The Origin of Snakes

Abstract

Summary 1. Current theories of ophidian evolution suggest that the snakes have been derived either from aquatic, above‐ground terrestrial, or burrowing ancestors. It is generally supposed that the ancestors of snakes, irrespective of their habits, were closely related to the platynotid lizards. In the absence of adequate palaeontological data, the respective merits of these assumptions must mainly be assessed on evidence obtained from study of living forms. 2. The close relationship between the snakes and lizards is pointed out in this article, and the main characteristics of the different families of fossil and living snakes are reviewed. 3. Many important features of the snakes as a group are described and their functional significance is discussed. Comparison with the corresponding condition in lizards is made. Ophidian characters regarded as being of particular significance include the bony investment of the forebrain, the platytrabic chondrocranium with absence of the interorbital septum and endocranial side‐walls, the absence of the epipterygoids and temporal arcades and the extreme mobility of the jaw skeleton in most forms; the presence of zygosphenal articulations throughout the vertebral column; the elaboration of the trunk musculature and ventral scales; the extreme concentration of alkali‐metal cation on the secretory side. (For reviews of evidence, see Crane (1950), Hokin & Rehm (1947), Rehm (1950).) (2) When the two sides of the mucosa are connected electrically, a current of relatively large magnitude can be maintained which requires the continuous performance of electrical work (Rehm, 1943; Rehm & Hokin, 1948; Crane, Davies & Longmuir, 1946, 1948 a; Crane & Davies, 19486). (3) There is a fall in the p.d. across the mucosa with the onset of secretion, but once secretion is well established further increases in the rate of secretion can occur with only small changes in the p.d. (cf. Crane, 1950; Rehm, 1944; Rehm, 1950). (4) The change from the non‐secretory to the secretory state and back is always associated with concomitant changes in the electrical parameters of the mucosa, whether these changes occur spontaneously or by drug action. In the absence of oxygen and in dead mucosae both p.d. and acid secretion are abolished (cf. Crane, 1950; Rehm & Hokin, 1947; Rehm, 1950). (5) Passage of electric current (c. 1 mA./cm.2) through the mucosa from an external source increases or decreases the rate of secretion of acid by a secreting mucosa according as the applied current enhances or opposes the spontaneous potential measured between calomel electrodes on either side of the mucosa (Rehm, 1945; Crane et al. 1946, 19486). (6) Passage of current does not initiate secretion in the dog (Rehm, 1945) but may do so in the frog (Crane et al. 1948 b). These results were obtained with intact gastric mucosa, and because of the technical difficulties involved it has not, so far, been possible to obtain direct information regarding the detailed electric changes in and around the oxyntic cells. A difficulty is that th...

The Cretaceous marine squamate Mesoleptos and the origin of snakes

“…More complete fossil finds, and thus, information on characters not obviously correlated with habitat and body form, are required before their phylogenetic relationships can be conclusively ascertained and the early evolution of snakes clearly understood. The fundamental questions investigated by Bellairs and Underwood (1951) and Underwood (1967) regarding the affinities and ecological origins of snakes still await convincing answers.…”

“…that snakes evolved from small elongate burrowing lizards (e.g. Janensch, 1906;Walls, 1940;Bellairs and Underwood, 1951;Underwood, 1967;Rieppel, 1988;Greene, 1997). Thus, few modern studies rigorously surveyed marine varanoids and marine ophiomorphs for possible relationships with modern snakes.…”

“…Mesoleptos emerges as on the stem lineage leading to snakes, lying phylogenetically between marine lizards (mosasauroids, dolichosaurs, Adriosaurus) and primitive limbed snakes (Pachyrhachis, Pachyophis, Haasiophis). Garth Underwood's earliest research interests included the origin and evolution of snakes, and he has contributed to possibly the two most influential papers on this topic (Bellairs and Underwood 1951;Underwood 1967). The current paper is thus a small contribution to a field of inquiry that Garth Underwood helped establish.…”

Hearing with an atympanic ear: good vibration and poor sound-pressure detection in the royal python,Python regius

“…The lack of an impedance-matching middle ear therefore implies that snakes should have poor pressure sensitivity and hence poor aerial hearing. Instead, the connection of the columella and the quadrate has been interpreted as a specialization for detecting substrate vibrations (Tumarkin, 1948;Bellairs and Underwood, 1951), consistent with a fossorial lifestyle, where vibration detection is likely to be advantageous compared with pressure hearing (Christensenregarding the ability of snakes to hear airborne sound [for an overview, see Young (Young, 2003)]. Manning (Manning, 1923) reported that rattlesnakes of the species Crotalus adamanteus, C. horridus and C. atrox were practically unresponsive to sound.…”

Evolution of Trophic Systems in Squamates

“…Perhaps as reversal during a burrowing ancestry, snake visual systems have been rebuilt to serve above ground activity (WALLS, 1942;BELLAIRS & UNDERWOOD, 1951). The skull too of snakes seems to be a rebuilt trophic .…”