DESCRIPTION OF THE CLASS LEPIDOSAUROMORPHA (BENTON 1983)

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Lepidosauromorpha (le-pi-do-SAR-o-MORF-a) is derived from three Greek roots meaning “scaled lizard forms” [scaled- lepi (λέπι); lizard- saura (σαύρα); form- morphi (μορφή)]. The name was coined by Benton (1983) to include the lepidosaurs and sauropterygians in a higher taxon.

INTRODUCTION TO THE CLASS LEPIDOSAUROMORPHA The lepidosauromorphs include two very different groups: the lepidosaurs (snakes, lizards and tuataras) and sauropterygians (from two Greek roots meaning “lizard wings”), extinct aquatic Mesozoic animals with four paddle-like legs. They evolved from a diapsid ancestor and are sisters to the Archosauromorpha (see Figures 1-A and 1-B). Though the groups are quite different from each other, Benton (2005) asserts that they are monophyletic based on the following four synapomorphies, only one of which do I view as a strong association: – reduction or loss of the supratemporal bone in the skull (weak association) – the dorsal intercentra are absent (weak association; see Figure 2 of Batrachomorpha for an illustration of the intercentrum) – the thyroid fenestra, a new opening in the pevic girdle (strong association) – no teeth on the lateral pterygoid flanges in the back of the mouth (weak association)

	FIGURE 1.A. Position of the Lepidosauromorpha in the gnathostomes according to Benton (2005) with two major groups: the extinct Sauropterygia and the extant Lepidosauria.
	MAJOR CLADES OF THE LEPIDOSAUROMORPHA 1. Sauropterygian Clade 2. Lepidosaur Clade 3. Sphenodont Clade 4. Squamata Clade 5. Serpentes Alternate Clades
FIGURE 1.B. Major Clades of the Lepidosauromorpha. The Lepidosaurs evolved from diapsid ancestors, though the lower temporal fenestra was lost in the Sauropterygian Clade. The Lepidosaurs (Clade 2) contain the Sphenodonts and Lizards and Snakes. The origin of snakes (Clade 5) is in dispute according to morphological evidence. Molecular evidence gives a very different picture (see below). The cladogram comes from Benton (2005) and Pough et al. (2009).

Sauropterygian Clade (1) Though of diapsid ancestry, the sauropterygians, like the ichthyosaurs, lost their lower temporal fenestra, a condition called euryapsid. Most of them seem to have been well-adapted to an aquatic existence and could not have moved about on land. Although some like the placodonts likely were shellfish eaters (Figure 2), most other sauropterygians had mouths filled with needle-like teeth adapted to catching fish. The most striking were the primitive nothosaurs (Figure 3), and the more derived plesiosaurs (Figure 4), which had small heads and very long necks, According to an analysis of the skeleton by Henderson (2006), the long-necked sauropterygians could not lift their heads out of water and seem to have been ambush predators. Nothosaurs, semi-aquatic animals of the Triassic, had recognizable legs and likely swam like a long-necked crocodile. Plesiosaurs had paddle-like legs and could have moved on land only with great difficulty. Also with paddles, the pliosaurs (Figure 5), short-necked animals within the Plesiosauria, had very large heads and powerful jaws which clearly were adapted to catch large fish and other aquatic animals, perhaps even ichthyosaurs and other sauropterygians.

FIGURE 2. Paraplacodus, a Triassic placodont. The animal looked like a large lizard and was marine, feeding as a walrus feeds on mollusks and other shellfish. Image by: ArthurWeasley Wikipedia	FIGURE 3. Nothosaurus a semi-aquatic animal of the Triassic had recognizable legs and feet rather than paddles. Likely, its legs were webbed and its tail was flattened. Its skeleton suggests that it could move only move sluggishly on land, much like a seal. Its teeth also suggest that it fed on fish. Image by: ArthurWeasley Wikipedia	FIGURE 4. Image of Elastodenta, an extinct plesiosaur of the Cretaceous. Note the paddle-like legs (front and back). Image from: http://www.biltek.tubitak.gov.tr/bilgipaket/jeolojik/Fanerozoik/Mezozoik/Kretase/KretaDeniz.htm	FIGURE 5. Kronosaurus, a pliosaur from the Jurassic and Cretaceous, was a powerful aquatic animal with a short neck and very large head. Image in the Public Domain

Lepidosaur Clade (2) and Sphenodont Clade (6) The lepidosaurs appeared in the Triassic period. The earliest lepidosaurs were the sphenodonts, which were lizard-like in appearance, but structurally much more primitive than true lizards. Sphenodonts had skull elements that were fused and immobile. In addition, both zygomatic arches were complete. Once a diverse and successful group, only Sphenodon (Figure 6), commonly known as tuatara and restricted to the islands of New Zealand, is a living remnant of the Sphenodontida.

Squamata Clade (4) Members of the Squamata are very successful and today occupy all but the polar terrestrial environments. In addition, some have exploited aquatic environments. Today, these include the lizards and snakes. The lizards, in a paraphyletic assemblage called Lacerta, seem to have appeared in the Triassic or early Jurassic period and are characterized by having highly reduced skull elements, which tend to allow the jaws to move independently of the cranium. Five assemblages of lizards are recognized: Iguania, Gekkota, Amphisbaena, Anguimorpha, and Scincomorpha. – The lizard families within Iguania include iguanas, chameleons, and anoles. Although most are of the squamates are carnivores or insectivores, a few are herbivorous (most notable are the Green Iguana and the Marine Iguana). The sexual dimorphism in this group is striking (see Figure 7 for a mature male Iguana). Benton (2005) states that the Iguania appeared in the Triassic or Jurassic and are the most primitive members of the Squamata and, therefore, the sister group to all of the other lizards and snakes. – The Gekkota according to Benton (2005) may have appeared in the upper Jurassic. Today, Geckos are distinctive lizards (Figure 8), usually small and insectivores. Many have the distinction of having specialized pads on their feet that allow them to scale almost any surface. When we were students in Oklahoma and Florida, House Geckos were common sights in some of the universities’ buildings, around outdoor lights during the warm months, and even in our respective apartments. – Amphisbaena is a collection of legless lizards that have a suite of adaptations to support a burrowing lifestyle. They have a specialized scale associated with the mouth, and no eyes. The Iberian Worm Lizard (Figure 9) shows some of the salient features of the group: the down-sloping, shovel-like head and the blunt tail. – Anguimorpha is a very diverse group which includes the anguids (legless lizards, Figure 10) and varanids. Mosasaurs (Figure 11), monstrous ocean-going varanid lizards of the late Cretaceous were similar to the short-necked pliosaurs, even to the point of having paddle-like legs. Today, the varanid, Komodo Dragon (Figure 12), is the largest living lizard. – The Scincomorpha dates to the middle Jurassic (Evans 1998). Skinks are elongate with a pointed snout (Figure 13). The head is covered with plates and the body is covered with smooth cycloid scales. The family Scincidae is the most speciose and diverse family of lizards, many of which are legless. Together with the geckos, the skinks make up more than half of the 4,765 species of lizards.

FIGURE 6. A photo of a living Sphenodon. Image from: http://www.zo.utexas.edu/courses/bio213/Sphenodon.jpg	FIGURE 7. A male Iguana with characteristic dewlap and other embellishments. Image in the Public Domain by Manuel de Corselas	FIGURE 8. Photo of a Leopard Gecko (Gekko), a lizard. From the Systematic Biology Biodiversity Archive.	FIGURE 9. An Iberian Worm Lizard, a member of the Amphisbaena, is legless and fossorial. Note how much it resembles a caecilian. Image in the Public Domain by Richard Avery
FIGURE 10. Anguis fragilis, the Glass Lizard is legless and superficially resembles a snake, but the ability to lose its tail and other characters identify it as a lizard. Image in the Public Domain by Marek	FIGURE 11. Mosasaurus, a large marine varanid lizard from the upper Cretaceous bore a striking resemblance to Kronosaurus. Image in the Public Domain by Dimitry Bogdanov	FIGURE 12. A Komodo Dragon, Varanus komodoensis. Image is in the Public Domain by Midori	FIGURE 13. A Blue-Tongued Skink. Image by Benjamint444, a contributor to Wikipedia

Serpentes Clade (5) Snakes emerged from within the lizard clade; however, the particular group is uncertain. Part of the problem is that the loss of legs has occurred many times in the lizards. In addition, there is debate (summarized in Vidal and Hedges 2004 and Benton 2005) as to the particular selective pressure that gave rise to the snakes (aquatic vs. terrestrial influences). Because most legless lizards are fossorial (burrowing), it would appear that burrowing lifestyle might have provided the selective pressure to give rise to modern snakes. Other possible ancestors include aquatic lizards like the mosasaurs. Morphological evidence seems to support either the aquatic or terrestrial theories. Molecular evidence, however, seems to exclude the varanids as a sister group and lends strong support to the terrestrial origin. Which terrestrial group is still up in the air. All groups of lizards except the Iguania have legless forms. Curiously, though, the Iguania seem to show the strongest sister relationship with the snakes (Vidal and Hedges 2004). Either way, all animals designated as snakes are members of the same successful line (>2,500 species). All snakes are carnivores (or insectivores). Many snakes catch and subdue their prey by injecting toxins or by immobilizing and constricting. Snakes swallow their prey whole and can consume animals several times their own diameter. They are aided in this by the mobile skull and a body that can expand to accommodate the meal. Some like the python can swallow animals as large as goats, pig or deer. There are three major groups of snakes: Typhlopoidea, Henophidia, and Xenophidia. – Typhlopoidea includes small blind, fossorial snakes (Figure 14). One group of them does retain remnants of the pelvic girdle. The similarities between the typhlopoids and the amphibaenids does lend support to that group as the sister to the snakes. – Boas, pythons (Figure 15), and anacondas are in the Henophidia. These are constrictors which can be very large and powerful and subdue their prey by constricting them until they asphyxiate. Some members of this group retain vestiges of the pelvic girdle and limbs. – Xenophidia is the largest group of snakes and includes all of the poisonous taxa. The venomous snakes have fangs on the front of the mouth (e.g. vipers, elapids, and sea snakes) or at the back of the jaw (most “nonpoisonous snakes”) with which they subdue prey before swallowing them. Examples of this group are cobra (Figure 16) and the Green Tree Snake (Figure 17), a front-fanged and a rear-fanged snake, respectively.

FIGURE 14. Rhamphotyphlops is a blind fossorial snake from the tropics. Image is in the Public Domain	FIGURE 15. Python molure, Indian Python. Image is in the Public Domain by Patric Jean	FIGURE 16. Indian Spectacled Cobra, Naja naja. Image by Kamalnv, a contributor to Wikipedia	FIGURE 17. A green tree snake, Dindrelaphis. From the Systematic Biology Biodiversity Archive.

LITERATURE CITED Benton, M. J. 1983. The Triassic reptile Hyperodapedon from Elgin: functional morphology and relationships. Philosophical Transactions of the Royal Society of London. Series B. 302: 605-720. Benton, M. J. 2005. Vertebrate Paleontology. Third Edition. Blackwell Publishing, Malden, MA. Evans, S. E. 1998. Crown group lizards (Reptilia, Squamata) from the Middle Jurassic of the British Isles. Paleontographica Abteilung A. 250: 123-154. Henderson, D. M. 2006. Floating point: a computational study of buoyancy, equilibrium, and gastroliths in plesiosaurs. Lethaia. 39: 227-244. Pough, F. H., R. M. Andrews, J. E. Cadle, M. L. Crump, A. H. Savitzsky, and K. D. Wells. 1998. Herpetology. Prentice Hall. Upper Saddle River, NJ. Pough, F. H., C. M. Janis, and J. B. Heiser. 2009. Vertebrate Life. 8th ed. Benjamin Cummings. New York. pp. 688. Vidal, N. and B. B. Hedges. 2004. Molecular evidence for a molecular origin of snakes. Proceedings of the Royal Society of London. B (supplement). 271: S226-S229.

By Jack R. Holt and Carlos A. Iudica. Last revised: 04/07/2013