The Fossil Species Anolis electrum Gets an X-ray Makeover

AA readers may remember from previous AA blog posts (here and here) that we have been tackling the field of anole palaeontology; the wonderful world of Amber Encased Anoles. This month, the first paper has been published in the Zoological Journal of the Linnean Society, on the Mexican amber fossil Anolis electrum (from the collection of UC Museum of Paleontology, Berkeley). And what a fossil!

The amber fossil (left) and x-ray CT reconstruction (right) of one half of the Mexican amber fossil, Anolis electrum.

The amber fossil (left) and x-ray CT reconstruction (right) of one of the two Mexican amber fossils of Anolis electrum. An ant (Azteca sp.) lies behind the right hindfoot. Part of the torso is also preserved (bottom of image). Morphobank images M323739 & M323741.

Anolis electrum was originally described by Skip Lazell in 1965, and has been highlighted before on AA because of its use in dating the anole tree of life. Rosario Castañeda, Jonathan Losos and I set out to study the two fossils (which appear to contain the front and back half of the same lizard) using cutting-edge technology that was not available to Skip in 1965. Using x-ray CT scanning to peer inside the amber, I created a 3D computer model of the lizard inside:

Here you can see the right hind leg, attached to the torso (no tail preserved). A fragment of the fourth toe of the left foot is preserved in front of the lower part of the right leg. The green is false coloured air-filled voids in the amber, while the white is bone and mineralised soft tissue.

Here you can see the head, upper body and two forelimbs stretched out to the side. As above, the green is air-filled voids and white is bone and mineralised soft tissue.

More images of the fossils can be found on MorphoBank.

What we found:

1) The two amber pieces seem to fit two halves of the same animal. Considered together, the lizard is definitely a juvenile, around 24mm in snout-to-vent length, and newly hatched – see the egg-sac scar on the belly in fossil UCMP68496!

Right forelimb of Anolis electrum. Morphobank image M323758

2) The limb scales are very well preserved, as imprints on the amber surrounding air-filled voids where the soft-tissue once was.

3) The animal has widely expanded toe pads with a lot of lamellae (see right), which may mean it was a more arboreal species.

4) The cracking of the amber resulted in the fossil being exposed to the air – which caused the fossil to mineralise. This means that many of the features of the skull are obscured by very dense matter, sadly.

Using morphological data coded from these scans, we aimed to infer the relationship of A. electrum to modern anole species. Rosario diligently coded as many morphological characters as she could from the scans. However, because of the preservation and that the lizard is very juvenile, we discovered that the specimen lacked so many of the informative characters that it is impossible to obtain a precise position in the anole evolutionary tree. In fact, electrum is placed in 14 alternative positions scattered throughout the tree!Though disappointing results, one thing is clear; the previously stated hypothesis by Nicholson et al. 2012 about the age of the group—based on a hypothesized placement of electrum as sister taxon to A. limifrons and A. zeus — is unfounded and likely to be a gross over-estimate. As a result, we suggest that the most likely hypothesis for the occurrence of anoles on the Caribbean islands is by overwater dispersal, and not through vicariance.

Still, it is a lovely fossil, don’t you think? Let’s hope more from Mexico are found.