Snake Pelvic Spurs

"Modern snakes have lost all hindlimb elements except for the femur.  Some alethinophidians (pythons and boids) have a single claw-like element of uncertain homology distal to the femur, used in courtship." — Michael Lee and Michael Caldwell, Philosophical Transactions of the Royal Society B, 1998¹

"Python embryos initiate formation of hindlimb buds, but leg development is not sustained, resulting in formation of a rudimentary femur and terminal claw." — Francisca Leal and Martin Cohn, Current Biology, 2016²

Summary

The parent article, Vestigial Structures, separates two claims: that a structure is useless, and that it is a reduced copy of something larger in an ancestor.  This article asks both of the pair of claws beside a python's vent, and finds that the first claim is not made by anyone who has dissected one, and that the second is conceded by nearly every creationist who has written on it.

• The spurs are hind limbs in miniature. Boas and pythons carry on each side of the vent a horny claw over a short femur, hung from a pelvic rod buried in the body wall. Lee and Caldwell's 1998 monograph on the fossil snake Pachyrhachis states the living condition: "Modern snakes have lost all hindlimb elements except for the femur. Some alethinophidians (pythons and boids) have a single claw-like element of uncertain homology distal to the femur, used in courtship."¹ Palci and colleagues' 2020 review of every snake pelvis known finds that the ancestor of living snakes "most probably retained all three pelvic elements and rudimentary hindlimbs (femoral spurs)," that the one rod left in blind snakes is the ischium and the one rod left in boas and pythons is the pubis.³ Over 85 percent of living snake species have no limb skeleton at all.⁴
• They work, and the anatomists have said so all along. The spurs "have musculature, innervation, and vascularisation and can move autonomously," and "are known to be used by males during combat, courtship, and mating."⁵ Male Bahamian boas carry spurs "nearly twice as long and notably more curved than females."⁶ Palci and colleagues read the spur, femur and pubis as "related functionally to the widespread use of the hindlimbs in mating and courtship, rather than the usual representation of the bones as non-functional vestiges."³ The useless-organ claim is not in the specialist literature.
• The embryo starts a leg and stops. Python embryos form a hind-limb bud with a real ridge and a real signalling centre; Sonic hedgehog switches on in a few cells and switches off within a day, the ridge decays, and cartilage condensations that "could be anlagen of the femur, tibia and fibula, and digital plate" form and then dissolve.² The cause is in the limb enhancer of Sonic hedgehog: three deletions shared by pythons and boas,² and a 17-base deletion, found in all six snakes Kvon and colleagues sequenced,⁴ that "is the only sequence that is deleted in all snakes but present in all examined limbed vertebrates and fish."⁴ A mouse given the python enhancer grows truncated limbs; put the 17 bases back and "the resulting knock-in mice with the modified python allele had normal limbs."⁴
• The fossils give legged snakes, not a lineage. Najash from Patagonia had "a sacrum supporting a pelvic girdle and robust, functional legs outside the ribcage";⁷ Pachyrhachis, Haasiophis and Eupodophis from the Cretaceous Tethys had femur, tibia, fibula and tarsals but "lack a sacral region."⁷ Their place on the tree has been disputed since 1997. The "four-legged snake" Tetrapodophis is probably a lizard.⁸
• The creationist literature concedes the reduction. Answers in Genesis in 2009: "creationists have generally understood this to mean that the original serpentine created kind had legs," and "The leglessness of snakes today represents a corruption, loss, or deactivation of that information."⁹ This answers the parent article's open question: for the snake pelvis, creationists do treat the structure as a reduced homologue of a leg. What they deny is that the legged ancestor was a lizard, and what they add is Genesis 3:14 as the occasion of the loss. The replies that list functions answer a claim the primary literature does not make.
• The verdict. The spurs pass all three steps of the live vestige test. The creationist position that fits the evidence, a legged snake kind whose legs were switched off, has to carry one enhancer deletion shared by boas, pythons and the advanced snakes, an enhancer that has piled up extra changes only in the snakes with no limb bones left, and legged fossils whose limbs are well formed rather than ruined.

Part 1: What the spurs are

Darwin listed them in 1859 among his rudimentary organs: in some snakes, he wrote, "there are rudiments of the pelvis and hind limbs."¹⁰

On either side of the cloaca of a boa or python sits a horny, curved claw.  Under the claw is a bone.  Lee and Caldwell, describing the modern condition as the baseline for their fossil, put it in one sentence: "Modern snakes have lost all hindlimb elements except for the femur.  Some alethinophidians (pythons and boids) have a single claw-like element of uncertain homology distal to the femur, used in courtship."¹  Leal and Cohn's 2016 embryology paper gives the adult result of python development as "a rudimentary femur and terminal claw."²  Romer's Osteology of the Reptiles says which snakes keep the thigh bone: the femur "is generally absent in snakes, but a vestige may persist in typical boids, leptotyphlopids, and Anilius."¹¹

Inside, the femur hangs from a pelvis.  Which bone of the pelvis is the question Palci, Hutchinson, Caldwell, Smith and Lee set out to settle in 2020, in a review of pelvic and hind-limb elements across living and fossil snakes.  Their abstract gives the result.  The most recent common ancestor of all living snakes "most probably retained all three pelvic elements and rudimentary hindlimbs (femoral spurs)."  Since then "there have been multiple losses of ossified pelvic and hindlimb elements and regaining of ossified pelvic elements."  And the two great branches of living snakes have reduced the pelvis by different routes: "The single remaining rod-like element in some scolecophidians is the ischium, whereas the single remaining rod-like element in many basal alethinophidians is the pubis."³  The scolecophidians are the blind snakes and thread snakes; the basal alethinophidians include the pipe snakes, boas and pythons.

What the pelvis has lost is its connection to the spine.  Lee and Caldwell list it as a character of modern snakes: "In modern snakes, the sacral contact is lost and the pelvic rudiments, when present, lie within the ribcage."¹  A python's pelvic rods are free bones in the muscle of the body wall, behind the last ribs, with a femur and claw on the end.  The whale's pelvic rod sits the same way.

How many snakes carry any of this?  Kvon and colleagues, choosing six genomes to span the range, describe the two ends: "basal snakes (boa and python) that retained a vestigial pelvic girdle and rudimentary hindlimbs," and "advanced snakes (viper, rattlesnake, king cobra, and corn snake) that completely lost all skeletal limb structures and represent the majority (>85%) of all extant snake species."⁴  Philip Bell of Creation Ministries International gives the same picture from the creationist side: "some types of snake (e.g. wormsnakes, pipesnakes, boas and pythons) do have a small pelvic girdle and tiny hind limbs.  However, the majority of today's snakes (1,600 species of the colubrid family) lack a pelvis."¹²

The spurs are not the same size in the two sexes.  Anzai and colleagues summarise the literature back to Hoge in 1947: "These pelvic spurs are generally larger in males than in females."⁵  Hoefer, Robinson and Pinou measured it in 32 Bahamian boas in 2021: "mean pelvic spur length in males being 2.6 mm (n = 16; range: 0.5 – 5.0 mm) compared to the 1.5 mm in females (n = 16; range: 0.5 – 3.0 mm)," and against body length "the proportion of spur length relative to SVL was 0.3% in males and 0.1% in females."⁶

Part 2: What they do

The spurs are moved by the snake.  Anzai and colleagues, writing in The Herpetological Bulletin in 2023, summarise sixty years of anatomy: "pelvic spurs have musculature, innervation, and vascularisation and can move autonomously," citing Hoge 1947, Bellairs 1950 and Carpenter and colleagues 1978 and 1979.  And they summarise the behaviour: "The pelvic spurs of boids and pythonids are known to be used by males during combat, courtship, and mating."⁵

Anzai and colleagues watched captive Boa constrictor at two Brazilian breeding facilities, six males and four females in 2019 and five males and three females in 2021.  "the males began to rub their spurs gently on the side of the females' bodies, moving in a posterior to anterior direction," and "The spurs repeatedly moved from a resting to an erect state."⁵  They read the movement as a stimulus that helps the female shift into position for cloacal contact, and add a reason it might matter: "Spur use may be particularly relevant for smaller males because they may not have enough strength to move the body of large females."⁵  Three of the females later gave birth to 51 young, and the authors set that against a 1983 report of failed births after matings with little spur use: "Our data would appear to reinforce the critical role of pelvic spurs for successful courtship and mating in red-tailed boas."⁵  That is a suggestion from two facilities, not a controlled test, and the authors' "would appear" is the right strength.

Pythons use the spurs harder.  Trivedi and colleagues watched a male Indian rock python with three larger females at a Gujarat zoo on 2 February 2024, and describe the male working the female "not with gentle rubbing but through more forceful poking actions."  "This behaviour, reminiscent of finger-walking but executed with the spurs, serves to provoke a reaction from the female, likely due to discomfort or pain caused by the spurs' sharpness."¹³  Their reading of why is the same as Anzai's: "it is plausible that this behaviour has evolved as a male strategy to navigate the challenges of mating with larger females."¹³

Senter, Harris and Kent put 33 courtship and combat behaviours onto a snake phylogeny in 2014.  Two of the 33 are spur behaviours.  The spur rub "is widespread enough in Boidae to infer that it is ancestral for the family," so that "courtship in Boidae included the spur rub by the Paleocene, the date of the earliest known crown-group boid."¹⁴  The spur poke in combat "is recorded only in the sister genera Eunectes and Epicrates," the anacondas and the West Indian boas.¹⁴  Their Boidae is the broad one that includes the pythons.

The function is real, sex-linked and old, and it is the kind the vestige argument allows.  The parent article shows that the modern argument was built on functional vestiges from the start.  Here the point is sharper than for the whale, because the people who make the homology claim are the same people who describe the use.  Lee and Caldwell's sentence calling the claw a hind-limb element ends "used in courtship."¹  Palci and colleagues, having traced the pelvis bone by bone from the oldest fossils to living forms, close their abstract with the function: "many basal alethinophidians share a distinctive configuration of cloacal spur (claw), femur and a sizeable pubis, which is likely to be related functionally to the widespread use of the hindlimbs in mating and courtship, rather than the usual representation of the bones as non-functional vestiges."³  The anatomists never held the popular view.

Part 3: The embryo starts a leg, and the enhancer that stops it

Cohn and Tickle opened this question in Nature in 1999.  In python embryos, "Hindlimb buds are initiated, but apical-ridge and polarizing-region signalling pathways that are normally required for limb development are not activated.  Leg bud outgrowth and signalling by Sonic hedgehog in pythons can be rescued by application of fibroblast growth factor or by recombination with chick apical ridge."¹⁵

Leal and Cohn went back to earlier embryos in 2016 and found that the python bud is closer to a leg than that.  Sonic hedgehog does switch on: "SHH mRNA was detected in a small domain of cells at the posterior margin of the hindlimb buds, but this ZPA-like expression pattern is transient, disappearing within 24 hr of oviposition."²  The ridge is there too: "A morphological AER is evident in python hindlimb buds at stage 1," and then "Shortly after termination of SHH, the FGF8 domain begins to degrade."²  Yet the distal genes keep going, and the bud lays down cartilage templates: the SOX9 domains in the python bud "could be anlagen of the femur, tibia and fibula, and digital plate," and "they are transitory structures, as distal hindlimb elements are not found in mature pythons."²  A python embryo sketches a shin and a foot and erases them.

The reason is a stretch of DNA about a million bases from the gene it controls.  Leal and Cohn describe the ZRS as "a limb-specific enhancer located ∼1 Mb upstream of SHH, in intron 5 of the LMBR1 gene."²  In pythons it carries three deletions, ΔA, ΔB and ΔC, each of which "occurred in HOXD13 binding sites,"² and the same three are in boas: "Our identification of the same three mutations in the ZRS of pythons and boas indicates that ΔA, ΔB, and ΔC were present in their last common ancestor."²

Kvon and colleagues, in Cell the same month, tested the enhancer in mice.  They compared six snake genomes with limbed vertebrates and fish, and found the boa and python ZRS still about 80 percent identical to a lizard's and changing at the ordinary rate for this enhancer, while "the ZRS of advanced snakes displays a substantially increased number of substitutions compared to other enhancers."⁴  Then they replaced the mouse enhancer with the snake versions.  The cobra sequence gave "a complete loss of Shh expression and a truncated limb phenotype, affecting both the fore- and hindlimbs, that is indistinguishable from the phenotype caused by deletion of the mouse enhancer."⁴  "The less diverged python ZRS resulted in a similar but a slightly milder phenotype."⁴  One change stood out: a 17-base deletion in a region conserved from fish to mammals, and "this deletion is the only sequence that is deleted in all snakes but present in all examined limbed vertebrates and fish."⁴  It removes a binding site for the transcription factor ETS1, and "all examined snakes have lost the E0 and E1 ETS motifs."⁴  Putting the 17 bases back into the python enhancer was enough: "the resulting knock-in mice with the modified python allele had normal limbs."⁴

So the boa and python enhancer is a damaged limb enhancer, not a different kind of element, and the damage is graded: light in the snakes that keep a femur, heavy in the snakes that keep nothing.

Not every limb enhancer decayed.  Infante and colleagues found in 2015 that "snake genomes retain many sequences orthologous to mammalian limb enhancers,"¹⁶ and showed why for one of them: the Tbx4 enhancer HLEB works in both hind limbs and genitals, and in snakes it has "functionally diverged along such a path with snake HLEBs losing hindlimb function but retaining activity within subdomains of the external genitalia."¹⁶  That is the finding Fazale Rana cites as shared design.  It does not extend to the ZRS, which is limb-specific.

Part 4: The fossil snakes with legs

Five fossils carry the argument, and their describers disagree about what they mean.

Pachyrhachis problematicus, from Cenomanian marine limestone near Ramallah, was reinterpreted by Caldwell and Lee in 1997 as "a primitive snake with a well developed pelvis and hindlimbs," which "retains a well developed sacrum, pelvis and hindlimb (femur, tibia, fibula, tarsals)," and which they placed as "the sister-taxon of all other snakes."¹⁷  Their 1998 monograph gives the pelvis: "The three elements of the pelvic girdle are not fused or sutured together.  Rather, they appear to have abutted each other, forming a very weak girdle,"¹ but the ilium "closely overlies the distal end of the sacral rib, indicating that the ilium was external to the ribcage and suggesting the presence of a sacral contact."¹  Below the femur, "Pachyrhachis is more primitive than modern snakes in retaining a normal tibia, fibula, astragalus and calcaneum."¹

Haasiophis terrasanctus, from the same beds, was described by Tchernov, Rieppel, Zaher, Polcyn and Jacobs in Science in 2000 as "the most extreme hindlimb development of any known member of that group, as it preserves the tibia, fibula, tarsals, metatarsals, and phalanges."  And they drew the opposite tree: their analysis "shows both to be related to macrostomatans, a group that includes relatively advanced snakes such as pythons, boas, and colubroids to the exclusion of more primitive snakes such as blindsnakes and pipesnakes."¹⁸  On that reading their legs are either retained or regrown.  Leal and Cohn's model takes the second option seriously, proposing that "Re-emergence of the tibia and fibula (zeugopod) and digits (autopod) in Tethyan snakes may have resulted from persistence of embryonic skeletal condensations into adulthood,"² the same condensations a python embryo forms and erases.

Eupodophis descouensi, from Lebanon, was scanned by synchrotron in 2011.  Houssaye and colleagues report that the scan "allowed a virtual exhumation of the second, hidden leg of the specimen," that "four tarsals are observed in each limb, but there are no traces of metatarsals or phalanges," and that the limb bones' inner structure "displays a microanatomical organization similar to that of extant terrestrial lizards.  This suggests that limb regression in Eupodophis was not due to a qualitative alteration of growth but, more likely, to a local decrease in growth rate or shortening of growth duration."¹⁹

Najash rionegrina, from the Upper Cretaceous of Patagonia, is the one with a hip that works.  Apesteguía and Zaher in 2006: "Here we report a fossil snake with a sacrum supporting a pelvic girdle and robust, functional legs outside the ribcage."  The three marine snakes "lack a sacral region"; Najash "is the most primitive (basal) snake known."⁷  New skulls and skeletons scanned by Garberoglio and colleagues in 2019 "provide robust support for an extensive basal radiation of early snakes with hindlimbs and pelves, demonstrating that this intermediate morphology was not merely a transient phase between limbed and limbless body plans."²⁰

Tetrapodophis amplectus was announced in Science in 2015 as a snake with four limbs, whose limbs "were adapted for grasping, either to seize prey or as claspers during mating."²¹  Caldwell and colleagues examined the specimen and its counterpart slab and in 2021 reported "new evidence refuting it as a snake"; four analyses "unambiguously recover Tetrapodophis as a dolichosaur," a marine lizard.⁸  Creationist writers were cautious about it from the start, and were right to be.

What the record supports, then, is this.  There were snakes with a sacrum, a hip outside the ribs and working legs (Najash); snakes with a weak, unfused hip still touching a sacral rib and a full lower leg (Pachyrhachis, Haasiophis); living snakes with free pelvic rods, a femur and a claw (boas, pythons); and living snakes with nothing.  Each stage keeps less of the same parts; it is a series, not a lineage.  Apesteguía and Zaher themselves warn that the old picture of "progressive loss of their limbs by gradual diminution of their use" has given way to "a more complex scenario of limb reduction, still poorly documented in the fossil record,"⁷ Palci and colleagues count "multiple losses" and "regaining" of pelvic bones,³ and the Tethyan snakes may be side branches whose legs are secondary.  The Jurassic snakes described in 2015 are known from fragments, and as Philip Bell noted for CMI, "No trace of limbs or even limb girdles (pelvis or pectoral) were found in the four 'oldest' fossils."²²  Fragments prove nothing either way.

Part 5: Does it pass the vestige test?

The parent article's live definition has three steps: homology with a fuller structure elsewhere, reduction, and a distribution that fits a tree built from other characters.

Step 1 is carried by the bones, the embryo and the enhancer together.  The pelvic rods are ischium or pubis by position and by their history in the fossils;³ the bone under the claw is a femur, and in Pachyrhachis a "normal tibia, fibula, astragalus and calcaneum" sit below it, bones that carry lizard names because they are lizard bones.¹  The python bud forms an apical ridge, a polarizing region, Sonic hedgehog, FGF8 and HOXD13 in the places a lizard's does, and condenses cartilage for a femur, tibia, fibula and digital plate.²  The limb enhancer is the lizard's at 80 percent, and drives a mouse leg when 17 bases are restored.⁴  Step 2, reduction, is conceded by every source on both sides; the creationist literature uses the words "tiny," "small" and "reduced" throughout.  Step 3 is the tree.  The legged snakes are Cretaceous; the sacrum-bearing Najash comes out basal on the analyses that include it;²⁰ the enhancer deletion is in every snake and no limbed animal;⁴ and the three python deletions are in boas too.²  The parts of step 3 that are contested, the placement of the Tethyan snakes and whether legs re-emerged, do not touch the spurs; they concern which fossil is nearest the root.

A creationist must say one of two things instead.  Either the spur is not a limb at all, which no creationist source found says; or it is a limb, reduced within the snake kind from a created snake that had larger ones, which nearly all of them say.  Part 6 states that position from its own sources and prices it.

Part 6: The creationist literature, in its own words

The text under everything is Genesis 3:14: "upon thy belly shalt thou go, and dust shalt thou eat all the days of thy life."²³  Answers in Genesis reads it with care: "While the Bible does not explicitly state that the serpent had legs, the divine curse implies a transformation from a previous condition, leading some to infer it might have had legs or a different form of movement before the fall," and whether it had legs "remains a point of theological speculation."²⁴  Sarfati and Batten's Creation Answers Book entry says only that "Genesis 3:14 suggests that snakes maybe once had legs."²⁵

The creationist position on the spurs has been consistent for twenty years, and it is a reduction position.  Sarfati in 2008, on Eupodophis: "We don't know exactly what God did to the serpent, but one possibility is that He turned off the genetic information to make the legs, hips and other features necessary to walk."²⁶  Answers in Genesis in 2009: "creationists have generally understood this to mean that the original serpentine created kind had legs.  Even if God 'muted' the genetic information for legs in snakes (so that none could truly walk), a mutation could cause some of the latent genetic information to be expressed in the form of an errant limb."⁹  Bell in 2009: "it is possible that some snakes had legs to begin with."¹²  Elizabeth Mitchell in 2016, on the 17-base deletion: "it is quite possible the 17 absent base pairs apparently responsible for the snake's signature slithery nature departed the snake genome sometime after snakes were created," and the ZRS papers "may have given us a peek into genetic events through which God may have implemented the Curse He spoke upon the serpent in Genesis 3:14, though we cannot be dogmatic about this."²⁷  Cserhati in 2019: "A possible creationist explanation is that God cursed the snake right after the Fall in such a way as to turn off certain genetic elements in its genome, which coded for limbs.  As to which species of snake this might have been, we don't know."²⁸  Brian Thomas at ICR: "For all we know, He may have made some snake kinds with no legs and others with the genetic ability to turn leg production on or off."²⁹

This answers the parent article's open question.  For the snake pelvis, the creationist literature does not answer the function claim and stop; it grants that snakes are descended from legged snakes, and that the spur and pelvic rods are what is left.  What it denies is the step before: "Snakes and lizards are different created kinds of animals, not biological cousins" (Mitchell),²⁷ "God specially created snakes—they did not evolve from other kinds of creatures" (Bell).¹²

The function replies sit beside this and answer a different claim.  Answers in Genesis in 2008: "these so-called 'legs,' which evolutionists admit were too short in this specimen to be used for ambulation, may have been used in copulation.  Perhaps no snakes ever 'walked' in any sense."³⁰  Troy Lacey in 2019: creationists "recognize the design feature of an anchoring pelvis and possible stimulators for reproductive structures."³¹  Rana, for the old-earth Reasons to Believe: "Snakes use their hind limbs—called anal spurs—to grasp during mating.  So, in that sense, these structures are functional and could be viewed as an archetypical design deployed by the Creator."³²  All of this is true and none of it is in dispute; Lee and Caldwell and Palci and colleagues say the same.  "Perhaps no snakes ever 'walked' in any sense" is the one claim here that the fossils contradict, since Najash had a sacrum and "robust, functional legs outside the ribcage."⁷

The position that fits, a created legged snake kind whose legs were switched off, has three costs.

First, the switch-off is one event at the base of most living snakes.  The 17-base deletion is in all six snakes Kvon and colleagues sequenced, the boa, the python, the king cobra, a rattlesnake, a viper and the corn snake,⁴ and the three HOXD13-site deletions are shared by boas and pythons.  All six are alethinophidians.  No blind snake, thread snake or pipe snake enhancer was sequenced in either 2016 paper, so whether the deletion goes back to the root of all snakes is an inference, not a measurement.  Leal and Cohn draw it from the bones, and mark it as predicted:  "Absence of a tibia, fibula, foot, and toes in scolecophidians (e.g., blind snakes) and alethinophidians suggests ZRS hypofunctionalization in the common ancestor."²  Blind snakes and thread snakes keep pelvic rods and, in the thread snakes, a femur.¹¹  On the sequenced snakes alone, the "serpentine created kind" that had legs has to take in the boas, pythons, vipers, cobras and colubrids, which together make up most of the some 4,000 living species, and the Curse has to fall on their one ancestor.  Cserhati's "we don't know" which snake is honest, and Thomas's several kinds with legs on or off would need the same deletion to have happened in each.

Second, the cost sits in the deletion and in the advanced snakes, not in the 80 percent figure.  The boa and python ZRS differs from a lizard's at about a fifth of its sites, but Kvon and colleagues found that it "shows a substitution rate similar to other vertebrate ZRS orthologs."⁴  On the boa and python branch, the enhancer has changed no faster than it has in other vertebrates.  A creationist who holds snakes and lizards to be separate created kinds can call that difference created, as for any other gene, and it counts against that view no more than ordinary snake-lizard differences do.  What stands out is narrower.  The 17-base gap falls in a stretch conserved from fish to mammals and removes an ETS binding site,⁴ in every snake sequenced.  And in the snakes that kept no limb bones at all, the ZRS "displays a substantially increased number of substitutions compared to other enhancers,"⁴ the pattern of a sequence that use no longer holds in check.  For scale only: the functional core Kvon and colleagues swapped into mice is 1.3 kilobases, and 2,000 generations of mutation at a mammalian per-site rate of one in a hundred million would be expected to change about 0.03 of its sites.  No model claims that a few thousand years produced the advanced snakes' extra changes, and none should.  A young-earth reading must therefore hold that the extra changes were created too, and that the Curse, if it acted here, was at most the 17-base deletion.  Mitchell's alternative, that snakes were "simply designed without those 17 base pairs in ZRS,"²⁷ gives up the Curse as a mechanism and leaves the parent article's question: why a created limbless animal carries a limb enhancer that works in a mouse when 17 bases are restored.

Third, the fossils do not look like the re-expression model's prediction.  Sarfati's mechanism for legged fossil snakes is a mutation that "turned some of this information back on, but incompletely," ruined by accumulated damage, so that "the only result is tiny deformed legs too small to walk on."²⁶  The legs that have been examined are small but not deformed: Eupodophis's bone microanatomy is that of a terrestrial lizard, grown for a shorter time;¹⁹ Pachyrhachis has a normal tibia, fibula, astragalus and calcaneum;¹ Haasiophis has a complete foot;¹⁸ Najash has a sacrum and legs its describers call functional.  Small, well-built legs are what reduced growth gives, not what a damaged program gives.

Objections and responses

Creationist: "The spurs have a function, so they are not vestigial."  The parent article answers this in general.  Here the sources answer it themselves: the 1998 sentence that calls the claw a hind-limb element ends "used in courtship," and the 2020 review reads the configuration as functional.  Function was never the claim.

Creationist: "Losing legs is devolution, not evolution."  Agreed, and the parent article grants it.  The vestige argument is about ancestry, not about gaining information.  The question is what the legged ancestor was.

Creationist: "Snakes and lizards are different kinds; a snake leg is not a lizard leg."  Then the created snake carried a lizard's three pelvic bones, a lizard's femur, tibia, fibula, astragalus and calcaneum, a lizard's limb-bud program, and a limb enhancer four-fifths the same as a lizard's.  Common design can be said of any of these, but it explains why the enhancer is present, not why it is broken in the same 17 bases in every snake sequenced.

Creationist: "Genesis 3:14 explains the loss."  The text says the serpent will go on its belly.  Answers in Genesis itself calls legs a "point of theological speculation."  Read as a mechanism, the Curse has to act once on the ancestor of every sequenced snake family, has to leave the rest of the boa and python enhancer changing at its ordinary rate, and has to be followed by legged fossils that Sarfati explains by re-expression and Thomas by created variation.

Skeptic: "Tetrapodophis shows a snake with four legs, so the series is complete."  It probably shows a dolichosaur.  Caldwell and colleagues re-examined the specimen and its counterpart and found "new evidence refuting it as a snake."  The creationist writers who doubted it in 2015 were right, and no part of this article rests on it.

Skeptic: "The fossils give a smooth sequence from legs to none."  They give stages, not a sequence.  The Tethyan snakes with the fullest legs have skulls that Tchernov and colleagues place among advanced snakes, so their legs may be retained or regrown; Palci and colleagues count losses and regainings of pelvic bones; the oldest snakes preserve no limb material.  The spurs are the strongest point on that tree because they are in the living animal.

Skeptic: "The enhancer's decay proves deep time on its own."  It shows that the divergence is not a few thousand years of mutation, which no young-earth writer claims.  A created difference is a design claim, and this article prices it rather than refutes it.

Issues and unanswered questions

• Palci and colleagues' 2020 paper was read only in abstract; its family-by-family table of pelvic elements is paywalled.
• The classic behavioural papers, Carpenter and colleagues 1978, Gillingham and Chambers 1982 and Slip and Shine 1988, are on JSTOR only and were not opened, and Cohn and Tickle 1999 was read in abstract only; their content is here through Anzai 2023, Trivedi 2024, Senter 2014 and Leal and Cohn 2016.
• Senter and colleagues' reconstruction makes the spur rub ancestral for Boidae in the broad sense. Whether pipe snakes or Anilius use their spurs, which would push the behaviour to the base of the alethinophidians, was not found in a primary.
• Kvon and colleagues and Leal and Cohn sequenced the ZRS only in boas, pythons and advanced snakes. Whether blind snakes, thread snakes and pipe snakes carry the 17-base deletion was not found in any primary; a ZRS sequence from a scolecophidian would test whether the deletion reaches the root of all snakes.
• No creationist source prices the shared 17-base deletion, the fast change of the advanced snakes' enhancer, or the well-formed legs of the fossils against the Curse model. Whether a baraminology study places all snakes in one created kind, which the shared deletion requires, was not checked.
• The relationship of the Tethyan legged snakes to the rest is an open mainstream dispute; its two answers are about 95 million years apart from a Flood year either way.

Verdict

The pelvic spurs of boas and pythons are working structures: a mobile claw on a femur, larger in males, used to court and to fight.  They are also a hind limb and a hip with the walking parts gone: the femur has lost its shin and foot, the pelvis has lost its joint with the spine, the embryo builds the missing parts and dissolves them, and the enhancer that should sustain the leg is broken in the same 17 bases in every snake sequenced.  Fossil snakes with a sacrum and working legs existed.  The creationist literature, unusually, agrees that snakes came from legged snakes and lost them.  What remains is whether the legged snake was made or descended, and on that the spurs bear through the enhancer they share with lizards and the leg bones their fossils share with lizards.  That evidence favours descent; the honest cost of the alternative is a created snake built with a lizard's limb program and a limb enhancer missing the same 17 bases across boas, pythons and advanced snakes.

Sources