Squamata

Squamata is the reptile lineage containing lizards, snakes and amphisbaenians. It belongs within lepidosaurs, whose other living branch is represented by the tuatara. Snakes are nested among lineages commonly called lizards, so excluding snakes does not leave a complete evolutionary branch.

The relationships require evidence beyond superficial similarity: elongated, limbless bodies can arise in different lineages. A 2015 analysis combined hundreds of anatomical characters with dozens of genes from living and fossil groups to compare competing arrangements.^[ref] This makes Squamata a useful example of how phylogenetics tests anatomical interpretations against molecular evidence while retaining fossils that have no recoverable sequence data.

Lacertoidea is a branch of Squamata containing Lacertidae, Teiidae, Gymnophthalmidae and amphisbaenians. It groups lineages by common ancestry, despite striking differences in their visible bodies. In particular, a burrowing, nearly limbless amphisbaenian can be more closely related to a normally limbed member of Lacertidae than superficial resemblance to a snake suggests.^[ref]

This grouping is a useful test of convergent evolution. Imagine recording only whether an animal has external legs: many otherwise unrelated burrowing forms would receive the same score. Recording hundreds of anatomical characters and comparing many genes supplies additional, partly independent evidence. The 2015 integrated analysis used 691 morphological characters and 46 genes, and included fossils as well as living species. Its combined analyses recovered Lacertoidea, while some deeper relationships depended on uncertain placements of fossil taxa.^[ref]

Gymnophthalmidae is a family of lizards within Lacertoidea, often called microteiids. It is distinct from Teiidae, despite that historical common name. Its members include forms with fairly conventional limbs and forms with strongly reduced limbs and elongated bodies, making the family useful for studying how a body plan changes during evolution.^[ref]

A morphological study distinguished modest reductions, such as losing one or two finger or toe bones, from much more extensive reductions that can leave no externally distinct adult limb. The investigators compared digit arrangements and also examined gene expression in developing reduced limbs. Adult anatomy and embryonic development consequently provide complementary observations of the same evolutionary problem.^[ref]

For an anatomical example, two feet can each have four visible digits while differing in which digit is absent or in the number of bones within each remaining digit. Merely counting visible toes collapses those possibilities into one score. Recording the detailed bone pattern and placing it on a phylogenetic tree gives a more informative test of whether superficially similar reductions share an evolutionary origin.

Teiidae is a family of lizards within Lacertoidea, including the whiptails of Aspidoscelis. It is a separate lineage from Gymnophthalmidae. Beyond their place in reptile classification, some whiptails offer a striking example of parthenogenesis: females produce offspring without fertilization.^[ref]

In a 2010 study of parthenogenetic Aspidoscelis, investigators asked how eggs could retain the parental complement of chromosomes and preserve heterozygosity. Chromosome imaging and fluorescent probes showed an extra premeiotic doubling, followed by pairing of genetically identical chromosome copies. The cells still underwent meiosis and chromosomal crossover; reproduction did not simply bypass the entire meiotic program.^[ref]

A simplified genetic example explains the significance. Begin with distinguishable chromosome versions $A$ and $B$. After copying, matching $A$ with another $A$ and $B$ with another $B$ permits exchange between equivalent copies while retaining both parental versions in the unreduced egg. This differs from mechanisms that discard one version and thereby lose heterozygosity. The finding concerns particular parthenogenetic species; it does not make every member of Teiidae an asexual animal.

Aspidoscelis is a taxonomic genus of whiptail lizards in Teiidae. It contains both sexually reproducing species and all-female lineages that reproduce by parthenogenesis. This variation makes a closely related set of animals useful for studying how reproduction and chromosome organization interact.^[ref]

In a 2011 experiment, researchers crossed a triploid parthenogenetic female lineage with males of a sexually reproducing lineage. The resulting four-copy hybrid offspring were checked with molecular and chromosome measurements. Some female offspring themselves reproduced by parthenogenesis, establishing laboratory lineages able to reproduce without repeating the original cross.^[ref]

A chromosome-set calculation clarifies the change: an unreduced egg carrying three sets can combine with a sperm carrying one set to make a four-set offspring, $3n+n=4n$. This is tetraploidy, rather than merely four copies of one selected gene. Producing one viable hybrid is also different from demonstrating that its descendants can reproduce. The experiment tested both steps, linking the immediate effects of a cross to the persistence of a new reproductive lineage.

Lacertidae is a family of lizards within Lacertoidea, distinct from Teiidae, Gymnophthalmidae and amphisbaenians. It provides an important comparison for the latter group: common ancestry can connect normally limbed lizards with highly modified burrowing forms, so a wormlike body is not by itself a reliable map of evolutionary relationships.^[ref]

Lacertidae is also a useful system for investigating thermoregulation. A 2019 comparative study related environmental temperatures to thermal physiology and to the evolutionary histories inferred from genomic data. Behavioral choices, such as basking or using shaded shelter, are relevant because the air temperature at a site is not automatically an animal's body temperature.^[ref]

For an illustrative measurement design, offer a lizard a range of warm and cool positions and record both its chosen position and body temperature. Repeating the assay at different background temperatures distinguishes an available environment from the temperature the animal actively selects. Comparisons between species then require the same measurement protocol and an account of their relatedness; otherwise a difference caused by the assay or shared ancestry can be mistaken for an independent adaptation.

A skink is a lizard in the family Scincidae, within Squamata. The family contains a wide range of body forms, from animals with four well-developed limbs to elongated forms with strongly reduced limbs or none externally visible. A long, legless body therefore does not establish that a reptile is a snake.^[ref]

A 2023 comparative dataset classified $22.8\%$ of its skink species as having reduced or absent limbs, using limb proportions relative to snout-to-vent length. This makes the comparison measurable: a short absolute leg length in a tiny animal is different from a short leg relative to the animal's body. Placing those measurements on a phylogenetic tree permits tests of repeated evolutionary changes instead of treating every similar body shape as one inherited event.^[ref]

Reproduction is diverse as well. Some skinks lay eggs and others give birth to live young. In the same dataset, about one third of the species with known reproductive mode were live-bearing; that qualification matters because unknown cases were excluded. A family name captures ancestry without requiring every member to share one body plan or reproductive mode.^[ref]

An amphisbaenian is a member of Amphisbaenia, a lineage of predominantly burrowing Squamata. The elongated body often has rings of scales, small eyes and a skull specialized for moving through soil. Most species lack external limbs, but Bipes retains forelimbs. Their wormlike appearance does not make them Annelida, and a limbless body does not make them snakes.

Their evolutionary relationships illustrate the problem of convergent evolution. Burrowing can favor similar reductions and skull modifications in separate reptile lineages, making an anatomy-only grouping misleading. An integrated 2015 study compared morphological characters, molecular sequences and fossil taxa, and recovered amphisbaenians with lacertids in Lacertoidea. Including DNA allowed the investigators to test similarities that might instead reflect similar modes of life.^[ref]

The example shows why a shared loss of limbs must be interpreted on a phylogenetic tree, alongside other evidence, before it is treated as evidence of close ancestry.

Bipes is a taxonomic genus of burrowing amphisbaenians distinguished by retaining external forelimbs despite an elongated body and absence of external hindlimbs. It is an informative comparison with other amphisbaenians, which lack external limbs: external appearance alone does not reveal how much internal anatomy remains.^[ref]

In a 2019 anatomical study, investigators used iodine-enhanced computed tomography to distinguish soft tissues and reconstruct shoulder muscles as well as bones. Bipes retained an almost complete shoulder girdle, while other sampled amphisbaenians had much more reduced bony structures. Nevertheless, all sampled amphisbaenian forms retained the full set of 17 shoulder muscles, sometimes fused or rearranged. Muscle attachment could shift from bone to surrounding soft tissue.^[ref]

The result challenges a tempting shortcut: losing a visible limb does not necessarily delete all its associated muscles at once. Bipes supplies the limbed comparison that helps reveal which internal components have persisted or changed during the evolution of the burrowing body form.