Caudal vertebrae
The description is organized from anterior to posterior position along the caudal series.
Specimen MCF-PVPH 917. Three isolated anterior and middle caudal vertebrae (Fig. 2). Only one belongs to the most anterior section of the tail, probably one of the first three elements (MCF-PVPH 917/1; Fig. 2A-C). This vertebra is incomplete, lacking its right transverse process, and with most of the posterior surface eroded. The centrum of this vertebra is dorsoventrally taller than anteroposteriorly long, and has a slightly concave anterior surface whereas the posterior one is poorly preserved. Considering that the middle caudal vertebra of the same specimen is amphicoelous (MCF-PVPH 917/2; Fig. 2D-F), it is likely that the posterior face is slightly convex or flat, as to the procoelous-opistoplatyan (see[35]) anterior vertebrae of Malarguesaurus[26]. In contrast, derived titanosaurs have procoelous caudal vertebrae (Aelosaurini, Saltasaurinae and Rinconsauria taxa). The slightly concave anterior surface also differs from the deeper concave surface present in anterior caudal vertebrae of a titanosaur from the same formation (MMS-PV-09 and MMS-PV-10;[36]). The lateral surface of the anteriormost caudal vertebra lacks fossae or ridges (Fig. 2B). This contrasts with the anterior caudal vertebrae with fossae or pits in many diplodocoids and some titanosauriforms[37, 38]. Also, the internal bone structure lacks of camerae or camellae in both the centrum and the neural arch, such as others titanosauriforms (e.g., Padillasaurus, Huabeisaurus, Chubutisaurus, Wintonotitan;[39–42]), which contrast with derived titanosaurs where caudal pneumaticity is present[43]. The lateral and ventral surfaces of the centrum are anteroposteriorly concave. The left preserved transverse process of MCF-PVPH 917/1 is posterolaterally oriented and transversely long, reaching the posterior face of the centrum (Fig. 2B), as in most titanosauriforms[44]. This transverse process tapers distally, in contrast to the wing-shaped transverse process of the anteriormost caudal vertebrae of diplodocoids[37]. A low prezygodiapophyseal lamina (PRDL) is present, faded anterodorsally to contact ventrally the prezygapophyseal process (Fig. 2B). The presence of this lamina reinforces the interpretation of this element as an anterior caudal vertebra. The neural arch is around 1.4 times as high as the centrum and located in the anterior half of the dorsal surface of the latter. This ratio is similar to that Malarguesaurus ([26]: Fig. 5). The neural spine is vertical and anteroposteriorly elongated having a flat dorsal margin in lateral view, different to the slightly convex dorsal margin present in the anterior caudal vertebra of Malarguesaurus[26]. The prezygapophyseal processes are robust and surpass the anterior face of the centrum (Fig. 2B). They are anterodorsally oriented with an angle of 45°, as in several titanosaurs (e.g., Narambuenatitan, Overosaurus, Aeolosaurus;[45–47]: Fig. 2). They are also curve slightly downward at their distal end and projected medially. On the lateral surface of the prezygapophyseal process there is a faint tubercle (Fig. 2A). The spinoprezygapophyseal lamina (SPRLs) are short and faded close to the base of the lateral surface of the neural spine. A PRSL is present as a long and broad lamina on the anterior surface of the neural spine (Fig. 2A). There is no spinodiapophyseal lamina (SPDL), anterior centrodiapophyseal lamina (ACDL), and posterior centrodiapophyseal lamina (PCDL), as in MMS-PV-09 and MMS-PV-10[36]. The postzygapophyseal facets are concave and oval in outline, and face ventrolaterally (Fig. 2C). Each postzygapophysis is supported ventrally by a broad centropostzygapophyseal lamina (CPOL) that also forms the lateral and dorsal margins of the neural canal. The poorly preserved posterior portion of the neural arch does not allow us to confirm the existence of a developed hyposphene. Both spinopostzygapophyseal laminae (SPOLs) are well developed and run dorsally along the full length of the posterior surface of the neural spine (Fig. 2B-C).
The middle caudal vertebra MCF-PVPH 917/2 only lacks of its left prezygapophyseal process, and is slightly deformed (Fig. 2D-F). The centrum is longer than high, and have a both anterior and posterior faces with a circular outline. The centrum is slightly amphicoelous with its anterior articular surface more concave than the posterior one. This is shared with Malarguesaurus [26] and other specimen from the Sierra del Portezuelo (MCF-PVPH 162;[48]) and contrast to the typical procoelous middle caudal vertebra of titanosaurs and the specimen MCF-PVPH 163[48] also from the same locality. The cross section of the centrum is circular as most Titanosauriformes, unlike the trapezoidal shape of several lognkosaurians (e.g., Uberabatitan, Trigonosaurus;[49]: Fig. 13;[50]: Figs. 28, 30). The ventral surface of the centrum is concave in lateral view (Fig. 2E) and lacks of a longitudinal hollow as in most neosauropods (e.g., Camarasaurus, Europasaurus, Chubutisaurus; BYU 9047;[5, 40]. On the dorsal margin of the posterior articular surface there is a notch (Fig. 2F), which is also observed in the specimen MCF-PVPH 916 (see below). The transverse processes are reduced to a low protuberance (Fig. 2E). As in Titanosauriformes, the neural arch of the middle caudal vertebra is placed in the anterior half of the dorsal surface of the centrum (e.g., Giraffatitan, Venenosaurus, Tastavinsaurus, Dreadnoughtus;[51];[35]: Fig. 11.4;[52]: Fig. 8;[53]). The height of the pedicels (below the level of prezygapophyses) is greater than that of other titanosauriforms (e.g., Lusotitan, Venenosaurus, Cedarosaurus;[54]: Fig. 6–7;[35]: Fig. 11.4;[55]: Fig. 4) but slightly less than the height observed in Malarguesaurus ([26]: Fig. 6). The neural spine is transversely ticker and rectangular in lateral view, being 1.5 times as long as high (Fig. 2E). The dorsal margin of the neural spine is slightly concave. As in Malarguesaurus, Tastavinsaurus ([52]: Fig. 8), and some Saltasauroidea (e.g., Epachthosaurus, Malawisaurus, Alamosaurus;[56]: Fig. 7;[57]: Fig. 2;[58]), the neural spine is vertical (Fig. 2E), contrasting with the slightly directed posteriorly neural spine in middle caudal vertebrae of most neosauropods. The prezygapophyseal process is nearly horizontal and projects beyond the anterior face of the centrum. Ventrally, this process is supported by a thick centroprezygapophyseal lamina (CPRL) that forms the lateral walls of the neural canal (Fig. 2D). The prezygapophyseal process is less than 40% of the anteroposterior length of the centrum, which is different from the elongated process of some titanosaur taxa (e.g., Epachtosaurus, Malawisaurus, Mendozasaurus, Bonitasaura;[56]: Fig. 7;[59]: Fig. 9;[60]: Fig. 7). The postzygapophyseal facet is circular and flat (Fig. 2F) and a deep fossa develops anteriorly to it in the lateral surface of the neural arch (Fig. 2E). This fossa seems to be also present in Malarguesaurus (IANIGLA-PV 110/3). In posterior view, there are two laminae around the postzygapophyses, one lateral and one medial. Both connect the postzygapophyses with the neural spine, thus we consider them as the lateral spinopostzygapophyseal lamina (LSPOL) and the medial spinopostzygapophyseal lamina (MSPOL; Fig. 2F). This arrangement of laminae is similar to that observed in a slightly more anterior caudal vertebra of Malarguesaurus ([26]: Fig. 5C).
The middle caudal vertebra MCF-PVPH 917/3 represent the most posterior element of the caudal series of the Last Day locality. It is damaged as it lacks the distal tips of its prezygapophyseal processes, and the anterior end of the neural spine (Fig. 2G-I). The features of this vertebra are similar to those of the anterior one, except for a lesser development of the transverse processes (of which only the base of the left is preserved) and the more posterior orientation of the neural spine (Fig. 2H). In this vertebra, a lateral fossa also develops in the neural arch, in front of the postzygapophyses, and an MSPOL above and medial to them is also present (Fig. 2I).
Specimen MCF-PVPH 916. This specimen is composed of three middle and two posterior caudal vertebrae (Fig. 3). All vertebrae from this specimen are very damaged. The first two middle caudal vertebrae are represented by incomplete centra. The middle caudal vertebra MCF-PVPH 916/1 (Fig. 3A-C) is amphicoelous with its anterior face slightly deeper than the posterior one, which contrast with the procoelous middle caudal vertebra of titanosaurs. Due to the poor preservation of this specimen, no chevron facets are recognized. The vertebra MCF-PVPH 916/2 is a ventral half portion of a centrum (Fig. 3D). As the anterior vertebra, the centrum is amphicoelous, with the anterior articular surface more concave than the posterior one. The ventral surface is markedly concave in lateral view, and transversely convex. There are no excavations or ridges on the lateral surfaces of this centrum. Also lacks the ventrolateral ridges and midline hollow that are widely present in somphospondylans and diplodocoids[37, 54, 61]. On the ventral surface there are facets for the articulation with the chevrons.
The caudal vertebra MCF-PVPH 916/3 lacks the anterior portion of the centrum and neural spine (Fig. 3E-F). Although incomplete, the centrum appears to be amphicoelous, and the neural arch would be located at the anterior end of the dorsal surface of the centrum, as in the middle caudal vertebrae of titanosauriforms[62]. The posterior articular surface is slightly wider than higher, although due to the state of preservation of the vertebra these dimensions could have been almost the same (Table 1). The transverse processes are not preserved in this vertebra.
The vertebra MCF-PVPH 916/4 could be one of the first posterior caudal vertebrae of the tail (Fig. 3G-I). This element lacks portions of the centrum margins, both prezygapophyseal processes, and the posterior portion of the neural spine. The centrum is longer than high, having an elongation index (EI, sensu[31]) of 1.3. Both anterior and posterior articular surfaces have similar measurements, being as wide as they are tall (Table 1). The lateral surfaces lack fossae or ridges (Fig. 3H). As the anterior vertebra, no transverse processes are present (Fig. 3H). The dorsal margin of the posterior articular surface is lipped dorsally in lateral view. As in the specimen MCF-PVPH 917, the neural arch is placed in the anterior half of the dorsal surface of the centrum. Given the morphology of the preserved portion of the neural spine, if it were complete, it would be posteriorly inclined, which, together with the absence of transverse processes, and a greater elongation of the centrum, would indicate a more posterior position in the caudal series. The postzygapophyses are flat, have a circular outline (Fig. 3H), and do not surpass the posterior surface of the centrum.
Table 1
Measurements of caudal vertebrae from the Portezuelo Formation. Abbreviations: ah, anterior height of the centrum; aw, anterior width of the centrum; EI, elongation index sensu [31]; lc, length of the centrum; ph, posterior height of the centrum; pw, posterior width of the centrum; sh, height of the neural spine; sl, length of the neural spine; sw, width of the neural spine. All measurements are in centimeters. * indicates that a measurement is estimated.
Specimen | cl | aw | ah | pw | ph | sl | sw | sh | EI |
MCF-PV-PH 916 | 1 | 12,5 | - | - | - | - | - | - | - | - |
| 2 | 13,0 | - | - | - | - | - | - | - | - |
| 3 | - | - | - | 10,5 | 9,5 | - | - | - | - |
| 4 | 11,0 | 9,0 | 8,5 | 9,0 | 8,5 | - | - | - | 1,3 |
| 5 | 10,5 | - | 7,5 | 7,5 | 7,5 | - | - | - | 1,4 |
MCF-PV-PH 917 | 1 | - | 17,5 | 17,5 | - | - | 8,0 | - | 10,5 | - |
| 2 | 12,5 | 9,5* | 11,5 | 9,5 | 11,5 | 9,0 | 2,0 | 6,0 | 1,1 |
| 3 | 12,5 | 9* | 11,5 | - | 11,5 | 8,5 | 2,0 | - | 1,1 |
The caudal vertebra MCF-PVPH 916/5 is represented by a nearly complete isolated centrum without the neural arch (Fig. 3J-L). The centrum is slightly amphicoelous, and the anterior and posterior articular surfaces are equally concave. This contrasts with the procoelous posterior caudal vertebrae of Malarguesaurus ([26]: Fig. 7) and Eutitanosauria. The centrum is more elongated, having an EI of 1.4 (Table 1), which is different from the very elongated centrum of diplodocoids (e.g., Apatosaurus, Lavocatisaurus, Nigersaurus; CM 3018;[63]: Fig. 2A6, A7; MNN GAD 512). The articular surfaces have a circular outline (Fig. 3J, L), contrasting some titanosaur that have dorsoventrally flattened posterior caudal centra (e.g., Saltasaurus, Rinconsaurus;[64]: Fig. 24–25;[65]). No lateral ridges, fossae, and transverse processes are present (Fig. 3K). As was described in the specimen MCF-PVPH 917, the dorsal margin of the posterior articular surface is lipped and has a notch at half width (Fig. 3L, 3K).
Phylogenetic analysis
The initial analysis retrieved 173 most parsimonious trees (MPTs) of 1603 steps. The second round of TBR branch swapping found 400,000 MPTs, resulting in an overflow of the memory tree space (consistency index = 0.33; retention index = 0.71). The strict consensus tree (Additional file: Figure S1) had the same polytomies seen in previous iterations of the phylogenetic dataset (Pérez Moreno et al., 2023; Simón and Salgado, 2023). Iter PCR identified the neosauropods Andesaurus, Puertasaurus, Nemegtosaurus, and Rayososaurus as unstable taxa that were pruned to give 53,138 MPTs. This allowed the internal nodes Eutitanosauria, Titanosauria and Lithostrotia to be resolved (Fig. 5).
A sister clade to Somphospondily, consisting of Tastavinsaurus, Tehuelchesaurus, Malarguesaurus and specimens MCF-PVPH-916 and MCF-PVPH 917 was identified in our analysis. Within this newly recognized Somphospondily sister clade, the taxa share specific morphological features, such as a large pedicel height below the prezygapophysis with a vertical anterior border on the middle caudal vertebrae, vertical orientation of the neural spines on the most posterior middle and anterior caudal vertebrae, and subequal relative lengths of the proximal ulnar condylar processes. Notably, specimens MCF-PVPH-916 and 917 share additional features, such as a procoelous-opistoplatyan anterior caudal centra and anterodorsally oriented (~ 45°) prezygapophyses in the middle caudal vertebrae.
Bremer support values show a support of 1 for most nodes, including the clade to which specimens MCF-PVPH 916 and 917 belong. Some nodes with values greater than 1 correspond to the clades such as Neosauropoda, Diplodocidae, Dicraeosauridae and Macronari, although there are also other unnamed monophyletic groups with a support higher than 1 (Figs. 5; Additional file: FiguresS2-S3). When analyzing the Bremer support without the influence of unstable taxa, the results are very similar, although other nodes with a support greater than 1 stand out, such as the clade corresponding to Saltasaurinae (Additional file: Figure S3).Regarding the Jackknife and Bootstrap indices (Additional file: Figures S4 and S5), It is observed that the values are low, except in well-conserved groups such as Diplodocidea where these are greater than 60% (Additional file: Figure S4). Support values do not increase or vary considerably when unstable taxa are excluded prior to the analysis (Additional file: Figure S5).
In summary, the clade to which specimens MCF-PVPH 916 and 917 belong is poorly supported in Bremer, Jackknife and Bootstrap indexes. Also this clade lacks a reliable character combination that distinguishes this group from other titanosauriform groups, since the synapomorphic characters of this could be attributed to serial anatomical variations within the caudal vertebral series. This lack of support is attributed to the presence of highly unstable taxa, such as Malarguesaurus, which are recovered in different positions in other studies[3, 32, 67]. This instability is mainly due to the paucity of anatomical information of the taxa that make up this clade. Thus, due to the aforementioned, we do not feel comfortable nominating this clade unless future studies with a larger number of taxa can strengthen its recovery.
In any case, the phylogenetic information of the new specimens studied here contributes significantly to unraveling the complex phylogenetic relationships among non-somphospondily titanosauriforms and sheds light on the distinctive anatomical adaptations of these sauropod taxa.