Whitlock and Wilson Mantilla (2020) recently published a redescription of the diplodocoid sauropod "Morosaurus" agilis Marsh, 1889, erecting the new genus Smitanosaurus for it and assigning it to the family Dicraeosauridae based on comparisons with other flagellicaudatan diplodocoids from the Morrison Formation and phylogenetic analysis. The holotype of Smitanosaurus agilis, USNM 5384, was originally considered referable to Diplodocus longus by Othniel Charles Marsh judging from archival records, but was later described as a new species of the genus Morosaurus by Marsh (1889). Later, Gilmore (1907) believed it to be allied with either Brachiosauridae or Haplocanthosaurus; Osborn and Mook (1919, 1921) formally synonymized Morosaurus with Camarasaurus, and Gilmore (1925:352) clearly stated that M. agilis was not congeneric with other Camarasaurus species. McIntosh (1990) followed the suggestion by Gilmore (1907) that "M." agilis could be either a brachiosaurid or Haplocanthosaurus. Subsequently, an abstract by Tidwell et al. (2005) concluding that "Morosaurus" agilis represented a Diplodocus specimen as initially proposed by Marsh, and that it was a junior synonym of Diplodocus longus based on comparisons with skulls of diplodocid taxa from the Morrison Formation, including skulls referred to Diplodocus (AMNH 969, CM 3452, CM 11161, USNM 2672, USNM 2673). However, AMNH 969 and USNM 2673 were referred to Galeamopus hayi and G. pabsti respectively by Tschopp et al. (2015) and Tschopp & Mateus (2017), raising doubts about whether or not "Morosaurus" agilis is synonymous with Diplodocus. Suuwassea has been assigned to Dicraeosauridae by Whitlock (2011), but the placement of Smitanosaurus as a dicraeosaurid by Whitlock and Wilson Mantilla reveals a hitherto hidden diversity of dicraeosauids from the Morrison. The genus name Smitanosaurus is quite tongue-twisting, for it pays homage to not only J. August Smith, the discoverer of the holotype of S. agilis (USNM 5384), but also the Smithsonian Institution in which USNM 5384 was reposited.
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| Photograph and lateral view drawing of the holotype of Smitanosaurus agilis (after Gilmore 1907) |
One anatomical aspect of Smitanosaurus agilis noted by Whitlock and Wilson Mantilla that deserves attention is the presence of a postparietal foramen on the skull roof. Woodruff et al. (2017) considered the presence or absence of this feature in diplodocoids to be a influenced by ontogeny, with juvenile specimens possessing the postparietal foramen and adults lacking it. However, as Whitlock and Wilson Mantilla point out, the diplodocine specimen CM 3452 as well as CM 11255 (probably Barosaurus; Melstrom et al. 2016) lack a postparietal foramen despite being juvenile, while the adult apatosaurine specimen BYU 17096 has this feature, as does juvenile specimen SMM.P.84.15.3. Therefore, the presence or absence of a postparietal foramen is a non-ontogenetic trait among Morrison diplodocoid specimens. As detailed in a previous post, I did not rule out the possibility that the dicraeosaurid taxon Suuwassea could be a distinct taxon from other Morrison diplodocoids despite its holotype being a juvenile (Woodruff and Fowler 2012) because a postparietal foramen is absence in the diplodocine skull CM 11255 in spite of the juvenile nature of that specimen. The presence of a postparietal foramen in the Smitanosaurus agilis holotype and recovery of Suuwassea as derived within Dicraeosauridae by Whitlock and Wilson Mantilla (2020) confirms that despite being a juvenile, the holotype of Suuwassea emilieae (ANS 21122) is a distinct taxon in its own right rather than a Diplodocus juvenile as proposed by Woodruff and Fowler (2012).
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| Cladistic analysis of Smitanosaurus from Whitlock and Wilson Mantilla (2020). Although not a diplodocid as concluded by Tidwell et al. (2005), the diplodocoid placement of Smitanosaurus as hinted as by Tidwell et al. is vindicated by the above cladogram. |
The recovery of Kaatedocus as a dicraeosaurid rather than a diplodocine diplodocid is quite surprising. This genus has been recovered as either closer to Barosaurus than to other diplodocines (Tschopp et al., 2015) or a basal diplodocine (Tschopp and Mateus 2017). Characters cited by Tschopp et al. (2015) as placing Kaatedocus in Diplodocinae include: (1) broad maxilla-quadratojugal contact (2) antorbital fenestra with concave dorsal margin; (3) ‘hooked’ prefrontal; (4) mandible without strong coronoid eminence; (5) direct crown–crown occlusion absent; (6) ‘box-like basal tubera;’ (7) 14–15 cervical vertebrae; and (8) posterior cervical neural spines with elongate lateral spine cavities. The first four of these characters are dismissed by Whitlock and Wilson Mantilla due to either the state of preservation in the holotype of Kaatedocus siberi (SMA 0004) or the fact that SMA 0004 does not preserve enough vertebrae. The concave dorsal margin of the antorbital fenestra is noted as present in the rebbachisaurid Nigersaurus, the ‘box-like’ basal tubera character is omitted by the authors from the cladistic analysis of Smitanosaurus "because the codified states conflate too many distinct morphologies in the current taxonomic scope", while a ‘hooked’ prefrontal is noted as absent in Kaatedocus and the presence of elongate cavities on the posterior cervical neural spines is inconsistently present among diplodocoid taxa. Whitlock and Wilson Mantilla (2020) assign Kaatedocus to Dicraeosauridae based on several characters, namely the presence of a postparietal foramen, an expanded crista prootica, a shelf overhanging the opening for cranial nerve V, and a flat medial margin of the prefrontal. The authors' assertion about the absence of dorsal vertebrae in the Kaatedocus siberi holotype that could provide a definite cervical/dorsal vertebral count is borne out by the fact that Apatosaurus louisae is the only apatosaurine diplocid for which 15 cervicals are preserved, while Diplodocus carnegii and Galeamopus pabsti are the only diplodocines known from specimens that preserve a complete cervical vertebral series (Tschopp et al. 2015). Moreover, given assignment of Suuwassea to Dicraeosauridae by Whitlock (2011), it is not hard to imagine four dicraeosaurid taxa in the Morrison Formation (Kaatedocus, Smitanosaurus, Suuwassea and MOR 592) because the Puesto Antigual Member of the La Amarga Formation has yielded two dicraeosaurids, Amargasaurus and Amargatitanis.
In retrospect, Othniel Charles Marsh was initially right in the first place about Smitanosaurus being a diplodocoid before changing his mind about the classification of USNM 5384, and although Tidwell et al. (2005) agree that USNM 5384 is a diplodocoid, doubts about their referral of Smitanosaurus agilis to Diplodocus longus due to the re-assignment of some putative skulls of D. longus to Galeamopus and the discovery of dicraeosaurid characters in USNM 5384 make clear that the diversity of Morrison non-diplodocid flagellicaudatans is greater than previously thought and that some traditional diplodocine synapomorphies are also present in some dicraeosaurids.
References:
Gilmore, C.W., 1907. The type of the Jurassic reptile Morosaurus agilis redescribed, with a note on Camptosaurus. Proceedings of the United States National Museum 32: 151–165.
Gilmore, C.W., 1925. A nearly complete articulated skeleton of Camarasaurus, a saurischian dinosaur from the Dinosaur National Monument, Utah. Memoirs of the Carnegie Museum 10:347-384.
Marsh, O.C., 1889. Notice of new American Dinosauria. The American Journal of Science and Arts, series 3 38:331-336.
pp. 345-401. In: Weishampel DB, Dodson P, Osmólska H, eds. The Dinosauria. Berkeley: University of California Press.
Melstrom, K.M., D’Emic, M.D., Chure, D.J., and Wilson, J.A., 2016. A juvenile sauropod dinosaur from the Late Jurassic of Utah, USA, presents further evidence of an avian style air-sac system. Journal of Vertebrate Paleontology e1111898.
Osborn, H. F., and Mook, C. C., 1919. Characters and Restoration of the sauropod genus Camarasaurus Cope from type material in the Cope Collection in the American Museum of Natural History. Proceedings of the American Philosophical Society 58 (6): 386-396.
Osborn, H. F., and Mook, C. C., 1921. Camarasaurus, Amphicoelias, and other sauropods of Cope. Memoirs of the American Museum of Natural History, New Series 3 (3): 247-387.
Tidwell, V., K. Carpenter, and C. Miles. 2005. A reexamination of Morosaurus agilis (Sauropoda) from Garden Park, Colorado. Journal of Vertebrate Paleontology 25 (supplement to 3):122A.
2015. A specimen-level phylogenetic analysis and taxonomic revision of Diplodocidae (Dinosauria, Sauropoda) PeerJ 3:e857 https://doi.org/10.7717/peerj.857
2017. Osteology of Galeamopus pabsti sp. nov. (Sauropoda: Diplodocidae), with implications for neurocentral closure timing, and the cervico-dorsal transition in diplodocids. PeerJ 5:e3179 https://doi.org/10.7717/peerj.3179
