Squamata
Squamata - lizards and snakes
Vertebrata;
Gnathostomata; Osteichthyes; Sarcopterygii; Tetrapoda; Amniota; Sauropsida; |
Squamates are a highly
successful and familiar group of reptilies including lizards (e.g. gekkotans,
skinks, chamaeleons), snakes, and amphisbaenians. With over 7000 species, they
are present on every continent except Antarctica, and have invaded marine environments
(e.g. sea snakes, mosasaurs), as well as diversifying into many different
specialised terrestrial forms, including burrowers (e.g. amphisbaenians),
gliders (e.g. Draco),
bipedal runners (e.g. Basiliscus),
climbers (e.g. chameleons), and active predators (e.g. Varanus).
In addition to the plesiomorphic reptilian
condition of egg-laying (oviparity), some groups give birth to live young
(viviparity). More surprisingly, at least eight groups contain a species with
only female members, reproducing by a process called parthenogenesis - the
development of unfertilised eggs into functional (female) offspring.
Squamates range in size from approximately 3 cm (pygmy chamaeleons, e.g. Brookesia minima) to several
metres (e.g. komodo dragon, anaconda, extinct marine mosasaurs). Limblessness
is common in squamates (incluing snakes, amphisbaenians, dibamids, and members
of each major lizard group), and is thought to have evolved as many as five
times independently. For more information on squamate diversity, please visit
the squamate subgroup pages visible in the table of contents to the left.
The Squamata is the sister group to the Rhynchocephalia (the Tuatara and
extinct relatives), sharing a common ancestor around 250 million years ago at
the start of the Mesozoic. Together, they form the monophyletic Lepidosauria. For lepidosaurian synapomorphies and phylogeny,
see the information on the rhynchocephalian page.
Synapomorphies of the Squamata
- Cranial kinesis - a high degree of
flexibility between the bones of the back of the skull, allowing relative
movements between them.
- Paired hemipenes (present in
all lepidosaurs) are fully eversible.
- Pleurodont dentition -
teeth set into the side of the inner surfaces of the jaws, and
periodically replaced.
- Loss of gastralia (ventral belly
ribs).
- Double-hooked fifth metatarsal,
functionally analagous to the mammalian heel.
- Further complexity to the mesotarsal
joint (in comparison to non-squamate lepidosaurs).
Phylogenetic relationships of the Squamata
Researchers in squamate phylogeny have
yet to reach a consensus on the evolutionary affinities of the group's
different lineages. Traditionally, the crown-group Squamata was divided into
Lacertilia (lizards and amphisbaenians) and Ophidia (snakes). Modern cladistic analysis,
however, has demonstrated that both snakes and amphisbaenians can be grouped
with certain lizards to the exclusion of other lizard groups (e.g. Estes et al. 1988), and thus the term 'lizard' is
not a natural grouping (i.e. not monophyletic). Therefore, formal terms like
Lacertilia should be avoided, as they are phylogenetically misleading. The term
lizard can be used informally to indicate any squamate that is not a snake or
amphisbaenian.
Studies based on cladistic analysis
of the morphological characteristics of both extant and
fossil taxa assert
that the primary split in the crown-group Squamata was between the Iguania and the Scleroglossa around 206 million years ago, at
the Triassic-Jurassic boundary. The Iguania contains the familiar iguanas and
chamaeleons, as well as the agamids, and its monophyly is
well supported. For more information see the Iguania page.
The first major phylogenetic analysis of the
Squamata (Estes et al. 1988) defined three major
scleroglossan clades- Gekkota, Anguimorpha, and Scincomorpha. The phylogeny of
Squamata as proposed by Estes et
al. (1998) is illustrated in
the cladogram below
(note the colour-coding of proposed clades, for comparison with conflicting
theories below):
While the monophyly of
Gekkota and Anguimorpha is usually supported by morphological studies, the monophyly of
the Scincomorpha is not always reconciled (e.g. Lee et al.1998), and the
interrelationships of these clades has
been difficult to resolve. In addition to the three principle lineages, there
are three other groups of limbless scleroglossans that have been difficult to
place into the phylogeny (see cladogram above). These are the snakes (Serpentes),
amphisbaenians (Amphisbaenia), and dibamids (Dibamidae). Snakes
are usually placed within the anguimorphs (e.g. Lee et al. 1998), although their origins are a
major subject of scientific dispute. Amphisbaenians and dibamids are, however,
something of a phylogenetic mystery. Often grouped as a clade (Amphisbaenia + Dibamidae), some authors
place them as sister to the Gekkota (e.g. Lee et
al. 1998), while others
suggest a close affinity with snakes (e.g. Rieppel & Zaher 2000).
Lee et al. (1998) suggested that the apparent
grouping of these three problematic taxa as
a clade within
the anguimorphs seen in many phylogenetic studies is a misleading result, as it
is almost exclusively supported by specialisations for a burrowing mode of
life, such as cranial consolidation, loss of limbs and elongation of the body,
which are known to convergently evolve in other tetrapods (e.g. caecilians).
They argue that the addition of fossil taxa is
crucial to understanding the evolutionary transition of these groups to
leglessness, in order to detect any convergence, or homoplasy. Indeed, their
inclusion of certain fossil taxa (namely
the large marine mosasauroids, the basal snake Pachyrhachis, and the limbed
amphisbaenian-like Sineoamphisbaenia)
produced an amphisbaenian-dibamid clade as
sister group to the Gekkota, away from snakes and anguimorphs. Snakes, however,
remained as anguimorphs, forming the clade Pythonomorpha with the large marine mosasaurs of
the Cretaceous, which is then sister group to the monitor lizards (family
Varanidae) in the Varanoidea.
In addition, Lee et al. (1998) questioned the monophyly of
the Scincomorpha, defined by (Estes et
al. 1988). First, they
proposed that scincids and cordylids are more closely related to anguimorphs
than to other "scincomorphs". Second, they placed the xantusiids in a
new clade, Nyctisaura,
as sister group to the Gekkota-Amphisbaenia-Dibamida clade. The remaining "scincomorphs"
persisted in the clade Lacertiformes.
The phylogeny of
squamates proposed by Lee et
al. (1998) is shown below (note the polyphyly of Scincomorpha, as well as
the positions of Serpentes, Amphisbaenia, and Dibamidae, and the addition of
mosasaurs):
More recently, the monophyly of
the Scleroglossa has been questioned by researchers studying squamate DNA
sequence data. For example, Townsend et
al.(2004), who compared around 4600 DNA base pairs in 69 squamate species,
placed the Iguania as well-nested within a consequently paraphyletic Scleroglossa
- contrary to the usual Iguania-Scleroglossa dichotomy. Instead, they proposed
that geckoes (plus relatives) and dibamids were the first squamate groups to
diverge. The monophyly of
the traditional Anguimorpha was supported, although it did not include snakes,
and refuted the monophyly of
Xenosauridae by proposing separate origins for the genera Xenosaurus and Shinisaurus. Again, the monophyly of
Scincomorpha was refuted, albeit in a different manner (see below) to that
proposed by Lee et al.
(1998). Amphisbaenians were placed in yet another position, this time embedded
within the Lacertiformes as sister group to the family Lacertidae. The authors
showed that amphisbaenians share almost identical multicodon deletions in a gene called c-mos with members of a particular lacertid
genus (Gallotia). This provided additional support to their sequence
comparison results that demonstrated that dibamids are not closely related to
snakes or dibamids, but to lacertids. Finally, the relationships between
Anguimorpha, Lacertiformes, Serpentes, and Iguania were not resolved. The cladogram below
illustrates the squamate molecular phylogeny sensuTownsend et al.(2004):
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