Sixty-six million years ago marked the conclusion of the Cretaceous period, a pivotal moment in Earth’s history. During this epoch-ending event, dinosaurs and approximately 90% of all species on our planet vanished, giving rise to longstanding debates among paleontologists. The central question: Was this extinction a gradual decline or a sudden, unforeseeable catastrophe?
In the early 19th century, Georges Cuvier, one of the pioneering paleontologists, championed the theory of geological “revolutions” causing swift waves of extinction. His perspective was partly influenced by the study of Mosasaurus, a massive sea lizard that thrived and perished at the Cretaceous boundary.
Contrastingly, Charles Darwin saw the end of the Cretaceous as a gradual process, where extinctions unfolded over millions of years, much like the slow transformation of landscapes through sedimentation and erosion.
For over a century, the debate raged on. However, the notion of catastrophic extinctions gained momentum as paleontologists amassed more fossil evidence, meticulously tracking the arrival and departure of species. The sheer volume of species disappearing rapidly worldwide at the Cretaceous’s end implied a severe and sudden cause—a global catastrophe.
Then, in 1980, physicist Luis Alvarez pinpointed a potential trigger for the extinctions: a colossal asteroid impact, later linked to the massive Chicxulub crater in Mexico. Debris ejected into the upper atmosphere blocked sunlight, halting photosynthesis and leading to plummeting temperatures.
Yet, this discovery did not conclude the discussion. Some contended that other events, such as volcanic eruptions, also played a role, or that dinosaurs were already in decline. In these scenarios, the asteroid strike acted as one of several factors contributing to the extinctions, possibly delivering the final blow to species already waning.
Recently, I’ve been part of a team of paleontologists delving into newly discovered marine reptiles from North Africa’s latest Cretaceous period. Our findings unveil a wealth of mosasaur species, close relatives of Cuvier’s Mosasaurus. Our research points to the notion that mosasaurs retained their diversity until the very end. Given their status as dominant predators of their era, their evolutionary history sheds light on the broader marine ecosystem, suggesting that it remained rich and varied until a sudden, catastrophic upheaval triggered by the asteroid impact.
Cretaceous marine reptiles
As the Cretaceous period neared its close, Earth’s landscape was dramatically different. Vast stretches of Africa were submerged beneath rising sea levels, with the Tethys Sea – the precursor to the Mediterranean – swallowing the Sahara, while the Atlantic Ocean expanded eastward to reach Morocco’s Atlas Mountains.
During this time, the prevailing trade winds directed their force eastward, much as they do today. These winds drove surface waters away from the coast, prompting nutrient-rich waters to surge upward from the ocean floor along the eastern Atlantic. This marine nutrient infusion sparked immense plankton blooms, kickstarting a vibrant food chain. Phytoplankton nourished zooplankton, which in turn fed small fish and ammonites, sustaining a cascading ecosystem of life.
At the apex of this intricate marine food web thrived a remarkable diversity of marine reptiles. Among them were colossal sea turtles, enigmatic long-necked plesiosaurs, and the formidable mosasaurs.
The remains of these creatures, along with fish skeletons and shark teeth, eventually settled to create expansive bonebeds. Today, these bonebeds, found in present-day Morocco, serve as a trove of fossilized evidence, offering a glimpse into the extraordinary marine ecosystem of the Cretaceous’s final days.
Contrary to the notion of decline, marine reptiles, particularly mosasaurs, experienced a flourishing of diversity. These mosasaurs exhibited a wide range of sizes, from a few meters to colossal giants exceeding 10 meters in length. Their evolution also led to a striking array of tooth adaptations, including hooks, spikes, cones, blades, and crushing molars.
Recent years have yielded the discovery of numerous new species, some bearing unique and specialized features. Take, for instance, Pluridens serpentis, equipped with a mouthful of small, hook-shaped, snake-like teeth, likely adapted for hunting small, soft prey like fish and squid.
In the realm of peculiarity, the tiny Xenodens stood out, with its blade-like teeth arranged edge-to-edge to create a saw-like cutting apparatus—an unprecedented trait among lizards and reptiles. Its teeth likely served for sawing apart larger prey or scavenging from carcasses.
Among the apex predators was the formidable Thalassotitan, boasting massive, conical teeth reminiscent of a killer whale’s. It feasted on plesiosaurs, sea turtles, and even its own kind—other mosasaurs.
And then, the most recently unearthed species, known as Stelladens, or “star tooth,” perplexed researchers. Unlike typical mosasaurs, Stelladens sported teeth adorned with two to four extra ridges, resembling a Phillips-head screwdriver or hex wrench. The function of these unique teeth remains a mystery, leaving scientists to wonder about its dietary habits and ecological role.
Resilience and adaptability
Mosasaurs, among the formidable inhabitants of prehistoric seas, offer a unique perspective into the dynamics of this ancient ecosystem. Their coexistence in great diversity stemmed from specialization—each species honed its hunting strategies and targeted distinct prey, avoiding direct competition.
To have such a rich tapestry of top-tier marine predators, it’s crucial to recognize the presence of varied prey species at lower trophic levels. The wealth of mosasaurs suggests that, in the final million years of the Cretaceous period, the marine ecosystem thrived in robust health and stability. This lends weight to the notion that the end-Cretaceous extinction resulted from a sudden and catastrophic event—the infamous asteroid impact—rather than a protracted decline.
Zooming out across the vast expanse of geological time, life’s story reveals astonishing resilience and adaptability, along with a certain orderliness. Species evolve, diversify, and refine their adaptations. However, even the most successful and finely tuned species remains a single catastrophe away from extinction—be it an asteroid, volcanic eruption, or ice age. This reality casts a disquieting shadow, suggesting that our own species, too, is not immune to such uncertainties. The next cataclysmic event could spell the end of our existence.
Yet, paradoxically, the same asteroid impact that erased marine reptiles and dinosaurs from the Earth’s canvas paved the way for the world we know today. The extinction of mosasaurs and plesiosaurs cleared space for the evolution of dolphins, whales, and seals. Similarly, the demise of dinosaurs opened the door for the emergence of creatures like horses, lions, elephants, and, ultimately, humans. One world was extinguished by catastrophe, but from that crucible, something new, extraordinary, and unforeseen—our world, with its diverse array of species—emerged.