Reichel et al, 2009

A NEW TRAVERSODONTID CYNODONT

(THERAPSIDA, EUCYNODONTIA) FROM THE

MIDDLE TRIASSIC SANTA MARIA FORMATION OF

RIO GRANDE DO SUL, BRAZIL

by MIRIAM REICHEL* , CESAR LEANDRO SCHULTZ� and

MARINA BENTO SOARES�*Department of Biological Sciences, University of Alberta, Edmonton, Alberta, T6G 1K7, Canada; e-mail: [email protected]

�Instituto de Geociencias – UFRGS, Universidade Federal do Rio Grande do Sul, Avenida Bento Goncalves, 9500. CEP 91509-900, Porto Alegre, RS, Brazil;

e-mail: [email protected]

�Instituto de Geociencias – UFRGS, Universidade Federal do Rio Grande do Sul, Avenida Bento Goncalves, 9500. CEP 91509-900, Porto Alegre, RS, Brazil;

e-mail: [email protected]

Typescript received 9 May 2008; accepted in revised form 17 September 2008

Abstract: Remains of a peculiar traversodontid cynodont,

Protuberum cabralensis gen. et sp. nov., are described herein.

The material was collected from two outcrops representing

the Therapsid Cenozone (Middle Triassic) of the Santa

Maria Formation, and consists of a cranium with most of

its dentition preserved and an associated postcranial skele-

ton. The upper postcanines have two sharp cusps that are

connected by a medial crest on unworn postcanines. The

specimens possess several autapomorphies, including: (1)

presence of thickened bone on the dorsal surface of the

skull; (2) thick dorsal ribs, with remarkable processes situ-

ated on their dorsal borders that decrease in size distally;

and (3) an iliac blade with a series of rugosities along its

dorsal border. The lumbar ribs bear overlapping costal

plates and have distally projecting rib shafts that differ from

the pattern observed in Thrinaxodon, Pascualgnathus and

Cynognathus.

Key words: traversodontid, protuberances, Santa Maria

Formation, Therapsid Cenozone, Middle Triassic, Brazil.

The family Traversodontidae (Therapsida, Eucynodon-

tia) was established by von Huene (1936) and is one of

the best known and most widespread families of non-

mammalian cynodonts. Members of this family occur in

rocks of Middle to Upper Triassic age from Africa

(Crompton 1972a; Kemp 1980; Gow and Hancox 1993;

Flynn et al. 2000), Asia (Chatterjee 1982), South America

(Bonaparte 1962; Romer 1967; Barberena 1981a, b;

Abdala et al. 2002; Abdala and Ribeiro 2002, 2003), North

America (Hopson 1984; Sues and Olsen 1990; Sues et al.

1992, 1994, 1999) and Europe (Tatarinov 1973; Hahn

et al. 1988; Godefroit and Battail 1997). The broad geo-

graphical and temporal distribution of this clade suggests

that it represents a successful adaptive radiation that may

have been facilitated by the development of many special-

ized features (e.g. precise dental occlusion).

The traversodontids were diverse herbivores that can be

characterized by their specialized dental morphology. The

postcanines are transversely enlarged in occlusal view,

providing the upper ones with a rectangular outline and

the lower ones with a square outline. This structure

allowed precise dental occlusion that resembles the pat-

tern observed in most mammals in some respects. The

external crest functioned to shear food items, while the

internal basin facilitated crushing. This dental configura-

tion of traversodontids represents an important evolu-

tionary step, representing a great improvement in terms

of food processing (Hopson 1984). Although travers-

odontids possess many derived (and mammal-like) fea-

tures, details of their teeth indicate that this family is not

close to mammalian origins (Hopson and Kitching 2001;

Rowe 1988; Luo 1994; Luo and Wible 2005).

There are no specific postcranial characteristics that

diagnose traversodontids that are not known in other

non-mammalian cynodonts, but this may reflect the poor

fossil record of their rather conservative postcranial skele-

tons. Rib specializations are known in some cynodonts

(Brink 1955; Jenkins 1971) and some mammaliaforms

(Ji et al. 2006), but these specialized features are not diag-

nostic of traversodontids and are also present in widely

variable forms in galesaurids, cynognathids and dia-

demodontids (Jenkins 1970). The presence of a series of

[Palaeontology, Vol. 52, Part 1, 2009, pp. 229–250]

ª The Palaeontological Association doi: 10.1111/j.1475-4983.2008.00824.x 229

processes along the ribs represents a remarkable feature of

the new cynodont taxon described herein: much needed

anatomical data on other parts of the cynodont postcrani-

um are also provided by this new taxon.

MATERIALS

The new taxon is based on specimens collected by Father

Daniel Cargnin at two outcrops in the state of Rio

Grande do Sul, southern Brazil (Text-fig. 1). Some pecu-

liar ribs and vertebrae were found in the Municipality of

Paraıso do Sul in 1977, in a taphocoenosis comprising

specimens of several taxa, including cynodonts (Masseto-

gnathus ochagaviae and Probelesodon kitchingii) and dic-

ynodonts (Dinodontosaurus sp.). In 1989, Father Cargnin

collected part of an articulated skeleton with the associ-

ated skull from an outcrop located in the Municipality of

Novo Cabrais. This specimen is housed at the Guido

Borgomanero Museum in Mata (Rio Grande do Sul).

The specimen from Novo Cabrais includes articulated

vertebrae and ribs. A skull and some additional thoracic

ribs were associated with the specimen. The vertebrae are

poorly preserved, but their general features can be

described. The cranium is also poorly preserved, with the

exception of the dentition. The specimen from Paraıso do

Sul is better preserved but, consists of only a few isolated

elements.

GEOLOGICAL SETTING

The holotype of Protuberum cabralensis (MGB 368 ⁄ 100)

was collected from the ‘Sıtio Cortado’ outcrop (S

29�44¢54.4¢¢ W 53�01¢49.4¢¢: Text-fig. 1), located in the

municipal district of Novo Cabrais. The paratypes were

collected from the ‘Rincao do Pinhal’ outcrop (S

29�43¢14.8¢¢ W 53�13¢46.8¢¢; Text-fig. 1) in the municipal

district of Paraıso do Sul. Both outcrops are composed of

thin layers (each only a few centimetres thick) of red

massive mudstones that alternate with amalgamated len-

ticular bodies of fine sandstones. Two layers of carbonate

concretions were described by Da Rosa et al. (2004) in

the former outcrop. These lithologies are typical of the

Alemoa Member of the Santa Maria Formation (Schultz

et al. 2000).

The tetrapod fauna of the Santa Maria Formation has

been divided into four biostratigraphic units (Text-fig. 2).

The lowermost, which includes the two outcrops yeilding

Protuberum material, represents the Ladinian aged Therap-

sid Cenozone (Rubert and Schultz 2004), and is dominated

by the herbivorous dicynodont Dinodontosaurus, with

TEXT -F IG . 1 . Map showing location

of the outcrops. Key to localities: 1, Sıtio

Cortado. 2, Rincao do Pinhal.

230 P A L A E O N T O L O G Y , V O L U M E 5 2

some occurrences of the cynodonts Massetognathus and

Probelesodon.

Institutional abbreviations. MCP, Museu de Ciencias e Tecnolo-

gia da Pontifıcia Universidade Catolica, Porto Alegre; MGB,

Museu Guido Borgomanero, Mata; UFRGS, Universidade Fed-

eral do Rio Grande do Sul, Porto Alegre.

SYSTEMATIC PALAEONTOLOGY

THERAPSIDA Broom, 1905

CYNODONTIA Owen, 1861

EUCYNODONTIA Kemp, 1982

TRAVERSODONTIDAE von Huene, 1936

Genus PROTUBERUM gen. nov.

Etymology. The generic name refers to the numerous protuber-

ances present on the ribs and ilia.

Diagnosis. As for the type and only species.

Protuberum cabralensis sp. nov.

Etymology. In reference to the Municipality of Novo Cabrais

where the holotype was collected.

Holotype. MGB 368 ⁄ 100, a skull with most of its dentition but

lacking the lower jaw and an articulated series of 19 vertebrae

(two cervicals?, nine thoracics, five lumbar and three sacral)

including four articulated thoracics, 10 lumbar ribs (five from

each side) and four sacral ribs (two from each side), a fragment

of the left ilium and seven isolated thoracic ribs.

Paratypes. The paratypes are represented by (1) UFRGS PV

0981T, a proximal fragment of a right cervical rib; (2) UFRGS

PV 0983T, an isolated vertebra; (3) UFRGS PV 0985T, an isolated

vertebra; (4) UFRGS PV 0986T, an isolated vertebra; (5) UFRGS

PV 1009T, a left cervical rib; (6) UFRGS PV 1010T, a left tho-

racic rib; and (7) UFRGS PV 1011T, a fragment of a thoracic rib.

Diagnosis. Protuberum is a large traversodontid in which

the upper postcanines have two main cusps (one labial

and one lingual) that are connected by a sharp transverse

crest. As in Luangwa, Pascualgnathus, Scalenodon, Andes-

cynodon and Traversodon, the postcanines of Protuberum

lack the shouldering pattern (in which the mesial of one

tooth ‘shoulders’ into the proper area of the preceeding

tooth) observed in Massetognathus and Exaeretodon. The

paracanine fossae are anteroposteriorly elongated and pos-

teriorly placed in relation to the upper canine. This fea-

ture is also present in Exaeretodon and Scalenodontoides.

The parietal crest is short, as in Scalenodontoides

macrodontes (Gow and Hancox 1993). Protuberum has

well-developed masseteric processes of the jugals, as in

Exaeretodon. The paraoccipital process is bifurcated in

Protuberum, as in tritylodontids, brasilodontids (Bona-

parte et al. 2005) and some early mammals (Kermack

et al. 1981; Crompton and Luo 1993), in contrast to the

unbifurcated condition present in other traversodontids.

Autapomorphies observed in the skull include: (1) incisive

foramina totally enclosed by the maxillae; and (2) a bony

thickening that forms wide crests on the dorsal surface

of the skull. The postcranium of Protuberum is robust and

the contacts between lumbar vertebrae, as well as between

the lumbar vertebrae and ribs, are very strong. The lum-

bar ribs of Protuberum have costal plates that differ from

those in Thrinaxodon, Pascualgnathus and Cynognathus, as

rib shafts are situated distal to all of the costal plates in

the new taxon. Other postcranial autapomorphies are: (1)

all ribs show very pronounced processes on their dorsal

border, the most proximal of these is generally the largest

and the others become smaller distally; and (2) the iliac

blade has a series of rugosities along its dorsal border.

Description

Cranium

The type specimen, MGB 368 ⁄ 100, includes an almost complete

cranium (Text-figs 3–10), with the right quadrate and

TEXT -F IG . 2 . Stratigraphic correlations between Brazil and

Argentina for the Middle–Upper Triassic. Modified from Rubert

and Schultz (2004), and Schultz and Soares (2006).

R E I C H E L E T A L . : N E W T R A V E R S O D O N T I D C Y N O D O N T F R O M S A N T A M A R I A F O R M A T I O N 231

A

B


quadratojugal preserved in their original positions and with

most of the upper dentition present; the right postorbital bar

and the lower jaws were not preserved. The cranium is some-

what distorted, especially in its dorsal portion. The total cranial

length (from the anterior border of the premaxilla to the con-

dyles) is approximately 200 mm, while the parietal crest mea-

sures about 15 mm, representing only 7.5 per cent of the

length of the cranium.

The cranium is heavily built, with thick bones in the preor-

bital region and in the lambdoid crests. The short parietal crest

is high and the temporal openings are wide. The occiput is

exposed in dorsal view with posteriorly projecting occipital con-

dyles. The internarial process of the premaxilla is not present, so

that the external naris is a single and confluent opening.

The canines are reduced in size, when compared to the pro-

portions observed in Exaeretodon. The incisors (which are of

similar size to the canines) and postcanines are slightly procum-

bent. In palatal view, the postcanine rows diverge posteriorly.

Facial region. The rostrum is short and wide. The anterodorsal

surfaces of the premaxillae are very similar to those of Exaereto-

don (Bonaparte 1962) as an ossified internarial process is absent

in both taxa. In Protuberum, however, there is no relict of a

conic appendix, in contrast to the condition in Exaeretodon

(Bonaparte 1962). The anterior margin of the premaxillae is

broad and the opening of the external naris is wide (Text-fig. 5).

The maxillary process of the premaxilla (Text-fig. 6A) is reduced

(it does not reach the nasals), as also occurs in Massetognathus

(Romer 1967) and Exaeretodon.

The margins of the maxillae are unclear, especially on the dor-

sal part of the cranium. In lateral view (Text-fig. 6), a depression

can be observed adjacent to the dorsal border of the maxilla.

A wide foramen is present near the anterior margin of the

maxilla, above the canine. The septomaxillary foramen, which

should be immediately dorsal to this foramen, is not preserved.

Another maxillary foramen opens lateral to the third postcanine.

The facial process of the septomaxilla (Text-fig. 6A) is well

TEXT -F IG . 3 . The skull of Protuberum cabralensis (MGB 368 ⁄ 100). A, dorsal, and B, palatal views. Scale bar represents 50 mm.

A

B

TEXT -F IG . 4 . The skull of Protuberum

cabralensis (MGB 368 ⁄ 100). A, lateral,

and B, occipital views. Scale bar

represents 50 mm.


developed in Protuberum. Its subtriangular shape is similar to

that of Exaeretodon (Bonaparte 1962). The premaxillae partici-

pate in the formation of the posterior and ventral borders of the

narial opening.

The margins of the nasals are not clear. Their middle portion

is constricted at the level of the paracanine fossae, forming a

bony thickening that is developed as an anteriorly directed ‘V’

shaped crest (Text-fig. 5). This crest continues posterolaterally

along the prefrontals and postorbitals, and connects with the dor-

sal margin of the orbit. The crest is symmetrical and well-defined

and is unlikely to be the result of post-mortem deformation.

Palate. Each premaxilla contains three incisors. The canine is

placed near the anterior border of the maxilla and only a small

space is present between the canine and the posteriormost inci-

sor, as also occurs in Exaeretodon (Bonaparte 1962), Massetogna-

thus (Romer 1967) and Probainognathus (Romer 1970). The

premaxillae participate in the borders of the incisive foramina

(Text-fig. 7), which are placed near the anteromedial border of

the maxillae and are totally enclosed by them. This feature has

not been observed in other traversodontids, but is present in

Diademodon (Brink 1955). The paracanine fossae (Text-fig. 7)

are positioned posterior to the canines (a feature observed in

only two other traversodontids: Exaeretodon and Scalenodon-

toides; see Abdala and Ribeiro 2003) and are deep and antero-

posteriorly elongate.

A marked ridge separates the dorsal depression of the maxillae

from their ventral surface. This ridge corresponds to the upper

limit of the area described as a maxillary bulge by von Huene

(1935–1942). This maxillary bulge is quite well developed in Pro-

tuberum, as in other traversodontids, with an accentuated lateral

ridge on the maxillae.

The posterior border of each maxilla reaches the subtemporal

fossa, by means of a narrow process that intervenes between the

anterior border of the jugal and the pterygoid, as in Exaeretodon.

The process of the jugal that extends between the posterior bor-

der of the maxilla and the pterygoid in Exaeretodon (Bonaparte

1962) could not be observed in Protuberum. The jugal does,

however, have a small medial process that is inclined towards

the pterygoid (Text-fig. 7), but which does not contact it.

TEXT -F IG . 5 . Reconstruction of the

skull of Protuberum cabralensis in dorsal

view. Scale bar represents 20 mm.


The ventral contact between the maxillae and the palatines

begins at the posterior border of the third postcanine along a

serrated suture and extends backwards, medial to the postcanine

tooth row, until reaching the pterygoids posteriorly. The pala-

tines extend for a considerably distance posteriorly, reaching the

level of the masseteric processes of the jugals (Text-fig. 7). No

palatal foramina could be observed.

A crest along the medial contact between the maxillae starts at

a point level with the anterior border of the paracanine fossae

and continues along the medial contact of the palatines to the

posterior terminus of the secondary palate (Text-fig. 7). This

crest is continuous with an ossified septum within the nasal cav-

ity. This septum is probably formed from the fused vomers and

is also observed in Traversodon (MR, pers. obs.) and Exaeretodon

(Bonaparte 1962). The opening of the internal choanae is tall, as

in Exaeretodon (Bonaparte 1962), and wide, as in Massetognathus

(Romer 1967).

The pterygoids of Protuberum form long descending processes

(Text-figs 5–7), resembling those in Exaeretodon (Bonaparte

1962), but they are more vertically directed and are proportion-

ally wider than in the latter taxon. Posterolaterally, the pteryg-

oids contact the epipterygoids.

Ventrally, the cultriform process of the parasphenoid (Text-

fig. 7) extends between the posterior ends of the pterygoids.

At this level, the pterygoids form a pronounced crest continu-

ous with the parasphenoid and the basipterygoid process of

the basisphenoid (Text-fig. 7). These features are similar to

those in Exaeretodon, but are not present in Massetognathus

(MR, pers. obs. UFRGS PV 0968T). Protuberum lacks an

interpterygoid vacuity (a feature that is variably present in

Massetognathus: Romer 1967; Reichel and Schultz 2004). Ante-

rolaterally, the pterygoids contact the maxillae, participating in

the anterior border of the subtemporal fossa. In contrast to

the condition in Massetognathus (Romer 1967) and Travers-

odon (Barberena 1981a), the pterygoids do not reach the jugals

laterally.

Orbital region and zygomatic arch. The lacrimal forms most of

the anterior border of the orbit and the anterior wall within the

orbit (Text-fig. 6). The limits of this bone inside the orbits are

not visible, but a clear serrated suture is present at the contact

with the maxilla. This suture has a semicircular outline. The lac-

rimal foramen is not preserved. A foramen located in the inter-

nal wall of the orbit is probably the posterior opening of the

infraorbital canal.

The limits of prefrontals and frontals are not clear (Text-

fig. 5). The region occupied by the frontals is quite depressed

because of the thickening of the lateral border of the prefrontal,

which forms the dorsomedial rim of the orbit. The postorbital

bar is preserved only on the left side of the skull. The dorsal

portion of the postorbital process is divided by an ascending

cuneiform projection of the jugal, as in Exaeretodon (Bonaparte

1962). However, the resulting ‘V’-shaped contact in Protuberum

is observed on the anterior and posterior faces of the postorbital

bar, differing from Exaeretodon, in which this contact is devel-

oped on the lateral and medial faces. In lateral view, the post-

A

B


skull of Protuberum cabralensis in lateral

view. A, complete, and B, without the

zygomatic arch. Scale bar represents

20 mm.



skull of Protuberum cabralensis in palatal

view. Scale bar represents 20 mm.


skull of Protuberum cabralensis in

occipital view. Scale bar represents

20 mm.


orbitals extend posteriorly covering most of the anterolateral

portion of the reduced parietal.

The parietals are fused dorsally to form the parietal crest

(Text-fig. 5), which is remarkably short in Protuberum. The pari-

etal foramen (Text-fig. 5) opens dorsally and slightly anteriorly.

It is located at the level of the posterior margin of the postorbi-

tals, which cover the anterior ends of the parietals.

The masseteric process of the jugal (Text-figs 6–7) can be

observed on the anteroventral portion of the zygomatic arch.

This process has also been described in Exaeretodon (Bonaparte

1962), Luangwa (Kemp 1980), Traversodon (Barberena 1981a),

Pascualgnathus (Bonaparte 1966) and Santacruzodon (Abdala

and Ribeiro 2002). In Protuberum it is ventrally directed, as in

Exaeretodon. The lateral view of the zygomatic arch resembles

that in Exaeretodon, although the dorsal prolongation of the

jugals does not project as far posteriorly as in the latter taxon.

The squamosals extend for a considerable distance anteriorly,

forming a cuneiform process that divides the posterior portion

of the jugals, and almost reaches the level of the postorbital bar,

as in Exaeretodon (Bonaparte 1962) and Massetognathus (Romer

1967).

Lateral wall of the braincase. In dorsal view, the posterior por-

tion of the squamosals resembles the condition in Exaeretodon.

The groove that separates the neurocranium and the zygomatic

arch is very deep (Text-fig. 5) and this contact is narrow, resem-

bling that of Massetognathus (Romer 1967). In lateral view, the

posterodorsal border of the portions of the squamosals that

form the lambdoid crests is rectangular (in contrast to that in

Massetognathus, in which this border is rounded) and extends

ventrally, outlining the external auditory meatus (Text-fig. 6A),

as occurs in Exaeretodon and Massetognathus.

The posteroventral articulation of the epipterygoid with the

prootic is preserved on the right side of the braincase. On this

border, a foramen is present for the exit of the maxillary (V2)

and mandibular (V3) branches of the trigeminal nerve (Text-

fig. 6B).

The pterygoparoccipital foramen appears to open laterally, but

this region is not well preserved. A similar condition is observed

in tritylodontids, in the tritheledontid cynodonts Riograndia

guaibensis and Pachygenelus (Luo 1994), and in the early mam-

mals Sinoconodon and Morganucodon (Wible and Hopson 1993),

but not in other traversodontids, in which the pterygoparoccipi-

tal foramen is enclosed by the squamosal. For most eucynodonts

(including other traversodontids), the laterally open pterygopa-

roccipital foramen is a derived feature (Luo 1994; Luo et al.

2002), but in mammaliaforms, this feature is variable and homo-

plastic. It is open in Sinoconodon and all morganucodontians

A BTEXT -F IG . 9 . Reconstruction of the

quadrate and quadratojugal of

Protuberum cabralensis (top) and

Massetognathus pascuali (bottom),

modified from Luo and Crompton

(1994). A, posteroventral, and B,

anterior views. Scale bar represents

10 mm.

A B

TEXT -F IG . 10 . The last left postcanine (unworn) of

Protuberum cabralensis. A, posterolateral view. B, occlusal view.

Scale bar represents 20 mm.


(Wible and Hopson 1993; Luo 1994) but closed in Adelobasileus

(Lucas and Luo 1993) and Hadrocodium (Luo et al. 2001).

The limits of the prootics are unclear (especially ventrally).

Laterally the sutures with the parietal and epipterygoid indicate

that this bone forms much of the sidewall of the braincase and

that it extends further dorsally than in Exaeretodon (Bonaparte

1962). In lateral view, the anteroventral limits of the prootic and

the epipterygoid are very clear, especially at the level of the tri-

geminal foramen. Posteroventrally, the prootic delimitates the

pterygoparoccipital foramen and posterodorsally the posttempo-

ral fossa (Text-fig. 6B). Both openings communicate through a

deep groove, in which only the portion closest to the posttem-

poral fossa (Brink 1955) is enclosed in a canal. A similar feature

is present in Massetognathus: in the latter taxon this short canal

was described as a lateral flange vascular canal by Rougier et al.

(1992). Conversely, in Exaeretodon the pterygoparoccipital fora-

men and the post-temporal fossa communicate through a closed

canal (Bonaparte 1962).

Basicranium. In ventral view, the squamosal bears a groove for

the external auditory meatus that rapidly decreases in diameter

medially and curves anteromedially (Text-fig. 7), terminating

behind the quadrate and the quadratojugal. The quadrate and

quadratojugal articulate with grooves in the squamosal. These

elements are tightly fused, as in Massetognathus (Luo and

Crompton 1994). Medial to the groove for the quadrate, a crest

of the squamosal encloses the quadrate into the concavity. The

squamosal does not make contact with the quadrate ramus of

the epipterygoid (Text-fig. 7). The dorsal plate of the quadrate

(Text-fig. 9) of Protuberum is similar to that of Massetognathus,

but it possesses a quadrangular anterior outline. The surface of

the dorsal plate is slightly concave and the dorsal angle is par-

tially covered by the squamosal, as in Massetognathus.

On the ventral area of both prootics, the cavum epiptericum

(Text-fig. 7) is open, as in most non-mammalian cynodonts

(Wible and Hopson 1993; Luo and Crompton 1994). In ventral

view, the paraoccipital process (Text-fig. 7) is better preserved

on the left side of the cranium. The process is bifurcated, with

an anterior and a posterior region, which is not observed in

other traversodontids. This is a derived feature, present in trityl-

odontids, brasilodontids (Bonaparte et al. 2005) and in early

mammals such as Morganucodon (Kermack et al. 1981) and

Sinoconodon (Crompton and Luo 1993). The posterior paraoc-

cipital crest (Text-fig. 7) is very clear and more strongly devel-

oped in Protuberum than in Brasilodon (Bonaparte et al. 2005)

and extends from the contact with the squamosal to the border

of the jugular foramen.

Although the preservation of the fenestra ovalis is not good

on either side of the braincase, their borders appear to have been

formed by a thickened ring of bone, which is a primitive feature

that is also observed in other traversodontids (Kemp 1980) and

non-mammalian cynodonts with the exception of Brasilitherium

(Bonaparte et al. 2003, 2005). This is otherwise a derived condi-

tion present in mammaliaforms (Luo et al. 1995). The jugular

foramina are better preserved and each is clearly confluent with

the fenestra rotunda.

The margins of the parasphenoid and basisphenoid are not

clear. In ventral view, the crest formed by the cultriform process

of the parasphenoid is very high, as observed in Exaeretodon

(MR, pers. obs., UFRGS PV0715T). Its anterior portion (extend-

ing between the pterygoids) is much shorter than in Massetogna-

thus (Romer 1967) and is more similar to that of Exaeretodon

(Bonaparte 1962). Posteriorly, the basisphenoid contacts the

basioccipital along a clear serrated suture (Text-fig. 7). Laterally,

the basisphenoid wing extends towards the fenestra ovalis (con-

tributing to its medial margin), but its contact with the prootic

is indistinguishable. Anteriorly, the basipterygoid process of the

basisphenoid forms the anterior margin of the ventral opening

of the cavum epiptericum. The foramina for the internal carotid

are absent in the basisphenoid, as also occurs in other travers-

odontids, Cynognathus and trytilodontids (Hopson and Barghu-

sen 1986).

It is not possible to determine if the basioccipital extends pos-

teriorly between the occipital condyles (Text-fig. 7), as it does in

Massetognathus (Romer 1967). No hypoglossal foramina are evi-

dent in the specimen, so they were probably confluent with the

jugular foramina (Text-fig. 7) as in most non-mammalian cyno-

donts, including traversodontids (Luo 1994; Luo et al. 2002). As

in most non-mammalian cynodonts, the occipital condyles of

Protuberum are mostly composed of the exoccipitals. The exocci-

pitals contact each other medially, but their articulation with the

basioccipital is not clear. The odontoid notch could not be

observed. The condyles project posteriorly, so that in ventral

view they are aligned with the posterior border of the lambdoid

crests, in contrast to the condition in Exaeretodon (Bonaparte

1962) and Massetognathus (Romer 1967). The condyles are pos-

teriorly placed in relation to the quadrates, as also occurs in

Massetognathus. The ventral parts of the condyles are very close

to each other, to a greater extent than occurs in Exaeretodon

frenguellii (Bonaparte 1962), but similar to their position in Exa-

eretodon riograndensis (Abdala et al. 2002). They are not as verti-

cally inclined as in the former and their form is bulbous, which

is the most primitive form among non-mammalian cynodonts,

including traversodontids (Lucas and Luo 1993).

Upper dentition

The dental formula of Protuberum consists of three incisors, one

canine and seven postcanines. Only the second and third incisors

are preserved on the left side and the second incisor on the right

side. They are slightly procumbent, but not as much as in Exa-

eretodon (Bonaparte 1962). Each incisor is similar in size, but

the second has a spatulate tip. It is possible to observe enamel

layers only on the labial face of incisors, which is also the case

in Exaeretodon (Chatterjee 1982; Abdala et al. 2002) and

Luangwa (Kemp 1980).

The right canine is small and approximately the same size as

the incisors: the canine is vertically oriented. As with the inci-

sors, the enamel layer can only be observed on the labial surface.

The paracanine fossa is placed in a posteromedial position in

relation to the upper canine, as is also observed in Exaeretodon

and Scalenodontoides. This fossa is very deep and anteroposteri-

orly elongate so that it almost fills the diastema that is present

between the canine and postcanines. The size of the lower canine


is unknown, but the anteroposterior length of the paracanine

fossa suggests that movement of the lower jaw occurred in a

posterior direction.

The postcanine rows diverge somewhat in their posterior half.

The first four teeth are implanted at right angles to the medial

plane. The posteriormost postcanines begin to diverge markedly,

reaching an angle of 50 degrees relative to the medial plane.

They increase in size posteriorly and are extremely worn, with

the exception of the posteriormost one (Text-fig. 10), which

shows no wear. The third and fourth left postcanines are free

from their alveoli. One of these became attached to the palate

during fossilization, while the other became associated with the

right paracanine fossa. A single root is visible on the former and

does not show any sign of division. Its length is the same as the

width of the crown of the tooth, so that the root shows the

shape of an inverted triangle in distal view. The empty alveoli

corroborate this observation.

The unworn teeth (Text-fig. 10) are similar in morphology to

the postcanines of Pascualgnathus. Two cusps can be observed,

one lingual and one labial. The lingual cusp is slightly higher

than the labial and a low transverse crest (that can be observed

only on the left unworn tooth) connects the cusps. There is no

cingulum. Wear increases from the back to the front along the

tooth row, as also observed in Luangwa (Kemp 1980), Masseto-

gnathus pascuali (Romer 1967) and other traversodontids. On

the fifth postcanine the transverse crest has been worn off, pro-

ducing two well-marked crests: a low one at the anterior border

of the tooth and a high one at the posterior border. These crests

enclose a deep concavity, so that the crown surface forms an

oval concave area that lacks internal features. The original differ-

ence in size between the lingual and labial cusps is accentuated

in comparison to the most anterior postcanines.

On the first four postcanines wear has produced a flat occlusal

surface, that is continuous anteroposteriorly. The lingual and

labial crests tend to decrease in size and no longer differ in size

from each other. The general morphology of the worn postca-

nines resembles that in Luangwa but is much simpler. The

shouldering of postcanines (an advanced feature for travers-

odontids, which is observed in Exaeretodon and Massetognathus,

for example) is absent in Protuberum, a characteristic it shares

with Luangwa, Pascualgnathus, Scalenodon, Andescynodon and

Traversodon. In occlusal view, the teeth are subrectangular in

outline and are nearly twice as broad as they are long anteropos-

teriorly, as also occurs in Massetognathus pascuali.

Axial skeleton

The axial skeleton of Protuberum (Text-figs 11–14) is peculiar

due to the presence of numerous processes on the ribs and neu-

ral spines with expanded distal ends that are similar to those

observed by Sues (1985) in the thoracolumbar vertebrae of Kay-

entatherium. The axial skeleton of Protuberum also resembles

that of Pascualgnathus (Bonaparte 1966), Thrinaxodon, Cynogna-

thus and Diademodon (Jenkins 1971), in its development of

costal plates.

In some non-mammalian cynodonts (e.g. Thrinaxodon, Cyno-

gnathus and Diademodon), posterior ribs with costal plates deli-

mit the intergirdle portion of the vertebral column into thoracic

and lumbar regions (Brink 1955; Jenkins 1971). In other cyno-

donts costal plates are absent and it is not possible to recognise

this division (e.g. Exaeretodon; Bonaparte 1963). There is also

another pattern, which may be represented by Massetognathus

(Jenkins 1970), in which specialized lumbar ribs occur; these are

not in the form of costal plates but appear to be reduced ver-

sions of these plates, lending a ‘Y’-shape to the distal end of ribs.

In Protuberum, the transition between the last thoracic and the

first lumbar rib is based on the presence ⁄ absence of overlapping

costal plates. On the lumbar ribs, the costal plates overlap with

adjacent ribs anteriorly and posteriorly, with each posterior plate

overlapping the anterior plate of the following rib.

The criteria proposed by Jenkins (1971) to distinguish

between thoracic and lumbar regions in non-mammalian cyno-

donts were (1) thoracic ribs possess a rib shaft that extends dis-

tal to the costal plates and (2) the first lumbar rib can be

defined as the first rib that bears a costal plate that contacts

adjacent costal plates. In the case of Protuberum, all of the ribs

possess a shaft distal to the costal plates. Therefore, the second

criterion applies better in Protuberum, but in this taxon the defi-

nition is modified slightly so that the first lumbar rib is identi-

fied as that which bears a costal plate that contacts adjacent

costal plates both anteriorly and posteriorly.

Vertebral column

There are 19 vertebrae in the holotype of Protuberum, consisting

of two cervicals, nine thoracics, five lumbars and three sacral

vertebrae. All of the vertebrae are amphicoelous and robust with

posteriorly inclined neural spines. Each neural spine bears pos-

terolateral expansions at its dorsal end, so that in dorsal view

the spines have a triangular outline. These expansions are more

robust than those observed in Kayentatherium (Sues 1985).

The transverse processes of the cervical and the first thoracic

vertebrae project posterolaterally, but from the third to the

eighth thoracic vertebra these processes extend laterally. From

the ninth thoracic (the last) to the third lumbar the transverse

processes project anterolaterally, but on the two last lumbars

(fourth and fifth), these processes project laterally again, as do

those of the sacral vertebrae. Cervical and anterior thoracic cen-

tra are anteroposteriorly short, while the centra of the posterior

thoracics, lumbars and sacral vertebrae increase in length and

become more spool shaped. This trend is especially marked in

the lumbar region where the centra have expanded anterior and

posterior borders.

Cervical series. The two first vertebrae (Text-figs 11, 12A) of the

holotype are considered to be cervicals, because they differ from

the next vertebrae (here considered to be thoracic) in several

aspects. The expansions at the extremity of the neural spine are

much smaller in the cervicals than in the first thoracic, even

though the expansions seen in the second cervical are larger than

those present on the first cervical. In addition, the cervical neu-

ral arches are much narrower than those on the thoracic verte-

brae and they lack the anterior lamina that is present in the


A

B


thoracics (see description, below). Moreover, the proportions of

the neural spines and vertebral centra are quite different, so that

the neural arches of the cervicals are proportionately much

higher than those of the thoracics, although the total height of

the cervical and anterior thoracic vertebrae remains almost the

same.

The cervical centra do not differ significantly from those of

the first thoracics. They are short anteroposteriorly and bear a

small laterally compressed ventral ridge. Cervical zygapophyses

resemble those of the thoracic vertebrae: the prezygapophyses

project beyond the anterior border of the vertebral centrum,

while the postzygapophyses terminate above the posterior border

of the centrum. A shallow anterior concavity is present on the

dorsal border of the centrum, ventral to the prezygapophyses,

which appears to act as an articular surface for a small tuberos-

ity that extends posteriorly from the preceding centrum (Text-

fig. 12A–B). This feature creates a very precise intervertebral

articulation.

Thoracic series. The largest neural spines expansions are present

in thoracic vertebrae 1–6 (Text-fig. 11), which might indicate

that they were subjected to some kind of additional mechanical

stress. The thoracic vertebrae (Text-fig. 12B) have anteroposteri-

orly elongated neural spines with an anterior lamina that pro-

jects into the posterior border of the anteceding vertebra. The

proportions of the neural arches and centra differ from those

observed in Exaeretodon (Bonaparte 1963) and Diademodon

(Jenkins 1971): however, the thoracic neural spines of Protube-

rum are generally low and resemble the pattern observed in Mas-

setognathus pascuali (Jenkins 1970). There are no clear sutures

between the neural arches and centra (in contrast to Exaereto-

don, in which those elements were not fused).

The anterior centra are anteroposteriorly short and com-

pressed laterally, so that a low ventral ridge is present. These

proportions change along the column and the posteriormost

vertebrae have centra are at least 50 per cent longer. The length

of the centra ranges from 18–30 mm. There is no evidence of

intercentra. The zygapophyses have the same pattern as

described above for the cervical vertebrae. Although it is hard to

observe the articular surfaces of the pre- and postzygapophyses

in the articulated vertebrae, it seems that these are almost verti-

cal on the anterior thoracic vertebrae but are oriented more hor-

izontally in the posterior vertebrae. The parapophyses are well

developed on the dorsolateral aspect of centrum, adjacent to the

articular surface. In Protuberum the facets for the thoracic rib

heads are situated intervertebrally, as in Thrinaxodon and

A B C D

TEXT -F IG . 12 . Reconstruction of the vertebrae of Protuberum cabralensis in lateral (top) and posterior (bottom) views. A, cervical,

B, thoracic, C, lumbar, and D, sacral vertebrae. Scale bar represents 20 mm.

TEXT -F IG . 11 . The articulated postcranium of Protuberum cabralensis (MGB 368 ⁄ 100). A, dorsal view, and B, ventral view. Striped

area represents sediment. Scale bar represents 40 mm.


Cynognathus (Jenkins 1971) and in the anterior dorsal vertebrae

of Massetognathus (Jenkins 1970). There are no indications of

anapophyses in MGB 368 ⁄ 100, although poor preservation and

the fact that most of the vertebrae are articulated in this speci-

men (with some matrix filling the spaces between them) could

have obscured this feature. In the isolated vertebra UFRGS

PV0985T (probably a thoracic), well-preserved anapophyses are

present.

Lumbar series. The lumbar vertebrae (Text-fig. 12C) have short,

robust and anteroposteriorly broad neural spines. No space is

present between adjacent neural spines, due to poor preserva-

tion. The expansions at the distal extremities of the neural spines

tend to decrease in size posteriorly.

The vertebral centra of the lumbar vertebrae are spool-shaped

in ventral view and their lengths range from 27–37 mm. The ante-

rior and posterior borders of the centra are expanded so that the

contact area between centra of adjacent vertebrae is enlarged. The

strength of their union can be confirmed by a postmortem incli-

nation of the posterior part of the column that caused the break-

age of the third, fourth and fifth lumbar vertebrae through the

middle portion of their centra, but did not disarticulate them. The

parapophyses are intervertebral, as in Massetognathus (Jenkins

1970). In Thrinaxodon, the last lumbar vertebrae have parapophy-

ses located on the centra (Jenkins 1971). From the third lumbar

vertebra and continuing posteriorly, synapophyses (fused parap-

ophyses and diapophyses) occur in an intervertebral position in

Protuberum, as also seen in the lumbars of Luangwa (Kemp 1980)

and Thrinaxodon and the sacrals of Massetognathus. Articulations

between lumbar pre- and postzygapophyses are unclear. The

transverse processes are robust and anteroposteriorly broad.

Sacrum. The sacral vertebrae (Text-fig. 12D) are identified on

the basis of contact between the ribs and the internal surface of

the iliac blade. The solid construction of the posterior part of

the vertebral column continues in this region. The first three

A

B

C

TEXT -F IG . 13 . Ribs of Protuberum cabralensis. A, cervical rib (UFRGS PV 1009T) in posterior view, B, thoracic rib (UFRGS PV

1010T) in anterior view, and C, thoracic rib (UFRGS PV 1010T) in posterior view. Striped area represents sediment. Scale bar

represents 20 mm.

TEXT -F IG . 14 . The sacral region of Protuberum cabralensis (MGB 368 ⁄ 100). A, lateral view of the articulated sequence of the

postcranial skeleton. B, detail of the fragment of the iliac blade, and C, reconstruction of the pelvis of Protuberum cabralensis.


A

B

C


sacral vertebrae are preserved in the type specimen (Text-

fig. 11), but the sacral region of Protuberum probably contained

more vertebrae, as Thrinaxodon has five sacrals (Jenkins 1971),

while Exaeretodon has seven (Bonaparte 1963).

The only preserved neural spine is that of the first sacral ver-

tebra. It is shorter than those of the lumbar vertebrae and is less

robust. The expansions at the distal extremity of the neural

spine are smaller than those on the lumbar vertebrae. Sacral

centra are spool-shaped (as in the lumbar vertebrae). The length

of the first sacral vertebra is about 32 mm, the second has a

severe fracture and dislocation of the centrum, making its mea-

surement difficult, but the third is approximately 24 mm long,

indicating a tendency towards shortening of the posterior sacral

vertebrae. The breakage of the second sacral vertebra occurs in a

similar way to that described for the third, fourth and fifth lum-

bar vertebrae (see above), with an intact intervertebral articula-

tion indicating strong fusion between the sacrals as well as the

lumbars.

Synapophyses are developed on the sacral vertebrae, but they

are placed on the anterior border of each vertebra (instead of

intervertebrally, in contrast to the lumbars). It is impossible to

distinguish the pre- and postzygapophyses, due to the poor pres-

ervation and the strong intervertebral attachment. The transverse

processes are anteroposteriorly narrower than those of the lum-

bar vertebrae.

Ribs

The presacral ribs of Protuberum (Text-figs 11, 13) are unique

among traversodontids, for having a series of distinctive knob-

like processes along their dorsal borders. These processes

decrease in size distally on each rib, so that the proximal pro-

cesses (close to the tuberculum) are generally the largest. On

some of thse processes, a dorsolateral concavity develops. The

middle thoracic ribs are the longest and bear up to eight pro-

cesses. The number of processes decreases in shorter ribs (i.e.

the posterior thoracic, lumbar and cervical ribs), but the pro-

cesses themselves do not change significantly in size wherever

they are found along the column. Regular spacing separates

each process and they form rows that are parasagittally

aligned. Similar processes occur on the dorsal margin of the

illium, with these processes in alignment with those on the

ribs (see below). This condition is comparable to Thrinaxodon

(Jenkins 1971), in which tubercles are present on the dorso-

medial edge of costal plates and also form parasagittally

aligned rows.

The surface of each process is smooth and they are composed

primarily of compact (probably pachyostotic) bone, suggesting

that they were not covered by cartilaginous tissue. All of the pre-

sacral ribs (including the cervical ribs) exhibit modest curvature

in transverse section, so the trunk of Protuberum was probably

wide and flat. The rib-vertebra contact is very strong on the pos-

terior thoracic and lumbar regions, making it hard to distinguish

the limits between the rib heads and their respective apophyses

on the vertebrae. All of the ribs on the left side are fractured

and somewhat displaced between the tuberculum and the first

protuberance (this fracture has affected all of the ribs from the

thoracic to sacral region) but their heads remain articulated with

the vertebrae. Conversely, the anterior portion of the trunk in

MGB 368 ⁄ 100 does not preserve any articulated ribs, indicating

a weaker articulation of the ribs on the vertebrae in this region.

The ventral and dorsal borders of ribs are convex to produce an

ellipsoid transverse cross-section. A dichocephalous condition

(Romer 1956, p. 277) is observed in most ribs, except for the

last lumbar and sacrals, where the tuberculum and capitulum

are fused.

Cervical ribs. Cervical ribs are rarely preserved in non-mamma-

lian cynodonts, so few specimens are available for comparison.

However, comparisons with Exaeretodon suggest that two ribs

referred to Protuberum represent the cervical region and that

they are probably from posterior cervicals. One of these ribs is

well preserved (UFRGS PV 1009T; Text-fig. 13A) and lacks only

the distal tip, while the other is represented by its proximal end

only (UFRGS PV 0981T).

UFRGS-PV1009T has three processes on its dorsal margin,

which decrease in size distally. These protuberances occur only on

the proximal half of the rib. The first protuberance has a distinct

morphology, being twice as elongate proximodistally as the other

two. The rib has a small ridge on the posterior surface, which

begins at the capitulum and ends 15 mm beneath the tuberculum.

In the same region, but on the anterior surface of the rib, a shal-

low groove is present. The capitulum is very broad in UFRGS PV

1009T and presents a slightly convex, almost flat articular facet.

UFRGS PV 0981T (a right proximal fragment) has a small capitu-

lum, that is approximately the same size as the tuberculum. The

larger size of the capitulum in the former is probably a conse-

quence of preservational distortion, so it is most likely that the

tuberculum and capitulum had similar sizes originally.

The curvature observed in UFRGS PV 1009T is not particu-

larly strong: this contrast with the morphology of the first cervi-

cal ribs of Exaeretodon, which have a ‘horseshoe form’

(Bonaparte 1963, page 20).

Thoracic ribs. No anterior thoracic ribs are preserved, so it is

not possible to observe the transition between the cervical and

thoracic ribs. The processes of the thoracic ribs are characterized

by a globular shape. The most proximal process sometimes dis-

plays a concavity dorsolaterally and is generally the largest and

anteroposteriorly longest process on each rib, although the sec-

ond process reaches a similar size in some ribs (e.g. in the last

left thoracic rib the second process is longer than the first one).

The number of processes present varies from eight (as seen on

the seventh thoracic rib) to five (on the ninth thoracic rib).

An anterior crest arises in the proximal region of some tho-

racic ribs (Text-fig. 13B), also occurs in Massetognathus (Jenkins

1970) and Cynognathus (Jenkins 1971). This crest begins below

the most proximal dorsal process. From the eighth thoracic rib

onward, another crest on the posterior margin of the ribs can be

observed. These crests do not overlap adjacent ribs. Both crests

become slightly larger on the ninth (last) thoracic, so that the

posterior crest of this rib overlaps the anterior costal plate of the

first lumbar rib. The crests abruptly enlarge in the lumbar region

(and are termed costal plates hereafter), whereas in Luangwa

(Kemp 1980), Cynognathus and Diademodon (Jenkins 1971),


these crests gradually increase in size along the thoracic and

lumbar series. The posterior surface of the shaft is marked by a

shallow groove (Text-fig. 13C), which is also observed on ribs

bearing a posterior crest and lies directly beneath this structure.

According to Jenkins (1971), this groove probably represents an

intercostal neurovascular sulcus.

The heads on the middle thoracic ribs consist of a short

tuberculum and an elongate and robust capitulum, as occurs in

some thoracic ribs of Exaeretodon (Bonaparte 1963). A small

ridge is present between the tuberculum and capitulum of

each thoracic rib. The heads of the posterior thoracic ribs are

comparable to those of many non-mammalian cynodonts (e.g.

Cynognathus, Diademodon, Luangwa, Massetognathus and Exaere-

todon), with an enlargement of the tuberculum and a shortening

of the capitulum; consequently, these structures converge in the

posterior region of the axial skeleton. The distal portion of each

thoracic rib decreases in size gradually, but the tips terminate

abruptly, presenting a peg-like morphology: in contrasts with

Exaeretodon in which the distal ends of ribs taper gradually.

Lumbar ribs. All ten lumbar ribs are articulated with the five

lumbar vertebrae in MGB 368 ⁄ 100 (Text-fig. 11). The costal

plates of the lumbar ribs are lanceolate in shape, and lack the

subrectangular outline described in Thrinaxodon, Diademodon

and Cynognathus (Jenkins 1971) and Pascualgnathus (Bonaparte

1966). An anteriorly directed process arises from the anterior

border of the last costal plate in ventral view.

The general morphology of the costal plates in Protuberum dif-

fers significantly from that of other non-mammalian cynodonts,

in which the costal plates of the lumbar ribs are characterized by a

reduction of the distal portion of the shaft of the ribs, so that just

the plate remains. In Protuberum the shafts of the ribs do not exhi-

bit such severe reduction, and a portion of the shaft continues dis-

tal to the costal plates. The dorsal processes of the lumbar ribs are

similar to these observed on the thoracic ribs. They differ in num-

ber, however, with four on each lumbar rib, except for the last rib,

which is short and bears only one process.

The lumbar ribs are fused to the vertebrae. The capitular pro-

cesses become progressively shorter toward the sacrum and shift

dorsally to be in closer proximity to the tuberculi. All capituli

articulate intervertebrally. Starting from the third lumbar rib,

the capitular and tubercular facets are essentially confluent. The

last lumbar rib has a fused capitulum and tuberculum.

The last lumbar rib shows a marked decrease in length. Its

posterior border has a broad contact with the anterior portion

of the iliac blade, which is facilitated by the lateral curvature of

the anterior portion of the iliac blade. The distal ends of the

lumbar ribs terminate abruptly, with a sub-quadrangular outline

in anterior view, and a small process is placed just dorsal to the

distal tip of the rib.

Sacral ribs. Only two pairs of sacral ribs are preserved in the

type specimen (Text-fig. 11). They are poorly preserved, but

some of their features can still be described. The sacral ribs lack

dorsal processes. Their general morphology resembles that of the

sixth and seventh sacral ribs of Exaeretodon (Bonaparte 1963).

The first sacral rib is the most robust, especially distally where it

contacts the iliac blade. The second rib has a smaller distal

expansion and a distal fragment of the third left sacral rib, which

is still attached to the iliac blade, is even more slender.

As in Thrinaxodon (Jenkins 1971) and the last lumbar rib of

Protuberum, the capituli and tuberculi are fused. The shaft is

short and posterolaterally oriented and the distal ends are

expanded anteroposteriorly and strongly fused to the iliac blade.

Ilium

A fragmentary left ilium (Text-figs 11, 14) is present in the type

specimen, but its acetabular and ventral portions are not pre-

served. It has a robust and elongate blade, with a lanceolate

anterior edge, which possesses rounded processes along its

dorsal border that are similar to those of the presacral ribs.

These processes are aligned with those on the ribs and form part

of the same parasagittal series (see above). The blade is laterally

concave, especially in its anterior half, where it curves laterally:

the anterior part of the ilium extends parallel to the last lumbar

rib at an angle of approximately 80 degrees to vertebral column.

DISCUSSION

Craniodental features

One of the most striking features of Protuberum is the

short parietal crest, which represents only 7.5 per cent of

skull length. In Exaeretodon (Bonaparte 1962), the parietal

crest represents about 30 per cent of skull length, while in

Luangwa sudamericana this figure is 28 per cent (Abdala

and Teixeira 2004: MCP 3284) and is 24 per cent in Mas-

setognathus (UFRGS PV 0968T). The thickened skull roof

is quite remarkable: its function is unknown, but it is

possible that it was associated with defense or burrowing.

The incisors of Protuberum have differential thicknesses

of enamel on their labial and lingual surfaces. Kemp

(1980) described a similar condition in Luangwa and

observed that this feature ensured that the ridges on the

teeth remained sharp throughout life. This self-sharpening

tooth could have been employed in gripping, cutting and

tearing off food. Moreover, the wide concave areas

formed on the crowns could be employed for plant crush-

ing. The postcanines of Protuberum are deeply worn

(except for the posteriormost teeth) as also occurs in

other traversodontids, such as Massetognathus (Crompton

1972a), Dadadon (Goswami et al. 2005) and Luangwa

(Kemp 1980). The latter possesses a pattern of tooth wear

that seems applicable to Protuberum (Text-fig. 15). The

mechanism proposed for the formation of the broad but

poorly matching concave wear facets present on the upper

and lower postcanines of Luangwa was a form of food-to-

tooth contact analogous with mammalian puncture-

crushing (Crompton and Hiiemae 1969), rather than

abrasion caused by occlusion. The wear pattern observed


in these traversodontids supports the hypothesis of a pos-

terodorsally directed power stroke of the mandible

(Crompton 1972a), which is reinforced in Protuberum by

the presence of anteroposteriorly elongate paracanine fos-

sae and the probable mobility of the quadrate.

Luo and Crompton (1994) and Crompton (1972b) also

interpreted proposed a posteriorly directed movement of

the lower jaw of power stroke in Massetognathus, based

on the morphology of the quadrate. The quadrate troch-

lea in Protuberum is not as well pronounced as in Mas-

setognathus, but its orientation is similar and consistent

with the same type of movement. An anteroposterior

movement was also suggested for the quadrate of Dia-

demodon (Brink 1955), which is closely related to travers-

odontids (Abdala and Ribeiro 2003). The quadrate in

Protuberum is not rigidly attached to the squamosal and

may, therefore, have had some freedom of movement as

also suggested for Exaeretodon (Allin 1975).

Postcranial features

Limited morphological diversity is apparent in the post-

cranial skeletons of Triassic non-mammalian cynodonts.

However, rib morphology does appear to be useful for

distinguishing traversodontid taxa. Pascualgnathus polan-

skii, the earliest traversodontid (Puesto Viejo Formation,

Late Olenekian–Early Anisian; Bonaparte 1966) for which

ribs are known, has a costal morphology similar to that

of Diademodon (Scythian–early Anisian; Kitching 1995),

Luangwa (Anisian; Brink 1963) and Cynognathus (Scyth-

ian–early Anisian; Kitching 1995). Massetognathus pascuali

(Chanares Formation, Ladinian; Jenkins 1970), which is

temporally intermediate between P. polanskii and Exaere-

todon, has rib specializations that are restricted to the

lumbar region, while Exaeretodon (Ischigualasto Forma-

tion, Carnian; Bonaparte 1963) lacks rib specializations.

This reversal to a non-specialized condition was inter-

preted as a derived feature by Jenkins (1970). Protuberum

(Ladinian), however, shows unique and distinctive rib

morphology: while other traversodontids, such as Exaere-

todon, tended to simplify rib structure, Protuberum deve-

loped more elaborate ribs.

The presence of costal plates of Protuberum allows rec-

ognition of separate thoracic and lumbar regions, as in

Diademodon, Thrinaxodon and Cynognathus (Jenkins

1971). This distinction points to a possible division into

thoracic and abdominal cavities, as suggested by Brink

(1955). In the case of Protuberum, the presence of both

dorsal processes and costal plates gives the lumbar region

an exceptionally robust appearence. This, in combination

with the simple curvature of the ribs, indicates that Pro-

tuberum had a distinctive trunk morphology. It is possible

that this robust and specialized postcranial structure may

have provided protection against predators, a conclusion

supported by some other features of the skeleton, such as

the thickened skull roof (see above).

The accentuated curvature of the anterior portion of

the iliac blade provided a wide surface for muscle inser-

tion in that region. Therefore, the hindlimb and associ-

ated musculature may have been very strong, especially

the m. iliofemoralis and m. iliotibialis, both of which origi-

nate on the iliac blade. Indeed, strong, robust limbs

would have been necessary to support the robust axial

skeleton, and could have been employed in burrowing or

digging for food.

Systematics

A phylogenetic analysis to investigate the relationships of

Protuberum to other cynodonts was conducted using the

28 craniodental characters proposed by Abdala and Ribe-

iro (2003). Our data matrix incorporates 16 taxa, includ-

ing Diademodon, Trirachodon and 14 traversodontids.

Analyses were performed using the beta version of PAUP

4.0 (Swofford 1998) with Diademodon as the outgroup.

The branch-and-bound search method was applied and

produced a single most parsimonious tree (tree

length = 58, Consistency Index = 0.569, Rescaled Consis-

tency Index = 0.418).

It is important to be cautious with the resulting clado-

gram (Text-fig. 16), because the data matrix used by

Abdala and Ribeiro (2003) focused on dental characters

(21 characters, with the remaining seven relating to the

rest of the cranium and mandible) and excluded

TEXT -F IG . 15 . Dental wear in postcanines of Luangwa (Kemp 1980). A, occlusion of postcanines in sagital section. B, mechanism

of development of the transversely concave wear facet of the upper postcanine by occlusion with a narrower lower postcanine.

Modified from Kemp (1980).


postcranial information. In addition, as the mandible of

Protuberum is currently unknown, our analysis is missing

data for eight lower dentition ⁄ mandibular characters.

However, this preliminary analysis places Protuberum in a

clade with Gomphodontosuchus, Exaeretodon, Scalenodon-

toides hirschsoni and Menadon. This clade shares charac-

ters such as large incisors (acquired convergently in S.

hirschsoni), the presence of three upper incisors (acquired

convergently in S. hirschsoni and Pascualgnathus), paraca-

nine fossae positioned posterior to the upper canine,

absence of the internarial bar and the presence of a well

developed posterior extension of the jugal above the squa-

mosal in the zygomatic arch (this condition is unknown

in Gomphodontosuchus).

CONCLUSIONS

Protuberum cabralensis is a new traversodontid cynodont

that can be diagnosed on the basis of several distinctive

features, particularly the presence of prominent processes

on the cervical, thoracic and lumbar ribs, similar protu-

berances on the anterodorsal margin of the ilium and in

its unique dental morphology. Protuberum possesses a

combination of characters thought to be derived (e.g. the

lack of the internarial process of the premaxilla, bifurca-

tion of the paroccipital process and the lateral opening of

the pterygoparoccipital foramen) and primitive (e.g. mor-

phology of the iliac blade and presence of rib specializa-

tions) among traversodontids: more work is needed to

establish the phylogenetic position of this taxon within an

expanded analysis of traversodontid relationships.

Acknowledgements. We would like to thank C. Salla Bortolaz

(technician at MGB) for completion of the arduous and skillful

preparation of the type specimen and for assistance on begin-

ning this work. Additional thanks to A. Battaglin Cafaro (Secre-

tary of Tourism and Culture in the Municipality of Mata) for

lending us the type specimen. Special thanks to J. F. Bonaparte,

E. Snively, R. C. Fox, T. Kemp and Z.-X. Luo for valuable com-

ments on earlier versions of the manuscript. D. Larson helped

with the phylogenetic analysis. L. Morato and T. Veiga de Olive-

ira also contributed important ideas. Conselho Nacional de

Desenvolvimento Cientıfico e Tecnologico (CNPq) provided

financial support.

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TEXT -F IG . 16 . Most parsimonious tree generated from the analysis of phylogenetic relationships of Protuberum cabralensis (see text

for further details).


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APPENDIX

Character list

Morphological characters used in this paper are as follows. Char-

acters are derived from Abdala and Ribeiro (2003). ‘0’ represents

the primitive state.

1. Number of upper incisors: four (0); three (1).

2. Incisor size: small (0), large (1).

3. Diastema between upper incisors and canine: present (0),

absent (1).

4. Upper canine size: large (0), reduced (1).

5. Lower canine size: large (0), reduced (1).

6. Position of paracanine fossae in relation to the upper canine:

anteromedial (0), medial (1), posteromedial (2).


7. Overall morphology of the upper postcanines: ovoid-ellipsoid

(0), rectangular-trapezoidal (1).

8. Shouldering in upper postcanines: absent (0), present (1).

9. Posteromedial inclination of the last upper postcanines:

absent or small (0), oblique (1).

10. Transverse crest of upper postcanines: central (0), anterior

(1), posterior (2).

11. Number of cusps in the transverse crest of upper postca-

nines: two (0), three (1).

12. Central cusp of upper transverse crest: midway between buc-

cal and lingual cusps (0), closer to the lingual cusp (1).

13. Posterior cingulum in upper postcanines: present (0), absent

(1).

14. External cingulum in the anterior portion of the upper post-

canines: absent (0), present (1).

15. Anterolingual cusp in upper postcanines: absent (0), present

(1).

16. Number of cusps in the sectorial border of the upper post-

canines: three (0), one (1), two (2).

17. Overall morphology of the lower postcanines: ovoid-ellip-

soid (0), quadrangular (1).

18. Transverse crest in lower postcanines: central (0), anterior

(1).

19. Number of cusps in the transverse crest of the lower postca-

nines: two (0), three (1).

20. Size of the anterior cusps in the lower postcanines: labial

lower than lingual (0), labial higher than lingual (1).

21. Cingulum in front of the transverse crest in the lower post-

canines: absent (0), present (1).

22. Internarial bar: present (0), absent (1).

23. Maxillary labial platform: absent (0), present (1).

24. Parietal foramen in adults: present (0), absent (1).

25. Zygomatic process of the jugal: conspicuously projected (0),

little projected (1), a ball-like process (2), absent (3).

26. Posterior extension of the jugal above the squamosal in the

zygoma: absent or with a small extension (0), well-developed

(1).

27. Coronoid process of the mandible: cover the last postcanine

(0), does not cover (1).

28. Dentary angle: not or weakly projected posteriorly (0), well

projected posteriorly (1).

Data matrix

12345 67891

0

11111

12345

11112

67890

22222

12345

222

678

Diademodon 00000 00000 000?? 0000? ?0000 000

Trirachodon 00000 00000 100?? 1001? ?0100 010

Andescynodon 00000 01001 0?001 11101 10113 0?0

Massetognathus 00111 11112 11100 01100 00111 000

Exaeretodon 11101 21112 0?101 01101 01110 101

Luangwa 00000 01002 11011 21100 10101 000

Scalenodon

angustifrons

00000 01000 11010 11100 1010? ?0?

Scalenodon

hirschsoni

1111? 010?2 11001 21100 101?? ???

Traversodon 00000 01012 11000 01100 00101 ?0?

Gomphodontosuchus 01111 11112 ??10? ?1101 001?? ?0?

Pascualgnathus 10000 01000 0?0?1 111?? ??111 000

Scalenodontoides 111?? 211?2 0?10? 01101 0111? 101

Menadon 11111 ?1112 0?101 2???? ?11?3 101

Dadadon 000?? 01112 11111 2???? ?01?2 ???

Santacruzodon 000?? 01112 11101 01101 001?2 ?00

Protuberum 1111? 21000 ??100 2???? ?1100 1??

Abbreviations in text-figures

An., anapophysis; Ant. Lam., anterior lamina; B. Th., bone thick-

ening; Bas. Pro. Bas., basipterygoid process of basisphenoid; Bas-

iocc., basioccipital; C., canine; Cap., capitulum; Cav. Ep., cavum

epiptericum; Cho., choana; Cont. F., contact facet; Cr., crest;

Cult. Proc. Parasph., cultriform process of parasphenoid; Desc.

Proc. Pt., descending process of pterygoid; Dor. Ang., dorsal

angle; Dor. Pl., dorsal plate; Epipt., epipterygoid; Ext. A. Meat.,

external auditory meatus; Fen. Ov., fenestra ovalis; For. Mag.,

foramen magnum; Fr., frontal; Gr., groove; I.c., posterior open-

ing of infraorbital canal; Inc. For., incisive foramen; Int., interpa-

rietal; Jug., jugal; Jug. For., jugular foramen; L., lacrimal; Lab.

Cusp, labial cusp; Lam. Cr., lambdoid crest; Lat. M., lateral mar-

gin; Lat. Tr. C., lateral trochlear condyle; Ling. Cusp, lingual

cusp; Mas. Proc. Jug., masseteric processes of jugal; Med. M.,

medial margin; Med. Proc. Jug., medial process of the jugal; Mx.,

maxilla; Mx. B., maxillary bulge; Mx. Proc. Prmx., maxillary pro-

cess of the premaxilla; N., nasal; Occ. Cond., occipital condyle; P.

Temp. Fos., post-temporal fossa; Pal. palatine; Par., parietal;

Para., parapophysis; Par. Cr., parietal crest; Paro. Cr., posterior

paraoccipital crest; Par. For., parietal foramen; Par. Fos., paraca-

nine fossa; Par. Proc., paraoccipital process; Po., postorbital; Prf.,

prefrontal; Prm., premaxilla; Pro., prootic; Pt., pterygoid; Ptp.

For., pterygoparoccipital foramen; Q. Ram. Ep., quadrate ramus

of the epipterygoids; Qj., quadratojugal; Qu., quadrate; R., ridge;

Sm., septomaxilla; Sph. Op., sphenorbital opening; So., supraoc-

cipital; Sq., squamosal; Syn., synapophysis; Tab., tabular; Tr.

For., trigeminal foramen; Trans. Crest, transverse crest; Tro. Tr.,

trochlear trough; Tuber., tuberosity; Tub., tuberculum.


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