Paliguana, a basal stem lepidosauromorph, and Sophineta, a crownward one
- Paliguana: (Late Permian - right) known from a skull
- Kuhneosauridae: (Late Triassic) Lizard-like reptiles with elongate ribs that supported a gliding membrane. (Analogous, but not homologous to the supports in Wiegeltisauridae and the living Draco volans.)
- Marmoretta (Middle Jurassic Evans, 1991) known from dermal skull roof
- Sophineta (Early Triassic Evans and Borsuk-Bialynicka, 2009) known from dermal skull roof.
Moving crownward we note trends:
- The lacrimal becomes small and is crowded by the maxilla, which has a tall dorsal process and broadly enters the orbit margin.
- The quadratojugal becomes small
- The quadrate flares laterally into a conch for support of the tympanum
- The area of overlap between the quadrate and pterygoid is greatly reduced. (Compare Dimetrodon to Tupinambis, a squamate.)
- The teeth become pleurodont - i.e. rather than being set in shallow sockets (subthecodont) they occupy a shelf and are buttressed laterally by bone.(Link to Sophineta maxilla.)
Lepidosauria: (Triassic - Quaternary)Synapomorphies:
- Keratinous overlapping scales: (contrast mojave rattlesnakes with false gavial, an archosaur)
- Modified mid-dorsal scale row.
- Paired hemipenis (pl: hemipenes): Unique intromittant organs derived from midline amniote penis.
- Caudal autotomy planes. Caudal autotomy facilitates escape. Note: this feature is lost among some derived members.
- The mandibular condyle (the articular surface of the jaw) is formed exclusively from the articular. (In other saurians, the dermal angular contributes.)
Gephyrosaurus bridensis from Conrad, 2008.
- A row of palatine teeth that parallel the marginal teeth
- A posterior process of the dentary that laps over the angular
- A squamosal that extends ventrally to buttress the quadrate
- Supratemporal lost
- The differentiation of the tooth row
- Acrodont dentition, in which teeth are fused to the bone of the jaw
- Loss of the lacrimal
- The re-evolution, in some, of the lower temporal bar.
- Pleurosauridae: (Jurassic) aquatic, with long, eel-like bodies and short limbs.
- Sphenodontidae: (Early Jurassic - Quaternary - right) The living Sphenodon and its closest fossil relatives are characterized by a return to the strong, slow bite we saw in ancestral amniotes. Sphenodon lives on a few islands off the shore of New Zealand. It feeds on small vertebrates (which it bites pieces off of). Noteworthy for being able to remain active at low temperatures (roughly 50 degrees) thus being literally cold-blooded.
Squamata:Lizards and snakes. (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:
- Hinged quadrate articulation to the skull roof is hinged, allowing the quadrate to rock back and forth. Compare Sphenodon, a rhynchocephalian ( above left) with Tupinambis (right), the tegu lizard. Associated synapomorphies:
- Overlap between the quadrate and pterygoid is minimal, allowing the ventral end of the quadrate to swing freely.
- Squamosal lacks a ventral process
- Quadratojugal is lost
- Jugal extends anteriorly in orbit margin
- Epipterygoid is reduced to a narrow column
Tupinambis negropunctatus from Estes et al. 1988
- Angular is short (not reaching the jaw articulation posteriorly)
- Coronoid is complex and forms a tall coronoid process.
Sphenodon punctatus a rhynchocephalian compared to Tupinambis teguixin, a squamate.
- Frontals and parietals meet in a straight line that forms a hinge. This is functionally similar to the intracranial hinge of basal sarcopterygians. Together with the mobile quadrate, squamate skulls enjoyed a degree of cranial kinesis that enabled them to optimize the leverage of the temporalis and petrygoideus muscles of the jaw.
- Pineal foramen moves to the fronto-parietal contact (but is often absent)
- Nasals reduced
- Premaxilla fused
- Frontals fused
- Parietals fused
- Parietals anteroposteriorly short, such that the occipital region is exposed in dorsal view.
- Opisthotics and exoccipitals fused
Sphenodon punctatus a rhynchocephalian compared to Morunsaurus annulatus from Estes et al. 1988
- Palatal teeth reduced
- Choanae recessed in choanal grooves
- Suborbital fenestrae expanded such that pterygoids form part of their medial border
- Enlarged interpterygoid vacuity, such that pterygoids barely touch, if at all
- Vertebral articulations procoelous (concave anteriorly)
- Accessory articulations of neural arches present
- Dorsal intercentra absent
- Coracoid emargination present
- Sternum present, facilitating rotation of coracoids.
- Thyroid fenestra in pelvis
- Astragalus and calcaneum fused to form astragalocalcaneum.
- Distal tarsals 1 and 2 are lost.
This is a bewildering list, however some major adaptations shine though:
- In contrast to rhynchocephalians, whose strong jaws enable them to bite pieces off of their prey, the anatomy of the kinetic squamate skull is adapted to seizing and holding prey, then swallowing it whole. In some, this is developed into an astonishing ability to wrap their heads around their prey.
- Squamate locomotion is improved, with increased rotation of forelimbs and robust consolidation of ankle bones. Link to video.
Huehuecuetzpalli mixtecus from Reynoso 1998 - ~3 cm.
- Premaxillae remain paired
- Vertebrae are amphicoelous
- Distal tarsal 2 is present
Diversity: Since the early days of cladistics, a short list of major monophyletic groups appears has typically appeared:
- Iguania: (Cretaceous - Quaternary) iguanas, anoles, fence lizards, horned lizards,
agamids; chameleons, etc.
- Gekkota: (Late Jurassic - Quaternary) Paedomorphic forms including Geckonidae and Pygopodidae.
- Anguimorpha: (Early Cretaceous - Quaternary) Anguidae, Helodermatidae, Varanoidea.
- Scincomorpha: (Jurassic - Quaternary) Scincidae, Lacertidae, Cordylidae, Teiidae, etc..
Squamate Phlogeny - 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:
- Iguania: whose members:
- use the tongue in prey capture
- Empahsize limb-propelled locomotion with large limbs (plesiomorphy?)
- Scleroglossa: characterized by:
- Scaly tongues
- Loss of the lepidosaurian modified mid-dorsal scale row.
Scleroglossans show a tendency toward the reduction of the limbs and contain several limbless groups.
This consensus evaporates when we look at some more specialized groups, beginning with minor groups:
- Amphisbaenia: (Early Cretaceous - Quaternary) Limbless (with one exception) burrowers with extremely strong skulls modified for burrowing.
Dibamus bogadeki from Reptiles of Hong Kong
- Dibamidae: (Quaternary) Nearly limbless burrowers in soil and leaf-litter.
Diplometopon zarudnyi from markoshea.info
Squamate superlatives I: Mosasauroidea: (Cretaceous)
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
- Retraction of the nares
- Development of a distinct hinge joint at the mid-length of the jaw.
- Organization of the palatal teeth into a single stout row.
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.
Squamate superlatives II: Serpentes
(Late Jurassic - Rec.) The most subtle of beasts.
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.
- The temporal region is long in comparison to the snout (a condition found in other "head hunters" like aistopods.)
- Braincase reinforced anteriorly by laterosphenoid ossification.
- Bones of the dermal skull roof and braincase fuse into a solid cylindrical unit.
- 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.
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:
- 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.
Amphisbaenians and skinks, also with fossorial adaptations, 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-length of the jaw.
- Reduction of limbs and limb girdles.
- 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.
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, dolichosaurs, and mosasaurs.
Bitter Realities of Squamate Phylogeny
The debate on the phylogenetic position of snakes has fed a larger controversy that deflates our confidence about squamate phylogeny: