Why Are Current Claims of Paleocene Non-Avian Dinosaur Fossils Still Unconvincing?

The Paleocene, the first epoch of the Paleogene, marks the beginning of the Cenozoic after the end of the Cretaceous. The phrase "Paleocene non-avian dinosaur" therefore challenges one of the most familiar boundaries in palaeontology: non-avian dinosaurs became extinct at the end of the Cretaceous, and no non-avian dinosaurs are known to have lived during the Paleocene. If non-avian dinosaurs truly survived into the Paleocene, that would reshape our understanding of the Cretaceous-Paleogene extinction event.

Since the nineteenth century, scattered reports of supposed Paleocene non-avian dinosaurs have appeared in the scientific literature. Although not numerous, these claims are geographically widespread, having been made in North America, South America, Europe, Africa, Asia, and the South Pacific. At first glance, this makes the question tempting. Yet once researchers examine the cases one by one, the situation quickly becomes more complicated. Many specimens are not dinosaur fossils at all; some are actually crocodilian remains. Other fossils may indeed be dinosaurian, but they come from strata whose ages were incorrectly interpreted. Still other cases involve an even more difficult issue: whether some Cretaceous fossils were geologically reworked and moved into younger Paleocene sediments, leading them to be mistaken for Paleocene non-avian dinosaurs.

This kind of reworking is especially likely in fluvial channels, shorelines, and other high-energy depositional environments. A river, for example, may cut into Cretaceous strata, erode dinosaur fossils out of those older beds, and then redeposit them in Paleocene channel deposits. For that reason, where a fossil is found today does not necessarily indicate when the animal lived. Reworking also does not always leave obvious traces on a fossil, which makes age assignments much more difficult to defend.

Several cases illustrate why supposed Paleocene non-avian dinosaurs remain problematic:

One European example comes from Asturias, Spain, where two teeth identified as theropod dinosaur teeth were once considered Cenozoic in age. Later re-examination based on archival photographs and similar specimens showed that these teeth were actually those of ziphodont mesoeucrocodylians. The case from Nigeria in Africa is similar. Vertebrae once described as belonging to a hadrosaurid were later shown to be crocodilian. When only fragmentary teeth or bones are available, dinosaurs and some crocodilians can indeed be confused, especially when crocodilian teeth resemble the serrated teeth of theropod dinosaurs.

Crocodilian tooth misidentified as a dinosaur tooth (specimen ACN 4809)（Image source：Museo Nacional de Ciencias Naturales de Madrid，CC0 1.0 ）

Another important historical episode was the Laramie problem in the American West, a debate that lasted for decades. Some coal-bearing strata contained dinosaur fossils together with fossil plants that early palaeobotanists regarded as Cenozoic in age, leading some to suspect that dinosaurs may have survived into the Cenozoic. The problem was later resolved when those plant-based age assignments were rejected. The mistake arose largely because Late Cretaceous angiosperm-rich floras in the western United States looked more "modern" than typical European Cretaceous floras and were therefore incorrectly compared with European Eocene floras.

Plant fossils from the Laramie Formation（Image source：Knowlton FH. (1922)，CC0 1.0 ）

The San Juan Basin of New Mexico is one of the most famous regions in the debate over Paleocene non-avian dinosaurs. Some levels of the Ojo Alamo Formation yield dinosaur fossils, while overlying parts contain Paleocene palynomorphs, and this association has been used to argue that dinosaurs persisted into the Paleocene. In reality, the stratigraphically highest in situ dinosaur fossils mainly come from the Naashoibito Member and are Late Cretaceous in age. Dinosaur bones in the overlying Kimbeto Member are mostly isolated, fragmentary, and abraded, so the most reasonable interpretation is that they were reworked from underlying Cretaceous strata. Even a relatively well-preserved hadrosaur femur found stratigraphically above Paleocene palynomorphs cannot serve as reliable evidence, because it is an isolated bone from a fluvial deposit capable of transporting large bones.

A hadrosaur femur from the Kimbeto Member（Image source：Fassett JE et al. (1987)，CC0 1.0 ）

In the Hell Creek Formation of Montana, channel deposits near the Cretaceous-Paleogene boundary have yielded early Paleocene mammal teeth, palynomorphs, and teeth of several dinosaur taxa in close association. Supporters argued that the dinosaur teeth were finely preserved, with clear serrations, suggesting that they had not been reworked. Critics, however, pointed out that these channels were incised into older Cretaceous strata, and that bones occurred near the base of the channels together with mudstone clasts. This is exactly the kind of setting expected if older strata were eroded and redeposited. Thus, although the dinosaur teeth are striking, reworking cannot be ruled out.

The Bolivian case in South America is a matter of mistaken stratigraphy. Early interpretations suggested that Paleocene mammals from Tiupampa occurred together with dinosaur footprints in the El Molino Formation. Later work showed that the Tiupampa mammals actually come from the Santa Lucía Formation, which overlies the El Molino Formation and is middle Paleocene in age. They are therefore not directly associated with the underlying Cretaceous dinosaur fossils.

The Chatham Islands case in the South Pacific is more subtle. In the Takatika Grit, several bones identified as theropod dinosaur remains were found in strata associated with microfossils ranging from the Late Cretaceous to the Paleocene. Some researchers suggested that small non-avian theropods may have survived the end-Cretaceous extinction event for a short time on a remote island. Yet the Takatika Grit is only about 10 meters thick while spanning an age range from the Campanian to the Paleocene, indicating a condensed deposit in which fossils of different ages were likely mixed. The bones are also mostly isolated, broken, and abraded, and they occur together with marine fossils and phosphatic nodules. In that setting, interpreting the dinosaur bones as reworked is more reasonable than accepting them as evidence that dinosaurs truly survived into the Paleocene.

Although there is still no definitive evidence that non-avian dinosaurs lived during the Paleocene, the idea is not theoretically absurd. Earth is vast and complex, and it is not impossible to imagine some organisms retreating into safer habitats after a mass extinction. Ammonites, once thought to have vanished completely at the end of the Cretaceous, include cases of early Paleocene survival, such as Hoploscaphites, Baculites, and Fresvillia. At present, however, all claims of Paleocene non-avian dinosaurs fall into three main categories: misidentified specimens, incorrect stratigraphic or age assignments, and reworked fossils. A convincing case would require a fossil that is clearly dinosaurian, clearly from Paleocene strata, and demonstrably not reworked. Such a fossil has not yet been found, but the search will almost certainly continue.

Author: Shui-Ye You

References:

1. Fassett JE et al. (1987). Dinosaurs, pollen and spores, and the age of the Ojo Alamo Sandstone, San Juan Basin, New Mexico. GeoScienceWorld.

2. Knowlton FH. (1922). The Laramie flora of the Denver Basin: with a review of the Laramie problem. U.S. Geol. Surv.

3. Lucas SG. (2025). Paleocene non-avian dinosaurs. Ital. J. Geosci.

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