Content uploaded by Mattia Antonio Baiano
Author content
All content in this area was uploaded by Mattia Antonio Baiano on Nov 14, 2023
Content may be subject to copyright.
Osteology of the axial skeleton of
Aucasaurus garridoi: phylogenetic and
paleobiological inferences
Mattia Antonio Baiano
1,2,3,4
, Rodolfo Coria
4,5
, Luis M. Chiappe
6
,
Virginia Zurriaguz
7
and Ludmila Coria
5
1Chinese University of Hong Kong, Hong Kong, Hong Kong
2Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET), Buenos Aires,
Argentina
3Museo Municipal Ernesto Bachmann, Villa el Chocón, Argentina
4Universidad Nacional de Río Negro, General Roca, Argentina
5Museo Municipal Carmen Funes, Plaza Huincul, Argentina
6Dinosaur Institute, Natural History Museum of Los Angeles County, Los Angeles, United States
of America
7Instituto de Investigación en Paleobiología y Geología (IIPG), General Roca, Argentina
ABSTRACT
Aucasaurus garridoi is an abelisaurid theropod from the Anacleto Formation (lower
Campanian, Upper Cretaceous) of Patagonia, Argentina. The holotype of
Aucasaurus garridoi includes cranial material, axial elements, and almost complete
fore- and hind limbs. Here we present a detailed description of the axial skeleton of
this taxon, along with some paleobiological and phylogenetic inferences.
The presacral elements are somewhat fragmentary, although these show features
shared with other abelisaurids. The caudal series, to date the most complete among
brachyrostran abelisaurids, shows several autapomorphic features including the
presence of pneumatic recesses on the dorsal surface of the anterior caudal neural
arches, a tubercle lateral to the prezygapophysis of mid caudal vertebrae, a marked
protuberance on the lateral rim of the transverse process of the caudal vertebrae, and
the presence of a small ligamentous scar near the anterior edge of the dorsal surface
in the anteriormost caudal transverse process. The detailed study of the axial skeleton
of Aucasaurus garridoi has also allowed us to identify characters that could be useful
for future studies attempting to resolve the internal phylogenetic relationships of
Abelisauridae. Computed tomography scans of some caudal vertebrae show
pneumatic traits in neural arches and centra, and thus the first reported case for an
abelisaurid taxon. Moreover, some osteological correlates of soft tissues present in
Aucasaurus and other abelisaurids, especially derived brachyrostrans, underscore a
previously proposed increase in axial rigidity within Abelisauridae.
Subjects Evolutionary Studies, Paleontology, Taxonomy, Zoology
Keywords Theropoda, Abelisauridae, Brachyrostra, Late Cretaceous, Anacleto formation,
Patagonia, Phylogeny, Pneumaticity
How to cite this article Baiano MA, Coria R, Chiappe LM, Zurriaguz V, Coria L. 2023. Osteology of the axial skeleton of Aucasaurus
garridoi: phylogenetic and paleobiological inferences. PeerJ 11:e16236 DOI 10.7717/peerj.16236
Submitted 25 May 2023
Accepted 14 September 2023
Published 14 November 2023
Corresponding author
Mattia Antonio Baiano,
mbaiano@unrn.edu.ar
Academic editor
Mathew Wedel
Additional Information and
Declarations can be found on
page 63
DOI 10.7717/peerj.16236
Copyright
2023 Baiano et al.
Distributed under
Creative Commons CC-BY 4.0
INTRODUCTION
Abelisauridae is among the best known groups of non-avian theropods that reached the
end of the Cretaceous (Bonaparte, 1985;Wilson et al., 2003;Krause et al., 2007;Novas et al.,
2010;Gasparini et al., 2015). Abelisaurids are mostly known from Gondwanan landmasses,
which have provided the best record in terms of abundance and specimen completeness
(e.g., Krause et al., 2007;Novas et al., 2013;Zaher et al., 2020). In contrast, the Laurasian
record is scant; it is mostly derived from the Cretaceous of France (Buffetaut, Mechin &
Mechin-Salessy, 1988;Le Loeuff & Buffetaut, 1991;Accarie et al., 1995;Allain & Suberbiola,
2003;Tortosa et al., 2014), although some putative abelisaurids have been reported from
the Cretaceous of Hungary and Spain (Ősi, Apesteguía & Kowalewski, 2010;Ősi &
Buffetaut, 2011;Isasmendi et al., 2022).
Since they were first discovered, abelisaurids were recognized as having a peculiar
cranial anatomy and striking differences in their appendicular and axial skeleton when
compared to other theropods. In particular, the axial skeleton shows traits, mostly in the
vertebrae, which are unique of this group. Among Gondwanan abelisaurids, several taxa
have preserved axial elements (e.g., Ekrixinatosaurus,Ilokelesia,Pycnonemosaurus;Coria
& Salgado (2000);Kellner & Campos, 2002;Calvo, Rubilar-Rogers & Moreno, 2004, but
only seven taxa have preserved complete portions (articulated or semi-articulated) of the
vertebral series: Aucasaurus,Eoabelisaurus,Carnotaurus,Majungasaurus,Skorpiovenator,
Spectrovenator, and Viavenator (Bonaparte, Novas & Coria, 1990;Coria, Chiappe &
Dingus, 2002;O’Connor, 2007;Canale et al., 2009;Pol & Rauhut, 2012;Filippi et al., 2016;
Zaher et al., 2020). Among them, detailed osteological descriptions of the vertebral column
have been provided for Carnotaurus (Méndez, 2014a), Majungasaurus (O’Connor, 2007),
and Viavenator (Filippi et al., 2018).
Here, we have carried out a detailed description of the axial skeleton of the holotype of
Aucasaurus garridoi (MCF-PVPH-236), which is the second detailed study of the anatomy
of this abelisaurid after the study of its braincase (Paulina-Carabajal, 2011). The axial
skeleton of MCF-PVPH-236 is composed of cervical, dorsal, and caudal vertebrae, cervical
and dorsal ribs, gastralia, and haemal arches. In spite of Coria, Chiappe & Dingus (2002)
proposing a valid diagnosis for Aucasaurus, after the discovery of new abelisaurid species
in the ensuing 20 years, we propose a new revised diagnosis using information from the
axial skeleton. An exhaustive comparison between Aucasaurus and other abelisaurids,
especially Argentinian specimens, has allowed us to detect several anatomical traits of the
axial skeleton shared by these taxa, thus strengthening the diagnosis of Abelisauridae and
adding new data for future phylogenetic analyses. We have also used computer
tomographic (CT) scans of some caudal vertebrae to visualize their internal structure.
We thus offer the first CT data of the axial skeleton of Abelisauridae, and investigate its
pneumaticity. Finally, our detailed study of the axial anatomy has revealed traits in
Aucasaurus and other brachyrostran abelisaurids that are functionally related to increased
rigidity of the axial skeleton.
Baiano et al. (2023), PeerJ, DOI 10.7717/peerj.16236 2/72
MATERIAL AND METHODS
The axial skeleton of the holotype of Aucasaurus garridoi (MCF-PVPH-236) includes the
atlas and fragments of the cervical vertebrae, the second to seventh dorsal vertebrae,
fragmentes of posterior dorsal vertebrae, the complete sacrum, the first to thirteenth caudal
vertebrae, posterior caudal vertebrae, cervical and dorsal ribs, gastralia, and the first to
thirteenth haemal arches (Fig. 1). We conducted a detailed comparison of MCF-PVPH-
236 with several theropods, particularly Argentinian abelisauroids. In the case of
specimens in which the position of the vertebrae was confidently identified, comparisons
used the same vertebral element. However, in those cases in which the position of specific
axial elements was not known with certainty, comparisons were carried out at a more
regional level: anterior, middle, and posterior (see Discussion). Table 1 shows all taxa used
in the present study (examined directly or whose data were taken from the literature).
We followed the anatomical nomenclature of Wilson (1999,2012) and Wilson et al. (2011)
to describe laminae and fossae. These structures are spelled out when first mentioned in
the text (plus acronym), subsequently they are cited only using their acronyms.
All measurements were taken using a digital calliper (Tables S1–S3) and images for
figures (both single pothographs and photogrammetry renderings) were captured using a
Nikon 3100 digital camera.
To test the phylogenetic position of Aucasaurus based on new axial information, we
carried out an analysis based on the most recently studies of Ceratosauria (Tortosa et al.,
2014;Filippi et al., 2016;Rauhut & Carrano, 2016;Baiano, Coria & Cau, 2020;Baiano
et al., 2021,2022;Aranciaga Rolando et al., 2021;Gianechini et al., 2021;Cerroni et al.,
2022). We added 11 characters (seven new and four from other sources) to the data
matrices of Baiano et al. (2022) and Cerroni et al. (2022); we also added three new taxa (i.e.,
Kurupi,Thanos, and MPM 99). The resulting data matrix consisted of 246 characters and
46 taxa (Data S1). Moreover, we provided 17 new scorings (either missing data or
previously scored characters) for Aucasaurus (characters 96, 98, 107, 112, 115, 116, 117,
119, 120, 121, 123, 123, 128, 133, 134, 136, 137). The data matrix (Data S2) was edited in
MESQUITE 3.61 (Maddison & Maddison, 2019). The analysis was performed using TNT
Figure 1 Axial skeleton of Aucasaurus garridoi.Lateral right view of the axial elements of the holotype
MCF-PVPH-236. Scale bar: 1 m. Silhouette modified from Scott Hartman (https://www.skeletaldrawing.
com). Full-size
DOI: 10.7717/peerj.16236/fig-1
Baiano et al. (2023), PeerJ, DOI 10.7717/peerj.16236 3/72
Table 1 Taxa used for anatomical comparisons.
Taxa examined directly Specimen no. First reference
Arcovenator escotae MHNA-PV-2011.12.5/198/213 Tortosa et al. (2014)
Aucasaurus garridoi MCF-PVPH-236 Coria, Chiappe & Dingus (2002)
Carnotaurus sastrei MACN-PV-CH 894 Bonaparte (1985)
Ekrixinatosaurus novasi MUC Pv 294 Calvo, Rubilar-Rogers & Moreno (2004)
Elemgasem nubilus MCF-PVPH-380 Baiano et al. (2022)
Eoabelisaurus mefiMPEF Pv 3990 Pol & Rauhut (2012)
Huinculsaurus montesi MCF-PVPH-36 Baiano, Coria & Cau (2020)
Ilokelesia aguadagrandensis MCF-PVPH-35 Coria & Salgado (2000)
Niebla antiqua MPCN-PV-796 Aranciaga Rolando et al. (2021)
Skorpiovenator bustingorryi MMCh-PV 48 Canale et al. (2009)
Tralkasaurus cuyi MPCA-PV 815 Cerroni et al. (2020)
Viavenator exxoni MAU-PV-LI 530 Filippi et al. (2016)
Xenotarsosaurus bonapartei UNPSJB-PV 612/1-2 Martínez et al. (1986) see also Ibiricu et al. (2021)
Abelisauridae indet. MACN-PV-RN 1012 Ezcurra & Méndez (2009)
Abelisauridae indet. MAU-Pv-LI 547 Méndez et al. (2018)
Abelisauridae indet. MAU-Pv-LI 665 Méndez et al. (2022)
Abelisauridae indet. MCF-PVPH-237 Coria, Currie & Paulina-Carabajal, 2006
Abelisauridae indet. MMCh-PV 69 Canale et al. (2016)
Abelisauridae indet. MPCN-PV-69 Gianechini et al. (2015) see also Baiano et al.
(2021)
Abelisauridae indet. MPM 99 Martínez, Novas & Ambrosio (2004)
Abelisauroidea indet. MPEF PV 1699/1-2 Rauhut et al. (2003)
Taxa drawn from literature Source First reference
Aerosteon riocoloradensis Aranciaga Rolando et al. (2022) Sereno et al. (2008)
Allosaurus fragilis Madsen (1976) Marsh (1877)
Camarillasaurus cirugedae Sánchez-Hernández & Benton (2012) Sánchez-Hernández & Benton (2012)
Ceratosaurus sp Gilmore (1920),Madsen & Welles (2000) Gilmore (1920)
Dahalokely tokana Farke & Sertich (2013) Farke & Sertich (2013)
Dilophosaurus wetherilli Welles (1984),Marsh & Rowe (2020) Welles (1954)
Elaphrosaurus bambergi Rauhut & Carrano (2016) Janensch (1920,1925)
Herrerasaurus
ischigualastensis
Sereno & Novas (1994) Reig (1963)
Kurupi itaata Iori et al. (2021) Iori et al. (2021)
Majungasaurus
crenatissimus
O’Connor (2007) Depéret (1896),Lavocat (1955)
Masiakasaurus knopfleri Carrano, Sampson & Forster (2002),Carrano, Loewen & Sertich
(2011)
Sampson, Carrano & Forster, 2001
Pycnonemosaurus nevesi Delcourt (2017) Kellner & Campos (2002)
Rahiolisaurus gujaratensis Novas et al. (2010) Novas et al. (2010)
Rajasaurus narmadensis Wilson et al. (2003) Wilson et al. (2003)
Sinraptor dongi Currie & Zhao (1993) Currie & Zhao (1993)
Spectrovenator ragei Zaher et al. (2020) Zaher et al. (2020)
Baiano et al. (2023), PeerJ, DOI 10.7717/peerj.16236 4/72
1.5 (Goloboff, Farris & Nixon, 2008;Goloboff & Catalano, 2016), conducting a traditional
search through 1,000 replicates of Wagner trees (saving 10 trees per replicate) followed by
tree bisection–reconnection (TBR) branch swapping. The memory to store all most
parsimonious trees (MPTs) was implemented to 50,000. The MPTs obtained were
submitted to a second round of TBR. All characters were weighted equally. To detect
possible unstable taxa, we performed the IterPCR procedure (Pol & Escapa, 2009), and
used Bremer support and Jackknife value through the pcrjack.run script to assess nodal
support (Pol & Goloboff, 2020).
We CT scanned six caudal vertebrae (i.e., first, fifth, sixth, ninth, twelfth, and thirteenth)
to investigate their internal structure. The CT scans was performed using a Toshiba
Aquilion Lightnight 16/32 scanner, in the Sanatorio Plaza Huincul in Plaza Huincul
(Neuquén Province, Argentina). The CT scans were carried out along the transversal,
coronal, and sagittal planes with the following settings: 120 kVp, 50 mA, and slices each
5-mm. The number of slices for each vertebra is: 36 coronal slices, 11 transversal slices, and
23 sagittal slices for the first caudal; 44 coronal slices, 12 transversal slices, and 23 sagittal
slices for the fifth and sixth caudals; 30 coronal slices, nine transversal slices, and 23 sagittal
slices for the ninth caudal; and 36 coronal slices, seven sagittal slices, and 19 sagittal slices
for the twelfth and thirteenth caudals. The slices were observed using the K-PACS software
produced by Ebit (ESAOTE).
SYSTEMATIC PALAEONTOLOGY
Dinosauria Owen, 1842
Saurischia Seeley, 1887
Theropoda Marsh, 1881
Ceratosauria Marsh, 1884
Abelisauroidea Bonaparte & Novas, 1985
Abelisauridae Bonaparte & Novas, 1985
Brachyrostra Canale et al., 2009
Aucasaurus Coria, Chiappe & Dingus, 2002
Etymology
The generic name was established by Coria, Chiappe & Dingus (2002); in reference to Auca
Mahuevo, the fossil locality in which the holotype was found, with the Greek suffix
-σaῦρος (sauros), lizard or reptile.
Table 1 (continued)
Taxa examined directly Specimen no. First reference
Thanos simonattoi Delcourt & Iori (2018) Delcourt & Iori (2018)
Tyrannosaurus rex Brochu (2003) Osborn (1905)
Abelisauroidea indet. CPP
893
Novas et al. (2008) Novas et al. (2008)
Baiano et al. (2023), PeerJ, DOI 10.7717/peerj.16236 5/72
Diagnosis
As for the species.
Aucasaurus garridoi Coria, Chiappe & Dingus, 2002
Type species and etymology
The name of the type species was erected in recognition to geologist Alberto Garrido, who
discovered the holotype.
Holotype
MCF-PVPH-236, Museo Carmen Funes (Plaza Huincul, Neuquén Province, Argentina), a
partial skeleton including cranial, axial, and appendicular elements (see Coria, Chiappe &
Dingus, 2002).
Locality and horizon
Auca Mahuevo paleontological site (Chiappe et al., 1998), near Mina La Escondida, in the
northeastern corner of Neuquén Province, Argentina. The holotype was recovered from
strata belonging to the Anacleto Formation (lower Campanian, Upper Cretaceous), Río
Colorado Subgroup, Neuquén Group of the Neuquén Basin. Sedimentological and
stratigraphic descriptions of these strata and of the Anacleto Formation are provided
elsewhere (see Dingus et al., 2000;Coria, Chiappe & Dingus, 2002;Garrido, 2010a,2010b).
Comments on the original diagnosis
The original diagnosis established by Coria, Chiappe & Dingus (2002) was largely based on
morphological comparisons with Carnotaurus and mentioning only one autapomorphy
(i.e., anterior haemal arches with proximally opened neural canal). Here, we expand the
diagnosis to include the following unique features of the axial skeleton: (1) atlas with a
subcircular articular surface; (2) interspinous accessory processes extended to sacral and
caudal neural spine; (3) presence of a tubercle lateral to the prezygapophysis of mid caudal
vertebrae (a similar structure is mentioned in Aoniraptor;Motta et al., 2016); (4) presence
of pneumatic foramina laterally to the base of the neural spine in the anterior caudal
vertebrae; (5) presence of a prominent tubercle and extensive rugosity on the lateral rim of
the transverse processes of caudal vertebrae fourth to twelfth; (6) presence of a small
ligamental scar near the anterior edge of the dorsal surface in the anteriormost caudal
transverse processes; (7) distinct triangular process located at the fusion point of posterior
middle gastralia. In addition, according to Coria, Chiappe & Dingus (2002), the skull of
Aucasaurus differs from that of Carnotaurus sastrei in having a longer and lower rostrum,
frontal swells instead of horns, and a sigmoidal outline of the dentigerous margin of the
maxilla. Several postcranial differences also distinguish Aucasaurus garridoi from
Carnotaurus sastrei: a less developed coracoidal process, a forelimb relatively longer, a
humerus with a slender and craniocaudally compressed shaft and well-defined condyles,
and a proximal radius lacking a hooked ulnar process.
Baiano et al. (2023), PeerJ, DOI 10.7717/peerj.16236 6/72
DESCRIPTION AND COMPARISONS
Cervical Vertebrae (Figs. 2 and 3): An almost complete atlas and several cervical fragments
are preserved. The most notable piece is a right neural arch that could belong to the fifth
cervical vertebra. The other remains are identified as part of isolated epipophyses.
Atlas (Fig. 2;Table S1): The atlas preserves the intercentrum with a fused portion of the
right neurapophysis (Figs. 2A–2C). In anterior view (Fig. 2A), the articular surface for the
occipital condyle is strongly concave and subcircular, which differs from the slightly
transversely wider than tall atlas of Skorpiovenator (Mattia A. Baiano, 2018, personal
observation on MMCh-PV 48) and Viavenator (see also Discussion, in particular the
paragraph on the autapomorphic axial traits of Aucasaurus), and from the strongly
dorsoventrally compressed atlas of Carnotaurus,Ceratosaurus, and some tetanurans (e.g.,
Allosaurus,Sinraptor). The concave dorsal edge preserves the odontoid process in
artculation. The right neurapophysis is directed dorsolaterally, and a hook-shaped process
directed anteromedially on its ventromedial part seems less developed than in
Ceratosaurus,Majungasaurus,Skorpiovenator,Viavenator, and Carnotaurus. The absence
of prezygapophyses suggests that Aucasaurus lacked a proatlas as in Majungasaurus,
Skorpiovenator,Viavenator, and Carnotaurus.
In posterior view (Fig. 2B), the articular surface is flat as in Viavenator, but different
from the convex surface in Ceratosaurus,Carnotaurus, and some tetanurans (e.g.,
Allosaurus,Sinraptor). The posterior articular surface is stepped due to two parapophyseal
Figure 2 Atlas of Aucasaurus garridoi MCF-PVPH-236. In anterior (A), posterior (B), right lateral (C),
ventral (D), and dorsal (E) views. amp, anteromedial process; ic, intercentrum; nrp, neurapophysis; od,
odontoid; vp, ventral process. Scale bar: 5 cm. Full-size
DOI: 10.7717/peerj.16236/fig-2
Baiano et al. (2023), PeerJ, DOI 10.7717/peerj.16236 7/72
processes located on the ventral edge. In this view, the pneumatic internal arrangement can
be visualized through a break in the odontoid process. There are several small chambers,
resembling a camellate condition.
In lateral view (Fig. 2C), the surface has a rectangular outline and is slightly
dorsoventrally concave, although it slightly narrows ventrally. The neurapophysis is firmly
fused to the intercentrum and there are no visible sutures. The posterior border of the
neurapophysis forms a ridge that ends ventrally in the intercentrum.
In ventral view (Fig. 2D), the surface presents two ventrally directed processes as seen in
Skorpiovenator,Viavenator, and Carnotaurus, which could be interpreted as
parapophysis-like structures for rib articulation. However, in Aucasaurus these processes
are separated by a more superficial groove than in Viavenator and Carnotaurus.
In dorsal view (Fig. 2E), the poor preservation of the neurapophyses prevents either the
evaluation of its extension, or an assessment of the morphology of the postzygapophysis
and medial process. The preserved portion of the neurapophysis has an oval cross-section,
although it narrows slightly anteriorly. The neurapophysis is slightly twisted with its
greater axis anteromedially-posterolaterally directed. A fragment of the odontoid process
Figure 3 Cervical vertebra fragments of Aucasaurus garridoi MCF-PVPH-236. In lateral (A, G and E),
ventral (B), dorsal (C), and medial (D and F) views. ape, anterior process of epipophysis; eprl, epipo-
physeal prezygapophyseal lamina; podl, postzygodiapophyseal lamina; poz, postzygapophysis; ppe,
posterior process of epipophysis; prz, prezygapophysis; sprl, spinoprezigapophyseal lamina; tp, transverse
process. Scale bar: 5 cm. Full-size
DOI: 10.7717/peerj.16236/fig-3
Baiano et al. (2023), PeerJ, DOI 10.7717/peerj.16236 8/72
is preserved on the dorsal part of the atlas. It has a triangular shape in dorsal view, different
from the more circular outline of this structure in Ceratosaurus,Masiakasaurus,Thanos,
and Carnotaurus, whereas Majungasaurus shows an intermediate condition between
Aucasaurus and other abelisauroids (see also Discussion, in particular the paragraph on
the autapomorphic axial traits in Aucasaurus). Therefore, the condition present in
Aucasaurus is here considered an autapomorphy of Aucasaurus. The dorsal surface of
odontoid is concave, while the lateral and ventral surfaces are strongly convex to fit in the
dorsal edge of the intercentrum.
Middle cervical vertebra (Cv-05?) (Figs. 3A–3C): Only the right lateral portion of the
neural arch is preserved. In anterior view, the prezygapophysis has a flat, dorsomedially
sloping facet as in Dahalokely,Carnotaurus,Ilokelesia,Majungasaurus,Skorpiovenator,
Viavenator, and MPM 99.
In lateral view (Fig. 3A), a well-defined epipophyseal-prezygapophyseal lamina (eprl)
connects the prezygapophysis with the epipophysis, separating the lateral part of the
transverse process from the dorsal part of the neural arch, as in other abelisauroids (e.g.,
Carrano & Sampson, 2008). This lamina, although broken in some parts, is straight as in
Majungasaurus,Viavenator, and Carnotaurus, but unlike Dahalokely where it is strongly
convex. Furthermore, in Aucasaurus, the posteriormost part of the eprl seems to be
dorsally directed, though we cannot assess if it was less dorsally inclined as in
Majungasaurus or oblique as in Carnotaurus. The transverse process is triangular in
outline and directed ventrally. It has a flat, lateral surface with a straight
prezygodiapophyseal lamina (prdl) and a concave postzygodiapophyseal lamina (podl).
The latter is developed as a faint crest (Fig. 3B), which is a condition observed in
abelisaurids such as Skorpiovenator and Ilokelesia. The postzygapophysis is partially
preserved and positioned 1.5 cm from the podl. The postzygapophysis has a flat articular
facet, is directed ventrolaterally, and is anteroposteriorly longer than mediolaterally wide
(Fig. 3B). However, the medial border is partially broken, suggesting that it also extended
medially with a teardrop-like outline. The base of an epipophysis is preserved dorsally to
the postzygapophysis.
In dorsal view (Fig. 3C), a slight depression separates the prezygapophysis from a robust
spinoprezygapophyseal lamina (sprl) that preserves only the base. This lamina has an
anterolateral-posteromedial orientation. The prezygapophysis shows a drop-like outline,
having the widest part located laterally as other abelisaurids (e.g., Dahalokely,Carnotaurus,
Ilokelesia,Majungasaurus,Viavenator).
Other cervical remains (Figs. 3D–3G): Several fragments of epipophyses are preserved.
Two of them contacting to each other (Figs. 3D and 3E). The dorsal edges of the
epipophyses are slightly convex, transversely thicker than the body and with a rough
surface. At least one epipophysis shows anterior and posterior processes as in Noasaurus,
Rahiolisaurus,Viavenator, and Carnotaurus, in contrast to other abelisaurids that present
only a posterior process (e.g., Ilokelesia,Skorpiovenator,Spectrovenator).
An epipophysis probably belonging to either the eighth or the ninth cervical vertebra is
preserved (Figs. 3F and 3G). It has an anteroposteriorly reduced posterior process. Beneath
it, the postzygapophysis is partially crushed. Most likely, the epipophyses had medially
Baiano et al. (2023), PeerJ, DOI 10.7717/peerj.16236 9/72
converging anterior processes. The hypertrophied epipophyses of Aucasaurus and other
abelisaurids (e.g., Viavenator,Carnotaurus) served as the point of origin of the m.
complexus (on the anterior process), and the attachment point of the m. longus colli
dorsalis (on the posterior process) (Snively & Russell, 2007;Méndez, 2012;González,
Baiano & Vidal, 2021).
Dorsal Vertebrae (Figs. 4–7): The preserved dorsal vertebrae are very fragmentary. A
series of articulated anterior dorsal vertebrae are regarded to range from the second to the
seventh dorsal based on the morphology of the neural spines and the position of the
parapophyses. In addition, a posterior dorsal vertebra, a posterior vertebral centrum, and
several distal fragments of posterior dorsal neural spines are also preserved.
Second dorsal vertebra (D2; Figs. 4A,4B and 5A–5D;Table S1): The second dorsal
vertebra is badly preserved. The centrum is severely cracked and transversely crushed. Part
Figure 4 (A–B) Photographs and line drawings of the anterior dorsal vertebrae of Aucasaurus
garridoi MCF-PVPH-236. In lateral (A) view. 2dns, second dorsal neural spine; 7dns, seventh dorsal
neural spine; acpl, anterior centroparapophyseal lamina; D2–D6, second to seventh dorsal vertebrae; iap,
interspinous accessory process; ilp, interspinous ligament process; pl, pleurocoel; pp, parapophysis; prz,
prezygapophysis; tp, transverse process. Scale bar: 5 cm. Full-size
DOI: 10.7717/peerj.16236/fig-4
Baiano et al. (2023), PeerJ, DOI 10.7717/peerj.16236 10/72
of the anterior articular surface and the lateral surface are missing. The neural arch is
almost entirely missing, except for the neural spine, which was posteriorly displaced.
The anterior articular surface is concave and dorsoventrally higher than transversely
wide, probably due to taphonomic deformation. The right parapophysis is partially
preserved. It is low and probably had a dorsoventral elliptical outline as in Carnotaurus,
Dahalokely,Skorpiovenator, and Xenotarsosaurus. The posterior articular surface seems to
be a little more complete than the anterior one (Figs. 4A and 4B). It is strongly concave and
shows an elliptical contour probably due lateral compression. The ventral surface shows
neither a groove nor a keel (Figs. 5A and 5B)asinDahalokely,Skorpiovenator, and
Xenotarsosaurus, but unlike Elaphrosaurus and Majungasaurus where there is a faint keel.
Conversely, Carnotaurus and Viavenator have two longitudinal crests converging
posteriorly.
The neural spine is transversely wider than anteroposteriorly long, being less than one
third of the centrum length as in Carnotaurus,Skorpiovenator, and Viavenator, but shorter
than in Dahalokely. The lateral surface of the spine is slightly concave anteroposteriorly
(Figs. 4A and 4B), thus the anterior and posterior edges are more laterally protuding.
The neural spine is distally thick and presents a reduced anterior process for the insertion
Figure 5 Photographs and line drawings of the anterior dorsal vertebrae of Aucasaurus garridoi
MCF-PVPH-236. In ventral (A and B), and dorsal (C and D) views. Abbreviations: D2–D7, second to
seventh dorsal vertebrae; iap, interspinous accessory process; ilp, interspinous ligament process. Scale bar:
5 cm. Full-size
DOI: 10.7717/peerj.16236/fig-5
Baiano et al. (2023), PeerJ, DOI 10.7717/peerj.16236 11/72
Figure 6 Posterior dorsal vertebrae of Aucasaurus garridoi MCF-PVPH-236. In anterior (A and G),
posterior (B and H), lateral (C, D, I and J), dorsal (E), and ventral (F and K) views. ns, neural spine; pl,
pleurocoel. Scale bar: 5 cm. Full-size
DOI: 10.7717/peerj.16236/fig-6
Figure 7 Distal fragments of dorsal neural spines of Aucasaurus garridoi MCF-PVPH-236. In dorsal
(A–C), and left lateral (D–F) views. iap, interspinous accessory process; ilp, interspinous ligament pro-
cess. Scale bar: 5 cm. Full-size
DOI: 10.7717/peerj.16236/fig-7
Baiano et al. (2023), PeerJ, DOI 10.7717/peerj.16236 12/72
of interspinous ligaments. This process is separated from the rest of the spine by two lateral
grooves. In dorsal view (Figs. 5C and 5D), a small process projects posteriorly.
Third dorsal vertebra (D3; Figs. 4A,4B and 5A–5D;Table S1): The third dorsal vertebra
is better preserved than the preceeding one, although it presents a significant transversal
deformation and several fractures.
The anterior articular surface of the centrum is slightly concave but its articulation with
the preceeding vertebra obscures other anatomical features. In lateral view (Figs. 4A and
4B), the anterior and posterior rims are parallel to each other. The parapophysis is
positioned more dorsally than the previous vertebra and is elliptical in outline as in
Eoabelisaurus,Majungasaurus,Skorpiovenator, and Carnotaurus, but its ventral part is
slightly narrower anteroposteriorly than the dorsal one. The long axis of the parapophysis
is slightly inclined posteriorly as in Carnotaurus and Masiakasaurus, but different from the
dorsoventrally oriented parapophysis of Eoabelisaurus and Majungasaurus.
Posterodorsally to the parapophysis and below the neurocentral suture, there is an
anteroposterior oval fossa on the lateral surface. In the anterior corner of that fossa, there is
a circular pleurocoel, which in turn is separated dorsally from two other small foramina by
a septum. An anterior pleurocoel is also present in Carnotaurus,Majungasaurus,
Xenotarsosaurus, and Skorpiovenator (the latter have also a posterior one). In posterior
view, the articular surface is covered by the centrum of the next vertebra. However, a
reduced part is exposed, showing a concave surface. In ventral view (Figs. 5A and 5B), the
surface has neither a keel nor a groove as Eoabelisaurus and Skorpiovenator; in contrast, a
faint keel is present in Elaphrosaurus.
The anterior surface of the neural spine has a dorsal process that protrudes anteriorly
for the anchorage of interspinous ligaments. In lateral view (Figs. 4A and 4B), the right
transverse process is not preserved. However, the anterior centrodiapophyseal lamina
(acdl), the posterior centrodiapophyseal lamina (pcdl) and the centrodiapophyseal fossa
(cdf) (or the centroparapophyseal fossa; cpaf) are visible. The neural spine is
anteroposteriorly longer than the previous one, with a square cross-section, but it is shorter
than the half of the centrum length as in Carnotaurus and Majungasaurus, whereas in
Eoabelisaurus is slightly longer. Laterally, the anterodorsal process for the interspinous
ligaments is visible. The two lateral grooves that separate this process from the rest of the
dorsal neural spine are deeper than in the D2 (Figs. 5C and 5D). The interspinous
ligamental process is also present in Carnotaurus and Eoabelisaurus, but more ventrally
positioned than in Aucasaurus and Skorpiovenator. Lateral to the interspinous ligamental
process, there is another process projected anteriorly as in Eoabelisaurus. In posterior view,
only the right postzygapophysis can be observed, which, despite being articulated with the
prezygapophysis of the next vertebra, seems to be anteroposteriorly longer than
transversely wide.
Fourth dorsal vertebra (D4; Figs. 4A,4B and 5A–5D;Table S1): The centrum of the
fourth dorsal vertebra is slightly anteroposteriorly larger than that of the D3 (Figs. 4A and
4B). Both articular surfaces are slightly concave and, despite the deformation, probably
were dorsoventrally taller than transversely wide. The lateral surface of the centrum
presents a wide fossa with a pleurocoel located more centrally than that of the D3, unlike
Baiano et al. (2023), PeerJ, DOI 10.7717/peerj.16236 13/72
Carnotaurus,Majungasaurus,Skorpiovenator,Viavenator, and MAU-Pv-LI 665, which
hold a more anterior pleurocoel, whereas Rajasaurus lacks pneumatic opening in the
centrum of this dorsal. The parapophysis is shifted more dorsally, between the centrum
and neural arch, as in Carnotaurus,Eoabelisaurus,Rajasaurus,Skorpiovenator, and MAU-
Pv-LI 665, but different than in Viavenator that holds parapophyses entirely on the neural
arch and more laterally projected. The ventral surface lacks keel or groove (Figs. 5A and
5B), as in Carnotaurus,Eoabelisaurus, but unlike Viavenator that has a shallow groove,
and Rajasaurus and MAU-Pv-LI 665 that hold a longitudinal keel.
In anterior view, only the neural spine is visible, which is transversely narrower than
that of the D3. The anterodorsal process of the neural spine for the interspinous ligaments
is conspicuous and has a rough surface, as in Viavenator but unlike Carnotaurus,
Eoabelisaurus,Majungasaurus where it is poorly developed, or even absent in
Skorpiovenator.
In lateral view (Figs. 4A and 4B), the ventral terminus of the right acdl and pcdl are
visible and diverge from each other, reaching the arch pedicels. These laminae frame a
triangular centrodiapophyseal (or centroparapophyseal) fossa. The right prezygapophysis
is articulated with the postzygapophysis of the D3, preventing to see its morphology.
However, it seems to be anteroposteriorly longer than mediolaterally wide and tilted
medially. The prezygapophysis does not have any ventral process, attributable as the lateral
wall of the hypantrum, such as the one present in Carnotaurus and Skorpiovenator. This
condition differs from Eoabelisaurus,Majungasaurus, and Viavenator that have an
incipient ventral process. The lateral surface of the neural spine is slightly concave and it is
the first neural spine that is longer than transversely wide, as in Eoabelisaurus,
Majungasaurus, and Skorpiovenator. This condition differs from the wider than long
neural spine of Carnotaurus, whereas in Viavenator is square in cross-section. The dorsal
end of the neural spine presents a transversal thickening and a marked anterodorsal
process for the interspinous ligaments. This structure is anteriorly projected, unlike the
neural spine of D3 where it protrudes dorsally over the dorsal surface of the neural spine.
The two grooves that separate it from the neural spine are deep, different from
Carnotaurus,Eoabelisaurus,Majungasaurus,Skorpiovenator, and Viavenator where there
are no grooves.
In posterior view, only the right postzygapophysis, articulated with the prezygapophysis
of D5, was preserved. As in the preceeding vertebrae, the postzygapophysis is longer than
wide and the articular facet is slightly ventrolaterally oriented, differing from the
horizontal postzygapophysis of Majungasaurus,Rajasaurus,Carnotaurus,Skorpiovenator,
Viavenator, and MAU-Pv-LI 665.
In dorsal view (Figs. 5C and 5D), the neural spine has a Y-shaped outline, due to the
lateral grooves separating the anterior process and a strong concavity between two partially
broken posterior processes. This morphology differs from that of other abelisaurids, since
these taxa either lack or have a reduced interspinous ligamental process. Furthermore, in
Aucasaurus the anterior process for the interspinous ligaments is anteroposteriorly longer
than in other abelisaurids.
Baiano et al. (2023), PeerJ, DOI 10.7717/peerj.16236 14/72
Fifth dorsal vertebra (D5; Figs. 4A,4B and 5A–5D;Table S1): In the fifth dorsal vertebra
the centrum is almost complete (although deformed), whereas the neural arch is
incomplete. Also, this vertebra presents an anterior diagenetical displacement of the neural
spine (Figs. 4A and 4B).
The anterior and posterior articular surfaces are concave and elliptical in outline with
their long axis directed dorsoventrally, as in Eoabelisaurus,Majungasaurus,
Skorpiovenator, and CPP 893, but different from Carnotaurus and Viavenator where the
centrum is subcircular. The lateral surface of the centrum holds a shallower fossa than in
D4, and it lack pleurocoels (Figs. 4A and 4B), as in Eoabelisaurus and Majungasaurus, but
in contrast to Carnotaurus,Skorpiovenator,Viavenator, and CPP 893 where there are
fossae with pleurocoels. The parapophysis is located on the neural arch, as in Carnotaurus,
Eoabelisaurus,Majungasaurus,Skorpiovenator,Viavenator, and CPP 893. The ventral
facet has neither a groove nor a keel (Figs. 5A and 5B), as in Eoabelisaurus,Skorpiovenator,
and Viavenator, but different from the longitudinal crest present in Carnotaurus.
In anterior view, similar to the preceeding vertebrae, the articulation prevents the
evaluation of various morphological characteristics of the neural arch. Ventrolateral to the
right prezygapophysis there is a shallow centroprezygapophyseal fossa (cprf). This fossa is
incipient in Carnotaurus and absent in Eoabelisaurus,Majungasaurus, and Viavenator.
The prezygapophysis is subquadrangular and the articular facet is directed slightly
dorsolaterally, as in Carnotaurus,Eoabelisaurus,Majungasaurus,Skorpiovenator,
Viavenator, and CPP 893. The prezygapophysis of Aucasaurus lacks the ventral columnar
process present in Carnotaurus,Majungasaurus,Skorpiovenator,Viavenator, and CPP
893. The anterior process for the interspinous ligaments of the neural spine is present, but
it is less developed than that of the D4.
In lateral view (Figs. 4A and 4B), the prezygapophysis lacks a ventral process, which is
present in Carnotaurus and Skorpiovenator. Despite both transverse processes are lost, the
anterior centroparapophyseal lamina (acpl) is visible. This lamina is robust and ends
dorsally into the parapophysis. The parapophysis is not located in its original position, due
to a dorsal and posterior displacement. However, it is a pendant structure as in other
abelisaurids. The parapophysis has an oval contour, as in Carnotaurus,Eoabelisaurus,
Skorpiovenator, and Viavenator. The neural spine, as mentioned above, is displaced
anteriorly. It is dorsoventrally taller than in the D4, and the thick distalmost portion is
separated from the rest of the spine by a subhorizontal step. The presence of several
anteroposteriorly directed ridges gives the surface of this area of the neural spine a rough
appearance. The process for the interspinous ligaments is located at the same level of the
dorsal rim of the neural spine, and the lateral grooves are shallower than in the D4, as in
Viavenator and CPP 893. In Carnotaurus this process is more ventrally located, whereas it
is absent in Eoabelisaurus,Majungasaurus, and Skorpiovenator. In posterior view, only the
surface of the neural spine can be seen; this has the same transverse thickness of the
anterior portion, and it becomes wider towards its distal end.
In dorsal view (Figs. 5C and 5D), the neural spine is transversely thick and
anteroposteriorly longer than that of the D4. The dorsal surface of the neural spine is
slightly convex transversely and rectangular in outline, with the lateral rims diverging
Baiano et al. (2023), PeerJ, DOI 10.7717/peerj.16236 15/72
slightly posteriorly. The posterior rim is concave, due to the presence of the base of two
posteriorly directed processes.
Sixth dorsal vertebra (D6; Figs. 4A,4B,5A–5D;Table S1): The sixth dorsal vertebra has
preserved part of the centrum and the neural arch. The centrum is as high as long and is
slightly larger than D2-D5 vertebrae, as seen in Carnotaurus and Majungasaurus.
The concavity of the anterior and posterior articular surfaces is even greater than in the
previous vertebrae, and they show an oval outline. The lateral fossa of the centrum (Figs.
4A and 4B), such as D5, is shallow and lacks pneumatic foramina, as in Majungasaurus,
but different from Carnotaurus and Skorpiovenator, which have lateral pleurocoels.
Ventrally (Figs. 5A and 5B), despite the deformation, no groove or keel are observed as in
Eoabelisaurus and Skorpiovenator, but unlike the D6 of Carnotaurus that has a
pronounced keel.
The neural arch is badly damaged and crushed. In anterior view, the neural spine is
transversely wider than the D5, and the anterior process for the interspinous ligaments
reaches the dorsal table of the spine. In lateral view (Figs. 4A and 4B), the surface is eroded
and only the parapophysis is distinguishable. It is partially broken and displaced
anterodorsally. The neural spine is fully displaced anteriorly, being positioned almost
entirely dorsally to the D5 centrum. It is anteroposteriorly long, exceeding half of the
length of the vertebral centrum as in Carnotaurus and Skorpiovenator, but different from
Majungasaurus where it is much smaller. The distal portion of the neural spine is
transversely expanded with faint lateral ridges directed anteroposteriorly. The anterior
process for the interspinous ligaments is partially broken; however, it is separated from the
spine table.
In posterior view, only the right postzygapophysis can be distinguished, which is
partially articulated with the next prezygapophysis. It seems to be longer anteroposteriorly
than transversely wide, and the articular facet is directed ventrally, as in Eoabelisaurus and
Skorpiovenator, but unlike Carnotaurus that has a ventromedially oriented
prezygapophysis. In dorsal view (Figs. 5C and 5D), the neural spine is transversely wider
and the lateral rims diverge more posteriorly than the D5. It shows a posterior concavity
that probably separated two posteriorly directed processes.
Seventh dorsal vertebra (D7; Figs. 4A,4B,5C and 5D:Table S1): Only the right
prezygapophysis and neural spine are preserved of this vertebra. The prezygapophysis is
partially articulated to the preceding postzygapophysis (Figs. 4A and 4B). It is longer than
wide, and the articular facet is slightly directed dorsolaterally, as in Carnotaurus and
Viavenator, but different than the horizontal prezygapophysis present in Majungasaurus,
or the dorsomedially oriented condition shown in Dahalokely. The neural spine shows the
same size as the neural spine of the D6, and the anterior process for the interspinous
ligaments is conspicuous (Figs. 4A and 4B). The distalmost portion of the neural spine is
thick and holds several longitudinal crests. In dorsal view (Figs. 5C and 5D), the neural
spine shows a triangular outline, and the right posterior process is visible.
Posterior dorsal vertebrae (Figs. 6 and 7;Table S1): Only some disarticulated elements
corresponding to the posterior portion of the dorsal series are preserved. Despite their
taphonomic deformation, some characteristics of the preserved centra and neural spines
Baiano et al. (2023), PeerJ, DOI 10.7717/peerj.16236 16/72
indicate that these elements belong to the most distal dorsal vertebrae. One isolated
centrum is spool-shaped (Figs. 6A–6F), with slightly concave and subcircular articular
surfaces (Figs. 6A and 6B). The lateral surface has a shallow fossa, and there is a pleurocoel
on each side (Figs. 6C and 6D). Dorsally, there are no signs of the neurocental suture
(Fig. 6E), thus the centrum was separated from the neural arch after their fusion.
The ventral surface lacks either a groove or keel (Fig. 6F).
Another vertebra (Figs. 6G–6K), probably more distal than the centrum described
above, preserves part of the centrum and neural arch. The anterior and posterior articular
surfaces are concave with a slightly oval outline (Figs. 6G and 6H). In lateral view (Figs. 6I
and 6J), there is a deep fossa, just below the neurocentral suture, without a pneumatic
foramen, as in the posterior dorsals of Dahalokely,Eoabelisaurus,Huinculsaurus,
Ilokelesia,Majungasaurus,Niebla, and Skorpiovenator but different than in Carnotaurus,
Viavenator, and MPCN-PV-69, in which central fossae bear pleurocoels. The ventral
surface lacks either a groove or a keel (Fig. 6K). The neural arch is crushed, and only the
neural spine was preserved, which is anteroposteriorly shorter than the neural arch (Figs.
6I and 6J).
Several isolated dorsal neural spines were found (Figs. 7A–7F), preserving
approximately their dorsal halves. All of them have a smaller anteroposterior extension
than the one observed in the seventh neural spine. Reduced neural spines in the posterior
portion of the dorsal series, especially in the last three ones, are also present in Carnotaurus
and Majungasaurus. All recovered neural spines have the anterior processes for the
interspinous ligaments (Figs. 7A–7C), which are separated from the dorsal table of the
neural spines by two shallow lateral grooves. Theses processes reach dorsally the distal rim,
as in Dahalokely,Majungasaurus,Skorpiovenator, and Viavenator. However, the posterior
dorsals of Carnotaurus have a more ventrally placed process. All neural spines have a
thickened distal end, with a marked lateral step and several lateral longitudinal ridges (Figs.
7D–7F). A similar condition is also present in Carnotaurus and Viavenator, whereas in
Dahalokely,Majungasaurus and Skorpiovenator this dorsal swallowness is less developed,
and absent in Eoabelisaurus. The dorsal surface is transversely and anteroposteriorly
convex. In dorsal view (Figs. 7D–7F), the neural spines seem to have a Y-like outline,
tapering anteriorly. In the posterior end, two lateral interspinous accessory processes are
present (completely preserved only in one neural spine). These processes are finger-like
shaped and posteriorly directed (Figs. 7B–7F). This structure was proposed as an
autapomorphic condition for Viavenator (Filippi et al., 2016) and considered as an
accessory interspinous articulation. This feature differs from the dorsal expansion of the
neural spines present in other abelisauroids such as Elaphrosaurus,Dahalokely, and
Huinculsaurus.
Sacrum (Fig. 8;Table S1): The sacrum is partially preserved and the vertebral centra
suffered some degree of deformation. The entire right side was found fused with the right
ilium, while the left side is fully exposed, except for the third vertebral centrum, which is
fused and covered by the pubic peduncle of the ilium and part of the iliac peduncle of the
pubis (Fig. 8A). The sacrum is composed of six vertebrae, as in Eoabelisaurus,Carnotaurus
Baiano et al. (2023), PeerJ, DOI 10.7717/peerj.16236 17/72
and Masiakasaurus, but different from the sacrum of Majungasaurus, and some
tetanurans, which includes only five vertebrae. Although partially deformed, all six
vertebral centra are fused forming an unique structure (Figs. 8A and 8B) as observed in
Ceratosaurus,Carnotaurus,Elaphrosaurus,Eoabelisaurus,Rahiolisaurus,Skorpiovenator,
and several Patagonian indeterminate abelisaurids (MAU-Pv-LI 547, MCF-PVPH-237,
MMCh-PV 69, MPCN-PV-69), and possibly Berberosaurus and Huinculsaurus. Other
abelisauroids, such as Majungasaurus (although adult individuals from that species are
unknown), Masiakasaurus,Rajasaurus, and Vespersaurus, have a partially fused sacrum.
Despite the deformation, the anterior surface of the first centrum is slightly concave and is
dorsoventrally higher and mediolaterally wider than the remaining sacral centra. From the
second to fifth sacral vertebra, the centra are transversally narrower and dorsoventrally
lower than the first and sixth sacral vertebra, as observed in almost all ceratosaurs (e.g.,
Berberosaurus,Ceratosaurus,Elaphrosaurus,Carnotaurus,Skorpiovenator), whereas in
Rahiolisaurus this constriction is present from the third sacral centrum backwards; such a
feature is apparently absent in Majungasaurus.Aucasaurus has apneumatic sacral centra,
and the lateral walls are flat or slightly concave, as in other abelisauroids.
Figure 8 Sacrum of Aucasaurus garridoi MCF-PVPH-236. In lateral (A and B), ventral (C), posterior
(D), and dorsal (E and F) views. Colored dashed lines marking the anterior and posterior rims of the third
to fifth transverse processes. 1sc–6sc, first to sixth sacral centra; 4sr, fourth sacral rib; 1stp–5stp, first to
fifth sacral transverse processes; IL, ilion; ns, neural spine. Scale bar: 10 cm.
Full-size
DOI: 10.7717/peerj.16236/fig-8
Baiano et al. (2023), PeerJ, DOI 10.7717/peerj.16236 18/72
In lateral view (Fig. 8A), the sacrum is arched giving a concave outline to the ventral rim
of the centra as in Berberosaurus,Carnotaurus,Elaphrosaurus,Masiakasaurus,
Skorpiovenator, and MAU-Pv-LI 547, whereas in Rahiolisaurus this arching is less defined.
Conversely, Eoabelisaurus,Majungasaurus, and Rajasaurus show a rather horizontal
ventral margin. The lateral surfaces of the centra have shallow longitudinal fossae lacking
pleurocoels, as in Carnotaurus, and Majungasaurus, and the indeterminate abelisaurids
MAU-Pv-LI 547, MMCh-PV 69, and MPCN-PV-69. The neural arches are partially
preserved and are fused to each other, creating a median axial wall. Unfortunately, the
right side is fused to the ilium preventing us from getting additional morphological
information, such as the presence or absence of fossae and laminae.
A fragment of the right rib of the first sacral vertebra was identified, and it is positioned
just beneath the transverse process. This portion of the rib is dorsoventrally taller than
anteroposteriorly long, different from the posterior sacral ribs, which are longer. Four left
sacral ribs have be identified, being the fourth one the best preserved (the other three are
poorly preserved). This rib is robust and holds a fossa on the ventral surface.
The neural spines of all sacral vertebrae are completely fused to one another forming a
continuous shelf, as in Skorpiovenator,Carnotaurus, MAU-Pv-LI 547, and possibly
Majungasaurus.Eoabelisaurus also possesses fused sacral neural spines, albeit it differs
from more derived abelisaurids in that it lacks a dorsal shelf. Moreover, the sacral neural
spines are transversely thin but with thicker distal ends. Several anteroposteriorly directed
grooves and ridges stand out on the laterodorsal edge of the spines. In Aucasaurus, the
fused neural spines are visible laterally above the dorsal edge of the ilium, as in
Eoabelisaurus,Majungasaurus,Carnotaurus, and MAU-Pv-LI 547, but unlike
Elaphrosaurus and Skorpiovenator where the sacrum is hidden by the ilia.
In ventral view (Fig. 8B), at least five of the sacral centra can be distinguished. In this
view, the transverse constriction of the middle portion of the sacrum is clearly visible.
The ventral surface of the vertebrae lack grooves or ridges, as seen in Eoabelisaurus,
Skorpiovenator, and Carnotaurus.
In posterior view (Fig. 8D), the sixth sacral centrum has a posterior articular surface that
is slightly concave and has an oval contour, being taller than wide. This vertebra has also
the largest posterior surface when compared to the other sacral vertebrae.
In dorsal view (Figs. 8E and 8F), the transverse processes of the second through the fifth
neural arches are fused to the ilium, two centimeters away from the dorsal rim, whereas the
first transverse process contact the medial wall more ventrally. Moreover, the second up to
the fifth sacral vertebra have transverse processes nearly horizontally directed. Conversely,
the transverse process of the sixth sacral is dorsally inclined, due to the ventral position of
this vertebra with respect the anterior ones. The transverse processes of the third through
the fifth sacral vertebrae are anteroposteriorly longer than the other sacral transverse
processes (Fig. 8F). In addition to be fused with the ilium, the transverse processes are
fused each other at their distalmost ends, leaving a medial passage (Fig. 8F), as in
Masiakasaurus and Skorpiovenator. The dorsal part of the neural spines form a continuous
co-ossified table and among them are visible two anterior and posterior interspinous
processes that contact each other, as in Carnotaurus,Skorpiovenator, and MAU-Pv-LI 547.
Baiano et al. (2023), PeerJ, DOI 10.7717/peerj.16236 19/72
Caudal vertebrae (Figs. 9–21;Table S1): MCF-PVPH-236 includes the articulated first
to thirteenth anterior vertebrae (with their corresponding haemal arches), two posterior
caudal vertebrae, and several isolated remains such as fragmentary neural spines and
transverse processes. In general, there is a reduction in the general size of the centrum
towards the posterior region, a transverse narrowing of the neural arch in the area of the
pedicels in the distal anterior elements (between the seventh and tenth vertebrae), and a
posterior displacement of the neural spine towards the rear of the tail. The transverse
processes are transversely wide, with a ratio higher than 1.3 with respect to the length of
the centrum. Sutures between neural arches and vertebral centra are completely obliterated
in all caudal vertebrae.
First caudal vertebra (Fig. 9;Table S1): The first caudal vertebra is well-preserved.
The centrum has a concave anterior surface and an oval outline with its major axis
dorsoventrally directed (Fig. 9A), as in Eoabelisaurus and Skorpiovenator, but different
from Carnotaurus in which the articular surface has a circular outline. In lateral view (Figs.
9B and 9E), a pleurocoel is absent and instead, there is an extensive anteroposterior
depression just beneath the neurocentral suture, as in Carnotaurus.InSkorpiovenator, this
depression is shallow, whereas it is absent in all caudal vertebrae in Eoabelisaurus and
MPM 99. In this view, the centrum has a parallelogram outline, since the anterior margin is
Figure 9 First caudal vertebra of Aucasaurus garridoi MCF-PVPH-236. In anterior (A), lateral (B and
E), dorsal (C), posterior (D), and ventral (F) views. acdl, anterior centrodiapophyseal lamina; apltp,
anterior process of lateral transverse process; cdl, centrodiapophyseal lamina; dr, dorsal roughness; ha,
hypantrum; hy, hyposphene; iap, interspinous accessory process; ldvc, lateral depression of vertebral
centrum; lrcdl, lateral ridge of centrodiapophyseal lamina; nc, neural canal; ns, neural spine; pcdl, pos-
terior centrodiapophyseal lamina; pf, pneumatic foramen; poz, postzygapophysis; prz, prezygapophysis;
spof, spinopostzigapophyseal fossa; spol, spinopostzigapophyseal lamina; sprf, spinoprezigapophyseal
fossa; tp, transverse process; vlrtp, ventrolateral ridge of the transverse process. Scale bar: 10 cm.
Full-size
DOI: 10.7717/peerj.16236/fig-9
Baiano et al. (2023), PeerJ, DOI 10.7717/peerj.16236 20/72
Figure 10 Second caudal vertebra of Aucasaurus garridoi MCF-PVPH-236. In anterior (A), lateral
(B and E), dorsal (C), posterior (D), and ventral (F) views. acdl, anterior centrodiapophyseal lamina;
apltp, anterior process of lateral transverse process; cdl, centrodiapophyseal lamina; ha, hypantrum; haaf,
haemal arch articular facet; hy, hyposphene; ldvc, lateral depression of vertebral centrum; lrcdl, lateral
ridge of centrodiapophyseal lamina; nc, neural canal; ns, neural spine; pcdl, posterior centrodiapophyseal
lamina; pf, pneumatic foramen; poz, postzygapophysis; ppltp, posterior process of lateral transverse
process; prz, prezygapophysis; spof, spinopostzigapophyseal fossa; spol, spinopostzigapophyseal lamina;
sprf, spinoprezigapophyseal fossa; tp, transverse process; vg, ventral groove; vlrtp, ventrolateral ridge of
the transverse process. Scale bar: 10 cm. Full-size
DOI: 10.7717/peerj.16236/fig-10
Figure 11 Third caudal vertebra of Aucasaurus garridoi MCF-PVPH-236. In anterior (A), lateral
(B and E), dorsal (C), posterior (D), and ventral (F) views. acdl, anterior centrodiapophyseal
lamina; apltp, anterior process of lateral transverse process; cdf, centrodiapophyseal fossa; cdl, cen-
trodiapophyseal lamina; dr, dorsal roughness; ha, hypantrum; haaf, haemal arch articular facet; hy,
Baiano et al. (2023), PeerJ, DOI 10.7717/peerj.16236 21/72
slightly concave and the posterior margin slightly convex, as in several abelisaurids
(Méndez, 2014b). The posterior surface is also concave and elliptical with the greater axis
dorsoventrally directed (Fig. 9D), as in Skorpiovenator, but unlike Kurupi and Carnotaurus
in which the surface is transversely wider than dorsoventrally high. The ventral end of the
posterior surface bears the articular facet for the first haemal arch. In ventral view (Fig. 9F),
the surface has a shallow depression, different from the flat surface observed in
Eoabelisaurus,Kurupi,Skorpiovenator, and Carnotaurus, or the grooved surface present in
Dilophosaurus,Ceratosaurus, and Majungasaurus.
Figure 12 Fourth caudal vertebra of Aucasaurus garridoi MCF-PVPH-236. In anterior (A), lateral (B
and E), dorsal (C), posterior (D), and ventral (F) views. acdl, anterior centrodiapophyseal lamina; apltp,
anterior process of lateral transverse process; cdf, centrodiapophyseal fossa; cdl, centrodiapophyseal
lamina; dr, dorsal roughness; haaf, haemal arch articular facet; hy, hyposphene; ldvc, lateral depression of
vertebral centrum; lrcdl, lateral ridge of centrodiapophyseal lamina; lrtp, lateral rugosity of transverse
process; nc, neural canal; ns, neural spine; pcdl, posterior centrodiapophyseal lamina; pocdf, post-
zygapophyseal centrodiapophyseal fossa; poz, postzygapophysis; prcdf, prezygapophyseal cen-
trodiapophyseal fossa; prz, prezygapophysis; spof, spinopostzigapophyseal fossa; spol,
spinopostzigapophyseal lamina; sprf, spinoprezigapophyseal fossa; vg, ventral groove. Scale bar: 10 cm.
Full-size
DOI: 10.7717/peerj.16236/fig-12
Figure 11 (continued)
hyposphene; ldvc, lateral depression of vertebral centrum; lrcdl, lateral ridge of centrodiapophyseal
lamina; lrtp, lateral rugosity of transverse process; nc, neural canal; ns, neural spine; pcdl, posterior
centrodiapophyseal lamina; pocdf, postzygapophyseal centrodiapophyseal fossa; poz, post-
zygapophysis; prcdf, prezygapophyseal centrodiapophyseal fossa; prz, prezygapophysis; spof, spi-
nopostzigapophyseal fossa; spol, spinopostzigapophyseal lamina; sprf, spinoprezigapophyseal fossa;
vg, ventral groove; vlrtp, ventrolateral ridge of the transverse process. Scale bar: 10 cm.
Full-size
DOI: 10.7717/peerj.16236/fig-11
Baiano et al. (2023), PeerJ, DOI 10.7717/peerj.16236 22/72
In anterior view (Fig. 9A), the neural canal shows an elliptical outline, different from the
circular shape seen in Carnotaurus. The hypantrum is transversely reduced and the
prezygapophyses are close to each other, as in Eoabelisaurus and Carnotaurus. It is likely
that the articulation between the last sacral vertebra and the first caudal vertebra allowed
limited lateral movements. The prezygapophysis (the right one is partially broken) has a
nearly vertical orientation, as in Eoabelisaurus and Carnotaurus. The prezygodiapophyseal
(prdl) and spinoprezygapophyseal laminae are lost due to weathering.
The spinoprezygapophyseal fossa (sprf) is deep but transversely narrow, different from the
shallower fossa present in Eoabelisaurus or the wider fossa observed in Kurupi. A septum
divides the sprf in two areas. Laterally to the prezygapophysis, the prezygapophyseal
centrodiapophyseal fossa (prcdf) is a shallow depressions. This fossa is also present in
Carnotaurus but forming a shallow concavity, whereas in Eoabelisaurus the surface is flat
without depression. In this view, the transverse process has a strong laterodorsal
inclination, at an angle of approximately 48,asinEoabelisaurus and Carnotaurus whereas
in Kurupi and Skorpiovenator the transverse process shows an inclination less than 30.
The neural spine is transversely thin; it widens distally forming a terminal bulge, as in
Figure 13 Fifth and sixth caudal vertebrae of Aucasaurus garridoi MCF-PVPH-236. In anterior (A),
lateral (B and E), dorsal (C), posterior (D), and ventral (F) views. 5 cv, fifth caudal vertebra; 6 cv, sixth
caudal vertebra; apltp, anterior process of lateral transverse process; cdl, centrodiapophyseal lamina; dr,
dorsal roughness; ha, hypantrum; har, haemal arch; haaf, haemal arch articular facet; hy, hyposphene;
iap; interspinous accessory process; lrcdl, lateral ridge of centrodiapophyseal lamina; lrtp, lateral rugosity
of transverse process; nc, neural canal; ns, neural spine; pf, pneumatic foramen; poz, postzygapophysis;
prcdf, prezygapophyseal centrodiapophyseal fossa; prz, prezygapophysis; spof, spinopostzigapophyseal
fossa; spol, spinopostzigapophyseal lamina; sprf, spinoprezigapophyseal fossa; tp, transverse process; vg,
ventral groove; vlrtp, ventrolateral ridge of the transverse process. Scale bar: 10 cm.
Full-size
DOI: 10.7717/peerj.16236/fig-13
Baiano et al. (2023), PeerJ, DOI 10.7717/peerj.16236 23/72
Eoabelisaurus and Carnotaurus. This terminal bulge appears absent in the caudal vertebrae
of Ceratosaurus.
In lateral view (Figs. 9B and 9E), the prezygapophysis and postzygapophysis do not
exceed the anterior and posterior rims of the centrum, respectively, as in Skorpiovenator
and Carnotaurus but unlike Dilophosaurus,Ceratosaurus, and Eoabelisaurus where they
are projected beyond the rims of the centrum. Ventrally, the transverse process exhibits a
centrodiapophyseal lamina (cdl) that splits ventrally in the acdl and pcdl that are poorly
developed, as in Kurupi.InAucasaurus and other abelisaurids, such as Skorpiovenator and
Carnotaurus, the first and the remaining caudal vertebrae lack pneumaticity ventral to
these laminae. The cdl ends laterally with a well-marked ridge, as in Skorpiovenator and
Carnotaurus, which is absent in Eoabelisaurus. A depression separates this crest from
another accessory ridge that is also directed anteroposteriorly, as in Carnotaurus.
The neural spine, in lateral view, it is almost perpendicular to the centrum and shows a
rectangular outline with the dorsal rim directed anterodorsally/posteroventrally.
In contrast, in Carnotaurus and Eoabelisaurus the neural spine is inclined posteriorly,
projecting beyond the posterior surface of the centrum. At the dorsalmost portion of this
vertebra, the neural spine presents anteroposteriorly directed ridges and furrows for
Figure 14 Seventh caudal vertebra of Aucasaurus garridoi MCF-PVPH-236. In anterior (A), lateral (B
and E), dorsal (C), posterior (D), and ventral (F) views. acdl, anterior centrodiapophyseal lamina; apltp,
anterior process of lateral transverse process; cdf, centrodiapophyseal fossa; cdl, centrodiapophyseal
lamina; dr, dorsal roughness; ha, hypantrum; haaf, haemal arch articular facet; hy, hyposphene; iap;
interspinous accessory process; lrcdl, lateral ridge of centrodiapophyseal lamina; lrtp, lateral rugosity of
transverse process; nc, neural canal; ns, neural spine; pcdl, posterior centrodiapophyseal lamina; pf,
pneumatic foramen; poz, postzygapophysis; prcdf, prezygapophyseal centrodiapophyseal fossa; prz,
prezygapophysis; spof, spinopostzigapophyseal fossa; sprf, spinoprezigapophyseal fossa; sprl, spinopre-
zigapophyseal lamina; vg, ventral groove. Scale bar: 10 cm.
Full-size
DOI: 10.7717/peerj.16236/fig-14
Baiano et al. (2023), PeerJ, DOI 10.7717/peerj.16236 24/72
ligamental anchorage. The neural spine is the half of the anteroposterior length of the
neural arch at its base, different from Ceratosaurus,Carnotaurus and Eoabelisaurus where
it is longest.
In dorsal view (Fig. 9C), the transverse process is posteriorly inclined with respect to the
neural spine, surpassing the posterior surface of the centrum, as in Eoabelisaurus,Kurupi,
Skorpiovenator, and Carnotaurus. Although partially broken, the transverse processes
hold, at the lateral edge, the anterior awl-like processes as in Carnotaurus. This process is
totally absent in all the caudal vertebrae of Eoabelisaurus and Majungasaurus. In the
posterodorsal portion of the transverse process, there is a V-shaped rugosity, also present
in Carnotaurus albeit much weaker. Between this scar and the lateral border of the
transverse process, the dorsal surface is slightly concave. The anterior rim of the transverse
process is concave, whereas the posterior one is almost straight, as in Carnotaurus and
Skorpiovenator but unlike Eoabelisaurus where both rims are straight. In the middle of the
anterodorsal surface of the transverse process, a possibly ligamentous scar is present,
different from the prominent spur observed in Kurupi. This trait is here considered
autapomorphic for Aucasaurus garridoi (see Discussion). There are two anteriorly
directed, dorsal processes of the neural spine absent in Eoabelisaurus and Carnotaurus.
Figure 15 Eighth caudal vertebra of Aucasaurus garridoi MCF-PVPH-236. In anterior (A), lateral (B
and E), dorsal (C), posterior (D), and ventral (F) views. apbns, anterior process of basal neural spine;
apltp, anterior process of lateral transverse process; cdl, centrodiapophyseal lamina; dr, dorsal roughness;
haaf, haemal arch articular facet; hy, hyposphene; lrcdl, lateral ridge of centrodiapophyseal lamina; lrtp,
lateral rugosity of transverse process; nc, neural canal; ns, neural spine; pocdf, postzygapophyseal cen-
trodiapophyseal fossa; poz, postzygapophysis; prcdf, prezygapophyseal centrodiapophyseal fossa; prz,
prezygapophysis; spof, spinopostzigapophyseal fossa; spol, spinopostzigapophyseal lamina; sprf, spino-
prezigapophyseal fossa; sprl, spinoprezigapophyseal lamina; vg, ventral groove. Scale bar: 10 cm.
Full-size
DOI: 10.7717/peerj.16236/fig-15
Baiano et al. (2023), PeerJ, DOI 10.7717/peerj.16236 25/72
In posterior view (Fig. 9D), the neural canal is wider dorsally than ventrally. There is a
small depression at the entry of the neural canal. The hyposphene is prominent and
formed by the union of the intrapostzygapophyseal laminae that arise ventrally to the
postzygapophyses, as in several ceratosaurs (e.g., Ceratosaurus,Carnotaurus,Kurupi).
Laterally to the hyposphene, the postzygapophyseal centrodiapophyseal fossa (pocdf) is
shallow and hold a pneumatic foramen (see Discussion). This fossa is also shallow in all the
anterior caudal vertebrae of Carnotaurus,Eoabelisaurus,Skorpiovenator, and Viavenator,
although they lack pneumatic foramina. Unlike Carnotaurus,Aucasaurus lacks
centropostzygapophyseal lamina (cpol) that delimit ventrally the pocdf.
The postzygapophyses are partially preserved, and the articular surfaces are directed
ventrolaterally, as in Ceratosaurus,Carnotaurus, and Skorpiovenator, whereas in
Dilophosaurus they are directed ventromedially. Laterally to the postzygapophysis, the
podl is low. Dorsal to the postzygapophyses, the spinopostzygapophyseal laminae (spol)
are robust and join dorsally on the posterior surface of the neural spine. Between these last
two laminae and the postzygapophyses the spinopostzygapophyseal fossa (spof) is
transversely narrow, as in Carnotaurus, whereas in Skorpiovenator this fossa is wider.
Second caudal vertebra (Fig. 10;Table S1): The second vertebra is almost completely
preserved, lacking only the anterior ends of the prezygapophyses and the distal half of the
Figure 16 Ninth caudal vertebra of Aucasaurus garridoi MCF-PVPH-236. In anterior (A), lateral (B
and E), dorsal (C), posterior (D), and ventral (F) views. apltp, anterior process of lateral transverse
process; cdl, centrodiapophyseal lamina; haaf, haemal arch articular facet; hy, hyposphene; lrtp, lateral
rugosity of transverse process; nc, neural canal; ns, neural spine; pf, pneumatic foramen; pocdf, post-
zygapophyseal centrodiapophyseal fossa; poz, postzygapophysis; prcdf, prezygapophyseal cen-
trodiapophyseal fossa; prz, prezygapophysis; spof, spinopostzigapophyseal fossa; spol,
spinopostzigapophyseal lamina; sprf, spinoprezigapophyseal fossa; sprl, spinoprezigapophyseal lamina;
vg, ventral groove. Scale bar: 10 cm. Full-size
DOI: 10.7717/peerj.16236/fig-16
Baiano et al. (2023), PeerJ, DOI 10.7717/peerj.16236 26/72
Figure 17 Tenth caudal vertebra of Aucasaurus garridoi MCF-PVPH-236. In anterior (A), lateral (B
and E), dorsal (C), posterior (D), and ventral (F) views. apltp, anterior process of lateral transverse
process; cdl, centrodiapophyseal lamina; dr, dorsal roughness; haaf, haemal arch articular facet; lrcdl,
lateral ridge of centrodiapophyseal lamina; lrtp, lateral rugosity of transverse process; nc, neural canal; ns,
neural spine; pf, pneumatic foramen; prcdf, prezygapophyseal centrodiapophyseal fossa; prz, pre-
zygapophysis; sprf, spinoprezigapophyseal fossa. Scale bar: 10 cm.
Full-size
DOI: 10.7717/peerj.16236/fig-17
Figure 18 Eleventh caudal vertebra of Aucasaurus garridoi MCF-PVPH-236. In anterior (A), lateral
(B and E), dorsal (C), posterior (D), and ventral (F) views. apltp, anterior process of lateral transverse
process; dr, dorsal roughness; haaf, haemal arch articular facet; lrtp, lateral rugosity of transverse process;
nc, neural canal; ns, neural spine; pf, pneumatic foramen; poz, postzygapophysis; prz, prezygapophysis;
vg, ventral groove. Scale bar: 10 cm. Full-size
DOI: 10.7717/peerj.16236/fig-18
Baiano et al. (2023), PeerJ, DOI 10.7717/peerj.16236 27/72
Figure 19 Twelfth and thirteenth caudal vertebrae of Aucasaurus garridoi MCF-PVPH-236. In
anterior (A), lateral (B and E), dorsal (C), posterior (D), and ventral (F) views. 12 cv, twelfth posterior
vertebra; 13 cv, thirteenth posterior vertebra; apltp, anterior process of lateral transverse process; dr,
dorsal roughness; haaf, haemal arch articular facet; ltprz, lateral tubercle of prezygapophysis; lrtp, lateral
rugosity of transverse process; nc, neural canal; ns, neural spine; poz, postzygapophysis; prz, pre-
zygapophysis; vg, ventral groove. Scale bar: 10 cm. Full-size
DOI: 10.7717/peerj.16236/fig-19
Figure 20 Caudal neural spines of Aucasaurus garridoi MCF-PVPH-236. In lateral (A and B) and
dorsal (C and D) views. iap, interspinous accessory process. Scale bar: 5 cm.
Full-size
DOI: 10.7717/peerj.16236/fig-20
Baiano et al. (2023), PeerJ, DOI 10.7717/peerj.16236 28/72
neural spine. The centrum has an elliptical anterior articular surface being taller than wide
(Fig. 10A), as in Eoabelisaurus and Skorpiovenator but different from Carnotaurus where it
is wider than tall. Ventrally to the anterior articular surface, a low rim represents the
contact area for the haemal arch. As in the first caudal vertebra, the lateral surface lacks
pleurocoels (Fig. 10B), although there is a depression below the neurocentral suture.
Conversely, the second caudal vertebra of Carnotaurus and Skorpiovenator lack such
depression on the lateral surface of the centrum. As in the first caudal vertebra, in lateral
view the centrum has a parallelogram-shaped outline. The posterior articular surface is
smaller than the anterior one (Fig. 10D), although it has the same oval outline, unlike
Carnotaurus that has an almost circular outline. The posterior contact surface for the
haemal arch is more extensive with respect to the anterior facet. The ventral surface has a
longitudinal groove that extends along the entire surface (Fig. 10F), and is laterally
bounded by two low ridges. While, in Carnotaurus the ventral surface is smooth without
groove or ridges.
In anterior view (Fig. 10A), the neural canal has a circular outline. The prezygapophyses
are almost completely lost, thus the shape cannot be observed. Although, they possibly
were oriented medially with an inclination of 60from the horizontal plane, as
Eoabelisaurus and Carnotaurus. The hypantrum is partially preserved, with an almost
Figure 21 Caudal transverse processes of Aucasaurus garridoi MCF-PVPH-236. In dorsal (A and B)
and ventral (C and D) views. Abbreviations: apltp, anterior process of lateral transverse process; cdl,
centrodiapophyseal lamina; dr, dorsal roughness; lrcdl, lateral ridge of centrodiapophyseal lamina; lrtp,
lateral rugosity of transverse process. Scale bar: 5 cm. Full-size
DOI: 10.7717/peerj.16236/fig-21
Baiano et al. (2023), PeerJ, DOI 10.7717/peerj.16236 29/72
complete right wall. This structure is wider than in the previous vertebra. In Aucasaurus,
laterally to the prezygapophysis there are neither foramina nor concavities, as in
Skorpiovenator. Despite the sprl are partially broken they seem low, delimiting a
dorsoventrally deep sprf. There is a median septum in the bottom of the sprf.
The transverse process continue to show a pronounced dorsal inclination (although the
right one is more dorsally inclined due to the diagenetic deformation), as in Eoabelisaurus
and Carnotaurus. In contrast, in Skorpiovenator the transverse process is approximately
horizontal. In Aucasaurus the neural spine is partially preserved and is transversely thin.
In lateral view (Figs. 10B and 10E), the lateral rims of the transverse process has a
pronounced roughness. Ventral to the transverse process there is a well-developed cdl that
occupies the entire surface, as Carnotaurus. This condition differs from Skorpiovenator
where the cdl is mainly developed in the anteroventral portion of the transverse process,
forming a shallow depression in the posterior portion. Moreover, this lamina is laterally
bounded by an anteroposteriorly directed ridge (as in the first caudal vertebra) as in
Carnotaurus. As observed in the first caudal vertebra, there is another accessory lateral
ridge located almost in the lateral edge of the transverse process. Ventral to the transverse
process there are no pneumatic foramina or fossae, holding only a shallow concavity that
separates the acdl from the pcdl, as in Carnotaurus and Skorpiovenator, while in
Eoabelisaurus these two laminae are poorly developed. The transverse process present a
considerable posterior inclination, since it projects beyond the centrum, as in
Skorpiovenator and Carnotaurus. Only the base of the neural spine is preserved, making it
impossible to observe the morphology of the dorsal region.
In dorsal view (Fig. 10C), the lateral rim of the transverse process have the typical
awl-shaped anterior process. Moreover, in this view the lateral rim is slightly convex and is
visible the lateral roughness. A small process is also present in the posterolateral end of the
transverse process, although it does not have the same development as the same process
present in some abelisaurids, such as Ekrixinatosaurus,Ilokelesia, and Skorpiovenator. This
reduced posterior process is absent in Carnotaurus. On the posterolateral end the
V-shaped scar is conspicuous, whereas in the second caudal vertebra of Carnotaurus it is
less-marked. The longitudinal scar on the middle of the transverse process is less
pronounced than the previous vertebra. The anterior and posterior rims of the transverse
process have a slightly sigmoid outline. The preserved portion of the neural spine is
transversely narrow with a leaf like contour in cross-section, being the posterior portion
wider than the anterior one. In Aucasaurus, the transverse process is less posteriorly
inclined than Carnotaurus.
In posterior view (Fig. 10D), the neural canal has a triangular outline and is
dorsovetrally taller than the first caudal vertebra. The hyposphene is lost, but it was
conspicuous. As in the first caudal vertebra, the pocdf is shallow and has a pneumatic
foramen, which is absent in Eoabelisaurus and Carnotaurus. The postzygapophyses are
partially broken, with the articular facets ventrolaterally oriented. The spol delimit a
rectangular spof that is transversely narrower and anteroposteriorly shallower than the
previous vertebra, unlike Carnotaurus where this fossa remains deep and wide.
Baiano et al. (2023), PeerJ, DOI 10.7717/peerj.16236 30/72
Third caudal vertebra (Fig. 11;Table S1): The third caudal vertebra was almost
completely preserved, lacking only the anterior ends of the prezygapophyses, part of the
neural spine, and the anterior and posterior ends of the lateral border of the left transverse
process. The anterior articular surface of the centrum is elliptical in outline with its long
axis oriented dorsoventrally (Fig. 11A), as in Eoabelisaurus and Skorpiovenator. This
morphology differs from Carnotaurus which has a circular contour. In lateral view
(Fig. 11B), the neurocentral suture is obliterated. The centrum has the depression just
below the neurocentral suture, which is absent in Carnotaurus. The anterior and posterior
margins of the centrum are slightly concave and convex, respectively, giving to it a
parallelogram-shaped outline, as in Eoabelisaurus and Carnotaurus. The posterior
articular surface is elliptical in outline with its long axis oriented dorsoventrally (Fig. 11D),
as in Carnotaurus. On the posteroventral end, the contact surface for the haemal arch is
wide and has an inclination of 40. In ventral view (Fig. 11F), the centrum holds a
longitudinal groove, which is absent in Carnotaurus,Eoabelisaurus, and Skorpiovenator.
In anterior view (Fig.