Determining the sex of fossils can be difficult, if not often impossible, especially if the material consists of isolated bones (ElShafie 2024). Occasionally sex can be identified in fossiliferous assemblages if sexual dimorphism can be detected (Dodson 1976; Cheng et al. 2009); in mammals, the males are often larger than females and some might have specialised armaments such as tusks, antlers and horns and this has been associated with sexual selection (Darwin 1859; Kodric-Brown et al. 2006; Hill 2015) and the genitalia of many groups includes the bony baculum, which exhibits a remarkable variation (Schultz et al. 2016). In non-avian dinosaurs, sexual dimorphism has proved to be more difficult to determine, although in some species of Stegosaurus, plate morphology clearly indicates two different forms, interpreted as sexual dimorphism (Saitta 2015; Benton 2023). Similarly, anatomical differences have been detected in pterosaurs, which remains some of the most compelling evidence for sexual dimorphism amongst fossil reptiles (Bennett 1992). In reptiles, identifying the sex of single fossil specimens, especially males, remains challenging, although additional clues may assist in determining females. In single fossil specimens, female sex may be inferred if the rare specimen is preserved with embryos (Wang and Evans 2011; Liu et al. 2017) or if medullary bone is detected in a sexually mature females, although this tissue is restricted to birds and some non-avian dinosaurs (Rick 1975; Schweitzer et al. 2007; O’Connor et al. 2018; Wang et al. 2020; see also Prondvai and Stein (2014) and O’Connor et al. (2018) for arguments regarding non-reproductive significance of medullary bone in pterosaurs); the presence of medullary bone has been used to determine the sex in fossils birds, including some with marked plumage differences such as the Mesozoic bird Confuciusornis sanctus where males have two elongated ornamental rectrices (Chinsamy et al. 2013). However, the temporary nature of both embryos and medullary bone prevents diagnosing female sex when the animal is not gravid. More frequently, osteological evidence that would indicate sex in fossil reptiles is not apparent and single specimens often remain unsexed.

Using skeletal morphology to determine sex is complex and is often taxon dependent. Amongst fossil neodiapsid choristoderes (e.g. Champsosaurus), these have three sacral vertebrae that can be fused or not and variation in the sacrum has been interpreted to be a sexual dimorphic character (Katsura 2007). Amongst lizards, males have generally larger heads (Kelly 2015; Olsson et al. 2002) and, in some species, there are marked sexual dimorphic traits like the long snout of male Anolis brunneus when compared with females (Sanger and Kircher 2017). In the veiled chameleon (Chamaeleo calyptratus), there is marked sexual dimorphism in body length and size of head casque (Bauerová et al. 2020). Amongst gekkotans, sexual size dimorphism (SSD) is variable; a survey of 43 species found that males were larger in 13 species and females in 26, while the sexes were equal in 4 (Fitch 1981). In the gecko Hemidactylus turcicus, intraspecific variation of the quadrate bone was not associated with sexual dimorphism (Paluh et al. 2018), but studies of size (snout-vent-length) in other species reported marked SSD, where females are larger than males. This is the case of the gecko Ptyodactylus guttatus (Johnston & Bouskila, 2007). In Sphaerodactylidae, the larger species are male biased and smaller species are female biased (Glynne and Adams 2024), although in some miniaturised species in this family, the males are larger (e.g. Sphaerodactylus verdeuzicola Díaz-Lameiro et al., 2022). In Paroedura vazimba, individuals of both sexes hatched at a similar size, but the early growth of females is more rapid, while in Paroedura picta, males and females grow at a similar rate, but males are larger (Kubička et al. 2022; Meter et al. 2024). These studies show that body size is variable and not generally reliable for estimating sex in fossil geckos because there is no clear clade-wide trend and it is still further complicated by a need for larger sample sizes.

Additional structures frequently associated with male geckos are post-cloacal bones (Rieppel 1976; Kluge 1982; Russell et al. 2016) and have been tentatively identified in fossils as indications of male sex (Evans 1988; Evans and Wang 2010). In the genus Aristelliger calcified hemibacula (Kluge 1982; Böhme 1988; Griffing et al. 2025) may also be present, although these have not yet been detected in any fossil lizards. Indirect evidence correlated to sex has also been suggested by the presence of exocranial, hypertrophied endolymphatic sacs in females (Kluge 1967; Lamb et al. 2017; Laver et al. 2020) and a strong association with development of eggs (Ineich and Gardner 1989). In anoles, there was no correlation between hypertrophied endolymphatic sacs and sex (Etheridge 1959) and, in some gecko species, these structures tend to be large, even in hatchlings (Eurydactylodes Bauer, 1989; Euleptes Wiedersheim, 1875; Duran 2025).

Soft tissues might display marked differences between sexes. This may include brightly coloured integument, sexual dichromatism, specific integumentary ornaments (such as horns or spurs). In the green anole, (Anolis carolinensis), it has been demonstrated that males tend to change colour during social displays, while females maintained their colour while displaying (Horr et al. 2023); also males of this species have larger postanal scales (Lovern et al. 2004). Sexual dichromatism is also frequent in many gekkotan species, especially diurnal species (Thomas and Schwartz 1966; Johnston and Bouskila 2007; Regalado 2015; Allen et al. 2020; Cavagnaro 2020; Powell and Uy 2023). Some gekkotans have enlarged femoral glands, precloacal organs or escutcheon scales (Camp 1923; Noble and Klingel 1932; Smith 1935; Thomas and Schwartz 1966; Maderson and Chiu 1970; Bauer 1990) and these structures are more developed in males (Kluge 1967; Vanzolini 1968; Bauer 1990), although they may also be expressed in females (Maderson 1972). The function of these organs and glands is poorly understood (Bauer 1990), but it is likely to help in reproduction or pheromone secretion (Greenberg 1943; Chiu and Maderson 1975).

Reptiles have internal testes and retract the intromittent organ into the body cavity and because of this, sex cannot be identified easily by viewing the animal externally. The male reproductive organs of squamates are divided into hemipenes or paired intromittent organs, which are a synapomorphy of Squamata (Montingelli et al. 2022) and their development is attributed to the lack of expression of Sonic hedgehog (Shh) signalling genes during embryological development (Leal and Cohn 2015). In many squamate reptiles, however, male sex may be determined by the presence of a hemipenal bulge located posteriorly to the cloaca which indicates that the hemipenes have been retracted into the base of the tail (Arnold 1986). While not apparent in all squamates (e.g. scincids), the hemipenal bulge is seen in most male lizards. The hemipenes and the associated bulge, varies in size and is characteristic of crown squamates and is absent in Sphenodon punctatus (Gredler et al. 2014; Sanger et al. 2015), the closest living relative to squamate reptiles. Sphenodon punctatus embryos possess a genital anlage homologous to the paired genital swellings seen in early stages of other amniotes (Sanger et al. 2015); the unique paired hemipenes of squamates develop directly from the hind-limb buds or the remnants of these in the case of limbless forms, while in mammals with unpaired external genitalia, the genital tubercle develops from the ventral and tail bud mesenchyme (Tschopp et al. 2014).

Here, we describe a fossil lizard preserved in mid-Cretaceous (99 Ma) amber from Myanmar. Like other squamates preserved in amber, the scales and portions of the integument are preserved in fine detail, outlining the shape of the body as it was in life, including structures that indicate male sex. Amber squamates previously described preserve fine details and delicate structures, like digital lamellae (Arnold and Poinar 2008; Sherratt et al. 2015), muscle tissue (Daza et al. 2018) and most commonly integument (e.g. Borsuk-Białynicka et al. (1999); Arnold et al. (2002); Perrichot and Néraudeau (2005); Daza et al. (2016); Fontanarrosa et al. (2018); Xing et al. (2018); Bolet et al. (2021); Wagner et al. (2021); Čerňanský et al. (2022); Daza et al. (2024)) in addition to skeletal elements. None has reported any sexually dimorphic structures that would indicate sex, although this was proposed as one possible argument to explain the marked differences between the two specimens assigned to Oculudentavis (Bolet et al. 2021).

The amber lizards described here are identified as a member of Pan-Gekkota and include one of the oldest fossil lizards that can be unequivocally identified as male. This hemipenal bulge in one of the fossils documents the presence of hemipenes in the Squamata, during the mid-Cretaceous, although their origin was probably much earlier likely with the origin of the group.

The phylogenetic placement of Carinasquama rusmithii is uncertain, but it shares many derived characters with geckos. In scansorial geckos, the penultimate phalanges are arcuate and the ungual is short, which contributes to the releasing grip mechanism of the toepad (Russell 2002), Carinasquama has small unguals and lacks both toepads and arcuate penultimate phalanges. The unguals are not recurved— this foot morphology suggests a more terrestrial than scansorial habit (i.e. arboreal, rupiculous), contrasting with other geckos in burmite that have a combination of scansorial traits: 1) remains of toepads; 2) paraphalangeal elements; 3) some phalanges slender proximally and with expanded distal epiphyses; and 4) arcuate penultimate phalanges (Daza et al. 2016; Fontanarrosa et al. 2018).

Amongst living gekkotans, it has been estimated that the ancestor lacked toepads and that these structures originated multiple times and with several subsequent reversals (Gamble et al. 2012); using the same data, but different methods, Harrington and Reeder (2017) argued that the ancestral character state for gekkotans was toepads present and the clade had undergone multiple losses. These criticisms were addressed in later works (Gamble et al. 2017; Russell and Gamble 2019) and it was pointed out that exclusion of detailed morphological evidence contributed to the single origin of gecko toe pads (there is not only a single type of toepad) and this has been corroborated by the demonstration that pad formation has occurred within multiple padless lineages (Higham et al. 2017; Russell and Gamble 2019; Riedel et al. 2021). Adhesive toe pads also developed convergently in other clades, namely anoles and skinks (Gamble et al. 2012; Griffing et al. 2024) and burmite fossils document padless and pad-baring gekkotans, including multiple pad bearing configurations during the mid-Cretaceous (Arnold and Poinar 2008; Daza et al. 2016; Fontanarrosa et al. 2018). This indicates that ground dwelling and scansoriality were already present during the Cretaceous, a time that has been estimated for the origin of the crown clade of geckos (Burbrink et al. 2020) and probably when the first transitions and reversals from pad-less to pad bearing took place. Earlier suggestions of scansoriality have been proposed for the Jurassic pan-gekkotans Eichstaettisaurus, Ardeosaurus and Limnoscansor (Simões et al. 2018; Meyer et al. 2023) and, although these fossils lack evidence of having toe pads, it has been proposed that they have developed some adaptations to climbing; including a symmetrical foot, and dorsoventrally expanded claws and elongated penultimate phalanges (Simões et al. 2018). Additionally, inspection of images in Simões et al. (2018) shows an enlarged lateral process of the astragalocalcaneum of Eichstaettisaurus schroederi and this trait has been discussed as a gekkotan feature (Higham et al. 2021; Russell et al. 2025).

Evidence of hemipenal structures seen here (Figs 1, 2) and in Lower Cretaceous lizards (cloacal bones described by Evans and Wang (2010)) hint at the early evolution of the hemipenes. The hemipenes are one of the most variable features of squamates and evolve at higher rates than other anatomical structures (Arnold 1986; Klaczko et al. 2015; Montingelli et al. 2022). Total group Squamata likely evolved in the Late Permian or Early Triassic and diversified greatly by the mid-Cretaceous, with many lizards from this time being either stem-squamates or basal members of extant clades (Evans and Jones 2010; Simões et al. 2018). During this time, there is an increase in morphological diversity within squamates, represented by variation in body size and form, scalation and novel adaptations (Bolet et al. 2021; Evans et al. 2006; Li et al. 2007; Daza et al. 2016). Today, squamates exhibit an extreme morphological hemipeneal variety and this could have been an additional factor associated with their increased diversification and speciation during the Cretaceous (Evans 2003; Herrera-Flores et al. 2021).

The ancestral state of the amniote penis was likely a single mid-line phallus (Sanger et al. 2015). This feature is lost along the lepidosaurian lineage, with squamates possessing paired hemipenes and Sphenodon (and possibly other extinct rhynchocephalians) lacks a phallus. The variation in lepidosaurian genitalia raises the question of: what was the ancestral state in Lepidosauria? Currently three hypotheses exist on this issue. First, the ancestral lepidosaur possessed a single phallus and squamates and rhynchocephalians diverged on this trait as the lineages split. Second, the ancestral state was the paired hemipenis, which was eventually lost at some point in rhynchocephalians. Third, the phallus was lost in lepidosaurs and then re-evolved in squamates. To address the question, we call for future work to pay close attention to mineralised genital structures like postcloacal bones and penal spines and evidence of soft tissues associated with sex in exceptionally preserved fossil specimens.