A new Early Cretaceous lizard in Myanmar amber with exceptionally preserved integument

Abstract

We here report on a well-preserved juvenile lizard specimen in Albian amber (ca. 110 mya) from the Hkamti site (Myanmar). This new taxon is represented by an articulated skull and the anterior portion of the trunk, including the pectoral girdle and forelimbs. The scleral ossicles and eyelid are also visible, and the specimen exhibits pristine detail of the integument (of both head and body). In a combined molecular and morphological analysis, it was consistently recovered as a scincoid lizard (Scinciformata), as sister to Tepexisaurus + Xantusiidae. However, the phylogenetic position of the new taxon should be interpreted with caution as the holotype is an immature individual. We explored the possibility of miscoding ontogenetically variable characters by running alternative analyses in which these characters were scored as missing data for our taxon. With the exception of one tree, in which it was sister to Amphisbaenia, the specimen was recovered as a Pan-xantusiid. Moreover, we cannot rule out the possibility that it represents a separate lineage of uncertain phylogenetic position, as it is the case for many Jurassic and Cretaceous taxa. Nonetheless, this fossil offers a rare opportunity to glimpse the external appearance of one group of lizards during the Early Cretaceous.

Figure 1

General view of Retinosaurus hkamtiensis (GRS 29689). (a) Photograph of the specimen within the amber resin in dorsal view. (b–d) HRCT rendering of the integument surface. Note that the integument is not visible in the photograph, as it may be preserved as a translucent layer. (b) dorsal view of the body, (c) ventral side of the head, (d) left lateral and (e) right lateral views of the head. Greenish colour indicates the skin, light brown indicates the bones preserved inside.

Figure 2

(a) Dorsal and (b) ventral views of Retinosaurus hkamtiensis (GRS 29689). The images show the skeleton of the specimen after removing the calcified material within the abdomen.

Figure 3

Skull of Retinosaurus hkamtiensis (GRS 29689) based on X-ray microcomputed tomography (synchrotron data). Individual bones segmented using VGSTUDIO MAX (Volume Graphics GmbH), rendering each bone as a volume. Cranium in dorsal (a), ventral (b), left lateral (c), right lateral (d), and posterior (g) views, Jaw in lateral (e) and medial (f) views. Ba braincase, Cb compound bone, Co coronoid, D dentary, Ec ectopterygoid, Ep epipterygoid, Fr frontal, J jugal, Mx maxilla, N nasal, P parietal, Pal palatine, Po postorbital, Pof postfrontal, Prf prefrontal, Pt pterygoid, Px premaxilla, Spl splenial, Q quadrate, Smx septomaxilla, Sq squamosal, St supratemporal, V vomer.

Figure 4

(a) Phylogenetic position of Retinosaurus hkamtiensis (GRS 29689) using a combined data set of morphological and molecular data, including all characters (i.e. not coding ontogenetically variable characters of Retinosaurus as ?), and treating characters as additive following Gauthier et al.. Terminals and groups numbered a–c were identified as wildcard taxa, therefore the strict consensus was calculated without them, and their alternative positions indicated in the tree. Using the combined data set, R. hkamtiensis was recovered as a stem-xantusiid. Numbers below nodes indicate Bremer support values. Letter at nodes indicate alternative positions of wildcard taxa (right). Nodes with no support values were collapsed when wildcards were included in the consensus. (b) Inset left side of R. hkamtiensis showing the preservation of the eyelids and circumorbital bones.

Figure 5

Retinosaurus hkamtiensis prior to being trapped in tree resin 110 mya (Scientific illustration by Stephanie Abramowicz).