How amber captures the history of life
Plants and animals trapped in amber preserve a frozen moment for tens of millions of years. Paul Willis reports.
In Greek mythology, they were thought to be the tears of the sisters of Phaëton, son of Helios, the Sun God. They were included in jewellery as far back as the Neolithic period. They first appear in the literature in the fourth century BCE in the writings of Theophrastus and, later, Pytheas. Their honey-gold transparent allure has transfixed people for thousands of years. And now amber, and its occasional cargo of fossil inclusions, is retelling the story of life in exquisite detail.
The origin of amber is much more prosaic than the tears of the gods. It started out as dribbles of plant sap, perhaps fallen to the forest floor in sticky blobs. Once buried in sediment, the sap undergoes molecular polymerisation, hardening under heat and pressure to form, first, an intermediate substance known as copal, and then amber itself. But it’s while still freshly exuded and viscous that it sometimes ensnares a hapless passing creature, trapping it forever.
The oldest amber dates back 320 million years to the Upper Carboniferous period. There are hundreds of localities all over the world that produce these glassy-golden stones, but only a few produce specimens with fossils trapped within.
The largest deposit of amber comes from the Baltic region, which covers eastern Germany, Poland and Russia. Recent analysis shows that the original resin came from a rare group of conifer trees, of which there is only one species alive today – the Japanese Umbrella pine (Sciadopitys verticillateI). Although recovered from marine sands dated as 25 million years old, analysis of the insects occasionally found in Baltic amber reveals that it is considerably older still, dating from between 44 and 49 million years ago.
The Dominican Republic in the Caribbean is renowned for producing yellow, gold and rare blue amber of exceptional clarity. It also produces more fossils, particularly of insects, than the Baltic variety. Dating to around 25 million years ago, the environment in which the sap was produced was a tropical forest filled with lianas, shrubs and large trees. The sap itself can be traced back to the now-extinct species of tree, called Hymenaea protera, from a genus that can still be found growing in the tropical Americas and the east coast of Africa today.
But most of the spectacular fossils recovered over the past decade or so have come from Myanmar – where almost a third of specimens unearthed contain all or part of a once-living creature. This amber has been mined for jewellery since the first century CE. Most of it comes from Hukawng Valley in Kachin State in the north of the country. Today there is a large, unregulated industry mining Myanmar amber and most of the prized fossil specimens are sold to private collectors, often putting them out of reach of scientific study.
Myanmar amber contains a huge variety of animals and plants from a 99-million-year-old tropical forest. This was a time, late in the Age of the Dinosaurs, where many types of animals and plants were undergoing significant changes, and we see the first appearance of several groups that are familiar to us today.
The beauty of amber as a medium for creating fossils is that it forms a perfect impression of the surface of the creature or plant entombed within. The body of the animal decays to carbon that can line the cavity created by the original form. The fidelity of the surface impression is so high – and the size of many of the creatures is so small – that the fossils can be scanned and then blown up to reveal the tiniest details.
In recent years several specimens of feathers, even partially preserved wings and whole birds, have been recovered from 99 million-year-old amber from Myanmar. The specimen shown here, first revealed in 2017, is an almost complete baby bird.
So far, all the Myanmar amber birds appear to belong to an early group known as the Enantiornithines, which are cousins to all the living birds.
This is an interesting stage in the evolution of birds from their dinosaur ancestors. Enantiornithines had similar flight feathers to modern birds, suggesting they were capable of skilled, powered flight, while still retaining the primitive teeth and wing-claws. Despite being a diverse and vibrant group in the Late Cretaceous period, the enantiornithines went the way of the other dinosaurs at the close of the Mesozoic.
As with other creatures trapped in amber, all that is left is an impression of the surface of the birds, but the detail and clarity are stunning. In this case, we can see each individual scale and claw on the legs, separate feathers, skin, muscles and many other intimate details, including the original colours of the feathers.
Even though this is only a hatchling, barely three and a half centimetres long, the fact that it is covered in well-developed feathers tells us individual enantiornithines developed in a different manner to their living relatives. Enantiornithines appear to have had fully-formed feathers at a very early stage of their development, while their modern counterparts are still only downy-covered chicks – details we would not have known if they had not had the misfortune of being encased in tree sap.
In 2006, the world was stunned by the announcement of a spectacularly preserved lizard in a piece of Baltic amber. This specimen, shown here, was named Yantarogekko balticus. At the time, it was the oldest gecko known to science except for some odd, fragmentary bones. While the toes of Yantarogekko revealed features not seen in any other lizard, it also had the expanded pads seen in modern geckos that enable their legendary climbing abilities.
Ten years later came news of much older specimens preserved in amber from Myanmar. This pushes back the fossil history of geckos to around 99 million years ago, a time when all the major groups of living lizards are thought to have evolved.
This stunning collection of 11 geckos all came from private fossil collections and included what was initially thought to be the world’s oldest chameleon – although it has since been re-identified as a kind of amphibian. All of the lizards were tiny, less than a couple of centimetres long, and required micro-computed-tomography scans to reveal the exquisite details of scales, teeth and claws.
The ancient animals also had the adhesive toe pads seen in modern geckos. One specimen has its tongue sticking out, revealing a narrow, extended tip not seen in any other lizard or snake. Lizards have also been recovered from Dominican amber. In 2015, 17 Anolis lizards were described and compared to a modern community of Anolis from the nearby Greater Antilles Islands in the Caribbean. Remarkably few differences were observed between the 20 million-year-old fossils and their modern counterparts.
Considering that amber originated from plant sap exuded in environments where other plants abounded, it has been observed that such plants are comparatively rare inclusions. Luckily, there are some breathtaking exceptions.
Flowering plants emerged some 99 million years ago, when Myanmar amber was still sticky tree sap in a tropical forest. Several flowers have been preserved, including one remarkable fossil of two flowers having sex! The male anther of one flower can clearly be seen inserted into the female stigma of another.
Flowers are the reproductive organs of plants and their evolution allowed for the more efficient transfer of pollen via agents such as bees and other insects. It was the flower-power revolution that lead to them becoming the dominant botanical group in the world today.
Flowers have also been recovered from the much younger Dominican amber. One specimen represents a group of plants known as the asterids, which today includes around a third of all flowering plants, comprising some 80,000 species, including sunflowers, coffee, peppers, potatoes and mint.
This single fossilised flower indicates that the asterids had made it to the New World by around 20 million years ago. Named Strychnos electri, it is thought to be closely related to the strychnine tree. In 2014, a report was published of part of a carnivorous plant perfectly preserved in Baltic amber. Its sticky tentacles closely resemble those of Roridula, a unique plant now known only to grow in South Africa. The fact that this fossil comes from the other end of the Earth indicates that this group once had a much wider distribution.
In 1993, the world was electrified by the release of the first Jurassic Park movie. The premise was that dinosaurs had been recreated from DNA recovered from mosquitoes trapped in amber. There are many flaws in such a scenario, not the least of which was the fact that Dominican amber, the source of the mosquitoes in the movie, is too young to contain dinosaur DNA.
Fast forward to 2017 and news that blood-filled ticks had been recovered from the much older Myanmar amber reignited hope that we just might get some dino DNA. Alas, it was not to be. DNA is a very fragile molecule and breaks down rapidly after an animal dies. Current estimates are that the most ancient DNA we can ever hope to recover is perhaps just a few tens of thousands of years old – a far cry from the 65 million years needed for dinosaur material.
But the amber fossil ticks are quite remarkable in their own right. One is tangled in a feather which possibly belonged to a dinosaur. Another is wrapped in a tiny death shroud of spider silk. And one was swollen to eight times its normal size, engorged with blood.
Several of the ticks have fine hairs from carpet beetles attached to them. The beetles are known to inhabit bird nests, in which they feed on the feathers and skin of chicks. Finding their defensive spiky hairs indicates that the ticks themselves had also been in nests just before they became entombed in the tree sap that became their amber coffins.
When the Myanmar amber was forming as tree sap 99 million years ago, the area was a tropical forest teeming with an abundant and diverse fauna. This is the perfect environment for snakes, so it was only a matter of time before one was found inside a piece of the sap. And that happy discovery occurred in the middle of 2018, when not one, but two specimens were revealed to the world.
One of these, pictured here, was assigned to a new species, called Xiaophis myanmarensis. The second is too incomplete to confidently determine if it also belongs to the same genus, or if it is something different.
Both are only babies, and may be among the first to have lived in a forest. Being such young individuals reveals key insights as to how ancient snakes grew from hatchlings to adults. It appears that Xiaophis had around 97 vertebrae, and is similar in size to hatchlings of several living snakes such as the Asian Pipe Snake (Cylindrophis ruffus). The similarity is so striking that it suggests very little has changed in the way that snakes have grown for over 99 million years.
The similarities in bone structures between Xiaophis and snakes from the ancient super-continent Gondwana indicates that this group originated in the southern hemisphere and dispersed into the northern super-continent of Laurasia. Once there, they stayed in a rather primitive form for tens of millions of years.