In May 1997, as The Lost World: Jurassic Park premiered in the U.S., the U.S. Postal Service released a stunning series of stamps featuring dinosaurs and extinct reptiles, exciting enthusiasts and paleontologists alike. Christopher Brochu, a paleontologist from the University of Iowa, recalls the buzz: “We all rushed out to get them.” Among the stamps was one depicting Goniopholis, a crocodyliform from the late Jurassic period. However, upon closer inspection, Brochu noticed discrepancies: the tail patterns, scale shapes, and tooth arrangements didn’t align with the fossil evidence. It became clear that the illustration was based on today’s Nile crocodile, not the ancient species.

Brochu reflects, “People assume that to evoke a primitive landscape, all you need is a crocodile — even a modern one.” This notion, often echoed in media and educational contexts, suggests that crocodiles have remained unchanged since the era of dinosaurs, labeling them as “living fossils.” However, this idea is fundamentally flawed, and the entire notion of “living fossils” is crumbling.

The term "living fossil" was first used by Charles Darwin in The Origin of Species to describe certain creatures, like the lungfish and platypus, which evolved early and have persisted to the present. Darwin viewed these animals as evidence of evolutionary transitions, but he cautioned that the term was fanciful. Despite this, it gained traction in both scholarly and popular discourse, eventually symbolizing species that seemed to have changed little over vast spans of time. The phrase morphed into a compelling concept influencing scientific perspectives, suggesting that these creatures could serve as windows into ancient life.

However, the idea that any species has ceased to evolve is a misconception. In recent years, scientists have reevaluated various species, including coelacanths and horseshoe crabs, liberating them from the “living fossil” label. Among these, crocodiles have undergone significant reexamination.

Early paleontologists recognized similarities between ancient crocodilians and their modern counterparts. Yet by the 1800s, researchers began to find fossilized crocodiles that differed in crucial ways from today’s species. Some scientists dismissed these differences, adhering to the belief that species remain static. Many of these researchers held biblical views on creation, believing all life was divinely crafted and unchanging. For instance, William Buckland, a theologian and early paleontologist, argued in 1836 that crocodilian fossils did not exhibit sufficient differences from contemporary species to merit detailed study.

By the late 19th century, the notion of living fossils gained popularity, especially concerning crocodiles. Paleontologists assumed that ancestral and modern crocodiles adhered to the same biological model: large, scaly ambush predators with specific physical traits. While fossil crocodilians were not ignored, they often received superficial attention, leading to a tendency to categorize any crocodile-like fossil as simply another variant of modern crocodiles.

The narrative began to shift in the 1980s with new fossil discoveries, enhanced classification techniques, and advanced molecular analyses. Brochu meticulously examined crocodilian fossils, uncovering a more complex evolutionary history than previously acknowledged. He realized that many fossils previously categorized as crocodyliforms were, in fact, distantly related reptiles, revealing a previously underappreciated diversity among these ancient creatures.

In a notable sidebar, Brochu highlights various ancient crocodilian relatives, such as Terrestrisuchus, an agile insect-eater; Pristichampsus, a group of swift terrestrial carnivores; and Simosuchus clarki, a small herbivore. Fossil evidence shows that crocodilians were once as diverse as dinosaurs, dominating the planet until a catastrophic event about 200 million years ago allowed dinosaurs to rise to prominence.

As Brochu and his peers began to unveil this diversity, they also challenged the timeline of when modern crocodilians evolved. Traditionally, it was believed that crocodilians emerged in Africa during the Cretaceous period and remained largely unchanged. Yet, molecular analysis revealed that immune system molecules across living crocodilians exhibited remarkable similarity, suggesting a more recent common ancestor.

Jamie Oaks, a post-doctoral researcher, collected DNA samples from all 23 living crocodilian species, revealing minimal genetic divergence. This led him to propose that modern crocodilians diverged only between 8 to 13 million years ago, significantly younger than previously thought.

Additionally, Oaks discovered unexpected genetic variations within Nile crocodiles, indicating the possibility of two distinct species. This finding coincided with a separate investigation by Evon Hekkala, who found notable genetic differences in crocodiles from Chad and Mauritania, leading to the identification of two species: Crocodylus niloticus in the East and the less aggressive Crocodylus suchus in the West.

Hekkala, Oaks, and their colleagues collectively reshaped the understanding of crocodilian evolution, definitively removing them from the “living fossil” category. The fossil record indicates that crocodilians survived a severe mass extinction around 15 million years ago, during a period of significant climate change. The survivors evolved into the 24 modern species recognized today, highlighting their resilience and adaptability.

The reasons for the survival of these crocodilians amid mass extinction events remain unclear, but their ecological niche and robust physical characteristics likely played a role. Modern crocodilians, younger and more dynamic than once believed, are far from static entities; they continue to adapt and evolve as part of a rich evolutionary narrative.

As Alan Turner argues, the term “living fossil” should be retired, as it often leads to oversimplified conclusions. Oaks concurs, emphasizing that resemblance to ancient fossils does not equate to a lack of evolution. The perception that certain species are stagnant diminishes the complexity of evolutionary processes.

Ultimately, the view that crocodiles represent a primordial lineage is misleading. Instead, they exemplify the ongoing story of evolution, showcasing a lineage that is as modern and dynamic as any other. The notion of “living fossils” misrepresents the true nature of evolutionary change. Life is inherently dynamic and continues to evolve, while fossils remain fixed in time.

Ferris Jabr is a writer based in New York City. His work has appeared in The New York Times, The New Yorker, Scientific American, Wired, New Scientist, Popular Mechanics, NOVA Next, and The Awl.

Originally published at Nautilus on March 19, 2015.