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100 million year old lizards entombed in amber reveal stability of tropical ecosystems

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When talking about amber fossils, there is usually one image that springs to mind: that perfectly preserved mosquito sitting on John Hammond’s cane in Jurassic Park, entombed there for millions of years.

Amber is fossilised tree resin. Because of the way it forms, it is usually constrained in size and not more than a few inches in diameter. So there’s not much chance it will ever capture a dinosaur (although feathers have been found preserved in amber!). But what it does capture is a unique part of ecosystems that is not usually preserved to us: that of tree dwelling organisms.

Squamates in amber from the four known localities. Color key: Black, stem Squamata; blue, stem Gekkota; green, Lacertoidea; red, Acrodonta; orange, Pleurodonta. Daza et al. (2016)
Squamates in amber from the four known localities. Color key: Black, stem Squamata; blue, stem Gekkota; green, Lacertoidea; red, Acrodonta; orange, Pleurodonta. Daza et al. (2016)

And it preserves them exquisitely too. This is really important, as often smaller animals that are more delicate are less likely to be preserved – they simply cannot withstand the fossilisation process as much as a chunkier animal, like a dinosaur or mammal, is able to.

So amber can give us an occasional glimpse into a new dimension for ancient ecosystems. New discoveries now from the Cretaceous of Kachin Province, northern Myanmar, radiometrically dated to around 99 million years old, provide evidence for the oldest lizard fossils preserved in amber we know of. Amber from here is called burmite, and reveals a huge diversity of early flowering plants, scorpions, roaches, termites, ants, and a whole range of other creepy crawlies.

Lizards preserved in mid-Cretaceous Burmese amber. Daza et al. (2016)
Lizards preserved in mid-Cretaceous Burmese amber. Daza et al. (2016)

They also preserve them in astounding detail, with soft tissues still intact over millions of generations. If you look at the fossils, entombed in space and time, you’d think they could have formed like that only yesterday. Certainly, I’ve seen fresh roadkill that doesn’t look as good as these fossils!

The newly discovered lizard fossils contain representatives of five major groups of lizards, which is quite a bit more than the fossil record usually divulges for these fragile animals. This is the most diverse lizard fauna ever discovered in amber, and makes quite the shining spectacle.

High-resolution x-ray computed tomography images ofmid-Cretaceous lizards in Burmese amber
High-resolution x-ray computed tomography images of mid-Cretaceous lizards in Burmese amber. Daza et al. (2016)

Perhaps most importantly, the high degree of preservation reveals stunning details about their soft tissues, which means they can be more easily related to modern lizards. This makes them very valuable, as it means we can calibrate with more certainty when modern groups arose, and what their ancestral forms would have looked like.

Different fossils were recovered by phylogenetic analysis to represent a variety of early squamates, gekkos, and chameleons. Squamata is the group that includes all modern lizards and snakes, some 9900 species, so is a pretty hefty and diverse group. Gekkota is less diverse, comprising around 1650 living species of gecko and pygopod. Only around 200 species of chamaeleonoid are around today, but anyone who’s ever met one of these will know their rarity just makes them even cooler! Other fossils represent groups such as Agaminae, known from the old world tropics, and Lacertidae, which includes new and old world representatives.

F3.large
Results from phylogenetic analysis, showing where the new fossil specimens fall out. Daza et al. (2016)

All of these groups are often found in tropical areas, which don’t leave their traces too well in the fossil record. As such, discovery of a 100 million year old tropical herp fauna is pretty awesome! Small size and a bad environment for fossilisation means that we know relatively little about fossil lizards. This is a shame, because small size might actually be one of the things responsible for their great evolutionary success!

The detail they preserve is amazing. Some even preserve the little adhesive toe pads for climbing trees we see in modern species. The chamaeleonid has facial features characteristic of their weird projectile tongue feeding strategy, indicating that these key evolutionary characteristics were in place early on in their evolution during the Cretaceous.

Photomicrographs of the integument of mid-Cretaceous lizards in Burmese amber. Daza et al. (2016)
Photomicrographs of the integument of mid-Cretaceous lizards in Burmese amber. Daza et al. (2016)

The presence of these groups in Myanmar at this time tells us more about their evolutionary success too. Many of them still have living representatives in Myanmar, which suggests that not only have those lineages been there for around 100 million years, but survived the great extinction 66 million years ago, when groups such as the non-avian dinosaurs went kaput. So lizards > dinosaurs.

This also suggests that tropical assemblages are pretty stable, if they can survive relatively intact for 10s of millions of years. Such a find supports the idea of the tropics being a ‘museum’ of biodiversity. Good job humans aren’t totally destroying millions of years of evolutionary heritage or anything then.

Reference

Daza et al. (2016) Mid-Cretaceous amber fossils illuminate the past diversity of tropical lizards. Science Advances, 2: e1501080. DOI: 10.1126/sciadv.1501080

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