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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Emma Sherratt
Indeed, as Jonathan has said the CT scan has revealed that bones have been preserved in both pieces, as shown by the phalanges in the image above. And as I showed before, the skin features are well preserved as air-filled voids in the amber. The ant also. But sadly the most diagnostic feature of alpha and beta anoles, the tail morphology, isn’t preserved. Anole annals contributer Rosario Castañeda and I will be reassessing the specimen in light of all the information that has accrued since it was published in 1965. Certainly an exciting time for anole paleontology!
Rich Glor
Regarding Nicholson et al.’s placement of this fossil, they say: “Anolis electrum (Lazell, 1965), a fossil dated to 28 mya, is believed to be a member of the previously recognized fuscoauratus group and most likely is related to A. limifrons or A. zeus (members of our Norops auratus group) based upon the description of the fossil; this date was used as a minimum age for the branch leading to these two sister species.” By using this placement and minimum age, Nicholson et al. reject the more recent age estimates for this fossil (15-20 mybp). Their precise placement of this fossil on the branch leading to A. limifrons and A. zeus also assumes that the ancestors of the species that are considered most morphologically similar to the fossil – A. fuscoauratus, A. maculiventris, and A. chloris – did not occur in Mexico 28 million years ago.
Kirsten Nicholson
We placed the fossil there because it made the most sense at the time give what Lazell had said and what we know about Mexican taxa in that clade; I can’t see where else you would place that given the info. I seriously doubt there would be a significant difference between using some date between 15 – 20 mya, vs. 28 mya, but you’ll be testing that soon. Also, using it as a minimum date makes sense given that the species likely arose before that time anyway. And yes, while its a hypothesis, I don’t think many people would strongly argue for N. fuscoauratus, N. maculiventris or D. chloris NOT being in Mexico, given their current distributions as being solely South American…
Rich Glor
Kirsten – Great to see you on Anole Annals! I hope that you’ll continue to weigh in during our ongoing discussions of your paper. Remember that you’re always welcome to contribute posts as well as comments.
220mya
Kirsten,
I think what is more important and I assume implied by Rich’s comment is that there don’t seem to be any preserved unambiguous synapomorphies that actually support placement of A. electrum on the branch leading to A. limifrons and A. zeus. Incorporating fossils into a phylogenetic analysis and/or identifying unambiguous synapomorphies are a critical step in properly using fossils in molecular dating (see Parham et al. 2012, online and available via open access since November 2011).
Levi Gray
Finally took a look at the paper and it certainly appears to resemble the limifrons group species that still occurs in the area, Anolis rodriguezi. The combination of smooth ventrals and granular dorsals rules out most local species. However, anoles of the schiedii group tend to have those characteristics and are also still present in the region. In fact, the type locality for Anolis hobartsmithi is ~20 km from Simojovel, assumed to be near where the fossil is from. I’m curious as to how some of the other characteristics would hold up, but I don’t have the information in front of me at the moment.
Skip Lazell
Thanks, all, for paying attention to it after all these years. I thought I had heaved the proverbial brick into the pool of treacle! I was a grad student at Harvard back then, under EE Williams who had received the specimen to do Something about. The two little amber blocks sat on the edge of the bookcase in the herp office and the months went by. Ben Shreve looked at them, but did not want to get involved. Finally, EEW turned the project over to me. I think he thought it was just going to be an onerous job of tedious work. It was a bit of that, but also very exciting to me…. There were/are no real data on the specimens’ origin. Is it possible to date amber? If so, please do…. Onwards!