Thoughts on Peterson et al. (2022) paper regarding diplodocid feeding mechanisms

Many scientific papers have published regarding the degree of tooth replacement and wear in diplodocoid and macronarian sauropods, namely diplodocids, Camarasaurus, brachiosaurids, and select titanosaurs for which skull material is known. However, very little attention until recently was paid the patterns of tooth replacement and wear in apatosaurine diplodocids, in no small part because the Apatosaurus louisae skull CM 11162, the undescribed Apatosaurus ajax specimen CMC VP-7180, and the apatosaurine specimen TATE 099 found at the Nail Quarry in Como Bluff, Wyoming in 1996, constitute the only apatosaurine skulls that preserve complete teeth (known cranial material for the Apatosaurus ajax holotype includes a braincase and two quadrates, but that's another story). Recently, Peterson et al. (2022) published a paper describing in detail for the first time TATE 099 from Nail Quarry, offering a comprehensive analysis of patterns of tooth wear and replacement for this specimen and further illuminating the nature of feeding mechanisms among flagellicaudatan diplodocoids. 

The history of the systematic placement of TATE 099 within Diplodocidae is rather interesting. It was first referred to as Apatosaurus sp. when first reported in an abstract by Connely and Hawley (1998), who suggested that Apatosaurus used its jaws in front to back sliding motion to aid in cropping and biting when eating plants. For his part, Bakker (1998) referred TATE 099 to Brontosaurus excelsus and used this specimen to claim that B. excelsus differed from Apatosaurus in having the basitubera situated far behind the occipital condyle, but provided no justification as to why TATE 099 was conspecific with B. excelsus, and due to a lack of overlapping material between TATE 099 and B. excelsus holotype, this taxonomic referral was basically ignored by many authors (e.g. Upchurch et al. 2004; Tschopp et al. 2015). In any case, Peterson et al. point out that the basitubera of Apatosaurus louisiae are positioned anterior to the occipital condyle, similar to the condition in TATE 099, and I'm heartened that the authors recognized Bakker's (1998) referral of TATE 099 to Brontosaurus excelsus as lacking basis, and instead assign it to Apatosaurus sp. based on a widely diverging basipterygoid process greater than 60 degrees and the absence of a basisphenoid/basipterygoid recess, both of which are listed as diagnostic characters for Apatosaurus by Tschopp et al. (2015).

CT scans of the maxillae and premaxillae (left) and dentary (right) of Apatosaurus specimen TATE 099 showing unerupted teeth (from Peterson et al. 2022). 3D scans of the unerupted premaxillary/maxillary and dentary teeth shown at the bottom of figures 10 and 11 in Peterson et al. (2022) are included for convenience.

The discussion section of the paper by Peterson et al. (2022) constitutes the real focus on deciphering the rate of tooth wear and replacement among diplodocids. The unerupted tooth counts for TATE 099 reported by the authors in their description of TATE 099 are 5-8 unerupted tooth crowns per alveolus in the premaxilla, 3-5 unerupted tooth crowns per alveolus in the maxilla, and 1-3 unerupted crowns per alveolus in the dentary. Even though Peterson et al. observe differences between diplodocoids and macronarians in the number of replacement teeth as well as tooth volume, shape, and replacement rate, it is quite noteworthy how the authors point out that the rebbachisaurid Nigersaurus lacks alveolar septae and has smaller replacement teeth than those of diplodocid taxa despite the both Nigersaurus and TATE 099 being comparable in the maximum number of replacement teeth. However, the authors don't comment on how the number of replacement teeth of the dentary in TATE 099 compares with that of the holotype of the rebbachisaurid Lavocatisaurus from late Aptian-early Albian deposits in Patagonia, because the latter taxon has several replacement teeth preserved in the dentary. When noting that the numbers of premaxillary/maxillary replacement teeth for TATE 099 as well as Apatosaurus premaxilla MWC 8430 and maxilla MWC 6002 described from Mygatt-Moore Quarry in Colorado by McHugh (2018) are similar, but that dentary of TATE 099 has a number of replacement teeth comparable to those of Dicraeosaurus and the macronarian Brachiosaurus, Peterson et al. (2022) propose that diplodocid taxa retained similar numbers of replacement teeth in the dentary, and interpret the higher numbers of replacement teeth in the upper jaw of Apatosaurus compared to those of diplodocines as supporting the hypothesis by McHugh (2018) that the upper jaw of Apatosaurus was more accustomed to crushing tough plant leaves than that of Diplodocus. In retrospect, the difference between apatosaurines and diplodocines in how they used their upper jaw to chew on plant material may provide another hint at morphological differences between these two diplodocid clades, because some distinguishing features between Apatosaurinae and Diplodocinae can be found in the cervical vertebrae (see Tschopp et al. 2015). Although the erupted tooth row of TATE 099 was unavailable for study by Peterson et al., the authors provide a succinct analysis of the degree of tooth replacement in this specimen based on examination of high-fidelity casts of the erupted tooth row and comparisons with the tooth rows of other sauropods because they note that the row-set tooth replacement patterns in diplodocid taxa like  Apatosaurus, Diplodocus, and Galeamopus contrast with the alternating tooth replacement patterns of the basal macronarian Camarasaurus, since the tooth row of TATE 099 has an uneven occlusal margin that creates an uneven distribution of wear facets on the teeth. 

References:

Bakker, R.T. 1998. Dinosaur mid-life crisis: The Jurassic-Cretaceous transition In Wyoming and Colorado. New Mexico Museum of Natural History and Science Bulletin 14:67-76.

Connely, M.V. and Hawley, R. 1998. A proposed reconstruction of the jaw musculature and other soft cranial tissues of Apatosaurus. Journal of Vertebrate Paleontology 18 (suppl. to volume 3): 35A.

McHugh, J.B. 2018. Evidence for niche partitioning among ground-height browsing sauropods from the Upper Jurassic Morrison Formation of North America. Geology of the Intermountain West 5:95-103.  https://doi.org/10.31711/giw.v5.pp95-103

Peterson, J. E., Lovelace, D., Connely, M., and McHugh, J.B., 2022. A novel feeding mechanism of diplodocid sauropods revealed in an Apatosaurine skull from the Upper Jurassic Nail Quarry (Morrison Formation) at Como Bluff, Wyoming, USA. Palaeontologia Electronica 25(2):a21. https://doi.org/10.26879/1216. palaeo-electronica.org/content/2022/3653-apatosaurine-feeding-mechanism

Tschopp, E., Mateus, O., and Benson, R.B.J. 2015. A specimen-level phylogenetic analysis and taxonomic revision of Diplodocidae (Dinosauria, Sauropoda). PeerJ 3:e857.  https://doi.org/10.7717/peerj.857

Upchurch, 

P., 

Tomida, 

Y., and 

Barrett, 

P.M., 2004. 

A new specimen of Apatosaurus ajax (Sauropoda: Diplodocidae) from the Morrison Formation (Upper Jurassic) of Wyoming, USA. 

National Science Museum Monographs 

26: 

1–

107

.