Thoughts on Woodruff et al. (2024) paper on the size of Supersaurus and Diplodocus hallorum

I recall that when the giant diplodocids Supersaurus and Diplodocus (=Seismosaurus) hallorum were unearthed in the 1970s and 1980s, they were hailed as being the biggest sauropods that ever lived, with Supersaurus being initially regarded as a larger brachiosaurid than Brachiosaurus and Giraffatitan, while "Seismosaurus" was estimated to have a maximum length of 170 feet (52 meters), only for later studies to revise its estimated size. Recently, Woodruff et al. (2024) have come out with a new paper demonstrating that the gigantic body size of referred Supersaurus specimen (WDC DMJ-021) and the holotype of the Diplodocus hallorum can best interpreted as being a function of maturity rather than rendering these taxa uniquely super-sized among diplodocine diplodocids based on histological analysis. Because very large diplodocid specimens are quantitatively rare when compared to specimens of other diplodocids from the Morrison Formation, I will devote this post to offering feedback on the paper by Woodruff et al. (2024).

In the introductory section of their paper, Woodruff et al. place the taxonomic history of Supersaurus and "Seismosaurus" in the context of previous studies weighing in on the reliability of the size of dinosaur specimens as a clue as to their ontogenetic status, noting that Camarasaurus specimens NMZ 1000002 and GPDM 220 were initially interpreted as sub-adult but later re-assessed by Woodruff and Foster (2017) as very old individuals despite being smaller than the biggest Camarasaurus specimens. They bear in mind that fact that the size of Supersaurus vivianae was estimated by James Jensen based on the length of the holotype scapulocoracoid, which led him to hail Supersaurus as being one of the biggest sauropods, but their suggestion that referred Supersaurus cervical vertebra BYU 9024 (which Taylor and Wedel 2016 consider possibly a specimen of Barosaurus lentus) represents an individual measuring more than 164 feet (50 meters) long does not appear tenable in my opinion because of the biological limits to sauropod body size discussed by Woodruff and Fowler (2014), which prompted Carpenter (2018) to revise size estimates for Maraapunisaurus to 99-105 feet (30-32 meters). It should be noted that Diplodocus specimens AMNH 223, DMNS 1494, and USNM 10865 were seen as a possible distinct species of Diplodocus by McIntosh and Carpenter (1998), only to be later referred to D. hallorum by Tschopp et al. (2015), eliminating large size as a diagnostic trait for D. hallorum. By agreeing with Tschopp et al. that the varying body sizes of the four D. hallorum specimens render very large body size untenable as a criterion for diagnosing D. hallorum or S. vivianae, Woodruff et al. set the stage for investigation into whether the huge size of the D. hallorum holotype and the S. vivianae specimen WDC DMJ-021 could be age-related.

Comparison of tibia osteohistology of Supersaurus vivianae specimen WDC DMJ- 021 (marked with black numerals) and femur core section of Apatosaurus sp. OMNH 01991 (marked with white Roman numerals) under plane polarized light (from Woodruff et al. 2024).

The results of ontogenetic analysis of the Diplodocus hallorum holotype as well as WDC DMJ-021 by Woodruff et al. (2024) raise interesting points about how the growth stages of these specimens deduced by the authors stack up against those assessed for giant titanosaurs from Patagonia but also a few other giant diplodocoid specimens from the Morrison Formation. Similarities noticed by the authors between the outer cortices of elements of the D. hallorum holotype and those for Patagotitan and apatosaurine specimens OMNH 1991 and OMNH 4020 from the Morrison Formation of western Oklahoma leading to their conclusion that the D. hallorum holotype reached skeletal maturity may lend some support to Paul's (2019) estimate of a length of 102 feet (31 meters) for Patagotitan. Given that the D. hallorum holotype is incomplete, the ontogenetic status for D. hallorum inferred by Woodruff et al. makes an upper size estimate of 110 feet (33.5 meters) quite reasonable. On the other hand, when Woodruff et al. conclude that referred Supersaurus vivianae specimen WDC DMJ-021 is an extremely old individual, they also happen to mention that Supersaurus specimens from the Dry Mesa Quarry in southwestern Colorado (including the holotype scapulocoracoid BYU 9025) happen to be about the same size as WDC DMJ-021, and even though the authors conduct no histological analysis of Supersaurus vivianae specimens from the Dry Mesa Quarry, they demonstrate that a correlation between the proportions and old age of WDC DMJ-021 might make Supersaurus vivianae a bit oversized compared to Diplodocus hallorum. The size estimate for Argentinosaurus by Paul (2019) would have to be tested by histological analysis of known remains of Argentinosaurus to determine the ontogenetic stage of holotype of that taxon relative to that of WDC DMJ-021, but if Curtice's (2021) size estimate for Supersaurus holds water, then Supersaurus vivianae might have slightly dwarfed Diplodocus hallorum and possibly the biggest Patagonian titanosaurs, after entering the sub-adult phase because no immature specimens of S. vivianae have been found so far. 

When commenting on recent suggestions that large diplodocoid specimens from the middle and upper parts of the Brushy Basin Member of the Morrison Formation tend to be much larger compared to those from lower in the formation, Woodruff et al. (2024) do a good job of expressing openness to the notion that gigantism in the Diplodocus hallorum holotype and Supersaurus is not endemic to the middle and upper parts of the Brushy Basin Member and instead could be the result of skeletal plasticity and sexual dimorphism. By noting that Apatosaurus specimen MOR 957 is from the Salt Wash Member in spite of being similar in size to NMMNH P-3690 and apatosaurine specimen OMNH 1670 from the Morrison Formation of western Oklahoma, the authors implicitly agree with the observation by Carpenter (1998) that extremely large dinosaur specimens are distributed across the Morrison Formation instead of being confined to higher stratigraphic sections of the formation.   

In summary, the paper by Woodruff et al. constitutes an important first step in determining whether the gigantic size of a few documented diplodocoid specimens from the Morrison Formation is indicative of their unique gigantism, age, or other factors, given that no histological analysis of gigantic diplodocoid specimens from the Morrison Formation was undertaken before. Gigantism in diplodocoid sauropods is clearly sporadically distributed over the stratigraphic span of the Morrison Formation, and Woodruff et al. (2024) demonstrate ontogenetically the holotype of Diplodocus hallorum is about as big as some of the largest Patagonian titanosaurs despite being slightly bigger than D. carnegii and that Supersaurus could be a truly colossal diplodocid. By revealing the ontogenetic stages of NMMNH P-3690 and WDC DMJ-201, the paper by Woodruff et al. will set a precedent for future studies to reveal the ontogenetic status of Supersaurus specimens in the Dry Mesa Quarry (e.g. holotype scapulocoracoid and the type specimens of Ultrasauros macintoshi and Dystylosaurus edwini).

References:

Carpenter, K., 1998. Vertebrate biostratigraphy of the Morrison Formation near Canon City, Colorado. Modern Geology 23: 407–426.

Carpenter, K, 2018. Maraapunisaurus fragillimus, N.G. (formerly Amphicoelias fragillimus), a basal Rebbachisaurid from the Morrison Formation (Upper Jurassic) of Colorado. Geology of the Intermountain West 5: 227–244.

Curtice, B., 2021. New Dry Mesa dinosaur quarry Supersaurus vivianae (Jensen 1985) axial elements provide additional insight into its phylogenetic relationships and size, suggesting an animal that exceeded 39 meters in length. Society of Vertebrate Paleontology 2021 annual meeting, p. 92.

McIntosh, J.S., and Carpenter, K., 1998, The holotype of Diplodocus longus, with comments on other specimens of the genus. Modern Geology 23: 85–110. 

Paul, G.S., 2019. Determining the largest known land animal: A critical comparison of differing methods for restoring the volume and mass of extinct animals. Annals of the Carnegie Museum 85 (4): 335–358. doi:10.2992/007.085.0403

Taylor, M.P., and Wedel, M.J., 2016. How big did Barosaurus get? 64th Symposium on Vertebrate Palaeontology and Comparative Anatomy, Meeting Proceedings, p. 30.

Tschopp, E., Mateus, O., and Benson, R.B., 2015. A specimen-level phylogenetic analysis and taxonomic revision of Diplodocidae (Dinosauria, Sauropoda). PeerJ 3: e857.

Woodruff, D.C., Curtice, B.D., and Foster, J.R., 2024. Seis-ing up the Super-Morrison formation sauropods. Journal of Anatomy 1–17. https://doi.org/10.1111/joa.14108

Woodruff, D.C., and Foster, J.R., 2014. The fragile legacy of Amphicoelias fragillimus (Dinosauria: Sauropoda; Morrison formation–latest Jurassic). Volumina Jurassica 12: 211–220.

Woodruff, D.C., and Foster, J.R., 2017. The first specimen of Camarasaurus (Dinosauria: Sauropoda) from Montana: the northernmost occurrence of the genus. PLoS One 12: e0177423.