Small dinosaurs almost never make it into the fossil record
Hace 150 millones de años, en lo que hoy es Colorado, un pequeño herbívoro del tamaño de un labrador recorría el mismo mundo que los estegosaurios y los diplodocus. Los paleontólogos han bautizado a esta nueva especie Enigmacursor mollyborthwickae —el 'corredor misterioso'— y su esqueleto casi completo nos recuerda que los ecosistemas del pasado estaban habitados no solo por gigantes, sino por criaturas ágiles y discretas cuya historia apenas comenzamos a descifrar.
- Un fósil casi intacto de un dinosaurio pequeño —algo extraordinariamente difícil de encontrar— emergió del yacimiento Morrison Formation entre 2021 y 2022, desafiando la tendencia de los cazadores de fósiles a priorizar especímenes de mayor tamaño.
- La fragilidad de los huesos pequeños y la acción de depredadores y carroñeros hacen que esqueletos como este rara vez sobrevivan completos, lo que convierte este hallazgo en una anomalía científica de primer orden.
- Investigadores del Museo de Historia Natural de Londres identificaron en el fémur del animal inserciones musculares únicas, confirmando que se trataba de una especie completamente desconocida para la ciencia.
- El esqueleto llegó al museo gracias a la mediación de una galería de arte londinense y la generosidad de una donante, Molly Borthwick, cuyo nombre ahora vive en la nomenclatura científica de la especie.
- A partir del 26 de junio, el fósil se exhibe en Londres, ofreciendo al público una ventana hacia un mundo jurásico más complejo y diverso de lo que los grandes dinosaurios por sí solos podrían revelar.
Entre 2021 y 2022, paleontólogos excavando en la Formación Morrison de Colorado desenterraron los huesos de un pequeño herbívoro que habitó la Tierra hace 150 millones de años, compartiendo paisaje con estegosaurios y diplodocus. Lo han llamado Enigmacursor mollyborthwickae —'corredor misterioso' en latín— y era aproximadamente del tamaño de un labrador retriever, con una cola que representaba la mitad de su longitud total y que le servía de contrapeso durante la carrera.
El camino del fósil hasta la ciencia fue inusual: una galería de arte londinense lo tenía en exhibición cuando los investigadores Paul Barrett y Susannah Maidment lo localizaron. Gracias a la donación de Molly Borthwick, el espécimen pudo ingresar al Museo de Historia Natural de Londres, institución que honró su generosidad incluyendo su apellido en el nombre de la especie.
Lo que hace excepcional este hallazgo es su integridad: el esqueleto está casi completo, faltando solo el cráneo y partes de la columna vertebral. Los dinosaurios herbívoros pequeños son notoriamente difíciles de encontrar; sus huesos frágiles suelen ser dispersados o destruidos por carroñeros antes de fosilizarse. El animal pesaba aproximadamente lo que un collie, caminaba erguido sobre largas patas traseras y era, según su morfología, un corredor veloz.
Al examinar los huesos, Barrett y Maidment identificaron en el fémur inserciones musculares sin equivalente en ningún otro dinosaurio conocido, confirmando que se trataba de una especie nueva. El estudio fue publicado en Royal Society Open Science. El espécimen parece haber sido un juvenil, aunque la historia de su preparación previa impide certezas absolutas. Su causa de muerte sigue siendo un misterio: los huesos no muestran señales de enfermedad ni trauma.
La especie está emparentada con Yandusaurus hongheensis, un dinosaurio chino de tres metros, lo que sugiere un rango geográfico amplio para este linaje. Aunque se descubre una nueva especie de dinosaurio aproximadamente cada semana, encontrar ejemplares pequeños y bien preservados es mucho más infrecuente. Cada uno de estos hallazgos completa el retrato de los ecosistemas antiguos, recordándonos que los mundos del pasado, como los del presente, pertenecían tanto a los pequeños como a los grandes.
Paleontologists working in Colorado's Morrison Formation between 2021 and 2022 unearthed the bones of a creature that walked the earth 150 million years ago—a small, swift herbivore that shared its world with the towering stegosaurs and long-necked diplodocuses we know from museum halls. They have named it Enigmacursor mollyborthwickae, and it was roughly the size of a labrador retriever.
The skeleton arrived at the Natural History Museum in London through an unusual path. A London art gallery had been exhibiting the fossil when researchers Paul Barrett and Susannah Maidment made contact. The two scientists worked with the gallery to secure a donor—Molly Borthwick—whose generosity allowed the specimen to come to the museum. The species name honors her contribution. The genus name, in Latin, translates to "mysterious runner," a fitting description for an animal that has left so little trace in the fossil record.
What makes this discovery significant is not just what the bones reveal, but how complete they are. The skeleton is nearly whole, missing only the skull and portions of the spine. This near-completeness is itself remarkable. Paul Barrett, the study's co-director, explained to reporters that small herbivorous dinosaurs of this kind are extraordinarily difficult to find. A practically intact skeleton, rather than scattered bone fragments, is uncommon enough that these creatures remain poorly understood by science. The skeleton will go on display at the museum beginning June 26.
The animal itself was built for speed. It was light-boned, weighing roughly as much as a collie, and walked upright on long hind legs. That body plan suggests it was a swift runner—an adaptation that would have served it well when predators appeared. Its tail made up about half its total length, a proportion that aided balance during rapid movement. The dinosaur was a plant-eater, grazing on the vegetation of its time.
When Barrett and Maidment examined the bones carefully, they identified features that set this species apart from all previously known dinosaurs. The femur, or thigh bone, was particularly distinctive, bearing muscle attachments unlike those found in any other dinosaur skeleton. This anatomical uniqueness confirmed they were looking at something new to science. The research was published Wednesday in the Royal Society Open Science.
The specimen appears to have been a juvenile, judging by vertebrae that had not yet fused together. Yet the fossil's preparation history—how it was cleaned and treated before arriving at the museum—makes certainty impossible. The cause of death remains a mystery. The bones show no signs of disease or injury that might explain how the animal came to rest in what is now Colorado.
Barrett noted that Enigmacursor mollyborthwickae is closely related to Yandusaurus hongheensis, a three-meter-long dinosaur discovered in China. This kinship across continents suggests the species had a wide geographic range, though other fossils from this lineage simply have not been found. The rarity of small dinosaur fossils owes partly to the bias of fossil hunters themselves, who tend to pursue larger, more impressive specimens. But there is also a taphonomic reason: small skeletons are fragile, easily scattered and destroyed by scavengers and predators after death.
New dinosaur species are identified roughly once a week, Barrett explained. But finding small dinosaurs like this one is far rarer. Each discovery of these overlooked creatures fills in gaps in our understanding of ancient ecosystems, revealing not just the giants that dominated the landscape but the smaller animals that moved through the same world, hunted by the same predators, eating from the same plants. The skeleton now heading to display will tell that fuller story.
Notable Quotes
Finding a practically complete skeleton rather than scattered bone fragments is uncommon, which is why these creatures remain poorly understood— Paul Barrett, Natural History Museum of London
Discovering these smaller dinosaurs gives us a more complete picture of how ancient ecosystems actually functioned— Paul Barrett
The Hearth Conversation Another angle on the story
Why does it matter that we found this particular dinosaur? We discover new species all the time.
Because small dinosaurs almost never make it into the fossil record. They get eaten, scattered, destroyed. Finding one nearly complete skeleton is like finding a letter from someone history forgot to mention.
But it's just a small herbivore. What did it do that was important?
It lived in an ecosystem alongside creatures we know well—stegosaurs, diplodocuses. By understanding what small animals were there, we understand how that whole world actually worked. It wasn't just about the giants.
The skeleton is missing its skull. Doesn't that limit what we can learn?
It does. We can't know its exact size or what its face looked like. But what we have—the bones, the proportions, the muscle attachments—tells us it was a runner, built for speed. That's a survival strategy we can read from the skeleton itself.
How did it end up in a London museum instead of staying in Colorado?
An art gallery was showing it. Two researchers saw it there, got interested, and found a donor willing to fund its transfer. Sometimes discovery is as much about chance meetings as it is about digging.
What's the next step? Will they keep looking for more of these?
They'll keep their eyes open. The fact that a related species exists in China suggests these animals were widespread. Other fossils might be out there, waiting in collections or still buried. But you have to know what to look for.