"...More subtil than any beast..."
The scary lecture
Warning: The word "lizard" is sometimes used to convey a sense of primitiveness. That's utter bunk.
- Popular texts insist that primitive amniotes "looked like lizards," for instance.
- Popular culture can barely distinguish salamanders from lizards.
Don't be afraid.
The synapomorphies of Squamata are legion. We will zero in on three:
- Hinged quadrate articulation to the rest of the skull is hinged, allowing the quadrate to rock back and forth. Compare Sphenodon, a rhynchocephalian (left) with Tupinambis (right), the tegu lizard. Comparing the living animal with the skull clearly shows that the quadrate, as well as being hinged, is bowed anteriorly to enable it to support the ear-drum.
- Dermal bones of the skull roof meet in a straight line that forms a hinge This is functionally similar to the intracranial hinge of basal sarcopterygians. (Although not homologous - congruence test!) Compare the mid-cranial sutures in the rhynchocephalian Sphenodon (left) and the lizard Tupinambis (right.)
This feature enables the snout to rock up and down slightly with respect to the braincase and posterior skull roof. That facilitates a wider range of feeding strategies.
Caudal autotomy in California alligator lizard from SoCalHerps
- Caudal autotomy: "Autotomy" means "self cutting." In squamates, this is the ability to detach one's tail, allowing escape from predators that would otherwise nail them. The twitching tail distracts the predator while the lizard escapes. Caudal autotomy is facilitated by fracture planes passing through the mid-length of the caudal vertebrae. When the tail autotomizes, a rod of cartilage grows from the broken vertebral surface, forming the skeleton of a new, albeit inferior tail. Note: If you pull the tail apart between vertebrae, no regeneration occurs.
Leopard gecko shedding from Fabs Reptiles
- Regular ecdysis: Squamates shed their skins at once on a regular basis.
- Origins: In the ancestral saurian, the opening of the ventral margin of the infratemporal fenestra to the outside has to have weakened the temporal region. The ancestors of Sphenodon responded by reevolving the closed fenestra and strengthening the bite.
The ancestral squamates, in contrast opted to control and exploit the incipient mobility of the skull elements.
- Jaw muscles exert maximum leverage over the jaw when they are pulling perpendicularly to the jaw. Thus, a gnathostome bites most powerfully when its jaws are almost closed, and least powerfully when they are wide open. We have already said that saurians were specialized for a wide gape and rapid, if weak bite. How might they mitigate the weakness of the bite without sacrificing speed and mobility?
- So far, by "jaw muscles" we have meant the muscles that originate on or near the braincase and insert on the jaw. A second group of muscles originates on the upper surface of the palate and inserts on the jaw. This group pulls at roughly right angles to the first group. When the jaw is wide open, this group of palatal muscles is pulling on it at roughly right angles.
- By being able to rotate around its articulation with the skull roof, the quadrate can mechanically be coopetd into the jaw (almost). When it rotates, the quadrate-skull joint becomes , in essence, the equivalent of the jaw joint. Adding the quadrate to the "jaw" lengthens the input lever of the palatal muscles, allowing them to act more powerfully.
- Thus, when the jaws are wide open, the palatal muscles pull powerfully, rotating oth the quadrate and the jaw around the quadrate- skull roof joint. When the jaws are nearly closed, the old temporal muscles take over, exerting maximum leverage as they rotate the jaw around the ancestral jaw joint.
- Pretty slick.
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.
- Iguania: Contains iguanas (duh) plus a great diversity of other groups such as:
The use of the tongue in prey capture. (Obviously, secondarily derived plant eaters, such as the green iguana don't do this.)
- The champs are chameleons, in which the tongue can be projected more then the length of the torso.
- The champs are chameleons, in which the tongue can be projected more then the length of the torso.
- Scleroglossa: Includes:
One synapomorphy is also the etymology of the name:
- A scaled tongue. In some, the front part of the tongue is specialized as a sense organ, but the iguanian facility for lingual prey capture is lost.
- Loss of modified misddorsal scale row.
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:
- Transformation of the limbs into flippers
- Elongation of the snout
- Development of a distinct hinge joint at the mid-length of the jaw.
- Organization of the palatal teeth into a single stout row.
Within the group we see a variety of different evolutionary adaptations:
- Limblessness: The loss of limbs has occurred at least three separate times in Scleroglossa, whereas it is unknown in Iguania. Notable examples include:
- Anguids (so called glass-lizards, because of the ease with which they autotomize their tails)
- Amphisbaenians (burrowing forms)
- Pygopodids (Australian limbless geckos.)
- Dibamids: (Nearly limbless tropical burrowing forms - right.)
Dolichosaurus from ammonit.ru
- Secondary return to the ocean: Marine life is almost absent in Iguania. Notable scleroglossan examples include:
- Sea snakes (a monophyletic group nested well within modern snakes
- Some primitive snakes of the Cretaceous (E.G. Pachyrachis)
- Mosasaurs: medium-sized to giant marine lizards of the Cretaceous. (E.G. Prognathodon) Beloved by the general public.
- Dolichasaurs: Smallish long-necked marine lizards of the Cretaceous. (E.G. Dolichosaurus - right) Known only to specialists.
Agamodon angeliceps from J R Blair's Biology 170 Animal Diversity San Francisco State University
- Burrowing: A fossorial (burrowing) life-style is next to unheard of among iguanians but highly developed in scleroglossan groups including:
- Scolecophidia - burrowing snakes (a monophyletic group near the base of the snake cladogram)
- Skinks, many of whom are burrowers with greatly reduced limbs.
- Amphisbaenians: burrowing limbless scleroglossans. (E.G. Agamodon angeliceps right)
- Dibamids: Nearly limbless tropical burrowing forms.
- Venom: Scleroglossans show various adaptations for making their bites more potent.
- Snakes: range from caustic saliva to potent specialized venom.
- Gila monsters: Potent tissue destroying venomous saliva which enters the mouth during a relatively (for a squamate) powerful bite.
- Varanids: Harmful bite effects have been attributed both to venom and to sepsis from oral bacteria.
- Mosasaurs: Fossil bones show pathologies resulting from infection of bites by other mosasaurs. Maybe they had a varanid-style infectious bite.
Dibamus bogadeki from Reptiles of Hong Kong
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:
- How are snakes different?
- Who might their relatives be?
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:
- Limbs reduced or absent.
- Lidless eyes protected by a transparent scale.
- A unique pattern of rods and cones in the retina resulting in a reduction of color vision (typically well-developed in sauropsids.)
- External ear opening lost.
- Bones of the dermal skull roof and braincase consolidate into a solid cylindrical unit, aided by the addition of a new braincase bone, the laterosphenoid.
- The quadrates, palate bones, maxillae and premaxillae become very loose and mobile, allowing manipulation and swallowing of large food items.
- The supratemporal, the skull bone to which the quadrate attaches, also becomes mobile with respect to the braincase.
- The teeth of the palate are organized into a pair of rows.
- The jaws are jointed at their mid-length and flexible, and in most cases, the two sides do not connect in front, allowing swallowing of even larger objects.
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:
- Reduction of the skull roof bones around the temporal region and orbits, along with bones of the palate essentially decouple the palate from the skull roof. As a result, movement of the quadrate and supratemporal around their joints allow the palate to move forward and backward.
- The presence of a mobile joint in the jaw and the absence of a connection at the front makes then exceptionally mobile.
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:
- The aquatic hypothesis: The ancestral snake's visual system was modified for the lower lighting underwater. Loss of the tympanum reflects lack of need for impedance-matching ear underwater.
- The fossorial (burrowing) hypothesis: The ancestral snake's visual system was reduced in an ancestor that mostly lived underground and had little to see. Reduction of tympanum reflects advantage, underground, of picking up vibrations with jaw.
Basal snakes: Proponents of the fossorial hypothesis derive comfort from the cladogram of living snakes.
- The basal branches of the snake tree, including Leptotyphlops, the blind snake and Cylindrophis, the pipe-snake are definitely fossorial.
- Although their skulls definitely display the derived features of snakes, they definitely lack the extreme modification of more derived snakes. (See Cylindrophis)
- These snakes lack the large flexible jaws and jaw-articulations characteristic of the "big-mouthed" macrostomatan 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)
- A single row of stout palatal teeth
- Mobile joint at the mid-lenght of the jaw.
- Reduction of limbs and limb girdles.
Pachyrachus problematicus, a Cretaceous marine snake from Dinopedia
- they were definitely marine
- they retained small but functional hindlimbs.
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:
- Mosasaurs and dolichosaurs were banished to a position near the base of Scleroglossa.
- Serpentes emerged as the sister taxon to the fossorial dibamids and amphisbaenids in a "fossorial group."
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:
- Varanoidea (monitors)
- Helodermatidae (Gila monsters)
Dolichosaurus by Olorotitan from www.ammonit.ru
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.