Lizards (including amphisbaenians) and snakes, form Squamata, a clade of primarily terrestrial reptiles. They show a diversity of foraging modes, antipredation strategies, and Baupläne. Such adaptations help them to be highly successful in an extended range of environments, and they are the largest group of non-avian reptiles1,2,3. Squamates have a history estimated to date back over 200 million years4,5,6,7. Their early Mesozoic record remains limited, but their Cretaceous fossil record has been improving, especially in the last few decades. Nonetheless, this Cretaceous record is heavily biased toward northern continents; far less is known of the Mesozoic squamate record in Gondwana, leaving many aspects of the evolutionary history and palaeobiogeography of lizards uncertain.
The amber mines of Katchin State, northern Myanmar form a series of deposits dated from ~110 Ma (Hkamti amber; i.e., the “new mine”) to ~72 Ma (Tilin site, see Xing and Qiu8 2020; Nyunt et al.9), whereas specimens from elsewhere in the Hukawng valley are dated to 99 Ma10. These amber deposits have yielded a significant number of mostly early Cenomanian squamates as inclusions, including some exquisitely preserved lizards11,12,13,14,15,16,17 and snakes18. Elsewhere in the world, a majority of Albian and Cenomanian terrestrial lizard fossils are represented by disarticulated and isolated elements19,20,21,22,23,24,25, whereas Myanmar amber is famous for the extraordinary preservation of articulated fossils with preserved integument, although many of these are limited to isolated limbs and/or tails12. Therefore, the amber deposits in Myanmar provide a unique window into the mid-Cretaceous world. Moreover, the Burma Terrane has been reconstructed as part of a Trans-Tethyan island arc at the time of amber deposition26,27, with the amber biota representing an endemic island fauna, possibly of Gondwanan derivation26. The best represented group is Gekkota, which today contains over 2000 extant species of geckos and pygopods3, and is distributed worldwide in warm temperate to tropical areas28,29. However, the skeletons of these lizards are often delicate and the global gekkotan fossil record is relatively poor, making the Myanmar amber deposits particularly significant for this clade11,12. Other lineages – including iguanians – may also be represented from these deposits12,16,17.
In this paper we report on a well-preserved juvenile specimen from the Albian amber of the Hkamti site (ca. 110 mya). The specimen is represented by the articulated skull and the anterior portion of the trunk, including the pectoral girdle and anterior limbs, and is characterized by pristine detail of the integument. In this paper we describe the specimen in detail using high-resolution X-ray microcomputed tomography (synchrotron data), and discuss the possible affinities of the new taxon.
Squamata Oppel, 181130
Scinciformata Vidal and Hedges, 200531
Scincoidea Oppel, 181130
? Pan-Xantusiidae Gauthier et al., 20122
Retinosaurus gen. nov.
Etymology. the Greek word "Retine" which is the general term for all resin liquids exuded from tree trunks (lithified as amber) and saurus meaning lizard.
Diagnosis. As for Retinosaurus hkamtiensis, the only known species.
Retinosaurus hkamtiensis gen. et sp. nov.
LSID for this species: zoobank.org:pub:87548612-CECB-4631-9076-07A65813A7B9
Etymology. hkamtiensis; after the locality Hkamti.
Holotype. GRS 29689, an amber-inclusion with a well-preserved skull, including the mandible, part of the hyoid (ceratobranchial 1), and a partial postcranial skeleton, as well as well-preserved skin tissues (Figs. 1-3, 4b and Supplementary Figs. 1–2). The specimen is housed in the Peretti Museum Foundation (PMF), Gem Research Swiss Laboratory (GRS) in Meggen Switzerland. The PMF fulfills all requirements to hold a legal collection under Swiss law and provides access to all bona fide researchers.
Type locality, horizon and material: The specimen was recovered from the Hkamti District at Patabum (Sagaing State), in close proximity of the Jade mines, 100 km to the southwest of the Hukawng Basin in the northern Myanmar Central Basin32.
Age. Amber from this mine at the Hkamti District has been dated to the Albian, ca. 110 million years ago (Ma), using zircon U-Pb isotopes8.
Diagnosis: A lizard differentiated from other named Myanmar fossil squamates in a combination of features including short jaws, absence of a lacrimal, and procoelous vertebrae (contra Oculudentavis khaungraee, O. naga), and normal limbs and body proportions (contra Barlochersaurus winhtini). Protodraco monocoli is an isolated limb, but differs from Retinosaurus in having finer scalation. Allowing for immaturity, Retinosaurus also differs from other roughly contemporaneous fossil squamates known from Europe, Asia, and the Americas in the following combination of characters: depressed skull; nasal process of unpaired premaxilla long, almost reaching frontals; anterior width of nasals exceeds nasofrontal joint width; elongate frontal plate only weakly emarginated by orbits (contra Eichstaettisaurus, Liushusaurus, Meyasaurus, Huehuecuetzpalli); anteriorly well-developed subolfactory processes that extend toward the ventral midline; interdigitated fronto-parietal suture (as in Yabeinosaurus, Sakurasaurus, but contra Huehuecuetzpalli), with parietal tabs underlying frontals; paired parietals lacking ventral fossa for supraoccipital processus ascendens (contra Yabeinosaurus, Sakurasaurus, Kuroyuriella, Hoyalacerta, Dorsetisaurus, Purbicella, Jucaraseps, Huehuecuetzpalli, paramacellodids, polyglyphanodontians); lacrimal bone absent (contra Purbicella); palatal dentition absent (contra e.g., Dalinghosaurus, Yabeinosaurus, Purbicella); ectopterygoid with hooked posterior process that is laterally exposed (contra Tepexisaurus); ectopterygoid contacts palatine to exclude the maxilla from the lateral margin of the suborbital fenestra; large, deeply recessed lateral opening of the recessus scalae tympani; jaw joint lies well anterior to level of occipital condyle (contra e.g., Huehuecuetzpalli, Tepexisaurus); homodont pleurodont dentition of moderately pointed and unicuspid tooth crowns (contra bicuspid as in Meyasaurus, Hakuseps, Pedrerasaurus; multicuspid in Asagaolacerta and many polyglyphanodontians including Kuwajimalla; robust and striated, as in Saurillodon; truncated with anteroposteriorly directed apical groove in contogeniids, or rounded in Gueragama); splenial short, not reaching mid-point of dentary; long straight retroarticular process (e.g., contra Meyasaurus, Tepexisaurus, Huehuecuetzpalli); zygapophysial facet between atlas and axis; first and second intercentrum small and not in contact with each other; cruciform interclavicle with long anterior process (contra rhomboid, as in Scandensia, or T-shaped in Huehuecuetzpalli); short robust ungual phalanges with a terminal hook (contra slender and elongated in e.g., Scandensia); phalangeal formula of manus 2:3:3:4:3, with the two intermediate phalanges on digit 4 unusually short; cephalic scales have a regular dorsal scutellation pattern of alternating single and paired scales from front to back on the skull roof; four large scales and several tiny scales covering the eyelid; and horizontal palpebral fissure present.
Specimen description. The specimen preserves the skull, anterior vertebral column, forelimbs and pectoral girdle of a small lizard, as well as the skin and scales covering the body (Fig. 1) and skull. The specimen is small (estimated Snout-Pelvis Length [SPL] is around 35 mm), and all indications are that this was a juvenile animal. The skull roof is incompletely ossified with a large fronto-parietal fontanelle (Figs. 2, 3); the sutures are not tightly connected (as shown by disarticulation and displacement of several skull bones); the vertebral neural arches are not fused in the dorsal midline; the scapula and coracoid are not co-ossified; the epiphyses of the forelimb bones are not ossified; and the carpals and phalanges appear weakly ossified. The immaturity of the specimen is an important consideration with respect to the description (for a detailed description of each skeletal element and illustrations of the virtually isolated braincase, inner ear, and head scaling, see Supplementary Data 1), as bone shapes and fusions may alter the appearance of the adult skull. The skull is depressed and anteroposteriorly somewhat elongate. It is widest in the region of the parietals and then gradually narrows into a rounded anterior tip. Its maximum width (at the level of the quadrates) is 5.8 mm, whereas its anteroposterior length (from the tip of the snout to the occipital condyle) along the midline is 9.5 mm. The pre-orbital region is short, whereas the post-orbital portion is extended posteriorly. There is a small, narrow supratemporal fenestra, and an elongate infratemporal fenestra. The jaw joint lies well anterior to the occipital condyle, giving the quadrate a strongly oblique orientation that is not simply due to postmortem compression. The specimen preserves parts of the anterior vertebral column, pectoral girdle, and forelimb. A total of 23 procoelous presacral vertebrae are preserved, with ten anterior vertebrae clearly visible (of which 7-9 may be cervicals), whereas a further 13 vertebrae and their ribs are enclosed in a calcite sheath that partially obscures their structure.
Integumentary structures are unusually well preserved. The cephalic scales are enlarged and well preserved and follow a regular dorsal scutellation pattern of alternating single and paired scales from front to back of the skull roof. Smaller scales ranging in shape from rectangular to hexagonal cover the dorsal surface of the body, but those on the ventral side of the head are larger and rectangular. The left side preserves scleral ossicles and eyelid. The eye has rectangular scleral ossicles, seven of which are visible in dorsal view, and assuming that the ventral ones are similar in size and spacing to the dorsal ones, the eye is estimated to have had at least 14 ossicles in total. The eyelids of the left eye are also preserved, defining a horizontal palpebral fissure, and the eye was clearly not covered by a brille or spectacle (inset, Figure 4).
Phylogenetic placement of GRS 29689 The specimen is fairly complete. In all analyses (see Supplementary Data 1 for details on phylogenetic analyses results), the strict consensus of all the trees recovered using the combined dataset (i.e. the morphological data matrix with a molecular partition, see Methods) yielded a highly unresolved tree, partly due to the unstable position of the Early Cretaceous Spanish (Las Hoyas) taxa Hoyalacerta sanzi33 and Jucaraceps grandipes34, and of the Cretaceous Polyglyphanodontia2. To increase the resolution of the tree, another strict consensus tree was calculated without the unstable taxa, and their alternative positions are indicated on the simplified consensus (Fig. 4).
In the combined evidence analysis, where we scored all possible morphological characters, and used the character ordering proposed by Gauthier et al.2 (See Methods), Retinosaurus hkamtiensis was consistently recovered as a scincoid lizard. Retinosaurus was placed as the sister taxon to Tepexisaurus (Early Cretaceous, Mexico35) + Xantusiidae. Relationships within Xantusiidae were poorly resolved. Cricosaura was recovered as sister to an unresolved clade formed by Xantusia and Lepidophyma, the extinct Palaeoxantusia (Eocene, North America36), Palepidophyma (Eocene, North America37), and Catactegenys (Late Cretaceous, North America38). The sister group relationship between R. hkamtiensis and the clade formed by Tepexisaurus and xantusiids (including Catactegenys) was supported by 12 characters (present unambiguously in all trees), namely: character 7: ethmoideal foramina exit via external naris (reversal to 0 state); character 24: nasals not in ventral contact beneath premaxillary internasal process except near the apex (1); character 137: lacrimal absent; character 141: lacrimal duct enclosed in the prefrontal except ventrally ; character 155: posterior process of jugal absent; character 220: vomeronasal nerve exit dorsal to vomer (reversal to 0 state); character 308: crista prootica extends onto basipterygoid process forming open or closed bony canal; character 324: dorsum sellae shallow and poorly differentiated with, at most, shallow fossa and low crista sellaris; character 334: distal end of basipterygoid process not expanded (reversal to 0 state); character 410: retroarticular process breadth relative to mandibular condyle narrow (reversal to 0 state); character 641: fleshy eyelids absent (i.g., eyelids are thin).
The holotype specimen of Retinosaurus is clearly immature. We explored the possibility of miscoding ontogenetically variable characters, by running alternative analyses scoring these characters as missing data (?) for Retinosaurus (see Table S1). The analyses using all characters unordered and and those of Retinosaurus coded as preserved (i.e. not scoring ontogenetically variable characters as unknown for R. hkamtiensis; see Methods), R. hkamtiensis was recovered as sister to amphisbaenians. All other analyses recovered Retinosaurus as a Pan-xantusiid (see Table S1).