Comment 2: How do we know the tyrannosaur came from
Mongolia? (and the related question, which I’ve decided to lump with this one: Why
does the auction company, and subsequent news stories, refer to the specimen as
Tyrannosaurus bataar while palaeontologists call it Tarbosaurus?
The first thing we need to address with these questions is
the concept of ‘species’. In high school, most of us will have learned that a
species is defined as a group of interbreeding individuals that can produce
fertile offspring. This is called the Biological Species Concept. What many
people may not know is that this is just one of many species concepts.
Palaeontologists can’t go back in time to check on which dinosaurs were
interbreeding, so the biological species concept can’t really be used in
palaeontology. Instead, we generally use the Morphological Species Concept,
which essentially boils down to ‘a species is a group of individuals that look like
each other and not like others’.
Much of the arguing and debate in palaeontology results from
the problems associated with the morphological species concept, and that is
because individuals from the same species can look different from each other,
and individuals from different species can look similar to each other.
Biologists working on still living (‘extant’) animals need to worry about three
main sources of morphological variation:
1. Changes
that occur during growth: An older individual of the same species can look
different from a younger one.
2. Sexual
dimorphism: Males and females of the same species may look different.
3. Normal
individual variation: Individuals within a species can look different from each
other because of normal variation resulting from small genetic differences and
environmental differences. As an example, think about all the different dog
breeds there are today. All dogs can interbreed (theoretically, although surely a Great
Dane crossed with a Chihuahua must be pushing the limits of what is practical...)
and produce fertile offspring, and belong to the same species, Canis
familiaris. On top of that, all dogs are descended from domesticated grey
wolves, and so many scientists regard them as a subset of the species Canis
lupus. In taxonomic terms, this is called a subspecies, and we would write it
as Canis lupus familiaris.
Via Wikimedia Commons.
Palaeontologists need to add one more source of variation because we work with fossils, and that is:
4. Changes
that occur during fossilization (‘taphonomy’): Most fossils preserve only the
skeleton, and so information from soft tissues like colour, integument, and
muscles is lost. Skeletons become disarticulated, and individual bones get
broken. All of this reduces the amount of information we have to work with.
Finally, the remaining bones can become squished and distorted because of the
extreme forces that occur as rocks form.
Disarticulated Argali sheep skeleton in Mongolia, 2010.
Finally, it is important to remember that the natural world
doesn’t always fit into discrete categories, and that the concept of species,
genus, etc. is something that humans have created to help sort living things
into workable categories. How many differences do two populations need to have
in order to call them different species? How many differences until we split
things into different genera? There isn’t really an answer to those questions,
and so deciding on what are ‘species-level’ differences vs. ‘genus-level’
differences is the source of much debate.
But that’s ok, because that is a big part of what is fun
about taxonomy and palaeontology!
So, moving on to Tyrannosaurus rex vs. Tyrannosaurus bataar
vs. Tarbosaurus bataar. Tarbosaurus bataar and Tyrannosaurus rex have several
morphological differences in the skull, but these are not necessarily obvious
if you’re not accustomed to anatomical terminology or used to looking at
skulls. Nevertheless, they are present, and there is an excellent, free to
access paper in Acta Palaeontologica Polonica that you can go download right now to see for yourself. In a nutshell, the skull of Tarbosaurus is narrower
than Tyrannosaurus when you look at it front on, and the nasal bones (the bones
along the top of the snout) are domed near the front of the eyes. Additionally,
the arms of Tarbosaurus are proportionately even shorter relative to the rest
of the body than those of Tyrannosaurus, which are already pretty short.
Tarbosaurus bataar partial skull at the Palaeontological Institute in Moscow.
Tyrannosaurus rex skull at the Royal Tyrrell Museum in Drumheller, Alberta.
It’s pretty clear that Tarbosaurus bataar and Tyrannosaurus
rex represent different species, because these differences are geographically
separated. Small-armed, narrow-skull tyrannosaurs are only found in Mongolia,
and longer-armed, wider-skull tyrannosaurs are found only in North America. You
could make an argument that perhaps the differences are insufficient to
warrant having two genera, in which case the genus Tyrannosaurus has priority
over Tarbosaurus because it was named first. However, most palaeontologists
accept both the genus Tarbosaurus and the genus Tyrannosaurus as being valid.
So why did the auction house consistently call their skeleton Tyrannosaurus
bataar? I can’t say for sure, but I suspect it is because Tyrannosaurus is much
more a household name than Tarbosaurus, and this makes the skeleton more
recognizable and interesting. It’s easier to tell someone your skeleton is a Tyrannosaurus than
to do what I’ve just done here and explain how there is a similar, but
different, tyrannosaur in Mongolia called Tarbosaurus.
So, we can look for anatomical features in the skull and
skeleton that indicate whether or not the tyrannosaur was a Tarbosaurus or a
Tyrannosaurus. But how do we know that the skeleton came from Mongolia? Well,
much like certain animals today are found only in certain places, some
dinosaurs were found only in some places and not others. Tarbosaurus has so far
only been recovered from the Nemegt Formation, a set of rocks that are only
known in the Gobi Desert of Mongolia. If the skeleton is shown to be a
Tarbosaurus, and not some other species of tyrannosaur, then the conclusion we
must come to without any additional information is that the specimen comes from
Mongolia. We can also use the colour and preservation of the bones to identify
where the skeleton came from, because different rock units will produce
differently coloured fossils (in Alberta, dinosaur bones are often brown or
black, and in Mongolia, they are often white or reddish).
If for some reason there was good evidence that the skeleton
did not, in fact, come from Mongolia, that would only make the specimen more
scientifically important, as it would extend the geographic or stratigraphic
range of the genus Tarbosaurus and would give us important new information
about that genus.
I had originally hoped to post this second part much sooner,
and so much has transpired since my original Tarbosaurus post. I’m particularly
pleased to see that a delegation of Mongolians and palaeontologists examined the auctioned Tarbosaurus this week as part of the
investigation into the specimen. You can read their report at the Painter Law Firm's website. I also highly recommend this article at the Guardian by palaeontologist Dave Hone, and many thanks to Brian Switek at Dinosaur Tracking for his kind words about my first post in this series.
Literature!
Hurum JH, Sabath K. 2003. Giant theropod dinosaurs from Asia and North America: skulls of Tarbosaurus bataar and Tyrannosaurus rex compared. Acta Palaeontologica Polonica 48:161-190.
