The Pliocene Adventure -- The Fossils

The next few posts in this series will describe the kinds of animals present in Colorado during the mid-Pliocene, 3.3 million years ago, based on the fossils that have been found there.



Finding a suitable reference of fossils was problematic. While there are many webpages that provide lists of prehistoric animals, most just concentrate on the sexy ones (cave bears, sabretooth cats, dire wolves, etc.), the big ones (mammoths, ground sloths, rhinoceroses, etc.), or mammals in general (horses, camels, antelope, etc.); most give no information on where they lived, except to say North America or The West; and most either give no dates or a very general date, like the mid-Pliocene, which can still cover a couple of million years. Finally, I stumbled upon a database that listed all fossils stored in museums, which included the locations where they were found and their proposed age ranges, and which covered the entire biological world, including all vertebrates and invertebrates. With that source I was able to compile a list of all the creatures that Differel and Team Girl were likely to encounter.

Even then, it wasn't that straightforward. To begin with, I decided to ignore the invertebrates. As it turned out, there were almost too many vertebrates to review in a convenient length of time; had I included invertebrates I might still be trying to compile a definitive list. Besides, there were no sexy invertebrates at that time, like giant spiders or scorpions or ants, and while they tended to be different species, for the most part the same kinds of invertebrates that exist today also existed back then. Nor did I want to invent a fictional invertebrate that would force me to handwave why it doesn't show up in the fossil record. This is not to say I won't mention invertebrates at all; Differel and Team Girl will have to contend with insects and arachnids, and their water source will have crustaceans. But other than mentioning what type they are, I won't go into detail on their taxonomical classification.

Even for the vertebrates, I concentrated on just the mammals, birds, reptiles, and amphibians, and even that last was pushing it. My story will be as much bestiary as adventure, so I will be describing the various animals Differel and the Girls encounter, especially those they interact with, and I justify some otherwise unlikely interactions by establishing that, as long as they are stuck in the past, they will do biological survey work for Mabuse, including trapping animals for collection and study.

Since their home includes a beaver pond that would also serve as a home for various amphibians, and the savannah they reside in has other amphibian-friendly environments, I figured some of their traps would catch the odd frog, toad, or salamander, so I couldn't justify (to myself) leaving amphibians out, even if all I might do is have Sunny pedantically mention the toad or frog that showed up in one of their traps. However, I decided to leave out fish. Partially because the kinds back then would pretty much be the same as the kinds we have now (though that would also be true of the amphibians), but mostly because the equipment Mabuse will provide them for animal collection will be land traps, not nets or similar water traps. As such, aside from anything caught while fishing, I won't have much to say about fish at all.

Next I decided to concentrate on listing genera (the plural of genus). All lifeforms, including plants and microorganisms, are classified by a two-name system, for example Homo sapiens, that goes Genus species. So Homo is the genus, while sapiens is the species. A genus is the lowest rank in taxonomy that indicates a distinct creature; that is, a creature that is recognizably distinct from all other creatures (though the distinction may be subtle). Species within a genus can be distinct, but don't have to be. Genus roughly corresponds to what many people call kinds or types of animals. For example, the ordinary house mouse belongs to the genus Mus, whereas the field mouse belongs to the genus Apodemus. Except for some special cases, I won't bore you with recitations of all the fossil genera I found in my blog posts; my purpose in listing genera was so that I could understand the diversity that existed during the mid-Pliocene.

The problem is, however, that many prehistoric lifeforms are only known by their taxonomic names, such as Tyrannosaurus rex, so I cannot avoid mentioning some taxonomic names. My point, though, is that I soon discovered that there were almost too many genera to list, and recording all the species as well seemed like overkill. In the end, I just wanted a list of the kinds of animals that lived back then, and frankly, if you've seen one field mouse, you've seen them all. The only time I recorded species names was in those rare cases when species within a genus were distinct enough to be considered separate kinds.

Even so, as much as possible I tried to use popular names when I could find them, or in some cases invent them, but be forewarned: saying "there were camels back then" doesn't quite cut it if there were four different kinds of camels living together, so listing their genus names becomes necessary.

All that allowed me to pare down the number of creatures I needed to review, but next I had to select which animals to record. My primary criteria were all animals that lived in Colorado 3.3 million years ago, but since the database consisted of a listing of fossils, it could not be comprehensive in the way I wanted. In other words, it could list that a grasslands big cat fossil was found in Kansas, but it would not say whether it might have also lived in Colorado if no fossils were found there. Logically we could assume that any big cat that lived on the Kansas prairie could also live on the Colorado prairie, but unless a fossil had actually been found in Colorado, the database would not list that big cat as being a resident of Colorado.

In fact, if I limited myself to just the fossils found in Colorado, I would have only had about a Baker's dozen of creatures. So I decided that any creature found in the surrounding states -- Kansas, Texas, New Mexico, Arizona, Utah, and Wyoming -- probably lived in Colorado as well, unless its environment was too specialized or didn't exist in the state. I tried not to fudge this too much; there were some creatures I wanted to have in my story, but if their fossils were only found in California or Montana or Mexico I ignored them. Alligators are a good example. During the Pliocene they ranged farther north than they do now, and theoretically they could have followed the larger rivers west through Kansas to Colorado, but since none of their fossils were found west of the Mississippi I had to abandon that idea.

In the end, the only two creatures that I shoehorned in despite a lack of fossils were giant anteaters and terror birds. The former lived in Mexico, but since the environment there was similar to that of Colorado, I saw no reason to believe that individuals could not have wandered north, though I decided not to have a breeding population. Fossils of the latter have only been found in Florida, but they came up from South America, and to get to Florida they had to pass through the southwest. Again, I saw no reason to believe that a breeding flock could not have gone as far north as Colorado, with the lack of fossils perhaps explained by the fact that the arid conditions would not be conducive to preserving delicate bird bones except under unusual conditions, especially if it was just one or a very few flocks. If you feel inclined to dismiss this as handwaving, ask yourself why there are no fossils in Texas, Louisiana, Mississippi, Alabama, or Georgia, states the birds had to cross through to reach Florida, or explain how they got to Florida without crossing through these states.

I should point out that I am not arguing that terror birds actually made it to Colorado, only that they could have, and only for the purpose of my fictional story.

I was more strict with dating. The problem with dates is that they are imprecise and often approximations. That doesn't mean they are mere guesswork; there is a strong logical and evidential basis for sedimentary and taxonomical dating. However, even a paleontologist will admit that a dating range for a single fossil is based more on uncertainty than fact. For example, stating a fossil has a range of 5 to 3 mya isn't necessarily a claim of how long that creature's species lived, just the calculation of the most likely age range in which that individual creature lived. In other words, the range is not saying that species lived between 5 and 3 mya, but that the individual lived anywhere between 5 and 3 mya. It's like saying that a man died between 10 and 12 years ago. That doesn't mean he was dying for two years, only that we cannot say exactly when he died, except it wasn't more than 12 years ago and it wasn't less than 10.

Then too, the extremes are often themselves uncertain. In our above example, the 3 mya date might have really been 3 plus or minus 200,000 years, putting the lower limit at anywhere between 3.2 and 2.8 mya. With that kind of uncertainty it's amazing that we can be accurate at all. Multiple fossils make the date range more precise, and moves it closer to being the actual lifespan of the species, but even then we cannot say that the species did not exist before 5 or after 3 mya, only that we haven't found any fossils outside that date range. Of course, if you add in the uncertainties associated with the upper and lower limits, we can't even say that, since the data set might include a couple of outliers from 5.7 and 2.5 mya, that skewed the distribution into its wide range but otherwise fall outside the tighter range of the rest of the fossils.

The point of all of this is that I could only take the age ranges of the fossils with a grain of salt, and decided to err on the side of caution. I assumed for the sake of convenience that an age range of 5 to 3 mya was the lifespan of the species (I really had no evidence to the contrary), and I accepted the upper and lower limits as rock-solid (the database did not state the uncertainties associated with these dates), but I also rigidly applied them to deciding whether a creature might have existed 3.3 million years ago. If the range included the magical 3.3 number, say 4.2 to 3.1 mya, I accepted that the creature probably lived at the time of my story. However, even if the date was off by only 0.1 million years, I rejected it. In other words, any creature with a lower limit of 3.4 or an upper limit of 3.2 I ignored.

That might seem arbitrary, even considering the above discussion, since these cut-offs are so close to the magical 3.3, but keep in mind that we're talking about a difference of 100,000 years. Species can go extinct or be newly evolve in shorter times than that. You could also argue that with the uncertainty my cut-offs might not be all that accurate, but in that case where do I put them? At 3.5/3.1? 3.6/3.0? More? And what justification would I have for choosing my cut-offs? None that would be more objective than my justification for accepting the upper and lower limits given in the database as accurate. In the end, I did fudge even this to include some small mammals I felt were necessary for a thriving ecosystem, and a few birds to round out my list (otherwise it would be too paltry), but I fully accepted that my decision of how to interpret the dates given in the database would be rather subjective.

However, this did mean that some creatures I really wanted to include I couldn't because they lived too late. For example, I was surprised to learn that there were no mammoths in North America during the Pliocene. The soonest they appear in the fossil record is at the beginning of the Pleistocene 2.588 mya, with the iconic wooly mammoth not appearing until 780,000 years ago. There also weren't any bison, wolves, or coyotes (though there were analogs of the latter two). Certain extinct prehistoric animals that I had hoped were present, such as American lions and cave bears, were not (though again, I found suitable analogs), but as I pointed out in a previous post, I was able to include some exotic creatures from previous ages that would go extinct before the Pleistocene. They are what gives the Pliocene it's unique character that sets it apart from the Pleistocene.

I should point out, however, that this database was not perfect. For one thing, it tended to be conservative regarding age ranges, such that it listed some creatures as being younger or older than they really were. To compensate for that, I used other sources, such as Wikipedia, to provide alternative dates in a few cases. If I hadn't, for example, I would not have had any horses, which would have been ridiculous since horses lived in North America until the end of the last ice age. Then too, the database would sometimes list a creature as belonging to the Blancan faunal stage even though it listed an upper age range of 1.8 mya. The Blancan ended 1.8 mya, so rightfully the creatures belonged to the next stage, but even so, at 1.8 mya they were still too young to have lived in the mid-Pliocene. The problem was, I sometimes included a creature because the database claimed it was Blancan, without checking the date. I have tried to correct these errors, but a few might have still slipped through.