Lepidosauromorpha


Spencer's monitor Varanus spenceri a squamate; the tuatara Sphenodon punctatus a rhynchocephalian.

Lepidosauromorpha: (Triassic - Rec.) All organisms more closely related to lepidosaurs than to archosaurs. Max ∇ (Lepidosauria ~ Archosauria). Membership includes:

Paliguana whitei, from Ford et al., 2021

Stem Lepidosauria

The membership of stem-Lepidosauria has been historically unstable. As of 2018, most of the fossil taxa placed on the lepidosauromorph stem have been evicted to other (and varying) parts of the diapsid tree (Simões et al., 2018, Ford et al., 2021) leaving only a handful of fragmentary potential stem lepidosauromorphs. Not really surprising given the fragmentary nature of these small animals' remains. In this part of the tree, we dine on scraps.

The earliest reliable lepidosauromorph: The Early Triassic Paliguana whitei, recently redescribed by Ford et al., 2021 and known from a single skull. Broadly resembles the similar-sized stem saurian Lanthanolania, but with potential synapomorphies with Lepidosauria:


In Ford et al.'s analysis, Paliguana forms a clade with the similar Fraxinisaura rozynekae (Late Triassic) and Marmoretta oxoniensis (Middle Jurassic). Martinez et al., 2021 report that the Late Triassic Taytalura alcoberi is also a stem-lepidosaurian lepidosauromorph. Ford et al. and Martinez et al. don't seem to have been aware of one another's work so neither group's results tell us the relative position of Paliguana and Tataylura, however: Thus, for GEOL431 we tentatively view Tataylura as the more basal lepidosauromorph taxon.
  • Sophineta (Early Triassic Evans and Borsuk-Bialynicka, 2009) known from dermal skull roof.


    Sophineta, a stem lepidosauromorph.

    Lepidosaur Polytomy

    Lepidosauria: (Late Triassic - Rec.) The last common ancestor of Squamata and Rhynchocephalia, and all of its descendants. The earliest fossils of each are Late Triassic in age, but calibrated molecular clocks indicate a divergence right around the Permo-Triassic extinction event. In our consensus cladogram, we follow Ford et al., 2021 in placing Squamata and Rhynchocephalia in a polytomy with:

    Each has been placed as both stem lepidosaurs and stem squamates. Whereever they go, they display trends of lepidosaurian evolution:

    Lepidosauria: (Triassic - Quaternary)

    Synapomorphies:

    Rhynchocephalia:


    Gephyrosaurus bridensis from Conrad, 2008.
    (Triassic - Quaternary) Total group - everything closer to Sphenodon than to Squamata. The single genus Sphenodon, the New Zealand tuatara is the sad remnant of a large group of lepidosaurs that was abundant and speciose during the Triassic and Jurassic. Ancestrally they were terrestrial, and basal forms like Gephyrosaurus (Early Jurassic Evans, 1980) were barely distinct from stem lepidosauromorphs and yet they are distinct in having: Among later rhynchocephalians, we see trends toward:


    Sphenodon punctatus
    While mostly small and "lizard-like" fossil rhynchocephalians encompass some disparity of form:

    Despite their derived features, rhynchocephalians are a conservative group that, in many aspects of their biology, resemble stem-lepidosaurs.


    Squamata:

    Lizards and snakes. (Late Triassic - Rec.) Thousands of species. Many interesting adaptations. There is no way to cover exhaustively if we had an entire semester.


    Sphenodon punctatus a rhynchocephalian compared to Tupinambis teguixin, a squamate.


    The synapomorphies of Squamata are legion. We focus on a conspicuous subset:



    Sphenodon punctatus a rhynchocephalian compared to Morunsaurus annulatus from Estes et al. 1988

    This is a bewildering list, however some major adaptations shine though:


    Diversity:


    Huehuecuetzpalli mixtecus from Reynoso 1998 - ~3 cm.
    This huge list of synapomorphies suggests a very long squamate stem, however we have only one definite well-known stem squamate:


    Living Crown Squamata

    Diversity: Since the early days of cladistics, a short list of major monophyletic groups appears has typically appeared:


    Fossil Crown Squamata





    Squamate Phlogeny 1986 - the naive version:

    Morphology-based analyses have, since the early days of cladistics, recovered the pattern at right where Squamata is divided into two large groups: The comparison of the skulls of the Nile monitor lizard and reticulated python (a snake showing a relatively ancestral serpentine morphotype) underscore these differences.

    Nile monitor (left) and reticulated python (right) with premaxillae (blue), nasals (yellow), frontals (brown) and quadrates (red) highlighted.

    These trends are even more pronounced in derived snakes like vipers.

    What kind of animal was the first snake?

    Two hypotheses have been proposed, both of which address snakes odd sensory systems:

    Basal snakes: Proponents of the fossorial hypothesis derive comfort from the cladogram of living snakes.

    Note: both of these groups are ancestral enough that they lack the broad belly scales that we typically associate with snakes.

    Systematists are drawn into this debate because the discovery that the sister taxon of Serpentes was either aquatic or fossorial would weigh heavily in larger disputes.

    Hypotheses of Snake Relationships


    Tylosaurus proriger, a mosasaur (right); Python sebae from BioLib (left)

    Pythonomorpha hypothesis: But since the 19th century mosasaurs, and their close relatives the dolichosaurs, have been proposed as sister taxa of snakes based on the morphology of their palate and jaws. Similarities include: A sister taxon relationship between snakes and mosasaurs was first hypothesized by Cope, 1869, who coined the group name Pythonomorpha, and proposed that they belonged to Anguimorpha.


    Pachyrachus problematicus
    Renewed debate: In the 1990s, gasoline was thrown onto the issue of snake origins and relationships by a volley of descriptions or redescriptions of primitive snakes from the Cretaceous, including Pachyrachus problematicus (Lee and Caldwell, 1997), Haasiophis terrasanctus, and Eupodophis descouensis. Stimulating because: At first glance, a snake with legs ought to be a basal primitive snake, and a marine basal primitive snake ought to cinch the marine ecology of the ancestral snake, and support the pythonomorph hypothesis. But not so fast.

    Their skulls proved to have the derived characters of macrostomatan snakes. Thus, despite the presence of legs, most cladistic analyses continue to place them well within the crown of living snakes. Apparently the presence of legs in them is a reversal. Strange! Of course, if they are derived snakes then they do not represent the ancestral condition.

    More recently, Caldwell et al., 2015 have revealed leggy snakes from the Late Jurassic. These, in contrast, to Pachyrhachis, occupy a basal position in Serpentes.

    The fact that for over a year, the four-legged Early Cretaceous Tetrapodophis amplectus was regarded as a basal snake (Martill et al., 2015) only later to be identified as a dolichosaur (Caldwell et al., 2016) emphasizes the strong similarities between snakes and mosasauroids.


    Thesis, Antithesis, and Synthesis

    The debate on the phylogenetic position of snakes has fed larger controversies about the relative importance of morphological and molecular data in phylogenetic analysis.Some landmarks in the unfolding debate:


    The Krypteia Hypothesis:

    Gauthier et al., 2012, attacked these an many other issues at once with an authoritative comprehensive phylogenetic analysis that assembled a huge and well-documented morphological data set with substantial new character information. Their result included two surprising elements:

    The fossorial hypothesis seemed to have won the day, yet subsequent publications to support the Pythonomorpha hypothesis. Why hasn't this issue gone away? Ongoing disagreement about the actual identity of skeletal elements of the animals involved including:

    But the more potent apple of discord has been the entry of molecular analyses into the debate.

    The Toxicofera Hypothesis:


    Squamate phylogeny from Reeder et al., 2015
    When molecular methods are applied, things get very interesting. Wiens et al., 2012 obtained a result in which Scleroglossa does not appear. Instead: Reeder et al., 2015 have confirmed this result with an analysis that: The result confirms Toxicofera and places Mosasauroidea as the sister taxon of Serpentes. Moreover, morphological synapomorphies are identified for Toxicofera.

    Before we dismiss this radical idea, the Toxicofera hypothesis cleans up a loose end: Fossil iguanians don't appear stratigraphically until well after members of other major squamate groups, despite their basal position in morphological phylogenies. Placing them in the crown of the tree puts them in a stratigraphically congruent position.


    2020s consensus Squamate phylogeny after Simões et al., 2018
    As subsequent analyses (Streicher and Weins, 2017, Simões et al., 2018, Simões et al., 2020) have continued to recover the Toxicofera pattern, it has become the leading hypothesis.

    For an outstanding review of the history of squamate systematics, see Simões and Pyron, 2021.

    Stay tuned.


    Additional reading: