Dinosaurs of the Cedar Mountain Formation

Thursday, June 19 2008 - Cedar Mountain Project

Early Cretaceous dinosaurs in North America are best known from a swath of sedimentary rocks, or strata, from Montana to Texas, as well as from parts of Maryland. One of these strata, the Cedar Mountain Formation in eastern Utah and western Colorado, has produced dinosaurs in abundance only in the past decade. Many of these discoveries are of new species, some of which have not yet been officially named. Still, many species have been named and their importance is discussed in the last section. A scientific summary of these dinosaurs is available here (large 1677 KB file).

 

 

 

Map showing the distribution of some dinosaur producing Lower Cretaceous strata in the United States.

 

Buckhorn Conglomerate - The Buckhorn conglomerate is so coarse that until recently no identifiable dinosaur bones were known. It was thought that the rapidly flowing water would wash any carcass or bones away, or that the tumbling gravel would batter the bones to fragments. Recently, however, a partial ankylosaur skeleton has been reported by Utah State Paleontologist Jim Kirkland. Unfortunately, most of the bone has crumbled out, leaving cavities where they were. Still, the bone cavities do show that the specimen is of an ankylosaur, and that the carcass was pretty much intact when originally buried. Several ankylosaurs are known from the Cedar Mountain Formation, so it is not known which it is, or if it is a new species.

 

Yellow Cat Member - Dinosaurs from the Yellow Cat are interesting because their closest relatives occur in the Upper Jurassic Morrison Formation and in the Lower Cretaceous strata (such as the Wealden) of southern England. The significance of this is discussed below in the section “What Do Similarities Mean?” Two of the most important sites are the Gaston Quarry and the Dalton Well Quarry. Here, numerous individuals of different dinosaurs were buried together.

 

 

 

 

Theropods - Without doubt, the most famous dinosaur from the Cedar Mountain Formation is Utahraptor. This dinosaurs is related to Velociraptor, the villainous star of Jurassic Park as the cunning and intelligent raptor. Scientifically, however, Utahraptor ostrommaysorum is not very well known because so little of its skeleton has been found and described. Fortunately, the sickle-claw of the hind foot is known, a distinctive feature of deinonychids. As a group, deinonychids get their name from Deinonychus from the Cloverly Formation of Wyoming and Montana. We can get a rough approximation of the size of Utahraptor by comparing the size of its bones with those of a Deinonychus skeleton. This indicates that Utaraptor was somewhere around 6.5-7 meters (21-23 feet) long.

 

Pair of Utahraptors in a riverine forest. Inset shows the shin (tibia) and ankle bone (astragalus) from the Gaston Quarry.

 

We know that the sickle-claw (the enlarged claw on the second toe) was used as a piercing weapon because of a very famous find in Mongolia. There, a skeleton of Velociraptor was found with its intended victim, a plant-eating Protoceratops. The lower arm of the Velociraptor is between the beak of the Protoceratops indicating that the Protoceratops was biting down on the Velociraptor and that the two animals were fighting when they died. But why were they fighting? A clue comes from the sickle-claw of the right-hind foot of the Velociraptor. The claw is extended into what was the throat region of the Protoceratops, so clearly the Velociraptor was trying to kill the Protoceratops, but the intended victim did not go without a fight (a scientific description is available in my PDF reprints here).

 

It is safe to assume then that Utahraptor used its sickle-clawed hind feet as a killing weapon. But what did Utahraptor feed upon? That is more difficult to know with certainty because predator and prey have not been found so closely associated like Velociraptor and Protoceratops. A safe bet might be the bipedal, plant-eating iguanodontids that also occur in the Yellow Cat (see below).

 

Running around at the same time as Utahraptor is the less famous Nedcolbertia. This small theropod is closely related to Ornitholestes from the Morrison Formation and Aristosuchus from the Wessex Formation of England. As yet no complete skeleton of Nedcolbertia justinhofmanni is known, but scaling it the skeleton of Ornitholestes against the Nedcolbertia bones suggests this theropod was over 3 meters (10 feet) long.

 

Another small theropod has recently been discovered at the base of the Yellow Cat Member near the town of Green River, Utah. The site has produced numerous individuals of one species (yet unnamed) that appears to be related to the therizinosaurids of Asia. This odd theropod was a herbivore that has been described to resemble a “gorilla on crutches” because of its long arms.

 

Sauropods - Although several different species of sauropods are known from the Yellow Cat, only Cedarosaurus weiskopfae has been named so far. The specimen was discovered by Billy Kinneer, one of my volunteers. The upper arm bone, the humerus, is as long as the thigh bone, or femur. This is a typical character of the brachiosaurids, the group that includes Brachiosaurus from the Morrison Formation. These “giraffe” sauropods are taller at the shoulders than in the middle of the back as in all other sauropods. Some dinosaur paleontologists argue that this feature allowed brachiosaurids to feed from treetops.

 

Bill Brooks (left) and Virginia Tidwell (right), two of the describers of Cedarosaurus weiskopfae.

 

Cedarosaurus also shares features with Pelorosaurus and Eucamerotus from the Lower Cretaceous of England. These features include the overall shape of some of the limb bones and of the back vertebrae. This significance of these similarities is discussed in the section “What Do Similarities Mean?”

 

At least two different sauropods are present at the Dalton Well Quarry. One is represented by vertebrae with forked or split spines. Normally the spine of the vertebra (you see or feel the tops of these in the middle of the back) is single, but in some sauropods these are paired or forked. As yet this sauropod has not been named. It was once thought to be related to Camarasaurus from the Morrison Formation, which also has forked spines. But more recent work suggests that this dinosaur is more closely related to a bizarre titanosaurid sauropod from Mongolia called Opisthocoelicauda. Originally this Mongolian sauropod, from the Upper Cretaceous, was thought to be closely related to Camarasaurus because of the forked spines. But a new method of numerical analysis, called cladistics, demonstrates that Opisthocoelicauda is more closely related to a group of advanced sauropods called titanosaurids that are especially common in Argentina. Similarities of this unnamed sauropod from the Yellow Cat with Opisthocoelicauda then, suggest it too is a titanosaurid. This Yellow Cat titanosaurid is estimated to have been over 21 meters (69 feet) long.

 

One other sauropod is known from a string of vertebrae collected near Arches National Monument. The specimen was discovered by one of my volunteers, Tim Seeber. The spines of the vertebrae are completely absent on three of the vertebrae (they are not broken off), and short, but expanded into a club-shape on the fourth. Similar neural spines are known on Malawisaurus, a primitive titanosaur from the Lower Cretaceous of Malawi in southern Africa. Thus, although a close relative of Malawisaurus is present in the Yellow Cat, the remains are too incomplete to be named, unfortunately.

 

Ornithopods - Bipedal plant-eating dinosaurs, the ornithopods, are known from the Yellow Cat. Some very tall-spined vertebrae of an as yet unnamed iguanodontid have been found at the Dalton Well Quarry. The spines are the tallest known among iguanodonts, save for the sail-backed Ouranosaurus from the Lower Cretaceous of Niger. The skeleton of this Yellow Cat iguanodontid is estimated to have been about 8 meters (26 feet). A second, smaller iguanodontid, also unnamed, is known from Dalton Well, and possibly the Gaston Quarry. Its vertebrae are characterized by shorter, more “normal” spines. In many features, this iguanodontid resembles the short-spined Iguanodon mantelli from England. The only named species of Yellow Cat ornithopod is Iguanodon ottingeri from the Dalton Well Quarry. The jaw fragment undoubtedly belongs to one of the two iguanodontids, but which? In addition, there is nothing distinctive about the specimen to separate it from other species of Iguanodon. Some paleontologists therefore consider it a nomen dubium (meaning a dubius, or invalid name).

 

The teeth of Iguanodon are closely abutted against one another and are reinforced on one side by a ridge or series of ridges. The wear surface of the teeth is broad and slightly angled so that the enamel along the higher side forms a slicing edge. These features suggest that iguanodontids ate rather tough plant parts, possibly even twigs.

 

One other ornithopod was found north of Arches National Monument by another of my volunteers, David Gilpin, near our camp. The partial skeleton has an ilium, the upper pelvic bone, with a distinct projection above the hip socket seen only in hadrosaurs, the duck-billed dinosaurs. The rest of the ilium and the other parts of the skeleton are little changed from its iguanodontid ancestor. This skeleton is in the process of being described and named.

 

Ankylosaurs - Only a single species of armored dinosaur is known from the Yellow Cat, the polacanthid Gastonia burgei. This ankylosaur shares many characters in common with Polacanthus from the Wealden of England. Some of this includes fusing together of the armor on the hips into a large shield. Gastonia is known from several thousand bones from the Gaston and Dalton Well quarries, so must have been fairly common during the Early Cretaceous.

 

Skeleton of Gastonia as reconstructed by Rob Gaston and a life restoration.

 

The teeth of Gastonia are small and shaped like a hand with the fingers together. Gastonia definitely did not eat the tough plant parts, as did the iguanodontids. The teeth, taken in conjunction with the broad, arched beak, suggest that Gastonia cropped low plants, possibly ferns. Gastonia is about 4 meters (15 feet long).

 

Poison Strip Sandstone - Isolated dinosaur bones occasionally have been found, as well as large, fossilized logs. At one place called Tony’s Site, discovered by my volunteer Tony DiCroce, a considerable amount of dinosaur bone was found in a pond deposit between two bodies of sandstone.

 

 

Theropods - Utahraptor is possibly represented by a single pelvic bone.

 

Sauropods - Venenosaurus dicrocei is known from a partial skeleton of an adult, as well as a few bones of a juvenile. This sauropod is more evolutionarily advanced than Cedarosaurus, showing a blend of brachiosaurid and titanosaurid characters. One distinct feature is that one of the pelvic bones, the ischium, is shorter than another pelvic bone, the pubis. This unusual feature is known from titanosaurid sauropods from Argentina.

 

Bones of Venenosaurus as found at Tony's Site, and a life restoration.

 

Ornithopods - Known from a partial skeleton of at least two individuals, Planicoxa venenica is a peculiar iguanodontid. The back portion of the ilium, one of the pelvic bones, no longer remains long and upright or vertical, but is short and folded over so as to be horizontal. Why Planicoxa did this is unknown, but it did result in a rearrangement of the hip muscles.

 

Bones of the odd iguanodontid Planicoxa and life restoration.

 

Ankylosaurs - Armored dinosaurs are known from two sites just west of Arches National Monument. One of these is the specimen found by Lin Ottinger that was excavated by Jim Jensen, and described by Norman Bodily as Hoplitosaurus. The specimen, as well as another found by Jim Kirkland a few kilometers away, is problematic. Although it does show some similarities with Hoplitosaurus, known from the Lakota Formation of the Black Hills, it also shows some differences as well. Jim Kirkland and I have referred the specimens to Sauropelta, an ankylosaur also known from the Cloverly Formation of Montana and Wyoming, but I now have my doubts.  Figuring out the identity of this ankylosaur from the Poison Strip will have to wait for a more complete specimen.

 

Ruby Ranch Member  - The Ruby Ranch Member is the thickest and most wide spread member in the Cedar Mountain Formation. Some of the most important sites occur south of Price Utah and are currently being excavated by the College of Eastern Utah. Other important sites include the Long Walk Quarry on the west side of the San Rafael Swell, and Lorrie’s Site north of Arches National Monument.

 

 

Theropods -  A single hand claw of Utahraptor is known from near the base of the Ruby Ranch at Lorrie’s Site (more below). The site has also produced a tibia, or shin bone of some other small theropod. The foot of Deinonychus is known from high in the Ruby Ranch at Dinosaur National Monument. The site, discovered by Robert Bakker, is not far from the Quarry building and is the only known occurrence of this theropod in the Cedar Mountain Formation. It was originally reported from the Cloverly Formation of Montana and more recently from the Antlers Formation of southeastern Oklahoma. Deinonychus then, was a cosmopolitan dinosaur that was widely distributed in North America.

 

Some large theropod teeth have been found at the Long Walk Quarry. These have some resemblance with those of Acrocanthosaurus, a large theropod that rivaled Tyrannosaurus in size (a scientific description of this theropod is available in my PDF reprints). Acrocanthosaurus was the largest predator of its day in North America, with a skull almost 1.25 meters (4 feet) long. These teeth were found among bones of sauropods and may have broken off when the animal was scavenging the carcasses.

 

The large carnivorous Acrocanthosaurus along a river bank. Although only known from fragmentary remains in the Cedar Mountain Formation, a nearly complete skeleton is known from Oklahoma. 

 

Sauropods - Except for one specimen low in the Ruby Ranch, most sauropods occur high within the member. The sauropod at the Long Walk Quarry is represented by several individuals. Unfortunately, the bones are in a very hard limestone and cleaning them is difficult. The specimens were collected by the University of Utah but are being cleaned in my lab so that they may be described by Virginia Tidwell, a research associate at the Denver Museum of Nature & Science. Of the few bones thus far cleaned, some show a resemblance with Pleurocoelus from the Arundel Formation of Maryland.

 

Other sauropods from the Ruby Ranch include several partial skeletons from Price River II Quarry south of Price being excavated by the College of Eastern Utah. The sauropod has not been named and has variously been referred to as a brachiosaurid, thus related to Cedarosaurus, or to a primitive titanosaur more closely related to Venenosaurus or one of the Argentinian species. The specimens are currently under study as a Master’s Thesis, so eventually its identity will be resolved.

 

Ornithopods - Several partial skeletons of a small ornithopod related to Hypsilophodon have been recovered from the middle portion of the member on the west side of Arches National Monument and also west of Green River. Unfortunately, the material is so badly preserved that little can be determined about this animal.

 

A section of tail vertebrae from south of Price has been identified as belonging to Tenontosaurus, a very long tailed iguanodontid from the Cloverly Formation of Montana and Wyoming, the Antler Formation of Oklahoma and the Twin Mountain Formation of Texas. Thus, like Deinonychus, Tenontosaurus was widespread. The specimens from Oklahoma and Texas, however, are a different species from the one from Montana. The Utah specimen is between the two areas, but is too incomplete to determine which species, if either, the specimen belongs to.

Tail vertebrae identified as Tenontosaurus from the Ruby Ranch Member. 

 

Ankylosaurs - Three different ankylosaurs are known from the Ruby Ranch. Near the base of the member, at Lorrie’s Site, a new species of the polacanthid Gastonia is being uncovered. The site was discovered by Lorrie McWhinney, a research associate at the Denver Museum of Nature & Science. Several thousand bones have been recovered, representing at least a dozen individuals. The site appears to preserve the largest known herd of ankylosaurs from anywhere in the world. This new species differs from Gastonia burgei from the underlying Yellow Cat Member in several features of the skull.

 

A specimen from high in the Ruby Ranch at Price River II Quarry is the earliest occurrence of a nodosaurid ankylosaur. Lacking the characteristic tail-club of the ankylosaurids, this new specimen is currently under study. It is one of the largest known nodosaurids and may be related to Sauropelta from the Cloverly Formation.

 

Reconstructed skull of a new, unnamed nodosaurid from the Ruby Ranch Member.

 

The other ankylosaur, Cedarpelta bilbeyhallorum, is the oldest known ankylosaurid. It is known from two partial skulls and skeletons from south of Price, thus very high in the Ruby Ranch Member. It is very closely related to Shamosaurus from Mongolia and Gobisaurus from north-central China in  having the same boxy skull. However, Cedarpelta is more primitive in that it has teeth in the front of the upper jaw, a feature lost in Gobisaurus, Shamosaurus and all later ankylosaurids. As yet a tail club is not known.

 

Comparison of the skulls of Cedarpelta from the Cedar Mountain Formation to those of related ankylosaurs from Asia. Note the boxy rear part of the skull. The teeth at the front of the skull (arrow) in Cedarpelta identify it as more primitive than the other two, which lack these teeth.

 

Mussentuchit Member - The greatest diversity in kinds of dinosaur species in the Cedar Mountain Formation occurs in the Mussentuchit Member. That is because unlike the other members, many of the specimens are of teeth recovered from microvertebrate sites. Most of this work has been done by Richard Cifelli and his crews from the Oklahoma Museum of Natural History and Jeffrey Eaton from Weber State University.

 

 

Theropods - Isolated teeth suggest that Deinonychus is present, as well as a troodontid. Troodontids are closely related to Deinonychids in having a sickle-like foot claw, but also have bird-like features in the skull. They are better known from the Late Cretaceous, although a skeleton has been referred to as a primitive troodontid from the Lower Cretaceous of China. An even older troodontid has been referred to from the Morrison Formation. Called Koparion, it is based on a tooth having large serrations along the back edge. Although possibly a troodontid, material other than a tooth is desirable to confirm this identification.

 

A therizinosaurid may also be present in the Mussentuchit based on a few teeth. A tyrannosaurid has also been reported from teeth.

 

Sauropods - Narrow, slightly spoon-shaped teeth indicate that sauropods were present. At one time it was thought that sauropods had become extinct in North America sometime during the Early Cretaceous, and migrated back into North America from South America during the Late Cretaceous. However, these teeth are from lower Cenomanian deposits, meaning that these sauropods lived just after the start of the Late Cretaceous, long after they were thought to have become extinct. But even younger sauropods are known from the Dakota Formation (see below). Unfortunately, these teeth lack anything that allows them to be identified.

 

Ornithopods - Isolated teeth resemble those of a hypsilophodontid called Zephyrosaurus from the Cloverly Formation of Montana. More than teeth are needed to verify this identification.

 

A very advanced iguanodon from the Mussentuchit was named Eolambia caroljonesa. Originally described as the most primitive lambeosaurine (“crested”) hadrosaur, it lacks some of the defining features of hadrosaurs, such as multiple rows of teeth and projection on the ilium. The teeth do have a strong ridge and shows wear indicating a diet of tough plants. Eolambia and several other specimens worldwide show that the transition from iguanodontid to hadrosaurid occurred during this time.

 

A few peculiar teeth have been tentatively identified as belonging to a pachycephalosaur, the dome-headed dinosaur. As yet nothing more of the skeleton is known.

 

Ankylosaurs - A small nodosaurid from the Carol Site northwest of the San Rafael Swell has the distinction of being found by its radiation. As described above, Ramon Jones found the specimen using a device to measure radiation. Nothing of the skeleton was visible on the surface but the natural accumulation of radiation in the bones lead to its discovery. This nodosaurid was called Animantarx ramaljonesi. It was about 3 meters (10 feet) long, rather small for an ankylosaur. A few teeth of an ankylosaurid are also known, but nothing of its skeleton, yet.

 

Some of the material of Animantarx includes a partial skull (seen in side view at the upper left), part of the pelvis (lower left), and thigh bone (right).  

 

Ceratopsians - A few teeth of an early relative of the horned dinosaurs, or ceratopsians, are known. A nearly complete skull of one has recently been found in the Cloverly Formation of Montana, but has not yet been described. The skull lacks the distinctive frill of bone along the back of the skull, nor does it have prominent horns. It does, however, have a triangular bone at the front of the snout called a rostral bone, a distinctive feature of ceratopsians. These specimens are the earliest occurences of ceratopsians in North America. The teeth have a strong reinforcing ridge on one side in a manner analogous to iguanodonts, so these ceratopsians too ate tough plant parts.

 

Dakota Formation - As mentioned in the web page “The Cedar Mountain Formation”, the Dakota Formation is included in this study because it represents the shore and near-shore environment of the Cedar Mountain Formation. Dinosaur remains are rare in the Dakota Formation and most consists of broken parts because of wave action or rapid flow of water. There is one exception, a partial sauropod skeleton. In a few places, however, screen-washing has produced some teeth.

 

 

Theropods - Theropod bones include partial foot bones, or metatarsals, and a pair of pelvic bones, the pubes. The bones are from different sites near Green River. They indicate medium-sized individuals (i.e., larger than Deinonychus and smaller than Acrocanthosaurus). Unfortunately, the material is too fragmentary to identify this theropod. Some teeth have been tentatively identified as belonging to dromaeosaurids, troodontids, tyrannosaurids, Richardoestia, and Paronychodon. These last two theropods are only known from teeth, the first resembling an isosceles triangle and the other has one surface that is very flat and ridged. What these dinosaurs looked like and who they were related to are unknown.

 

Pelvic bones of a possible primitive tyrannosaurid as found in the Dakota Formation, and a hypothetical restoration.

 

Sauropods - The front limb of a sauropod has been found by one of my volunteers, Julia Van Pelt. Called Julie’s Site, the material so far recovered include part of the arm. The upper hand bones, or metacarpals, are very long and slender, suggesting that the individual may be a brachiosaurid. If true, then it is the last surviving individual known.

 

Ornithopods - Some foot bones and parts of a vertebra from different individuals are known. A few small teeth have been tentatively identified as hypsilophodontid, and others as hadrosaur.

 

Ankylosaurs - some ankylosaurid teeth and the much larger nodosaurid teeth are known. The nodosaurid teeth very much resemble those from later in the Cretaceous.

 

Ceratopsians - Some large, thin sheets of bone are tentatively identified as ceratopsian frill fragments. If correctly identified, this material is the first occurrence in the Dakota Formation.

 

What Do Similarities Mean?

As mentioned above, some of the dinosaurs from the Cedar Mountain Formation resemble species from England or Asia. For example, Gastonia from the Yellow Cat Member is closely related to Polacanthus from the Wealden of the Isle of Wight, England; and Cedarpelta from the top of the Ruby Ranch is related to Gobisaurus and Shamosaurus from the Early Cretaceous of Asia. What do these similarities mean?

 

Beginning in the Jurassic, the giant supercontinent formed by North America, Europe and Asia, began to separate like a zipper from south to north. Today, traces of that separation mark the mid-Atlantic ridge. As long as there was some connection, dinosaurs could migrate back and forth. Jim Kirkland has used this argument to explain the similarities between the dinosaurs of the Yellow Cat Member and those of the Lower Cretaceous of England. In fact, based on the ages of the dinosaurs in England, Jim believes the Yellow Cat Member is Barremian in age (127-121 mya; see the Cedar Mountain web page for an explanation of this term). Eventually, the two continents were completely separated, halting all migration of dinosaurs.

 

As the North America continental plate drifted west, it collided with the Pacific Plate and this began crumpling the western margin into mountains. It is from these mountains that the gravel and pebbles of the Buckhorn Conglomerate and Dakota Formation originated. Volcanism resulting from the subduction of the Pacific Plate poured ash towards the east. At least two major episodes of eruption occurred: an early sporadic one that lasted for millions of years added ash to the sediments that would become the Brushy Basin Member of the Morrison Formation. The second sporadic episode lasted only a few million years and this deposited ash in what would become the Mussentuchit Member.

 

As the North American Plate drifted westwards, it eventually collided with Asia producing a bridge of land across where Siberia and Alaska are today. This bridge allowed dinosaurs to migrate now between North America and Asia. Because Cedarpelta is more primitive than Gobisaurus or Shamosaurus in having teeth at the front of its mouth, ankylosaurids most likely originated in North America and migrated (evolving as they went) to Asia. In contrast, the oldest ceratopsian in Asia is over 121 million years old, but does not appear in North America until around 110 million years.

 

 

The importance of the dinosaurs of the Cedar Mountain Formation, then, is that they document the severing of the North American-European dinosaur migration pattern, and a later establishment of the North American-Asian one.

Based on the dinosaurs of the Yellow Cat Member, dinosaurs could migrate back and forth to Europe during the Barremian (~127-121 m.y.a.). By the late Albian (~103-99 m.y.a.), the land connection with Europe was severed and replaced with one with Asia.

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