Earliest Eutherian Ear Region: A Petrosal Referred to Prokennalestes from the Early Cretaceous of Mongolia

44 Pages·2001·0.4 MB·English

by  John R. Wible

PUBLISHED BY THE AMERICAN MUSEUM OF NATURAL HISTORY
CENTRAL PARK WEST AT 79TH STREET, NEW YORK, NY 10024
Number3322,44pp.,5figures February 27,2001
Earliest Eutherian Ear Region: A Petrosal Referred
to Prokennalestes from the Early Cretaceous of
Mongolia
JOHN R. WIBLE,1 GUILLERMO W. ROUGIER,2 MICHAEL J. NOVACEK,3
AND MALCOLM C. McKENNA3
ABSTRACT
A right petrosal from the ?Aptian or Albian Khoobur locality is referred on the basis of
sizeandmorphologytoProkennalestestrofimovi,theearliesteutherianpreviouslyknownonly
from dentigerous elements. The petrosal shows a mosaic of primitive and derived features,
bearing on the purported therian and eutherian morphotypes. Among the primitive features
shared with the Early Cretaceous prototribosphenidan Vincelestes and other more basal taxa
thataremodifiedinlatereutheriansandmetatheriansarethepatternofbasicranialarterialand
venous circulation, including a prootic canal and an intrapetrosal inferior petrosal sinus; a
verticalparoccipitalprocess;andafenestrasemilunaris,anincompletewallbetweenthecavum
epiptericum and cavum supracochleare. Among the derived features shared with therians is a
cochlea coiled through a minimum of 360(cid:56), with Prokennalestes extending the range of the
oldest occurrence of such a coiled cochlea by at least 10 million years. Shared with Late
Cretaceous eutherians is a shallow internal acoustic meatus with a thin prefacial commissure.
The petrosal referred to Prokennalestes is intermediate in having a reduced anterior lamina
and lateral flange, both of which are well developed in Vincelestes and essentially lacking in
later eutherians and metatherians. Features previously held to be part of the therian and eu-
therian morphotypes, such as the absence of the anterior lamina and lateral flange, may have
been lost independently in metatherians and in post-Prokennalestes eutherians.
1Research Associate, Division of Vertebrate Zoology, American Museum of Natural History; Associate Curator,
Sectionof Mammals,CarnegieMuseumofNaturalHistory,5800 BaumBoulevard,PittsburghPA15206.
2Research Associate, Division of Paleontology, American Museum of Natural History; AssistantProfessor,Depart-
mentofAnatomicalSciencesandNeurobiology,SchoolofMedicine,UniversityofLouisville,LouisvilleKY40292.
3Curator,Divisionof Paleontology,AmericanMuseumofNaturalHistory.
2 AMERICAN MUSEUM NOVITATES NO. 3322
INTRODUCTION (Trofimov, 1978), symmetrodonts (Trofimov,
1980, 1997), cladotherians (Dashzeveg,
Remains of eutherian mammals from the
1979, 1994), and tribosphenidans (Dashze-
Early Cretaceous are exceedingly rare. The
veg,1975;DashzevegandKielan-Jaworows-
only undoubted eutherians from this interval
ka, 1984).
are Prokennalestes from the ?Aptian or Al-
In 1995, Wible et al. described a well-pre-
bian of Mongolia (Kielan-Jaworowska and
served petrosal bone (PSS-MAE 104) from
Dashzeveg, 1989; Sigogneau-Russell et al.,
Khoobur yielded through screen washing by
1992) and BobolestesfromthelateAlbianof
the MAE. They concluded (Wible et al.,
Uzbekistan (Nessov, 1985; Nessov and Kie-
1995: 10), based on its size and the results
lan-Jaworowska, 1991; Nessov et al., 1994).
of their cladistic analysis, that this specimen
The former is known from nearly complete
belonged ‘‘to either an as yet unknown tri-
lower dentitions and fragmentary uppers
conodontortoaprimitiveholotherian,which
(Kielan-Jaworowska and Dashzeveg, 1989;
in the context of the known Khoobur fauna
Sigogneau-Russell et al., 1992), whereas the
would be the symmetrodont Gobiodon infin-
latter is known from a maxillary fragment
itus . ... Holotheria includes the common
with two molars (Nessov, 1985). Other pur-
ancestor of Kuehneotherium and therians
ported Early Cretaceous eutherians known
plus all its descendants.’’ In a subsequent
from incomplete lower jaws include Ausktri-
contribution, Rougier et al. (1996a) reported
bosphenos from the Aptian of Australia
a second petrosal (PSS-MAE 129), which
(Rich et al., 1997, 1998, 1999) and Endo-
was identified as the sister group of PSS-
therium from not later than the Aptian of
MAE 104 in their cladistic analysis. How-
northeastChina(Shikama,1947;Wangetal., ever, the triconodont versus holotherian re-
1995). The eutherian designation of Ausktri- lationships of both specimens were left un-
bosphenos has been contested by Kielan-Ja- resolved. More recently, another well-pre-
worowska et al. (1998) and Rougier and No- served, smaller petrosal has been found in
vacek (1998), and the original specimen of the MAE screen wash collection from
Endotherium has been lost. Other possible Khoobur. A preliminary announcement of
Early CretaceouseutheriansincludeMontan- this specimen (PSS-MAE 136) was made in
alestes from the Aptian-Albian of Montana Wible et al. (1997). These authors identified
(Cifelli, 1999) and, according to Kielan-Ja- thisasthepetrosalof?Prokennalestes.Afull
worowska (1992), Tribotherium known from description of this specimen, the earliest
four isolated, incomplete upper molars from knowneutherianearregion,isprovidedhere.
the ?Berriasian of Morocco (Sigogneau-Rus-
sell, 1991, 1995).
METHODS
Khoobur (also variously spelled Khobur,
Khoboor,andKhovboor),GuchinUsSomon, For the descriptions of PSS-MAE 136, we
Mongolia is the ?Aptian or Albian locality employ the anatomical terminology that we
yielding Prokennalestes(Kielan-Jaworowska have used in reports on basicrania of other
and Dashzeveg, 1989). Discovered by the Mesozoic mammals (e.g., Wible, 1990;
Joint Soviet-Mongolian Paleontological Ex- Rougier et al., 1992, 1996a; Wible et al.,
peditions (Beliajeva et al., 1974), additional 1995).Inadditiontodescribingtheoutersur-
collecting atKhooburhasbeenconductedby faces of the petrosal, we report some of the
the Geological Institute of the Mongolian internal features of the PSS-MAE 136 as re-
Academy of Sciences, and in the summer of vealed through radiographic analysis. The
1991, 1997, and 1999, by the Mongolian specimen was digitally imaged at the Uni-
Academy of Sciences–American Museum of versity of Louisville School of Dentistry us-
Natural History Expeditions (MAE). In ad- ing a small dental intraoral charged-coupled
dition to Prokennalestes, a diverse mamma- device (RadioVisioGraphy Model PCi sen-
lian fauna has been reported from fragmen- sor; Trophy Radiography, Vincennes,
tary mandibular remains, including multitu- France) and a dental X-ray generator oper-
berculates (Trofimov, 1980; Kielan-Jawo- ating at 70 kVp, 7mA at an exposure timeof
rowska et al., 1987), gobiconodontids approximately 0.1 seconds. Multiple projec-
2001 WIBLE ET AL.: PETROSAL REFERRED TO PROKENNALESTES 3
tions of the specimen were taken and resul- oventral pars cochlearis, enclosing the co-
tant images enhanced in Adobe Photoshop. chlea, and the more posterodorsal pars can-
We also reconstruct the major vessels and alicularis, enclosing the vestibule and the
nervesassociatedwiththepetrosalbone.Our semicircular canals. We describe the petrosal
research on the anatomy of recent mammals of Prokennalestes here in three views—ven-
(e.g., Novacek, 1986, 1993; Wible, 1986, tral, dorsal, and lateral—with the orientation
1987, 1990; Rougier et al., 1992; Wible and based on the presumed position in the skull.
Hopson, 1995) serves as background for this However, given that the specimen is isolated
vascular and nervous reconstruction.Recent- and incomplete, its precise orientation in the
ly, several authors (e.g., Bryant and Russell, skull is subject to interpretation. We have
1992; Witmer, 1995) have offered explicit provided an estimate in figure 1, but must
methods for reconstructing soft tissues in admit that the angulation to the midline may
fossils and for evaluating levels of confi- be considerably different depending on the
dence in those inferences. In formulatinghy- proportions of the surrounding cranial ele-
potheses about soft-tissue reconstruction ments. Following our descriptions, the mor-
here, we accept that PSS-MAE 136 is attrib- phology of the osseous inner ear as revealed
utable to Prokennalestes and, following the through radiographic analysis and a recon-
recentphylogeneticanalysisbyRougieretal. struction of the courses of the major vessels
(1998),identifyProkennalestesasabasaleu- andnervessuggestedbygrooves,canals,and
therian (fig. 5). Consequently, under the ter- foramina on PSS-MAE 136 are presented.
minology proposed by Witmer (1995), the PSS-MAE 136 is a right petrosal (fig. 1).
extant phylogenetic bracket (minimally, the The pars cochlearis is largely intact,withthe
first two extant outgroups) for Prokennales- only substantive damage being to the antero-
tes consists of placentals and marsupials. In- medial surface, which has exposed spongy
ferences that are based on soft-tissue struc- bone within thepetrosal.Incontrast,thepars
tures and osteological correlates occurringin canalicularis has suffered considerable dam-
both extant outgroups are considered more age such that roughly the lateral,posterodor-
decisive than those occurring in only one. sal half of it is missing. Of the three semi-
circular canals, the lateral one is almost
INSTITUTIONAL ABBREVIATIONS whollyenclosedinbone,asisthebulkofthe
posterior one. However, little remains of the
AMNH Department of Vertebrate Pale-
anterior semicircular canal or the bone be-
ontology, American Museum of NaturalHis-
tween it and the other two canals.
tory, New York
MACN Museo Argentino de Ciencias
Naturales ‘‘Bernardino Rivadavia’’, Buenos VENTRAL VIEW
Aires (fig.1A,D)
MAE Collections of joint Mongolian
In ventral view, the pars cochlearis is rep-
Academy of Sciences–American Museum of
resentedchieflybytheovalpromontoriumof
Natural History Paleontological Expeditions
the petrosal. The shape and topography of
PSS Paleontological and Stratigraphic
the promontorium closely reflect the en-
Section of the Geological Institute, Mongo-
closed cochlear duct (fig. 2), which does not
lian Academy of Science, Ulaan Baatar
coil in a single plane but in a ventrally di-
UCMP University of California, Muse-
rected spiral. Consequently, the ventralmost
um of Paleontology, Berkeley
bulge of the promontorium,whichissituated
posteromedially, underlies the end of the
coil. Extending from the anterior and medial
DESCRIPTIONS
border of the promontorium is anarrow,flat-
In Recent mammals, the petrosal houses tened shelf. The full extent of the anterior
the organs of hearing and equilibration. For partofthisshelfisuncertainbecauseofdam-
descriptivepurposes,thetherianpetrosalhis- age, but the medial partis intact.Weidentify
torically(e.g.,Voit,1909;Fawcett,1918)has this shelf as an epitympanic wing following
been divided into two parts: the more anter- MacPhee(1981),whousedthattermforout-
4 AMERICAN MUSEUM NOVITATES NO. 3322
Fig. 1. Three views of right petrosal referred to Prokennalestes trofimovi, PSS-MAE 136. A, D,
Ventral view. B, E, Dorsal view. C, F, Lateral view. Given that the petrosal is both isolated and
incomplete,providingthepreciseorientationfortheboneinthecompleteskullisdifficult.Oneestimate
is shown here. Scale (cid:53) 1 mm.
growths from any basicranial bones contrib- nestra vestibuli is recessed slightly from the
uting to the tympanic roof. surface of the promontorium; this is most
Twolargeaperturesopenintotheposterior pronounced along the lateral border. As a re-
aspect of the promontorium. The anterolat- sultofbreakage,theposteromedialsectionof
eral and larger of the two is the fenestraves- the rim of the fenestra vestibuli is missing.
tibuli or oval window, which in life accom- Despite the damage, the shape and orienta-
modated the footplate of the stapes. The fe- tion of the fenestra can be reasonably recon-
2001 WIBLE ET AL.: PETROSAL REFERRED TO PROKENNALESTES 5
Fig. 1. Continued.
6 AMERICAN MUSEUM NOVITATES NO. 3322
structed. As a measure of shape, and assum- Although the endpoint of this canal cannot
ing that the outline of the fenestra vestibuli be confirmed, we believe that its terminus is
reflects the stapedial footplate morphology, in the spongy bone exposed via damage at
Segall’s (1970) stapedial ratio (length/width the anteriormost surface of the pars coch-
of the oval window or footplate) is approxi- learis (visible in dorsal and lateral view, fig.
mately 1.7, that is, somewhat elliptical. Re- 1B, C). As discussed in the vascular recon-
garding orientation, the opening is directed struction, and following similar examples in
ventrolaterally and slightly posteriorly. The other Mesozoic mammals, we interpret this
other large aperture is into the posterior as- canal as for the inferior petrosal sinus.
pect of the promontorium. This is the fenes- Two moderately developed vascular sulci
tra cochleae or round window, which was run nearly the length of the ventral surface
closed by thesecondarytympanicmembrane of the pars cochlearis. The more medial one
in life. We identify this aperture as a round starts posteriorly at the medial rim of the fe-
window and not a perilymphatic foramen as nestra vestibuli and extends anteromedially
in monotremes (Kuhn, 1971; Zeller, 1985, to the end of the epitympanic wing. This is
1989, 1991), because PSS-MAE 136 has a the transpromontorial sulcus for the internal
separate canal for the perilymphatic duct as carotid artery (Wible, 1986). The sulcus
inRecenttherians(‘‘cochlearcanaliculus’’in notches the medial rim of the fenestra ves-
fig. 1E). The bone flooring this canal in ex- tibuli, with the occupant of this notch inter-
tant therians is derived from the processus preted to be the stapedial artery (Wible,
recessus of the chondrocranium (De Beer, 1987; Rougier et al., 1992). The second sul-
1937; Zeller, 1985). In PSS-MAE 136, the cus runs along the lateral edge of the pro-
processus recessus is the narrow bridge of montorium, beginning just in front of the fa-
bone between the medial border of the fe- cial foramen and extending onto the epitym-
nestra cochleae and the jugular notch. The panic wing. The major occupant of this
fenestra cochleae is directed posteriorly and groove is interpreted to be the ramusinferior
somewhat ventrally, and is subcircular, of the stapedial artery (Wible,1987;Rougier
slightly wider in the horizontal plane than et al., 1992). About halfway along thelength
vertically. Separating the round and oval of the sulcusfortheramusinferior,asecond,
windows is a narrow, near vertical bar of much smaller groove flows into it from the
bone, the crista interfenestralis. endocranial surface. This would have trans-
There are two smaller openings into the mitted thegreaterpetrosalnerve(palatinera-
outer contour of the ventral surface of the mus) of the facial nerve. Continuous poste-
pars cochlearis. Anterior to the fenestra ves- riorly with the well-developed sulcus for the
tibuli is a posteriorly directed, oval aperture, ramus inferior and anterior to the facial fo-
the facial foramen, which transmitted the ramen, there is what might be a very faint
main or hyomandibular branch of the facial sulcus (see Vascular and Nervous Recon-
nerve into the middle-ear space. Running struction).
posteriorly from the facial foramen is a very The pars canalicularis in ventral view is
short, shallow facial groove that ends just in roughly L-shaped, with the short arm poste-
front of theovalwindow.Theotheropening, rior to the promontorium and the long arm
barely visible in ventral view,liesatthepos- lateral to the posterior half of the promon-
teromedial corner of the pars cochlearis, just torium. The outer contour of both arms of
in front of the jugular notch, the petrosal’s the L is raised as crests and eminences,
contribution to the border of the jugular fo- whereas the inner contour is depressed as
ramen. As preserved, there are actually two troughs and fossae. For descriptivepurposes,
openings in this area: a smaller, circularpos- we treat the short and long arms separately.
terior one and a larger, oval anterior one.We Thecrestontheposterioredgeoftheshort
believe that the latter is a product of damage arm, the caudal tympanic process (MacPhee,
and only the smaller, circular foramen was 1981), forms the back wall of the middle-ear
present during life. Visible through both space.Itdoesnotextendmediallyalltheway
openings is spongy bone laterally and a nar- to the jugular notch, but fades out posterior
rowcanalintheepitympanicwinganteriorly. to the middle of the fenestra cochleae. The
2001 WIBLE ET AL.: PETROSAL REFERRED TO PROKENNALESTES 7
small section immediately behind the fenes- malleus and incus. Forming the medial wall
tra cochleae is the only undamaged part of of the fossa incudis is the crista parotica,
the caudal tympanic process. It is lower than continuous with the paroccipital process be-
the broken lateralmost part of the caudal hind. Only the broken base of this crest is
tympanicprocess,butslightlyhigherthanthe preserved, and it also contained a pneumatic
intervening middle part. The broken base of space, likely continuous with that in the par-
the caudal tympanic process, which con- occipital process. In extant mammals, the
tained a pneumatic space, extends laterally crista parotica provides the point of attach-
from behind the round window and connects ment to the embryonic auditory capsule of
with the raised outer edge of the long arm of Reichert’scartilage(thesecondorhyoidarch
the L. Between the caudal tympanic process cartilage), the proximate segment of which
and the fenestra cochleae is a flattened shelf, often ossifiestoformatympanohyalelement
which forms the posterolateral border of the in the adult (De Beer, 1937). Although not
jugular notch. Lateral to this shelf is a de- preserved in PSS-MAE 136, the point on the
pression containing a broad, flat, oval fossa, crista parotica where the tympanohyal
which is roughly twice the area of the fenes- (whether cartilaginous or osseous) would
tra vestibuli. This fossa housed the origin of have attached is indicated by slight notching
the stapedius muscle. The bone roofing the in the medial wall of the crest. This stylo-
stapedius fossa is thinner than that surround- mastoid notch transmitted the facial nerve
ing it and post mortem damage has opened from the middle-ear space. Thetympanohyal
a hole in the fossa that connects to the en- wouldhaveattachedtothecristaparoticaim-
docranial surface of the petrosal. It is appar- mediately in front of the notch. Forming the
entfromstudyoftheendocranialsurfacethat lateral wall of the fossa incudis and epitym-
the rim around the stapedius fossa is formed panic recess is a thickened, rounded ridge,
by the bone containing the lateral semicir- continuous with the paroccipital process be-
cular canal. Consequently, the caudal tym- hind. Judged by the surface texture, this
panicprocess,whichformstheposteriorwall ridge also likely contained a pneumatic
of the stapedius fossa, lies directly ventralto space. Moreover, it is apparent that the bulk
the lateral semicircular canal. of this ridge was covered in life by another
The morphology of the lateral edge of the bone, presumably the squamosal, which
long arm of the L is more complex than that therefore was the major element in the wall
of the short arm. Posteriorly is the broken lateral to the mallear–incudal articulation. In
base of a broad eminence, which contained front of the epitympanic recess, this ridge
apneumaticspacecontinuouswiththatinthe narrowsandcontinuestotheanteriorlimitof
adjacent caudal tympanic process. This em- the pars canalicularis. As described below,
inence is equivalent to what is called the lat- the narrower anterior section of this ridge
eral section of the caudal tympanic process floors several prominent vascular foramina.
of the petrosal in various placentals (Mac- Because of its continuity with the paroccip-
Phee, 1981) or the paroccipital process in ital process and crista parotica, we identify
more basaltaxa,suchastheprototribosphen- this ridge as the lateral flangeofthepetrosal,
idan Vincelestes from the Early Cretaceous as occurs in more basal taxa (Rougier et al.,
of Argentina (Rougier et al., 1992). We em- 1992; Wible and Hopson, 1993).
ploy the latter term here. Although the ven- In the interval between the promontorium
tralextentoftheparoccipitalprocessinPSS- on the one side and the paroccipital process,
MAE136isuncertain,itapparentlywaswell cristaparotica,andlateralflangeontheother,
developed and vertical in orientation. Ante- the pars canalicularis is marked by a broad,
rior to the paroccipital process is a triangular smooth-walled, longitudinal trough. This
depression, whose apex points posteriorly. trough ends at a subcircular aperture at the
The deepest part of this depression is at the anterior limit of the pars canalicularis. This
apex and in life housed the crus breve of the aperture is directed anterosuperiorly and
incus. Anterior to the fossa incudis is the slightly laterally, and leads into a short, near
shallower and broader epitympanic recess, vertical prootic canal opening on the endo-
which housed the articulation between the cranial surface of the petrosal. Lateral to the
8 AMERICAN MUSEUM NOVITATES NO. 3322
prootic canal, above the lateral flange, is a and medial to the internal acoustic meatus is
figure-eight-shaped aperture, which we inter- broad and flat. The surface posterior to the
pret as having accommodated two arteries. meatus slopes posterodorsally into the pars
The smaller posterior opening of the figure canalicularis (see below). The lateral wall of
eight was for the stapedial artery; the larger the meatus is formed by a thin bar of bone,
anterior aperture was for the ramus inferior. the prefacial commissure. The aspect of the
The figure-eight-shaped aperture, which is pars cochlearis lateral to the prefacial com-
directed laterally, leads into a canal that missure slopes steeply ventrally and is more
bends superiorly to open on the lateral sur- fully visible in lateral view (fig. 1C). This
face of the petrosal. This canal is equivalent smooth-walled surface formed the postero-
inpositionandgeneralorientationtotheven- medial wall of the cavum epiptericum, the
tral ascending canal in more basal taxa,such extradural space housing the trigeminal gan-
as Vincelestes (Rougier et al., 1992). glion and other nervous and vascular struc-
tures (Gaupp, 1902, 1905; Kuhn and Zeller,
DORSAL VIEW 1987). The only other feature on the endo-
cranial surface of the pars cochlearis is just
(fig.1B, E)
lateral to the jugular notch, where there is a
As in the case of the ventral view, the depression with two dorsomedially directed
shape of the pars cochlearis in dorsal view foramina. The larger ventral one transmitted
closely reflects that of the enclosed cochlear theperilymphaticduct.Thisforamenisoften
duct (fig. 2). The most prominent feature is called the cochlear aqueduct, but following
theinternalacousticmeatusforthefacialand the Nomina Anatomica Veterinaria (1994,
vestibulocochlear nerves, which is an ovoid 4th ed.) we refer to it as the cochlear cana-
openingoffsetlaterallyfromthecenterofthe liculus. The smaller dorsal foramen likely
pars cochlearis. Within the meatus are two transmitted a vein accompanying the peri-
unequal-sized, oval apertures separated by a lymphatic duct.
low transverse septum. The smaller, lateral Less than half of the endocranial surface
aperture, the foramen acusticum superius, is of the pars canalicularis is preserved; it rises
directed ventrolaterally and ends in two sub- steeply posterodorsally from the pars coch-
equal-sized, circular structures. The larger learis. The area just behind the internal
anterior one is a canal transmitting the facial acoustic meatus housed the vestibule of the
nerve. The smaller posterior one is a blind inner ear, and projecting from that were the
pit with tiny perforations in it. This is the three semicircular canals (fig. 2). The most
cribriform dorsal vestibular area for the pas- prominent feature on the pars canalicularisis
sage of bundles of the vestibular nerve. The a deep depression in the preserved posterior
larger, medial aperture in the internal acous- edge.Whenclosedbythecompleteparscan-
tic meatus, the foramen acusticum inferius, alicularis, this depression would have been
is directed ventrally into a pit, whose ante- the anterior part of a very wide, deep subar-
rior, medial, and posterior walls have three cuate fossa, which housed the paraflocculus
irregular apertures into the inner ear, which of the cerebellum. The loss of the posterior
we believe are the result of damage. The edgeofthesubarcuatefossahasmadevisible
spicules of bone between these three open- parts of the bony housing for all three semi-
ings have a rough, pitted surface resembling circular canals.
that in the cribriform dorsal vestibular area. The only complete canal preserved is the
The remaining surfaces in the foramen acus- lateral (horizontal) semicircular canal,which
ticum inferius are smooth. We interpret this lies in the floor of the subarcuate fossa. The
rough surface as evidence of another cribri- bone between the lateral semicircular canal
form area, in this case the spiral cribriform and vestibule is very thin and is perforated
tract (tractus spiralis foraminosus), tiny per- by a jagged, irregular opening, the artifact
forations in aspiral beltthattransmitthefas- within the stapedius fossa described above.
cicles of the cochlear nerve in other therians A bulge between the lateral terminus of the
(Meng and Fox, 1995a, 1995b). lateral semicircular canal and the vestibule
The surface of the pars cochlearis anterior reflects the underlying lateral ampulla. The
2001 WIBLE ET AL.: PETROSAL REFERRED TO PROKENNALESTES 9
posterior (inferior) semicircular canal lies in anterior pole with spongy bone exposed by
the medial wall of the subarcuate fossa, and post mortem damage. Posterior to that is the
that for the anterior (superior) would have smooth, laterally facing surface that contrib-
formed most of the rim of the now incom- uted to the posteromedial part of the cavum
plete opening into the subarcuate fossa.Ven- epiptericum described above. The ventral
trally the posterior canal connects with the edge of this smooth surface bears a narrow,
lateral one just distal to a bulge representing ventrolaterally directed ridge, except where
the posterior ampulla. Dorsally the posterior isitnotchedbyanarrowgroove(seebelow).
canal is broken open, exposing a groove and The edge of this ridge is flattened,servingas
two openings. It is in this broken area, inthe a facet for contact with another bone, pre-
medial rim of the subarcuate fossa, that the sumably the alisphenoid. Just above the pos-
posterior and anterior canals join to form the terior end of this ridge is a subcircular de-
crus commune. From there the crus com- pression, within which is an oval foramen.
mune continues forward in the anteromedial The foramen opens into a small spacewithin
rim of the subarcuate fossa to connect with the petrosal that has two other points of
the vestibule. Along the ventral surface of
egress: the facial foramen in the middle ear
the bone enclosing the crus commune is a
and the canal for the facial nerve in the in-
small posterodorsally directed foramen, the
ternal acoustic meatus. The space in the pe-
vestibular aqueduct for passage of the en-
trosal is the cavum supracochleare (Voit,
dolymphatic duct. The anterior semicircular
1909), which housed the geniculateganglion
canal is broken open just distal to the bulge
of the facial nerve. We name the opening
over the anterior ampulla in the anterolateral
into the cavum supracochleare visible in lat-
rim of the subarcuate fossa.
eral view the ‘‘fenestrasemilunaris’’,follow-
The surface of the pars canalicularis an-
ing Rougier et al. (1992). Running antero-
teromedial to the subarcuate fossaissmooth.
ventrally from the fenestra semilunaris is a
Often in eutherians, this area has a sulcus
narrowgrooveinterpretedtobeforthegreat-
transmitting the sigmoid sinus to the jugular
er petrosal nerve, a branch of the facial
foramen.Posteriortothissmooth-walledsur-
nerve, which notches the ridge bearing the
face is a roughened, medially facing, cres-
alisphenoid facet and, therefore, would have
centic facet for contact with another bone,
been closed as a foramen between the petro-
presumably the exoccipital. The surface of
sal and alisphenoid in life. The groove con-
the pars canalicularis lateral to the vestibule
tinues onto the ventral surface of the pars
containsaposterodorsallydirected,roundfo-
cochlearis and merges with the groove for
ramen; this is the endocranial aperture into
the ramus inferior of the stapedial artery.
the prootic canal. Leading into this foramen
Two distinct regions of the pars canalicu-
from above and behind is a broad sulcus for
laris are visible in lateral view, a ventrolat-
the prootic sinus, which likely continued
eralone(seebelow),andamoredorsomedial
onto the missing posterior part of the pars
one exposed through post mortem damage.
canalicularis. Ventrolateral to this sulcus, the
The latter is the deep, medial portion of the
endocranial aperture into the ventral ascend-
subarcuatefossa,rimmedbythesemicircular
ing canal is visible. As with the tympanic
aperture, the endocranial one is somewhat canals. Forming the anteroventral rim of the
figure-eight shaped, with the posteroventral subarcuate fossa is the crus commune, the
openinglargerthantheanterodorsalone.The conjoined anterior and posterior canals. The
shape of this aperture is fully visible only in remainder of the rim is formed by the pos-
an oblique dorsal view (not shown). Antero- teriorsemicircularcanal,includingitsbroken
lateral to thisapertureisasmooth,crescentic portion at the dorsal apex.
surface exposed on the lateral braincasewall The ventrolateral region of the pars can-
(see below). alicularis in lateral view shows postmortem
damage, and it is uncertain how much of the
LATERAL VIEW originalbonehasbeenlostthroughbreakage.
(fig.1C, F) The ventralmost part of what is preserved
The parscochlearispresentstwomainsur- has a small, bowed, crescentic surface that
facesinlateralview.Anteriorlyisthebroken we believe was exposed on the sidewall of
10 AMERICAN MUSEUM NOVITATES NO. 3322
the braincase. Consequently, we identify this
as an anterior lamina such as occurs in ex-
tinct non-therian mammaliaforms (Kermack
and Kielan-Jaworowska, 1971; Wible and
Hopson, 1993). The ventral edge of the an-
terior lamina is the lateral flange (see above
underVentralView,fig.1A,D).Theanterior
lamina in Prokennalestes was largerthanthe
preservedcrescenticsurface,becausethereis
evidence of breakage along most of its an-
teriorand dorsalborder.However,theunbro-
ken edges that are preserved, especiallypos-
terodorsally, reveal that, overall, the anterior
lamina in Prokennalestes was smaller than
that in extinct non-therian mammaliaforms.
Immediately behind the anterior lamina in
Fig. 2. Reconstruction of the osseous laby-
PSS-MAE 136 is a deep notch (visible in
rinth of the right petrosal referred to Prokenna-
ventral view lateral to the lateral flange, fig.
lestes trofimovi, PSS-MAE 136, in dorsal view,
1A, D). Most of the bony surface of this
based on radiographs. Bone housing lateral semi-
notch is roughened for contact with another circular canal and enclosing subarcuate fossa is
bone, presumably the squamosal. The only missing.
exception is the surface adjacent to the an-
terior lamina, which is smooth and may not
Transmitted via this foramen was a ramus
have been covered.
temporalis to the temporal musculature.
Running adjacent to the anterior lamina
are two major vascular channels.Immediate-
ly medialto theanteriorlaminaistheventral OSSEOUS INNER EAR
ascending canal, and anterior to it the sulcus Figure 2 shows our restoration of the os-
for the prootic sinus. Both of these channels seous labyrinth, cavities hollowed out within
run at about a 45(cid:56) angle to the horizontal, the petrosal that in life contained perilymph
sloping posterodorsally. The sulcus for the in which the membranous labyrinth wassus-
prootic sinus ends ventrally at the endocra- pended. Our restoration is based on radio-
nial aperture of the prootic canal, which is graphic analysis of PSS-MAE 136 along
situated between the anterior lamina and fe- with the surface topography as well as the
nestra semilunaris. Two major vessels were internal morphology exposed through post-
transmitted by the ventral ascending canalin mortem damage. The osseous labyrinth con-
light of grooves emanating from this canal’s sists of three parts: the cochlea, which con-
figure-eight-shaped endocranial aperture. tained thecochlearduct;thevestibule,which
The larger posteriorgroove,interpretedtobe contained the utricle and saccule; and the
the posttemporal groove for the arteria di- semicircular canals, which contained the
ploe¨tica magna, bends posteromedially, and semicircular ducts.
thesmalleranteriorone,identifiedasthedor- The mostprominentfeatureoftheosseous
sal ascending groove for the continuation of labyrinth isthecochlea.Itisacoiled,hollow
the ramus superior, runs dorsally. A third tube of uniform diameter that occupies the
smaller vessel apparently arose from the ar- majority of the available space in the par
teria diploe¨tica magna. The posterodorsal cochlearis with littleroomto spare.Thecon-
border of the anterior lamina (and thefigure- nection between the cochlea and vestibule is
eight-shapedapertureintotheventralascend- at the posteromedial aspect of the pars coch-
ing canal) has a shallow, smooth, concave learis. Anteromedialtoitsorigin,thecochlea
edge that we interpret as contributing to the is joined by the short, narrow cochlear can-
ventral border of a foramen on the sidewall aliculus, which transmitted the cochlear aq-
of the braincase. Completing the borders of ueductoftheperilymphaticduct.Beyondthe
this small foramen was likely thesquamosal. cochlear canaliculus, the cochlea coils in a