Anolis Electrum: The Amber Anole From Mexico

A recent photograph by Emma Sherratt. Amber tends to darken over time, which explains the deep orange color.

We’ve had previous posts on fossil anoles in amber. Emma Sherratt is currently studying them and has examined some three dozen specimens. All of these are from the Dominican Republic. Except the first one ever discovered, a Mexican piece described by Skip Lazell in 1965. Anolis electrum, as it was named, has had a pretty quiet scientific life. Now middle aged, the species has not been the subject of any subsequent research in the 47 years of its existence. But now it’s in the spotlight, as its phylogenetic position and dating may be pivotal for the recent calculation by Nicholson et al. that anoles originated more than 100 million years ago. In this post, I summarize what is known about A. electrum (examine the short original paper for yourself!). No doubt, we’ll be hearing more soon about the relevance of this species–specifically its phylogenetic placement and age–for dating anole diversification.

As you can see for yourself in the photo above, there are actually two pieces, a front half of a lizard and a back half a lizard. Since they were found together (or at least made it to the Paleontology Museum at UC-Berkeley together) and are matching in size, it seems like too much of a coincidence for them not to come from the same animal. Various aspects of the animal’s scalation are discernible, including some nicely visible toepads. Lazell stated that all that was left was skin, or the impression of skin, the bones having been eaten away, but Emma’s cat scanning has shown that this is not quite correct (see below).

Based on the specimen, what can be said about its phylogenetic placement? All anoles in Mexico today are from the Norops clade. Unfortunately, the primary character for identifying Norops is the shape of the caudal vertebrae, which cannot be discerned in this tailless specimen. Lazell compared the scalation of this specimen (a 26 mm juvenile) to various species, and found that the scalation was unlike most species. He concluded that electrum was most similar in scalation to A. fuscoauratus, A. maculiventris, and A. chloris, and among species found in Mexico, to A. limifrons (full quotations at the bottom of this post).

What should we make of all of this? It’s important to remember that this paper was published in 1965, prior to the description of many extant anole species and a year before Willi Hennig’s classic introduction to cladistic analysis was translated into English. This is a purely phenetic comparison of the amber baby lizard to known species, clearly non-phylogenetic and utilizing characters that now are recognized to generally have little higher level systematic utility in anoles. And the conclusion is that it is either a Norops clade anole (fuscoauratus, limifrons or maculiventris) or a Dactyloa species (chloris).

The other question one might have is: how old is this fossil? Dating amber is notoriously difficult. Solórzano Kraemer reviewed all of the data on Mexican amber bearing deposits in the 2010 volume Biodiversity of Fossils in Amber from the Major World Deposits and concluded: “In summary, it can be said that Mexican amber can be correlated with Dominican amber, with an age of approximately 15-20 million-years-old.” In other words, Mexican and Dominican amber anoles were contemporaneous.

Did anyone notice anything odd on the fossil of the lizard posterior (B, above)? Like, maybe, something munching on the toepads? Note that some bone is still present in the specimen.

CT-scan courtesy Emma Sherratt.

One last note: from whence the name “electrum“? One might think it’s referring to the electrifying nature of the first find of an amber anole, but in reality, electrum means amber in Latin.

Hey, Skip Lazell! Got any stories to tell about how this paper came to be?

Here’s the text of Lazell’s comparison of electrum to living species:

“There are, therefore, few living forms in which the combination of characters seen in Anolis electrum is at all closely approached. For example, living species like tigrinus, solitarius, and nasofrontalis, though they have middorsals, ventrals, and limb scales basically similar to those of electrum, possess strikingly enlarged and otherwise modified scales around the interparietal. Species like limifrons, polylepis, ortoni, antoni, and leptoscelis, though they resemble the fossil in the scales around the interparietal, ventrals, and limb scales, possess distinctly enlarged and other modified middorsals. Some, like leptoscelis, differ as well in having narrow digital dilations and low lamellae counts.

Both Anolis fuscoauratus and A. maculiventris are very close to A. electrum. There is virtual agreement with respect to middorsals, ventrals, scales around the interparietal, and limb scales. Each of these, however, differs from A. electrum in having comparatively narrow digital dilations and consistently less than 19 lamellae under the second and third phalanges of the fourth toe.

Anolis chloris, on the other hand, resembles A. electrum in ventrals, scales around the interparietal, limb scales, digital dilations, and the number of subdigital lamellae. As in the fossil, the middorsals of A. chloris are not notably enlarged; these middorsals are, however, aligned in a distinct double row. Despite this difference, it is entirely possible that Anolis chloris is the closest living relative of Anolis electrum.

…Of the three forms apparently closest to Anolis electrum, however, none occurs in southern Mexico or Central America today. A. fuscoauratus is widely distributed over northern South America east of the Andes. A. maculiventris and A. chloris are both forms from the Choco region of coastal Colombia, though the latter occurs southward into Ecuador. Of all the anoles closely comparable to electrum, only one, A. limifrons, occurs in southern Mexico today; it is very close to A. fuscoauratus and extends southward to Panama. In this form the dorsals enlarge gradually towards the midline, where they are aligned to form a noticeable, double, middorsal row. All other species of Anolis currently known from Mexico differ so markedly from A. electrum comparison is not called for.”

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Jonathan Losos

Professor of Biology and Director of the Living Earth Collaborative at Washington University in Saint Louis. I've spent my entire professional career studying anoles and have discovered that the more I learn about anoles, the more I realize I don't know.

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