Morphology of the Auditory Region in Paramys copei and Other Eocene Rodents from North America

16 Pages·2000·0.16 MB·English

by  John H. Wahlert

PUBLISHED BY THE AMERICAN MUSEUM OF NATURAL HISTORY
CENTRAL PARK WEST AT 79TH STREET, NEW YORK, NY 10024
Number 3307, 16 pp., 4 figures, 1 table 13 December, 2000
Morphology of the Auditory Region in
Paramys copei and Other Eocene
Rodents from North America
JOHN H. WAHLERT1
ABSTRACT
Themorphologyoftheexternalsurfaceofthepetrosalanditsjunctionwiththebasioccipital
in Paramys copei is described. Comparison with an outgroup of the Cretaceous Vincelestes
and Recent Didelphis and Solenodon reveals that the auditory region retains many primitive
features.TheEoceneparamyinesfromNorthAmericashowonlyslightdifferences.Sciuravus
is set apart by the lack of a ventral petrosalsinus canal between thepetrosalandbasioccipital
and by the facial nerve and stapedial artery sharing a common foramen in the petrosal. Eu-
ropean theridomyids, too, are not as primitive as Paramys but share with it and Sciuravus a
ridge on the promontorium that separates the transpromontorial continuation of the internal
carotid artery fromthe origin ofthetensortympanimuscle.Twelvecharactersoftheauditory
region are analyzed and summarized in a data matrix for use in future studies. Relativeprim-
itivenessof the auditory region inalloftheseEocenerodentssuggeststhatderivedcharacters
in later rodent taxa may represent intraordinal relationships but only homoplasy with nonro-
dents.
INTRODUCTION in publications on cranial anatomy. Thus, a
wealth of additional characters exists thatcan
Rodent auditory regions are sufficiently
be describedandevaluatedforpolarity(prim-
known for some characterstobeusedinphy-
itive or derived). Paramys (family Ischyro-
logenetic analyses. However, many visible
myidae) is a key taxon in this category. It is
structural details are not routinely described
College, City University of New York; member of doctoral faculty, CUNY Ph.D. program in biology: ecology,
evolutionarybiology,andbehaviorsubprogram.
2 AMERICAN MUSEUM NOVITATES NO. 3307
one of the oldest fossil rodents in which suf- 1962, figs. 13 and 14; Wahlert, 1974, fig. 2.
ficient detail has been preserved, and, in Similarly, independent versions of the audi-
North American paleontology, the genus has tory region have been published: Wood,
beenplacednearthebaseoftherodentfamily 1962, fig. 14a; Wahlert, 1974, fig. 4; Parent,
tree as a kind of structural ancestor or ex- 1980, pl. 2, figs. 2, 4; Lavocat and Parent,
ample of primitiveness (Gregory, 1951: fig. 1985, fig. 1c. However, I found that there
19.40; Wood, 1962: fig. 90). One expects it was still matrix obscuringmorphologyinthe
to be almost wholly primitive for rodents. auditory region and even a matrix-covered
Other contenders for structural primitiveness petrosaldisplacedposterolaterallyonAMNH
among rodents are the North Americangenus 4755. To sit at a microscope and carefully
Sciuravus, which McKenna and Bell (1997: use the sharpened tip of a thin beading nee-
186)putinanewanddifferentsuborderfrom dle to push away the fine covering matrix
the Ischyromyidae, and the European theri- gives one an intimate feel for the shapes and
domyids (Lavocat and Parent, 1985: 338). Li edges as they are revealed. Identification of
et al. (1989) described the auditory region of these features and comparisons with other
Cocomys, a rodent from the early Eocene of living and extinct mammals were madefrom
China; future restudy of this specimenshould the literature mentioned below and from the
yield important comparative details. ontogenetic studies by MacPhee (1981) and
My primary purpose in this paper is to de- Wible (1984).
scribe and illustrate the auditory region in Among thefirst majorcomparativestudies
Paramys copei, AMNH 4755 (holotype) and on auditory regions in mammals was that of
4756. Thesetwoskullspreservethefinestde- van Kampen (1905), who built primarily on
tail that I have seen in any fossil rodent, and the fine German work in anatomy and de-
theirearlyEoceneage,approximately51mya velopment. Van der Klaauw (1931) made
(million years ago), makesthemimportantby fossil mammals a part of his monograph; he
antiquity. The rodent branch of mammalian was able to take advantage of the descrip-
phylogeny may have been separate alreadyin tions and collections of fossils from the
Asia in the early Paleocene (Heomys Li, American West that had accumulated since
1977);undisputedrodentsareknownfromthe the late 1800s. Recent publications provide
late Paleocene: Alagomyidae in Asia and character analyses and detailed information
North America and Ischyromyidae in North about early mammals (Wible, 1990; Rougier
America. In North America, the appearance et al., 1992; Wible and Hopson, 1993; Wible
of ischyromyid rodents defines the beginning et al., 1995; Rougier et al., 1996; McKenna
of the Clarkforkian Mammal Age (Woodbur- et al., 2000) and insectivores (McDowell,
ne, 1987: 61) about 56 mya (McKenna and 1958; Novacek, 1986). The pioneering com-
Bell, 1997: fig. 1). The early Eocene saw si- parative studies on rodent auditory regions
multaneous diversification of rodents in Asia, are relatively recent (Oaks, 1968; Parent,
North America and Europe. My second pur- 1980;LavocatandParent,1985).Manystud-
poseistocomparetheauditoryregionofPar- ies of living rodents have focused on taxa
amys copei with an outgroup of thoroughly with inflated bullae (Webster, 1961, 1962,
described and illustrated mammals including 1975;Pye,1965;WebsterandWebster,1971,
the early mammal Vincelestes, the marsupial 1975, 1977, 1984; Lay, 1972, 1993; Wahlert,
Didelphis, and the insectivoreSolenodon,and et al., 1993). Segall (1971)illustratedgliding
with other Eocene rodents. My goals are to and non-gliding sciurids. Information is also
describecharacterpolaritiesasastartingpoint available about specific extinct taxa (Lavo-
forcomparisonofrodentauditoryregionsand cat, 1967; Meng, 1990; Wahlert, 1974,1977,
to assess the relative primitiveness of Para- 1978, 1983; Carrasco and Wahlert, 1999).
mys auditory morphology.
ThetwospecimensofParamyscopeiwere
SPECIMENS EXAMINED
collected by Jacob L. Wortman in 1880 for
Edward D. Cope. Images of the skulls have Specimens are listed according to the
beenindependentlyillustratedandpublished: classification of McKenna and Bell (1997);
Cope, 1884, pl. 24a, figs. 1, 1a, 2, 2a; Wood, genericassignmentsofparamyinespeciesare
2000 WAHLERT: AUDITORY REGION IN PARAMYS COPEI 3
according to Korth (1984, 1985, and 1994). Suborder Sciuravida, Family Sciuravidae
Abbreviations: AMNH, American Museum
Sciuravus nitidus, AMNH 12531 and 12551,
of Natural History; USNM, United States USNM17683and22477,BlacksForkMem-
National Museum of Natural History. ber, middle Eocene, Bridger Formation,
Bridger Basin, Wyoming
Class Mammalia, Legion Cladotheria, Suble-
gion Zatheria, Family Vincelestidae
Vincelestes neuquenianus La Amarga Forma- AUDITORY REGION OF
tion, early Cretaceous, Southern Neuque´n PARAMYS COPEI
Province, Argentina (information from
The late early Eocene species Paramys
Rougier et al., 1992)
copei (AMNH 4755, 4756, and 103390) pre-
Sublegion Zatheria, Supercohort Theria, Cohort serves the most ancient, complete example
Marsupialia, Magnorder Ameridelphia, Order
of auditory region morphology that is avail-
Didelphimorphia,FamilyDidelphidae,Subfam-
ableforrodents.Idescribeitbelowasastan-
ily Didelphinae
dard, and then note the differences of other
Didelphis virginiana, AMNH 2070, Recent,no Eocene rodent taxa. The region is illustrated
data. in slightly oblique ventral view in figures 1,
Sublegion Zatheria, Supercohort Theria, Cohort 2, and 3.
Placentalia, Magnorder Epitheria, Superorder In ventral viewthejunctionofthepetrosal
Preptotheria, Grandorder Lipotyphla, Order with the basioccipital is oblique, and the ba-
Soricomorpha sioccipital is widest posteriorly. The anterior
Solenodon paradoxus, AMNH 28270, Recent, part of the basioccipital and adjacent basi-
Haiti sphenoid appears swollen and may be pneu-
matized in the anteriorportion.Theposterior
Sublegion Zatheria,SupercohortTheria,Grandor-
partisaflangethatfloorstheventralpetrosal
derAnagalida,MirorderSimplicidentata,Order
sinus.Theposteriorlacerateforamenisalen-
Rodentia. All specimens are of Eocene age.
ticular gap medial to the posterior partof the
Suborder Sciuromorpha, Family Ischyromyidae,
auditory chamber and situated between the
Subfamily Paramyinae:
petrosal and the occipital.Abroadascending
Tribe Paramyini keelofthepetrosalpartlydividestheanterior
part of the gap, which transmitted the sig-
Paramys copei, AMNH 4755 (holotype) and
4756;LostCabinMember,lateearlyEocene, moidsinus,fromtheposterior,fornervesIX,
Wind River Formation, Wind River Basin, X, and XI. A prominent dimple in the petro-
Wyoming; AMNH 103390, late early Eo- sal just anterior to the keel is the opening of
cene, San Jose Formation, San Juan Basin, the cochlear canaliculus. A broad, covered
New Mexico passagebetweenthebasioccipitalandthepe-
Paramys delicatus, AMNH 12506 and 13090, trosalmaymarkthecourseoftheventral(in-
BlacksForkMember,middleEocene,Bridg-
ferior) petrosalsinus.Itopensjustanteriorto
er Formation, Bridger Basin, Wyoming
the posterior lacerate foramen. The sigmoid
Notoparamys costilloi (type species) ((cid:53) Lep-
sinus and ventral petrosal sinus joined here
totomus costilloi), AMNH 55110 and 55111
to form the internal jugular vein. A deep,
(holotype), Lost Cabin Member equivalent
(Wind River Formation), late early Eocene, horizontal flange of the basioccipital sepa-
HuerfanoFormation,HuerfanoBasin,south- ratestheventralpetrosalsinusfromthebrain
eastern Colorado (AMNH 4755, left side).
Thepetrosalendsanteriorlyatthepiriform
Tribe Manitshini
fenestra; an anterior prong from the petrosal
Pseudotomus hians (type species), AMNH
meets a posterior spine from the basisphe-
5025 (holotype), Blacks Fork Member, mid-
noid, and together they divide off the medial
dle Eocene, Bridger Formation, Bridger Ba-
part of the fenestra as an anterior carotid fo-
sin, Wyoming
Pseudotomus petersoni ((cid:53) Ischyrotomus peter- ramen. The alisphenoid and possibly a nar-
soni) AMNH 2018 (holotype), Myton Mem- row part of the squamosal form the rest of
ber, late Eocene,UintaFormation,UintaBa- the anterior edge of the fenestra. The dorso-
sin, Utah lateral part of the petrosal abuts the squa-
4 AMERICAN MUSEUM NOVITATES NO. 3307
Fig. 1. Paramys copei (AMNH 4755) auditory region diagram with major features labeled;slightly
oblique view of right auditory region; anterior is to the right. Abbreviations: 2ff secondary facial fo-
ramen; acf anterior carotid foramen; as alisphenoid; bo basioccipital; bp bullar process of petrosal; bs
basisphenoid; cf cochlear fossula; chc cochlear canaliculus; etr epitympanic recess; fi fossa incudis; fo
foramenovale;fvfenestravestibuli;hFhiatusFallopii;hyhypoglossalforamen;mmastoid;memastoid
eminence;ostfossafororiginofstapediusmuscle;pirpiriformfenestra;plfposteriorlacerateforamen;
pr promontorium; sq squamosal; stc stapedial artery channel; stf stapedial foramen; sty stylomastoid
foramen; vpc ventral petrosal sinus canal at posterior lacerate foramen.
Fig. 2. Paramys copei right auditory region Fig. 3. Paramys copei right auditory region
slightly oblique view. with arteries and nerves restored. Abbreviations:
acc accessory nerve; cc common carotid artery;
ecexternalcarotidartery;fafacialnerve;glglos-
mosal, and the mastoid, the posterior part of
sopharyngealnerve;gpgreaterpetrosalnerve;hy
the petrosal, ascends and narrows to a point hypoglossal nerve; ic internal carotid artery; icp
between the squamosal and occipital at the transpromontorial continuation of internal carotid
back of the skull. artery; st stapedial artery; vg vagus nerve.
The promontorium, which houses the co-
chlea, bulges anterolaterally. The fenestra
vestibuli faces laterally; it is oval, about two tongue-shaped flange of the petrosal. Pro-
times as long as wide, and the long axis is cesses, canals, and basins permit one to re-
nearly horizontal. The cochlear fossula, construct something of the nature of soft tis-
which contains the fenestra cochleae, faces sues.
posteriorly. Dorsolateral to the promonto- The ventral surface of the promontorium
rium, the roof of the tympanic cavity is bears evidence of the course of the internal
domed as a fossa and bordered laterally by a carotid artery and its stapedial branch. The
2000 WAHLERT: AUDITORY REGION IN PARAMYS COPEI 5
internal carotid artery would have entered a
posterior carotid foramen between the bulla
and petrosal; it ran laterally, posterior to a
change of surface angle that may mark the
extent of bullar overlap, and then within a
shallow groove that lies anteroventral to the
cochlear fossula; finally it turned anteriorly,
and thestapedialarterybranchedoff.Adeep
gutterlike anterior sulcus on the promonto-
rium may mark the transpromontorialcourse
of the internal carotid artery. The sulcus is
bounded laterally by a stout ridge; the ridge
runs anteriorly from the middle of the lateral
side of the promontorium. The origin of a
moderately large tensor tympani muscle
probably extended on the lateral surface of
this ridge to the crest. A low swelling forms
the medial side of the sulcus, and there is a
hole anteriorto thesulcusbetweenthepetro-
Fig. 4. Sciuravus nitidus (AMNH 12531)
sal and the pterygoid region that is inter-
ventral view diagram of auditory regions and en-
preted as the anterior carotid foramen. Evi-
docranial cast (with mechanical dot pattern). An-
dence of the transpromontorial continuation terioristowardthetopofthepage.Abbreviations:
of the internal carotid artery is stronger in acf anterior carotid foramen; fo foramen ovale; f/
Sciuravus nitidus (USNM 17683) (fig. 4); a s foramen shared by facial nerve and stapedial
shallow groove in a similarly situated sulcus artery;hyhypoglossalforamen;icinternalcarotid
joinsthetransversegrooveoftheinternalca- artery channel; icp transpromontorial internal ca-
rotid (Wahlert, 1974: fig. 8). rotid artery channel; ost fossa for origin of sta-
pedius muscle; otc orbitotemporal canal (con-
The stapedial artery branched from the in-
tainedsupraorbitalbranchofstapedialartery);plf
ternal carotid and passed through the inter-
posterior lacerate foramen; ri ramus inferior of
crural foramen of the stapes (missing). The
stapedial artery (cast); rs ramus superior of sta-
arterycrossedbelowthefacialnerve,ranan-
pedial artery (cast); sf sphenofrontal foramen; st
teriorly, above the medially projectingflange stapedial artery (cast); sty stylomastoid foramen.
from the lateral tongue of the petrosal, and
then entered the cranial cavity at the lateral
side of the piriform fenestra. Temporal fo- tended ventromedially onto the ridge along
ramina, which are present in the squamosal- the internal carotid sulcus. The bone that
parietal suture and in the parietal, probably roofs the fossa alongside the promontorium
transmitted temporal branches from the su- may have been formed as the epitympanic
perior ramus of the stapedial artery. In Sci- wing of the petrosal (MacPhee, 1981: 52), a
uravus nitidus (AMNH 12531) a stapedial separate ossification from the tonguelike lat-
groove on the promontorium and an endo- eral flange. Schrenk (1989) found the fossa
cranial cast of the stapedial artery channel andlateralflangetobepartsofasinglestruc-
are visible (fig. 4). The artery divided into ture in Ctenodactylus. The flange continues
inferior and superior rami shortly after en- anteriorlyfromthecristaparoticaasdoesthe
tering the cranium; the inferior ramus di- embryonic tegmen tympani (Beer, 1985:
vergedanteriorlyandthesuperiorramusdor- 391). The considerable breadth and domed
soanteriorly. shape of the fossa and lateral flange suggest
In ventral view the petrosal is very wide dorsolateral inflation of the auditory cham-
alongside the promontorium, and it is sub- ber.
dividedintotwoparts,ashallowmedianfos- Dorsolateral to the anterior end of the fe-
sa that widens anteriorly and a complex lat- nestra vestibuli, the facial nerve emerged
eral flange. The origin of the tensor tympani from the fallopian canal into the middle ear
muscle occupied part of the fossa and ex- through the posteriorly facing secondary fa-
6 AMERICAN MUSEUM NOVITATES NO. 3307
cialforamenattheposteriorendofthefossa. triangle is at the paroccipital process, which
The nerve ran posteriorly in the facialsulcus is formed by the occipital bone and but-
with the crista parotica on its lateral side; tressed by the mastoid at its base. Novacek
then it turned ventrally, where it was accom- (1986: fig. 20) labeled a similar feature in
modated in a broad groove on the medial Leptictis as the sulcus for origin of the di-
side of the mastoid eminence. It emerged gastricmuscle.Therearenomastoidcavities.
from a probably incomplete or primitivesty- The posterolateral extension of the mastoid
lomastoidforamen.Asmallcanalforthema- contains a large chamber that housed the
jor (greater) petrosal nervedivergesfromthe paraflocculus of the cerebellum; the smooth
fallopian canal and passes through the roof infilling of this chamber is exposed in
of the fossa; it ends at an anteriorly directed AMNH 4755. The tympanohyal is presumed
foramen,thehiatusFallopii,andiscontinued to be fused to the crista parotica.
by a groove to the anterior edge of the pe- The auditory chamber in all rodents is en-
trosal at the piriform fenestra. The canal is closed by the petrosal and ectotympanic;
exposed in AMNH 4756 by damage; it is there is no participation by the squamosal or
widest near the facial nerve as would be ex- other elements in its wall. Traces of the au-
pected, since it marks the branching of the ditory bullae can be found in a few fossil
major petrosal nerve from the geniculate specimens; bullae are not preserved in Par-
ganglion. amys and most of its Bridgerian (middle Eo-
Thenearlyverticaloutersurfaceofthelat- cene) relatives. The anteromedial part of the
eral flange of the petrosal was probably ex- promontorium bears a broad facet in Para-
posed between the squamosal bone and the mys. The lateral margin of this facet is a low
tympanic bulla (missing). The flange forms swell along the edge of the sulcus for the
an anteroposterior arch lateral to the fenestra anterior continuation of the internal carotid
vestibuli.Itsventrallyfacingsurfacecontains artery; its posterior margin is formed by a
a shallow, anteroposteriorly elongated basin, change in surface angle that is just anterior
the epitympanic recess. Just posterior to the to the channel for the internal carotid chan-
recess the fossa incudis is indicated by a nel. The facet approximately matches the
slight pocket in the anterior surface of the area where the dorsomedial curve ofthebul-
mastoid eminence. la overlaps the promontorium in Reithropar-
The stapedius muscle arose posterolateral amysandSciuravus.Atthelateralsideofthe
to the promontorium in a basin within the petrosal, the flange derived from the tegmen
ring of thelateralsemicircularcanal.Thean- tympani bears a ventral facet or seat at its
terior boundary of the basin isformedbythe anterior end, presumably for articulation of
ridgeenclosingthelateralsemicircularcanal; the anterior crus of the ectotympanic.
the ridge extends from above the posterior
end of the fenestra vestibuli to the mastoid ANALYSIS OF EAR CHARACTERS IN
eminence. The canal passes posteromedially PARAMYINES AND OUTGROUPS
around the depression for muscle origin and
then through the robust bridge from mastoid Asummaryofcharacterstatesispresented
to the promontorium. If the bulla in para- intable1.Thesedataarepresentedasastart-
myines was similar to that in Sciuravus ing point for future studies; they do not re-
(Dawson, 1961: pl. 3; Wahlert, 1974: fig. 7) veal any pattern of relationships within the
its bony posterior crus did not entirely cover rodents and, when processed with PAUP,
this area, although the membrane that en- give a simple comb structure to a clade of
closedthechambermayhaveextendedmuch the paramyines and Sciuravus. Some char-
farther posteriorly. acters are included simply because they are
In ventral view there is a triangular de- important if other groups of mammalsorlat-
pression in the mastoid posterior to the sta- er rodents are included in a comparative
pediusbasin.Thebaseofthetriangleextends study.DataforVincelestesarearetakenfrom
from the vertically projecting mastoid emi- Rougier, et al. (1992).
nence medially to the posterior end of the Character1: Anteriorpartofbasioccip-
posterior lacerate foramen. The apex of the ital and adjacent basisphenoid in ventral
2000 WAHLERT: AUDITORY REGION IN PARAMYS COPEI 7
TABLE1 ridges abutting the petrosal, and the junction
CharacterStatesinOutgroupMammalsandPara- ofbasioccipitalandbasisphenoidisprimarily
myineandSciuavidRodents flat; Sciuravus also lacks the ventral petrosal
sinus between these bones and the petrosal.
Character 2: Ventral (inferior) petrosal
sinus is transmitted by a canal (enclosed
passage) in the petrosal (0); is transmitted
by a partly shielded passagebetweentheba-
sioccipital and the petrosal, and a short ba-
sioccipital flange may be present (1); is
transmitted by a canal that deeply grooves
the side of the basioccipital, and the sinus is
separated from the brain by a deep,horizon-
tal flange of the basioccipital (2); is trans-
view are not swollen laterally and may meet mittedendocranially,andnocanalispresent
in a flat surface (0); appearswollenlaterally between the basioccipital and the petrosal
and may project ventrally (1). I do not know (3). The venous sinus passes through a pe-
the morphology of this area in Vincelestes. trosal canal in Vincelestes (Rougier et al.,
In Didelphis the middle of the basioccipital 1996). Wible (1990: fig. 4) illustrated the
swells anteriorly to meet a similarly swollen ventral petrosal sinus in Didelphis as enter-
basisphenoid,butthereisnolateralswelling; ing a canal anterior to the posterior lacerate
a similar increase in thickness can also be foramen. In the adult specimen that I have
observed internally. In Solenodon there isno on hand, the canal deeply indents the lateral
lateral swelling, and the bones meet in a flat side of the basioccipital and enters the cra-
surface. nium, as shown by Wible. McDowell (1958:
The lateral swellings present in paramy- 139) stated: ‘‘As in other lipotyphlan insec-
ines are very different from the condition in tivores, the ventral petrosal sinus of Neso-
Didelphis.InParamyscopeilateralswellings phontes and Solenodon is exposed ventrally,
of the basioccipital begin at the anterior end owing to the failure of the petrosal and ba-
of the posterior lacerate foramen as a floor sioccipital to form a suture beneath the sinus
under the ventral petrosal sinuses; the swell- immediately anteriortothejugular(posterior
ings expand anteriorly so there is only a me- lacerate)foramen.Thisexposureextendsfor-
dial trough separating the pair at the anterior ward from the jugular foramen, with which
end of the bone; this is continued anteriorly it is continuous, approximately to the level
on the basisphenoid, and the relief flattens of the basioccipital-basisphenoid suture.’’
out at the level of the foramen ovale. The The medial edge of the petrosal forms a par-
character is present in P. delicatus, but the tial roof above the sinus canal.
swelling is not as broad. The junction of ba- In Paramys copei and P. delicatus thereis
sioccipital and basisphenoid in Paramys ap- a wedge-shaped gap between the basioccip-
pearsinsideviewasabroadlyobtuse,down- italandpetrosalfortheventralpetrosalsinus.
ward projecting angulation of the medial el- It is widest at the posterior lacerate foramen
ements of the basicranium. and narrows anteriorly. Thechannelbetween
Swellings in Pseudotomus hians and P. thetwobonesisseparatedfromthebraincase
petersoni begin posteriorly with a low boss by a broad, long dorsal flange of the basi-
somewhat anterior and medial to theanterior occipital. This same character is present in
end of the posterior lacerate foramen; the PseudotomusandNotoparamys.Petrosalsare
swelling continues only as far as the basi- missing from Pseudotomus hians, and the
occipital-basisphenoid suture. The basicran- channel incises on the lateral face of the ba-
ium is not angled in side view. Dorsoventral sioccipital; it bends sharply dorsal at about
flattening of specimens of Notoparamys cos- the anterior end of the petrosal. The sinus
tilloi makes it impossible to evaluate the must have entered the cranium at the fora-
character. Sciuravus has a very slight, if any, men that could also have been forpassageof
anterior increase in breadth of the lateral the transpromontorial continuation of the in-
8 AMERICAN MUSEUM NOVITATES NO. 3307
ternal carotid artery (anterior carotid fora- monotremes, marsupials and some insecti-
men). In Sciuravus thepetrosaliscloselyap- voresandbats)orhelpingpartlytomarginate
pressed to the basioccipital, and there is no these openings at the middle lacerate foram-
room for the ventral petrosal sinus between ina (in more advanced eutherians)’’ (ibid.:
them. 56). Separate nameable foramina in this re-
Character 3: Piriform fenestra ((cid:53) mid- gion have been partitioned off from the pir-
dle lacerate foramen of some authors) is iform fenestra by changes in the extent of
small and medial (0); is large, and prongsof bones.
the petrosal and pterygoid separate the an- In Solenodon, the hiatus Fallopii is in the
terior carotid foramen from the lateral part anterior edge of the petrosal at the fenestra,
ofthefenestra(1).InVincelestes,anaperture whereas in paramyines it is in the middle of
identified as the ventral opening of the ca- thefossaforthemuscleorigin.Thissuggests
vum epiptericum may be the same, at least that the petrosal does not extend as far an-
in part, as the piriform fenestra. In Didelphis teriorly in Solenodon as it does in rodents;
itisasmallopeningatthetipofthepetrosal, the piriform fenestra in rodents may be cre-
and the carotid foramen is anterior to it. It is ated by failure of the alisphenoid and adja-
largeinSolenodon,paramyines,andSciuravus. cent basisphenoid to grow posteriorly rather
than of the petrosal to extend anteriorly.An-
I use the term, piriform fenestra, instead other explanation is that in rodents the an-
ofmiddlelacerateforamenasinmyprevious teriorpartofthefossaformuscleorigindoes
publications, and I haveused anteriorcarotid not descend far enough (that is, it slopes too
foramen as the name for the place where the steeply) to enclose the entire course of the
transpromontorial continuation of the inter- nerve.
nalcarotidarteryentersthecranium.Thepir- Character 4: Internal carotid artery en-
iform (also pyriform (cid:53) pear shaped)fenestra ters the middle ear, runs transversely, gives
is a name coined by McDowell (1958: 128) off the stapedial artery, and then turns an-
for the ‘‘large vacuity in the roof of the mid- teriorly across the promontorium (0); does
dle ear (corresponding to the area of origin not enter the ear (1). Rougier et al. (1992:
of the tensor tympani) of the Soricidae and fig. 3) reconstructed the internal carotid ar-
Solenodontidae.’’ He restricted theuseoffo- tery in Vincelestes. It ran laterally, gave off
ramenlacerummediumtotheentranceofthe the stapedial artery, and then turned anteri-
carotid artery into the skull and pointed out, orly in a channel on the anterior slope of the
however, that Story (1951) applied the term promontorium to the carotid foramen.
in carnivores to the foramen for the large The internal carotid artery does not enter
vein from the ventral petrosal sinus that the ear in adult Didelphis. This is apparently
drains to the pharyngeal and pterygoid plex- a derived characteristic of marsupials. Con-
uses (McDowell, 1958: 124). MacPhee cerning the morphology of the two isolated
(1981: 58) broadened the meaning of piri- petrosals from late Cretaceous didelphoids,
form fenestra ‘‘to include the large gap, seen Wible concluded (1984: 310): ‘‘In particular,
in all fetal and a few adult mammals, that no vascular grooves appear on the ventral
liesbetweentheauditorycapsuleanditsdor- surface of the promontorium for either the
sal outgrowths (tegmen tympani, epitympan- internal carotid or stapedial arteries. This
ic wing of petrosal) on the one hand and the suggests that the medial position of the in-
epitympanicwingsofthesphenoidandsqua- ternal carotid and the loss of the stapedial
mosal on the other.’’ Moore (1981: 41) said artery are ancient traits among Marsupialia.
that ‘‘posterior to the ala is the middle Also, a large groove along the medialborder
lacerate foramen which lies close to the of the petrosal’sdorsalsurfaceprobablycon-
entrance of the internal carotid artery into tained an intracranial ventralpetrosalsinus.’’
the cranial cavity .... The extension of the In conclusion about the living marsupials,
basisphenoid posteriorto thepituitaryresults Wible stated (1984: 312): ‘‘The internal ca-
in this bone totally enclosing the openings rotid artery follows an extrabullar pathway
for the internal carotid arteries (as occurs in medial to the tympanic cavity and promon-
typical amphibians and reptiles as well as in toriumofthepetrosal.... Theproximalsta-
2000 WAHLERT: AUDITORY REGION IN PARAMYS COPEI 9
pedial and its three branches, the posterior, ridge instead of a low one (2). The anterior
superior, and inferior rami, are absent.’’ The slope of the promontorium is smooth in Vin-
internal carotid artery enters the ear of So- celestes, Didelphis, and Solenodon. A prom-
lenodon; it turns and continues anterome- inent ridge is present in paramyines (no ev-
sially across the sloping anterior surface of idence is retained in Pseudotomus). It sepa-
the promontorium to the anterior border of rates the fossa for origin of the tensor tym-
the tympanic cavity. Its lateral branch, the pani muscle from a gutter that ascends
stapedial artery, passes between the crura of anteriorly on the medial part of the promon-
the stapes and divides at the anterior edge of torium. The ridge may have one or both of
the petrosal into superior and inferior rami; two functions: to delimit the channel for the
the ramus superior passes through the pos- anterior extension of the internal carotid ar-
terolateral part of the piriformfenestratobe- tery, and to enlarge the site of origin of the
come the middle meningeal artery withinthe tensor tympani muscle. Sciuravus (USNM
cranium (McDowell, 1958: 140). 17683 and 22477, and Parent, 1980: pl. 3,
Paramyines and Sciuravus have a channel fig. 3) has a slight ridge.
that runs transversely across the promonto- Character 6: Stapedial artery is exposed
rium to the fenestra vestibuli; it indicatesthe in the middle ear along its entire course (0);
presence of the stapedial artery.InSciuravus enters a foramen anteroventral to the sec-
a channel runs anteromedially from it on the ondary facial foramen (1); enters an aper-
anterior slope of the promontorium and rep- ture together with the facial nerve and im-
resents the transpromontorial continuationof mediatelypasseslaterallythroughaforamen
the internal carotid artery to the anterior ca- into the cranium (2); is absent (3). The sta-
rotid foramen. All paramyines have a large, pedial artery is absent in Didelphis. The sta-
gutterlike channel in this position that is pedial artery is exposed in the middle ear of
bounded on the medial side by the abutment Vincelestes and Solenodon. In Vincelestes,
of the petrosal against the basioccipital and Rougier et al. (1992: fig 3) reconstructedthe
laterally by a crest that may have been part artery as splitting into a ramus superior that
of the origin of the tensor tympani muscle. I exits dorsally through the petrosal and an in-
haveinterpretedthismorphologyasevidence ferior branch that continues anteriorly in a
for the anterior continuation of the internal channel. In Solenodon it branches into su-
carotid artery within the ear. However, in perior and inferiorramiattheanteriorendof
paramyines the internal carotid does not the petrosal (McDowell, 1958: fig. 6).
mark the promontorium adjacent to the pos- In paramyinesthestapedialarteryispartly
terior lacerate foramen, and the stapedial orentirelyenclosedinachannelorcanalthat
channel is only clear close to the fenestra begins a short distance anterolateral to the
vestibuli; the arteries may not have touched intercruralforameninthestapes.InParamys
the medial part of the promontorium. On the copei it is partly covered by a medially pro-
right side of one specimen of Paramys deli- jecting flange from the lateral tongue of the
catus (AMNH 12506) I imagine that I see a petrosal. It enters a foramen that is antero-
faint, anteriorly running channelthatascends lateral to the secondary facial foramen in
to the large anterior carotid foramen at the Paramys delicatus, Pseudotomus petersoni,
anteromedial end of the petrosal; it could be andpossiblyNotoparamyscostilloi(crushing
a channel for the internal carotid artery, and makes character assessment uncertain). A
there seems to be adequate room for both an specimen of Sciuravus nitidus (AMNH
artery and the ventral petrosal sinus to pass 12531) shows a complete picture of the ar-
through the foramen. However, this vague terial pattern. The basicranium and auditory
feature is not conclusive evidence of the ar- regions are relatively intact, but the bone of
tery. the cranium is gone, and a clear endocranial
Character 5: Anterior part of the pro- cast remains. The stapedial artery passed
montorium is smooth (0); has a low ridge through the intercrural foramen in the stapes
that separates the course of the internal ca- (missing), turned anterolaterally into a fora-
rotid artery from the fossa for origin of the men that also transmitted the facial nerve,
tensor tympani muscle (1); has a prominent and continued in an anterolateral direction
10 AMERICAN MUSEUM NOVITATES NO. 3307
through the petrosal into the braincase. A squamosal, the jaw joint is far anterior and
cast of the course of the artery and its not directly supported by the petrosal, and
branches is visible on the left side of the en- the epitympanic recess is entirely within the
docranial cast (fig. 4). The inferior ramusdi- petrosal [the squamosal is grooved ventrally
vergedanteriorlyjustinsidethecranium.The to receive the dorsal part of petrosal] (2).
superior ramus continued laterally and then Thepetrosalparticipatesinthesupportofthe
turned anteriorly; the course of the supraor- glenoid fossa in Vincelestes, and it appears
bital branch in the orbitotemporal canal is tobeexposedinlateralviewbelowthesqua-
marked by a cast that makes a broad arch mosal (Rougier et al., 1992: figs. 1, 2); the
that descends to the sphenofrontal foramen epitympanic recess is within the petrosal.
in a tiny bit of bone preserved in the back The petrosal is hidden in lateral view by the
of the orbit; the artery was presumably ac- squamosal in Didelphis and Solenodon, and
companied by a vein. the squamosal makes the lateral side of the
Character 7: Petrosal is narrow in width epitympanic recess in both; I regard this as
and length alongside the promontorium (0); theprimitiveconditionoftheoutgroupmam-
is wide alongside the promontorium and the mals. In paramyines and Sciuravus the dor-
fossa in which the tensor tympani muscle solateral part of the petrosal (the lateral
arose, extends, and broadens anteriorly (1). flange) is exposed below the squamosal, and
The fossa in which the tensor tympani mus- the epitympanic recess is entirely within the
cle arose is narrow and long in Vincelestes, petrosal; this appears to be a different and
narrow and short in Didelphis, and narrow derived condition compared to that in Vin-
and tapered anteriorly alongsidethepromon- celestes, in which the petrosal directly sup-
torium in Solenodon. In paramyines and Sci- ports the jaw joint.
uravus it is wide and broadens anteriorly as Character 10: Epitympanic recess is a
a shallow broad channel. The muscle origin shallow channel (0); is deepened and en-
leaves no distinct marks, so its actual extent larged (1). Vincelestes and Didelphis have
in the fossa is not known with certainty. only shallow epitympanic recesses. The re-
Character 8: Hiatus Fallopii is present cess is slightly enlarged dorsally in Soleno-
in the medial part of the fossa in which the don. The epitympanic recess is no morethan
tensor tympani muscle arose (0); is present a shallow groove in the lateral flange of the
at the front edge of the petrosal fossa for the petrosal in all of the paramyines and Sciura-
tensor tympani muscle (1). In Vincelestesthe vus; a slight pocket in the posterior end
hiatus Fallopii is in the fossa alongside the marks the fossa incudis.
promontorium and not close to the anterior Character 11: Alisphenoid bone makes
end of the bone. The hiatus Fallopii is pre- no part of the bulla (0); makes a part of the
sent at the anterior edge of the petrosal in bulla (1). The alisphenoid forms no part of
DidelphisandSolenodon.ThehiatusFallopii a bulla in Vincelestes. A posteriorly concave
is in the medial part of the fossa for the ten- process of the alisphenoid forms the anterior
sor tympani muscle in Paramyscopei,P.de- part of the bulla in Didelphis. The bulla in
licatus, Pseudotomus petersoni, Notopara- Solenodon is formed by a somewhat inflated
mys costilloi, and Sciuravus nitidus (AMNH butincompletetympanicring,andthereisno
12551, right side); the region is not pre- contribution fromthe alisphenoid;thecaseis
served in Pseudotomus hians. the same in paramyines and Sciuravus.
Character 9: Petrosal exposure inlateral Character 12: Shallow basin posterior
view:thepetrosalisvisible,supportsthejaw to the crista interfenestralis (between the
joint, and contains the epitympanic recess fenestra vestibuli and the fenestra cochle-
(0); the dorsolateral edge of the petrosal is ae) is simple and encircled by the lateral
hidden by a downward overlap of the squa- semicircularcanal(0);isenlargedbyashal-
mosal, the jaw joint is anterior and not di- low depression that extends posteromedially
rectly supported by the petrosal, and the from the basin ventral to the rim made by
squamosal makes the lateral side of epitym- the lateral semicircular canal (1). Some part
panic recess (1); the dorsolateral part of the of the basin is the fossa in which the stape-
petrosal is exposed between the bulla and dius muscle arises. In Vincelestes the basin