 92697, USA b Department of Integrative Biology and Museum of Vertebrate Zoology, 3060 VLSB, University of California, Berkeley, CA 94720-3140, USA Received 28 July 2005; received in revised form 13 September 2005; accepted 14 September 2005 Abstract Caecilians have two functionally separate sets of jaw closing muscles. The jaw adductor muscles are parallel fibered muscles positioned close to the jaw joint and their lever mechanics suggests they are well suited to rapidly closing the jaws. A second set of muscles, the hypaxial interhyoideus posterior (IHP), levers the jaws closed by pulling on the retroarticular process (RA) of the lower jaw. Models of the lower jaw point out that the angle and length of the RA has a profound effect on the closure force exerted by the IHP. The caecilian skull is streptostylic – the quadrate-squamosal apparatus (QSA) moves relative to the rest of the skull, a condition that seems at odds with a well-ossified cranium. Modeling the contribution of this streptostylic suspension of the lower jaw shows that rotational freedom of the QSA amplifies the force of the IHP by redirecting force applied along the low axis of the lower jaw. Measurements from several species and life stages of preserved caecilians reveal a large variation in predicted bite force (as a multiple of IHP force) with age and phylogeny. r 2005 Elsevier GmbH. All rights reserved. Keywords: Caecilians; Jaw mechanics; Bite force Introduction Caecilians are fossorial, limbless amphibians with a circumtropical distribution (Taylor, 1968). The archi- tecture of the caecilian skull appears particularly well suited to burrowing, exhibiting tight sutures and fusion of skeletal elements (Wake, 1993). In spite of their robust skull there is a surprising degree of cranial kinesis (streptostyly) in the quadrate-squamosal apparatus (Wake and Hanken, 1982), and though the quadrate does not rotate to any great degree, it may be relatively free to do so (Fig. 1). Streptostyly has evolved several times in tetrapods, serving to increase gape in lizards and snakes, and also allowing the lower jaw to move relative to the origin of the adductor muscles in order to alter the leverage of these jaw closers (Gans, 1961, 1966; Herrel et al., 2000). The development and functional significance of streptostyly in caecilians has been speculated on (Marcus et al., 1933; Wake and Hanken, 1982; Straub, 1985; Wilkinson and Nussbaum, 1997; summarized by Wake, 2003), but specific tests of the effects of streptostyly have not been done. Neither of the two reasons mentioned above for the evolution of streptostyly would seem to pertain to caecilians: most do not eat particularly large prey items ARTICLE IN PRESS www.elsevier.de/zool 0944-2006/$-see front matter r 2005 Elsevier GmbH. All rights reserved. doi:10.1016/j.zool.2005.09.007 Ã Corresponding author. E-mail address: asummers@uci.edu (A.P. Summers).