By examining lizards trapped in amber for the last 20 million years, researchers have revealed how their diversity is surprisingly similar to that of today’s lizards. The findings, published in Proceedings of the National Academy of Sciences this week, suggest that the structure of ecological communities can stay stable over many millennia – despite huge environmental change.
Whether or not communities remain stable over long macroevolutionary timescales is something that biologists have long debated. Recent studies found lots of differences between glacial and postglacial communities, in plants and mammals alike. Those studies seem to suggest that the structure and function of species assemblages (groups of various species in the same habitat) can change in just thousands of years. But then there are these Caribbean Anolis lizards...
These days, anoles occupy every island in the Greater Antilles, and each island has a similar set of habitat specialists (or ecomorphs). They’ve developed the best-suited body shapes and proportions based on where they forage. The trunk-crown ecomorph, for example, is found high up on tree trunks and in the canopy, so they have short limbs and long, narrow heads. Trunk-ground species, which scurry around the bases of trees, have long hindlimbs, medium-length forelimbs, and short, broad heads. Because the same ecomorphs are replicated on different islands, anole communities aren’t just haphazard sets of species that happen to occupy the same place at the same time.
To see if their ecological stability is a longstanding one, a team led by University of New England's Emma Sherratt analyzed 20 Anolis lizards, preserved in fossilized tree resin, that date back 15 million to 20 million years. They compared them to 15 modern species, paying close attention to their skeletons, body length, toes, and lamellae – tiny structures on the undersides of toes that add surface area for scaling walls and for not sinking into sand.
Exceptionally well-preserved fossil anole lizards in amber with 3D reconstructions. E. Sherratt et al./PNAS 2015.
Using X-ray micro-CT scanning, the team reconstructed complete skeletons and articulated skulls in 3D, based on the amber fossils. The lizards’ external surfaces were outlined in the amber by air-filled voids, revealing exceptional details of even their scales and lamellae.
The team discovered that amber fossils of anoles from the Caribbean island of Hispaniola have very similar morphological diversity to that of modern Hispaniolan anoles.
The fossils of trunk-crown and trunk-ground ecomorphs share the same traits as their modern-day counterparts. The same goes for members of the trunk ecomorphs, who forage on tree trunks with their short, broad heads, long limbs, and hindfoot toepads with intermediate lamella numbers. It seems that anole community structures are the same across islands and across vast timescales. If 20-million-year-old communities were made of the same types of habitat specializations that anoles have today, this means that these main elements were already in place – and operative – in the Miocene Epoch, 23 to 5.3 million years go.