"...More subtil than any beast..."


The scary lecture
Squamata: (Triassic - Rec.)

Warning: The word "lizard" is sometimes used to convey a sense of primitiveness. That's utter bunk.

Nothing could be more misleading. Squamates are actually highly evolved, derived, and intrinsically interesting creatures. Today we focus on some particularly interesting manifestations:

Don't be afraid.


Squamata

The synapomorphies of Squamata are legion. We will zero in on three:

  • Biological meaning:


    Squamate diversity: There are hundreds of living squamate species. All of their diversity falls into two major groups:

    These groups appear in the Early Jurassic, with essentially modern looking forms occurring in the Cretaceous.



    Tylosaurus hunts Hesperornis by Dan Varner from Oceans of Kansas

    Mosasauroidea - the mosasaurs: (Cretaceous)

    Mosasaurs, a particularly spectacular scleroglossan group, appeared and thrived in the second half of Cretaceous, but were extinguished by K-T extinction. During that interval, they ranked among the oceans' dominant predators. Mosasaurus was the first fossil reptile to be studied scientifically by Georges Cuvier, who identified it as a lizard.
    A look at the quadrate of Tylosaurus (right) clearly shows the apomorphic squamate quadrate. Other interesting mosasaurian features include:

    Despite this long history of study, mosasaurs continue to surprise us. E.G. the mosasaur tail has always been reconstructed as laterally flattened but similar to other lizard tails. A 2013 description of soft tissue preservation demonstrated that the mosasaur Prognathodon, at least, had a full-blown tail fin. Could other mosasaurs have been that different?


    Opetiosaurus buccichi
    All agree that mosasaurs are monophyletic and derived from "Aigialosaurs," a paraphyletic group of semi-aquatic medium-sized scleroglossans. Their close relatives are Dolichosauridae, containing medium-sized marine scleroglossans with long necks.

    Scleroglossan adaptations:

    Within the group we see a variety of different evolutionary adaptations:


    The Enigma: In each list above, which group keeps popping up? Why might this be?

    Answer: Serpentes - snakes - (Cretaceous - Rec.) The most subtle of beasts, who are the focal point of this lecture.

    For over a century, these animals, with their extreme derivations, have stimulated a debate about the ecologies of their ancestors that has at times been pursued with real venom (Yes. I said it.) Before considering this, we must ask:

    For phylogenetic purposes, snakes are just another group of scleroglossan squamate, part of a frustrating polytomy of derived scleroglossans (right). But snakes are as speciose as almost all other squamates combined. Obviously, they have hit on a particularly successful biological strategy. To explore it, consider their extremely derived condition:

    Rather than giving a synapomorphy list, we will note major evolutionary trends:

    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.

    Note two big consequences: These trends are even more pronounced in derived snakes like vipers.

    Now the enigma: What was the life style of the first snakes?

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

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

    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.

    No surprise that amphisbaenians and skinks have both been proposed as sister taxa of snakes.


    Tylosaurus proriger, a mosasaur (right); Python sebae from BioLib (left)
    Morphology: 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: This would imply an aquatic ancestry for snakes.


    Pachyrachus problematicus, a Cretaceous marine snake from Dinopedia
    Renewed debate: In the 1990s, gasoline was thrown onto this issue by a volley of descriptions or redescriptions of primitive snakes from the Cretaceous, including Pachyrachus problematicus (right), 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 its likely relationship to mosasaurs, dolichosaurs, and their kin. 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.


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

    If this is not convincing, then Martill et al., 2015, described Tetrapodophis amplectus, a proper snake with fore and hind limbs and fossorial specializations.

    The fossorial hypothesis seems to have won the day, and yet as recently as Palci et al., 2013, publications in support of the marine hypothesis and mosasaurian relationship have continued to appear.

    Why hasn't this issue gone away?

    Answer: Ongoing disagreement about the actual identity of the elements of the skull of the Cretaceous leggy snakes. Alternate hypotheses of the identity of bones that might be, for instance, the jugal (a dermal skull-roof bone) or the ectopterygoid (a dermal bone of the palate) greatly influence how characters in a taxon-character matrix are scored. With only a few changes in scoring, for instance, mosasaurs and dolichosaurs can be returned to the crown of the Scleroglossan tree and united with Serpentes.

    A vivid example of this uncertainty concerns the identity of the presacral vertebral column. According to Palci et al., 2013, the anterior third of presacral vertebrae in Haasiophis display unfused intercentra. Intriguingly, unfused cervical intercentra are seen in:

    If the free intercentra of Haasiophis are homologous to these (not crazy) it would imply that the front third of its "torso" is actually its neck (not easy to determine in a snake.)


    Dolichosaurus by Olorotitan from www.ammonit.ru
    What else has such a vertebral column? Dolichosaurs - closely related to mosasauroids in most recent analyses. If Palci et al. are correct on this and other interpretations of homology, the possibility of a close relationship between snakes and dolichosaurs can't be dismissed lightly.

    Are Palci and his confederates right, or are their interpretations of homology a subconscious attempt to administer artificial resuscitation to a hypothesis that has actually been properly falsified? - We can only stay tuned and see what happens next.

    Toxicofera: The molecular result, however, is different yet again. Reeder et al., 2015, in a combined molecular and morphological analysis, recover snakes, dolichosaurs, and mosasaurs as the sister taxa of Iguania (!) with Scleroglossa being paraphyletic (!)

    God knows when the serpent will cease to deceive us.