Crocodiles’ ancestral split and slow growth revealed

In the exploration of crocodiles’ deep and varied evolutionary history, researchers are delving into the origins of the surviving species. Presently, there are approximately 28 living species of crocodilians scattered throughout tropical and sub-tropical regions across the globe. However, this represents only a fraction of the once-diverse array of crocodile species.

Two new papers have undertaken the task of unraveling this rich evolutionary tapestry, tracing the group’s origin and global dispersion. It has been revealed that the larger, modern group of crocodilians likely made its first appearance in Europe around 145 million years ago. Subsequently, the ancestors of crocodiles and alligators diverged in North America. The crucial factor was the crocodile’s ability to tolerate saltwater, enabling them to spread extensively worldwide. This discovery was published in the Royal Society Open Science.

Professor Paul Barrett, a paleontologist at the Natural History Museum involved in both studies, explains, “It seems most likely that the ancestors of today’s alligators and crocodiles evolved in North America, and then subsequent to that alligatorids [which includes alligators and caimans] stay more or less within the Americas while crocodiles get everywhere else.”

The ability to traverse saltwater bodies has allowed crocodiles to disperse widely, including in tropical oceans, whereas alligators are confined to freshwater and unable to reach certain areas. Different crocodile subgroups appear to have prospered and originated in different regions.

Additionally, Paul and his colleagues discovered that the slow growth rate of crocodiles was a secondary adaptation not present in their distant relatives. Published in Current Biology, this research also reveals that crocodiles and birds, being each other’s closest living relatives, have adopted completely opposite physiological strategies for over 220 million years.

Diverse crocodilians

Contrary to the perception of crocodiles as unchanged relics from the dinosaur era, their evolutionary history is diverse and dynamic. Within the larger group Crocodylomorpha, encompassing living crocodiles, alligators, and gharials, there were once hundreds of now-extinct species. These included not only meat-eating, freshwater-adapted predators but also small, agile creatures preying on insects and herbivorous crocodilians with mammal-like teeth.

Paul explains, “Crocodiles and their relatives were really experimenting with lots of different ways of life.” The spectrum ranged from large predators hunting dinosaurs to small marine species with flippers and agile runners akin to reptilian whippets, such as Terrestrisuchus. This contrasts sharply with the limited, predominantly predatory, semi-aquatic lifestyle of present-day crocodiles restricted to tropical regions.

The diversity among ancient crocodilians allowed researchers to delve into their varied life histories, exploring how these creatures lived, grew, and behaved. Despite the common perception of large reptiles as slow-moving and slow-growing, the study reveals the complexity of crocodiles’ life history. This challenges assumptions about the pace of growth in enormous animals, akin to some dinosaurs, suggesting that rapid growth was more prevalent than previously thought. In essence, the current image of living crocodiles represents only a fraction of the vibrant diversity witnessed by their extinct relatives throughout history.

Secondarily slow

Transporting ourselves back to the Triassic Period unveils a stark contrast to the commonly perceived image of sluggish crocodilians. Fossil evidence indicates that ancient relatives of crocodiles were, in fact, swift-growing and active creatures. The mystery lies in pinpointing when and why these creatures transitioned to a more leisurely pace.

Paul poses the crucial questions: “Is the growth slowdown linked to crocodiles’ aquatic habits, or does it precede their semi-aquatic lifestyle? And when in crocodile evolution does the shift from their ancestrally high metabolism to a seemingly more primitive slow-growth state occur?”

The discovery of a 220-million-year-old crocodile-like relative challenges the notion that aquatic life prompted the slowdown. This ancient land-dweller exhibited a reduced growth rate, predating the aquatic phase of crocodiles. The reasons behind this metabolic shift remain speculative, with environmental resource scarcity being a potential factor.

Paul highlights an intriguing parallel: “They were living directly alongside the other big branch of archosaur evolution: the dinosaurs.” In the Late Triassic, crocodilians adopted a slow-growth strategy, contrasting with the fast-growing approach of dinosaurs, a dichotomy persisting to this day between slow-growing crocs and fast-growing birds.

The research also traces the roots of Crocodylomorpha, revealing its emergence in what is now modern Europe. Subsequently, the ancestors of crocodiles and alligators diverged in North America. Salt-adapted crocodiles gained a global foothold, colonizing Africa, Asia, and Oceania, while alligators and kin remained primarily in the Americas. The evolutionary journey of crocodiles unfolds, challenging preconceived notions and illuminating the complex interplay of factors influencing their growth and lifestyle.

Source: Natural History Museum

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