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.
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| 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:
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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
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