I lifted Vivian’s hide box to take the photo, and she was mildly perturbed, so she defensively hid her head in her coils.
The Department of Defense Partners in Amphibian and Reptile Conservation has sent out a great set of links for World Snake Day, put together by my friend and colleague Rob Lovich. There’s loads of stuff in these links– look around. I’ve brought to the top of the list a shutterfly album of a great diversity of snakes. If you don’t have time for more, open up that album click on the slideshow, and enjoy! (It works best if you have dual monitors, one to work one, and one for snake pix.)
Tomorrow [i.e. today] (July 16th) is World Snake Day! In celebration of this event and the important ecological value snakes play in the ecosystems of military lands, we would like to highlight some snake-focused DoD Partners in Amphibian and Reptile Conservation (DoD PARC) products below.
We hope you enjoy learning about snakes through the various DoD PARC products below.
The Timber Rattlesnake (Crotalus horridus) was chosen on our logo to reflect the long-standing relationship DoD and the Military Services have with protecting both our nation and its resources, including snakes. Ultimately, the use of this species is meant to represent how DoD protects the natural resources with which it has been entrusted, and how those resources in turn provide for and protect the military’s ability to prepare for its war-fighting and peace-keeping duties.
If you’re wondering why the military has a unit devoted to amphibians and reptiles, the military must follow environmental and conservation laws (unless specifically exempted); there are practical issues for the military involving venomous reptiles; and recall that Darwin traveled around the world largely by courtesy of the Royal Navy. The U.S. Navy published, with the assistance of the American Society of Ichthyologists and Herpetologists, the classic Poisonous Snakes of the World:
Greene, H.W. 1997. Snakes: The Evolution of Mystery in Nature. University of California Press, Berkeley.
Minton, S.A., H.G. Dowling & F.E. Russell. 1965. Poisonous Snakes of the World: A Manual for Use by U.S. Amphibious Forces. NAVMED P-5099. Bureau of Medicine and Surgery, Department of the Navy, Washington, D.C.
World Snake Day was this past Monday, July 16, and I missed it! I didn’t find out till Tuesday, and so a little snake catch up today. I did in fact, have two snake encounters on Monday. First, with Vivian, my 20+ year old ball python (Python regius), whom I see almost every day. It was just a “Hi, how are ya”, since it wasn’t time for feeding, and her water bowl didn’t need refilling. Here’s Vivian at a reptile demonstration at an alumni event at the University of Wisconsin-Parkside a few years ago. Vivian often participates in such public events, and is usually the star of the show. Ball pythons are probably the best choice for a reptile pet.
I also checked in on Hissy, a bullsnake (Pituophis catenifer sayi), for my colleague Chris Noto. Bullsnakes do not make as good pets as ball pythons– Hissy is pretty ‘bitey’. The reason this one is kept is that it is an escaped captive that was recaught, and, though native to Wisconsin, the species is not from this area, and thus there was no known locality to which Hissy could be returned.
As a parting tribute to World Snake Day, here’s Bill Haast, late director of the Miami Serpentarium. He was bitten by venomous snakes over 100 times, and had developed antibodies to a variety of venoms that enabled him to donate blood as a treatment to other snake-bite victims. Despite his many bites, he lived to be 100! I saw this near life-size photo of him in the Miami airport during a visit last March.
The tw**t doesn’t say where the picture was taken, but I’m guessing somewhere in the American southwest. It does say it’s a longnosed snake (Rhinocheilus lecontei), but it doesn’t look to be one to me. I’m pretty sure it’s a common kingsnake (Lampropeltis getula). [Added later: it is a longnosed– see comment below by rattlesnakeguy.]
The interest in this snake is that it’s banded fore and aft, but striped amidships, which is pretty unusual. The kingsnake is usually banded, but striped ones are known from southern California and Baja– this snake has both! Snakes with unusual and partial patterns are popular amongst herpetoculturalists, and I recall from grad school that one of my cohort working on snake development got stripey patterns from eggs incubated at the wrong temperature. Some quick checking revealed some definite evidence of low temperature incubation leading to striping in pythons (here and here), and some vaguer rumblings in the cornsnake forums, but I could not find any scientific papers on the subject. I did find some more reliable evidence that incubation temperature does not influence pattern in Australian eastern brown snakes.
Snake patterns are thought to have an influence on their detectability and catchability by visual predators; bands are often camouflagey, while stripes tend to make a predator (me!) grab behind the snake, as the longitudinal stripe obscures the forward motion of the snake, and the grab is mistimed.
The videographer, Nicky Bay, is most interested in the pulsation of a dark region below the ‘eyes’, but to me it’s the resemblance to a snake that is most striking– the ‘nostrils/loreal pits’ (note the number/arrangement is not perfect), and the ‘preocular and supralabial scales’. There are at least two species of pit vipers on Singapore. The mangrove pit viper is dark, and not at first glance an appealing candidate for a a model. Wagler’s pit viper, though, has potential.
Mimicry of snakes by arthropods is not common, but not unknown. Jerry posted here at WEIT earlier about a lepidopteran pupa mimicking a snake, and Matthew posted on a caterpillar mimicking a snake, as well as a frog said to mimic a bird dropping. I mention the latter because Bay considers that the spider may be a bird dropping mimic, which doesn’t seem to be the case to me. He gives it a vernacular name of “bird dropping spider”, but says, “Gave me the impression of a snake’s head though!” The taxonomic identity of the spider is not clear; there are additional photos on his website.
It’s a bit late in the day, but I must announce that today is, at least in the USA, National Reptile Awareness Day. Happy Reptile Day to all! I must admit, as a herpetologist who joined all the major American herpetological societies while still in high school (1975), I had never heard of National Reptile Awareness Day before today. Reptiles Magazine, a fanciers outlet, is the only group I can find who are promoting it, although even they admit not to know how or when it started. Despite its obscurity, we’ll celebrate with a few reptile pictures.
First, a wild red-eared slider, a southern US turtle popular in the pet trade, and often released, but less often established, in places outside its native range, like Wisconsin.
Next, my ball python Vivian, whom I’ve had for about 18 years.
A snapping turtle from UW-Parkside, at the same alumni event as Vivian.
And we’ll finish up with a series of eastern massasauga (Sistrurus catenatus catenatus) pictures; I believe all the pictures were taken in Cass, Michigan. They were taken by my former student Eric Hileman, who did his Ph.D. at Northern Illinois University with Rich King. Eric successfully defended his dissertation on the population ecology of massasaugas just this past Wednesday, and I was privileged to be able to attend. So we can all take this National Reptile Awareness Day as a day to send this joyous message to Eric: “Congratulations. Now get back to work.”
As the capstone to Snake Week, let’s take a closer look at how the squirrel-like mammal being eaten by Tetrapodophis in Julius Csotonyi’s striking reconstruction died. In my earlier post, I took note of the fact that the describers of the newly discovered four-legged fossil snake had inferred from its skeleton that it was a constrictor (and thus the earliest known constricting snake, implying that constriction is an ancestral characteristic of snakes), and included Csotonyi’s lovely reconstruction showing the four-legged Tetrapodophis doing in and beginning to swallow a squirrel-like mammal. Here’s a reprise of the picture.
The snake killed the ‘squirrel’, so we know who killed it, but what killed the ‘squirrel’? In police procedural talk, we’ve got the murderer, but we want to know the cause of death for the coroner’s report. Coincidentally, a new paper in the Journal of Experimental Biology, appearing at almost the same time as the description of Tetrapodophis, asks exactly that question, and shows via straight-forward and well-done experiments, what, in fact, is the ‘squirrel’s’ cause of death.
It was long thought that constricted prey died of suffocation, but it had also been suggested that the prey died of cardiac arrest due to drop in blood pressure. This had been suggested, in part, by the rapidity with which prey died, seemingly more rapidly than they would suffocate. What Scott Boback and colleagues have shown, using anesthetized rats fed to boa constrictors, with a set of catheters and probes in them to record their heart rates and blood pressures, is that there is a sharp and sudden drop in peripheral arterial pressure, an increase in central venous pressure, and a slowing of the heart rate. They conclude:
[S]nake constriction induces rapid prey death due to circulatory arrest.
I’m not sure if their experiments quite exclude asphyxia as a contributing cause, but it certainly shows the importance of the circulatory crisis caused by constriction.
Some of the media coverage has overstated the novelty of this result. For example, National Geographic headlined “Why We Were Totally Wrong About How Boa Constrictors Kill”, while Science, somewhat less over the top, headlined “Surprise: Snakes don’t kill by suffocation“. However, as Boback et al. note, circulatory collapse was first suggested over 80 years ago, and has been a viable idea for quite a while. Harry Greene, our foremost student of snake natural history, taking an ecumenical approach to the cause of death, wrote in his fine Snakes, in 1997, that constriction acted by “interfering with breathing and blood circulation so that the victim is immobilized within a minute or so”, while in a later, standard, herpetology text, Laurie Vitt and Janalee Caldwell (2009) wrote, “The tightening continues, and ultimately, circulatory failure causes death.” So Boback and colleagues have done a fine and needed study, but don’t believe the (media) hype!
In writing this post, I wondered what to call the prey in Csotonyi’s reconstruction. It could not be a rat, as in Boback’s study, as there were no rats, or rodents of any kind, in the Cretaceous. On the other hand, it does look like a squirrel (a rodent as well, again not possible for the Cretaceous), so I settled on ‘squirrel’, with scare quotes. A likely mammal for Tetrapodophis to have eaten is some sort of multituberculate, an extinct type of mammal found in the Cretaceous, and convergent on rodents in their dentition (gnawing incisors with a diastema before the molariforms). And, some of them showed arboreal adaptations, as do tree squirrels, but even more so, having prehensile tails. In the reconstruction below, accompanying a paper by Farish Jenkins and David Krause, the multiberculate Ptilodusis shown to be quite squirrel-like, except for its opossum-like prehensile tail.
Boback, S.M., K.J. McCann, K.A. Wood, P.M. McNeal, E.L. Blankenship and C. F. Zwemee. 2015. Snake constriction rapidly induces circulatory arrest in rats. Journal of Experimental Biology 218:2279-2288. abstract
Greene, H.W. 1997. Snakes: The Evolution of Mystery in Nature. University of California Press, Berkeley.
Jenkins, F.A. and D.W. Krause. 1983. Adaptations for climbing in North American multituberculates (Mammalia). Science 220:712-715. abstract (pdf of JEB commentary)
It has long been known that some snakes are two-legged, because many modern species have two legs– externally visible hind limbs– a fact we’ve noticed here at WEIT before.
These small external legs, capped by keratinous claws, are supported internally by vestigial femurs and a vestigial pelvis. They are larger in males, and are used during courtship. In the fossil record, snakes with much larger hind limbs have been known since Georg Haas described Pachyrachis in 1979 and Ophiomorphus in 1980 from the early Late Cretaceous (about 95 mya). In these the legs were less rudimentary than in modern snakes, having, in addition to the femur and pelvis, a distinct tibia and fibula, and tarsal bones. That legless tetrapods would have legged ancestors is of course expected, and for the caecilians, a modern group of legless amphibians, a four-legged progenitor was described by Farish Jenkins and colleagues several years ago.
In a new paper paper in Science, David Martill, Helmut Tischlinger, and Nicholas Longrich describe a four-legged snake from the late Early Cretaceous (about 120 mya), of Brazil, giving it the rather aptly descriptive name Tetrapodophis, “four-legged snake”. The fore and hind legs are small, but well developed, with five digits on each. The limbs are suggested to have been used during prey capture. They also interpret it as being fossorial. This is significant, as the two major theories of snake origin are that they came from fossorial (burrowing) ancestors, or that they came from marine ancestors (some of the closest known relatives of snakes are extinct aquatic lizards, and Haas’s specimens come from marine sediments).
A long, flexible body, recurved teeth, and intramandibular joint (for opening the mouth wide) all suggest to Martill and colleagues that Tetrapodophis was a constrictor, preying on other vertebrates. This also has significance for what it says about the origin of snakes. Under the fossorial theory, the earliest snakes should have been insectivorous or eating other small prey (as are many supposedly primitive burrowing snakes today). Under the marine theory, the earliest snakes should have been predators of prey “bigger than their heads“, having large, extensible mouths, and associated adaptations of the skeleton and musculature– such snakes are called macrostomatan (literally, ‘large mouthed’). Haas’s two-legged marine snakes are macrostomatan. Martill and colleagues have found what they consider to be a very early macrostomatan, yet fossorial, snake– a cross between the two theories.
Almost immediately upon its publication, the paper became enmeshed in a series of overlapping controversies, which, while nothing compared to the brouhaha over the insults traded between Nicki Minaj and Taylor Swift, has created quite a stir in the small world of science social media. There are at least three areas in which questions have been raised, so let’s take them one at a time.
Where (and thus when) is the type specimen of Tetrapodophis from? It turns out that the authors of the paper don’t actually know the provenance of the specimen. They have made an inference about it (and they may very well be right), but the fact that they do not even mention the assumed nature of the specimen’s provenance in the paper is shocking. A specimen’s provenance is absolutely crucial information in systematic biology; it is especially so for fossil specimens, because in most instances it is only by examining the geological context of the discovery (the associated fossils within the bed, and the nature of the over- and underlying beds) that we can date the fossil. In this case, we are not really sure where the specimen came from, and thus we cannot be certain of when the specimen died and was entombed in sediment.
I read the paper, and did not realize the provenance was uncertain. The uncertainty, and the argument for why the authors felt they could identify the provenance, are buried in an online supplement. To some it may seem like I’ve been a bit “hey you damned kids, get off of my lawn” about it”, but I’ve long complained of the growing practice of journals, especially Science, of burying key details of papers in ephemeral online sources, and in this case such warnings have come home to roost. Where (and thus when) it’s from is just about the most basic thing you can say about a fossil, and to hide the fact that in this case it’s unknown in an online supplement is unconscionable.
In the online material, Martill and colleagues state that “no notes as to its [the specimen’s] acquisition or provenance are available.” However, in an interview with the BBC, Martill says that he first saw the specimen at the Museum Solnhofen in an “exhibition of Brazilian fossils”, so some notes on provenance seem to have been available to the Museum. Another source states that the exhibit was of fossils specifically from the “Crato Formation”.
Is the fossil, as the authors claim, from the Nova Olinda member of the Crato Formation of Ceara, Brazil? It might well be. Martill is an expert on the formation. Certain fossil localities do have a distinctive lithology and preservation– I can (usually) recognize Green River Formation fossils myself. But to not mention this up front, and provide the justification for the assignment to provenance in the paper, is beyond the pale.
Is Tetrapodophis even a snake? In a news posting on Science‘s website, Michael Caldwell alleges that the specimen is not a snake; in fact, he says, it’s not even a reptile. Rather, the article says, Caldwell thinks it might be a surviving member of a “group of extinct amphibians that died out during mass extinctions about 251 million years ago, long before Tetrapodophis appeared on the scene.” (I’m not sure what amphibians he’s thinking of– perhaps microsaurs or lysorophids?) This would be astonishing– that a group survives 150 million years without a fossil record and then reappears (which does, though, have a partial precedent in the coelacanth), and that the authors and reviewers of a paper in Science could be so wildly off in the identification of the subject of the paper. This of course is not impossible, but it would be surprising. I have only seen the published figures, but Martill and colleagues do discuss and defend the characters by which they assign the specimen to the snakes. Caldwell has published on early snakes and should know their morphology, but he has not seen the specimen either, so it’s hard to give full credence to his views. We’ll have to wait till others get to look at the specimen more closely, or perhaps for a monographic treatment by Martill and colleagues. This is the scientifically most important controversy (although the first controversy is right up there, because much of its significance as a four-legged snake depends on its supposed time of occurrence).
Should fossil collecting and/or exporting require a permit or license? Since the specimen has no collecting data accompanying it, it is unclear if the specimen was collected legally. Fossil collecting in Brazil has required a license since 1942 (or perhaps 1988– recent sources diverge on the date). This is of course not a scientific question, but a question of public policy that has implications for science. To a great extent, the controversy is an old one in paleontology– does amateur and commercial collecting enhance or retard the growth of scientific knowledge? There are strong opinions on both sides. In the United States, this argument flared up over Tyrannosaurus Sue, which was discovered and collected by commercial collectors, but eventually seized from them without recompense (one even went to jail). The downside of such regulation is that many specimens will never come to light, or, if found, will be tossed aside and left to degrade, as their possession would be illegal. A commercial black market may develop, in which the best fossils may be found, but then disappear, unstudied, into private collections. The upside is that specimens will have known provenance, and be of maximal scientific value. Martill has long argued (also here) that in Brazil the permitting system has become so corrupt that scientists are driven out of the field, and that, through bribery, commercial trade flourishes, while many fossils are left to erode and break, as no one may legally save them. He also says it was not always so– for years he worked successfully under Brazil’s regulations. His view:
‘Protecting fossils’ criminalises palaeontologists. Laws banning fossil collecting and private fossil collections deter amateur palaeontologists, drive them underground and stifle curiosity. Fossils left in the ground weather away and are lost. Banning commercial collecting loses tax revenue.
A group of Brazilian paleontologists led by Max Langer, in a strong riposte to one of Martill’s pieces, wrote
Instead, the Brazilian perspective is that taking fossils out of the country is depleting its scientific resources. Brazil has a growing, but still minor scientific community. For palaeontology, keeping the fossils in the country is a way of promoting scientific opportunities. International partnerships are most welcome, but simply allowing fossils to leave Brazil to be studied by foreign scientists mostly helps science in the other countries.
On the issue of regulation, my own view is an in-between one. An analogy can be made (one which Martill disputes) with wildlife conservation. Many species and natural areas require protection. It is often scientists who are at the forefront of advocating for such protections, even though it will add to the difficulty of doing scientific work on the protected species and habitats. It is true that sometimes such regulations can be over-zealously enforced against scientists (in part because they are so visible and have no economic clout), while ignoring the truly endangering factors. But when scientifically informed and sensibly applied, these protections are welcomed by scientists. And, indeed, Martill describes a formerly good relationship with the Brazilian authorities. I do not know enough about the situation in Brazil to have an informed opinion on whether Brazilian policy and practice on this matter has achieved the right balance to encourage discovery and scientific research, while maintaining proper stewardship of their resources.
Another analogy with wildlife conservation issues is what to do with illegally collected specimens. It is standard for wildlife enforcement agencies to donate such materials to museums or educational institutions (although their value as scientific specimens is lowered by the frequent lack of provenance). More controversial is what to do with seized specimens of commercial, but no scientific, value. For example, some advocate the destruction of seized elephant ivory, while others argue that that only drives up prices, leading to more poaching. In the case of fossils, what scientific value there is in them can be extracted by describing them (again subject to the constraints of knowledge of provenance) and placing them in museums (not, by any means, destroying them!).
Regarding the issue of whether fossils should be exported, I am sympathetic to the need to develop scientific institutions throughout the world, and thus to build local collections and relationships between foreign and local institutions and researchers. This must be tempered by recognition that not all places are in a position to care for important collections or engage in collaboration. In this regard, I would note that Brazil has at least one distinguished student of early snake evolution, Hussam Zaher, and at least some excellent museums, although I do not know enough about the situation with the Crato fossils to have an informed opinion on that specific case. I would point to the recent return of Tiktaalik to the Canadian Museum of Nature after 11 years of study in the U.S., and Costa Rica’s policy of a division of collected specimens among foreign and Costa Rican institutions as policies that seem to be working.
The Brazilian journalist Herton Escobar has conducted an email interview with Martill. In it, Martill’s frustration with being denied further access to his field sites in Brazil is evident. He asserts (correctly, in my opinion) that the scientific value of the fossil is not affected by the legality of its collection (to which, I should add, there is no suggestion that Martill or his colleagues were involved in its collection– he first saw it in Museum Solnhofen during a class field trip); its value is affected by the lack of certain provenance. In response to a question as to whether he had sought a Brazilian collaborator, Martill said he did not. Not seeking a Brazilian collaborator is fair enough– he worked with colleagues in England and Solnhofen, close to him and the fossil, and he had at best difficult personal relationships with Brazilian workers at the time. But Martill also goes off on an odd rant about ethnic and sexual diversity in research groups– not at all what Escobar was asking about.
Haas, G. 1979. On a new snakelike reptile from the Lower Cenomanian of Ein Jabrud, near Jerusalem. Bulletin du Museum national d’Histoire naturelle 4: 51–64.
Haas, G. 1980. Remarks on a new ophiomorph reptile from the lower Cenomanian of Ein Jabrud, Israel. pp. 177–192. In Jacobs, L.L.,
ed., Aspects of Vertebrate History. Museum of Northern Arizona Press, Flagstaff, Arizona.
Martill, D.M., H. Tischlinger, and N.R. Longrich. 2015. A four-legged snake form the Early Cretaceous of Gondwana. Science 349:416-419.
Islands that have never been connected to a continent, often called oceanic islands, must receive their flora and fauna over water, by what Darwin termed “occasional means of transport”. Such means include floating (e.g. coconuts), wind (e.g. spiders), rafting (e.g. iguanas), ice floes (e.g. arctic foxes), and, of course, flying (e.g. birds and bats). Because the ability to disperse is rather unevenly distributed across a continental fauna, the animals of oceanic islands are usually a rather distinctive subset of what is found on the nearest continents. Insular faunas have bats and birds, often lizards and snakes, occasionally mice and rats, but only very rarely amphibians or larger terrestrial mammals.
Another feature of insular faunas is that they are rich in endemics (forms peculiar to the island), because of the rarity of gene flow from the mainland. Unfortunately, these endemic forms, having evolved in isolation with an unusual fauna around them (lacking in predators, for example), often succumb to the environmental changes wrought by man when their islands are discovered and colonized. There have been some cases where island species thought extinct have been rediscovered alive, most famously perhaps the case of the giant lacertid lizards of the Canary Islands. A case from the Revillagigedo Islands in the Mexican Pacific poses an interesting twist on the rediscovery story.
In a paper in PlosOne, Daniel Mulcahy and colleagues from the Instituto de Ecología in Xalapa, Veracruz, México and the U.S. National Museum (including my old friend and mentor George Zug) report the rediscovery of an extant population of the Clarion Island nightsnake (Hypsiglena ochrorhyncha unaocularus). The only previous specimen known had been collected by the intrepid naturalist William Beebe in 1936, and had been scientifically described as an endemic form on the basis of this specimen by Wilmer Tanner in 1944.
What makes this case different from the more usual rediscovery is that in 1955, because no further specimens had been found, Bayard Brattstrom suggested that the original specimen had come from the Mexican mainland, and that the locality data on Beebe’s specimen was in error. Thus the Clarion nightsnake disappeared not into the roll of the extinct, but into the roll of the never existed! So, for nearly 80 years, until 2013, no one had found a Clarion nightsnake, and for most of that time no one thought there even was such a thing.
Mulcahy and colleagues did two things. First, rereading Beebe’s writings about his Clarion expedition, it was clear to them that Beebe had not made an error in labeling where his snake was from– he was very explicit about having found the snake on Clarion, and not the mainland. Second, in 2013 they went to Clarion, and armed with Beebe’s book, they quickly found the right place, and the snakes. They found eleven in all, collected five, and took blood samples and photos of the rest. Thus, knowing the right place and time of day to look, the species proves to be locally common.
Based on their morphological and DNA analyses, Mulcahy and colleagues have raised the Clarion snake from a subspecies to a species, but as we’ve discussed before on WEIT the ranking of divergent allopatric forms is a judgment call, and not really the take home message here. Rather it’s a genuine rediscovery (not a shift in taxonomic rank) of an island endemic, which is potentially threatened by several factors, including introduced animals. Mulcahy and colleagues make several recommendations to help insure the snake’s survival.
Beebe, C.W. 1938. Zaca Venture. Harcourt, Brace, New York.
Brattstrom, B.H. 1955. Notes on the herpetology of the Revillagigedo Islands, Mexico. American Midland Naturalist 54:219-229.
Mulcahy, D.G., J.E. Martínez-Gómez, G. Aguirre-León, J.A. Cervantes-Pasqualli, and G.R. Zug. 2014. Rediscovery of an endemic vertebrate from the remote Islas Revillagigedo in the eastern Pacific Ocean: the Clarión nightsnake lost and found. Plosone 9(5): e97682 (8 pp). pdf
Tanner, W.W. 1944. A taxonomic study of the genus Hypsiglena. Great Basin Naturalist 5: 25–92. pdf
JAC Warning: This stuff is graphic, so if you are a d*g lover you may not want to watch.
by Greg Mayer
A video of a python disgorging a dog on a street in Bangkok is making the rounds, and has been the subject of an article in the Daily Mail.
The python appears to be a reticulated python (Python reticulatus). The video dramatically illustrates the flexibility and movability of snakes’ jaws. In most reptiles, the lower jaw articulates with the quadrate bone (q in the picture below), a firm part of the upper jaw. In snakes, the quadrate is only loosely attached to the skull, and there are other points of mobility in the skull. In the lower jaw, the anterior tooth bearing bone on each side, the dentary (d in the picture below), does not have a bony suture with its contralateral partner (as you do– feel your chin just below your lower incisors)– but only a soft tissue connection which is quite stretchable, allowing the two sides of the lower jaw to be widely separated.
The dog, of course, is quite dead, having been constricted before being swallowed. Constriction compresses the thoracic cavity, and leads to cessation of blood flow, killing the prey even before suffocation occurs. The saliva coating and compression of the dog’s body help it slide out backwards, instead of having the limbs get “stuck” somewhere in the snake’s alimentary canal.
My guess would be that the snake disgorged the prey because it was being harassed or bothered by people in the street. If it had been in the forest, it would have found a quiet nook in which to digest.
The cantil (Agkistrodon bilineatus), or Mexican moccasin, is a pit viper closely related to the water moccasin and copperhead of the United States. Like a number of other snakes, it moves its tail in a manner thought to attract the attention of prey, enticing them to come closer or look away from the snake’s business end, a behavior called caudal luring.
In juvenile cantils, the effect is accentuated by the bright color of the tail tip; this color fades with age.
There are several subspecies, distributed discontinuously from northern Mexico to northern Costa Rica. I’m not sure which subspecies the ones pictured here are.