Since the announcement(1) of the discovery of upper Eocene mammals in the Sespe deposits north of the Simi Valley, California, excavations have been continued in these beds, particularly at Locality 150 C. I. T. Vert. Pale., by field parties of the California Institute. Recently an opportunity has been afforded the writer to compare a number of the mammalian types found here with related forms known from the late Eocene and Oligocene. This has permitted a more precise determination of the genera and species than was attempted in the original statement. It is planned therefore to present in this and in subsequent papers brief descriptions of individual types, reserving a fuller discussion for a monographic treatment of the Sespe faunas

Stock, Chester

English

Caltech Authors - Main

HYAENODONTIDA. OF THE UPPER EOCENE OF CALIFORNIA
By CHESTER STOCK
BALCH GRADUATE SCHOOL OF THE GEOLOGICAL SCIENCES, CALIFORNAL INSTITUTE OF
TECHNOLOGY
Communicated February 17, 1933
Since the announcement' of the discovery of upper Eocene mammals
in the Sespe deposits north of the Simi Valley, California, excavations
have been continued in these beds, particularly at Locality 150 C.I.T.
Vert. Pale., by field parties of the California Institute. Recently an
opportunity has been afforded the writer to compare a number of the
mammalian types found here with related forms known from the late
Eocene and Oligocene. This has permitted a more precise determination
of the genera and species than was attempted in the original statement.
It is planned therefore to present in this and in subsequent papers brief
descriptions of individual types, reserving a fuller discussion for a mono-
graphic treatment of the Sespe faunas.
CREODONTA
Family Hyeenodontidoe
Hyaenodon vetus, n. sp.
Type Specimen.-Skull with upper dentition, No. 1243 C.I.T. Coll.
Vert. Pale., plate 1, figure 1, textfigures 1 and 2.
Locality.-Sespe Upper Eocene, north of Simi Valley, Ventura County,
California; Locality 150 C.I.T. Vert. Pale.
Specific Characters.-Larger than Hycnodon crucians and approaching
H. montanus and H. cruentus in size. Skull slender with postorbital con-
striction relatively more pronounced than in Oligocene species. De-
pression of frontal surface between temporal ridges broad and relatively
deep. Union of temporal ridges with sagittal crest slightly in advance
of level of deepest portions of postorbital constriction. Posterior position
of postglenoid processes like that in crucians; processes situated slightly
farther back than in paucidens and in mustelinus but not so far back as in
cruentus and in horridus. Backward extent of tooth-row not so great as
in American Oligocene species. Anterior premolars closely spaced but
not crowded. P1 with two well developed roots.
Remarks.-The beautifully preserved skull of Hyenodon vetus represents
the first specificaUly determinable material of this genus to be recorded
from an American Tertiary horizon older than the White River. No.
1243 exhibits several characters in which it may be regarded as more primi-
tive than representatives of the genus in the American Oligocene. The
434 GEOLOG Y.- C. STOCK PROC. N. A. S.
GEOLOGY: C. STOCK
position of the posterior choanae varies in skulls of the latter. In H.
vetus the floor to the postnarial passage apparently does not extend back-
ward quite so far as in the Oligocene hyeenodons. In No. 1243 the medial
walls of the palatines are virtually in contact, in part due perhaps to
lateral crushing which has affected this portion of the postnarial passage,
but a narrow cleft appears to prevail between them extending forward
farther than in the Oligocene forms. Likewise the pronounced postorbital
constriction and possibly the lack of a noticeable backward extension of
the tooth-row may be regarded as characters in which H. vetus is less
advanced.
Hyxenodon (Protohy=nodon) exiguus, n. subgen. and n. sp.
Type Specimen.-A maxillary fragment with P3 -M2, No. 928 C.I.T.
Coll. Vert. Pale., plate 1, figures 3, 3a.
FIGURE 1
Hyanodon vetus, n.sp. Skull, No. 1243, lateral view; X 1/2. C.I.T. Coll. Vert. Pale.
Sespe Upper Eocene, California.
Paratypes. Maxillary fragment with P4, M1 and M2 and a ramus with
teeth, No. 1140 C.I.T. Coll. Vert. Pale., plate 1, figures 2, 2a and 4, 4a.
Locality.-Sespe Upper Eocene, north of Simi Valley, Ventura County,
California; Locality 150 C.I.T. Vert. Pale.
Subgeneric Characters.-P4 with distinct inner cusp. Ml with tiny
protocone. Slight bifurcation of tip of principal antero-external cusp in
M2.
Specific Characters.-Distinctly smaller than Hyenodon mustelinus and
comparable in size to individuals of H. vulpinus from the Phosphorites of
Quercy. Approaches closely in size the American species ?Pseudopterodon
minutus. Width of P4, relative to length, greater than in latter species.
Remarks.-Protokyarnodon exiguus, in the several characters stated
above, is distinctly more primitive than typical Hyanodon. In the former
the inner lobe of P4 projects inwardly farther' than in the Oligocene species
VOL. 19, 1933 435
GEOLOGY: C. STOCK
and a distinct cusp is present. This region is better developed in the
Simi form than in ?Pseudopterodon minutus. The inner face of the prin-
cipal cusp facing the lobe is concave. In the American Oligocene species,
where the inner cusp is not so well defined or is absent, the inner surface
slopes upward from the tip of the prin-
cipal cusp to the side of the lobe. In
other words, the lobe supporting a
cusp of its own serves Iess as a buttress
to the principal cusp in P. exiguus than
in the Oligocene forms. In Ml of P.
exiguus a tiny but distinct protocone is
present and is defined from the inner
',)-v/<: twall of the antero-external cusp by
a small furrow. The presence of an
antero-internal cusp has been noted
by Matthew in Ml of ?Pseudopterodon
minutus. In the Sespe species an
4 11lll,Al}'5^ // antero-internal cusp is suggested in
M2, 'but is not so well marked as in
Ml. At the tip of the principal cusp
in M2 of the type specimen of P.
exiguus is a slight indentation of the
enamel surface on both the external
and internal sides, separating it into
two minute parts. The separation of
the tip of this cusp into two parts is
to be noted in Ml of the American
Oligocene species but I have not ob-
served it in M2.
!I, M3 is absent, in which respect P.
exiguus is like Hyenodon. In the
paratype (Fig. 4a) minute depressionsFIGURE 2 or perforations have been noted to the
Hyamnodon vetus, n.sp. Skull, No. inner side of the posterior part of M2,1243, dorsal view; X 1/2. C.I.T. Coll.
Vert. Pale. Sespe Upper Eocene, Cali- but it appears unlikely that these rep-
fornia. resent the remnants of the alveolus for
a third molar. Moreover, the width
of the maxillary wall to the inner side of M2 in the type specimen is certainly
not great enough to accommodate a third molar in that specimen.
Among the American species, H. minutus, described by Douglass2 from
the Pipestone beds of Montana, appears to show closest relationship to
Protohyanodon exiguus. The maxillary fragment with a portion of the
superior dentition later recorded by Matthew3 from these deposits furnished
436 PROC. N. A. S.
GEOLOG Y:. C. STOCK
the basis for assigning Douglass' species tentatively to Pseudopterodon.
Some doubt prevails regarding the validity of Schlosser's type for, ac-
cording to Matthew, Scott has expressed belief that Pseudopterodon is
based on deciduous teeth of Hya?nodon. Martin4 likewise regards this
genus as having been based on the milk dentition of Hyanodon. In
each of the upper carnassial teeth of Pseudopterodon the two cusps forming
the antero-external region of the crown are widely spaced and their sepa-
ration is much better defined than in our form. Protohyanodon exiguus
differs from Matthew's specimen in larger size, in the proportions of P4
and in the presence of a distinct inner cusp in this tooth.
Among European species, Hycenodon vulpinus from the Phosphorites
(Quercy) resembles our type in size. This species evidently displays
noteworthy .differences in size. In the upper dentition of vulpinus P3 is
elongate anteroposteriorly, for the region of the crown in front of the
principal cusp is extended farther forward than in American species.
P4 in vulpinus (No. 11462 Prin. Univ. Coll.) has a distinct inner cuspule.
P. exiguus is more like H. vuipinus in this character than are the American
Oligocene species. The protocone in Ml of vulpinus tends to be more
prominent than in the latter, in which respect our form is again more
like the former.
In at least some specimens of vuipinus the anterior basal tubercle of
P4 is more prominently developed than in No. 1140 from the Sespe. In
P. exiguus a tubercle is absent but a small cingulum is present at the
antero-internal base of the principal cusp. The most noteworthy differ-
ence in the lower dentition occurs, however, in M3. Relative to the length
of either Ml or M2, this tooth has a greater length in H. vulpinus than
in P. exiguus.
Pterodon californicus, n .sp.
The genus Pterodon, now recorded for the first time from an early
Tertiary horizon in North America, is represented by incomplete rami
of lower jaws of three individuals. In two specimens, Nos. 933 and 1213
C.I.T. Coll. Vert. Pale., the deciduous dentition is present with Mi and
M2. No. 941, however, represents an older individual in which the
permanent dentition includes not only the first and second molars, but
also the third and fourth premolars.
All the specimens from the Simi Sespe show the characteristic structure
of the molars in which Pterodon differs from Hycenodon. In size of talonid
and size of paraconid blade in the molars these specimens are like repre-
sentatives of the Pterodon group and differ from Hyanodon.
Type Specimen.-Fragment of left ramus with P3 - M2, No. 941
C.I.T. Coll. Vert. Pale., plate 1, figure 6.
Paratypes.-Two left rami with deciduous teeth and first and second
molars, No. 1213 (plate 1, Fig. 5) and No. 933.
437VOL. 19, 1933
438 GEOLOGY: C. STOCK PROC. N. A. S.
4
J
PLATE 1
(Description on opposite page.)
xN.t...
"v-
.. I
I...
S.I "
T
,3a.
Locality.-Sespe Upper Eocene, north of Simi Valley, Ventura County,
California; Locality 150 C.I.T. Vert. Pale.
Specific Characters.-Pterodon californicus approaches most closely in
size the species P. leptognathus from the FayCim, but is distinctly smaller.
P3 and P4 distinctly more robust and anterior molars relatively smaller
than in P. leptognathus. Decidedly smaller than Hemipsalodon grandis.
Remarks.-The three specimens available show the characters of the
lower deciduous dentition and the acqhisition of the permanent teeth.
The youngest specimen, in terms of development of the dentition, is
No. 933. In this jaw the milk incisors, canine and milk premolars are
present. The milk incisors are crowded in transverse row with DI 2
displaced slightly backward. DI 2 and DI 3 are of subequal size. The
crown of a permanent incisor is appearing behind the deciduous teeth and
immediately adjacent to the symphyseal surface. Dp i and Dp 2 are
two-rooted, with the anterior root considerably more reduced than the
posterior. With reduction in size of the anterior root and procumbent
position of crown, the posterior root in Dp 1 extends obliquely backward.
The single alveolus for Dp 1 in No. 1213 may have accommodated not
only the principal root but also the reduced forward fang. The crown of
Dp 1 extends very close to the canine, flanking the lower posterior border
of this tooth. The posterior root of Dp 2 likewise projects backward in
the jaw. In this tooth the highest point reached by the principal cusp
is situated farther posteriorly than that of Dp 1, and the crown resembles
more that of Dp 3.
The structure of Dp 3 is like that of P4, although the crown is more
compressed than in the latter tooth. The principal cusp, which does
not stand so high as in P4, is slightly recurved. A tubercle of minute
size is present at the antero-internal side of the base of the principal cusp,
while a much larger posterior basal cusp is also present. Dp 4 resembles
somewhat the first molar. It differs, however, from the latter tooth in
a number of characters. Dp 4 is longer and more compressed, the para-
conid and protoconid are not so blade-like and the paraconid shear shows
less obliquity with reference to the anteroposterior axis of the tooth than
in the first molar. A distinct talonid is present. In specimen No. 1213
the partly developed permanent premolar is seen in the jaw beneath Dp 4.
In the type specimen, No. 941, considerable disparity in size exists
DESCRIPTION OF PLATE 1
Figure 1, Hyenodon vetus, n .sp. Type specimen, No. 1243. Figures 2, 2a, 3, 3a, 4,
4a, Hyenodon (Protohyacnodon) exiguus, n. subgen. and n. sp.; figures 3, 3a, type speci-
men, No. 928; figures 2, 2a and 4, 4a, paratypes, No. 1140. Figures 5 and 6, Pterodon
californicus, n. sp.; figure 6, type specimen, No. 941; figure 5, paratype, No. 1213.
All figures X 8/4.
California Institute of Technology Collections. Sespe Upper Eocene, California.
GEOLOG Y.- C. STOCKVOL. 19, 1933 439
between P3 and P4. P3 possesses a broad base with principal cusp
anterior in position. No well developed posterior basal tubercle is present.
In P4 the principal cusp stands high and a posterior basal cusp was pre-
sumably present although the enamel surface is marred in this region.
The crown of this tooth does not tilt backward. The obliquity of the
paraconid shear in the molars, relative to the fore and aft axis of the
individual tooth, does not appear to be as great as that in P. leptognathus
and approaches more that seen In P. dasyuroides. The posterior edge of
the protoconid reaching upward for nearly half the distance between the
level of the talonid and the tip of the cusp projects noticeably backward
and can be readily noted in the illustration (plate 1, Fig. 5). In the more
adult specimen, No. 941, this portion of the edge is worn. A character-
istic enamel ridge extending for a short distance longitudinally on the
antero-external surface of the base of the paraconid marks laterally the
border of the area against which the tooth in front is pressed.
The posterior end of the mandibular symphysis in No. 941 reaches a
point below the anterior end of P4. In this specimen, also, the large
posterior mental foramen lies below the anterior root of P4. A distinct
groove extends from this opening anteriorly, reaching a point below the
anterior end of P3. The very large ramus of the mandible of Hemipsalodon
grandis, described by Cope from the White River Oligocene of Swift
Current Creek, Canada, is widely removed specifically from the Cali-
fornian form and is a later type.
1 Stock, C., Proc. Nat. Acad. Sci., 18, 518-523 (1932).
2 Douglass, E., Trans. Amer. Philos. Soc., 20, 255 (1901).
8 Matthew, W. D., Bull. Amer. Mus. Nat. Hist., 19, 208-209 (1903).
' Martin, R., Rev. Suisse de Zool., 14, 575 (1906).
440 GEOLOG Y: C. STOCK PROC. N. A. S.


Hyaenodontidae of the upper Eocene of California

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Hyaenodontidae of the upper Eocene of California

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