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Jeffrey Martz
Poncha Springs CO USA

This is a blog about paleontology (the study of the history of life on Earth through the fossil record) with an emphasis on vertebrate paleontology, the study of extinct vertebrates (animals with backbones). The methodology and findings of paleontology will be discussed, as well as related issues such as evolutionary theory. The blogger is a vertebrate paleontologist specializing in the Triassic Period, the Beginning of the Age of Dinosaurs.

My posts are presented as opinion and commentary and do not represent the views of LabSpaces Productions, LLC, my employer, or my educational institution.

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Monday, February 21, 2011

Tom Holtz, a well-known expert on tyrannosaurs, recently posted a blog entitled "What Should Everyone Know About Paleontology?" It is worth checking out.

The next few posts will discuss the subjects of taxonomy and systematics.  In biology and paleontology, the word “taxonomy” refers to a system of naming living organisms, and (more importantly for this discussion), figuring out how to classify them (group them together).  “Systematics” is a broader subject which includes taxonomy, but also considers the evolutionary relationships and history of organisms.

The word “species” refers to one of the most fundamental groups of living things.  Intuitively, it isn’t hard to grasp more or less what a species is: a group of living things that are the exact same “kind” of thing.  Apples are one species, oranges are another.  Humans are one species, chimpanzees are a different species.  However, saying that all humans are the same “kind” of thing is a little vague; what exactly does that mean?

There are several different definitions which have been proposed for species, but I’ll just mention a couple here.  The “biological species concept” is arguably the most well-known.  It claims that a species is a group of living things that can interbreed with each other to produce viable offspring (offspring which can also reproduce, as opposed to being sterile), but which cannot produce viable offspring with other species.  A human being from Sumatra could have a baby with a human being from Canada (which could grow up to have a baby itself), so they are the same species.  Neither could have a baby with a chimpanzee (and hopefully wouldn’t try), so humans and chimpanzees are different species.

Distinct biological species aren’t always physically incapable of having offspring, but might have other mechanisms in place to prevent them from trying; for example, different species may have different particular behaviors to encourage mating that appeal only to members of their own species.  Birds illustrate this very nicely; males of different species living in the same area have different color patterns and use different songs and displays which attract only females of their own species, even through in many cases a male and female of different species would be biologically capable of producing offspring if they tried.  Below are males of two different birds of paradise (the Greater Bird of Paradise is on the left, the Lesser Bird of Paradise is on the right), both of which live in New Guinea, but are just different looking enough that females of their respective species know who to mate with. Other bird of paradise species differ even more radically in appearance and behavior, for the same reason.

Birds of Paradise

Alternately, different biological species may be prevented from interbreeding by being geographically separated, such as living on different islands, or in areas separated by a mountain range (and in fact, isolation is probably a requirement for new species evolving in the first place, but that is another subject).

The biological species concept isn’t perfect, and these mechanisms aren’t foolproof.  Groups which normally only mate with their own “kinds” may, in some circumstances, interbreed.  Lions usually only mate with lions, and tigers only with tigers, and this was apparently true even in past centuries when lions and tigers lived in the same areas. Historical records of “tigons” (the offspring of a male tiger and female lion) and “ligers” (the offspring of male lions and females tigers) in the wild are unknown, and these usually only produced when human beings decide (for whatever bizarre reason) to encourage them. 

Tigon

The degree to which closely related “species” are capable of interbreeding varies. Some closely related species are capable of interbreeding to produce viable offspring but don’t because of behavioral differences and geography, some (such as horses and donkeys) can produce offspring which are usually sterile (mules), and some can’t produce offspring at all.  One of the most interesting cases is that of a "ring species", in which several closely related species occur at varying geographic distances apart, with the ability to interbreed being stronger in species which live close together.  In other words, if species A and species C live some distance apart, and species B lives in an area in between, species B may be able to interbreed with both species A and C...but species A and C cannot interbreed with each other.  All the exceptions and equivocations can make identifying different biological species a somewhat tricky and arbitrary exercise.

A very important point to make is that members of different biological species usually look and act differently...and that this is how they are usually identified in the first place.  If tigers and lions looked and acted exactly the same, it probably wouldn’t occur to us to even ask if they were different biological species.  We would just assume that they were the same species.  There are cases of groups of organisms in the wild which all look the same but in which certain members interbreed only with each other; we refer to these as “cryptic species” and can be detected by examining their DNA (and noting some members of the supposedly single species have genetic differences from the others), or by noting differences in their behavior which were initially missed.

The reality is that the vast majority of species (both modern and extinct) have been identified by looking different from other species, not because biologists spent years stalking individuals in the wild and scrupulously observing who every single individual is mating with.  With the advent of genetic technology, we can also look at the DNA of organisms to distinguish them (as in the case of cryptic species)…but the majority of species are, and always have been, distinguished from each other based on observable differences in morphology (shape).  For this reason, the "morphological species concept", which groups organisms to together if they pretty much look the same, is relied on more in real-life taxonomy than the biological species concept.

The biological species concept hits a brick wall in paleontology.  We have absolutely no way of knowing for certain if fossils which we identify as being the same species (for example, all the fossils which we call Tyrannosaurus rex) were really the same biological species or not.  The evidence is purely circumstantial.  If the fossils all look pretty much the same, all come from the same geographic area (in the case of T. rex, western North America) and from the same strata in that area, indicating that they are about the same age (in the case of T.rex, strata deposited during the last few million years of the of the Cretaceous Period of the Mesozoic Era), this certainly suggests that they were (maybe? probably?) the same biological species.

Tyrannosaurs mating

Below are two Tyrannosaurus rex skulls (the one on the left is a replica; here is the link if you just have to have one).  They look pretty similar, right?

Tyrannosaurus skulls

However, this does not clinch the case.  Remember that lions and tigers once had geographic ranges which overlapped and are certainly living at the same time, and their skeletons are also extremely similar; an extraterrestrial paleontologist visiting Earth millions of years in the future examining lion and tiger fossils might well assign them to the same species.

Tiger and Lion skulls

There are additional complications when looking at fossil species which are a bigger problem than for modern species.  One is individual variation (physical differences between members of the same species).  You can see a lot of subtle differences in the proportions of different parts of the skeleton in modern species.  Looking at the two T.rex skulls, one can detect slight differences; the one on the left has a slightly taller snout for example.  Is this just variation between members of the same species, the same way you might have a bigger or smaller nose than another human you pass on the street, or did these two skulls belong to different biological species?

Sexual dimorphism (physical differences between males and females) is another tricky problem; it is not uncommon for males of a species to be generally larger and more heavily built than females (this is true of humans as well as other animals), and in some cases, bear distinct structures absent in the female (again, this is common in birds, where males often have elaborate feather displays for attracting females).  Below are a couple examples of sexual dimorphism; in both cases the male is the larger individual, and also has a striking structure (the big nose in the elephant seal, the long tail feathers in the pheasant) absent in the female.

Sexual dimorphism

Sexual dimorphism is not a huge problem for modern biologists, since they can observe living animals and see males and females of the same species interacting.  But what about extinct species? If you find a bunch of pretty similar fossils from the same area that lived at the same time, and notice that there are slight differences between some of them, are they closely related species, or males and females of the same species?  It is a tricky question, and a difficult one to answer definitively for most extinct animals (one pretty compelling case is discussed here and here).  The problem is even more acute given that sexual dimorphism in modern animals often involves soft tissues (like noses and feathers) not usually preserved in the fossil record, and that even differences detectable in the skeleton can be pretty subtle.

Finally, there is ontogeny (physical changes that an animal undergoes as it grows up).  One example is the recent hoopla about whether or not the horned dinosaurs Triceratops and Torosaurus were actually the same (these are actually genera, which I will talk about in the next blog, but the basic problem is the same).  Questions about sexual dimorphism and ontogeny have also plagued tyrannosaur taxonomy.  Adult skeletons of T.rex have some subtle differences from each other which some paleontologists have suggested represent males and females, and others have suggested represent slightly different species. Moreover, there are skulls similar to adults of T.rex which are known from the same rocks in western North America, but which are much smaller have a few other differences (such as more teeth in the jaws).  There is debate as to whether these little specimens are juveniles of T. rex, or a separate species.

In summary, we aren’t even sure exactly sure what we mean by species for living organisms, even with genetic technology and observations on behavior to help with the issue, and paleontologists have a worse problem: not only are we not sure exactly what a species is, we have a hard time identifying one even when we have a pretty good idea what we mean by it.

Having completely failed to clearly establish exactly what a species is, we now move on to the genus (a group of species), which is even more poorly understood.

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Blog Comments

Brian Krueger, PhD
Columbia University Medical Center
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I always learn so much reading these posts.  Great stuff.


yannisguerra
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Agree with Brian. Thanks for taking the time to explain the little simple things to those of us that have no previous knowledge in this field


Suzy
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Great read!


Alchemystress
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Very imformative!! Thanks for taking the time to explain. I always wondered about these things!

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