Chapters 3 - Lizards That Turned Into Snakes
It is therefore quite probable that the ancestor of snakes belonged to some unknown group of early Mesozoic lizards which were not closely related to any of the existing saurian families at all…
Angus Bellairs, 1970
Walking along a sandy beach at the forest edge on the island of Tobago, I caught a glimpse of a dark, fast-moving form. I jumped toward it and my hands landed on a small, slender lizard know to the locals as the “ground puppy” and to science as Bachia sp. Its body and tail were long and covered with scales arranged in neat rings encircling the trunk and tail; it had four legs, but they were tiny, so small in fact they looked all but useless. This lizard appeared to be in the process of losing its legs.
The history of vertebrate animals contains many dramatic transformations of body forms. Most vertebrate animals living today are fishes, a highly successful clade adapted to life in virtually every aquatic habitat. Tetrapods are firmly nested in the sarcopterygian fishes that were present in the Devonian, about 420 MYA and the tetrapod limbs are the result of evolution tinkering with fins. But not all tetrapods have four limbs. Some tetrapods have just two; they may be in the front or in the back. Some limbs have become flippers or wings, and others have limbs that have disappeared. Some tetrapod bodies have greatly increased in size while others have decreased in size. Some have heads that increase in size, others have miniaturized heads. These dramatic changes are examples of large-scale evolution, macroevolution. It is interesting to note that while mutations sometimes produce an individual with more than four limbs, no living tetrapods have evolved more than four limbs, but reduction and modification of limbs are common.
Reduction of legs and feet and loss of complete limbs has happened numerous times in many tetrapod lineages. Today we see reduced limb and no-limbed salamanders, the worm-like caecilians with no limbs, many lizards with reduced or no limbs, flightless birds with no wings, and mammals with reduced limbs, like whales. Figure 3–1 shows the major tetrapod clades and the approximate number of living species in each.
INSERT FIGURE 3-1.
Figure 3–1. A phylogenetic tree showing major tetrapod clades with the approximate numbers of species in each group. The Squamata (lizards and snakes) and the Rhynchocephalia (tuataras) make up the Lepidosauria.
For many years I kept a Three-toed Amphiuma (Amphiuma tridactylum), one of the largest North American salamanders; stretched out she looked like a large salami with tiny legs, and could almost touch both ends of her three-foot long aquarium. Students would bring in worms, crayfish, and fish, and it was not long before I had to put her on a diet. Folds of fat covered her tiny limbs. Modern amphiumas have 50 MY old fossil relatives that had shorter bodies and more well-developed limbs. Salamanders are amphibians and, like many groups of lizards, some show trends towards an elongated body and reduced limbs.
Animals living today are the descendants of animals that lived in the past, and they are carrying clues to their ancestry in their structure. Bones, organs, proteins, and DNA contain clues to the past of all vertebrate animals. Snakes are descended from a lizard, the question is, which one?
Snake skulls follow the basic patterns seen in lizard skulls but most snake’s skulls contain more moveable parts, adaptations for swallowing large prey. Lizards tend toward broad skulls with strong, rigid jaws with the ability to puncture, crush, and tear. Snakes tend to have narrow skulls with short snouts and an elongated temporal region with forward set eyes. While snakes share a rigid braincase with lizards, the snake jaw is much more mobile, flexible, and loosely attached. The upper cheekbone is gone in snakes and this leaves the quadrate bone free to move. The palate and the front skull bones are also loosely set, allowing them to move independently of the braincase. In the lower jaw, the dentary bone carrying teeth at the front can move against the bones at the back of the jaw, and the right and left halves of the lower jaw are held together only by ligaments where they meet at the chin. While snakes with their narrow skulls lack the crushing bite force seen in lizards, and the ability to chew or tear their food, jaw elasticity makes the snake skull all about movement.
No living snake is known to have front limbs or a pectoral girdle. However, Takanobu Tsuihiji and colleagues have found muscles associated with the pectoral girdle in multiple snake families (as well as in lizards that have lost their legs) with the posterior edge of the muscles extending from the third through the eleventh vertebrae in different clades. This supports the view that snakes have a short, virtually absent, neck, and are more appropriately considered to have a greatly elongated thorax.
Interestingly, traces of the pelvic girdle and hind limbs are still present in many snake families. Science first became aware of remnant hind legs in snakes in 1799 when Johann G. Schneider described the structures in the Boa constrictor in his Historie Amphibiorum. Schneider did not recognize the significance of the structures, but a detailed description was later published by German anatomist August Mayer in 1825, and he recognized the anatomy for what it was. The remnant pelvis is not attached to the vertebral column in living snakes, but floats within the rib cage, and it is best developed in the thread snakes (family Leptotyphlopidae). Boas and pythons have a remnant ball-and-socket joint formed between an acetabulum (the socket) on the pelvis and the head (ball) of a greatly reduced femur. A terminal digit lies outside the body in many species and is usually larger and better developed in males; five limb muscles attach to the femur and pelvis and are used by male boas and pythons to vibrate the terminal keratinized digit during courtship. Vibrations of the digit against the female’s body may increase her interest in mating or may be used to assess the male’s fitness. Because the hind limb remnants still serve a purpose in male boids they are not considered vestigial in the true sense, similar remnants in the blind snakes and thread snakes (Typhlopidae and Leptotyphlopidae) are vestigial because they are completely internal and serve no known function [Figure 3–2].
INSERT FIGURE 3-2.
Figure 3–2. Remnant hind limbs in the Texas Worm Snake, Rena [Leptotyphlops] dulcis (top) and in the Burmese Python, Python molurus. Specimens have been cleared and stained.
Lizards and snakes share a Jacobson’s organ (also called the vomeronasal organ or vomeronasal system, VNS) to analyze chemical information obtained from molecules collected on the tongue, and they are transferred to the organ on the roof of the mouth. Information about prey, mates, and predators is obtained using the VNS. Snakes and some lizards have a long, slender, forked tongue associated with the Jacobson organ. The tongue collects chemical information and allows the squamate to follow chemical trails from mates and prey.
Snake eyes have numerous modifications that are often cited as the result of being descended from an ancestor adapted to life underground. Moveable eyelids are absent in snakes. Instead, the cornea is protected by a clear scale, the brill, which is shed with the skin. While lizards usually have three mobile eye-lids (the nictitating membrane, upper lid, and lower lid), burrowing lizards have lost these to varying degrees. Lizards focus their eye with muscles attached to the lens and the muscles alter the focal length to focus the image. Snakes have no muscles attached to the lens and must focus the image by moving the entire lens.
Snake ears differ from lizard ears as well. Most lizards have an eardrum (tympanic membrane); a bony rod (the stapes) connecting to the eardrum; and an inner ear (the cochlea). Snakes lack the tympanum, the stapes and the cavity that holds them, but they retained the cochlea. The notion that snakes cannot hear airborne sounds is widespread but mistaken.
Elongating the snake’s body also required modifications to internal organs from the lizard body plan. The gut is a straight tube, with the intestine the only section that shows coiling. The right lobe of the liver and the right kidney are greatly elongated, while their counterparts are reduced, staggered, and positioned more posteriorly. The testes and ovaries are similarly staggered with the organs on the left being set more posteriorly, and the left oviduct may be missing completely. Some snakes (boas and pythons) have two lungs, but most have a greatly enlarged right lung and a reduced or absent left lung. Increased gas exchange capacity has been attained in some snakes by turning a portion of the trachea (windpipe) into a lung-like surface, and in some aquatic snakes, the lung may extend the entire length of the body. Snakes have a urinary bladder only during embryonic development when the embryo is producing toxic urea. Most of the bladder disappears before birth, and snakes excrete nitrogen waste in the form of the relatively solid, non-toxic uric acid.
Perhaps the single characteristic that links lizards and snakes beyond any reasonable doubt is the morphologically unique penis shared by all squamates. Lizards and snakes store their divided penis in the base of their tail. During courtship, the male erects the divided organ by turning each half inside out but can insert only one of them (right or left) into the female. Snakes and anguimorph lizards have added ornamentation to their hemipenes: spines, hooks, and papillae adorn the surface of these organs, and these have been useful in determining relationships between species. The odd shapes and ornaments may also serve to reduce the chances of hybridization. The ornamentation and odd shapes of the hemipenes also aid the male in retaining his position in the female, increasing the probability of successfully transferring sperm.
Alan Greer surveyed limb loss trends in lizards and found that limb reduction and limb loss have occurred 62 times in 53 lineages. The apparent loss of digits, leg bones, and complete legs has been common during lizard history. Evolution tinkers and bodies change.
Australia is home to about 91 species of slider skinks in the genus Lerista. Sliders are terrestrial and use leaf litter, grassy habitats and burrows in dry environments. The genus is remarkable in that it contains species representing the full range of limb development, from those with four fully developed legs with five digits to no limbs at all.
Recently, Adam Skinner and colleagues looked at the loss of digits and limbs in various clades within this genus and found fingers and toes disappearing throughout the genus, providing evidence for trends toward limbless. They estimated slider skinks arose 13.4 MYA and that hands and feet with five complete digits can be completely lost within 3.6 MY. Slider skinks have undergone at least 10 independent reductions from five digits to fewer, another seven reductions from four digits to fewer, and four times slider skinks lost all of their limbs.
Vertebrae are small repeating bones that protect the spinal cord and support the body; these extend into the tail if one is present. Snakes usually have 110 or more vertebrae in their bodies and another 20 or more in their tail. One species of New Guinea worm snake (Ramphotyphlops depressiceps) has 653 total vertebrae, and another species from New Caledonia and the Solomon Islands (R. angusticeps) has 620 vertebrae. These two snakes probably have more vertebrae than any other tetrapod. Neck or cervical vertebrae lack ribs and usually have a process that extends off the ventral side of the bone. Thoracic vertebrae have ribs. The transition from cervical to thoracic vertebrae occurs at the pectoral girdle (the collarbone and shoulder blades), which is absent in all snakes.
Michael Caldwell examined tetrapod limb reduction and realized once the number of body vertebrae increased to 35–40 the number of digits (fingers and toes) tended to decrease. Notably, Caldwell also found the increase in vertebrae was not gradual; instead, it jumped from 40 to 60–70. Once a species reaches 60–70 vertebrae, limb loss becomes extreme like those of the amphiuma. Ninety or more vertebrae and the front legs are completely gone and the back legs, if present, are greatly reduced. Following Caldwell’s correlation snakes are not expected to have legs.
Vertebrates usually develop from a fertilized egg, and after an egg has picked up the DNA from a sperm, the fused cell undergoes mitosis and cell division to form an embryo. Cells multiply, some genes get turned on, and others may get switched off. Cells migrate around inside the growing embryo, some cells die while others differentiate into various cell types. Embryos grow under the control of those pieces of DNA we call genes.
In the early 1980s fruit fly studies discovered a small group of genes controlling the development of specific regions of the fly’s body. Soon after, similar genes were found in the mouse. Unexpectedly, the same genes controlling wing development in the fly controlled rib development in mammals. Hox genes make molecules that activate other genes. And, astonishingly, Hox genes control development in all animals, even snakes, and humans.
When Martin Cohn and Cheryll Tickle tested python embryos for Hox activity, they found that the HoxC6 and HoxC8 genes were turned on for the length of the body. Front limbs normally develop at the cervical-thoracic junction and correlate to HoxC6 expression. Because the junction has been moved forward in snakes, it has confused the point of front leg development and as a consequence, the front legs do not develop. Early in python development, HoxC6 is not turned on unless HoxC8 is turned on, so the embryo does not receive the signal to make front legs. The combination of HoxC6 and HoxC8 signals the embryo to make vertebrae with ribs. However, the loss (or at least the reduction) of hind limbs are not the result of their failure to grow but rather of arrested growth after they have started to grow to suggest – living snakes still carry the genes to make legs. The hind limb buds start to form in python embryos but they make only a small femur. This arrested growth is due to the lack of a signal from another gene named after the video game and cartoon character, Sonic Hedgehog (or Shh). In mice there is a mutation where the Shh gene is suppressed, producing a structure in the mouse embryo, that looks remarkably like the limb buds seen in pythons. The first pair of ribs often occurs on the third vertebrae in snakes, but ribs occur on the second vertebrae (the axis) in at least some boas, pythons and their relatives, and on the fourth vertebrae in some homalopsid and natricid snakes.
How is it that snakes possess the most vertebrae of any tertapod, and where do they come from? Timing is everything. When a gene is turned on, how long it stays on, and when it is turned off can have dramatic effects on an organism’s appearance. Vertebrae, as well as muscles and other tissues, grow out of paired segments called somites that are generated from the embryonic presomite mesoderm, a growth region that lies posterior to the previously formed somite in the tail region of the embryo. Development starts anteriorly, but the presomatic cells are derived from another growth zone, the tail bud. In other words, cells move through an assembly line process from tail bud to presomitic mesoderm, to somites. Formation of the somites is regulated by a cyclic wave of gene expression that is in turn regulated by a segmentation clock, a "clock-and-wavefront mechanism.” The segmentation clock regulates the process by spreading from tail to head in a wave. The cycle is stopped when it meets a wave of maturation that flows through the presomite mesoderm in the reverse direction, from head to tail. Each cycle produces a pair of somites, and subsequently a vertebra [Figure 3–3].
INSERT FIGURE 3-3.
Figure 3–3. A vertebrate embryo showing the position of the somites and presomite mesoderm. (A) The oldest somite. (B) The most recently made somite. (C) A somite being produced. (D) The horseshoe-shaped presomatic mesoderm (PSM). The bands on the PSM represent the waves-of-clock gene expression. Redrawn from Vonk and Richardson (2008).
Céline Gomez and colleagues compared vertebrate formation in the zebrafish, chicken, mouse, and corn snake embryos and found the somite formation process is halted by the shrinking presomite mesoderm. Their study revealed somite mesoderm develops more slowly in snakes than in short bodied species, but the segmentation clock in snakes ticks faster, four times faster, than in shorter-bodied vertebrates such as chickens. Snakes make smaller, but more numerous vertebrae which then allow the body length to be readily selected for various ecological niches as they become available.
Freek Vonk and Michael Richardson described the clock-and-wavefront mechanism as a 'hotspot' for natural selection, and wrote,
...because changes in vertebral number can greatly affect fitness. The number of vertebrae an animal has can influence such factors as locomotor speed and even fecundity...and may underlie the evolution of some species groups."
But other changes were required in the evolution of the snake. Eyelids and ear openings had to disappear; the jaws needed to become more flexible, and internal organs needed to elongate. These changes were most likely gradual, occurring over time in a single ancestral line of lizards that eventually gave rise to the lineage we now recognize as snakes. Keep in mind these are not rare events; these mutational changes were also taking place in other groups of lizards. For example, some groups of geckos lost their eyelids, others lost their ear openings, another group lost their legs and eyelids, but these species were not becoming snakes, just snake-like. Today, the snake-like geckos compose the Australian-New Guinea flap-footed lizards, the Pygopodidae. Similar processes were going on in skinks and anguid lizards as well. Evolution tinkers.
Lizards evolved toward leglessness in at least three situations. First, some lizards swim through leaf litter or grass, like the North American and Eurasian glass lizards (Ophiosaurus). Swimming through vegetation is a lifestyle that does not require limbs. In fact, limbs may interfere with side-to-side movement as the body pushes itself through the vegetation. And while it seems likely that a long tail aids the lizard in this side-to-side movement, tails may also serve as a site for fat storage, be useful in distracting a predator, or be used for grasping. Limb loss (and loss of digits) is correlated with a longer total length and thus a longer tail in most legless lizards. Having an exceptionally long tail may also increase the probability that a predator will seize the tail, an organ that can be shed quickly, rather than the lizard’s body. The sand swimmers, like the many of the slider skinks discussed earlier, are included here. Sand swimmers do exactly what their name suggests; they swim through sand which acts like a fluid rather than a solid. Sand swimmers are thought to use their relatively long tails actively in swimming, a motion quite distinct from burrowing [Figure 3–4].
INSERT FIGURE 3-4.
Figure 3–4. Top: The Sand-swimming Skink, Plestiodon reynoldsi, with greatly reduced limbs (front legs not visible) from Florida. Bottom: Percival’s Burrowing Skink, Acontias percivali, without limbs from South Africa. The arrows mark the approximate base of the tail.
Lizards may also lose digits and limbs when they become burrowers. The burrowing species have shortened tails, a trend also seen in burrowing snakes. Short tails may be advantageous to burrowing squamates. Short tails save space in a burrow and reduce the chance that a predator will seize it as it descends into its refuge. Perhaps the best known and most extreme burrowers are the worm lizards, the amphisbaenas. About 165 species inhabit loose sandy soils of tropical and warm temperate regions of the world. Worm lizards have squarish scales arranged in rings around their bodies, and each ring corresponds to a vertebrae. They burrow with their head, thrusting it forward to move and compress soil and, while most are limbless, one particularly odd group retains its front legs and hands (Bipes). The others have lost all limbs.
Another legless clade is the unusual and ancient blind snake-lizards (Dibamidae) with short tails, no ear openings, and greatly reduced eyes. They have a remnant pelvis with the upper leg bone (femur) located in the body cavity and fused lower leg bones (fibula and tibia), along with a partial heel bone (tarsus) in the degenerated hind limbs that forms small rear appendages. About 20 species of blind snake-lizards are known, one is found in Mexico and 19 others occur in Southeast Asia. They are so snake-like that even herpetologists may be temporarily fooled into thinking they are snakes. Upon close inspection, however, they have tiny flaps where their hind legs should be.
There is a third situation that favors limb reduction, limb loss, and an elongated body ― an aquatic lifestyle. Relatively few living lizards can be considered semi-aquatic and none are totally aquatic. Marine iguanas (Amblyrhynchus) graze on algae while submerged along the Galapagos Island shorelines but spend much of their time basking on rocks and deposit eggs in terrestrial nests. The common green iguana (Iguana iguana) will not hesitate to jump into the water to escape predation but, like the Amblyrhynchus, lays eggs in terrestrial nests and spends much of its time in trees. Some Southeast Asian and Australian agamid lizards (Physignathus and Hydrosaurus) live a green iguana-like lifestyle along streams and use water as an escape route. Other lizards use water for foraging or escaping predators (Asian water skinks in the genus Tropidophorus; some teiids, like the crocodile lizard, Crocodilurus, and the caiman lizard, Dracaena; monitor lizards, Varanus); but none of these have what could be considered greatly reduced limbs. There are tiny tropical forest microteiid lizards (Neusticrus, Proctoporus), using water and forest leaf litter habitats, with elongated bodies, some limb reduction, and moderately long tails. But it is unclear if the limb reduction resulted from selection for a leaf-litter swimming or an aquatic lifestyle since these lizards are using both of these microhabitats.
A fossilized jaw bone found in 1766 in St. Pieter's Mountain in The Netherlands belonged to a creature no human had seen. In 1780, near the same locality a mostly complete, 1.18 meter skull was excavated from a chalk quarry near the Meuse River. Dr. J. A. Hoffman, a retired military surgeon in the town of Maastricht, purchased the skull from quarry workers. Sometime later, the local minister, the Reverend Godding, claimed ownership of the fossil based on his title to the land above the rock mine and took Hoffman to court. Hoffman lost possession of the skull and was forced to pay court costs; Godding did not hold the fossil for long. Suspecting the invading French army would confiscate the famous fossil, Godding had it hidden. The French army offered a reward of 600 bottles of wine for the fossil and soon confiscated the skull and sent it off to Paris, where it remains in the Natural History Museum to this day. Several 18th-century researchers believed the skull belonged to a crocodile, but Dutch Anatomist Pieter Camper proposed it was a whale. In 1800, the skull’s identity was most accurately noted by Pieter's son, Adriaan Gilles Camper, who demonstrated it came from an animal related to monitor lizards and iguana-like lizards. The Reverend W. D. Conybeare named the skull Mosasaurus, Latin for the “Meuse lizard,” later the species was named in honor of Dr. Hoffman.
Mosasaurs have a skull-to-body ratio of about 1:10, so the 1780 mosasaur skull belonged to an animal about 13 meters long. Another specimen of Mosasaurus hoffmani suggested this species may have reached a length of 18.2 meters, making it the largest known marine reptile, and certainly the largest known lizard.
Fossil collections in the 19th and 20th centuries produced more mosasaurs specimens and today they are known from all continents, including Antarctica. The earliest known specimens were dated about 98 MYA, the most recent were present about 65 MYA. They have been organized into one family containing three subfamilies and about 40 species. Some mosasaurs were specialized for deep water, open-ocean environments and had their limbs modified into paddles. Others were adapted for shallow water coastal environments, with limbs that could be used on land or in the water. Mosasaurs ate large prey, things like sea turtles, belemnites (squid-like animals), and of course fish. So what do mosasaurs have to do with snakes?
Edward Drinker Cope, master naturalist of the 19th century, wrote 1,395 papers and books and named 1,282 genera and species of North American fossil vertebrates during his 39-year career. Cope noticed the mosasaurs’ flexible lower jaw was composed of several different bones, a flexible bone arrangement similar to the one in snakes. Also, mosasaurs, like snakes, have six rows of teeth: one row on each of the lower and upper jaw bones (right and left sides) and two rows on the roof of the mouth (pterygoid teeth). Mosasaurs have a reduced pelvic girdle and an elongated body, as do snakes. Cope considered snakes and mosasaurs closely related and he created a group for them, the Pythonomorpha.
Shared unusual characters, such as the six rows of teeth and the arrangement of the jaw bones for extreme flexibility, continued tooth replacement, socket-set teeth, a reduced number of premaxillary teeth, free-set lower jaw bones, and a reduced number of neck vertebrae are the type of characters that suggest close relationships. When he created the group Pythonomorpha, Cope was suggesting snakes and mosasaurs shared an ancestor. Recent support for the idea of Cope’s Pythonomorpha and a close relationship between mosasaurids and snakes has been supported and disputed.
However, shared characters in mosasaurs and snakes could be the result of convergence as opposed to a shared common ancestor because both have elongated bodies and eat large prey. In the past, dentition and the shape of the braincase and the intra-mandibular joint have been used to link mosasaurs and snakes. Convergent evolution is well documented in plant and animal lineages and squamates are certainly no exception. Convergence results from organisms finding similar solutions to problems because they live in similar environments and share similar lifestyles, not because they share a close ancestor. While one test of relationships supported the snake-mosasaur relationships, another test using many of the same characters suggested the mosasaurs to be the sister to the monitor lizards (Varanidae).
Mosasaurs, along with the monitor lizards, the heloderms (gila monster and beaded lizards) and their fossil relatives are often called platynotans or varanoids. The fossil platynotans are also represented by one or more radiations with small to medium bodies from the Cretaceous. Some are terrestrial, but others were marine and freshwater lizards with elongated bodies and reduced limbs. One or more of them may be the sister(s) to mosasaurs or snakes. There are at least 30 species of Cretaceous marine lizards mostly known from the Cenomanian-Turonian (99.6–89.3 MYA), in the late Cretaceous.
Unfortunately, many of these Cretaceous marine lizards are known on the basis of incomplete fossils and calculating how they are related to snakes or mosasaurs without complete skeletons, soft tissue, and DNA is like trying to construct a jigsaw puzzle with many, many missing pieces. This has not stopped paleontologists from trying. Various authors have considered mosasauroids related to monitor lizards, anguid lizards, and snakes but their relationships to each other and existing squamates is not yet completely resolved.
The fossil marine lizards appear in the Late Cretaceous, the Cenomanian (about 99.6 MYA) and they disappear at the Turonian-Coniacian boundary (about 89.3 MYA). The limited distribution in time is most likely due to the availability of the known fossil record and not an accurate picture of when these lizards lived. Collectively, the marine lizards had a global distribution at a time when sea level was perhaps at its maximum in Earth’s history. Sea level may have been 150 m above present levels and sea surface temperatures of 20–30º C extended far to the north and south (45ºN and 65ºS latitudes) of where they do today.
Some of these marine squamates show pachyostosis, a thickening and compacting of the bone composing the vertebrae and ribs. The increase in the bone’s mass is not the result of an abnormal condition but appears to be an adaptation to an aquatic lifestyle. It is often restricted to the anterior mid-trunk vertebrae suggesting it has a function. Alexandra Houssaye and colleagues found this condition results from greatly reduced bone remodeling. Instead of breaking down old bone and replacing it with new bone, the old bone material accumulates. The thicker, denser bone can then act as ballast which offsets the buoyancy of the lungs, making slow swimming or resting in the water column more energy efficient. Pachyostosis is unknown in living squamates, but it does occur in other marine tetrapods known to have evolved from recent terrestrial ancestors, such as manatees and dugongs.
Dolichosaurs, the now extinct long-necked lizards, probably grew to a meter in length; they had necks with as many as 19 cervical vertebrae, narrow heads, and short limbs. At least some lacked the thickened bone condition seen in some of the other Cretaceous lizards discussed here. Long-necked lizards inhabited shallow, coastal marine environments of the northern and southern Tethys Sea in what is now the Mediterranean, the ancient Mexican Gulf, and the shallow seas of western North America. They had a global distribution, and some were pelagic, inhabited open, deep water possibly hundreds of kilometers from shore. Long-necked lizards may have fed in the crevices and holes of coral reefs similar to many modern-day sea snakes, and they swam with side-to-side lateral undulations as do living snakes. Their small limbs were of little use in swimming but may have been useful for steering and changing direction [Figure 3–5a].
INSERT FIGURE 3-5.
Figure 3–5. Artist’s conceptions for the small to medium sized Late Cretaceous aquatic lizards with elongated bodies and reduced limbs. A. Dolichosaurus longicollis redrawn from Caldwell, 2000. B. Kaganaias hakusanensis redrawn from Evans et al. 2006. C. Adriosaurus suessi redrawn from Lee and Caldwell, 2002. D. Mesoleptos redrawn from Lee and Scanlon, 2002.
The Kaga Water Lizard (Kaganaias hakusanensis) from Hakusan City, Japan was described by Susan Evans and colleagues. It is 40–20 MY older than the other marine species discussed and lived in the early Cretaceous [Figure 3–5b]. The Kaga Water Lizard inhabited continental floodplain swamps that were presumably freshwater and was discovered at a locality that was least 100 kilometers from the sea. Its deep, narrow body, thick ribs, and small hind limbs support the idea that it was aquatic, although Evans and co-workers don’t consider it exclusively aquatic, and suggest it left the water and moved over land. Unfortunately, the species is known from a single specimen that is missing its skull and much of its neck. The authors consider the Kaga Water Lizard a dolichosaur.
Kramberger’s lizards, the aigialosaurs, were semi-aquatic and aquatic. Some had the ability to enter open waters, others had limbs that could probably support their weight on land, but most known forms had relatively small limbs and a total length that was probably less than a meter. Some members of this group have been considered near the base of the mosasauroid lineage. Krambergers’ lizards are known from Eastern Europe, Italy, Israel and Mexico suggesting they too had a global distribution in the late Cretaceous.
The Adriatic lizards, Adriosaurus were small, elongated, highly aquatic marine lizards with a laterally compressed body (straight ribs), a skeleton of thickened bone (pachyostosis) and a flattened tail. The feet and hands were relatively large compared to the rest of the limbs and the digits may have been webbed. Adriosaurus was most likely a slow swimming species of coastal environments, and it had a mouth full of sharp, recurved teeth, suggesting it was a predator. Adriosaurus microbrachis was described in 2007 from the Cenomanian of Slovenia. It is of particular interest because of a greatly reduced pectoral girdle and front limbs which provides insight into its locomotion. Alessandro Palci and Michael Caldwell commented on its movement out of water comparing it to slider skinks and wrote,
When out of the water, it likely locomoted in a similar manner to modern scincids such as Lerista…which display a range of forelimb reduction from absent to fully limbed; these animals are cryptic, often use the large hindlimbs as friction points and locomote using anguilliform undulations of the entire body.
They regard this lizard the sister to the limbed snakes (see below) and all modern serpents [Figure 3–5c].
The thin-bodied lizard, Mesoleptos [Figure 3–5d] is known from three specimens from Hvar Island, Croatia. Mesoleptos was about 1.5 m long with straight ribs, suggesting a laterally compressed body; gracile limbs; 7–9 cervical vertebrae that formed a narrow neck, and presumably supported a narrow head. Michael Lee and John Scanlon considered this marine squamate the sister to the limbed snakes (see below) and modern serpents.
Relationships of these fossil lizards are not well understood; various theories differ widely on how they might be related to snakes. However, the mosasaurids and the aigialosaurids may be sisters, and the dolichosaurids, Adriosaurus, and Mesoleptus all appear related and may be related to the snakes. Moreover, all appear nested within the anguimorph lizards which includes the helodermatids (gila monster, etc.) the anguids (glass lizards and relatives), and the varanids (monitor lizards).
The oldest known fossil snakes come from Spain and Algeria and are known only from vertebrae. The Algerian species was probably terrestrial, while the species from Spain was most likely aquatic. These earliest fossils reveal little about the origin of snakes, other than snakes were living on land and in the water 112–98 MYA. Thus, snakes must have evolved from lizards at an earlier time, possibly in the Jurassic about 166.4 (194–167) MYA as suggested by the DNA clock.
Vertebrae are the most frequently found snake remains, but occasionally complete or near-complete skeletons are discovered. While fossils document some existing snake families, other families of living snakes lack a fossil record, and yet other snake families are known only from the rocks.
George Haas, professor of zoology at The Hebrew University of Jerusalem, had the habit of buying fossils from quarry workers. In the late 1940’s, Haas purchased a fossil at the home of a man who worked in the Ein Yabrud quarries, about 12 miles north of Jerusalem. In 1979, Haas described the fossil as a “snakelike reptile” naming it Pachyrhachis problematicus. The genus (Pachyrhachis) refers to the thick bones the animal had; the second part of the name refers to the fact that Haas really did not know how to classify the fossil because it had some characters he considered not found in snakes, like back legs. However, its teeth were very similar to those of a snake. Haas obtained two specimens of this animal. Both were mostly complete skeletons and one had the remains of a marine fish in it body, suggesting the snake with legs was spending at least some time in the ocean hunting food and that it was digesting a meal when it died.
Twenty years later, Michael Caldwell and Michael Lee took a second look at Haas’ problem reptile with the thick ribs and realized that even though it had hind limbs and feet it was, in fact, a snake. Further investigation showed that it had a small head and a body that was compressed side to side. Therefore, a cross-section of its body would be shaped more like a book than a cylinder―a body shape similar to those Cretaceous marine lizards and modern-day marine snakes. The limestone quarries at Ein Yabrud are rich in vertebrate fossils, most of them representing animals from a marine environment that were associated with a coral reef ecosystem in the Late Cretaceous.
The head and neck of Haas’ marine snake have a light, slender build and a body form suggesting it was a slow swimmer. The neck vertebrae have a projection off the bottom that likely served as an attachment for muscles used in rapid underwater strikes, possibly similar to that used by the modern day snakes that hang in the water column and wait for fish to swim within striking distance. Alternatively, the fossil hind limb snake may have fed in crevices of coral reefs as do some modern sea snakes. If this were true, the hind legs could have been used to push against the walls of the crevice, giving the snake some support as it struggled with its prey.
The small but well developed hind limbs may have served other purposes, such as courtship or mating. The thickened, heavy ribs in Haas’ problem snake are similar to those found in the Late Cretaceous marine lizards previously discussed, and probably assisted in obtaining neutral buoyancy in the water.
At the turn of the 21st century, two more fossil snakes with hind legs were described; one was found in the same quarry as Pachyrhachis. It was named Haasiophis terrasanctus in honor of George Haas and the “Holy land” (terrasanctus). Figure 3–6 shows the hind limb bones in this snake. The third limbed snake came from Lebanon and was named Eupodophis descouensi.
INSERT FIGURE 3-6.
Figure 3–6. The hind limb of the snake Haasiophis terrasanctus. A. femur. B. tibia. C. fibula. D. metatarsals. Courtesy, The Field Museum, DigitalID GEO86483c.
The discovery of fossil snakes with hind legs began a lively discussion with two opposing hypotheses. Michael Lee and colleagues viewed the small and large Cretaceous marine lizards as the closest relatives to snakes. Harry Greene and David Cundall proposed a different one. They argued that the similarities between snakes and mosasaurs are the result of convergence, citing the lack of pterygoid teeth in the most ancient living snakes, as well as some fossil species as evidence. When Eitan Tchernov and colleagues described Haasiophis terrasanctus, they proposed that their new species and Pachyrhachis problematicus were not transitional between lizards and snakes, but were nested in the large-gape macrostomatan snakes. Macrostomatan snakes are able to expand their mouths to swallow large prey; the name is derived from the Greek macro meaning large and stomata meaning mouth [Figure 3–7]. The tree, presented by Tchernov, showed the limbed snakes as the sister to the pythons, and they wrote, “…the presence of well-developed hindlimbs optimizes unequivocally as a reversal…” meaning snakes reversed evolution and re-evolved hind legs.
INSERT FIGURE 3-7.
Figure 3–7. A Puff-faced Water Snake (Homalopsis bucatta) displaying its gape. Macrostomate snakes are large gape species capable of opening their mouth about 180º. Most species of living snakes are large gape species, capable of opening their mouths to this extent.
Hussam Zaher and Olivier Rieppel laid out two alternative hypotheses that could explain the limbed snakes. First, the limbed snakes are primitive and the large gape evolved twice, once in the primitive snakes with hindlimbs and again in the modern snakes. The second possibility is that a large gape evolved very early in snakes and that some groups of snakes living today have lost their large gape when they adapted to a burrowing lifestyle. A snake with a large, bulky prey item in its digestive system does not fit well in a burrow. Zaher and Rieppel make the point that if the second hypothesis is correct, it means that several groups of snakes independently lost their legs or that the legs reappeared in the limbed fossil species as Tchernov and colleagues suggest.
Jean-Claude Rage and François Escuillé reviewed the Cenomanian snakes and found that while three species were known to have legs, five others appeared closely related and they, too, probably had hind limbs. All of these limbed snakes were from the middle to late Cretaceous and were all geographically restricted to Western Europe, the Middle East, and North Africa.
Much has been written and said about the relationship of Haas’ problem snake and the other limbed snakes, but in 2005 Michael J. Polycn and co-workers used Computed Tomography (CT) scanning to look at the skull of Pachyrhachis. CT scanning uses x-ray images and computer technology to build two and three-dimensional views of objects that cannot be seen using traditional optical instruments such as light microscopes. The results suggested that Haas’s problem snake was most similar to the large gape snakes. The CT images of this snake are available for all to see at the University of Texas’ Digital Morphology Website. It seems likely the marine fossil snakes with legs may be advanced; more similar to modern snakes with the ability to consume large prey and that Pachyrhachis maybe not particularly useful in understanding snake origins.
Fossils are frequently discovered with novel anatomy. In 2006, a snake with robust hind legs (femur and tibia) and a sacrum (a modified rib that forms a support between the vertebral column and the pelvis and helps support the body on land) was described by Sebastián Apesteguía and Hussam Zaher. This snake came from a terrestrial deposit, presumably lived on land, and possibly used a burrow. It was also from the early Late Cretaceous, and named Najash rionegrina. Najash is from Hebrew and means legged biblical snake, and rionegrina refers to Rio Negro Province of Argentina where it was found. Najash appears to be the most primitive snake found to date. It is known only from pieces, but it is definitely a snake. In a follow-up paper, Zaher and co-workers describe Najash in greater detail and considered it the sister to all living snakes [Figure 3-8]. The right and left mandibles of the lower jaw appear to have a well- developed connection and the quadrate bone is low, resembling that found in living pipe snakes. With this morphology, Najash cannot be considered a macrostomatan; it was not able to dramatically gape its mouth to swallow large prey.
INSERT FIGURE 3-8.
Figure 3–8. A phylogenetic tree showing the positions of snakes with limb remnants. Snakes shown in large type have external limbs. Clades marked with * have remnant limbs, fossil snakes with limbs are in larger typeface and marked with an L. After Tchernov et al., (2000) and Apesteguía and Zaher (2006).
There have been three basic scenarios proposed for lizards turning into snakes. In 1923, Charles Camp suggested snakes evolved from a grassland anguimorph lizard. Today the anguimorph lizards remain potential candidates for the snake ancestor and there are living grass-swimming anguimorphs. Anguimorphs and snakes both share the paired male reproductive organs with ornamentation.
However, Beni Charan Mahendra suggested that the snake ancestor was a burrowing lizard, a hypothesis that later gained wide support and remains the most well-accepted idea regarding the snake’s ancestral lifestyle. In 1938, Mahendra noted that the eye in most advanced snakes was surrounded by a ring of scales. Burrowing forms like the worm snakes and blind snakes (Leptotyphlopidae and Typhlopidae) and the shield-tailed snakes (Uropeltidae), and the Amazonian pipe snake (Anilius) have the eye covered by a large single scale. Because he felt that all of these snakes were primitive, or basal to more advanced snakes, he placed them at the base of the evolutionary tree and concluded that the ancestral snake must have been a burrower. Today, most of these snakes are considered to be old lineages, particularly the burrowing dawn snakes, thread snakes and blind snakes, the scolecophidians.
Wider acceptance of the burrowing ancestor hypothesis was due to Gordon Walls’ work on snake eye anatomy. In a list of differences between snake and lizard eyes, Walls makes it clear that snake eye anatomy is quite distinct from lizard eye anatomy. Perhaps the most dramatic difference between the lizard and snake eyes is the inability of snakes to change the shape of their lenses in order to focus an image onto the retina. Muscles attached to the lens for focusing in lizards are absent in snakes; therefore, snakes must move the lens towards or away from the retina to focus using the enlarged peripheral iris muscles. Also, most snakes lack a fovea, although some Asian tree snakes have re-evolved the structure. Walls also explained the cone-like structure of rods in snake retinas (and those of other tetrapods) as resulting from the loss of rods in response to diurnal activity and then secondarily redeveloping rods from cones. He thought cone cells were transformed into rod cells, and this became known as the transmutation hypothesis. The modifications Walls found in snake eye anatomy suggested to him they were the result of a burrowing lifestyle. He wrote,
"…consistent with the hypothesis that the early snakes were subterranean and their eyes underwent wholesale degeneration into a condition fairly well represented by the modern Amphisbaenidae, from which they have recovered by a remarkable evolutionary ‘come back.’ It was during a long period of Amphisbaena-like existence that the ophidian type finally crystallized, later to blossom into a ‘respectable’ group in which wholesale substitutions were necessitated in order to restore the eye to decent usefulness and support an active, above-ground, and diurnal habit."
Edward D. Cope suggested snakes had an aquatic ancestor when he placed snakes and mosasaurs in Pythonomorpha, a hypothesis that has been championed by others and maintains its viability. An aquatic ancestor has been most recently been suggested by Michael Lee, Michael Caldwell, and others who have worked on the Cretaceous marine lizards. Evidence for an aquatic origin of snakes cannot be easily dismissed. Consider the fossil aquatic lizards with long bodies and reduced limbs from the Upper Cretaceous; the fossil aquatic snakes with legs; the ability of living snakes to exchange gases through their skin (an ability also found in some other tetrapods); and the aquatic and semi-aquatic snake species that occur in most major lineages of snakes. All are traits that connect snakes and water.
Christopher Caprette and colleagues, analyzed snake eye anatomy, comparing eye structure in a wide variety of vertebrates. Caprette and colleagues placed coded vertebrate eye data in a character matrix, the animals were divided into aquatic, amphibious, terrestrial, and fossorial categories. Computer anaylsis associated most snakes with the aquatic vertebrate group. The burrowing worm lizards and blind snake-lizards associated with burrowing vertebrates. Lizards associated with terrestrial vertebrates. Of interest, snakes (both modern snakes and the basal thread snakes) also associated with the worm-like caecilians which have both burrowing and aquatic lifestyles. The authors write,
None of the characters supporting the fish-caecilian-snake cluster are shared with exclusively burrowing taxa. In contrast, three characters are shared with exclusively aquatic or amphibious taxa: the flattened cornea, thickened corneal margin, and spherical lens.
In a study of visual pigment in two snakes thought to represent ancient lineages – the Sunbeam Snake (Xenopeltis unicolor) and the Ball Python (Python regius) – Wayne Davies and co-workers reported both species express two cone pigments (opsins), one sensitive to ultraviolet short-wave lengths and one sensitive to long wavelengths, suggesting snakes could have dichromatic color vision. However, they also found these snakes have rod photoreceptors with the usual rod pigment (Rh1). Walls’ transmutation hypothesis is not supported by these two lower snake taxa, but some caenophidian snakes such as garter snakes (Thamnophis) which have no rods, but four distinct types of cones, do support Walls’ hypothesis.
Richard Shine and Michael Wall examined body plans in limbless lizards and found they evolved for burrowing or surface activity, and that each has a distinctive morphology. Their analysis produced two eco-morphs: (1) burrowing lizards with elongated trunks, small heads, short tails and constant body widths; and (2) surface active legless species with short trunks, wider heads, longer tails, and more variable body widths.
They suggested that a long tail in surface dwellers is useful for escaping predators if the tail can be autonomized and that long tails are probably useful for locomotion when the animal is using lateral undulations (side to side movement) to move. Burrowing lizards, on the other hand, need to be able to push their way through the substrate, and this requires a different arrangement of muscles and bones in which the rib cage and trunk muscles form a rigid, but flexible, cylinder. This alternate morphology transfers force to the digging head, and a long tapering tail is not well suited for this life style.
Shine and Wall compared tail lengths as a percent of body length for burrowing lizards, litter-swimming lizards, surface active lizards, and snakes and found snakes tend to have shorter tails than many burrowing lizards. In regard to the aquatic origins of snakes, Shine and Wall note living semi-aquatic lizards have well developed limbs and long tails and that the tails are important in swimming and, therefore, are unlikely to be lost. Many living lizard species are burrowing, and by implication, Shine and Wall infer that snakes were statistically more likely to have evolved from a burrowing ancestor as opposed to an aquatic one.
Elongation of the trunk in burrowing species pre-adapts them to ingesting larger meals. In the ancestral snake this means not a single large prey, but multiple smaller prey. Life in a burrow does not favor a bulging body. Life in a burrow may result in the degeneration of eyes and ear openings that would collect debris. Early snakes or their lizard ancestor may have relied more on chemical sensing and used the VNS and then re-evolved eyes when they returned to the surface, as suggested by Gordon Walls. Burrowing squamates would then be expected to have a small, solid head, rigid rib cage similar to the living blind snakes and thread snakes (Typhlopidae and Leptotyphlopidae), and take numerous small prey (as do the blind snakes and thread snakes).
Shine and Wall proposed the following scenario for the origin of snakes: A burrowing lizard underwent limb reduction. It fed upon social insects (like termites) in nests where the ancestral snake could collect many food items at one time. A shift to larger prey was accompanied by ambush-hunting behavior and a low metabolism. Yet another shift to feeding on large, elongated prey such as earthworms would require some modifications, but earthworms are soft and easily compressed into a small digestive system. Selection for feeding on larger prey with hard parts likely included the evolution of venom and constriction behavior to handle prey that could retaliate. Flexible heads and jaws followed to allow the ancestral line to feed on yet larger prey as they returned to the surface.
Given that all of the scolecophidians [Figure 3–9a] are highly specialized for burrowing and considered the most basal snakes, it is not a difficult mental leap to conclude that the ancestral snake was a burrower. The analysis of Shine and Wall favors of a burrowing ancestor, and it seems probable the lizard ancestor to the snakes was indeed burrowing. But not necessarily to the degree seen in the worm snakes and blind snakes, they are extreme burrowers and highly specialized for feeding on ants and termites.
INSERT FIGURE 3-9.
Figure 3–9. Top: A worm snake in the genus Epictia [formerly Leptotyphlops] sp. from Chapala, Mexico. Bottom: The Asian Pipe Snake, Cylindrophis ruffus from southern Thailand in a defensive posture. Both of these snakes use burrows and represent old lineages that may provide some clues as to how the ancestral snake lived.
According to the Vidal et al. timetree, lizards last shared an ancestor with snakes 166.4 MYA. Other authors using DNA estimated this ancestor to have lived 194–145 MYA. Using the Timetree dates, the scolecophidians (thread snakes, blind snakes, and dawn snakes) shared ancestor with all other snakes 155.6–151.9 MYA. These dates of divergence are relatively close (10–15 MY), but other studies have them more widely separated. Andrew Hugall and co-workers placed the snake-Anguimorph divergence at about 155 MYA and the divergence of the scolecophidians from the rest of the snakes at about 105 MYA. Kate Sanders and Michael Lee place the anguimorph-snake divergence date at about 140 MYA and the divergence of the scolecophidians from the rest of the snakes at about 110 MYA. Given their highly specialized anatomy and the later divergence date from the rest of the snakes, it seems improbable that the scolecophidians were in the direct line of evolution for more advanced snakes. Therefore, they probably don’t make a good model for a snake ancestor. [Note that DNA clock dates for the divergence of various clades of snakes are summarized in Appendix 3. Appendix 4 provides common names and distributions for these snake clades.]
Shallow, turbid water with a muddy substrate is a habitat often overlooked by herpetologists, but, it is one exploited by some living snakes. Many of the Asian mud snakes (family Homalopsidae) use this environment, and, while they are near the base of the advanced snakes (Colubroidae and Elapoidea); they are not remotely close to an ancestral snake in morphology. More ancient living snakes using this habitat are the Asian pipe snakes (genus Cylindrophis) [Figure 3-9b], and the Amazonian pipe snake (Anilius scytale). The DNA suggests these lineages date to 102–78 MYA, still not close to the 194–145 MYA dates suggested by the DNA clock.
The Amazonian pipe snakes (Anilius), the Asian Pipe snakes (Anomochilidae and Cylindrophis) and the shield-tailed snakes (Uropeltidae) have long been considered small gape, Althenophidians snakes. But, the molecular work of Vidal and Hedges, and David Gower and colleagues suggests all of these snakes are, in fact, nested in the Macrostomata. They evolved from large-gape ancestors and lost the ability to feed on large prey when they returned to a burrowing existence. Nicolas Vidal and colleagues placed the Amazonian Pipe Snake (Aniliidae) with their New World sister, the Tropidophiidae, in the novel Amerophidia clade. Amerophidia now contains the small-gape Amazonian Pipe Snake and the large-gape wood snakes (Tropidophiidae), which are also sometimes known as dwarf boas. The validity of this clade has been challenged by Felipe Grazziotin and colleagues, who suggest conflicting molecular results are from the relictual nature of these snake families that have few surviving species and very diverse living members.
The Amazonian and Asian Pipe Snakes may make far better models for an early snake than the worm snakes and blind snakes. Bill Lamar reported that Anilius does not constrict its caecilian prey; instead, it bites hard. Alan Savitzky wrote,
Thus, at least some primitive living snakes (and perhaps also extinct ones) do not constrict, and apparently rely on a simple snapping of the jaws to subdue prey. Such a “snap-stun” approach seems feasible in snakes such as Anilius, in which the skull is relatively rigid (as the result either of its primitive nature, its fossorial adaptations, or both)…
Examine the literature on Asian pipe snakes (Cylindrophis), and the authors describe them as fossorial, living in burrows and leaf litter. However, they are quite aquatic. Daryl Karns , Harold Voris and I collected Asian Pipe Snakes (Cylindrophis ruffus) in funnel traps that were half-submerged in water and baited with dead fish, and in gill nets used by fishermen. We once put a radio transmitter in a large southern Thailand pipe snake, released it at the site of its capture, and checked its location and body temperature several times a day for eight days. It stayed within a few square meters, but underwater. It most likely was using a burrow, but it was a burrow covered by shallow water.
An alternative scenario for the lizard-to-snake transition is an ancestral lizard that was aquatic and burrowing [Figure 3-10]. A lizard that used burrows partially or completely covered by water, with an elongated body and reduced limbs may make a better snake ancestor. Such a lizard could burrow through soft mud or use a burrow previously constructed by a crustacean or other animal. Such a lizard would be expected to have reduced eyes from living in turbid water and burrows, and it would have a short tail. Shallow aquatic habitats are often very productive and contain high concentrations of small invertebrates, small fish, and larval amphibians that would allow an aquatic, burrowing predator to stay at or near the mouth of a burrow and feed. Or, the aquatic burrowing lizard could have been a scavenger, feeding on carrion washed up along shorelines. An aquatic-burrowing lizard would not have to undergo the radical specializations of a solid cranium and rigid trunk for burrowing on land and then re-evolve a more flexible skull for living on the surface. An aquatic-burrowing lifestyle may also explain why snake eye anatomy grouped with aquatic vertebrates rather than fossorial vertebrates in the Caprette study.
INSERT FIGURE 3-10.
Figure 3–10. A hypothetical lizard that inhabits burrows in shallow, turbid water, locates prey with the VNS, feeds on small prey and scavenges carrion, has a tail that becomes shorter as it specializes for using burrows, and evolves reduced eyes, and ear openings. Illustration by Judy Erwin.
Two fossil snakes, Najash rionegrina with hind limbs and Dinilysia patagonica, which is not known to have hind limbs, are both from South America’s Cretaceous and have been shown by by Michael Caldwell and Jorge Calvo to share a large number of common skull characteristics. They suggest these two snakes may form an ancient clade of Gondwanan snakes and, further note that Najash is probably not linked to the scolecophidians. Thomas Frazzetta described Dinilysia as semi-aquatic, with a lifestyle similar to the modern anacondas.
Burrowing and aquatic are not mutually exclusive lifestyles for snakes. A burrowing and aquatic, elongated, reduced leg or legless, short-tailed lizard may make an excellent model for the ancestral serpent. Unfortunately, none of these are in existence today and the fossil evidence is still missing, giving herpetologists plenty of latitude for wild speculation.
Just to return to Angus Bellairs’ prophetic statement at the start of this chapter, Mesozoic lizards of an unknown family most likely gave rise to snakes, but the evidence now points to a family of anguimorphs, some of which may have had an aquatic, burrowing lifestyle.
