The study, published in Evolution by scientists from the American Museum of Natural History and Princeton University, examines how body size evolved across vertebrates. Body size influences nearly every aspect of an animal’s biology, including metabolism, growth rate, reproduction, movement, vulnerability to predators, and the amount of food required for survival. Using mathematical models based on energy intake and reproductive output, the researchers tested whether physiological constraints could predict the range of body sizes found in different animal groups. The models successfully reproduced the size distributions of mammals, birds, and turtles, but they failed to explain why non-avian dinosaurs remained so much larger than expected.

The smallest known non-avian dinosaurs weighed approximately 450 grams, or about one pound—roughly the size of a large rabbit. That may sound small compared with a multi-ton Apatosaurus, but it is enormous by the standards of modern vertebrate diversity. The bee hummingbird, the world’s smallest bird, weighs only about 1.75 grams, while the Etruscan shrew, the smallest living mammal, weighs approximately 1.8 grams. A dwarf gecko can weigh as little as 0.15 grams. Around 75 percent of living mammal species and 90 percent of living bird species are smaller than the smallest known non-avian dinosaur.

The apparent absence of mouse-sized dinosaurs cannot be dismissed simply as a failure of fossil preservation. Very small animals are often difficult to discover because their skeletons are fragile and easily destroyed before burial. However, many fossil deposits that preserve dinosaur remains also preserve delicate bones belonging to mammals, lizards, amphibians, and other small vertebrates. If tiny dinosaurs had occupied these environments in significant numbers, researchers would expect at least some of their fossils to appear alongside those of other small animals. The fossil record instead suggests that genuinely miniature non-avian dinosaurs were uncommon or absent.

To investigate this pattern, the researchers used models describing how natural selection might shape body size according to the efficiency with which animals acquire energy and convert it into offspring. These models account for physiological relationships such as metabolic demands, growth, and reproductive investment. In principle, a species should evolve toward body sizes that provide the greatest reproductive advantage under its environmental conditions. The fact that these models could explain the size ranges of some vertebrate groups but not dinosaurs indicates that additional forces were operating. Ecology, rather than biology’s energy budget alone, appears to have imposed a lower limit.

One likely force was competition. Early mammals had already evolved small-bodied forms by the time dinosaurs became ecologically dominant. These mammals could exploit food resources, shelter, and lifestyles that may have been difficult for dinosaurs to enter without displacing established competitors. Small animals also tend to have rapid generations, specialized diets, and the ability to live in protected spaces such as burrows or dense vegetation. In this view, mammals may have occupied the miniature-animal niches so effectively that dinosaurs were prevented from evolving into equivalent forms. Dinosaurs, in turn, appear to have restricted mammals from reaching the largest body sizes for much of the Mesozoic.

This ecological separation may help explain why body size distributions were so uneven during the age of dinosaurs. Non-avian dinosaurs occupied a vast range of sizes, from roughly rabbit-sized animals to species weighing more than 80 tons, but the lower end of that range remained surprisingly high. Predation and habitat structure may also have reinforced the pattern. Small dinosaurs could have been vulnerable to predators, while larger bodies may have provided advantages in competition, mobility, or access to food. The researchers emphasize that no single explanation has yet been proven, but their findings show that physiological models alone cannot account for the pattern.

The study also sheds light on the remarkable emergence of birds. Birds evolved from theropod dinosaurs, yet modern birds include species far smaller than any known non-avian dinosaur. The transition happened relatively rapidly after the first birds appeared during the Early Cretaceous. According to the researchers, powered flight may have changed the rules governing dinosaur body size. Flight allowed early birds to exploit aerial habitats, move through environments inaccessible to terrestrial animals, and use new food sources and nesting sites. By escaping the ecological competition that constrained ground-dwelling dinosaurs, birds may have gained the freedom to evolve miniature bodies.

The findings offer a broader lesson about evolution: the organisms present in nature are shaped not only by what their bodies can physically achieve, but also by which ecological opportunities are available. Dinosaurs were biologically capable of producing descendants that became very small, as demonstrated by birds, and their distant ancestors included small-bodied forms. Yet for most of dinosaur history, competition, predation, and habitat structure appear to have blocked that evolutionary path. Understanding why certain body sizes flourish while others remain vacant could help scientists explain the structure of modern biodiversity—and reveal that evolutionary absences can be as informative as spectacular giants.

Subject of Research: Evolution of body size in dinosaurs and other vertebrates

Article Title: Why Dinosaurs Never Became Truly Tiny

Web References: https://doi.org/10.1093/evolut/qpag117

References: Evolution, DOI: 10.1093/evolut/qpag117

Image Credits: Alvaro Keding / © American Museum of Natural History

Keywords: dinosaurs, body size evolution, evolutionary biology, paleontology, mammals, birds, miniature dinosaurs, natural selection, ecology, competition, powered flight, vertebrate evolution

The fossil emerged from the Chorrillo Formation, a geological unit renowned for its diverse fossil record, including dinosaurs, amphibians, and early mammals. Dating to the Maastrichtian, this formation reflects a seasonally humid, freshwater floodplain environment that fostered rich biodiversity. Southern Patagonia during this period was a warm refuge hosting intricate food webs, where Kostensuchus atrox likely occupied the upper trophic levels. The discovery adds crucial information about crocodyliforms—relatives of modern crocodiles and alligators—that thrived alongside dinosaurs, filling ecological niches often overshadowed in popular reconstructions of prehistoric life.

Technically, Kostensuchus atrox belongs to the Peirosauridae family within Crocodyliformes, a group that diverged from the ancestors of today’s crocodilians but shared many anatomical and ecological traits. This species is distinguished by its large size—roughly 3.5 meters in length—and robust morphological adaptations, such as a wide, powerful jaw lined with large, serrated teeth optimized for hypercarnivory. The skull and other skeletal elements provide a window into the biomechanics of predation, demonstrating how this ancient predator could have subdued formidable prey, including medium-sized dinosaurs, thereby controlling herbivore populations and influencing ecosystem structure.

The etymology of the species name, Kostensuchus atrox, is culturally and scientifically resonant. “Kosten” references the strong Patagonian wind known locally in the Tehuelche language, embodying the fierce natural forces of the region. “Suchus” nods to the Egyptian crocodile-headed deity Souchos, symbolizing power and predation, while “atrox” is Latin for “fierce” or “harsh.” Together, the name encapsulates the animal’s imposing ecological role and the cultural heritage of the land where it was found, connecting natural history with human history through scientific nomenclature.

The fossil specimen stands out not only for its preservation but also for its completeness. Most peirosaurid crocodyliform fossils are fragmentary, hampering detailed anatomical and ecological reconstructions. Here, researchers have access to a nearly complete set of cranial bones, jaws, and multiple postcranial elements, facilitating comprehensive morphological analyses. Such data allow for advanced biomechanical modeling to estimate bite force, locomotion capabilities, and predatory behavior, shedding light on how Kostensuchus atrox thrived in a competitive environment crowded with large theropod dinosaurs and other formidable predators.

From an evolutionary standpoint, Kostensuchus atrox exemplifies an adaptive radiation event among crocodyliforms during the Late Cretaceous. Peirosaurids, unlike their modern counterparts, displayed a range of feeding strategies and ecological roles, including terrestrial hunting and complex interactions with other fauna. This discovery adds empirical evidence to hypotheses about crocodyliform diversity and their ability to exploit various ecological niches before the end-Cretaceous extinction, altering previous conceptions that mostly portrayed these reptiles as marginal players overshadowed by dinosaurs.

An especially notable aspect of the study is the multidisciplinary approach combining paleontology, ecology, and biomechanics. The researchers employed detailed stratigraphic mapping to contextualize the find within the Chorrillo Formation’s sedimentary layers, ensuring precise temporal placement. Advanced imaging techniques, including micro-CT scanning of the skull, revealed internal bone structures and teeth occlusion patterns, essential for understanding feeding mechanics. These methods, paired with comparative anatomy using related species, provide a rich framework to interpret how Kostensuchus atrox functioned within its ancient environment and how crocodyliforms evolved niche specialization.

The paleoenvironmental reconstructions paint a vivid portrait of the habitat inhabited by Kostensuchus atrox. Seasonal rainfall patterns fostered expansive floodplains dotted with freshwater bodies, creating corridors teeming with diverse organisms. This landscape supported a complex trophic system, balancing aquatic and terrestrial food webs. As a large hypercarnivore, Kostensuchus atrox likely influenced prey populations and competed with other predators for resources. Understanding these dynamics sheds light on ecosystem resilience and species interactions before the Cretaceous-Paleogene boundary dramatically reshaped Earth’s biosphere.

Importantly, Kostensuchus atrox holds significance beyond its palaeobiological implications. It deepens understanding of crocodyliform phylogeny and contributes to knowledge about morphological innovations enabling survival in variable climates and competition with dinosaurs. The findings support the view that crocodyliforms were ecological generalists displaying remarkable adaptability over evolutionary time, which may explain the lineage’s survival past the mass extinction event that eradicated dinosaurs. Thus, Kostensuchus atrox serves as a critical puzzle piece in reconstructing the evolutionary narrative of reptiles.

The international collaboration behind the discovery highlights the global importance of paleontological research in Argentina. The study’s leading author, Fernando Novas, together with colleagues from Portugal and Japan, brings diverse expertise to decipher the complexities of Kostensuchus atrox. This partnership emphasizes the pivotal role of cross-border scientific exchange in uncovering Earth’s deep history and underscores how fossil discoveries in remote regions can illuminate worldwide evolutionary processes. The support from major institutions and funding agencies reflects the priority placed on uncovering biodiversity origins at the twilight of the Age of Dinosaurs.

With this discovery, the scientific community gains a new and powerful lens through which to view the Maastrichtian ecosystems of southern South America. Kostensuchus atrox represents an apex predator that coexisted with dinosaurs and other fauna, yet occupied a distinct ecological niche shaped by robust anatomical adaptations and predatory prowess. Its emergence from the fossil record challenges assumptions that crocodile-line archosaurs were secondary players, instead positioning them as influential agents within their ecosystems. As more detailed analyses unfold, this species will undoubtedly become a cornerstone for understanding predation and survival strategies before Earth’s Cretaceous curtain fell.

Moreover, the fascinating picture painted by Kostensuchus atrox encourages a reevaluation of convergent evolutionary trends within Archosauria. The hypercarnivory and robust skull structure parallel those seen in theropod dinosaurs, suggesting similar selective pressures and ecological drivers across distinct clades. This convergence underscores how ecosystem architecture influences morphological and behavioral evolution. The paleoecological context of southern Patagonia offers an exceptional laboratory to investigate such phenomena, with Kostensuchus atrox standing as a flagship discovery illustrating evolutionary innovation amid environmental change.

The discovery is also a testament to the importance of fossil sites like the Chorrillo Formation, which continue to yield significant finds that challenge and refine paleontological models. Prior to this find, crocodyliform fossils had not been documented in this unit, making Kostensuchus atrox the first known specimen from the region. Its completeness and preservation quality open new avenues for targeted excavation and future research, motivating intensified exploration that may uncover further crocodyliform diversity, providing more pieces to the Late Cretaceous puzzle and filling gaps in our understanding of prehistoric biodiversity and biogeography.

In conclusion, the identification and analysis of Kostensuchus atrox revolutionize current knowledge about predator-prey dynamics at the end of the Cretaceous in Southern Patagonia. Its large size, specialized morphology, and ecological role emphasize the diversity and complexity of crocodyliforms, reinforcing their significance in ancient terrestrial ecosystems. This discovery not only enriches the fossil record but also inspires fresh perspectives on evolutionary resilience and paleoecological interactions during one of Earth’s most transformative periods, shedding light on the enduring legacies of ancient life.

Subject of Research: People

Article Title: A new large hypercarnivorous crocodyliform from the Maastrichtian of Southern Patagonia, Argentina

News Publication Date: 27-Aug-2025

Web References:

• http://plos.io/47w520t
• https://doi.org/10.1371/journal.pone.0328561
• https://www.nationalgeographic.org/society/
• https://www.faperj.br/
• https://www.gov.br/cnpq/pt-br

References:
Novas FE, Pol D, Agnolín FL, Carvalho IdS, Manabe M, Tsuihiji T, et al. (2025) A new large hypercarnivorous crocodyliform from the Maastrichtian of Southern Patagonia, Argentina. PLOS One 20(8): e0328561. https://doi.org/10.1371/journal.pone.0328561

Image Credits: Gabriel Diaz Yanten, CC-BY 4.0 (https://creativecommons.org/licenses/by/4.0/)

Keywords: Kostensuchus atrox, crocodyliform, Peirosauridae, Maastrichtian, Late Cretaceous, Southern Patagonia, hypercarnivore, apex predator, fossil discovery, paleoecology, Chorrillo Formation, evolutionary biology