Osteology of Andescynodon (Cynodontia: Traversodontidae) from the Middle Triassic of Argentina

2009·9.3 MB·English

by  Jun Liu

PUBLISHED BY THE AMERICAN MUSEUM OF NATURAL HISTORY
CENTRAL PARK WEST AT 79TH STREET, NEW YORK, NY 10024
Number 3674, 19 pp., 16 figures, 1 table November 30, 2009
Osteology of Andescynodon (Cynodontia:
Traversodontidae) from the Middle Triassic
of Argentina
JUN LIU1 AND JAIME POWELL2
ABSTRACT
All available specimens of Andescynodon mendozensis and Rusconiodon mignonei are examined
andrestudied,andthelatterspeciesisregardedasajuniorsynonymoftheformer.Thepostcranial
skeleton of this species is described for the first time; it generally shows primitive features of
traversodontids.Andescynodonmendozensisisasmall-tomedium-sizedtraversodontidcharacter-
izedby9,11upperpostcaninesinadults(fewerinlargerindividuals),thetransverseridgeofupper
postcanineslyinganteriorlyonthetooth,paracaninefossapenetratingtheskullroofinadult;itis
differentiated from Pascualgnathus by relatively flatten skull, shorter and narrower temporal
region, presence offourrather thanthree upper incisors.
INTRODUCTION are traditionally viewed as occurring in the
Rio Mendoza Formation and being among
Traversodontids are a common member of themostbasaltraversodontids(Godefroitand
the terrestrial Triassic fauna, especially in Battail, 1997; Abdala and Ribeiro, 2003; but
South America. The family name comes seeAbdalaetal.,2006).However,recentwork
from the buccolingually expanded postcanine hasshownthatthesecynodontsactuallycome
teeth, which are linked to an herbivorous or from the Cerro de las Cabras Formation
omnivorous diet. Andescynodon mendozensis (Middle Triassic) (Zavattieri and Arcucci,
and Rusconiodon mignonei are early represen- 2003), co-occurring with Cromptodon mami-
tativesoftheTraversodontidae,andbothtaxa feroides. Andescynodon and Rusconiodon were
1Key Laboratory of Evolutionary Systematics of Vertebrates, Institute of Vertebrate Paleontology and
Paleoanthropology, Chinese Academy of Sciences, Beijing, China 100044 ([email protected]); Division of
Paleontology,AmericanMuseumofNaturalHistory.
2PaleontologiadeVertebradosLillo,UniversidadNacionaldeTucuman,Argentina.
CopyrightEAmericanMuseumofNaturalHistory2009 ISSN0003-0082
2 AMERICAN MUSEUMNOVITATES NO. 3674
established by Bonaparte (1967, 1970), who postzygophysis; pp, paroccipital process; ppf,
gave a brief description of the skull and pterygo–paroccipital foramen; Pro, prootic;
dentition of each genus. Subsequently, Qui- prz, prezygophysis; Pt, pterygoid; ptf, post-
roga (1980) studied endocasts of Ande- temporalfenestra;pu,pubis;pup,pubicplate;
scynodon, whereas Gon˜i (1986) and Gon˜i Q,quadrate;rs,ribshaft;Sa,surangular;Sm,
andGoin(1987,1988)studiedthepostcanines septomaxilla; sp, neural spine; Sq, squamosal;
in detail. Gon˜i and Abdala (1989) briefly sr, sacral rib; th, trochlear; trf, transverse
describedtheskull,lowerjaw,anddentitionof flange of the pterygoid; tr mj, trochanter
Rusconiodon. In this paper, we restudy all the major;trmn,trochanterminor;trp,transverse
material pertaining to these taxa, especially process;tt(c),tuberosityfororiginofcoracoid
the postcranial skeleton. head of triceps.
RusconiodonissimilartoAndescynodoninits
skull and tooth morphology, except for the SYSTEMATIC PALEONTOLOGY
canine hypertrophy and supposed dorsal and
labial perforation of the snout by paracanine
fossae in Rusconiodon (Gon˜i and Abdala, THERAPSIDA BROOM, 1905
1989). In all the collected material from the
CYNODONTIA OWEN, 1861
Cerro de las Cabras Formation, only PVL
3834,PVL3840,PVL3892a,b,c,d,andPVL
TRAVERSODONTIDAE VON HUENE,
3900 have ever been referred to Rusconiodon
1936
(Gon˜iandAbdala,1989;Abdala,2000).Given
themorphologicalsimilarityofthetwospecies ANDESCYNODON BONAPARTE, 1967
and their identical geographic and temporal
SYNONYM: Rusconiodon Bonaparte, 1970.
distributions, we want to test the hypothesis
TYPE SPECIES: Andescynodon mendozensis
thatthespeciesrepresentasingletaxon.
Bonaparte, 1967.
DIAGNOSIS (revised): small- to medium-
ABBREVIATIONS sized traversodontid characterized by 9,11
upper postcanines in adults (fewer in larger
INSTITUTIONAL: MLP, Museo de La Plata, individuals), transverse ridge of each upper
Argentina; PVL, Coleccio´n de Palaeontolog´ıa
postcanine lying anteriorly on the tooth,
de Vertebrados, Instituto Miguel Lillo, Uni-
paracanine fossa penetrating the snout in
versidadNacionaldeTucuma´n,Argentina.
adult; differentiated from Pascualgnathus by
ANATOMICAL: A, angular; acr, acromion relativelyflattenedskull,shorterandnarrower
process;ae,anterioredgeofthemedialendof
temporal region, presence of four rather than
the clavicle; ap, anapophysis; Bo, basioccipi-
three upper incisors.
tal; Bs, basisphenoid; C, canine; cf, clavicular
flange; cp, capitulum; D, dentary; dc, delto-
Andescynodon mendozensis Bonaparte, 1967
pectoral crest; ec, ectopicondyle; en, entepi-
condyle; Eo, exoccipital; Ep, epipterygoid; F, SYNONYM: Rusconiodon mignonei Bona-
frontal; f ec, ectopicondylar foramen; f en, parte, 1970.
entepicondylar foramen; fj, foramen jugulare; HOLOTYPE: PVL 3833, incomplete skull.
fo, fenestra ovalis; f spc, supracoracoid fossa; STRATUM TYPICUM: Cerro de las Cabras
g, glenoid; gr, groove in dorsal edge of the Formation.
ischium; gt, greater tuberosity; h, head; I, LOCUSTYPICUS: 5 kmtothewestofVillade
incisor; icl, interclavicle; if, intertrochanteric Potrerillos,ProvincedeMendoza,Argentina.
fossa;il,ilium;is,ischium;isr,ischialridge;J, DIAGNOSIS: As for genus, of which this is
jugal; jf, jugular formamen; L, lacrimal; lt, the only known species.
lesser tuberosity; M, maxilla; N, nasal; ob f, AGE: Middle Triassic.
obturatorforamen;Op,opisthotic;P,parietal; REFERRED SPECIMENS: All specimens of
pcf,paracaninefossa;pe,posterioredgeofthe Andescynodon and Rusconiodon in PVL as
medialhalf of the clavicle; Pf, prefrontal; Pm, identified by Abdala (2000). The following
premaxilla; Pl, palatine; Po, postorbital; poz, specimens were used in this study. Skull and
2009 LIUAND POWELL:OSTEOLOGY OFANDESCYNODON 3
Fig. 1. The holotype of Andescynodon mendozensis (PVL 3833) in A, dorsal; B, ventral; and C,
lateralviews.
mandible: PVL 3833 (holotype of Andescy- 3894, one isolated vertebra, vertebral column
nodon mendozensis, fig. 1), 3834, 3840 (holo- with associated ribs, clavicles, portion of left
type of Rusconiodon mignonei, fig. 2), 3890, pelvic girdle, humerus, femur; PVL 4390
3892(a,b,c,d),3894,3894-1,3899,3900,4390. (juvenile), three articulated vertebrae and one
Postcranial skeleton: PVL 3890, part of scap- isolated vertebra, ribs, part of pelvis, right
ula, pelvis, two humeri, left ulna, femur; PVL ilium; PVL 4423, scapula; PVL 4424–4427,
4 AMERICAN MUSEUMNOVITATES NO. 3674
Fig. 2. The holotype of Rusconiodon mignonei (PVL 3840) in A, dorsal; B, ventral; C, lateral; and D,
occipital views.
2009 LIUAND POWELL:OSTEOLOGY OFANDESCYNODON 5
Fig.3. ReconstructionoftheskullandlowerjawinA,dorsalview;B,ventralview;andC,lateralview.
Abbreviationsas intext.
four humeri; PVL 4428, right pectoral girdle; small to medium sized. The basicranial length
PVL4432,oneincompletevertebra;PVL6226, reaches 137 mm in PVL 3900, but in most
incompletepelvis. specimens is less than 100 mm (table 1).
The snout is only slightly expanded, and its
MORPHOLOGICAL DESCRIPTION length is subequal to that of the temporal
region. The zygomatic arch flares posteriorly
SKULL and the skull’s maximum width occurs near
the posterior end. The external nares are
The sutures are not well preserved in any separated by an internarial bar. The premax-
specimen, and the drawings (fig. 3) are com- illa is overlapped by the nasal and terminates
posites of different specimens. The skull is between the external nares. This part of the
6 AMERICAN MUSEUMNOVITATES NO. 3674
TABLE1
Measurements ofPVLspecimens (in mmexceptfor UTN)
(SL,skulllengthfromthetipofthesnouttotheoccipitalcondyle;TEL,totallengthoftemporalregion;SW,
skull maximumwidth, occurringin temporal region; OH,occipital height; UCH,upper canine height,
definedastheprominenceofthecaninebeyondthealveolarmargin;LCH,lowercanineheight;UTN,upper
postcanine teethnumber.)
Specimen SL TEL SW OH UCH LCH UTN
3833 86 35 72 22 — — 10
3834 76 30 60 20 9.6 — 10
3836 65 28 53 18 — — 8
3840 91 41 62 22 — — 9
3890 86 40 — — 11 — —
3892a 103 42 — 36 18 19 9
3894 79 27 — 7.7 10
3894-1 87 32 57 21 — 10 11
3899 69 26 57 24 6 — —
3900 137 44 82 33 19 .16 —
4390 55 4 7
snout is broken in most specimens, but the the maxilla in lateral view by a laterally open
internarial bar is complete in PVL 3899 paracanine fossa (fig. 2C); this state was
(fig. 4A).Theseptomaxillaliesonthepostero- suggested to be a distinct character of
ventral side of the external naris. Rusconiodon by Bonaparte (1970), Gon˜i and
The incisive foramen is mostly enclosed by Abdala(1989),andAbdala(2000).Amongthe
the premaxilla, although the posterior border specimens placed in this genus by Abdala
is formed by the maxilla. A deep paracanine (2000), only the holotype and PVL 3892c
fossa lies anteromedial to the large upper possess this condition, which may in fact be
canine. In some specimens, it is a rather large the result of breakage. In better-preserved
opening. The fossa is surrounded by the specimens assigned to Rusconiodon, such as
premaxilla and the maxilla in the palate. In PVL 3900 (fig. 4B), the premaxilla extends
PVL 3840 (the holotype of Rusconiodon posteriorly as a thin plate to contact the
mignonei), the premaxilla is separated from maxilla just anterior to the upper canine. The
thin plate forms the labial wall of the
paracanine fossa and conceals the lower
canine in lateral view. Accordingly, the labial
fossaisboundedbyalateralwallinPVL3900,
despite Bonaparte’s (1970) assertion that a
laterally open fossa is characteristic of
Rusconiodon. The upper perforation of the
paracanine fossae is present in all the speci-
mens classified as Rusconiodon by Abdala
(2000),butinotherspecimenseithertheupper
perforation is absent or the condition is
unknown. When the perforation is present, it
lies between the septomaxilla and nasal and
the maxilla.
As is usual in traversodontids, the maxilla
curves inward and downward from the lateral
Fig.4. A,anterolateralviewoftheskullofPVL
surface of the snout, forming a platform
3899, showing the nasal region and incisors; B,
lateralviewoftheskullandmandibleofPVL3900, lateral to the postcanine teeth. A depression
showing the completeness of the lateral margin of is present on the dorsalmost portion of the
the paracanine fossa.Abbreviations asin text. maxilla,closetothesuturewiththenasal,and
2009 LIUAND POWELL:OSTEOLOGY OFANDESCYNODON 7
just anterior to the orbit. The depression is
rounded and well preserved in PVL 3894-1,
similar to the condition in Pascualgnathus.
Abdala et al. (2006) state that an equivalent
fossa is also present in Langbergia, Cyno-
gnathus, Diademodon, and some traversodon-
tids. Based on our observations, this feature
canbefoundinallwell-preservedspecimensof
gomphodont cynodonts although it is shallow
in some, e.g., Massetognathus, and deep in
others,e.g.,Pascualgnathus.
The skull is slightly depressed in the middle
of the interorbital region, where the posterior
portionofthefrontalsislocated.The frontals
extend backward into the temporal region.
Behind them, a long parietal crest is formed
mostly by the parietals, although the post-
orbitals contribute to the anterior part of the
crest. The squamosals and parietals extend
well backward, concealing the occipital plate Fig. 5. Specimen PVL 3899 in ventral view.
in dorsal view. Abbreviations asin text.
The posterior margin of the skull roof is
third and fourth postcanines from the rear;
formed by the conjoined parietals and squa-
however, it can extend to the level of the
mosals. No parietal foramen is present. The
second(theleftsideofPVL3840)orfifth(the
squamosals extend laterally and slightly pos-
left side of PVL 3894-1) tooth from the rear.
teriorly away from the parietal crest, and the
The palatines contribute more than one-third
rightandlefthalvesoftheposteriormarginof
of the length of the osseous secondary palate.
theskull generallycome together inan obtuse
Posterior to the secondary palate, the
angle in dorsal view. The acute angle in PVL
central area of the palate is bounded by a
3892 and 3900 is possibly unnatural, formed
pair of pronounced ridges that run poster-
by the lateral compression of the specimens.
omedially, defining a shallow midpalatal
A small jugal process is developed slightly
vault.Theposteriorpartofthepaired vomers
anterior to the level of the posterior border of
forms the roof of the vault, but more
the orbit in PVL 3894-1. Perhaps because of
anteriorly the vertical ridge of the vomers is
poor preservation, this feature cannot be
concealed by the secondary palate.
observed in other specimens. As in Masse-
The lateral crests extend from palatine to
tognathus pascuali, the lower margin of the
pterygoid, ending in the anterior portion of
zygomatic arch curves slightly dorsally, the the thin basicranial girder as distinct medial
anterior part is nearly straight, and slightly tubercles.Oneitherside,thelateralmarginsof
higher than the ventral margin of the maxilla. the pterygoids extend backward as ridges that
This differs from the strongly dorsally curved terminate in sharply pointed flanges directed
zygomatic arch in Pascualgnathus. The zygo- strongly ventrally and slightly backward and
maticarchisdeepdorsoventrally,hidingmost laterally (fig. 5). The thin basicranial girder is
oftheparietalregioninlateralview.Thejugal formed by the conjoined pterygoids and
extends nearly to the posterior end of the parasphenoid, and the basicranium is of the
zygomaticarch,andthesquamosalisexposed type commonly seen in cynodonts. The
in a narrow area on the posterodorsal and basioccipital and basisphenoid-parasphenoid
posterior part of the arch. form a triangular excavated area bounded by
The secondary palate occupies 39%–46% of a pair of posterolaterally directed ridges, with
the basal skull length; the proportion usually a thin, low crest running along the midline of
is lower in larger specimens. The secondary the excavation. On either side, there is a large
palate generally extends to the level of the cavum epiptericum, which is bounded medi-
8 AMERICAN MUSEUMNOVITATES NO. 3674
Fig. 6. Specimen PVL 3892ain lateralview.Abbreviationsas in text.
ally by the basisphenoid and laterally by the Pascualgnathus, this border is convex. The
epipterygoid. Posterolaterally, a pterygopar- lower edge turns smoothly upward towards
occipital foramen is bounded posteriorly by the posterior end of the dentary, and the
the lateral flange of the prootic. The squamo- angular process does not develop as in
sal contacts the quadrate ramus of the Diademodon. The coronoid process is directed
epipterygoid anteriorly and the lateral flange posterodorsally,atalargeanglewiththemain
of the prootic medially. Medial to the pter- body. The posterodorsal terminus of the
ygoparoccipital foramen, the fenestra ovalis coronoid process is slightly convex.
and jugular foramen lie close to the posterior Thesplenialcanbeobservedinventralview
endoftheridgeofthebasisphenoid. Distalto in PVL 3899 and 3900. It is a thin lamina,
the epipterygoid bar, a right quadrate with a extending anteriorly to the symphyseal region
troughliketrochleargrooveisstillpreservedin and posteriorly to the angle of the dentary.
situ in PVL 3899, and this element articulates The angular is widely exposed posterior and
with the articular bone of the mandible. ventral to the dentary, and forms a concave
Inoccipitalview(fig. 2D),theskullshowsa lateral surface. The surangular lies poster-
well-developed V-shaped notch between the odorsal to the angular and is exposed below
lambdoidal crest and the zygomatic arch. A the posterior process of the dentary.
round posttemporal fenestra lies in the con-
cave area above the paroccipital process.
DENTITION
Other features are obscured due to poor
preservation. The incisors are poorly preserved in most
specimens.Theirnumbercanbedeterminedin
a few good specimens like PVL 3899 (fig. 4),
MANDIBLE
in which there are four upper and three lower
Manymandibleshavebeenfound,although incisors of moderate size, with a somewhat
in most cases the postdentary bones are not chisellikeshape.Boththeupperandthelower
preserved. However, a few complete mandi- incisors are slightly procumbent (PVL 3892,
bles have been found associated with skulls 3894-1), contrary to the observations of Gon˜i
(figs. 5, 6). The dentaries are fused at the and Goin (1988); the upper incisors pass
symphysis. The lower edge is nearly straight, lateral to the lower incisors in occlusion.
although the anterior part turns slightly The canines vary in height among individ-
downward to form a chinlike structure. In uals of different sizes, being hypertrophied in
2009 LIUAND POWELL:OSTEOLOGY OFANDESCYNODON 9
some specimens. The maximum height above are developed posterior to and slightly sepa-
thealveolarridgereachesapproximately2 cm, ratedfromtheanterolabialcusponthelateral
forthecaninesofPVL3892aand3900.When ridge, which is a nearly straight ridge that
the mandibles are occluded with the skull, the slopes upward posteriorly. These cusps are
upper canines can extend down to or beyond poorly preserved, and it seems that only one
the ventral edges of the mandibles, whereas cusp is present in most postcanines. A basin
the lower canines can penetrate the skull and occupiestheposteriorhalfofeachtooth,lying
areexposedindorsalandlateralviews.Thisis posterior to the anterior transverse ridge and
probablyanaturalstateratherthanartifactof lingual to the labial ridge.
preservation because this feature is well Wear occurs more prominently on the
preserved in some specimens. This kind of anterolingual cusp, the anterior half of which
canine is also developed in Pascualgnathus. isusuallyentirelyabsent.Thetransverseridge
The size of a vertebrate tooth usually cannot therefore seems to be displaced toward the
increase after eruption, so increases in tooth center of the tooth, rather than lying at the
size can generally occur only by replacement. anterior margin. With further wear, the
However, the large canines of Andescynodon transverse ridge becomes worn away (PVL
and some other cynodonts seem to represent 3840), so that the crown surface iscontinuous
an exception. The canine total height far anteroposteriorly for successive teeth but
exceeds the depth of the jaw, so the canine concave as seen in posterior view. This
must continue to grow following eruption as resembles the last stages of wear in other
in modern pigs and presumably in dicyno- traversodontids, such as Pascualgnathus and
donts. Serrations are observed on the posteri- Massetognathus (personal obs.).
or margins of the upper canines of PVL 3899. Each lower postcanine is subquadrate in
The postcanine teeth vary in number and crown view, with length and width subequal
shape in different individuals. The number (fig. 8). The anterior part forms a high
ranges from 8 to 11 upper postcanines (10 for transverse ridge, partly subdivided into two
most countable specimens) and 7 to 10 lower cusps. The labial cusp is the higher, but the
postcanines, with maximum values tending lingualcuspiswiderandmoresteeplyinclined
to occur in middle-sized specimens like posteriorly. In a freshly erupted tooth, small
PVL 3894-1. The second largest specimen, cuspules lie anterior to the anterolabial cusp;
PVL3892a, has 9 postcanines, while the posteriorly, two minor cusps follow the main
number isunavailableinthelargestspecimen, labialcusp,andcombinewiththemaincuspto
PVL 3900. Although all postcanines are formalateralridge.Themiddlecuspisusually
gomphodont in shape in adults, subgompho- worn away, leaving only two well-separated
dontteetharepresentintheanteriormostpart cusps on the lateral portion of the tooth. A
ofthejawinjuvenileslike PVL4390,whereas basin lies in the middle of the tooth, bounded
the last three postcanines in these specimens by the ridges. As in other traversodontids, the
are sectorial (Gon˜i and Goin, 1988). internal side of the basin is lower than the
In crown view, each gomphodont upper external side, and following wear the internal
postcanineissubquadrate,around1.5timesas sideformsadeepU-shaped opening.
broad labiolingually as long mesiodistally
(fig. 7). Two major cusps develop on the POSTCRANIAL SKELETON
anterior half to form a V-shaped area in
mesial view. The labial cusp is the higher of Because two genera and species were
thetwo,butthelingualcuspisnearlytwiceas thought to have coexisted in the Cerro de las
wideasthelabialcusp.Theanteriorcuspsare Cabras Formation, it was previously difficult
slightly recurved posteriorly, and the lingual to identify some postcranial materials recov-
member of the pair occupies approximately eredfromtheformationtothelevelofspecies.
half of the lingual side of the tooth. Posterior However, as the two genera and species are
to the cusp, the lingual margin turns postero- here regarded to be synonymous (see discus-
labially. The posterior margin is protruded sion later), all postcranial skeletons are
slightly anteriorly. One or more small cusps described together.
10 AMERICAN MUSEUMNOVITATES NO. 3674
Fig.7. Uppercanineandpostcaninesonrightside(PVL3894-1)inA,crownview;B,labialview;Cand
D,lingualview.Dfromfigure 1ofGon˜iandGoin(1988).AnteriorisatrightexceptinB,inwhichanterior
isat left.
VERTEBRAEANDRIBS the pedicle, and the long axis of the tubercu-
lum facet is directed anteroventrally. The low
DORSAL REGION: One isolated vertebra of neural spine is transversely slender and re-
PVL 3894 looks similar to an anterior dorsal curved posteriorly, tapering abruptly toward
ofMassetognathus(Jenkins,1970:fig. 1C)and the apex to assume a nearly triangular shape
is therefore identified as an anterior dorsal in lateral view. The prezygapophyses are
(fig. 9A, B). This element is almost complete, directed anterodorsally, extending slightly
although the anteroventral part of the cen- beyond the centrum anteriorly. The articular
trum is damaged. The amphicoelous centrum facet of each prezygapophysis is composed of
is rather long, with a length approaching a nearly vertical wall laterally and a nearly
8 mm, about double the height. The rim horizontal floor medially. Two sectors are
surrounding the notochordal concavity is separated medially by a ridge that is an
swollen, increasing the concave curvature of anterior continuation of the neural spine.
the lateral and ventral edges. The pedicles are The postzygapophyses protrude posteriorly
slightly shorter than the centrum, and bear from the base of the neural spine and extend
very short transverse processes. Each trans- beyond the rim of the centrum. Their ventral
verse process arises from the middle part of edges are nearly horizontal. The distance