What’s counted counts

It is springtime in Cleveland, and animals are all over the place. Tree squirrels, chipmunks, and eastern cottontail rabbits are the most conspicuous mammals during the day, though I can often glimpse a shrew scurrying through the groundcover if I am out in my yard long enough. In the evening, white-tailed deer are a common sight, and it isn’t unusual to see a raccoon, striped skunk, woodchuck, or Virginia opossum. Bats are usually visible in the night sky, and a few times I have even glimpsed a flying squirrel. Foxes and coyotes pass through the area but are more difficult to spot. Mice undoubtedly abound but generally go unnoticed.

What mammals do you see most often where you live? And which animals are most abundant in your local ecosystem? Are they one and the same?

As a paleomammalogist, I often contemplate these same questions when I am in the field. The remains of some extinct mammals are quite common and others are quite rare. Does this accurately reflect which species were most abundant millions of years ago in the area, or are the remains that are preserved simply due to chance?

Mesothere teeth.jpg
A partial skull of a mesothere notoungulate eroding out of the rock. The front of the skull was once sticking out of the hill but has long since disappeared. The two sides of the lower jaw are visible toward the right, and some of the left upper teeth are visible toward the left. Photo by D. Croft. Reuse permitted under CC BY-NC-SA 2.0.

To a first approximation, the most commonly found fossils do generally correspond to the most abundant species, and the rarest species are usually represented by few or no specimens. However, if you collect ten specimens of one species and only one of another, it doesn’t mean the first species was ten times as abundant. Many other factors affect the probability of a species’ remains entering the fossil record, meaning that the data are not as precise as those generated by censusing a modern mammal community.

I was thinking about this last week when I was searching for fossil mammals at a site in southern Bolivia known as Nazareno that dates to the middle Miocene epoch (probably about 15 million years ago). Nazareno isn’t a particularly rich site, and we only collected 68 specimens, but assuming its fossil record faithfully reflects the mammals that were living there, what did the community look like?

The most abundant mammals were wombat-like plant-eaters (notoungulates) known as mesotheres, which account for about half of all specimens.

The next most abundant mammals were armadillos, which account for about one of every ten specimens.

Chunk of armadillo armor.jpg
Fossil remains of armadillo armor (in white). These bones, known as osteoderms, come from the flexible bands located in the middle part of an armadillo’s shell. Part of one of these bands is visible toward the left. Photo by D. Croft. Reuse permitted under CC BY-NC-SA 2.0.

Moderately common mammals, each represented by a handful of specimens, included other types of notoungulates (cat-sized interatheres and hegetotheres, cow-sized toxodonts), sloths (at least two different types), and rabbit-size rodents related to the modern plains viscacha (Lagostomus maximus).

Toxodont tooth.jpg
A partial upper molar of a large (cow-sized) toxodont notoungulate. The fossil appears to be perched on a small pedestal because much of the softer surrounding rock has eroded away.  Photo by D. Croft. Reuse permitted under CC BY-NC-SA 2.0.

Less common mammals, each represented by a single specimen, included longer-legged hoofed mammals (gracile and robust litopterns of the families Proterotheriidae and Macraucheniidae, respectively), horned armadillos, giant armadillo-like glyptodonts, and tiny rodents distantly related to modern spiny rats (family Echimyidae).

Litoptern bone.jpg
A lower arm bone (radius) from a long-legged native ungulate known as a litoptern. The specimen is composed of six separate pieces that will be glued together in the lab.  Photo by D. Croft. Reuse permitted under CC BY-NC-SA 2.0.

In total, we collected the remains of some 14 different mammal species – not bad, but certainly far fewer than were living in the area at the time. How do we know that? For one, there are very few places in the world today that are home to so few species of mammals: only some of the harshest environments on earth. A typical temperate habitat like Ohio is home to at least a couple dozen mammal species. The same was undoubtedly true in the past. It must be the case that we just didn’t collect the remains of many species that were living there – either because their remains didn’t fossilize or we simply didn’t find them.

One way to test this idea would be to go back to the site again and look for more fossils. I won’t be doing that until at least next year, but I can go back and compare our recent collection to what we found there nearly a decade ago. I can also compare it to what other researchers have collected there.

What do those other collections look like? They mostly include the same species that we found this year. However, a few different ones are represented, including a close relative of modern chinchillas, a second type of tiny rodent, a dog-sized meat-eating marsupial, and perhaps an astrapothere, a rhino-sized animal with a trunk: four additional species in total. That brings the total number of Nazareno mammal species to 18: better, but probably still shy of the actual number.

What makes me think that we are still missing species? A couple things.

Tiny rodent.jpg
A partial upper jaw of a tiny rodent with three teeth. Very small specimens such as this are quite challenging to spot in the field, particularly if you are not searching on your hands and knees, a technique that is usually only worthwhile in areas that preserve many tiny specimens. Photo by D. Croft. Reuse permitted under CC BY-NC-SA 2.0.

Again, based on analogy with modern mammal communities, there tend to be many more species of small-bodied mammals in any particular area than large-bodied ones. Just think about the mammals found where you live. There are probably many types of rodents, shrews, and other small mammals but relatively few large hoofed mammals and large carnivores. Conversely, remains of small mammals are less likely to be preserved in the fossil record, as they tend to be fragile and therefore more easily destroyed. Even when they are preserved, their small size makes them difficult to spot  unless special techniques such as screen-washing are used to sort small bones and teeth from sediments of similar size. The average body mass of a mammal alive today is about two pounds (about 1 kg), and only a couple of the species we have found at Nazareno thus far were likely that size or smaller. Therefore, a variety of small rodents and marsupials were probably living at Nazareno that we haven’t yet discovered.

The other peculiar aspect of Nazareno is that nearly all the mammals we have found are plant-eaters and/or insectivores rather than carnivores (predators). This isn’t entirely surprising – carnivores are less common than mammals with other dietary habits in modern ecosystems, too – but there certainly wasn’t just one type of meat-eating mammal at Nazareno. Others were undoubtedly present; we just haven’t yet discovered their remains.

Although a handful of specimens can provide some interesting insights about abundances in past ecosystems, we have to be cautious in interpretations. Fortunately, knowing that there are species missing from our sample always gives us paleontologists an excuse to go back to the field!

Further Reading:

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