Discussion on just how many Tyrannosaurus species there were

[andrewsaurus rex]

Given the diversity seen in T. rex fossil remains, it has been proposed by some that there were as many as 3 Tyrannosaurus species.  T.rex, T.imperator and T.regina. 

The main differences noted in adult Tyrannosaurus skeletons is between gracile and robust bone structure and tooth morphology.  Imperator is considered to be the oldest 'species' and had a robust build with thick thigh bones. Later came Regina and Rex.  Regina was the gracile form with more slender thigh bones and a lighter overall build while Rex was the most robust of the '3' species with thick thigh bones and the heaviest overall build.

Since gracile and robust forms seem to have coexisted, it has been proposed that they represent male and female of the same species.  On the other hand it's also possible that they represent 2 different species in a sort of Lion and Leopard scenario.  Or they could simply be normal variation within one species.

Given that Tyrannosaurus was by far the most dominant predator in Late Cretaceous Laramidia, and occupied the entire range, it certainly makes sense that sub populations would spring up in pockets  throughout the large area, and that these sub populations would then form new species through interbreeding, passing on the traits that were best suited to the particular area of Laramidia they inhabited and over time changing noticeably in appearance from Tyrannosauruses in other areas.

It's also possible that Tyrannosaurus was one large unified species spanning the whole of Laramidia, filling virtually every predatory niche as they grew, from pursuit predator to ambush predator to bully stealing the kills of smaller Tyrannosauruses.  They may have even had the ability to change colour to suit the local environment, the way crocodiles can today.

I'd be interested to hear others' thoughts.

[Torvosaurus]

There is also T. mcraeensis.

I've read a bit about different species in Tyrannosaurus, and one issue is that they've all been defined as t-rex for so long that the lines between possible different species and T. rex has been blurred over the decades. Nobody is quite sure where t-rex stops and the other species begin.

Its a complicated matter, but personally I think there were more than one species over the 6 million years (more or less) that the genus existed.

Torvo

[crazy8wizard]

The "3 species" paper is very sloppily produced and the defining characteristics of the 3 species aren't even confident in their own diagnosis. One of them describes the species of T. imperator as "Generally robust". None of the diagnostic traits laid out that define the 3 are consistent amongst the proposed species and almost seem to be based purely on a once-over rather than specific ontogeny.

In short, T. imperator and regina are most likely not valid species, and many other paleontologists have come to the same conclusion due to how flimsy the reasoning is.

[andrewsaurus rex]

part of the problem is the way species are defined in paleontology, out of necessity. If there are 3 (or 4) Tyrannosaurus species eventually settled on, the reality is all of these 'species' may have freely interbred back in the day, 66 million+ years ago. So that would mean that they are just one species, as they are defined for extant animals, with variations in morphology that is 'normal' for that species.

But without the luxury of being able to determine the breeding habits of Tyrannosaurus, morphological differences can become overly magnified in significance and researchers can begin to dwell on minutia.

[Ajax88]

part of the problem is the way species are defined in paleontology, out of necessity. If there are 3 (or 4) Tyrannosaurus species eventually settled on, the reality is all of these 'species' may have freely interbred back in the day, 66 million+ years ago. So that would mean that they are just one species, as they are defined for extant animals, with variations in morphology that is 'normal' for that species.

But without the luxury of being able to determine the breeding habits of Tyrannosaurus, morphological differences can become overly magnified in significance and researchers can begin to dwell on minutia.

Being able to breed does NOT make a species. Mallard ducks can produce viable offspring with just about any duck species in North America. The modern species concept is far more nuanced.

[crazy8wizard]

Plus in paleontology, species aren't determined by interbreeding for obvious reasons but by diagnostic skeletal traits. It depends on the animal but it has to be over a certain number of uniquely distinct skeletal traits to be classified as a new species, of which T. regina and imperator are hazy on.

[Ajax88]

Let us not forget, species are a made up thing humans came up with to satisfy our need to organize things. There is no true definition of a species, genus, family, etc.

[crazy8wizard]

Let us not forget, species are a made up thing humans came up with to satisfy our need to organize things. There is no true definition of a species, genus, family, etc.

I mean, genetics says otherwise but the naming conventions are what we decided.
If it isn't real because humans described it then nothing is real.

[andrewsaurus rex]

Since T.rex grew in size so dramatically, from birth to adult, and its body proportions also changed i wonder what else changed as it grew.  As it aged and got bigger it would have inhabited different niches and as a result prey choices would have changed, no doubt necessitating different approaches to how it secured that prey.  When the size of Jane, it would have probably been fairly quick and agile and been an active hunter.  When it got to the size of Bucky, it was about 85 to 90 percent of adult size.  It could take on larger prey but that prey would have to be slower, larger and less agile than before.  Perhaps at that stage  a few T.rex hunted together to cooperate in taking down the larger prey.

By the time it was fully grown and approached the size of Sue it was indeed massive. And probably slow and unagile.  Perhaps relying on ambush from forested areas, relying on its enormous size and strength to quickly subdue prey.  It may have also specialized in bullying smaller T.rex individuals away from their kills.  With the significant changes in hunting techniques that probably occurred during its life, almost transforming into different animals, i wonder if colouration might have even changed.  Perhaps going from a lighter colour that would be better suited for being out in the open to a darker colour that would hide it better in the forested regions for ambush.

Without any modern equivalent to this dramatic change in size from birth to adulthood, it's difficult to speculate with any certainty, as with so many aspects of dinosaur life and behaviour.

[stargatedalek]

Let us not forget, species are a made up thing humans came up with to satisfy our need to organize things. There is no true definition of a species, genus, family, etc.

I mean, genetics says otherwise but the naming conventions are what we decided.
If it isn't real because humans described it then nothing is real.

Deciding a species by genetics is one of the worst way to go about trying to define a species. Anatomical differences can be extreme in animals still capable of hybridization, sometimes to far greater extents than between animals that are unable to hybridize. To say a chihuahua, a coyote, and a grey wolf are all the same species because they can hybridize but a red-eared slider and painted turtle are different genera entirely because they are unable to hybridize... that may be "less arbitrary" in the sense that there is a singular "hard line", but it undermines the purpose in categorizing species in the first place.

It's also just not how we actually do it, since coyotes and grey wolves aren't typically considered the same species. Nor are most ducks despite it feeling like almost all of them have mallard hybrids at this point.

[andrewsaurus rex]

well it seems to me that we have to look at what is the whole point of defining 'species' in the first place.  Nature doesn't know anything about species nor does it care about species; life just happens the way it happens.

So what is the purpose of we humans defining 'species'?  Once that question is answered, the rest pretty much falls into place.

[Ajax88]

Let us not forget, species are a made up thing humans came up with to satisfy our need to organize things. There is no true definition of a species, genus, family, etc.

I mean, genetics says otherwise but the naming conventions are what we decided.
If it isn't real because humans described it then nothing is real.

Lots of things are real, rocks, trees, math, stars. Species however are totally made up. lol My cladistics professor's favorite quote was "Lock a biologist, a paleontologist, and an entomologist in a room until they agree on what a species is, and all you'll get is three skeletons." There are no two species on earth that are perfectly equivalent. It's an arbitrary assignment that follows a few general rules some of the time.

[The Templar of the Past]

The "3 species" paper is very sloppily produced and the defining characteristics of the 3 species aren't even confident in their own diagnosis.

I remember the first time Andrea Cau heard about the study, he said it would probably be something that could be seriously commented only by experts of North American tyrannosaurids, just to completely change his mind the day after once he did read it, saying and I quote: "I was wrong. I have overestimated the scientific depth of that article. I'm sorry to be so direct, but after having read the article, I have to conclude the hypothesis suggested by Paul et al. (2022) is truly weak and immediately rejectable, even without being an expert specialized in Tyrannosaurus".

Like damn, the study was that bad. 

[JohannesB]

I like to read a bit of "On the Origin of Species" now and then, to get my head cleared up