Paleo Bios
Museum of Paleontology
University of California, Berkeley
Number 36                                                                                           July 17, 1981
PLIOMETANASTES PROTISTUS  (EDENTATA, MEGALONYCHIDAE)
FROM KNIGHT'S FERRY, CALIFORNIA WITH DISCUSSION OF
EARLY HEMPHILL I AN MEGALONYCHIDS
by
SUE ELLEN HIRSCHFELD1
ABSTRACT
A well-preserved, partial skeleton of Pliometanastes protistus
Hlrschfeld and Webb, 1968, Is described from the Siphon Canal locality
(Early Hemphillian) in the Mehrten Formation near the Knight's Ferry,
Stanislaus County, California. A tuff 4 meters overlying the sloth
has been K-Ar dated at 8.19 + .16 m.y.b.p. (Hugh Wagner, personal
communication, 1980) making this the earliest, most precisely dated
and westernmost occurrence of P. protistus.
The oldest ground sloth in North America, from the Oshkosh Quarry,
Nebraska, dated at 9 - 9.5 m.y.b.p. (Marshall et.  al.,  1979) and
identified as Megalonyx  (Schultz and Stout, 1948, Marshall et.  al.,
1979) is reevaluated and is now considered an indeterminate mega-
lonychid. The earliest occurrence of Megalonyx  is Late Hemphillian.
The closeness of the relationship between Pliometanastes  and Mega-
lonyx  is still in question.
INTRODUCTION
A partial skeleton representing the first associated and most
complete remains of one individual of Pliometanastes protistus  Hirsch-
feld and Webb 1968, has been recovered near Knight's Ferry,
Stanislaus County, California, in the foothills of the Sierras.  This
is the westernmost identification of the species, extending its range
across the United States.
The skeleton was collected at the Siphon Canal locality in the
Mehrten Formation, an extensive series of M?o-Pliocene volcanics,
tuffs and continental deposits derived from eruptive centers near the
Department of Geological Sciences,
Hayward , California  9i»5it2.
California
State University,
2
PALEOBIOS
NO. 36
crest of the ancestral Sierras to the east.  A tuff lying k  meters
stratigraphically above the Knight's Ferry sloth has been radiometri-
cally dated at 8.19 ;+ 0.16 m.y.b.p. (Hugh Wagner, personal communica-
tion, 1980) indicating an Early Hemphillian age.  This is the earliest
and most precisely dated occurrence of Pliometanastes.
The fine skull and type of Megalonyx mathisi  Hirschfeld and Webb
1968 was collected at Black Rascal Creek, in the upper part of the
Merhton Formation, near Merced, Merced County, about 90 km south of
Knight's Ferry.  Based on the associated fauna, this locality is
correlated with the Pinole Tuff site, Contra Costa County, California
which has a K-Ar date of 5-2 my, considered Late Hemphillian (Hugh
Wagner, personal communication, 1980).
Three genera of ground sloths, Megalonyx  and Pliometanastes  of the
Family Megalonychidae and the mylodont Thinohadistes  first appear in
North America in the Hemphillian.  They are the earliest immigrants
from South America, having arrived in North America at least six
million years before the connection of the Panamanian land bridge
about three million years ago and the major faunal interchange that
followed in the Late Blancan (Marshall et.   al.,   1979). Pliometanastes
is the most primitive of the North American megalonychids, retaining
a predental spout and small triangular caniniform teeth worn to sharp
cutting blades. Megalonyx  is distinguished from Pliometanastes  and all
other megalonychids, 1) by having long ovate, horizontally worn canini-
form teeth which are larger than the first molariforms, and are
oriented obliquely (except M.  mathisi),  and 2) with the mandible
terminating just anterior to the caniniform teeth.
ACKNOWLEDGEMENTS
My thanks go to Mr. and Mrs. Raymond Willms and family for
permission to collect on their ranch and their generous hospitality.
I wish to acknowledge the Museum of Paleontology, University of Cali-
fornia, Berkeley, for use of their facilities, access to their collec-
tions, and illustrations for this paper, and the Department of Geologi-
cal Sciences, California State University, Hayward for assistance in
preparation of this manuscript.  My thanks to Ms. Pat Lufkin, staff
artist of the Museum of Paleontology, Berkeley, for the fine illustra-
tions.  Dr. S. David Webb and Mr. Greg McDonald (University of
Florida, Gainesville), and Dr. Gordon Edmond (Royal Ontario Museum,
Toronto) critically reviewed the manuscript.  Sincerest thanks are
extended to them for their helpful comments and criticisms.
METHODS
Measurements - all measurements given are in millimeters.
Abbreviations - K-Ar - Potassium-Argon
Mc - metacarpal
Mt - metatarsal
m.y.b.p. - million years before present
UCMP - University of California Museum of Paleontolo-
gy, Berkeley
1981                                     PLIOMETANASTES PROTISTVS                                          3
UF-FSM - University of Florida, Florida State Museum,
Gainesvi1le
V-0000 - UCMP fossil locality number.
Anatomy -  Interpretation of muscle attachments are based on data
gathered from the writer's dissection of the lesser anteater Tamandua
tetradaetyla.
SYSTEMATIC PALEONTOLOGY
Family Megalonychidae Zittel 1892
Subfamily Megalonychinae Trouessart \S0k
Pliometanastes  Hirschfeld and Webb 1968
Type species: Pliometanastes protistus  Hirschfeld and Webb 1968
Type locality: McGehee Farm, Alachua County, Florida
Age:  Early Hemphillian
Material :  UCMP 97371 from V-72004, partial skeleton of single indivi-
dual, Siphon Canal locality, near Knight's Ferry, Stanislaus County,
California.  Previously undescribed skeletal elements include epitym-
panic ring, scapula, unciform, lunar, magnum, and cuneiform.
Revised diagnosis:  Dental formula 5A.  A relatively small megalony-
chid approximately the size of Megatonyx mathisi,  the next larger
megalonychid being the Blancan Megatonyx tevtostomous.     Mandible with
long deep and relatively wide symphyseal spout, posterior end of
symphisis below caniniform tooth. Caniniforms subtriangular in cross-
section, sharp pointed wear surfaces on posterior side of upper and
anterior side of lower.  Dorsal ridges on cranium may be united to form
sagittal crest. M.   rectus eapitus ventralis  excavations lateral
rather than postero-medial to basilar tubercles.  Dorsal border of
occipital condyles nearly confluent with occiput.  Palate with long
narrow canals for nutritive foramina.  Shaft of ulna straight on
anterior side.  Metacarpal II with prominent dorso-external process
of proximal end.  Patella proportionally longer than in Megatonyx.
Fibular shaft not curved dorso-medially to distal end in region of
M.  peroneus brevis  as in Megatonyx.     Tuber eatais  and its neck inter-
mediate in development between Megatonyx  and Hapalops.     Astragalus
lacking V-shaped tibial notch.  Metatarsals slender with restricted
muscle scars.
Description:
Skull - The entire basicranial region, including the epitympanic ring,
approximately half of the dorsal region (Figure 1), portions of the ,
nasal, frontal, lacrymal, maxilla, jugal, and molariform teeth M -M
are preserved.
lcm

o
CO
Figure 1. Pliometanastes protistus  skull, UCMP 97371. A. dorsal view; B. ventral view; C. occi-
pital view.

1981
PLIOMETANASTES PROTISTUS
5
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PALEOBIOS
NO. 36
Greater variation in the structure of the posterior portion of
the skull is now recognized necessitating revision of the diagnosis of
the species.  Primary differences from the type UF-FSM 9*»79: occiput
proportionately wider medio-laterally, supraoccipital crest very
prominent projecting outward and posteriorly, and a single median
sagittal crest is present. Variation in these crests may be due to
age since no cranial sutures are discernible in the Knight's Ferry
skull but some sutures are visible in the type. Prominent depres-
sions on either side of the median occipital ridge in the type are not
developed and the surface of the occiput is less flattened dorsal to
the occipital condyles in the Knight's Ferry skull.
The Knight's Ferry cranium is almost equal in size to Megalonyx
mathisi.     Basicranial differences are very slight.  In ventral view,
the occipital condyles are medio-laterally more elongate and not as
deep dorso-ventrally as in M.  mathisi.     However, dorsally, the occipi-
tal condyles are quite similar and in the Knight's Ferry skull are
nearly confluent with the occiput. The stylohyal process is located
farther below (dorsal to) the level of the occipital condyles than in
M.  mathisi.
The Knight's Ferry skull is similar to the type and both differ
from M.  mathisi  in the more lateral excavation of the basilar tubercles
for attachment of the M.  M.   pectus eapitus ventralis.     The basilar
tubercles are also less protrusive than in M.  mathisi.     Although this
character is consistent in the two skulls of P.  protistus, its varia-
bility is still not known and should not by itself be given much weight
as a taxonomic character.
The stylomastoid foramen is as usual, antero-lateral to the base of
the stylohyal process and communicates via the sulous facialis  with the
facial foramen. Another foramen is seen on the right side posterior to
the stylomastoid foramen; while on the left, it is represented by an
open canal running dorso-laterally from a point on the postero-lateral
base of the stylohyal process to join a foramen which enters the
occiput.  On the right side, the foramen at the postero-lateral base of
stylohyal process runs dorsally through the squamosal bone of the
occiput and exits via a more dorsally situated foramen. This extreme
variability in individual skulls of Pliometanastes  and Megalonyx  make
these characters of little taxonomic value although much attention was
previously focused on them (Hirschfeld and Webb, 1968, p. 2kk).
The epitympanic ring is firmly sutured and fused with the ento-
tympanic medially and the squamosal dorsally.  It is horseshoe shaped,
broader ventrally where it contacts the squamosal.  It is inclined
mesially and very slightly anteriorly.
The upper molariform teeth (Figure 3) closely resemble those of
Megalonyx  in shape. Teeth M and M are so similar they cannot be
differentiated from one another.  Both are subtrapazoidal, convex
along their vertical anterior margin, with strongly concave surfaces
on their vertical posterior margins, but with slightly concave indenta-
tions on their lingual sides.  In both, the highest parts of crown are
at the antero-1ingual and labial corners, the most deeply excavated
portion being at the postero-1ingual margin. A flat sickle-shaped
cutting surface forms the anterior surface of the crown. Tooth M^ is
smaller than the anterior molariforms and has slight concavities along
1981
PLIOMETANASTES PROTISTUS
7
its vertical margins on the antero-1ingual, labial and posterior sides.
The posterior surface of crown is the highest part of the tooth. The
antero-1ingual margin of crown is also raised above the excavated
central portion of the crown.                           „   ,
The Knight's Ferry molariform teeth M - M are similar in cross-
section but are slightly larger and more elongate medio-laterally than
are the molariform teeth of Pliometanastes, UCMP 57808, from Klipstein
Ranch, V-5684, Upper Caliente Formation, Caliente Range, California
(Hirschfeld and Webb, 1968, p. 285).
Measurements of skull: height of the occiput from the dorsal
margin of the foramen magnum 47.0 mm; height of the occiput from the
basicranium 68.2 mm; maximum width of the occiput across the mastoid
process 118.8 mm; maximum width across the occipital condyles 67-0 mm;
minimum orbito-temporal width 71-5 mm.
Mandible - This is the most complete mandible and lower dentiti'tion of
this taxon (Figure 2), lacking only a portion of the ascending ramus.
The pre-dental spout is slightly longer (h]  mm) anterior to the
caniniform alveolus (measured at midline) than in the Florida Withla-
coochee 4a mandibular symphysis, UF-FSM 11941, measuring 38 mm (Hirsch-
feld and Webb 1968, p. 258). The spout maintains the same width from
its tip to just anterior to caniniform alveoli. The distance between
the caniniform and the first molariform is greater (24.5 mm compared to
16.5 mm in UF-FSM 119^*1).
The horizontal ramus is deepest, dorso-ventrally, in the region of
M., as in Megalonyx.     The angular process is not as deep and the
coronoid process is proportionately broader than in Megalonyx.    The
articular condyle is not raised as high above the tooth row as in
Megalonyx,  and projects farther posteriorly producing an acute angle
with the angular process. The articulating surface of condyle has two
parts:  the outer portion is widest, sloping postero-laterally; the
inner surface is smaller and slopes medially. The left condyle
measures 34.6 mm, medio-laterally.
The caniniform is much smaller than the molariform teeth, is sub-
triangular in cross-section, and has a sharp-pointed wear surface.
Its occlusal surface is highest at the anterio-1ingual margin and
slopes postero-labially.
The highest portion of the occlusal surfaces of the mandibular
molariform teeth are at the antero-1ingual and postero-labial margins,
opposite the highest and lowest portions of the upper molariforms. The
antero-labial margins bear the most deeply excavated wear surfaces.
Tooth M. is subrectangular, narrow anteriorly and broader posteri-
orly. M2 is subtrapezoidal and its anterior vertical surface is con-
cave. M. is very similar to the M, of Megalonyx curvidens   in which the
posterior vertical side is distinctly concave with a lesser vertical
depression along the labial side.
Measurements of the mandible:  length from anterior end of the
symphysis to the midline between the posterior end of the condyles
225.0 mm; depth of the horizontal ramus at midline of the left diastema
55-2 mm; length of the symphysis from the right caniniform alveolus to
the tip of the spout 44.5 mm; height of the condyle above the ventral
border of the angular process 76.5 mm; least depth of the horizontal
PALEOBIOS
NO. 36
1 cm
2 4
Figure 3. Pliometanastes protistus,  right M -M , UCMP 97371
ramus at M, 45.5 mm; maximum antero-posterior diameter of the right
cuneiform alveolus 15-3 mm; maximum medio-lateral diameter of the
right caniniform alveolus 10.4 mm.
TABLE 1.  Measurement of dentition (in millimeters).
left
right
Maxi
Maxi
Maxi
Maxi
Maxi
Maxi
Maxi
Maxi
Maxi
mum antero-posterior diameteic M *
mum medio-lateral diameter M'* ,
mum antero-posterior diameter M *
mum medio-lateral diameter M** .
mum antero-posterior diameter M
mum medio-lateral diameter M4
mum antero-posterior diameter lower caniniform
mum medio-lateral diameter lower caniniform
mum antero-posterior diameter M.
2     3
* M and M identification uncertain
	14.3
	20.5
	15-4
	22.2
	11.0
	17.0
14.2	
11.2	
15.0	15.2
1981
PLIOMETANASTES PROTISTVS
TABLE 1.  Continued
left
right
Maximum medio-lateral diameter M.
Maximum antero-posterior diameter M„
Maximum medio-lateral diameter M_
Maximum antero-posterior diameter M,
Maximum medio-lateral diameter M,
20.9	21.1
15.0	15-5
22.3	22.3
16.2	16.9
19.0	19-6
lam
Figure h.    Pliometanastes protistus,   left humerus UCMP 97371.
anterior view; B. posterior view
10
PALEOBIOS
NO. 36
Scapula - The fragmentary left scapula (Figure 5) includes the glenoid
articulation and part of the coraco-acromial arch.
The glenoid fossa is ovate as in Megalonyx  but is proportionately
narrower medio-laterally than in M.   jeffersoni   (Leidy, 1855)-  The
coraco-acromial arch is thick, rugose, and pitted by nutritive foramina
below the level of the glenoid cavity. The subscapular surface contains
at least two prominent ridges which extend toward the dorsal border.
The posterior margin is marked by a prominent ridge for attachment of
the M.  M.   teres  and txn-oeps.
Measurements of the scapula: greatest antero-posterior diameter of
the glenoid cavity 58.5 mm; greatest medio-lateral width of the glenoid
cavity 36.4 mm.
Figure 5- Pliometanastes protistus,   left scapula, UCMP 97371. A.
outer view;  B. view of proximal end.
Humerus - The left humerus (Figure k),  which lacks a third of the
posterior side of shaft, is proportionately less robust and the shaft
is relatively slimmer, with narrower epiphyses, than in Megalonyx.     The
deltoid and pectoral crests extend about 2/3 of the way down the shaft,
eis in Megalonyx.     The median ridge between the deltoid and pectoral
ridges is not as prominent and curves more toward the pectoral crests
1981
PLIOMETANASTES PROTISTUS
11
than in the type.  The bridge of bone roofing the entepicondylar
foramen is much narrower than in either type or in Megalonyx  (Stock,
1925, pi- 19).  The proximal and distal ends are similar in shape to
those of Megalonyx.
Measurements of the humerus:  greatest length 332.5 mm; width
across the tuberosities approximately 85.0 mm; greatest width at the
middle of the shaft 47.8 mm; thickness of the shaft at the end of the
deltoid crest, 47.1 mm; greatest width of the distal end 135.2 mm;
width of the distal troclea 74.4 mm.
Carpals - A complete right cuneiform, lunar and unciform, and a
damaged magnum are preserved.
Cuneiform - The cuneiform (Figure 6) is irregularly pyramidal,
approaching cuboidal, but is less elongate than in Megalonyx jeffersoni
(Leidy, 1855, pi. VIII).  Unl ike Megalonyx,   the ulnar facet is concavi*
rather than convex. The pisiform facet is larger and in closer proxi-
mity to the lunar facet than in Megalonyx.
Measurements of the cuneiform:  greatest dorso-palmar diameter
parallel to the ulnar facet 32.0 mm; greatest depth of the dorsal
margin measured perpendicular to the ulnar facet 26.7 mm.
Lunar - The shape of the lunar (Figure 6) is similar to that of M.
jeffersoni   (Leidy, 1855, pi. VIII).  The major difference is the more
elongate projection of the bone on its palmar side which articulates
distally with the magnum and medially with the scaphoid.
Measurements of the lunar: greatest diameter across the radial
facet 35-4 mm; greatest width across the radial facet 24.8 mm;
greatest diameter from the radial facet to the palmar side of the
magnum facet 39-0 mm.
Unciform - The shape of the unciform (Figure 6) is similar to that
of M.   jeffersoni   (Leidy, 1855, pi. VIII). The facet for the cuneiform
is more angular than in Megalonyx.     A slight transverse ridge crosses
the length of the cuneiform facet.  This slight angularity is also
reflected in the unciform facet of the cuneiform. The internal articu-
lar facet for magnum is quite narrow. The facet for Mc III is similar
to that of Megalonyx,   but the Mc IV facet is wider dorsally.
Measurements of the unciform: greatest medio-lateral diameter
across the cuneiform facet 32.5 mm; greatest depth from the cuneiform
facet to the palmar side of the metacarpal facet 28.5 mm.
Magnum - The magnum (Figure 6) is long and narrow   in the dorso-
palmar direction as in Megalonyx.     The unciform facet   is narrow and
presents a sigmoid surface along the contact with the   lunar facet.  The
magnum is too incomplete for further comparisons.
Metacarpals - Right Mc II and Mc IV are complete and are similar to the
McGehee metacarpals UF-FSM 31526 in all essential characters.
12
PALEOBIOS
NO. 36
1 cm
Figure 6. Pliometanastes protistus,   right carpals, UCMP 97371.  A.
palmar view, lunar, magnum, cuneiform and unciform; B.
radial view lunar;  C. proximal view cuneiform;  D. inter-
carpal view of cuneiform facet of unciform.
TABLE 2.  Measurements of the metacarpals (in millimeters).
Maximum length
Maximum depth of proximal end
Maximum width of proximal end
Minimum depth of shaft
Minimum width of shaft
Maximum width of distal end
Maximum depth of distal end
carpal   11	metacarp.
60.2	75.1
32.5	32.3
28.2	24.0
15.2	16.5
18.4	11.6
22.8	21.7
33.3	33.9
Patella - Both are present.  Compared with the patella from McGehee,
UF-FSM 9774, they are proportionately wider medio-laterally and
narrower dorso-ventrally across the femoral facet.  Also the medial
and lateral sides of the femoral facet are not as symmetrical and the
ventral projection distal to the femoral facet is narrower.  The inter-
nal side extends farther distally than the external side.
Measurements of the patella:  greatest length - left 83.5 mm,
right 84.5 mm; greatest width across the femoral facet - left 48.4 mm,
right 50.8 mm.
Tibia - The distal third of the right tibia lacks the astragalar spine
and the postero-medial projections and grooves for the flexor tendons.
The Knight's Ferry tibia is very similar in size and shape to the
tibia from the Hemphillian Mulhoi land site 2, V-3611, Contra Costa
1981
PLIOMETANASTES PROTISTUS
13
County, California (Stirton, 1939; Hirschfeld and Webb, 1968). The
articular facet for the astragalar spine on the tibia and the tibial
notch on the astragalus has been used to separate Pliometanastes  and
Megalonyx  (Hirschfeld and Webb, 1968).  Since the Knight's Ferry tibia
is missing the astragalar spine and no astragalus is preserved, the
variability in development of these characters in Pliometanastes  is
still not certain.  Since the Knight's Ferry tibia is so similar to the
Mulholland tibia which has a prominent spine, generic identification
should not be based on these characters alone.
Measurements of the tibia: greatest medio-lateral width of the
distal end 85.0 mm; greatest antero-posterior width of the distal end
44.5 mm.
Fibula -The complete left and distal end of the right fibulae are
preserved.  The shaft is essentially straight rather than curved pos-
teriorly, distal to the region for attachment of M.  peroneus brevis  as
in the McGehee fibula, UF-FSM 9446.  In Megalonyx,   this muscle scar is
not as strongly developed.  The Knight's Ferry fibula differs from the
McGehee fibula and resembles Megalonyx  in the shape of the proximal
half of the shaft which is triangular, with angles at the anterior,
lateral and posterior sides. The proximal end is antero-posteriorly
elongate and oval in shape, and is divided into three distinct regions
by muscle scars.  Posteriorly, the proximal end has a deeply depressed
area of muscle attachment presumably for M.  M.  peroneus  and soleus.
Externally there is a prominent raised area for the lateral ligament
and insertion of the M.  biceps femoris.    Antero-internally, the shaft
below the tibial facet is slightly depressed by the presumed scar of
fibular origin of M.   tibialis antiaus.     The distal end of the Knight's
Ferry fibula is similar in shape and proportions to the McGehee fibula.
Compared with the fibula of Megalonyx,  UCMP 40102, (Hempillian)
from the Hemme Hills, V-5053, Contra Costa County California, the
Knight's Ferry fibula differs as follows:  it is slightly shorter and
considerably less robust; the shaft is straight and narrower; the
proximal and distal ends are considerably less massive; and the distal
tibial facet forms a more acute angle with the astragalar facet. The
shape of the proximal end is similar in the two specimens. However, in
UCMP 40102, the scar for lateral ligament and M.   biceps femoris   is
twice as large as in the Knight's Ferry fibula.
Measurements of the left fibula:  greatest length 222.0 mm; maximum
antero-posterior diameter of the proximal end 42.3 mm; transverse dia-
meter of the proximal end perpendicular to the tibial facet 33-5 mm;
antero-posterior diameter of the distal end 45-3 mm; transverse diame-
ter of the distal end 47.1 mm.
Navicular - The left navicular is similar in size and shape to the
McGehee navicular UF-FSM 9474, but is slightly more pointed on plantar
side.  As in UF-FSM 9474, the ectocuneiform facet does not meet the
mesocuneiform facet.  The median notch between the cuneiform facets is
more prominent and the mammilate surface of the astragalar facet is
more rounded than in UF-FSM 9474.  The medio-lateral diameter of the
Knight's Ferry navicular is slightly less than the dorso-plantar dia-
meter.
14
PALEOBIOS
NO. 36
Measurements of the navicular: maximum diameter across the cunei-
form facets 42.4 mm; greatest length 44.2 mm; maximum thickness through
the astragalar convexity 21.5 mm.
Cuboid - The right cuboid (Figure 7) is irregularly cuboidal with
quadrilateral dorsal and plantar surfaces for ligamentous attachment,
Figure 7- Pliometanastes protistus, right cuboid, UCMP 97371. A. dor-
sal view;  B. distal view;  C. calcaneal view.
as in Megalonyx.     The calcaneal facet is ovate, approaching circular,
and meets the astragalar facet which is slightly concave and subtrian-
gular.  The inner border of the cuboid is formed by a narrow, poorly
defined navicular facet which is continuous with the astragalar facet.
This surface is also continuous with the ectocuneiform facet. The
astragalar facet is set off from the ectocuneiform facet by a distinct
angle.  Distally, the facet for metatarsals IV and V is triangular and
slightly concave. The articulating surface for the metatarsals is
deeper than wide, while in Megalonyx  it is wider than deep.
Measurements of the cuboid: maximum diameter from the astragalar
to the metatarsal facets 30.0 mm; maximum diameter across the meta-
tarsal facets 33-5 mm.
Metatarsal IV - The complete left Mt IV is nearly identical to UF-FSM
11572 from McGehee, differing only in the slightly shorter and more
rounded facet for Mt III. The facet for Mt V is not raised above the
postero-plantar surface but is flush with it.  The distal carina is
slightly longer than in UF-FSM 11572.
Measurements of the metatarsal IV: greatest length 78.1 mm;
greatest depth of the proximal end 38.0 mm; greatest width of the
proximal end 30.0 mm; minimal depth of the shaft 17.5 mm; minimum width
of the shaft 16.7 mm; greatest width of the distal end 24.0 mm;
greatest depth of the distal end 39-8 mm.
EARLIEST OCCURRENCES OF PLIOMETANASTES  AND MEGALONYX
Marshall et.   at.   (1979) provide an excellent review of the occur-
1981
PILOMETANASTES PBOTISTUS
15
rence of Hemphillian ground sloths of North America.  The earliest
recorded megalonychid is from the Oshkosh Quarry, Garden County,
Nebraska.  No datable tuffs occur at this site.  However, the Oshkosh
site is considered stratigraphically lower than the Dalton Quarry site
at Greenwood Canyon, Cheyenne County, Nebraska for which a date of
9.310.8 m.y.b.p. was obtained (Marshall et.  al. ,   1979).
The Oshkosh Quarry specimen is a single tooth collected by the
South Party at Pit 1 in 1937 and given a field number 55-10-7-37 SP,
but no permanent number was assigned (George Corner, personal communi-
cation, 1979). Schultz and Stout (1948) note the occurrence in their
faunal list, but no description of the specimen or illustration has
been given.  Unfortunately, the tooth has been lost (George Corner,
pe rsona1 commun i ca t i on, 1979)•
Identification of the Oshkosh Quarry tooth as Megalonyx  is very
questionable.  Since Pliometanastes  was not yet recognized, Megalonyx
was the only identified North American megalonychid.  The tooth was
probably a molariform because a caniniform of either Megalonyx  or Plio-
metanastes  would be sufficiently different from Pleistocene Megalonyx
caniniforms that it would surely have been noted and described.  If it
were a molariform, the similarity to Megalonyx  is so striking that it
probably would not be recognized as a different genus or even distin-
guishable from Pliometanastes.    The Oshkosh Quarry sloth must therefore
be considered as an indeterminate megalonychid.  Unless the tooth is
found and can be reexamined, the Knight's Ferry sloth is the earliest
undoubted occurrence of Pliometanastes  at 8.19 +. 0.16 m.y.b.p. (Hugh
Wagner, personal communication, 1980).  At present, Pliometanastes
occurs earlier than any positively identified Megalonyx which appears
in the Late Hemphillian. M.  mathisi  is the most conservative species
of the genus.  Because so few of the early occurrences of North
American megalonychids have been generically determined, there is still
a question of how close the relationship is between Megalonyx  and Plio-
metanastes,  whether Pliometanastes  is the ancestor of Megalonyx  or
whether they are sister taxa derived from a pre-Hemphi11ian megalony-
chid.  It is hoped that future discoveries and more precise absolute
dating will provide the answer.
16
PALEOBIOS
NO. 36
LITERATURE CITED
Hirschfeld, S.E. and S.D. Webb, 1968.  PIio-Pleistocene megalonychid
sloths of North America.  Fla. State Mus., Biol. Ser., Bull.  12:
213-296.
Leidy, J., 1855.  A memoir on the extinct sloth tribe of North America.
Smithson. Contrib. Knowl.  7:1-68.
Marshall, L.G., R.F. Butler, R.E. Drake, G.H. Curtis and R.H. Tedford,
1979. Calibration of the Great American Interchange. Sci. 204:
272-279.
Schultz, C.B. and T.M. Stout, 1948.  Pleistocene mammals and terraces
in the Great Plains.  Geo!. Soc. Amer. Bull.  59:553-577.
Stirton, R.A., 1939-  Cenozoic mammalian remains from the San Francisco
Bay region.  Univ. Calif. Publ. Geol. Sci., Bull.  24:399-410.
Stock, C., 1925.  Cenozoic gravigrade edentates of western North Ameri-
ca.  Publ. Carnegie Inst. Wash.  331:1-206.