By Steven Wade Veatch
My journey in search of fossils led to the Morrison
Formation, which crops out over a vast area in Colorado. Though famed for its
giant sauropods, it unexpectedly coughed up a critical piece of the
paleontological puzzle: at least six different spots containing both Jurassic
eggshells and whole eggs, a true rarity for a time period between 155 to 148
million years ago. As one of the world’s premier Late Jurassic fossil sites,
the Garden Park Fossil Area yielded the first discovered remains of iconic
dinosaurs like Stegosaurus, Diplodocus, and Allosaurus. It
famously served as a primary battleground for the 'Bone Wars,' the legendary
19th-century rivalry between paleontologists Othniel Charles Marsh and Edward
Drinker Cope.
The local action centered in the Garden Park Fossil area
of Fremont County, Colorado. The whole thing kicked off in 1991, when Kenneth
Carpenter of the Denver Museum of Natural History found a well-preserved
eggshell fragment while collecting fossil snails (Hirsh 1994). That initial find
paved the way for more fragments and eventually led paleontologists to the site
that became the focus of scientific inquiry: Egg Gulch (Figure 1.).
Figure 1. General location of Egg Gulch in Garden Park, north and west of Cañon City, Colorado.
At Egg Gulch, weathering and relentless erosion had stripped away layers of rock, revealing ancient eggshell fragments scattered across a rugged, 10-meter slope (Figure 2.). The presence of tiny fossilized freshwater plants called charophytes established the site's age as early Kimmeridgian (157.3 to 152.1 million years ago), making it the oldest dinosaur eggshell site in North America (Alf, 1998).
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| Figure 2. Under a brilliant spring sky, the crack team of volunteers from the Dinosaur Depot and the author are in the field, exploring Egg Gulch and painstakingly unearthing rare Jurassic dinosaur eggshells. This 1998 trip followed several years after the discovery of the nest. The Bureau of Land Management protects this area. You can’t dig here or take anything without a permit. We had such a permit as the collection of these fragments was for scientific purposes. Photo date 1998 by S. W. Veatch. |
The initial surface collection in the early 90s produced hundreds of shell pieces. But the biggest prize was the discovery of an embryonic dinosaur bone at Egg Gulch—proof that life and death had played out right here (Alf, 1998). An entire nest of eggs was collected there (Figure 3).
Figure 3. Dinosaur egg clutch from Garden Park, Colorado. Now on exhibit at the Denver Museum of Nature and Science. Photo date 1997 by S. W. Veatch.
In 1989, after the dinosaur nest with eggs was found, I was part of a field party from Cañon City’s Dinosaur Depot (Figure 2.). We wanted to see if we could find something new. It was time to get serious. I felt the true weight of that when I visited Egg Gulch.
We couldn't just aimlessly wander; we implemented a systematic approach to finding eggshells by spreading out and working our way up the slope, much like a TV search party looking for evidence at a crime scene. The fragments were larger and more concentrated closer to their source. Each shell fragment was bottled and meticulously labeled. I noticed the fragments were mostly concentrated on the flat areas and small ravines, washed out during intermittent rainstorms.
I remember finding my first piece of eggshell. I froze,
the heat of the summer day instantly forgotten, my breath catching in my throat
as I peered into the deep, spiky shadow cast by a cactus. My heart hammered a prehistoric rhythm against
my ribs. There, nestled right against the face of a sandstone outcrop, was not
just another bleached pebble, but something impossibly rare and ancient: a
tiny, curved shard of a shell. Its color resembled that of a black crow’s wing mixed with
desert dust, while its faintly bumpy texture hinted at something hidden
beneath. With trembling fingers, I
carefully plucked the Jurassic dinosaur eggshell fragment from its hiding
place, feeling a silent, electric surge
connecting me to a world ruled by thunderous giants, a treasure that had
patiently waited, buried under the silent sun, for 150 million years.
Back in the lab, the shells underwent rigorous
microscopic analysis. Radial thin sections, viewed under polarized light,
showed the internal architecture, and highlighted that most fragments were
heavily eroded. Lab technicians also used a technique called cathodoluminescence
(CL), which causes the material to emit light when exposed to an electron beam (Alf, 1998; Boggs,
& Krinsley, 2006; Götze, 2012; Götze, Plötze, & Habermann, 2001;
Marshall, 1988; Pagel, Barbin, Blanc, & Ohnenstatter, 2000). This showed that the
original shell material in some fragments had been replaced by silica.
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| Figure 4. Microscopic cross-section of a Jurassic dinosaur eggshell recovered at the Egg Gulch locality. According to Alf (1998) laboratory analysis revealed extensive recrystallization and diagenetic alteration in several shell samples. Siliceous replacement likely occurred as a result of the decomposition of the shells' organic matter. Photo courtesy of the Dinosaur Depot. |
Because it is not known which species laid the eggs, paleontologists use an artificial classification system called parataxonomy. The most common eggshell type was assigned to the family Prismatoolithidae, and named Prismatoolithus coloradensis (Hirsch,1994). I also found a second, rarer, very thin dinosaur eggshell that was too altered to classify.
Remarkably, the Egg Gulch shell structure is similar
to the Orodromeus eggshell from Cretaceous sites. Orodromeus was
an ornithopod (bird-hipped) dinosaur. In the Garden Park area, the only known
ornithopods are Dryosaurus and Othnelia. All the evidence points
to one of these two as the likely parent of the P. coloradensis eggs.
Figure 5. Discovering Jurassic age dinosaur eggshell fragments right on the surface of the Morrison Formation is incredibly exciting because these delicate, ancient remnants offer a rare, tangible link to the reproductive lives of the prehistoric reptiles that once roamed this landscape. It's a thrill to realize that just by walking across these rocks, you're picking up direct evidence of where a dinosaur mother may have nested over 150 million years ago. Photo date 2023 by S. W. Veatch.
Collecting these rare Jurassic dinosaur
eggshells marked a highlight of my summer’s fieldwork. The dinosaur eggshells
unlocked a story of multiple nesting seasons and preservation, all providing
invaluable data on a globally scarce fossil resource. The resulting story
provides a tangible connection to life 150 million years ago, a whisper from
the past made loud by patient, detailed science.
Figure 6. Nodular patterns or ornamentation, as seen in this image, is primarily hypothesized to have served in functions like shell strengthening, and may have also been related to respiratory gas exchange through specialized pore arrangements. Photo date 2023 by S. W. Veatch.
Acknowledgments:
Some of the information presented in this report was gained from a number of field trips undertaken by the author to the Garden Park Fossil Area and from former Dinosaur Depot personnel Donna Engard, curator, and Phil Wilder, program coordinator. The author is grateful to Bob Carnein for his thoughtful suggestions and insightful review of an earlier version of this manuscript, which led to significant improvements. Special thanks are also due to Sawyer Blizard for creating the accompanying map and for sharing valuable time on-site examining and discussing Jurassic age dinosaur eggshells.
References and further reading:
Alf, K, 1998. Preliminary study of an eggshell site in the Morrison Formation of Colorado: Modern Geology, Vol. 23, Part 2., pp 241-248.
Boggs, S., Jr., & Krinsley, D. H. 2006. Application of cathodoluminescence imaging to the study of sedimentary rocks. Cambridge University Press.
Carpenter, K., pers. comm.: Department of Earth Sciences, Denver Museum of Natural History, March 17, 1999, (telephone interview).
Götze, J., Plötze, M., & Habermann, D., 2001. Origin, spectral characteristics and practical applications of the cathodoluminescence (CL) of quartz — a review. Mineralogy and Petrology, 71(3-4), 225–250.
Götze, J. 2012. Application of cathodoluminescence microscopy and spectroscopy in geosciences. Microscopy and Microanalysis, 18(6), 1270–1284.
Hirsch, K.F., 1994. Eggshells
from the Western Interior of North America.
In Carpenter, K., K.F. Hirsch, and J.R. Horner, eds, 1994, Dinosaur Eggs and Babies: New York:
Cambridge University Press, pp. 137 – 150.
Marshall, D. J., 1988. Cathodoluminescence of geological materials. Unwin Hyman.
Pagel, M., Barbin, V., Blanc,
P., & Ohnenstatter, D. (Eds.). 2000. Cathodoluminescence in geosciences.
Springer-Verlag.
