The Jurassic wasn't all dinosaurs

kw: book reviews, nonfiction, monographs, paleontology

After reading a monograph about paleontology in northern Egypt, published by the New Mexico Museum of Natural History and Science, I saw the museum has a series of monographs available freely as e-books. I followed up with their Bulletin 23, Paleoecological Analysis of the Vertebrate Fauna of the Morrison Formation (Upper Jurassic), Rocky Mountain Region, U.S.A., by John R. Foster. The contents of the monograph, published in 2003, form the major part of Dr. Foster's PhD dissertation.

This is not light reading for "escape", unless you are like me and can wade through the necessary details that are the stock in trade for scientific monographs. Paleontology is one of my loves, and who doesn't love dinosaurs? The Morrison Formation, composed of rock layers with ages from 155 to 148 million years, is the most famous dinosaur fossil source in the world. Thousands of dinosaur specimens have been recovered from a great many quarries around the Rocky Mountain region that includes portions of the states of Arizona, Colorado, Montana, New Mexico, Oklahoma, South Dakota, Utah, and Wyoming.

I have a particular fondness for the Morrison Formation because it makes up one "ring" of exposures around the Black Hills, where I spent several years during graduate school. Although my study area required rock samples of the Precambrian rocks in the core of the Hills (granite, gneiss, graywacke, and schist, primarily), I hiked all over the area and enjoyed seeing the spectacular cliffs of Morrison mudstones and limestones.

After an Introduction, the author proceeds in standard form, beginning with Methods and closing with Results and Discussion, and Conclusions. The three chapters between these bookends describe the localities and quarries from which specimens were obtained, the species studied, and the ecological categories found in the various areas where the Formation crops out.

Boiling down the science-speak, I can share a few of many interesting matters. Firstly, the number of species of all sizes is impressive. While big dinosaurs get all the press, and had a combined biomass that totaled perhaps half of all animal biomass, smaller dinosaurs, lizards, turtles, frogs, salamanders, mammals of many kinds (all small), one snake specimen, and several kinds of fish were the components of a complex food web, or rather several food webs in environments ranging from semiarid to stream-and-lake areas.

The Camarasaurus and Allosaurus shown in the image above were the most common larger dinosaurs. However, the more familiar Brachiosaurus, at 44 tons, was much larger than the Camarasaurus, which weighed 14 tons (these are averages for large adults). A grown Allosaurus weighed around one ton. It is unlikely that an Allosaurus, or even a pack of them, could overcome a grown Camarasaurus, let alone one of the larger sauropods. Juveniles, when they could be separated from a herd, were more likely prey, as were smaller herbivores and smaller carnivores such as Velociraptor. This is a similar to the situation today with lions, that leave mature elephants alone, but will sometimes go after a young one. Lions prefer animals their size or smaller. A grown wildebeest (gnu) weighs about the same as a lioness.

Secondly, the variety of mammals was greater than I had considered before: 29 genera, frequently with more than one species represented, although the author studied them only at the genus level. The mammals were numerous but small. I had to learn some new terms to get a clear impression of the mammal ecology. For example, I was rather intrigued by the name Triconodont, which means "three-cone-tooth." The molars of modern carnivorous and omnivorous mammals (including humans) have either two or four cusps. These small carnivores (half the weight of a small house cat, maybe 1.5-2.5 pounds) had molars in one jaw with three cusps, such as the four on the left illustrated here, and a single cusp in the opposing jaw, which fit between the three, as seen in the three teeth on the right.

This reconstruction of Triconodon mordax, with neutral coloring (I had to hunt for something not fanciful and stripey; this is from Encyclopedia Britannica), shows an animal a little smaller than a common opossum. It probably had a similar diet, unless it ate no plant matter at all; opossums are omnivorous, but prefer meat whenever possible.

Rather than go on and on, I'll close here with the note that the author writes well, with a readable style that comes through the required stodginess of scientific monograph writing. This Bulletin is worth reading to learn about what was living all around the dinosaurs in their day.