My Prehistoric Saga: Did T. rex Appear During the Campanian?

Whenever I hear or read something about the timing of when T. rex appeared, I always hear 66, or 68-66, million years ago (late Maastrichtian) (as stated in Urban and Lamanna, 2006, p. 231). This is right at the very end of the Maastrictian. However, I've always imagined that T. rex had to have evolved at the beginning of the Maastrictian, which started 72 million years ago. In fact, not only do I seem to have been right, but it appears that T. rex appeared earlier than I ever thought... (At the very least, the genus Tyrannosaurus did, anyway. More on that below.)

Specimens:
1. CM 9401:
In 2006, a paper by Michael Urban and Matthew Lamanna describe an "isolated right lacrimal." This specimen, called CM 9401, was discovered in 1903 and is said to have probably originated from Willow Creek in the Judith River Formation of Fergus County, about 75 million years ago (pp. 231-232). Urban and Lamanna identified the specimen as ?Tyrannosaurus sp. (p. 232). Interestingly, the size of the lacrimal is about the same as the T. rex holotype's, CM 9380 (p. 233). I got a length of 41 feet (12.5 meters) for this specimen, compared to 42 feet (12.9 meters) for CM 9380.

CM 9401 (Urban and Lamanna, 2006, p. 232):

However, Urban and Lamanna do consider the possibility that the lacrimal might have been collected from the Hell Creek Formation in Montana, which is late Maastrictian in age. This is due to the fact that the location of where this specimen was collected from is not available, since no field documents of the expedition exist. If it was found in Montana, then chances are that this specimen dates back to the late Maastrictian. Still, the paper dates the fossil to the Campanian because the preservation of the fossil is consistent with other fossils collected from Willow Creek (Judith River) in 1903, and is not consistent with Hell Creek material from other Carnegie Museum collections. Despite this, Urban and Lamanna caution that the specimen's age might not ever be solved (p. 234).

However, according to Yun (2017), paleontologist Dr. Thomas Carr says that some of the lacrimal's preservation is artificial, and is not different from T. rex. (p. 3). Therefore, the lacrimal might belong to a new species of Tyrannosaurus, or it could be T. rex (p. 3). Dalman et al., (2018) says that the lacrimal matches the bones found in the Two Medicine Formation of Montana, which is older than the Judith River Formation, at 81-75 million years old (pp. 126 and 136). Therefore, CM 9401 seems to definitely come from the Campanian.

Update (7/1/20): Carr (2020) says that CM 9401 is more than likely Tyrannosaurus rex (Character polarity, para. 3; Wildcard specimens, para. 2; Table 1).

2. YPM VPPU 023469:
For a little while, I couldn't say with certainty that Tyrannosaurus lived during the Campanian. Then, I learned about a Tyrannosaurus tooth that is said to have come from the Campanian as well (Yun, 2017, p. 3). Eventually, I was able to find the paper that this tooth was described in. In Dalman et al., (2018), a premaxillary tooth named YPM VPPU 023469 was discovered in the Willow Creek Anticline of Teton County, Two Medicine Formation of Montana in 1980. The Two Medicine Formation is Campanian in age, 81-75 million years old (pp. 125-126).

YPM VPPU 023469 (Dalman et al., 2018, pg. 128 Figure 3 E-H):

The "morphology of the tooth matches the premaxillary teeth of T. rex because it, and T. rex's teeth, lacks the characteristic vertical ridge on the distal (back/rear) surface as seen in other tyrannosaurid teeth" (pp. 134 and 136), and "the distal surface at the base is convex and slightly flat at the tip" (p. 134). This, along with the lacrimal (CM 9401), seems to indicate that the genus Tyrannosaurus is older than previously estimated (p. 136).

Just to be safe, I decided to look up the geological setting/age of the Judith River and Two Medicine Formations. Just like Urban and Lamanna (2006), and Dalman et al., (2018), said, The Judith River (Arbor and Evans, 2017, Occurrence and Geological Setting) (Sankey et al., 2002, Abstract; Materials and Methods) (Donald B. Brinkman, 1990, Abstract), and Two Medicine (Jackson and Varricchio, 2009, Abstract) (Raymond R. Rogers, 1998, pp. 615, 617, and 620) (National Geological Database) Formations, are Campanian in age. According to the International Chronostratigraphic Chart (2019), the Campanian was 84-72 million years ago.

3. TSJC 2008.1:

Recently, I've found a paper, Berry (2008), that describes a fragmentary T. rex specimen from Colorado. The age of the fossil's location is dated to 71 Ma. The fossils consist of a partial tooth, and a fragment of the femur. The closeness of the LAGs indicate that the specimen is mature. The tooth is blunt, and has 12 denticles per 1 cm. This is classic for T. rex. The specimen is estimated to be 5.8 tons (5.23 metric tons), using Erickson et al., (2004)'s growth curve and based on the specimen's femur circumference (pp. 12-13). The weight of the animal could have been more.

Figure 1: Tooth (A), denticles (B), and femur fragment (C) (p. 12):

Figure 2 (p. 13):

Age (p. 12):

4. NMMNH P-1013/P-3698:
Specimen from Larson and Carpenter (2008) (p. 42):

I've known about this specimen for a while now. However, I didn't know that the date of this specimen has been changed. I first noticed this in Dalman et al., (2021). The McRae Formation, the location where this specimen was originally described from Gillete et al., (1986), is now the McRae Group (Lucas et al., 2019, p. 4 Figure 3). The Hall Lake Member, where the specimen was discovered from within the McRae Formation, is now the Hall Lake Formation (Lucas et al., 2019, p. 4 Figure 3; p. 27). The Hall Lake Formation is now 73.2 Ma (73.9-66 Ma in total) (Amato et al., 2017, p. 1213 Figure 3; p. 1218) (Lucas et al., 2019, p. 28) (Dalman et al., 2021, Geological Setting, para. 3). Lucas et al., (2019) does question this, but Dalman et al., (2021) says that more Campanian-aged radioisotopes have been collected (Geological Setting, para. 3).

Dalman et al., (2021) on Hall Lake Formation Campanian Age (Geological Setting, para. 3):

Dalman et al., (2021) calls the specimen cf. Tyrannosaurus sp., but there's little reason to suggest that this specimen isn't T. rex. Carr (2020) plots this specimen on the T. rex growth chart, and gives it an age of 24 (Figure 12 Number 20). The tooth count in the dentary (12), and a serration count of 6-10.5 per 5 mm (Gillete et al., 1986, pp. 235-236; p. 238 Table 1), places this specimen clearly within the genus T. rex. Carr and Williamson (2000) also places this specimen within T. rex (pp. 126 and 134).

We now have a good specimen of T. rex that came from the Campanian, which helps to solidify T. rex as a Campanian-Maastrichtian tyrannosaurid.

Invasive/Rare Species:
Very few fossils of T. rex (or just Tyrannosaurus) have been found from the Campania-era formations of North America. This could be due to a possible hypothesis that T. rex was an invasive species from Asia. According to paleontologists Dr. Carr and Steven Brusatte, T. rex is closely related to Zhuchengtyrannus and Tarbosaurus, and a land bridge between Asia and North America opened up 67 million years ago. This would have allowed T. rex to enter into North America (Geggel, 2016, para. 1-6). However, the presence of Tyrannosaurus fossils from the Campanian seems to show that T. rex appeared a lot earlier than that.

However, the fossils mentioned in this post indicate that T. rex was around during the Campanian. Other tyrannosaurids, like Gorgosaurus and Albertosaurus (or Albertosaurus libratus and sarcophagus), would have been its competition. Nanuqsaurus (or Albertosaurus/Tyrannosaurus hoglundi) could have been its competition as well. Better yet, Tyrannosaurus would have been its own enemy. Other Tyrannosaurus species, such as Daspletosaurus (or Tyrannosaurus torosus and (?)horneri), Bistahieversor (or Tyrannosaurus sealyi), Teratophoneus (or Tyrannosaurus currei), and Lythronax (or Tyrannosaurus argestes), would have competed with T. rex throughout the Campanian.

However, when the late Maastrichtian came along (68-66 Ma), T. rex might have driven the other tyrannosaurs into extinction, since it was bigger and more derived, and there are not other tyrannosaur genera around 68 Ma. This was also hinted by other paleontologists in 2016 (Geggel, 2016, para. 13). Dinosaurs also seemingly were going into decline around 68 Ma as well ().

Update (11/15/21):
In all honesty, there were other tyrannosauroids that coexisted with T. rex in the late Maastrichtian. One of them is Dryptosaurus (this includes "Nanotyrannus"), and a possible new tyrannosauroid that Dalman is currently describing. With T. rex/Tyrannosaurus appearing during the Campanian of North America, it seems that the genus may have spread from North America to Eurasia, or vice versa.

Conclusion:
It seems that the genus Tyrannosaurus, perhaps the species Tyrannosaurus rex in general, appeared during the Campanian of North America, 84-72 million years ago. This is much older than the 68-66-million-year time frame that T. rex, or Tyrannosaurus the genus, is usually given. Also during the Campanian, T. rex seems to have been a rare genus of tyrannosauroid, but it would become the top predator when the Maastrichtian came around, seemingly kicking other tyrannosauroids into extinction in the process.