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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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Wednesday, March 2, 2011

A genus is a group of very similar species (the plural is genera).  The practice of naming genera and species is called alpha taxonomy. When we name a species, we say both the genus and species names together. For Tyrannosaurus rex, “rex” is the actual species. “Tyrannosaurus” is a genus (notice that both words are italicized; genera and species names can also be underlined). This means that you could have a bunch of closely related species which are all grouped under Tyrannosaurus. In fact, there is a form from Mongolia called Tarbosaurus bataar, which lived just a few million years before Tyrannosaurus rex, and which is almost identical to it. Some paleontologists think should be considered a species of Tyrannosaurus: Tyrannosaurus bataar. It is acceptable to give the genus name as just the first letter followed by a period, for example T. rex (but not as T-rex, T-Rex, as it is sometimes given). So, we could say that there are two species of Tyrannosaurus: T. rex and T. bataar.

Unfortunately, we are even vaguer on how to recognize a genus as how to recognize a species. As with the morphological species concept, it is pretty much based on similarity. But how similar? The genus Tyrannosaurus belongs to a group of theropods (meat-eating dinosaurs) called tyrannosaurids, which all have a lot of features in common.  Here is a lineup of the well-known tyrannosaurids modified from a 1992 paper by Kenneth Carpenter (the skull drawings further down are also from the same paper).  It is a bit out of date, as there have been several new genera and species added to the tyrannosaur family tree in the last twenty years; two of them, Teratophoneus curriei and Bistahieversor sealyi, were just named in the last year or so.

Tyrannosaur skeletons


Here is a link to slightly more up-to-date page on tyrannosaurids by Tom Holtz.  Under "characteristics" he provides a list of things that tyrannosaurids have in common (these are called "synapomorphies"; a term I'll explain in more detail in the future).  However, some tyrannosaurids are more similar to each other than others (here is a linked page discussing relationships among tyrannosaurs in more detail), and Tyrannosaurus rex and Tarbosaurus bataar share a bunch of characteristics which distinguish them even from other tyrannosaurids; if you scroll down that page, they are listed below the photographs of the their skulls.

So...Tyrannosaurus rex and Tarbosaurus bataar are much more similar to each other, and probably therefore more closely related to each other, than to other tyrannosaurids like Daspletosaurus and Gorgosaurus (both genera; Daspletosaurus torosus and Gorgosaurus libratus are species). It is also accepted that all these tyrannosaurs are more similar to each other, and probably therefore more closely related to each other, than to other meat-eating dinosaurs like Allosaurus and Velociraptor. But how do we decide whether the name “Tyrannosaurus” should just include T. rex, or whether it should also include T. bataar? Why not just say that Gorgosaurus and Daspletosaurus are Tyrannosaurus as well? How similar is similar enough to call two species the same genus?

Funnily enough, no one seems to know, in spite of the fact that paleontologists can get awfully opinionated about it. Grouping species into genera seems to be a matter of individual preference and what feels right to the researcher, and nothing more.

In alpha taxonomy, we often talk about "splitting" and "lumping."  A "splitter" is someone who tends to divide up specimens into more genera and species; for example treating Tyrannosaurus and Tarbosaurus as separate genera, and possibly even splitting Tyrannosaurus rex itself into more than one species (in the last blog, I discussed some of these issues surrounding T. rex).  A "lumper" on the other hand tends to combine specimens into fewer genera and species; for example, dumping the species Tarbosaurus bataar (and even, it has been suggested, Daspletosaurus torosus) into Tyrannosaurus, and placing specimens in as few species as possible (in this case, T.rex, T. bataar, and T. torosus).  In the case of lumping, we would stop using the names Tarbosaurus and Daspletosaurus and say that they have been "sunk" into Tyrannosaurus.  

One more issue relating to genera and species is that of "priority".  When I say that Tarbosaurus bataar and Tyrannosaurus rex should be the same genus, why should that genus be Tyrannosaurus instead of Tarbosaurus?  It has to do with the order that names were first published in.  The name Tyrannosaurus was created in 1909, while the name Tarbosaurus was created in 1946.  Therefore, we say that Tyrannosaurus has priority over Tarbosaurus, and "sink" Tarbosaurus into Tyrannosaurus (IF we think they are the same genus).

The case of Tyrannosaurus and Tarbosaurus is actually a little more complicated, and nicely illustrates how the issues of distinguishing species, splitting, lumping, and priority tie together.  The genus and species names "Tyrannosaurus rex" were created in the same paper by a paleontologist named H.F. Osborn in 1909 for specimens from North America.  It should be noted that when a new species is named, a particular specimen is chosen as the holotype, and that specimen becomes the official name-bearer for the species.  What this means is that if you want to prove that other specimens belongs to the same species, you have to prove that they are the same species as the holotype.  The reason why holotypes are important is because it avoids a lot of confusion about applying names if someone revises the genus or species.  For example, let's say that when Osborn named Tyrannosaurus rex, he identified several different specimens as belonging to the species, but didn't specify a holotype.  Moreover, let's say that years later, someone looked at these specimens and decided that they were actually two or three different species.  Which specimens do we call Tyrannosaurus rex, and which do we call something different?  By picking a holotype in the beginning, this is resolved immediately.

Anyway, the first specimen the Mongolian form was originally published as the holotype of Tyrannosaurus bataar in 1955 by Soviet paleontologist Evgeny Maleev who recognized how similar it was to Tyrannosaurus rex.  Later the same year however, Maleev named three new species and one new genus from other Mongolian specimens (all from the same rocks in the same area of Mongolia): he thought one specimen was different enough from other tyrannosaurids to make it the holotype of a new genus and species, Tarbosaurus efremovi, but he thought that two other specimens were more similar enough to the North American species Gorgosaurus libratus to assign them to the same genus (as the holotypes of G. lancinator and G. novojilovi; Gorgosaurus incidentally was named in 1914).  Below are drawings of the skulls, again from Ken Carpenter's 1992 paper:

Tarbosaurus skulls


The differences between these Mongolian specimens had partly to do with size (the specimens assigned to "Gorgosaurus" were smaller than the other specimens, and Tarbosaurus efremovi was slightly smaller than Tyrannosaurus bataar), and in 1965 another Soviet paleontologist named Rozhdestvensky decided that they were probably all individuals of the same species at different stages in growth (the "Gorgosaurus" specimens were juveniles, Tarbosaurus efremovi was mostly grown, and Tyrannosaurus bataar was an adult).  However, he also decided that this species was different enough from both Tyrannosaurus rex and Gorgosaurus libratus that it should be a distinct genus from either.  SO...what to call this genus and species? Well, the oldest GENUS name that was not Tyrannosaurus or Gorgosaurus was Tarbosaurus.  The oldest SPECIES name given to these specimens was bataar, so we get Tarbosaurus bataar, a name combination which hadn't existed before.

It gets even more complicated though.  In his 1992 paper, Ken Carpenter decided that MOST of the Mongolian specimens were similar enough to Tyrannosaurus rex to lump into the same Tyrannosaurus bataar.  However, he thought that the holotype one of the species named in 1955 (Gorgosaurus novojilovi) was different enough from any of these other specimens that it wasn't a juvenile of Tyrannosaurus, Tarbosaurus, or Gorgosaurus, and was a distinct genus and species.  So what do we call THAT specimen?  Well, Ken couldn't use any of the other genus names...which means there wasn't a genus name available, so he had to create a new one: Maleevosaurus.  The specimen already had a species name, novojilovi, so it became the holotype of Maleevosaurus novojilovi.

To make it even MORE complicated, in 1995 a dinosaur enthusiast named George Olshevsky published a paper in which he thought that nearly ALL of these specimens were different genera (he is a big-time "splitter").  He thought that "Gorgosaurus" lancinator was a juvenile of Tyrannosaurus bataar, and that this was a different genus and species from Tyrannosaurus rex, Tarbosaurus efremovi AND Maleevosaurus novojilovi (which were also all different from each other) he had to name a new genus for Tyrannosaurus bataar: Jenghizkhan (after Ghengis Khan), so the species became Jenghizkhan bataar, and "Gorgosaurus" lancinator was sunk into it. 

Most other tyrannosaur experts today seem to think at least all the Mongolian forms are the same genus and species (as Rozhdestvensky did in 1965), and sink Maleevosaurus novojilovi and Jenghizkhan into Tarbosaurus (or Tyrannosaurus) bataar...but there is still debate.

One more thing about names, and then I swear I will stop.  It was a pretty common practice in the 19th and early 20th century to give new names to just about every dinosaur specimen that got pulled out of the ground that looked even a little different from other specimens.  Unfortunately these specimens were often so scrappy (just a few incomplete bones) that they were "not diagnostic"...meaning that we cannot really tell if they were different genera or species from other specimens or not.  We call names given to such specimens nomen dubia (plural; the singular is nomen dubium), and don't use them anymore.

There are a lot of other complicated legalistic issues about naming, but they can get awfully boring, so enough about names.  In the next blog (or two) I'll talk more about systematics, and what that tells us about evolution.

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Blog Comments
Dave Hone

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Great stuff Jeff. One thing I would add is that you will almost inevitably see in the media "A new dinosaur species Whateverugenus was today named by researchers" and while this is true, there is of course a new genus AND a new species being named.

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Awesome, you had me at dinosaur:) This was very helpful and interesting. Thanks for posting. I had no idea about the history of naming species like the Tyrannosaurus. It did always seem a little garbbled to me in the way things were named and hard to make a distinction between species and genus etc.

Brian Krueger, PhD
Columbia University Medical Center
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Do you think that the splitters are doing it out of pride?  For example, they like splitting into multiple genera just so that when they make a new discovery they have complete control over the naming convention and don't get "stuck" with someone else's genus?

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Nice article. Is the holotype section kind of what happened with the whole Brontosaurus/Apatosaurus fiasco?

Jeffrey Martz
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Splitting has certainly been a pride issue in the past, most notoriously during the late 19th century between two vertebrate paleontologists, O.C. Marsh of Yale, and E.D. Cope of Philidelphia.  The two loathed each other and were locked in a competition to collect and describe more dinosaurs (and other fossil vertebrates) from the western U.S.  Although thier compeition resulted in a bonaza of dinosaur fossils (including some of the most famous, such as Triceratops, Stegosaurus, and Allosaurus), they tended to apply a new name to everything that they found.  In fairness, this was a common practice.  At the time, most dinosaur genera and species were new discoveries, and paleontologists did not have a good handle on what variation within genera was like, or what types of bones would turn out to be identifiable as belonging to a valid genus or species, and which were not (ribs for example can look pretty similar in a lot of different species, so you wouldn't want to name a specimen for which only a few ribs were known).  Marsh allegedly named about 80 new species of dinosaur, while Cope named only 56; not being an expert on dinosaur taxonomy, I don't know how many of these are actually valid.  Many of them(possibly even the majority) have either been sunk into other taxa, or are so scrappy that we can't tell if they are the same as other genera and species or not, and are therefore nomen dubia.

Personally, I tend to be a bit of a splitter, although I see it as a practical issue.  Names are intended to convey information, so I prefer to use more rather than fewer names to keep that information.  Some paleontologists prefer to lump because of circumstantial evidnece that two slightly different forms might be males and females of the same species, or just showing individual variation.  However, as I discussed in the species, the case for this will always be equivocal.  I like keeping separate MORPHOLOGICAL species names for different forms to convey anatomical information.  If I say the species name, you know right away what characterisitics it has to distinguish it from the other form(s), even if they might in reality have been the same BIOLOGICAL species.

The holotype of Apatosaurus was named a couple years (1877) before that of Brontosaurus (in 1879) both by Marsh.  Already by 1903, it was recognized by another paleontologist (Elmer Riggs) that the two were probably the same genus.  However, the name Brontosaurus was well-established in popular culture by then, partly due to a published skeletal drawing Marsh published in 1895 (one of the first ever drawn), and partly because of the skeleton mounted at Yale and labelled Brontosaurus in 1905; one of the first dinosaur skeletons ever mounted.  The name Brontosaurus therefore became stuck in popular usage until about the 1980s (it was still popular when I was a kid).  These days, the name Apatosaurus has become sufficently well-known that there isn't much of an issue about it, even with the public.

Another example of priority was that of Triceratops and Torosaurus last year.  Both of these horned dinosaurs were also named by O.C. Marsh in 1889 (Triceratops) and 1891 (Torosaurus).  Both come from the same rocks from the very end of the Mesozoic Era in westrern North America.  Last year, paleontologists and Jack Horner and John Scannella suggested that Torosaurus, which is slightly larger than Triceratops  and has a more exaggerated frill, was actually a mature individual, and that Triceratops was immature.  Fortunately, Triceratops (the better known name by far) would have priority in this case.  However, this argument has not been met with too much enthuisiasm, and most dinosaur paleontologists think they were distinct genera.

Abanoub Marcus

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Jeffrey Martzsaid:

 These days, the name Apatosaurus has become sufficently well-known that there isn't much of an issue about it, even with the public.

I completely disagree with that. At least where I live, almost everybody knows about the Brontosaurus, and has no idea that it is really named Apatosaurus. And whenever I tell anyone about that, they are very surprised, as if the idea that Brontosaurus has a different name had never even occurred to them, before. 


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oth of these horned dinosaurs were also named by O.C. Marsh in 1889 (Triceratops) and 1891 (Torosaurus). Both come from the same rocks from the very end of the Mesozoic Era in westrern North America. Last year, paleontologists and Jack Horner and John Scannella
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