Biodiversity was booming in the early Cretaceous Period, and not just among dinosaurs. The oceans also teemed with life, including some monstrous predators unlike anything alive today.
In a new study, researchers dive deep into one intriguing ecosystem from the early Cretaceous, where bus-length marine reptiles like pliosaurs preyed on other large carnivores, forming a rare seventh trophic level in their food web.
Modern oceans, for comparison, max out at trophic level five or six, represented by apex predators like orcas, sperm whales, and great white sharks.
A trophic level indicates a position within a food web, with lower levels signifying species closer to the bottom. Level one features primary producers like algae and plants, while level two includes primary consumers like herbivores.
Level three is for carnivores that eat herbivores, known as secondary consumers, and higher levels are for carnivores that prey on fellow carnivores.
The new study focuses on ancient inhabitants of the Paja Formation, an early Cretaceous geologic formation located in present-day Colombia. Sea levels were higher at the time, and much of the area was covered by a warm, shallow sea.
That sea apparently bustled with life at all trophic levels, the study’s authors note, part of a broader biodiversity surge during the Cretaceous driven largely by warm climates and the ongoing breakup of the supercontinent Pangaea.
It takes a diverse, healthy ecosystem to support high-level apex predators, and the vibrancy of this prehistoric sea allowed some incredible leviathans to evolve.
The Paja ecosystem was home to massive dolphin-like reptiles known as ichthyosaurs, crocodile-shaped beasts called teleosaurs, and long-necked pliosaurs, some more than 10 meters long.
The potential for a seventh trophic level illustrates how rich and intricate the Paja ecosystem must have been 130 million years ago, the study’s authors say.
To shed light on this ancient habitat, the researchers rebuilt its ecosystem network – a model of interactions among species within an ecosystem – using data from all known animal fossils found at the Paja Formation.
They factored in key details like the fossilized animals’ body sizes and feeding adaptations, and used analogs from modern wildlife to help them infer some unfossilized information.
This approach has mainly been used to study present-day ecosystems, the authors point out, but there is a recent trend toward applying it to fossil data.
The researchers drew inspiration from an existing marine-ecosystem network based on contemporary Caribbean ecosystems, they write, and used this reference model to help them test and fine-tune their Paja network.
When it was finished, their network provided an unprecedented look back at one of the liveliest marine food webs known to science, the authors say.
“Our study is the first to examine these possible ecological interactions,” says McGill University biologist Dirley Cortés.
And while that’s interesting in its own right, it also informs our wider understanding of ecology, both ancient and modern, she adds.
“Understanding this complexity helps us trace how ecosystems evolve over time,” Cortés says, “shedding light on the structures that support today’s biodiversity.”
The Paja Formation has become known for its imposing marine reptiles, Cortés and her colleagues point out, but it’s unlikely those apex predators could have evolved without a robust food web to support them.
Relatively little is known about the habitat’s broad paleoecological structure, the researchers write, including the many fish, ammonites, and other important creatures from lower trophic levels.
In addition to spotlighting this one incredible Cretaceous community, the study’s authors say their work should help answer broader questions about the evolution of marine ecosystems – including the origins and influence of “exceptionally large predators” like those at Paja.
“These findings illuminate how marine ecosystems developed through intense trophic competition and shaped the diversity we see today,” says McGill University biologist Hans Larsson.
Not many fossil ecosystems have received the kind of scrutiny this study applied to the Paja Formation, the researchers note, but given the wealth of data already available in the fossil record, that may soon change.
The study was published in the Zoological Journal of the Linnean Society.
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