Young animals limited predatory dinosaur diversity

This illustration illustrates the void in the medium-sized predators of the dinosaur era compared to the biodiversity of modern carnivores in Africa. (Image: UNM Biology Department)

Fearsome giants dominate the image of the predatory dinosaurs – in fact, they were also typical of the era, because the biodiversity was particularly low in the medium-sized predators, researchers report. As can be seen from their study, this was apparently due to the fact that the young animals of Tyrannosaurus and Co pushed the smaller species out of their typical ecological niches. As a result, the patterns of biodiversity at that time differed from those of today’s ecosystems, say the scientists.

They dominated the earth for more than 150 million years and produced many different species – yet the dinosaur kingdom appears to be comparatively poor in species. Above all, there were noticeably few representatives weighing less than 60 kilograms. This differs from the patterns of today’s animal world: Since small and medium-sized animals can often adapt particularly well to different ecological niches, they have typically produced the greatest diversity in the ecosystems. They mostly cover the whole spectrum of orders of magnitude. But that was not the case with the dinosaurs. “Even taking into account possible distortions due to a lack of fossil finds, it becomes apparent that the bottom line was that there were rather few dinosaur species,” says Felisa Smith University of New Mexico in Albuquerque.

On the trail of low biodiversity

In their study, Smith and her colleagues have systematically investigated the question of what the rather low biodiversity of the dinosaurs was all about. To do this, they first analyzed data from hundreds of dinosaur species from 43 sites around the world. The fossils came from the Jurassic and Cretaceous periods with a total time span of 136 million years. The paleontologists categorized the animals according to their diet and body size – they specifically analyzed the proportions of small, medium and large dinosaur species in each of the former ecosystems.

In this way, they were initially able to make it fundamentally clear that the comparatively low overall species diversity among the dinosaurs is mainly due to the lack of rather smaller species among the predatory dinosaurs. In detail it became clear: “In the ecosystems typical of that time with very large predatory dinosaur species at the top, this gap was particularly pronounced – there were noticeably few carnivorous dinosaur species weighing between 100 and 1000 kilograms,” says first author Katlin Schroeder from the University of New Mexico.

The scientists then specifically explored a possible explanation for this loophole: “We checked the idea that the juveniles of large predatory dinosaurs assumed the role of multiple species as they grew, which limited the number of species that could coexist in an ecosystem could, ”says Schroeder. The background: since dinosaurs hatched from eggs, giants like T. rex were inevitably born small – about the size of domestic cats. This meant that they went through many stages of size until they reached the size of a city bus and weighed up to eight tons. Paleontologists have long suspected that they changed their behavior and their prey in the process. This is also supported by findings that the features of the skulls of many large predatory dinosaurs changed noticeably strongly in the course of adolescence.

Young animals replaced species

In order to test the theory that the juveniles of T. rex and co. Have taken on the role of several species, the scientists reconstructed the former ecosystems using models, taking these special “small predators” into account. They also calculated on the basis of fossil findings what proportion of the large predatory dinosaur species could have consisted of young animals. From these assessments it emerged: “The young animals formed an extremely large group of individuals of a species and thus they probably had a significant influence on the resources available in the ecosystems,” says Schröder. “The young animals of the megatheropods fit exactly into the gap in the missing medium-sized predatory dinosaurs,” the paleontologist sums up.

As the researchers further report, this gap was interestingly more pronounced in the Cretaceous ecosystems (145 to 65 million years ago) than in the Jurassic (200 to 145 million years ago). There is also a plausible explanation for this, says Schröder: “Jurassic mega-predatory dinosaurs did not change as much – their young were more like the adults, which presumably allowed more space for the development of different carnivore species. The Cretaceous period, on the other hand, is completely dominated by tyrannosaurs and abelisaurs, which change their characteristics significantly in the course of their growth ”. Apparently, their young animals filled the ecological niches that are typically occupied by medium-sized predator species particularly intensively, according to the study.

Source: University of New Mexico, specialist article: Science, doi: 10.1126 / science.abd9220

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