Elsevier

Cretaceous Research

Volume 128, December 2021, 104957
Cretaceous Research

Osteology, paleohistology and phylogenetic relationships of Pellegrinisaurus powelli (Dinosauria: Sauropoda) from the Upper Cretaceous of Argentinean Patagonia

https://doi.org/10.1016/j.cretres.2021.104957Get rights and content

Abstract

Pellegrinisaurus powelli is a large titanosaurian sauropod from the Upper Cretaceous of northern Patagonia (Río Negro Province, Argentina). The holotype of this species comprises an incomplete femur, four dorsal and 26 caudal vertebrae collected from the Lago Pellegrini locality. A single middle caudal vertebra from the nearby Cinco Saltos locality has been more recently referred to Pellegrinisaurus. With the purpose of increasing our knowledge about the titanosaur faunas of the Upper Cretaceous of Patagonia, here we provide a detailed redescription of the holotype and referred specimens of Pellegrinisaurus, reviewing its diagnosis and phylogenetic position. A histological analysis of the femur of the holotype is also carried out. The diagnostic characters originally proposed for Pellegrinisaurus are here considered invalid because they either correspond with diagenetic deformations of the specimen or are present in other titanosaurs. The new diagnostic features are related with the presence of large lateral blind fossae with distinct dorsal rims in the anterior caudal centra and with a low longitudinal ridge on the posterior side of the femur shaft originated from the trochanteric shelf and extending to the distal third of the shaft. The phylogenetic analysis recovers Pellegrinisaurus as a non-saltasaurid lithostrotian, closely related with Alamosaurus sanjuanensis. The bone histology indicates that the holotype corresponds to a subadult (i.e. sexually mature but still growing) individual. Finally, the caudal vertebra previously attributed to Pellegrinisaurus cannot be referred to this genus and is here assigned to Titanosauria indet.

Introduction

Pellegrinisaurus powelli is a large titanosaurian sauropod from the Upper Cretaceous of northern Patagonia (Río Negro Province, Argentina). The remains of the holotype specimen were discovered in a sand quarry located at the south margin of the Pellegrini Lake and recovered by the crew of the Museo Carlos Ameghino of Cipolletti (Río Negro Province) in 1975 (Salgado 1996; Powell 2003). Although the original description of Salgado (1996) was based on four dorsal centra, 26 caudal vertebrae and an incomplete femur, the individual actually preserved many more elements when it was discovered (Fig. 1), including several complete dorsal and sacral vertebrae, which were found in articulation (Powell 2003). Pellegrinisaurus powelli was initially assigned to Epachthosaurus sp. on the basis of dorsal vertebrae characters (Powell 1986). Later, Salgado (1996) assigned its remains to a new genus and species, proposing autapomorphies based on the relative proportions of the dorsal centra and in the morphology of the neural arches of the mid-posterior and posterior caudal vertebrae. More recently, Salgado and Bonaparte (2007) assigned an isolated middle caudal vertebra from the Cinco Saltos locality (Río Negro Province) to Pellegrinisaurus. Illustrations of this specimen were however not provided, as well as the anatomical features supporting its taxonomical assignment.

Regarding its phylogenetic affinities, Salgado (1996) considered Pellegrinisaurus powelli as the probable sister taxon of Saltasaurinae (=Saltasaurini sensu Salgado and Bonaparte 2007), but it was not included in a phylogenetic analysis to support this hypothesis. The first contribution that included Pellegrinisaurus in a phylogenetic analysis corresponds to Upchurch et al. (2004). There, this species was recovered as the sister taxon of Alamosaurus sanjuanensis, with this clade recovered as the sister group of Saltasauridae. More recently, Gorscak and O'Connor (2016, 2019) recovered Pellegrinisaurus as the sister group of the clade Alamosaurus sanjuanensis + Baurutitan britoi. In this data set Pellegrinisaurus is recovered as a Saltasauridae, with this position the result of an unusual position of the specifier taxa of this clade (Saltasaurus loricatus and Opisthocoelicaudia skarzynski). As such, Saltasauridae, in that study, includes several taxa that are usually excluded from this clade (e.g., Lognkosauria and basal titanosaurs such as Epachthosaurus sciuttoi; Carballido et al., 2017; González Riga et al., 2018; Mannion et al., 2019). The matrix of Gorscak and O'Connor (2016) was used by Sallam et al. (2018) to perform a phylogenetic analysis based on both Bayesian and parsimony approaches. Bayesian analysis recovered Pellegrinisaurus as the sister group of a clade formed by Dreadnoughtus schrani + (Alamosaurus sanjuanensis + Baurutitan britoi), in a similar topology to that recovered by Gorscak and O'Connor (2016, 2019). On the other hand, the parsimony analysis recovered Pellegrinisaurus in a slightly more basal position, being the sister group of Ampelosaurus atacis + (Paludititan nalatzensis + (Dreadnoughtus schrani + (Alamosaurus sanjuanensis + Baurutitan britoi))). A basal position of Pellegrinisaurus amongst titanosaurs was recovered by Bandeira et al. (2016). In sum, most analyses coincide in depicting Pellegrinisaurus as a non-Saltasauridae lithostrotian, except that of Gorscak and O'Connor (2016) and data sets derived from it, in which the “Saltasauridae relationships” of Pellegrinisaurus are caused by the unusual distant position of Opisthocoelicaudia and Saltasaurus, as was mentioned above.

In this paper, we provide a comprehensive redescription of the holotype material Pellegrinisaurus powelli. Our main goals are: to expand the anatomical description of Pellegrinisaurus powelli, to revise its current diagnosis, to describe and discuss the assignation of the specimen mentioned by Salgado and Bonaparte (2007), to determine the ontogenetic stage of the holotype specimen using bone histology, and to revise the phylogenetic position of this taxon. With regard to this last objective, we test previous hypothesis of the phylogenetic affinities of Pellegrinisaurus. This study is the first that test the phylogenetic position of Pellegrinisaurus based on first hand study of the material.

Institutional abbreviations: MCS-Pv, Museo de Cinco Saltos, colección de paleontología de vertebrados, Río Negro, Argentina; MPCA-Pv, Paleovertebrate collection of the Museo Provincial ‘Carlos Ameghino’, Cipolletti, Río Negro, Argentina.

Section snippets

Materials

The materials analyzed comprise the holotype of Pellegrinisaurus powelli (MPCA-Pv 1500), which consists of four incomplete dorsal vertebrae (MPCA-Pv 1500/1-4), 26 fragmentary caudal vertebrae (MPCA-Pv 1500/5-30) and an incomplete left femur (MPCA-Pv 1500/6-31). Unfortunately, other discovered parts of the skeleton (e.g. sacrum, dorsal ribs, neural arches of the dorsal vertebrae) were badly ruined in the field during the excavation procedures. Different from the original description of Salgado

Systematic paleontology

  • Dinosauria Owen 1842

  • Saurischia Seeley 1887

  • Sauropoda Marsh, 1878

  • Titanosauria Bonaparte and Coria 1993

  • Lithostrotia Upchurch et al., 2004

  • Pellegrinisaurus powelli Salgado 1996

  • Emended diagnosis. Lithostrotian titanosaurian characterized by the following combination of characteristics (autapomorphies highlighted with asterisks): presence of a prominent blind lateral fossa (“pleurocoel”) with well-developed dorsal rim in the anterior caudal centra∗; femur with longitudinal ridge located on the

Dorsal vertebrae (MPCA-Pv 1500/1-4)

Four incomplete dorsal vertebrae centra are preserved (Fig. 3). There is not current information regarding to their relative position in the column. Also, it is not possible to determine if they are continuous elements. Salgado (1996) assigned these elements to posterior dorsal vertebrae based on the absence of parapophyses. For descriptive purpose, we assign tentatively these elements to seventh (MPCA-Pv 1500/1), eighth (MPCA-Pv 1500/2), ninth (MPCA-Pv 1500/3) and tenth (MPCA-Pv 1500/4) dorsal

Femur histology

The samples show a poor preservation; however, the main histological features are discernible. In transverse section, the posterior side of the sample displays both compact and cancellous bone (Fig. 9A). The medullar region exhibits thick trabeculae enclosing intertrabecular spaces of varying sizes (Fig. 9B). These cavities are surrounded by centripetally deposited layers of secondary lamellar bone. The trabecular bone contains remains of secondary osteons. Some primary osteons are also

Phylogenetic analysis

In order to better understand the phylogenetic position of Pellegrinisaurus powelli amongst Titanosauria, we include the taxon into the data matrix recently published by Gallina et al. (2021), expanding the taxon sampling with the incorporation of four genera (Nullotitan glaciaris, Diamantinasaurus matildae, Choconsaurus baileywillisi and Pellegrinisaurus powelli), two new characters (419 and 420), and the inclusion of an additional state in two already used character (236 and 251) (see

Anatomical description of caudal vertebra MCS-Pv 53

This specimen consists of a single element, interpreted here as a middle caudal, which includes an almost complete centrum and an incomplete neural arch (Fig. 11A-C). Natural fractures reveal that pneumatic cavities are absent in both the centrum and the neural arch. The vertebral centrum is strongly procoelous, with a deep concave anterior face that is slightly wider than high. Excluding the articular condyle, the centrum is slightly longer than high. The posterior articular face has a

Diagnostic features of Pellegrinisaurus powelli

The anatomical review of the holotype of Pellegrinisaurus powelli provided here allows us to review the original diagnosis of Salgado (1996). Considering dorsal vertebrae, Salgado (1996) proposed two diagnostic features: 1- dorsal centra strongly depressed and, 2- transverse width of the centrum of the posterior dorsal vertebrae approximately twice the maximum dorsoventral depth. These characters, however, are only recorded in two of the dorsal vertebrae analyzed (MPCA-Pv 1500/2 and MPCA-Pv

Conclusions

A complete preparation and detailed study of the holotype specimen of Pellegrinisaurus powelli MPCA 1500 provides a wealth of new information on the anatomy, diagnosis, and phylogenetic position of this taxon. The diagnostic characters originally proposed by Salgado (1996) are here considered invalid because, either they are due to diagenetic deformations, or they are widely distributed amongst titanosaurs. The new diagnostic features are the presence of large lateral blind fossae with distinct

Acknowledgements

The authors would like to thank Carlos Muñoz (MPCA), Natalia Cides (MCS) and all the staff of Comisión Amigos del Museo of Cinco Saltos for access to the specimens under their care. Jaime Powell (R.I.P) provided unpublished pictures of the Pellegrinisaurus excavation. We thank José Aravena for the preparation of the material. Lorenzo Novak found and brings us the attention of specimen MCS-Pv 53. Jorge Blanco has skillfully made the illustrations on Fig. 5, Fig. 6, Fig. 8. The constructive

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