Basal Snakes Research Articles

Southernmost lepidosaur (Reptilia) assemblage from the Late Cretaceous of Patagonia

ABSTRACT The aim of the present contribution is to describe new materials of lepidosaur reptiles coming from the late Cretaceous (Maastrichtian) Chorrillo Formation, at Santa Cruz province, Argentina. The lepidosaur assemblage is composed by four different snakes (belonging to basal snakes, madtsoiids, and ‘anilioids’) and a tuatara sphenodont. The latter is a new genus and species represented by an incomplete maxilla that shows strong ties to extant Sphenodon. The snakes are represented by isolated vertebrae that indicate they belong to basal forms. Both are very apomorphic and in all probability are the representatives of poorly known lineages. In contrast with recent claims, the fact that most members of this lepidosaur assemblage are highly apomorphic may be indicative of some biogeographical isolation from other Cretaceous lepidosaur associations reported from northern Patagonia.

Convergent Evolution of Elongate Forms in Craniates and of Locomotion in Elongate Squamate Reptiles.

Synopsis Elongate, snake- or eel-like, body forms have evolved convergently many times in most major lineages of vertebrates. Despite studies of various clades with elongate species, we still lack an understanding of their evolutionary dynamics and distribution on the vertebrate tree of life. We also do not know whether this convergence in body form coincides with convergence at other biological levels. Here, we present the first craniate-wide analysis of how many times elongate body forms have evolved, as well as rates of its evolution and reversion to a non-elongate form. We then focus on five convergently elongate squamate species and test if they converged in vertebral number and shape, as well as their locomotor performance and kinematics. We compared each elongate species to closely related quadrupedal species and determined whether the direction of vertebral or locomotor change matched in each case. The five lineages examined are obscure species from remote locations, providing a valuable glimpse into their biology. They are the skink lizards Brachymeles lukbani, Lerista praepedita, and Isopachys anguinoides, the basal squamate Dibamus novaeguineae, and the basal snake Malayotyphlops cf. ruficaudus. Our results support convergence among these species in the number of trunk and caudal vertebrae, but not vertebral shape. We also find that the elongate species are relatively slower than their limbed counterparts and move with lower frequency and higher amplitude body undulations, with the exception of Isopachys. This is among the first evidence of locomotor convergence across distantly related, elongate species.

Flap-Footed Lizards (Gekkota: Pygopodidae) Have Forelimbs, Albeit During Embryonic Development

Limb reduction among squamate clades is common; limbless (forelimbs, hindlimbs, or both) forms have had at least 25 independent origins among snakes and lizards. The general evolutionary pattern is the gradual loss of limb elements proceeding from the toes to the shoulder or hip. In contrast, forelimbs are not known to have ever existed in snakes. Forelimbs are not known for either stem or crown snakes; snake fossils lack forelimbs and the embryos of modern snakes lack the signaling pathways that initiate forelimb development. Both fossil and developmental data thus imply an abrupt loss of forelimbs. In contrast, the well-developed hindlimbs of the oldest fossil snakes and vestigial hindlimbs of some basal clades of modern snakes imply the historical presence of hindlimbs, and their gradual reduction during snake evolution. In contrast, observations on limb-reduced lizards are in accord with gradual reduction of both fore- and hindlimbs; embryos of even limbless species initiate limb development. The question then arises, have any clades of limbless lizards become limbless abruptly, as have snakes? To address this question, I examined limb development in the family Pygopodidae. Like basal snakes, pygopodids are characterized by the absence of forelimbs and by vestigial hindlimbs. My observations on the development of forelimb buds in Delma molleri support gradual limb reduction, as do observations on other limbless lizards. Nonetheless, many clades of limbless lizards are yet to be studied and hence provide a wealth of opportunities to address modes of limb reduction and loss in squamate reptiles.

A new specimen with skull and vertebrae of Najash rionegrina (Lepidosauria: Ophidia) from the early Late Cretaceous of Patagonia

The limbed snake Najash rionegrina from the Cenomanian (early Late Cretaceous) of the La Buitrera Palaeontological Area (LBPA), northern Patagonia is a key taxon in any study of the origin and early evolution of snakes. The original concept of the taxon was based on the holotype and a number of referred specimens including an isolated partial skull; a conservative rebuttal argued this concept was too broad due to the lack of association between the holotype elements and the referred specimens. Here we describe a new snake specimen consisting of a partial skull and closely associated vertebrae from the La Buitrera Locality, one of many productive snake localities within the LBPA. The analysis of the vertebrae of the new specimen identifies distinct features shared with the vertebrae of the N. rionegrina holotype; the partial skull of the new specimen is also identical to the partial skull that formed part of the original concept of N. rionegrina. Phylogenetic analysis, using data from the type and referred specimens, and new materials described here, reconstructs N. rionegrina as a basal snake in a ‘madtsoiid’ clade outside of crown-group Serpentes. These new morphologies, and autapomorphies recognized in character analyses, permit the construction of a new and expanded diagnosis of N. rionegrina.

Palaeoecological inferences for the fossil Australian snakes Yurlunggur and Wonambi (Serpentes, Madtsoiidae).

Madtsoiids are among the most basal snakes, with a fossil record dating back to the Upper Cretaceous (Cenomanian). Most representatives went extinct by the end of the Eocene, but some survived in Australia until the Late Cenozoic. Yurlunggur and Wonambi are two of these late forms, and also the best-known madtsoiids to date. A better understanding of the anatomy and palaeoecology of these taxa may shed light on the evolution and extinction of this poorly known group of snakes and on early snake evolution in general. A digital endocast of the inner ear of Yurlunggur was compared to those of 81 species of snakes and lizards with known ecological preferences using three-dimensional geometric morphometrics. The inner ear of Yurlunggur most closely resembles both that of certain semiaquatic snakes and that of some semifossorial snakes. Other cranial and postcranial features of this snake support the semifossorial interpretation. While the digital endocast of the inner ear of Wonambi is too incomplete to be included in a geometric morphometrics study, its preserved morphology is very different from that of Yurlunggur and suggests a more generalist ecology. Osteology, palaeoclimatic data and the palaeobiogeographic distribution of these two snakes are all consistent with these inferred ecological differences.

Developmental, genetic, and genomic insights into the evolutionary loss of limbs in snakes.

The evolution of snakes involved dramatic modifications to the ancestral lizard body plan. Limb loss and elongation of the trunk are hallmarks of snakes, although convergent evolution of limb-reduced and trunk-elongated forms occurred multiple times in snake-like lizards. Advanced snakes are completely limbless, but intermediate and basal snakes have retained rudiments of hindlimbs and pelvic girdles. Moreover, the snake fossil record indicates that complete legs were re-acquired at least once, suggesting that the potential for limb development was retained in some limb-reduced taxa. Recent work has shown that python embryos initiate development of a transitory distal leg skeleton, including a footplate, and that the limb-specific enhancer of the Sonic hedgehog gene, known as the zone of polarizing activity regulatory sequence (ZRS), underwent gradual degeneration during snake evolution. In this article, we review historical and recent investigations into squamate limblessness, and we discuss how new genomic and functional genetic experiments have improved our understanding of the evolution of limblessness in snakes. Finally, we explore the idea that pleiotropy of cis-regulatory elements may illuminate the convergent genetic changes that occurred in snake-like lizards, and we discuss a number of challenges that remain to be addressed in future studies.

A new basal snake from the mid-Cretaceous of Morocco

Fossil snakes are relatively well represented in the Upper Cretaceous of northern Africa, with material known from Morocco, Sudan, Egypt, Libya, Algeria, and Niger. The Moroccan Kem Kem beds have yielded a particularly diverse snake assemblage, with Simoliophiidae, Madtsoiidae, ?Nigerophiidae and several unnamed taxa co-occurring. These fossils are important for our understanding of the early evolutionary history of snakes, and may shed light on the ecology and initial diversification of basal snakes. We describe a new taxon, Norisophis begaa gen. et sp. nov., from the Kem Kem beds of Begaa, in southeast Morocco. It is characterised by a marked interzygapophyseal constriction, parazygantral foramina, an incipient prezygapophyseal process, and an anterio-posteriorly short centrum. Several characteristics shared with Najash, Seismophis, Madtsoiidae, and Coniophis suggest that Norisophis is a stem ophidian. N. begaa further increases the diversity and disparity of snakes within the Kem Kem beds, supporting the hypothesis that Africa was a mid-Cretaceous hotspot for snake diversity.

Postnatal ontogeny and the evolution of macrostomy in snakes.

Macrostomy is the anatomical feature present in macrostomatan snakes that permits the ingestion of entire prey with high cross-sectional area. It depends on several anatomical traits in the skeleton and soft tissues, of which the elongation of gnathic complex and backward rotation of the quadrate represent crucial skeletal requirements. Here, the relevance of postnatal development of these skull structures and their relationship with macrohabitat and diet are explored. Contrary to the condition present in lizards and basal snakes that occupy underground macrohabitats, elements of the gnathic complex of most macrostomatan snakes that exploit surface macrohabitats display conspicuous elongation during postnatal growth, relative to the rest of the skull, as well as further backward rotation of the quadrate bone. Remarkably, several clades of small cryptozoic macrostomatans reverse these postnatal transformations and return to a diet based on prey with low cross-sectional area such as annelids, insects or elongated vertebrates, thus resembling the condition present in underground basal snakes. Dietary ontogenetic shift observed in most macrostomatan snakes is directly linked with this ontogenetic trajectory, indicating that this shift is acquired progressively as the gnathic complex elongates and the quadrate rotates backward during postnatal ontogeny. The numerous independent events of reversion in the gnathic complex and prey type choice observed in underground macrostomatans and the presence of skeletal requirements for macrostomy in extinct non-macrostomatan species reinforce the possibility that basal snakes represent underground survivors of clades that had the skeletal requirements for macrostomy. Taken together, the data presented here suggest that macrostomy has been shaped during multiple episodes of occupation of underground and surface macrohabitats throughout the evolution of snakes.

Postovipositional development of the sand snake Psammophis sibilans (Serpentes:Lamprophiidae) in comparison with other snake species

AbstractCharacterizing and comparing developmental progress across different species helps to interpret how different or similar body forms evolved. We present an embryonic table for the oviparous African Sand Snake Psammophis sibilans from the Lamprophidae family, describing its postovipositional in ovo development. Psammophis is a good model of a genus that is widely distributed in Africa and Asia and includes 22 species. We describe ten embryonic stages based on the development of externally visible morphological characteristics such as; pharyngeal arches, facial processes, eyes, scales, body pigmentation and body colour pattern development. This study discusses the development of this snake and compares it with that of the closely related brown house snake Lamprophis fulliginosus (Lamprophidae) and the medically important venomous cobras Naja haje haje and Naja kaouthia from the sister lineage Elapidae. The distantly related basal snake Python sebae, which displays different morphology and behaviour, was chosen for deeper insight into the evolution of body structures within the snake clade. We found interspecific differences in the relative stage of development of embryonic structures at the time of oviposition and during postovipositional embryonic development. One of the outcomes of this study is that embryonic structures such as the pharyngeal processes, eye pigmentation and scales are interspecifically conservative in regard to timing of morphodifferentiation, while body pigmentation, colour and colour pattern are interspecifically plastic in their temporospatial development.

A blindsnake that decapitates its termite prey

AbstractAlmost all snakes swallow their prey whole. However, a few species are known as exceptions. Two species ofAsian crab‐eating snakes tear off crab legs and ingest them one at a time to eat prey that is otherwise too large. Two species of leptotyphlopid blindsnakes break off the head of termites or suck abdominal contents of termites, discarding remains of termites. Here, we show that a typhlopid blindsnakeIndotyphlops braminusdecapitates its termite prey and consumes only the thorax and abdomen. In our feeding trials,I. braminusdecapitated a median of 47% of termites they eat, while swallowing the remaining termites whole. Decapitation did not affect ingestion speed, and therefore, it is unlikely that decapitation assists for fast ingestion. Ingested termite heads often remain undigested in the feces, implying that the decapitation functions to remove an indigestible part of termites. Decapitation might also be related to circumvention of chemical defenses of termites because termite heads often contain toxic compounds. These observations showed unusual feeding behavior used by a basal snake, which could be associated with removal of indigestible and/or toxic parts of prey items.

Emended diagnosis and phylogenetic relationships of the Upper Cretaceous fossil snake Najash rionegrina Apesteguía and Zaher, 2006

ABSTRACT—The fossil snake Najash rionegrina, from the Cenomanian–Turonian (Upper Cretaceous) of Argentina, is reinterpreted after examination of the type and referred material. The current diagnosis is emended in the light of important considerations that cast doubt on the attribution of type and referred specimens (a braincase, a quadrate, and two dentary/lower jaw fragments) used to systematize this taxon. Alternative interpretations of the anatomy of the sacrum and hind limbs are proposed. Following the reevaluation of the anatomy of the type specimen and the removal from this taxon of the above-mentioned referred material, the phylogenetic position of N. rionegrina was tested in a series of maximum parsimony analyses that included all groups of extant snakes, all best-known fossil snakes (i.e., Pachyrhachis, Haasiophis, Eupodophis, Madtsoiidae, and Dinilysia), and alternative outgroups. Regardless of the outgroup used to polarize the character-state transformations, our phylogenetic analyses found no support for the hypothesis that Najash rionegrina occupies a position as the most basal snake. Depending on the outgroup, Najash is placed (1) in a position basal to all living snakes, but more derived than other fossil forms (most notably Pachyrhachis, Eupodophis, and Haasiophis); or (2) as the most basal representative of a clade of fossil snakes that is the sister group of living snakes; or (3) as the most basal representative of a clade of fossil snakes that is located between the Scolecophidia and the Alethinophidia. SUPPLEMENTAL DATA—Supplemental materials are available for this article for free at www.tandfonline.com/UJVP

Predation upon hatchling dinosaurs by a new snake from the late Cretaceous of India.

Author SummarySnakes first appear in the fossil record towards the end of the dinosaur era, approximately 98 million years ago. Snake fossils from that time are fragmentary, usually consisting of parts of the backbone. Relatively complete snake fossils preserving skulls and occasionally hindlimbs are quite rare and have only been found in marine sediments in Afro-Arabia and Europe or in terrestrial sediments in South America. Early snake phylogeny remains controversial, in part because of the paucity of early fossils. We describe a new 3.5-m-long snake from the Late Cretaceous of western India that is preserved in an extraordinary setting—within a sauropod dinosaur nest, coiled around an egg and adjacent the remains of a ca. 0.5-m-long hatchling. Other snake-egg associations at the same site suggest that the new snake frequented nesting grounds and preyed on hatchling sauropods. We named this new snake Sanajeh indicus because of its provenance and its somewhat limited oral gape. Sanajeh broadens the geographical distribution of early snakes and helps resolve their phylogenetic affinities. We conclude that large body size and jaw mobility afforded some early snakes a greater diversity of prey items than previously suspected.

Dissociation of somatic growth from segmentation drives gigantism in snakes

Body size is significantly correlated with number of vertebrae (pleomerism) in multiple vertebrate lineages, indicating that change in number of body segments produced during somitogenesis is an important factor in evolutionary change in body size, but the role of segmentation in the evolution of extreme sizes, including gigantism, has not been examined. We explored the relationship between body size and vertebral count in basal snakes that exhibit gigantism. Boids, pythonids and the typhlopid genera, Typhlops and Rhinotyphlops, possess a positive relationship between body size and vertebral count, confirming the importance of pleomerism; however, giant taxa possessed fewer than expected vertebrae, indicating that a separate process underlies the evolution of gigantism in snakes. The lack of correlation between body size and vertebral number in giant taxa demonstrates dissociation of segment production in early development from somatic growth during maturation, indicating that gigantism is achieved by modifying development at a different stage from that normally selected for changes in body size.

Morphological integration and adaptation in the snake feeding system: a comparative phylogenetic study

A long-standing hypothesis for the adaptive radiation of macrostomatan snakes is that their enlarged gape--compared to both lizards and basal snakes--enables them to consume "large" prey. At first glance, this hypothesis seems plausible, or even likely, given the wealth of studies showing a tight match between maximum consumed prey mass and head size in snakes. However, this hypothesis has never been tested within a comparative framework. We address this issue here by testing this hypothesis in 12 monophyletic clades of macrostomatan snakes using recently published phylogenies, published maximum consumed prey mass data and morphological measurements taken from a large sample of museum specimens. Our nonphylogenetically corrected analysis shows that head width--independent of body size--is significantly related to mean maximum consumed prey mass among these clades, and this relationship becomes even more significant when phylogeny is taken into account. Therefore, these data do support the hypothesis that head shape is adapted to prey size in snakes. Additionally, we calculated a phylogenetically corrected morphological variance-covariance matrix to examine the role of morphological integration during head shape evolution in snakes. This matrix shows that head width strongly covaries with both jaw length and out-lever length of the lower jaw. As a result, selection on head width will likely be associated with concomitant changes in jaw length and lower jaw out-lever length in snakes.

Phylogenetic relationships of the dwarf boas and a comparison of Bayesian and bootstrap measures of phylogenetic support

Four New World genera of dwarf boas ( Exiliboa, Trachyboa, Tropidophis, and Ungaliophis) have been placed by many systematists in a single group (traditionally called Tropidophiidae). However, the monophyly of this group has been questioned in several studies. Moreover, the overall relationships among basal snake lineages, including the placement of the dwarf boas, are poorly understood. We obtained mtDNA sequence data for 12S, 16S, and intervening tRNA–val genes from 23 species of snakes representing most major snake lineages, including all four genera of New World dwarf boas. We then examined the phylogenetic position of these species by estimating the phylogeny of the basal snakes. Our phylogenetic analysis suggests that New World dwarf boas are not monophyletic. Instead, we find Exiliboa and Ungaliophis to be most closely related to sand boas (Erycinae), boas (Boinae), and advanced snakes (Caenophidea), whereas Tropidophis and Trachyboa form an independent clade that separated relatively early in snake radiation. Our estimate of snake phylogeny differs significantly in other ways from some previous estimates of snake phylogeny. For instance, pythons do not cluster with boas and sand boas, but instead show a strong relationship with Loxocemus and Xenopeltis. Additionally, uropeltids cluster strongly with Cylindrophis, and together are embedded in what has previously been considered the macrostomatan radiation. These relationships are supported by both bootstrapping (parametric and nonparametric approaches) and Bayesian analysis, although Bayesian support values are consistently higher than those obtained from nonparametric bootstrapping. Simulations show that Bayesian support values represent much better estimates of phylogenetic accuracy than do nonparametric bootstrap support values, at least under the conditions of our study.

Snake phylogeny based on osteology, soft anatomy and ecology.

Relationships between the major lineages of snakes are assessed based on a phylogenetic analysis of the most extensive phenotypic data set to date (212 osteological, 48 soft anatomical, and three ecological characters). The marine, limbed Cretaceous snakes Pachyrhachis and Haasiophis emerge as the most primitive snakes: characters proposed to unite them with advanced snakes (macrostomatans) are based on unlikely interpretations of contentious elements or are highly variable within snakes. Other basal snakes include madtsoiids and Dinilysia--both large, presumably non-burrowing forms. The inferred relationships within extant snakes are broadly similar to currently accepted views, with scolecophidians (blindsnakes) being the most basal living forms, followed by anilioids (pipesnakes), booids and booid-like groups, acrochordids (filesnakes), and finally colubroids. Important new conclusions include strong support for the monophyly of large constricting snakes (erycines, boines. pythonines), and moderate support for the non-monophyly of the trophidophiids' (dwarf boas). These phylogenetic results are obtained whether varanoid lizards, or amphisbaenians and dibamids, are assumed to be the nearest relatives (outgroups) of snakes, and whether multistate characters are treated as ordered or unordered. Identification of large marine forms, and large surface-active terrestrial forms, as the most primitive snakes contradicts with the widespread view that snakes arose via minute, burrowing ancestors. Furthermore, these basal fossil snakes all have long flexible jaw elements adapted for ingesting large prey ('macrostomy'), suggesting that large gape was primitive for snakes and secondarily reduced in the most basal living foms (scolecophidians and anilioids) in connection with burrowing. This challenges the widespread view that snake evolution has involved progressive, directional elaboration of the jaw apparatus to feed on larger prey.

Varanoid-Like Dentition in Primitive Snakes (Madtsoiidae)

Several recent studies (Estes et al., 1988; Schwenk, 1988; Cooper, 1997; Lee, 1998; Lee and Caldwell, 2000) have suggested that snakes are related to anguimorph lizards and, in particular, are nested within varanoids. However, there are characters that contradict this arrangement. ror instance, the teeth of all varanoid lizards exhibit a distinctive infolding of dentine and enamel near the base of the tooth crown. This results in a characteristic appearance, with regular vertical grooves and ridges around the entire circumference of the tooth base (Bullet, 1942). The occurrence of this has previously been interpreted as a derived character uniting varanoid lizards as a monophyletic group, to the exclusion of other squamates such as snakes (Pregill et al., 1986; Estes et al., 1988; Lee, 1998). It characterizes all extant varanoid lizards (Varanus, Lanthanotus, and Heloderma) and extinct terrestrial forms such as Estesia, Until recently, plicidentine has not been observed in nonvaranoid lizards, apart from a weak development in some extinct anguids and necrosaurids, which are closely related to varanoids (Estes et al., 1988). It is also absent in amphisbaenians and has been regarded as absent in all primitive living snakes (e.g., Estes et al., 1988). However, an enigmatic Eocene snake (Archaeophis) and some advanced snakes (caenophidians) possess externally fluted or grooved teeth, which are superficially similar to those of varanoids (Janensch, 1906; Bogert, 1943; Klauber, 1956; Vaeth et al., 1985). Nevertheless, despite these occurrences, the apparent absence of plicidentine in more basal snakes (scolecophidians and anilioids) has led to the widespread assumption that it is primitively absent in snakes. Here, we describe jaw fragments of two species of primitive snakes (from the extinct family Madtsoiidae) that clearly exhibit varanoid-like plicidentine. These observations, together with other recent studies of primitive snakes (see later), suggest that plicidentine

Prey Transport Mechanisms in Blindsnakes and the Evolution of Unilateral Feeding Systems in Snakes1

Most snakes ingest and transport their prey via a jaw ratcheting mechanism in which the left and right upper jaw arches are advanced over the prey in an alternating, unilateral fashion. This unilateral jaw ratcheting mechanism differs greatly from the hyolingual and inertial transport mechanisms used by lizards, both of which are characterized by bilaterally synchronous jaw movements. Given the well-corroborated phylogenetic hypothesis that snakes are derived from lizards, this suggests that major changes occurred in both the morphology and motor control of the feeding apparatus during the early evolution of snakes. However, most previous studies of the evolution of unilateral feeding mechanisms in snakes have focused almost exclusively on the morphology of the jaw apparatus because there have been very few direct observations of feeding behavior in basal snakes. In this paper I describe the prey transport mechanisms used by representatives of two families of basal snakes, Leptotyphlopidae and Typhlopidae. In Leptotyphlopidae, a mandibular raking mechanism is used, in which bilaterally synchronous flexions of the lower jaw serve to ratchet prey into and through the mouth. In Typhlopidae, a maxillary raking mechanism is used, in which asynchronous ratcheting movements of the highly mobile upper jaws are used to drag prey through the oral cavity. These findings suggest that the unilateral feeding mechanisms that characterize the majority of living snakes were not present primitively in Serpentes, but arose subsequently to the basal divergence between Scolecophidia and Alethinophidia.

Muscle Physiology and the Evolution of the Rattling System in Rattlesnakes

Muscle Physiology and the Evolution of the Rattling System in Rattlesnakes

Adriosaurus and the affinities of mosasaurs, dolichosaurs, and snakes

The poorly-known, long bodied, limb-reduced marine lizard Adriosaurus suessi Seeley, 1881, is reassessed. Adriosaurus and a number of other marine lizards are known from Upper Cretaceous (Upper Cenomanian-Lower Turonian) marine carbonate rocks exposed along the Dalmatian coast of the Adriatic Sea, from Komen, Slovenia, to Hvar Island, Croatia. A revised vertebral count reveals 10 cervical, 29 dorsal, and at least 65 caudal vertebrae. The projections previously interpreted as hypapophyses are instead transverse processes. Openings on the anterior part of the skull, previously described as external nares, are probably internal nares. Important features not noted previously include accessory articulations on all presacral vertebrae, pachyostosis of dorsal vertebrae and ribs, and the presence of two pygal vertebrae. Phylogenetic analysis of 258 osteological characters and all the major squamate lineages suggests that Adriosaurus and dolichosaurs are successive sister-taxa to snakes. This is consistent with their long-bodied, limb-reduced morphology being intermediate between typical marine squamates (e.g., mosasaurs) and primitive marine snakes (pachyophiids). The analysis further reveals that up to five successive outgroups to living snakes (pachyophiids, Adriosaurus, dolichosaurs, Aphanizocnemus, and mosasauroids) are all marine, suggesting a marine (or at least, semi-aquatic) phase in snake origins. These phylogenetic results are robust whether multistate characters are ordered or unordered, thus refuting recent suggestions that snakes cluster with amphisbaenians and dibamids (rather than aquatic lizards) if multistate characters are left unordered. Also, the recent suggestion that Pachyrhachis shares synapomorphies with advanced snakes (macrostomatans) is shown to be poorly supported, because the reinterpretations of the relevant skull elements are unlikely and, even if accepted, the character states proposed to unite Pachyrhachis and advanced snakes are also present in more basal snakes and/or the nearest lizard outgroups, and are consequently primitive for snakes.