Tyrannosaurid Ecology and Ontogeny and the Invalidity of Nanotyrannus

Welcome back to my second installment of my tyrannosaur chronicles (I am so sorry for that pun)! The first post if you haven't read it yet basically went over the evolution of integument in tyrannosauroids and my thoughts on it. However, this post we will completely ignore all of that nonsensical dribble and talk about an actually important aspect of their biology; their ecological roles and how that changed throughout their ontogeny. Ontogeny is really interesting, especially in dinosaurs. And furthermore, really interesting in tyrannosaurids. Yes, sadly, I’m only discussing this in tyrannosaurids this time, but that's just to make it easier on me.

A Brief Overview of Ontogeny in Dinosaurs

Since my last post went too in depth about sauropods, they will be excluded from this. From what we know of dinosaur ontogeny, they grew up very quickly, and reached sexual maturity rather early on in their lives, far before they reached skeletal maturity. Some of the more well known cases of dinosaur ontogeny include Triceratops, hadrosaurs, and pachycephalosaurs (all ornithischians). These all show rather drastic ontogenetic changes. In Triceratops, the young were born with tiny frills, small parietal fenestrae, a shortened snout, and tiny nubs over their eyes, which in time would turn into their horns. All hatchling dinosaurs display larger proportioned skulls and larger proportioned eyes, and depending on the dinosaur either ridiculously tiny limbs or ridiculously large ones.

Anyways, as Triceratops grew, they had a few basic trends. One is the horn shape, which in juveniles was curved backwards, then become more straight in subadults, and finally curved downwards in adults. Strangely, in the earlier stages of ontogeny (likely during the juvenile phase), the horns were actually very dense, much like those of ox and other large horned mammals. However, the horns became hollow as it reached adulthood, and actually became part of the sinus passages as well, an interesting feature that is still unpublished. The frill of Triceratops was very boxy and short in hatchlings, but became more flattened as it aged, and also reabsorbed the parietal fenestrae. The snout also became longer in older Triceratops specimens, possibly hinting at a change of diet? This is just one example of how drastic dinosaur ontogeny can be.

Pachycephalosaurs also have some cranial changes in ontogeny that are drastic. They seemed to have had flat domed heads, and as they grew they formed the bony dome. Thus, “genera” such as “Homalocephale” most definitely belong to others such as Prenocephale. In the case of Pachycephalosaurus wyomingensis, they started out with long skulls which were flat, but also filled with many ornamental horns and spikes and bumps on their skull as well. As they grew, the domes formed, and their spikes and ornamentation apparently gets reabsorbed by the surrounding bone as well. So, we do have fairly confident data that drastic ontogenetic changes did occur in dinosaurs. Because…

… this Post is All About Tyrannosaurid Ontogeny

Which in some cases is pretty drastic. I will be taking a look at the available data on the ontogeny for these animals. The SVP2016 abstract list included an abstract which talks about tyrannosaurid ontogeny and ecology and how these two go hand in hand. Basically it proposes that tyrannosaurids that lived alongside other large theropods did not change much during ontogeny, while those that were the only large predators in their environment did. And when looking closely it's fairly obvious. I’ll go through four different localities where we have a good idea of tyrannosaurid ontogeny and what ontogenetic changes occurred in the genera there.

Nemegt Formation

Juvenile A. remotus. By Emily.

Our first stop is the Nemegt Formation in Mongolia (which we talked about on the last tyrannosaur post). And here we have two large theropods (both tyrannosaurids) which coexisted with one another; Tyrannosaurus (Tarbosaurus) bataar, and Alioramus remotus/altai. Unfortunately we only have juvenile remains of A. remotus (a possible adult is known, but it's in private hands sadly), but I will use its sister taxa A. sinensis to help out with this.

Tarbosaurus bataar

T. bataar is known from a plethora of remains, including adults, subadults, and juveniles. The youngest Tarbosaurus specimen is 2 or 3 years old, and show a few differences from the adults; their tails seems to be a bit shorter, and their legs were also very long proportionally. Their skulls were slightly different as well, and their teeth were thin. These all suggest that the juveniles were doing something different from the adults. The long legs and thin teeth could be an adaptation to hunting small, swift prey such as lizards and mammals, which would make sense and thus isn't that surprising.

As Tarbosaurus grew into an adult, their bodies did become robust, their legs shortened, and their tail did lengthen. But otherwise they stayed very much the same throughout their ontogeny. Adult Tyrannosaurus bataar can be distinguished from Tyrannosaurus rex in several ways;

A) There is a higher tooth count in T. bataar and not in T. rex (making it an oddity among tyrannosaurids).

B) Looking at the metacarpals, Tarbosaurus does seem to have much smaller forelimbs than T. rex.

C) The skull bones on the upper and lower jaw of T. bataar show them to be far more rigid than in T. rex.

D) The back of the skull in T. bataar is not very expanded as in T. rex, which probably means it did not have binocular vision. It also makes the dorsal view of the skull somewhat reminiscent of carcharodontosaurids, but only superficially.

Tarbosaurus is indeed the largest predator in the Nemegt Formation, and thus was an apex predator. Most predators today will often feed on smaller prey more often than larger prey. In the case of Tarbosaurus, the limited binocular vision might have made it more difficult to go after smaller, swifter moving prey, although it definitely fed on them quite a bit. But the limited binocular vision might hint at an adaptation for going after larger prey, such as the derived titanosaur sauropods and the saurolophine hadrosaurs in the locality. The rigid upper and lower jaws also might be an adaptation for feeding on such large animals.

Restoration of a superadult Tarbosaurus. By Christian Halliwell.

A few bites marks are known from several of the large herbivores from the Nemegt Formation. These include bite marks from isolated bones from various hadrosaurs and titanosaurs which are presumably from Tarbosaurus, as well as bite marks on the gastralia of the holotype Deinocheirus. The snout is still pretty wide and the teeth are pretty thick, so they can still deliver a strong bite force, although given the lack of the expansion at the back of the skull, probably did not have exceedingly powerful bites like Tyrannosaurus.

Alioramus remotus

Now that we’ve talked about Tyrannosaurus (well, one of them), we can move on to its contemporary, the mysterious Alioramus. Only known from juvenile specimens, Alioramus remotus is still a very big mystery. It was actually assumed these were juvenile Tarbosaurus, until we found remains of juvenile Tarbosaurus. It usually includes two species; A. remotus and A. altai. However, naming an entire new species off juvenile remains is very problematic (see why later in the post), and A. altai does indeed look like a younger A. remotes.

Restoration of an adult A. sinensis. By Christian Halliwell.

Anyways, Alioramus is famous for its long snout and the bumps along the nasal ridge. The only adult Alioramus is A. sinensis, but this is still not from Nemegt and probably is a distinct species of Alioramus. However, like A. remotus, A. sinensis does have a long snout, which suggests that they changed very little through ontogeny. While Tyrannosaurus was the top predator of the ecosystem, Alioramus was likely smaller, and unfortunately we know very little about its ecology. A. sinensis does have long legs like A. remotus, so perhaps it was going after smaller, faster moving prey? After all, they didn't have the powerful mouths that other tyrannosaurines possess, so my bet is a focus on small prey and thus the need for long legs.

What This Means

Here we clearly see two tyrannosaurine species in the same place, both with different ecological roles, and changed very little through ontogeny. Alioramus being a fast predator of small things, and Tyrannosaurus being a robust predator… well, everything? Most things. Including large sauropods and hadrosaurs. This seems like this hypothesis has some merit to it.

Two Medicine Formation

Next up is the Two Medicine Formation, which hails for the mid Campanian of Montana, USA. Several theropod dinosaurs coexisted here, including Saurornitholestes langstoni, and Daspletosaurus horneri. Otherwise there is a distinct lack of theropods here, and here we have only on tyrannosaurid and one dromaeosaurid. How did these two interact?

Adult D. horneri skull.

Saurornitholestes langstoni

Saurornitholestes is a genus of dromaeosaurid, known from Alberta, Canada, and Montana, USA, from the Dinosaur Park and Two Medicine Formations, respectively. Saurornitholestes is average sized for a dromaeosaurid, only 2 metres long and weighing about 10 kilograms. While dromaeosaurs are depicted as pack hunting animals like wolves, taking down prey orders of magnitude larger than their size, the evidence has been very dubious and not well substantiated in recent years. Instead, it seems like mid sized dromaeosaurs like Saurornitholestes where mainly eating small mammals, lizards, baby dinosaurs, and on occasion scavenge and maybe take on prey slightly larger than itself. Thus, Saurornitholestes would've been a predator of small bodied prey.

Daspletosaurus horneri

Daspletosaurus horneri were the largest, and only large predator in the Two Medicine Formation, and thus would've been hunting far larger prey than Saurornitholestes. The adults are big, 8-9 metres and weighing 3-4 tonnes. Juvenile D. horneri specimens are known, although the only well preserved juvenile D. horneri skull is still unpublished. The juvenile remains show that when younger, D. horneri had a more gracile, narrow snout, with teeth which are flattened side to side. This is in contrast to the more robust adults, who have taller, more robust skulls, and are very robust, with incredibly short legs, the shortest known for any tyrannosaurid. This suggests that adult D. horneri were likely ambush predators, using their powerful bodies to hunt larger animals. The teeth and skull of juvenile D. horneri on the other hand, suggests a diet of smaller, probably fast moving prey.

Juvenile D. horneri skull, frustratingly unpublished.

What This Tells Us

Daspletosaurus is clearing the only large theropod from Two Medicine, the next largest predator being the rather abruptly small Saurornitholestes. Due to skull shape and dental differences in Daspletosaurus point to a change in niches as they grew, the younger juveniles possibly going after smaller, faster moving prey, and the robustly built adults going after larger prey, in addition to hunting juvenile dinosaurs, also going after adult Centrosaurus and Brachylophosaurus on occasion as well. Thus, this fits the pattern of tyrannosaurids not coexisting with large theropods having greater ontogenetic change.

Dinosaur Park Formation

We go to our third tyrannosaur fossil locality to look at another environment, another series of ontogeny, and two tyrannosaurids. While they predominantly lived without one another, they briefly overlapped in the middle Dinosaur Park Formation at least a million years or so. These tyrannosaurids are, of course, Albertosaurus libratus and a new unnamed species of tyrannosaurine. They were accompanied by two dromaeosaur species; Dromaeosaurus albertensis, and Saurornitholestes langstoni. Dromaeosaurus is the same length as Saurornitholestes, but much more robustly built, and thus probably heavier. Dromaeosaurus, due to their more robust nature, may have been ambush predators and tackled larger prey than Saurornitholestes. This itself shows niche partitioning in dromaeosaurs, but what about niche partitioning in the tyrannosaurids?

Albertosaurus libratus

I’ve briefly discussed A. libratus in the last post, but here I will go more in depth. Juvenile Albertosaurus are pretty gracile, as is the case in more juvenile tyrannosaurs, and have certain “babified” proportions such as larger orbits, longer legs, and shorter tails. Juvenile Albertosaurus also had teeth which were flattened side to side. This suggests that they were prey of fast, small moving prey, as seems to be the case in all tyrannosaur juveniles. Albertosaurus grew rather slowly and steadily, and within 14 years of age reached reproductive maturity. Adult A. libratus are indeed more robust than the juveniles, and despite being the same length as A. sarcophagus, is far heavier, around 2.5-3 tonnes as opposed to the 1.5 tonnes in A. sarcophagus.

However, despite being more robust, adult Albertosaurus did not differ much from juveniles. Yes their tails were longer, and their tooth count went down and the teeth were replaced by the typical conical esque teeth of tyrannosaurids, but their skull shaped changed very little, and they still retained the long hindlimbs the juveniles had. This perhaps suggests that they didn't change much in their ecology. Adult Albertosaurus may still have been ambush predators, but the long limbs suggest that speed was important, and thus may have relied less on ambush. Their diet would’ve included basically everything it can hunt and kill.

Dinosaur Park tyrannosaurine

Skull restoration of a juvenile specimen. Created by Ville Sinkkonen.

The next species is a tyrannosaurine. Initially reported as a species of Daspletosaurus by Carr, Currie has since then pointed out differences which do not put it in any known Daspletosaurus species, and may itself not even be a species of Daspletosaurus. Thus, for the sake of uncertainty, I shall refer to it as the Dinosaur Park tyrannosaurine. Regardless on what it is, the Dinosaur Park tyrannosaurine is very robust compared to the contemporary Albertosaurus, probably relying more on ambush and may have taken on larger prey than Albertosaurus. One juvenile specimen, TMP 94.143.1., clearly shows adult proportions, with the typical thickly built robust skull shape, and very much like an adult. This, as is in the case of Albertosaurus, shows the Dinosaur Park tyrannosaurine changed little throughout ontogeny.

What This Tells Us

Once again we have two tyrannosaurids in the same area at the same time which show evidence of little ontogenetic change, so as to avoid competition. However, there is an interesting proposal in regards to how the two coexisted. Albertosaurus remains are much more common in northern formations than the Dinosaur Park tyrannosaurine, which is seen in other dinosaur groups. So this further allowed the two to coexist with one another.

Hell Creek Formation

The Death(s) of Nanotyrannus

Before I proceed, I must take this time and use it wisely to just put an end on this stupidly stretched out debacle. Because the amount of wrong this is is unbelievably staggering. The “holotype” specimen of this dubious taxa is CMNH 7541, first described in 1946 as a new species of Gorgosaurus/Albertosaurus; “Gorgosaurus lancensis”. In 1988 the specimen was redescribed by Bakker et al., and was given a new genus name; “Nanotyrannus lancensis”. They thought it was an adult of around 5.2 metres, but this was rebutted in a 1999 paper by Carr, which strongly suggested it to be a juvenile specimen, and specifically proposed it was a juvenile Tyrannosaurus.

Then in 2001, a new specimen similar to CMNH 7541 was found, BMRP 2002.4.1. Nicknamed “Jane”, the specimen measured 6.5 metres and weigh anywhere from 639 to 1,269 kilograms. Even the colleagues in the 1988 redescription of CMNH 7541, like Phil Currie, and Michael Williams, used BMRP 2002.4.1. as evidence to suggest that “Nanotyrannus lancensis” is indeed a juvenile Tyrannosaurus rex. Since then many, many papers have come out about the validity of this taxa, and we’ll go through each argument for its validity and scrutinize it.

CMNH 7541 skull. By Witmer et al. (2010).

1. Nanotyrannus is an adult; when Bakker, Currie, and Williams redescribed CMNH 7541, they postulated it to be a small adult. However, upon reexamination of the skull of CMNH 7541, Carr failed to find anything suggesting to be an adult, citing out the lack of fusion in the areas Bakker et al. (1988) and Gilmore (1946) identified to be fusions. In addition, Carr found immature bone grain, and due to other similarities with juvenile Albertosaurus, such as flattened teeth, and a shallow dentary, showed CMNH 7541 to be a juvenile specimen. In addition, a 2015 study Holly Woodward Ballard looked at the growth rings in BMRP 2002.4.1., and found it to be around 11 years old, and that it too was an immature specimen that still had a lot of growing left to do. Another cited piece of evidence for maturity in “Nanotyrannus” is the presence of a small foramen in the quadratojugal is also found in the mature specimens of Daspletosaurus horneri, but these have since been rebutted as individual and/or ontogenetic variations, and in addition, not to be a good indicator of maturity in archosaurs.
2. Nanotyrannus had a higher tooth count than Tyrannosaurus; this argument is commonly used as evidence for validity in “Nanotyrannus”. In the two immature specimens we have show 14-15 teeth in the maxilla, and 17 teeth in the dentary, as opposed to the 11-12 teeth in the maxilla and 12-14 teeth in the dentary of mature Tyrannosaurus. Indeed, we do not seem to find this sort of tooth loss in Tyrannosaurus bataar. However, as Hone cited in his book, The Tyrannosaur Chronicles, T. bataar does seem to be an oddity in tyrannosaurids, with most tyrannosaurids, including A. libratus, having a similar but not as drastic tooth loss during ontogeny. And this goes even beyond tyrannosaurids. A 2017 paper described the ontogenetic growth of the ceratosaurian dinosaur Limusaurus. The young were born with a small beak at the tip of its snout, with a set of long teeth lining the jaws. As they grew, Limusaurus individuals slowly had the beak envelop their entire mouths, and in the process lost all of their teeth and filled their tooth sockets. Yes they aren't tyrannosaurids, but if you're using the argument that tooth loss can't explain the number of teeth in “Nanotyrannus” and Tyrannosaurus specimens, you clearly have no idea that the tooth loss in Limusaurus is by far way more drastic than what people propose for Tyrannosaurus. So does that mean baby Limusaurus are en entire different genera? To deny tooth loss in Tyrannosaurus you have to deny tooth loss in Albertosaurus and especially Limusaurus, and the latter is very much true.
3. Difference in brain shape; in 2009, Witmer and Ridgely published a paper where they CT scanned the braincase of CMNH 7541 and compared it to not only Tyrannosaurus but other theropods as well. What they found showed that the brain of CMHN 7541 was not only different from Tyrannosaurus, but different from all other tyrannosaurids. They also found that CMHN 7541 had its head pointed downwards in an alert position, unlike other tyrannosaurs as well. However, it is worth noting that; A) The skull itself is very crushed and distorted, B) this would make sense because they are going to mainly be hunting smaller things, and C) allows them to look over their snout so the snout length does not impair their vision. In addition, many other differences are found in the brain, but we also see those differences throughout the ontogeny of crocodylians and birds. Thus, the argument that the brain is too different from Tyrannosaurus is problematic.
4. Narrower skull; this isn't so much a debate in the palaeontological community as it is among less learned people. There is a misconception that “Nanotyrannus” has a generally narrower skull, but it is only the snout that is narrower, and not the back portion of the skull. Just wanted to get that out of the way briefly.
5. Different body proportions; another aspect that people who support “Nanotyrannus”’s validity point to, such as the long cursorial hindlimbs, the different shaped skull bones, and overall gracile body. First off, a gracile body is to be expected in younger animals, and they do get more robust as they get bigger. The long cursorial hindlimbs are not distinguishing features, as even juvenile Tarbosaurus have them. As for the skull shape, it is still not that drastic. From what we have of early juvenile Tyrannosaurus, they started out with small, short of shortened snouts. The snouts then elongated as they entered into late adolescence, and then became tall and robust as they reached adulthood. This remarkably similar to the ontogenetic stages of crocodylians today; baby crocodiles have short, snub snouts, which elongated as they age, then become progressively more robust as they approach adulthood. Because we even see a similar ontogenetic change in the skulls of extant archosaurs, this further supports the validity of “Nanotyrannus” being a juvenile Tyrannosaurus.
6. The Dueling Dinosaurs; in 2011, the Black Hills Institute unveiled a fossil which was originally excavated in 2006, which supposedly shows a Nanotyrannus specimen over 9 metres preserved with a new, undescribed ceratopsian. Larson also claimed this “Nanotyrannus” specimen had larger arms proportionally, and even stated they are larger than adult Tyrannosaurus arms. He used this to argue to demonstrate that “Nanotyrannus” was distinct from Tyrannosaurus. And then the fossil was sent off to sell at auction. Wait, what? That actually happened? Yes. Yes it actually did. The biggest problem is that it is no longer available for scientific study, and thus anything claimed by the BHI or Larson is untrustworthy, especially given the ethics of Larson and the BHI (for further reading you can look at these articles). Even if the arms are bigger, this is not that extreme of an ontogenetic change, and thus an invalid argument. Furthermore, the specimens aren't even confirmed to be from Hell Creek, and it is much more likely that they are from separate, earlier deposits. Unfortunately it is likely that Larson was hyping the fossil up for when it was auctioned (and fortunately not sold) at 6 million USD. But the fact that it wasn't sold is still disappointing, as there is still no way for it to be examined by palaeontologists. So, any mention of the Dueling Dinosaurs is instantly to be ignored.
7. Nanotyrannus convergently evolved juvenile Tyrannosaurus features; absolutely everything I’ve discussed about Nanotyrannus has thoroughly shown its invalidity and by extension, explains all of the supposed differences. But there is one more hypothesis that people have used to support the validity of “Nanotyrannus”. This one postulates that “Nanotyrannus” is a valid taxon which convergently evolved features of juvenile Tyrannosaurus. I'm not sure how to react to this… other than it's pure special pleading with absolutely no evidence backing it. First off, if it convergently evolved juvenile Tyrannosaurus traits, then how do the two niche partition? Oh, my fellow readers, do not worry, they have an explanation for this. The authors state that the orbits of “Nanotyrannus” are large and thus suggest nocturnality in “Nanotyrannus”, allowing the two to coexist. We know both BMRP 2002.4.1 and CMNH 7541 are juveniles so large orbits are to be expected in juvenile dinosaurs (juvenile animals in general), and if they weren't juveniles, large orbits still doesn't mean they are nocturnal, so the argument is illogical either way. They did not think that “Nanotyrannus” was a juvenile, citing in the specimen MOR 1125 as an example of juvenile Tyrannosaurus, as it is very young. Here's the issue; even though MOR 1125 is 14-16 years old, it is still older than BMRP 2002.4.1. by 3-5 years, and it still fits well within Tyrannosaurus growth patterns. By age 11-12, Tyrannosaurus began to grow rapidly, and despite cranial differences between BMRP 2002.4.1. and MOR 1125, the rapid change in skull build is easily explainable by the sudden growth spurt.
8. Almost forgot about the dental grooves!; another line of evidence that researchers use to support the validity of “Nanotyrannus” is the fact that the “Nanotyrannus” specimens supposedly have a deep groove along the tooth, which adult Tyrannosaurus supposedly lack and that albertosaurines have them, thus further validating it. The dental groove runs right into the neurovascular foramina, where the nerves control the blood vessels. However, a recent paper by Carr et al. (2016) showed that these dental grooves are in fact present on Tyrannosaurus, and even found to be widespread in not just tyrannosaurids, but even tyrannosauroids. During ontogeny, the groove is at first a sharp, deep impression, which as the animal ages turns into a more shallow indentation, proving it to be ontogenetically variable, again, destroying the argument that dental grooves support “Nanotyrannus” validity.

So yes, absolutely every single piece of evidence supposedly supporting “Nanotyrannus” as a distinct genus from Tyrannosaurus is either dubious at best, downright special pleasing at worst, and can easily be shot down with a few lines about ontogeny. I did spend a rather larger amount of time on this yes, but it is something that people still believe in, misinforming people when the evidence absolutely does not support its validity. In the media it is made out as super controversial, but the evidence in recent years have only strengthened its claim as a juvenile Tyrannosaurus, and not a distinct genus. The only true holdouts are Larson and his team, but their credibility has dropped since the auction of the Dueling Dinosaurs.

With That Out of the Way…

We can now take a look at the final tyrannosaurid fossil site, the infamous Hell Creek Formation, which covers Montana, North Dakota, and South Dakota, in the United States, which covers a million years, right up until the very end of the Maastrichtian. The environment was a coastal woodland floodplain, bordered to the west by the Rocky Mountains, and bordered to the east by the Western Interior Seaway. Such flora included Metasequoia, and beeches, as well as ferns, horsetails, and a relative of the modern day Araucaria. In the earlier part of the formation, it was warm temperate, about 7-11°C annually, but once the Deccan Traps started erupting around 66.2 million years, global warming rendered the MAT of Hell Creek to be 20°C, turning it into a more subtropical place.

Here, there are three predatory dinosaurs; Acheroraptor temertyorum, Dakotaraptor steini, and the infamous Tyrannosaurus rex. Acheroraptor is the smallest of the bunch, but Dakotaraptor is of considerable size. Thus, we will compare Tyrannosaurus to Dakotaraptor and how Tyrannosaurus ontogeny happened as a result of their coexistence.

Dakotaraptor steini

Despite Hell Creek having been studied well over a century, it only took 99 years into its research to find fossils of Dakotaraptor, which hints it at being a very rare part of the ecosystem. In the past, remains of “giant dromaeosaurs” have been known (I recall hearing this from the lead Saurian dev but I cannot recall, so take this statement cautiously), but with the description of Dakotaraptor, these remains are probably from Dakotaraptor itself. Dakotaraptor is only known from a few isolated teeth, as well as some limb bones and a few vertebrae. It is estimated that Dakotaraptor measured about 5.5-6 metres and weighed 140-230 kilograms. While not much is known about Dakotaraptor, phylogenetic placements puts it as the sister taxa of Dromaeosaurus. Dromaeosaurines are known to be very robust dromaeosaurids, and thus Dakotaraptor was likely taking on prey equivalent to or slightly larger than itself, so animals like Thescelosaurus and juvenile Edmontosaurus were definitely on the menu, as well as smaller animals if it could catch them.

Tyrannosaurus rex

Tyrannosaurus is by far not only the most well known dinosaur in media, but also the most well known and extensively studied dinosaur we know of. And known from many well preserved specimens. The youngest known Tyrannosaurus is ‘Biter’, but is unpublished. The next youngest published specimen is MOR 6625, only 2 years old and already 30 kilograms. Unfortunately, MOR 6625 is very fragmentary, only known from a few maxillary(?) pieces, and a more complete but still very much fragmentary dentary. So much of what we believe hatchling Tyrannosaurus looked like is based off its close relative, Tarbosaurus (for a description of what hatchling Tarbosaurus look like, see further up the post).

Fragments from MOR 6625 with an adult tooth for scale. Courtesy of Witmer Lab.

Then there is LACM 28741, also known as “Jordan”, which is about three years old, and slightly larger than MOR 6625. By this time they are already lithe animals with a long, narrow snout. For much of their juvenile stage, they were lithe animals with very long limbs, possibly being the fastest species in Hell Creek, with BMRP 2002.4.1. having an estimated speed of 50-56 kilometers an hour. Yes, they would be hunting small animals like juvenile dinosaurs, but they could've also hunted fast prey, such as the equally fast ornithomimids present.

A gravid MOR 1125. By Nik Holly

By age 14, Tyrannosaurus reached sexual maturity (for females anyways), which is correlated with a growth spurt at the same time, and also morphological changes. The skull (and overall body) becomes more robust, and beyond that as they reach adulthood. Adulthood isn't reached until about 19-20 years, and as they enter past the adult stage into the superadult stage (so around their thirties), they become increasingly robust. While FMNH PR 2081 is often cited as the largest and oldest Tyrannosaurus, is only potentially rivaled by “Scotty” from the Frenchman Formation. While unpublished, it appears to even more robust than FMNH PR 2081, which is already 8.5 tonnes, so the possibility of “Scotty” being around 9 tonnes is likely. A preliminary analysis also suggests “Scotty” to be 35 years old, 7 years older than FMNH PR 2081.

As an adult, Tyrannosaurus becomes much slower, and seemingly rely more heavily on ambush. Their teeth also change from thin and bladelike, to a more conical dentition, designed to crush through prey. Adult Tyrannosaurus probably ate absolutely everything in their environment, whether it be ankylosaurus, ceratopsians, or hadrosaurs. An interesting thing to note, is that juvenile Tyrannosaurus briefly fill the mid sized predator niche, putting it into competition with Dakotaraptor. With Dakotaraptor seemingly the rarest dinosaur in the environment, these interactions were probably few and far between, but when they did, it may have been as a way to get rid of one another's competition, in an act called intraguild predation.

Tyrannosaurus ontogeny by Christian Halliwell; 2 year old represents MOR 6625; 3 year old represents LACM 28741; 11 year old represents BMRP 2002.4.1.; 13 year old is based off an unnamed juvenile from the Los Angeles Natural History Museum; 18 year old is based off BHI 3033; 29 year old is based off FMNH PR 2081.

What This Tells Us

This tells us that while Tyrannosaurus was in competition with Dakotaraptor for a brief time, once reaching subadult to adulthood and onwards, they were the top predators. This once again shows the extreme ontogenetic changes in environments with only one large predator, Tyrannosaurus as a juvenile being the lithe, fast moving predator, and the adults being the slow robust ambushers.

Conclusions

With this analysis of the different tyrannosaurid fossil sites and species, the case for more drastic ontogeny when there are no other large predators around seems very probable, and thus, I strongly agree with the basic argument that this abstract was stating. This also further invalidates “Nanotyrannus” (as if there was anything more to invalidate), and thus explains while CMNH 7541 and BMRP 2002.4.1. are so different from FMNH PR 2081 and BHI 3033. This is a very interesting hypothesis and I do wonder if this extends to other large theropods as well. It does also appear to be basal to tyrannosauridae, since Bistahieversor also seems to show this drastic ontogeny too.

In a way, this also makes me think about Saurian; the developers considered the possibility that the reported “gracile and robust morphs” represent anagenesis, with specimens such as BHI 3033 and MOR 980 which are reportedly “gracile” seem to be from later deposits in Hell Creek. I used to think that way, until I dug deeper. After doing the extensive researching for putting this post together, I no longer consider this idea to be plausible. The main issue is that these “gracile morphs” like BHI 3033 are actually not fully grown, some being late subadults/early adults, only 17-18 years in age. In addition, no academic papers have yet performed such a survey, so we can't really say anything super confidently.

Anyways, this concludes my post! I might or might not do a third post in tyrannosaurids, maybe now, maybe later, maybe later. Really depends on what I feel like doing. Again I’d like to thank Christian for his constant amazing reconstructions he made specifically for this post, and my good friend Emily for making that adorable Alioramus restoration (sorry, it is a picture not a scan), and for my other friend Nik for making that gravid B-Rex illustration. You can find all them on deviantart.

References

• Horner, J. R., & Goodwin, M. B. (2006). Major cranial changes during Triceratops ontogeny. Proceedings of the Royal Society B: Biological Sciences, 273 (1602), 2757-2761.
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