Roughly 130 million years ago, in an area within what is now central Colombia, the ocean was filled with a diversity of species unseen today. Within that water swam several massive apex predators that are the stuff of nightmares. These marine reptiles could reach lengths of 2 to 10 meters (about 6 to 32 feet), some with enormous mouths filled with teeth, others with relatively small heads (also filled with teeth) attached to long, snake-like necks.
These giants shared the ocean with countless smaller species, many of them predators themselves. These included ichthyosaurs—dolphin-like reptiles—as well as turtles, fish, ammonites, crabs, mollusks, sharks, and at least one species of crocodyliform.
Allowing all these creatures to thrive must have required a flourishing ecosystem at all levels. Thanks to discoveries in what’s called the Paja Formation, a treasure trove where fossils are abundantly and exquisitely preserved, researchers are now beginning to figure out how the ecosystem supported so many apex predators. And they may find hints of how it flourished so soon after a mass extinction brought the Jurassic to a close.
Who ate what?
Dirley Cortés is a PhD candidate at the Redpath Museum of McGill University, a predoctoral fellow at the Smithsonian Tropical Research Institute, and a researcher at the Centro de Investigaciones Paleontológicas(CIP). She presented data she and her team have been working on from the Paja Formation at the 2022 annual meeting of the Society of Vertebrate Paleontologists (SVP), held this past November in Toronto.
The team’s goal is to dive much deeper into the role each species played in ancient oceans. In other words, from apex predator to the tiniest species within the sea, they hope to determine each species’ ecological niche. It’s mind-boggling, given the gaps of information they have to overcome. Not all species fossilize, for example, and few fossils offer gut contents to show what they ate. So how can scientists recreate an extinct ecosystem?
Acknowledging these limits of their study, the team compared the size of each species, aspects about their respective teeth, and other attributes to analyze where they fell within this early Cretaceous food chain. “This,” Cortés explained, “is a quantitative analysis. It is a starting point to develop energy flow models.”
“This trophic food web is quantitatively reconstructed based on inferred trophic interactions of marine producers, consumers, and large-apex-predators,” she added.
Layers upon layers
One of the things they found was that there were more trophic levels, meaning longer food chains, in this ancient sea than there are in today’s oceans.
This, she explained, “means greater complexity in the ecosystem. The more levels, one could suppose that there is more space for links between the species that occupy each of the trophic levels. An interesting question is whether higher levels imply greater stability of the ecosystem. What has been studied so far is that the base of marine systems has remained relatively stable for hundreds of millions of years. Studying the trophic web of the Paja Formation in Colombia could broaden this discussion at higher levels.”
That complexity stems, in part, from the diversity of predators within this ancient sea. Apex predators such as the pliosaur Monquirasaurus—a short-necked marine reptile that could reach lengths of about 10 meters long (32 feet)—made up one trophic level. But a separate one was composed of smaller, approximately 2-meter (6-foot) pliosaurs such as Stenorhynchosaurus and Acostasaurus and ichthyosaurs. Sea turtles and elasmosaurs (long-necked reptiles) made up yet another.
It’s tempting to assume that, because of their size, pliosaurs may have feasted on everything that swam among them, but there are still many unknowns regarding the pliosaur diet. Studies of their skulls indicated they may not have had a bite force comparable to today’s crocodiles, a strength that would have enabled them to grasp, twist, and jerk their prey into submission. Stomach contents reveal a regular diet of cephalopods, but some also include sharks, fish, turtles, ichthyosaurs, other marine reptiles, and even dinosaur dermal scutes.
The remarkably long necks of elasmosaurs have prompted several hypotheses about how they may have aided in predation. Could they have used their necks like today’s snakes: coiling back, then striking at prey? Could they have used them to help scoop up nutrients and food from the bottom of the sea (benthic feeding)? Or, did they simply swim with their necks fully extended, striking out and ambushing the prey they pursued? These, too, are questions yet to be answered, but their teeth seem to suggest a diet of fish.
An ecosystem in transition
“We are beginning to see that the Paja ecological network was very complex and diverse,” Cortés noted, adding that the “top of the network was dominated by these apex predators that feed on large prey such as large fish and other comparatively smaller marine reptiles, and also ammonites.”
We don’t have ammonites in our oceans today; the closest thing we have to some species of ammonite may be the Nautilus. Ammonites are ancient cephalopods that lived in thick shells, many of them tightly coiled; they're found in fossil deposits worldwide. Some could be as tiny as a few centimeters, but others were almost 3 meters wide (9 feet). Over 100 different species of ammonites have been found in the Paja Formation—ammonite fossils are so common that one species has become a regional symbol.
“The material from the Paja Formation is providing useful insights to investigate the dynamics of Mesozoic marine systems,” Cortés stated, “and ultimately how these systems responded to biotic and abiotic factors during the transitional Early Cretaceous period.” That transitional period refers to the recovery from the ecological disasters and extinctions that marked the end of the Jurassic.
What was presented at SVP is just the beginning. A paper outlining their work is expected this year, and the next steps involve determining “what trophic players are missing, and, ultimately, generate energy flow models.”
“Paleoecological network theory,” she concluded, “is relatively new in paleontology. Perhaps one of the most challenging parts is that there are few sites from the Mesozoic to compare broadly our data. However, this research has been exciting in terms of bringing new ideas into Mesozoic marine ecosystem evolution and ecological networks.”
Jeanne Timmons (@mostlymammoths) is a freelance writer with a strong passion for paleontology. Based in New Hampshire, she writes about paleontology (and some archaeology) on her blog mostlymammoths.wordpress.com.
