Non-ophidian ophidiomorphs — the Cretaceous marine-lizard assemblage conventionally called dolichosaurs, long treated as the stem group of snakes — are known from a severely undersampled fossil record, and their evolutionary history remains unresolved. Campbell Mekarski, Pierce and Caldwell (2019) synthesized the record and posed a linked set of open questions: whether dolichosaurs form a monophyletic family, a paraphyletic grade toward snakes, or an assemblage not especially close to snakes at all; when and where their lizard ancestors became aquatic (Tethyan-origin, Western-Pacific-origin, and multiple-origins models all remain live); whether their Cenomanian–Turonian decline was driven by the oceanic Cenomanian–Turonian Boundary Event or by the rise of mosasauroids; and why Cenomanian Tethyan assemblages were so endemic. Later work (2021–2026) has added key fossils — Tetrapodophis reinterpreted as a dolichosaur, the Middle Jurassic parviraptorid Breugnathair, a new Carentonosaurus — and sharpened rather than settled the central disagreement, with one new phylogenetic analysis explicitly rejecting a close mosasaurian–snake relationship. The problem — establishing the interrelationships, spatiotemporal origin, and extinction dynamics of this group and thereby the stem-lineage context of snake origins — remains open.

Background

Squamata (lizards and snakes) is the largest living clade of terrestrial vertebrates. Within it, the interrelationships of the earliest lineages — and in particular the origin of snakes (Serpentes) — have been contentious for over a century. One long-standing hypothesis, going back to Nopcsa in the early twentieth century, links snakes to a group of small, elongate, semi-aquatic lizards called dolichosaurs (the 'non-ophidian ophidiomorphs': ophidiomorphs are the snake-line clade, and these are its members that fall outside snakes proper). Dolichosaurs are diagnosed by elongate necks (more than about ten cervical vertebrae), elongate trunks (more than about 35 presacral vertebrae), accessory zygosphene–zygantra articulations between vertebrae, and reduced forelimbs; they are generally interpreted as small (under a metre) semi-aquatic piscivores of shallow Cretaceous seas, historically known from Cenomanian (~100–94 million years ago) deposits of the Tethys Ocean (the Mesozoic seaway between Laurasia and Gondwana) and the Western Interior Seaway of North America. They sit near the mosasauroids (the clade including aigialosaurs — primitive, still fully limbed mosasauroid lizards — and the large marine mosasaurs), and much of the debate concerns whether snakes' closest relatives are these marine lizards (the 'pythonomorph' or 'ophidiomorph' hypotheses) or instead terrestrial varanoid lizards (varanoids are the monitor-lizard lineage; the 'varanoid hypothesis').

Computer-based cladistic analyses through the 2000s, as reviewed by Campbell Mekarski, Pierce and Caldwell (2019), disagreed: some recovered a monophyletic dolichosaur family, while others recovered the named dolichosaur genera as successive sister groups to snakes — a paraphyletic evolutionary 'grade' along the road to snakes. The description of four tiny snake fossils from the Middle Jurassic to Lower Cretaceous of Europe by Caldwell and colleagues in 2015 (including the Middle Jurassic Eophis, then the oldest known snake) implied, under the grade interpretation, that non-ophidian ophidiomorphs must have originated by the Middle Jurassic — tens of millions of years before their first clear body fossils, which appear only in the Lower Cretaceous and, confoundingly, in three isolated freshwater localities (Japan, Spain, Australia) far from the marine Tethyan heartland. On this basis, Campbell Mekarski, Pierce and Caldwell (2019) assembled all known occurrences and evaluated two competing origin models — a Tethyan origin versus a Western Pacific origin — while noting that multiple independent lineages could not yet be ruled out.

Campbell Mekarski, Pierce and Caldwell (2019) also identified three macroevolutionary puzzles. First, dolichosaur diversity drops sharply across the Cenomanian–Turonian boundary, coinciding both with the Cenomanian–Turonian Boundary Event (a major global oceanic anoxic event about 94 million years ago that restructured marine ecosystems) and with the radiation of mosasauroids in the Tethys, so the relative roles of environmental crisis versus ecological replacement cannot be separated. Second, Cenomanian Tethyan dolichosaur faunas show pronounced endemism (regionally restricted genera) despite the absence of obvious physical barriers to dispersal. Third, many records — isolated vertebrae and teeth from South America, Central Asia, and elsewhere — are only tentatively identified, and the group's history in the Southern Hemisphere is essentially unknown.

Subsequent literature has deepened, rather than settled, these questions. Caldwell and colleagues (2021) showed that the famous four-limbed 'snake' Tetrapodophis from the Early Cretaceous of Brazil is actually a dolichosaur with aquatic adaptations, adding a Gondwanan record and a further instance of convergent snake-like body evolution. Benson and colleagues (2025) described Breugnathair, a Middle Jurassic (~167 million-year-old) Scottish squamate placed in the enigmatic family Parviraptoridae, whose mosaic anatomy yields conflicting phylogenetic results — either an early toxicoferan (Toxicofera: the clade comprising snakes together with iguanian and anguimorph lizards) potentially on the snake stem, or a stem squamate that convergently evolved snake-like jaws and teeth. Meanwhile, Augusta, Zaher, Polcyn, Fiorillo and Jacobs (2022), sampling non-mosasaurid mosasaurians densely, concluded that a particularly close relationship between mosasaurians and snakes can be rejected (the varanoid hypothesis), and Head, Howard and Müller (2022) argued that the elongate snake body plan is no older than about 100 million years — implicitly denying that the Jurassic parviraptorids are snakes at all. The stem-lineage context of snake origins therefore remains one of the most actively disputed problems in squamate palaeontology.

Problem Statement

Establish the evolutionary history of the non-ophidian ophidiomorphs (dolichosaurs) — the early-diverging snake-line (or putatively snake-line) Cretaceous marine lizards — as one coherent objective with four linked components addressing the overarching questions of the group's phylogenetic position, spatiotemporal origin, radiation (including its pronounced Cenomanian Tethyan endemism), and decline, and thereby the stem-lineage context of snake origins:

  1. Interrelationships: determine the phylogenetic relationships of the named dolichosaur genera to one another — monophyletic Dolichosauridae, paraphyletic grade toward snakes, or neither — and the relationship of the group as a whole to mosasauroids and to snakes, thereby deciding whether the snake stem passes through the dolichosaurs at all (the pythonomorph/ophidiomorph hypotheses versus the varanoid hypothesis).
  2. Origin and transition: determine the timing and geographic location of the origin of non-ophidian ophidiomorphs and of the transition from terrestrial lizard ancestors to the first aquatic forms, discriminating between the Tethyan-origin model, the Western-Pacific-origin model, and the possibility of multiple independent origins — a discrimination that presently requires new Jurassic and Early Cretaceous fossils, especially articulated specimens from underexplored regions such as the Southern Hemisphere and Central Asia.
  3. Decline: determine the relative roles of the Cenomanian–Turonian Boundary Event (and its oceanographic and ecological consequences) versus the radiation of mosasauroids in the decline and near-extinction of dolichosaurs across the Cenomanian–Turonian boundary.
  4. Endemism: explain the pronounced provinciality of Cenomanian Tethyan dolichosaur assemblages in the absence of obvious physical barriers to dispersal.

A complete answer must address all four components. For any component whose competing models cannot be discriminated with the existing evidence, the answer must say so explicitly and specify the decisive test or material that would discriminate them — such a component counts as addressed, not as unresolved, and only components that are skipped or left unexamined make an answer partial. Answers are expected to work from the existing fossil record together with revised phylogenetic, stratigraphic, and palaeobiogeographic analyses, addressing all four components as far as that record allows.

References

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  2. Caldwell MW, Nydam RL, Palci A, Apesteguía S (2015). The oldest known snakes from the Middle Jurassic-Lower Cretaceous provide insights on snake evolution. Nature Communications 6:5996. DOI 10.1038/ncomms6996. https://doi.org/10.1038/ncomms6996
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