While most of the dinosaurs grew larger and larger in the course of the Cretaceous Period between 145 and 66 million years ago, an extraordinary line of dinosaurs shrank: the Alvarez sauroids. Researchers have now found a plausible explanation for this trend towards increasing miniaturization. Accordingly, the reduction in body size was accompanied by the fact that the Alvarez sauroids changed their diet and specialized in ants and termites.
The Cretaceous Period brought forth some of the largest land animals that have ever lived on earth, the titanosaurs. At the same time, however, individual dinosaur lines also developed in the opposite direction. The ancestors of the birds are known to reduce their size in adaptation to their newly acquired ability to fly. But there is also evidence for another flightless dinosaur family, the Alvarez sauroids, which lived around 160 to 70 million years ago, that they became smaller and smaller during the Cretaceous period. The reasons for this were unclear, however.
Sudden miniaturization
A team led by Zichuan Qin from the University of Bristol in Great Britain has now measured numerous representatives of the Alvarez sauroids from different times and has proven that miniaturization has actually taken place. “My calculations show how the body sizes went up and down in the 90 million years in which the Alvarez sauroids existed, from turkey to ostrich size with an average weight of 30 to 40 kilograms,” says Zichuan. “95 million years ago, her body weight suddenly dropped to five kilograms, and her claw shape changed from adapting to grasping and cutting to striking.”
When measuring the fossils, the researchers made sure that only adult specimens were included in the analyzes. Using bone sections with age rings, they estimated the age of the individuals. “From the number of growth rings in the bones, we could tell that some skeletons came from babies and young animals and therefore left them out of the calculations,” explains co-author Qi Zhao from the Chinese Academy of Sciences in Beijing.
Adaptation to changed diet
But even the fully grown specimens from the later Cretaceous period were noticeably small compared to their earlier living ancestors. In order to determine the causes of this miniaturization, the researchers combined findings on the anatomy of the Alvarez sauroids with further information on developments during the Cretaceous period. “The Cretaceous Period was a time when ecosystems developed rapidly. The biggest change was the increasing distribution of flowering plants, ”explains Zichuan’s colleague Michael Benton. “Flowering plants completely changed the nature of the landscape, and yet the dinosaurs mostly did not feed on these new plants. But they led to an explosion of new types of insects, including ants and termites. “
The changed claws of the younger specimens indicate that the Alvarez sauroids developed from less fussy predators to specialized ant and termite eaters. One example is the Mononykus, a four to five kilogram representative of the Alvarez sauroids from the Upper Cretaceous Period. “His arm was short and stocky and he had lost all fingers except one, which was modified into a short stinger. It looked like a sturdy little arm – not good for grabbing things, but ideal for punching a hole in the side of a termite mound, ”said Washington University co-author James Clark. Digging for insects living in the ground was also possible with this claw shape.
New ecological niche
“Perhaps competition with other dinosaurs intensified in the Cretaceous,” says Benton. A smaller body size opened up a new ecological niche in which the food consisted of small, concentrated sources of proteins and fats – for example insects. In the course of the Cretaceous this niche became larger and larger and provided space for different species of the Alvarez sauroids. In fact, the biodiversity within the family tripled at this time.
In the ancestors of the birds, the size reduction has so far been attributed to adaptations to flying. “We have now identified a second miniaturization event – but that was not for flying, but to enable a completely new diet, the switch from meat to termites,” says co-author Xing Xu from the Chinese Academy of Sciences. “Anatomical features that small Alvarez sauroids have in common with birds could therefore be more related to a shift in the ecological niche than to adaptations to flying,” the authors say.
Source: Zichuan Qin (University of Bristol, UK) et al., Current Biology, doi: 10.1016 / j.cub.2021.06.013