On a dicraeosaurid specimen from the Mulichinco Formation (Valanginian, Neuquén Basin) of Argentina and phylogenetic relationships of the South American dicraeosaurids (Sauropoda, Diplodocoidea)

The osteology of Pilmatueia faundezi Coria, Windholz, Ortega & Currie, 2019, a dicraeosaurid sau-ropod from the Lower Cretaceous of Patagonia, is reassessed from the perspective of a new specimen (MLL-Pv-010) that provides additional information on the axial skeleton and the pectoral girdle. The specimen MLL-Pv-010 is composed of three articulated anterior-middle cervical vertebrae (with their respective ribs in position and an associated fourth rib), seven dorsal vertebrae with associated dorsal ribs, a distal caudal vertebra, a left scapula and the proximal end of a right scapula. The new specimen shows features, especially in the middle cervical vertebrae, that link it to Pilmatueia faundezi . Additionally, the specimen MLL-Pv-010 shows features previously unknown for the species, such as the morphology and orientation of the bifid neural spines of the anterior and mid-cervical vertebrae. The information obtained from the specimen MLL-Pv-010 allows us to propose an expanded diagnosis of Pilmatueia faundezi . Moreover, two phylogenetic analyses focusing on South American dicraeosaurids show that Pilmatueia Coria


INTRODUCTION
Dicraeosauridae Huene, 1927 (Sauropoda, Diplodocoidea) is a clade of neosauropod dinosaurs that has been defined as all diplodocoids closer to Dicraeosaurus Janensch, 1914 than to Diplodocus Marsh, 1878(Sereno 1998. The biochron of this clade corresponds to the Middle Jurassic -Lower Cretaceous, and its records come from Africa, Asia, North America, and South America (Janensch 1914;Salgado & Bonaparte 1991;Upchurch 1995;Wilson 2002;Harris & Dodson 2004;Rauhut et al. 2005;Apesteguía 2007;Whitlock 2011;Tschopp et al. 2015;Gallina 2016;Xu et al. 2018;Coria et al. 2019;Gallina et al. 2019). Interestingly, in the Lower Cretaceous of Patagonia in Argentina, this group was a conspicuous component of the terrestrial ecosystems. Its diversity is represented so far by four species: Amar-  (Coria et al. 2019) is a recently reported dicraeosaurid sauropod from the Lower Cretaceous (Valanginian) of the Neuquén Basin, Argentina. It was originally based upon a posterior dorsal vertebra (holotype), a posterior cervical vertebra (paratype), a mid-cervical vertebra, a middorsal neural arch, and two mid-caudal vertebrae (referred material). A new dicraeosaurid specimen (MLL-Pv-010) collected from the same site and geological formation as the type materials of Pilmatueia Coria, Windholz, Ortega & Currie, 2019 is described here. The new specimen is a partially articulated trunk and includes presacral vertebrae (three articulated anterior-middle cervical vertebrae, with their respective ribs in position and an associated fourth rib, and seven dorsal vertebrae with associated dorsal ribs), a posterior caudal vertebra and both scapulae.
The new specimen MLL-Pv-010 shares with Pilmatueia the presence of anteriorly and posteriorly bifid ventral keels in the anterior and mid-cervical vertebrae, which is an unidentified character in other dicraeosaurids that justifies reference of the new specimen (MLL-Pv-010) to this species. Simultaneously, the cervical vertebrae of MLL-Pv-010 show features unknown to date for Pilmatueia, such as the morphology and orientation of the bifid neural spines. Furthermore, a distal caudal vertebra and elements from the appendicular skeleton (in this case, both scapulae) expand the anatomical knowledge of this species.
Historically, dicraeosaurid diversity consisted of a handful of taxa (Janensch 1914;Salgado & Bonaparte 1991). Recently, the number of species described has grown substantially, which has promoted the inclusion of dicraeosaurid taxa in various phylogenetic analyses (Gallina 2016;Xu et al. 2018;Coria et al. 2019;Gallina et al. 2019;Whitlock & Wilson Mantilla 2020). There is some agreement in previous phylogenies that show Amargasaurus Salgado & Bonaparte, Our phylogenetic hypotheses recover Pilmatueia faundezi well nested within Dicraeosauridae. In one of them, this taxon is recovered phylogenetically closer to Amargatitanis and Bajadasaurus, while in the other one, it is sister taxon to Amargatitanis and Suuwassea. Also, in one of our analyses the South American dicraeosaurids were recovered phylogenetically far from each other, as in previous contributions (Rauhut et al. 2005;Gallina 2016;Xu et al. 2018;Coria et al. 2019;Gallina et al. 2019;Whitlock & Wilson Mantilla 2020), whereas, in the other one these taxa cluster together in a natural group (Appendix 1).

MATERIAL AND METHODS
The identifications of the presacral vertebrae were based upon the relative positions of the parapophyses on the vertebral centra, compared with the complete presacral vertebral series of the holotype specimen of Dicraeosaurus hansemanni Janensch, 1914(Janensch 1929 and personal observations on Amargasaurus cazaui . For the osteological description we mainly followed the nomenclature present in Romer (1956) and Upchurch et al. (2004a). Also, the  nomenclature of the vertebral laminae and fossae proposed by Wilson (1999Wilson ( , 2012 and Wilson et al. (2011) was followed. The specimen MLL-Pv-010 was recovered approximately 100 m from the paratype and referred specimens of Pilmatueia, and about 1000 m from the holotype (Fig. 1). All the elements come from the same quarry, and were found at the same stratigraphic level (Fig. 2). The degree of association (some bones were articulated), the comparative sizes (Table 1), and the absence of repeated bones show that the elements described belong to a single specimen .

Institutional abbreviations
expAnDeD DiAgnosis. -Pilmatueia faundezi is unique in having anterior and mid-cervical vertebrae with anteriorly and posteriorly forked ventral keels (new, MLL-Pv-010); cervico-dorsal vertebrae with dorsoventrally oriented ridges on the anterior surfaces of anterior centrodiapophyseal laminae; posterior dorsal vertebrae with deep fossae located posteriorly at the bases of the bifid neural spines separated by thick, low, sagittal laminae; the proximal end of the mid-cervical rib has a foramen in its medial surface (new, MLL-Pv-010); and a prominent crest on the medial surface of the scapula is close to the acromial process (new, MLL-Pv-010). Furthermore, after the phylogenetic analysis using TNT, Pilmatueia shows the presence of bifurcated posterior centrodiapophyseal laminae in midcervical vertebrae (also present in some diplodocids and Giraffatitan, character 194); accessory laminae in region between posterior centrodiapophyseal lamina and posterior centroparapophyseal lamina in posterior dorsal vertebrae (unknown in Bajadasaurus, present in Demandasaurus and some macronarians, character 267); and three infrahyposphenal laminae in each dorsal vertebra (also present in Apatosaurus ajax, character 493).
locAlity AnD horizon. -The Pilmatué locality is 9 km northwest of Las Lajas, Neuquén Province, Argentina ( Fig. 1) and includes extensive outcrops of the Mulichinco Formation (Stipanicic et al. 1968;Schwarz 2003;Schwarz et al. 2011). The fossil accumulation is hosted in a sandy body composed of sets of lenticular beds that conform co-sets of tabular geometry, composed of fine conglomerates to coarse-grained sandstones with tangential cross-bedding (Pino et al. 2021).

cervicAl eleMents
The new specimen (MLL-Pv-010) has three articulated anterior-middle cervical vertebrae (putatively the fourth, fifth and sixth ones) in articulation with their respective left ribs (Fig. 3). Also, another cervical rib on the left side was preserved, possibly from a subsequent cervical vertebra (cervical rib 7) (Fig. 4). The vertebrae are well preserved, although transversely crushed. The parapophyses are located in the anteroventral regions of the centra.
In anterior view, the vertebral centrum of the fourth cervical vertebra is taller than wide. The parapophyses are robust and project lateroventrally, as in the fourth cervical vertebra of Amargasaurus (MACN-N 15). Both the centroprezygapophyseal and anterior centrodiapophyseal laminae frame a wide prezygapophyseal centrodiapophyseal fossa that has a triangular outline. The articular surfaces of the prezygapophyses are small in comparison with those of other diplodocoid taxa, and likely diverged dorsolaterally (before being deformed) as in the mid-cervical vertebra MLL-Pv-004 of Pilmatueia faundezi. In other diplodocoid sauropods like Dicraeosaurus, Diplodocus and MMCH-Pv-49 (Rebbachisauridae indet.), the articular surfaces of the prezygapophyses are more developed and dorsomedially oriented (Hatcher 1901;Janensch 1929). In anterior view, the bifid neural spine is straight and projects dorsally. In lateral view, the vertebral centra of the fourth and fifth cervical vertebrae are strongly opisthocoelous, as in the cervical vertebrae of other sauropods, such as Amargasaurus (MACN-N 15), Lavocatisaurus (MMCH-Pv 63) and Leinkupal Gallina, Apesteguía, Haluza & Canale, 2014 (MOZ-Pv1232; Gallina et al. 2014). Each is slightly longer than tall and has a wide lateral depression. In the anterior region of each lateral depression is a deep cavity, like a pleurocoel, reminiscent of those present in the cervical vertebrae of Pilmatueia (MLL-Pv-004) and Amargasaurus (MACN-N 15). The lateral depressions lack foramina, indicating absence or a low degree of pneumaticity in the middle region of the neck, as previously suggested for Pilmatueia (Windholz et al. 2019). The centrum of the fourth cervical vertebra bears a small accessory fossa posteroventral to the lateral depression, which is a feature shared with other flagellicaudatans such as Pilmatueia The neural arches bear the typical configurations of laminae and fossae previously described in other sauropods, with welldeveloped anterior and posterior centrodiapophyseal (acdl, pcdl), prezygodiapophyseal (prdl), and postzygodiapophyseal (podl) laminae. These laminae define deep centrodiapophyseal (cdf ), prezygapophyseal centrodiapophyseal (prcdf ), postzygapophyseal centrodiapophyseal (pocdf ) and spinodiapophyseal (sdf ) fossae, all of which have triangular outlines.
The centroprezygapophyseal laminae are broad, robust and project anterodorsally, whereas the centropostzygapophyseal laminae extend dorsoventrally and are column-like.
The bifid neural spines of the fourth and fifth cervical vertebrae are at least twice as high as their respective vertebral centra, although their distal ends are missing. The ventral surfaces of the centra are poorly preserved due to weathering. However, the ventral surface of the centrum in the fourth cervical vertebra is deeply excavated by a large, anteroposteriorly long depression. In the deepest part of this depression are two small foramina, separated by an anteriorly and posteriorly bifurcated ventral keel. Ventral keels are widely distributed features within Sauropoda. Diplodocoids tend to have simple keels, as in Kaatedocus (Tschopp & Mateus 2013), and a posteriorly bifid keel occurs in Dicraeosaurus (Janensch 1929). Thus, an anteriorly and posteriorly bifurcated ventral keel is only known to occur in MLL-Pv-004 and MLL-Pv-010, both specimens referred to Pilmatueia.
The cervical ribs are robust and lack most of their distal shafts (Fig. 3), with the exception of one cervical rib (Fig. 4). This cervical rib is short, as in other diplodocoid sauropods such as Amargasaurus 1929). The entire medial surface of the rib bears a prominent longitudinal lamina. As a result, the bone has a triangular outline in cross-section. The tuberculum is robust and has a kidney-shaped articulation, whereas the articular surface of the capitulum has an elliptical outline. The proximal end of the rib has a foramen ventral to the capitulum. This is a possible nutrient foramen opening, due to the lack of external correlates that indicate a certain degree of pneumaticity in the mid-cervical vertebrae. The presence of foramina in cervical ribs is unusual in Dicraeosauridae, because Amargasaurus (MACN-N 15), Brachytrachelopan (MPEF-PV 1716) and Dicraeosaurus (Janensch 1929) lack them. The edges of this foramen show no indications that suggest it is a preservational artifact. On the contrary, the edges are smooth and complete. Thus, the presence of this foramen seems to be a feature that differentiates Pilmatueia (MLL-Pv-010) from other dicraeosaurid taxa. cAuDAl vertebrA MLL-Pv-010 includes a well preserved distal caudal vertebra (Fig. 5). In anterior view, the centrum is as wide as high, and has a rectangular outline. The entrance to the neural canal is elliptical in outline, and is wider than tall. The neural arch is considerably lower than the centrum, and the neural spine is very small. Both prezygapophyses diverge dorsolaterally. In lateral view, the centrum is approximately twice the length of its height. The lateral surface lacks fossae and foramina, and is concave anteroposteriorly. The dorsal sector of the centrum has a prominent crest along its entire lateral surface, as in the distal caudal vertebrae of other flagellicaudatans such as Dicraeosaurus (Janensch 1929) and Diplodocus (Hatcher 1901). The neural arch is lower and shorter than the associated centrum. The neural spine is low and anteroposteriorly long, as in the distal caudal vertebra of Amargasaurus (MACN-N 15).

DorsAl eleMents
In posterior view, the articular surface of the centrum is as wide as tall, with a subquadrangular outline. The neural canal has an elliptic outline and is transversely wider than high. Both postzygapophyses are reduced and the articular surfaces at the base of the neural spine are oriented ventrolaterally. In dorsal view, the prezygapophyses are prominent and diverge anterolaterally. The neural spine is singular and anteroposteriorly long. In ventral view, the centrum is somewhat constricted in its middle part, which makes it slightly hourglass-shaped. Its ventral surface is slightly anteroposteriorly and transversely concave as in the distal caudal vertebrae of other flagellicaudatans, such as Amargasaurus (MACN-N 15), Dicraeosaurus (Janensch 1929) and Diplodocus (Hatcher 1901).  The new specimen (MLL-Pv-010) shares characters with other dicraeosaurids, such as extremely tall bifid neural spines in the cervical vertebrae (Fig. 7), and the position of the acromial process of the scapula proximally above the glenoid position ( Fig. 8) (Janensch 1929(Janensch , 1961Harris 2007;Tschopp et al. 2015;MACN-N  The fact that MLL-Pv-010 comes from the same formation and geographic proximity as the holotype, paratype, and referred specimens of Pilmatueia faundezi prompts a detailed review. The overlapping elements are the cervical vertebrae, although they probably do not correspond exactly to the same position (MLL-Pv-010 preserved articulated fourth, fifth and sixth cervical vertebrae, while MLL-Pv-004 is a seventh cervical vertebra and MLL-Pv-002 is a twelfth cervical vertebra) and posterior dorsal vertebrae, although these elements in MLL-Pv-010 are poorly preserved, which makes the comparisons difficult. However, MLL-Pv-010 shares with Pilmatueia faundezi the presence of anteriorly and posteriorly bifid ventral keels in the anterior and mid-cervical vertebrae (diagnostic character for Pilmatueia). Also, the posterior dorsal bifid neural spines are posterodorsally oriented both in MLL-Pv-010, as in the Pilmatueia holotype (MLL-Pv-005); this feature is also present in Amargasaurus (MACN-N 15).
This new specimen expands the diagnosis proposed by Coria et al. (2019) for Pilmatueia faundezi, adding new three autapomorphic characters (Fig. 9). The anterior and midcervical vertebra of Pilmatueia has a prominent anteriorly and posteriorly forked ventral keel. Ventral keels are widely distributed features within Sauropoda, for example diplodocoids taxa tend to have simple keels, as occurs in Kaatedocus (Tschopp & Mateus, 2013), or posteriorly bifid keels as occurs in Dicraeosaurus (Janensch 1929). An anteriorly and posteriorly bifurcated ventral keel is only known to occur in Pilmatueia (Fig. 9A, B). In addition, the proximal end of the mid-cervical rib has a foramen in its medial surface (Fig. 9C). The presence of cervical rib foraminae have been described  for some diplodocids taxa, but, this feature is not known in the other dicraeosaurid ribs. Finally, the scapula of Pilmatueia (MLL-Pv-010) has a prominent crest on its medial surface close to the acromial process (Fig. 9D), which is a character that differentiates it from other dicraeosaurids.  In Gondwanan dicraeosaurids (Dicraeosaurus (Brachytrachelopan (Amargasaurus (Pilmatueia, Amargatitanis, Bajadasaurus)))) the maximum diameter of each supratemporal fenestra is less than 1.2 times the largest diameter of the foramen magnum (character 74, unknown in Pilmatueia, Amargatitanis and Brachytrachelopan, present in the rebbachisaurid Limaysaurus); the neural spines are higher than the neural arch in the mid-cervical vertebrae (character 172, unknown in Pilmatueia and Amargatitanis); the angles formed by postzygodiapophyseal and spinopostzygapophyseal laminae are straight in the mid-cervical vertebrae (character 174,unknown in Pilmatueia,Amargatitanis and Bajadasaurus,present in Isisaurus); the posterior dorsal neural spines each has a midline cleft along the dorsal surface (character 291, unknown in Pilmatueia, Amargatitanis and Bajadasaurus, present in some diplodocines and Camarasaurus); the distal transverse expansion of the ulna is less than 1.3 times the minimum width of the diaphysis (character 402, unknown in the dicraeosaurids Pilmatueia, Amargatitanis, Bajadasaurus and Brachytrachelopan, present in some apatosaurines and Shunosaurus); the prefrontal length/height ratio (in lateral view) is greater than 3.0 (character 490, unknown in the dicraeosaurids Pilmatueia, Amargatitanis and Brachytrachelopan); the paroccipital processes are at least twice as long as wide (in dorsal view) (character 491,unknown in Pilmatueia,Amargatitanis and Brachytrachelopan,present in Camarasaurus).
Dicraeosauridae: (Suuwassea (Lingwulong (Dicraeosaurus (Brachytrachelopan (Amargasaurus (Pilmatueia, Amargatitanis, Bajadasaurus)))))), share the presence of a prominent projection (ventrally directed) at the posteroventral margin of the squamosal (character 60, unknown in Pilmatueia, Amargatitanis and Brachytrachelopan, present in the diplodocid Kaatedocus siberi); distinctive sagittally arranged nuchal crest in the supraoccipital (character 77, unknown in Pilmatueia, Amargatitanis and Brachytrachelopan, present in the diplodocids Galeamopus hayi and Kaatedocus siberi, and Mamenchisaurus); a supraoccipital foramina (character 78, unknown in Pilmatueia, Amargatitanis and Brachytrachelopan, present in the diplodocid Kaatedocus and the macronarian Giraffatitan); subtriangular outline in the cross section of the dentary symphysis (character 108, unknown in Pilmatueia, Amargasaurus, Amargatitanis, Brachytrachelopan and Lingwulong); presence of a tuberosity on the labial surface of the dentary, near the symphysis (character 109,unknown in Pilmatueia,Amargasaurus,Amargatitanis,Bajadasaurus,Brachytrachelopan and Lingwulong); the width/ height ratio in cervical vertebrae is less than 0.5 (character 131, unknown in Amargatitanis; shared with the macronarian Camarasaurus; Lingwulong has the apomorphic state); the anterolateral corner of the tooth row of the dentary, in occlusal view, is labially displaced (character 145,unknown in Pilmatueia,Amargasaurus,Amargatitanis,Brachytrachelopan and Lingwulong;present  For almost nine decades, the Dicraeosauridae was limited to two species of Dicraeosaurus (Janensch 1914) from the Middle Jurassic of Africa. Near the end of the 20 th century, Salgado & Bonaparte (1991)   Kaatedocus as a successive sequence of sister taxa (Fig. 10B). As in the phylogenetic hypothesis proposed by Whitlock & Wilson Mantilla (2020), a tendency is observed for the United States taxa to occupy the most basal positions within the Dicraeosauridae (with the exception of Suuwassea) suggesting a possible North American origin for this family. In this analysis, the South American dicraeosaurids are not recovered as a monophyletic group, unlike the analysis carried out previously. The different results could be explained, in principle, due to a difference in taxonomic sampling, character selection, and the presence of fragmentary and incomplete taxa. Nevertheless, we think that our first hypothesis is a better fit, because the appearance of biogeographic barriers in the Jurassic (Scotese 1991; Benedetto 2010; Ding et al. 2019) possibly generated vicariance phenomena between continental vertebrate faunas. Thus, it would be reasonable to recover a South American dicraeosaurid clade that eventually may be confirmed in the future by additional evidence. Furthermore, the phylogenetic position recovered for Suuwassea seems to be somewhat doubtful. This conclusion, is not only due to different time range with its sister taxon Amargatitanis, but also because of the high number of characters that are unknown for both. Finally, Pilmatueia in a position close to Bajadasaurus would seem to be the most logical hypothesis, because these taxa come from close geographical areas and two partially synchronous geological formations (lowermost Cretaceous levels from Patagonia).

CONCLUSIONS
A new dicraeosaurid specimen MLL-Pv-010 from the Mulichinco Formation, (Valanginian, Lower Cretaceous) of Patagonia, Argentina is characterized by having extremely tall bifid neural spines in the presacral vertebrae and an acromial process of the scapula that is positioned proximally above the glenoid. Both of these features show that MLL-Pv-010 is unequivocally a dicraeosaurid sauropod, which is also supported by two phylogenetic analyses. Also, MLL-Pv-010 shares with Pilmatueia the presence of anteriorly and posteriorly bifid ventral keels in the cervical vertebrae, which is a character unidentified in other dicraeosaurids. Such a result allows us to refer the new specimen MLL-Pv-010 to Pilmatueia faundezi, which is, indeed, a more parsimonious conclusion than having a diversity of dicraeosaurids at a single locality in the same stratigraphic unit. One of our phylogenetic hypotheses differs from previous contributions in showing South American dicraeosaurids as a monophyletic group. However, in the second hypothesis these are not recovered as a natural group. We think that it is reasonable to recover a South American dicraeosaurid clade that eventually will be best supported as more evidence is collected. The apomorphic state was divided into two new states to differentiate the forms whose bifid neural spines reach the most posterior dorsal vertebrae. Within Diplodocoidea, bifid neural spines in the presacral vertebral series occur only in the flagellacaudatans (Dicraeosauridae + Diplodocidae). In diplodocids, completely bifid neural spines include the most anterior dorsal vertebrae; while in Brachytrachelopan (MPEF-PV 1716) and Dicraeosaurus (Janensch 1929) dicraeosaurids, they reach the middle dorsal vertebrae. The most derived forms of Dicraeosauridae, such as Pilmatueia (MLL-Pv005) and Amargasaurus (MACN-N 15), have completely bifid neural spines up to the most posterior dorsal vertebrae (Appendix 6).
A new character was added to discriminate the degree of development of the paraoccipital processes present in dicraeosaurids, which is different than in other sauropods (Appendix 9).
A new character was added to differentiate the neural spines anterodorsally oriented, present in the anterior cervical vertebrae of Bajadasaurus (MACN-N 15) and Pilmatueia (MLL-Pv-010). These structures are posterodorsally oriented in other dicraeosaurid sauropods.
A new character was added to consider the presence of the median tubercle in the mid-cervical vertebrae of Dicraeosaurus (Janensch 1929) and Pilmatueia faundezi (MLL-Pv-004) (Appendix 10).