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What is the most likely attributable identity for the Tyrannosauroid remains at Phoebus Landing of the Tar Heel Formation?

 

What I am referring to specifically are bones documented in Baird and Horner's 1979 paper which speaks of a distal third of a right femur that is attributed to cf. Dryptosaurus and is smaller than the holotype of D. aquilunguis. It is also compared with Albertosaurus which shows similarities as well but that's expected with Eutyrannosaurs. 

 

Also there is another distal left femur of a tyrannosauroid shown to be found in a Hypsibema bonebed and originally attributed to the Hadrosaur taxon as a tibia but later revealed to be part of the femur of Tyrannosauroide

 

Keep in mind at the time they were attributed to tyrannosauridae until dryptosaurus was shown to be part of an outgroup from the main family.

 

My only question is, are these specimens possibly referrable to Appalachiosaurus instead of Dryptosaurus? It could be possible as the time of description of these specimens Appachiosaurus was not described yet.

 

There are a few flags that could make such bones the cf. Appalachiosaurus instead, like the more pronounced medial condyle than that of Dryptosaurus which is smaller and less noticeable. This I noticed with specimen ANSP 15330. Although it is overrall smaller than the Dryptosaurus holotype this could be just a sign of it being in a juvenile ontogenetic stage. And despite being smaller not only is it's medial condyle larger, the politeal pit and the intercondylar fossa are deeper and more prominent. 

Screenshot_20210714-002215_Drive.jpg

Although I cannot say the same for the larger left femur Cope mistakenly thought was a Hypibema tibia. This femur is larger than the Dryptosaurus holotype however it's fossa and processes aren't as pronounced. Although this could be attributed to the fact it is largely abraded and weathered.

Screenshot_20210714-005528_Drive.jpg

 

Here is the subadult Appalachiosaurus right femur from the holotype (ignore the left tibia included below it);

 

Screenshot_20210714-011111_Drive.jpg

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A very recent paper that reviews our current knowledge of material from Appalachia and includes the Tar Heel Fm.  Not sure you've seen it but makes comments on the bones from that area.

 

Excerpt from paper:

The theropod dinosaurs also left behind an extensive record at Phoebus Landing. Hindlimb material comparable to Dryptosaurus aquilunguis was noted by Baird and Horner (1979). The femoral material compared to D. aquilunguis and figured by Baird and Horner (1979) may show an autapomorphic feature of this taxon. This is the presence of an ovoid fossa on the medial surface of the femur just above the distal condyles (Brusatte etal., 2011). More recently, Weishampel and Young (1996) also regarded Dryptosaurus aquilunguis as present at Phoebus Landing. This large tyrannosauroid theropod is known from a holotype specimen from the New Egypt Formation of New Jersey (e.g., Brusatte et al., 2011). In addition to Dryptosaurus aquilunguis, material assigned to the tyrannosauroid dinosaur Appalachiosaurus montgomeriensis was reported from Stokes Quarry by Schwimmer et al. (2015). Some of these elements may be from juvenile individuals (Schwimmer et al., 2015). Therefore, two large tyrannosauroid dinosaurs were present in the Tar Heel-Coachman non-avian dinosaur fauna.

 

https://www.researchgate.net/publication/323059336_The_biogeography_and_ecology_of_the_Cretaceous_non-avian_dinosaurs_of_Appalachia

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AlexSciChannel
10 hours ago, Troodon said:

A very recent paper that reviews our current knowledge of material from Appalachia and includes the Tar Heel Fm.  Not sure you've seen it but makes comments on the bones from that area.

 

Excerpt from paper:

The theropod dinosaurs also left behind an extensive record at Phoebus Landing. Hindlimb material comparable to Dryptosaurus aquilunguis was noted by Baird and Horner (1979). The femoral material compared to D. aquilunguis and figured by Baird and Horner (1979) may show an autapomorphic feature of this taxon. This is the presence of an ovoid fossa on the medial surface of the femur just above the distal condyles (Brusatte etal., 2011). More recently, Weishampel and Young (1996) also regarded Dryptosaurus aquilunguis as present at Phoebus Landing. This large tyrannosauroid theropod is known from a holotype specimen from the New Egypt Formation of New Jersey (e.g., Brusatte et al., 2011). In addition to Dryptosaurus aquilunguis, material assigned to the tyrannosauroid dinosaur Appalachiosaurus montgomeriensis was reported from Stokes Quarry by Schwimmer et al. (2015). Some of these elements may be from juvenile individuals (Schwimmer et al., 2015). Therefore, two large tyrannosauroid dinosaurs were present in the Tar Heel-Coachman non-avian dinosaur fauna.

 

https://www.researchgate.net/publication/323059336_The_biogeography_and_ecology_of_the_Cretaceous_non-avian_dinosaurs_of_Appalachia

I believe I am aware of the paper. However it seems that if this was the case that would mean Dryptosaurus would've lived from the Early Campanian to the Late Maastrichtian which is pretty extraordinary and almost unheard of such an occurrence where a Cretaceous terrestrial taxa has lived a through two full ages without going extinct. On top of that Dryptosaurus has only been found in Maastrichtian aged rocks and as far as I know this is the only instance where an Early-Mid Campanian specimen was attributed to it. It just seems pretty unlikely to me.

 

To add on there's somewhat conflicting with information of previous papers some of which it references. for example the paper by Baird and Horner make no mention of a ovoid fossa above the medial condyle, which can be understandable since there were no other known Tyrannosaurs from Appalachia at the time so finding autopomorphic features may not have seemed necessary. But then we get to (Brusatte et al. 2011). In this paper this I believe is the first mention of this feature being autopomorphic however the paper mainly focuses on the Dryptosaurus holotype and only sparsely mentions the Phoebus Landing specimen and when it does, it is intended to be distinguished from the holotype.

 

(Brusatte et al. 2011)

Beginning in the middle of the 20th century, and especially gaining steam in the 1970s, paleontologists began to recognize similarities between Dryptosaurus and the characteristic Late Cretaceous tyrannosaurids Albertosaurus and Tyrannosaurus from western North America (e.g., Gilmore, 1946; Steel, 1970; White, 1973). Baird and Horner (1979) formally referred Dryptosaurus to Tyrannosauridae, but this was based on the tyrannosaurid affinities of a femur from North Carolina that was tentatively referred to Dryptosaurus, not the holotype.

___________________________________________________________________________________________________________________________________________________________________________________________________

This excerpt was used to disprove any referral of Dryptosaurus to tyrannosauridae just because a separate bone conferred to Dryptosaurus was described to have similarities to Albertosaurines. And of described tyrannosauroid material in Appalachia, femoral morphology that are most conferred to Albertosaurines is Appalachiosaurus (Carr et al. 2005).

 

for comparison here are the distal ends of the Dryptosaurus holotype, Phoebus Landing specimen, Gorgosaurus libratus and the Appalachiosaurus holotype. All shown through the medial view as that's where it seems agreed upon the most diagnostic features of Appalachian Eutyrannosaurs occur.

 

Not to scale

501206983_DryptosaurusHolotypecast.PNG.86777b5b392839703381957d2cb4a0d5.PNG 1471337117_AppalachiosaurusHolotype.PNG.59eac0cc8db1d319ea391965dcb0e8fd.PNG 458238180_PhoebusLandingEutyrannosaur.PNG.7f30e55b942005611ca51cbe5587a7c0.PNG

Left Femur, Dryptosaurus aquilunguis ANSP 9995 (Holotype)    Right Femur, Appalachiosaurus montgomeriensis RMM 6670 (Holotype)    Right Femur, Phoebus Landing Eutyrannosaur ANSP 15330

 

1107498163_GorgosaurusLibratus.PNG.b6ea741f24142428e9bc532d9943b1c3.PNG

   Left Femur, Gorgosaurus Libratus Source; T. Cullen

 

As you can see despite some being left associated and right associated, the Dryptosaurus holotype has different morphological tendencies when compared to the other three. On the anterior end it seems pretty homologous. The anteromedial process on each is smaller than the crest on the anterolateral side or the same size in the case of the Gorgosaurus. But only the Dryptosaurus anterior concaves inward. It is more noticeable on the cast than in the real fossil from (Baird an Horner et al. 1979) due to the original suffering from pyrite's disease. The cast also better preserves the anterolateral crest. But I digress, on none of the other three does the anterior end concavely. Not only that but on the ventral end we get some real differing features.

 

Lets start with Appalachiosaurus, as seen from the pictures as well as descriptions (Carr et al. 2005) the holotype femur has a greatly exaggerated internal medial condyle to the point where the lateral condyle ventrally seems almost nonexistent. The Phoebus Landing tyrannosaur also shares this pattern where it's intermedial condyle is noticeably larger although obviously not as pronounced as the RMM 6670 holotype, although this could be simply attributed to being an earlier ontogenetic phase if they are indeed the same taxa. They both also have a deeper popliteal fossa no doubt from the larger medial condyle. Gorgosaurus Libratus also has this same occurrence. But for Dryptosaurus something very wonky is going on. The medial inner condyle is short and interestingly it's most elevated point is less distal and closer to the shaft. When crossing diagonally across the shaft to merge with the lateral condyle it is noticeably a more prominent ridge buttressing the lateral crest on the anterior side (Baird Horner 1979).

 

And when it comes to the ovoid fossa along the medial surface (Brusatte et al. 2011) I am skeptical that it is specifically an autopomorphic feature to Dryptosaurus. As not only does Gorgosaurus have this feature, there isn't enough described tyrannosaur femoral material from Appalachia to make a precise assessment. Even the femur from the Appalachiosaurus holotype, it's nearly impossible to tell whether this genus possessed the same fossa due to the heavily disarticulated nature of the specimen.

 

My personal conclusion is that the Phoebus Landing tyrannosaur femur doesn't belong to Dryptosaurus. It's of an Appalachiosaurus of a younger ontogenetic stage or it's an as of yet undescribed taxa that is phylogenetically either in the same clade as Appalachiosaurus and Tyrannosauridae, more derived from D. aquilunguis or shares a separate clade with Appalachiosaurus itself.

 

My educated guess for a phylogenetic tree:

2069562390_Eutyrannosauriaphylogenetictree.PNG.175ecf1ebb20e9da289f0f58c6450dd6.PNG

 

Please let me know if I missed something here. I could very well be wrong and ANSP 15330 is actually a Dryptosaurus or perhaps a different species or part of a theoretical Dryptosauridae family along with D. aquilunguis that is less derived from Appalachiosaurus.

 

 

 

Edited by AlexSciChannel
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Suggest you take you hypothesis to the author of that paper or someone very familiar with eadtern tyrannosauroids.

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