Squamates are grouped within the paraphyletic “Reptilia,” with Testudines (turtles), Crocodylia (crocodiles, alligators, and gavials), and Rhynchocephalia (tuataras). Rhychocephalia is the closest related to the Squamata, being its sister-group (clade Lepidosaura) and sharing many characters, as the skin covered by scales/plates that are partially impermeable and changed periodically, and a transversal cloacal slit. Squamata is the largest and most diversified clade of extant reptiles, comprising about 95% of its current diversity, with around 6500 lizard species, 3700 snake species, and 190 amphisbaenian species (Uetz and Hosek 2018).
It is not hard to recognize a squamate and morphologically distinguish it from other reptiles. The basic corporeal plan of a lizard, with an elevated head, short neck, four limbs, and long tail, distinguish them from most other reptiles (McDiarmid 2012). The differences between Squamata and Rhynchocephalia are subtler – the main difference regards the presence of an ancestral skull in tuataras, with complete temporal bars defining the upper and lower temporal openings. Lizards lost the lower temporal bar, while snakes lost both (Kardong 2014). Many lizards and amphisbaenians, as well as all snakes, evolved the apodal elongated body, which is also distinct from other reptiles. Some important morphological features among snakes are the absence of eyelids and external ear, numerous vertebrae with modification on its connections, many paired internal organs elongated and some modified or lost (McDiarmid 2012). Moreover, all squamates have paired copulatory organs defined as hemipenis.
Squamates are extremely diverse and can be quite specialized, not only with respect to their habitat or locomotion type, but also their morphology (Klein and Gorb 2014). Both snakes and amphisbaenians are legless elongated reptiles, even though a few snake species (blindsnakes and a few boas) exhibit rudimentary pelvic girdles that project externally as spurs, and a few amphisbaenians (Bipes spp.) exhibit reduced forelimbs. Additionally, there are different degrees of reduction in the number of limb elements to complete limblessness. The tail tends to be long and slender, but some snakes and lizards might exhibit bulbous or laterally expanded, or dorsoventrally or laterally compressed tails (Zug et al. 2001; Lillywhite 2014).
Several external morphological features associated to their sensory system include a parietal eye on the top of the head in a few lizards, distinct by the presence of a modified head scale, which is important for their photosensitivity. Additionally, a few snakes exhibit sense organs that respond and detect infrared radiation, very useful at night when visible light is usually unavailable. Such receptors are present as modified lip scales (= labial pits) in boids and as a facial pit (=loreal pits), located between the eye and the nostril, in pitvipers. Additionally, all snakes, amphisbaenians, and several lizards present biphid tongues, which are extended to sweep air in front of them. The tongue collects airborne particles and is posteriorly retractes to perceive chemical signals, transferred to the vomeronasal organ located on the roof of the mouth. Tongues may also serve for prey capture, as in some chamaeleonids lizards (Kardong 2014).
The eyes of most squamates are large and conspicuous, especially in terrestrial and arboreal species. However, several fossorial species might present extremely reduced/degenerate eyes that can lie under the scales. Additionally, eyes of lizards exhibit bony plates (scleral ossicles) embedded in the sclera and surrounding the cornea (Zug et al. 2001). Pupils may vary from round to elliptical and are usually oriented vertically, even though it can be horizontally oriented in a few species. Extreme specializations in the squamate eye include the protrude lateral eyes of chameleons, with distinct anatomy of nodal and center points, in which the amplitude of movement is very large and the eyes move and focus independently (Zug et al. 2001; Lillywhite 2014).
Amphisbaenians exhibit the most conserved external morphology amongst Squamates, being characterized by their elongated bodies that are usually less than 150 mm long, with an extreme reduction (Bipes spp.) or complete loss of the limbs, and rudimentary/reduced eyes. Most of their distinctive features regard the dramatic modification of the head shapes, which is functionally correlated with specific tunneling behaviors. Heads may vary from (1) “shovel-headed,” with snouts dorsoventrally flattened with a strong craniofacial angle; (2) “keel-headed” forms; (3) “spade-headed” forms; or (4) “round-headed” forms – representing the most common form amongst amphisbaenians (Kearney 2003).
Squamates in general exhibit a water-conserving integument (Withers and O’Shea 1993) with the skin modified into scales, which might be named as plates, tubercles, lamellae, etc., depending on the taxonomic group and location of the scales (Zug et al. 2001). Scales in various squamates have evolved in different sizes, geometries and gross structure, also varying regionally in the body, what is extremely relevant for systematics (Lillywhite 2014). The scales of a few lizards might be underlain by bony plates, called osteoderms. The integument of Amphisbaenians is characteristic in representing a disconexion to the trunk, enhancing its underground locomotion (Pough et al. 2018).
The basic form of the squamate skull has the quadrate suspension with reduced dorsal and ventral processes of squamosal; loss of quadratojugal; fusion of the parietals; reduction of palatal dentition and anterior plate of the pterygoid; enclosure of the Vidian canal; shortening of the maxilla/dentary; among others (Evans 2008). Additionally, the snake skull seems to be paedomorphic in relation to lizards as claimed by several ontogenetic and anatomical studies. Considering the limbs, 15 synapomorphies are known for Squamata, and some of it are: elongate, gracile limbs; specialized joints (ulna-ulnare and radius-radiale; first metacarpal-wrist; locked tibio-atragalar; ankle); intermedium reduced or absent; and second distal tarsal absent (Russel and Bauer 2008).
The morphological diversity of the lizards’ axial skeleton reflects the wide extent of their morphological specializations, as previously shown. The number of presacral vertebrae varies from 16 in some Chamaleonidae to as much as 116 in some Dibamidae. The Sauria (lizards) shows two main morphological types of vertebrae, those with an amphicoelous centrum and those with a procoelous centrum. Sauria (=lizards) ribs are usually called holocephalous (one of the two articulations between the rib and the vertebra has disappeared), but the first cervical ribs of Varanus may still be dichocephalous. On the following vertebrae, and generally in the cervical and anterior trunk of lizards, the two articulations fuse to form a single synapophysis, but the latter remains oblong, keeping a vestige of dichocephaly. In the posterior part of the trunk, the single articular facet becomes hemispherical and the ribs clearly holocephalous. The vertebral column is divided into cervical, trunk, sacral/cloacal, and caudal regions (Hoffstetter and Gasc 1969).
In amphisbaenians, the number or precloacal vertebrae varies from 64 to 145. Each vertebra is generally depressed with transversely widened condyle, and flat ventral face of the centrum with parallel lateral borders. Neural spines are lacking in the trunk region, and prezygapophysial processes are more or less clearly present. The division of the column into regions is difficult. There is generally a vestigial pectoral girdle with no connections with the ribs, and a pelvic girdle, also without connection with vertebral column – thus, there is no differentiated sacrum. A series of vertebrae that bear forked ribs may be identified as cloacals. The caudal region is always short, comprising fewer than 30 vertebrae (Hoffstetter and Gasc 1969). Among amphisbaenians, only one family/genus presents limbs (Bipedidae, Bipes), and it is restricted to the forelimbs.
The absence of pectoral girdle hampers the definition of the boundary between the cervical and trunk regions. As in amphisbaenians, the site of the cloaca and associated organs is reflected in the vertebral morphology. The vertebral number is always high, varying from 160 to more than 400 (precloacal ranging from 120 to over 320). There are no fundamental differences between the vertebral morphology of snakes and the other squamates, but there are a certain number of typically ophidian characters. The centrum bears a large anterior cotyle (glenoid cavity) faced ventrally, and a dorsally turned posterior condyle, which widely overlaps the transverse section of the centrum. The ventral surface of the centrum is limited on both sides by a more or less clear crest (which are completely lacking in some families). The hypapophyses arise from the posterior part of the centrum in the midventral line. In the middle and posterior parts of the trunk, the hypapophyses may be reduced to a simple haemal keel, disappearing completely in some burrowing families. In the caudal region, they are replaced, when present, by paired haemapophyses which fuse to the centrum. The neural arch consists of a roof and walls. The neural crest is often trilobate in section. The ribs are completely ossified and generally robust with a double articular facet – each part articulated with one of the two areas of the synapophyses (Hoffstetter and Gasc 1969).
Squamates, as well as other amniotes, exhibit two major sets of muscles: the cranial (jaw and pharyngeal musculature and extrinsic eye muscles) and postcranial muscles (appendicular and axial) (Kardong 2014). The loss of the temporal arches in the skull, as well as the great differences in the degree of cranial kineticism, is directly reflected in the wide variation of the squamate cranial muscles. Differences in the position and configuration of the upper temporal arch and the relative width of the dorsal temporal fossa might have caused radical changes in the arrangement of the external and internal adductor muscles. Several fossorial squamates exhibit vestigial or absent extrinsic eye muscles that are possibly directly associated to the reduced eyes (Haas 1973). The muscles of the head of snakes differ strikingly and in many ways from those of lizards and are somehow closely and similar to amphisbaenians. However, the latter has developed an extremely strong biting apparatus, while in the microphagous snakes, cranial kinesis and horizontal movements of the jaws are more important. The process to swallow preys in ophidians is aided by the complex constrictor internus dorsalis group of muscles, which effect the protraction and retraction of the bony palate, and the complicated and diversified concomitant movement of the maxillae and bones of the snout (Haas 1973).
In Squamate (as well as in other amniotes), the horizontal septum that splits the epaxial and hipaxial muscles is lost or indistinct, although the supply by the dorsal and ventral rami of the spinal nerve still aids on the identification of such muscles. In lizards (except legless ones), even though lateral undulations of the vertebral column contribute to locomotion, limbs become more important in providing propulsive forces. Therefore, the epaxial muscles associated to vertebral column are reduced, and the musculature associated to the appendices are much more conspicuous. In snakes, amphisbaenians, and limbless lizards, the axial muscles are very important on providing propulsive forces and, therefore, are prominently developed. The hypaxial muscles form much of the body wall and are associated to breathing as it is attached to the rib cage. Most epaxial and hipaxial muscles in squamates split into several layers forming many differentiate muscles that span several segments (Kardong 2014).
Lungs represent the main respiratory surface of squamates, although some degree of cutaneous respiration might be found in several species (Zug et al. 2001; Kardong 2014). The lungs of snakes and most lizards typically include a single central air chamber into which faveoli open (Kardong 2014). Thoracic aspiration is used to ventilate the lungs, and, in lizards, intercostal muscles between the ribs contract and force the ribs forward and outward (Zug et al. 2001). In several snakes, the faveoli may be reduced in the posterior part of the lung, leaving it as a nonexchange region, traditionally named as air sacs or saccular lungs. In monitor lizards, the single central air chamber is subdivided into numerous internal chambers that receive air from the trachea.
The elongated body plan leads to several rearrangements in the organ topography in snakes, amphisbaenids, and elongated-legless lizards. Among the snake organs, the respiratory system exhibits the highest level of specialization of squamates. In primitive snakes, the lungs are paired, but in many advanced snakes the left lung is reduced and often entirely lost (Kardong 2014). A few snakes (scolecophidians and some advanced snakes) exhibit a tracheal lung, which is characterized by the presence of a vascular portion of the lung located anterior to the heart (Wallach 1998).
Squamates show a typical double circulation, which includes a three chambered heart, with the retention of the aortic arches III, IV, and VI. The ventral aorta is subdivided forming the left and right aortic arches and the pulmonary trunk. Such configuration in the aortic arches provides one pulmonary circuit and two systemic circuits, each of which arising independently from the heart. Although the ventricle is given as a single and undivided cavity, three interconnected compartments are present and separated from each other by a muscular ridge. Thus, a few authors might assign the squamate heart as five-chambered, composed of two atria and three compartments of the ventricle, or six chambers if the sinus venosus is counted (Kardong 2014).
Two types of tooth implantation are present in squamates: acrodonty (a few lizards) and pleurodonty (lizards, snakes and amphisbaenids). Additional features of the squamates digestive system include a buccal cavity lacking a secondary palate and several oral glands that support food digestion. In both snakes and lizards, such head glands have evolved independently several times to venom glands that support not only food ingestion, but prey capture. The alimentary canal is very diverse in size and regionalization, mostly depending on type of food ingested. However, its general pattern consists on a short esophagus, with gradual transition to stomach, which is followed by the small and large intestines that finally open in the cloaca. The cloaca is partially differentiated into the coprodeum, a chamber into which the large intestine empties, and the urodeum, into which the urogenital system empties. The alimentary tract of herbivores, particularly the large intestine, is consistently longer and more voluminous than that of similar sized carnivores (O’Grady et al. 2005). In some herbivorous lizards, a cecum is present between the small and large intestines (Kardong 2014). The body elongation on snakes resulted on an elongate digestive system, with inconspicuous folds in the small intestine.
All squamates bear metanefric kidneys, with primary uricotelism as their nitrogen waste. However, nephron structure can be quite different from one taxonomic group to the next and may appear at first to have no obvious correlation with the phylogenetic position, but most likely to their environmental demand. The kidney typically lacks the distinct color and functional distinction of the mammalian cortex and medulla. The kidneys are paired, lobular (weakly so in some lizards), elliptical, pink or red structures that are located retroperitoneally (extracoelomically). Kidneys are posterior to the level of the ilial crest in most lizards and usually lack distinct lobes. Snake kidneys are distinctly lobed and elongated, found in the posterior third of the bodies, with the right kidney occurring anterior (approximately 69–77% of snout-vent length [SVL]) to the left kidney (~74–82% of SVL). Snakes lack urinary bladders; nitrogenous wastes are refluxed from the cloaca into the rectum, where uric acid is stored and further ion reclamation may occur. Several species of lizard lack urinary bladders or develop just a vestigial bladder, including some varanids, agamids and a few gekkonids (Wyneken 2013).
The gonads (ovaries and testes) are located dorsally in the body cavity, posterior to the lungs (except in elongated individuals, where lungs might overcome the right ovarium), and ventral to the kidneys and peritoneal wall. The female reproductive tract is composed of paired ovaries, and oviducts (that might be missing in a few snake species) supported by mesenteries. In lizards, at least the caudal part of each ovary is attached to the peritoneum along the ventromedial surface of each kidney. In some lizards with highly modified lungs, such as chameleons, the ovary may extend cranial between the two lungs. In snakes the ovaries tend to be well posterior to the lung(s) and saccular lung and anterior to the kidneys, attached to the dorsal body wall by the mesovarium, while the testes can be round or fusiform in shape (Mader and Wyneken 2002).
Unlike other reptiles, Squamata have a pair of hemipenis, an intromittent copulatory organ that originates at the junction of the cloacal vent and the base of the tail. Although squamates exhibit two hemipenis, during copulation, only one of them is inserted in the cloaca of the female (Kardong 2014). Each hemipenis is usually grooved (sulcus spermaticus) to allow sperm transport. A retractor muscle returns each hemipenis to the body, a process called invagination, storing the organ at a pocket located at the base of the tail, posterior to the vent. During erection, muscle action and hemotumescence force each hemipenis through the cloaca, expanding it out through the vent, as a process named evagination (Kardong 2014).
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