Iguania - iguanas, chamaeleons, and agamids

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Diversity and Lower Taxonomy

Like most other squamate groups, the taxonomy of the over 1550 species-rich Iguania has long been a topic of dispute. An early classification of the Iguania by Charles Camp in 1923 defined three families:

• Iguanidae - Approximately 956 species of iguanas, anoles, horned lizards, collared lizards, and relatives.
• Chamaeleonidae - Approximately 178 species of chamaeleon.
• Agamidae - Approximately 416 species of agamid, such as the gliding Draco, and the thorny devil Moloch horridus.

This simple classification remained for over 50 years, but there became a strong need to readdress these relationships in a phylogenetic framework, i.e. one that uses cladistics to understand evolutionary history by identifying monophyletic groups, or clades. This led Etheridge and de Queiros (1988) to examine the contents of the family Iguanidae and, through analysis of morphological characteristics, identify eight major groups within. These were the subfamilies Corytophaninae (helmet lizards), Crotaphytinae (collared lizards),Hoplocercinae (dwarf and spiny tail iguanas), Iguaninae (iguanas), Oplurinae (Madagascan iguanas), Phrynosomatinae (spiny lizards, horned lizards, and relatives), Polychrotinae (anoles), andTropidurinae (neotropical ground lizards).

After further study, however, Frost and Etheridge (1989) failed to find sufficient support for the monophyly of Iguanidae, and so proposed to raise the eight subfamilies to family status (all had the same name but with a -dae suffix rather than a -nae suffix). While some authors still use this taxonomy, many refute it. For example, Macey et al. (1997) found strong support for the monophyly of the traditional Iguanidae using a combined morphological and DNA sequence analysis, and proposed that the families promoted by Frost and Etheridge (1989) be returned to their initial statuses as subfamilies of Iguanidae. This was corroborated by more comprehensive combined analyses by Schulteet al. (1998, 2003), who also brought into question the monophyly of two of the subfamilies of the Iguanidae, the Tropidurinae and Polychrotinae. The monophyly of all the remaining subfamilies was strongly supported, although the phylogenetic interrelationships between these subfamilies failed to be reconciled.

The sister group to the Iguanidae is the monophyletic Acrodonta (Estes et al. 1998). This contains the remaining iguanians: the chamaeleons and agamids. Within the Acrodonta, studies such as Maceyet al. (1997, 2000) have found little statistical support for the monophyly of the traditional family Agamidae. That is, some agamids may be more closely related to the chamaeleons than to other agamids. Nonetheless, it is useful to use the term Agamidae in the meantime, as what is termed ametataxon - a traditionally recognised group whose monophyly is statistically uncertain. Macey et al.(2000) defined six agamid subfamilies:

• Agaminae - the African-West Asian clade of agamids, containing 6 genera, including Agama.
• Amphibolurinae - the clade comprising the Chinese water dragon, Physignathus cocincinus, plus all Australian or New Guinean agamids, in 14 genera.
• Draconinae - the South Asian clade of agamids, containing 14 genera, including the gliding Draco.
• Hydrosaurinae - the sailfin lizards, Hydrosaurus.
• Leiolepidinae - the butterfly lizards, Leiolepis.
• Uromastycinae - the spiny-tailed lizard, Uromastyx.

The following cladogram illustrates the evolutionary history of these taxa, as proposed by Macey et al.(2000). While the monophyly of the subfamilies was well supported, the among group phylogenetic relationships and the monophyly of Agamidae shown in the diagram received little statistical support.

The chamaeleons, on the other hand, remain in the monophyletic clade Chamaeleonidae. This family is divided into two subfamilies, Brookesiinae (containing three genera, including Brookesia) andChamaeleoninae (containing 6 genera, including Chamaeleo).


Description

The iguanids.

Living agamids are a highly diverse group of average to large-sized lizards with a wide range of specialisations. As a result, it would be difficult to describe the appearance of a "typical" agamid. For example, members of the South Asian arboreal genus Draco (subfamily Draconinae) have evolved extremely elongate, protruding ribs, which stretch the skin out into two wing-like patagial membranes, allowing individuals to perform extensive glides between trees (recorded as far as 60 m, with only a 10 m descent). Some groups possess modified spiny scales, which may cover the body (as in the Australian thorny devil, Moloch horridus) or the tail (as in members of the genus Uromastyx), while another group, the Southeast Asian sailfin lizards (Hydrosaurus), possess a laterally compressed dorsal extension of the tail, making them proficient swimmers.

Most agamids are diurnal, feeding mainly on insects and other small prey. A few, e.g. Uromastyx, are partly herbivorous. They are all oviparous, except for members of the genus Phrynocephalus, which give birth to live young (viviparous).

The chamaeleons are a familiar and charismatic group of lizards, with a large suite of unique physical characteristics making them extremely distinct. Ranging in size from around 2.5 cm (e.g. Brookesia spp.) to over 50 cm (e.g. Furcifer oustaleti), they are probably most famous for their ability to change colour - made possible by the presence of specialised chromatophores (cells containing pigment that reflect light) in the skin - which is often used in social signalling. The body of a chamaeleon is laterally compressed, and the head often bears many horns and ridges. These features are likely to play a role in sexual selection, as males are usually considerably more ornamented than females.

Chamaeleons are highly specialised for arboreal life. Not only do they have a strong grasping prehensile tail, but their feet are zygodactylus - meaning that two digits face forwards and two face rearwards, allowing a firm grip on a branch. In addition, they have an extremely long, rapidly protractable tongue, with which insect prey can be seized efficiently from a distance often as far away as the length of their own body. This kind of accuracy is afforded by the positioning of their eyes, which are bulbous and protruding, allowing the two fields of vision to overlap and result in stereopsis(depth perception). The eyes can also be moved independently. The tip of the tongue is highly muscular and covered in mucus, forming a suction cup that is extremely difficult for prey to escape.


Distribution and Habitat

Iguanids

Agamids are distributed throughout Asia, Oceania, Africa, and Europe. Thus, they are a strictly Old World group. Inhabiting areas from desert to tropical rainforest, they are mostly terrestrial, although some groups, e.g. Draco, are abrboreal.

Chamaeleons are mainly found in Africa and Madagascar, but a few species occur in Southern Europe, the Middle East, and Asia. Like agamids, chamaeleons are an Old World group. They are, for the most part, an arboreal group, again inhabiting regions as diverse as desert to tropical rainforest.


Conservation Status (IUCN)

There are 223 species of iguanid present on the IUCN Red List. Almost 25% of these are recognised as either Critically Endangered (CR; 5%), Endangered (EN; 7%), or Vulnerable (VU; 12%). Thankfully, the majority (55%) are considered Least Concern (LC). The remainder are either Near Threatened (NT; 4%) or Data Deficient (DD; 17%).

Only 13 of the 416 species of agamid are present on the Red List. Of these, 2 are Critically Endangered (CR), whilst 2 are Endangered (EN), and 3 are Vulnerable (VU). Of the remaining 6 species, 2 are Data Deficient (DD), and the rest are Lower Risk (LR) - either Near Threatened (NT) or Least concern (LC).

There are 9 species of chamaeleon on the Red List. Smith's dwarf chamaeleon, Bradypodion taeniabronchum, is the only Critically Endangered (CR) species. Of the rest, 2 are Endangered (EN), 4 are Vulnerable (VU), and 2 are Near Threatened (NT).


Synapomorphies of the Iguania and Acrodonta

• Iguania:
  • Postfrontal either reduced or absent.
  • Anteroventral margin of the orbit formed by the jugal.
  • Ridges near the orbital margin.
  • Frontal shelf underlying the nasal.
  • Pineal foramen located on the frontoparietal suture or within frontals.
  • Articular separate from prearticular and surangular.
  • A contact between the jugal and squamosal along the upper temporal arch.
• Acrodonta
  • Acrodont dentition.
  • No replacement teeth.
  • Postfrontal absent.
  • Contact between palatines across the entire midline.
  • No intravertebral fracture planes for caudal autotomy.
  • No depression on ventral surface of the pterygoid.
  • Unique mitochondrial gene order, involving the rearrangement of two tRNA genes - a molcular synapomorphy.