Dinosaurs debuted on Earth’s stage in the aftermath of the Permian-Triassic mass extinction event, and survived two other Triassic extinction intervals to eventually dominate terrestrial ecosystems. More than 231 million years ago, in the Late Triassic Ischigualasto Formation of west-central Argentina, dinosaurs were just getting warmed up. At this time, dinosaurs represented a minor fraction of ecosystem diversity. Members of other tetrapod groups, including synapsids and pseudosuchians, shared convergently evolved features related to locomotion, feeding, respiration, and metabolism and could have risen to later dominance. However, it were dinosaurs that radiated in the later Mesozoic most significantly in terms of body size, diversity, and global distribution. Elevated growth rates are one of the adaptations that set later Mesozoic dinosaurs apart, particularly from their contemporary crocodilian and mammalian compatriots. When did the elevated growth rates of dinosaurs first evolve? How did the growth strategies of the earliest known dinosaurs compare with those of other tetrapods in their ecosystems? In new research, paleontologists studied bone histology of an array of early dinosaurs alongside that of non-dinosaurian contemporaries from the Ischigualasto Formation in order to test whether the oldest known dinosaurs exhibited novel growth strategies. Their results indicate that the Ischigualasto vertebrate fauna collectively exhibited relatively high growth rates.
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Herrerasaurus ischigualastensis. Image credit: Kristina Curry Rogers.
Dinosaurs grew up fast, a feature that likely set them apart from many other animals in their Mesozoic (252 to 66 million years ago) ecosystems.
Some paleontologists have proposed that these elevated growth rates were key to the global success of dinosaurs, but little is known about the growth strategies of the earliest dinosaurs.
In a new study, Macalester College researcher Kristina Curry Rogers and colleagues performed histological analysis, examining patterns of bone tissue growth in the fossilized leg bones of an array of animals in one of the earliest known Mesozoic ecosystems.
The studied fossils come from the Ischigualasto Formation of Argentina and date between 231 and 229 million years old.
Sampled fossils include several of the earliest known dinosaurs as well as several non-dinosaur reptiles and one early relative of mammals.
“Being able to look at the same area of the same bones in all of the animals we sampled was an important part of our study,” the researchers said.
“Most of the times when we study growth rates in any fossil animal, our samples are opportunistic. Often this might mean that samples come from very fragmentary specimens that don’t hold a lot of anatomical value otherwise.”
“You may only be able to sample a shin bone from one animal, a thigh bone from another, and a rib from yet another. But since each of these bones does a different job in an organism’s body, such a diverse sample might be telling you a slightly different story about that animal’s growth history.”
“To be very specific about comparing growth patterns among different kinds of creatures, it’s best to sample the same bone, in the same place, and from the same environment.”
“For this study, we sampled only the middles of the thigh bones, where we have the longest record of growth history preserved for each animal we studied.”
“We sampled only the largest known skeletons of these animals in hopes of getting as much of a complete growth record history that we could.”
“We also constrained our sample to a very narrow time window in the ancient past, so that we weren’t dealing with a background paleoenvironmental change that might impact what we were seeing in the bone tissue.”
“Those controls gave us a really unique perspective on comparability across this dinosaur and non-dinosaur group.”
The authors found that most of the examined species had elevated growth rates, more similar to some modern-day mammals and birds than to living reptiles.
The early dinosaurs all exhibited particularly fast growth, but they weren’t alone in this, as similar growth rates were seen in several of the non-dinosaur reptiles as well.
These results show that the earliest dinosaurs were already fast growers, supporting the idea that this feature was important to their later success.
But apparently dinosaurs were only one of multiple lineages evolving with elevated growth rates during the Triassic period (252-201 million years ago), suggesting that this feature is only part of the story of dinosaurs’ eventual global prosperity.
“Future studies could expand on these preliminary results by sampling a wider variety of ancient animals from additional early Mesozoic fossil sites,” the paleontologists said.
“Our sample comes from a time in which dinosaurs were the new kids on the block, restricted to relatively small, basic body plans, and evolving within a world rich with a diverse array of more specialized, non-dinosaur reptiles.”
“We tackled the question of how all of these animals grew, and found that the earliest dinosaurs grew quickly, and that these rapid growth rates probably played a significant role in dinosaurs’ subsequent ascent within Mesozoic ecosystems; but dinosaurs weren’t unique — many of their non-dino sidekicks shared rapid growth 230 million years ago.”
The study was publlished in the journal PLoS ONE.
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K. Curry Rogers et al. 2024. Osteohistological insight into the growth dynamics of early dinosaurs and their contemporaries. PLoS ONE 19 (4): e0298242; doi: 10.1371/journal.pone.0298242
