In the late 1870s Jurassic dinosaurs were discovered in Morrison, Colorado. The animals excavated by Arthur Lakes and his field crew formed the foundation for Upper Jurassic paleobiology in North America. Dinosaurian icons, like the spike-tailed Stegosaurus and enormous long-necked Apatosaurus were exhumed in or near the tiny foothills town at the type locality of the famed Morrison Formation.
The Morrison Formation forms part of the base of a prominent asymmetrical ridge known as the Dakota Hogback. Made of Mesozoic sediments, the Upper Jurassic (Kimmeridigian - ?Tithonian) Morrison Formation outcrops on the western face of the lower third of the hogback, beneath the early Late Cretaceous (Aptian) South Platte Formation and the Lower Cretaceous Lytle Formation, (Peterson and Turner, 1998). Both Cretaceous Formations are bundled in the Dakota Group, (Peterson and Turner, 1998). Below the Morrison Formation is the (?)Oxfordian –Kimmeridigan age Ralston Creek Formation, (Peterson and Turner, 1998).
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| Morrison, Colorado circa 1899. The Morrison Formation outcrops on the hillside behind the village. Photo copyright Denver Public Library |
Lakes shipped crates of Morrison fossils to New Haven, Connecticut into the hands of O. C. Marsh at the Yale Peabody Museum, (Kohl and McIntosh, 1997). Yale-sponsored crews discovered at least a dozen sites where fossils could be collected on the surface, (Kohl and McIntosh, 1997). Between 1877 and 1879, four of these sites were excavated, two in massive sandstones, Quarry 1 and 5, and two mudrock quarries, Quarry 8 and 10, (Kohl and McIntosh, 1997, Ostrom and McIntosh, 1966, McIntosh, 1995).
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| Location of a pair of historic dinosaur quarries with local geological Formations for context. Interpretation by Bryan Turner. Photo copyright Denver Public Library |
Fossil Vertebrates from Lakes' Quarries: Reassessment and Discovery
Each site is discussed below in context of what the Yale crew found with modern interpretation of significance. Modern quarry names are borrowed from Ostrom and McIntosh, 1966, while Lakes' nomenclature is preserved in quotes. Only historical specimens are discussed in this section, although modern interpretations are offered with recently collected specimens when relevant.
Lakes' first site, Quarry 1, is historically significant and a glimpse at unusual fauna. Disappointingly, both species recovered there remain obscured by a lack of material. The only indisputable non-dinosaur recovered by Lakes was found in a sandstone talus boulder: the skull of the goniopholid mesoeucrocodylian "Diplosaurus" felix. No other remains have been unearthed at any site, historic or otherwise, in Morrison.
Sauropod remains were surface collected from the talus blocks Lakes called "Saurian 1." The genoholotype of Atlantosaurus montanus (YPM-1835/YPM 1835A) is based on a weathered, but uncrushed sacrum with ambiguous characters - the size matches that of a large apatosaur but the pinching of the vertebral bodies suggests diplodocine affinity. To date, no other Morrison Formation sauropod sacrum displays the same unique features.
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| Quarry 5 Sandstone (center photo) near Alameda Parkway. The original site worked by Lakes is center, right near the steel railing of the cement overlook. Photo copyright M. T. Mossbrucker / MNHM |
To the south of Quarry 1, another fluvial (river) sandstone harbored fossils. Quarry 5 produced some of the most significant fossils from the Morrison area, such as the genoholotype of Stegosaurus armatus, YPM-1850, the specimen that defined the genus.
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| Reconstruction of Stegosaurus armatus, YPM 1850. Highlighted bones in the tail are some YPM 1850 specimens. The remainder of the body is based upon the National Museum of Natural History specimen USNM 6646. Reconstruction copyright R. T. Bakker & M. T. Mossbrucker |
The Quarry 5 sandstone dips into the hillside, making in situ collection difficult as several meters of sandstone and mudrock cap the bone-bearing layer. Lakes collected specimens in loose blocks as indicated by his field journal "On removing the loose masonary of rock above it... [w]e broke open the block in which it (fossil bone) lay and exposed twelve long black enameled spines [Stegosaurus armatus]" (Kohl and McIntosh, 1997). Lakes' watercolor of work at Quarry 5 depicts a sledgehammer-wielding gentleman cracking open a hard sandstone block. Perhaps this man is the passing farmer who "lent his powerful arm" to cleave the loose boulder for examination by Lakes and crew.
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| Unprepared proximal tail vertebrae from Stegosaurus armatus, YPM 1850. Photo coypright MNHM |
In 1993, some of the historic material was loaned to the newly founded Morrison Natural History Museum (MNHM) via the Yale Peabody Museum of Natural History. About half of the material of Stegosaurus specimen YPM 1850, and Apatosaurus specimen YPM 4676 were entrusted to MNHM for study, display, and preparation. Robert O'Donnell, a retired United States Geological Survey fossil preparator began the cleaning of YPM 1850, as the material had never been prepared for study. Although a proximal caudal (tail) vertebra was fully cleaned and returned to Yale, the remainder of the specimen was placed on display in Morrison. The nature of the specimen –hard matrix enveloping softer bone plagued with microfractures – prevented further preparation at the time.
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| Prepared proximal tail vertebrae from Stegosaurus armatus, YPM 1850. Photo coypright MNHM |
The specimen YPM 1850 is currently undergoing preparation and evaluation at the Morrison Natural History Museum. Preliminary reports (Mossbrucker, et al., 2009) indicate that S. armatus is unlike any other North American stegosaur in that the mid-tail neural spines are taller and more robust than known stegosauria.
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| Vertebrae from the mid-tail region from YPM 1850, the type specimen of Stegosaurus armatus. Photo copyright M. T. Mossbrucker / MNHM |
Sauropod remains were discovered to be miscataloged as YPM 1850 by Matthew Mossbrucker. The nature of Quarry 5 bone deposits are jumbled isolated bones from various taxa, with the exception of S. armatus remains which occur in partially articulated caudal series. The sauropod remains cataloged with YPM 1850 appear to belong to Apatosaurus cf. ajax: a robust pedal ungual (I or II) and a large, left prezyagapophosis and partial neural arch from a dorsal vertebra. A matched pair of diplodocid premaxillae (MNHM-999), belonging to a large individual might match the dentaries of Diplodocus lacustris (YPM 1922). It is more likely that these skull components belong to Apatosaurus, not Diplodocus, as the former is the only diagnosed sauropod from Quarry 5.
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| Apatosaurus cf. ajax claw, Quarry 5. Photo copyright M. T. Mossbrucker / MNHM |
The holotype for Diplodocus lacustris (specimen YPM-1922) (Marsh, 1884) are based upon a partial dentary with teeth and are nondiagnostic to the generic level and should be considered nomen dubium.
Quarry 8 produced beautifully preserved partial forelimbs and scapula of Apatosaurus ajax, specimen YPM 4676 which are on display currently at the Morrison Museum. The style of preservation is very similar to that of the upper "black streak" mudstone Quarry 10 bones. Lakes field notes and illustrations demonstrate that Quarry 8 was a mudstone quarry (Kohl and McIntosh, 1997). Test trenches made by MNHM staff near Quarry 8 and the green mudstone with orange-stained calcareous concretions match the descriptions by Lakes. This lithology is present far to the south, near Bear Creek, in several layers immediately below Lakes Quarry 10 but these southern outcrops have not produced bones.
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| Quarry 10 in 2001. The majority of fossils from this site were discovered in the mudrock below the prominent sandstone beds. Photo copyright Sally White / MNHM |
Quarry 10 was the most prolific dig site in Morrison during the late 1870s. Bones are crowded into two superimposed layers of dark grey siltstone, about a meter thick, separated by a meter thick light green siltstone. Each bone-bearing mudstone layer contains a well preserved specimen of Apatosaurus ajax. Lakes reported a third bone layer in the "ceiling sandstone" above the upper bone bearing mudstone, (Kohl and McIntosh, 1997).
The genoholotypic Apatosaurus ajax, YPM 1860 was collected in the lower siltstone bed. From the upper dark siltstone bed YPM 1861 (Apatosaurus laticollis) and YPM 1840 (Atlantosaurus immanis) were recovered. It is likely that the remains represent at least two individuals, as the composite left "femur" is actually a pair of femora and do not associate. The distal femur at maximum width is narrower than the proximal shaft and head, which is a condition not observed in other diplodocid femora. It appears that these remains were reworked from the top dark grey siltstone into the ceiling sandstone and represent specimens YPM 1861/1840, not another individual, (McIntosh, 1995).
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| Life restoration of Apatosaurus ajax. Model by Shane Foulkes and Ewan Sanko. Photo copyright MNHM. |
Arthur Lakes records seven shed allosaur teeth were around YPM 1860, suggesting feeding behavior at Quarry 10, (Bakker and Bir, 2004). One crown exists in the YPM collection, specimen YPM 4845. Lakes also notes the presence of turtle carapace/plastron fragments, (Ostrom and McIntosh, 1966), but the specimens are not cataloged at the Yale Peabody Museum of Natural History, nor have any chelonian fragment been unearthed in the Quarry 10 "spoil pile" of displaced siltstone by the Morrison Natural History Museum crew.
Arthur Lakes' excavations at Morrison, Colorado proved fruitful. O. C. Marsh, who used the Morrison sample to name a series of genera that became the foundation for Late Jurassic paleobiology.
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| Morrison, Colorado. The buff sandstone outcrop in the middleground of the photo is 280 million year old Lyons Formation sandstone. The Dakota Hogback, or "Dinosaur Ridge" is pictured in the background. Photo copyright M. T. Mossbrucker / MNHM |
Lakes' Morrison Discoveries at Morrison, Colorado | |||
Name | Specimen Number | Quarry | Notes |
Atlantosaurus montanus | YPM 1835 | Quarry 1 | Marsh initially dubbed the species Titanosaurus montanus (Marsh, 1877{a})but the genus name was pre-occupied and hence Marsh substituted Atlantosaurus, (Marsh, 1877{b}), |
Atlantosaurus immanis | YPM 1840 | Quarry10, upper | Partial skeleton from the upper black streak of Quarry 10, and was mixed with specimen YPM 1861, (Marsh 1878). |
Apatosaurus laticollis | YPM 1861 | Quarry10, upper | Based upon a single posterior cervical vertebrae, (Marsh, 1879). |
Apatosaurus ajax | YPM 1860 | Quarry10, lower | First species of the genus, specimen YPM 1860 (Marsh 1877{b}) based upon excellent material from the lower "black streak" mudstone at Quarry 10, (McIntosh 1995). |
Stegosaurus armatus | YPM 1850 | Quarry 5 | The first defined specimen of North American stegosauria (Marsh 1877{c}). |
Diplodocus lacustris | YPM 1922 | Quarry 5 | Long after Lakes had stopped forwarding material from Morrison, Marsh used the sample to define this new species, (Marsh, 1884). |
Diplosaurus felix | YPM 517 | Quarry 1 | Lakes quarries at Morrison produced one non-dinosaurian vertebrate: Marsh erected a new genus of goniopholid mesoeucrocodylian, (Marsh, 1877{d} |
"YPM" is the institutional abbreviation for the Yale Peabody Museum of Natural History. No undisputed mammal, pterosaur, chelonian, or osteichthyian remains were identified by Marsh from the Lakes' collection. Minor specimens, an allosaur tooth (YPM 4845 ) and the Apatosaurus cf. ajax YPM 4833 are not listed here due the incomplete or diagnostic nature of the finds. | |||
Nota Bene:
This is a modified excerpt from M.T. Mossbrucker and R.T. Bakker "A Guide to the Paleontology of the Upper Jurassic Morrison Formation of Morrison, Colorado: New Interpretations and Discoveries." Bulletin of the Morrison Natural History Museum, Volume 1, page 2.
References:
Anonymous, 1899. 'Western History and Geneology' Denver Public Library April 14 2010, web October 27 2010. http://photoswest.org:8080/cgibin/cw_cgi?fullRecord+29130+594+773493+130+0
Bakker, R.T. and Bir, G., 2004, Dinosaur crime scene investigations; Theropod behavior at Como Bluff, Wyoming, and the evolution of birdness. – In Currie, P. J., Koppelhus, E. B., Shugar, M. A., and Wright, J. L., (Eds.): Feathered Dragons: Studies on the transition from dinosaurs to birds Indiana University Press, Bloomington, p. 301-342.
Kohl, M. and McIntosh J. S. (Eds.), 1997, Discovering Dinosaurs in the Old West: The Field Journals of Arthur Lakes. Washington D.C., Smithsonian Institution Press.
Marsh, O.C. 1877 {a}. Notice of a new gigantic dinosaur. American Journal of Science. Series 3, Vol. 14, pp.87-88. –named Titanosaurus montanus
Marsh, O.C. 1877 {b}. Notice of some new dinosaurian reptiles from the Jurassic formation. American Journal of Science, Series 3, Vol.14: pp. 514-516. –named Apatosaurus ajax & renamed Atlantosaurus montanus
Marsh, O.C. 1877 {c}. A new order of extinct Reptilia (Stegosauria) from the Jurassic of the Rocky Mountains. American Journal of Science, Series 3, Vol.14: pp. 513-514. –named Stegosaurus armatus
Marsh, O. C. 1877 {d}. Notice of some new vertebrate fossils. American Journal of Science. Series 3, Vol. 14, 249-256. –named Diplosaurus felix
Marsh, O.C. 1878. Notice of new dinosaurian reptiles. American Journal of Science, Series 3, Vol.14: pp. 241-244. –named Atlantosaurus immanis
Marsh, O.C. 1879. Principal characters of American Jurassic dinosaurs. Part II. American Journal of Science, Series 3, Vol.17: pp. 86-92. –named Apatosaurus laticollis
Marsh, O.C. 1884. Principal characters of American Jurassic dinosaurs. Part VII. American Journal of Science, Series 3, Vol.27: pp. 161-167. –named Diplodocus lacustris.
McIntosh, J. S. 1995. Remarks on the North American sauropod Apatosaurus Marsh. In A. Sun and Y. Wang (Eds.), Sixth Symposium on Mesozoic Terrestrial Ecosystems and Biota, Short Papers, pp. 119-123. Beijing: China Ocean Press.
Mossbrucker, M. T., Bakker, R. T., and Prueher, L., 2009, 'New information regarding the holotype of Stegosaurus (Marsh 1877).' Symposium on Stegosauria Abstracts, Aathal, Switzerland, p. 9.
Ostrom, J.H. and McIntosh, J.S., 1966. Marsh's Dinosaurs: The Collections from Como Bluff, Yale University Press, New Haven, CT.
Peterson, Fred, and Turner, C.E., 1998. Stratigraphy of the Ralston Creek and Morrison Formations (Upper Jurassic) near Denver, in Carpenter, Kenneth, Chure, D.J., and Kirkland J.I., editors, The Upper Jurassic Morrison Formation – an interdisciplinary study: Modern Geology Vol.23, pt. 1, p. 3-38.