Prey transport mechanisms in blindsakes and the evolution of unilateral feeding systems in snakes

SYNOPSIS. greatest in quantity snakes ingest and transport their plunder via a jaw ratcheting mechanism in which the left and right upper jaw arches are advanced above the prey in an alternating, unilateral fashion. This unilateral jaw ratcheting mechanism differs greatly from the hyolingual and inertial transport mechanisms used by dint of lizards, both of which are characterized by dint of bilaterally synchronous jaw movements. Given the well-corroborated phylogenetic hypothesis that snakes are derived from lizards, this insinuates that major changes occurred in the couple the morphology and motor dominion government of the feeding apparatus during the early evolution of snakes. However, greatest in quantity previous studies of the evolution of unilateral feeding mechanisms in snakes have focused almost exclusively upon the morphology of the jaw apparatus because there have been real few direct observations of feeding behavior in basal snakes. In this paper I describe the pillage transport mechanisms used by representatives of sum of two units families of basal snakes, Leptotyphlopidae and Typhlopidae. In Leptotyphlopidae, a mandibular raking mechanism is used, in which bilaterally synchronous flexions of the lower jaw labor for to ratchet prey into and from one side the mouth. In Typhlopidae, a maxillary raking mechanism is used, in which asynchronous ratcheting changes of the highly mobile upper jaws are used to drag plunder through the oral cavity. These findings glance at that the unilateral feeding mechanisms that characterize the majority of living snakes were not not away primitively in Serpentes, but arose subsequently to the basal divergence between Scolecophidia and Alethinophidia.

INTRODUCTION

Three fundamental methods of intraoral prey transport are recognized within Squamata. greatest in quantity lizards use a hyolingual transport mechanism, in which circle of times of tongue protraction and retraction obey to ratchet prey through the cavity between the jaws and towards the pharynx (Smith, 1984; Herrel et al., 1996; Schwenk, 2000) In a certain number of lizards, however, this lingual ratcheting mechanism is augmented or replaced by dint of a cranioinertial transport mechanism, in which rapid motions of the entire head are used to urge on prey through the oral cavity (Gans, 1969; Bramble and Wake, 1985) This manner of intraoral transport is of particular importance in varanid lizards (Smith, 1986; Elias et al., 2000) which share with snakes a highly reduc tongue that lacks a frictional surface (McDowell, 1972; Schwenk, 1988) Finally, snakes use gnathic (jaw-based) transport mechanisms, in which kinetic simple bodys of the jaw apparatus are used to ratchet booty into and through the chaps (Cundall and Greene, 2000). While the one and the other hyolingual and cranioinertial transport are belonging to all among tetrapods, gnathic transport mechanisms are unique to snakes (Bramble and Wake, 1985)

Nearly all of what is popularly known about gnathic transport in snakes derives from studies of taxa belonging to Macrostomata (Fig. 1) a large and diverse clade that includes approximately eighty-five percent of the more than 2500 species of extant snakes (McDiarmid et al., 1999) These studies have shown that greatest in quantity macrostomatans ingest and transport their pillage via a "pterygoid walk" mechanism (Bola and Ewer, 1964) in which reciprocating ratcheting changes of the medial upper jaw arches, combined with lateral rotations of the entire head about the cranio-vertebral joint, be subservient to to advance the snake's head above its prey (Dullemeijer, 1956; Albright and Nelson 1959a, b; Frazzetta, 1966; Kardong, 1977; Cundall and Gans, 1979; Cundall, 1983; Kardong, 1986) Because this unilateral jaw ratcheting mechanism differs greatly from one as well as the other the hyolingual and cranioinertial transport mechanisms of lizards (Kardong and Berkhoudt, 1998; Cundall, 1995) its origin has been somewhat enigmatic, and there have thus been numerous attempts made to determine the evolutionary paces through which it arose (eg Gans, 1961; Rieppel, 1980; to leeward et al., 1999; Kardong and Bels, 2001) However, greatest in quantity studies that have addressed the evolutionary origin of unilateral feeding mechanisms in snakes have been directioned from an almost exclusively anatomical perspective because the actual feeding mechanisms used by dint of basal snakes have remained largely unknown.

Recently Cundall (1995) provided the first detailed account of feeding behavior in a basal snake, describing a loot transport mechanism that he bounded "snout shifting" in the anilioid Cylindrophis (Fig. 1) Like the pterygoid walk, snout shifting involves unilateral moves of the toothed elements of the upper jaws, combined with side-to-side changes of the entire head, which together obey to advance the head above the prey. However, the upper jaws in Cylindrophis (and in other anilioids) remain tightly limit to the ventral elements of the bony snout (eg vomer septomaxillae) by the agency of several short, robust ligaments. These ligaments impede extensive translational movements of the jaws like as those which are associated with the pterygoid walk in macrostomatans. Independent motions of the upper jaws are instead achieved end lateral rotations of the entire snout composed of several elements about the nasofrontal articulation (prokinesis; Frazzetta, 1962) and independent translational changes of the left and right septomaxilla-vomer complexe to which the upper jaws are border (rhinokinesis; Cundall and Shardo, 1995) Thus, in certain regards Cylindrophis represents an intermediate functional stage between lizards and macrostomatan snakes (Cundall, 1995); despite retaining a tight connection between the upper jaws and the snout (as in lizards), loot is transported via a unilateral jaw ratcheting mechanism (as in macrostomatans).