How old are dinosaurs?

Question

What is a fossil,?

Anwer

From: Phillip Currie and E.B. Kopplehus, 101 Questions about Dinosaurs, Dover Publications, 1996 (Philip Currie is Curator of Dinosaurs at the Royal Tyrell Museum in Drumheller, Alberta, Canada and an Adjunct Professor in the Departments of Biology, Geology and Geophysics at the University of Calgary).

What is a fossil? Although many definitions of "fossil" (derived from the Latin word for "dug up") have been published, one of the best is that fossils are simply any evidence of extinct life. Bones do not have to be "turned into stone" to be fossils, and usually most of the original bone is present in a dinosaur fossil. Footprints, the impressions left in mud, as well as dung and eggs, are some of the other common types of dinosaur fossils. Although the horny beaks of hadrosaurs (duckbilled dinosaurs), cartilage at the ends of long bones, and skin usually decompose before they have an opportunity to fossilize, these things are sometimes pre-served. Most remarkable, perhaps, is the recent discovery that ancient genetic material, including DNA, can be extracted from dinosaur teeth, bones and eggs.


How long does it take for a bone to become "fossilized"?

From 101 Questions about Dinosaurs (emphasis not in the original):

Fossilization is a process that can take anything from a few hours to millions of years. "Fossilized" bone is usually considered to be that which has "turned to stone." The process is called permineralization. When a bone becomes buried, groundwater seeps through all of the pores and openings to invade the spaces inside. Minerals carried by the water are deposited inside these spaces, and over time can fill them in completely. This is similar to what happens in the sediments around the bone, where minerals carried in solution are deposited in the spaces to cement sand grains together to form sandstones, silts into siltstones, and so on. The amount of time that it takes for a bone to become completely permineralized is highly variable. If the groundwater is heavily laden with minerals in solution, the process can happen rapidly. Modern bones that fall into mineral springs can become permineralized within a matter of weeks. The preservation of soft tissues in some dinosaurs also suggests that, under exceptional circumstances, fossilization can occur within days. Some of the more common minerals found in permineralized bone are cal-cite, iron, phosphate and silica. Virtually any mineral that can be carried in solution in water can be found in fossilized hones, however. Dinosaur bones found in northeastern Colorado contain enough uranium to be located by a geiger counter. Dinosaur bones are not always permineralized, however. In Dinosaur Provincial Park in Alberta, dinosaur hones were sometimes encased in ironstone nodules shortly after they were buried 75 million years ago. The nodules prevented water from invading the bones, which for all intents and purposes cannot he distinguished from modern bone. A more spectacular example was found on the North Slope of Alaska, where many thousands of hones lack any significant degree of perrnineralization. The bones look and feel like old cow bones, and the discoverers of the site did not report it for twenty years because they assumed they were bison, not dinosaur, bones.

We note that fossils of dinosaurs can form rapidly, that their bones are discovered on or near the surface, that the bones look fresh and could even be confused with bison or cow bones (both large mammals). This means that dinosaur bones (ancient reptiles) and large mammals are found in the same layer.

From: Duck-bill dinosuars (Hadrosauridae, Ortnithiscia) from the North Slope of Alaska. KYLE L. DAVIES. Journal of Paleontology, v. 61, no.1, January 1987, pages 198-200.

(Davies did this work while at the Vertebrate Paleontology Laboratory, Balcones Research Center,
10100 Burnet Road. Austin. Texas 78758).


HADROSAUR BONES have been found on the Colville River north of Umiat on the North Slope of Alaska. This find represents the first report of dinosaur bones in Alaska and their northernmost reported occurrence. ...

The hadrosaur bones were collected in 1961 by the late R. L. Liscomb while working for Shell Oil Company. Renewed research around the Colville River led to the bones being sent to the U.S. Geological Survey in Menlo Park, where they were tentatively identified as hadrosaurian by C. Repenning. He sent the bones to the Texas Memorial Museum for further investigation, and the Shell Oil Company was kind enough to donate the bones to TMM for safekeeping. The bones were found on the Colville River at approximately 70N, 151 W (Figure 1), and the site was relocated in 1984 by two U.S. Geological Survey field parties (C. Repenning, personal common.). The site is easily accessible by float plane or helicopter, common means of transportation in the area, and to deter possible vandalism it is felt best not to reveal the exact location of the site. Precise locality information is on file at TMM.

The site occurs in what is mapped as undifferentiated Upper Colville Group of Late Cretaceous age, which is overlain by the Plio(?)-Pleistocene Gubik Formation (Brosge and Whittington, 1966). Recent studies. however, have indicated Paleogene rocks in this region (Carter et al., 1977; Nelson, 1981: Marincovich et al., 1983). Pending resolution of the stratigraphy, beds underlying the Gubik Formation in the region are being referred to simply as "pre-Gubik" (Carter et al., 1977; R. V. Emmons, personal commun.). Hadrosaurs are exclusively Late Cretaceous and their presence limits the age of the pre-Gubik rocks. "This datum has allowed H. J. Clippinger to interpret somewhat contaminated pollen samples and establish a Maastrichtian or possibly Campanian age for the strata 28 feet above and 12 feet below the dinosaur bed. Foraminifera 2 feet below the bone bed indicate a shallow marine environment" (Shell Oil Company memorandum, with per-mission of R. V. Emmons and H. J. Clippinger).

Shell Oil Company's stratigraphic section of the locality shows the fossil bones come from a thin sandstone in pre-Gubik silty sands. Matrix still adhering to the bones is a soft, brown, sandy silt.

The bones, catalogued as TMM 42475-1. apparently represent a quick surface collection by Liscomb, and consist of fragments of limb bones, ribs, and vertebrae. The quality of preservation is remarkable. The bones are stained a dark red brown but otherwise display little permineralization, crushing, or distortion. None of the diagnostic cranial or pelvic bones were recovered, so determination below family level is not possible.