A 34-million-year-old snake discovered by scientists alters our perception of evolution.

Sorting through a mess of tiny backbones is typically required to find a fossilised snake. One vertebra here, a piece of ribs there, and a lot of speculation regarding the animal's other parts. Palaeontologists are therefore excited by an almost complete snake skeleton.

Field workers in western Wyoming discovered four snake fossils this spring that were so complete that their ribs, tails, and skulls were all in almost perfect alignment.

Approximately 38 million years ago, during the early Oligocene, when cooler climes were encroaching on North America, the quartet was formed from sediment. It appears that the snakes died within hours of each other since they were kept together in a thin covering of fine mudstone.

Subtle characteristics quickly suggested something new, but early laboratory work connected the fossils to two recognised genera. A portion of the snake family tree would be altered by such clues.

Four Oligocene snake fossils

In-depth examinations revealed that the Wyoming snakes were different from the species they initially resembled, Ogmophis and Calamagras, in terms of vertebral keels, tooth arrangement, and jaw form. Because of the continuous characteristics between the four individuals, scientists have named a new species, Hibernophis breithaupti.

Each of the creatures was only a few feet long, and the largest one was twice as long as its fellows. A unique side-by-side view of juvenile and adult phases within a single species is provided by that size spread.

The state of the fossils astounded Michael Caldwell of the University of Alberta's Faculty of Science. There are most likely close to a million disarticulated snake vertebrae in museum collections worldwide. You can easily locate them. However, locating the entire snake? It's uncommon," Caldwell exclaimed.

Researchers can observe how each portion evolves from head to tail thanks to the four skeletons, which provide a continuous map of more than 200 vertebrae (live snakes of similar size can carry 200 to 400), as well as ribs and fragile skull bones. This is something isolated bones can never display.

Family relationships are shown by DNA.

To determine evolutionary links, the team integrated DNA sequences from living snakes with the anatomical data. According to their findings, Hibernophis was closely related to the boas of today, a diversified family that includes enormous constrictors, sand-swimmers, and tree-climbers.

Caldwell observed, "We learn quite a bit more about Boidae evolution in the broad sense." It's noteworthy to note that they most likely began as rather small-bodied snakes.

The fact that the new genus is located outside of the branches that contain contemporary boa species indicates that by the early Oligocene, the boid family had already started to divide into several lineages.

Another hint is provided by growth patterns. The largest Wyoming snake has fused sutures and larger jaw bones, while the smallest snake has a skull that is just about half an inch long.

The notion that ancestral boas were little before some lineages developed into the giants of today is supported by that discovery. Researchers can associate individual vertebrae with particular life stages because Hibernophis adds an articulated spine to the database.

In their winter habitat, hibernating

The four snakes were huddled together in what looked to be a hibernaculum, a communal winter haven. "This is unusual for reptiles," Caldwell said, adding that the layout "represents social behaviour in snakes, which is something that we don't often see."

None of the over 15,000 species of reptiles that are now extant hibernate like garter snakes do. Garter-like strategies were already in use millions of years ago, as the fossil cluster demonstrates.

The fossilised skeleton of the recently found snake species Hibernophis breithaupti, which existed in western Wyoming 38 million years ago, provides information about the social behaviour and evolutionary history of its contemporary offspring. To enlarge the image, click it. Credit: South Australian Museum/Flinders University/Michael Caldwell

Because they are unable to control their body temperature, they must create big masses to retain as much heat as possible over the winter, according to Caldwell. Hundreds of modern garter snakes can congregate at times.

There have also been reports of opportunistic rattlesnakes sneaking into the crowd to steal warmth. According to the Wyoming discovery, communal sheltering may have been common among ancient snakes and emerged early.

Snake fossils can be found in ash clouds.

While the snakes sought refuge below, the surrounding volcanoes were spewing clouds of ash, which is why the remains were in such immaculate condition.

Burrows were sealed in an airtight blanket by the fine particles that collected throughout the floodplain, slowing the rate of deterioration. According to Caldwell, "they were preserved in a very unusual circumstance, geologically speaking."

The skeletons were locked into what geologists refer to as the White River Formation, a fossil-rich strata that crosses multiple Great Plains states, by alternating layers of ash with pulses of mud carried in by seasonal rains.

Before prospectors cut the rock, a little flood quickly filled the burrow with silt, preventing the snakes from being disturbed. "Fossilisation is a difficult process that needs the exact right conditions to be preserved."

The quartet illustrates how vertebrae move down the spine while all of the bones are still present; this reference may cause museums to reevaluate the labels they place on their loose bones.