Jurassic West: The Dinosaurs of the
Morrison Formation and Their World
(Life of the Past) by John Foster
Wonderfully Comprehensive Overview Of A Fascinating Ecosystem
The famous bone beds of the Morrison Formation, whose rocks are
exposed from Wyoming down through the red rock region of the American
Southwest, have yielded one of the most complete pictures of any ancient
vertebrate ecosystem in the world. After more than a century of
exploration, the Morrison continues to yield new discoveries about a time
so different from our own that it almost seems imaginary. Aimed at the
general reader, Jurassic West tells the story of the life of this ancient
world as scientists have so far been able to reconstruct it. The book
recounts the discovery of many important Late Jurassic dinosaurs such
as Apatosaurus, Allosaurus, and Stegosaurus. But dinosaurs comprise
barely a third of the more than 90 types of vertebrates known from the
Formation, which include crocodiles and turtles, frogs and salamanders,

dinosaurs and mammals, clams and snails, and ginkgoes, ferns, and
conifers.
Personal Review: Jurassic West: The Dinosaurs of the Morrison
Formation and Their World (Life of the Past) by John Foster
_Jurassic West_ by John Foster is a comprehensive overview of the
geology and biota of the Morrison Formation of Jurassic western North
America (roughly 150 million years ago). In this well-illustrated book with
many pages of references, the author recounted what the formation is, the
types of rocks it is found preserved in, what the environment was like, the
history of the study of the Morrison (one of the most intensively studied of
all Mesozoic ecologies, it has been explored for over 125 years), and what
to me is the heart of the book, a complete list and description of all
vertebrate animals found there and the ecology of that region back in the
Mesozoic.
So what was the Morrison? It was an immense flat basin between
about 30 degrees and 40 degrees north latitude, a floodplain located
between highlands to the west and far to the east. Thanks to the western
mountains, it existed in something of a rain-shadow and was semi-arid and
mostly savanna, though did receive a fair amount of water from surface
rivers from the mountains, some regions of rather high groundwater levels,
and seasonal storms. Foster wrote that geologist Ralph Moberly proposed
that the Gran Chaco Plain of northern Argentina is a very good analog for
the Morrison, as it is a vast alluvial plain, well-vegetated with patches of
forest scattered through savannas and between lakes and freshwater
swamps (some of these bodies of water are seasonal). Most of the rain
occurs in the spring and summer during the rainy season, while during the
winter dry season some bodies of water become sun-baked mud flats. Of
course, the Morrison's savannas didn't have grass, as like other flowering
plants they had yet to evolve; instead there were open plains of ferns
(more than 80 types are known) and cycadophytes and the scattered lone
trees and patches of woodland (mostly around bodies of water) were
_Sequoia_ (related to modern redwoods), araucarian conifers (similar to
Norfolk Island pines), _Podozamites_ (similar to the kauri tree of New
Zealand), ginkgoes, tree ferns, seed ferns, cycads, and horsetails.
Foster recounted the 90 known vertebrate species from the
Morrison, including fish, frogs, salamanders, turtles (which were quite
abundant), sphenodontians (related to the modern tuatara), lizards, a
possible snake, champsosaurs (a crocodile-like group of reptiles - distantly
related - that existed as late as the Paleocene), crocodiles (the most
interesting was "Fruitachampsa," a long-legged, terrestrial housecat-sized
predator), pterosaurs, and mammals, but the main focus is on the
dinosaurs, ranging from the predatory _Allosaurus_ (most abundant
theropod of the formation, nearly 75% of the theropod specimens) to the
huge _Saurophaganax_ (a nearly tyrannosaur-sized allosaurid, only two
individuals have been found and it appears to have been quite rare) to
smaller theropods such as _Ornitholestes_ (possibly feathered) to the

huge sauropods (ranging form _Camarasaurus_, the most abundant
dinosaur of the formation to the apparently rare _Brachiosaurus_) to
stegosaurs, the rare ankylosaurs (not uncovered until the 1990s), and
smaller fleet-footed ornithopods like _Dryosaurus_ and _Camptosaurus_.
Foster wrote that the diversity of mammals was so rich that there
were nearly as many mammal genera as there were all groups of
dinosaurs combined, showing a real diversity in lifestyles, prey, and
habitats. Some forms (_Docodon_) appear to have been semiaquatic like
muskrats while others (_Fruitafossor_) were burrowers, showing many
similarities to the much later evolved aardvarks and armadillos, with peg-
like teeth, spade-like claws, and a robust humerus.
There was an unusually high diversity of large carnivores, both when
compared with today and with other past ecosystems. Though the famed
Cleveland-Lloyd Quarry in Utah is dominated by _Allosaurus_, five other
genera of theropods were found there, which lived in the same place at the
same time. Foster compared the theropods of the tyrannosaur-dominated
Late Cretaceous Hell Creek Formation and found that the Morrison
Formation showed "greater species diversity, less body mass range from
smallest to largest, and lesser disparity from one species to next smaller"
than at Hell Creek. While Morrison had six species greater than 100 kg
(220 lbs) in weight, only three were at Hell Creek (and two of them - an
ornithomimid and oviraptorosaur - lacked teeth, so the next "pure
carnivore" after _Tyrannosaurus_ was a 35 kg or 77 lb. troodontid).
_Tyrannosaurus_ weighs more than 10 times as much as the next
theropod, while no such extreme disparity exists at Morrison, where the
difference as shown on a chart is "strikingly gradual and steplike." Foster
did speculate that perhaps different age groups of _Tyrannosaurus_
functioned in a similar ecological role as the many genera of Morrison.
Foster wondered why this "freak of the Late Cretaceous," an "anomalous
Mesozoic carnivore on steroids," was so big; did it really need to be that
huge to prey upon hadrosaurs and ceratopsians? Why wouldn't the
gargantuan sauropods of Morrison produce "behemoth carnivores?" Foster
speculated that adult sauropods and even older juveniles were practically
immune from predation so theropods focused on stegosaurs, ornithopods,
and smaller sauropods.
How did the theropods of Morrison survive together? Analyzing body size,
tooth size, forelimb build, and relative abundance of specimens, Foster
proposed that _Allosaurus_ was something of a generalist predator
(perhaps even feeding at times on aquatic life such as large lungfish
according to Bob Bakker), feeding on a wide variety of prey items, while
the larger and more robust _Torvosaurus_ feed on larger dinosaurs and
the smaller, more gracile but large-toothed _Ceratosaurus_ fed on
midsized dinosaurs.

In contrast to the gradual progression in size ranges in theropods, the
herbivores were generally either all very large or rather small, showing a
marked "bimodality." Richard Stucky proposed that given the already open
terrain of the Morrison and the probable habitat modification thanks to the
feeding habits of the sauropods, herbivores were under evolutionary
pressure to either evolve into extremely large sizes to be immune to
predation or be small to hide in what little understory there was.
The book also has an excellent section on sauropod metabolism where he
reviewed concepts like inertial homeothermy and fermentative
endothermy.
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