Ardetosaurus and implications for degree of diplodocine diversity in the Morrison Formation

Since the publication of the paper by Tschopp et al. (2015), a number of individuals (e.g. Prothero 2019) have had a number of misgivings about the taxonomic scheme for Morrison diplodocids in Tschopp et al. (2015) by arguing that ecosystems in western North America was not suited to support a great many taxa of diplodocids because of Morrison diplodocids being oversplit. However, this opinion overlooks the fact that the Morrison Formation itself spans 10 million years and that the presence of Haplocanthosaurus in the type locality as Supersaurus (Boisvert et al. 2024) demonstrates the co-existence of diplodocids with basal diplodocoids in the lower part of the Brushy Basin Member of the Morrison Formation. Recently, van der Linden et al. (2024) have described a new taxon Morrison diplodocine diplodocid, Ardetosaurus viator, from the Howe-Stephens Quarry in northern Wyoming, increasing the diversity of diplodocines in the Morrison Formation to eight. Therefore, I will take the liberty of the bearing of Ardetosaurus upon the question of the true number of diplodocine diplodocids from the Morrison Formation.

Skeletal reconstruction of Ardetosaurus viator (after van der Linden et al. 2024)

In their paper, van der Linden et al. neglect to provide a detailed cladistic analysis because they note that the description of Ardetosaurus viator is part of a broader collaborate project regarding the systematics of Diplodocoidea. It is understandable that the authors refrain from testing the cladistic position of the new taxon because their planned collaborative phylogeny of Diplodocoidea incorporates new diplodocoid specimens, but exhaustive comparison by the authors of Ardetosaurus with other diplodocines reveals hints about possible cladistic position of Ardetosaurus relative to other members of Diplodocinae. For instance, as noted by van der Linden et al., Ardetosaurus is similar to Barosaurus lentus in having elongated cervical vertebrae, so there is a possibility that Ardetosaurus itself could be more closely related to Barosaurus and Supersaurus than to other Morrison diplodocines. If so, then those three taxa (and possibly Dinheirosaurus) most likely form a distinct clade of diplodocines with elongated necks, and diplodocines from the Morrison Formation might have exhibited a degree of cladistic diversity with respect to evolving a variety of neck lengths in response to the emergence of different types of ferns and cycads in Morrison times. After all, the presence of four different taxa of dicraeosaurids in the Morrison Formation (see Whitlock and Wilson Mantilla 2020) may suggest a similar diversity pattern of diplodocine diplodocids in the Brushy Basin Member of the Morrison Formation as a result of dynamism of the Morrison ecosystems by the late Kimmeridgian.

Given the assignment of Ardetosaurus to Diplodocinae by van der Linden et al. (2024), it is important to note that the interpretation by Maidment and Muxworthy (2019) of the Morrison Formation as being more dynamic, more spatially varied, and temporally changing over the time it was deposited begs the question of the possible upper limit of diplodocine diversity in the Morrison. While the diversity of Morrison diplodocines exceeds the number of valid apatosaurine species, van der Linden et al. note that ontogeny and stratigraphy of the Morrison Formation may affect estimates of diversity of diplodocines throughout the Morrison Formation. Indeed, the type locality of Ardetosaurus viator is low in the upper part of the Salt Wash Member of the Morrison Formation, whereas Supersaurus vivianae has only been found in the lower part of the Brushy Basin Member, and Galeamopus remains occur in both the Salt Wash and lower Brushy Basin Members. The holotype of Ardetosaurus viator is assessed by van der Linden et al. (2024) as mature, in contrast to the holotype of Kaatedocus siberi being a juvenile, so the low stratigraphic position of Howe-Stephens Quarry compared to localities which have yielded known remains of Barosaurus, Diplodocus, Galeamopus, and Supersaurus may indicate that diplodocine diversity in the Salt Wash Member was low compared to a gradual increase over the timespan of the Brushy Basin Member. In other words, the evolution of Morrison ecosystems noted by Maidment and Muxworthy (2019) over the timespan of the Morrison Formation indicates that the relative growth in the diversity of Morrison diplodocines increased over time in response to changes in ecosystems in western North America over the span of time in which the Morrison Formation was deposited. For instance, a growth in Morrison diplodocine diversity during the late Kimmeridgian-Tithonian might have filled ecological niches left by the disappearance of Haplocanthosaurus from the fossil record by the late Kimmeridgian.

References:

Boisvert, C., Curtice, B., Wedel, M., and Wilhite, R., 2024. Description of a new specimen of Haplocanthosaurus from the Dry Mesa Dinosaur Quarry. The Anatomical Record: 1–19. http://doi.org/10.1002/ar.25520

Maidment, S.C.R., and Muxworthy, A., 2019. A chronostratigraphic framework for the Upper Jurassic Morrison Formation, western U.S.A. Journal of Sedimentary Research 89 (10): 1017–1038. https://doi.org/10.2110/jsr.2019.54

Prothero, D., 2019.The Story of the Dinosaurs in 25 Discoveries. New York, NY: Columbia University Press.

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 

van der Linden, T.T.P., Tschopp, E., Sookias, R.B., Wallaard, J.J.W., Holwerda, F.M., and Schulp, A.S., 2024. A new diplodocine sauropod from the Morrison Formation, Wyoming, USA. Palaeontologia Electronica 27(3):a50. https://doi.org/10.26879/1380 

Whitlock, C., and Wilson Mantilla, J., 2020. The Late Jurassic sauropod dinosaur 'Morosaurus' agilis Marsh, 1889 reexamined and reinterpreted as a dicraeosaurid. Journal of Vertebrate Paleontology DOI: 10.1080/02724634.2020.1780600

   

Thoughts on phylogenetic analysis of Qunkasaura by Mocho et al. (2024)

The description of the derived titanosaur Abditosaurus by Viel et al. (2022) upended the long-standing view that all titanosaur taxa described from the Campanian-Maastrichtian of the Ibero-Armorican island (now northeastern Spain and southern France) were dwarf, insular forms because known specimens of Abditosaurus kuehnei are larger than Ampelosaurus, Lirainosaurus, Magyarosaurus, and other dwarf titanosaurs from the Late Cretaceous of Europe. Now, Mocho et al. (2024) have described yet another non-dwarf titanosaur from the Late Cretacrous of Spain, Qunkasaura pintiquiniestra, which hails from the type locality of the co-eval titanosaur Lohuecotitan pandafilandi in Lo Hueco, south-central Spain. While Qunkasaura constitutes the newest addition to an already-burgeoning inventory of lithostrotian titanosaurs from the Ibero-Armorican realm, there are aspects of the phylogenetic analysis of this taxon by Mocho et al. (2024) that are quite novel and worth discussing.

Phylogenetic analysis of Qunkasaura pintiquiniestra by Mocho et al. (2024) showing bifurcation of Saltasauroidea into Lohuecosauria and a Rapetosaurus+Overosaurus clade.

The most tangible taxonomic action concocted by Mocho et al. as a result of their cladistic analysis of Qunkasaura involves a shake-up of the phylogenetic nomenclature for and interrelationships of taxa outside South America assigned to Saltasauridae. The creation of the new name Lohuecosauria for a clade within Saltasauroidea comprising Lirainosaurinae, Saltasauridae, and Isisaurus by the authors constitutes a bold effort to tackle the interrelationships of Saltasaurus-like lithostrotians because the cladistic analysis by Mocho et al. (2024) differs from the phylogeny of Diez Díaz et al. (2018) in recovering Lirainosaurinae as closely related to Saltasauridae rather than as sister to Lognkosauria, suggesting that lirainosaurines may have evolved in Gondwana prior to the Maastrichtian and became the dominant titanosaur clade in Europe by the Maastrichtian. In particular, Carballido et al. (2022) noted that the Sallam et al. (2018) phylogeny recovering Opisthocoelicauda within Lirainosaurinae (which would have meant Opisthocoelicaudiinae having priority over Lirainosaurinae) in contrast to the results by Diez Díaz et al. rendered the exact cladistic position of Lirainosaurinae, and by finding Lirainosaurinae to be basal to Isisaurus and Saltasauridae, Mocho et al. do an outstanding job of tackling the question of whether Opisthocoelicaudia is closely related to members of Lirainosaurinae, because the saltasaurid Ibirania described from the late Santonian-early Campanian of southeastern Brazil by Navarro et al. (2022) is a dwarf taxon like many members of Lirainosaurinae, implying that the dwarfism evolved independently within different clades of Lohuecosauria during the Santonian-Maastrichtian interval.    

The recovery of Nemegtosaurus as closely related to Lognkosauria by Mocho et al. (2024) is quite remarkable. Diez Díaz et al. (2018) and Sallam et al. (2018) also find Nemegtosaurus to be the sister taxon of Rinconsaurus, Muyelensaurus, Lognkosauria, Aeolosaurus, and Gondwanatitan, but the placement of Nemegtosaurus within Colossosauria as closely related to Lognkosauria should be taken with a grain of salt because of marginal overlap between the few postcranial elements assigned to the holotype of N. mongoliensis from the type locality of this taxon by Currie et al. (2018) and postcranial remains of lognkosaurian taxa but also the younger age of Nemegtosaurus compared to lognkosaurian taxa included in the phylogeny by Mocho et al. except Garrigatitan and Puertasaurus. Still, Mocho et al. (2024) agree with Wilson et al. (2016) and Navarro (2019) that Rapetosaurus does not group with Nemegtosaurus to the exclusion of other derived lithostrotians, especially as their cladistic analysis finds Rapetosaurus and Tapuiasaurus to be distantly related despite overlap between known skeletal elements for those taxa. As a matter of fact, cladistic analyses by Navarro et al. (2022) and Gorcsak et al. (2023) recover Nemegtosaurus and Opisthocoelicaudia within Saltasauridae, which makes sense when considering that few postcranial elements are known for Nemegtosaurus despite overlap with those of Opisthocoelicaudia.

Mocho et al.'s note of caution on the cladistic positions of Garrigatitan and Normanniasaurus as well the Algora titanosaur within Colossosauria is quite reasonable. While I agree that newly discovered titanosaur material from Cenomanian-age deposits in Algora, Spain may shift the cladistic position of the Algora taxon to a different spot within Lithostrotia, the placement of Normanniasaurus within Colossosauria could still stand because a number of topologies in Mannion et al. (2019) happen to recover Normanniasaurus as sister to Lognkosauria and Aeolosaurus despite the incompleteness of the N. genceyi holotype. Although Mocho et al. (2024) call into question the tentative referral of some specimens to Garrigatitan, making their cladistic recovery of Garrigatitan as a lognkosaurian tenuous, Garrigatitan could still be a member of Colossosauria despite being based on incomplete material because the cladistic results in Mocho et al. (2024) demonstrate the co-existence of two distinct clades of saltasauroids in the Campanian-Maastrichtian of Europe.   

References:

Carballido, J.L., Otero, A., Mannion, P.D., Salgado, L., and Moreno, A.P., 2022. Titanosauria: A Critical Reappraisal of Its Systematics and the Relevance of the South American Record. pp. 269-298. In Otero, A.; Carballido, J.L.; Pol, D. (eds.). South American Sauropodomorph Dinosaurs. Record, Diversity and Evolution. Cham, Switzerland: Springer. doi:10.1007/978-3-030-95959-3. 

Currie, P. J., Wilson, J.A., Fanti, F., Mainbayar, B., and Tsogtbaatar, K., 2018. Rediscovery of the type localities of the Late Cretaceous Mongolian sauropods Nemegtosaurus mongoliensis and Opisthocoelicaudia skarzynskii: Stratigraphic and taxonomic implications. Palaeogeography, Palaeoclimatology, Palaeoecology 494:5–13. doi:10.1016/j.palaeo.2017.10.035. 

Díez Díaz, V., Garcia, G., Pereda-Suberbiola, X., Jentgen-Ceschino, B., Stein, K., Godefroit, P., and Valentin, X., 2018. The titanosaurian dinosaur Atsinganosaurus velauciensis (Sauropoda) from the Upper Cretaceous of southern France: New material, phylogenetic affinities, and palaeobiogeographical implications. Cretaceous Research. 91: 429–456. doi:10.1016/j.cretres.2018.06.015

Gorscak, E., Lamanna, M. C., Schwarz, D., Díez Díaz, V., Salem, B. S., Sallam, H. M., and Wiechmann, M. F., 2023. A new titanosaurian (Dinosauria: Sauropoda) from the Upper Cretaceous (Campanian) Quseir Formation of the Kharga Oasis, Egypt. Journal of Vertebrate Paleontology e2199810. doi:10.1080/02724634.2023.2199810.

Mannion, P. D., Upchurch, P., Jin, X. and Zheng, W. 2019. New information on the Cretaceous sauropod dinosaurs of Zhejiang Province, China: impact on Laurasian titanosauriform phylogeny and biogeography. Royal Society Open Science 6: 191057. doi:10.1098/rsos.191057

Mocho, P., Escaso, F., Marcos-Fernández, F., Páramo, A., Sanz, J. L., Vidal, D., and Ortega, F., 2024. A Spanish saltasauroid titanosaur reveals Europe as a melting pot of endemic and immigrant sauropods in the Late Cretaceous. Communications Biology 7: 1016. doi:10.1038/s42003-024-06653-0

Navarro, B.A., Ghilardi, A.M., Aureliano, T., Díaz, V.D., Bandeira, K.L. N., Cattaruzzi, A.G.S., Iori, F.V., Martine, A.M., Carvalho, A.B., Anelli, L.E., Fernandes, M.A., and Zaher, H., 2022. A new nanoid titanosaur (Dinosauria: Sauropoda) from the Upper Cretaceous of Brazil. Ameghiniana 59(5): 317–354. doi:10.5710/AMGH.25.08.2022.3477

Sallam, H., Gorscak, E., O'Connor, P., El-Dawoudi, I., El-Sayed, S., and Saber, S., 2018. New Egyptian sauropod reveals Late Cretaceous dinosaur dispersal between Europe and Africa. Nature 2 (3): 445–451.

Vila, B., Sellés, A., Moreno-Azanza, M., Razzolini, N.L., Gil-Delgado, A., Canudo, J.I., and Galobart, A., 2022. A titanosaurian sauropod with Gondwanan affinities in the latest Cretaceous of Europe. Nature Ecology & Evolution 6: 288-296. doi:10.1038/s41559-021-01651-5.

Wilson, J. A., D. Pol, A. B. Carvalho, and H. Zaher. 2016. The skull of the titanosaur Tapuiasaurus macedoi (Dinosauria: Sauropoda), a basal titanosaur from the Lower Cretaceous of Brazil. Zoological Journal of the Linnean Society 178(3):611–662.

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.

Opisthocoelia: first effort towards recognition of sauropods as a distinct reptile group

The history of the classification of the eusauropod Cetiosaurus is well-known because it was initially seen by its describer, Richard Owen, as representing either an unknown race of aquatic reptiles or a gigantic crocodyliform, until it was recognized as a dinosaur in the 1870s. However, buried in the very superfluous process of the classification of Cetiosaurus within Reptila during the 1840-1880 interval is the fact that Owen himself reinforced his perception of Cetiosaurus as a gigantic marine crocodile when he unintentionally planted the seeds for long-term recognition of sauropods as a distinct reptile group by including Cetiosaurus in a new group, christened Opisthocoelia (Greek for "posterior hollows"), in a paper published by him in 1859.

Since the publication of his 1842 paper whereby the name Dinosauria was coined, Richard Owen knew that the proportions of the caudal vertebrae that he named Cetiosaurus medius and C. brevis (the latter now Pelorosaurus) discriminated Cetiosaurus from other Mesozoic crocodyliform taxa known to him, but he needed to buy time to determine whether including Cetiosaurus in a new clade within Crocodilia was warranted. Mantell (1850) had coined the new genus Pelorosaurus for Cetiosaurus conybeari (a junior objective synonym of brevis) after recognizing that the humerus NHMUK 28626 found at the type locality of the syntype caudals of C. brevis in Cuckfield, West Sussex, belonged to the same animal as the caudals, and while Pelorosaurus was the first-named sauropod to be recognized as terrestrial by its describer, Owen (1859a) classified Pelorosaurus as a gigantic crocodile on the grounds that Mantell (1850) had NHMUK 28626 incorrectly oriented. In a paper working out the classification of both extant and fossil reptiles, Owen (1859b) erected a new suborder within Crocodilia to include Cetiosaurus and the tetanuran theropod dinosaur Streptospondylus (which is now recognized as a theropod), which he named Opisthocoelia. According to Owen (1859b), Opisthocoelia could be discriminated from his other crocodilian suborders, Procoelia and Amphicoelia, in having opisthocoelous dorsal vertebrae with concave articulations at the rear end of the dorsal centra. Although the material known for Cetiosaurus was still extremely limited and not enough to illuminate the true appearance of this genus relative to extant and extinct crocodyliforms, this did not deter Owen (1859b) from taking the first crucial step to formally place Cetiosaurus in a systematic grouping of its own.

Following recognition of Cetiosaurus as a member of Dinosauria by Huxley (1870) and Phillips (1871), Owen (1875) admitted that his initial classification of Cetiosaurus within Crocodilia was untenable and realized that Cetiosaurus was a dinosaur after noting numerous similarities between the dorsal vertebrae of Cetiosaurus oxoniensis (referred by him to the Late Jurassic nominal species 'C. longus') and those of Hylaeosaurus, Iguanodon, and Scelidosaurus in their opisthocoelous nature. However, he did not touch upon whether or not he still regarded Streptospondylus as a close relative of Cetiosaurus even though Hulke (1872) came to regard Streptospondylus as also a dinosaur and possibly related to not just Cetiosaurus but also Ornithopsis and Eucamerotus. Seeley (1874), agreeing with the re-assessment of Cetiosaurus as a dinosaur by Phillips (1871), included Cetiosaurus and his new genus Craterosaurus in a distinct grouping within Dinosauria, utilizing the name Cetiosauria (first mentioned in Owen 1859c*) for it. When Edward Drinker Cope described the genera Amphicoelias and Camarasaurus from the Morrison Formation, he recognized them as forming a grouping with Ornithopsis, but did not mention Opisthocoelia in his papers naming Amphicoelias and Camarasaurus. His arch-rival during the Bone Wars, Othniel Charles Marsh, got the chance to apply a name to a grouping of exclusively long-necked sauropodomorphs when he coined the higher-level taxonomic name Sauropoda for the dinosaur group formed by long-necked sauropomorphs in a paper published by him in 1878, unaware of the names Opisthocoelia and Cetiosauria. Cope (1883) treated Opisthocoelia as a senior synonym of Sauropoda, basing his diagnosis for Opisthocoelia on some of the characters used by Marsh (1878) to define Sauropoda. 

* Although Cetiosauria was first used by Owen (1859c) and not Seeley (1874), Owen did specify whether his Cetiosauria was a provisional name for the group he named Opisthocoelia should future study judge Cetiosaurus was too morphologically distinct from crocodyliforms to be classified within Crocodilia. Osborn (1898) used Cetiosauria as a senior synonym of Sauropoda but incorrectly attributed Cetiosauria to Seeley (1874) rather than Owen (1859c). Hatcher's (1903) comments about Cetiosauria having subordinal rank only within Sauropoda leave room open for using Cetiosauria to phylogenetically define a eusauropod clade including Neosauropoda, Mamenchisauridae, Turiasauria, Cetiosaurus, and any other eusauropods more closely related to Neosauropoda than to Shunosaurus.  

Given that Sauropoda has had universal currency in the literature, it is important to note that although Riggs (1903) and Matthews (1915) both used Opisthocoelia as a senior synonym for Sauropoda, the taxonomic content of Opisthocoelia listed by Owen (1859b), while paving the way for Cetiosaurus and Pelorosaurus to be one day recognized as constituting previously unknown clade of dinosaurs, would seemingly make the name Opisthocoelia equivalent to Saurischia (as stated by Mickey Mortimer) given that Streptospondylus is now classified as a theropod and has opisthocoelous dorsal vertebrae, as does Eustreptospondylus. Also, the dorsal vertebrae of Camarasaurus and Amphicoelias are opisthocoelous and amphicoelous respectively as noted by Cope (1877), and the presence of opisthocoely in both the cervical and dorsal vertebrae has an uneven distribution among sauropods (Fronimos 2016). Thus, the name Opisthocoelia can be considered not as a synonym of Sauropoda but as a prelude to eventual recognition of sauropods as distinct reptile ground in their own right by taking into account dorsal vertebral features distinguishing sauropods from all extinct and extant crocodyliforms.

References:

Cope, E.D., 1877. On Amphicoelias, a genus of saurians from the Dakota Epoch of Colorado. Paleontology Bulletin 27: 1–5.

Cope, E.D., 1883. On the characters of the skull in the Hadrosauridae. Proceedings of the Philadelphia Academy of Natural Sciences 35: 97-107.

Fronimos, J.A., 2016. Functional Mechanics of Concavo-convex Articulations and Neurocentral Sutures in the Vertebral Column of Sauropod Dinosaurs. Unpublished dissertation, University of Michigan.

Hatcher, J. B., 1903. Osteology of Haplocanthosaurus with description of a new species, and remarks on the probable habits of the Sauropoda and the age and origin of the Atlantosaurus beds. Memoirs of the Carnegie Museum 2: 1–72.

Hulke, J. W., 1872. Appendix to a "Note on a new and undescribed Wealden Vertebra," read 9th February 1870, and published in the Quarterly Journal for August in the same year. Quarterly Journal of the Geological Society 28 (1–2): 36–38.

Huxley, T.H., 1870. On the Classification of the Dinosauria, with observations on the Dinosauria of the Trias. Quarterly Journal of the Geological Society of London 26: 32–51.

Mantell, G. A. 1850. On the Pelorosaurus: an undescribed gigantic terrestrial reptile, whose remains are associated with those of the Iguanodon and other saurians in the strata of Tilgate Forest, in Sussex. Philosophical Transactions of the Royal Society of London 140: 379–390.

Marsh, O.C., 1878. Principal characters of American Jurassic dinosaurs. Part I. American Journal of Science, Series 3 16: 411–416.

Matthew, W. D. 1915. Dinosaurs, with Special Reference to the American Museum Collections. American Museum of Natural History, New York.

Osborn, H. F. 1898. Additional characters of the great herbivorous dinosaur Camarasaurus. Bulletin of the American Museum of Natural History 10: 219–233.

Owen, R., 1859a. Monograph on the fossil Reptilia of the Wealden and Purbeck Formations. Supplement no. II. Crocodilia (Streptospondylus, &c.). Palaeontographical Society Monograph 11: 20–44.

Owen. R., 1859b. On the orders of fossil and recent Reptilia, and their distribution in time. Report of the British Association for the Advancement of Science 29:153–166. 

Owen, R., 1859c. On the Classification and Geographical Distribution of the Mammalia. London, UK: John W. Parker and Son.

Owen, R., 1875. Monographs of the fossil Reptilia of the Mesozoic formations, part II (genera Bothriospondylus, Cetiosaurus, Omosaurus). Palaeontographical Society Monographs 29:15–93.

Phillips, J. 1871. Geology of Oxford and the Valley of the Thames. Oxford, UK: Clarendon Press.

Riggs, E. S. 1903. Structure and relationships of opisthocoelian dinosaurs. Part I, Apatosaurus Marsh. Field Columbian Museum, Geological Series 2: 165–196.

Seeley, H. G. 1874. On the base of a large lacertian cranium from the Potton Sands, presumably dinosaurian. Quarterly Journal of the Geological Society, London 30: 690–692.

Implications of Averianov et al. (2023) paper for evolution of sacral vertebral count in early titanosaurs

In their paper describing the Diamantinasaurus matildae specimen AODF 0906, Poropat et al. (2023) weighed in on whether the presence of five sacral vertebrae in Diamantinasaurus is plesiomorphic for Titanosauria or an evolutionary reversal in Diamantinasaurus among basal titanosaurs given that sacral material is not known for many early-diverging titanosaurs and a five-vertebrae sacrum is common among basal members of Somphospondyli. Recently, Averianov et al. (2023) have described a new specimen comprising caudal vertebrae (KOKM A) which they refer to the somphospondyl Sibirotitan astrosacralis based on comparisons of this specimen with referred S. astrosacralis caudal vertebra KOKM 26786 from the type locality of this taxon, and they recover Sibirotitan as a non-lithrostrotian titanosaur in spite of the presence of strongly procoelous articulations of the anterior caudal being used as a synapomorphy for Lithostrotia. In light of the cladistic results in Averianov et al. (2023), it should be worth discussing what the five-vertebrae sacrum and strongly procoelous caudals of Sibirotitan mean for interpreting the evolution of the number of sacral vertebrae in early titanosaurs.

Cladistic analysis by Averianov et al. (2023) showing the phylogenetic placement of Sibirotitan as closely related to Lithostrotia within Titanosauria.

As noted by Poropat et al. (2023), the six-vertebrae sacrum common for titanosaurs (especially the most derived clades) is not exclusive to Titanosauria and has also been described for Klamelisaurus, some specimens of Camarasaurus, the neosauropod "Apatosaurus" minimus, and several somphospondyl titanosauriform taxa, while a sacrum with five vertebrae is recognized as the plesiomorphic condition for somphospondyls. Although strong procoely of the anterior caudals of referred Sibirotitan specimens KOKM A and KOKM 26786 is shared with Hamititan (Wang et al. 2021), the sacrum of referred specimen PM TGU 120/8-Sh1-1 has five vertebrae despite the strong procoely of the anterior caudals of Sibirotitan differing from slightly procoelous anterior caudals in Andesaurus and Ninjatitan as well as the amphicoelous nature of the caudal vertebrae of Diamantinasaurus (Gallina et al. 2021; Poropat et al. 2023). Given that Stephen Poropat (pers. comm. to me, Feb. 13, 2024) now doubts that the sacrum of adult Diamantinasaurus had five vertebrae and because Sibirotitan is Barremian in age, the recovery by Averianov et al. (2023) of Sibirotitan as a non-lithostrotian titanosaur closer to Lithostrotia than to any titanosaur with non-procoelous anterior caudals could suggest that irrespective of the condition of the articulations of the anterior caudal vertebrae, a five-vertebra sacrum was probably plesiomorphic for the clade Titanosauria due to homoplasticity of six sacral vertebrae within Macronaria. For instance, a complete sacrum is not preserved for some titanosaur taxa from the Early-Middle Cretaceous of East Asia, but given that Averianov et al. (2023) recover Daxiatitan as sister to Sibirotitan, it is probable that the ancestral morphological condition for basal titanosaur clades comprised not only a five-vertebra sacrum but also non-procoelous anterior caudals, and that anterior caudal articulation morphologies diversified in those clades as the Early Cretaceous progressed. Also, the placement of Huanghetitan as sister to Titanosauria in the phylogenetic analysis by Han et al. (2024) further supports my opinion that in early titanosaurs, the five-vertebra sacrum preceded the acquisition of strongly procoelous anterior caudals because the anterior caudal of the holotype of H. liujiaxiaensis is slightly procoelous.

Given the presence of five sacral vertebrae in Sibirotitan and the non-lithostrotian titanosaur placement of this taxon by Averianov et al. (2023), one question arises: why did titanosaurs eventually evolve more than five sacrals after initially retaining the plesiomorphic five-vertebra sacrum as the Middle and Late Cretaceous progressed? Frankly, an increase in body size cannot account for a slight increase in the number of sacral vertebrae in titanosaurs because the saltasaurid Neuquensaurus is a small-sized genus despite being distinguishable from other lithostrotians in having seven sacral vertebrae (Salgado et al. 2005) and small size evolved in more than one clade of derived titanosaurs (Navarro et al. 2022). Given individual variation in the sacral vertebral count of Camarasaurus by Tidwell et al. (2005), but also the fact that no sacral remains are known for the dwarf saltasaurid Ibirania, it is possible that the emergence of the six-vertebra sacrum as a synapomorphic condition for titanosaurs by the Middle Cretaceous might have had something to do with environment-related paleobiological factors, and that some dwarf titanosaurs from Europe probably had either five or six sacral vertebra.

References:

Averianov, A., Podlesnov, A., Slobodin, D., Skutschas, P., Feofanova, O., and Vladimirova, O., 2023. First sauropod dinosaur remains from the Early Cretaceous Shestakovo 3 locality, Western Siberia, Russia.  Biological Communications 68 (4): 236–252. doi:10.21638/spbu03.2023.404.

Han, F., Yang, L., Lou, F., Sullivan, C., Xu, X., Qiu, W., Liu, H., Yu, J., Wu, R., Ke, Y., Xu, M., Hu, J., and Lu, P., 2024. A new titanosaurian sauropod Gandititan cavocaudatus gen. et sp. nov., from the Late Cretaceous of southern China. Journal of Systematic Palaeontology 22 (1): 2293038. doi: https://doi.org/10.1080/14772019.2023.2293038.

Navarro, B. A., Ghilardi, A.M., Aureliano, T., Díaz, V.D., Bandeira, K.L.N., Cattaruzzi, A.G.S., Iori, F.V., Martine, A.M., Carvalho, A.B., Anelli, L.E., Fernandes, M.A., and Zaher, H., 2022. A new nanoid titanosaur (Dinosauria: Sauropoda) from the Upper Cretaceous of Brazil. Ameghiniana 59 (5): 317–354. doi:10.5710/AMGH.25.08.2022.3477.

Poropat, S. F., Mannion, P. D., Rigby, S. L., Duncan, R. J., Pentland, A. H., Bevitt, J. J., Sloan, T., and Elliott, D. A., 2023. A nearly complete skull of the sauropod dinosaur Diamantinasaurus matildae from the Upper Cretaceous Winton Formation of Australia and implications for the early evolution of titanosaurs.Royal Society Open Science 10(4): 221618. https://doi.org/10.1098/rsos.221618 

Salgado, L., Apesteguía, S., and Heredia, S. 2005. A new specimen of Neuquensaurus australis, a Late Cretaceous saltasaurine titanosaur from North Patagonia. Journal of Vertebrate Paleontology 25: 623634.  

Tidwell, V., Stadtman, K., and Shaw, A., 2005. Age-related characteristics found in a partial pelvis of Camarasaurus; pp. 180-186, In: Tidwell, V., and Carpenter, K. (eds.), Thunder-Lizards: The Sauropodomorph Dinosaurs. Indiana University Press: Bloomington, IA.

Wang, X., Bandeira, K. L. N., Qiu, R., Jiang, S., Cheng, X., Ma, Y., and Kellner, A.W.A., 2021. The first dinosaurs from the Early Cretaceous Hami Pterosaur Fauna, China. Scientific Reports 11:14962. doi:10.1038/s41598-021-94273-7.

Bearing of Han et al. (2024) paper on the inclusion of Andesaurus in titanosaur phylogenetic definitions

The Patagonian titanosaur Andesaurus delgadoi has been universally recognized as a basal titanosaur from the time of its description in 1991 because it has anterior caudal vertebrae with slight procoely in marked contrast to the strongly procoelous anterior caudal vertebrae of derived titanosaurs, hence its inclusion in phylogenetic definitions for Titanosauria following Salgado et al. (1997) and Wilson & Upchurch (2003). However, the holotype of A. delgadoi is rather incomplete, comprising only caudal vertebrae, four dorsal vertebrae, a few limb bones, pelvic elements, and rib fragments. Carballido et al. (2022) noted that a few recent cladistic analyses have found Andesaurus to occupy a rather unstable phylogenetic position within Somphospondyli or basal Titanosauria, stressing the need for further testing of the cladistic instability of Andesaurus to see whether a redefinition of Titanosauria is required. Han et al. (2024) have recently described a new titanosaur from the Cenomanian-Turonian of southern China, Gandititan cavocaudatus, recovering this form and Andesaurus in a basal titanosaur clade which also includes Abdarainurus, Baotianmansaurus, Dongyangosaurus, and Huabeisaurus. Given the results of the cladistic analysis by Han et al., it is imperative to discuss the impact of the basal titanosaur placement of Gandititan on continued use of Andesaurus in phylogenetic definitions employed for Titanosauria.

As I have mentioned previously, Ninjatitan not only is currently the oldest titanosaur genus described so far but is also similar to Andesaurus in having slightly procoelous anterior caudal vertebrae, and the phylogenetic results in Wang et al. (2021) indicate that titanosaurs found in East Asia evolved a diverse array of morphologies of the articular surfaces of the anterior caudal vertebrae during the Barremian-Albian interval given that Hamititan has strongly procoelous anterior caudals unlike Andesaurus and Ninjatitan. Although Ninjatitan is recovered within Titanosauria as either a basal form or a member of Lognkosauria in the different topologies obtained by Gallina et al. (2021), a basal position for this genus is most likely because the anterior caudals of this taxon have slight procoely as in Andesaurus and Ninjatitan is far older than known lognkosaurian taxa. However, Han et al. (2024) did not include Ninjatitan in their cladistic analysis despite its Berriasian-Valanginian age because of the paucity of known material for the N. zapatai holotype even though doing so would have tested the phylogenetic placement of Ninjatitan within the basal titanosaur grouping formed by Abdarainurus, Andesaurus, Baotianmansaurus, Dongyangosaurus, Gandititan, and Huabeisaurus. The strict consensus cladogram in Mannion et al. (2013) applies the name Andesauroidea to the basalmost titanosaur clade that includes Andesaurus, but the cladistic analysis by Han et al. places some of the taxa included in that clade by Mannion et al. outside Titanosauria while keeping Andesaurus and Baotianmansaurus in Titanosauria, so even if Andesaurus is younger than Ninjatitan, a redefinition of Titanosauria to exclude Andesaurus is unwise because both taxa exhibit discrete titanosaur synapomorphies despite being known from sparse axial and appendicular material.

A time-calibrated cladistic analysis of Titanosauria showing Gandititan in a basal titanosaur clade also comprising Abdarainurus, Andesaurus, Baotianmansaurus, Dongyangosaurus, and Huabeisaurus (after Han et al. 2024)

When compartmentalizing the results of the cladistic analysis by Han et al. (2024) with the tendency by many cladistic analyses to root titanosaur phylogenetic trees with Andesaurus, a number of important things ought to be emphasized when it comes to continuing to include Andesaurus in a phylogenetic definition for Titanosauria. First, the opisthocoelous nature of the caudal vertebrae in Gandititan (which is convergent in the saltasaurid Opisthocoelicauda) differs from the slightly procoelous anterior caudals of Andesaurus and occurs in Abdarainurus, while the caudals of Huabeisaurus, Baotianmansaurus, and Dongyangosaurus are amphicoelous. Abdarainurus and Huabeisaurus are recovered as late-surviving basal titanosaurs by Wang et al. (2021) and Poropat et al. (2023), so it is prudent to surmise that if the clade formed by Abdarainurus, Andesaurus, Baotianmansaurus, Dongyangosaurus, Gandititan, and  Huabeisaurus in the Han et al. (2024) cladistic analysis is supported by future papers, then Andesaurus-like titanosaurs evolved different caudal articulation morphologies and Andesauroidea could be used for this clade. Second, the recovery of the two nominal Huanghetitan species and Diamantinasauria just outside Titanosauria by Han et al. (2024) might further preclude omitting Andesaurus from future phylogenetic definitions of Titanosauria because it is unclear if Andesaurus and Baotianmansaurus had six sacral vertebrae as in Dongyangosaurus, Gandititan, and Huabeisaurus, or if they possessed the five sacral vertebral count noted by Poropat et al. (2023) for Diamantinasaurus and Huanghetitan due to  Andesaurus delgadoi preserving no sacral remains and the Baotianmansaurus henanensis holotype preserving one and a half sacrals. Irrespective of discussion in Poropat et al. (2023) as to whether the sacral vertebral count for Diamantinasaurus places Diamantinasauria just outside Titanosauria or merely reaffirms the position of this clade within Titanosauria by virtue of being plesiomorphic for titanosaurs, the systematic position of Andesaurus inside Titanosauria still appears secure enough for continued inclusion of this genus in a phylogenetic definition for Titanosauria.

References:

Gallina, P. A., Canale, J. I., and Carballido, J. L., 2021. The Earliest Known Titanosaur Sauropod Dinosaur. Ameghiniana 58 (1): 35–51. doi:10.5710/AMGH.20.08.2020.3376.

Han, F., Yang, L., Lou, F., Sullivan, C., Xu, X., Qiu, W., Liu, H., Yu, J., Wu, R., Ke, Y., Xu, M., Hu, J., and Lu, P., 2024. A new titanosaurian sauropod, Gandititan cavocaudatus gen. et sp. nov., from the Late Cretaceous of southern China. Journal of Systematic Palaeontology 22 (1): 2293038. doi: https://doi.org/10.1080/14772019.2023.2293038.

Mannion, P. D., Upchurch P., Barnes R. N., and Mateus O., 2013. Osteology of the Late Jurassic Portuguese sauropod dinosaur Lusotitan atalaiensis (Macronaria) and the evolutionary history of basal titanosauriforms. Zoological Journal of the Linnean Society 168: 98–206.

Poropat, S. F., Mannion, P. D., Rigby, S. L., Duncan, R. J., Pentland, A. H., Bevitt, J. J., Sloan, T., and Elliott, D. A., 2023. A nearly complete skull of the sauropod dinosaur Diamantinasaurus matildae from the Upper Cretaceous Winton Formation of Australia and implications for the early evolution of titanosaurs. Royal Society Open Science 10(4): 221618. https://doi.org/10.1098/rsos.221618 

Salgado, L., Coria, R.A., & Calvo, J.O. 1997. Evolution of titanosaurid Sauropods. I: Phylogenetic analysis based on the postcranial evidence. Ameghiniana 34: 3-32.

Wang, X., Bandeira, K. L. N.,  Qiu, R., Jiang, S., Cheng, X., Ma, Y., and Kellner, A.W.A., 2021. The first dinosaurs from the Early Cretaceous Hami Pterosaur Fauna, China. Scientific Reports 11:14962. doi:10.1038/s41598-021-94273-7.

Wilson, J.A., and Upchurch, P. 2003. A revision of Titanosaurus Lydekker (Dinosauria – Sauropoda), the first dinosaur genus with a ‘Gondwanan’ distribution. Journal of Systematic Palaeontology 1(3): 125–160.

Is it time to utilize Antarctosauridae for square-jawed titanosaurs?

The Mesozoic era is replete with instances of distantly related terrestrial tetrapod clades convergently evolving similar morphological traits, exemplifed by the convergence of cranial Bauplan of toothless ostrich mimic dinosaurs and shuvosaurids as well as the similar carnivorous dentitions of both predatory 'rauisuchians' and large-bodied theropods. Although it is well known that the discoveries of Rapetosaurus and Tapuiasaurus led to Nemegtosaurus and Quaesitosaurus being reclassified as titanosaurs and their cranial similarities to those of flagellicaudatan diplodocoids easily recognized as a product of convergent evolution despite some morphological similarities, another instance of convergent evolution between members of both Diplodocoidea and Titanosauria in the cranial region worthy of discussion is a square-shaped mandibular symphysis being long considered diagnostic for Rebbachisauridae until the description of the lithostrotian titanosaur Bonitasaura by Apesteguía (2004) demonstrated that a number of titanosaurs convergently evolved this morphological trait, refuting suggestions that either Antarctosaurus belonged to Diplodocoidea or the jawbone of this taxon was from a rebbachisaurid rather than a titanosaur. Recently, Filippi et al. (2024) have described a new titanosaur with a square-shaped jaw on the basis of complete remains, Inawentu oslatus, from the Santonian-age Bajo de la Carpa Formation in Neuquén Province, Argentina, and their cladistic analysis recovers this form along with Antarctosaurus as part of a previously unrecognized clade also including Rinconsauria and Aeolosaurini. Due to Antarctosaurus being the type genus of the family-group name Antarctosauridae Olshevsky, 1978, I'm pondering the question of whether Antarctosauridae should be used for the titanosaur grouping "Clade A" recovered by Filippi et al. (2024).

Semi-detailed aspects of the osteology of Antarctosaurus wichmannianus have already been given by Powell (2003) and Novas (2009), but a few salient morphological features of A. wichmannianus will be elaborated here to illuminate the history of the higher-level classification of Antarctosaurus. Until the 1970s, there was no consensus on the exact position of titanosaurs within Eusauropoda, with some authors (e.g. von Huene 1929; Stromer 1932; Gilmore 1946) and others (Nopcsa 1928; Tatarinov 1964; White 1973) placing Antarctosaurus and other titanosaur genera in either Cetiosauridae, Diplodocidae, or Morosauridae (=Camarasauridae). Despite classifying Antarctosaurus as a diplodocid, White (1973) hinted that this genus and other Cretaceous peg-toothed sauropods with procoelous anterior caudals should be placed in their own family. Olshevsky (1978) erected the new family Antarctosauridae to include Antarctosaurus and some other titanosaurs, and Powell (1986, 2003) treated Antarctosauridae as a distinct subfamily within Titanosauridae, Antarctosaurinae. However, a number of similarities of the mandible of the Antarctosaurus wichmannianus holotype to the jaws of rebbachisaurids, especially the square-shaped mandibular symphysis and the proportionally small and slender teeth at the front of the jaw, led several authors (e.g. Jacobs et al. 1993; Sereno et al. 1999; Upchurch 1999) to suggest that the either the holotype is a chimera of titanosaur and rebbachisaurid remains or A. wichmannianus is a late-surviving rebbachisaurid. However, Chiappe et al. (2001) noted that Antarctosaurus differs from rebbachisaurids in having a sharp angle between the surface of the wear facet and the longitudinal axis of the tooth as well as an angle of about 90 degrees between the main axes of the mandibular symphysis and mandible. Apesteguía (2004) further refuted suggestions about the A. wichmannianus jaw being a rebbachisaurid by pointing out similarities of the mandible of Antarctosaurus with that of the newly described titanosaur Bonitasaura. Irrespective of Bonitasaura demonstrating that the square-shaped mandibular symphysis in Antarctosaurus evolved convergently with that of rebbachisaurids, Wilson (2005) raised uncertainty over whether the cranial elements of Antarctosaurus wichmannianus are associated with the postcranial remains catalogued under MACN 6904 given that the material was collected from more than one locality.

Phylogenetic analysis of Titanosauria showing the cladistic position of Antarctosaurus, Bonitasaura, and Inawentu (after Filippi et al. 2024). Under ICZN rules, Antarctosauridae hypothetically should be applied to "Clade A" recovered in the phylogeny. 

While the description of Bonitasaura put to rest any suggestions that the jaw belonging to the holotype of Antarctosaurus wichmannianus could have come from a rebbachisaurid, Apesteguía (2004) did not carry out a cladistic analysis to test whether the stark similarities of the jaw of Antarctosaurus with that  of Bonitasaura could translate into a sister relationship between the two taxa within Titanosauria. A paper by Gallina and Apesteguía (2011) detailing the cranial osteology of Bonitasaura carried out the first cladistic analysis of Antarctosaurus, recovering it and Bonitasaura as members of the titanosaur clade Colossosauria. The cladistic analyses of Dongyangosaurus and Jiangshanosaurus by Mannion et al. (2019) also found varying placements for Antarctosaurus within Colossosauria although they did not include Bonitasaura, and thus Santucci and Filippi (2022) assigned Antarctosaurus to Colossosauria incertae sedis. In their cladistic analysis of Inawentu, Filippi et al. (2024) confirm that the squared-shaped mandibular symphysis of Antarctosaurus and Bonitasaura places these taxa as more closely related to each other than to members of Lognkosauria, Rinconsauria, and Aeolosaurini, yet at the same time also find Antarctosaurus, Baalsaurus, Brasilotitan, Bonitasaura, Inawentu, Narambuenatitan, and Uberabatitan to form a distinctive clade within Eutitanosauria, which they term "Clade A".

The cladistic analysis by Filippi et al. (2024) raises a critical question about the interrelationships of non-lithostrotian eutitanosaurs: should be the name Antarctosauridae be utilized for "Clade A" recovered by Filippi et al. (2024)? Since Brasilotitan has the square-shaped jaw of Antarctosaurus,  Baalsaurus, Bonitasaura, and Inawentu despite being more closely related to both Aeolosaurini and Rinconsauria, it could be tempting to use the name Antarctosauridae for the titanosaur clade which Filippi et al. term "Clade A" because Antarctosaurus is the type genus of Antarctosauridae and the cranial material of the Inawentu oslatus holotype (MAU-Pv-LI-595) overlaps with that known for Antarctosaurus, Bonitasaura, Brasilotitan, Muyelensaurus, Narambuenatitan, and Rinconsaurus as well as the holotype specimen of Baalsaurus mansillai. However, no mandibular material is known for colossosaurian titanosaurs which are not recovered within "Clade A" by Filippi et al., and Carballido et al. (2022) note that the varying positions of Aeolosaurini depending on different cladistic analyses conducted over the past decade might result from a lack of a detailed osteology for Aeolosaurus rionegrinus. Therefore, it may be premature for future papers to apply Antarctosauridae to Filippi et al.'s "Clade A"  and phylogenetically define it as "all taxa closer to Antarctosaurus wichmannianus von Huene, 1929 than to Saltasaurus loricatus Bonaparte and Powell, 1980". Nevertheless, Filippi et al. (2024) briefly mention before the end of their paper that cranial material of the Diamantinasaurus matildae specimen described by Poropat et al. (2023) is morphologically distinct from that of the taxa recovered as part of "Clade A" within Eutitanosauria. Given that members of Diamantinasauria have a distinct cranial morphology that that of more derived titanosaurs, the possibility that the square-jawed eutitanosaur clade recovered by Filippi et al. (2024) could hold water in future phylogenies, albeit with a slightly restricted content (Antarctosaurus, Baalsaurus, Brasilotitan, Bonitasaura, and Inawentu), should not be ruled out, in which case Antarctosauridae Olshevsky, 1978 would be applied to this clade in a phylogenetic context. 

Apesteguía, S., 2004. Bonitasaura salgadoi gen. et sp. nov.: a beaked sauropod from the Late Cretaceous of Patagonia. Naturwissenschaften 91(10):493–497.

Carballido, J.L., Otero, A., Mannion, P.D., Salgado, L., and Moreno, A.P., 2022. Titanosauria: A Critical Reappraisal of Its Systematics and the Relevance of the South American Record. pp. 269-298. In Otero, A.; Carballido, J.L.; Pol, D. (eds.). South American Sauropodomorph Dinosaurs. Record, Diversity and Evolution. Cham, Switzerland: Springer. doi:10.1007/978-3-030-95959-3. 

Chiappe, L.M., Salgado, L., and Coria, R.A., 2001. Embryonic Skulls of Titanosaur Sauropod Dinosaurs. Science 293: 2444-2446.

Filippi, L.S., Juárez Valieri, R.D., Gallina, P.A., Méndez, A.H., Gianechini, F.A., and Garrido, A.C., 2024. A rebbachisaurid-mimicking titanosaur and evidence of a Late Cretaceous faunal disturbance event in South-West Gondwana. Cretaceous Research 154: 105754. doi:10.1016/j.cretres.2023.105754.

Gallina, P.A. and Apesteguía, S. 2011. Cranial anatomy and phylogenetic position of the titanosaurian sauropod Bonitasaura salgadoi. Acta Palaeontologica Polonica 56 (1): 45–60.

Gilmore, C. W., 1946. Reptilian fauna of the North Horn Formation of central Utah. U.S. Geological Survey Professional Paper 210-C: 29-51.

Jacobs, L., Winkler, D.A., Downs, W.R., and Gomani, E.M., 1993. New material of an Early Cretaceous titanosaurid sauropod dinosaur from Malawi. Palaeontology 36:523–534.

Mannion, P. D., Upchurch, P., Jin, X., and Zheng, W., 2019. New information on the Cretaceous sauropods of Zhejiang Province, China: impact on Laurasian titanosauriform phylogeny and biogeography. Royal Society Open Science 6(8):191057. doi:10.1098/rsos.191057.

Nopcsa, F., 1928. Paleontological Notes on Reptiles. Geologica Hungarica, Series Palaeontologica 1  (1): 1-102

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