Small and omnivorous—two traits that proved advantageous for survival
Setenta e cinco milhões de anos antes do presente, uma criatura do tamanho de um hamster atravessava as florestas da atual Baixa Califórnia, indiferente ao asteroide que ainda tardaria nove milhões de anos a chegar. A descoberta de Cimolodon desosai — identificada a partir de fósseis raros que incluem crânio, mandíbula e ossos dos membros — oferece à ciência uma janela para compreender como a pequenez e a versatilidade alimentar permitiram que certos mamíferos sobrevivessem à extinção que varreu os dinossauros. No grande arco da vida na Terra, esta espécie representa um elo entre o mundo mesozoico e os mamíferos que hoje o habitam.
- A extinção Cretáceo-Paleogénico eliminou três quartos de toda a vida na Terra há 66 milhões de anos — e a questão de quem sobreviveu e porquê continua a moldar a nossa compreensão da evolução dos mamíferos.
- Um único dente a emergir da rocha em 2009 poderia ter sido apenas mais um fragmento anónimo, mas a persistência da equipa revelou um conjunto fóssil excecional: crânio, mandíbula, fémur e ulna intactos.
- A tomografia computorizada de alta resolução permitiu comparar os dentes com outras espécies conhecidas de Cimolodon, confirmando que se tratava de uma espécie nova e ancestral das formas que sobreviveriam ao impacto.
- O pequeno tamanho e a dieta omnívora de C. desosai — traços partilhados com os seus descendentes — revelam-se como as chaves evolutivas que abriram caminho aos mamíferos modernos após o colapso do mundo dos dinossauros.
- O fóssil foi batizado em memória de Michael de Sosa VI, o assistente de campo que primeiro avistou o dente e que faleceu durante o estudo — um lembrete de que a ciência é também feita de perdas humanas.
Há setenta e cinco milhões de anos, uma criatura não maior do que um hamster-dourado percorria as florestas da atual Baixa Califórnia, caçando insetos e colhendo frutos. Chamamos-lhe hoje Cimolodon desosai. Os seus descendentes sobreviveriam ao asteroide que, nove milhões de anos mais tarde, obscureceria o céu e extinguiria três quartos da vida na Terra.
Uma equipa da Universidade de Washington, liderada pelo paleontólogo Gregory Wilson Mantilla, publicou os resultados desta descoberta no Journal of Vertebrate Palaeontology. O que começou em 2009 com um único dente a emergir da rocha tornou-se algo muito mais raro: ao examinar a fenda com mais cuidado, os investigadores recuperaram um crânio, uma mandíbula, dentes adicionais, um fémur e uma ulna. Estes fragmentos permitiram ir muito além da análise dentária habitual, revelando o tamanho real do animal e a forma como se movia.
C. desosai pertencia aos multituberculados, um grupo de mamíferos que já existia há mais de 100 milhões de anos quando o asteroide atingiu a Terra e que persistiria por mais 100 milhões de anos depois. A sua pequenez reduzia as necessidades calóricas; a sua dieta omnívora garantia opções quando o mundo mudou abruptamente. Estes traços, partilhados com os seus descendentes, terão sido decisivos para a sobrevivência.
O fóssil foi nomeado em honra de Michael de Sosa VI, o assistente de campo que primeiro avistou aquele pequeno dente na rocha e que faleceu durante o decorrer do estudo. Wilson Mantilla descreveu-o como um excelente trabalhador de campo e como um irmão mais novo. Este sobrevivente do tamanho de um hamster, morto há 75 milhões de anos, carrega agora o nome de quem o trouxe de volta à luz.
Seventy-five million years ago, a creature no larger than a golden hamster scurried through the forests of what is now Baja California, hunting insects and plucking fruit from branches. Its name, assigned long after its bones had turned to stone, was Cimolodon desosai. It would never know the asteroid that was still nine million years away—the one that would darken the sky and erase three-quarters of all life on Earth. But its descendants would survive it.
A team from the University of Washington, led by paleontologist Gregory Wilson Mantilla, has published findings about this newly identified species in the Journal of Vertebrate Palaeontology. The discovery matters because Cimolodon desosai belongs to a lineage that endured one of the planet's most catastrophic events. Understanding how these small, omnivorous mammals made it through the Cretaceous-Paleogene extinction—the one that killed the dinosaurs—offers clues to how modern mammals came to inherit the world.
The fossil itself is a small miracle of preservation. In 2009, a field assistant working in Baja California spotted a tooth jutting from rock. What might have ended there—a single specimen, valuable but limited—became something richer. When the team examined the crevice more closely, they found more bone. Eventually they recovered a skull, a jaw, additional teeth, a femur, and an ulna. These pieces, seemingly modest, transformed the study from dental analysis alone into something that could reveal the animal's actual size, its gait, the way it moved through its world.
Wilson Mantilla explained the rarity of such finds. At this particular site, complete skeletal material is exceptionally scarce. Paleontologists of that era typically named species based on tooth characteristics alone, leaving many fossils nameless when teeth were absent. The Baja California specimen was different. Using digital imaging and high-resolution CT scanning, the researchers compared the teeth against other known Cimolodon species and confirmed they had found something new.
Cimolodon itself was a common mammal during the Late Cretaceous, the final chapter of the age of dinosaurs. Fossils of the genus have turned up across western North America, from western Canada down through Mexico. But this particular species, C. desosai, appears to have been ancestral to the forms that would survive the extinction event. Both this species and its descendants shared traits that proved advantageous: they were small, and they were omnivorous. Size meant lower caloric needs. Dietary flexibility meant options when the world changed.
The multituberculates—the larger group to which Cimolodon belonged—had already been on Earth for over 100 million years by the time the asteroid struck. They would persist for another 100 million years after, eventually giving way to other mammalian lineages. By studying animals like C. desosai, researchers can begin to map the pathway from the Mesozoic survivors to the mammals alive today.
The fossil was named in honor of Michael de Sosa VI, the field assistant who first spotted that small tooth in the rock. De Sosa died during the course of the study. Wilson Mantilla called him an excellent field worker and spoke of him as a younger brother. In paleontology, as in science broadly, the names we give to ancient things often carry the weight of present loss. This hamster-sized survivor, dead for 75 million years, now carries the name of a man who helped bring it back to light.
Notable Quotes
This new species was the predecessor of the species that survived the extinction. Both this and its descendants were relatively small and omnivorous, two characteristics that proved advantageous for survival.— Gregory Wilson Mantilla, University of Washington paleontologist
He was an excellent field assistant and was like a younger brother to me. It is a great honor to be associated with him.— Gregory Wilson Mantilla, on Michael de Sosa VI
The Hearth Conversation Another angle on the story
Why does a single new species of extinct hamster-sized mammal warrant publication in a major journal?
Because it's not really about the hamster. It's about understanding a survival strategy. When 75 percent of life dies, the ones that make it through are telling you something about what works. This animal was small, it ate almost anything, it lived in trees and on the ground. Those traits mattered.
But we already knew mammals survived the extinction. What's new here?
The completeness of the skeleton. Before this, you could only look at teeth and guess at the rest. Now you can see how big it actually was, how it moved, what its proportions tell you about its life. That changes how you reconstruct the whole genus and the whole ecosystem.
The fossil was found in 2009 but published in 2026. Why the long delay?
Paleontology is slow work. You have to compare it against every other known species, run it through imaging technology, write it up carefully. And sometimes life interrupts. The field assistant who found it died before the work was finished. That matters too—it's not just data.
What does this tell us about modern mammals?
That we're here because our ancestors were small and flexible. The dinosaurs were big, specialized, locked into their niches. When the world changed, they couldn't adapt. The mammals that made it through were the ones that could eat anything and didn't need much. We inherited that.
Is there more to find in Baja California?
Possibly. But the site is difficult. Finding even one complete skeleton is rare. That's why this one matters so much—it's a window that doesn't open often.