Modern sharks trace their roots back more than 400 million years, yet the branches that eventually produced today’s species originated during the Age of Dinosaurs. The oldest fossils representing these modern lineages date to about 135 million years ago. These early sharks, known as lamniforms, were small and may have measured only around 1 m (~3 feet) in length. Over millions of years they evolved into much larger forms, including the enormous ‘Megalodon’, which may have exceeded 17 m (56 feet), and the modern Great White shark, an apex-predator that reaches about 6 m (20 feet).

Why Shark Fossils Are Mostly Teeth

Because sharks possess skeletons made of cartilage, their bodies rarely fossilize. As a result, most of what survives are their teeth, which are shed continuously and accumulate on ancient sea floors. These teeth commonly appear in sedimentary rocks alongside the bones and teeth of fishes and the giant marine reptiles that dominated many prehistoric ocean ecosystems.

The rocky shores near today’s city of Darwin in northern Australia were once part of the mud-covered floor of the Tethys ocean, a vast body of water that extended from Gondwana (now Australia) to the island chains of Laurasia (now Europe). Numerous remains of ancient marine life have been uncovered here, including plesiosaurs (long-necked marine reptile resembling the popular image of the Loch Ness monster), ichthyosaurs (‘fish-lizards’), and large bony fish. Among the most remarkable discoveries are several huge vertebrae that indicate the presence of a very large lamniform shark.

Revealing the Early Giant: Cardabiodontid Characteristics

Five vertebrae have been recovered, each partly mineralized, which helped preserve them. Their structure is almost identical to that of today’s Great White shark. However, while adult Great Whites have vertebrae about 8 cm wide, the vertebrae from the Darwin shark exceed 12 cm in diameter. Their features clearly link them to a cardabiodontid, a group of giant predatory sharks that inhabited the oceans around 100 million years ago. The Darwin specimen is especially notable because it is roughly 15 million years older than other known cardabiodontids and had already reached the extremely large size typical of the group.

To determine the body size of this early modern shark mega-predator, a team of researchers from multiple fields collaborated on a detailed analysis. The group included paleontologists and specialists in tomography from the U.S.A. Sweden, and Australia, as well as ichthyologists from South Africa and the U.S.A.

The findings appear in the Nature portfolio journal Communications Biology. Fossils of ancient sharks from the Age of Dinosaurs can be viewed by the public at the Swedish Museum of Natural History.

“We discovered that deep faults can heal themselves within hours,” said Amanda Thomas, professor of earth and planetary sciences at UC Davis and corresponding author on the paper. “This prompts us to reevaluate fault rheological behavior, and if we have been neglecting something very important.”

Slow Slip Events and Shifting Stress

Thomas, UC Davis colleague Professor James Watkins and their team investigated slow slip events, or SSEs, which resemble extremely slow earthquakes.

Regular earthquakes happen when stresses that accumulate as tectonic plates grind together over centuries or millennia are suddenly released, creating intense shaking that lasts only seconds.

Around 2002, Thomas said, researchers identified a different kind of seismic activity. In a slow slip event, stresses that build for months to years are relieved in movements of only a few centimeters that occur gradually over days, weeks or months.

Repeating Slip in the Cascadia Subduction Zone

To better understand these deep events, the team examined seismic data from the Cascadia Subduction Zone in the Pacific Northwest, where the Juan de Fuca plate is sliding beneath the North American plate. Slow slip events here do not behave like typical earthquakes. The same fault segment can slip again within hours or days, which indicates the fault has partially regained strength and that stress has returned very quickly.

Thomas noted that even small tidal forces reveal how rapidly stress can rebuild. The gravitational pull of the Sun and Moon affects the Earth’s crust just as it influences ocean tides. In addition, the shifting weight of seawater also applies pressure to the rocks below.

The remaining question is how the fault manages to recover so quickly.

High-Pressure Experiments Reveal Rapid Healing

Watkins, a geochemist who specializes in the behavior of minerals at high temperature and pressure, used laboratory equipment capable of simulating the conditions found deep in the crust or beneath a volcano.

To recreate the aftermath of a slow slip event, Watkins and Thomas packed powdered quartz into a silver cylinder, sealed it, and placed it under 1 Gigapascal of pressure (10,000 times atmospheric pressure) at 500 degrees Celsius.

“We’re simulating what happens in the aftermath of a slow slip event,” Watkins said. “We cook it and look at it.”

The researchers measured the speed of soundwaves traveling through the treated quartz, then opened the cylinders and examined the samples using electron microscopy.

They found that the mineral grains had welded back together during compression.

“It’s like quick set fault glue,” Thomas said. “It’s really fast and you can get significant strength recovery.”

Cohesion May Play a Larger Role Than Expected

This ability of faults to regain strength, known as cohesion, may be significant in other tectonic environments as well, including shallower systems and regions responsible for large earthquakes.

“Cohesion is neglected in most models,” Thomas said. “Under certain conditions, cohesion may be more important than we thought.”

Thomas and Watkins recently received a new National Science Foundation grant to expand their investigation of cohesion in earthquake faults.

“It links events on the microscopic scale to major thrust earthquakes on a scale of hundreds of kilometers,” Watkins said.

Additional contributors to the study include Nicholas Beeler, U.S. Geological Survey; Melodie French, Rice University; Whitney Behr, ETH Zürich, Switzerland and Mark Reed, University of Oregon.

Previous research has already established that smoking raises the likelihood of cardiovascular disease, but the connection between smoking intensity and specific health consequences has been harder to clarify, particularly for light smokers. As more individuals smoke fewer cigarettes than in past decades, understanding both the risks tied to low-intensity smoking and the long-term benefits of quitting has become increasingly important, even for people who do not consider themselves heavy smokers.

Large Multi-Study Review Reveals Long-Term Damage

Blaha’s group examined information from more than 300,000 adults who participated in 22 longitudinal studies (which follow individuals over extended periods) for as long as 19.9 years. During that time, more than 125,000 deaths and 54,000 cardiovascular events were recorded, including heart attacks, strokes and heart failure. The results showed that smoking only two to five cigarettes per day was linked to a 50 percent higher risk of heart failure and a 60 percent higher risk of death from any cause compared with people who had never smoked. The greatest reduction in cardiovascular risk occurred within the first 10 years after quitting and continued to improve the longer a person remained smoke-free. Even so, former smokers still had higher risk levels than lifelong non-smokers for as long as three decades after they quit.

Quitting Completely Offers the Strongest Protection

Because even occasional or low-level smoking can sharply increase the chance of heart disease and premature death, the researchers emphasize that stopping entirely at a younger age is the most effective way to reduce long-term harm. Simply cutting back on the number of cigarettes smoked each day does not provide the same protective benefits. These findings support long-standing public health recommendations that encourage early and complete cessation and highlight the need for robust smoking prevention efforts.

Researchers Stress the Impact of Early Cessation

The authors add, “This is one of the largest studies of cigarette smoking to date using the highest quality data in the cardiovascular epidemiology literature. It is remarkable how harmful smoking is — even low doses of smoking confer large cardiovascular risks. As far as behavior change, it is imperative to quit smoking as early in life as possible, as the among of time passed since complete cessation from cigarettes is more important prolonged exposure to a lower quantity of cigarettes each day.”