Tyrannosaurus rex size estimation (part 1)

    Even thought FMNH PR 2081 is almost 90% complete, there is still a lot of dispute about the definite dimensions of this specific Tyrannosaurus rex specimen. Brian Switek estimates Sue to be approximately 40 feet (ca. 12.1m) long in axial length, and astounding 9.5 tons heavy. Hutchinson, Bates, Molnar and Makovicky gave Sue 12.3m in axial length in their 2011 study “A Computational Analysis of Limb and Body Dimensions in Tyrannosaurus rex with Implications for Locomotion, Ontogeny, and Growth”. Hartman gave her 12.4m in axial length and 8.4 tons in mass. Mickey Mortimer gave her 12.8m in length and 5.8 tons in mass. Darren Naish gave her 13m in his book “Build a T. rex” (Note: Although a children’s book, Naish provides the precise hip height of Sue in the book (3.66m), so there’s no need to think that 13m is an approximate length). Burnie seems to agree with this, as he uses this length in his “Great Encyclopedia of Dinosaurs”, although he does make clear it’s an approximate measurement.

    So, it seems like we can definitely agree Sue was between 12 and 13m long. Only definite thing about Sue is her hip height – 3.66m. To me personally, Mortimer’s estimate seems to be the most parsimonious, so I will use it in this small research of mine.

    One other part that we can all agree on – Sue was definitely not a healthy individual. Her skeleton shows multiple pathologies that could definitely influence her general health and her age at the moment of death. She had:

    ·        A severely damaged scapula;

    ·        Torn tendon in the right front limb;

    ·        3 broken ribs (although there are evidences of these pathologies being healed);

    ·        Left fibula has double larger diameter than the right one (likely an infection);

    ·        Multiple holes resembling bite marks on the rostrum (most likely from an infestation of an ancestral form of Trichomonas gallinae, a protozoan parasite that infests birds and ultimately leads to death by starvation due to internal swelling of the neck)

    ·        Several caudal vertebrae are fused as a consequence of arthritis;

    ·        A gout on its phalanx

    Something I find quite funny about this specific specimen is that some people believe that it’s age is the maximum possible age of an average T. rex individual. Sue is estimated to have died when approximately 27-28 years old. Larson and Donnan concluded that she most likely did not die of natural death. That is, needless to say, very obvious. Some of these pathologies are so severe that it is hard to even imagine they would not ultimately result in an immediate death.

    It is pretty much very parsimonious that this individual either died from an infestation by the previously mentioned evolutionary ancestor of Trichomonas gallinae, or was killed, in a clash with another Tyrannosaurus rex individual or its potential prey. How is it considered to be the oldest possible Tyrannosaurus individual by some? T. rex could most certainly grow much older. Then why are T. rex specimens older than Sue still not found? Natural selection, regardless which type of ecosystem it’s taking place in, is always extremely rigorous. It does not tolerate any form of disadvantage, whether it was morphological, psychological, or in any other possible form. Animals that die of natural death are extremely rare in nature. Chances of finding a T. rex specimen that died of natural death are absurdly small. That would imply that individual lived relatively peacefully, which, taking into consideration T. rex’ probably hypercarnivorous type of behavior, is either impossible, or that specific individual is extremely efficient at hunting. Maximum age in case of natural death is not an important factor in nature. The important factors are hunting efficiency, defense efficiency, social behavior and probably luck. Sue’s numerous pathologies very negatively influenced both her hunting and defense efficiency, as well as general health condition. An animal with this bad health condition and this many severe pathologies definitely cannot be the oldest possible T. rex specimen.

    However, in case Horner’s hypothesis about T. rex being an opportunistic predator is correct, then it would be highly likely for us to find a T. rex specimen that died of natural death – because opportunists do not need to come in physical contact with living prey. They feed on what is left by other predators in that ecosystem, and on animals that died under distinct circumstances, including natural death. The fact that we do not find any T. rex specimens that died of natural death can also be used as an argument against this hypothesis.

    However, the fact that it is really difficult to find a T. rex specimen that died of natural death does not mean that it is impossible to find a specimen whose age in the moment of death was closer to the maximum age of an average Tyrannosaurus rex individual in case of natural death – meaning that the specimen older than Sue does not have to die of natural death. And such specimens definitely exist.

    Why is it important for us to know that there are older specimens than Sue? T. rex has a very unique growth curve. It’s growth curve is “s-shaped”, implying that the animal grew very slowly in the juvenile ontogenic phase, and very rapidly in subadult ontogenic phase, with ontogenic mass increase per year being equivalent to ca. 1 metric ton. When it reaches adulthood at around age of 20, ontogeny in a T. rex individual dramatically slows down, reducing its ontogenic mass increase per year to around 300kg, according to Horner’s study from 2009 and a study by Makovicky et al. from 2004. As Sue was 27/28 years old, making her a fully developed adult, a specimen just 4 years older than Sue would be approximately 1.2 tons heavier than her. So, knowing that Sue is most definitely not the oldest possible T. rex specimen, it is consequently definitely not the largest as well. What do other paleontologists think about this? Brian Switek and perhaps one of the biggest experts for Tyrannosauroidea out there, Thomas Holtz, both seem to agree that there were individuals larger than Sue, as Brian Switek writes this on his blog „Laelaps“, in a post called „My T. rex is bigger than yours“, while also quoting Thomas Holtz:

    Who is the king of all T. rex is trickier to answer. "Sue does seem to be the largest one, or at least the largest one we can clearly determine the size for," Holtz says. This famous T. rex stretched approximately 40 feet long and is estimated to have weighed about nine and a half tons. But the largest T. rex may have been even bigger still.

     

    Based on clues inside the microstructure of the dinosaur's bones, Holtz says, Sue was fully grown at the time of death. But individuals vary in how large they can get, and chances are that Sue represents the average full-grown T. rex rather than an extreme example. Given the way that animals vary in terms of size and growth, Holtz suggests that "it is very reasonable to suspect that there were individuals that were 10, 15, or even 20 percent larger than Sue in any T. rex population."

    Holtz gives the maximum possible size of a T. rex specimen 20% larger than that of Sue. Let’s see how much that is, based upon Mortimer’s size estimate:

    ·        Total body length: (12.8/100)*20+12.8= 0.128*20+12.8= 2.56+12.8= 15.36

    ·        Hip height: (3.66/100)*20+3.66= 0.0366*20+3.66= 0.732+3.66= 4.392

    ·        Cranial length: (1.45/100)*20+1.45= 0.0145*20+1.45= 0.29+1.45= 1.74

    David Hone, another paleontologist very experienced in Tyrannosauroid paleontology, stated this in his recent 2016 paper “The Tyrannosaur Chronicles”:

    "...Sue was certainly massive--considerably larger individuals are likely to have existed...I would be very surprised if individuals did not exist that weighed well over 10 tonnes and were more than 15 metres in total length, and quite possibly a fair bit more."

    So the ~15m estimate definitely credible. However, are there any fossil evidences that might support the idea that there were T. rex individuals larger than Sue? Earlier in the text I said that Sue is currently the biggest and the oldest Tyrannosaurus rex specimen found. However, that might not be the case.

    Jack Horner discovered 2 T. rex specimens which he claims are both larger than Sue. MOR 008 and MOR 1126, also known as Celeste or C-rex are both estimated to be up to 13.5m long in linear dimension. MOR 008 represents a complete cranium, while MOR 1126 consists of left prearticular and surangular, 20 dorsal ribs (some of which are fragmentary), 3 dorsal vertebrae, and a chevron. According to Horner’s estimates, both specimens were 10% bigger than Sue (however, Horner based his estimates upon the 12.3m estimate, which corresponds with ~13.54m length for both).

    Although many remained skeptical about Horner’s estimates (considering that the measurements were made by a Museum of The Rockies worker, and not by Horner himself), I personally do not see a reason for that, because Horner was recently awarded with Romer-Simpson prize, which is probably the highest possible award a paleontologist can get, making him unformally the best paleontologist in the World.

    Some corrections were made on Horner’s MOR 008 estimate, primarily based on a claim that the cranium was axially compressed, shifting the mandible slightly backwards and making the skull look longer. When corrected, the cranium would reportedly be slightly shorter in linear dimension, according to some, even shorter than Sue’s, resulting in total body length of 12.57m when based on AMNH 5027, Tyrannosaurus rex holotype specimen. However, there were no serious publications made on this topic, although Scott Hartman did state he personally agrees with this estimate on his DeviantArt account.

    Mickey Mortimer estimated MOR 1126 to ~12.3m, although she did not specify how she got to that conclusion.