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Monday, February 12, 2018

Basics of snake skulls

This article will soon be available in Spanish!

Snakes have a lot more bones than we do, but they have a lot fewer types of bones. Aside from a few boas, pythons, pipesnakes, and blindsnakes with vestigial femurs, most snakes just have a few hundred vertebrae with one pair of ribs each (except in the neck & tail), and a skull.

The snake skull is a remarkable structure. Snake skulls are highly kinetic, with a lot more moving parts than our skulls. Human skulls have just one movable part: the temporomandibular joint, which opens and closes your mouth. Snake skulls have many joints and moving parts; they can move the left and right sides of their jaws independently, as well as the outer (maxilla) and inner (palatine+pterygoid) parts of their upper jaws. Many bones that are tightly knit together in the skulls of most animals are loosely connected by stretchy ligaments in snakes, allowing them to stretch their jaws over huge prey (pardon the goofy music in the linked video). Contrary to the popular phrase, snakes cannot actually "unhinge" their jaws (Harry Greene explains this very well in this video).

The right side of the skull of an alethinophidian snake (nose pointing to the right).
Bones with teeth are the maxilla (mx), palatine (pal), pterygoid (pt), and dentary (d).
From Cundall & Irish 2008. For a key to all abbreviations, click here.
The bones or parts of bones that are shaded are not present in all snake species.
Most snakes have teeth on four pairs of bones, two of which are the same as pairs of bones where humans do: the maxilla (most of our upper jaw) and the dentary (our lower jaw). In addition, almost all snakes have teeth on two bones that in humans form part of the roof of the mouth: the palatine and the pterygoid1, which are connected one in front of the other. This means that snakes have two upper jaws on each side: an outer one (the maxilla) and an inner one (the palatine+pterygoid). If a snake has fangs, they are always on the maxilla2. Some snakes, such as pythons, also have teeth on the premaxilla, where we humans have our incisors, although in most snakes the premaxilla is a part of the snout, has no teeth, and does not act as part of the jaws.
The right half of the skull of a snake, looking up from the bottom (nose pointing to the right).
Bones with teeth are the maxilla (mx), palatine (pal), pterygoid (pt), and dentary (d). The premaxilla (pmx) has no teeth.
From Cundall & Irish 2008. For a key to all abbreviations, click here.The bones or parts of bones that are shaded are not present in all snake species.
Tooth marks left by a
python bite (upper jaw
above, lower jaw below).
You can sometimes see this pattern of tooth marks left behind when a non-venomous snake lets go after biting something, and in fact many resources suggest that you can use the tooth pattern to determine3 whether or not a bite has come from a venomous snake (a viper at least, which are responsible for >99% of venomous snakebites in the USA), since most dangerously venomous snakes have different tooth patterns on account of their fangs, and most of their non-fang teeth don't usually come into contact with the target. I mentioned above that fangs are always on the maxilla, and that's because the maxilla is the primary prey-catching bone in the snake skull. As far as we know, fangs evolved only once, as enlarged teeth at the back of the maxilla in the ancestor of all living colubroid snakes about 75 million years ago. In many living species of snakes, this is still the situation, and the vast majority of these are not dangerous to humans (although some can inflict painful bites if allowed to chew for a few minutes, and a few can be deadly). In at least three cases (vipers, elapids, and atractaspidids), the fangs have moved up to the front of the maxilla, through the developmental suppression of the front part of the maxilla (and its teeth) in the snake embryo. I covered this and the evolution of grooved and hollow fangs in more detail in my article about snake fangs.
The right half of the skull of a snake, looking down from the top (nose pointing to the right).
No teeth are visible. From Cundall & Irish 2008. For a key to all abbreviations, click here.The bones or parts of bones that are shaded are not present in all snake species.
Although most people are most interested in the teeth and fangs, the rest of the snake skull is no less fascinating. The outer and inner upper jaw are connected by a toothless upper jaw bone called the ectopterygoid, which works like a lever to transfer muscular power from the muscles attached to the pterygoid out to the maxilla, which has no muscles of its own. When a snake is eating, the entire upper jaw (inner and outer parts) is raised and moved slightly backward, alternating the left and right sides and pulling the prey into the mouth: the characteristic "jaw-walking" or "pterygoid walk" motion of feeding snakes. So, the front of the pterygoid is attached to the back of the palatine, the ectopterygoid hangs off the outside of the pterygoid, and the maxilla hangs off of the other end of the ectopterygoid. In vipers, whose fangs fold, the maxilla and its fang are pushed forward by the ectopterygoid and pterygoid.

Roughly the same fang movements are made during striking and swallowing. Supratemporal (st), quadrate (q), mandible (ma), pterygoid (pt), ectopterygoid (ec), palatine (pa), prefrontal (pf), maxilla (mx). From Kardong 1977

The independent left and right movement
of the upper jaws of a viper.
Abbreviations as above. From Kardong 1977.
Amazingly, in most snakes there is no direct connection between the upper jaws and the braincase4. Instead, the palatine and maxilla are connected to the braincase by long ligaments, which give them great freedom of motion. The front end of the palatine is connected more firmly to the snout, albeit still with some freedom to move. The rear end of the maxilla is also connected by a ligament to the lower jaw. It's really the movements of the palatine and pterygoid that swallow the prey. The lower jaws mainly press the prey against the upper jaws, and the teeth on the dentary and maxilla rarely contact the prey and play little active role in swallowing.

The lower jaws or mandible participate in the process of feeding as well, and unlike in humans they have a loose attachment of the lower jaws to each other at the front of the dentary bones. The dentary bones are connected firmly at the back to the articular bones, which are connected to the quadrate bones at a flexible joint, which are connected to the back of the braincase by the supratemporal bones, also at a somewhat moveable joint. Together with the flexible palato-maxillary apparatus ("upper jaws"), this three-part lower jaw allows snakes to open their mouths very wide, walk their heads over, and consume things that are as big as they are without breaking them into smaller pieces or using their non-existent hands. The quadrate also attaches to the columella, which is the sole inner ear bone in reptiles; thus, the lower jaw also conducts sound to the ear.

So there you have it. The snake skull is divided into four functional units: the braincase, the snout, the palato-maxillary apparatus ("upper jaws") and the mandibular apparatus ("lower jaws"), each of which can move independently (well, except for the braincase, which is relatively stationary). The upper jaws are divided into two partially separated structural-functional units, a medial swallowing unit and a lateral prey capture unit, both of which work with the lower jaws to accomplish their tasks.

From Frazzetta 1970Click for larger size.
A quick note about a special case: one of my favorite snakes, and one of the first I wrote about on this blog, Casarea dussumeri, are often called Round Island boas, although I chose to use the more apt "splitjaw snakes" in my article. As if the usual kinesis of the snake skull isn't enough, these snakes have a maxilla that is uniquely subdivided into two movable parts, called the anterior and posterior maxilla. The anterior maxilla has 10 teeth and the posterior maxilla has 12. It is thought that the divided maxilla evolved through incomplete development, because the maxilla of other snakes forms in two parts before fusing together in the embryo, and the function is thought to be to help Casarea encircle hard, cylindrical prey such as skinks.

We still have a lot left to learn about snake skulls. We didn't even cover half of the bones in this article. You don't actually so much find snake skulls as you do carefully prepare them. The individual bones are so small and light and fragile that they tend not to fossilize well, nor can they easily be found among the other bones of a snake's skeleton. Even normal cleaning and preparation methods can damage the fragile bones of tiny snake skulls. Thus, there is much left to discover about how they work!

Skull of Natrix natrix from Andjelković et al. 2017. Mobile connections marked with red dashed arrows and circles.Paired bones are shown in yellow (pa – palatine, pt – pterygoid, ec – ectopterygoid, mx – maxilla, st – supratemporal,q – quadrate, cp – compound bone, d – dentary, pf – prefrontal), unpaired bones are shown in green or grey (pmx – premaxilla, na – nasal, b – braincase, smx – septomaxillae & vomers).



1 Although the pterygoids are stand-alone bones in the roof of the mouth of many vertebrates, in humans they are called the pterygoid processes of the sphenoid bone because they are fused to the sphenoid bone.



2 There is one very strange snake, Pythonodipsas carinata from Africa, that has an ungrooved fang on the palatine bone. They aren't any studies of their functional morphology so we don't really know exactly how they use their palatine fangs, but they use constriction to subdue their prey.



3 I don't necessarily recommend this, partly because if you've been bitten then it's too late, and partly because it's better just to learn the few venomous snake species that live in your area than it is to try to follow some "rule" that inevitably has exceptions.



4 Atractaspidids have a ball-and-socket joint between the prefrontal (part of the braincase) and the maxilla, which along with a gap, bridged by a ligament, between the pterygoid and palatine, allows them to "strike" with their fang backwards, with a closed mouth, using just the fang on one side, a useful if terrifying adaptation for envenomating prey in underground burrows. A hook-like ridge on the fang increases the size of the wound, presumably enhancing the absorption of venom.




ACKNOWLEDGMENTS

Thanks to gibby for the use of their photograph.

REFERENCES

Albright, R. G. and E. M. Nelson. 1959. Cranial kinetics of the generalized colubrid snake Elaphe obsoleta quadrivittata. I. Descriptive morphology. Journal of Morphology 105:193-239.

Albright, R. G. and E. M. Nelson. 1959. Cranial kinetics of the generalized colubrid snake Elaphe obsoleta quadrivittata. II. Functional morphology. Journal of Morphology 105:241-291.

Andjelković, M., Tomović, L., & Ivanović, A. 2017. Morphological integration of the kinetic skull in Natrix snakes. Journal of Zoology, 303:188-198 <link>

Cundall, D. 1983. Activity of head muscles during feeding by snakes: a comparative study. American Zoologist 23:383-396.

Cundall, D. and H. W. Greene. 2000. Feeding in snakes. Pages 293–333 in K. Schwenk, editor. Feeding: Form, Function, and Evolution in Tetrapod Vertebrates. Academic Press, San Diego, CA.

Cundall, D. and F. Irish. 2008. The snake skull. Pages 349-692 in C. Gans, A. S. Gaunt, and K. Adler, editors. Biology of the Reptilia. Volume 20, Morphology H. The Skull of Lepidosauria. The University Of Chicago Press, Chicago, Illinois, USA <link>

Frazzetta. T. 1970. From hopeful monsters to bolyerine snakes? The American Naturalist 104:55-72 <link>

Frazzetta, T. 1971. Notes upon the jaw musculature of the Bolyerine snake, Casarea dussumieri. Journal of Herpetology 5:61-63

Irish, F. and P. Alberch. 1989. Heterochrony in the evolution of bolyeriid snakes. Fortschritte der Zoolologie 35:205.

Juckett, G. and J. G. Hancox. 2002. Venomous snakebites in the United States: management review and update. American Family Physician 65:1367-1375 <link>

Kardong, K. 1974. Kinesis of the jaw apparatus during the strike in the cottonmouth snake, Agkistrodon piscivorus. Forma et functio 7:327-354.

Kardong, K. V. 1977. Kinesis of the jaw apparatus during swallowing in the cottonmouth snake, Agkistrodon piscivorus. Copeia 1977:338-348 <link>

Lombard, R. E., H. Marx, and G. B. Rabb. 1986. Morphometrics of the ectopterygoid in advanced snakes (Colubroidea): a concordance of shape and phylogeny. Biological Journal of the Linnean Society 27:133-164 <link>

Maisano, J. A. and O. Rieppel. 2007. The skull of the Round Island boa, Casarea dussumieri Schlegel, based on high-resolution X-ray computed tomography. Journal of Morphology 268:371-384 <abstract>

Raynaud, A. 1985. Development of Limbs and Embryonic Limb Reduction. Pages 59-148 in C. Gans and F. Billett, editors. Biology of the Reptilia. Volume 15. Development B. John Wiley & Sons, New York <link>

Rieppel, O. 2012. “Regressed” Macrostomatan Snakes. Fieldiana Life and Earth Sciences 5:99-103 <link>

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Life is Short, but Snakes are Long by Andrew M. Durso is licensed under a Creative Commons Attribution-NonCommercial-NoDerivs 3.0 Unported License.

2 comments:

Science IT and Leisure said...

informative...
thank you for sharing

doll said...
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