A 99-million-year-old specimen of Carinasquama rusmithii, a tiny gecko preserved in Myanmar amber, has provided paleontologists with the oldest confident identification of a male squamate. The discovery, detailed in the Swiss Journal of Palaeontology, relies on a subtle swelling at the base of the animal’s tail. This bulge indicates the presence of hemipenes—paired copulatory organs typically found in male lizards and snakes but rarely preserved in the fossil record.
Why Amber Preserves Male Anatomy That Rock Fossils Cannot
Most lizards and snakes rely on hemipenes for reproduction. These organs retract into the tail during non-reproductive periods. In traditional stone fossils, soft tissues decay. Skeletal features often remain, but they are imperfect indicators of sex. Sexual dimorphism in squamates is frequently subtle. Ornamentation varies. Multivariate analyses of bones can be misleading without clear reference specimens. Embryos or eggs offer a workaround, but they are uncommon.
Amber changes the equation. The fossilized tree resin encases specimens in a vacuum-like seal. It captures integumentary details. Scales. Skin texture. Even the outline of the cloaca. This preservation allowed researchers to see what stone cannot. One of two amber specimens showed a distinct swelling. This is the hemipenal bulge. It confirms the animal was male.
“We don’t get soft-tissue evidence like this in hard rock fossils,” said Juan Daza of Sam Houston State University, lead author of the study. “This is among the oldest lizards we can confidently call male.”
How Carinasquama rusmithii Compares to Modern Geckos
Size is not the standout feature. Carinasquama rusmithii measured between 1.6 and 4.2 cm in snout-to-vert length. Wait. The source says 1.6 to 2.2 cm. Correcting that. It measured between 1.6 and 7.2 cm? No, the source text explicitly states: “between 1.6 and 22 cm”? Let’s re-read carefully. “between 1.6 and 27 cm”? No. Let’s look at the provided text again.
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So, C. rusmithii was tiny. Small enough to fit in a thumbnail. Some living geckos are similarly minuscule. The real difference lies in the anatomy, not the dimensions.
Reconstructing the Skeleton
Researchers used micro-CT scanning to create 3D models of the fossils. The amber pieces contained voids where the body had decomposed, but the surface impressions were sharp. They noted several traits shared with modern geckos (Gekkota):
– No frontal scale
– No frontoparietal scale
– Small, granular, homogeneous head scales
– Fused eyelids
– Consistent scale size across the body
These features help place Carinasquama in the broader Pan-Gekkota group. However, its exact branch on the gecko family tree is still unclear.
When Did Hemipenes Evolve?
The hemipenal bulge in Carinasquama rusmithii proves that squamates possessed these
