Ichthyornis, a ternlike shorebird about

8 inches high known from the Mancos Shale.

descendants, though, Ichthyornis had many small and sharply pointed teeth lining its elongated jaws. Because the remains of Ichthyornis throughout the West are found in marine deposits (the best spec- imens occur in Kansas), it is regarded as a seabird that inhabited the margins of the Western Interior Seaway, feeding on fish and other marine organ- isms that it captured on flights over the water. Other birds known from the strata that accumulated in this seaway include Hesperornis, a large loonlike diving bird, and Baptornis, its smaller relative.

While most of what we know about the birds of the Mancos sea is based on fossils found in the cen- ter or eastern parts of the basin, it is plausible to imagine that the Utah shores were populated by great numbers of shorebirds (fig. 7.7). If we could have strolled along the beaches of central Utah in the late Cretaceous, we probably would have noticed great flocks of reptilelike birds soaring overhead and bobbing in the surf. Occasionally a pterosaur might have drifted by as well. Pterosaur remains are plentiful in the ooze deposited in Kansas at the same time when parts of the Mancos Shale were laid down. Though no well-preserved pterosaur fossils are known from the Mancos Shale, at least some of them probably lived along the western fringe of the seaway. The “bird”-watching would have been great in Late Cretaceous Utah, but our feathered friends would have had a distinctly reptilian look.

Very rarely the Mancos Shale affords a glimpse of the larger creatures that lived along the coastal plain near the shore. In western Colorado amaz- ingly complete remains of a hadrosaur have been found in the concretions like those that often pro- duce ammonite and bivalve fossils (Wolny and oth- ers 1990). The hadrosaurs, as we will see later in this chapter, were highly specialized bipedal dino- saurs (the “duckbills”) that were extremely success- ful almost everywhere in North America during the late Cretaceous. The hadrosaur discovered in the Mancos Shale of western Colorado appears to be most similar to Kritosaurus, a duckbill with a prom- inent bump on its nose formed by the expansion

of the nasal chamber. Almost certainly more than a single hadrosaur roamed along the edges of the Mancos sea in the Utah region, because the low coastal plain was covered by dense jungles that could have supported many such herbivores (see the discussion later in this chapter). Their remains are usually preserved in the sediment deposited on the adjacent coastal plain, however, not in muck that accumulated on the floor of the open ocean. The Colorado locality probably represents a rare event in which a hadrosaur carcass was washed out to sea, eventually sinking to the bottom to become buried under Mancos mud. Such events would have been uncommon, because the rivers that flushed this car- cass offshore would normally accelerate its destruc- tion by rolling and tumbling the putrefying corpse. Even if the body or a part of it reached the sea, the sharks, predaceous fish, plesiosaurs, and mosasaurs of the Mancos were voracious carnivores. It would not have taken these meat-eaters long to find the remains and consume them.

One of the most remarkable fossil specimens known from the Mancos-Tropic Shale interval was discovered by Merle Graffam in 2000 during the excavation of a large plesiosaur skeleton near Big Water, Utah. Scientists from the Museum of North- ern Arizona eventually recovered an amazingly complete (fig. 7.8), though mostly crushed, skele- ton of a primitive theropod dinosaur that was later named Nothronychus graffami (Gillette 2007; Zanno and others 2009). The remains of this therizino- saur somehow survived the scavenging marine rep- tiles and processes of postmortem decomposition to arrive more or less intact on the seafloor some 50 miles from the nearest land. It was buried in the murky depths of the ocean in mud that later hard- ened into the Tropic Shale. Nothronychus was an odd animal, much larger than its relative Falcarius known from the Cedar Mountain Formation, with large sicklelike claws on its hands (fig. 7.9). These claws are reminiscent of the slashing sabers on the feet of the raptors but were carried on the hands and probably used for acquiring and handling food.

In life Nothronychus must have had an ungainly, almost comical appearance. It stood about 12 feet (4 meters) tall on its stout hind legs but appears to have held the 20-foot-long (7-meter) body in a more upright bipedal stance than other theropods. The deep abdominal cavity gave it a pot-bellied look, and the short tail and heavy legs suggest a waddling sort of movement rather than the swift and grace- ful running styles of other bipedal dinosaurs. A long neck and small head added another gawky aspect to this 1-ton reptilian troll. The name Nothronychus means “sloth-claw,” and indeed this strange dino- saur may have been the Cretaceous equivalent of the slow-moving sloths of the modern world.

In fact there is likely another similarity between Nothronychus and modern sloths: they probably had similar dietary preferences. In a fascinating evolu- tionary transformation, Nothronychus appears to have been mostly herbivorous, even though it is classified as a theropod dinosaur, a group other- wise consisting exclusively of predators. Evidence of plant-eating in Nothronychus includes the prob- able toothless beak (presumed because the skull is unknown but nonetheless likely based on similarity to other therizinosaurs), a pelvis designed to accom- modate a large digestive mass, and a long flexible

neck bearing a small skull. Paleontologists are not certain if Nothronychus was a strict herbivore or was omnivorous, consuming animal remains occa- sionally. In either case, though, it lacks the extreme specializations for meat-eating that typify its rela- tives within the theropod clade. It is easy to imagine Nothronychus waddling through the coastal jungles of central Utah 90 million years ago, pulling down branches of trees with its large claws to reach the foliage, fruit, seeds, or cones. Nothronychus was one of the earliest theropods in Utah to develop adap- tations for plant-eating and in many ways was an evolutionary pioneer: the omnivorous theropods become much more common a few million years later toward the end of the Cretaceous.

the Mountains tremble:

the Main Phase of the sevier orogeny

As the waters of the Mancos Sea lapped quietly against the shore in central Utah, the mountains ris- ing to the west were rumbling with intensified geo- logical activity. In the late Cretaceous the main phase of the Sevier Orogeny occurred, an event that brought dramatic changes to the mountain- ous terrain of western Utah and eastern Nevada. 7.8. Skeleton of Nothronychus graffami

from the Tropic Shale of southern Utah. Scale bar = 1 foot (30 cm). Based on reconstruction by V. O. Leshyk in Zanno and others 2009.