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A restoration of hopelessly mired Kaatedocus in what is now Wyoming's Jurassic Howe Quarry. Art courtesy Sauriermuseum Aathal.

Jurassic Boneyard Yields Hidden Dinosaur

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A restoration of hopelessly mired Kaatedocus in what is now Wyoming's Jurassic Howe Quarry. Art courtesy Sauriermuseum Aathal.

The Late Jurassic was the heyday of dinosaur giants. Apatosaurus, Barosaurus, Brachiosaurus, and Diplodocus were gargantuan, long-necked sauropods that reached lengths in excess of 70 feet, and some, such as Supersaurus and the diplodocid formerly known as “Seismosaurus”, stretched even longer. But what’s even more startling is that many of these massive dinosaurs were neighbors who trod among fern-covered floodplains dotted by swaths of conifer forest. These habitats, preserved in the American west’s Morrison Formation, must have been some of the most productive ecosystems of all time.

But the enormous sauropods didn’t enjoy an edenic existence. Part of the reason we know so much about them is because of vast graveyards strewn with the jumbled, disarticulated remains of Late Jurassic dinosaurs. Scattered from eastern Utah to southern Montana, these mass death assemblages reflect a harshly seasonal Jurassic climate. Thirsty dinosaurs died in droughts, only to have their dessicated bodies preserved by floodwater-carried sediment when the rains returned.

Dinosaur National Monument, straddling the Utah-Colorado border, is the most famous Jurassic graveyard. But other bonebeds preserve different assemblages of dinosaurs. Wyoming’s Howe Quarry, for one, has traditionally thought to be the resting place of multiple Barosaurus.Well-preserved, articulated sauropod hands and feet hint that the parched herbivores came to this place looking for water, but ended up stuck in the muck. As testified by shed teeth, Allosaurus and other scavengers tore the sauropods apart, and what remained of the unfortunate dinosaurs was quickly buried soon thereafter.

Despite being painstakingly excavated by the famous paleontologists Barnum Brown, Roland T. Bird, and their American Museum of Natural History crews during the 1930s, the Howe Quarry dinosaurs received little attention from succeeding generations of paleontologists. The site seemed to be yet another slurry of sauropod bones from already-known dinosaurs. Yet, in a Journal of Systematic Palaeontology paper, Emanuel Tschopp and Octávio Mateus propose that they have found a previously-unknown dinosaur hiding within the chaotic mix of sauropod bones.

Barnum Brown’s crew was the first to systematically work the Jurassic rock around Shell, Wyoming for dinosaur bones, but they were not the last to do so. “Big Al”, a young Allosaurus that belongs to an as-yet-undescribed species, was found in close proximity to the Howe Quarry, and excavations by Switzerland’s Sauriermuseum Aathal turned up additional parts of the original site. Among the bones recovered by the Swiss team were a series of neck vertebrae associated with a partial, disarticulated skull. These were previously considered to be from a juvenile Diplodocus or Barosaurus, but Tschopp and Mateus argue that the bones represent a new diplodocid dinosaur they have named Kaatedocus siberi.

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The reconstructed head and upper neck of Kaatedocus. Photo courtesy Sauriermuseum Aathal.

In general appearance, Kaatedocus wasn’t much different from Diplodocus and Barosaurus. All of these dinosaurs were long-tailed sauropods with long, low skulls with muzzles tipped with peg teeth. The features that make Kaatedocus distinct, Tschopp and Mateus suggest, are subtle characteristics in the skull and vertebrae, such as a U-shaped notch between the frontal bones and the minor degree of separation between the neural spines along some neck vertebrae, among other details. Based on the suite of apparently unique characteristics, the researchers suggest that their Kaatedocus specimen was a relatively small, subadult animal that was about 46 feet long when it perished.

Yet Tschopp and Mateus also point out that the skull of Kaatedocus resembles that of a juvenile diplodocid sauropod. The orbit, or eye opening, is large, and the dinosaur had a rounded muzzle with an expanded tooth row also seen in the skull of a juvenile Diplodocus described by John Whitlock and coauthors in 2010. The skull of Kaatedocus is shallower and more slender than the juvenile Diplodocus skull, but, given the size of the animal and the juvenile characteristics, I have to wonder if Kaatedocus is really a relatively young individual of Barosaurus or another previously-named sauropod. What might seem to be a distinctive combination of characteristics could be a sign that the dinosaur was still changing as it approached maturity.

Kaatedocus could be a distinct form of sauropod that inhabited Jurassic environments in the little-known northern part of what is now preserved in the Morrison Formation. Or maybe it’s an immature Barosaurus or Diplodocus. Context will be essential to testing these ideas.

The authors of the new study argue that Kaatedocus was found relatively low in the Morrison Formation, earlier than 147 million years ago. If this is accurate, perhaps Kaatedocus seems archaic because it actually lived before derived sauropods like Barosaurus and Diplodocus in a swath of northern ecosystems that evolved as the ancient Sundance Sea drained off the continent. When paleontologists Jerry Harris and Peter Dodson described Suuwassea emilieae – a cousin of Diplodocus and possibly a contemporary – from bones found in Montana, they proposed that the “northern end of the Morrison Formation depositional basin may contain a unique fauna from a heretofore unrecognized paleoecosystem that contrasts with the general portrait of the formation’s fauna based on material from outcrops south of the Bighorn Basin.” Then again, geologic studies have yet to fully resolve the ages and relationships of the various Howe Quarry excavations, and the rarity of definite Barosaurus material hampers attempts to compare Kaatedocus with the previously-discovered species.

The perplexing nature of Kaatedocus underscores the fact that, despite their familiarity, there’s much we still don’t know about Morrison Formation dinosaurs and their relationships. In fact, how these Jurassic environments could have supported such startling arrays of dinosaurs is one of the biggest mysteries of all. Bonebeds concentrate clues that may help us understand this ecological enigma, and better understand a time when titans roamed the earth.

References:

Foster, J. 2007. Jurassic West. Bloomington: Indiana University Press. pp. 90-92

Harris, J., Dodson, P. 2004. A new diplodocoid sauropod dinosaur from the Upper Jurassic Morrison Formation of Montana, USA. Acta Palaeontologica Polonica 49, 2: 197–210

Wilborn, B. 2001. Two new dinosaur bonebeds from the Late Jurassic Morrison Formation, Bighorn Basin, WY: An analysis of the paleontology and stratigraphy. Unpublised master’s thesis, Virginia Polytechnic Institute and State University. i-44.

Whitlock, J., Wilson, J., Lamanna, M. 2010. Description of a nearly complete juvenile skull of Diplodocus (Sauropoda: Diplodocoidea) from the Late Jurassic of North America. Journal of Vertebrate Paleontology. 30, 2: 442:457