Scientists date back fossil of largest raptor dinosaur to 135 million years

"We determined the age of the dinosaur Utahraptor and found that it was much older than previously supposed. That finding has important implications for the evolutionary history of dinosaurs,” said Gregory Ludvigson, emeritus senior scientist with the Kansas Geological Survey, who was part of this study, in an official release.

The research, led by University of Kansas researchers, has been published in the journal Geosciences.

Estimating the exact age

The fossils were found at the well-known Utahraptor Ridge site in Utah. This is the location of Stikes Quarry, part of the Cedar Mountain Formation. Interestingly, from this location, experts have uncovered several peculiar dinosaur fossils, and most of them were found to be in a good state.  

The Utahraptor fossils were found encased in the Yellow Cat Member rock layer of the Cedar Mountain Formation. The team examined this rock layer again to evaluate the age of the rock as well as the age of the fossils. 

According to the study, uranium/lead dating of zircon crystals was conducted to estimate the exact date. The researchers also looked at changes in the "relative abundance of two types of stable carbon isotopes found in buried organic matter."

The findings revealed that the rocks at Stikes Quarry are at least 135 million years old, whereas it was previously estimated to be around 125 million years old.

"Before, we had a 25-million-year gap between them. That's a third of the age of mammals, more than double the length of time for hominid evolution. It's a big chunk of time. Anything can happen over 25 million years if you have no record of what's going on. We've plugged that record, for the most part," said Jim Kirkland, a paleontologist who first identified the dinosaur Utahraptor ostrommaysi in 1993.

The team also discovered that the rock strata from the Stikes Dinosaur Quarry were deposited during a Weissert Event that occurred approximately 133 million years ago. It is regarded as one of the most significant palaeoceanographic events of the Early Cretaceous.

Study Abstract:

The Early Cretaceous Yellow Cat Member of the terrestrial Cedar Mountain Formation in Utah, USA. has been interpreted as a “time-rich” unit because of its dinosaur fossils, prominent paleosols, and the results of preliminary chemostratigraphic and geochronologic studies. Herein, we refine prior interpretations with: (1) a new composite C-isotope chemostratigraphic profile from the well-known Utahraptor Ridge dinosaur site, which exhibits δ13C features tentatively interpreted as the Valanginian double-peak carbon isotope excursion (the so-called “Weissert Event”) and some unnamed Berriasian features; and (2) a new cryptotephra zircon eruption age of 135.10 ± 0.30/0.31/0.34 Ma (2σ) derived from the CA-ID-TIMS U-Pb analyses of zircons from a paleosol cryptotephra. Our interpretations of δ13C features on our chemostratigraphic profile, in the context of our new radiometric age, are compatible with at least one prior age model for the “Weissert Event” and the most recent revision of the Cretaceous time scale. Our results also support the interpretation that the Yellow Cat Member records a significant part of Early Cretaceous time.