The Origin and Early Evolutionary Radiation of Dinosaurs
Many studies suggest that many of today's most diverse groups of animals and plants originated through evolutionary radiation—a process in which a lineage rapidly diversifies into a wide variety of species over a relatively short period of time.
Research on Archosauria conducted in the twenty-first century supports this view. Previous studies have proposed that archosaurs experienced multiple episodes of evolutionary radiation, including the rapid diversification of major archosaur lineages during the Early to Middle Triassic, as well as the radiation of modern birds following the Cretaceous–Paleogene (K–Pg) mass extinction about 66 million years ago.
The earliest undisputed dinosaur fossils currently known date to the early Late Triassic, approximately 230 million years ago, suggesting that dinosaurs may have appeared after the initial large-scale radiation of archosaurs. Moreover, compared with other contemporary archosaurs, early dinosaurs appear to have been relatively conservative in their morphology. These observations indicate that the earliest archosaur radiation was probably only indirectly related to the origin of dinosaurs.
Because the evolutionary relationships among early dinosaur lineages remain controversial, and because large and small carnivorous, omnivorous, and herbivorous dinosaurs all appeared within roughly the same time interval, researchers have suggested that dinosaurs themselves may also have undergone an independent evolutionary radiation during their origin.
To test this hypothesis, a research team used recently compiled morphological datasets of early dinosaurs to perform phylogenetic analyses and investigate the timing of dinosaur diversification and the rate of morphological evolution.
The researchers compiled several morphological datasets focusing on early dinosaurs and selected the major lineages involved in dinosaur origins, including Ornithischia, Sauropodomorpha, Theropoda, Silesauridae, Herrerasauria, and their close relatives. Taxa with insufficient sampling or little relevance to the study were excluded to minimize potential biases.
Species younger than approximately 170 million years were also excluded because their long evolutionary branches could artificially influence estimates of evolutionary rates during the earliest stages of dinosaur evolution.
The team then analyzed morphological evolutionary rates, divergence times, branch lengths, and phylogenetic support across different time intervals to determine whether dinosaurs experienced a period of unusually rapid morphological evolution early in their history.
The analyses indicate that dinosaurs originated sometime between 251 and 230 million years ago, during the Early to Middle Triassic. Subsequently, from the late Middle Triassic to the early Late Triassic, the major dinosaur lineages—including Silesauridae, which may themselves represent dinosaurs—diversified rapidly within a relatively short period.
One of the clearest findings is that dinosaurs experienced a pronounced burst of morphological evolution between approximately 240 and 220 million years ago. Evolutionary rates then gradually declined and became relatively stable after the Jurassic began.
Notably, if dinosaur origins are inferred solely from the oldest known fossil evidence rather than the older origin estimated by the evolutionary models, the same amount of morphological change would have had to occur within an even shorter time span. This suggests that the true rate of early dinosaur evolution may have been even faster than estimated here, making the current results relatively conservative.
Furthermore, this rapid morphological evolution was concentrated during the divergence of the major dinosaur lineages rather than after those lineages had already become established. Although Ornithischia, Sauropodomorpha, and Theropoda continued to evolve afterward, their overall evolutionary rates were substantially lower than those observed during the earliest phase of dinosaur evolution.
The study also found that Ornithischia followed a somewhat different evolutionary pattern from other dinosaurs. Their major branches show greater uncertainty in divergence times and are separated by relatively long ghost lineages, indicating that the early evolutionary history and relationships of ornithischians remain less certain.
The timing of dinosaur origins has long been debated. One reason is that the earliest known dinosaur fossils already represent several distinct groups that were widely distributed geographically, implying that the true origin of dinosaurs likely predates the oldest fossil evidence currently available.
If dinosaurs indeed originated during the Early to Middle Triassic, this would also be consistent with the recently proposed Pangaean biogeographic hypothesis. According to this hypothesis, an extensive arid belt stretching across the low latitudes of Pangaea may have limited the dispersal of dinosaurs and other archosaurs. Only after the Carnian Pluvial Episode, when rainfall increased dramatically, did this barrier begin to disappear, allowing dinosaurs to expand into new regions.
The researchers further predict that even older dinosaur fossils are more likely to be discovered in Early and Middle Triassic deposits from the southern regions of Pangaea rather than the north. If Silesauridae truly belong within Dinosauria, this prediction may already have received partial support.
Overall, the results match the three classic characteristics of an evolutionary radiation. First, the major dinosaur lineages diversified rapidly within roughly five million years after the origin of dinosaurs. Second, the burst of morphological evolution occurred before many of the characteristic body plans became fully established. Third, although dinosaur diversity continued to increase afterward, the rate of morphological evolution declined rapidly by the Late Triassic. Together, these findings support the hypothesis that dinosaurs experienced an evolutionary radiation during the Middle to Late Triassic.
As for what triggered this radiation, the researchers argue that many previously proposed explanations do not match the timing. For example, the origin of dinosaur endothermy remains highly controversial, while the Carnian Pluvial Episode and other Triassic climatic changes all occurred after the initial burst of rapid morphological evolution. These events were therefore more likely to have influenced later dinosaur evolution rather than serving as its original driving force.
Instead, the researchers suggest that the numerous ecological opportunities left behind by the end-Permian mass extinction were probably the primary driver of dinosaur diversification. However, this does not necessarily mean that dinosaurs gained dominance by directly outcompeting other reptiles. Rather, they may simply have diversified rapidly by exploiting the many vacant ecological niches created by the mass extinction.
The study uses the African cichlids of Lake Victoria as an example. Although more than 500 cichlid species evolved there in less than 20,000 years, they were neither the first fishes to colonize the lake nor initially the dominant group. Instead, their rapid diversification was likely driven by a combination of key innovations and ecological opportunity.
The researchers also argue that dinosaur radiation was not an isolated event but rather part of a series of nested radiations. Following the end-Permian mass extinction, archosaurs as a whole underwent a major evolutionary radiation, while the radiation of dinosaurs represented a secondary radiation nested within this broader archosaur diversification.
During the approximately 240-million-year evolutionary history of dinosaurs, morphological evolution did not proceed at a constant pace but instead consisted of several episodes of rapid change. However, the study found no clear increase in morphological evolutionary rates following the Triassic–Jurassic mass extinction. This may reflect the currently limited fossil sampling, or it may indicate that the most rapid evolution occurred later within theropod lineages such as Coelurosauria and Averostra. Additional fossil evidence will be needed to test these possibilities.
Finally, the researchers conclude that phylogenetic analyses can reveal evolutionary events that cannot be recognized from the fossil record alone, providing a more complete picture of the early evolutionary history of dinosaurs.。
（Author: Bai Leng）
Reference：
Brownstein, C. D., Griffin, C. T. (2026). An early burst of skeletal evolution at the origin of dinosaurs. Proceedings of the Royal Society B: Biological Sciences.
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