Cranial morphology of Pterodaustro guinazui (Pterosauria: Pterodactyloidea) from the Lower Cretaceous of Argentina

19 Pages·2000·15.2 MB·English

by  Luis M Chiappe

Cranial Morphology of Pterodaustro guinazui
(Pterosauria: Pterodactyloidea) from the
Lower Cretaceous of Argentina
Luis M. Chiappe 1 Alexander W. A. Kellner 2
, ,
David Rivarola 3 Sergio Davila 3
, ,
and Marilyn Fox4
ABSTRACT.Withhundredsoffilamentliketeethinitsmandiblesandseveralotheruniquecranialfeatures,
the Argentine Early Cretaceous Pterodaustro guinazui ranks among the most specialized of pterosaurs.
Based on newly collected specimens, this study provides a detailed description ofthe peculiar skull mor-
phology of Pterodaustro and discusses its phylogenetic position within pterosaurs. An overview of the
stratigraphy, sedimentology, and chronology ofthe Lagarcito Formation from which Pterodaustrocomes
is provided, along with an interpretation ofthe paleoenvironment. Cranial morphology corroboratesthe
sister-taxon relationship between Pterodaustro and the LateJurassic Ctenochasmaproposed bymostpre-
vious authors.
RESUMEN. Con cientos de dientes filamentosos en sus mandibulas y varios otros caracteres craneanos
distintivos, Pterodaustro guinazui del Cretacico temprano de Argentina, es uno de los pterosaurios mas
especializados. Sobre la base de ejemplares recientemente colectados, este estudio provee una description
detallada de la peculiar morfologia craneana de Pterodaustro y analiza sus relacionesfilogeneticasdentro
de los pterosaurios.—Tambien se provee una resena estratigra—fica, sedimentologica, y cronologica de la
FormationLagarcito laformationportadoradePterodaustro juntoconunainterpretationdelpaleoam-
biente.LamorfologiacraneanacorroboralarelationdegrupohermanoentrePterodaustroyCtenochasma,
delJurasicotardio, que fuera propuesta por la mayoria de los autores previos.
INTRODUCTION gentina. When discovered, Pterodaustro represent-
ed onlythe secondrecord ofpterosaursfromSouth
tPhaeleloantteol1o9g6i0caslaenxdpeedairtliyon1s9l7e0dsbmyaJ.dFe. Baonsaigpnairftiecainnt America (Price, 1971). Most importantly, Ptero-
daustro provided strong evidence that pterosaurs
contribution to the knowledge of pterosaur evolu-
hadevolvedafilter-feedingmorphologyuncommon
tionwith the discovery ofthe spectacularpterodac-
among tetrapods.
tyloid Pterodaustro guinazui Bonaparte, 1970
(original spelling emended by Wellnhofer [1978] Twenty-five years after Bonaparte’s exploratory
following the guidelines of the ICZN). Collected expeditions to the Mesozoic deposits ofthe central
from the banks of a creek cutting through beds of Argentine province of San Luis, three large-scale
the Lagarcito Formation at a site known today as excavations (in 1994, 1996, and 1998; see Chiappe
Loma del Pterodaustro, in what is now the Parque et al., 1998a, b) were conducted at the quarry in
Nacional Sierra de las Quijadas (Fig. 1), Ptero- the Loma del Pterodaustro where the first speci-
daustro was the first pterosaur to be found in Ar- mens ofPterodaustrowerecollected (Fig. 1). These
excavations produced hundreds ofskeletalremains
of this pterosaur, including adult, juvenile, and ne-
1. Department of Vertebrate Paleontology, Natural
History Museum ofLos Angeles County, 900 Exposition onate specimens (Chiappe et ah, 1998b).
Boulevard, Los Angeles, California 90007, USA. Despite the availability ofmaterial ofPterodaus-
2. CNPq Fellow, Departamento de Geologia e Palen- tro (even before the newlycollectedspecimens) and
tologia, Museu Nacional/Universidade Federal do Rio de the fact that this multitoothed pterosaur has con-
Janeiro, Quinta da Boa Vista, Sao Cristovao, Rio de Ja- sistently been used as a startling example of mor-
neiro, RJ 20.940-040, Brazil. phological specialization within this group (e.g.,
3. DepartamentodeGeologia,UniversidadNacionalde Benton, 1990; Wellnhofer, 1991; Chiappe and
San Luis, Chacabuco y Pedernera, (5700) San Luis, Ar- Chinsamy, 1996), the cranial anatomy of Ptero-
gentina.
4. Department of Vertebrate Paleontology, Peabody daustro has received little attention beyond early
MuseumofNaturalHistory,YaleUniversity,NewHaven, descriptions by Bonaparte (1971) and Sanchez
Connecticut06520, USA. (1973). The recent expeditions to Loma del Ptero-
Contributionsin Science, Number 483, pp. 1-19
Natural HistoryMuseum ofLos Angeles County, 2000
0 10 20 Km
Figure 1 Geographic distributionofthe Lagarcito Formation and location ofthe Loma del Pterodaustro fossil site
daustro collected several skulls and jaws of adult the breakdown ofGondwana (Ramos, 1990). Lith-
individuals. These are described here in detail. We ologically, this basin corresponds to a typical con-
also review previous anatomical data on this taxon tinental sequence of red beds exceeding 1,000 me-
in light of these specimens and discuss the phylo- ters in thickness. These rocks crop out in several
genetic position of Pterodaustro among pterodac- ranges grouped under the name Cordon de Serran-
tyloids. ias Occidentales, which extendin a north-southdi-
Institutions are abbreviated as follows: PVL,Sec- rection over roughly 300 kilometers (Fig. 1).
cionPaleontologia deVertebrados,InstitutoMiguel Stratigraphically, the Lower Cretaceous rocks of
Lillo (San Miguel de Tucuman, Argentina); and San Luis are divided into the Gigante Group and
MHIN-UNSL-GEO, Museo de Plistoria Natural, the LagarcitoFormation (FloresandCriadoRoque,
Universidad Nacional de San Luis (San Luis, Ar- 1972; Fig. 2), which comprise the entire duration
gentina). oftwo cycles ofinfilling ofa continental basin (Ri-
varola, 1994). These cycles represent two deposi-
GEOLOGICAL SETTING OF THE
tional megasequences involving environments that
LAGARCITO FORMATION
range from alluvial fans associated with alluvial
The LowerCretaceous rocks oftheArgentineProv- plains tofluvialplains andlacustrineenvironments.
ince of San Luis form a geotectonic unit known as Rocks of the Gigante Group form most ofthe two
the San Luis Basin (Flores and Criado Roque, megasequences, and they are composed of con-
1972). This basin has been interpreted as a rift ba- glomerates, sandstones, claystones, and evaporites.
sin resulting fromcortical stressesgeneratedduring The Lagarcito Formation forms the top section of
2 Contributionsin Science, Number 483 Chiappe et al.: PterodaustroSkull
[ I
Legend
Claystone
\^r-J Siltstone
iTiMl Sandstone
llvXl Conglomerate
|v vl Basalt
Irregular
1 bedding
Laminated
I I
Massive
F.i Facies1
^
Fluidscape
Ripples
”2- Slumps
k Intraclasts
Bioturbation
Pterodaustro
Pleuropholidae
Semionotidae
•
Conchostraca
Claystones-Basalts—3 Claystones-—1 I ~ Tracefossils
Siltstones J Siltstones
FineSandstones FineSandstones $ Plantremains
MediumSandstones ^
Conglomerate Anura
O
Ostracoda
Figure2 StratigraphicsectionandfossiloccurrencesoftheLomadelPterodaustrofossilsite (fromChiappeetah, 1998b)
the second megasequence, and it is formed by fine stones, and very fine sandstones, which representa
sediments developed under fluvio-lacustrine envi- thickening andcoarseningupward sequence. Facies
ronments associated with limited development of 1-3 have been interpreted as debris flows, sheet-
aeolian dunes. floods deposited in a sand-flat near the shore, and
In the Sierra de Las Quijadas, the Lagarcito For- a typical lake sequence, respectively. With veryfew
mation is best exposed at Quebrada de Hualtaran exceptions, all fossils from Quebrada de Hualtaran
(Fig. 1). In this area, 45 meters of continuous sed- come from Facies 3 (Chiappe etah, 1998a, b). This
iments of sandstones and mudstones, representing facies has been subdivided into three subfacies of
at least three fluvio-lacustrine sequences, rest over low-energy deposits (Fig. 2). One of them (F3.1;
a basaltic flow dated between 107.4 and 109.4 Ma Fig. 2) is composed of laminated, very fine sedi-
(Yrigoyen, 1975). ments entombing the majority of fossils. This su-
Most fossils from the Lagarcito Formation have bfacies was interpreted as forming in the offshore
been excavated at a small site (—50 m2). This site portion of a lake.
has become known as Loma del Pterodaustro
(Chiappe et ah, 1995). The Loma del Pterodaustro BIOTA, PALEOENVIRONMENT,
fossil site corresponds to the basal 8 meters of the AND CHRONOLOGY OF
section ofthe Lagarcito Formation at Quebrada de THE LAGARCITO FORMATION AT
Hualtaran (Fig. 2). A detailed sedimentological LOMA DEL PTERODAUSTRO
study of these deposits has been presented else-
where (Chiappeetah, 1998a).Threelithofaciescan The fine sandstones andclaystones oftheLagarcito
be recognized from base to top: Facies 1, inversely Formation atLoma delPterodaustrohaveprovided
graded, massive sandstones tomassive,matrix-sup- abundant fossil remains (Chiappe et ah, 1995,
ported conglomerates with lenticulargeometryand 1998a, b). These include a diverse array of trace
disordered fabric; Facies 2, fine-grained sandstone fossils, plant remains, conchostracans, ostracods,
with a flat top and base, and asymmetric ripples; and various vertebrates. Amongthe vertebrate fau-
and Facies 3, massive to laminated claystones, silt- na aresemionotidandpleuropholidfishes,anurans,
Contributionsin Science, Number 483 Chiappe et ah: PterodaustroSkull 3
andabundantpterosaurremains,mostofwhichare are broken, thus complicating identification be-
probably of Pterodaustro. Preservation of delicate tween sutures. In MHIN-UNSL-GEO-V-57 and
structures such as the needlelike mandibular teeth MHIN-UNSL-GEO-V-135 the skull is exposed on
of Pterodaustro and the imprints of stems and re- its left side; only a few right bones are visible. In
productive plant structures led to the classification MHIN-UNSL-GEO-V-57 the left mandible is ex-
of these beds as a Konservat Lagerstatte sensu Sei- posed laterally, whereas in MHIN-UNSL-GEO-V-
lacheretal. (1985) (see also Seilacher, 1990;Chiap- 135 the right one is exposed. MHIN-UNSL-GEO-
pe et ah, 1995, 1998a). V-175 exposes its left side.
The facies association ofthe lower section ofthe
Lagarcito Formation, along with the absence ofev- SKULL
siudgegnecsetotfhastubraoercikaslaetxpLoosmuaredaelndPteevraopdoaruistterloevceolrs-, The skull ofPterodaustro is characterized by a re-
markably long, slender, and upwardlycurvedpre-
respond to a fluvio-lacustrine sequence oflong du-
orbital region, whichcomprises more than 85 per-
omiranxatdiliiclocynab.tosettPsrrtatetohsimafeitrtevdtha,hatetailopnanrdkeeoviftewnlmataasemydiantdhaleatesivatoesrntusdcpeetivrineioolsnduoiobpcffeaadtlchliaeyenstlhaa3em.nr1--- caGelEnmOto-soVtf-t25h97e cs(kmFu.ilgl.Tlh3ei)nsghtiahss(aFapipgstr.ootx3ai,lm4ac)tr.aeMnliHyaIl2N0l-eUnpgeNtrShceLno-tf
larger than that of PVL-3860, the mostcommonly
inations and favored preservation ofdelicate struc-
figured specimen of Pterodaustro (Sanchez, 1973;
tures. Bonaparte, 1978; Wellnhofer, 1991). MHIN-
QuTehberaldoawedreseHcutailotnaorfant,heisLaignatrecriptroetFeodrmasataiocno,ma-t UceNnSt)L-laGrEgeOr-tVh-a1n3M5HI(FNig-.UN5)SLis-sGoEmOe-wVh-a5t7(.—T1h0epnear--
plete sequence oftransgression and expansion ofa
perennial lake over an alluvial sandy flat, followed ssomaanltlor(bKeiltlanlerf,en1e9s9t5ra),inretahcishitnagxobnetiwseceonmp1a0raatnidve1l2y
by its gradual infilling and shallowing during a
percent of the total skull length. Because the skull
high-standperiod ofthe lacustrine system. Thepre-
dominant climate during deposition of the Lagar- is flattened, no detailed information regarding the
cito Formation is interpreted as semiarid and sea- temInpolraatleraolpevniienwg,stihseavsanioluatblec.urves gently upward
sonal (Chiappe et ah, 1998a). (Figs. 3-5). A long premaxilla and maxilla, whose
Originally, the Lagarcito Formation was placed
suture is not discernible in any of the available
in the Tertiary (Flores, 1969). After the discovery
specimens, form the snout. As in other pterosaurs
of Pterodaustro guinazui, Bonaparte (1970) placed
(Wellnhofer, 1978), the slender premaxilla forms
this unit within the Upper Jurassic on the basis of
the entire dorsal margin ofthe snout, approaching
similarities betweenthispterosaurandLateJurassic the rostral margin of the orbit. In MHIN-UNSL-
pterodactyloids. Subsequently, with the discovery
of an Aptian-Albian palynoflora in the underlying GEO-V-57 it is uncertain whether the premaxilla
was toothed or not because the mostrostral endof
La Cantera Formation (Gigante Group) and the
Early Cretaceous dates for the basalts at Quebrada tthhee srneomuatiniisnngotknproewsnervsekdu.llUsnfcolarrtiufnyattehliys,inssoune.e oInf
de Hualtaran, Yrigoyen (1975) placed the Lagar- MHIN- UNSL-GEO-V-57 both premaxillae seem
cito Formation within the Upper Cretaceous, and
to be fused to each other for most of their length.
Bonaparte (1978) allocatedthosestratatotheLow-
Sanchez (1973) reported a thin, caudal tongue of
er Cretaceous. Recent interpretations, combining
the maxilla contacting the nasal and excluding the
sedimentological, stratigraphical, and paleontolog-
premaxilla from the nasoantorbital fenestra (Figs.
ical data, have adjusted the chronology ofthis lith-
ostratigraphic unit, supporting an Albian age (see 6A, 7). This condition, which is differentfromthat
Chiappe et ah [1998a] for a more extensive discus- of most pterosaurs (e.g., Wellnhofer, 1978; Well-
nhofer and Kellner, 1991), has not been corrobo-
sion of the age of these beds).
rated by either ofthe new specimens, in which the
CRANIAL ANATOMY OF premaxilla forms the dorsal margin of the nasoan-
PTERODAUSTRO GUINAZUI torbital fenestra (Figs. 6B, 8, 9). Alsodifferingfrom
previous interpretations (Sanchez, 1973), the pre-
This studyismostlybasedonrecentlycollectedma- maxilla isextendedfarthercaudallyovertherostral
terial, including a skull and jaw (MHIN-UNSL- half of the orbit (cf. Figs. 6A, B, 8).
GEO-V-57), another skull and jaw missing their The maxilla forms most of the lateral surface of
rostral halves (MHIN-UNSL-GEO-V-135), and an the snout (Figs. 3-5, 7). This bone tapers rostrally
isolated, nearly complete jaw (MHIN-UNSL-GEO- from the rostral margin of the nasoantorbital fe-
V-175). As in other known skulls of Pterodaustro, nestra. In all available specimens the suture be-
the new crania and jaws are flattened, preserved tween this bone and the rostral end of the jugal is
essentially in two dimensions. Many of the bones not clear. In fact, these two bones appear to be
Figure3 SkullandjawsofPterodaustroguinazui(MHIN-UNSL-GEO-V-57) inleftlateralview.Scalebar = 5centimeters
4 Contributionsin Science, Number483 Chiappe et al.: PterodaustroSkull
Contributionsin Science, Number 483 Chiappe et al.: PterodaustroSkull 5
ring
sclerotic
SR,
squamosal;
SQ,
supraorbital;
SO,
6 Contributionsin Science, Number483 Chiappe et al.: PterodaustroSkull
Figure 5 Skull and jaws of Pterodaustro guinazui (MHIN-UNSL-GEO-V-135). Skull in left lateral view; mandible in
right lateral view
fused to each other. However, a thin tongue ofthe right nasal and jugal in MHIN-UNSL-GEO-V-57
maxilla projects caudally beneath the jugal, reach- (Fig. 6B) is not natural and can be regarded as a
ingthecaudalmarginofthenasoantorbitalfenestra result of deformation during crushing. The mor-
(Figs. 6B, 8). The maxilla bears hundreds of tiny phology of the ventral nasal process is similar to
teeth, which in MHIN-UNSL-GEO-V-57 extend that of archaeopterodactyloids sensu Kellner,
(
from the rostral end ofthe maxilla to near the ros- 1996, 1997) such as Pterodactylus Cuvier, 1809,
tral margin of the nasoantorbital fenestra (Fig. 4). and Germanodactylus Young, 1964. All more de-
Thus, the caudal extension of the maxillary tooth rived pterodactyloids, which are members of the
row ofthis specimen extends much more thanthat Dsungaripteroidea (Young, 1964; Kellner, 1996),
illustrated by Sanchez (1973) in her skull recon- have either displaced this nasal process medially
struction (Fig. 7). (e.g., Anhanguera Campos and Kellner, 1985) or
The dorsocaudal corner ofthe nasoantorbitalfe- have almost lost it (e.g., Quetzalcoatlus Lawson,
nestra is formed by a subtriangular nasal (Figs. 6- 1975) (see Kellner and Langston, 1996).
9). This elementhas a comparativelylong,thinros- The lacrimal of Pterodaustro is wedged between
tral process that reaches the center of the nasoan- the nasal,prefrontal, andjugal. This bonehasthree
torbital fenestra, underlying the premaxilla (Figs. distinctprocesses: a short rostral one, a longerand
6B, 9). Ventrally, the nasal tapers into a thin pro- wider caudal one, and a hook-shaped ventral one
cess that fails to reach the jugal, although it ends (Figs. 6B, 8, 9). Although now flattened, in life the
very close to it (Sanchez, 1973). The contact ofthe ventralprocessprobablyextendedslightlylateralto
Contributionsin Science, Number483 Chiappe et al.: PterodaustroSkull 7
QJ?
Figure 6 Camera lucida drawings of the postorbital region of Pterodaustro guinazui. A, from Sanchez (1973) (PVL-
3860). B, MHIN-UNSL-GEO-V-57. Abbreviations: ITF, infratemporal fenestra; OR, orbit; PF, prefrontal; QJ, quadra-
tojugal; SA, surangular; STF, supratemporal fenestra. Other abbreviations as in Figure 4
the orbit, as seen in some other pterosaurs known is, infact, the dorsalexposureofthecaudalprocess
fromless distortedspecimens (e.g.,Anbanguera).In of the lacrimal (Fig. 9).
MHIN-UNSL-GEO-V-57 (Fig. 6B), this process is Rostrally, both frontals are united by a suture
significantly more robust than the process illustrat- that becomes indistinguishable in the caudal half.
ed by Sanchez (1973; Fig. 6A), which agrees more Caudally, thefrontalsarewedgedbetweenthepost-
with that of MHIN-UNSL-GEO-V-135 (Fig. 9). frontals and parietals (Figs. 6, 8).
MHIN-UNSL-GEO-V-135 clearly shows that the Theparietals in bothspecimens areverycrushed,
bone identified as a prefrontal by Sanchez (1973) making their edges difficult to interpret,particular-
8 Contributionsin Science,Number 483 Chiappe et al.: PterodaustroSkull
Figure 7 Sanchez’s (1973) cranial reconstructionofPterodaustroguinazui. Abbreviations as in Figures 4 and 6
ly in their caudal region. As far as they can be ob- Pterodaustro from other pterosaurs (Wellnhofer,
served, both elements are fused to each otheralong 1978; Wellnhofer and Kellner, 1991).
the midline. Although the boundaries of this bone The jugal is a conspicuous bone that forms the
were not clearly illustrated by Sanchez (1973; Fig. ventrocaudal and ventral margins of the nasoan-
7), the new specimens indicate that the parietal torbital fenestra and the orbit, respectively, andthe
forms the medial margin of the supratemporal fe- rostroventral corner of the infratemporal fenestra
nestra (Fig. 6B), as in most archosaurs (Romer, (Fig. 6A). The jugal has four distinct processes
1956). Interestingly, the caudal region of the pari- (Figs. 8, 9). The rostral process forms the ventral
etals ofall available specimens, includingthosepre- margin of the nasoantorbital fenestra and, in
viouslydescribed byBonaparte (1971) andSanchez MHIN-UNSL-GEO-V-135, projects rostrally be-
mm
(1973), is slightly extended as a thin bony lamina yond the nasoantorbital fenestra for about 9
(Figs. 6, 8, 9). Although some of this appearance (Fig. 9). This extension is shorter than interpreted
results from crushing, it cannot be ruled out that by Bonaparte (1971). The dorsal process contacts
the parietals projected caudally as a small crest. the lacrimal, forming a bony bar that separates the
Even if this interpretation is correct, this parietal orbitfromthe nasoantorbitalfenestra.Thisprocess
crest would have been very small and not as devel- is robust and tapers to a sharp point (Fig. 9). Cau-
oped as in some other pterosaurs (e.g., Gallodac- dally, the jugal forks into dorsocaudal and ventro-
tylus Fabre, 1974). caudal processes. The former is overlain by the
Caudal to the lacrimal, there is a flattened bone, postorbital and takes part in the rostral margin of
here interpretedastheprefrontal. BasedonMFilN- the infratemporal fenestra. The second partially
UNSL-GEO-V-57, this bone would correspond to overlies the quadrate (displaced in MF3IN-UNSL-
the rostral portion of what Sanchez (1973) inter- GEO-V-57).
preted as the supraorbital (Fig. 6A). The slender The tetraradiate jugal ofall knownPterodaustro
supraorbital does indeed line the dorsal margin of specimens (Figs. 6, 9) differs markedlyfromthetri-
the orbit and precludes the frontal from the orbital radiate shape ofthe jugal ofother pterodactyloids,
margin (Sanchez, 1973), but it does not appear to including Pterodactylus and Ctenocbasma Meyer,
extend rostrally to reach the lacrimal (Fig. 6B). 1851, resemblingmorethemorphologyofthejugal
In MHIN-UNSL-GEO-V-57, the postfrontal is a in some basal pterosaurs (Wellnhofer, 1991). The
suboval element that contacts the frontal medially differenceappearstobemainlyontheventrocaudal
and the parietal caudally and is partially overlain portion, where the jugal of Pterodaustro bears an
by the postorbital (Fig. 6B). A caudally oriented extended process overlying the quadrate (Figs. 6B,
process ofthe postfrontal, as illustrated bySanchez 8). Thiscontactbetweenthe quadrate andthejugal
(1973), was not observed in the new specimens. is known only for this taxon. Furthermore, Ptero-
The postorbital is a triradiate element (Fig. 6B). daustro also differs from some derived pterodac-
Its dorsal process is slightly expanded and abuts tyloids (e.g., Anhanguera, Pteranodon Marsh,
against the parieto-postfrontal suture. The ventral 1876) in that its jugal lacks a pronounced caudo-
process overlays the dorsocaudal process ofthe ju- dorsal ridge near the nasoantorbital fenestra.
gal, whereas the caudal process overlies the lateral The quadratojugal is very difficult to discern in
surface ofthe squamosal. The unusualconditionof most specimens. In MFiIN-UNSL-GEO-V-57, a
having the postorbital completely separated from thinandincompletebone,displacedventrallytothe
the frontal by the postfrontal (Figs. 6B, 8), also in- quadrate, is tentatively identified as the left quad-
dicated by Sanchez (1973; Fig. 6A), distinguishes ratojugal (Fig. 6B). According to Sanchez (1973),
Contributionsin Science, Number483 Chiappe et al.: PterodaustroSkull 9
Figure 8 Nasoantorbital, orbital, and postorbital regions of Pterodaustro guinazui (MHIN-UNSL-GEO-V-57), in left
lateral view. A, general view. B, detail ofthe jugal and surrounding bones
this element forms all of the ventral margin of the proximal end ofthe quadrate appears to articulate
infratemporal opening. However, such a configu- with the squamosal.
ration has not been observed in any other ptero- The squamosalhas anexpanded,ventrolaterally
dactyloid (Wellnhofer, 1978). rounded body (Figs. 6B, 8, 9). As shown by San-
The quadrate is not well preserved in any ofthe chez (1973), this bone forms the entire ventral
new specimens. This bone is rostroventrally tilted margin of the supratemporal fenestra (Fig. 6), re-
(more than 150° relative to the ventral margin of stricting the postorbital to the rostral margin of
the skull; Fig. 6), a feature common to all Ar- this arcade. The squamosal has a thin, tapering
chaeopterodactyloidea (Kellner, 1996, 1997), as caudomedial process thatabuts the braincase (Fig.
well as the Azhdarchidae (Unwin and Junchang, 6A). Rostroventrally, the squamosal bears a sharp,
1997). Rostrally, the main body of the quadrate thin process that runs through the proximalhalfof
projects mediad as a broad pterygoid flange. The the caudal margin of the quadrate.
10 Contributionsin Science, Number483 Chiappe et al.: PterodaustroSkull