Tag: computed tomography

Cranial Ornamentation in Anolis baleatus

On August 28, 2019

When I first encountered Anolis baleatus, this Hispaniolan crown-giant was mostly an inconvenience. At the time I was gathering data for my doctoral thesis by cycling preserved anoles through a µCT-scanner. Most of the adult specimens of A. baleatus were just too large to easily fit into the scan chamber, so it took a lot of patience and creativity to acquire any decent images of the appendicular girdles, which are the body parts I was interested in.

During that process I also acquired radiographic images of the head skeleton, and found unusual patterns of crenulation in this species. The cranium of Anolis baleatus displays a great degree of seemingly asymmetrical (or at least somewhat irregular) ornamentation across its dorsal surface. This is especially pronounced on the prefrontal and frontal bones, and completely obscures all superficial distinction between them in adult lizards. In adults, cranial ornamentation is also borne by the paired nasals, maxillae, and postorbitals, and the parietal (see figure).

Both Steven Poe (1998) and Susan Evans (2008) mentioned this ossified garnish, but a thorough account of their variation among anoles remains absent from the primary literature. Richard Etheridge and Kevin de Queiroz (1988) were probably the first to report on skull ornaments in anoles (as part of a discussion of several iguanian lizards with similar cranial adornments), and remarked that the distribution patterns of dermal rugae may reflect those of the topographically associated epidermal scales.

Overall, this ornamentation appears to be relatively uncommon among anoles, especially to the degree expressed in Anolis baleatus (and several other crown-giant ecomorph anoles). Considering the osteologically robust appearance of crown-giants, even at early stages of ontogenetic development, this gives rise to questions regarding the development of these ornamental patterns. Thanks to the collection efforts of Luke Mahler (University of Toronto), and a postdoctoral position in his lab, I was able to acquire CT-image data representing an ontogenetic series of this species, ranging from very young juveniles to skeletally mature adults.

While parts of the paired frontals of juveniles are covered in modest eminences, prominent cranial ornamentation is absent from small specimens (see figure). Likely, growth of these ornaments begins very late during ontogenetic development. Ornaments on the prefrontals and parietal are only evident in specimens that, to the best of our judgement, are approaching sexual maturity. We looked at fifteen specimens per sex, representing a range of juvenile and subadult sizes, and this general pattern is consistent throughout the image data. Schwartz (1974) inferred that anoles in the ricordii group reach sexual maturity between 100 and 110 mm snout-vent length (SVL), and we observed the first prominent ornaments at sizes between 90 and 95 mm SVL. Assuming that differences in size directly represent ontogenetic growth, these findings imply that Anolis baleatus starts to grow elaborate ornamentation as it approaches sexual maturity, and that expansion and growth of these ornaments then continues into skeletal maturity. Interestingly, both males and females appear to develop them at roughly the same body size.

The function and evolutionary cause of these structures remain unknown, and these are questions we are currently investigating. Body size is an important correlate for the occurrence of cranial ornaments, but these structures may also conceivably play roles in defense, feeding, or intraspecific agonistic interactions. Stay tuned!

Videos

A. baleatus, female, 55 mm SVL

A. baleatus, female, 65 mm SVL

A. baleatus, female, 96 mm SVL

A. baleatus, female, 126 mm SVL

References

Etheridge, R. & de Queiroz, K. (1988): A phylogeny of Iguanidae.─ [In:] Estes, R.D. & Pregill, G.K. (eds.): Phylogenetic Relationships of the Lizard Families: Essays Commemorating Charles L Camp, 283-367; Stanford: Stanford University Press.

Evans, S. (2008): The skull of lizards and tuatara.─ [In:] Gans, C., Gaunt, A.S. & Adler, K. (eds.), Biology of the Reptilia, vol. 20:1-347; Society for the Study of Amphibians and Reptiles, Ithaca, New York.

Poe, S. (1998): Skull characters and the cladistic relationships of the Hispaniolan dwarf twig Anolis.─ Herpetological Monographs, 12:192-236; The Herpetologists’ League.

Schwartz, A. (1974): An analysis of variation in the Hispaniolan giant anole, Anolis ricordi Dumeril and Bibron.─ Bull. Mus. Comp. Zool., 146:89-146.

20-Million-Year-Old Fossils Reveal Ecomorph Diversity in Hispaniola

By Emma Sherratt

On July 27, 2015

In Anole Videos, Lizards in an Evolutionary Tree, New Research

Twenty exquisitely preserved anole fossils in 20 My old Dominican Amber have been reported on in a paper out in Proceedings of the National Academy of Sciences (PNAS) this week.

Previously on AA, I reported that the search was on to find anole fossils in order to piece together the anole family tree. We were extremely fortunate to find in the end 38 amber fossils with anole inclusions, sourced from museums such as the Staatliches Museum für Naturkunde Stuttgart, Germany, American Museum of Natural History, and Naturhistorisches Museum, Basel Switzerland, as well as from generous private collectors.

All of the fossils were exquisite, stunningly-preserved anoles in Dominican Amber. Sometimes just a foot or tail was preserved, sometimes a whole limb or two, or an isolated head, but occasionally a whole lizard was preserved laid out as if it has been pressed into resin just moments before.

Modified from Figure 1 of Sherratt et al. 2015 PNAS.

Using micro-CT scanning to peer inside the fossils, we were delighted to find well-preserved skulls and skeletons. We were surprised to find that many of the amber pieces had air-filled pockets representing where the lizard body had once been (but subsequently mostly rotted away), and the scales had left their impression on the amber. This allowed us to view the scales of the limbs and toepads in the greatest of detail.

The forelimb lying atop belly scales of a trunk-ground fossil, specimen M of Sherratt et al. 2015.

Twenty of these fossils were complete enough, or preserved with the right body parts (limbs with a pelvis, or toepads with countable lamellar scales) to study qualitatively. I micro-CT scanned 100 modern specimens from the Harvard MCZ collection, representing adults and juveniles of all the ecomorphs in Hispaniola. With these data, I build up a dataset of measurements of the limbs, skulls and pelvic girdles that could be used to compare with the fossils. Working fossil by fossil, I used discriminant function analysis to assess the probability that the fossil matched each of the modern ecomorphs.

The fossil twig anole, from Jose Calbeto of Puerto Rico.

The results were very exciting. We found evidence for four of the six ecomorphs in the amber. Trunk-crown were the most abundant, but there was also one that fell within the twig anoles, two that fell with trunk and two with trunk-ground anoles. Not all the fossils could be assigned to an ecomorph with high probability. Though, my gut feeling is that there is a second twig anole (specimen P) based on the distinct few lamellar scales on its widely-expanded toepads, but sadly it didn’t have enough skeleton and no hind limbs preserved to add to the analysis.

We didn’t find any fossils that resembled crown-giants or grass-bush anoles. Why?

The Hi-Tech World of Anole Paleontology

By Emma Sherratt

On March 6, 2012

In All Posts, Lizards in an Evolutionary Tree

Previously, I reviewed what we currently know about anole fossils – these fossils are preserved in amber, a fossilised tree sap/resin from Mexico and the Dominican Republic (like the one pictured right). Today, I want to share how I have been using high resolution x-ray computed tomography, a.k.a CT scanning to look at these fossils and so peer into the past.

Background to CT scanning Amber

CT scanning involves x-raying an object from many angles, and then compiling these x-rays to reconstruct 3D models of the object (more detailed description here). CT scanning works when the object being scanned is made of different materials that each absorb x-rays differently. Think of a medical x-ray; skin absorbs far fewer x-rays than bone, so the two show up as different shades of grey on the developed x-ray.

The inclusions in amber are usually subfossils, where organic material still remains (e.g., bone).

Piecing Together The Anole Family Tree: Anole Fossils

By Emma Sherratt

On February 15, 2012

In Lizards in an Evolutionary Tree

Our knowledge of the evolution of anoles comes primarily from studying living forms and using information about how species are related (phylogenetic trees) to predict how traits such as their head shape have changed over time. Scientists often use this approach because there may be few (or no) actual fossils representing those stages in the evolutionary past. For anoles, this is no exception; the fossil history of our favourite lizards is sparsely recorded. Here I shall give you, Anole Annals readers, a brief overview of what we do know about anole ancestors and what we can learn from studying these fossils.

Fossil hunting history

In the box below I summarise the five papers that have published upon fossils of the genus Anolis.

Anolis – Now in 3D!

By Emma Sherratt

On November 18, 2011

In Research Methods

Jumping on the 3D bandwagon that has infested Hollywood, I wanted to introduce the Anole Annals community to the newest tool being employed to study Anole diversity and evolution, High Resolution X-ray Computed Tomography, or CT scanning for short.

HRXCT is a tool that uses x-rays to visualize the internal geometries of opaque objects. It is similar to the CAT scan you would get at a hospital, but with high-power x-rays so higher resolution. It is perfect for museum specimens because it is non-destructive; you can study skeletal morphology without removing skin or flesh, unlike the skeletonizing or clearing and staining methods as previously described here. In this first blog post on HRXCT of anoles, I shall explain how the scan process works and how the data are collected.