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Snake skeleton

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Asnakeskeletonconsists primarily of theskull,vertebrae,andribs,with onlyvestigialremnants of the limbs.

Skeleton of a snake at theNatural History Museum

Skull

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The skull ofPython reticulatus.

Theskullof asnakeis a very complex structure, with numerousjointsto allow the snake to swallow prey far larger than its head.

The typical snake skull has a solidlyossifiedbraincase,with the separatefrontal bonesand the unitedparietal bonesextending downward to thebasisphenoid,which is large and extends forward into arostrumextending to theethmoidalregion. The nose is less ossified, and the pairednasal bonesare often attached only at their base. Theoccipital condyleis eithertrilobateand formed by thebasioccipitaland theexoccipitals,or a simple knob formed by the basioccipital; thesupraoccipitalis excluded from theforamen magnum.The basioccipital may bear a curved ventral process orhypapophysisin thevipers.

Theprefrontal boneis situated, on each side, between the frontal bone and themaxilla,and may or may not be in contact with the nasal bone.

Thepostfrontal bone,usually present, borders theorbitbehind, rarely also above, and in thePython sasupraorbital boneisintercalatedbetween it and the prefrontal bone.

Thepremaxillary boneis single and small and as a rule, connected with the maxillary only byligament.

The pairedvomeris narrow.

Thepalatine boneandpterygoidare long and parallel to the axis of the skull, the latter diverging behind and extending to thequadrateor to thearticular extremityof the mandible; the pterygoid is connected with the maxillary by theectopterygoidortransverse bone,which may be very long, and the maxillary often emits a process towards the palatine, the latter bone being usually produced inwards and upwards towards the anterior extremity of thebasisphenoid.

The quadrate is usually large and elongate, and attached to the cranium through the supratemporal (often regarded as thesquamosal).

In rare cases, (Polemon) the transverse bone is forked and articulates with the two branches of the maxilla.

The quadrate and maxillary andpalatopterygoid archesare more or less movable to allow for thedistensionrequired by the passage of prey, often much exceeding the size of the mouth. For the same reason, theramiof the lower jaw, which consist ofdentary,splenial,angular,andarticularelements, with the addition of acoronoidin theboasand a few other small families, are connected at thesymphysisby a very extensible elastic ligament.

Thehyoid apparatusis reduced to a pair of cartilaginous filaments situated below the trachea, and united in front.

There are various modifications according to thegenera.A large hole may be present between thefrontal bonesand the basisphenoid (Psammophis,Coelopeltis); the maxillary may be much abbreviated and movable vertically, as in theViperidae;the pterygoids may taper and converge posteriorly, without any connection with the quadrate, as in theAmblycephalidae;the supratemporal may be much reduced, and wedged in between the adjacent bones of thecranium;the quadrate may be short or extremely large; the prefrontals may join in amedian suturein front of the frontals; the dentary may be freely movable, and detached from the articular posteriorly.

The deviation from the normal type is much greater still when we consider the degraded wormlike members of the familiesTyphlopidaeandGlauconiidae,in which the skull is very compact and the maxillary much reduced. In the former this bone is loosely attached to the lower aspect of the cranium; in the latter, it borders the mouth and issuturallyjoined to the premaxillary and the prefrontal. Both the transverse bone and the supratemporal are absent, but the coronoid element is present in the mandible.

Joints of the snake skull

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Lateral view of the skull of aBurmese Python,with visible kinetic joints labeled. Red =highly mobile,green =slightly mobile,blue =immobile.
  • Red A:thejointbetween themandibleand quadrate. It isanalogousto the joint in mammalian jaws.
  • Red B:the joint between the quadrate and thesupratemporal.It is highly mobile in most directions, allowing a wider gape (i.e., the snake can open its mouth wider) and greater jaw flexibility.
  • Red C:the joint between the prefrontal andmaxilla.It allows the maxilla to pivot in the plane of the photograph, and while it does not increase gape, it does facilitate the complex action by which the snake draws prey into its mouth.
  • Green A:the joint between thefrontal boneandnasal bone.It allows thenoseto upturn slightly, increasing gape and assisting inswallowing.
  • Green B:allows the lower jaws to bow outwards, further increasing the gape.
  • Blue:the joint between the supratemporal and parietal. Immobile, except forDasypeltis.

Snake dentition

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In mostsnakes,teethare located on thedentaryof thelower jaw,themaxilla,thepalatine boneand thelateral pterygoid plate.The latter form an "inner row" of teeth that can move separately from the rest of the jaws and are used to help "walk" the jaws over prey. Several snake lineages have evolvedvenomwhich is typically delivered by specialized teeth calledfangslocated on themaxilla.

Most snakes can be placed into one of four groups, based on their teeth, which correlate strongly with venom and lineage.

Aglyph

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Anaglyphoussnake. ABurmese Pythonskull (Python bivittatus)

Aglyphoussnakes (lacking grooves) have no specialized teeth; each tooth is similar in shape and often size. When teeth vary in size, as in some bird eaters, they do not vary in shape. Most aglyphous snakes are non-venomous; some, likeThamnophis,are considered mildly venomous. The feature is not asynapomorphy.

Opisthoglyph

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Anopisthoglyphoussnake. Ahognose snakeskull (Heterodon nasicus)

Opisthoglyphous( "rearward grooves" ) snakes possess venom injected by a pair of enlarged teeth at the back of themaxillae,which normally angle backward and are grooved to channel venom into the puncture. Since these fangs are not located at the front of the mouth, this arrangement isvernacularlycalled "rear-fanged". In order to envenomate prey, an opisthoglyphous snake must move the prey into the rear of its mouth and then penetrate it with its fangs, presenting difficulties with large prey although they can quickly move smaller prey into position. The opisthoglyphous dentition appears at least two times in the history of snakes.[1]The venom of some opisthoglyphous snakes is strong enough to harm humans; notably,herpetologistsKarl SchmidtandRobert Mertenswere killed by aboomslangand atwig snake,respectively,[2][3]after each underestimated the effects of the bite and failed to seek medical help. Opisthoglyphous snakes are found mostly in the familiesColubridaeandHomalopsidae.

Proteroglyph

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Aproteroglyphoussnake. Aking cobraskull (Ophiophagus hannah)

Proteroglyphoussnakes (forward grooved) have shortened maxillae bearing few teeth except for a substantially enlarged fang pointing downwards and completely folded around the venom channel, forming a hollow needle. Because the fangs are only a fraction of an inch long in even the largest species, these snakes must hang on, at least momentarily, as they inject their venom.[4]Somespitting cobrashave modified fang tips allowing them to spray venom at an attacker's eyes. This form of dentition is unique toelapids.

Solenoglyph

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Asolenoglyphoussnake. Arattlesnakeskull (Crotalussp.)

Solenoglyphoussnakes (pipe grooved) have the most advanced venom delivery method of any snake. Each maxilla is reduced to a nub supporting a single hollow fang tooth. The fangs, which can be as long as half the length of the head, are folded against the roof of the mouth, pointing posteriorly. The skull has a series of interacting elements that ensure that the fangs rotate into biting position when the jaws open. Solenoglyphous snakes open their mouths almost 180 degrees, and the fangs swing into a position to allow them to penetrate deep into the prey. While solenoglyph venom is typically less toxic than that ofproteroglyphs,this system allows them to deeply inject large quantities of venom. This form of dentition is unique tovipers.

Exceptions

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A few snakes do not conform to these categories.Atractaspisis solenoglyphous but the fangs swing out sideways, allowing it to strike without opening its mouth, perhaps allowing it to hunt in small tunnels.Scolecophidia(blind burrowing snakes) typically have few teeth, often only in the upper or lower jaw.

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Common names for the various types of snake dentition originate largely from older literature, but still are encountered in informal publications. Aglyphous snakes are commonly calledfangless;opisthoglyphous snakesrear-fangedorback-fanged;and both Proteroglyphous and Solenoglyphous snakes are referred to asfront-fanged.[5][6]

Taxonomic key of skull modifications

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Modifications of the skull in the European genera:

  • I. Quadrate articulating with the cranium, supratemporal absent; mandible much shorter than the skull, with coronoid bone; maxillary small, on lower aspect of cranium; pterygoids not extending to quadrate; nasals forming long sutures with the premaxillary, prefrontals, and frontal:Typhlops.
  • II. Quadrate suspended from the supratemporal; mandible at least as long as the skull; pterygoids extending to quadrate or mandible.
  • A. Mandible with coronoid bone; nasals in sutural contact with frontals and prefrontals; transverse bone short, not projecting much beyond cranium; maxillary not half as long as mandible, which is not longer than skull (to occiput):Eryx.
  • B. No coronoid bone; nasals isolated.
  • 1. Maxillary elongate, not movable vertically.
  • a. Maxillary half as long as mandible.
  • Supratemporal half as long as skull, projecting far beyond cranium; mandible much longer than skull:Tropidonotus.
  • Supratemporal not half as long as skull, projecting far beyond cranium; mandible much longer than skull:Zamenis.
  • Supratemporal not half as long as skull, projecting but slightly beyond cranium; mandible much longer than skull:Coluber.
  • Supratemporal not half as long as skull, not projecting beyond cranium; mandible not longer than skull:Coronella,Contia.
  • b. Maxillary not half as long as mandible, which is longer than skull; supratemporal not half as long as skull, projecting beyond cranium.
  • Quadrate longer than supratemporal; maxillary much longer than quadrate, nearly straight in front of prefrontal; a large vacuity between the frontal bones and the basisphenoid:Coelopeltis.
  • Quadrate not longer than supratemporal; maxillary little longer than quadrate, strongly curved in front of prefrontal:Macroprotodon
  • Quadrate longer than supratemporal; maxillary little longer than quadrate, nearly straight in front of prefrontal:Tarbophis
  • 2. Maxillary much abbreviated and erectile; supratemporal not half as long as skull; mandible much longer than skull; basioccipital with a strong process.

Vertebrae and ribs

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A snake has from 175 to more than 400 vertebrae in its backbone. The means by which vertebrae are secured are twofold: either a ball and socket joint, or zygopophyses, which stick out from each vertebra to poke rear-pointing projections from the vertebrae ahead of it. This results in a spine well-adapted to the snake's method of movement.[7]

Thevertebral columnconsists of anatlas(composed of two vertebrae) withoutribs;numerousprecaudal vertebrae,all of which, except the first or first three, bear long, movable, curved ribs with a small posteriortubercleat the base, the last of these ribs sometimes forked; two to ten so-calledlumbar vertebraewithout ribs, but with bifurcatetransverse processes(lymphapophyses) enclosing thelymphatic vessels;and a number of riblesscaudal vertebraewith simple transverse processes. Whenbifid,the ribs or transverse processes have the branches regularly superposed.

Thecentrahave the usualball and socket joint,with the nearly hemispherical or transversely ellipticcondyleat the back (procoelousvertebrae), while theneural archis provided with additional articular surfaces in the form of pre- and post-zygapophyses,broad, flattened, and overlapping, and of a pair of anterior wedge-shaped processes calledzygosphene,fitting into a pair of corresponding concavities,zygantrum,just below the base of the neural spine. Thus the vertebrae of snakes articulate with each other by eightjointsin addition to the cup-and-ball on the centrum, and interlock by parts reciprocally receiving and entering one another, like themortise and tenonjoints. The precaudal vertebrae have a more or less high neural spine which, as a rare exception (Xenopholis), may be expanded and plate-like above, and short or moderately long transverse processes to which the ribs are attached by a single facet. The centra of the anterior vertebrae emit more or less developed descending processes, orhaemapophyses,which are sometimes continued throughout, as inTropidonotus,Vipera,andAncistrodon,amongEuropeangenera.

In the caudal region, elongate transverse processes take the place of ribs, and the haemapophyses are paired, one on each side of thehaemal canal.In therattlesnakesthe seven or eight last vertebrae are enlarged and fused into one.

Vestigial limbs

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Skeleton of aBoelens Pythonshowing the bones inside the anal spurs

No living snake shows any remains of thepectoral arch,but remains of thepelvisare found in:

References

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  1. ^Bruna Azara, C. (1995). "Animales venenosos. Vertebrados terrestres venenosos peligrosos para el ser humano en España".Bol. SEA11:32-40.
  2. ^As herpetologists, both kept a record of the effect up to thir death, Schmidt for a couple of days:https://people.wku.edu/charles.smith/chronob/SCHM1890.htm
  3. ^Mertens kept a diary of his dying for full 18 days: cf Beolens, Bo; Watkins, Michael; Grayson, Michael (2011).The Eponym Dictionary of Reptiles.Baltimore: Johns Hopkins University Press. xiiiISBN 978-1-4214-0135-5. ( "Mertens", p. 176; "Robert", p. 223; "Robert Mertens", p. 223).
  4. ^LD50 for various snakes
  5. ^Rose, Walter; The reptiles and amphibians of southern Africa; Pub: Maskew Miller, 1950
  6. ^Engelmann, Wolf-Eberhard. Snakes (No. 05352). Publisher Bookthrift 1982.ISBN978-0896731103
  7. ^Campbell, Sheldon; Shaw, Charles E. (1974).Snakes of The American West.New York:Alfred A. Knopf.ISBN978-0-394-48882-0.
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  • Snake AnatomyExternal and Internal snake anatomy with postmortem images.
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