The annual meeting for the Australian Society of Herpetology (ASH) is wrapping up here today in the lovely town of Eildon, Australia. Just because we’re a continent away from the native distribution of anoles doesn’t mean that anoles were not represented at the meeting. Yesterday afternoon Emma Sherratt, new faculty at the University of New England in Armidale, Australia, presented some of her post-doctoral work on fossil anoles preserved in amber. Emma began by saying that Caribbean anoles represent one of the oldest examples of extant adaptive radiations. Despite the age of this radiation, most of the work on the Caribbean anoles (and other adaptive radiations, for that matter), has focused primarily on living species, with historical inferences drawn from DNA analyses. She pointed out that historical insights based on analyses of extant species only should be treated with caution, unless there is corroborating information from the fossil record.
We know, she said, that islands are typically inhabited by a single lineage of ecomorphs (with subsequent diversification within ecomorphs). The fact that most ecomorph groups are represented by a single lineage on an island suggests that once an ecomorph niche is filled, it cannot be replaced, an idea known as ‘niche incumbency’. She argued that we can use fossils to assess that hypothesis – if fossil anoles pertain to same lineages of ecomorphs (e.g., the cristatellus clade of trunk-ground anoles, or the carolinensis group of trunk-crown anoles), then that would support the idea that ecomorph niches were only filled once. If extinct anoles fell into different lineages of ecomorphs, distinct from those that are extant today, then that would support the idea that ecomorphs could be replaced on islands, which would suggest that niche incumbency need not be occurring. Of course, it could also be possible for niche incumbency to have occurred if there were two lineages of the same ecomorph present on the same island, as long as the incumbent lineage drove the more recent one to extinction. But the hypotheses proposed by Emma were certainly a reasonable first pass to understand the origin of ecomorphs on the Caribbean islands.
Anoles have been fossilized in Hispaniolan amber, which we know to be about 15-20 million years old. All you folks who are anxiously awaiting the next installment of Jurassic Park be advised – this means that the famous amber used to get dinosaur DNA is far too young, as the dinosaurs (save for birds, of course) went extinct about 62 million years ago. For her study, Emma accessed an impressive 38 anole fossils preserved in amber. By far this is the largest data set of fossilized amber anoles ever examined. And, beyond their utility for understanding the process of diversification, anoles caught in amber are stunning fossils and the high resolution reconstructions that Emma makes using x-ray CT scans are equally impressive.
Emma found strong evidence that Hispaniolan fossil anoles fall into known ecomorph categories. To determine this she compared morphological details from extant species to the fossil anoles. Overall she found substantial morphological variation in the fossils, particularly in 20 of the best preserved and most complete fossils. Amazingly, Emma found that some of the fossils fell very clearly into the trunk-crown, trunk, trunk-ground, and twig ecomorph classes! She was further able to determine that the trunk-crown fossils fell into the chlorocyanus group of extant Hispaniolan lizards, and, with less confidence, evidence that the trunk-ground lizards fell into the cybotes group of extant Hispaniolan lizards. Thus, the results are suggestive that, once an ecomorph niche is filled, it prevents other lineages from evolving into it, which is consistent with niche incumbency. Obviously it is not possible to fully rule out the alternative – that species of other ecomorph lineages existed in the past – but certainly the results are a tantalizing glimpse into the processes that forged the current Caribbean fauna. In short, she found that most ecomorphs recognized today are not only present in the Miocene fauna, but also are represented by members of the same clades. Together, her results were consistent with the idea that niche incumbency occurred in the Caribbean radiation of anoles, which would indicate that interspecific interactions have regulated morphological diversity for millions of years.