Sunday, April 21, 2019

Your Friends The Titanosaurs, part 11: Drusilasaura, Elaltitan, and Epachthosaurus

It's an all-Argentina installment this time around, including two large titanosaurs that seem to have gotten lost in the flood of titanosaur taxa (Drusilasaura deseadensis and Elaltitan lilloi) and a more modestly sized stalwart of the clade (Epachthosaurus sciuttoi).

Drusilasaura deseadensis

Drusilasaura deseadensis is kind of your basic titanosaur: represented by a few bones primarily from the vertebral column, from Upper Cretaceous rocks of Argentina, not widely known except to specialists. It was named for excavation volunteer Drusila Ortiz de Zárate, with the species name referring to the valley of the Deseado River, where the specimen was found, making this roughly "Drusila Ortiz de Zárate's lizard from the Deseado River valley". The discovery locality was on the south side of the river on the María Aike ranch in northern Santa Cruz Province, southern Argentina. Stratigraphically, the find came from Cenomanian–Turonian rocks of the Upper Member of the Bajo Barreal Formation. The type material, catalogued as MPM-PV 2097/1 to 2097/19 (Museo Padre Molina, Paleontología de Vertebrados, Río Gallego, Santa Cruz, Argentina), consists of four dorsal vertebrae, a sacral, six caudals, the left scapula, a rib fragment, a couple of chevrons, and bits and pieces, and as you might guess the genus and species are defined based on vertebral characteristics (Navarette et al. 2011). The specimens had been briefly mentioned a few years before, in Navarette et al. (2008).

D. deseadensis is a "sneaky big" titanosaur, a large species generally unknown to the public. The (rather beat up) scapula is 143 cm long (56.3 in), and described by Navarette et al. (2011) as 30% larger than than of Mendozasaurus neguyelap, now known to be a mid-sized titanosaur but thought to be larger at the time. As a large South American titanosaur, D. deseadensis gets a guest pass to Club Lognkosauria, and Navarette et al. (2011) suggested that features in the caudals indicated it belonged to that group. The most telling feature was the presence of transversely expanded caudal neural spines (i.e., the spines on the tail vertebrae are expanded side-to-side). The authors also suggested that it could be close to Baurutitan britoi (Navarette et al. 2011).

Like some of the other titanosaurs that have already been featured, D. deseadensis is the kind of titanosaur that so far has only received brief mentions outside of its original description. It has occasionally appeared in phylogenetic analyses. In one case it turned up joined with Baurutitan, but oddly just outside of Titanosauria (see also comments to the Baurutitan post linked above) (Bandeira et al. 2016). It has also been found in Lognkosauria (Carballido et al. 2017), just as Navarette et al. (2011) suspected.

Elaltitan lilloi

Elaltitan lilloi goes back quite a while, even if the name only goes back to 2012. It is based on PVL 4628/MACN-CH 217 (PVL = (Colección de Paleontología de Vertebrados de la Fundación Instituto Miguel Lillo, Tucumán; MACN = Museo Argentino de Ciencias Naturales Bernardino Rivadavia, Buenos Aires) a partial skeleton. This specimen was first mentioned by Bonaparte and Gasparini (1979), as Antarctosaurus sp., then was moved to Argyrosaurus by Powell (1986, 2003), where it remained until Mannion and Otero (2012) whittled Argyrosaurus down to size. In the interim, it frequently stood in for Argyrosaurus in the literature.

The type and only known specimen was found on the south bank of the Río Senguerr in southeastern Chubut Province. Originally it was catalogued as PVL 4628, but some of the vertebrae later went to the MACN, explaining the other catalog number (Mannion and Otero 2012). The identification of the source formation has changed a few times; Bonaparte and Gasparini (1979) attributed the specimen to the Laguna Palacios Formation, whereas authors in the 1990s and early 2000s preferred the Lower Member of the Bajo Barreal Formation (e.g. Mannion and Otero 2012), and it is now attributed to the lower Lago Colhue Huapi Formation (Casal et al. 2016). This also means it was not contemporaneous with Argyrosaurus superbus (well, what's left of it) (Casal et al. 2016). The genus name refers to Elal, the Tehuelche creator god (Mannion and Otero 2012). (No, not the Israeli airline.) The species name honors Miguel Lillo. I'm not exactly sure of the best way to translate the name, specifically which name should go where; perhaps something like "Miguel Lillo's titan of Elal"?

PVL 4628/MACN-CH 217 is an associated partial skeleton including three dorsals, two caudals, most of the left arm and shoulder except for the coracoid and hand, right ulna, right pubis, the proximal half of the right femur, the distal left tibia, most of the left fibula, and the right astragalus and calcaneum. The calcaneum, the ankle bone associated with the fibula, is otherwise practically unknown in titanosaurs, but given the small size of the bone, we may have taphonomy to blame for its rarity. That inventory might sound like a lot, but there's also a lot of titanosaur to cover, both figuratively and literally; the partial femur measures 115.0 cm long (45.3 in), which, if we double, puts us at 230.0 cm (90.6 in). That's "Antarctosaurus" giganteus territory, and Mannion and Otero (2012) noted that the half we have of the E. lilloi femur is morphologically similar to the "A." giganteus femur to boot. Based on humeral and femoral circumferences, Lacovara et al. (2014) estimated the type individual of E. lilloi at 42.8 metric tons, and even if you think that's drastically oversized and reduce it by, say, half, that's still a heckuva slab of titanosaur beef. Like D. deseadensis, E. lilloi is another "sneaky big" titanosaur.

E. lilloi is distinguished from other titanosaurs by the anatomy of the laminae in the dorsal vertebrae, details of the ankle, and unusual features of the proximal caudal vertebra: an unusually tall, fore-aft-shortened, and anteriorly cheated neural arch. The forelimb bones don't compare all that well to Argyrosaurus superbus (absent ridges and different shape for the humerus, robust ulna instead of gracile, distal radius expanded and with a different cross-section in E. lilloi). In fact, E. lilloi is rather more similar to D. deseadensis, although Mannion and Otero (2012) did point out some salient differences. Also like D. deseadensis, E. lilloi is an "only mentioned, never more" taxon. Curiously, it doesn't show up in phylogenetic analyses despite being represented by more bones than many titanosaurs, apparently having been blackballed as a traitor to Argyrosaurus or something. Mannion and Otero (2012) regarded it as a lithostrotian.

Epachthosaurus sciuttoi

I'm not all that fond of Epachthosaurus sciuttoi. I think it's the genus name; my eye trips over that "chth" business. I know it's completely irrational, but we all play favorites for stupid reasons. (For me it also suffers from memory corruption. Do you have an actor incorrectly associated with a movie, or a band incorrectly associated with a song, or a player on the wrong team, and you can't get that wrong connection out of your head even though you know it's wrong? For some reason my mental files think E. sciuttoi is known from several skeletons, but it's basically just the one nice referred skeleton.)

E. sciuttoi first appeared as a scientific entity in 1986, when it showed up in Powell's dissertation. He formally named the genus and species in Powell (1990). The genus name means "heavy lizard", with the species name honoring Dr. J. C. Sciuttoi, who reported the locality, so we've got "J. C. Sciuttoi's heavy lizard." The type specimen, MACN-CH 1317, is a posterior dorsal vertebra from the Upper Member of the Bajo Barreal Formation, Ocho Hermanos Ranch, Chubut Province, Argentina. This means that all three guests in today's entry are from or were thought to be from that formation. It's just a coincidence; we won't be seeing the Bajo Barreal again until we get to Sarmientosaurus musacchoi, unless a new titanosaur comes out of it.

This was not the end of the material. Powell (1990) assigned a second specimen, MACN-CH 18689, from Ocho Hermanos as a "paraplastotype": basically, a cast of some fossils that could not be removed. These consisted of six articulated dorsals, a partial sacrum, and a fragment of the ilium. This specimen has sat somewhat uneasily in E. sciuttoi, with some authorities supporting the assignment (e.g. Martínez et al. 2004) and others regarding it as from a different taxon (e.g. Apesteguía 2005). He also had the rather more famous articulated skeleton UNPSJB-PV 920 (Universidad Nacional de la Patagonia "San Juan Bosco" Paleovertebrados, Comodoro Rivadaria, Argentina), which again came from the Ocho Hermanos site. Finally, he had most of a tail from a sauropod from the Allen Formation, which he had assigned to cf. E. sp. back in 1986. Salgado (1996) made this specimen the type specimen of Pellegrinisaurus powelli, but even though Powell was included in the species name he still considered it cf. E. sp. years later (Powell 2003). Casal and Ibiricu (2010) have since described a couple more E. sciuttoi specimens (vertebrae and pelvic bones) from the Ocho Hermanos site.

UNPSJB-PV 920 was the subject of a more detailed description by Martínez et al. (2004), which mentioned that one of the authors was preparing a monograph on the species. Unfortunately, it has not appeared to date. After all of the "couple of dorsal and caudal verts and a long bone" we've seen, it's refreshing to get an essentially complete and articulated skeleton from the shoulders back. The only parts missing are the skull, neck, and some anterior dorsals. (They may well have been there at one time but became lost to erosion, because erosional damage is concentrated at the anterior end of the skeleton. So close! Do you realize the odds against a nearly complete and articulated sauropod?) No osteoderms were found, and given the condition of the skeleton we can be reasonably certain that this individual did not have any. As for the hands, the only evidence of phalanges is a "vestigial element fused to the distal surface of metacarpal IV". No carpals were found in the wrist or calcaneum in the ankle, so they may have been cartilaginous. The foot is reduced to a phalangeal formula of 2-2-3-2-0. The limb bones are not described as notably robust or gracile, except for the "slender" tibia and fibula. The better-preserved right femur measures 109.5 cm long (43.11 in), which is about in the middle for titanosaurs. As in any good titanosaur the hips flare across traffic, so E. sciuttoi would have taken up a fair amount of space. The sacrum features a strip-like ossified-tendon-type structure positioned above the sacral neural spines, also described in Olga et al. (2008). The preservation of the hips and hind legs has permitted a detailed reconstruction of the muscles of this end of the animal (Ibiricu et al. 2018).

Epachthosaurus sciuttoi getting up on its hind legs at the Egidio Feruglio Museum in Argentina, probably not something you saw every day in Bajo Barreal times. Photo by Gastón Cuello, found on Wikimedia Commons (with some ceiling trimmed by me). CC-BY-4.0.

One of the more unusual features possessed by E. sciuttoi, at least from the standpoint of a "normal titanosaur", is its possession of hyposphene–hypantrum articulation or something like it in the dorsal vertebrae. (In fact, E. sciuttoi goes over and above the call of duty in this area by having H–H articulation in many of the caudals as well; Martínez et al. 2004.) We looked at this kind of articulation in some detail back with Andesaurus delgadoi, so suffice it to say here that titanosaurs in general made do without H–H, unlike almost all other sauropods. Unlike Andesaurus, E. sciuttoi had well-developed procoelous caudals. The combination suggests it was more derived than Andesaurus while still being less derived than most other titanosaurs, but we should always be cautious when looking at a small number of features.

You know, E. sciuttoi isn't as bad as I thought.

References

Apesteguía, S. 2005. Evolution of the hyposphene-hypantrum complex within Sauropoda. Pages 248–267 in V. Tidwell and K. Carpenter, editors. Thunder-Lizards: the sauropodomorph dinosaurs. Indiana University Press, Bloomington and Indianapolis, Indiana.

Bandeira, K. L. N., F. Medeiros Simbras, E. Batista Machado, D. de Almeida Campos, G. R. Oliveira, and A. W. A. Kellner. 2016. A new giant Titanosauria (Dinosauria: Sauropoda) from the Late Cretaceous Bauru Group, Brazil. PLoS ONE 11(10):e0163373. doi:10.1371/journal.pone.0163373.

Bonaparte, J. F., and Z. B. Gasparini. 1979. Los saurópodos de los grupos Neuquén y Chubut y sus relaciones cronológicas. Actas V Congreso Geológico Argentino, Neuquén 2:393–406.

Carballido, J. L., D. Pol, A. Otero, I. A. Cerda, L. Salgado, A. C. Garrido, J. Ramezani, N. R. Cúneo, and J. M. Krause. 2017. A new giant titanosaur sheds light on body mass evolution among sauropod dinosaurs. Proceedings of the Royal Society B: Biological Sciences 284(1860):20171219. doi:10.1098/rspb.2017.1219.

Casal, G., and L. Ibiricu. 2010. Assignable material to Epachthosaurus Powell, 1990 (Sauropoda: Titanosauria), from the Bajo Barreal Formation, Upper Cretaceous, Chubut, Argentina. Revista Brasileira de Paleontologia 13(3):247–256.

Casal, G. A., R. D. Martínez, M. Luna, and L. M. Ibiricu. 2016. Ordenamiento y caracterización faunística del Cretácico Superior del Grupo Chubut, Cuenca del Golfo San Jorge, Argentina. Revista Brasileira de Paleontologia 19:53–70.

Ibiricu, L. M., R. D. Martínez, and G. A. Casal. 2018. The pelvic and hindlimb myology of the basal titanosaur Epachthosaurus sciuttoi (Sauropoda: Titanosauria). Historical Biology in press. doi:10.1080/08912963.2018.1535598.

Lacovara, K. J., L. M. Ibiricu, M. C. Lamanna, J. C. Poole, E. R. Schroeter, P. V. Ullmann, K. K. Voegele, Z. M. Boles, V. M. Egerton, J. D. Harris, R. D. Martínez, and F. E. Novas. 2014. A gigantic, exceptionally complete titanosaurian sauropod dinosaur from southern Patagonia, Argentina. Scientific Reports 4:6196. doi:10.1038/srep06196.

Mannion, P. D., and A. Otero. 2012. A reappraisal of the Late Cretaceous Argentinean sauropod dinosaur Argyrosaurus superbus, with a description of a new titanosaur genus. Journal of Vertebrate Paleontology 32(3):614–618.

Martínez, R. D., O. Giménez, J. Rodríguez, M. Luna, and M. C. Lamanna. 2004. An articulated specimen of the basal titanosaurian (Dinosauria: Sauropoda) Epachthosaurus scuittoi from the early Late Cretaceous Bajo Barreal Formation of Chubut Province, Argentina. Journal of Vertebrate Paleontology 24(1):107–120

Navarrete, C., G. Casal, and R. Martínez. 2008. Nuevos materiales de Titanosauridae (Lognkosauria?) de la Formación Bajo Barreal, Cretácico Superior, Santa Cruz, Argentina. Congreso Latinoamericano de Paleontología de Vertebrados 3, Libro de Resúmenes:176.

Navarrete, C., G. Casal, and R. Martínez. 2011. Drusilasaura deseadensis gen. et sp. nov., a new titanosaur (Dinosauria-Sauropoda), of the Bajo Barreal Formation, Upper Cretaceous of north of Santa Cruz, Argentina. Revista Brasileira de Paleontologia 14(1):1–14. doi:10.4072/rbp.2011.1.01.

Olga, G., L. Salgado, and I. A. Cerda. 2008. Osteohistología de la viga supraneural del sacro del titanosaurio Epachthosaurus sciuttoi (Cretácico Tardío del Chubut). Naturalia Patagónica 4(1):111–117.

Powell, J. E. 1986. Revision de los Titanosauridos de America del Sur. Dissertation. Universidad Nacional de Tucumán, Tucumán, Argentina.

Powell, J. 1990. Epachthosaurus sciuttoi (gen. et sp. nov.) un dinosaurio sauropodo del Cretácico de Patagonia (provincia de Chubut, Argentina. Actas del Congreso Argentino de Paleontologia y Bioestratigrafia 5:123–128.

Powell, J. E. 2003. Revision of South American titanosaurid dinosaurs: palaeobiological, palaeobiogeographical, and phylogenetic aspects. Records of the Queen Victoria Museum 111.

Salgado, L. 1996. Pellegrinisaurus powelli nov. gen. et sp. (Sauropoda, Titanosauridae) from the Upper Cretaceous of Lago Pellegrini, northwestern Patagonia, Argentina. Ameghiniana 33(4):355–365.

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