|The Vertebrates||Pythonomorpha: Scolecophidia|
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SQUAMATA |--Iguania Scleroglossa | Pythonomorpha |--Mosasauroidea `--Serpentes |--Scolecophidia | |--Anomalepididae | `--+--Leptotyphlopidae | `--Typhlopidae `--Alethinophidia |--Aniloidea `--Macrostomata |--Boidae `--Caenophidia |--Acrochordoidea `--Colubroidea |--Colubridae `--Viperidae
Leptotyphlops dulcis, the Texas blindsnake. Image © 1999 by Nathan Kley and reproduced by permission.
Range: fr lwPc.
Characters: Small (10-100 cm long & down to 2 mm in width), fossorial; small, blunt heads tipped with rostral shield; integrated skull; $ rostral shield formed by enlarged nasals reinforced at margins by prefrontals and premaxilla; small jaw gape; $ dentaries are joined; reduced dentition with no teeth on pterygoid or palatine; dentaries or maxillae (but not both) highly mobile; eyes reduced and permanently covered with a scale, or absent; head contains numerous glands of unknown function; up to 600 vertebrae; short, blunt tails (1-18% of length); spine at end of tail, probably used to push off in locomotion; traces of pelvic girdle; spurs attached to pelvic girdle in some; 2 common carotid arteries; multi-lobed liver; round, polished uniform scales; feed on soft invertebrates or insect larvae; fossorial down to 2 m, but commonly hunt nocturnally above ground or even in trees; commonly follow insect pheromone trails; one species (Ramphotyphlops braminus) appears to be entirely female.
Digging them out. Blind snakes look a great deal like earthworms and have about as many distinguishing features. This, and their secretive, nocturnal and underground life style have made them notoriously difficult to differentiate and to study. However, they are at the base of the snake clade -- if it is a clade. The monophyly of the snakes is very likely, but not certain. See discussion in Cundall et al. (1993). Thus blind snakes ought to be of serious evolutionary interest. However, they have historically been dismissed as "degenerate" forms, perhaps unworthy of serious study*. It is only in the last thirty years or so that enough information about the evolution of snakes has accumulated to ask the appropriate questions. As recently as 1997, Greene wrote that no serious study of blind snake feeding behavior had been conducted at all. This was perhaps an exaggeration, and is certainly untrue now. See, e.g. Kley & Brainerd (1999). But the study of snake evolution is certainly still in its infancy and there are still a remarkable number of matters that have not been addressed.
Leptotyphlops dulci feeding on ant larvae. © 1999 by Nathan Kley and reproduced by permission.
The blind snakes use a shovel instead of a pick. The scolecophidian nasal is broadly expanded to support a rostral blade. Instead of central reenforcement, the nasal is supported at the margins, and the force of digging is ultimately carried by the premaxilla, prefrontals and frontals. While no detailed study has been done, this arrangement probably sacrificies strength for speed. It may be related to the scolecophidian habit of attacking (even nesting in!) the colonies of social insects, largely ants and termites. This is earth that has already been turned. It requires no special strength to dig through. The premium is on speed to attack and ingest eggs and larvae before the small snake is overwhelmed by the insects themselves. This hypothesis is consistent with recent studies of feeding behavior in blind snakes whch also conclude that their highly specialized feeding mechanisms are designed for high-speed, hit-and-run tactics. Kley (1998); Kley & Brainerd (1999).
Dig In! The feeding studies mentioned above are available in great detail, with film clips, at Nathan Kley's web site. According to Kley & Brainerd (1999) and other work noted there, the Scolecophidia have a variety of distinct feeding styles which seem to involve substantial active kinesis of one jaw or the other and an assortment of unusual jaw joints. The common denominator may be the use of one highly mobile jaw element (either dentary or maxilla, but not both) to scrape or push food into the throat. However, the fossorial scolecophidian life constrains the skull to be relatively solid in order to assist in digging. By contrast, the Alethinophidia have a more kinetic skull overall, but use the kinesis passively: to allow large objects to be swallowed, rather than to rake small objects into the throat as in scolecophidians. The alethinophidian feeding style may be related to, and may have evolved from, locomotion.
Trench Warfare. All of this may have interesting implication for the origin of the snakes. The classical explanation for snakes is that they descended from lizards via some fossorial intermediate. This explains the fossorial habits of basal snakes and such peculiar adaptations as the ophidian eye, which has a number of unique features including the absence of a fovea centralis, the presence of double cones, and the absence of colored oil droplets. Further, unlike lizards (but somewhat like fish), snakes focus the eye by moving the lens rather than distorting the shape of the lens. These and other changes convinced early evolutionary biologists that snakes had evolved from some dark-adapted underground group which had re-evolved the eye after a period during which vision was of limited utility.
More recently, a number of workers have championed a marine varanid origin for snakes, even suggesting that snakes are survivors of the mosasaurs. One of the strong points in favor of this hypothesis is that the fossil record of snakes contains no fossorial forms, but does contain apparently marine species such as Pachyrachis. Most recently, however, a contemporary of Pachyrachis (Haasiophis) -- from the same formation -- has been described as a macrostomate (advanced, non-fossorial snake) which had secondarily become aquatic in the manner of modern sea snakes (Elapids) Tchernov et al. (2000). The same authors question the previous interpretation of Pachyrachis as primitive. The only other well known Cretaceous snake is Dinilysia. Dinilysia is clearly a terrestrial snake. Its detailed affinities are unclear, but the structure of its skull, as described by Estes et al. (1970), suggests aniliid affinities. That is, it has a noticeable "pick" and may well have derived from a fossorial form.
Digging Out from Under. We may now assemble a nice story along the following lines. Snakes did in fact begin as very basal varanids, with whom they share the forked tongue and other characters. However they did not come by sea, but underground as in the classical interpretation. (Possibly the development of a fossorial life was related to the early Cretaceous development of birds and mammals which could effectively prey on small lizards). Originally these lizards had a variety of shovel-type fossorial adaptations, represented today by the weird variation in scolecophidians (see the Kley site). However, some developed the pick-style, aniliid, centrally-supported rostra and a feeding style that coordinated with snake-like locomotion. This freed up the jaw and peripheral skull elements to begin the development of the extreme cranial kinesis seen in all advanced snakes. During the later Cretaceous, these advanced forms became large and strong enough to abandon fossorial life and experiment with terrestrial or marine existence. Dinilysia, Pachyrachis, and Haasiophia are all examples of this radiation, which resulted in the variety of terrestrial and marine forms we see today.
Nice story, but time -- as always -- will eventually tell. ATW 000429.
Note added in disproof: A great deal has happened in this area since the Note was originally written, and another essay will follow shortly. Pending that, the better story may now be that the scolecophidians are a very early and very derived group who are not closely related to any of the other living groups of snakes. In fact, one may question (not too seriously) whether they are snakes at all. But that is a story for another day. ATW 010901
* The use of the dismissive term "degenerate" in 19th and early 20th century biology is itself an interesting subject, and perhaps not at all as simple as it might seem. However, it is well beyond the scope of this essay.
Anomalepididae: Medium sized (20-75 cm) fossorial snakes
Range: R of SAm.
Characters: premaxilla without teeth; retroarticular process elongate; vomerine process of palatine narrow; transverse process of pterygoid curved & receives ectopterygoid dorsally; transverse process of premaxilla straight and lateral; postorbital absent; supratemporal process of parietal not developed; supratemporal present; quadrate suspension shifted anteroventrally to near prootic - basisphenoid suture; stapes shaft not straight; stapes shaft thick & shorter than width of footplate.
References: Tchernov et al. (2000) [T]
Leptotyphlopidae: Very small to small (10-25 cm) fossorial snakes.
Range: SAm, NAm, Eur, Afr & SW Asia.
Characters: premaxilla without teeth; vomerine process of palatine narrow; transverse process of pterygoid curved & receives ectopterygoid laterally; transverse process of premaxilla straight and lateral; postorbital absent; supratemporal process of parietal not developed; supratemporal absent; quadrate suspension shifted anteroventrally to near prootic - basisphenoid suture; stapes shaft not straight; stapes shaft thick & shorter than width of footplate.
References: Tchernov et al. (2000) [T]. 000428.
Typhlopidae: Small to medium fossorial snakes
Characters: premaxilla without teeth; retroarticular process elongate; vomerine process of palatine narrow; transverse process of pterygoid curved & receives ectopterygoid laterally; transverse process of premaxilla straight and lateral; reduced eyes; postorbital absent; supratemporal process of parietal not developed; supratemporal absent; quadrate suspension shifted anteroventrally to near prootic - basisphenoid suture; stapes shaft not straight; stapes shaft thick & shorter than width of footplate.
References: Tchernov et al. (2000) [T]. 000428.
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