The Fossil Record Of Horned Dinosaurs
With ceratopsians, we are rather fortunate. Horned dinosaurs have one of the best fossil records of any group of dinosaurs. There are close to four hundred specimens in museum collections around the world. Twenty-three genera and perhaps thirty species have been described to date, with more to come.11 There is an average of more than 30 specimens per genus. Even if the extremely abundant Psittacosaurus and Protoceratops were eliminated from this tally, we would still be left with a respectable 9.4 specimens per genus, which is well above the average of rougHy 7 specimens per genus for all dinosaurs. Only six of the twenty-three genera (26 percent) are known from fewer than five specimens. By contrast, some 80 percent of all dinosaurs are known from fewer than five specimens, and 50 percent are known from only a single speci-men.12 Where the fossil record of horned dinosaurs really shines is in completeness of material. Nearly two-thirds of the twenty-three genera are based on essentially complete skulls and skeletons (whether or not these have been adequately described), and all of them have essentially
complete skulls. By contrast, the giant sauropods have a fossil record that includes some forty-five genera, fewer than a dozen of which have skulls and only five of which are based on complete material.13
Why is ceratopsian material so abundant? Part of the answer is that these dinosaurs were restricted in time to the Cretaceous Period—in fact, except for Psittacosaurus, to the Late Cretaceous Period. A large volume of Cretaceous sediments has been preserved. This phenomenon has been termed the "pull of the recent," by which we mean that there has been less time for the physical processes of this dynamic earth we live on to wreak their havoc.14 In effect, the closer we come to the present, the greater is the volume of sediment available, and the richer (potentially) is the fossil record. Many horned dinosaurs also had the good judgment to live in areas close to the sea, where active sedimentation was occurring. Another explanation is that horned dinosaurs simply seem to have been abundant, successful animals. As I shall discuss later, it is quite likely that many also were gregarious, social animals. So many of them were fossilized because there were so many of them, clustered in relatively limited areas, to fossilize.
A peculiarity of the fossil record of ceratopsians is that fossils are restricted to western North America and eastern Asia (Maps 1.1 and 1.2).15 In the Late Cretaceous, some 65-80 Ma, the Rocky Mountains did not yet exist in their present form. A vast seaway extended from the Gulf of Mexico to the Arctic Ocean and at various times covered a large part of what is now the Great Plains. Giant marine reptiles frolicked in the waves, 4-m "sardines" ploughed the waters, and pterosaurs screamed through the skies over what is now western Kansas and southern Manitoba, and areas in between.16
The earth was stirring: volcanoes started to belch sulfurous soot into the air, and the proto-Rockies began to elevate. For several tens of millions of years, a titanic struggle ensued between land and sea, along a linear swatch of land that stretched from Alaska to northern Mexico. At times, the forces of land prevailed. The land surface rose to the west, and muddy streams and rivers flowed eastward, carrying life-giving, nutrient-laden sediment that sustained lush, swampy lowlands near the inland sea where dinosaurs and other living things flourished. At other times, the groans of the earth in labor quieted, and the land sank beneath the surface of the sea. Habitats that once nourished the dinosaurs were drowned as the sea pushed 500 km or more westward. Marine organisms—including unusual clams called inoceramids, and beautiful, straight or coiled mollusks with squid-like tentacles project-
• Pachyrhinosaurus OTri ceratops •Leptocerat ops
• A n i h i t f r a I o p s, A r rhinoiei a I ops, & Pachyrhinosaurus ATriceratops & Torosaurus VMontanoceratops
O Toros aurus
• Centrosaurus
• Pentaceratops VBrachyceratops
• Einiosaurus
A Achelousaums AChismosaurus
• Styracosaurus
MAP 1.1. Horned dinosaur localities, United States and Canada.
MAP 1.2. Horned dinosaur localities, eastern Asia.
ing from their floating dwellings—swept silently over the hardening tombs of the dinosaurs. Again Gaia stirred, and the seas retreated eastward. A fresh cast of dinosaurs, descendants of the previous ones, tenanted the verdant lowlands.
About 65 Ma, the cycle was broken, and the seas drained off the major part of the western land surface for good. Dinosaurs and many of the creatures with which they had shared the earth disappeared forever.
Marine and freshwater sediments of Cretaceous age hardened to rock (albeit often rather soft, readily eroded rock) deep beneath the sea. Eventually the land rose, and the sediments of western North America where we find dinosaurs today are frequently 1,000 m or more above sea level. When we collect dinosaur fossils today in the arid West, and the July sun beats pitilessly down on our backs, when the nearest cool beverage seems an eternity away, it is amusing to recollect that 75 Ma, the very spot we toil in was irrigated with ample fresh water and bathed in leafy, sun-dappled shade.
The Late Cretaceous dinosaur faunas of western North America represent a rather brief interval of geological time. The beds are arranged in three time-successive units. These formations are clearest in the Red Deer River Valley of Alberta. The Judith River Formation of Montana and Alberta is generally dated between about 77 and 74 Ma. The Horseshoe Canyon Formation of Alberta is a little younger, say 72-69 Ma, and the Scollard Formation is youngest, 68-65 Ma. The Judith River Formation of Alberta has the best-known vertebrate fauna of this time interval and has given its name to the interval: Judithian. Characteristic horned dinosaurs of the Judith River include Monoclonius, Centrosaurus, Sty-
racosaurus, and Chasmosaurus. The intermediate time interval is not very well understood in the United States, although dinosaur-bearing formations in New Mexico and Texas are probably correlative. This time interval is named Edmontonian, in reference to the formation that was once called the Edmonton Formation in the Red Deer River Valley, but which is now called the Horseshoe Canyon Formation of the Edmonton Group. Its characteristic horned dinosaurs are Anchiceratops and Pachyrhinosaurus. The youngest time interval is well known in Wyoming from the Lance Formation and in Montana from the Hell Creek Formation. It is accordingly called the Lancian interval. Lancian beds are also known in Alberta, Saskatchewan, Colorado, and the Dakotas. Characteristic horned dinosaurs include the great Triceratops and Tor-osaurus and, ironically, the diminutive Leptoceratops. The terms Judithian, Edmontonian, and Lancian will frequently be used in this book.
In Mongolia, there are three time-successive formations that are roughly equivalent in time to the Judithian, Edmontonian, and Lancian intervals. Only the oldest of these, the Djadochta Formation, has horned dinosaurs in abundance, notably Protoceratops, a possible ancestor of North American ceratopsids. The oldest horned dinosaur of all, Psittacosaurus, comes from Mongolian deposits dated at about 100 Ma, from the end of the Early Cretaceous and the beginning of the Late
Cretaceous. Protoceratops and Psittacosaurus are both found in China as well as in Mongolia.
In North America, we can document a variety of habitats for horned dinosaurs. In the Judith River Formation of Montana and Alberta, dinosaurs lived in wet lowland environments within a few tens of kilometers of the sea. In the Horseshoe Canyon Formation of Alberta, marine influence was very strong, and the sea was close. In the Hell Creek Formation of Montana, water energy was lower than in the Judith River Formation, judging by the finer grains of sediment, and low-energy, flat, coastal floodplains are envisioned.17 In the Two Medicine Formation of Montana, environments were higher and drier. Drought may have been an agent of mortality.18
In Mongolia, proto-horned dinosaurs lived 1,000 km or more from the sea, and aridity was a significant factor. There was water, but lakes may have been alkaline. Some dinosaurs were buried in dune sands. In fact, protoceratopsid fossils are not found in the Nemegt Formation, whose pale sediments were laid down in well-watered conditions that most resemble the Judith River Formation, but in the dry beds of the Djadochta Formation. At Bayan Mandahu in nearby Chinese Inner Mongolia, small dinosaurs appear to have been overcome by blowing sand.19
Average user rating: 5 stars out of 1 votes
Post a comment