Pycnonemosaurus nevesi Kellner & Campos, 2002

Delcourt, Rafael, 2017, Revised morphology of Pycnonemosaurus nevesi Kellner & Campos, 2002 (Theropoda: Abelisauridae) and its phylogenetic relationships, Zootaxa 4276 (1), pp. 1-45 : 2-15

Pycnonemosaurus nevesi Kellner & Campos, 2002

Holotype. The holotype consists of an incomplete post-cranial skeleton ( DGM 859 -R) housed in the Museu de Ciências da Terra ( Earth Sciences Museum ), Rio de Janeiro, Brazil. It comprises: two incomplete caudal vertebrae, four caudal centra, three caudal transverse processes, rib remains, a right proximal pubis, right tibia, right distal fibula and unidentified elements. Kellner & Campos (2002) did not recognize the three transverse processes as belonging to Pycnonemosaurus . Nevertheless, additional material, including a distal caudal vertebra, one isolated transverse process, an isolated neural spine and some unrecognized elements were allocated to the same accession number as the holotype. Thus, here I recognized the transverse processes as belonging to Pycnonemosaurus nevesi (see description and discussion).

In the original description, Kellner & Campos (2002) assigned five incomplete teeth to the holotype (DGM 859-R) of Pycnonemosaurus with only a few features described and the authors wrote that the “detail description (with illustration) of this material will be presented elsewhere”. In the same month of Pycnonemosaurus ’s publication Bittencourt & Kellner (2002) published a detailed description of five teeth assigned to DGM 859-R. According to these authors there are nine teeth, four that are very fragmentary and consist of just the root and five others that are better preserved and only lacking portions of the enamel. However, these teeth were not assigned as Pycnonemosaurus nevesi by Bittencourt & Kellner (2002) and this assumption was made in 2011 when Bittencourt & Langer mentioned materials of Parecis Group’s abelisaur. As the teeth were found isolated from any dentigerous element (i.e. premaxilla, maxilla and dentary) and the Pycnonemosaurus holotype is based on caudal vertebrae and hind limb elements, it is thus difficult to assign them to the holotype. For these reasons, I consider the DGM 859-R teeth as belonging to an indeterminate abelisaurid instead of Pycnonemosaurus nevesi .

Type Locality. Pycnonemosaurus nevesi was collected in the Fazenda Roncador , Mato Grosso, Brazil, and according to Kellner & Campos (2002), this locality is in the Bauru Group. However, recent work has shown that this locality is actually part of the Parecis Group and that it is of Campanian-Masstrichitian age ( Weska , 2006; Bittencourt & Langer , 2011). In this geological context then, many sauropods and theropods remains have been discovered over the last years (see Bittencourt & Langer, 2011 for a wide revision).

Revised Diagnosis. Pycnonemosaurus nevesi can be distinguished from other abelisauroids on the basis of the following autapomorphies:

1) a pubis with small rounded foot and a ventrally bowed anterodistal end, 2) the posterior caudal transverse process is hook-shaped with the anterodistal expansion being short and bowed (modified from Kellner & Campos, 2002) and 3) a well-developed lateral malleolus of tibia that is ventrally expanded.

Kellner & Campos (2002) diagnosed Pycnonemosaurus as having “tibia with hatchet-shaped cnemial crest” and “caudal vertebrae showing moderate distal expansion of the transverse process”. Nevertheless, as seen in the description section, these features are shared among other abelisaurids species, thereby requiring the amended diagnosis above.

Description and Comparisons. Axial skeleton. The six caudal vertebrae of Pycnonemosaurus nevesi are assigned to the caudal series due to the absence of pneumaticity, as observed in other abelisaurids ( Méndez, 2014) (Measurements in Table 1). In addition, just two vertebrae preserve the neural arch ( Figs. 1–2 ) and similarity of size between them suggests that all vertebrae belong to the anterior position in the tail.

All caudal centra of Pycnonemosaurus are larger than other known abelisaurids ( Figs. 1–3 ) ( Bonaparte et al., 1990; O’Connor, 2007; Grillo & Delcourt 2017) and the articular facets are round like the those in Ceratosaurus ( Madsen & Welles, 2000) , Eoabelisaurus , Masiakasaurus , Carnotaurus, Aucasaurus, Ekrixinatosaurus, Ilokelesia and Skorpiovenator. By contrast, Majungasaurus, Arcovenator and Rajasaurus have articular facets with an elliptical shape ( Wilson et al., 2003; O’Connor, 2007; Tortosa et al., 2014). The caudal centra are spool-shaped and without pleurocoels, as in other abelisaurids. However, the lateral surface just ventral to the neural arch suture is slightly depressed with foramina. These depressions are present in Masiakasaurus (FMNH PR 2133), Carnotaurus, Aucasaurus, Ekrixinatosaurus and probably in Skorpiovenator (poor preservation in the latter prevents a more definitive statement regarding this character). Some foramina are large, being located on the neural arch suture near the articular surface. These may be associated with the attachment of the caudofemoralis muscle (Persons & Currie, 2011). The centra are amphicoelous as in other abelisaurids ( Méndez, 2014), Masiakasaurus and Ceratosaurus ( Madsen & Welles, 2000, plate 17) with the anterior surfaces slightly more concave than the posterior ones. The chevron facets are ventrally prominent and the ventral surfaces have a shallow longitudinal sulcus, as seen in Masiakasaurus, Eoabelisaurus, Aucasaurus , Carnotaurus, Ilokelesia, Viavenator ( Filippi et al., 2016) and Majungasaurus ( O’Connor, 2007) whereas Ekrixinatosaurus and Rajasaurus have a low ventral keel.

The neural arches generally resemble those of other South-American abelisaurids with a dorsally inclined transverse process and an anterior distal projection. The pre- and post-spinozygapophyseal lamina limit the pre- and post-spinal fossae anteriorly and posteriorly, respectively. The spinoprezygapophyseal fossa of Pycnonemosaurus resembles that of the sixth caudal vertebra in Carnotaurus , being larger and wider than the condition in Eoabelisaurus, Skorpiovenator and Aucasaurus (particularly if we consider the more incomplete vertebra).

The prezygapophyses form a right angle with one another and are inclined nearly 45° from vertical, as in Carnotaurus (from the third vertebra), Eoabelisaurus, Viavenator ( Filippi et al., 2016: fig. 7A) and Majungasaurus ( O'Connor, 2007: fig. 15). More acute angles can be seen in the caudal vertebrae of Ekrixinatosaurus, Aucasaurus, Skorpivenator and Arcovenator ( Tortosa et al., 2014: fig. 5). In Pycnonemosaurus nevesi , the prezygapophysis is slightly more elongated than in Carnotaurus, Aucasaurus and Skorpiovenator , being also similar in length to those in Eoabelisaurus , Majungasaurus ( O'Connor, 2007: fig. 15) and Arcovenator ( Tortosa et al., 2014: fig. 5), exceeding the limit of vertebral centrum such as in Majungasaurus, Eoabelisaurus, Viavenator ( Filippi et al., 2016: fig. 7C) and Arcovenator . In Skorpiovenator, Aucasaurus and Carnotaurus , the extent of the prezygapophysis reaches the margin of the vertebral centrum. In Pycnonemosaurus , the more incomplete neural arch of the prezygapophysis are very abraded and does not show this condition. In addition, the postzygapophyses are spaced, as seen in the first and second caudal vertebrae of Carnotaurus , in the sixth caudal vertebra of Skorpiovenator and in the fifth caudal vertebra of Eoabelisaurus . However, this structure differs from that of Aucasaurus , in which all caudal postzygapophyses are near to each other. The hyposphene-hypantrum articulations are present in Pycnonemosaurus , similar to the condition in Ceratosaurus ( Madsen & Welles, 2000) , Masiakasaurus, Eoabelisaurus, Viavenator ( Filippi et al., 2016) , Carnotaurus, Aucasaurus, Ekrixinatosaurus and Skorpiovenator , but differing from that in Majungasaurus ( O’Connor, 2007) and Arcovenator ( Tortosa et al., 2014) .

Both neural spines are incomplete, lacking the distal portion and located dorsal to the posterior half of the centrum. Nevertheless, the anteroposterior proportion of the neural spine in the articulated vertebrae is similar to anterior caudal of Carnotaurus and Majungasaurus , suggesting a rod-like condition for Pycnonemosaurus as in other abelisaurids ( Rauhut, 2003). The ventral half of the neural spine is directed posteriorly as in other abelisaurids. However, due to the fragmentary condition it is impossible to infer exact degree of inclination.

Additionally, there are three isolated transverse processes ( Figs 4-6 ). The dorsal inclination of the articulated processes is similar to those of furileuraurian abelisaurids, such as Carnotaurus, Aucasaurus and Skorpiovenator and in contrast to the condition in Majungasaurus, Arcovenator and Eoabelisaurus. The length ratio of the transverse process to the length of vertebral centrum is greater than 1.3 in abelisaurids ( Rauhut, 2003). Due to incomplete preservation (i.e. loss of the distal end of the transverse process), it is not possible to assess this state in Pycnonemosaurus . The three isolated transverse processes in Pycnonemosaurus nevesi exhibit an anterior projection at the distal end, similar to furileuraurian abelisaurids ( Filippi et al., 2016). The smaller transverse process is hook-shaped with a short and bowed tip ( Fig. 6 ). This condition is different from Aucasaurus, Ilokelesia, Ekrixinatosaurus and Skorpiovenator in which the distal end of the posterior caudal transverse process is more elongated and straight. The ventral surface of the transverse process is convex and has anterior and posterior centrodiapophyseal lamina, similar to other abelisaurids. These laminae delimit the three fossae on the lateral surface; the prezygapophyseal centrodiapophyseal fossa, the centrodiapophyseal fossa and the postzygapophyseal centrodiapophyseal fossa (sensu Wilson et al., 2011). Interestingly, the left side of the most complete vertebra, the postzygapophyseal centrodiapophyseal fossa has three small hollows unlike the right side, which only has two hollows ( Fig 1 C). This condition is unique among abelisaurids and probably is an individual condition of this specimen of Pycnonemosaurus nevesi .

Appendicular skeleton. The appendicular elements of Pycnonemosaurus are represented by a right pubis, right tibia and the distal end of the right fibula (Measurements in Table 1).

Pubis. The pubis of Pycnonemosaurus resembles the non-abelisaurinae abelisaurs with only the right shaft and the distal pubic foot preserved, whereas the proximal articulation has been lost ( Fig 7 ). The shaft is proximally sub-circular and becomes more compressed anteroposteriorly toward the distal end. There is an anteroposterior expansion immediately distal to the first constriction of the shaft, and then the shaft becomes compressed again. In the anterior view, the shaft is laterally bowed and the distal portion is rectangular shaped. The shaft shows a medial dorsoventral shelf that becomes the pubic apron and extends at the pubic foot as seen in Masiakasaurus (Carrano et al., 2002) , MCF-PVPH-237 (Coria et al., 2006) and Carnotaurus . This shelf is quite rugose, particularly at the caudal end and suggests a strong contact with the left pubis, however, it clearly was not fused as exhibited by Masiakasaurus ( Carrano et al., 2011) and Aucasaurus . The lateral surface of the shaft is flat and has scars that were interpreted as belonging to the muscle ambiens insertion. The distal foot is not elongated anteroposteriorly as in Carnotaurus and Aucasaurus , and the anterior end is anteroventrally inclined with a flat dorsal surface, whereas the posterior end is dorsally inclined. The distal pubis of Eoabelisaurus is short as in Pycnonemosaurus , however, the former has a straight foot base. The foot base is thick and robust as generally seen in other abelisaurids.

Tibia. The tibia of Pycnonemosaurus nevesi has a strong, nearly straight shaft and subcircular transverse section this is slightly compressed anteroposteriorly ( Fig 8 ). There are some incomplete parts with some surfaces covered with plaster. It is very large and thick, rivalling in size with those of large theropods such as Mapusaurus (Coria & Currie, 2006) , Allosaurus (Madsen, 1976) and Tarbosaurus ( Christiansen & Fariña, 2004) and it is larger than in other abelisaurids ( Grillo & Delcourt, 2017). Apart from its unusual size, the tibia has many abelisaurian characteristics. The shaft of the tibia of Pycnonemosaurus is gently curved laterally in lateral view, with the curvature being almost inconspicuous as in Ekrixinatosaurus, Quilmesaurus and Rahiolisaurus and distinguishing it from the tibial shaft of MCT 1783-R (Cambará taxon), Skorpiovenator, Xenotarsosaurus, Aucasaurus and Arcovenator in which the curvature is much more pronounced.

In the dorsal view, the lateral condyle is subcircular and robust. This shape is also present in Carnotaurus, Aucasaurus, Skorpiovenator, Ekrixinatosaurs and Arcovenator ( Tortosa et al., 2014: fig. 6) whereas in Quilmesaurus the lateral condyle is straight and in Eoabelisaurus , Majungasaurus, Cambará taxon and Xenotarsosaurus , it is square-shaped. The latter characteristic is even more pronounced in MTC 1783-R and Eoabelisaurus . The intercondylar groove is deep as in Cambará taxon ( Machado et al., 2013) and Aucasaurus compared with other taxa in which it is shallower.

The cnemial crest of Pycnonemosaurus is strongly pronounced anteriorly, inclined further proximally than the condyles with a dorsoventral constriction, giving it a hatchet-shaped similar to the condition in Aucasaurus . Its internal (i.e. lateral) surface is concave with an anteroposterior bulge in the dorsal portion, making an expanded dorsoventrally lateral fossa like in Ekrixinatosaurus, Aucasaurus and Quilmesaurus ( Fig. 8 B). The distal border is thicker than the body of the crest, suggesting strong insertion of muscles.

The fibular crest of Pycnonemosaurus is incomplete and filled with plaster, but appears to reach the ventralmost level of the cnemial crest and ending approximately at the first one-quarter of the bone’s shaft as seen in Majungasaurus ( Carrano, 2007) . The preserved portion is oriented vertically as in Masiakasaurus, Velocisaurus and Eoabelisaurus , whereas in Carnotaurus, Aucasaurus, Skorpiovenator and Xenotarsaurus the fibular crest is slightly anteroposteriorly inclined.

The lateral malleolus is well developed and ventrally expanded with an inclination of nearly 45° to the horizontal axis. In Pycnonemosaurus this structure is more pronounced than in the other abelisaurs. Quilmesaurus, Ekrixinatosaurus , Majungasaurus and Skorpiovenator also have ventral expansion but to a lesser degree than Pycnonemosaurus .

The articular facet of the astragalus of Pycnonemosaurus ( Fig. 9 ) is tongue-shaped and is not fused with the tibia as seen in the unfused astragalus of Quilmesaurus, Cambará taxon ( Machado et al., 2013), Rajasaurus ( Wilson et al., 2003) and Majungasaurus ( Carrano, 2007) . Its surface is narrower than figured by Kellner & Campos (2002) and the scars suggest a small ascending process with subparallel margins like in Ekrixinatosaurus ( Calvo et al., 2004; Rauhut, 2012). This condition is quite different to Rajasaurus and Quilmesaurus who have the articular facet broader and triangular.

Fibula. The right fibula is incomplete preserving only the distal portion. The shaft is flattened mediolaterally, gently posteriorly bowed and distally expanded, with a slight twist and many foramina on its surface ( Fig 10 ). In anterior view the distal end is marked with a medial dorsoventral bulge as seen in the fibulae of Aucasaurus, Skorpiovenator and Masiakasaurus (Carrano et al., 2002: fig. 15A). In Xenotarosaurus and Eoabelisaurus this structure is not preserved. The articular surface is semi-circular-shaped and robust, as seen in Skorpiovenator, Xenotarsosaurus, Aucasaurus and Eoabelisaurus. However, in Eoabelisaurus , the distal articulation has a cranial projection on the distal end. The distal end of the fibula of Masiakasaurus is square-shaped (Carrano et al., 2002: fig. 15B) unlike the one in Pycnonemosaurus nevesi , in which the distal margin is rounded.

Other DGM 859-R elements. The Pycnonemosaurus nevesi holotype described by Kellner & Campos (2002) did not include the isolated neural spine, the three transverse processes, a distal caudal centrum and an unidentified incomplete bone with the same number (DGM 859-R). The authors refer to these materials as “several unidentified incomplete bones”. Here, I have identified three transverse processes as belonging to an abelisaurid dinosaur, as described above. The isolated neural spine and distal caudal centrum as belonging to a sauropod dinosaur, the incomplete bone perhaps pertaining to a sauropod with the other isolated transverse process being unidentified taxa. Here, I described these specimens as the Parecis taxon.

The neural spine ( Fig 11 A) differs from sauropod titanosaurs, such as Trigonosaurus pricei , Baurutitan britoi and Uberabatitan ribeiroi . In these taxa the neural spines are distally broad in the mid-caudal vertebrae ( Salgado & Carvalho, 2008: fig. 14). However, the proportion of the neural spine present in Parecis taxon is similar to the mid and posterior caudal vertebrae of Aeolosaurus maximus ( Santucci & Arruda-Campos, 2011: fig. 4). In these neural spines, the proximal bases are broad, whereas the distal ends are laterally narrow. In the posterior view, the isolated neural spine is wide, whreas in the anterior view it is narrow as similarly seen in Aeolosaurus maximus ( Santucci & Arruda-Campos, 2011: fig. 4). These proportions are clearly different from those of Carnotaurus and Aucasaurus . Therefore, this neural spine is assigned to an indeterminate sauropod and not to an unidentified bone of Pycnonemosaurus nevesi .

The distal caudal centrum ( Fig 11 B) is abraded and is procoelous as seen in titanosauran sauropods (Novas, 2009), differing from the amphicoelous condition of Majungasaurus ( O’Connor, 2007) , Carnotaurus, Aucasaurus, Ilokelesia and Ekrixinatosaurus. This condition suggests that the Parecis taxon distal caudal belongs to a titanosaur sauropod.

The isolated transverse process ( Fig 11 C) belongs to unidentified taxa. It differs from the other three, that have straight borders. This element is very abraded and too incomplete to assign to another taxon with certainty.

The unidentified bone is a large element with a tip marked with sulci. This bone exhibits pneumaticity, but it is not possible to assign it to a theropod. Pneumaticity in theropods (e.g. Aerosteon ; Sereno et al., 2009), is gracile with narrow laminae, whereas in Parecis taxon it is robust with wide laminae. Neither abelisaurid theropod shows pneumatisation in the post-cranial skeleton as observed in this unidentified bone. For this reason, I cannot assign this bone to Pycnonemosaurus .

FIGURE 13. Hypothetical reconstruction of the Brazilian Upper Cretaceous fauna. Pycnonemosaurus nevesi (left side); a titanosaurian sauropod (centre-right on side); a medium-sized maniraptoran (near the neck of the sauropod) and a megaraptorid (behind the sauropod). Art by Pedro Rodrigues Busana.

Figures

FIGURE 1. Anterior and most complete caudal vertebra of Pycnonemosaurus nevesi (DGM 859 - R) in anterior (A), posterior (B), right lateral (C), dorsal (D) and left lateral (E) views. Abbreviations: hyf, hyposphene; hyp, hypanthrum; nc, neural canal; ns, neural spine; poz, postzygapophysis; prz, prezygapophysis; tp, tranverse process. Scale bar equals 100 mm.

FIGURE 2. Anterior and less complete caudal vertebra of Pycnonemosaurus nevesi (DGM 859 - R) in anterior (A), left lateral (B), posterior (C), right lateral (D) and dorsal (E) views. hyf, hyposphene; nc, neural canal; ns, neural spine; poz, postzygapophysis; tp, tranverse process Scale bar equals 100 mm.

FIGURE 3. Four anterior caudal centra of Pycnonemosaurus nevesi (DGM 859 - R) in left lateral (A 1 - D 1), anterior (A 2 - D 2) and posterior (A 3 - D 3) views. Scale bar equals 100 mm.

FIGURE 4. Largest isolated transverse process of Pycnonemosaurus nevesi (DGM 859 - R) in dorsal (A) and ventral (B) views. Scale bar equals 100 mm.

FIGURE 5. Medium-sized isolated transverse process of Pycnonemosaurus nevesi (DGM 859 - R) in dorsal (A) and ventral (B) views. Scale bar equals 100 mm.

FIGURE 6. Smallest isolated transverse process of Pycnonemosaurus nevesi (DGM 859 - R) in dorsal (A) and ventral (B) views. Scale bar equals 100 mm.

FIGURE 7. Right pubis of Pycnonemosaurus nevesi (DGM 859 - R) in anterior (A), lateral (B), posterior (C) and medial (D) views. Abbreviations: dpf, distal pubic foot. Scale bar equals 100 mm.

FIGURE 8. Right tibia from Pycnonemosaurus nevesi (DGM 859 - R) in anterior (A), lateral (B), posterior (C), medial (D), dorsal (E) and ventral (F) views. Abbreviations: cn, cnemial crest; fcr, fibular crest; lct, lateral condyle; lf, lateral fossa; lm, lateral malleolus; mm, medial malleolus. Scale bar equals 100 mm.

FIGURE 9. Distal end of the tibia of Pycnonemosaurus nevesi (DGM 859 - R) in anterior view. The ellipse shows the scar of ascending process of the astragalus. Without scale bar.

FIGURE 10. Right fibula of Pycnonemosaurus nevesi (DGM 859 - R) in anterior (A), lateral (B), posterior (C) and medial (D) views. Scale bar equals 100 mm.

FIGURE 11. Parecis taxa elements undescribed by Kellner & Campos (2002), and identified here as an isolated neural spine belonging to a sauropod dinosaur in right lateral view (A); a distal caudal centrum belonging to a sauropod dinosaur in left lateral view (B) and an isolated transverse process of an unidentified taxon in dorsal view (C). Although labelled as DGM 859 R, they are not belonging to the holotype of Pycnonemosaurus nevesi. Scale bar equals 100 mm.