Written from 8/3/22-2/9/23.
Link:
https://www.academia.edu/96574784/Nanotyrannus_is_Dryptosaurus_An_Abstract
“Nanotyrannus” is Dryptosaurus: An Abstract
This is an updated version of an abstract written in 2022.
Dryptosaurus aquilunguis is a tyrannosauroid from the late Maastrichtian of Eastern North America, also known as Appalachia. So far, only one good specimen, the holotype ANSP 9995, has been found for the genus. A few teeth have been assigned, but no relatively complete specimens have been described yet. However, after an exhaustive examination of the controversial “Nanotyrannus”/juvenile Tyrannosaurus rex specimens, this author is going to introduce a new hypothesis: Dryptosaurus lived in Appalachia and Laramidia towards the end of the Maastrichtian. The tyrannosauroid specimens previously labeled as “Nanotyrannus” are either cf. Dryptosaurus aquilinguis, or a sister taxon to Dryptosaurus and the two genera form a clade within tyrannosauroidea. This author tends to lean more towards “Nanotyrannus” being cf. Dryptosaurus aquilinguis. Both Dryptosaurus and “Nanotyrannus” lived during the same time. Numerous publications have suggested that Laramidia and Appalachia reconnected when the Western Interior Sea subsided in the Maastrichtian. Both Laramidia and Appalachia seemed to have had similar fauna: lambeosaurs, ceratopsians, and mosasaurs. Ceratopsids, in particular, were thought to have not existed in Appalachia. However, a ceratopsian tooth has been found in the Maastrichtian-aged Owl Creek Formation, which is in Appalachia. If animals in Laramidia can be found in Appalachia, and vice versa, then Dryptosaurus could’ve migrated into Laramidia. Dryptosaurus and “Nanotyrannus” share many physical characteristics, and the “Nanotyrannus” specimens have many traits not seen in T. rex. A few traits include:
1. “Nanotyrannus” had a first maxillary tooth that was incisiform. This morphology is also seen in premaxillary teeth. The “Nanotyrannus” specimens lack serrations, and have two carinae on the distal side of the premaxillary, and first maxillary, teeth. This trait is not present in T. rex. Specimen UCMP 119853, a 8-mm long tooth that likely belongs in the premaxillary position, has one serrated carina that is located on the labial and lingual positions of the crown. This morphology is seen in larger T. rex specimens, showing that T. rex’s tooth morphology didn’t change during ontogeny. More than likely, the first maxillary tooth in young T. rex specimens wasn’t similar to the “Nanotyrannus” or Dryptosaurus specimens.
2. The maxillary, and perhaps dentary, teeth in both Dryptosaurus and “Nanotyrannus” were pinched on the labial and lingual sides in cross-section, and the denticle morphology was “hook-shaped.” T. rex’s cross-sections were oval-shaped, and the denticles were square/rectangular in shape.
3. The dentary in the “Nanotyrannus” specimens had the lingual bar covering the first alveoli on the medial side, as seen in more basal tyrannosauroids like Appalachiosaurus, Gorgosaurus, Albertosaurus, and Bistahieversor. Since Dryptosaurus is a basal tyrannosauroid as well, it’s more than likely that it had this trait too. The derived tyrannosaurinae taxa T. rex, T. bataar, and Daspletosaurus had the lingual bar covering the first two alveoli. It seems that the lingual bar’s position can help differentiate between basal tyrannosauroids and tyrannosaurids from the derived tyrannosaurinae. The 3-4-year old T. rex specimen “Baby Bob” had the lingual bar covering the first two alveoli, as seen in the adult specimens. This alone places the “Nanotyrannus” specimens in basal tyrannosauroids or tyrannosaurids, not tyrannosaurinae.
4. The tooth count in the dentaries of baby-juvenile specimens of T. rex were the same as the adults, and the maximum count seems to be fifteen. This is also seen in T. bataar. The 3-4-year old specimen “Baby Bob” had 12 teeth in its dentary, as in the slightly larger baby/juvenile specimen BHI 6439. The 18-year old specimen BHI 3033 had 13. The 23-year old specimen CM 79057 ("Samson"), and PARC-TD-11-094/FDM-xx? (perhaps a subadult/adult individual), had 15. “Nanotyrannus” had 16-17. There is no sign of tooth loss during ontogeny.
5. The “Nanotyrannus” specimens have a non-incisiform, conical-shaped first dentary tooth with carina/carinae on the mesial and distal positions. This is also seen in Albertosaurus, but not in Gorgosaurus. T. rex’s first dentary tooth has the same morphology seen in the premaxillary teeth (incisiform, with a serrated carina on the labial and lingual faces). This is seen in the 3-4-year old “Baby Bob,” and the the 18-year old BHI 3033. The first dentary tooth never changed morphology as the T. rex individual aged, just like the premaxillary teeth.
6. Both genera have similar caudal vertebrae morphologies. In terms of the vertebrae count, it seems to be about 25 or so, as suggested by Cope. T. rex and Tarbosaurus/Tyrannosaurus bataar have 40 or more caudal vertebrae, and this is also seen in the young T. bataar specimen PIN 552-2. Whether or not this number is permanent doesn’t concern this author, but the morphology of the vertebrae does. From the middle to the distal portion of the tail, both Dryptosaurus and “Nanotyrannus” have elongated caudals that also lack transverse processes. T. rex’s caudals became shorter up to the very last caudal, and most of the caudals up until the last distal members had transverse processes. The middle-distal caudals were longer in Dryptosaurus than T. rex’s, and “Nanotyrannus’” looks to be the same.
7. Morphology of the arms of both genera are identical. The deltopectoral crest is positioned lower from the proximal end of the humerus, and faces different directions when viewed in multiple positions than in T. rex’s and other tyrannosauroids. For example, in proximal view, the deltopetoral crest in Dryptosaurus and “Nanotyrannus” faced anteriorly compared to the laterally-facing crest in T. rex. The manual phalanx 1-1 of Dryptosaurus and “Nanotyrannus” are extremely elongated, and this is an autapomorphy of Dryptosaurus. However, Megaraptor has this as well. Other tyrannosauroids like T. rex (9.85 cm), Gorgosaurus (9.8 cm), Albertosaurus (8.5 cm), and T. bataar (5.4 cm) have smaller manual phalanx 1-1 bones compared to Dryptosaurus’ (16 cm) or “Nanotyrannus’.” The manual unguals of the two genera are large and comparable in morphology and size, contra to T. rex’s and T. bataar’s short manual unguals.
8. Both Dryptosaurus and “Nanotyrannus” were agile tyrannosauroids throughout their biogenic existence, based on their hindlimb proportions. The tibia in the “Nanotyrannus” specimens are longer than the femur. The Dryptosaurus holotype seems to have a femur and tibia that were either equal in length, or the tibia was slightly longer than the femur. Dryptosaurus’ tibia is slightly eroded, so it might’ve been longer in life. Regardless, the femur and tibia of the “Nanotyrannus” specimens may have grown to become equal in length when they reached maturity, as possibly seen in Dryptosaurus. Or, both Dryptosaurus and “Nanotyrannus” had a tibia longer than the femur. Baby-juvenile specimens of T. rex and T. bataar had femora and tibiae that were about equal in length to each other, while the adults had longer femora. The 3-4-year old ”Baby Bob” demonstrates this. Body mass could account for this. Dryptosaurus and “Nanotyrannus” were small-medium-sized tyrannosauroids, so having a longer tibia, or a femur and tibia of equal length, could’ve meant a speedier lifestyle for the two genera. Another example of this is the adult Qiazhousaurus/Alioramus sinensis specimen, which also had a longer tibia compared to its femur. Larger adult tyrannosauroids had more massive bodies, which resulted in larger femora to support their weight. Indeed, baby-juvenile specimens of large tyrannosauroids had the femora and tibiae at about an equal length, but this changed during ontology. This doesn’t seem to be the case for Dryptosaurus, nor “Nanotyrannus.”
The fact that there are baby-juvenile specimens of T. rex that have traits not seen in the “Nanotyrannus” specimens, but are seen in the older specimens, show that “Nanotyrannus” doesn’t belong in the genus Tyrannosaurus. The stratigraphic and geographic correlations, as well as the similarities in characteristics with Dryptosaurus, show that “Nanotyrannus” is a more basal tyrannosauroid than a derived tyrannosaurinae. Further analyses will be conducted in the future to explore these characteristics in more explicit detail.
