Eurasian Miocene snake taxa, localities, stratigraphy, palaeogeography, and palaeoenvironment are reviewed. Palaeogeographic evolution of Paratethys facilitated communication between European and Asiatic faunas since the early Oligocene, with at least two main routes from Asia or Africa into Europe. The early Burdigalian saw spreading of non-erycid Booidea and the first ‘Oriental vipers’ in Europe, which dispersed substantially within Eurasia during late Ottnangian warming. This warm climate, culminating as the Miocene Climatic Optimum, was associated with the middle Burdigalian first appearance of highly thermophilic Naja and Python in Europe. Python disappeared in Europe at the end of the Langhian due to rapid cooling, but Naja and ‘Oriental vipers’ persisted until the late Pliocene and early Pleistocene, respectively. Communication among mid-latitude Asian and European assemblages occurred across the early–middle Miocene, but this Eurasian fauna was heterogeneous, at least since the middle Miocene. Miocene S and SE Asian snakes resemble those of today. Increasing end-Miocene aridity and Eurasia–Africa connection facilitated invasion into Eurasia of African and SW Asian taxa.

Females of many squamates maintain viable sperm in their reproductive tract after insemination. This 'female sperm storage' (FSS), has several advantages and clear implications for squamate evolution by dramatically influencing life histories, mating systems, and sexual selection and conflict. In this chapter, we summarize the literature on the anatomy of FSS and reconstruct the evolution of sperm-storage location in squamate reptiles. Our major aim is to provide insights into the evolution of FSS in squamates, with a particular focus on the origin and early evolution of snakes. Various lizard lineages store sperm exclusively in crypts or tubules in the posterior oviduct. Most ‘basal’ lizards (gekkotans) and all ‘basal’ snakes (scolecophidians) studied thus far store sperm in tubules in the anterior oviduct. A few gekkotans and most alethniophidian snake studied store (or potentially store) sperm in both oviductal regions. Some snakes apparently have evolved morphological adaptations to hold sperm in the posterior oviduct. Based on ancestral state reconstructions, we elaborate a scenario for the evolution of FSS in snakes.

Squamate hemipenes have yielded much systematic data, but there have been few, if any, attempts to infer changes across the lizard–snake transition. We assess external morphology of hemipenes of major extant squamate lineages. Summarizing information across Squamata from mostly published data is challenging because of (i) patchy coverage, (ii) uncertainty as to whether described organs are fully everted or inflated, (iii) interpreting mixed text, photographs and drawings, (iv) non-standardized terminology, (v) shifting views of squamate phylogeny. However, we provide suggestions towards a unified terminology for hemipenial morphology, and score 24 lineages for 10 traits. We infer likely ancestral states as follows. (1) Ancestral toxicoferan: slightly bilobed hemipenis; simple, flared sulcus spermaticus; lack of spines; possibly flounce-like transverse flanges on body. (2) Ancestral snake: simple, flared sulcus with closely spaced, symmetrical lips; lack of spines; lack of lobular calyces; possibly unilobed hemipenis. (3) Ancestral alethinophidian: moderately to deeply bilobed hemipenis; lobular flounces; lack of spines; centripetal, bifurcate sulcus reaching tips of lobes.

Despite recent advances, key events in snake evolution have remained difficult to resolve, including their position in the squamate tree and several ingroup relationships. Comparative genomics has unrealised potential for phylogenetic inference and may advance understanding of snake evolution. This chapter reviews the history of snake molecular phylogenetics up to the current genomics revolution. This work has often corroborated phylogenetic inferences from morphology but also discovered relationships not previously considered or supported. We discuss properties of snake nuclear genomes, considering their potential for phylogenetic inference. Using data from 30 available squamate genomes, we provide preliminary examples applying both cumulative and non-cumulative frequency coding to genome size, GC content, and 14 repetitive element characteristics. Cumulative frequency coding outperforms non-cumulative coding and recovers most, but not all, well-known snake clades. We describe how the relationships of some snake lineages remains poorly supported despite their inclusion in large genomic-scale datasets, and suggest possible avenues of future research using comparative genomics.

Some snakes are the only vertebrates able to engulf prey with cross-sectional areas several times larger than the area encompassed by the snake’s jaws at peak gape. This ability is conferred by modifying soft tissues ventral to the axial musculoskeletal system for extraordinary extensibility between the mandibles and stomach. Moving large prey into the gut depends on structural decoupling of toothed jaws from the braincase. In all living snakes, kinetic jaws form mobile ratchets. In scolecophidians, transverse maxillary or dentary ratchets have evolved to move small prey into the gut. In alethinophidians, longitudinal palatopterygoid ratchets move the head and body of the snake over the prey. Evidence from extant snakes shows that streptostyly, prokinesis, rhinokinesis and loss of all ventral skeletal elements connected to the axial skeleton were critical to evolution of the upper-jaw ratchet on which macrostomy is based. The existing fossil record gives tantalizing clues that suggest the ancestor of snakes might have been macrostomous. Resolution of this issue will require structural details of the snout, braincase, and toothed ratchets in both ‘basal’ extant snakes and fossils.

The origin and early evolution of snakes has long been studied, but little research has focused on soft-tissue organs such as the brain. I report data from dissections and 3D reconstructions of the endocasts of diverse species, including the Cretaceous stem snake Dinilysia patagonica in order to provide a comparative evolutionary framework for the snake brain. Snakes are a special case among reptiles because the braincase almost entirely encloses the whole brain, so endocasts provide realistic representations of brain size and shape. Diversity of brain gross anatomy among snakes is remarkable, encompassing two major cerebrotypes occurring in surface-dwelling and burrowing species. The repeated acquisition of the burrowing cerebrotype in different and phylogenetically distant snake clades suggests that brain gross anatomy is surprisingly evolutionary labile in snakes. Brain gross anatomy and other features such as body size and the absence of any unequivocal osteological feature related to burrowing is interpreted as evidence that D. patagonica was surface-dwelling, and that at least some of the early history of snakes occurred above ground.

Sea-serpent sightings were popular subjects of nineteenth century fictional tales. One of the most famous sightings, the 1817 appearance in the harbor of Gloucester (Massachusetts), generated a report published by the Linnean Society of New England. In 1869, ED Cope introduced a new reptilian order, Pythonomorpha, comprising large Upper Cretaceous marine lizards (mosasaurs) that he thought rather well captured in historical depictions of sea-serpents. The name Pythonomorpha emphasized the many striking features that Cope found mosasaurs to share with snakes. Cope’s Pythonomorpha was resurrected in the late 1990s, as a clade including mosasaurs plus snakes. This was supported by the placement of mid-Cretaceous marine snakes with well-developed hindlimbs as evolutionarily intermediate between mosasauroids and snakes. Critics pointed to features indicating that those fossil snakes are instead evolutionarily advanced, which would imply that hindlimbs of these fossil snakes re-developed from rudiments such as occur in pythons. Recent molecular developmental studies confirmed that the embryonic limb bud of the python hindlimb conserves the genetic program to generate a complete limb.

Oral glands underwent substantial modification during the origin and diversification of snakes. Oral glands have provided rich data for snake systematics, and for informing evolutionary scenarios about the adaptive radiation of snake feeding. However, sampling has been patchy, and many questions remain about gland homology, function and evolution. This chapter addresses labial (supra- and infralabial), temporomandibular, rictal, sublingual, premaxillary, accessory and dental (= venom and Duvernoy’s) glands. We review and synthesize developments and data and present new histological sections and high-resolution tomography of some snakes and lizards, providing descriptions and illustrations of oral glands and associated structures. We comment on labial and dental glands of some toxicoferan and non-toxicoferan lizards, and report the first observation of a possible infralabial gland in a dibamian lizard. There are insufficient data to resolve all outstanding questions about gland homology across lizards and snakes, but the ancestral snake possibly had rictal and lacked dental (venom) glands, the latter perhaps evolving only within colubroidean caenophidians.

Genomic studies have elucidated some molecular underpinnings for adaptations during the early history of snakes, but studies of dietary adaptations remain sparse. Snakes differ from most other squamates by tending towards diets of vertebrate prey (carnivory), whereas arthropods are common in diets of most other squamates (insectivory). To test whether a shift from insectivory to carnivory occurred early in snake history, I examined chitinase genes (CHIAs) in 19 squamates. Previous studies on mammals found that contraction in the number of CHIAs, which enzymatically digest arthropod chitinous exoskeletons, correlates with transitions from insectivory to carnivory or herbivory. I found evidence that CHIAs have a long history in Squamata, with at least seven paralogs inferred in their last common ancestor. Retention of these CHIAs seems to be commonplace for arthropod-eating squamates, but snakes likely lost six CHIAs between diverging from other toxicoferans and the origin of afrophidian snakes. This genomic signal corresponds with an inferred major shift towards carnivory during the origin and evolution of early snakes, which may have contributed to their successful radiation.

We give a review of all published Palaeogene snake taxa from all localities worldwide. Several conceptual and material advances in the past two decades—a focus on apomo+P31rphies, greater attention to variation, quantification of morphology, and new fossil discoveries—have vivified the fossil record. Particularly noteworthy have been new fossils from Gondwanan continents and complete, articulated skeletons. Species known only from vertebrae are unlikely to be placed precisely phylogenetically, but a high number of vertebrae is a strong indication that cranial remains are present, which in turn allow more precise phylogenetic placement. Extrapolations of snake palaeodiversity are of the same order of magnitude as rough calculations of cumulative lineage diversity in the Palaeogene, raising the prospect that palaeontological morphospecies may more closely approximate biological species than is commonly conceived. As their interrelationships become better known, Palaeogene fossils will increasingly help elucidate the early evolution of snakes.

This brief chapter introduces the book. The rationale, scope, and coverage are summarized, including mention of topics that are not covered. Aspects of debate, disagreement, and consensus in the field are summarized before the chapter is concluded with a look to future potential progress.