Compound osteoderms preserved in amber reveal the oldest known skink

Abstract

Scincidae is one of the most species-rich and cosmopolitan clades of squamate reptiles. Abundant disarticulated fossil material has also been attributed to this group, however, no complete pre-Cenozoic crown-scincid specimens have been found. A specimen in Burmite (99 MYA) is the first fossil that can be unambiguously referred to this clade. Our analyses place it as nested within extant skinks, supported by the presence of compound osteoderms formed by articulated small ostedermites. The specimen has a combination of dorsal and ventral compound osteoderms and overlapping cycloid scales that is limited to skinks. We propose that this type of osteoderm evolved as a response to an increased overlap of scales, and to reduced stiffness of the dermal armour. Compound osteoderms could be a key innovation that facilitated diversification in this megadiverse family.

Figure 1

Comparison among higher-level classifications of skinks and relatives as used in recent studies.

Figure 2

Electroscincus zedi. Fossil in ventral (a) and dorsal (b) views. Detail of the right foot (c, e) and osteoderms (d). X-ray of the whole specimen showing the skeletal remains, and several articulated and scattered osteoderms (f). Scale bar applies to the entire amber piece.

Figure 3

Electroscincus zedi, pectoral and pelvic girdle in ventral view (a, b). Numbers indicate digit number.

Figure 4

Schematic representations of scincoid osteoderms, depicting the osteodermite arrangement in Electroscincus zedi, the gular osteoderms of gerrhosaurs and the nuchal osteoderms of representative genera from all scincid subfamilies. Scale bar equals 0.5mm.

Figure 5

(a) Bayesian timetree of Squamata based on a combined molecular/morphological data set. Posterior probability and bootstrap support values are given at key nodes. Phylogenetic position of Electroscincus zedi in the Maximun Likelihood (b) and parsimony (c) analyses. Bremer support values on the nodes of the parsimony analysis.

Figure 6

Cleared and stained subadult specimen of the sphaerodactylid gecko Sphaerodactylus townsendi from the University of Puerto Rico, Río Piedras Collection (UPRRP 006400). Note that the pedes exhibit proportionally less cartilage (blue) than the manūs. Photograph courtesy of Elyse Howerton.

Figure 7

Life reconstruction of Electroscincus zedi. Areas of the lizard not represented in the material available are depicted as blurred. Illustration by Stephanie Abramowicz.