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Tanner, L.H., Spielmann, J.A. and Lucas, S.G., eds., 2013, The Triassic System. New Mexico Museum of Natural History and Science, Bulletin 61.
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A COMPREHENSIVE SURVEY OF TRIASSIC STEREOSPONDYLS FROM SOUTHERN
BRAZIL WITH COMMENTS ON THEIR OVERALL SIGNIFICANCE
SÉRGIO DIAS-DA-SILVA1 AND ELISEU VIEIRA DIAS2
Universidade Federal do Pampa, Laboratório de Paleobiologia, Campus de São Gabriel, Avenida Antônio Trilha, 1847,
São Gabriel, Rio Grande do Sul, CEP 97.300-000, Brazil, email: sergiosilva@unipampa.edu.br;
2
Universidade Estadual do Oeste do Paraná – UNIOESTE – Centro de Ciências Biológicas e da Saúde; Rua Universitária n° 2069.
CEP 85819-110, Cascavel, PR, Brazil, email: diaseliseu@yahoo.com.br/ eliseu.dias@unioeste.br
1
Abstract—The fossil record of Triassic stereospondyls from southern Brazil has expanded significantly in the last
27 years, increasing their overall significance among fossil assemblages from the Rosário do Sul Group (Triassic of
Paraná Basin). They are quite abundant in Lower Triassic strata (Sanga do Cabral Formation), comprising presumed lydekkerinids, undoubted rhytidosteids (Sangaia lavinai), mastodonsauroids, plagiosaurine plagiosaurids,
and unnumbered dermal fragments attributed to Stereospondyli indet. So far, they are not represented in the
Middle Triassic (Ladinian of the Santa Maria Formation), probably due to either insufficient prospecting or
taphonomic biases. Upper Triassic occurrences (Carnian of the Santa Maria Formation) include very large fragmentary remains of uncertain affinities and a juvenile chigutisaurid, regarded as a second specimen of the Indian
genus Compsocerops. A single postcranial element (an interclavicle) of a putative mastodonsauroid was recovered
from the Caturrita Formation (Norian). The record of this group in the South Brazilian Triassic clearly shows that
they were much more diverse and abundant in this part of Gondwana than previously noted. Additionally, they
serve as tools for biostratigraphic correlation as well as palaeobiogeographic interpretations, especially when
taking into consideration diagnostic Lower Triassic specimens and Compsocerops sp.
INTRODUCTION
Temnospondyl fossils are known from all continents. This demonstrates a remarkable capability of dispersion (Schoch and Milner,
2000). Basal members of stereospondyl temnospondyls survived the
Permo-Triassic extinctions in a refuge probably located in the landmass
that now comprises Australia. Subsequently, they radiated to other parts
of Pangaea, reaching their highest distribution and diversification during
the Early Triassic (Warren et al., 2000). Temnospondyls are among the
most frequently encountered tetrapods in Triassic continental deposits,
predominating in both fluvial and lacustrine facies (Milner, 1990; Schoch
and Milner, 2000; Dias-da-Silva et al., 2006a).
Terrestrial temnospondyls possess thick bones and large limbs,
whereas aquatic taxa are usually dorsoventraly flattened with slender
limbs. In most aquatic taxa, carpals and tarsals had become cartilaginous,
so these elements are usually absent even in the rare occurrences of
completely articulated skeletons. Some taxa present longirostrine or
alligatorid/crocodile-like skulls, whereas in others the skull had become
wide and/or had a parabolic outline (Schoch and Milner, 2000).
We present here an up-to-date synthesis of the knowledge on
Triassic stereospondyls from Southern Brazil, all of which come from
the Rosário do Sul Group (Triassic of Paraná Basin) (Fig. 1). It is important to point out that temnospondyls (non-stereospondyls and
stereospondyls) are also well represented in Permian strata from Brazil.
As Permian occurrences from western Gondwana are beyond the scope
of the present contribution, comments and a complete set of references
regarding this subject can be encountered in Dias-da-Silva (2012).
The first Brazilian record of Triassic stereospondyls was based
upon scarce and fragmentary remains from the Lower Triassic Sanga do
Cabral Formation and are tentatively attributed to “Lydekerinidae” and
Rhytidosteidae (Lavina and Barberena, 1985). Ever since, their record
has grown considerably in abundance, diversity, and temporal range. At
present, five different less inclusive taxonomic groups are represented in
both the Sanga do Cabral and Santa Maria formations (Rosário do Sul
Group, Triassic of Paraná Basin). The specimens reviewed here are
presented first in stratigraphic order, and second in order of their publication date (see below). Table 1 presents an updated list of published
Triassic stereospondyl records from southern Brazil. For the sake of
objectivity, we illustrate only the more representative specimens previously described.
Regarding tetrapod-based biostratigraphy, it is important to point
out that in spite of the use of tetrapods to correlate nonmarine Triassic
units for more than 130 years (Cope, 1875; Broom, 1906, 1907, 1909;
Watson, 1914a, b; Kitching, 1970; Rubidge et al., 1995; Romer, 1975;
Ochev and Shishkin, 1989; Anderson and Cruickshank, 1978; and many
others), it has long being recognized that linking terrestrial tetrapod faunas to marine age assignments is problematic. Fortuitous and infrequent
exceptions are discoveries of continental tetrapod taxa in well-dated
marine deposits (Lucas and Heckert, 2000).
Lucas (1990) recognized the necessity of a nonmarine tetrapodbased Triassic biochronology. As a result, Lucas and co-authors presented a number of slightly varying biostratigraphic schemes based on
continental tetrapods (Lucas, 1990, 1993, 1998, 2010; Lucas and Hunt,
1993; Huber et al., 1993; Lucas and Schoch, 2002; Lucas et al., 2007).
These schemes based upon tetrapod occurrences divide the Triassic into
eight Land Vertebrate Faunachrons (LVFs). The starting point of each
LVF is characterized by the First Appearance Datum (FAD) of a specific
tetrapod taxon and a series of index taxa have been proposed to characterize each LVF. These would permit cross-correlation between assemblages. The duration of each LVF is defined as the time between the first
appearance of the FAD taxon for that LVF and the first appearance of the
FAD taxon for the succeeding LVF. The more recent LVFs for the Triassic are the following: Lootsbergian = uppermost Changshingian, Induan
and possibly earliest Olenekian; Nonesian = much of the Olenekian;
Perovkan = most of the Anisian; Berdyankian = latest Anisian? and
Ladinian; Otischalkian = early to late Carnian; Adamanian = most of the
late Carnian; Revueltian = early–middle Norian; and Apachean = late
Norian–Rhaetian (Lucas, 2010).
According to Lucas (2010), the global Triassic timescale based on
tetrapod biochronology (e. g. based on LVFs) is a robust tool for both
regional and global age-assignment and correlation. However, for different reasons, this approach has been challenged and subjected to discussion (Lucas et al., 2007, 2010; Parker, 2006; Rayfield et al., 2005, 2009).
According to Rayfield et al. (2005, 2009), the LVF concept would be
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FIGURE 1. A, Sequence stratigraphy where Triassic Brazilian units are summarized modified from Zerfass et al. (2003). B, Geological map of Triassic
Brazilian units modified from Dias-da-Silva and Da-Rosa (2011). C, Simplified biostratigraphic chart of Brazilian Triassic Assemblage Zones modified from
Soares et al. (2011a).
problematic because of the following issues: (1) the faunachron concept
itself; (2) taxonomic uncertainty; (3) endemism; (4) uncertainty in temporal ranges. Other authors accept the LVF concept but reject its use in
a global framework (Langer 2005a, b; Schultz, 2005, among others). For
instance, they claim that, in Argentina, the Otischalkian and Adamanian
cannot be distinguished from each other, so they should be abandoned
and replaced by a single LVF, the Ischigualastian (Langer, 2005b).
In the present work, we avoid the use of LVFs (despite its possibly being an innovative and promising approach), due to the discussion
surrounding its generalized use. Thus, we use here several recent tetrapod-based biostratigraphic frameworks published for the Rosário do Sul
Group (Triassic of Paraná Basin), calibrated (despite their acknowledged
problems) against a marine geochronology (for references, see further
sections).
Stereospondyl temnospondyls are biostratigraphic markers of
considerable value as several taxa (above specific level) have short temporal distribution and even small fragments are easily identifiable, due to
their characteristic sculpturing of dermal bones (Cosgriff, 1969, 1984;
Anderson and Cruickshank, 1978; Kitching, 1978; Shishkin, 1994;
Groenewald and Kitching, 1995; Schoch and Milner, 2000; and many
others).
Institutional abreviations. MCN, Museu de Ciências Naturais
da Fundação Zoobotânica do Estado do Rio Grande do Sul; UFRGS,
Universidade Federal do Rio Grande do Sul; UFSM, Universidade Federal de Santa Maria; ULBRA, Universidade Luterana do Brasil; UMVT,
Museu de História da Vida e da Terra da Universidade do Vale do Rio dos
Sinos; UNIPAMPA, Universidade Federal do Pampa; PV, paleovertebrate
collection; T, Triassic;
Anatomical abbreviations. apj, alar process of jugal; ar, ascending ramus of pterygoid; ex, exoccipital; f, frontal; is, infraorbital sulcus; j,
jugal; l, lacrimal; mx, maxilla; p, parietal; paf, parietal foramen; pf, prefrontal; po, postorbital; pof, postfrontal; pp, postparietal; ppp,
postparietal process, pqf, paraquadrate foramen; pr, palatine ramus of
pterygoid; psp, parasphenoid; pt, pterygoid; ptf, posttemporal fenestra; q, quadrate; qj, quadratojugal; qr, quadrate ramus of pterygoid; sh,
palatal shagreen; smx, septomaxilla; sos, supraorbital sulcus; sq, squamosal; st, supratemporal; t, tabular; X, vagus foramen.
TRIASSIC RECORD OF STEREOSPONDS
IN SOUTHERN BRAZIL
Sanga do Cabral Formation (Lower Triassic)
Geological Setting and age
The Sanga do Cabral Formation comprises the whole Sanga do
Cabral Supersequence (Zerfass et al., 2003) and is the lowermost sequence within the Rosário do Sul Group. This second-order sequence is
a 50- to 100-m thick unit that crops out in southern Brazil and
unconformably overlies the Permian Rio do Rasto and Pirambóia formations (Andreis et al., 1980; Scherer et al., 2000). Massive to trough crossbedded, intraformational conglomerates and horizontally-bedded sandstones were deposited by braided river systems with poorly confined
channels that were developed on a low-gradient alluvial plain. Argillaceous lenses in the sequence provide indirect evidence of lakes and ponds
related to the alluvial plains (Zerfass et al., 2003).
The bone-bearing levels of this unit yield disarticulated and fragmented remains, including a single putative actinopterygian scale,
procolophonids, putative cynodonts, archosauromorphs,
archosauriforms, and a single stapes tentatively attributed to a lystrosaurid
dycinodont (see Da-Rosa et al., 2009; Dias-da-Silva and Da-Rosa, 2011
for a complete set of references). This unit is considered Lower Triassic
as it is correlated with the Lystrosaurus Assemblage Zone of South
Africa, based upon the presence of Procolophon trigoniceps, and a definite rhytidosteid (Sangaia lavinai). Rhytidosteids are a group with
Pangean occurrence whereas P. trigoniceps is a taxon of exclusive
Gondwanan distribution (Dias-da-Silva and Marsicano, 2006; Dias-daSilva et al., 2006a; Cisneros, 2008). The latter is widespread in Lower
Triassic Gondwanan deposits and Rhytidosteidae comprises almost exclusively Early Triassic taxa (Cisneros, 2008; Dias-da-Silva and
Marsicano, 2011). Among rhytidosteids, the sole exception is
Trucheosaurus major, a Late Permian rhytidosteid from Australia (see
Marsicano and Warren, 1998; Schoch and Milner, 2000).
Stereospondyl content
Lavina and Barberena (1985) described the first stereospondyl
remains for this unit based on scattered and fragmentary dermal bones
with the so-called “spider-web” pattern of sculpting. They identified
two different groups, the “Lydekkerinidae” and Rhytidosteidae. The
argument for the identification of rhytidosteids was based on the presence of nodes or pustules at the points of junction and bifurcation of
crests and ridges of the dermal bones (see Cosgriff and Zawiskie, 1979).
The remaining fragments were assigned to “lydekkerinids” based on the
lack of such feature and also because this group is abundant in Lower
Triassic deposits of South Africa. Later, Dias-da-Silva et al. (2005) reviewed these specimens and described several new ones. These authors
maintained the attribution of most material to Rhytidosteidae, but discarded the possibility that the others belonged to ‘Lydekkerinidae’, arguing that the pattern of ornamentation alone was not compelling evidence
to justify such assignment. Hence they ascribed them to Temnospondyli
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TABLE 1. Complete list of published stereospondyls from the Rosário do Sul Group.
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TABLE 1. Continued. Complete list of published stereospondyls from the Rosário do Sul Group.
* In a less inclusive possible level
indet. However, for biostratigraphic reasons, we herein ascribe these
elements to Stereospondyli indet. because non-stereospondyl
temnospondyls are quite rare in Triassic strata. Two mandibular fragments (UFRGS PV 0506 T and UFRGS PV 0651 T) preserving both
glenoid and postglenoid areas (PGA sensu Jupp and Warren, 1986) were
attributed to Temnospondyli indet. They possess short PGAs, which is
inconsistent with those encountered in preserved mandibles of
rhytidosteids. Due to biostratigraphic considerations, we now consider
these elements as Stereospondyli indet.
One of the newly described specimens by Dias-da-Silva et al.
(2005; MCN PV 2606) was referred to an undescribed new rhytidosteid
taxon, described later (Sangaia lavinai Dias-da-Silva et. al., 2006a). S.
lavinai was at first attributed to the genus Cabralia, latter found as being
preoccupied by a Brazilian butterfly (Moore, 1882). Accordingly, Diasda-Silva and Marsicano (2006) changed it to Sangaia. Its syntype consists of two skull fragments: UMVT 4302, a left half of a skull in which
the premaxilla is missing; and UMVT 4303, a partial right fragment of a
palate, which includes part of the parasphenoid and the pterygoid (Fig.
2). S. lavinai is ascribed to the Rhytidosteidae based on the presence of
lateral orbits placed very close to the skull margins; an otic notch reduced
to a slender embayment; reduced and broad based tabular horns; a straight
posterior margin of the palate; skull sculpture with nodules and pustules; and a “twisted” quadrate ramus of the pterygoid. Based on the
presence of a prominent lacrimal bone in UMVT 4302, Dias-da-Silva et
al. (2006a) suggested a possible a basal position of S. lavinai within
Rhytidosteidae and its close relationship with the Indian Indobrachyops
panchetensis, because all other rhytidosteids lack this bone (a feature
considered a derived condition shared with several other trematosaurians).
Recently, a phylogenetic reappraisal of Rhytidosteidae (Dias-da-Silva
and Marsicano, 2011) did not support this assumption. In this analysis,
carried out with 28 taxa and 87 morphological characters, Sangaia was
found to be the sister taxon of Mahavisaurus dentatus from Madagascar.
Both share a unique character: contribution of the squamosal to the
tabular horns. This character was first reported by Dias-da-Silva et al.
(2006a). They suggested that this could relate Sangaia to Mahavisaurus,
which was corroborated later by Dias-da-Silva and Marsicano (2011).
Conversely, I. panchetensis is now considered (in spite of possessing a
lacrimal) as belonging to the less inclusive group Derwentiinae, occupying a derived position within this clade (see Dias-da-Silva and Marsicano,
2011, figs. 5B and 6).
No postcranial specimens of Triassic stereospondyls from southern Brazil were described until 2008. In that year, Dias-da-Silva and
Schultz (2008) provided the first contribution dealing with scapular and
pelvic specimens and appendicular elements as well. Due to the lack of
informative features that could help to ascribe them to a less inclusive
group within stereospondyls, they were also assigned to Stereospondyli
indet.
A specimen (comprising dermal skull bones) with a conspicuous
pustular ornamentation was described by Dias-da-Silva and Ilha (2009).
After its detailed comparison with all taxa known to possess pustular
sculpturing (namely Peltobatrachus, chroniosuchians, plagiosaurids, and
amphibamids) these authors assigned the specimen to ?Plagiosauridae
indet. A year later, based on newly published information (Damiani et al.,
2009), Dias-da-Silva and Milner (2010) reassessed this specimen and
found compelling arguments to include it within a less inclusive group,
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FIGURE 2. Sangaia lavinai, a rhytidosteid from the Sanga do Cabral Formation (Lower Triassic of Paraná Basin, Southern Brazil). A-C, Photographs of
holotype UMVT4302 in dorsal, occipital and lateral views respectively. D-E, Photographs of paratype UMVT4303 in dorsal and ventral views respectively.
G-I, Interpretative drawings of holotype UMVT4302 in dorsal, occipital and lateral views respectively. J-K, Interpretative drawings of paratype
UMVT4303, in dorsal and ventral views respectively. Scale bars represent 2 cm. Photographs by Sérgio Dias-da-Silva and interpretative drawings modified
from Dias-da-Silva et al. (2006a). See anatomical abbreviations in text.
Plagiosterninae (Fig. 3). However, they could not ascribe it to any known
genus within this group.
Two recently described fossiliferous sites (Bica São Tomé and
Granja Palmeiras) from the Sanga do Cabral Formation (Da-Rosa et al.,
2009; Dias-da-Silva and Da-Rosa, 2011) have yielded new material ascribed to Temnospondyli. From the Bica São Tome outcrop, well-preserved cranial material preliminarily assigned to Mastodonsauroidea
(sensu Damiani, 2001) (see Feltrin et al., 2008) was recovered. At this
point this material is being studied and represents a new taxon for the
Lower Triassic of Southern Brazil (Piñeiro et al. in progress). From the
Granja Palmeiras outcrop, badly preserved fragmentary remains were
recovered (Dias-da-Silva and Da-Rosa, 2011). Five specimens (see Table
1) were ascribed to Temnospondyli indet, they lack diagnostic features
for a less inclusive assignment within this order. However, due to biostratigraphic reasons, we are herein amending their taxonomic status to
Stereospondyli indet.
Santa Maria Formation (Ladinian to Carnian)
Geological setting and age
This unit belongs to the Santa Maria Supersequence and comprises the whole Santa Maria 1 Sequence (Ladinian to Carnian) and the
Lower Portion of the Santa Maria 2 Sequence (Carnian) (see Zerfass et
al., 2003; Langer et al., 2007). The Santa Maria 1 sequence is a 50 m thick
third-order sequence comprising a high energy coarser lowstand systems
tract, of a low sinuosity river system. It is interpreted as being developed
under a semi-arid climate. A transgressive system tract composed of
massive or laminated mudstones concentrates the fossil vertebrates (Holz
and Scherer, 2000; Zerfass et al., 2003). These vertebrate-bearing strata
correspond to deposits of either shallow lakes (Zerfass et al., 2003) or
floodplains of an anastomosed fluvial system (Scherer et al., 2000).
The bone-bearing levels of this unit yield three different sets of
98
FIGURE 3. A, schematic line drawing of Plagiosternum granulosum from
the Middle Triassic of Germany and Russia, modified from Damiani et al.
(2009), the gray area shows the presumable anatomic correspondence
between MCN PV 1999 and P. granulosum. Not to scale. B, Line drawing of
MCN PV 1999, Plagiosterninae indet. from the Lower Triassic Sanga do
Cabral Formation (Paraná Basin, Southern Brazil). C, Photograph of MCN
PV 1999 with its presumable anatomic correspondence to P. granulosum.
After Dias-da-Silva and Milner (2010).
tetrapods that, from bottom to top, allowed the proposition of three
distinctive Assemblage Zones (AZ): (1) Dinodontosaurus AZ (Correlated with the Chañares Formation from Argentina - Ladinian), in which
the most abundant components is the dicynodont Dinodontosaurus,
followed by rauisuchian archosaurs, and several cynodonts (Abdala and
Ribeiro, 2012), among other groups; (2) Santacruzodon AZ (Correlated
with the Isalo II unit from Madagascar - Ladinian), in which the most
abundant component is the traversodontid cyndont Santacruzodon and,
in less extent, Massetoganthus, Menadon, Chiniquodon, and an unnamed
proterochampsid; and (3) Hyperodapedon AZ (Correlated with the
Ischigualasto Formation from Argentina – Carnian) in which the most
abundant component is the rhynchosaur Hyperodapedon and the
traversodontid Exaeretodon. Lower and Upper boundaries between different AZs from Southern Brazil are object of slightly diverse interpretations beyond the scope of the present contribution (For a better understanding of the chronology, faunal content and global correlations of the
Brazilian Triassic, see contributions by Langer et al., 2007, Soares et al.,
2011a, b, Abdala and Ribeiro 2012, and others).
Stereospondyl content
Until very recently, no stereospondyl temnospondyls were known
from this unit. This scenario has suddenly changed with the discovery of
two different sets of fragmentary remains of both cranial and postcranial
elements: one belonging to a giant adult individual ascribed to
Stereospondyli indet. (Dias-da-Silva et al., 2011) and, another belonging
to a juvenile specimen ascribed to the chigutisaurid Compsocerops sp.
(Fig. 4). (Dias-da-Silva et al., 2012). This genus is previously known
from strata in India (Sengupta, 1995, 2003). They were both recovered
from the Sítio Buriol fossiliferous site from São João do Polêsine
(Hyperodapedon Acme Zone sensu Langer et al., 2007, of Carnian age).
According to Dias-da-Silva et al. (2011) the giant specimen (comprising
a partial clavicle and interclavicle, a dermal skull fragment, a palatal
fragment, and a doubtful appendicular element) could belong to
trematosaurids, metoposaurids or mastodonsauroids. Unfortunately, the
lack of informative characters is an obstacle to the identification of the
specimen in any less inclusive level than Stereospondyli. The juvenile
FIGURE 4. Compsocerops sp. A-B, Photographs, in dorsal and
lateral views respectively, of ULBRA PVT 059a, a cranial
fragment comprising the orbital series. C-D, Interpretative
drawings in dorsal and lateral views, respectively. E-F,
Photographs, in dorsal and occipital views respectively, of
ULBRA PVT 059b, a cranial fragment comprising part of the
postorbital region. G-H, Interpretative drawings in dorsal and
occipital views, respectively. I-K, Photographs of ULBRA PVT
060, a partial mandibular fragment in dorsal, labial and lingual
views, respectively. L-M, Photographs of ULBRA PVT 061, a
partial clavicle in ventral and dorsal views, respectively. N,
Photograph of ULBRA PVT 062, a partial humerus in ventral
view. Scale bars represent 10mm. Photographs and interpretative
drawings modified from Dias-da-Silva et al. (2012). See
anatomical abbreviations in text.
specimen from the same outcrop was ascribed to the Indian chigutisaurid
genus Compsocerops sp. due to the presence of a long, straight and
pointed tabular horn, which runs parallel to the skull midline towards its
tip, and a distinctive projection in the posterior border of the postparietal
(Dias-da-Silva et al., 2012), both features present in Compsocerops
cosgriffi (Sengupta, 1995). Three distinctive and combined characters
suggest that the Brazilian chigutisaurid would be a distinctive species:
the presence of an alar process of the jugal in the ventral margin of the
orbit; jugal does not extend well beyond the anterior margin of the orbit;
and tabular does not contact the parietal. However, these characters
might reflect its immature ontogenetic stage as well.
Caturrita Formation (Norian)
Geological setting and age
This unit belongs to the Santa Maria Supersequence and comprises the superior half of the Santa Maria 2 Sequence and possibly the
entire Santa Maria 3 Sequence (Zerfass et al., 2003). A single Assemblage
Zone (Riograndia, formerly called Mammaliamorpha) occurs in this
unit. According to Soares et al., (2011a, fig. 5), the Lower and Upper
limits of the Riograndia AZ are uncertain. Still, this unit is considered
Upper Triassic (Norian in age) as it is (at least in part) correlated with the
Lower portion of the Lower Elliot Formation from South Africa and with
the Los Colorados Formation from Argentina. This unit yields several
basal dinosaurs (Unaysaurus, Guaibasaurus, Sacisaurus), a dicynodont
(Jachaleria – according to Lucas 1993, 2001 a synonym of Ischigualastia,
but according to Vega-Dias and Schultz 2004, Vega-Dias et al., 2004 both
are two distinct taxa, a statement also accepted by Langer et al., 2007 and
others), a procolophonid (Soturnia), a sphenodontid (Clevosaurus), a
lepidosaur (Cargninia), a presumed pterosaur (Faxinalipterus), an indeterminate phytosaur, and five advanced non-mammaliaform cynodonts,
including the common Riograndia (for the complete set of references
regarding the faunal content and biostratigraphy of this unit, see Soares
et al., 2011a).
Stereospondyl content
So far, a single incomplete interclavicle (UFRGS PV 1059 t) was
recovered from this unit. It is rhomboidal, slightly longer than wide, and
typical of stereospondylomorphs. Its center of ossification is presumably located behind the level of the maximum width of the interclavicular
blade, a condition quite similar to that found to the neighbor Argentinean
Promastodontosaurus (Bonaparte, 1963). Therefore, it is reasonable to
assume that this group might have been present in Upper Triassic environments from Southern Brazil. Further corroboration provided by more
complete material is necessary to reinforce this hypothesis.
DISCUSSION AND CONCLUSIONS
Stereospondyls are the most common fossil content in deposits
from the Lower Triassic (Induan/Olenekian) Sanga do Cabral Formation.
Unfortunately, most comprise fragmentary dermal skull elements, identifiable solely by their characteristic type of sculpturing. So far, only a
single rhytidosteid new taxon was described (Sangaia lavinai). Also, an
indeterminate plagiosaurinae plagiosaurid was recently reported (see Diasda-Silva et al., 2006a; Dias-da-Silva and Ilha, 2009; and Dias-da-Silva and
Milner, 2010). A second specific taxon (a mastodonsaurid) was recently
discovered (Feltrin et al., 2008; Da-Rosa et al., 2009). At this point, this
specimen is currently under study (Piñeiro et al., in progress). It is
important to point out that both S. lavinai (rhytidosteid) and the
plagiosteninae plagiosaurid belong to one of the major groups of derived
Mesozoic stereospondyls, the Trematosauria (sensu Yates and Warren,
2000), whereas the undescribed mastodonsaurid belongs to the other
major group, the Capitosauria (sensu Yates and Warren, 2000), or
Mastodonsauroidea (sensu Damiani, 2001). Moreover, the presence of a
plagiosternine plagiosauroid in this unit is one of the oldest records of
99
this group and the first one in Gondwana (see Dias-da-Silva and Ilha,
2009; Dias-da-Silva and Milner, 2010). It is now clear that a rich and
diversified fauna of stereospondyls inhabited this portion of Gondwana
during the Early Triassic. Unfortunately, the mode of preservation of
vertebrates in the Sanga do Cabral Formation (with intense reworking of
its deposits) constrains the discovery of better-preserved vertebrates, as
they usually occur scattered and fragmentary. Nevertheless, it is quite
possible that with the ongoing increase in prospecting efforts in this unit,
new specimens complete enough to achieve a less inclusive taxonomic
assignment and the erection of new specific taxa as well may be discovered. It is important to point out that nearby deposits from Uruguay of
the Buena Vista Formation, which according to Piñeiro et al. (2012)
brackets the Permo-Triassic Boundary, yield several occurrences of basal
temnospondyls, such as the laidlerid Uruyiella liminea, considered a
non-stereospondyl temnospondyl by Piñeiro et al. (2007a), Arachana
nigra, a rhinesuchid-like temnospondyl (Piñeiro et al., 2012), and indeterminate remains of mastodonsaurids and a single dvinosaurid (Marsicano
et al., 2000; Piñeiro et al., 2007b). These occurrences of basal
temnospondyls in Uruguay fully corroborate the idea that the Brazilian
Sanga do Cabral Formation is younger than the Uruguayan unit, and
possesses much more derived temnospondyls in comparison (for the age
comparison and discussion of both units, see Dias-da-Silva et al., 2006b;
Modesto and Botha-Brink, 2010; Piñeiro et al., 2007a, b, c, 2012).
At present, there is no record of stereospondyls in Ladinian and
Norian deposits from the Santa Maria Formation (Dinodontosaurus and
Santacruzodon AZs), probably due to either insufficient prospecting or
taphonomic bias. Nonetheless, we trust that the discovery of
stereospondyl temnospondyls in the lower portion of this unit is just a
matter of time.
Until recently, stereospondyl temnospondyls were also not recognized in the upper portion of the Santa Maria Formation, the Upper
Carnian/Lower Norian Hyperodapedon Acme Zone (sensu Langer et al.,
2007). This scenario changed dramatically with intense prospecting in
outcrops located at the municipality of São João do Polêsine, in the state
of Rio Grande do Sul. Initially, Dias-da-Silva et al. (2011) reported large
fragmentary remains of a fully grown stereospondyl. Those authors
considered and suggested the presence of either mastodonsauroids,
trematosaurids or metoposaurids in the Carnian of the Santa Maria Formation and excluded the possibility that these remains would belong to
chigutisaurids because these specimens are very thick and robust. They
argued that chigutisaurids, even the largest specimens, usually possess
slender skeletons in comparison. Later, Dias-da-Silva et al., (2012) presented, from the same outcrop, a new chigutisaurid specimen of juvenile
individual ascribed to Compsocerops sp., a genus previously known
only from Norian deposits in the upper portion of the Maleri Formation
(see Kutty and Sengupta, 1989; Sengupta, 2003) from the PranhitaGodavari basin of central India. Due to this new discovery, the authors
concluded that if new material corroborates the presence of either (or
both) mastodonsauroids or metoposauroids in Carnian strata from Brazil, these groups would have lived and shared habitats with chigutisaurids
during the Carnian in the Paraná Basin. Moreover, according to Sengupta
(2003), an important aspect of the Triassic faunal succession of the
Pranhita-Godavari basin is the displacement of the metoposaurid
Buettneria by both chigutisaurids Compsocerops and Kuttycephalus by
the end of the Carnian. Worldwide, occurrences of metoposaurids and
chigutisaurids are mutually exclusive. The only unit where they are encountered together is the Maleri Formation, but in this unit they do not
overlap in time. If the presence of metoposaurids is confirmed in the
same locality and stratigraphic levels from where the new chigutisaurid
specimen was recovered, it is logical to consider that they shared habitats
during the Carnian of Southern Brazil. Later, probably during the early
Norian, metoposaurids were replaced by chigutisaurids, which survived
as relict occurrences until the Early Cretaceous.
As other chigutisaurids from South America occur in Norian deposits from Argentina, their presence in the Carnian of South America
raises noteworthy speculations regarding their evolutionary and
100
paleobiogeographic pathways across western Gondwana. Wordwide, so
far, they have been reported in the Lower Triassic, Lower Jurassic and
Lower Cretaceous of Australia (Warren, 1981; Warren and Hutchinson,
1983; Warren et al., 1997), the Upper Triassic of Argentina (Bonaparte,
1975; Marsicano, 1993, 1999) and India (Sengupta, 1995), and in the
Upper Triassic and Lower Jurassic of South Africa (Warren and Damiani,
1999). Both Indian and Argentinean taxa are dated as Norian (Warren and
Marsicano, 2000). So the presence of an earlier occurrence in South
America suggest that after evolving in the landmass that now comprises
Australia, they reached Southern Brazil and then spread into both Argentina and India. However, this hypothesis is probably a result of biases
caused by the non-preservation of chigutisaurids in older deposits from
western of Gondwana. Hence, the possibility that they also inhabited
Antarctica and Madagascar cannot be dismissed. Furthermore, close relatives of chigutisaurids (Bothriceps, Keratobrachyops, brachyopids,
rhytidosteids, tupilakosaurids and dvinosaurids) were reported in older
Gondwanan deposits from the Upper Permian and Lower/Middle Triassic (Yates and Warren, 2000). Consequently, ghost chigutisaurid lineages
in Gondwana cannot be dismissed, and they could be more diverse and
widespread than their fossil record suggests, given that they are still the
main component of the South American temnospondyl fauna during the
Triassic (Marsicano, 2005).
Regarding the Norian-age Caturrita Formation (Riograndia AZ), a
single fragmentary interclavicle attributed to an indeterminate (and still
doubtful) mastodonsauroid (Dias-da-Silva et al., 2009), supports the
statements provided above. As it does not add any new insight/informa-
tion to the discussion, we do not discuss it in the present contribution.
Again, it is important to point out that we believe that the discovery of
more complete and diagnostic stereospondyls in this unit is also just a
matter of time.
In a span of only 27 years, the record of stereospondyls in the
Rosário do Sul Group, Triassic of the Paraná Basin, rose from absent to
a large number of specimens. In the Sanga do Cabral Formation, as in
other Lower Triassic units worldwide, they comprise the main faunal
component, identifiable even from fragmentary dermal elements. The
situation in the remaining units from the Rosário do Sul Group (Santa
Maria and Caturrita formations) is still incipient, but very promising. In
an interval of four years, their status changed from non-existent to positive. Moreover, in spite of their modest presence in both units, their
taxonomic status, in spite of being tentative, indicates that a diversified
fauna of stereospondyls inhabited the landmass that now comprises
Southern Brazil. In conclusion, there is no reason not to assume that new
discoveries in the near future will substantially improve the knowledge
of this important group of basal tetrapods in western Gondwana.
ACKNOWLEDGMENTS
We are greatly indebted to Marina Bento Soares, who permitted
the use and modification of figures 4 and 5 published in Soares et al.
(2011a). We also thank Átila Augusto Stock da Rosa, who allowed the
use and modification of figure 1 published in Dias-da-Silva and Da-Rosa
(2011). CNPq Research Grant 500919/2009-7 to SDS supported this
research.
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