The Challenge of Dinosaurs in Virginia

Most of my fossil hunting has been in Maryland with an occasional foray into other states, including neighboring Virginia, Delaware, and West Virginia.  So I thought I had some familiarity with the ground covered by geologist Robert E. Weems' new book:  The Age of Dinosaurs in Virginia and Nearby States (2022).  Actually, it turns out I didn't have a real clue as to the central challenge Weems faced in writing this book.

Weems tackles a sweeping and complex topic in this well-written, accessible volume.  (The Background Note below provides additional information about the author and describes a connection I have with him.)  The book is organized chronologically, describing the flora and fauna of the region across most of the Mesozoic Era that encompassed the Triassic, Jurassic, and Cretaceous Periods running from 252 to 66 million years ago (mya).  The many tables, graphs, maps, and illustrations that grace the book are clear and informative.  Sadly, though, the volume is missing an index which makes revisiting topics difficult.

Among the most important and welcome aspects of the book is Weems’ account of the deep impact that geological forces over time have had on the area’s geography and climate, factors shaping its flora and fauna.  This includes the influence of the ever northward movement of Virginia (that is, the geographic region we now call “Virginia”) from near the equator in the early Mesozoic, to the “low latitude desert belt” in the late Triassic, and to the “subtropical belt” in the Cretaceous.  (p. 14)  I think it is important to always keep in mind these broad, powerful forces shaping the environment within evolution is at work.

The highlight for me of this focus on the geological influence on the composition of the flora and fauna is his discussion of the late Cretaceous.  He delineates the effect of the separation of the Appalachia continent (of which the Virginia region was a part) from the Laramidia continent during the Cretaceous as shown in the map below of North America during late Campanian Stage of the Cretaceous (about 75 mya).  (This map is taken from Scott D. Sampson, et al., New Horned Dinosaurs from Utah Provide Evidence for Intracontinental Dinosaur Endemism, PLOS One, September 22, 2010.  It was modified from one prepared by Ron Blakey.  It is available on Wikimedia Commons and used here under Creative Commons Attribution 4.0 International License.)

According to Weems, this isolation of Appalachia in the late Cretaceous had important consequences for its terrestrial species, particularly its dinosaurs.  He posits that during the Campanian, the dinosaurs of Appalachia “diverged markedly” from those of Laramidia whose extensive dinosaur fauna, I would note, included the iconic Tyrannosaurus and Triceratops.  (p. 89)  As Weems writes

Because Appalachia was a small continent and isolated from the rest of the Campanian world, its dinosaurs were less diverse and on average smaller than the dinosaurs found elsewhere.  In most other parts of the Campanian world, dinosaurs continued to diversify and on average grow ever larger.  (p. 98)

Evolutionary changes marking the dinosaurs of Laramidia, such as the rise of the ceratopsians, could not be represented in Appalachia during much of the late Cretaceous when there was no terrestrial link.

The key challenge Weems set for himself in writing this book is significant:  focusing this story on Virginia.  An initial reflection of the impact of choosing this topic comes with the discussion early in the book of the phrase “Age of Dinosaurs.”  Clearly, it falls within the Mesozoic because that’s when dinosaurs lived and the most logical ending point for Weems and the majority of the rest of us is with the mass extinction at the end of the Cretaceous, 66 million years ago.  (Admittedly, the present day existence of birds – avian dinosaurs – muddies the water of the ending just a bit.)  An important question is:  when did it begin?  With the appearance of the first animal considered a dinosaur or when dinosaurs came to dominate the landscape?  My own take on it squares with that of paleontologist Steve Brusatte who argues that the “Age of Dinosaurs” started in the Jurassic when dinosaurs were no longer bit players, but clearly in line to be faunal stars.  (The Rise and Fall of the Dinosaurs:  A New History of Their Lost World, 2018, p. 99)  Yes, there were dinosaurs before that, during the Triassic, but they didn’t seem destined to larger roles until the mass extinction at the end of Triassic opened up vast niches into which dinosaurs stepped.  In contrast, Weems posits that the "Age" has its origins about 243 mya in the Triassic when the first dinosaurs came on stage, even though he acknowledges that those first representatives were “relatively small and unimposing animals” (p. 4)  So, why look that far back?  His answer is simple.  It’s because “a large part of that early record is represented in the rock record of Virginia.”  (p. 4)

This is an acceptable and understandable decision because those early years helped shape what came later.  It is also true that, without the Triassic fossil record, the Virginia-based evidence of the kinds of flora and fauna that lived here during the "Age of Dinosaurs" is dramatically limited.  Even with reaching back into the Triassic, Weems had no choice in telling this story but to look beyond Virginia to "nearby states."  Why?  Because the exposed geological formations of the Mesozoic in Virginia are much too limited.  The map of Virginia (Figure 3, p. 10-11) he includes showing where Mesozoic strata are exposed makes the situation abundantly clear.  There are 15 relatively small Mesozoic exposures in the state:  nearly all (12) from the Upper Triassic (it's no wonder Weems starts the story there), a single one from the Lower Jurassic, none from the Middle and Upper Jurassic, 2 from the Lower Cretaceous, and none from the Upper Cretaceous.  This map reflects the lengthy “unconformities” in the stratigraphic record of the state, that is, gaps where exposed geological strata are missing and, so, the fossil record in Virginia is absent there as well.  A figure depicting graphically the stratigraphic record of the Mesozoic in Virginia (Figure 2, p. 6) makes this starkly evident:  most of it is blank.

Weems is up front about this situation and his strategy for dealing with it:  use information where available about the flora and fauna of neighboring states to fill in the picture, arguing that what was going on there likely was true for Virginia as well.  That’s a perfectly reasonable approach, though one that he has to turn to repeatedly throughout the volume.  That does raise the question of why he choose Virginia as the ostensible focus in the first place, and not, say, a broadly defined Mid-Atlantic region as a whole.  In fact, his book is an excellent account of the "Age of Dinosaurs" as it played out in that broader region, albeit in the slightly uncomfortable guise of a story about Virginia.

Background Note

Robert Weems is a highly respected, much published geologist who had a long career with the U.S. Geological Survey.  His publications cover a wide variety of geological and paleontological topics.  Many can be downloaded in PDF format from the website of the Maryland Geological Society (MGS).  In the interest of full disclosure, I would note that Weems and I have both been members of the MGS, a club composed of amateur and professional geologists and paleontologists.  The club helped Weems defray part of the cost of publishing The Age of Dinosaurs.