Your Friends The Titanosaurs, part 36.5: Australotitan

After three years and dozens of posts, the titanosaurs have issued a lovely parting gift before the wrap-up. (Isn't the title of this series "Your Friends The Titanosaurs", after all?) Here we have Australotitan cooperensis from the Winton Formation of Australia, joining Diamantinasaurus matildae, Savannasaurus elliottorum, and part-time titanosaur Wintonotitan wattsi.

Genus and Species: Australotitan cooperensis; the genus name shouldn't be too difficult to figure out after numerous "-titans". "Australo", derived from the Ancient Greek word for southern, is a reference to Australia, so you could translate this strictly as "southern titan" or more loosely as "Australian titan". Meanwhile, "cooperensis" is a triple reference to the Cooper-Eromanga Basin, Cooper Creek channel system, and "Cooper Country" (Hocknull et al. 2021). It's also appropriate because the nickname of the type specimen is "Cooper", a specimen mentioned just a couple of entries ago under the erstwhile informal name "Bananabendersaurus".

Citation: Hocknull, S. A., M. Wilkinson, R. A. Lawrence, V. Konstantinov, S. Mackenzie, and R. Mackenzie. 2021. A new giant sauropod, Australotitan cooperensis gen. et sp. nov., from the mid-Cretaceous of Australia. PeerJ 9:e11317. doi:10.7717/peerj.11317.

Stratigraphy and Geography: The type specimen comes from EML011(a) (Eromanga Natural History Museum Locality, Eromanga Natural History Museum, Eromanga, Queensland, Australia), while referred specimens come from EML010 and EML013. These sites are in southwestern Queensland, in a cluster 80–90 km (50–56 mi) west-southwest of Eromanga. All localities are in the Winton Formation, dating to the early Late Cretaceous (Cenomanian–Turonian time frame, about 96–92 million years ago). (The type localities for the other three Winton Formation sauropods are some distance north, in central Queensland.) A southwest-draining river system (the "Ceduna River" existed here at that time. EML011(a) has the "(a)" because the site is known to include three sauropod specimens on three discrete levels. A group of dorsal vertebrae and teeth designated EFM103 comes from (b), and EMF111 comes from (c). The site is also notable for dinoturbation caused by sauropods (Hocknull et al. 2021).

Figure 7 from Hocknull et al. (2021) shows EML011. The original caption reads "(A) Site overview showing excavation pit, distant weathered geochemically weathered Glendower (Gl) and more proximal weathered Winton (Wf) and Quaternary alluvial (Qa) deposits. (B) Semi-articulated pubes and ischia from A. cooperensis gen. et sp. nov. with mediodorsal surfaces of each pubis facing upwards with the dislocated ischia in close articular approximation (arrows indicate d, distal; p, proximal; rd, right dorsal and ld, left dorsal). (C) In situ ovo-lobate deformation (def) of pubis. (D) Cross-section (a-b) of sediment beneath pelvis showing downwardly deformed laminations (lam) of the siltstone (slt) above E. (E) a lower surface-scoured sandstone (ss) layer. (F) Associated humerus (hum), ulna (uln) and scapula (sca) of A. cooperensis gen. et sp. nov. within the shallow stratigraphy of the site, including the surface vertosol (blacksoil, bs) that transitions into underlying Winton Formation siltstone (slt) with the bonebed (bb). A thin sandstone (ss) layer occurs below the siltstone and bonebed. Scale bars = 10 cm (C–E) and 100 cm (B & F)." CC BY 4.0.

Holotype: EMF102 (Eromanga Natural History Museum Fossil), which includes a partial left scapula, partial left humerus, complete right humerus, right ulna, both pubic bones and ischia, and partial right and left femora. Indeterminate bone material (originally attributed to osteoderms) is also associated. The bones were found over an area of approximately 480 square meters (5,200 square feet). The bones are oriented roughly NW–SE or, less frequently, perpendicular, indicating NW–SE water flow. They are buried in fine siltstone–mudstone, with woody fragments. Bone surfaces are weathered (Hocknull et al. 2021).

Figure 23 of Hocknull et al. (2021) shows the femora from the type specimen and referred specimen EMF105. The colors are explained in the text. The original caption is "(A–C) EMF102, left proximal femur head in proximal (A), posterior (B) and anterior (C) views. (D–G) EMF102, right near complete femur in proximal (D), posterior (E), anterior (F) and distal (G) views. (H–K) EMF105, right femur in proximal (H), posterior (I), anterior (J) and distal (K) views. 3-D image rendering methods used included, natural, B, D, F, I, K (left), A, C, E, G, H, J (right); ambient occlusion with radiance scaling, A-K (middle); coloured schematic (see Fig. 8), B, D, F, I, K (right), A, C, E, G, H, J (left). Arrows indicate direction (a, anterior; l, lateral, m, medial, p, posterior). Feature abbreviations: fic, fibular condyle; ft, forth trochanter; gtr, greater trochanter; hd, femoral head; lb, lateral bulge; lec, lateral epicondyle; lic, linea intermuscularis cranialis. Scale bars = 20 cm." CC BY 4.0

A few other specimens including limb bones were also referred to the species: EMF105, a complete femur; EMF164, composed of fragments of a presacral centrum and ribs, and more substantial but still fragmented ulna and femur; and EMF165, the distal end of a humerus. EMF164 came from EML010 along with remains of a smaller indeterminate sauropod (EMF106), and was heavily weathered. EMF105 and 165 came from EML013, along with metacarpal EMF166 and an unprepared large limb element. Their level is overlain by a bed of woody debris, and underneath is a thin mudstone lens fragments of gymnosperms, pteridophytes, and a possible cycadeoid. These and other plant sites of the south-central Winton exposures differ from those to the north that have horsetails, ginkgos, cycads, and angiosperms, suggesting a paleoenvironmental difference that could account for some of the titanosaur diversity in the Winton Formation (Hocknull et al. 2021).

A. cooperensis differs from the other Winton Formation sauropods in several ways, such as: several features of the scapula, such as its relatively elongate, straight parallel-sided blade, and possession of a ventral ridge on the lateral side of the blade; the relatively elongate and gracile humerus; the ulna has a relatively long anteriomedial process and an additional ridge, but relatively little bone between processes, so it is not as "thick" as the ulnae of the other sauropods; and its greater size and relatively elongate limbs. Comparisons are made throughout using digital surface models, created to make it easier to do such things without messing around with the weighty physical objects (Hocknull et al. 2021).

Figure 9 from Hocknull et al. (2021) compares scapulae of three Winton Formation taxa. The left column is Australotitan, the middle column is Wintonotitan, and the right column is Diamantinasaurus. The original caption is "Each element is rendered using four methods from top to bottom, natural; ambient occlusion with radiance scaling; coloured schematic (see Fig. 8); and orthogonal outline edge detection. (A & B) 3-D model of A. cooperensis gen. et sp. nov. left scapula in lateral (A) medial (B) views. (C & D) 3-D model of W. wattsi left scapula in lateral (C) and medial (D) views. (E & F) 3-D model of D. matildae right scapula (mirrored) in lateral (E) and medial (F) views. (G–I) Proximoventral views showing mid scapular blade cross-sectional profile in A. cooperensis gen. et sp. nov. (G), W. wattsi (H) and mirrored in D. matildae (I). Arrows indicate direction (d, dorsal; di, distal; p, proximal; v, ventral). Feature abbreviations: cr, central ridge of scapular blade; mt, medial tuberosity; pvp, proximoventral process; vr, ventral ridge of scapular blade. Scale bar = 20 cm." CC BY 4.0.

A. cooperensis is the largest Australian dinosaur known from bones (Hocknull et al. 2021). (The "bones" qualification is necessary because of a gigantic track.) The more complete right humerus is close to 1.5 m long (4.9 ft), and the most complete femur is nearly 1.9 m long (6.2 ft), but a more fragmentary femur is estimated to have exceeded 2.1 m long (6.9 ft) (Hocknull et al. 2021). This would certainly put A. cooperensis in the upper tier of titanosaurs. Overall, though, the build seems to have been on the gracile side.

Phylogenetic analyses of A. cooperensis put it in a clade with Diamantinasaurus and Savannasaurus (Diamantinasauria). Wintonotitan also shows a tendency to join. The position of the clade in the wider context of Titanosauria is uncertain, though: a rotating cast of Asian and/or South American sauropods are allied to them in various analyses, including one (with post-Turonian sauropods excluded) in which the diamantinasaurs are outside of Titanosauria (Hocknull et al. 2021).

Incidentally, there is a yet better specimen from these sites, EMF109 from EML012, which appears to include much of the torso and tail, although the exact material will not be known until preparation is complete (concretions are in the way). All that can be definitely said about it at this point is that it is not Wintonotitan wattsi (Hocknull et al. 2021).

This crop from Figure 6 of Hocknull et al. (2021) shows EMF109 in the field. "Articulated sauropod skeleton from EML012 preserved within a siltstone concretion, including the torso and tail. Identifiable elements include ribs (rib), dorsal vertebrate (dor), pelvic elements (pel) and caudal vertebrae (cdl)." CC BY 4.0.

References

Hocknull, S. A., M. Wilkinson, R. A. Lawrence, V. Konstantinov, S. Mackenzie, and R. Mackenzie. 2021. A new giant sauropod, Australotitan cooperensis gen. et sp. nov., from the mid-Cretaceous of Australia. PeerJ 9:e11317. doi:10.7717/peerj.11317.