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)
A commenter on my post about the four-legged snake, Tetrapodophis, quoted a line from Genesis about the snake in the Garden of Eden going on its belly in retribution for bamboozling Eve; I thought (and still think) it was a poe. But apparently there are real people who think this fossil demonstrates the truth of the Adam and Eve story. An alert reader sends a link to “New Fossil Discovery Provides Massive PROOF of Bible’s Story of Adam and Eve” in the “Conservative Tribune: In Defense of Western Civilization”, and there’s a link in that piece to a more extensive story at a site called “shoebat.com“. (The latter reminded me of a similar word, except the syllables are reversed, and the two letters “oe” need to be replaced; adding the word “crazy” is also clarifying.) I didn’t want to soil my cursor by clicking around at these sites, but some of the most horrifying sponsored content and ads I think I’ve ever seen (hint to Nissan: if you want to maintain your corporate image, you may want to exercise a bit more discretion about the company your ads are keeping), ALL CAPS FOR IMPORTANT WORDS, and clauses lacking needed parts of speech, all point to the sort of earnest incompetence that marks sincerity, so I don’t think they are poes.
It’s really amazing– how you get from a four-legged snake (that went on its belly!) to confirming the existence of Adam and Eve is incomprehensible. The one site even says this proves that Moses and Joshua knew snakes once had legs! (It reminds me a bit of claims that the Koran contains all or much of modern science, just expressed cryptically– see FvF, p. 105.) The Tribune gets a little modest at the end, admitting “This discovery may not prove the entire biblical account of Adam and Eve outright”. You can say that again.
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.